Combustion research currently aims to reduce emissions, whilst improving the fuel economy. Burning fuel in excess of air, or lean-burn combustion, is a promising alternative to conventional combustion, and can achieve these requirements simultaneously. However, lean-burn combustion poses new challenges, especially for internal combustion (IC) engines. Therefore, models used to predict such combustion have to be reliable, accurate and robust. In this work, the flamelet approach in the Reynolds-Averaged Navier- Stokes framework, is used to simulate flames relevant to spark-ignition IC engines. A central quantity in the current modelling approach is the scalar dissipation rate, which represents coupling between reaction and diffusion, as well as the flame front dynamics. In the first part of this thesis, the predictive ability of two reaction rate closures, viz. strained and unstrained flamelet models, are assessed through a series of experimental test cases. These cases are: spherically propagating methane- and hydrogen-air flames and combustion in a closed vessel. In addition to these models, simpler algebraic closures are also used for comparison. It is shown that the strained flamelet model can predict unconfined, spherically propagating methane-air flames reasonably well. By comparing spherical flame results with planar flames, under identical thermochemical and turbulence conditions, it is shown that the turbulent flame speed of spherical flames are 10 to 20% higher than that of planar flames, whilst the mean reaction rates are less influenced by the flame geometry. Growth of the flame brush thickness in unsteady spherical flames have been attributed to turbulent diffusion in past studies. However, the present analyses revealed that the dominant cause for this increase is the heat-release induced convective effects, which is a novel observation. Unlike methane-air flames, hydrogen-air flames have non-unity Lewis numbers. Hence, a novel two degrees of freedom approach, using two progress variables, is used to describe the thermochemistry of hydrogen-air flames. Again, it is shown that the strained flamelet model is able to predict the experimental flame growth for stoichiometric hydrogen-air flames. However, none of the models used in this work were able to predict lean hydrogen-air flames. This is because these flames are thermo-diffusively unstable and the current approach is inadequate to represent them. When combustion takes place inside a closed vessel, the compression of the end gases by the propagating flame causes the pressure to rise. This is more representative of real IC engines, where intermittent combustion takes place. The combustion models are implemented in a commercial computational fluid dynamics (CFD) code, STAR-CD, and it is shown that both strained and unstrained flamelet models are able to predict the experimental pressure rise in a closed vessel. In the final part of this work, a spark-ignition engine is simulated in STAR-CD using the flamelet model verified for simpler geometries. It is shown that this model, together with a skeletal mechanism for iso-octane, compares reasonably well with experimental cylinder pressure rise. Results obtained from this model are compared with two models available in STAR- CD. These models require some level of tuning to match the experiments, whereas the modelling approach used in this work does not involve any tunable parameters.

Lean premixed combustion is at present one of the most promising methods to reduce emissions and to maintain high efficiency in combustion systems. As the emission legislation becomes more stringent, modelling of turbulent premixed combustion has become an important tool for designing efficient and environmentally friendlier combustion systems. However, in order to predict these emissions reliable predictive models are required. One of the methods used for predicting pollutants is the conditional moment closure (CMC), which is suitable to predict pollutants with slow time scales. Despite the fact that CMC has been successfully applied to various non-premixed combustion systems, its application to premixed flames is not fully tested and validated. The main difficulty is associated with the modelling of the conditional scalar dissipation rate (CSDR) of the conditioning scalar, the progress variable. In premixed CMC, this term is an important quantity and represents the rate of mixing at small scales of relevance for combustion. The numerical accuracy of the CMC method depends on the accuracy of the CSDR model. In this study, two different models for CSDR, an algebraic model and an inverse problem model, are validated using two different DNS data sets. The algebraic model along with standard k-ε turbulence modelling is used in the computations of stoichiometric and very lean pilot stabilized Bunsen flames using the RANS-CMC method. A first order closure is used for the conditional mean reaction rate. The computed nonreacting and reacting scalars are in reasonable agreement with the experiments and are consistent with earlier computations using flamlets and transported PDF methods for the stoichiometric flames, and transported PDF methods for the very lean flames. Sensitivity to chemical kinetics mechanism is also assessed.

