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The catalytically stabilized combustion (CST) of a lean (equivalence ratio ⌽ ϭ 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 CH 4 and O 2 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 ϭ 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 H 2 O, 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 ϭ 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 H 2 O. Product formation promotes homogeneous ignition due to a shift in the relative importance of the reactions H ϩ O 2 ϩ M 3 HO 2 ϩ M and HCO ϩ M 3 CO ϩ H ϩ M.

The turbulent catalytically stabilized combustion of lean hydrogen-air premixtures is investigated numerically in plane channels with platinum-coated isothermal walls. The catalytic wall temperature is 1220 K and the incoming mixture has a mean velocity of 15 m/s and a turbulent kinetic energy of 1.5 m 2 /s 2 . A two-dimensional elliptic model is developed with elementary heterogeneous and homogeneous chemical reactions. The approach is based on a two-layer k-ε model of turbulence, a Favre-average moment closure, a presumed-shape (Gaussian) probability density function for gaseous reactions, and a laminar-like closure for surface reactions. Gaseous combustion is confined close to the catalyst surface due to the diffusional imbalance of the limiting reactant (hydrogen). In addition, the peak rms temperature and species fluctuations are always located outside the extent of the homogeneous reaction zone indicating that thermochemical fluctuations have no significant influence on gaseous combustion. Turbulence is significantly suppressed by gaseous combustion resulting in higher turbulent transport for the leaner mixtures, a successive push of the gaseous reaction zone towards the wall, incomplete combustion, and subsequent catalytic conversion of the leaked fuel. Comparison between turbulent and laminar cases having the same incoming properties shows that turbulence inhibits homogeneous ignition due to increased heat transport away from the near-wall layer.

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