On the roles of humidification and radiation during the ignition of ammonia–hydrogen–air mixtures

Abstract

It is well established that radiation plays an important role in the combustion process of premixed combustible mixtures, which significantly affects the flame temperature and chemical reaction rates during flame propagation. However, the impact of radiation, especially the radiation reabsorption effect, on minimum ignition energy (Emin) still lacks thorough research and understanding. In this study, the ignition processes of spherical NH3/H2/H2O/O2/N2 flames were simulated. Three different computational models, the adiabatic model (ADI), the optically thin model (OTM) and the statistical narrow-band model (SNB), were employed to determine the radiation effect (including both radiative heat loss and reabsorption) on Emin. Results indicated that the water content in the premixture suppressed ignition; characteristically, the addition of vol. 9.63% H2O in the premixture resulted in a nearly 40% increase of Emin under adiabatic condition. Both radiative heat loss and reabsorption effects increased Emin. The radiation effects on Emin, quantified as the relative differences between the OTM-/SNB- and ADI-obtained Emin, were up to 16.47% (for radiative heat loss) and 17.65% (for radiation reabsorption), respectively. It was further demonstrated that radiation increased Emin through the combination of three aspects: thermal effect, chemical effect and flame structure effect. The thermal effect, induced by the outward radiative emission, reduced the ignition kernel temperature significantly. The reduced temperature decelerated chemical reactions and then suppressed the chemical heat release, i.e., the chemical effect as reflected by the concentrations of H, O and OH radicals. Furthermore, the larger flame thickness obtained by the radiation models slowed down the diffusion of fuels, which weakened the chemical heat release and eventually increased the Emin. Moreover, the reaction-diffusion region simulated by SNB was enlarged due to the radiation reabsorption, and thus, Emin obtained by SNB was higher than that by OTM.