Methane/air premixed and partially premixed turbulent flames at high Reynolds number are characterized using Direct Numerical Simulations (DNS) with detailed chemistry in a spatially evolving slot Bunsen configuration. Two sets of simulations are performed. A first set of simulations with fully premixed inlet conditions is considered in order to assess the effect of turbulence on the flame. Four simulations are performed at increasing Reynolds number and up to 22400, defined based on the bulk velocity, slot width, and the reactants' properties, and 22 billion grid points, making it one of the largest simulations in turbulent combustion. The simulations feature finite rate chemistry with a 16 species mechanism. To perform these simulations, few preliminary steps were required: (i) two skeletal mechanisms were developed reducing GRI-3.0; (ii) a convergence study is performed to select the proper spatial and temporal discretization and (iii) simulations of fully developed turbulent channel flows are preformed to generate the inlet conditions of the jet. The study covers different aspects of flame-turbulence interaction. It is found that the thickness of the reaction zone is similar to that of a laminar flame, while the preheat zone has a lower mean temperature gradient, indicating flame thickening. The characteristic length scales of turbulence are investigated and the effect of the Reynolds number on these quantities is assessed. The tangential rate of strain is responsible for the production of flame surface in the mean and surface destruction is due to the curvature term. A second set of simulations with inhomogeneous inlet conditions is performed to study how partial premixing and turbulence interact with the flame and with each other. The jet Reynolds number is 5600, and a 33 species mechanism is used. The effect of the inlet fluctuations is reflected on heat release rate fluctuations, however the conditional mean is not affected. The flames show thickening of the preheat zone, and for the lowest level of mixing a slight thickening of the reaction zone is observed. The effect of partially mixed mixture on the NOx formation is analyzed and no major impact was found.

<p> In premixed turbulent combustion, the normal speed of propagation of the flame front is enhanced by the turbulent velocity field. This project will focus on the method of computing the normal speed of propagation of the flame front in the Majda-Souganidis model of turbulent combustion. Solving this problem involves computing the eigenvalue of a nonlinear cell problem. Discussed in this thesis is a new, simple and direct numerical method for approximating the eigenvalue, also called the effective Hamiltonian.</p> / Thesis / Master of Science (MSc)

Producer gas derived from biomass is one of the most environment friendly substitutes to the fossil fuels. Usage of producer gas for power generation has effect of zero net addition of CO2 in atmosphere. The engines working on producer gas have potential to decrease the dependence on conventional fuels for power generation. However, the combustion process is governed by complex interactions between chemistry and fluid dynamics, some of which are not completely understood. Improved knowledge of combustion is, therefore, of vital importance for both direct use in the design of engines, and for the evolution of reliable simulation tools for engine development. The present work is related to the turbulent combustion of producer gas in closed vessels and engine cylinders. The main objective of the work was multi-dimensional simulation of turbulent combustion in the bowl-in-piston engine operating on producer gas fuel and to observe the flame and flow field interaction. First, the combustion model was validated in constant volume combustion chamber with experimental results. Experimental turbulent combustion data of producer gas (composition matching with engine operating conditions) was presented. The required data of laminar burning velocity of producer gas was computed and used in the simulation of turbulent combustion in closed vessel. The effect of squish and reverse squish flow on flame propagation in the bowl-in-piston engine cylinder was described. Laminar burning velocity of unstretched flame was computed using flame code which was developed earlier in this laboratory. One dimensional computations of unstretched planar flame were made to calculate laminar burning velocity of the producer gas-air mixture at pressures (1-10 bar) and temperatures (300-600 K). A correlation of laminar burning velocity of producer gas as a function of pressure and temperature was fitted and compared with experiments. A fixed composition and equivalence ratio of producer gas-air mixture, typical of the engine operating conditions, was considered. The correlation was used in simulation of turbulent combustion in closed vessel. The turbulent combustion experiments with producer gas-air mixture were conducted in a closed vessel. The aim of experiments was to generate pressure-time data, in closed vessel during turbulent flame propagation, which was required to validate turbulent combustion models. Determination of (ST /SL) was made from pressure-time data which requires corresponding laminar combustion data with same initial conditions. For this purpose a set of laminar combustion experiments was conducted. Experimental setup consists of a constant volume combustion chamber of