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1.
  • Edman, Jonas, 1973 (författare)
  • Modeling Diesel spray combustion using a Detailed Chemistry Approach
  • 2005
  • Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
    • The rapid development of computer hardware during the past decade has contributed substantially to advances in almost all branches of science. Computer modeling is being applied to increasingly small physical scales and increasingly large domains, facilitating the generation of advanced phenomenological models and models based on first principles. These developments have been especially valuable in fields where highly complex micro-scale events are observed or modeled, such as combustion studies, allowing (for instance) the incorporation of complex chemical combustion kinetics into engine spray combustion models. The crude models and global curve fits that were previously used to represent combustion phenomena have now been largely replaced by models based on "first principles". These modeling developments have coincided fortuitously with a shift in the focus of combustion concepts, from mixing-oriented combustion modes like Diesel and stratified charge Otto combustion to the kinetically controlled combustion modes usually referred to as Homogeneous Charge Compression Ignition (HCCI). The driving forces behind the development of the HCCI concept are environmental considerations, manifested in the form of emission legislation. Theoretically, HCCI combustion (characterized by fuel lean mixtures and low peak temperatures) has the potential to reduce soot and NOx emissions to current emission legislation levels even without after-treatment systems. In practical production engine applications, due to current drawbacks such as poor high load capability, the capacity to switch to conventional mode at high load operation is required. For the above reasons, computer modeling that is capable of describing both old and new combustion modes is required. In the work underlying this thesis, CFD modeling was applied to the passenger car Dl Diesel engine operated in both HCCI and conventional Diesel combustion modes. The aim was to couple chemical combustion kinetics and turbulent mixing in order to capture relevant phenomena related to ignition and emission formation for both modes. The resulting, coupled model is referred to as the Partially Stirred Reactor model (PaSR), and is the main component in the Detailed Chemistry Approach currently utilized in combustion modeling at Chalmers University of Technology (CTH). Other essential components of the Detailed Chemistry Approach are the Reference Species Technique (used to determine the relevant chemical timescales) and the Diesel fuel surrogate model (constructed to facilitate realistic treatment of the fuel in both liquid and gaseous states). The gaseous kinetic treatment of the Diesel fuel surrogate model, represented by a blend of aliphatic and aromatic components, consists of a chemical kinetic mechanism considering -75 chemical species participating in -330 elementary or global reactions describing n-heptane and toluene oxidation. Although most of the modeling was done in the CFD code KIVA-3V rel2, the development and validation of the chemical kinetic combustion mechanism was done using the SENKIN code and the CHEMKIN package. The chemical kinetic modeling has provided a kinetic mechanism for Diesel combustion that is capable of reproducing experimental ignition delay characteristics of both n-heptane and toluene oxidation in both low and high pressure regimes. In addition, it reproduces the negative temperature coefficient behavior that is an important feature of commercial Diesel fuels. It has also been able to reproduce cool flame phenomena, which play important roles in HCCI combustion. Results from the constant volume spray modeling have shown that the spray development, liquid and gas penetration and ignition characteristics observed in high pressure Diesel spray experiments are properly reproduced. Furthermore, major combustion variables such as ignition timing, heat release and pressure traces generated in engine simulations have satisfactorily reproduced experimental data acquired in tests using a single cylinder engine at Chalmers University of Technology.
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2.
  • Kyprianidis, Konstantinos, et al. (författare)
  • Dynamic performance investigations of a turbojet engine using a cross-application visual oriented platform
  • 2008
  • Ingår i: Aeronautical Journal. - 0001-9240. ; 112:1129, s. 161-169
  • Tidskriftsartikel (refereegranskat)abstract
    • This paper presents the development of visual oriented tools for the dynamic performance simulation of a turbojet engine using a cross-application approach. In particular, the study focuses on the feasibility of developing simulation models using different programming environments and linking them together using a popular spreadsheet program. As a result of this effort, a low fidelity cycle program has been created, capable of being integrated with other performance models. The amount of laboratory sessions required for student training during an educational procedure, for example for a course in gas turbine performance simulation, is greatly reduced due to the familiarity of most students with the spreadsheet software. The model results have been validated using commercially available gas turbine simulation software and experimental data from open literature. The most important finding of this study is the capability of the program to link to aircraft performance models and predict the transient working line of the engine for various initial conditions in order to dynamically simulate flight phases including take-off and landing.
