Yearly report (1/7 2003-30/6 2004) for the Centre of competence combustion processes at Lund University Bengt Johansson [email protected] CONTENTS INTRODUCTION............................................................................................................................................... 1 LAYOUT OF THIS REPORT................................................................................................................................... 1 BASIC FACTS: PARTICIPATING UNIVERSITY DIVISIONS AND INDUSTRY PARTNERS, STAFF, AND ECONOMIC ACCOUNTING ..................................................................................................................................................... 1 BUDGET ............................................................................................................................................................ 5 CENTRE MANAGEMENT ..................................................................................................................................... 6 HCCI ENGINE RESEARCH (ATAC ENGINE) ............................................................................................. 7 SUMMARY ......................................................................................................................................................... 7 BASIC ENGINE STUDIES ..................................................................................................................................... 8 Personnel..................................................................................................................................................... 8 Multipoint Ion Current Measurements ........................................................................................................ 8 LASER DIAGNOSTICS ....................................................................................................................................... 12 Personnel................................................................................................................................................... 12 Background................................................................................................................................................ 12 High speed formaldehyde visualization ..................................................................................................... 14 Optical Diagnostics of Laser-Induced and Spark Plug-Assisted HCCI Combustion ................................ 19 References.................................................................................................................................................. 31 Papers published ....................................................................................................................................... 31 COMBUSTION CONTROL .................................................................................................................................. 32 Persons involved........................................................................................................................................ 32 Experimental setup .................................................................................................................................... 32 Studies performed during the year............................................................................................................. 33 Future work ............................................................................................................................................... 35 CHEMICAL KINETICS MODELING ACTIVITIES ................................................................................................... 36 Personnel................................................................................................................................................... 36 Full cycle simulations with coupled kinetics ............................................................................................. 36 Modeling and investigation of exothermic centers in HCCI combustion .................................................. 36 Calculations of hydroxyl radicals and formaldehyde and comparisons to LIF-measurements................. 37 NO, NO2 and N2O in HCCI combustion................................................................................................... 37 UNORTHODOX OTTOENGINE ................................................................................................................... 42 PERSONNEL ..................................................................................................................................................... 42 BACKGROUND ................................................................................................................................................. 42 EXPERIMENTAL SETUP .................................................................................................................................... 42 LOAD REGIME ................................................................................................................................................. 43 INFLUENCE OF INTAKE TEMPERATURE............................................................................................................. 45 3000rpm..................................................................................................................................................... 45 4000rpm..................................................................................................................................................... 46 CYLINDER TO CYLINDER VARIATIONS AND CYCLIC INFLUENCES ................................................................... 47 SPARK ASSISTANCE ......................................................................................................................................... 49 PAPERS ............................................................................................................................................................ 51 LASER DIAGNOSTICS IN CAR ENGINE................................................................................................... 52 PISTON TEMPERATURE MEASUREMENT BY USE OF THERMOGRAPHIC PHOSPHORS ........................................... 52 Introduction ............................................................................................................................................... 52 Experimental.............................................................................................................................................. 52 Results........................................................................................................................................................ 54 Conclusion ................................................................................................................................................. 57 REFERENCES ................................................................................................................................................... 57 TWO DIMENSIONAL EQUIVALENCE RATIO IMAGING IN FLAMES ....................................................................... 58 Calibration measurements......................................................................................................................... 58 Two dimensional imaging – Dual burner .................................................................................................. 59 Two dimensional imaging – Single burner ................................................................................................ 60 PRACTICAL DIAGNOSTICS ........................................................................................................................ 62 ENHANCEMENT OF SPATIAL RESOLUTION WITH OBSCURATION FOR LINE-OF-SIGHT TECHNIQUE ..................... 62 Experimental setup .................................................................................................................................... 62 Two dimensional measurements ................................................................................................................ 63 Point measurements................................................................................................................................... 63 Increased distance ..................................................................................................................................... 64 Conclusions ............................................................................................................................................... 64 FORMALDEHYDE MEASUREMENTS .................................................................................................................. 65 Introduction ............................................................................................................................................... 65 Combined formaldehyde and OH measurements....................................................................................... 65 Formaldehyde LIF spectroscopy ............................................................................................................... 68 Formaldehyde measurements in a DME diffusion flame........................................................................... 70 Formaldehyde detection during the pyrolysis of wood.............................................................................. 71 List of publications .................................................................................................................................... 72 BIOLÅG............................................................................................................................................................. 73 SUMMARY ....................................................................................................................................................... 73 FIXED BED COMBUSTION PROCESS IN BIOMASS BOILERS ............................................................................... 74 Personnel................................................................................................................................................... 74 Analysis of fixed bed boiler performance – fuel residence time ................................................................ 74 Modeling of biomass combustion in the fuel bed....................................................................................... 77 MODELING OF RADIATION HEAT TRANSFER IN FIXED BED BOILERS ................................................................. 79 Personnel................................................................................................................................................... 79 Development and validation of radiation heat transfer models................................................................. 79 INTERACTION BETWEEN THE TWO SUBPROJECTS ............................................................................................. 81 PUBLISHED RESULTS ....................................................................................................................................... 81 REFERENCES ................................................................................................................................................... 82 COMBUSTION IN NEW ATMOSPHERES.................................................................................................. 83 SUMMARY ....................................................................................................................................................... 83 BACKGROUND ................................................................................................................................................. 83 PRE-COMBUSTION PROCESSES:........................................................................................................................ 83 STATUS ........................................................................................................................................................... 84 Assessment of new atmospheres ................................................................................................................ 84 Experiments in atmospheric combustor..................................................................................................... 85 Experiments in High pressure facility ....................................................................................................... 85 Interaction with other projects .................................................................................................................. 86 Description of the test facilities ................................................................................................................. 86 THERMOACUSTICS ...................................................................................................................................... 89 SUMMARY ....................................................................................................................................................... 89 COMBUSTION INSTABILITY MODELING:........................................................................................................... 89 COMBUSTOR INSTABILITY MEASUREMENTS: ................................................................................................... 91 SHEAR-LAYER INSTABILITIES:......................................................................................................................... 91 Report Year 9 Competence centre combustion processes at Lund University (1/93) ____________________________________________________________________________ Introduction This is a report of the activities within the centre of competence combustion processes, KCFP, at Lund Institute of Technology at Lund University for the period 1/7 2003 to 30/6 2004. This is the ninth year of the ten year period of KCFP and the first year of the final twoyear contract period. The competence centre combustion processes is an organization funded by three equal partners; the Swedish energy administration, STEM, Lund University, LU and industry. Layout of this report This report has a description of the projects running within the centre. The activities for senior researchers are not explicitly given as they are involved in the projects with supervision of students and development of support systems. Each project is descried in some detail and some results from the time period are given. In summary all projects are running well and generate much interesting and in may cases world leading results. It is in some respect the harvesting time of the centre now after many years of basic development and some initial struggles to find a well working concept or work between industry and academia. Basic facts: Participating university divisions and industry partners, staff, and economic accounting University divisions The participating University Divisions, approximate number of employees, the heads of the divisions and a very short note on the respective research field follows: Combustion Physics - about 40 employees Head: Professor Marcus Aldén The Combustion Physics Division develops methods for laser diagnostics in combustion like rotation and vibration CARS, laser induced fluorescence, and polarization spectroscopy. It also deals with spark phenomena. A sub-group for Chemical Kinetics is headed by professor Fabian Mauss. Fluid Mechanics - about 20 employees Head: Professor Laszlo Fuchs The Fluid Mechanics Division research is directed towards numerical methods for modeling laminar/turbulent compressible/incompressible reacting/non-reacting flows. LES, Large Eddy Simulation is a specialty. Combustion Engines - about 25 employees Head: Professor Bengt Johansson The Combustion Engines Division performs Otto, Diesel, HCCI and Stirling engine research. It covers in-cylinder flow, fuel distribution, combustion and emission formation and their correlation to performance and efficiency. Report Year 9 Competence centre combustion processes at Lund University (2/93) ____________________________________________________________________________ Thermal Power Engineering - about 20 employees Head: Professor Tord Torisson In the Thermal Power Engineering Division new possibilities are studied for the use of fuel energy to generate power and heat. Steam and gas turbines play an important role, as well as low-emission combustion in furnaces, ovens and combustors. Promising combinations of fuel cells and gas turbines are studied. Heat Transfer - about 20 employees Head: Professor Bengt Sundén Heat exchanger modeling and design, especially with modified surfaces for enhanced heat transfer Participating industrial partners The participating industry partners are listed below, and their areas of interest in the Centre are indicated. In Sweden Sydkraft is the second largest electric power company, Siemens and Volvo Aero are the two largest gas turbine manufacturers, Volvo Powertrain and Scania the two largest heavy-duty truck engine manufacturers, Volvo Technology is Volvo’s research company, Volvo Car is the largest car engine manufacturer and Saab Automobile Powertrain is the other. Caterpillar and Cummins are well-known engine manufacturers in USA, and Toyota Motor Corporation, Nissan and Hino Motors likewise well-known engine manufacturers in Japan. Siemens combustion in large gas turbines for power generation Caterpillar Inc. ATAC engines Cummins Inc. ATAC engine Hino Motors, Ltd. ATAC engines in heavy-duty trucks Nissan Motors ATAC engines Saab Automobile Powertrain AB ATAC engines in passenger cars SCANIA CV AB ATAC for heavy-duty trucks and buses Sydkraft AB emissions and efficiency in thermal power stations Toyota Motor Corporation ATAC engines in passenger cars Volvo Aero Corporation combustion in small gas turbines for power generation and vehicles Volvo Car Corporation development of laser diagnostics for passenger car engines, ATAC engines and combustion in Otto engines Report Year 9 Competence centre combustion processes at Lund University (3/93) ____________________________________________________________________________ AB Volvo Penta ATAC in cogeneration sets Volvo Technology improved understanding of combustion phenomena in engines, development of laser diagnostics in engines Volvo Powertrain Corporation ATAC for heavy-duty trucks and buses Report Year 9 Competence centre combustion processes at Lund University (4/93) ____________________________________________________________________________ University research staff 1 Department of Physics Division of Combustion Physics Marcus Aldén professor Fabian Mauss professor Zhongshan Li research assistant Frederik Ossler research assistant Felix Barreras guest researcher Xiao Bai PhD student Per Amnéus PhD student Axel Franke PhD student Mattias Richter PhD student Martin Rupinski PhD student 30% 55% 100% 30% 100% 100% 100% 20% 100% 50% Department of Heat and Power Engineering Division of Fluid Mechanics Laszlo Fuchs professor Xue-Song Bai professor Johan Revstedt senior researcher Fabrice Guillard senior researcher Doru Caraeni senior researcher Mirela Caraeni researcher Jonas Holmborn PhD student Vladimir Milosavlevic ABB Alstom Power Lars-Erik Eriksson Volvo Aero 25% 30% 85% 60% 20% 100% 100% 20% 10% Division of Combustion Engines Bengt Johansson professor Per Tunestål assistant professor Roland Pfeiffer PhD Student Andreas Vressner PhD Student Håkan Persson PhD Student Jan-Ola Olsson PhD student Tommy Petersen electronic engineer Bertil Andersson mechanical engineer Tom Hademark engineer Jan Erik Everitt research engineer 100% 100% 100% 100% 100% 50% 25% 100% 50% 15% Division of Thermal Power Engineering Tord Torisson professor Rolf Gabrielsson adjunct professor Jens Klingmann assistant professor Fredrik Hermann PhD student 20% 10% 60% 100% 1 As of end of phase 3 i.e. start of 2003 Report Year 9 Competence centre combustion processes at Lund University (5/93) ____________________________________________________________________________ Budget During Phase 4 the following companies will financially support the Competence Centre for Combustion Processes in the following way: Support/yr in kSEK for 2 yrs Company Specification Cash In kind Alstom Power Sweden AB Thermoacoustics in gas turbines 250 600 Combustion in new atmospheres 100 Caterpillar Inc. ATAC engine 244* (€26.6) Cummins Inc. ATAC engine 244* (€26.6) Hino Motors, Ltd ATAC engine 244* (€26.6) Fiat-GM Powertrain ATAC engine 133 Nissan ATAC engine 551 Scania CV AB ATAC engine 133 150 Sydkraft AB Practical laser diagnostics 480 Thermoacoustics in gas turbines 250 Combustion in new atmospheres 100 Biomass furnace low value gas 300 200 Toyota Motor Corporation ATAC engine 244* (€26.6) Volvo Aero Corporation Combustion in new atmospheres 125 150 AB Volvo Penta ATAC engine 44 Volvo Powertrain Corp. ATAC engine 133 Volvo Car Corporation Unorthodox methods for compression ignition 150 200 ATAC engine 133 LIF in Otto engines 500 Volvo Technology AB PIV development 350 _______________________________________________________________________ Sum 4 258 1 750 * with exchange rate 9.18 SEK/€ as of May 19, 2003 Important remark #1: The contributions to the ATAC engine project are 44 % of the respective company contribution. The rest, 56 %, goes formally to a parallel project sponsored by other government money through the Swedish Gas Center. Important remark #2: In-kind contributions with question mark after are not confirmed by the respective company. Content can be own work in direct connection with the Centre project, engines, parts, laboratory equipment, etc Report Year 9 Competence centre combustion processes at Lund University (6/93) ____________________________________________________________________________ Centre management The Centre board consists of Lars Sjunnesson Sydkraft AB Marcus Aldén, professor Lund University Tord Torisson, professor Lund University Christian Troger ALSTOM Power Sweden AB Sören Udd Volvo Powertrain Corporation Ove Backlund Volvo Car Corporation Rolf Gabrielsson Volvo Aero Corporation Tommy Björkqvist Saab Automobile Powertrain AB Jörgen Held 2 Swedish Energy Agency, STEM Centre Director is Bengt Johansson, professor of Combustion Engines Project managers are Marcus Aldén, professor