cubical shape and size 80 x 80 x 80 mm3 . The initial mixtures pressure and temperature were 1 bar and 300 K respectively. A fixed composition and equivalence ratio of producer gas-air mixture, typical of the engine operating conditions, was used. The composition of producer gas was H2 -19.61%, CO2 -19.68%, CH4 -2.52%, CO2 -12.55% and N2 -45.64% on volume basis. Fuel-air mixture was ignited with electric spark at the center of the cube. Initial turbulence in the chamber was created by moving a perforated plate with specified velocity. Perforated plate was placed in chamber so that the central hole in the plate passes over the spark electrodes as it sweeps across the chamber. Two geometrically similar plates with hole diameter of 5 and 10 mm were used. The new experimental setup constructed as a part of this work was first tested with one set of experiments each with methane and propane data of SL and ST /SL from the literature. Maximum turbulent intensity (u’) achieved was 1.092 ms−1 . The ratios of turbulent to laminar burning velocity (ST /SL) values were determined at six different turbulence intensity levels. Laminar combustion experiments were extended to elevated initial pressures 2-5 bar and temperature 300 K. The value of SL was calculated from the pressure-time history recorded during laminar stretching flame propagation inside closed vessel. These SL values were compared with computed SL,∞ after accounting for stretch. Turbulent combustion simulations were carried out to validate combustion models suitable for multi-dimensional CFD simulation of combustion in constant volume closed chamber. Two models proposed by Choi and Huh, based on Flame Surface Density (FSD) were tested with the present experimental results. User FORTRAN code for the source terms in transport equation of FSD was implemented in ANSYS-CFX 10.0 software. First model called CFM1, grossly under-predicted the rate of combustion. The second model called CFM2, predicted the results satisfactorily after replacing the arbitrary length scale with turbulent integral length scale (lt) having a limiting value near the wall. The modified CFM2 model was able to predict the propagation phase of the developed flame satisfactorily, though the duration for initial flame development was over-predicted by the model. CFD simulation of producer gas engine combustion process was carried out using ANSYSCFX software. Mesh deformation option was used to take care of moving boundaries such as piston and valve surfaces. The fluid domain expands during suction process and contracts during compression process. In order to avoid excessive distortion of the mesh elements, a series of meshes at different crank angle positions were generated and checked for their quality during mesh motion in the solver. For suction process simulation, unstructured meshes having 0.1 to 0.3 million cells were used. During the compression and combustion process simulations, structured meshes having 40,000 to 0.1 million cells were used. k-ε model was used for turbulence simulation. The suction, compression and combustion processes of an SI engine were simulated. Initial flame kernel was given by providing high flame surface density in a small volume comparable to the spark size at the time of ignition. The flame surface density model, CFM-2, was adapted with the modification of length scale tested against constant volume experiments. A suitable limiting value was used to avoid abnormal flame propagation near the wall. The limiting value of integral length scale (lt) near the wall was determined by linear extrapolation of the integral length scale in the domain to the wall. Engine p - θ curves of three different ignition timings 26°, 12° and 6°before top dead center (TDC) were simulated and compared with earlier experimental results. The effects of flow field on flame propagation have been observed. A comparison of the simulated and experimental p - θ diagram of the engine for all above cases gave mixed results. For the ignition timing at 26° before TDC case, predicted peak pressure value was 17% higher and at 3° earlier than those of the experimental peak. For the other two cases, the predicted peak pressure value was 28% lower and 5° later than those of the experimental peak. The reason for under-prediction of the pressure values could be due to the delay in development of initial flame kernel. Simulated pressure curves have offset about 3-4° compared to the experimental pressure curves. It was observed that in all predicted p - θ cases, there was a delay in the initial flame development. It is evident from the under-prediction of pressure values, especially in the initial flame kernel development phase and it also affects the p - θ curve at later stage. The delay was about 3-4° of crank angle rotation in various cases. The delay in predicting the initial flame development needs to be corrected in order to predict the combustion process properly. The proposed FSD model seems to have capability to predict p - θ values fairly in the propagation phase of developed flame. Reasonably good match was obtained by advancing the ignition timing in the computation by about 3-4° compared to the experimental setting. In the bowl-in-piston engine cylinders, the flow in the cylinder is characterised by squish and reverse squish when the piston is moving towards and away from the top dead center (TDC) respectively. The effect of squish and reverse squish flow