Role of bromine doping in freely-propagating hydrogen-oxygen flames

The separate and coupled chemical and transport effects on the freely-propagating H₂-O₂-Br₂ flames in the doubly-infinite domain are studied for fixed equivalence ratio and different bromine fractions. Contrary to previous studies, it is shown that the laminar flame speed monotonically decreases with the increasing bromine fraction, and slight flame enhancement is observed in the mass burning velocity. On the other hand, stronger chemical inhibition and transport enhancement have been respectively identified on the lean and rich sides, accounting for the rich-shift of the peak flame speed. Substantial preferential diffusion effects due to the co-existence of heavy bromine species and light hydrogen species can enhance and inhibit flame propagation. Sensitivity analysis indicates that, compared to reactions in the bromine sub-mechanism, key reactions in the hydrogen-oxygen mechanism H + O₂ = O + OH (R1) may exert stronger influences on the laminar flame speed. Furthermore, the chemical and thermal structures exhibit multiplicity depending on the equivalence ratio and bromine concentration. Bromine reactions introduce additional reaction layers situated in the preheat zone close to the upstream unburnt mixtures. Reaction Br₂ + H = Br + HBr (R41) with the largest reaction rate releases considerable heat, imposing considerable ambiguity on the definition of flame thickness based on the maximum temperature gradient. HBr + H = H₂ + Br (R33) reverses in the leading layer in cases with relatively lower reactivity, while in the middle reaction zone R33 generates H₂ and hence increases the local equivalence ratio.