Numerical investigation on the inhibition effect of HBr in stoichiometric hydrogen/air mixtures on head-on flame quenching (HoQ)

Abstract

This paper investigates the effect of the flame retardant hydrogen bromide (HBr) on premixed hydrogen/air flames undergoing head-on quenching at an inert cold wall. Numerical simulations with full spatio-temporal resolution and detailed treatment of chemical reactions and molecular transport are employed to study this configuration. The simulations reveal how the addition of HBr affects the species profiles and the heat release rate in the flame, particularly at the quenching time, defined as the point of maximum heat loss to the wall. It is found that the accumulation of HO${}_2$ and H${}_2$O${}_2$ at the wall, which is significant for pure hydrogen/oxygen flames, becomes much weaker in the presence of HBr. Instead, with HBr the near-wall accumulation of Br and Br${}_2$ becomes more pronounced. Further investigation shows that the recombination reaction $\text{Br+Br+M}\to {\text{Br}}_2+\text{M}$ is the dominant exothermic reaction contributing to the heat release rate at the wall. Moreover, heat losses from the flame to the wall have been compared for different fuels. For stoichiometric H${}_2$ air, the wall heat loss (and therefore the thermal stress exerted onto the wall by the flame) is around two times higher than that for a conventional fuel like, e.g. methane. However, when 2% HBr are added to the combusting mixture, the heat loss is similar to methane. HBr addition can therefore efficiently reduce the thermal load for walls exposed to hydrogen combustion down to the levels of the conventional fuel CH${}_4$.