Deviation from Thermochemical Equilibrium and Aerodynamic Heating Assumptions in the Ablation Process of Camphor

A heritage approach to simulating the thermal response and ablation of protective materials relies on assumptions of equilibrium thermochemistry and film coefficient theory, which drive the heat and mass transfer at the surface. With the growing maturity of finite-rate models for gas-surface reactions and coupled approaches between the flow and material domains, the traditional assumptions can be critically evaluated and potentially improved. In this study, we utilize our previous work on the fully-coupled simulation of camphor ablation to explore the observed differences in material response when simulated with the equilibrium-based uncoupled approach. Using the available data from the coupled simulations, we demonstrate that the significant deviation of the uncoupled approach occurs as a result of unequal heat and mass transport in the boundary layer, as well as an incorrect estimation of the heat flux reduction. By adjusting only two parameters in the film coefficient approach, we can significantly enhance the uncoupled results for the cases studied in this work. The derived corrections are demonstrated to be applicable to the stagnation point ablation of camphor on spherically-blunted geometries, and their generality is showcased. Furthermore, we evaluate the applicability of the approach to spatial locations along the geometry and different free-stream conditions.