Non-monotonic extinction behavior in cracked ammonia premixed counterflow flames

Abstract

Blow-off behavior has been observed to vary in premixed bluff-body stabilized flames using different ammonia/hydrogen/nitrogen blends. These variations underscore their different responses to the strain rate and highlight the role of rapid $H_2$ consumption. The dimensionless extinction strain rate initially increases with the cracking ratio but decreases at higher cracking ratios. This non-monotonic behavior of the resilience to strain-induced blow-off is investigated in this study. This phenomenon may be attributed to the Lewis number effect, where the effective Lewis number of the unburnt mixture is not equal to 1, as well as to the preferential diffusion effect, which arises from differing Lewis number values for individual species. In the present study, the extinction strain rates for premixed counterflow ammonia/hydrogen/nitrogen/air flames, under varying ammonia cracking ratios, were calculated using one-dimensional simulations. The findings reveal that the dimensionless extinction strain rate reaches a peak at an intermediate ammonia cracking ratio. By artificially altering species’ Lewis numbers, the contributions of the Lewis number effect and the preferential diffusion effect were disentangled. Simulations using these modified transport models indicate that preferential diffusion primarily drives the peak in the dimensionless extinction strain rate. Although preferential diffusion reduces the laminar burning velocity, it significantly shifts $H_2$ consumption upstream to the unburnt side in flames with intermediate cracking ratios, while having less impact at very high or very low cracking ratios. This upstream shift of $H_2$ consumption causes the flame to move upstream so that the incomplete reaction occurs at higher strain rates.