Catalytic Combustion in a Stagnation Point Boundary Layer

Abstract

In order to examine the relative contribution of surface and gas phase reactions to the exothermic conversion of a fuel-air gas mixture flowing over a catalytic surface, we have carried out a theoretical and experimental study of catalytic combustion under stagnation point flow conditions. In the presence of exothermic surface reaction the theoretical model provides an analytical solution; in the presence of both reaction modes, homogeneous and heterogeneous, computer solutions were obtained for the total heat flux at the surface and the distribution of temperature, reactant, and product concentrations in the stagnation point boundary layer. Having available kinetic data on the platinum-catalyzed oxidation of propane and on the gas phase reaction between propane and air, we selected this chemical fuel system for experimental and theoretical study. Experimentally the flow of the propane-air mixture (1 vol percent C3Hs) was directed at a quartz plate whose surface was coated with thin strips of vacuum-deposited platinum, that served both as catalyst and resistance thermometer. During an experiment the temperature of each strip was maintained constant (± 5 K.) by adjusting the electrical heat input as monitored by resistance measurements. By this procedure the heat released by exothermic reaction of the fuel-air mixture could be determined from the difference in electrical power required to keep each Pt strip at its original temperature. With the aid of the theoretical analysis we were able to compute the fractional contributions of catalytic and gas phase combustion to the total heat flux conducted to the catalytic surface. The relative contribution of each combustion mode' depended on catalyst activity, volumetric flow rate, and fuel-air ratio. Under our experimental conditions with transport limited surface reaction an increase in reactant flow rate enhances the surface-catalyzed contribution to the total heat release rate. The results obtained clearly indicate the conditions under which, either combustion mode predominates.