Fabian Mauss, professor Bengt Johansson, professor Rolf Gabrielsson, adjunct professor Laszlo Fuchs, professor Xue-Song Bai, professor 2 Jörgen Held was replaced by Christina Bergström when Jören left STEM (chairman) Report Year 9 Competence centre combustion processes at Lund University (7/93) ____________________________________________________________________________ HCCI engine research (ATAC engine) Summary This is an activity report for the period 1/7 2003 to 30/6 2004 for the HCCI engine project with the centre of competence combustion processes at Lund University. The project started 1/7 1997 and did a restart 2000 and 2003. Thus this is a report of the first year of this third period of research. The project is divided into four areas: Basic engine studies, laser diagnostics, engine control and chemical kinetics modelling. Each area has a full time student and the control area also have a post-doc. For the basic engine studies the focus has been on ion current signal and multiple ion measurements within a single cylinder have been conducted. The student, Andreas Vressner, has also spent some time running the optical engine during those experiments. The laser diagnostics have been on laser and spark assisted HCCI. It was shown that some form of pre-reactions exists with spark or laser triggering. High speed LIF of formaldehyde has used to distinguish between traditional HCCI and triggered. The control work has focused on MIMO system description and some work on the effects of ethanol on low temperature reactions has been done. Jan-Ola Olsson got his Ph.D. in February 2004 and this successor, Roland Pfeiffer has been working mainly with studies and getting to know the engine system. The kinetic simulations have been very productive with results in four different areas. Simulation of the inhomogeneous location of formaldehyde and OH could give some understanding of the process and could well predict the results seen in the laser experiments. The formation of N2O was modelled and also the formation of exothermic centres. Coupling between kinetic simulations and full engine cycle was also performed. The results during the year were presented at two reference group meetings, January 17 and August 19 with attendance from the sponsors. The PowerPoint’s from those meetings can be found at the unofficial homepage of the project: http://130.235.81.104/ce/sufm with the password l1m1ted This webpage also contains the reports from all the previous meetings since 1997. Report Year 9 Competence centre combustion processes at Lund University (8/93) ____________________________________________________________________________ Basic engine studies Personnel This work was conducted by Ph.D. student Andreas Vressner under supervision of Professor Bengt Johansson. and Ass. Prof. Per Tunestål and Anders Hultqvist. Multipoint Ion Current Measurements Interest in ion current sensing for HCCI combustion arises when a feedback signal from some sort of combustion sensor is needed in order to determine the state of the combustion process. Ion current sensors in the form of spark plugs can be used instead of expensive piezoelectric transducers for HCCI combustion sensing. However, the ion current phenomenon is local compared to the pressure which is global in the combustion chamber, depending on the state of combustion on the location of the spark plug. In order to investigate these differences between different locations in the combustion chamber, ion current were measured at seven locations simultaneously using a spacer placed between the engine block and the engine head. In addition to the seven ion current sensors an piezo electric pressure transducer was placed in the centre of the combustion chamber in order to compare the pressure history with the ion current signals. The experimental setup with 6 spark plugs mounted in the spacer with an angular division of 60 degrees can be seen in Figure 1. Figure 1: Location of the 6 spark plugs mounted in the spacer. One additional spark plug and one pressure transducer were mounted in the engine head. The engine used for these experiments was a Volvo TD100 diesel engine in single cylinder operation. Rich mixtures with the dilution of EGR were used in order to achieve high ion current signal strengths. Modified spark plugs were used with the side electrode removed in order to measure ion current over a larger gas volume resulting in higher signal strength. To ensure that there were no manufacturing defects or variances in the spark plugs themselves, the spark plugs were switched to different locations and replaced. Report Year 9 Competence centre combustion processes at Lund University (9/93) ____________________________________________________________________________ 50 Ion Channel 1 Ion Channel 2 Ion Channel 3 Ion Channel 4 Ion Channel 5 Ion Channel 6 Ion Channel 7 45 Pressure 0.8 0.6 0.4 40 0.2 35 0 30 0 5 10 Crank Angle Position 15 Ion Current [μA] Pressure [Bar] 55 -0.2 20 Figure 2: Example of seven ion current traces and a pressure trace over CAD for a single cycle. Figure 2 shows the difference in signal between the different ion current measuring positions, both in maximum amplitude and in timing. As seen the differences are significant. Ion current timing is referred to the crank angle position where half of the peak amplitude is located. 50 Channel 1 Channel 2 Channel 3 Channel 4 Channel 5 Channel 6 Channel 7 2 1.5 40 30 EGR rate 1 20 0.5 10 0 1 EGR Rate [%] Ion Current Amplitude [μA] 2.5 0 2 2.5 3 λ Figure 3: Ion current amplitude over Lambda for a EGR sweep with constant amount of fuel. Mean values of 500 cycles. 1.5 In Figure 3 the ion current amplitude over lambda can be seen. The fuel amount was kept constant and lambda was decreased by the dilution of EGR. The ion current amplitude slowly increases with decreasing lambda but when approaching stoichiometric proportions the amplitude rises rapidly. This behavior can be either temperature or chemistry related or Report Year 9 Competence centre combustion processes at Lund University (10/93) ____________________________________________________________________________ both. It can also be seen that there are channels that seem to have higher signal strength than others depending on location and since these values are based on the mean of 500 cycles the conclusion is that there are certain positions in the combustion chamber that are more appropriate for good ion current measurements with high signal strength. The ion current timing over CAD can be seen in Figure 4 where the crank angle of 50% burned is also plotted. As can be seen there are a good correlation in constant offset between the ion current timing and the combustion phasing in terms of CA50 depending on spark plug position. The same channels that have the highest amplitude also have the earliest timing. Ion Current Timing [CAD] 8 7 6 5 4 Channel 1 Channel 2 Channel 3 Channel 4 Channel 5 Channel 6 Channel 7 CA50% 3 2 1 1.5 2 2.5 3 λ Figure 4: Ion current timing over Lambda for a EGR sweep with constant amount of fuel. Mean values of 500 cycles. In Figure 5 the peak ion current amplitudes are shown versus CA50 for a timing sweep from 8 to 0 CAD ATDC. As can be seen the amplitudes increases when the auto-ignition starts earlier in the combustion process. It is however not clear why the amplitude decreases again when CA50 is advanced beyond 1-2 CAD ATDC. A hypothesis could be that the maximum gas temperature peaks when CA50 is at 1-2 CAD ATDC and that the most efficient combustion timing is at this CAD. Figure 6 shows the ion current timing compared to CA50 for the timing sweep from 8 to 0 degrees. As have been seen before there is an almost constant offset between the ion current timing and CA50. Depending on the ion current measuring locations the offset is about 1 to 1.5 degrees to CA50. Report Year 9 Competence centre combustion processes at Lund University (11/93) ____________________________________________________________________________ 1.8 Channel 1 Channel 2 Channel 3 Channel 4 Channel 5 Channel 6 Channel 7 Ion Current Amplitude [μA] 1.6 1.4 1.2 1 0.8 0.6 0.4 0.2 0 2 4 Timing [CAD] 6 8 Figure 5: Ion current amplitudes for a timing sweep without EGR. Mean values of 500 cycles. Ion Current Timing [CAD] 10 8 6 4 Channel 1 Channel 2 Channel 3 Channel 4 Channel 5 Channel 6 Channel 7 CA50% 2 0 -2 0 2 Desired Timing 4 [CAD] 6 8 Figure 6: Ion current timing information for a timing sweep without EGR Mean values of 500 cycles. Report Year 9 Competence centre combustion processes at Lund University (12/93) ____________________________________________________________________________ Laser diagnostics Personnel The multi-Yag experiments were conducted by Jimmy Olofsson and Hans Seyfried. The laser assisted experiments with schlieren and chemiluminescence imaging were conducted by Martin Weinrotter, Max Lackner and Kurt Iskra. The engine was run by Andreas Vressner. Background The Scania D12 Engine The engine used for the laser diagnostics experiments was an inline six-cylinder, 2.0 liter/cylinder Scania D12 diesel engine, converted to single cylinder HCCI operation. The engine was run on 80% iso-octane and 20% n-heptane. The compression ratio was 11.2:1 and the inlet air was preheated using an electrical heater. In order to provide for optical access the engine was equipped with an elongated piston and a 30 mm high quartz liner. The engine was also modified to operate with port-fuel injection, which generates a principally homogeneous charge. A picture of the engine can be seen in Figure 7. Figure 7: Experimental setup with the Scania D12 engine with the Bowditch extension. Multi-YAG laser cluster The laser cluster consists of four standard flash-lamp pumped Nd:YAG lasers (BMI/CSFThomson, France). The laser cluster is illustrated in Figure 8. Each laser consists of a Q- Report Year 9 Competence centre combustion processes at Lund University (13/93) ____________________________________________________________________________ switched oscillator and a single amplifier. By switching the Q-switch twice during the flashlamp pumping each of the Nd:YAG lasers is fired twice with a short time separation. The beams from the four lasers are combined using the scheme illustrated in Figure 8. The many optical components in the beam path of the first lasers introduces energy losses and beam profile degradation, however, the energy losses can be compensated for by increasing the flash-lamp pumping intensity of these lasers. After the beam combining optics there is an option to insert a fourth harmonic generation crystal, converting 532 nm to 355 nm. The four individual lasers can be fired in series with any time delay up to 100 ms, corresponding to the overall repetition frequency of 10 Hz. The time separation between two double pulses from one of the lasers can be adjusted from 25 μs to 145 μs. It is limited by the length of the flash-lamp pumping pulse and the gain build up time in the cavity. By interleaving the double pulses from the four lasers the time separation between pulses can be reduced down to 6.25 μs (=25μs/4) when equal time separations are desired. The laser-pulses do not have to be equally spaced, however, since each laser timing is individually controllable. Figure 8: This illustration shows the principal layout of the multi-yag laser and its patented beam combining system. Detector The detector is a custom modified high speed CCD camera (Imacon 468, DRS Hadland, UK). Extra triggering features and an optional image intensifier at its input have been added. The high framing speed is achieved by exposing eight individual CCDs sequentially, using short exposure times. A schematic overview of the camera is shown in Figure 9. The high speed camera employs a single optical input with subsequent split up to the individual CCD detectors. An eight-facet pyramid beam splitter is used to divide the incoming light into eight separate optical paths. The individual CCD modules consist of an image intensifier and a CCD image sensor. The signals from the CCDs are digitized to 8 bits and then stored in digital framestores within the camera, and transferred to the controlling computer via a fiber optic link. The possibility of using different gains for different CCDs allows events to be captured where the signal intensity varies strongly in time, as the dynamic range between images is increased. Report Year 9 Competence centre combustion processes at Lund University (14/93) ____________________________________________________________________________ The optional image intensifier is attached at the front of the optical system. It increases the sensitivity of the camera by up to two orders of magnitude. It also makes it UV sensitive by converting the incoming UV light to visible light, which is transmitted and detected by the original optical system. Figure 9: The high-speed Hadland framing camera. The schematic layout of the eight ICCD cameras is shown to the left. High speed formaldehyde visualization High-speed laser diagnostic was utilized for single-cycle resolved studies of the formaldehyde distribution in the combustion chamber of an HCCI engine. Planar Laser Induced Fluorescence, PLIF, with an excitation wavelength of 355 nm, was used for visualization of the formaldehyde molecule. By using laser pulses with time separations as short as 70 μs (1/2 CAD), cycle-resolved image sequences of the formaldehyde distribution were obtained. Thus, with this technique it is possible to follow the formaldehyde formation and consumption processes within a single cycle. For a more complete spatial analysis formaldehyde images were acquired in both vertical and horizontal cross sections of the cylinder. Formaldehyde is usually formed as an intermediate species when combusting hydrocarbons. The formation occurs through low temperature oxidation in an early phase of the ignition process. The generated formaldehyde is then being consumed in the following combustion process. Formaldehyde is also associated with the low temperature reactions that occur when certain mixtures of hydrocarbon fuels (In this case nheptane) and air are close to the explosion limit. Hence, the low temperature reactions (coolflames) and the early phase of the main heat release in an HCCI engine can be investigate by probing the formation and consumption of formaldehyde. The cycle to cycle variations in terms of in-cylinder pressure are known to be reasonably small, but the spatial variations of the combustion from one cycle to another can be quite significant. The heat release when running HCCI is also known to be very rapid. This combination of fast consumption and jittering combustion phasing introduces an averaging effect when performing conventional LIF measurements where one image is recorded per cycle. In order to really capture the details of such processes it is necessary to follow a single cycle event. Optical setup For the formaldehyde measurements in the Scania D12 engine, the laser beam from the Nd:YAG laser cluster was formed into a horizontal sheet by using a combination of a Report Year 9 Competence centre combustion processes at Lund University (15/93) ____________________________________________________________________________ negative cylindrical lens (f=-100 mm) and a spherical lens (f=+500 mm). The dimensions of the laser sheet, which was positioned in the center of the quartz cylinder liner, were 40x0.4 mm2. The energy per pulse was approximately 45 mJ at 355 nm, and had a pulse separation of 69 μs (1/2 CAD). The fluorescence signal from the formaldehyde was imaged perpendicular to the sheet, through the quartz piston via a UV enhanced mirror in the piston extension onto the high-speed framing camera. In front of the camera two optical filters were mounted, a long-pass filter, GG385, to eliminate the laser scattering at 355 nm, and a shortpass filter with a cut-off wavelength at 500 nm to avoid interference from the remaining 532 nm laser light. Also an achromatic quartz lens (f=100 mm, f#=2) was placed in front of the framing camera. The basic setup is in illustrated in Figure 10. Figure 10: Schematic experimental setup for High-speed Formaldehyde PLIF imaging in the optical HCCI-engine. Results Seven pulse LIF sequences of the formaldehyde distribution within one cycle in the HCCI engine were recorded. The combustion development was studied in terms of the rate of formaldehyde formation and consumption for different load conditions, i.e. by varying the stoichiometry (lambda value). In Figure 11 image sequences of seven images showing the formaldehyde consumption within one cycle event for λ=4.5, λ=4.0, and λ=3.5, respectively are presented. In the images the formaldehyde consumption can be followed within the main combustion process for the different running conditions. Regions where formaldehyde consumption has occurred appear dark, while bright regions indicate the presence of formaldehyde. From the images it is evident that the formaldehyde consumption becomes more rapid as the fuel mixture becomes richer. This is in agreement with traditional heat release analysis which shows an increased rate of heat release for richer mixtures. It can also be seen that the consumption process Report Year 9 Competence centre combustion processes at Lund University (16/93) ____________________________________________________________________________ occurs at an earlier stage for fuel richer mixtures. From earlier experiments it is known that the ignition temperature decreases slightly with lambda. This implies that the combustion phasing becomes more advanced for richer mixtures if the inlet temperature is kept constant. The spatial structure of the formaldehyde distribution during the consumption phase looks quite similar to what was found in an earlier investigation using high-speed fuel tracer LIF [1]. In that paper it was shown how the combustion progressed through distributed reactions throughout the entire bulk volume and this without traditional flame front propagation. Supporting those results, the formaldehyde images also clearly show that the ignition occurs at multiple points simultaneously and as can be seen, also the consumption of formaldehyde is lacking normal flame propagation. a b c Figure 11: A sequence of seven PLIF images showing the formaldehyde consumption is depicted for each the three stoichiometries λ=4.5 (a), λ=4.0 (b), and λ=3.5 (c). For the purpose of studying the formaldehyde formation and consumption in the engine, the surface fraction of LIF signal covering the imaged area was determined. To determine the surface fraction binary images, rather than grayscale images, are desired. Binary images were accomplished by applying a threshold to the grayscale images, marking image areas containing LIF signal as white, and leaving areas lacking signal as black. In Figure 12 an example is shown for the formaldehyde formation running the engine at λ=4.5. It should be noted that thresholding does not always provide a perfect representation of the signal areas in the images. However, even in the case of λ=4.5, which is the leanest mixture used in the present study and thus the formaldehyde LIF signal to noise ratio is the lowest, thresholding still gives an acceptable result. Report Year 9 Competence centre combustion processes at Lund University (17/93) ____________________________________________________________________________ Figure 12: Binary images (b) were accomplished by applying a threshold to the grayscale images (a), marking image areas containing LIF signal as white, and leaving areas lacking signal as black. An example is shown for the formaldehyde formation running the engine at λ= 4.5. The cycle-resolved surface fraction was plotted together with the corresponding rate of heat release curve for each of the four stoichiometries studied. In Figure 13 this is shown for the formaldehyde formation. Figure 14 shows the cycle-resolved surface fractions and the corresponding heat release curves for the formaldehyde consumption. In Figure 13 one can see how the phasing of the cool flame changes with lambda. With increased amount of fuel the cool flame combustion phasing advances until a certain point about 18- 20 CAD BTDC where it remains. Note that the combustion duration for the cool flame seems to be constant independent of load. Since more fuel is injected the heat release rate during the cool flame increase. In spite of this, the amount of heat released during the cool flame period compared to the heat released during the main combustion is lower with higher load. In Figure 14 it can be seen how the surface fraction with formaldehyde signal shrinks with increasing rate for lower lambda values. Earlier experiments performed with one image captured per engine cycle revealed similar trends. However, by employing high-speed single-cycle resolved measurements the averaging effect introduced by the jittering of the combustion phasing from cycle-to-cycle is avoided. Report Year 9 Competence centre combustion processes at Lund University (18/93) ____________________________________________________________________________ Figure 13: The cycle-resolved surface fraction acquired at formaldehyde formation is shown together with the corresponding rate of heat release curve for each of the four stoichiometries studied. Figure 14: The cycle-resolved surface fraction acquired at formaldehyde consumption is shown together with the corresponding rate of heat release curve for each of the four stoichiometries studied. Report Year 9 Competence centre combustion processes at Lund University (19/93) ____________________________________________________________________________ Optical Diagnostics of Laser-Induced and Spark Plug-Assisted HCCI Combustion HCCI, laser-assisted HCCI and spark plug-assisted HCCI combustion was studied experimentally in a modified single cylinder truck-size Scania D12 engine equipped with a quartz liner and quartz piston crown for optical access. The aim of this study was to find out how and to what extent the spark, generated to influence or even trigger the onset of ignition, influences the auto-ignition process or whether primarily normal compression-induced ignition remains prevailing. By using optical diagnostic methods, namely PLIF (Method described in the section above), Schlieren photography and chemiluminescence imaging, differences in the combustion process could be evaluated. Ignition Laser A Q-switched, flashlamp