Assessment of combustion submodels for turbulent nonpremixed hydrocarbon flames

Data bases generated by direct numerical simulation (DNS) of nonpremixed combustion are used to evaluate stationary laminar flamelet and conditional moment closure (CMC) models of turbulent combustion. The chemical kinetics used for the simulation and modeling is a systematically reduced two-step mechanism for hydrocarbon combustion. Heat release effects on the chemistry are included but a constant density assumption is used. Three different Reynolds numbers and a range of Damkohler numbers are considered. Two different versions of stationary laminar flamelet models are considered. In one version, the instantaneous turbulent scalar dissipation rate at stoichiometry is used to match the laminar flamelets, whereas in another version the conditional average of scalar dissipation at stoichiometry is used. In the CMC calculations, turbulent mixing is modelled by a presumed beta function pdf with the mixture fraction variance being the only input quantity and this is obtained from the DNS. CMC predictions of major and minor species are excellent and are always within 6% despite the presence of some local extinction. Both versions of flamelet models predict the major species with much less accuracy than this. The minor species predictions, and hence the reaction rate predictions, are even less accurate.