The mechanism of mutual annihilation of stoichiometric premixed methane-air flames

The mechanism of head-on quenching of two stoichiometric premixed methane-air flames by mutual annihilation is investigated numerically using detailed chemistry. The mutually annihilating dames initially accelerate before quenching as observed by other studies involving reduced chemistry. The mechanism of dris acceleration is investigated by comparing the balance between transport and reaction of O-2 at different times. The primary contribution to the enhanced flame propagation is attributed to a change in the balance between reaction and diffusion. This effect is further enhanced by a decrease in the concentration gradients of the reactants during diffusional interactions of the mutually annihilating flames. The rates of fuel consumption and oxidation of H-2 and CO are significantly enhanced before the merging of the various consumption/oxidation layers. The diffusion of He from the reaction zone to the unburned reactants is reversed, resulting in a buildup of H-2 concentration in the reaction zone. H-2 plays a key role in enhancing the chemistry before the merging of the various consumption layers because of its high mass diffusivity and its importance in the production of radicals. In particular, the accumulation of H-2 in the reaction zone results in the enhancement of reactions that produce H from the H-2/O-2 system and in a buildup of radicals including H, O, and OH. The increased contribution of the H-2/O-2 system continues until the onset of quenching of the H-2 oxidation layer. CO oxidation then becomes the dominant contribution to H-radical production. During the flame deceleration phase, H-radical production is significantly reduced. The key reactions governing the production of radicals shift from the fuel (HCO and CH3) to H-2 and CO oxidation. Radical recombination reactions, which play a key role in flame-wall quenching, are insignificant until all fuel and H-2/CO oxidation layers are quenched.