A direct numerical simulation study on NO formation in lean premixed flames

There is considerable interest in the development of ultra-low emission fuel-flexible combustion systems for power generation, which are typically operated under lean premixed conditions. In this work, NO formation in turbulent premixed combustion is investigated via a direct numerical simulation of a temporally evolving methane/air jet flame at a jet Reynolds number of 9000. A detailed chemical mechanism, which in addition to the methane chemistry contains comprehensive NO kinetics is employed in a three-dimensional, shear driven, premixed turbulent configuration computed using 3 billion mesh points. After the fidelity of the DNS database and physical properties of the simulated flame are discussed, the results are used to investigate the interaction of NO chemistry and turbulence in lean premixed flames. To this end, the heat release and NO formation rates are analyzed. These indicate a decorrelation of the heat release and the NO production under highly turbulent conditions. In order to explain and understand this behavior, statistics of individual formation pathways and elementary reactions of the computed turbulent jet flame are compared to an unstretched premixed flame, which could be considered a simple approach used for modeling. Specifically, the formation via the NNH pathway is found to be important for flame generated NO and significantly affected by the turbulence, which is shown to be a consequence of differential diffusion.