pumped Nd:YAG solid-state laser (5 ns pulse duration, 25 mJ pulse energy) was employed to generate a plasma inside the combustion chamber of the engine for each cycle. The laser pulse could be released at arbitrary crank angle positions and any rotation speeds by applying a special trigger interface. However, normally the speed was kept constant at 1200 rpm during the whole measurement requiring a constant pulse repetition rate of 10 Hz. A three-lens system (Extension-(1), collimating-(3), focusing-lens (4)) focused the beam through a quartz liner into the center of the cylinder, about 1 mm below the cylinder head (see Fig. 15). An additional cylindrical lens (2) had to be employed to compensate the effect of the quartz liner. Schlieren Imaging Schlieren photography was conducted in the plane of the focal spot of the igniting laser or the spark plug. As the index of refraction in gases is strongly dependent on density, areas with gradients of temperatures or pressures have considerable effect on the propagation of light. As a consequence, the refraction angle is proportional to the first derivate of these parameters. The experimental setup I is depicted in Fig. 15. Collimated light from a high pressure Mercury discharge lamp was directed through the combustion chamber. The focusing lens (11) has two functions: i) it focuses the unscattered part of the incoming parallel light and ii) it images a real and inverted picture of the scattered light on the CCD chip of an intensified camera. Now the parallel part of the beam is cut out by placing an aperture (12) in the focal region in a way that it covers the focused light but enables the scattered part to pass around the focal area. Hence, regions with high temperature or pressure refract the parallel light and it can pass the aperture. To compensate the negative cylindrical lens effect of the quartz liner providing optical access to the engine, a compensating positive cylindrical lens (6) was inserted in the beam path before the cylinder. The difficulties due to engine motion restricted optical access and distortion resulted in a somewhat reduced image size and quality, but nevertheless the effect of laser ignition on the onset of combustion could be clearly observed. Report Year 9 Competence centre combustion processes at Lund University (20/93) ____________________________________________________________________________ Figure 15: Experimental setup I; Schlieren / PLIF (vertical laser sheet): 1-Extension lens, 2-Cylinder lens(Ignition laser), 3-Collimating lens, 4-Focusing lens(f=100mm), 5Filter (long pass cutoff 600nm), 6-Cylinder lens (Schlieren), 7-Cylinder lens (PLIF), 8Focusing lens (PLIF), 9-Beamsplitter (reflecting 400-500nm), 10-Beamsplitter (reflecting 670nm), 11-Focusing lens (Schlieren), 12-Schlieren aperture (Knife-edge) Figure 16: Experimental setup II; Chemiluminescence / PLIF (horizontal laser sheet): 1-Extension lens, 2-Cylinder lens (Ignition laser), 3-Collimating lens, 4-Focussing lens (f=100mm), 7-Cylinder lens (PLIF), 8-Focussing lens (PLIF), 13-Beam splitter (reflecting 308nm) Report Year 9 Competence centre combustion processes at Lund University (21/93) ____________________________________________________________________________ Chemiluminescence imaging Chemiluminescence imaging was performed by viewing through the quartz bottom of the piston and a 45° mirror using the Bowditch extension scheme (see Figure 7). A schematic drawing of the experimental setup II can be found in Figure 14. The imaging system consisted of an intensified camera system equipped with UV optics and a dielectric coated beamsplitter (13), reflecting chemiluminescence emission around 308 nm while allowing passage for the fluorescence light of formaldehyde lying in the spectral range 400 nm and longer. The chemiluminescence images were recorded with an exposure time of 100 µs (0.72 CAD (Crank Angle Degree)) unless otherwise stated. Spectral investigation of the emitted radiation revealed a distinct signature around 308 nm originating in the A-X emission band of OH and a broad underlying unstructured spectrum which can be attributed to carbon monoxide. Therefore, the images reveal not the exact location of the flame front but give a good overview of the occurrence and development of flame kernels in the HCCI combustion process. The fuel used for all these tests was 80% iso-octane and 20% n-heptane. Unsupported HCCI A sequence of pictures recorded by PLIF imaging with vertical laser sheet (experimental setup I) during one particular cycle can be seen in Figure 17. The pictures reveal the multipoint flame development structure during a combustion sequence. In all picture sequences of unsupported HCCI the formaldehyde consumption, which can be taken as a marker of the starting combustion, begins near the cylinder walls (left and right side of the pictures) like it can be observed in Figure 17. Figure 17: PLIF picture sequence; unsupported HCCI; vertical laser sheet (experimental setup I); λ = 2.8; first picture at 11.4° CA ATDC (After Top Dead Center); interval 0.5° CA Report Year 9 Competence centre combustion processes at Lund University (22/93) ____________________________________________________________________________ Figure 18 shows the corresponding heat release (solid line) to the PLIF picture sequence in Figure 17 measured during one cycle. The dots on the solid line indicate the CAD when the PLIF pictures were taken. The dashed line shows the average heat release of 100 cycles; 50 cycles before the heat release for the PLIF pictures (solid line) was taken and 50 cycles after. In this picture it can be clearly seen, that the cycle to cycle variations have been quite large. As a result of this no clear difference in combustion characteristics (combustion timing, maximum pressure) could be observed between unsupported-, laser assisted- and spark plugassisted HCCI combustion mode. Figure 18: Heat release; unsupported HCCI; λ=2.8; solid line corresponds to the PLIF picture series in Figure 17; dots indicate the CAD when the PLIF pictures are taken; dashed line: average of 100 cycles, 50 taken before and 50 after the heat release indicated by the solid line The resulting density and temperature gradients throughout the volume cause the even more grainy structure in the Schlieren image presented in Figure 19. Such images were recorded one by one for every cycle, shifting the delay between consecutive images. The images of the Schlieren setup cover a region of approximately 2x2 cm where the spark plug or the laser plasma is located later on. Figure 19: Schlieren picture; unsupported HCCI; (experimental setup I); λ = 2.8; image size: 2 x 2 cm; at 13° CA ATDC Report Year 9 Competence centre combustion processes at Lund University (23/93) ____________________________________________________________________________ Figure 20 shows a PLIF picture sequence viewed through the quartz piston crown via the 45° mirror, like depicted in the experimental setup II in Figure 16 applying a horizontal PLIF laser sheet. Hence these pictures are perpendicular to the ones in Figure 17. Again, it can be seen that the combustion (formaldehyde consumption, dark regions) starts near the cylinder walls and proceeds through the unburned mixture. Figure 20: PLIF picture sequence; unsupported HCCI; horizontal laser sheet (experimental setup II; λ=2.8; first picture at 11.4 CA ATDC; interval 0.5°CA Through the beamsplitter (13 in Figure 16) it was possible to take chemiluminescence pictures at the same time like the PLIF pictures were taken as shown e.g. in Figure 20. In Figure 21, the whole combustion area can be seen from the bottom via the 45° mirror. The pictures are yielding just averaged intensity information over the line of sight (i.e. the vertical axis of the cylinder). The combustion starts at some points mostly close to the wall like in the PLIF pictures (Figure 20), but the exact locations show erratic behavior from Report Year 9 Competence centre combustion processes at Lund University (24/93) ____________________________________________________________________________ cycle to cycle. Consecutively, flame development starts at more and more points until the whole mixture is consumed. Both, the arbitrary locations of ignition and the fast heat release typical for HCCI are clearly visible in these sequences. Figure 21: Chemiluminescence pictures (308 nm); unsupported HCCI; (experimental setup II); image size: 14x14 cm; λ=2.8 Report Year 9 Competence centre combustion processes at Lund University (25/93) ____________________________________________________________________________ Laser-assisted HCCI The second part of the experiments involved HCCI combustion for which starting was attempted by a laser-induced plasma at 25°CA BTDC. Earlier ignition timing was also tested, but no effect could bee seen in the images. The laser pulse energy was about 25 mJ and the pulse length was 5 ns. The air/fuel equivalence ratio λ had the value 2.8. With leaner mixtures than λ=2.8 no effect of the laser or spark plug ignition could be seen. Figure 22 depicts a PLIF picture sequence with a vertical laser sheet (experimental setup I). In contrast to the unsupported HCCI case in Figure 17, a clear “flame front” structure can be seen, which can be attributed to be caused by the ignition plasma. This effect of the laser plasma on the onset of combustion was not as pronounced at all cycles. The PLIF pictures showed all degrees of influence of additional ignition from strong flame front propagation to no effect at all. This erratic behavior is a result of a high COV value like mentioned above and obscured the effect of laser ignition in the rate of heat release curves. Two reasons might contribute to these somewhat faint results: The ignition timing of the laser was too early, a fact determined by the limits of optical access. This problem could be overcome by using a laser entrance window in the cylinder head which was not available at that time. And the second feature unfavorable for pronounced supported ignition is the low octane number of the fuel used in these experiments. Figure 22: PLIF picture sequence; laser-assisted HCCI; vertical laser sheet (experimental setup I); λ=2.8; first picture at 11.4 °CA ATDC; interval 0.5°CA; ignition time: -25°CA BTDC Report Year 9 Competence centre combustion processes at Lund University (26/93) ____________________________________________________________________________ Figure 23: Heat release; laser-assisted HCCI; λ = 2.8; ignition time: -25° CA BTDC; solid line corresponds to the PLIF picture series in Fig. 17; dots indicate the CAD when the PLIF pictures are taken; dashed line: average of 100 cycles, 50 taken before and 50 after the heat release indicated by the solid line The corresponding heat release (solid line) to the above depicted PLIF picture sequence can be found in Figure 23. Like in Figure 18, the dots on the solid heat release indicate the CAD when the PLIF pictures were taken. The dashed line shows the average heat release of 100 cycles; 50 cycles before the heat release for the PLIF pictures (solid line) was taken and 50 cycles after. The PLIF picture sequence in Figure 24 taken through the quartz bottom piston via the 45° mirror shows again like the PLIF pictures in Figure 22 an expanding “flame front” structure. Besides that, also “normal” HCCI combustion starts from the left cylinder wall. Report Year 9 Competence centre combustion processes at Lund University (27/93) ____________________________________________________________________________ Figure 24: PLIF picture sequence; laser-assisted HCCI; horizontal laser sheet (experimental setup II); λ=2.8; first picture at 11.4 CAD ATDC; interval 0.5 CA; ignition time: -25°CA BTDC Report Year 9 Competence centre combustion processes at Lund University (28/93) ____________________________________________________________________________ In the chemiluminescence pictures in Figure 25 viewed through the quartz piston, also some kind of flame front can be observed. In comparison to the unsupported HCCI combustion case in Figure 21 where the combustion starts mainly near the cylinder walls, in these images an intensity peak can be observed in the middle of the cylinder. But it has to be pointed out that these pictures are taken from different cycles because the used intensified camera was only able to take one picture in a cycle. Figure 25: Chemiluminescence pictures (308 nm); laser-assisted HCCI; (experimental setup II); image size: 14x14 cm; λ=2.8; ignition time: -25°CA BTDC Report Year 9 Competence centre combustion processes at Lund University (29/93) ____________________________________________________________________________ Figure 26 shows Schlieren pictures through the quartz liner (experimental setup I). Again, it has to be taken into account that just one picture per cycle could be taken. But also in the Schlieren pictures it can be observed that the combustion is starting mainly from the middle of the combustion chamber in comparison to unsupported HCCI were the combustion starts mostly near the cylinder walls. Figure 26: Schlieren pictures; laser-assisted HCCI; (experimental setup I); image size: 2x2 cm; λ=2.8; ignition time: -25°CA BTDC Spark plug-assisted HCCI Beside laser-assisted HCCI also spark plug-assisted HCCI was investigated. The spark plug was positioned in the center of the cylinder head. The ignition timing was the same as for the laser-assisted case (25°CA BTDC) and the same air/fuel equivalence ratio of 2.8 was used. Like in the laser-assisted case, no effects of the spark in the images could be found if an earlier ignition time was chosen. Again something like an “expanding flame front structure” can be seen in the PLIF picture sequence with the vertical laser sheet (experimental setup I) in Figure 27. No big difference to the laser-assisted HCCI combustion case in Figure 22 could be observed. But again as a result of a big COV-value sometimes nearly no effect of the spark could be observed and sometimes it was possible. The same behavior for the spark plug-assisted HCCI can be found in the chemiluminescence pictures in Figure 28. Report Year 9 Competence centre combustion processes at Lund University (30/93) ____________________________________________________________________________ Figure 27: PLIF picture sequence; spark plug-assisted HCCI; vertical laser sheet (experimental setup I); λ=2.8; first picture at 11.4° CA ATDC; interval 0.5 CA; ignition time: -25°CA BTDC Figure 28: Chemiluminescence pictures (308 nm); spark plug-assisted HCCI; (experimental setup II); image size: 14x14 cm; λ=2.8; time: -25 CA BTDC Report Year 9 Competence centre combustion processes at Lund University (31/93) ____________________________________________________________________________ Finally the heat release of the spark plug-assisted HCCI combustion is depicted in Figure 29. Again, like in the other cases a big COV (Coefficient of Variation) can be observed. Hence, the conclusion of the heat release date’s is that the COV was too big so no clear difference in the combustion characteristics of the three different combustion modes can be extracted. Figure 29: Heat release; spark plug-assisted HCCI; λ=2.8; ignition time: -25°CA BTDC; solid line corresponds to the PLIF picture series in Figure 17; dots indicate the CAD when the PLIF pictures are taken; dashed line: average of 100 cycles, 50 taken before and 50 after the heat release indicated by the solid line References [1] Anders Hultqvist, Magnus Christensen, Bengt Johansson, Mattias Richter, Jenny Nygren, Johan Hult, Marcus Aldén: "The HCCI Combustion Process in a Single Cycle – High-Speed Fuel Tracer LIF and Chemiluminescence Imaging", SAE2002-01-0424 Papers published 1 A.Vressner, Andreas Vressner, Petter Strandh, Anders Hultqvist, Per Tunestål & Bengt Johansson: “Multiple Point Ion Current Diagnostics in an HCCI Engine”, SAE 2004-010934 2. J. Olofsson, H. Seyfried, M. Richter, M. Aldén, A. Vressner, A. Hultqvist ,B. Johansson, K. Lombaert: “High-Speed LIF Imaging for Cycle-Resolved Formaldehyde Visualization in HCCI Combustion”, SAE 05P-364 3. M. Weinrotter, E. Wintner, K. Iskra, T. Neger, J. Olofsson, H. Seyfried, M. Aldén, M. Lackner, F. Winter, A. Vressner, A. Hultqvist, B. Johansson: “Optical Diagnostics of LaserInduced and Spark Plug-Assisted HCCI Combustion “, SAE 05P-183 Report Year 9 Competence centre combustion processes at Lund University (32/93) ____________________________________________________________________________ Combustion Control Persons involved The combustion control work was mainly conducted by two PhD students, Jan-Ola Olsson and Roland Pfeiffer, under supervision by Assistant Professor Per Tunestål and Professor Bengt Johansson. Jan-Ola Olsson successfully defended his PhD thesis on February 27 2004 and now works for Volvo Car Corporation in Gothenburg. All experiments have been performed on the Scania D12 6-cylinder engine schematically represented by Figure 30. The engine is equipped with heaters and coolers for the intake air and a double port fuel injection system for combustion timing control. It also has a system to supply cooled EGR to the intake. Experimental setup All experiments have been performed on the Scania D12 6-cylinder engine schematically represented by Figure 30. The engine is equipped with heaters and coolers for the intake air and a double port fuel injection system for combustion timing control. It also has a system to supply cooled EGR to the intake. The figure shows the engine with two liquid fuel injection systems. During the year it was converted to operation with natural gas and n-heptane instead of ethanol and n-heptane as was previously the case. INTER COOLER WATER SEPARATOR TEMP VALVE Q AIR FLOW METER HEATERS TURBOCHARGER EGR COOLER EGR VALVE n-C7H16 C2H5OH ENGINE Figure 30: The Scania D12 6-cylinder engine converted to HCCI operation. Report Year 9 Competence centre combustion processes at Lund University (33/93) ____________________________________________________________________________ Studies performed during the year System identification Model based control design requires simple dynamic models relating measured variables as a function of time to control variables as a function of time. For some systems these dynamic models can be derived from first principles. In the case of HCCI ignition and combustion this is not the case. Detailed physical models require thousands of chemical kinetic mechanisms and species, and the models are too computationally expensive to use for control system design. Instead the approach of data driven modelling was taken. State-of-the-art system identification methods were applied to input/output data in order to identify the dynamics from fuel ratio changes, intake temperature, engine speed and FuelMEP to measured combustion timing (using pressure based heat release). The method involved superimposing PRBS (Pseudo Random Binary Sequence) disturbances of suitable amplitude on the steady state control values for each operating point. PRBS is a deterministic binary sequence with frequency characteristics resembling white noise. A dynamic model could then be extracted from the input/output data for each operating point. Figure 31 shows measured CA50 (combustion timing) during PRBS excitation of all inputs. The nonlinear behaviour of the system is indicated in the figure by the fact that the mean CA50 has changed from 12 to 13 degrees ATDC even though the excitation is symmetric. 