Effects of fracture density, roughness, and percolation of fracture network on heat-flow coupling in hot rock masses with embedded three-dimensional fracture network

Abstract

This paper presents a model to reveal the heat transfer mechanism and simulate the heat-flow coupling process in fractured rock masses. Specifically, effects of fracture density, roughness, and percolation on heat-flow coupling are investigated systematically. The fractured rock masses are composed of a discrete fracture network and a rock matrix. Regarding the mesh discretisation for finite element analysis, the rock matrix is discretised into three-dimensional (3D) solid elements, whereas the discrete fractures are modelled by zero-thickness elements. Fracture and matrix elements share the same nodes. Considering the effect of temperature on fluid density and dynamic viscosity, a heat-flow coupled model of fractured rock masses is established with an embedded 3D fracture network. The reliability of the model is verified by comparing it with the analytical solution of a two-dimensional single-fracture heat-flow coupling problem. The effects of fracture density and diameter on percolation probability are studied based on Monte-Carlo tests with each group for 10,000 times. Finally, numerical samples of the 3D discrete fracture network with different geometric parameters are generated to characterise fractured rock masses, and heat-flow coupling numerical simulation is conducted simultaneously. Results show that the percolation of the fracture network is the decisive factor affecting heat-flow coupling. The average outlet flow rate of the percolation network under the same fracture density is much larger than that of the nonpercolation fracture network, which results in a more rapid decrease in the outlet temperature. Other factors such as fracture roughness are also investigated. It is discovered that the effect of fracture roughness on heat-flow coupling is almost negligible for the nonpercolation fracture network model.