Modeling of structure and NO formation in an oscillating stagnation flame

To further study the unsteady NO formation behavior in the premixed flames, numerical simulation using CHEMKIN packages and GRI Mech. 3.0 is performed on the flat flame in the stagnation flow field subject to unsteady straining induced by periodically changing the flow rate (case 1) and the distance between plate and nozzle exit (case 2). The resultant flame thickness and T_max show either nearly no response to strain rate variation in high frequency cases or coincident with the steady curve without significant phase delay in low frequency cases. Unlike the diffusion flames presented by Im et al. (1999), the major axis of the phase-delay oval for the NO concentration in premixed flames does not coincide with the steady curve. The inclined angle between the major axis and the steady curve is larger in the low frequency cases. The effect of NO slow reaction becomes significant in low frequency cases. The value of T_max and relative NO concentration is higher in case 2 than that in case 1. When the distance L is oscillatory (case 2), after a peak response before 90° the normalized T_max location from the plate, d/L, and the normalized flame thickness, f/L, almost remain constant at 0.2 and 0.52 respectively. The NO formations are dominated by the mechanism of thermal and NNH pathways. The reason is that higher T_max is obtained in case 2 and the dominant mechanism in case 2 is thermal, instead of prompt pathway in case 1.