The Thermal Conductivity of Glass Sieves: II. Liquid and Gas-Saturated Frits

Abstract

The thermal conductivity of liquid and gas-saturated glass sieves (frits) of porosities between 20 and 48 % is presented as measured at room temperature and ambient pressure. The saturating fluids cover a range in thermal conductivity from 0.018 W⋅m⁻¹⋅K⁻¹ to 0.598 W⋅m⁻¹⋅K⁻¹. The experimental results are fitted to a simple two-dimensional composite model for the heat transfer in porous media. The runs were carried out using a transient hot-bridge (THB) measuring instrument of an expanded uncertainty 3 % to 5 %. It turned out that (1) in order to correctly describe the experimental findings, a so-called thermal porosity has to be introduced that differs from the stated porosity of the frits. (2) There is not only a smaller-than-predicted thermal conductivity of gas-saturated frits as is known since a couple of decades but also a larger-than-predicted conductivity and, of course, a continuous transition between both effects. The whole latter effect can be attributed to the impact of dissolved He on the thermal conductivity of the matrix and be mathematically described in terms of the thermal porosity in the framework of the above-mentioned composite model.