Radiation heating tests to evaluate low-emissivity porous ceramics coatings for stand-off thermal protection systems

Abstract

Diffuse reflection of thermal radiation transfer by Mie scattering can provide a thermal barrier without conversion leading to conduction heat transfer. Applying such a system to internal thermal insulating materials of a stand-off thermal protection systems (TPS) provides an efficient means of preventing thermal radiation transfer. Porous MgAl2O4 ceramics have high thermal resistance because of their low radiation absorption over a wide range from visible to near-infrared wavelengths. For this study, after porous MgAl2O4 ceramics was coated onto a fibrous insulator surface, we investigated its application to stand-off TPS, especially targeting reusable space transportation systems. Direct infrared heating tests were conducted in vacuum. Indirect infrared heating tests were done using graphite as a surface panel in an arc-heated wind tunnel with a high heat flux rate. The apparent thermal conductivity was obtained from the unsteady-state thermal history of the infrared heating experiment using an arc-heated wind tunnel to evaluate the radiation thermal barrier effects of the porous MgAl2O4 ceramics. The low emissivity of porous MgAl2O4 ceramics suppressed heat input and maintained a low surface temperature. Results suggest that thermal radiation transfer was suppressed and that the apparent thermal conductivity of the fibrous insulator was reduced.