Unsteady strain rate and curvature effects in turbulent premixed methane-air flames

Turbulent premixed stoichiometric methane-air flames modeled with reduced kinetics have been studied using the direct numerical simulation (DNS) approach. The simulations include a four-step reduced mechanism for the oxidation of methane and the molecular transport is modeled with Lewis numbers for individual species. The effects of strain rate and curvature on the intermediate radical concentrations and heat release rate are evaluated. The topology of the flame surface is interpreted in terms of its propagation and statistics. The correlation of radical species with strain rate and curvature is found to be strongly dependent upon their individual mass diffusion rates; hence, a global Lewis number representation of all of the species may be inadequate in predicting the heat release rate and evolution of the flame surface. It is found that highly diffusive and fast reactive species, H and H-2, are well correlated with curvature, while less diffusive species, CO, with a slow oxidation rate is more susceptible to unsteady strain rate effects. The global response of the flame is presented in terms of volumetric heat release and fuel consumption rates. The contributions of flame surface wrinkling and flame structure are identified.