18 14 o CA50 ( ATDC) 16 12 10 8 0 20 40 60 Cycle Index (−) 80 100 Figure 31: Raw CA50 data during excitation at 1500 rpm, net IMEP 8 bar. The straight lines shows the set point of 12° ATDC. Figure 32 to Figure 35 show Bode magnitude plots of the CA50 response to changes in fuel ratio at 1500 rpm and two different loads (3 and 8 bar IMEP) and two different combustion timings (early and nominal respectively). Particularly at the low load it can be seen that the low frequency gain increases as the combustion timing is retarded. This corresponds well to experience. Combustion gets more sensitive with later combustion timing. It is for example more difficult to manually dial in an operating point with retarded combustion timing than with advanced combustion timing. Another thing that can be observed is a slight increase in the high frequency gain with retarded timing. This means that the system reacts faster to changes in fuel ratio. One possible explanation is that there is less heat transfer to the walls with retarded timing and thus some of the slow wall heating dynamics is removed and a more direct response to input changes is observed. Magnitude Report Year 9 Competence centre combustion processes at Lund University (34/93) ____________________________________________________________________________ 10 10 0 −1 −2 10 −1 10 −1 Frequency [Cycle ] Figure 32: Bode magnitude plot from fuel ratio to CA50 at 1500 rpm, net IMEP 3 bar, CA50 -1°ATDC. Magnitude 0 10 −1 10 −2 10 −1 10 −1 Frequency [Cycle ] Figure 33: Bode magnitude plot from fuel ratio to CA50 at 1500 rpm, net IMEP 3 bar, CA50 3°ATDC. Report Year 9 Competence centre combustion processes at Lund University (35/93) ____________________________________________________________________________ Magnitude 0 10 −1 10 −2 10 −1 10 −1 Frequency [Cycle ] Figure 34: Bode magnitude plot from fuel ratio to CA50 at 1500 rpm, net IMEP 8 bar, CA50 7°ATDC. Magnitude 0 10 −1 10 10 −2 −1 10 −1 Frequency [Cycle ] Figure 35: Bode magnitude plot from fuel ratio to CA50 at 1500 rpm, net IMEP 8 bar, CA50 10°ATDC Future work In the next step the identified dynamic models will be used for model based control system development using systematic methods such as LQG (Linear Quadratic Gaussian) and MPC (Model Predictive Control) methods. Work in progress on improving the intake temperature control will also be completed next year. Report Year 9 Competence centre combustion processes at Lund University (36/93) ____________________________________________________________________________ Chemical kinetics modeling activities Personnel The kinetic and modeling work during 2002-07 to 2003-07 was performed by Dr. Per Amnéus, with support from PhD student Martin Tunér, both under executive supervision from Prof. Fabian Mauss Full cycle simulations with coupled kinetics The homogeneous HCCI model gives a description of the in-cylinder pressure and temperature, given an initial in-cylinder pressure and temperature for a certain crank angle position. However, there is no way of determining the in-cylinder temperature, pressure and fill-rate after valve closing from the inlet temperature and boost pressure. To allow for more advanced modeling of the fluid dynamics of the running engine cycles, meanwhile detailed chemical simulations of the HCCI ignition process is performed, the homogeneous HCCI model was coupled to the LTH-developed ESIM [1] as well as the commercial tool GTPower[2]. The work, which was started during 2002, was finalized during the end of 2003, showing some additional interesting characteristics. Figure 36 shows the measured and calculated pressure traces, proving a good agreement between measurements and calculations, although the heat release is much too fast, a property inherent in all homogeneous models. Figure 37 confirms this, showing the measured and calculated pressure-volume curve. Figure 38 and Figure 39 show a sweep over different PRF mixtures using a constant air fuel ratio. Figure 40 shows the calculated and measured indicated mean effective pressures plotted versus lambda (solid) and RON (dotted) for a sweep where the air-fuel ratio was varied between l = 2.34-5.93, meanwhile the ignition timing was kept constant by altering the octane number in a PRF mixture between RON = 0-95. Here the image shows some discrepancy between measurements and calculations note however that the calculations behave rather logically versus fuel amount in the cylinder, something the measurements do not. Figure 41 shows the calculated brake efficiency versus lambda and RON for the above mentioned set of cases. Figure 42 shows the calculated ignition timing for the 15 coupled engine cycles in the calculations. These starts at cycle number 86 since the first 85 cycles are uncoupled to ensure that the turbocharged system stabilizes. As seen, the first coupled cycle does quite often not ignite at all. Stable operation is obtained first at the third or fourth coupled cycle. In some cases bimodal cyclic variations are present, these has also been identified to occur in measurements under certain conditions. The reason for this behavior is a chemical feedback through chemically active species, most likely NO and H2O2, stored in the internal EGR during engine operation. Modeling and investigation of exothermic centers in HCCI combustion Investigations have been made using a stochastic reactor model of 200 particles on variations in the cylinder wall temperature, and its effects on gas temperature inhomogeneities. Subsequently the impact on gas temperature variations on self-ignition of the gas was investigated. The work describes a stochastic wall interaction model where cooling occur through stochastic interaction with other particles and the combustion chamber walls. By attributing different wall temperatures on different particles, the different wall temperatures of cylinder walls, piston and exhaust valve was described. Figure 43 and Figure 44shows the Report Year 9 Competence centre combustion processes at Lund University (37/93) ____________________________________________________________________________ individual particle temperatures for two different cases long before ignition (Figure 43, 80 CAD before TDC) and during ignition (Figure 44, 7 CAD before TDC). The first case (blue) has a uniform cylinder wall temperature, whereas in the second case (red) part of cylinder wall area has essentially higher temperature, which in a real engine could originate from for example a hotter exhaust valve. This work is under development [3]. Calculations of hydroxyl radicals and formaldehyde and comparisons to LIF-measurements In this work [4] calculations of concentrations of hydroxyl radicals and formaldehyde, performed using homogeneous and stochastic reactor models of an iso-octane / n-heptane fuelled HCCI engine, were compared to those obtained through LIF-measurements. A distinct peak of each of the two species was obtained, and the calculated separation in time between the two peaks was in qualitative good agreement with experiments, as seen in Figure 45. Time- and the initial temperature resolved concentrations of formaldehyde and hydroxyl radicals are presented, shown in Figure 46. Here it is shown that the formaldehyde concentrations grows slowly, contrary to the hydroxyl concentrations which are fast developed. The formaldehyde is locally fast consumed once the high temperature chemistry has started. The highest concentrations of formaldehyde were found in the cases when the low-temperature chemistry was never transitioned to high-temperature ignition. Particle resolved distributions of the concentrations of the two species were visualized through probability density plots (PDP) where a 15x15 mesh was set up, and the respective particles were sorted into a suitable slot in this mesh depending on its concentration of formaldehyde (x-wise) and hydroxyl radicals (y-wise), see Figure 47. The same procedure was done for the LIF-images, but here the 15x15 mesh was divided into intervals of rising signal intensity, and before sorting, the images were cleaned from noise using arithmetical conditional filtering. Thus the concentration PDPs could be compared to LIF-signal PDPs, showing that the agreement was as good.Once the model was validated, the combustion analysis that was performed through the LIF-measurements could be extended to very low concentrations of both analysed species, exceeding the 10% of maximum signal that is the resolution of the LIF-measurements. This allows for a deeper access the very early and very late stages of each phase of the combustion, where PDPs using local maxima as in Figure 48 could be analyzed and give their contribution to our understanding of the ignition process. NO, NO2 and N2O in HCCI combustion NOx chemistry was investigated for a low NOx operating HCCI engine [5]. It was found that for moderate NOx levels, N2O reactions play an important role in the NOx formation (Figure 50), while Zeldovich reactions as well as prompt NOx reactions rises in importance with rising peak temperature (Figure 51). N2O is also likely to appear in the exhaust gases of HCCI engines at levels up to several ppm. The levels of NOx in the exhausts are highly sensitive to peak temperature (Figure 52), however N2O has a weak negative dependence on temperature. Once the temperature effects of fuel rich operation are decoupled, the fuel rich conditions in itself has a favorable effect on low-NOx engine operation (Figure 53). It appears that conversion from NO to NO2 is favored by hydroperoxy radicals. Thus the high proportions of NO2 found in from some HCCI engines seems to be an effect from the simultaneous occurrence of hydrocarbons and NO, thus the high NO2 proportions can be traced back to high temperature inhomogeneities, poor mixing and slow overall combustion, properties often connected to poor combustion efficiency, as seen in Figure 54 showing the same calculation case using different wall temperatures in a stochastic reactor model. Report Year 9 Competence centre combustion processes at Lund University (38/93) ____________________________________________________________________________ 7 3000 Experimental pressure (99 cycle average) Calculated pressure 6 dH [J/dCAD] Pressure [Pa] 1 10 Heat release 8 10 2000 T [K] T 0D 56 T 0D 61 T 0D 66 T 0D 69 T 0D 75 T 0D 78 T 0D 82 T 0D 86 1800 1600 1400 2000 6 6 10 1200 1000 6 4 10 800 1000 600 -30 6 2 10 0 -50 0 -25 0 Crank angle 25 50 -25 -20 -15 -10 -5 CAD 0 5 10 Figure 38: Temperature trace from an octane sweep using homogeneous reactor model and GT-Power. 2000 T [K] T [K] T [K] T [K] T [K] T [K] T [K] 1800 Figure 36: Calculated and measured cylinder pressures at a region near top dead center. An average of 99 engine cycles is used for the experimental pressure values. Shown in the figure is also the heat release. 1600 56 66 75 78 82 86 1400 1200 1000 800 100 Cylinder pressure [bar] Measured pressure (99 cycle average) Calculated cylinder pressure 600 -30 -25 -20 -15 -10 -5 CAD 0 5 10 Figure 39: Temperature trace from an octane sweep using stochastic reactor model and GT-Power. 10 lo g λ 3 4 5 6 e x p e rim e n ta l IM E P c a lc u la tio n a l IM E P e x p e rim e n ta l IM E P c a lc u la tio n a l IM E P vs. R O N vs . R O N vs. λ vs . λ IMEP 5 4 1 0 0,5 1 1,5 3 Cylinder volume [dm ] 2 Figure 37: PV-diagram, comparison of calculated and experimental pressures for the entire engine cycle 3 2 RON 80 60 40 20 0 Figure 40: IMEP as functions of lambda (solid) and octane number (dotted) for experiments (blue) and calculations (red) Report Year 9 Competence centre combustion processes at Lund University (39/93) ____________________________________________________________________________ 2 3 lambda 4 Particle temperatures at -80 CAD 6 5 628 626 624 622 T[K] calculated brake efficiency [%] 35 30 620 618 616 brake efficiency vs. octane brake efficiency vs. lambda 614 25 0 20 40 60 octane num ber [RO N] Figure 41: Calculated brake efficiency as functions of RON number and fuel air ratio. 2 ignition crank angle 0 100 80 0 100 no 150 200 Figure 43: Individual particle temperatures in SRM model during compression phase for uniform wall temperature (blue) and cylinder wall area containing hotter regions (red). In the latter case some particles are heated by wall collisions, whereas in the former, all particles are cooled. λ = 5.93 λ = 3.72 λ = 3.25 λ = 2.86 λ = 2.34 -2 50 Particle temperatures at -7 CAD 1300 -4 T[K] 1250 -6 85 90 95 cycle number in calculations 100 Figure 42: Ignition timing (CA 50) of all coupled cycles during calculations. As seen, the bimodal cyclic variations known from experiments are detected even here. 1200 1150 0 50 100 150 200 no Figure 44: Individual particle temperatures in SRM model during ignition for uniform wall temperature (blue) and cylinder wall area containing hotter regions (red). Report Year 9 Competence centre combustion processes at Lund University (40/93) ____________________________________________________________________________ norm OH, exp. norm CH2O, exp. 7 norm OH, calc. norm. CH2O, calc. 6 5 4 3 2 1 0 -20 -10 0 10 20 Figure 45: Calculated (dotted) and measured (solid) OH (red) and CH2O (blue) Figure 47: Example of calculated PDP using total concentrations maxima normalized concentrations 30 OH CH2O 1 hea t re lea se 0,1 0 ,0 1 0 ,0 0 1 0,00 0 1 -2 0 -15 -1 0 -5 0 cra n k a ngle 5 10 [C A D ] 15 Figure 48: The verified model extends the attainable information from the LIFmeasurements 20 18 16 14 12 10 8 6 4 2 Figure 46: Start temperature and crank angle resolved concentration profiles of formaldehyde (blue) and hydroxyl (red) 2 4 6 8 10 12 14 16 18 Figure 49: Example of calculated PDP using local concentrations maxima 20 molefractions [ppm] Report Year 9 Competence centre combustion processes at Lund University (41/93) ____________________________________________________________________________ -5 3 10 -5 2 10 λ = 3.33 λ = 2.22 λ = 1.66 λ = 1.33 λ = 1.11 λ = 1.05 λ=1 λ = 0.95 -5 1 10 0 Figure 50: Flow analysis, φ=0.42 0 50 100 time [CAD] molefractions Figure 53: NOx levels for different air fuel ratios when the temperature effects are decoupled by using a read-in temperature trace NO case 3b NO2 case 3b NO case 3c NO2 case 3c -6 8 10 -6 6 10 -6 4 10 concentration [ppm] Figure 51: Flow analysis, φ=0.45, other parameters equal to previous case. -6 2 10 0 -100 10 -5 NOx, φ=0.42 N2O, φ=0.42 NOx, φ=0.45 N2O, φ=0.45 10 -6 0 50 time [CAD] 100 150 Figure 52: NO and N2O concentrations for the cases considered in Figure 15 and 16. -50 0 50 100 crank angle [CAD] 150 Figure 54: NO and NO2 for a calculation case in stochastic reactor model, using two different cylinder wall temperatures, proving that high NO2 proportions may origin from for example large inhomogeneities in temperature. Report Year 9 Competence centre combustion processes at Lund University (42/93) ____________________________________________________________________________ Unorthodox Ottoengine Personnel This work was conducted by Ph.D. student Håkan Persson under supervision of Professor Bengt Johansson. and Ass. Prof. Anders Hultqvist. Background Spark Ignition (SI) engines struggle with low efficiency at part load operation. A feasible way to improve low load efficiency for cars currently running with SI engines would be to use an engine that can run in HCCI mode at part load and switch to SI at high load6. Studies have reported high efficiency and low NOx emissions compared to the SI-engine 7. In this project an SI engine is run in HCCI mode through negative valve overlap (NVO). The work has resulted in two papers, one published in the SAE world congress 2004 and one submitted to the SAE world congress 2005. Experimental Setup The test engine (Figure 55) is an in-line six-cylinder engine with a total displacement of 2.9L (Table 1). It is a naturally aspirated engine with port fuel injection. Camshafts comprising low lift and short duration are used. The engine is equipped with cam phasing mechanisms, making it possible to phase both intake and exhaust cam during engine operation. This makes it possible to make mode switches from SI to HCCI by increasing the negative valve overlap, thus increasing the amount of residuals, and vice versa from HCCI to SI. The engine is equipped with an air heat exchanger installed upstream from the throttle. The heat exchanger can be operated with either hot or cold tap water making it possible to either heat or cool the intake air. Table 1 Engine data Displacement Number of cylinders Bore Stroke Compression ratio Valve lift Inlet/Exhaust valve duration VVT/ CAM Fuel 2922 cm3 6 83 mm 90 mm 10.3:1 3 mm 150 CAD 60 CAD RON 98 The engine is controlled by an in-house developed management system with the ability to set ignition timing, dwell, injection timing, injection duration, cam timing and inlet air temperature to desired values. The in-cylinder pressure is monitored separately for each cylinder using Kistler 6053C60 pressure transducers. The exhaust temperature is measured individually for each cylinder approximately 10 cm downstream of the exhaust ports, using type K thermocouples. The emission analysis equipment consists of a Flame Ionization Detector (FID) for measuring HC, a ChemiLuminecence Detector (CLD) for measuring Nitrogen Oxides (NOx), a Non Dispersive Infra Red (NDIR) detector for measuring carbon dioxide (CO2) and carbon monoxide (CO) and a paramagnetic detector for measuring oxygen (O2). Emissions are Report Year 9 Competence centre combustion processes at Lund University (43/93) ____________________________________________________________________________ measured individually, although not simultaneously for each cylinder close to the thermocouples as shown in Figure 55. Fuel consumption is measured by a fuel balance. Figure 55 Test engine with emission measurements separately for each cylinder Load regime To get the engine running in HCCI mode the amount of residuals must be sufficient to obtain auto ignition temperature. Therefore the engine must be started in SI mode and then a switch is done to HCCI combustion by phasing the camshafts to generate a larger negative valve overlap and thus increasing the amount of residuals. The negative valve overlap is accomplished by closing the Exhaust Valve (EVC) early in the exhaust stroke trapping hot residuals, followed by late Intake Valve Opening (IVO). In the initial tests (Figure 56) no variable valve timing (VVT) equipment was available so the camshafts couldn’t be phased during operation. At low speed, high load HCCI was achieved by starting the engine in SI mode with some negative valve overlap and then advancing the timing until the engine ran in spark assisted HCCI mode. When starting at higher speed the first cycle was still spark ignited without residual gas, however in the next cycle the cylinder contained enough trapped hot residuals so the engine ran directly in spark assisted HCCI. At high speed the temperature was high enough for HCCI combustion without spark assistance. Report Year 9 Competence centre combustion processes at Lund University (44/93) ____________________________________________________________________________ 4.5 4 4.5 HCCI Spark assisted HCCI 3.5 IMEPnet [Bar] IMEP (bar) 3.5 3 2.5 3 2.5 2 2 1.5 1.5 1 500 Unassisted HCCI Spark assisted HCCI 4 1000 1500 2000 2500 3000 3500 4000 Engine Speed (rev/min) Figure 56 Early operating regime 1 1000 1500 2000 2500 3000 Speed [rpm] 3500 4000 Figure 57 Operating regime with VVT With the VVT system it is possible to do a transition from SI to HCCI combustion at speeds up to about 2000rpm. For higher speed the amount of residuals for a given negative overlap increases and a smaller NVO than the VVT system can give is needed to run SI. The possible operating regime with VVT is presented in. Figure 57. The HCCI operating regime is limited by several factors. The combustion must be slowed down to avoid damaging in-cylinder pressure and pressure derivatives; this is done by diluting the mixture compared to the ordinary SI engine. In this case by trapping hot residuals in the cylinder. The short intake cam duration gives the HCCI engine a disadvantage at very high loads. Therefore HCCI operation is not very useful at high speed where the engine usually is run at high load. In this study measurements are made up to 4000rpm. It is most likely possible to obtain HCCI at higher speed, but already at 4000 rpm the indicated load is very low and comparable to the friction work. The lower load limit is set by high cycle-to-cycle variations and misfire. Passing the limit for spark assisted HCCI may result in misfire, if the mixture is highly diluted it will not ignite at all. As a result the next cycle will not have any residual gas but some of the fuel will still be present in the cylinder. The result is very fast combustion also known as knock. The reason for misfire in the low load region is basically that the residual gas is too cold for the mixture to self ignite when it is diluted, especially at low speed. At higher load the problem is the opposite, high load is restricted by the fast reaction rate. A negative valve overlap from 145 to 205 Crank Angle Degrees (CAD) is used for HCCI operation. For the points run with a large overlap the exhaust cam is phased somewhat further than the intake cam, not to predate on the compression stroke. An increased offset from a symmetric NVO is used for lower speed due to lower amounts of residuals for a given NVO. A cam with shorter duration would make it possible to run lower load at low speed, and then higher amounts of residuals could be trapped without predating on the compression stroke. Figure 58 shows Pumping losses (PMEP) that increase when asymmetric NVO is used at lower speed. A great difference between EVC and IVO lowers efficiency. Figure 59 shows the increased pumping work at low speed as a result of asymmetric NVO. It can also be seen that pumping losses increase also for symmetric NVO at very high speed. Report Year 9 Competence centre combustion processes at Lund University (45/93) ____________________________________________________________________________ 0.3 0.28 0.26 0.24 0.22 0.2 0.18 0 Low Load Medium Load High Load 0.28 PMEP [Bar] PMEP [Bar] 0.3 Low Load Medium Load High Load 0.26 0.24 0.22 0.2 5 10 15 20 EVC + IVO [CAD] 25 30 Figure 58 Offset in CAD from symmetric negative valve overlap 0.18 1000 1500 2000 2500 3000 Speed [rpm] 3500 4000 Figure 59 Pumping losses for different speed and load Influence of intake temperature The effect of changes in the intake temperature is studied to simulate realistic variations in ambient temperature. The effects on combustion stability and emissions are specially studied. Several points are tested by changing the intake temperature in the span from 15 to 50°C, tests are made both with spark assisted HCCI and without spark assistance. The intention is to study the effect of intake temperature at a given load. The points compared have the same load and negative overlap, but as a result of the change in temperature the AFR differs somewhat due to different air density. All points are run with a lean air/fuel mixture with lambda from 1.20 to 1.33. 3000rpm When running unassisted HCCI at 3000rpm, 2.6 bar IMEP with wide open throttle (WOT) and a 200 CAD negative valve overlap without the VVT the 35° decrease in the ambient temperature can be enough for misfire if the load is to be kept constant without enriching the mixture. Of course a richer mixture keeps the engine going but it also causes higher amounts of NOx, another problem is the fast reaction rate. For this reason there is a limit to how rich the mixture can be even if the emissions are disregarded. Figure 60 shows the later combustion timing for spark assisted HCCI with lowered intake temperature. For Unassisted HCCI at 50 degrees intake temperature CA50 was about 8.5. It is clearly shown in Figure 63 that the combustion stability is severely affected when lowering the inlet temperature also for the spark assisted mode. At this point when operating unassisted, some cylinders misfired and stopped when lowering the intake temperature, this is an indication that small cylinder to cylinder variations are much more influential close to the border of the load region. Figure 61 shows the later combustion and the lower ROHR with lower intake temperature for spark assisted HCCI. Figure 62 shows the same spark assisted points for 50 and 15 degrees inlet temperature, but also the single point without spark assistance run at 50 degrees intake temperature. Although this point is run at slightly higher load, it still has later timing and lower ROHR than spark assisted HCCI. This indicates that the spark can stabilize and govern the conditions for HCCI combustion. In Figure 63 the increase in COVimep and COVpmax is shown for spark assisted HCCI. Report Year 9 Competence centre combustion processes at Lund University (46/93) ____________________________________________________________________________ 40 Spark Assisted HCCI Cylinder Pressure [bar] CA50 [CAD ATDC] 9.5 9 8.5 8 60 50°C 30°C 15°C 30 45 20 30 10 15 7.5 20 30 40 Inlet Temperature [°C] 0 −15 50 Figure 60 CA50 as a function of intake temperature for spark assisted HCCI operation at 3000 rpm, 2.6 bar IMEP. 60 50°C 15°C 50°C Without Spark −5 0 5 10 Crank angle [CAD] 11 30 45 20 30 10 15 20 0 25 COVimep COVpmax 10 15 Rate of Heat Release [J/CAD] Cylinder Pressure [bar] 40 −10 Figure 61 Pressure traces and ROHR for cylinder 1 with different intake temperature at 3000 rpm, 2.6 bar IMEP for spark assisted HCCI. 9 8 COV 7 10 Rate of Heat Release [J/CAD] 10 7 6 5 4 3 0 −15 −10 −5 0 5 10 Crank angle [CAD] 15 20 0 25 Figure 62 Pressure traces and ROHR for cylinder 1 with different intake temperature at 3000 rpm, 2.6 bar IMEP, showing later timing without spark assistance. 