Effects of mixture inhomogeneity on the auto-ignition of reactants under HCCI environment

As an attempt at providing insight to develop better modeling strategies for HCCI engines, the ignition and propagation of a reaction front in a premixed fuel/air stream mixed with hotter exhaust gases is computationally investigated using the opposed-flow configuration. The effects of heat and radical transport are studied by imposing various mixing rates on the system. The results show that the scalar dissipation rate and mixture inhomogeneities have a significant effect on auto-ignition of the local mixture. Two different modes of front propagation are identified; the spontaneous propagation mode and the diffusion controlled mode. In the former case, mixing slows down ignition due to loss of radicals from the ignition kernel, while in the latter the transport of radicals and heat is mainly responsible for propagation of the front. A criterion to distinguish the two ignition modes is suggested based on the characteristic time scales of the auto-ignition and diffusion. The results show that lower mixing rate and higher pressure in general favors ignition in the spontaneous propagation mode. A parametric mapping of the ignition delay as a function of the mixture fraction and scalar dissipation rate suggests a simplified modeling strategy in multi-dimensional simulation of HCCI engines.