The effects of non-uniform temperature distribution on the ignition of a lean homogeneous hydrogen-air mixture

To characterize the ignition process in homogeneous charge compression ignition engines, high fidelity simulations are performed to study the effects of different initial temperature distributions on the autoignition of a turbulent homogeneous mixture at high pressure. The effects of the initial temperature distribution on the ignition and subsequent heat release are studied by comparison of simulations with three initial random temperature fields having different skewness. It is found that the scalar mixing and turbulence have a significant influence on the initial location and further evolution of the ignition kernels. A comparison of the integrated heat release rates shows that the presence of a hot core leads to early ignition and increased duration of burning, while a cold core leads to a dormant end gas, which is consumed by slow combustion. The extent of flame fronts is quantified by a temperature gradient cut-off, revealing distinct behavior in the appearance of flame fronts for the three cases. Finally, two distinct ignition regimes, namely the spontaneous propagation and the deflagration regimes, are identified, and a predictive criterion is defined based on the spontaneous propagation speed and deflagration speed at the local mixture conditions. The predictions are found to be consistent with the observed results, suggesting a potential strategy in the modeling of HCCI combustion process.