on flame propagation has been assessed. For the more advanced ignition case, i.e., 26° before TDC, The flame propagation did not have favorable effect by the flow field. The direction of flame propagation was against the squish and reverse squish flow. This resulted in suppressed peak velocities in the cylinder compared the motoring process. Hence the burning rate was not augmented by the turbulence inside the cylinder. For the ignition 12° before TDC case, the flame propagation did have favorable effect by the flow field. During the reverse squish period, the flame had reached the bowl wall. At this stage, the flame was pushing the reactants out and this augments the reverse-squish flow, and hence the maximum reverse-squish velocity was increased to 2.03 times the peak reverse-squish velocity of motoring case. The reverse-squish flow was distorting the flame from spherical shape and the flame gets stretched. Flame surface enters the cylindrical region faster compared to the previous case. The stretched flame in the reverse-squish flow may be considered as reverse squish flame, as was proposed earlier by Sridhar G. The burn rate during the reverse squish period may be 2 to 2.5 times the normal burn rate. For the ignition 6° before TDC case, the flame was very small in size and it did not affect the flow in squish period. During the reverse squish period, the flame radius was moderate compared to the bowl radius. The flame was pushing the reactants out and it increased the maximum reverse-squish velocity to 1.3 times by the flame. In this case, the reverse-squish flow moderately affecting the flame shapes. The results of this study could give an idea of what ignition timing must be kept for favorable use of flow field inside the engine cylinder. Main contributions from the present work are: Multi-dimensional simulation of combustion process inside the engine cylinder operating on producer gas was carried out to examine flame/flow field interactions. Two models based on FSD were first tested against present experimental results in constant volume combustion chamber. In CFM2 model; a modification of replacing the arbitrary length scale by integral length scale with a limiting value near the wall was suggested to avoid prediction of abnormally large turbulent burning velocity near the wall. This combustion model has been implemented in ANSYS-CFX10. The required data of laminar and turbulent burning velocities of producer gas-air mixture has been determined by experiments and computations at varied initial pressures and turbulent intensities. Finally, the simulated engine pressure data has been compared with earlier experimental data of the engine operating on producer gas. The proposed FSD model has the capability to match well with the experimental results except for the initial flame kernel development phase. Even though this issue needs to be resolved, the work has brought out the important interaction between the flame propagation and flow field within the bowl-in-piston engine cylinder.

In the last 10-15 years, large-eddy simulation (LES) has become well established for non-reacting flows, and several successful models have been developed for the transfer of momentum and kinetic energy to the subfilter-scales (SFS). However, for reacting flows, LES is still undergoing significant development. In particular, for many premixed combustion applications, the chemical reactions are confined to propagating surfaces that are significantly thinner than the computational grids used in practical LES. In these situations, the chemical kinetics and its interaction with the turbulence are not resolved and must be entirely modelled. There is, therefore, a need for accurate and robust physical modelling of combustion at the subfilter-scales. In this thesis, modelled transport equations for progress variable and flame surface density (FSD) were implemented and coupled to the Favre-filtered Navier-Stokes equations for a compressible reactive thermally perfect mixture. In order to reduce the computational costs and increase the resolution of simulating combusting flows, a parallel adaptive mesh (AMR) refinement finite-volume algorithm was extended and used for the prediction of turbulent premixed flames. The proposed LES methodology was applied to the numerical solution of freely propagating flames in decaying isotropic turbulent flow and Bunsen-type flames. Results for both stoichiometric and lean flames are presented. Comparisons are made between turbulent flame structure predictions for methane, propane, hydrogen fuels, and other available numerical results and experimental data. Details of subfilter-scale modelling, numerical solution scheme, computational results, and capabilities of the methodology for predicting premixed combustion processes are included in the discussions. The current study represents the first application of a full transport equation model for the FSD to LES of a laboratory-scale turbulent premixed flame. The comparisons of the LES results of this thesis to the experimental data provide strong support for the validity of the modelled transport equation for the FSD. While the LES predictions of turbulent burning rate are seemingly correct for flames lying within the wrinkled and corrugated flamelet regimes and for lower turbulence intensities, the findings cast doubt on the validity of the flamelet approximation for flames within the thin reaction zones regime.

