Numerical Analysis of a Methane-Air Bunsen Flame. 2nd Report. Structures near the Burner Rim and at Inner and Outer Cone Tips.

Abstract

A methane-air premixed flame with and equivalence ratio of 1.25 was simulated with a model including 91 pairs of elementary reactions occurring among 29 species. In this second part, the flame structures at the base of the flame and at inner and outer cone tips are discussed. Around the burner rim, the entrainment of air from the surroundings is seen to be the main reason for the activation of chemical reactions by promoting large production of O atoms and OH radicals. As a consequence, the rate of heat release there is very large, allowing the flame temperature to increase steeply and the flame to be held on the burner. At the inner cone tip, also, chemical reactions proceed with large rates, due to flame stretch and to preferential diffusion of H2. H2 is consumed there, so that the fuel burning at the inner cone tip is composed of a mixture of CH4+H2. CH3 radicals remain above the inner cone tip and, around the tip of the outer cone, oxidation via C1 route leads to formation of CO, which is oxidized into CO2, as in any other point of the outer cone. H2, however, is formed by reduction of water above the inner cone tip. This H2 formation continues even within and above the outer cone tip, so that respect to H2 the outer cone is found to have no tip.