Numerical Analysis of a Methane-Air Bunsen Flame (1st Report, Structures of Inner and Outer Cones

A methane-air premixed flame with an equivalence ratio of 1.25 was simulated with a model including 91 pairs of elementary reactions occurring among 29 species. In this first work, the structure of a representative transversal section of the flame is presented and the mechanisms of combustion at the inner and outer cones are discussed in detail. CH₄ and O₂ in the mixture both react with H atoms, but the reaction between CH₄ and H occurs preferentially to that between O2 and H at the fuel side of the inner cone. This results in a lack of oxygen atoms at the position where CH₄ breaks into CH₃ radicals and the reactions belonging to the C₂ route proceed with higher velocities than those belonging to the C₁ route. The consumption of O₂, being posterior to the consumption of CH₄, produces O atoms at the center of the inner cone. These O atoms, though not abundant, are sufficient to oxidize the last stable product of the C₂ route, C₂H₂, preventing soot production. The diffusion of H atoms from the flame front to low-temperature regions and the subsequent production of OH radicals were found to be essential to the stability of both inner and outer cones, by triggering the combustion chain reactions.