Effects of radiation reabsorption on the flame speed and NO emission of NH3/H2/air flames at various hydrogen ratios

Ammonia has attracted extensive attention in the direct combustion applications due to its carbon-neutral properties. In this study, the numerical simulations of NH3/H-2/air one-dimensional premixed laminar flame under the stoichiometric condition with different H-2 ratios (eta = X-H2/(X-NH3 + X-H2) = 0.2-0.7) were conducted to explore the influence of radiation reabsorption on laminar flame speed and NO emission. Also the effect of radiation reabsorption on NO generation and emission in NH3/H-2/air mixture when eta = 0.6 with different equivalence ratios was discussed. The results showed that the radiation reabsorption effect on flame speed decreased monotonically with the H-2 ratio increasing from 0.2 to 0.7, and the maximum effect was 17%. The effect on flame speed was regulated by the combination of direct radiation effect and chemical effect, which was influenced by reaction R257 NH2 + NO = NNH + OH via OH radical when eta = 0.2-0.7 and phi = 1.0. The promotion effect of radiation reabsorption on NO generation and emission decreased monotonically with the increasing H-2 proportion. The enhancement of NO generation caused by radiation reabsorption was mainly controlled by R222 N + OH double left right arrow NO + H. On the other hand, R220 N + NO double left right arrow N-2 + O, R240 NH + NO = N2O + H and R242 NH + NO = N-2 + OH affected the NO emission in the post-flame zone. At eta = 0.6 and phi = 0.65-1.5, the promotion of NO generation by radiation reabsorption was non-linear, which was mainly affected by R257, R241 NH + NO = N2O + H and R222 reactions. The NO emission was mainly influenced by R241 and R233 in the post-flame zone at phi = 0.65-1.05. And R241, R242, R256 NH2 + NO = N-2 + H2O and R257 in the post-flame zone dominated the NO emission when phi > 1.05.