Autoignition of Hydrogen-Air Mixture with Temperature and Composition Inhomogeneities

Autoigntion and front propagation behavior in Low Temperature Combustion (LTC) engine environments is investigated in this paper. First, non-reacting 3D engine simulations are conducted to investigate different mixture formation scenarios that might exist in LTC engines prior to autoignition. It is found that depending on the timing of fuel spray injection and the level of wall heat loss, two different scenarios might exist close to top-dead center (TDC): (1) early start of injection for which the intake stroke results in largely uncorrelated temperature (T)-equivalence ratio (φ) fields mostly due to wall heat loss; (2) late start of injection for which the compression stroke results in negatively correlated T-φ fields mostly due to evaporative cooling. Small-scale effects of these different mixture formation scenarios on the autoignition and subsequent front propagation are then studied using high-fidelity direct numerical simulation (DNS). For this purpose, high pressure hydrogenair mixture in constant volume with isotropic turbulence is investigated with detailed chemistry. Three cases with different initial T-φ fields are studied: case (A) baseline case with just temperature inhomogeneities and a uniform equivalence ratio field, case (B) uncorrelated T-φ fields and case (C) negatively-correlated T-φ fields. Numerical diagnostics are developed based on Damköhler number defined using the sensible enthalpy equation and an appropriately defined passive mixing time scale to decipher the different modes of heat release. It is found that majority of heat release in uncorrelated case and the baseline case occurs via premixed flame propagation, whereas the negatively correlated case ignites pretty much homogeneously.