Modeling of Influence of Gas Atmosphere and Pore-Size Distribution on the Effective Thermal Conductivity of Knudsen and Non-Knudsen Porous Materials

Abstract

Application of porous insulation materials in various gas atmospheres changes their effective thermal conductivity due to many structural and thermal influences. One of those is the pore-size distribution of the material. In this paper a study of the influence and modeling of the modification of the effective thermal conductivity of various materials is presented by exchanging the gas atmosphere. For this purpose 11 different materials with various pore-size distributions are investigated. The experimental work included thermal conductivity measurements along with an analysis of the porous structure. Two additional important effects of thermal accommodation and radiation will be clearly identified as caused by the exchange of the filling gas. Modeling the gas thermal conductivity for different polydisperse systems is carried out based on kinetic theory and the Knudsen effect of rarefaction of the gas in a wide range of the Knudsen number. Development of the single pore model of the gas thermal conductivity to a new predictive model with an exponential parameter gives good agreement with the experimental results.