Uncertainty Analysis of Reaction Rates in a Finite Rate Gas-Surface Model

A finite–rate–catalytic wall boundary condition incorporated into hypersonic flow simulations is investigated. Benchmark simulations of hypersonic flow over a cylinder are presented using the finite–rate–catalytic model parameterized with a test air–silica chemical model comprising the gas–surface reaction mechanisms and their associated rates. It is demonstrated that backwards recombination rates should not be arbitrarily set but must be consistent with the gas–phase thermodynamics, otherwise a drift from the equilibrium state may occur. The heat flux predicted by the finite rate model lies between non–catalytic and super–catalytic limits depending on the surface temperature. It is found that even for a constant surface temperature, the oxygen recombination efficiencies determined by the model are not only a function of temperature, but also a function of the surface coverage, where recombination efficiencies are seen to rise as coverage decreases. Monte Carlo uncertainty analysis is performed to correlate the influence of individual mechanisms to the stagnation point heat flux and the expected progression of dominant mechanisms is found as the surface temperature is raised. Additionally, it is found that increased surface reactivity increases the chemical heat flux while also altering the boundary layer in a manner that decreases the conductive heat flux. Finally, efforts to use computational chemistry to reduce the uncertainty in individual rates of dominant oxidation mechanisms for oxygen–carbon interactions will be summarized.