Thermal conductivity of sand–tire rubber mixtures as a function of tire chip fraction, size ratio, void ratio and applied vertical stress

Abstract

Sand–tire rubber mixtures are promising materials for thermal insulation. However, studies evaluating the impact of applied stress on the thermal conductivity (kt) of these mixtures are limited, despite the fact that energy storage tanks are typically located at deep depths where the sand–tire rubber mixtures may experience changes in the connectivity between sand particles under increasing stress. Therefore, in this study, thermal needle probe tests were conducted on sand–tire rubber mixtures with various size ratios (SR = 0.3, 1.4, and 5.2) and tire chip fractions (TF = 0.0, 0.1, 0.2, 0.4, and 1.0). To separate the impact of porosity and that of applied stress on kt of the tested sand–rubber mixtures, kt was measured as a function of porosity at very low stress levels. The kt values of the mixtures were then measured according to the applied vertical stress. The results of the tests performed at low stress levels demonstrated that kt of the tested mixtures decreased with increasing TF and decreasing SR because of the decrease in the number of sand-to-sand contacts. All mixtures showed a decrease in kt with increasing porosity; however, the dependence of kt on porosity was affected by TF and SR. With an increase in the applied stress, kt of all the mixtures increased. In particular, the tested mixtures with smaller SR showed even greater kt than pure sand at an applied vertical stress of 460 kPa, highlighting the significance of the applied stress on kt of the tested mixtures. Most notably, this study developed a novel thermal conductivity model that considers both the packing impact and pure stress impact, providing a more comprehensive framework for predicting the thermal conductivity of sand–tire rubber mixtures with varying SRs, TFs, porosities, and applied stresses.