| In the field of modern rock engineering, such as the disposal of high level radioactive nuclear waste, the deep mining of oil, gas and solid mineral resources, and the hot dry rock geothermal energy extraction system, the multi-field compled problemes of thermal, hydrological, mechanical and chemical behavior in sparsely fractured rocks should be considered. This paper studies the semi-analytical calculation theory of heat transfer and thermal stress in sparsely fractured rocks with water flow, and analyses the features of temperatures, thermal stress and displacement in the near field of nuclear waste repositories with the proposed method. The main contents and results of this thesis are follows:(1) The semi-analytical method and analysis of influencing factors for temperatures in single-fracture rock with3-D heat transfer and saturated water flow:A simplified conceptual model, taking into account of distributed heat source and saturated single-fracture rock of infinite extent, is proposed for three-dimensional water flow and heat transfer, and a semi-analytical method is developed to to solve the transient temperature distributions in fracture water and rock matrix, and a solution program is coded. Two numerical examples with special flow field are provided for illustration of the proposed method with comparison of an analytical solution based on ID rock thermal conduction, and other numerical examples with distributed heat sources are extended for characteristics of flow and heat transfer in single-fracture rock and the sensitivities to flow velocity, rock thermal conductivity and heat source intensity. The temperature of water in the fracture calculated by using the semi-analytical method is lower in upstream and higher in downstream than the analytical solution, due to the fact that the former method takes into account3D thermal conduction in the rock matrix, whereas the latter assumes ID conduction. The effects of the distributed heat source on the temperatures of fracture water and rock are more sensitive in downstream as a result of the heat advection of fracture fluid flow.(2) The semi-analytical method and analysis of examples for temperatures in sparsely fractured rocks with3-D heat transfer and saturated water flow:A simplified conceptual model, taking into account of distributed heat source and saturated sparsely fractured rocks, is proposed for three-dimensional water flow and heat transfer, and a semi-analytical method is developed to to solve the transient temperature distributions in fracture water and rock matrix, and a solution program is coded. Three example calculations are presented to illustrate the workability of the developed approach. The calculations found that the temperature distribution and evolvement in sparsely fractured rock mass may be significantly influenced by geometric characteristics of fractures. In addition, it may be observed that the early-time transfer of heat in the rock matrix adjacent to only one particular fracture is dominated by the heat diffusion perpendicular to the fracture walls, whereas a similar statement could not made if the rock matrix is close to more than one fractures.(3) The semi-analytical method and analysis of examples for thermal stresses and displacement in single-fracture rock with3-D heat transfer and saturated water flow: Taking the simplified conceptual model of sparsely fractured rocks with3-D heat transfer and saturated water flow as the subject of study, the thermoelastic governing differential equations of rocks are formulated. According to Goodier’s thermoelastic displacement potential method, the thermal stress and displacement are divided into two portions:the particular solution caused by temperature evolution (temperature stresses and displacement) and the general solution for the boundary conditions (constraint stresses and displacement). The semi-analytical method for both the temperature stress (displacement) and constraint stresses (displacement) is developed, and the solution program is coded. A numerical example with special flow field are provided for illustration of the proposed method with comparison of an analytical solution based on ID rock thermal conduction, and another numerical example with distributed heat sources are extended for characteristics of thermal stresses and displacement in single-fracture rock. The thermal stresses on the fracture surface calculated by using the semi-analytical method is very close to the analytical solution. There is small difference between the semi-analytical solution and the analytical solution, due to the fact that the former method takes into account3D thermal conduction in the rock matrix, whereas the latter assumes1D conduction. The temperature stress near the heat source is compressive as the heating expansion of the rock, and the temperature stress far away from the heat source turns into tensile due to the strain compatibility requirement in3D elasticity.(4) The semi-analytical method and analysis of examples for thermal stresses and displacement in sparsely fractured rocks with3-D heat transfer and saturated water flow A semi-analytical method for thermal stresses and displacement in sparsely fractured rocks with3-D heat transfer and saturated water flow is developed, and a solution program is coded. Numerical examples are provided for characteristics of thermal stresses and displacement in sparsely fractured rocks under the influence of heat source(s). The result shows that the thermal stresses in sparsely fractured rock mass may be significantly influenced by geometric characteristics of fractures and heat source(s) distribute, and the peak normal primary temperature stresses appear at the vertical projection of the heat source on the fracture surface.(5) The semi-analytical analysis of thermo-hydro-mechanical processes for nuclear waste repositories in sparsely fractured rocks:Using the proposed semi-analytical method for temperatures and thermal stresses and displacement in sparsely fractured rocks with3-D heat transfer and saturated water flow, temperature evolution and thermal stresses and displacement of the nuclear waste repository are analysed, and the influence of the fracture water flow velocity and the distance between the heat sources on the temperature evolution and thermal stresses and displacement are studied. The calculations show the following observations:Influenced by the heat sources, temperature, thermal stresses and displacement increase rapidly and get the peak, then decrease slowly duo to the decay of the heat sources. The water flux in the fractures may decrease the rate of temperature rise in regions close to the distributed heat source and increase the rate of temperature rise in regions downstream away from the distributed heat source, and the influence on temperature, thermal stresses and displacement is more significant when the fracture water flow velocity becomes larger. In the near filed of the nuclear waste repository, the temperature and the normal primary temperature stresses and displacement on fracture surfaces increase as the distance between the heat sources decreases. If the distance between the heat sources decreases is too small, the peak of normal primary temperature stresses and displacement on fracture surfaces will be significantly enhanced as the heat transfer superposition of different heat sources. |
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