Multiscale simulation of coupled fluid flow, thermal and heterogeneous chemical reactions in fibrous porous media during ablation

Abstract

High-accuracy prediction of the thermal response for ablative material is significant for the reliability of thermal protection system (TPS). Multiscale approach development is still required to simultaneously consider the coupled fluid flow, thermal diffusion, heterogeneous chemical reactions and the pore-scale structure evolution during the thermal ablation process. In this work, the ablation process of carbon fibrous porous media is investigated under the atomic oxygen (AO) flow considering the high-temperature gas non-equilibrium effect. To deepen our understanding of both heterogeneous chemical reactions of fibrous porous media at the atomic scale and its effect on the heat and mass transfer at the pore scale, Reactive Molecular Dynamics (RMD) method is used to explore the chemical reaction kinetics at the gas-solid interface, which is employed to Darcy–Brinkman–Stokes (DBS) model to reveal the development of porous flow, thermal, and chemical reactions simultaneously. The effect of volumetric temperature, incoming AO flow concentration, Péclet (Pe) number, and the initial pore structure on the thermal ablation process are further explored. The results show that during heterogeneous reactions at the gas-solid interface between AO and carbon surface, the oxidation reaction is found to be dominant with an activation energy of 106.402 ± 5.75 kJ/mol. Considering the exothermic oxidation reaction in the porous medium, higher incoming AO flow concentrations accelerates the ablation of carbon fibrous porous media. Under identical porosity conditions, the ablation recession rate remains relatively consistent regardless of the homogeneous distribution of carbon fibers within the porous medium. However, at elevated Péclet numbers, the influence of initial porous structural variations on ablation morphology becomes pronounced, with dual-porosity structures developing a higher surface roughness significantly. This proposed multiscale simulation work can potentially provide valuable pore-scale structure evolution insights during the ablation of porous medium, enhancing the prediction accuracy of the material thermal response for TPS applications.