  The flamelet approach offers a viable framework for combustion modeling of
homogeneous charge compression ignition (HCCI) engines under stratified mixture
conditions. Scalar dissipation rate acts as a key parameter in flamelet-based
combustion models which connects the physical mixing space to the reactive
space. The aim of this paper is to gain fundamental insights into turbulent
mixing in low temperature combustion (LTC) engines and investigate the modeling
of scalar dissipation rate. Three direct numerical simulation (DNS) test cases
of two-dimensional turbulent auto-ignition of a hydrogen-air mixture with
different correlations of temperature and mixture fraction are considered,
which are representative of different ignition regimes. The existing models of
mean and conditional scalar dissipation rates, and probability density
functions (PDFs) of mixture fraction and total enthalpy are a priori validated
against the DNS data.
