Comparison of NOx prediction methodologies for gas turbine combustor simulations

Requirements to reduce the emissions of nitrogen oxides (NO_x ) from gas turbines used in aircraft propulsion and stationary power generation have led to consideration of several low-emission combustor concepts. As part of the engineering research, design and development of these combustors, multi-dimensional combustion computational fluid dynamics (CFD) calculations are increasingly being utilized. To examine the ability of combustion CFD to accurately predict emissions, a series of calculations have been performed with STAR-CD, a general purpose commercial CFD package capable of treating three-dimensional flows with turbulence, sprays and chemical reactions. Some of the calculations shown in this paper make use of a new coupled n-step chemistry solver combining features of CHEMKIN with STAR-CD which is being developed jointly by Reaction Design and adapco. The NO_x predictions are made via two techniques. In the first, a joint scalar PPDF thermal NO_x post-processor is employed in conjunction with a two-step combined timescale combustion model. In the second approach, a reduced 26-step finite rate kinetics mechanism is utilized. Here, the combustion chemistry is combined with the NO_x formation chemistry based on the extended Zel´dovich mechanism for thermal NO_x and a surrogate species approach for prompt NO_x. Comparisons to experimental data show that both approaches can produce reasonable predictions