Large Eddy Simulation of a Methane Diffusion Flame: The effect of the Chemical Mechanism on NOx Emissions

The accurate prediction of the mass fraction of NOx and OH in turbulent combustion is one of the challenging problems. A large eddy simulation (LES) of a CH4/H-2/N-2 diffusion flame "DLR Flame A" was carried out at a Reynolds number of 15200, and special emphasis was placed on NOx predictions. A steady state flamelet model was used for combustion closure model. However, the steady state flamelet model is not appropriate for the prediction of NOx. In the present study, a transport equation for NOx was solved, and the source term was estimated from the flamelet tables. In LES, the inflow boundary conditions influence the entire flow field, and the effects of the boundary conditions become more important during combustion. The effect of inflow boundary conditions was studied, and the results were quantified in terms of the nozzle diameter. NOx predictions are dependent on the chemical mechanism; thus, the GRI-Mech 3.0, GRI-Mech 2.11 and San Diego mechanism were studied. The results of the flamelet model were in good agreement for the temperature and major species for all the reaction mechanisms. However, for NOx, the San Diego mechanism performed better than the other reaction mechanisms. The results of the present study showed that the steady flamelet model could accurately predict kinetically controlled reactions, such as the formation of NOx