A simplified approach to the numerical description of methane-air diffusion flames

Starting with a three-step reduced chemistry description that employes H-2 and CO as the only intermediates not in steady state, a simplified formulation aimed at facilitating numerical computations of non-premixed methane-air systems is developed. The analysis retains finite rates for radical recombination and CO oxidation but assumes infinitely fast fuel consumption taking place in a diffusion-controlled manner in an infinitely thin reaction sheet. To remove stiffness associated with the fast fuel consumption, the conservation equations for the major species and the temperature are written in terms of generalized coupling functions that for predictive accuracy permit species diffusivities that differ from the thermal diffusivity. The resulting formulation, which automatically determines the position of the fuel-consumption layer without necessity of front tracking or further interface approximations, can be used for analytical, computational, and modeling studies of both laminar and turbulent flows, removing stiffness difficulties associated with highly disparate chemical time scales. Comparisons of results of the simplified formulation in the counterflow mixing layer with those obtained with detailed chemistry and transport descriptions indicate that the proposed formulation applies with good accuracy to strain conditions ranging from weakly strained, robust flames to near-extinction flames.