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3.
  • Golovitchev, Valeri, 1945, et al. (författare)
  • CFD COMBUSTION AND EMISSION FORMATION MODELING FOR A HSDI DIESEL ENGINE USING DETAILED CHEMISTRY
  • 2006
  • Ingår i: ASME 2006 Internal Combustion Engine Division Fall Technical Conference, ICEF 2006. ; , s. 349-358
  • Konferensbidrag (refereegranskat)abstract
    • In order to comply with current emissions regulations, a detailed analysis of the combustion and emission formation processes in the Diesel engines accounting for the effect of the main operating parameters is required. The present study is based both on 0D and 3D numerical simulations by compiling 0D chemical kinetics calculations for Diesel oil surrogate combustion and emission (soot, NOx) formation mechanisms to construct a φ-T (equivalence ratio - temperature) parametric map. In this map, the regions of emissions formation are depicted defining a possible optimal path between the regions by placing on the same map the engine operation conditions represented by the computational cells, whose parameters (equivalence ratio and temperature) are calculated by means of 3D engine modelling. Unlike previous approaches based on static parametric φ-T maps to analyze different combustion regimes and emission formations in Diesel engines, the present paper focuses on a construction of dynamic φ-T maps, in which the pressures and the elapsed times were taken in compliance with those calculated in the 3D engine simulations. The 0D chemical kinetics calculations have been performed by the SENKIN code of the Chemkin-2 library. In-cylinder conditions represented by computational cells with known φ and T are predicted using KIVA-3V code. When cells are plotted on the map, they identify the trajectories helping to navigate between the emissions regions by varying hardware and injection parameters. Sub-models of the KIVA-3V, rel. 2 code has been modified including spray atomization, droplet collision and evaporation, accounting for multi-component fuel vapor coupled with the improved versions of the chemistry/turbulence interaction model and new formulation of the combustion kinetics for the diesel oil surrogate (consisting in 70 species participating in 310 reactions). Simulations were performed for the HSDI 1.300 Fiat Diesel engine at optimized engine operating conditions and pilot injections. Finally, numerical results are compared with the experimental data on in-cylinder pressure, Rate of Heat Release, RoHR, and selected species distributions.
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4.
  • Heyne, Stefan, 1979, et al. (författare)
  • Numerical simulations of a prechamber autoignition engine operating on natural gas
  • 2009
  • Ingår i: ECOS 2009 - 22nd International Conference on Efficiency, Cost, Optimization, Simulation and Environmental Impact of Energy Systems. ; , s. 1969-1978
  • Konferensbidrag (refereegranskat)abstract
    • Small to medium scale cogeneration engines are a common means of power production in remote areas. Reducing emissions of this type of power generation equipment-while maintaining high efficiencies-is an effective way of reducing greenhouse gas emissions both on a local and global level. At out laboratory extensive research has been conducted on the conversion of conventional Diesel cogeneration engines to operation on natural gas and biogas. By equipping the engines with a prechamber, Swiss emission limits could be kept without exhaust gas treatment while keeping high efficiencies. Recent research has focused on further improving the prechamber concept by converting the spark ignited prechamber to a prechamber operating in autoignition mode. In the framework of this research, a numerical simulation of a prechamber autoignition gas engine has been performed based on an experimental test case. With a simplified finite-rate/eddy-dissipation model for the combustion of natural gas, it was possible to properly reproduce the experiment considering the combustion duration, ignition timing and overall energy balance. However the predefined empiric constant of the eddy-dissipation model had to be increased by a factor of 10. A modification of the original cylindrical-conical prechamber geometry to a simpler cylindrical one was tested with the simulation model. The influence of burnt gases inside the prechamber was assessed simulating the mixture formation inside the prechamber. The simulations showed little effect of taking into account the non-homogeneities in the gas phase on the combustion duration. The simulation showed that the new and cylindrical geometry envisaged did not show any improvement in the combustion homogeneity inside the prechamber and its volume (limited by the real engine geometry) is in fact not sufficient to properly ignite the main chamber. The model can be used to further guide design modifications of the prechamber engine to improve performance.
