ASYMPTOTIC ANALYSES OF STOICHIOMETRIC AND LEAN HYDROGEN-AIR FLAMES

The asymptotic structure of premixed hydrogen-air flames is analyzed using a reduced two-step mechanism, involving the reactants H-2 and O2, the product H2O and the intermediate species H. The principal rate of the first step of this two-step mechanism is the rate of the branching reaction H + O2 --> OH + O, and that of the second step is the rate of the recombination reaction H + O2 + M --> HO2 + M. This reduced mechanism was deduced from a starting mechanism containing 13 elementary reactions involving 7 species. In the analysis the overall flame structure is subdivided into four zones-an inert preheat zone with thickness of order unity, an inner layer with thickness of order delta, an oxidation layer with thickness of order epsilon, and a thick post-flame zone having uniform properties. The analysis is performed for delta << epsilon << 1. In the inner layer finite rates of both overall steps are considered, whereas in the oxidation layer the branching step H + O2 reversible OH + O maintains partial equilibrium. The structures of these layers were resolved numerically and the results were used to calculate the burning velocities. The structure of the inner layer was also resolved in the limit of large values of the effective activation energies of the two overall steps, under which conditions the chemical reactions in the inner layer were found to occur in two distinctly separated layers, each governed by one of the global reactions. Asymptotic analysis in this limit provided analytical formulas for calculating the burning velocity. The predictions of the burning velocities by these two methods are found to show good agreement with results of previous detailed numerical calculations for stoichiometric and lean flames at pressures from 0.2 to 64 atm and initial temperatures from 300 to 800 K