Avec l’accroissement considérable de la puissance de calcul, les simulations aux grandes échelles (SGE) sont maintenant utilisées de façon routinière dans de nombreuses applications d’ingénierie. Les modèles de combustion usuels utilisés dans les SGE sont le plus souvent basés sur une hypothèse d’équilibre entre le mouvement des structures turbulentes et le plissement de la surface de la flamme. Ils s’écrivent alors sous forme d’expressions algébriques fonctions de grandeurs connues aux échelles résolues ainsi que de paramètres dont l’ajustement est à la charge de l’utilisateur selon la configuration étudiée et les conditions opératoires. Le modèle dynamique récemment développé ajuste automatiquement au cours du calcul les paramètres de modélisation qui peuvent alors dépendre du temps et de l’espace. Cette thèse présente une étude détaillée d’un modèle dynamique pour la simulation aux grandes échelles de la combustion turbulente prémélangée. L’objectif est de caractériser, explorer les avantages et les inconvénients, appliquer et valider le modèle dynamique dans plusieurs configurations. / Large eddy simulation (LES) is currently applied in a wide range of engineering applications. Classical LES combustion models are based on algebraic expressions and assume equilibrium between turbulence and flame wrinkling which is generally not verified in many circumstances as the flame is laminar at early stages and progressively wrinkled by turbulent motions. In practice, this conceptual drawback has a strong consequence: every computation needs its own set of constants, i.e. any small change in the operating conditions or in the geometry requires an adjustment of model parameters. The dynamic model recently developed adjust automatically the flame wrinkling factor from the knowledge of resolved scales. Widely used to describe the unresolved turbulent transport, the dynamic approach remains underexplored in combustion despite its interesting potential. This thesis presents a detailed study of a dynamic wrinkling factor model for large eddy simulation of turbulent premixed combustion. The goal of this thesis is to characterize, unveil pros and cons, apply and validate the dynamic modeling in different flow configurations.

L’industrie automobile se trouve aucentre des préoccupations environnementalesactuelles. Les moteurs essence à injection directeopérés en condition pauvres offrent un fortpotentiel en terme de réduction des émissions depolluants. En contrepartie, ils sont sujets à uneforte variabilité cyclique de combustion (CCV)qui ne peut être que partiellement étudiéeexpérimentalement. La simulation aux grandeséchelles (SGE) apparait comme une approchenumérique adaptée pour étudier de telsphénomènes, du fait de sa capacité naturelle àcapter les phénomènes instationnaires. Laprésente thèse se propose d’une part d’estimer lacapacité de la SGE à reproduire les CCVobservées expérimentalement, et d’autre part decontribuer à une meilleure compréhension deleur apparition. Dans ce contexte, un effortparticulier est mis sur la modélisation desphénomènes proche paroi. En particulier, unmodèle de paroi adapté à l’étude del’aérodynamique interne de configurationindustrielles est proposé. Il est validé sur desconfigurations de complexités variées. Enfin,l’étude porte sur le cas du moteur M256 qui estétudié en s’appuyant sur une solide base dedonnées expérimentales. Les causes de CCV sontexplorées, et notamment, l’impact de lavariabilité de l’écoulement généré pendant laphase d’admission sur la propagation du front deflamme est clarifié. / The automotive industry finds itselfat the center of current environmental concerns.Modern direct injection engines, operated underlean condition have the potential to reducepollutant emissions. As a drawback, they aresubject to large cyclic combustion variability(CCV), that can be explained only partially byexperimental measurements. Large-EddySimulation (LES) appears as an adapted tool tocomplement experiments, due to its naturalability to capture unsteady phenomena. Thepresent PhD thesis first aims at reproducing theCCV, and at contributing achieving a betterunderstanding of their occurrence.In this context, a special effort is put on thereproduction of near-wall phenomena, throughthe proposal of a new wall boundary conditionthat is validated on cases of differentcomplexity. Then the focus is put on the M256case, for which an extensive experimentaldatabase is available. The causes of CCV areexplored, and in particular, the impact of thevariability of the intake flow on the flame frontpropagation is clarified.

