The search for clean and efficient combustors is motivated by the increasingly stringent emissions regulations. New gas turbine engines are designed to operate under lean conditions with inhomogeneous reactants to ensure cleanliness and stability of the combustion. This ushers in a new mode of combustion, called the inhomogeneous reactants premixed combustion. The present study investigates the effects of inhomogeneous reactants on premixed combustion, specifically on the interactions of an initially planar flame with field of inhomogeneous reactants. Unsteady and unstrained laminar methane-air flames are studied in one- and two-dimensional simulations to investigate the effects of normally and tangentially (to the flame surface) stratified reactants. A three-dimensional DNS of turbulent inhomogeneous reactants premixed combustion is performed to extend the investigation into turbulent flames. The methaneair combustion is represented by a complex chemical reaction mechanism with 18 species and 68 steps. The flame surface density (FSD) and displacement speed S_d have been used as the framework to analyse the inhomogeneous reactants premixed flame. The flames are characterised by an isosurface of reaction progress variable. The unsteady flames are compared to the steady laminar unstrained reference case. An equivalence ratio dip is observed in all simulations and it can serve as a marker for the premixed flame. The dip is attributed to the preferential diffusion of carbon- and hydrogen- containing species. Hysteresis of S_d is observed in the unsteady and unstrained laminar flames that propagate into normally stratified reactants. Stoichiometric flames propagating into lean mixture have a larger S_d than lean flames propagating into stoichiometric mixtures. The cross-dissipation term contribution to S_d is small (~~10%) but its contribution to the hysteresis of S_d is not (~~50%). Differential propagation of the flame surface is observed in the laminar flame that propagates into tangentially stratified reactants. Stretch on the flame surface is induced by the differential propagation, which in turn increases the flame surface area.

Large Eddy Simulation (LES) of bluff body stabilized premixed and partially premixed combustion close to the flammability limit is carried out in this thesis. The LES algorithm has no ad-hoc adjustable model parameters and is able to respond automatically to variations in the inflow conditions. Algorithm validation is achieved by comparison with reactive and non-reactive experimental data. In the reactive flow, two scalar closure models, Eddy Break-Up (EBULES) and Linear Eddy Mixing (LEMLES), are used and compared. Over important regions, the flame lies in the Broken Reaction Zone regime. Here, the EBU model assumptions fail. The flame thickness predicted by LEMLES is smaller and the flame is faster to respond to turbulent fluctuations, resulting in a more significant wrinkling of the flame surface. As a result, LEMLES captures better the subtle effects of the flame-turbulence interaction. Three premixed (equivalence ratio = 0.6, 0.65, and 0.75) cases are simulated. For the leaner case, the flame temperature is lower, the heat release is reduced and vorticity is stronger. As a result, the flame in this case is found to be unstable. In the rich case, the flame temperature is higher, and the spreading rate of the wake is increased due to the higher amount of heat release Partially premixed combustion is simulated for cases where the transverse profile of the inflow equivalence ratio is variable. The simulations show that for mixtures leaner in the core the vortical pattern tends towards anti-symmetry and the heat release decreases, resulting also in instability of the flame. For mixtures richer in the core, the flame displays sinusoidal flapping resulting in larger wake spreading. More accurate predictions of flame stability will require the use of detailed chemistry, raising the computational cost of the simulation. To address this issue, a novel algorithm for training Artificial Neural Networks (ANN) for prediction of the chemical source terms has been implemented and tested. Compared to earlier methods, the main advantages of the ANN method are in CPU time and disk space and memory reduction.

A large eddy simulation (LES) model has been developed and validated for turbulent non-premixed and partially premixed combustion systems. LES based combustion modelling strategy has the ability to capture the detailed structure of turbulent flames and account for the effects of radiation heat loss. Effects of radiation heat loss is modelled by employing an enthalpy-defect based non-adiabatic flamelet model (NAFM) in conjunction with a steady non-adiabatic flamelet approach. The steady laminar flamelet model (SLFM) is used with multiple flamelet solutions through the development of pre-integrated look up tables. The performance of the non-adiabatic model is assessed against experimental measurements of turbulent CH4/H2 bluff-body stabilized and swirl stabilized jet flames carried out by the University of Sydney combustion group. Significant enhancements in the predictions of mean thermal structure have been observed with both bluff body and swirl stabilized flames by the consideration of radiation heat loss through the non-adiabatic flamelet model. In particular, mass fractions of product species like CO2 and H2O have been improved with the consideration of radiation heat loss. From the Sydney University data the HM3e flame was also investigated with SLFM using multiple flamelet strategy and reasonably fair amount of success has been achieved. In this work, unsteady flamelet/progress variable (UFPV) approach based combustion model which has the potential to describe both non-premixed and partially premixed combustion, has been developed and incorporated in an in-house LES code. The probability density function (PDF) for reaction progress variable and scalar dissipation rate is assumed to follow a delta distribution while mixture fraction takes the shape of a beta PDF. The performance of the developed model in predicting the thermal structure of a partially premixed lifted turbulent jet flame in vitiated co-flow has been evaluated. The UFPV model has been found to successfully predict the flame lift-off, in contrast SLFM results in a false attached flame. The mean lift-off height is however over-predicted by UFPV-δ function model by ~20% for methane based flame and under-predicted by ~50% for hydrogen based flame. The form of the PDF for the reaction progress variable and inclusion of a scalar dissipation rate thus seems to have a strong influence on the predictions of gross characteristics of the flame. Inclusion of scalar dissipation rate in the calculations appears to be successful in predicting the flame extinction and re-ignition phenomena. The beta PDF distribution for the reaction progress variable would be a true prospect for extending the current simulation to predict the flame characteristics to a higher degree.

