The computation of stretched laminar methane-air diffusion flames using a reduced four-step mechanism

Abstract

We demonstrate that a systematically reduced four-step mechanism is able to provide the essential properties of methane-air diffusion flames such as the structure and extinction limits in a way similar to a full kinetic mechanism. The reduction is based on the C 1-chain for methane oxidation. By looking at the magnitude of the intermediate species we argue that a “steady state” assumption is justified for OH, O, HO 2, CH 3, CHO, and CH 2O. Furthermore, by considering which reactions are fast, we are able to assume partial equilibrium of the reactions OH + H 2 ⇄ H 2O + H and OH + OH ⇇ H 2O + O. Even though the maximum mole fraction of the hydrogen atom H is of the same order of magnitude as that of OH, it is not assumed to be in steady state because of its larger diffusion coefficient. The resulting mechanism, (I) CH 4+2 H+H 2O=CO+4H 2, (II) CO+H 2O=CO 2+H 2 , (III) 2H+M=H 2+M , (IV) O 2+3H 2=2H+2H 2O , is global, and thereby defines the stoichiometry, but it employs combinations of unmodified elementary reaction rates. For the stagnation point diffusion flame of Tsuji and Yamaoka there is a good agreement with measurements as well as with previous calculations.