The energy transfer catalytic recombination coefficient for nitrogen and oxygen recombination on the surface coating of high-temperature reusable surface insulation (HRSI) is inferred from stagnation point heat flux measurements in a high-temperature dissociated arc jet flow. The resulting catalytic recombination coefficients are correlated with an Arrhenius model for convenience, and these expressions may be used to account for catalytic recombination effects in predictions of the heat flux on the HRSI thermal protection system of the Space Shuttle Orbiter during reentry flight. Analysis of stagnation point pressure and total heat balance enthalpy measurements indicates that the arc heater reservoir conditions are not in chemical equilibrium. This is contrary to what is usually assumed for arc jet analysis and indicates the need for suitable diagnostics and analyses, especially when dealing with chemical reaction phenomena such as catalytic recombination heat transfer effects.

Catalytic recombination of nitrogen and oxygen on high-temperature reusable surface insulation

The lecture discusses the meaning of catalysis and its relation to aerodynamic heating in nonequilibrium hypersonic flows. The species equations are described and boundary conditions for them are derived for a multicomponent gas and for a binary gas. Slip effects are included for application of continuum methods to low density flows. Measurement techniques for determining catalytic wall recombination rates are discussed. Among them are experiments carried out in arc jets as well as flow reactors. Diagnostic methods for determining the atom or molecule concentrations in the flow are included. Results are given for a number of materials of interest to the aerospace community, including glassy coatings such as the RCG coating of the Space Shuttle and for high temperature refractory metals such as coated niobium.

Wall catalytic recombination and boundary conditions in nonequilibrium hypersonic flows - With applications

Slip/jump boundary conditions for rarefied reacting/non-reacting multi-component gaseous flows

An efficient and accurate method for solving the viscous shock layer equations for hypersonic flows over long slender bodies is presented. The two first order equations, continuity and normal momentum, are solved simultaneously as a coupled set. The flow conditions included are from high Reynolds numbers at low altitudes to low Reynolds numbers at high altitudes. For high Reynolds number flows, both chemical nonequilibrium and perfect gas cases are analyzed with surface catalytic effects and different turbulence models, respectively. At low Reynolds number flow conditions, corrected slip models are implemented with perfect gas case. Detailed comparisons are included with other predictions and experimental data.

/pdf/viscous-shock-layer-analysis-of-hypersonic-flows-over-long-371ui1f0dk.pdf

Viscous-shock-layer analysis of hypersonic flows over long slender vehicles

The influence of change in the precursor region flow properties on the entire shock layer flow phenomena around a Jovian entry body was investigated. The flow in the shock layer was assumed to be steady, axisymmetric, and viscous. Both the chemical equilibrium and the nonequilibrium composition of the shock layer gas were considered. The effects of transitional range behavior were included in the analysis of high altitude entry conditions. Realistic thermophysical and radiation models were used, and results were obtained by employing the implicit finite difference technique in the shock layer and an iterative procedure for the entire shock layer precursor zone. Results obtained for a 45 degree angle hyperboloid blunt body entering Jupiter's atmosphere at zero angle of attack indicates that preheating the gas significantly increases the static pressure and temperature ahead of the shock for entry velocities exceeding 36 km/sec. The nonequilibrium radiative heating rate to the body is found to be significantly higher than the corresponding equilibrium heating. The precursor heating generally increases the radiative and convective heating of a body. That increase is slightly higher for the nonequilibrium conditions.

/pdf/effects-of-precursor-heating-on-radiative-and-chemically-3xx66v430l.pdf

Effects of precursor heating on radiative and chemically reacting viscous flow around a Jovian entry body

The influence of wall slip and catalytic atom-recombination on the flowfield and wall heat flux are calculated for high-altitude flight and arcjet-flow conditions. Boundary equations, which include velocity slip, temperature jump, and wall catalytic atom-recombination, are coupled to the viscous reacting multicomponent NavierStokes equation. These equations are solved using a time-dependent finite difference technique applied to spheres in an arcjet flow (Reynolds number of 550) and a high-altitude flight case representative of the space shuttle orbiter (Reynolds number of 450). The results indicate that catalysis strongly influences the temperature jump, but not the velocity slip. Slip increases the atom fraction and temperature at both the wall and in the flowfield. Likewise, the shock stand-off distance, the wall heat flux, and friction coefficient are increased over the nonslip cases. The reacting gas calculations confirm the chemically frozen nature of the shock layer in arcjet flows. design the reusable space shuttle orbiter for highaltitude, low Reynolds number, atmospheric entry necessitates a more comprehensive treatment and understanding of the interaction between a high enthalpy gas flow and a relatively cold surface. At low densities the continuum model of the gas breaks down in regions of large gradients such as those near a cold body. Corrections to the equations for the boundary conditions are then required for the flow. These influences are reflected in the calculations of surface heat-transfer rate and chemical composition of the flow near the wall. The aim of this paper is to quantify how these low density phenomena interact, how they influence interpretation of test data obtained on thermal protection systems, and how they alter the predictions of heating rates and performance of the space shuttle orbiter thermal protection system (TPS) during its long high-altitude entry. The approach taken here is to obtain finite difference solutions to the reacting Navier-Stokes equations for the flow around spheres in both space shuttle flight and arcjet environments. The wall boundary conditions for these solutions are obtained from slip and jump relations for a nonequilibrium multicomponent gas mixture. They include the effects of catalytic atom recombination. From the solutions one can assess the effects. of the boundary conditions on the flow properties, the heat flux, etc. At low densities, the continuum-flow equations are no longer adequate close to the wall because the mean free path becomes long compared to characteristi c lengths associated with significant changes in macroscopic-flow parameters. The flow in a region next to the wall having a thickness on the order of a mean free path (the Knudsen layer) cannot be described by the Navier-Stokes description (Kogan1). To determine the flow properties within the Knudsen layer requires the direct solution of the Boltzmann equation matched to the solutions for the outer flow (Navier-Stokes equation) and the wall boundary condition. This is most conveniently done through the use of a slip model in which slip and jump properties are used for the boundary conditions for the Navier-Stokes equations.

Carl D. Scott

Papers

Reacting Shock Layers with Slip and Catalytic Boundary Conditions