2 10 20 30 40 Inlet Temperature [°C] 50 Figure 63 Increase of the COV as a function of lowered temperature for spark assisted HCCI at 3000rpm, 2.6 bar IMEP. 4000rpm At 4000 rpm the engine runs smoothly both with and without ignition and the effect of the inlet temperature decrease is small due to the higher amount of residuals, partially because of lower volumetric efficiency despite a decrease of the negative overlap to 190 degrees. CA50 is approximately between 5 and 5.5 ATDC as shown in Figure 64. Figure 65 shows ROHR for unassisted HCCI. The ROHR decreases for lower intake temperature, but the difference is very small. The difference between spark assisted and unassisted HCCI are more or less insignificant, this is also the case when comparing COV for spark assisted and unassisted HCCI, shown in Figure 66. Both COVimep and COVpmax are low and never exceed 3%. No increase in COV can be seen when lowering the inlet temperature at 4000rpm, this can be explained by the high amount of hot residuals that raises the temperature well above the Report Year 9 Competence centre combustion processes at Lund University (47/93) ____________________________________________________________________________ critical temperature. The emissions of NOx shown in Figure 67 are also relatively constant when the intake temperature is changed. Spark Assisted HCCI Unassisted HCCI 30 Cylinder Pressure [bar] CA50 [CAD ATDC] 6.5 6 5.5 5 4.5 4 20 30 40 Inlet Temperature [°C] Figure 64 CA50 as a function of intake temperature for both spark assisted and unassisted HCCI operation at 4000rpm, 1.6 bar IMEP. 4 2.5 2 20 20 15 15 10 10 5 5 −10 −5 0 5 10 15 Crank Angle [CAD] 0.06 ISNOx [g/kWh] COV 3 25 20 0 25 Figure 65 Pressure traces and ROHR for cylinder 1 with different intake temperature at 4000 rpm, 1.6 bar IMEP, without spark assistance COVimep Spark Assisted HCCI COVimep Unassisted HCCI COVpmax Spark Assisted HCCI COVpmax Unassisted HCCI 3.5 30 25 0 −15 50 35 50°C 30°C 15°C Rate of Heat Release [J/CAD] 35 7 Spark Assisted HCCI Unassited HCCI 0.05 0.04 1.5 1 20 30 40 Inlet Temperature [°C] 50 Figure 66 COV as a function of intake temperature for both spark assisted and unassisted HCCI operation at 4000 rpm, 1.6 bar IMEP. 0.03 20 30 40 Inlet Temperature [°C] 50 Figure 67 NOx as a function of intake temperature for both spark assisted and unassisted HCCI operation at 4000rpm, 1.6 bar IMEP. Cylinder to Cylinder Variations and Cyclic Influences The effect of cylinder-to-cylinder deviations and also the cycle-to-cycle coupling and variations are important issues because these effects limit the obtainable working region. In this work, the effects are studied in an engine with low compression ratio that relies on trapped hot residuals to attain auto ignition temperature. Due to the hot residuals the combustion in one cycle can affect the next in a much more extensive way than an HCCI engine run with little or no residual gas. Sweeps have been made both in Speed and load; Figure 68 shows two sweeps in speed for different loads. The difference in IMEP is between 0.1 and 0.3 Bar, from cylinder to cylinder. For all cases cylinder 5 has a slightly higher load while cylinder 3 runs at a slightly lower load. In general it can be seen that the difference in load is small despite that no cylinder balancing Report Year 9 Competence centre combustion processes at Lund University (48/93) ____________________________________________________________________________ is applied. Figure 69 shows CA50 for all cylinders, it can be seen that there is a greater spread between the cylinders at lower speed. At low speed and load the end cylinders has the latest timing, this can be explained by lower wall temperature The effect is however not as prominent for cylinder 1 and not for any of the above at higher load and speed. This could be explained by tuning effects also having great impact, combined with a higher charge temperature at higher speed, the earlier timing also indicates this. Cyl 1 Cyl 2 Cyl 3 Cyl 4 Cyl 5 Cyl 6 IMEP net [Bar] 4 3.5 3 2.5 2 Cyl 1 Cyl 2 Cyl 3 Cyl 4 Cyl 5 Cyl 6 10 8 6 4 2 1.5 1 1000 12 CA50 [CAD] 4.5 1500 2000 2500 3000 Speed [r/min] 3500 Figure 68 IMEP for all cylinders for a sweep in speed at two different loads 4000 0 1000 1500 2000 2500 3000 Speed [rpm] 3500 4000 Figure 69 CA50 for all cylinders for a low load sweep in speed When the temperature conditions are low for HCCI with trapped residuals, periodic behavior has been observed, i.e. a late burning cycle due to low charge temperature generates high charge temperature for the next cycle resulting in early combustion timing generating low charge temperature and so on. One way of observing this phenomenon is to look at a correlation coefficient between two consecutive cycles. The Correlation Coefficient (R) is a linear correlation coefficient between two parameters (i,j), in this paper its between CA50 from cycle i to CA50 in cycle j. C is the covariance. If R = 0 then no correlation is found. If R = -1 then a perfect inverted linear correlation is found which implies that a high value is followed by a low etc. C (i, j ) R (i, j ) = C (i, i ) × C ( j , j ) Figure 70 Shows R for the whole sweep in speed at low load. Between 2000 and 3500rpm some correlation can be seen. Especially at 3000rpm the correlation for cylinder 2, 4 and 5 is very high, these cylinders also have high COVimep. Cylinder 3 has on the other hand also high COVimep, but a low R, so a high COV does not have to imply a high S correlation, but vice versa A correlation investigation is also done with an increased offset in cycles for a vector containing 100 consecutive measured cycles; this is shown in Figure 71. The figure shows cylinder 5 for the measuring point at 3000rpm with an R of approximately –0.8, which is a strong negative correlation. Here an auto correlation is done for an offset of up to 55 cycles. It is obvious that the largest correlation is for an offset of one cycle, which is the same as shown in Figure 70. For an increased offset in cycles the correlation effect is dampened out. Although the combustion timing fluctuates back and forth from one cycle to the next some slower variations in combustion timing due to things like wall temperatures will decrease the correlation. Still a higher correlation is recognized again for higher offsets. This is an indication of a disruption in the cycle-to-cycle behaviour, i.e. the periodic behaviour ceases and starts again, Report Year 9 Competence centre combustion processes at Lund University (49/93) ____________________________________________________________________________ and can then be in counter phase to the earlier periodic behaviour. It should be noted though, that the correlation for higher time separation is overall low. The reason for the self-extinction of the periodic phenomena is that when the changes in combustion timing gets so high, the very late combustion timing will no longer increase the charge temperature for the next cycle. Instead the next cycle will also burn late. Here the phenomena can be extinct or start again in counter phase to the earlier behaviour. If the amplitude gets even higher it will not be self stabilizing, instead of a very late combustion, misfire will occur. A cross-correlation between the different cylinders for an increased time separation in cycles is conducted, also looking at CA50. The correlation from one cylinder to another cylinder in the next cycle is negligible. No communications between the cylinders have been observed in terms of combustion timing. 0.6 Correlation [CA50] 0.4 0.2 0 −0.2 −0.4 −0.6 −0.8 1000 Cyl 1 Cyl 2 Cyl 3 Cyl 4 Cyl 5 Cyl 6 1500 2000 2500 3000 Speed [rpm] 3500 4000 Figure 70 Correlation between cycle n and cycle n+1 Figure 71 Correlation between cycle n and cycle n+k, k= 1:55 Spark assistance At lower speed heat losses increase, therefore the lower load limit is at a higher IMEP. Under some conditions, the working region can be extended by spark assistance. The spark-assisted area of the load regime is shown in Figure 57. Effects of spark assistance have been observed up to 3000 rpm. On the borderline between where ordinary HCCI combustion and spark assisted HCCI takes place the temperature after compression is at the limit for reaching auto ignition. Figure 72 shows the influence on CA50 by going from unassisted HCCI to spark assisted HCCI with advanced spark timing. Unassisted HCCI is to the left in the figure and advanced timing further to the right. This sweep is done for different speeds at the limit for unassisted HCCI. The point at 1000rpm is spark assisted only. The effect is more prominent at lower speed, which is also at a higher load with a lower amount of residuals. For the sweep at 2500 and 3000 rpm there is no obvious change in CA50, however, at 2500rpm, the load could be lowered further with a spark timing of 40 CAD BTDC. Without spark assistance the combustion timing retarded with random early burning cycles followed by misfire. Of course the lack of trapped residuals then hinder the charge from reaching auto ignition temperature again. At 1000rpm, which is past the point where it is possible to run without spark assistance, later spark timing than 20 CAD BTDC was not possible due to misfire from random cylinders from time to time, this due to cold in-cylinder conditions. Figure 72 shows a great advance in CA50 from 20 to 40 CAD, and then from 40 to 50 the effect is weaker. The possibility to run some cylinders without spark assistance for a couple of cycles suggest that a large part of the combustion is in HCCI mode. This is supported by the case with spark set off at 40 CAD Report Year 9 Competence centre combustion processes at Lund University (50/93) ____________________________________________________________________________ BTDC, generating conditions for HCCI combustion with a fast reaction rate. Rate of heat release is shown in Figure 73. For the case with late spark timing the reaction rate is slower due to later combustion timing. Also for the case with very early timing the rate of heat release is lower suggesting more effect of spark assistance, and some initial SI combustion. CA50 [CAD ATDC] 14 12 50 1.7bar @ 3000rpm 2.1bar @ 2500rpm 3.1bar @ 2000rpm 3.4bar @ 1500rpm 3.4bar @ 1000rpm Rate of Heat Release [J/CAD] 16 10 8 6 20° 40° 50° 40 30 20 10 0 4 − 0 20 Spark Angle [CAD] 40 50 Figure 72 Influence on engine average on CA50 for unassisted HCCI and for spark assisted HCCI with advanced spark timing −20 −10 0 10 20 Crank Angle [CAD] 30 40 Figure 73 Rate of heat release for spark assisted HCCI at 1000rpm, 3.4 Bar IMEP There is a substantial increase in load for advanced spark timing, mainly due to the very late combustion with late spark timing; this is shown in Figure 74. The deviation from cylinder to cylinder in terms of load is below 6 %. An early spark doesn’t only improve efficiency it also lowers COVpmax and COVimep, COVimep shown in Figure 75. From this it can be established that an optimal ignition angle exists. With more measuring points it can be more well established, but from the measured points an ignition angle of 40 CAD BTDC seems to be the optimum. By using cylinder individual spark timing combustion timing for all cylinders can be balanced when running spark assisted at lower speed. 3.5 10 8 COVimep [%] IMEPnet [bar] 3.4 3.3 Cyl 1 Cyl 2 Cyl 3 Cyl 4 Cyl 5 Cyl 6 3.2 3.1 20 40 Spark Angle [CAD BTDC] 6 Cyl 1 Cyl 2 Cyl 3 Cyl 4 Cyl 5 Cyl 6 4 2 50 Figure 74 IMEP as a function of spark angle for spark assisted HCCI at 1000rpm 0 20 40 Spark Angle [CAD BTDC] 50 Figure 75 COVimep as a function of spark angle Report Year 9 Competence centre combustion processes at Lund University (51/93) ____________________________________________________________________________ Papers H. Persson, M. Agrell, J-O. Olsson, B. Johansson, H. Ström: “The Effect of Intake Temperature on HCCI Operation Using Negative Valve Overlap” SAE Paper 2004-010944 H. Persson, R. Pfeiffer, A. Hultqvist, B. Johansson , H. Ström: “Cylinder to Cylinder Variations and Cyclic Influences on HCCI Operation with Trapped Residuals”, Submitted to SAE WC 2005, Detroit Report Year 9 Competence centre combustion processes at Lund University (52/93) ____________________________________________________________________________ Laser diagnostics in car engine Piston temperature measurement by use of thermographic phosphors Introduction Different piston geometries are used in efforts to achieve more optimal mixing of fuel and air in Diesel engines. More efficient combustion can be achieved through better air utilisation by optimizing the shape of the combustion bowl. At the same time, it is highly important that the piston is designed so that the peak temperature does not become too high especially in the squish area, where cooling is particularly difficult. A conventional way of measuring temperatures on the piston is by mounting surface thermocouples at different fixed positions. This has certain drawbacks, however, such as the thermocouples being expensive to manufacture, their being located at only fixed positions and the crown geometry of the piston being difficult to change. Also the transfer of signals from the piston either by mechanical linkage or by telemetry is complicated. Through use of thermographic phosphors, the above mentioned drawbacks to piston temperature measurements in Diesel engines can be partially avoided. The aim of this investigation is to demonstrate how such measurements can be performed and to discuss differences between this method and the conventional thermocouple method. Experimental Experiments were conducted using a single-cylinder heavy-duty research engine, configured with a low compression ratio, a low swirl and a CR (Common Rail) fuel-injection system. An AVL 501 single cylinder research engine with the same cylinder head geometry as the Volvo Trucks D12C engine was used in the experiments. To allow for optical measurements to be performed, one exhaust valve was removed and replaced by a window. Note that this will impose some effects on the gas exchange and residual gas fraction in the cylinder. Two production pistons were used in the study. One, with a compression ratio of 18.5:1, was provided with thermocouples on the surface of the piston bowl and in the squish area. The second piston had a similar omega-shaped piston bowl but had no thermocouple instrumentation and had a lower compression ratio of 12:1. This piston was chosen for the experiments, using an endoscope and optical fibre access, because it allows for a fairly large engine operation range with very low soot emissions, i.e. only limited amounts of soot deposits on the window. The engine was configured with a common-rail fuel-injection system and an eight-hole orifice nozzle. Report Year 9 Competence centre combustion processes at Lund University (53/93) ____________________________________________________________________________ Figure 76 Experimental setup: The laser light is directed through the optical access to illuminate the phosphor coating on the top of the piston. A photomultiplier detector registers the phosphorescence signal through the same optical access A pulsed Nd:YAG laser (Spectra physics) was used to excite the phosphor particles. The third harmonic from the laser, at 355 nm, was directed into the combustion chamber with the aid of optical prisms and a dichroic mirror, see Figure 76. The mirror, positioned at a 45 degrees angle, strongly reflects the 355 nm laser light and transmits the visible light effectively. After excitation, the emission, which then occurred, was detected by a photomultiplier detector (Hamamatsu) and was digitised by a TDS 620 oscilloscope (Tektronix). The data was then stored on a PC for subsequent processing. To infer the temperature from the laser-induced phosphorescence (LIP) signal, the timeresolved signal was first fitted to an exponential decay curve to allow the lifetime to be estimated. The temperature could be determined then from previous calibration measurements and from the resulting lifetime. Report Year 9 Competence centre combustion processes at Lund University (54/93) ____________________________________________________________________________ Results Figure 77 Piston temperatures at a crank angle of 270 CAD. The temperature becomes stabilized about 2 min after the start of combustion Figure 78 Variations in piston temperature in case 2 as measured by the thermocouples Figure 77 shows the surface temperatures as measured with the Mg3FGeO4:Mn phosphor. The thermographic phosphor used has broad temperature sensitivity. It is suitable for a preliminary investigation of the actual temperature prior optimising the measurement accuracy with a more suitable phosphor. The temperature of the piston was measured in time at a fixed crack angle, 270 CAD. In figure 3 results from the thermocouple is presented. Comparing the two different measurement techniques in terms of the compression and expansion stroke values obtained for Report Year 9 Competence centre combustion processes at Lund University (55/93) ____________________________________________________________________________ the two different engine-load cases, indicates the results to be similar, the temperatures obtained using the phosphors being in the temperature region between the values obtained using thermocouples B1 and A2, respectively as can be seen in Figure 78. Figure 79 Temperature versus crank angle using La2O2S:Eu. Each box plot represents the data for 50 samples Figure 80 A RoHR curve showing how close combustion if is possible to perform measurements using the La2O2S:Eu phosphor Figure 79 shows the results for piston temperature using La2O2S:Eu. Use of this phosphor made it possible to come closer to combustion, up to 350 CAD and from 400 CAD on, making it possible to estimate the piston temperature close to the thermocouple peak temperatures of Report Year 9 Competence centre combustion processes at Lund University (56/93) ____________________________________________________________________________ about 380 CAD. The highest temperature was found to be about 263-264 oC at 400 CAD. After 400 CAD, the piston temperature decreased slowly to 450 CAD. For crank angles of 270 to 350 the piston temperature increased due to compression. The standard deviation of the data shown in figure 4 was between 0.5 and 1.5 oC. Figure 80 shows the rate of heat release calculated from the pressure trace sampled simultaneously with obtaining the 400 CAD phosphor measurements. Use of La2O2S:Eu allowed measurement to be performed close to combustion, at 400 CAD without getting disturbance from the high luminescent flames background. Figure 81 Fibre bundle used to transmit light into and out of the engine Figure 82 Signal-to-noise ratios and the evaluated temperatures at 270 CAD The potential of using optical fibres to transmit the laser light and the emission in and out was also tested. To this end, three fibres were used for excitation and another two for transmitting the subsequent emission to the detector, see Figure 81. The use of optical fibres largely facilitates the alignment of the optics and removes the need of an optical window. The detection limit was investigated, the photomultiplier gain being set to a constant value Report Year 9 Competence centre combustion processes at Lund University (57/93) ____________________________________________________________________________ throughout the tests. The tests showed the optical fibres to have successfully collected the signals emitted by the engine during a 450 s period, see Figure 82. The soot deposits began to be clearly visible after 60 seconds, the intensity of the signal decreasing continuously over time. The signal-to-noise ratio and the evaluated piston temperatures during the first 450 seconds were found to be acceptable. The gap visible in the figure is due to the sudden deposit of a large quantity of soot. In contrast, it was sometimes observed that the soot deposits on the window were reduced, resulting in the signal intensity increasing. After the first 450 seconds the signal-to-noise ratio deteriorated decreasing the measurement accuracy, only an increase in the photomultiplier gain being able to improve the results. The present investigation indicates thermographic phosphor technique to be potentially useful, despite the possibility of deposits on the collecting optics, although the measurement time possible is limited. Conclusion Piston temperature measurements in a heavy-duty Diesel engine were performed both by use of laser-induced phosphorescence and of thermocouples. The phosphorescence technique was applied to the engine by use of an optical window, and also by use of endoscopic access and of optical fibres. The endoscope and the optical fibres showed the potential of replacing the optical window, thus simplifying the experimental setup and allowing the experiments to be run continuously for a longer period of time. A comparison of measurements obtained using thermocouples and using the optical technique showed temperatures measured to be in a good agreement. The differences found can be attributed to measurement errors, soot deposits and the non-uniformity of the piston temperatures. Thermographic phosphor thermometry could be a viable alternative to the thermocouple method since it has the flexibility allowing it to be applied to various piston geometries. Both endoscopes and optical fibres make it possible to use the thermographic phosphor method in production-type engine applications, optical fibres in particular. In further development of the optical technique it could be fruitful to use optical fibres for the simultaneous temperature monitoring of a multi-cylinder engine. References Husberg, S.Girja, I.Denbratt, A.Omrane, M. Aldén and J. Engström: "Piston temperature measurement by use of thermographic phosphors and thermocouples in a heavy-duty Diesel engine run under partly premixed conditions", submitted to SAE World Congress 2005. F. Jarl "Enhancement of Spatial Resolution with Obscuration and Two Dimensional Equivalence Ratio Imaging", LRCP 99, Lund University, 2004. Report Year 9 Competence centre combustion processes at Lund University (58/93) ____________________________________________________________________________ Two dimensional equivalence ratio imaging in flames In this technique a linear dependence between OH*/CH* ratio and the equivalence ratio (φ) of hydrocarbon flames are used for determination of the equivalence ratio in a combustion process. Measurements were performed for two different burner configurations. The first configuration involved two Bunsen burners that were placed close to each other at an angle with flows that could be regulated separately with flow controllers. This made it possible to have two different equivalence ratios in the respective flame. In the other configuration a single Bunsen burner was used for the measurements. In order to produce two-dimensional pictures of the equivalence ratio, it is necessary to calibrate the technique and find a mathematical relationship between the equivalence ratio and the OH*/CH* ratio. Therefore the flame investigations were preceded by calibration measurements. Calibration measurements The calibration was performed with a McKenna burner using a premixed methane/air mixture between φ=0.7 and φ=1.2. The experimental setup is illustrated in Figure 83. A CCD camera equipped with a UV objective and a stereoscope was focused along the line-of-sight of a McKenna burner. By using a stereoscope it was possible to produce two identical images of the same object. For the measurements the stereoscope was equipped with two narrowband interference filters, centered at 310 nm and 430 nm respectively, for simultaneous imaging of the OH* and CH* emission. Stereoscope IF 310nm (OH) IF 430nm (CH) Aperture McKenna burner Figure 83. Schematic of the experimental setup for the calibration measurements. Figure 84 shows the recorded relation between the OH* (a) and CH* (b) emission and the equivalence ratio. Report Year 9 Competence centre combustion processes at Lund University (59/93) ____________________________________________________________________________ 1 1 OH Normalized emission intensity - CH Normalized emission intensity - OH 0.98 0.96 0.94 0.92 0.9 0.88 0.86 0.84 0.82 0.7 CH 0.95 0.9 0.85 0.8 0.75 0.7 0.65 0.6 0.55 0.75 0.8 0.85 0.9 0.95 1 1.05 1.1 1.15 0.5 0.7 1.2 0.75 0.8 0.85 0.9 Equivalence ratio 0.95 1 1.05 1.1 1.15 Equivalence ratio Figure 84. Relationship between the chemiluminescence emission intensities and the equivalence ratio, for CH (a) and OH (b). In Figure 85 the OH*/CH* intensity ratio as a function of the equivalence ratio is presented. The dependence of the OH*/CH* ratio on the equivalence ratio is approximated by a linear fit and then used to acquire two-dimensional images of the equivalence ratio. 