Mathematical Modeling of Catalytic-Surface Combustion of Reacting Flows

Abstract

Catalytic combustion of methane-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 NOx. The present paper is concerned with the numerical computation of heat transfer and chemical reactions in flowing methane-air mixtures impinging on a platinum coated hot plate. Chemical reactions are included in the gas phase and in the solid platinum surface. In the gas phase, 16 species are involved in 49 elementary reactions. On the platinum hot surface, additional surface species are included that are involved in 24 additional surface chemical reactions. The platinum surface temperature is fixed, while the properties of the reacting flow are computed. The flow configuration investigated here is impinging jet. Finite-volume equations are obtained by formal integration over control volumes surrounding each grid node. Up-wind differencing is used to ensure 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. In 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 existing data of similar geometry. The obtained agreement is fairly good for all main combustion products, while differences are observed for the unstable species such as OH, HO2 and H2O2. The computational results for heat and mass transfer at the gas-surface interface are correlated by non-dimensional relations. These relations can be used as sub-models for the more complicated catalytic burners.