Flamelet-based modeling of H2/air auto-ignition with thermal inhomogeneities

Homogeneous-Charge Compression Ignition (HCCI) engines have been shown to have higher thermal efficiencies and lower NO_x and soot emissions than Spark Ignition engines. However, HCCI engines experience very large heat release rates which can lead to the occurrence of damaging engine knock. One method of reducing the maximum heat release rate is to introduce thermal inhomogeneities, thereby spreading the heat release over several crank angle degrees. Direct Numerical Simulations (DNS) with complex H₂/Air chemistry by Hawkes et al. (2005) showed that both ignition fronts and deflagration-like fronts are present in systems with such inhomogeneities. Here, an enthalpy-based flamelet model is presented and applied to the four cases of varying initial temperature variance presented in Hawkes et al. (2005). This model uses a mean scalar dissipation rate to model the mixing between regions of higher and lower enthalpies. The predicted heat release rates agree well with the heat release rates of the four DNS cases. Although this model does not treat ignition fronts and deflagration-like fronts differently, here it is shown to be capable of capturing the combustion characteristics for both the case in which combustion occurs primarily in the form of spontaneous ignition fronts and for the case dominated by deflagration-type burning. The flamelet-based model shows considerably improved agreement with the DNS results over the popular multi-zone model, particularly, where both deflagrative and spontaneous ignition are occurring, that is, where diffusion is important.