0.6 Experimental data Linear fit 0.55 Emission - OH/CH 0.5 0.45 0.4 0.35 0.3 0.25 0.7 0.75 0.8 0.85 0.9 0.95 1 1.05 1.1 1.15 Equivalence ratio Figure 85. Chemiluminescence emission intensity ratio of OH/CH. The solid line represents linear fit. Two dimensional imaging – Dual burner Figure 86 illustrates a image taken by the stereoscope of the two slanted Bunsen flames burning with approximately the same equivalence ratio, here φ is equal to 1.3 (left) and 1.2 (right). Splitting and warping the picture makes it possible to do a pixel-to-pixel comparison and visualize the OH*/CH* ratio. This ratio can then be converted using the mathematical relationship from the calibration to a two-dimensional image of the equivalence ratio (Figure 87). 1.2 Report Year 9 Competence centre combustion processes at Lund University (60/93) ____________________________________________________________________________ IF 430nm IF 309nm CH OH Figure 86. The two flames burning with φ=1.3 (left) and φ=1.2 (right). The CH* concentration is on the left hand side and the OH* concentration on the right hand side. 1.5 1.25 1 0.75 0.5 0.25 0 Figure 87. A two dimensional image of the equivalence ratio with the two slanted Bunsen burners, φ=1.3 (left) and φ=1.2 (right). Two dimensional imaging – Single burner A measurement of the OH* and CH* emission was conducted for a single Bunsen flame burning with equivalence ratios of 1.0 and 1.2. The flame was assumed to be laminar, so the stereoscope was left out and the interference filters were instead shifted in front of the camera objective. Figure 88 depicts the equivalence ratio for a flame burning with approximately Φ=1.0 (left) and Φ=1.2 (right). 1.6 1.2 1.4 1 0.8 1.2 1 0.8 0.6 0.6 0.4 0.4 0.2 0 0.2 0 Figure 88. Two dimensional picture of the equivalence ratio for 1.0 (left) and 1.2 (right). Report Year 9 Competence centre combustion processes at Lund University (61/93) ____________________________________________________________________________ The measured values of the equivalence ratio show good agreement with the actual running conditions. The left figure has approximately an average value of 1.1 in the reaction zone, and the right an average value of 1.3. This is a good result, since the calibration curve determines the appearance of the two-dimensional equivalence ratio image, and a small variation of this curve will cause large variation of the equivalence ratio. Conclusions It has been shown that the OH*/CH* ratio and the equivalence ratio has a nearly linear relationship between φ=0.7 and φ=1.2 for a methane/air mixture. Since this relationship is well known and the result is repeatable this technique has the potential of measuring equivalence ratio in different applications by just detecting the chemiluminescence emission from the OH*- and CH* radicals. The technique is advantageous compared to laser-based techniques were often three optical windows are needed for beam insertion and detection. Report Year 9 Competence centre combustion processes at Lund University (62/93) ____________________________________________________________________________ Practical diagnostics Enhancement of spatial resolution with obscuration for line-of-sight technique In this work enhancement of the spatial resolution has been made for a line-of-sight technique by placing an obscuration disk covering the central portions of the light-collecting lens. Both two dimensional and point measurements have been investigated and the results indicate that the spatial resolution can be enhanced for point measurements, but harder to implement in two dimensions. The disk, termed an obscuration disk is placed in front of the central part of the lens to block the low-angle contribution (Figure 89). Figure 89. Higher spatial resolution can be achieved by placing an obscuration disk in front of the lens. This result in that angles less than those subtended by the obscuration disk are completely blocked. The depth of field becomes narrower and higher spatial resolution can be achieved. Experimental setup In an attempt to increase the spatial resolution when focusing on an object, an experimental setup similar to Figure 90 was used. A white light emitting diode mounted on a translation stage served to simulate an isotropic scattering source. The top of the diode was cut to make the light more isotropic. The light collection optics was a UV-lens with 100mm focal length. An ICCD camera was used for the detection and a 430 nm interference filter was placed in front of the camera to suppress the impact of chromatic aberrations. Obscuration disk Diode Figure 90. Schematic of the experimental setup for enhancement of the spatial resolution with an obscuration disk. The diode is placed on a translation stage and can be moved in and out of focus. Report Year 9 Competence centre combustion processes at Lund University (63/93) ____________________________________________________________________________ Several different lenses and light sources were also tested. The first light source was the end of a fiber optic cable. However, this light source was not isotropic enough, giving the light a directionality that affected the alignment. Other lenses were also tested but suffered from high chromatic aberrations, which made it hard to find the focus. The distance between the light source and the detection system for the experiment described above was approximately 50 cm. Experiments were also performed for a larger distance, approximately 3 m, because of the commercial interest of high spatially resolved point measurements. For this measurement, the collecting optics was replaced with a much larger lens to compensate for the much smaller detection angle. Two dimensional measurements Experiments were first performed in two dimensions, i.e., the light source was considered to be approximately the same size as the detector. Several different approaches were made for this particular experiment to study how the spatial resolution was affected by moving the light source in- and out of focus. Figure 91 (a) shows a comparison of the spatial resolution with and without an obscuration disk in two dimensions. The ratio is the diameter of the obscuration disk divided by the diameter of the lens, i.e., if the ratio is 0.50 the obscuration disk covers 25% of the lens, which also results in a 25% lower, signal. There is a slight improvement with the obscuration disk present. The FWHM is approximately 33% smaller for a ratio of 0.50. You would expect the width to decrease for higher ratios, but a ratio of 0.75 does not affect the width considerably. A lot of effort was put into making the two dimensional measurements work better, and even though the results above indicate an enhancement of the spatial resolution, most of the measurements showed no enhancement at all. Ratio: FWHM: 1.0 0.5 NORMALIZED INTENSITY Unobscured Obscured - Ratio: 0.50 Obscured - Ratio: 0.75 0.4 0.2 NORMALIZED INTENSITY 1.0 Unobscured: Obscured - Ratio: 0.50 Obscured - Ratio: 0.75 0.9 25.4139 16.8184 16.4864 a) 0.8 Unobscured Obscured - Ratio: 0.50 Obscured - Ratio: 0.75 Ratio: FWHM Unobscured: Obscured: O/D=0.50 Obscured: O/D=0.75 43.5418 26.1567 23.7077 0.8 b) 0.6 0.7 0.6 0.4 0.3 0.0 0.2 -30 -20 -10 0 10 DISTANCE FROM FOCUS [MM] 20 30 -60 -40 -20 0 20 40 60 DISTANCE FROM FOCUS [MM] Figure 91. Two dimensional measurement (a) and point measurement (b) of the spatial resolution with and without obscuration Point measurements Figure 91 (b) presents a comparison of the spatial resolution with and without an obscuration disk for a point measurement. The FWHM in this case is approximately 40% smaller for a ratio of 0.50. As in the case with two dimensional measurements, there is no considerable improvement for higher ratios compared to the loss of signal. Report Year 9 Competence centre combustion processes at Lund University (64/93) ____________________________________________________________________________ Increased distance Achieving point measurements with high spatial resolution is of great interest for industrial applications. A point measurement was therefore performed to investigate how the distance affects the spatial resolution. It is quite obvious that the distance will affect the measurements because of the much smaller space angle and the difficulty to align. Since the diameter of the lens decides the space angle and affects the light gathering capacity, a larger lens was used for this experiment. Figure 92 shows a comparison of the spatial resolution at a larger distance. The FWHM is approximately 17% smaller for a ratio of 0.50 and increases to 30% for a ratio of 0.75. In this case, the resolution increases when the ratio is higher then 0.50, but the signal strength will decrease. 1.1 NORMALIZED INTENSITY 1.0 Ratio: Unobscured Obscured - Ratio: 0.50 Obscured - Ratio: 0.75 0.9 FWHM Unobscured: 73.4376 Obscured: O/D=0.50 61.2082 Obscured: O/D=0.75 52.8499 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 -60 -40 -20 0 20 40 60 DISTANCE FROM FOCUS [MM] Figure 92. Point measurement performed at a larger distance. The distance was approximately 3m between the CCD camera and the light source. Conclusions Line-of-sight measurements require a high and well-defined spatial resolution. It has been shown that an obscuration disk placed in the central regions of a lens will increase the spatial resolution with over 40% for point measurements. The result in two dimensions is not equally overwhelming, even though the result show signs of improvement. High resolution point measurements are nonetheless a very attractive feature and have great commercial interest. Report Year 9 Competence centre combustion processes at Lund University (65/93) ____________________________________________________________________________ Formaldehyde measurements Introduction Formaldehyde (CH2O) is an intermediate species of interest in many combustion applications and can be detected with the laser-induced fluorescence (LIF) technique. One possibility for this is to use the wavelength of 355 nm for excitation. This alternative has been investigated and some technical developments for combustion diagnostics and a flame study are presented in this report. The technical developments include a combination of formaldehyde fluorescence measurements with LIF measurements of OH using a setup with a single Nd:YAG/OPO system. Also, the excitation of formaldehyde using the wavelength 355 nm has been investigated in more detail by scanning the single-mode wavelength of the Nd:YAG laser in a small range around 355 nm. Formaldehyde measurements using 355 nm have also been part of a flame study where diffusion flames of dimethyl ether burning in a counter flow burner have been characterized using different laser based techniques. Formaldehyde has also been detected with LIF in a setup where the pyrolysis of wood particles has been investigated. Combined formaldehyde and OH measurements The experimental setup is illustrated in Figure 93 and is based on a single-mode Nd:YAG laser with an optical parametric oscillator (OPO). D 283 nm THG SHG P P F G P 355 nm IF OPO unit 532 nm I2 cell Diode Nd : YAG G D D 355 nm A CL CCD Burner Stereoscope Figure 93 Experimental setup for combined measurements on formaldehyde and OH and high-resolution excitation scan experiments on formaldehyde. Report Year 9 Competence centre combustion processes at Lund University (66/93) ____________________________________________________________________________ The 355 nm laser beam from the Nd:YAG laser is mainly used for pumping the OPO, but a fraction of it is used for the LIF experiments on formaldehyde. The OPO provided a beam with the wavelength 283 nm tuned to the Q1(8) transition of OH. With this additional laser beam combined measurements of formaldehyde and OH could be performed. The two beams were aligned together and directed to the measurement object. The generated LIF signals were detected with a single CCD equipped with a dual image separator (stereoscope) and suitable filters for spectral isolation of the two signals. Measurements of the two species were performed in a premixed conical flame of dimethyl ether (DME) and air. The flame measurements were followed by measurements in a small single-cylinder four-stroke engine modified for optical access. Figure 94 shows two simultaneously detected LIF images of formaldehyde and OH in the premixed flame. Both signals can be seen to follow the cone contour of the flame. The formaldehyde is located in a thin layer along the cone and the OH is found in the flame-front region along the cone as well as in the burned gas outside the cone. Figure 94 Simultaneous fluorescence images of formaldehyde (left) and OH (right) detected in a premixed dimethyl ether-air flame. Figure 95 shows four pairs of LIF images of formaldehyde and OH detected in the engine at different crank angles in the combustion cycle. A photo of the combustion chamber viewed through the detection window is also included in the figure. Report Year 9 Competence centre combustion processes at Lund University (67/93) ____________________________________________________________________________ Figure 95 Simultaneous formaldehyde and OH images detected in a spark-ignition engine at four different crank angles. Below the fluorescence image pairs a photo of the combustion chamber view is illustrated. The engine valves can be seen in the photo and the fluorescence measurement region is indicated by the dashed rectangle. Also, the spark-plug position in the lower right corner is indicated. Report Year 9 Competence centre combustion processes at Lund University (68/93) ____________________________________________________________________________ The spark plug is located in the lower right corner of the photo and the flame mainly propagates from right to left in the images. For the image pair detected at 3 CAD only a small amount of formaldehyde signal can be observed and no OH signal is visible. From the fluorescence images detected at 13 CAD and 18 CAD it can be seen that the formaldehyde is detected in the left part of the images, which is in front of the propagating flame, in the unburned mixture. The OH signal is mainly detected to the right in the images, in the flame front region and the burned gas. For the image pair detected at 23 CAD the combustion has progressed so that no formaldehyde signal is observed and an OH signal can be observed along the entire laser sheet indicating the presence of burned gas. Formaldehyde LIF spectroscopy The fluorescence from formaldehyde was also investigated in laboratory flames when the laser wavelength was tuned in a small range around 355 nm. In these experiments the second harmonic at 532 nm was used as a wavelength marker for the laser. Figure 96 shows an excitation scan spectrum of formaldehyde detected in a premixed DME/air flame. 18789 18790 Intensity (a.u.) Pos. 3 Pos. 1 1 0.8 Pos. 4 Pos. 2 0.6 0.4 0.2 0 28183 28184 28185 Frequency (cm-1) Figure 96 High-resolution excitation spectrum of formaldehyde obtained in a premixed DME-air flame. The upper graph shows an iodine spectrum obtained simultaneously with the formaldehyde spectrum, that has been used to set a frequency scale for the formaldehyde spectrum. A not fully resolved line structure can be observed in the spectrum. Note that the laser has a line width of approximately 0.005 cm-1 and does not limit the resolution of the spectrum. The upper graph of Fig. 4 shows an iodine signal used for wavelength calibration of the scanned spectrum. The arrows pointing at different spectral positions in the scanned spectrum indicate wavelengths where LIF emission spectra have been detected. Report Year 9 Competence centre combustion processes at Lund University (69/93) ____________________________________________________________________________ The photo of Figure 97 shows a DME/air diffusion flame burning on a counter-flow burner. Fuel and air are supplied through two opposite nozzles and a flame is stabilized between them. The flame can be seen on the photo as a luminous disc located in the upper half of the burner gap. A scan with similar features of that illustrated in Fig. 4 was also obtained in this flame. Figure 97 Photo of a DME-air diffusion flame burning in a counterflow burner. The dashed lines indicate positions where formaldehyde emission spectra have been detected. Fluorescence emission spectra of formaldehyde have been detected in this flame at three different positions indicated on the photo by white dashed lines. The distances from the upper nozzle are also given. The strongest formaldehyde signal was obtained at the 6 mm position and emission spectra have been measured at this position for the different wavelengths indicated in the scan of Figure 96. The spectra are illustrated in Figure 98 and all of them show peaks that can be identified as formaldehyde emission. Furthermore, the shapes of the spectra are similar when normalized to the maximum intensity. This confirms that the signal obtained in b)1 a)1 Intensity (a.u.) Intensity (a.u.) T=700 K Pos. 3 Pos. 1 0.8 0.6 Pos. 2 Pos. 4 0.4 T=600 K 0.6 0.4 T=450 K 0.2 0.2 0 350 0.8 400 450 500 λ (nm) 550 600 0 350 400 450 500 550 600 λ (nm) Figure 98 a) Formaldehyde emission spectra detected at the 6 mm position in the flame depicted in the photo of Fig. 5. The spectra have been measured with the laser set at four different wavelengths indicated by the arrows in the scan of Fig. 4. b) Formaldehyde emission spectra detected at the three different spatial positions indicated in Fig. 5. A temperature has been estimated for each position in the flame using Rayleigh scattering, 450 K at 6.8 mm, 600 K at 6 mm, and 700 K at 4.8 mm. Report Year 9 Competence centre combustion processes at Lund University (70/93) ____________________________________________________________________________ the scan originates from formaldehyde. Similar results were obtained for sets of spectra measured at 4.8 mm and 6.8 mm. Figure 98b shows three emission spectra, one for each measurement position in the counterflow burner, obtained with the laser set at the wavelength corresponding to Pos. 3 in the scan. A comparison between the spectra shows that the line structure becomes less resolved when measurements are made closer to the middle of the burner gap. This could be interpreted as added background fluorescence. However, for each measurement position in the flame, the spectral shape remained the same for the four different excitation wavelengths of the laser. Therefore we interpret the spectra as formaldehyde signal. The less resolved line structure obtained closer to the middle of the burner could be caused by a broadening of the spectral lines due to temperature since the population would be distributed over more rotational levels at elevated temperature. The temperature has been estimated by Rayleigh scattering to 450 K at 6.8 mm, 600 K at 6 mm, and 700 K at 4.8 mm. These temperature values have been assigned to the spectra of Figure 98b. Formaldehyde measurements in a DME diffusion flame Figure 99 shows results from measurements of formaldehyde in the flame illustrated in the photo of Figure 97. Figure 99a, shows the formaldehyde fluorescence signal versus distance from the upper burner nozzle. the unfilled circles and the line shows a profile not corrected for the influence of temperature and quenching effects. Correction for temperature was possible using the temperatures estimated by Rayleigh scattering (cf. Figure 98b). The compensation for quenching required the measurement of the effective fluorescence lifetime in the flame. The experimentally obtained lifetimes for different positions in the flame are presented in Figure 99b. The solid circles in the graph of Figure 99a give the formaldehyde profile obtained after correction for the influence of temperature and quenching. a) b) 20 0.8 CH2O lifetime (ns) CH2O signal (a.u.) 1 0.6 0.4 0.2 0 15 10 5 0 0 2 4 6 Distance from upper burner (mm) 8 0 2 4 6 8 Distance from upper burner (mm) Figure 99 Results from formaldehyde measurements in a DME-air diffusion flame. a) Formaldehyde profiles where the unfilled circles and the line shows an uncorrected profile and the filled circles shows the result when the dependence on fluorescence quenching and temperature have been taken into account. b) Measured effective lifetimes used for the quenching correction of the formaldehyde profile in Figure 99a. Report Year 9 Competence centre combustion processes at Lund University (71/93) ____________________________________________________________________________ Formaldehyde detection during the pyrolysis of wood Formaldehyde has also been detected by means of laser-induced fluorescence during pyrolysis of wood particles. The experiments were carried out in an oven in a non-oxidative environment of nitrogen gas heated to around 400 °C. Investigations were made on birch wood particles having masses of around 180 mg. Figure 100 shows a top view of the oven and the laser beam path through it. The laser beam at 355 nm was shaped into a horizontal 20 mm wide laser sheet and directed through the oven above the particle. The generated laser-induced fluorescence was collected through a window at 90 degrees angle relative to the laser sheet propagation direction. The signal was collected to a spectrometer connected to an intensified CCD camera. Laser-induced fluorescence spectra were detected during the pyrolysis. 