Two-dimensional modelling for catalytically stabilized combustion of a lean methane-air mixture with elementary homogeneous and heterogeneous chemical reactions

The catalytically stabilized combustion (CST) of a lean (equivalence ratio Φ = 0.4) methane-air mixture was investigated numerically in a laminar channel flow configuration established between two platinum-coated parallel plates 50 mm long and 2 mm apart. A two-dimensional elliptic fluid mechanical model was used, which included elementary reactions for both gaseous and surface chemistry. Heat conduction in the solid plates and radiative heat transfer from the hot catalytic surfaces were accounted for in the model. Heterogeneous ignition occurs just downstream of the channel entrance, at a streamwise distance (x) of 4 mm. Sensitivity analysis shows that key surface reactions influencing heterogeneous ignition are the adsorption of CH₄ and O₂ and the recombinative desorption of surface-bound O radicals; the adsorption or desorption of radicals other than O has no effect on the heterogeneous ignition location and the concentrations of major species around it. Homogeneous ignition takes place at x = 41 mm. Sensitivity analysis shows that key surface reactions controlling homogeneous ignition are the adsorption/desorption of the OH radical and the adsorption/desorption of H₂O, the latter due to its direct influence on the OH production path. In addition, the slope of the OH lateral wall gradient changes from negative (net-desorptive) to positive (net-adsorptive) well before homogeneous ignition (x = 30 mm), thus exemplifying the importance of a detailed surface chemistry scheme in accurately predicting the homogeneous ignition location. The effect of product formation on homogeneous ignition was studied by varying the third body efficiency of H₂O. Product formation promotes homogeneous ignition due to a shift in the relative importance of the reactions H + O₂ + M → HO₂ + M and HCO + M → CO + H + M.