NOx formation in stretched premixed flames established far from extinction

Numerical simulations of axisymmetric stretched premixed laminar twin flames burning methane in air are conducted in order to understand the formation of NOx. A previously developed chemical kinetic mechanism is employed after relevant simplification, the adopted scheme involving 27 gas-phase species and 224 elementary reaction steps. The results indicate that the peak concentration of NO declines with increasing strain rate, and with decreasing equivalence ratios. An interesting result is obtained, namely that if the NO concentration is observed as a function of temperature in the flame, at the same temperature the NO concentration is higher for slightly lean flames than for stoichiometric flames. As lean flames correspond to a lower peak temperature, the maximum NO concentration is always lower for lean flames than for those established under stoichiometric conditions. Similarly, as the stretch rate is increased, the NO concentration is observed to be higher at lower temperatures for flames established at a stretch rate of 500 s−1 as compared to those established at a stretch rate of 100 s−1. The peak NO concentration falls with increasing stretch rates.