The stringent requirement to reduce the NO_x from various combustion devices while maintaining the combustor performance is of paramount motivation for the current study. Partially premixing, fuel blends and fuel diluents represents promising approaches for reducing NO_x from flames. A detailed two dimensional, transient numerical study on the later aspect of fuel diluents (helium) to reduce the NO_x emission in a back ward facing step geometry has been carried out using CFX 5.7. The equivalence ratio has been varied from 0.85-1, to study the effect of excess air on NO _xemission. The methane flame is simulated using Eddy break-up model with Zeldovich thermal mechanism for NO_x formation and the turbulence model is w-RSS (Reynolds Shear Stress). The simulation results indicate that considerable reduction in NO_x emission and increase in flame stability can be achieved for a predetermined addition of inert in to the fuel air mixture.

KEYWORDS: Premixed Combustion, Fuel diluent, Flame stability.