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5.
  • Husberg, Tobias, 1975, et al. (författare)
  • Analysis of advanced multiple injection strategies in a heavy-duty diesel engine using optical measurements and CFD-simulations
  • 2008
  • Ingår i: SAE Technical Papers. - 400 Commonwealth Drive, Warrendale, PA, United States : SAE International. - 0148-7191.
  • Tidskriftsartikel (refereegranskat)abstract
    • In order to meet future emissions legislation for Diesel engines and reduce their CO 2 emissions it is necessary to improve diesel combustion by reducing the emissions it generates, while maintaining high efficiency and low fuel consumption. Advanced injection strategies offer possible ways to improve the trade-offs between NOx, PM and fuel consumption. In particular, use of high EGR levels ( > 40%) together with multiple injection strategies provides possibilities to reduce both engine-out NOx and soot emissions. Comparisons of optical engine measurements with CFD simulations enable detailed analysis of such combustion concepts. Thus, CFD simulations are important aids to understanding combustion phenomena, but the models used need to be able to model cases with advanced injection strategies. Thus, in the study presented here, engine tests were performed with settings selected to simplify CFD simulation, with long dwell times between the injections and only injection changes between engine settings in test cases presented in this paper. The key to reducing both soot and NOx emissions by applying pilot injections is that the pilot injected fuel should not ignite before sufficient mixing/lean-out has occurred. Hence, substantial heat releases prior to the main injection must be prevented. Thus, high EGR levels are needed to increase the bulk gas mass and reduce the temperature so that there is sufficient time for the pilot injections to mix and become locally lean before ignition. Copyright © 2008 SAE International.
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6.
  • Orbay, Raik, 1974, et al. (författare)
  • Off-Design Performance Investigation of a Low Calorific Value Gas Fired Generic-Type Single-Shaft Gas Turbine.
  • 2008
  • Ingår i: Journal of Engineering for Gas Turbines and Power. - : ASME International. - 1528-8919 .- 0742-4795. ; 130:3
  • Tidskriftsartikel (refereegranskat)abstract
    • When low calorific value gases are fired, the performance and stability of gas turbines may deteriorate due to a large amount of inertballast and changes in working fluid properties. Since it is rather rare to have custom-built gas turbines for low lower heating value (LHV) operation, the engine will be forced to operate outside its design envelope. This, in turn, poses limitations to usable fuel choices. Typical restraints are decrease in Wobbe index and surge and flutter margins for turbomachinery. In this study, an advanced performance deck has been used to quantify the impact of firing low-LHV gases in a generic-type recuperated as well as unrecuperated gas turbine. A single-shaft gas turbine characterized by a compressor and an expander map is considered. Emphasis has been put on predicting the off-design behavior. The combustor is discussed and related to previous experiments that include investigation of flammability limits, Wobbe index, flame position, etc. The computations show that at constant turbine inlet temperature, the shaft power and the pressure ratio will increase; however, the surge margin will decrease. Possible design changes in the component level are also discussed. Aerodynamic issues (and necessary modifications) that can pose severe limitations on the gas turbine compressor and turbine sections are discussed. Typical methods for axial turbine capacity adjustment are presented and discussed.
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7.
  • Ask, Jonas, 1970, et al. (författare)
  • Flow and dipole source evaluation of a generic SUV
  • 2007
  • Ingår i: 13th AIAA/CEAS Aeroacoustics Conference (28th AIAA Aeroacoustics Conference). - Reston, Virigina : American Institute of Aeronautics and Astronautics.
