Microscale Knudsen Effect over the Transverse Thermal Conductivity of Woven Ceramic Fabrics Under Compression

Abstract

Woven-fiber ceramic materials have shown remarkable results in the design of insulative lay-up structures for flexible thermal protection systems. A deeper understanding of heat transfer through the different insulation layers is key for predicting the performance of heat shields. In this article, a thermo-mechanical multiscale model is developed to predict the out-of-plane thermal conductivity at the micro- and meso-levels of transversely loaded two-dimensional woven ceramic fabrics. Knudsen effects within the multiscale structure are studied by adjusting gas pressure conditions. Alumina-based Nextel-BF20 and silicon carbide Hi-Nicalon with a 5-harness satin weave pattern are modeled by finite-element analysis. The computational results are validated experimentally by applying the anisotropic transient plane source method. We find that out-of-plane thermal conductivity decreases significantly with gas pressure due to Knudsen effects in the confined air within the fibrous structure. The dependence of thermal conductivity of fabrics on fiber volume fraction is shown to decrease markedly with pressure reduction. The proposed multiscale modeling approach yields a notable accuracy improvement, with respect to simplified series-parallel models, when compared with our experimental measurements. The FEA model is applicable to other fabric materials and loading conditions and presents an opportunity to study how changes at the fiber level affect the overall thermal behavior of woven fabrics.