Numerical simulations have proven itself as a significant and powerful tool for accurate prediction of turbulent premixed flames in practical engineering devices. The work presented in this thesis concerns the development of simulation techniques for premixed turbulent combustion of three different fuels, namely, CNG, LPG and Hydrogen air mixtures. The numerical results are validated against published experimental data from the newly built Sydney combustion chamber. In this work a newly developed Large Eddy Simulation (LES) CFD model is applied to the new Sydney combustion chamber of size 50 x 50 x 250 mm (0.625 litre volume). Turbulence is generated in the chamber by introducing series of baffle plates and a solid square obstacle at various axial locations. These baffles can be added or removed from the chamber to adapt various experimental configurations for studies. This is essential to understand the flame behaviour and the structure. The LES numerical simulations are conducted using the Smagorinsky eddy viscosity model with standard dynamic procedures for sub-grid scale turbulence. Combustion is modelled by using a newly developed dynamic flame surface density (DFSD) model based on the flamelet assumption. Various numerical tests are carried out to establish the confidence in the LES based combustion modelling technique. A detailed analysis has been carried out to determine the regimes of combustion at different stages of flame propagation inside the chamber. The predictions using the DFSD combustion model are evaluated and validated against experimental measurements for various flow configurations. In addition, the in-house code capability is extended by implementing the Lewis number effects. The LES predictions are identified to be in a very good agreement with the experimental measurements for cases with high turbulence levels. However, some disagreement were observed with the quasi-laminar case. In addition a data analysis for experimental data, regarding the overpressure, flame position and the flame speed is carried out for the high and low turbulence cases. Moreover, an image processing procedure is used to extract the flame rate of stretch from both the experimental and numerical flame images that are used as a further method to validate the numerical results. For the grids under investigation, it is concluded that the employed grid is independent of the filter width and grid resolution. The applicability of the DFSD model using grid-independent results for turbulent premixed propagating flames was examined by validating the generated pressure and other flame characteristics, such as flame position and speed against experimental data. This study concludes that the predictions using DFSD model provide reasonably good results. It is found that LES predictions were slightly improved in predicting overpressure, flame position and speed by incorporating the Lewis number effect in the model. Also, the investigation demonstrates the effects of placing multiple obstacles at various locations in the path of the turbulent propagating premixed flames. It is concluded that the pressure generated in any individual configuration is directly proportional to the number of baffles plates. The flame position and speed are clearly dependent on the number of obstacles used and their blockage ratio. The flame stretch extracted from both the experimental and numerical images shows that hydrogen has the highest stretch values over CNG and LPG. Finally, the regime of combustion identified for the three fuels in the present combustion chamber is found to lie within the thin reaction zone. This finding supports the use of the laminar flamelet modelling concept that has been in use for the modelling of turbulent premixed flames in practical applications.