  • Konferensbidrag (refereegranskat)abstract
    • Accurately predicting both average flow quantities and acoustic sources at the front side window of today's ground vehicles is still a considerable challenge to automotive companies world-wide. One of the most important aspects for obtaining trustworthy results, but also the most tedious one and therefore perhaps overlooked, is the control and outcome of the mesh generation process. Generating unstructured volume meshes suitable for Large Eddy Simulations with high level representation of geometrical details is both a time consuming and an extremely computer demanding activity. This work investigates two different mesh generation processes with the main aim to evaluate their outcome with respect to the prediction of the two dominating dipole sources in a temporal form of the Curie's equation. Only a handful of papers exists with high level representation of the vehicle geometry and the aim of predicting the fluctuating exterior noise sources. To the author's knowledge no studies have been conducted in which both these source terms are evaluated quantitatively against measurements. The current paper investigates the degree to which the amplitude of these two source terms can be predicted by using the traditional law-of-the-wall and hex-dominant meshes with isotropic resolution boxes for a detailed ground vehicle geometry. For this purpose the unstructured segregated commercial FLUENT Finite Volume Method code is used. The flow field is treated as incompressible, and the Smagorinsky-Lilly model is used to compute the sub-grid stresses. Mean flow quantities are measured with a 14-hole probe for 14 rakes downstream of the side mirror. Dynamic pressure sensors are distributed at 16 different positions over the side window to capture the fluctuating pressure signals. All measurements in this work were conducted at Ford's acoustic wind tunnel in Cologne. All simulations accurately predict the velocity magnitude closest to the side window and downstream of the mirror head recirculation zoner. Some variations in the size and shape of this recirculation zone are found between the different meshes, most probably caused by differences in the detachment of the mirror head boundary layer. The Strouhal number of the shortest simulation was computed from the fundamental frequency of the mirror lift force component. The computed Strouhal number agrees well with the corresponding results from similar objects and gives an indication of an acceptable simulation time. Dynamic pressure sensors at 16 different locations at the vehicle side window were also used to capture the levels of the two dipole source terms. These results are compared against the three simulations. With the exception of three positions, at least one of the three simulations accurately captures the levels of both source terms up to about 1000Hz. The three positions with less agreement as compared to measurements were found to be in regions sensitive to small changes in the local flow direction.
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8.
  • Delattin, Frank, et al. (författare)
  • A Comparison Between the Combustion of Natural Gas and Partially Reformed Natural Gas in an Atmospheric Lean Premixed Turbine-Type Combustor
  • 2008
  • Ingår i: Combustion Science and Technology. - : Informa UK Limited. - 0010-2202 .- 1563-521X. ; 180:8
  • Tidskriftsartikel (refereegranskat)abstract
    • A small-scale combustor was set up to analyze the combustion of natural gas and two mixtures of partially reformed natural gas. The partially reformed mixtures can be formed using biomass to feed the endothermic reforming reactions. Before combusting these mixtures in a gas turbine, experimental work was done on a primary zone combustion chamber to examine the combustor behavior when switching from natural gas to the wet and dry hydrogen-rich mixtures. Temperature profiles, flame location and ignition limits have been investigated for a variety of stoichiometries and several air temperatures. Possible problems concerning blow-off, flashback, increased pollutant products and excessive liner wall temperatures were analyzed. It was concluded that the switch in operation from natural gas to these wet and/or dry partially reformed natural gas mixtures lowers the blow-off limits while maintaining similar liner wall temperature profiles. Furthermore, no significant changes in pollutant production were observed. Flame area, shape and position display considerable differences in combustion regime for the three tested fuel types.
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9.
  • Kyprianidis, Konstantinos G., et al. (författare)
  • Thermo-Fluid Modelling for Gas Turbines-Part I: Theoretical Foundation and Uncertainty Analysis
  • 2009
  • Ingår i: ASME TURBO EXPO 2009 Proceedings, GT2009-60092.