1 3 5 2 2 4 Figure 100 Experimental setup for the investigation on wood pyrolysis using laser-induced fluorescence. The figure shows a top view of the oven with the heated tube (1), the inlets for nitrogen gas, the inserted particle (3), the laser beam path (4), and the detection window (5). Figure 101 shows an averaged LIF emission spectrum detected during the pyrolysis. A line structure is observable that can be identified as the emission spectrum of formaldehyde. For comparison purposes, a formaldehyde spectrum detected in a premixed dimethyl ether (DME)-air flame is also included in the graph. Similar to the formaldehyde emission spectra detected in flames, see Figure 98, the lines of the spectrum detected during the pyrolysis process are not completely resolved. For the pyrolysis case the spectrum probably also includes additional fluorescence from other species, which add a spectral profile without distinct features to the detected signal This is also evident from the fact that, as the pyrolysis process progressed, the spectral lines disappeared, the shape of the spectrum becoming more featureless. This observation is in agreement with the results from previous investigations. Report Year 9 Competence centre combustion processes at Lund University (72/93) ____________________________________________________________________________ 1 Intensity (a.u.) 0.8 0.6 0.4 0.2 0 350 400 450 500 550 600 λ (nm) Figure 101 LIF emission spectrum detected during the pyrolysis of birch wood particles. The spectrum was obtained using 355 nm excitation and the signature of formaldehyde can clearly be observed. For comparison a spectrum detected in the premixed DME-flame has also been included in the graph (the dashed line). List of publications C. Brackmann, Z. Li, M. Rupinski, N. Docquier, G. Pengloan, and M. Aldén, Strategies for formaldehyde detection in flames and engines using a singlemode Nd:YAG/OPO laser system , recommended for publication in Applied Spectroscopy. C. Brackmann, J. Bood, G. Pengloan, Ö. Andersson, and M. Aldén, Quantitative Measurements of Species and Temperature in a DME-Air Counterflow Diffusion Flame using laser diagnostic methods, submitted to Combustion Science and Technology F. Jarl "Enhancement of Spatial Resolution with Obscuration and Two Dimensional Equivalence Ratio Imaging", LRCP 99, Lund University, 2004. Report Year 9 Competence centre combustion processes at Lund University (73/93) ____________________________________________________________________________ Biolåg Summary This is an activity report for the period 1/7 2003 to 30/6 2004 for the biolåg (low heating value biogas combustion) project with the competence centre of combustion processes at Lund University. The project is divided into two areas: modelling of combustion of fixed bed boiler and modelling of radiation heat transfer in fixed bed boilers. Each area has a half-time student. For the fixed bed biomass combustion process studies the focus has been on the modelling of the biomass combustion process in the bed and volatile combustion in the boiler above the bed. The student, Torbern Klason, has also spent some time on analysis of industrial boiler performance. The boiler under systematic investigation is the Sydkraft värme syd AB’s Flintrännan boiler, located in Malmö city. NOx and CO emissions and fuel residence time in the boiler are targeted in the investigation, and the implementation of ECO tubes, which is to regular secondary air supply has been systematically examined. The results are very helpful for the boiler operation and improvement. The radiation heat transfer has been studied by Thomas Nilsson. The main focus has been on the development of accurate radiation heat transfer models accounting for the combustion condition in fixed bed boilers. Several models have been developed, including the P1 approximation model, the Discrete ordinate method (DOM) and the finite volume method (FVM). These developments provide a solid basis for the analysis of the combustion process in utility boilers. Effort of combining the two areas has been made with success. Bed combustion interact with the radiation heat transfer, gas combustion coupled with various radiation heat transfer models have been systematically investigated. The methods are applied to the Flintrännan boiler. The accuracy of different models is evaluated. This improves the accuracy of the boiler performance analysis. Report Year 9 Competence centre combustion processes at Lund University (74/93) ____________________________________________________________________________ Fixed Bed Combustion Process in Biomass Boilers Personnel This work was conducted by Ph.D. student Torbern Klason, under the supervision of Professor Xue-Song Bai. Analysis of fixed bed boiler performance – fuel residence time The biolåg project has been focused on analysis of practical boiler performance. In practical boilers, some problems exist such as, emissions of unburned hydrocarbons (UHC), CO, NOx, dioxins and particles, etc. Unburned hydrocarbons and CO are originated from incomplete combustion of the volatile and to some extent small char particles. Fly ash, small char particles and soot particles emitted from the furnaces are not only strongly coupled with the combustion process, but also with the flow and turbulence structures in the furnace. An important parameter, the flow residence time, can be singled out to characterize the combustion process. The particles and pollutant species are transported with the flow to the outlet of the boiler. If the residence time is not sufficiently long, then the hydrocarbons (volatile gases from the devolatilization of biomass fuel in the fuel bed) will not be fully oxidized. They leave the boiler as unburned. NOx and dioxin are originated from the nitrogen elements and trace elements of chloride in the fuel. The formation and destruction of them are related to the residence time, flame temperature and air distribution. Dioxin is an aromatic compound resulting from low temperature combustion, 300-400°C. The molecule becomes unstable above 850°C and is easily oxidized with sufficiently long residence time (e.g. 2 seconds). As regulations on the emissions continue to be stricter, the energy producers are subjected to higher demands on producing energy in a more environmentally friendly and efficient way. In order to meet the new emission standard, the residence time of gas/solid particles in the boiler should be long enough and the combustor temperature should be high enough. Eleven different possible operation conditions, with different load and wood chip moisture content, have been investigated. To illustrate the performance of the boiler, the standard operation case at full load and fuel moisture content 40 wt% is briefly shown below. Figure 102 shows the fuel particle position at different residence times. To help understanding the particle distribution the flow streamlines in the middle vertical plane is shown in Fig.2a. The dense streamline zone is the zone with high flow speed. This zone is found near the air curtain, in which there are two rows of opposing jets supplying air to further oxidize the volatiles. Above the air curtain, two main recirculation zones are found, near the left and right walls, respectively. The ECO tubes affect the flow streams, as evidenced in Fig.2a: the streamlines turn aside towards the walls, thereby they affect the size and form of the two large recirculation zones. Finally the streamlines reach the top of the furnace and turn around and end at the outlet of the furnace. From Figure 102 and Figure 103, it can be noted that in the lower part of the furnace, below the air curtain, the flow speed is low. Therefore during the first one second, the fuel parcels travel rather slowly. After 1.5 second, most fuel parcels have passed the air curtains, as shown by the parcels in Figure 102. The parcels follow the flow streamlines, and mostly concentrated in the narrow zone in the middle of the furnace. After 1.75 seconds, the fuel parcels start to spread wider in the furnace due to the two large recirculation zones above the air curtain. After 2 Report Year 9 Competence centre combustion processes at Lund University (75/93) ____________________________________________________________________________ Case 1 t=1s t=1.5s t=1.75s t=2s Figure 102: Fuel particle distribution in the Flintrännan boiler at four different residence times seconds some parcels have reached to a height of 9 m in the furnace, and the spreading of the parcels is even wider. Figure 103b shows the flow streamlines in a horizontal cross section at the height about 8 m, where the secondary air (four jets) generates a strong recirculation motion in the horizontal plane. Considering Figure 103a and Figure 103b together, it can be understood that the flow streamlines are of helical shape. Due to the helical motion of the flow streamlines the fuel parcels also move helically upwards in the furnace. This helical motion increases the residence time of the fuel parcels. Since the ECO tube redirected the flow streams towards the side walls, the fuel parcel residence time is also increased. Changing fuel and moisture load leads to changing of the combustion process and the flow field. This affects the fuel particle distribution in the furnace. Figure 104 shows the parcel height in the furnace at different parcel residence time. Three different fuel loads are shown in the figure. Several important observations can be made from Figure 104. Firstly, in all cases (30 – 50 MW) the particles move up continuously with the increase of the residence time. This has been explained in the text above for the standard case. Secondly, the flow speed in the boiler becomes lower with decreased load. This results in a slowdown of the fuel gas parcels, which in return increases the parcel residence time in the furnace. Thirdly, at the 30 MW load the flow speed is so low that after 2 second of residence time, the parcels have still not Report Year 9 Competence centre combustion processes at Lund University (76/93) ____________________________________________________________________________ completely passed the air curtain. Fourthly, decreased load leads to decreased temperature inside the boiler. ECO tubes Air curtain (a) (b) Figure 103: Flow streamlines in two cross-sections of the Flintrännan boiler Furnace height 8 t=1s t=1.5s t=1.75s t=2s 6 4 2 0 30 40 50 Fuel Load (MW) with 40% moisture Figure 104: Particle position (at different furnace height) at different load, with wood chip moisture content 40 wt%. Report Year 9 Competence centre combustion processes at Lund University (77/93) ____________________________________________________________________________ We can conclude that the load affects the parcel residence time. Decrease of load leads to an increase of the residence time of the parcels. The gas temperature decreases when the load is decreased slightly (to not too low load), yet, large parts of the boiler still have high temperatures. Further details about the work is given in [8,9]. Modeling of biomass combustion in the fuel bed The Flintrännan boiler fuel bed is studied. The bed is sketched in Figure 105. It is a fixed bed (wood chips moving rather slowly), which is fed with wet wood chips from the top left corner. The wood chip size is about 6 mm – 100 mm. Ash is removed from the bed at the bottom of the right corner. The bed is divided into four different zones. The first two zones are the drying zones where the wet fuel is dried. The main gaseous species found in these zones are water vapor, nitrogen and oxygen. In the second zone the fuel particles are heated up and volatile (mainly tar and hydrocarbons, HC) are produced. In the third zone devolatilization process continues where the fuel is further broken down and the products that leave the bed are H2, CO, CO2, CH4 and tar. The fourth zone is the char combustion zone and most of volatile has left the fuel, here CO and CO2 are the main products. Radiation Wood Chips H2O, O2 H2O, HC 1 O2, tar CO, CO2, Tar CO, CO2 CH4, H2, 2 Inlet 1: Flue gases Inlet 2: Air 3 4 Inlet 5: Air & flue gases Inlet 3: Air & flue gases Inlet 4: Air Figure 105: The biomass bed in the 50 MWe boiler (xy-direction) The bed is modeled here as one-dimensional, with the same length and equivalent volume as the real bed, see Figure 5. The modeled bed in Figure 106 is counter-current with fuel supplied from the top and air and flue gases from the bottom. The mass, composition and temperature of the gases leaving the bed is used as boundary conditions to the boiler calculations. The mass and gas species obtained from the bed model are distributed over the four zones of the bed. The effects of radiation are taken into account by adding heat source to the side of the bed (Qin). As the fuel moves down the 1-D bed it is assumed to go through four stages before it reaches the bottom, where it is removed as ash. The fuel is first dried and then devolatilization starts where the fuel is broken down into volatile and char. In the gasification zone the char is gasified by products from the char combustion and the recirculated flue gases. Lastly, the remaining char is oxidized to CO and CO2 and only ash left the bed at the bottom. Report Year 9 Competence centre combustion processes at Lund University (78/93) ____________________________________________________________________________ The FG-DVC (functional group, depolymerization, vaporization and cross-linking) model used in the 1-D bed simulation describes the volatile and char formation processes, and it consists of two sub-models. The FG sub-model predicts how the functional groups in the fuel are broken down and form light gases. The DVC sub-model assumes the fuel as a large network and as the fuel is heated it breaks down and form fragments through bridge breaking and cross-linking processes. Wood Chips Volatile Moisture Drying Devolatilization Qin Qin Gasification Oxidation Flue Gas Ash Air Figure 106: Modeled 1-D bed 45 CO CO2 CH4 H2 H2O O2 40 Species [Mole %] 35 30 25 20 15 10 5 0 0 1 2 3 4 Bed Length [m] Figure 107: Species distribution along the bed length 5 6 7 Report Year 9 Competence centre combustion processes at Lund University (79/93) ____________________________________________________________________________ To illustrate the bed modeling, Figure 107 shows the distribution of several gaseous species at different length of the fuel bed. To the left of the figure, three species are found, namely, H2O, CO2 and O2. They are from the primary air and flue gas recirculation. At the bed length 4.2m, the reaction layer consumes all the oxygen, and forming CO, CH4, H2 and other combustion products. At the right hand of the figure, the gaseous species leave the bed and enter into the furnace where volatile combustion continues. The overall performance of the furnace depends on the proper modeling of the bed combustion process. This work is to be continued in the coming years. Some results are reported in [1,2]. Modeling of radiation heat transfer in fixed bed boilers Personnel This work was conducted by Ph.D. student Thomas Nilsson, under the supervision of Professor Bengt Sundén. Development and validation of radiation heat transfer models Analysis of practical boiler performance (and for future design of environmental friendly boilers) depends on the capability of combustion and heat transfer models. In the biolåg heat transfer modeling subproject, the main focus has been to develop and identify the most appropriate radiation models for biomass fired boilers. Different radiation models can be employed in the analysis of heat transfer in boilers. Some models for example, the optically thin (OT) and the Rosseland models, are simple to implement and computational cheap to use. The OT model has been widely used in simple jet flame calculations. However, these simple models may be not accurate enough for certain applications, for example, for calculation of the heat flux between the freeboard and the boiler bed. To investigate this, several different models are employed, including the P1 approximation model, the discrete ordinates method (DOM) and the finite volume method (FVM). DOM (SN-method) is a tool to transform the radiative transfer equation (RTE) into a set of simultaneous partial differential equations, just like the spherical harmonics method (PNmethod). The natural step to improve DOM is to move to a fully finite volume approach, in space as well as in direction. Thus, the FVM will be the result if one uses finite ”solid angle volumes”, instead of discrete direction like in DOM, for the direction discretization. FVM is fully conservative, i.e., satisfaction of all full- and half-moments can be achieved for arbitrary geometries. The computational cost for the FVM is higher than that for DOM and P1 model. OT model is the cheapest to use. To illustrate the performance of some these models, we have applied the models to simulation of the Flintrännan boiler. It was found that the temperature distributions in the boiler are less sensitive to the model variations and model parameters such as the mean beam length. This can be explained by using Figure 108. Here, absorption coefficient (κ) and incident radiation (G) are plotted along the furnace height at the center of the boiler, as a function of Lm. For both FVM and P1 the increase in κ is compensated by the increase of G. This makes the temperature profile calculated from both P1 model and FVM insensitive to a change of Lm (the Report Year 9 Competence centre combustion processes at Lund University (80/93) ____________________________________________________________________________ mean beam length) and thus, the two models are superior in engineering calculations (such boiler calculations), where Lm is usually unknown or difficult to be accurately predicted. 2 Absorption Coefficient [1/m] Absorption Coefficient [1/m] The amount of heat radiated back to the bed from the free board, calculated using different models and soot volume fractions (svf) are shown to be rather sensitive to the model variations, as seen in Figure 109. NS stands for no soot in the calculation; the soot volume P1 STD P1 Grid P1 Middle P1 Large 1.5 1 0.5 0 0 2 4 8 10 6 Furnace Height [m] 12 6 1.5 1 0.5 0 0 2 4 8 10 6 Furnace Height [m] 12 0 2 4 8 10 6 Furnace Height [m] 12 1.0×10 5 5 8.0×10 G [W/m^2] 8×10 G [W/m^2] FVM STD FVM Grid FVM Middle FVM Big 6 1×10 5 6×10 5 4×10 5 5 6.0×10 5 4.0×10 5 2×10 0 2 2.0×10 0 2 4 8 10 6 Furnace Height [m] 12 0.0 Figure 108: Absorption coefficient (κ) and incident radiation (G) as function of Lm for P1 and FVM, taken at x=3.26 m and z=2.9 m. Report Year 9 Competence centre combustion processes at Lund University (81/93) ____________________________________________________________________________ 2 P1 [15] 1 NS S 1e-7 S 1e-6 Heat radiated to the Bed [MW] 0 -1 S 1e-5 S 1e-6 S 1e-7 NS -2 -3 -4 S 1e-6 -5 -6 -7 -8 -9 NS S 1e-7 Optical Thin P1-Approximation Radiation Cases Finite Volume Method Figure 109: Radiation heat transfer to the bed for different models. fraction varies from 1e-7 to 1e-5. The OT model predicts negative heat flux to the bed, i.e., heat is radiated from the bed to the furnace upper volume; the FVM, on the other hand, predicts that about 1 MW heat is transferred from the flame to the fuel bed. The P1 model predicts rather low heat fluxes to/from the bed. This shows that the heat flux calculation strongly demands accurate models. More results are presented in [3-6]. By coupling the radiation modeling with the fuel bed modeling, we found that FVM results are more physically correct – if heat is lost from the bed to the environment, the bed combustion will be quenched. More results are given in [1-3]. Interaction between the two subprojects The two subprojects, combustion modeling and radiation modeling, have been in close collaboration. The combustion modeling provides detailed species distribution and heat release; the radiation modeling provides additional energy balance in the combustion modeling. The two subtasks together make it possible for the analysis of industrial boilers with comprehensive accuracy. Several joint publications have been issued thanks to the close collaboration. Published results Five papers have been presented at international conferences [1-5]; one of these papers is being extended and revised, and will be submitted to International Journal of Heat and Mass Transfer [3]. A Ph.D. thesis has been defended (Thomas Nilsson, December 2003) [6], and a Report Year 9 Competence centre combustion processes at Lund University (82/93) ____________________________________________________________________________ licentiate thesis is scheduled to be defended this fall (Torbern Klason, November 2004) [7]. In addition, a report for Sydkraft Värme Syd AB, describing detailed numerical investigation of Flintrännan boiler has been delivered [8]. Some of the results have been submitted to an international journal [9]. Extension of biofuel combustion modeling to pulverized flame (PF) mode has made progress in the summer. A paper is to be submitted to a journal [10]. Application of the tool to Sydkraft boilers (8 MW wood powder fired steam boiler in Hanaskog, near Kristianstad) is in progress (Bai and Mats Åbjörnsson, Sydkraft). References 1. Klason, T., Nilsson, T., Bai, X.S. and Sunden, B.: Simulation of Biomass Combustion and Radiation Heat Transfer in Fixed Bed Boilers, 7th Int. Conf. On Techn. and Combustion for a Clean Environment, July, 2003, Portugal. 