High-speed flow with discontinuous surface catalysis

In a reacting gas flow both gas-phase chemical activity and surface catalys is can increase the rate of heat transfer from the gas to a solid surface. In particular, when there is a discontinuous change in the catalytic proper ties of the surface, there can be a very large increase in the local heat t ransfer rate. In this study numerical simulations have been performed for t he laminar high-speed how of a high-temperature, non-equilibrium reacting g as mixture over a flat plate. The surface of the plate is partly catalytic, with the leading region non-catalytic, and a discontinuous change in the c atalytic properties of the surface at the catalytic junction. The surface i s assumed to be isothermal, and cold relative to the free stream. The gas i s assumed to be a mixture of molecular and atomic forms of a diatomic gas i n an inert gas forming a thermal bath, giving a three-species mixture with dissociation and recombination of the reactive species. The calculations ar e performed for a gas with atomic and molecular oxygen in an argon bath, bu t a full range of gas-phase chemical and surface catalytic effects is consi dered. Kinetic schemes with frozen gas-phase chemistry, and partial or full recombination of atomic oxygen in the boundary layer are investigated. The catalytic nature of the surface material is given by a catalytic recombina tion rate coefficient, which varies from zero (non-catalytic) to one (fully catalytic), and the effects on the flow and the surface heat transfer of m aterials which are non-, partially, or fully catalytic are considered. A se lf-similar thin-layer analytical model of the change in the gas composition downstream of the catalytic junction is developed. For physically realisti c (O(10(-2))) values of the catalytic recombination rate coefficient, the p redictions from this model of the surface values of the atomic oxygen mass fraction and the catalytic surface heat transfer rate are excellent when th e only change in the composition of the gas comes from the surface catalysi s, and reasonable when there is partial, recombination of the gas in the bo undary layer due to the gas-phase chemistry. In contrast, when the surface is fully catalytic, the streamwise diffusion terms play a significant role, and the model is not valid. These results should apply to other situations with an attached boundary layer with recombination reactions. A comparison is made between the calculated and experimental measurements of the heat t ransfer rate at the catalytic junction. With a kinetic scheme which allows partial recombination in the boundary layer, good agreement is found betwee n the experimental and predicted values for surface materials which are ess entially non-catalytic. For a catalytic material (platinum), the experiment al and numerical heat transfer rates are matched to estimate the value of t he catalytic recombination rate coefficient. The values obtained show a con siderable amount of scatter, but are consistent with those found in the lit erature.