  • Konferensbidrag (refereegranskat)abstract
    • In this two-part publication, various aspects of thermo-fluidmodelling for gas turbines are described and their impact onperformance calculations and emissions predictions at aircraftsystem level is assessed. Accurate and reliable fluid modellingis essential for any gas turbine performance simulation softwareas it provides a robust foundation for building advanced multidisciplinarymodelling capabilities. Caloric properties forgeneric and semi-generic gas turbine performance simulationcodes can be calculated at various levels of fidelity; selection ofthe fidelity level is dependent upon the objectives of thesimulation and execution time constraints. However, rigorousfluid modelling may not necessarily improve performancesimulation accuracy unless all modelling assumptions andsources of uncertainty are aligned to the same level. Certainmodelling aspects such as the introduction of chemical kinetics,and dissociation effects, may reduce computational speed andthis is of significant importance for radical space explorationand novel propulsion cycle assessment.This paper describes and compares fluid models, based ondifferent levels of fidelity, which have been developed for anindustry standard gas turbine performance simulation code and an environmental assessment tool for novel propulsion cycles.The latter comprises the following modules: engineperformance, aircraft performance, emissions prediction, andenvironmental impact. The work presented aims to fill thecurrent literature gap by: (i) investigating the commonassumptions made in thermo-fluid modelling for gas turbinesand their effect on caloric properties and (ii) assessing theimpact of uncertainties on performance calculations andemissions predictions at aircraft system level.In Part I of this two-part publication, a comprehensiveanalysis of thermo-fluid modelling for gas turbines is presentedand the fluid models developed are discussed in detail.Common technical models, used for calculating caloricproperties, are compared while typical assumptions made influid modelling, and the uncertainties induced, are examined.Several analyses, which demonstrate the effects of composition,temperature and pressure on caloric properties of workingmediums for gas turbines, are presented. The working mediumsexamined include dry air and combustion products for variousfuels and H/C ratios. The errors induced by ignoringdissociation effects are also discussed.
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10.
  • Kyprianidis, Konstantinos G., et al. (författare)
  • Thermo-Fluid Modelling for Gas Turbines-Part II : Impact on Performance Calculations and Emissions Predictions at Aircraft System Level
  • 2009
  • Ingår i: ASME TURBO EXPO 2009 Proceedings, GT-2009-60101. ; , s. 483-494
  • Konferensbidrag (refereegranskat)abstract
    • In this two-part publication, various aspects of thermo-fluidmodelling for gas turbines are described and their impact onperformance calculations and emissions predictions at aircraftsystem level is assessed. Accurate and reliable fluid modellingis essential for any gas turbine performance simulation softwareas it provides a robust foundation for building advanced multidisciplinarymodelling capabilities. Caloric properties forgeneric and semi-generic gas turbine performance simulationcodes can be calculated at various levels of fidelity; selection ofthe fidelity level is dependent upon the objectives of thesimulation and execution time constraints. However, rigorousfluid modelling may not necessarily improve performancesimulation accuracy unless all modelling assumptions andsources of uncertainty are aligned to the same level. Certainmodelling aspects such as the introduction of chemical kinetics,and dissociation effects, may reduce computational speed andthis is of significant importance for radical space explorationand novel propulsion cycle assessment.This paper describes and compares fluid models, based ondifferent levels of fidelity, which have been developed for anindustry standard gas turbine performance simulation code and an environmental assessment tool for novel propulsion cycles.The latter comprises the following modules: engineperformance, aircraft performance, emissions prediction, andenvironmental impact. The work presented aims to fill thecurrent literature gap by: (i) investigating the commonassumptions made in thermo-fluid modelling for gas turbinesand their effect on caloric properties and (ii) assessing theimpact of uncertainties on performance calculations andemissions predictions at aircraft system level.In Part II of this two-part publication, the uncertaintyinduced in performance calculations by common technicalmodels, used for calculating caloric properties, is discussed atengine level. The errors induced by ignoring dissociation areexamined at 3 different levels: i) component level, ii) enginelevel, and iii) aircraft system level. Essentially, an attempt ismade to shed light on the trade-off between improving theaccuracy of a fluid model and the accuracy of a multidisciplinarysimulation at aircraft system level, againstcomputational time penalties. The results obtained demonstratethat accurate modelling of the working fluid is not alwaysessential; the accuracy/uncertainty for an overall engine modelwill always be better than the mean accuracy/uncertainty of the individual component estimates as long as systematic errors arecarefully examined and reduced to acceptable levels to ensureerror propagation does not cause significant discrepancies.Computational time penalties induced by improving theaccuracy of the fluid model as well as the validity of the idealgas assumption for future turbofan engines and novelpropulsion cycles are discussed.
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