Assessment of air-pyrolysis gas reactions on the wall heat flux during atmospheric re-entry

The goal of this study is to evaluate the effects of chemical reactions between air and pyrolysis gases on the modification of convective–diffusive heat flux, for a carbon/polymer composite material performing an atmospheric re-entry from low Earth orbit. CFD simulations are carried out for a 2D axisymmetric sphere of diameter 1 m, with an uniform blowing condition over the entire wall exposed to the flow, and considering air-pyrolysis reactions. In the hypersonic community, the emission of ablation species is usually assumed to reduce heat transfer. However, this study shows that for moderate blowing rates, highly exothermic air-pyrolysis gases reactions take place sufficiently close to the wall to contribute to a significant increase in wall heat flux. At high blowing rates, convective effects dominate, and the heat transfer decreases with respect to the blowing rate. Competition between reactive and convective effects is demonstrated. The mixture composition of outgassed species is shown to have a major influence on this phenomenon. The effects of species diffusion coefficients modeling fidelity is studied by comparing two approaches: calculation of the coefficients using the Schmidt number or collision cross-section data. A deviation up to 15 % is observed between the two methods, increasing with respect to blowing rate and flow enthalpy. Simulations for a fuel tank geometry are carried out to study the effects of object’s shape on the evolution of heat transfer in the presence of blowing. • Effects of blowing on wall heat flux with air-pyrolysis gas chemistry are analyzed. • Exothermic air-pyrolysis gas reactions near the wall can increase the wall heat flux. • Competition between reactive and convective regimes is demonstrated. • Effects of composition, diffusion modeling, and object shape are investigated.