Estimating Effective Radiative Properties and In-Depth Radiative Heating of Porous Ablators

High entry speeds and exotic planetary gases can result in significant radiative heat loads on space capsules. The mechanism behind the transport of radiative signatures is fundamentally different from the conductive mode of energy transport. Radiative heating significantly depends on the effective radiative coefficients of the thermal protection systems (TPS) material. The radiative coefficients of carbon-based and silica-based fibrous materials have been computed as function of wavelength using the photon path length Monte Carlo method. Significant variations in the radiative coefficients are observed at wavelengths that are relevant to shock-layer emissions. Although carbon-based fibrous materials exhibit higher absorption coefficients, in comparison to silica-based systems, the absorption coefficients drop by two orders of magnitude in the wavelength range of 100-200 nm. The radiative coefficients for the silica system demonstrate significant transmittivity in the range of 100-1000 nm, which corresponds to most shock-layer emissions. The radiative coefficients are used to solve the radiative transport equation using the P-1 approach to obtain radiative heat flux. The total energy equation for decomposing TPSmaterials is solved with the radiative heat flux from the P-1 model, and the conductive heat flux using the Fourier’s law. It is observed that peak temperatures inside the material are higher when radiative transport is explicitly accounted for through the P-1 model. For the silica-based materials, it is observed that the peak temperature varies by 100 K for a mere change in the absorption coefficient from 80 m−1to 34 m−1. The temperature distribution inside the material is also broader with low absorption coefficient, and the density profiles are also influenced by the radiative heat flux. This study demonstrates that it is important to include radiative transport in material response solvers and the need to accurately compute the effective radiative coefficients of the materials.