Simulation of porous carbon preform ablation in a chemically reacting environment

The intense aerothermal conditions encountered during atmospheric entry require the application of thermal protection materials to shield the moving vehicle. The porous nature of these materials leads to complex interactions between the boundary layer and the material structure. These interactions may involve penetration of the reactive species into the material, excessive heating, in-depth oxidation, and eventual weakening of the material. In this study, we simulate the ablation of a sphere-cone sample made of porous carbon preform in an arc-jet environment. We investigate the chemical composition inside the material as a result of the gas-surface interactions and in-depth chemistry. Furthermore, we evaluate the effect of modeling the multi-species environment in the material on the accuracy of the thermal response prediction and surface recession. Finally, we explore the simulation accuracy by modeling the material environment using a single bulk gas with an estimated elemental composition. In the simulation of the problem, we employ a coupled framework between the hypersonic flow solver CHAMPS NBS-Cart and a material response solver KATS-MR. To account for the complex interactions, the material environment is modeled with a high-fidelity approach, accounting for finite-rate gas-surface chemistry, multi-species gas transport through the material, and in-depth gas-phase chemical reactions. Gas-solid reactions beneath the surface are not modeled at this stage. The diffusive species transport is modeled using Modified Fick's law and multi-component diffusion coefficients. The in-depth gas-phase reactions are modeled with finite-rate or equilibrium-based chemistry. The obtained results point to a substantial amount of atomic oxygen penetrating beneath the material surface, indicating the potential occurrence of an in-depth oxidation regime. The chemical environment inside the material appears closely approximated by the equilibrium-based rates showing only minor differences with the finite-rate model. Finally, it is shown that the in-depth multi-species environment in the carbon preform has a negligible effect on the surface heating and in-depth energy transport. The same problem, if not accounting for the in-depth oxidation, can be accurately simulated with a single bulk composition.