Effects of radiation reabsorption on laminar NH3/H2/air flames


Ammonia combustion, owing to its zero greenhouse gas CO2 emission, is attracting attention for energy utilization. However, the thermal radiation of NH3 has not been reported and involved in the numerical simulations of ammonia flames, which may cause serious errors in estimating the laminar flame speeds. In this study, the effects of radiation reabsorption on the laminar flame speed at different equivalence ratios and elevated pressures were numerically investigated using planar NH3/H2/air flames. The Statistical Narrow-Band (SNB) model parameters for NH3 were generated and used for simulations of NH3/H2/air flames, considering the radiation reabsorption. It was found that the radiation reabsorption exhibited a non-monotonic behavior at φ = 0.65-1.6, with the maximum enhancement of flame speed up to 15.6%. The effects of radiation reabsorption were controlled by both radiation and chemistry. The preheat-induced chemical effect dominated at φ = 0.65-1.25 and the enhancement of flame speed was mainly influenced by H, OH and NH2 radicals, which were primarily controlled by the reactions R36, R257 and R246, respectively. In contrast, the direct radiation effect dominated at φ = 1.25-1.6 and the enhancement of flame speed was mainly affected by the increasing mole fraction of NH3 . With increasing pressures, the preheat-induced chemical effect dominated at P = 1-10 atm and the enhancement of flame speed were mainly impacted by H and NNH radicals, which were controlled by the reactions R44 and R257, respectively. At higher pressures above 10 atm, direct radiation effect was dominating and the enhancement of flame speed was mainly controlled by the increasing optical thickness.

In Combustion and Flame