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

The structure of an edge flame in a mixing layer of two uniformly flowing pure CH₄ and pure O₂ 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 H₂ 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 C₂H_m 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 C₂H_m 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.