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, and penetration of radiative signatures depend on the effective radiative properties (coefficients) of the thermal protection systems (TPS) material that protects the space capsule. 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, absorption coefficient of carbon based material drops by two orders of magnitude in the range of 100-200 nm. The radiative properties of carbon-based fibrous material is seen to be dominated by scattering and absorption with minimal transmission. However, the transmission coefficients for the silica system dominated the radiative coefficients 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 the in-depth radiative heat flux. The total energy equation for decomposing porous TPS materials 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 through the P-1 model. Small variations in the absorption coefficient of the silica-based materials also affected the in-depth temperature profiles. Additionally, a broader temperature distribution is obtained inside the material with low absorption coefficient, and the density profiles are also influenced by the radiative heat flux. This study demonstrates that it may be important to include radiative transport in material response solvers, and the need to accurately compute the effective radiative coefficients of the TPS materials.