The present thesis considers several important problems of combustion theory, which are closely related to each other: turbulent burning, flame interaction with walls in different geometries, flame acceleration and detonation triggering. The theory of turbulent burning is developed within the renormalization approach. The theory takes into account realistic thermal expansion of burning matter. Unlike previous renormalization models of turbulent burning, the theory includes flame interaction with vortices aligned both perpendicular and parallel to average direction of flame propagation. The perpendicular vortices distort a flame front due to kinematical drift; the parallel vortices modify the flame shape because of the centrifugal force. A corrugated flame front consumes more fuel mixture per unit of time and propagates much faster. The Darrieus-Landau instability is also included in the theory. The instability becomes especially important when the characteristic length scale of the flow is large. Flame interaction with non-slip walls is another large-scale effect, which influences the flame shape and the turbulent burning rate. This interaction is investigated in the thesis in different geometries of tubes with open / closed ends. When the tube ends are open, then flame interaction with non-slip walls leads to an oscillating regime of burning. Flame oscillations are investigated for different flame parameters and tube widths. The average increase in the burning rate in the oscillations is found. Then, propagating from a closed tube end, a flame accelerates according to the Shelkin mechanism. In the theses, an analytical theory of laminar flame acceleration is developed. The theory predicts the acceleration rate, the flame shape and the velocity profile in the flow pushed by the flame. The theory is validated by extensive numerical simulations. An alternative mechanism of flame acceleration is also considered, which is possible at the initial stages of burning in tubes. The mechanism is investigated using the analytical theory and direct numerical simulations. The analytical and numerical results are in very good agreement with previous experiments on “tulip” flames. The analytical theory of explosion triggering by an accelerating flame is developed. The theory describes heating of the fuel mixture by a compression wave pushed by an accelerating flame. As a result, the fuel mixture may explode ahead of the flame front. The explosion time is calculated. The theory shows good agreement with previous numerical simulations on deflagration-to-detonation transition in laminar flows. Flame interaction with sound waves is studied in the geometry of a flame propagating to a closed tube end. It is demonstrated numerically that intrinsic flame oscillations coming into resonance with acoustic waves may lead to violent folding of the flame front with a drastic increase in the burning rate. The flame folding is related to the Rayleigh-Taylor instability developing at the flame front in the oscillating acceleration field of the acoustic wave.

Autoignition of non-premixed methane-air mixtures is investigated using first-order Conditional Moment closure (CMC). In CMC, scalar quantities are conditionally averaged with respect to a conserved scalar, usually the mixture fraction. The conditional fluctuations are often of small order, allowing the chemical source term to be modeled as a function of the conditional species concentrations and the conditional enthalpy (temperature). The first-order CMC derivation leaves many terms unclosed such as the conditional scalar dissipation rate, velocity and turbulent fluxes, and the probability density function. Submodels for these quantities are discussed and validated against Direct Numerical Simulations (DNS). The CMC and the turbulent velocity and mixing fields calculations are decoupled based on the frozen mixing assumption, and the CMC equations are cross-stream averaged across the flow following the shear flow approximation. Finite differences are used to discretize the equations, and a two-step fractional method is implemented to treat separately the stiff chemical source term. The stiff ODE solver LSODE is used to solve the resulting system of equations. The recently developed detailed chemical kinetics mechanism UBC-Mech 1.0 is employed throughout this study, and preexisting mechanisms are visited. Several ignition criteria are also investigated. Homogeneous and inhomogeneous CMC calculations are performed in order to investigate the role of physical transport in autoignition. Furthermore, the results of the perfectly homogeneous reactor calculations are presented and the critical value of the scalar dissipation rate for ignition is determined. The results are compared to the shock tube experimental data of Sullivan et al. The current results show good agreement with the experiments in terms of both ignition delay and ignition kernel location, and the trends obtained in the experiments are successfully reproduced. The results were shown to be sensitive to the scalar dissipation model, the chemical kinetics, and the ignition criterion.