Effective conductivity of porous ceramics in a radiative environment

The knowledge of the effective thermal conductivity, taking into account conduction and radiation, is crucial for the accurate prediction of the thermal behavior of porous foams, especially in an environment where radiation is dominating. We present a combined experimental-numerical method for the quantification of the temperature-dependent effective thermal conductivity of porous ceramics. The experiments include transient and spatially-resolved temperature measurements of ceramic foam samples with porosities between 75% and 92%, exposed to high radiative fluxes (peak of 1500 kW.m(-2)). Maximum surface temperatures of 1900K for high purity alumina samples and of 1500K for alumino-silicate (AS) samples were measured, and nonlinear temperature profiles through the sample along the main temperature gradient direction were measured. A 3D numerical model based on an OpenFOAM toolbox was developed and used to determine the thermal conductivity, using the experimental temperature measurements as input. We quantified the effective thermal conductivities (ETCs) in a temperature range of 288-1473K and showed that the ETC of AS samples can significantly increase in the presence of incoming and internal radiation. Alumina with 2 to 5 times smaller typical pore sizes (80 mu m vs 190 mu m, 230 mu m, and 400 mu m) exhibited an ETC dominated by conduction and relatively unaffected by radiation. This work illustrates that the ETC of porous materials exposed to radiative fluxes do not only depend on material properties or morphology but also on the operational conditions.