Theoretical modeling and experimental validation for the effective thermal conductivity of moist silica aerogel

In this paper, the influence of water content on insulating performance of moist silica aerogel nano-porous composite material is studied theoretically and experimentally. A new fractal-intersecting sphere with inhomogeneous water film structural model is proposed. It considers the microstructures in aerogel and analysis of condensation process in moist environment based on surface physics theory. The effective thermal conductivity of composite aerogel material has been also derived using the equivalent circuit method. The nano-scale heat transfer effect and the influence of micron-scale additives such as opacifiers and reinforced fibers are involved in the present theoretical model. The theoretical model is semi-empirical because some parameters are introduced to better predict the effective thermal conductivity based on partial experimental data at a reference temperature. Experiments were conducted to measure thermal conductivities of nano-porous aerogel composites within temperature range of 15-45 degrees C and water uptake range of 0-110 mg/g by the transient plane source method. Validation study for both current samples and material in Zhang et al. (2015) was carried out, where the relative errors between experimental thermal conductivity and theoretical calculations are less than 7% and 8%, respectively. The prediction accuracy of present model for this super insulating material is much higher than that of Bjurstrom's model which are widely used to calculate effective thermal conductivity for common porous media containing moisture. In addition, the results show that the increase of thermal conductivity with water uptake is linear at the same temperature. Lower porosity would result in a greater sensitivity to moisture content for aerogel material. Comparing to the influence of various additives, the properties of silica aerogel matrix is the most dominant factor in determining the effective thermal conductivity under different water content. (C) 2019 Elsevier Ltd. All rights reserved.