Structure of the edge flame in a methane-oxygen mixing layer

Abstract

The structure of an edge flame in a mixing layer of two uniformly flowing pure CH4 and pure O-2 streams has been investigated numerically by employing a detailed methane-oxidation mechanism in order to investigate the influence of using pure oxygen, instead of air, as the oxidizing agent. The results exhibited similar behaviour to the CH4-air counterpart in the premixed-flame front, through which the carbon-containing compound leaked mainly in the form of CO and H-2 instead of HC compounds. However, while passing through the rich premixed-flame region, the most pronounced distinction of using pure oxygen was that the major route for CO production, in addition to the direct CH4 decomposition, is C2Hm compound formation followed by their decomposition into CO, thereby giving continuous CO production, contrary to the rich CH4-air premixed-flame region in which CO consumption existed. In the downstream region from the rich premixed flame front, CO is further oxidized into CO2 in a broad diffusion-flame-like reaction zone located around the moderately fuel-rich side of the stoichiometric mixture by the OH radical produced from the oxygen leakage from the fuel-lean premixed-flame front. Since the secondary CO production through C2Hm decomposition has a relatively strong reaction intensity, an additional heat-release branch appears and the resulting heat-release profile can no longer be treated as a tribrachial structure.