Le fonctionnement en régime très pauvre ou avec forts taux d'EGR des moteurs à allumage commandé (MAC) permet de réduire efficacement les émissions de CO2 et de Nox ; cependant ces stratégies se heurtent à l'augmentation des variabilités cycliques. Ces dernières sont principalement dues à la phase d'allumage qui devient critique de dilution. Le modèle ECFM-LES actuellement utilisé à IFPEn, basé sur la notion de densité de surface de flamme, est insuffisant pour décrire l'allumage dans ces conditions critiques. Dans ces travaux, l'approche TF-LES est adoptée, l'allumage étant alors décrit par un emballement cinétique des réactions chimiques lors d'une élévations locale de la température. Ces travaux définissent et évaluent une stratégie de simulation pour TF-LES en configuration moteur, qui permette une prédiction fine des allumages critiques et de la propagation turbulente de la flamme, afin de décrire le cycle moteur complet.Dans une première partie, des DNS d'allumages turbulents ont été réalisées, en modélisant la phase d'allumage par un dépôt d'énergie thermique (Lacaze et al., (2009)). Les calculs ont simulé les expériences d'allumage de Cardin et al. (2013), dans lesquelles l'énergie minimum d'allumage (MIE) d'un mélange mtéhane-air a été mesuré, pour différentes richesses pauvres et sous différentes intensités turbulentes. L'objectif principal des simulations a été de déterminer les paramètres numériques et physiques du modèle permettant de reproduire les allumages de l'expérience. Deux types de schémas cinétiques ont été évalués : un schéma simplifié et un schéma analytique (ARC), ce dernier reproduisant et les délais d'auto-allumage et la vitesse de flamme laminaire. Les résultats ont permis de définir des critères d'allumage et de mettre en évidence les différentes prédiction d'allumage avec les deux types de schémas cinétiques. Les résultats ont été également démontré que l'approche choisie permettait de prédire les bons niveaux d'énergie pour les allumages laminaires et à faible nombres de Kalovitz (Ka<10). Aux plus hauts nombres de Karlovitz, il a été montré que le modèle ED était insuffisant pour prédire les énergie d'allumage et qu'une description plus fine du dépôt d'énergie est nécessaire.Dans la seconde partie des travaux, un modèle de plissement dynamique (Wang et al., 2012) a été étudié, afin de décrire le développement hors-équilibre de la flamme dans la phase de propagation turbulente. Des études sur des flammes sphériques laminaires ont d'abord été menées. Ensuite, les premiers tests de configuration moteur ayant révélé des incompatibilités du modèle, des modifications ont été proposées. Le modèle de plissement dynamique modifié a été finalement évalué sur la configuration moteur ICAMDAC. Les résultats obtenus ont été comparés aux résultats obtenus par Robert et al. (2015) avec le modèle ECFM-LES, qui utilise une équation de transport de densité de surface de flamme décrivant le plissement hors-équilibre de la flamme. Les résultats obtenus avec le plissement dynamique sont en très bon accord avec ceux du modèles ECFM-LES, démontrant ainsi la capacité du modèle dynamique à prédire des valeurs de plissement hors-équilibre. D'autre part, le modèle dynamique s'ajustant automatiquement aux conditions de turbulence de l'écoulement, nul besoin n'est d'ajuster la constante de modélisation en fonction du régime moteur, comme c'est le cas pour l'équation de transport de la densité de surface de flamme. / The use of lean equivalence ratios or high EGR rates in spark ignition engines (SIE) enables to optimize CO2 and NOx emissions; however too important dilution rates leads to increased cycle-to-cycle variability. These latter are mostly due to the ignition phase, which becomes critical when dilution rates are important and requires high ignition energy. The ECFM-LES model currently used in IFPEN, which is based on the flame surface density concept, is not sufficient to describe ignition in these critical conditions. The TF-LES approach was chosen in this study, principally because it directly resolved chemistry and can thus model ignition via a local raise of the temperature. The present work defines and evaluates a simulation strategy for TF-LES in SIE configurations, that enables a fine prediction of critical ignitions and of the turbulent flame propagation.In the first part, DNS of turbulent ignition were performed. The ignition phase was modeled using a thermal energy deposit (ED model, Lacaze et al.). Simulations reproduced the ignition experiments of Cardin et al. who determined the minimum ignition energy (MIE) of lean premixed methane/air mixtures, for different turbulence characteristics. The main purpose of the study was to determine the numerical and physical model parameters, which enable to reproduce Cardin et al. experiments. Two types of kinetic schemes were evaluated: a simplified kinetic scheme and an analytical kinetic scheme (ARC), that can predict both the auto-ignition delays and the laminar flame speed, while keeping affordable CPU times. Results analysis enabled to define ignition criteria and to highlight the differences in terms of ignition prediction using the two kinetic schemes. Results also demonstrated that the chosen approach could recover correct levels of ignition energy for laminar and low Karlovitz number cases (Ka<10). For higher Karlovitz number cases, the ED model was found to be insufficient to predict the ignition and a finer description of the energy deposit is required.In the second part, a dynamic wrinkling model (Wang et al., 2012) was studied to describe the out-of-equilibrium behavior of the flame during the propagation phase. Studies on laminar spherical flames were first performed, to assess the laminar degeneration of the model. Then, as first tests in an engine configuration have revealed incompatibilities of the model, modifications were proposed. The modified dynamic model was finally tested in the ICAMDAC engine configuration. Results of the simulations were compared against previous results of Robert et al. obtained with the ECFM-LES model using a transport equation for the flame surface density that can describe the out-of-equilibrium wrinkling of the flame. Results obtained with the dynamic model are in very good agreement with the ones of Robert et al., thus demonstrating the ability of the dynamic model to predict out-of-equilibrium values in the engine configuration. Besides, the dynamic model self-adapts to the turbulence conditions, hence does not require any model parameter adjustment, as is it the case for models based on the flame surface density transport equation.

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