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The natural fracture system exerts a significant influence on the physical and mechanical properties of rock masses. The heat transfer in fractured rock masses is much more complicated than that of intact rock due to the thermal contact resistance induced by natural fractures with varying size, aperture, roughness, and orientation. This work describes a method for determining the representative element volume (REV) of heat transfer and the 3D thermal conductivity tensor of fractured rock masses. The method is based on the element-wise averaging of temperature and heat flux, which can be obtained from a finite element solution to a steady state heat transfer problem with specific boundary conditions. A case study was conducted based on the geological data of a potential area for high level radioactive waste disposal in China. The results indicate that the size of heat transfer REV of the rock masses in the study area is approximately 18 m × 18 m × 18 m. 3D thermal conductivity tensor, the magnitudes and directions of the three principal thermal conductivities, and anisotropy of the fractured rock masses were obtained. Influences of main fracture parameters including density, disc diameter, and thermal contact resistance on the heat transfer characteristics were investigated. The research results in this work can provide a better understanding of the discontinuous heat transfer processes in fractured rock masses and their homogenization methods. [ABSTRACT FROM AUTHOR]

In this work, thermal damage effect on the thermal conductivity inhomogeneity of granite was experimentally investigated. To study the influence of different cooling methods, granite samples were heated at different temperature levels (from 100°C to 1000 °C), then cooled in air and water, respectively. Distributed thermal conductivity of granite samples were measured using optical scanning method before and after thermal damage treatment. Besides, some non-destructive tests (open porosity and P-wave velocity) were performed. The results indicate that thermal damage will lead to the decrease of thermal conductivity and the increase of thermal inhomogeneity degree. The decrease degree of thermal conductivity values increases with the treatment temperature. The maximum decrease can be 54.0% (after heated at 1000 °C). When the treatment temperature is higher than the phase transition temperature of quartz (573 °C), the thermal inhomogeneity degree of the damaged samples increased obviously. After heated at each temperature level, the water cooled samples exhibit greater thermal conductivity decrease and greater thermal inhomogeneity factor increase than air cooled samples. This may be attributed to the fact that rapid cooling has extra effects on the damage evolution of rock, which is induced by the high temperature gradient generated when temperature changes occur in a very short time. A thermal damage model was proposed to describe the evolution characteristics of thermal conductivity with treatment temperature. Research results in this work can provide better knowledge to the evolution mechanism of distributed thermal conductivity of engineering rock masses experienced high temperature environment, and the influence of different cooling methods. [ABSTRACT FROM AUTHOR]