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Simultaneous Inverse Characterization of Thermal Properties of Semitransparent Porous Materials and its Application in Thermal Protection Materials

Kolloquium der Abteilung 7

Reusable launch vehicles (RLVs), such as the space shuttle experience extreme temperatures upon reentry into the atmosphere. To guarantee the vehicles safety in mission, thermal protection system (TPS) is utilized to prevent the substructure of the vehicle from exceeding a certain maximum temperature. Light-weight porous materials are very effective insulation for thermal protection applications. For instance, rigid ceramic tiles, flexible fibrous blankets, ceramic foams, aerogel composite material, and multilayer insulation materials, etc., have been developed as reusable insulations for different Vehicles’ TPS. These thermal protection materials consisting essentially of solid particles and gas share common nature of semitransparent, strongly scattering, and high porosity. Owning largely to the effects of radiation absorption and scattering, the heat transfer within the material subjected to severe aerodynamic heating often exhibits distinctive features: non-stationary, non-linearity, and conjugate nature. In order to accomplish successful design and optimization of thermal protection system subjected to extreme aerodynamic heating, reliable thermal properties and their possible dynamic evolution with heating conditions are of primary concern, since a little overheating of primary structure of vehicle may lead to unexpected disasters.

Due to a few limitations of direct theoretical modeling method of thermal properties for semitransparent porous materials in practical engineering applications, this presentation will introduce general inverse determination procedures of thermal properties for three typical thermal protection materials, in close combination of their microstructure changes during heating in use. In the first part, on basis of measured high temperature transmission spectra for fibrous insulation, the radiative properties were modified taking into account anisotropic scattering within the material. An inverse conduction–radiation analysis in an absorbing, emitting and scattering medium was conducted for the simultaneous estimation of the conductive and radiative properties using the experimentally measured temperature responses for external temperatures up to 980K. After the estimated properties were validated by comparing the predicted and measured results under transient and steady-state condition, this characterization method was used to quantify thermal properties degradation, and then correlated with microstructure evolution for this material subjected to intense heat over prolonged periods of time, which raised by reuse purpose of thermal protection system. In the second part, a hierarchical inverse reconstruction method was proposed to determine multiple high temperature thermal properties and relevant parameters of interest for ceramic foams. Some unknown or uncertain fundamental properties and model parameters in thermal analysis were determined by bringing the inverse identified properties into a set of equations. The relation of conductive and radiative properties with temperature, cell density and different solid phases of foams, as well as the reliability of the determined parameters was discussed. In the last part, the general idea of my ongoing research project on joint inverse characterization of thermal properties at different wavelengths and temperatures for aerogel-fiber composite material will be introduced. After a detailed dynamic tracking of microstructure evolution for the material subjected transient heating conditions, the thermal properties at different heating stages as well as phase transformation parameters for the aerogel-fiber material would be simultaneously retrieved on basis of conduction/radiation/phase transformation involved heat transfer models. A draft plan to collaborate with PTB from our side will be referred at the end of this presentation.