Characterization of Carbon Ablation Models Including Effects of Gas-Phase Chemical Kinetics

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Characterization of Carbon Ablation Models Including Effects of Gas-Phase Chemical Kinetics

Candler, Graham V. / Alba, Christopher R. / Greendyke, Robert B.

The modeling of the oxidation and sublimation of carbon-based ablative thermal protection system materials remains an area of active research. In this paper, two gas–surface interaction models for carbon are studied at representative reentry conditions. One model is based on the widely used $B^{'}$ approach with an equilibrium saturated state assumption for the surface composition, and the second uses a detailed finite-rate chemical kinetics model for the gas–surface reactions. Key modeling parameters are varied in the finite-rate model to assess its sensitivity to modeling uncertainties. The gas-phase chemical kinetics models are also varied to characterize their effects on the ablation process. The models were evaluated using a generic sphere–cone geometry at four representative reentry conditions. It is found that there are notable differences in the predicted overall surface mass flux, and particularly in the details of the individual species mass fluxes to and from the surface. In addition, the gas-phase kinetic model is found to have important effects on the surface kinetics and the flux of individual species at the surface. From this study, it is clear that more detailed measurements of surface gas evolution under tightly controlled conditions are required to validate and improve the mechanism-based gas–surface interaction model for carbon. The $B^{'}$ approach does not capture the interaction of the nonequilibrium state of the gas interacting with the surface, making that approach highly suspect for many of the conditions studied.