L'objectif de ce travail de thèse est le développement d'un modèle numérique prédictif pour la simulation des phénomènes de postoxydation dans la ligne d’échappement d’un moteur à combustion interne. Le modèle a été écrit pour reproduire le processus d'auto inflammation des hydrocarbures durant la postoxydation, mais également l'évolution des polluants et des produits de combustion en général. Ceci a nécessité de mettre au point un schéma cinétique détaillé qui tienne compte de la chimie à basse température des hydrocarbures et de l'influence sur cette chimie des différentes espèces majeures présentes dans les gaz brûlés à postoxyder. Ces espèces sont le CO2, le H2O et le N2, qui agissent comme diluants, mais également des polluants tels que le CO ou les NOx. Ces derniers, même en faibles concentrations, peuvent avoir un effet important sur l’oxydation des hydrocarbures qui doit aussi être prise en compte dans le modèle chimique.Afin de considérer, en plus de la chimie, et les phénomènes physiques de la postoxydation, tels que la turbulence et les effets de mélange, ce schéma cinétique a été couplé à un modèle de combustion turbulente adapté à l'utilisation dans un code CFD 3D moteur. Ce couplage a été effectué via une tabulation a priori de la chimie, méthode qui permet de réduire considérablement le temps de calcul, tout en décrivant l'ensemble des phénomènes liés à la chimie détaillée. Une technique de tabulation de la cinétique chimique a donc été développée et implantée dans un code CFD. Une configuration permettant de représenter les phénomènes caractéristiques de la postoxydation dans la ligne d'échappement d'un moteur à combustion interne a été simulée. Les résultats permettent de mieux appréhender ces phénomènes et de proposer des solutions technologiques visant à leur optimisation / The aim of this PhD thesis is the development of a predictive numerical model capable of simulating hydrocarbon postoxidation in an IC engine exhaust line. The model should reproduce the auto-ignition of hydrocarbons, as well as the evolution of pollutants and combustion products under postoxidation conditions. For this purpose, a detailed kinetic reaction model was developed. It should be valid at low temperatures and under highly diluted conditions. The model should also take into account the effects of the major components of engine exhaust gas on hydrocarbon postoxidation. These are CO2, H2O, and N2, acting as diluting species, but also CO and NOx, which even in small amounts, may strongly impact hydrocarbon oxidation kinetics. These species must hence be considered for postoxidation modelling.In order to gather chemical and physical effects such as turbulence and mixing, the chemical kinetic mechanism was coupled with a turbulent combustion model designed for CFD 3D engine computations. An a priori tabulation methodology was developed, minimizing computational effort and the developed tabulation technique was validated under postoxidation conditions in an IC-engine exhaust line. The coupled chemical kinetics tabulation and turbulent mixing model was implemented in the CFD code IFP-C3D. Simulations were performed on a configuration representative of the physical phenomena characteristic of hydrocarbon postoxidation in exhaust lines. Results improved the understanding of postoxidation phenomena in an IC-engine exhaust line and propose technical solutions for an enhanced postoxidation control

In this study, combustion performances and emission characteristics of olive cake and olive cake+coal mixture are investigated in a bubbling fluidized bed of 102 mm inside diameter and 900 mm height. The average particle sizes of coal and olive cake used in the experiments were 1.57 mm and 1.52 mm, respectively. Flue gas concentrations of O2, CO, SO2, NOx, and total hydrocarbons (CmHn) were measured during combustion experiments. Operational parameters (excess air ratio, secondary air injection) were changed and variation of pollutant concentrations and combustion efficiency with these operational parameters were studied. The temperature profiles measured along the combustor column was found higher in the freeboard for olive cake than coal due to combustion of hydrocarbons mostly in the freeboard. The location of the maximum temperature in the freeboard shifted to the upper part of the column, as the volatile matter content in the fuel mixture increased. Combustion efficiencies in the range of 83.6-90.1% were obtained for olive cake with the excess air ratio of 1.12-2.30. The corresponding combustion efficiency for coal was 98.4-99.7% under the same conditions. As the CO and hydrocarbon concentration in the flue gas increased, the combustion efficiency decreased. Also co-combustion experiments of olive cake and coal for various mixing ratios were carried out. As the amount of olive cake in the fuel mixture increased, SO2 emissions decreased because of the very low sulfur content of olive cake. In order to increase the combustion efficiency, secondary air was injected into the freeboard which was a good solution to decrease the CO and hydrocarbon emissions, and to increase the combustion efficiency. For the setup used in this study, the optimum operating conditions with respect to NOx and SO2 emissions were found as 1.35 for excess air ratio, and 30 L/min for secondary air flowrate for the combustion of 75 wt% olive cake and 25 wt% coal mixture. Highest combustion efficiency of 99.8% was obtained with an excess air ratio of 1.7, secondary air flow rate of 40 L/min for the combustion of 25 wt% olive cake and 75 wt% coal mixture.