Mathematical modeling of impinging hydrogen-air flows augmented by catalytic surface reactions

Catalytic combustion of hydrogen-air mixtures involves the adsorption of the fuel and oxidant into a platinum surface, chemical reactions of the adsorbed species and the desorption of the resulting products. Re-adsorption of some produced gases is also possible. The catalytic reactions can be beneficial in porous burners that use low equivalence ratios. In this case the porous burner flame can be stabilized at low temperatures to prevent any substantial gas emissions, such as nitrogen oxides. The present paper is concerned with the numerical computation of heat transfer and chemical reactions in hydrogen-air mixtures which impinge perpendicularly on a platinum coated hot plate. Chemical reactions are included in the gas phase as well as in the platinum layer. In the gas phase, eight species are involved in 24 elementary reactions. On the platinum hot surface, additional surface species are included that are involved in 14 additional surface chemical reactions. The platinum surface temperature is fixed, while the properties of the reacting flow are computed. The flow configuration investigated in the present paper, is that of impinging jets. Finite-volume equations are obtained by formal integration over control volumes surrounding each grid node. Upwind differencing is used to insure that the influence coefficients are always positive to reflect the real effect of neighboring nodes on a typical central node. The finite-volume equations are solved, iteratively, for the reacting gas flow properties. On the platinum surface, surface species balance equations, under steady-state conditions, are solved numerically. A non-uniform computational grid is used, concentrating most of the nodes near the catalytic surface. The computed species concentrations and temperature are compared with numerical data of similar geometry. A surface temperature of 1150 K caused fast reactions, on the catalytic surface and in the flowing gas, for some species such as OH, HO2 and H 2O2. The computational results for mass transfer and surface reaction rate at the gas-surface interface are correlated by non-dimensional relations. These relations can be used as a sub-model for the more complicated catalytic reactors.