THE ASYMPTOTIC STRUCTURE OF HYDROGEN AIR DIFFUSION FLAMES

Abstract

A two-step global reduced mechanism has been identified from a detailed starting mechanism to simplify descriptions of structure and extinction of hydrogen-air diffusion flames. Based on this two-step scheme and on results from previous numerical integrations of conservation equations, an asymptotic analysis is performed to derive analytical expressions for predicting extinction strain rates. The flame structure consists of three regions: a thin radical-production zone in which H atoms are generated by the global step 3H2 + O2 --> 2H2O + 2H whose rate is largely determined by a high-activation-energy chain-branching elementary reaction, flanked by two thick radical-consumption layers in which the H is consumed by the overall step H + H + M --> H-2 + M, the rate of which is controlled by different elementary reactions on the lean and rich sides. The structure is similar to the premixed-flame regime of diffusion-flame structure as defined by Linan with H-2 but not O2 leakage.