Effects of Differential Diffusion on the Mutual Annihilation of Two Premixed Hydrogen-Air Flames

Abstract

The unsteady process of head on quenching of two laminar premixed hydrogen-air flames in one-dimension by mutual annihilation is investigated numerically using a detailed chemical mechanism and realistic transport. The process of annihilation through interactions is inevitable in highly corrugated turbulent flames, and contributes to turbulent flame shortening. Processes leading to mutual annihilation involve interactions that take place in the following stages: (1) interaction of preheat zones, which corresponds to the transport of heat and reactants, (2) interactions of the reaction layers as the flames merge, and finally (3) the process of burnout. The primary objective of this work is to study the effects of differential diffusion during the various events that occur during the unsteady process of annihilation. For the stoichiometric condition two cases are considered namely; a case where transport is based on prescribing non-unity Lewis numbers for all the species and a case with unity Lewis numbers prescribed for all the species. The latter case provides with a reference problem for the other flames considered. Because of the importance of differential diffusion during thermo-diffusive interactions, which are owed to the transport properties of H2, relative to temperature and the oxidizer, two additional cases are considered. They correspond to lean and rich hydrogen-air flames. The results show that differential diffusion of H2 plays an important role in determining the composition of the reacting mixture and thus, affects the final temperature and composition of the products. The differential diffusion of H2 causes a deficiency of the fuel for the stoichiometric and lean cases thereby altering the rates of reactions involving H2 while merger. For the rich case the deficiency caused by the differential diffusion is offset by the presence of excess H2 in the reaction mixture. Due to these conditions for the rich flames and non-unity Lewis number case for the stoichiometric flame there is an increased production of the species O towards the end of merger.