A numerical study of laminar methane/air triple flames in two-dimensional mixing layers

Triple flames formed in methane/air mixing layers with three different mixture fraction gradients were investigated by numerical simulation. The primitive variable method, in which the fully elliptic governing equations were solved with detailed chemistry and complex thermal and transport properties, was used. Radiation heat transfer from CO2, CO and H2O was calculated using the discrete-ordinates method coupled to a statistical narrow band correlated-K based wide band model. The results show that with the increase of the mixture fraction gradient, the combustion intensity in the diffusion flame branch of a triple flame is enhanced. In the near-stoichiometric mixture fraction region, the local burning flux of a triple flame is reduced when the mixture fraction gradient is increased. However, when the mixture fraction is significantly different from the stoichiometric value, the local burning flux increases as the mixture fraction gradient is increased. The correlation of the burning speed versus stretch rate established from conventional homogeneous premixed flames cannot completely explain the phenomena observed in triple flames. The interaction between the diffusion and premixed flame branches significantly affects the local burning velocity in regions where the mixture fraction is far from the stoichiometric value in a triple flame. This interaction is caused by both conduction heat transfer and radical exchange. Radiation has negligible effect on local burning properties in a triple flame.