A computational study of auto-ignition and flame propagation in stratified mixtures relevant to modern engines.

Abstract

Numerical simulations are performed to study the nature of auto-ignition and flame propagation in a stratified mixture. The results of this study are expected to provide a fundamental understanding of the combustion occurring in direct injection spark ignition (DISI) and homogeneous charge compression ignition (HCCI) engines. In the first part, the effect of time varying composition on a premixed methane-air flame is studied using a counterflow configuration and the concept of dynamic flammability limit is established to quantify the extension in flammability limit under unsteady situations. In addition, the effects of blending hydrogen to methane are studied as a possible means to improve the stability of lean premixed combustion. It is found that hydrogen blending substantially affects the diffusive-thermal stability while the dynamic response is unchanged. The second part of the dissertation is devoted to a fundamental study of ignition characteristics relevant to HCCI engines. Models at various levels of complexity are attempted, ranging from a homogenous reactor model to direct numerical simulation (DNS). First, the mixing of exhaust gas recirculation (EGR) on HCCI combustion are investigated for their benefit of knock reduction. Results obtained using a homogenous reactor model suggest that the effects of EGR is predominantly thermal than chemical for the conditions under study. This leads to a closer examination of the thermo-physical aspects of EGR on HCCI combustion due to incomplete mixing and mixture stratification. High-fidelity DNS studies are thus performed to assess the effects of the initial temperature distribution on ignition and subsequent heat release. For the three test cases considered, the presence of hotter core gas leads to early ignition and increased duration of burning, while a cold core leaves dormant end gas which is consumed by slow combustion. Finally, as a more extensive parametric study to quantify the effects of mixing rate on HCCI ignition, the ignition and propagation of a reaction front in a premixed fuel/air stream mixed with hotter exhaust gases is investigated using the counterflow configuration. The results provide a systematic framework to identify two distinct regimes of ignition, namely the spontaneous propagation and the deflagration regimes. A criterion based on the ratio of the time scales of auto-ignition and diffusion is proposed to identify the transition between these two regimes. Implications of the different regimes in the development of submodels for HCCI modeling are discussed.