2. Klason, T. and Bai, X.S.: FG-DVC Modeling of Biomass Combustion in Fixed Bed, Third Mediterranean Combustion Symposium, 8-13, June, 2003, Marrakech. 3. Nilsson, T.K., Klason, T., Bai, X.S. and Sundén, B., Thermal radiation heat transfer and biomass combustion in a large-scale fixed boiler, Proceedings of IMECE'03, IMECE 2003-42249, 2003, Extended version to be submitted for journal publication. 4. M. Bahador, T.K. Nilsson and B. Sunden, On heat load calculations in gas turbine combustors, in Advanced Computational Methods in Heat Transfer VII, (Eds. B. Sunden, C. Brebbia, A. Mendes), 345-357, WIT Press, UK, 2004. 5. M. Bahador and B. Sunden, Modeling of the absorption coefficient in the exponential wide band model, in Radiative Heat Transfer IV (Eds. M.P. Menguc, N. Selcuk), 563571, Begell House, Inc., 2004. 6. Nilsson, T.K., Modeling and simulation of thermal radiation in biomass combustion, Ph.D. thesis, Dept. of Heat and Power Engineering, LTH, Dec. 2003. 7. Klason, T., Modeling of biomass combustion in boilers, Licentiate thesis scheduled to be finished in November 2004. 8. Klason, T. and Bai, X.S. A study of gas residence time in FFC Boiler (the ‘two-second 800 C hot zone rule’ for future dioxin emission legislation), report delivered to Malmö Värme Syd AB, 2004. 9. Klason, T. and Bai, X.S., Combustion Process in a Fixed Bed Biomass Fired Industry Furnace: A CFD Study, submitted to Progress in Computational Fluid Dynamics, 2004. 10. Elfasakhany, A., Klason, T. and Bai, X.S. Modeling of pulverized wood combustion using a functional group model. submitted for publication, 2004. Report Year 9 Competence centre combustion processes at Lund University (83/93) ____________________________________________________________________________ Combustion in new atmospheres Summary This is an activity report for the period 1/7 2003 to 30/6 2004 for Combustion in New Atmospheres for Sustainable Energy Systems within the centre of competence combustion processes at Lund University. The project started 1/7 2003 but is to a large extent a continuation of Combustion in Humid Air during previous phase of the centre. Background The emission of carbon dioxide (CO2) from the burning of fossil fuels has been identified as the major contributor to global warming and climate change (International Panel on Climate Change, http://www.ipcc.ch/). However, for the immediate term over the next 20 – 30 years at least, the world will continue to rely on fossil fuels as the source of primary energy. The challenge now is to find cost-effective solutions that will reduce the release of CO2 into the atmosphere. Principally CO2 emission can be reduced by increasing efficiency, firing bio fuels or by CO2 capture/sequestration. During phase 3 of the Competence centre, combustion in humid air has been studied at atmospheric conditions. These studies will continue at high-pressure conditions during phase 4. However, there are a number of processes, which have been proposed for increased cycle efficiency and CO2 capture/sequestration in power production processes. These processes can be brought together in three groups: Pre-combustion processes: Fuel reforming or utilization of bio-mass derived fuels: • • • Hydrogen rich fuels. Reformed fossil fuels with water shift reaction where CO is converted to CO2, which results in an energy rich mixture of hydrogen and CO2 from which CO2 can be removed. Combustion in an atmosphere of H2/N2/O2 or H2/N2/O2/ rest-CO2 Weak gas from SOFC LCV gas. Combustion in “new” atmospheres: • • • • Humid air and ethanol in humid air Oxygen rich atmosphere. Example: AZEP membrane reactor producing O2. Combustion in O2/ H2O/CO2 atmosphere with H2O/CO2 recirculation. (O2 produced in the process) CO2 atmosphere. Example: Air separation process producing O2. Stoichiometric combustion in O2/ CO2 atmosphere with CO2 recirculation (O2 produced outside the process). CO2 rich atmosphere. Chemical Looping Combustion, CLC. Post-combustion processes: • CO2 removal from the exhaust gases. The 2 first categories imply major changes in the combustor inlet conditions. Not only inlet temperature and pressure are changed but also the composition of the reacting mixtures. Other consequences of these changes that are outside the scope of this project include changes in heat transfer and turbo machinery behavior due to changed fluid properties. Report Year 9 Competence centre combustion processes at Lund University (84/93) ____________________________________________________________________________ Status Assessment of new atmospheres Focus has been put in wet combustion and combustion in CO2 rich atmospheres (O2 fired processes). Figure 110 shows typical examples of these cycles. In the EvGT the water is added to the cycle in an evaporator upstream of the combustor. This reduces the temperatures and makes recuperation more attractive. In the O2-fired cycle show here, no nitrogen is present in the combustion and the exhaust gases consist mainly of CO2 and H2O. CO2 removal is obtained by cooling the exhaust gases to condense the water vapor. Liquid water Air Evaporator N2 ASU EGR O2 Expander Compressor Expander Compressor Figure 110: EvGT (left) and O2 fired cycle (right). Due to the relatively high efficiency losses in CO2 capturing cycles, combinations of these have also been investigated. O2/CO2 recycle combustion has been studied in heat balance programs for (not yet published): • • • Simple cycle (SC) Steam injected turbines (STIG) Evaporative gas turbine (EvGT) The results indicate that the turbine outlet temperature will get high unless combustor outlet temperatures are kept low (below 1400 C.). Optimal pressures are fairly high (30 < Π < 35). A summery of the conditions expected in a large machine under these circumstances is given in Table 2. Expected values for the Advanced Zero Emission Plant (AZEP) have been included for reference. Table 2: Combustor conditions for different cycles cycel Tin P SC Recup. EvGT O2-SC O2-STIG O2-EvGT AZEP [ 3 ] (K) 700 854 800 630 650 890 723-1123 (Bar) <35 10 35 35 35 35 20 3 Fuel (%w) 2.32 NG 1.76 NG 3.31 NG 2.65 NG 3.62 NG 2.27 NG 3.01 CH4 O2 (%w) 23.013 22.91 18.9 11.603 12.677 9.78 12.04 N2 (%w) 75.1 74.8 61.72 0 0 0 0 CO2 (%w) 0.0487 0.0485 0.04 87.78 64.914 76.53 46.35 H2O (%w) 0.63 1.033 18.4 0.62 22.41 13.69 38.60 φ (1) 0.48 0.35 0.65 0.94 0.94 0.94 1 T. Griffin, D. Winkler, M. Wolf, C. Appel, J. Mantzaras, ”Staged Catalytic Combustion Method for the Advanced Zero Emissions Gas Turbine Power Plant“, ASME GT2004-54101, 2004. Report Year 9 Competence centre combustion processes at Lund University (85/93) ____________________________________________________________________________ Experiments in atmospheric combustor The atmospheric combustor rig used in phase 3 has been redesigned to increase optical access, see descriptions of test facilities below. Also changes in mixture preparation have been made in order to facilitate CFD modeling of the combustor. Emission measurements using highly humidified air have been made previously [8, 9]. Computational support to understand the emission formation has been given by adjacent projects [10, 11], see Interaction with other projects below. Emission measurements of combustion of DNG in CO2 / air mixture have been made [12]. Lean blow off limits have been deduced from these, see Figure 111. The figure shows the equivalence ratio at minimum CO emission at different inlet temperatures for DNG in air and in air with 20%(mass) CO2 added to the air. Reducing the equivalence ratio from this point results in rapid increase of UHC and CO and generally this value is not far from flame extinction or lean blow-of. The lean flammability limit of pure methane in air is included for reference 4 . 0.800 0.750 0.700 φ (min CO) 0.650 0.600 0.550 0.500 0.450 without CO2 Omega CO2 around 0.2 0.400 0.350 CH4 Lean flammbility limit 0.300 0 100 200 300 400 500 600 700 T in (C) Figure 111: Lean blow of limits taken as minimum CO emissions in DNG/CO2/Air combustion Further experiments with increased dilution rates are planned for the spring 2005. Experiments in High pressure facility Commissioning of the HP facility has been delayed. However, experiments in a limited range of inlet conditions have been performed. The range of inlet conditions is constantly improved, but at presents it not possible to reach the desired inlet temperatures desirable for this project. Nevertheless, experiments in humid air are scheduled for January 2005. The planned experiments include both premixed and diffusion flames. In summery: 4 H. F. Coward and G. W. Jones, "limits of flammability of gases and vapors", Bureau of Mines, Bulletin 503, 1952. Report Year 9 Competence centre combustion processes at Lund University (86/93) ____________________________________________________________________________ • • • The test duct has been designed and manufactured A steam boiler (120 kW) has been purchased, installed and commissioned. Emission measurements in humid air are scheduled for January 2005. Interaction with other projects The phase 3 project “combustion in humid air” had strong interaction with STEM:s “Termiska processer för elgenerering”. Here, both CFD and Chemical kinetics computations were applied to combustion in humid air. Some of the techniques used will be applied in the present project as well [13, 14]. The current project has interactions with Cecost2 Gas turbine program “Investigation of flames from biogas”. The focus of this project is to investigate details of the flame front and the flowfield in biogases using the atmospheric combustor. Description of the test facilities Both test facilities will, within this project, be operated with a test section based on the swirler package of the Volvo VT40 gas turbine. Minor modifications of the swirler package (exclusion of ternary swirl) have been made for the atmospheric combustor to make it more attractive for CFD work. Both combustors are equipped with steam boilers. A summary of the mentioned combustors is given in Table 3. Table 3: summery of the different combustor parameters Pressure (bar) Mass flow air (g/s) (primary zone) Inlet temperature (deg. C) VT40 original 4 110 601 Atmospheric combustor 1 <25 <600 HP facility <16 <1300 <700 HP facility with VT40 duct 2-6 50- 150 300-400 The atmospheric combustor The atmospheric combustor is based on the swirler package of the Volvo VT40 gas turbine, which was developed for premixed combustion of mainly liquid fuels. The burner is also equipped with a pilot that enables diffusion flame experiments. Within this project, the flame tube is replaced by an optically accessible duct with square cross section, see Figure 112. Report Year 9 Competence centre combustion processes at Lund University (87/93) ____________________________________________________________________________ Figure 112: Atmospheric combustor. Left image: Premixed air and fuel enters the combustor through the concentric slot (painted red). The nozzle for the pilot flame is mounted in the centre below the dump plane. Right image: Premixed combustion of natural gas and air at φ ≈ 0.5 and inlet gas temperature, Tin ≈ 400o C . The flame itself can be seen to be faint blue and all yellow/red light is from blackbody radiation of the metal construction. The DESS / High Pressure facility The DESS/High pressure facility (HP facility) is capable of supplying air at more conditions that are more realistic in gas turbine combustion. A more detailed description can be found at http://www.vok.lth.se/%7Etpe/research/dess_high_pressure_combustion_more.html. Figure 113, taken from the above link, shows an overview of the high pressure facility. Figure 113: The DESS/ high pressure facility Report Year 9 Competence centre combustion processes at Lund University (88/93) ____________________________________________________________________________ Budget and schedule Duration: 1 July 2003 - 30 June 2005 Personnel: PhD student Raik Orbay 50% PhD student Fredrik Hermann 50% Budget, SEK 03/04 04/ 05 total PhD student 600 600 1200 Equipment 250 250 500 Alstom in kind 100 100 200 Volvo Aero in kind 150 150 300 Sum 1100 1100 2200 In Kind Cash Total Financier: Alstom/Siemens 200 Volvo Aero 300 200 250 550 Sydkraft 200 200 LTH 110 110 STEM 1140 1140 1700 2200 Sum 500 Time schedule Overview of relevant atmospheres Atmospheric experiments Design and manuf. of pressurized combustor Pressurized experiments HT03 VT04 HT04 VT05 Report Year 9 Competence centre combustion processes at Lund University (89/93) ____________________________________________________________________________ Thermoacustics Summary The work has been focused on the following: 1. Developing numerical methods for combustion instabilities in GT combustors. 2. Measuring combustion instabilities in industrial combustors. 3. Setting up a rig for measuring shear layer instabilities (injection of fuel in cross-flow). Combustion instability modeling: Combustion instabilities can be induced by flow instabilities or by flame-flow interaction and flame-acoustic interaction. The first is driven by the non-linearity of the flow field and the instability acts as a pace maker for the combustion also. The main issue in these cases is the separation of the coherent structured that are associated with the instability from the turbulent structured. If the frequency of the unstable mode is within the range of turbulent structures the separation is impossible. This is usually the case for GT combustors. Therefore, one has to use Large Eddy Simulations (LES) even though this requires substantial computational effort. An example for such a situation is the GT-combustor of SYDKRAFT (Hamlstad) where under certain condition, flow induced vibrations occur Figure 114and Figure 115. Figure 114: The geometry of the burner (left). The vortex shedding behind the swirler blades (left) acts as pace-maker and “locks in” all the unsteady motions at a relatively low frequency (of about 120 Hz) Report Year 9 Competence centre combustion processes at Lund University (90/93) ____________________________________________________________________________ Figure 115: Instability modes in the combustor shown in Figure 115. A second mechanism is due to the interaction between the flow and the flame. This interaction is non-linear. Flow instability leads to local fluctuations in equivalence ratio and local temperature fluctuations which in turn lead to fluctuations in the reaction rate. An typical example is the instability formed by annular swirling flows. A typical modern GT combustor work in the so called Lean Prevaporized Premixed (LPP) mode. Thus, too large variation in equivalence ratio has direct effect on the emissions from the combustor. An example of flow induced combustion instability is the AEV burner, where small inlet perturbation with a frequency corresponding to the shear-layer instability are applied, leading to resonance Figure 116. Figure 116: The frequencies related to the shear-layer instability of the AEV burner (left). The flame instability induced by inlet perturbations (right). The third mode is related to the flame-acoustic interaction. This interaction is related to the classical Raleigh instability which states that when the pressure- and heat-release fluctuations are in phase, the system is unstable. This statement has been deduced for inviscid and laminar and 1-D flows. The situation in 3-D real flows is more complex. The mechanism can be understood also in such cases since it is enough that the local pressure fluctuations lead to higher temperature that the heat-release is enhanced which implies increase in pressure source. We have developed numerical tools for handling the propagation of acoustical waves and the acoustical sources from combustion. A typical example is the acoustical instability in a Report Year 9 Competence centre combustion processes at Lund University (91/93) ____________________________________________________________________________ combustion chamber in which an AEV burner is placed. Figure 117 shows an instantaneous picture of the acoustical waves. Figure 117: The acoustic modes generated by the flame (solid line) in a combustion chamber. The Ph.D. student working on the acoustical coupling is Mihai Mihaiescu. He shall complete his Ph.D. in March 2005. Dr. Ch. Duwig has been working on LES modeling issues of GT combustors. The results have been submitted to journal publications. Combustor instability measurements: The ALSTOM (now Siemens) industrial student (Jonas Holmborn) carried out, together with Dragan Stankovic (Ph.D. student) measurements in Finspång on the AEV and GT10B burners. These results have not been released fully for publications. Parts of the results are included in Jonas Holmborn’s thesis (defended in September 2003). Shear-layer instabilities: Injection of fuel into a GT combustor is done into a shear-layer so as to enhance the break-up, evaporation and mixing of the fuel with incoming air (see for example Figure 114). To better understand the interaction between the injected fuel-jet and flame instabilities, with Siemens approval, we started working on this issue this year. The work includes experimental and computational (LES) work. Some preliminary results are shown in Figure 118. Report Year 9 Competence centre combustion processes at Lund University (92/93) ____________________________________________________________________________ Figure 118: Streamline (left) and vorticity (right) fields induced by injection of a jet into a cross-flow. Note the presence of instability in the jet and near the wall behind the jet. Dragan Stankovic is to complete his TeknL. In February 2005. Publications (July 2003- June 2004): 1. D. Moroianu, M. Mihaescu and L. Fuchs – Numerical computation of the acoustical field around wind-turbines. 10th Int. Congress on Sound and Vibration, July 2003. 2. M. Mihaescu, R. Szasz and L. Fuchs – Modelling of the acoustical field due to a jet engine with ground effects. 10th Int. Congress on Sound and Vibration, July 2003. 3. C. Duwig and L. Fuchs - Numerical Study of excited turbulent Bunsen flames: Vortex control. Proceedings of the Tenth European Turbulence Conference. H. I. Andersson & P.A. Krogstad (Eds.), 2004. 4. A.Secareanu, V. Milosavljevic, J. Holmborn, D. Stankovic and L. Fuchs - Experimental Investigation of Airflow and Spray Stability in an Air-Blast Injector of an Industrial Gas Turbine. GT2004-53961, ASME TURBO EXPO, 2004. 5. M. Mihaesu, R-Z. Szasz and L. Fuchs – Noise computation of a turbo-engine jet exhaust based on LES and Lighthill’s acoustic analogy. GT2004-53580, ASME TURBO EXPO, 2004. 6. C. Duwig and L. Fuchs - Study of a gas turbine combustion chamber: influence of the mixing on the flame dynamics. GT2004-53276, ASME TURBO EXPO, 2004. Report Year 9 Competence centre combustion processes at Lund University (93/93) ____________________________________________________________________________ References 1 Olof Erlandsson, PhD thesis, Heat and Power Dep., Lund Inst. Of Technology, 2002, 2 Giuseppe Cantore, Luca Montorsi, Fabian Mauss, Per Amnéus, Olof Erlandsson, Bengt Johansson, and Thomas Morel, “Full Cycle Simulations of a Truck-size Turbo HCCIengineusing detailed che-mical kinetics”, under revision for submitting to ASME Transactions 3 Martin Tunér, Per Amnéus, Fabian Mauss, Marcus Kraft, Anders Hultkvist, Bengt Johansson., ” Modeling and investigation of exothermic centers in HCCI-combustion”, Planned publication 4 Per Amnéus, Martin Tunér, Fabian Mauss, Robert Collin, Jenny Nygren, Mattias Richter, Marcus Aldén, Markus Kraft, Leif Hildingsson and Bengt Johansson, ”Formaldehyde and hydroxyl radicals in an HCCI engine – calculations and LIFmeasurements”, Work finished, paper under final revising before submitting 5 Per Amnéus, Fabian Mauss, Markus Kraft, Andreas Vressner, Bengt Johansson, “NOx formation and its impact on HCCI ignition”, Submitted for SAE 2005 Detroit conference, 6 L. Koopmans, H. Ström, S. Lundgren, O. Backlund, I. Denbratt: “Demonstrating a SIHCCI-SI Mode Change on a Volvo 5-Cylinder Electronic Valve Control Engine”, SAE Paper 2003-01-0753 7 M. Christensen, P. Einewall, B. Johansson: ”Homogeneous Charge Compression Ignition (HCCI) Using Isooctane, Ethanol and Natural Gas – A Comparison to Spark Ignition Operation”, SAE Paper 972874 8 F. Hermann and J. Klingmann, “Computational and Experimental Investigation of Emissions in a Highly Humidified Premixed Flame”, ASME GT2003-38337, 2003. 9 F. Hermann, T. Lindquist, and J. Klingmann, “Combustion in Humid Air”, First Biennial meeting of the Scandinavian-Nordic Section of the Combustion Institute, Gothenburg, 2001. 10 U. Engdar, F. Hermann, P. Nilsson, and J. Klingmann, “Investigation of Turbulent Combustion in Humid Air using a Level-set Flamelet Library Approach”, ASME GT2004-53364, 2004. 11 F. Hermann, T. Zeuch, and J. Klingmann, “Numerical Modeling of NOx Emission in a Highly Diluted Laminar Premixed Flame”, Joint meeting of the Italien and Scandinavian-Nordic Sections of the Combustion Institute, Ischia 2003. 12 F. Hermann, R. C. Orbay and J. Klingmann, ”Emission measurements in an atmospheric preheated premixed combustor with CO2 dilution”, submitted to European Combustion meeting 2005. 13 U. Engdar, F. Hermann, R. Gabrielsson and J. Klingmann, “CFD Investigation of the Effects on Different Dilutions on the Emissions in a Swirl Stabilized Premixed Combustion System”, Submitted to ASME. 14 F. Hermann, T. Zeuch, and J. Klingmann, “The Effect of Diluents on the Formation Rate of Nitrogen Oxide in a Premixed Laminar Flame”, ASME GT2004-53841, 2004.