Dependence of Zel'dovich number on pressure and temperature in lean hydrogen-air mixtures

Simulations of unperturbed lean hydrogen-air flames were performed using three state-of-the-art chemical mechanisms under various conditions: pressure 1 <= P <= 50 atm, unburned gas temperature 300 <= Tu <= 900 K, and the equivalence ratio 0.3 <= Phi <= 0.5. Multicomponent diffusion and Soret effect were considered. The computed results show that, under certain conditions, (i) differently defined Zel'dovich numbers decrease with increasing P and (ii) sensitivity coefficients of Ze to the rates of the most important chain-branching reaction (R1) H + O2--OH+O and chain-terminating reaction (R9) H + O2+M=HO2+M change their signs from negative and positive, respectively, to positive and negative, respectively, at high P. Analysis of the computed data shows that this transition occurs when the rates of the chain-terminating reaction (R14) 2HO2--H2O2+O2 and the chainbranching reaction (R15) H2O2+M = 2OH+M are almost equal. Under such conditions, these two rates are much higher than a rate of another reaction that involves H2O2 in the largest parts of flame reaction zones. Moreover, in the vicinity of the upstream boundaries of the reaction zones, these two rates are significantly higher than a rate of another bimolecular reaction that involves HO2. Accordingly, in such a flame zone, whose location controls Zel'dovich number, almost all H2O2 formed in reaction (R14) is immediately converted to two radicals OH via reaction (R15). Therefore, the entire root of reactions (R9)->(R14)->(R15) becomes chainpropagating root, with its rate being significantly increased by P, because reactions (R9) and (R15) are termolecular ones. The emphasized effects are mitigated by both Tuand Phi, i.e., they are observed at higher pressure if Tuor Phi is increased. The explored negative pressure-dependence of Zel'dovich number should be considered when analyzing experimental or numerical data obtained from lean hydrogen-air turbulent flames under elevated pressures or when modeling such flames.