| Engineering constructions such as oil and gas resources exploitation,geothermal energy development,nuclear waste disposal,high and low temperature tunnels generally involve water-heat coupling and multiphase flow problems in rock fractures.A large number of cracks in natural rock mass are the main channels of groundwater flow,which greatly enhances the permeability of rock mass.As the basic unit to study the seepage problem of rock fracture,the single fracture is also an important matrix to study the seepage heat transfer and multiphase flow of rock fracture under temperature.Therefore,in order to study the seepage and heat transfer characteristics of rock fracture and the permeability characteristics of multiphase flow,this paper takes granite fracture as the research object,and uses theoretical and experimental methods to carry out the seepage and convective heat transfer tests of granite fracture under temperature,and analyzes the seepage and convective heat transfer characteristics of rock fracture under temperature.Based on the self-similarity of capillary,a fractal model of relative permeability of rough fractures is proposed,and the effectiveness of relative permeability of fractures is verified by combining fractures with different opening distributions.The main research contents are as follows:（1）The nonlinear seepage characteristics of rock fracture under temperature are studied.A visual experimental system of rock fracture seepage-temperature coupling was developed.The seepage experiments of granite rough fracture under different temperature conditions were carried out.The nonlinear relationship between flow rate and seepage pressure under different temperature conditions was analyzed.The critical point of fluid from laminar flow to turbulent flow was characterized by critical Reynolds number,and the influence mechanism of fracture surface roughness and temperature on nonlinear seepage characteristics was revealed.The results show that the flow rate and osmotic pressure follow a linear relationship when the flow rate is small.When the flow rate increases to a certain extent,the flow rate and osmotic pressure show a significant nonlinear relationship.At the same time,the change of temperature will change the dynamic viscosity coefficient of water and the elastic modulus of rock mass,and ultimately affect the permeability characteristics.In the range of 20℃～50℃,the dynamic viscosity of water plays a dominant role.With the increase of temperature,the dynamic viscosity coefficient of water decreases continuously,and the seepage flow increases under the interlinked seepage pressure.In the range of 50℃～80℃,the elastic modulus of rock mass plays a leading role.With the increase of temperature,the elastic modulus of rock mass decreases and the rock mass expands,which further reduces the fracture opening.Under the same pressure gradient,the seepage flow decreases.（2）Based on the temperature-seepage test system of rock fracture,the convective heat transfer test of granite fracture was carried out.Based on the test conditions of’hot water and cold rock’,the effects of geometric characteristics of fracture surface,temperature and flow velocity on the seepage and heat transfer characteristics of granite fracture were analyzed.The results show that when the water flow with higher temperature flows through the rock fracture at room temperature,the temperature of rock mass increases due to convective heat transfer,the temperature difference between water and rock decreases continuously,and the convective heat transfer intensity decreases with time.Under the premise of constant temperature,the flow rate and convective heat transfer coefficient are positively correlated.Under the same flow rate,the greater the solid-liquid temperature difference,the greater the convective heat transfer intensity;when the roughness of the fracture surface increases,the contact area of convective heat transfer increases,and the convective heat transfer intensity also increases.With the progress of convective heat transfer,the heat exchange between water flow and rock fracture will eventually reach equilibrium.（3）Based on the cubic theorem and the self-similarity of tortuous capillary,the fractal model of relative permeability of rough fracture is derived by theoretical analysis.Combined with the normal distribution and logarithmic normal distribution of fracture aperture,the mathematical relationship between fracture relative permeability and aperture is established.The validity of the proposed model is verified by comparing the experimental data with the numerical simulation results.The results show that for the kr-Sw relationship of Gaussian aperture distribution fractures with different tortuosity fractal dimensions and minimum apertures,when the minimum aperture bmin remains 0,for Gaussian aperture distribution fractures with fractal dimension DT close to 2,the relative permeability kr and saturation Sw change more obviously,and the relative permeability decreases rapidly with the increase of DT.This is because when DT is large,the seepage path becomes more tortuous.Based on the cubic theorem,it can be concluded that the flow velocity decreases.When the tortuosity fractal dimension DT remains unchanged,as bmin increases,the residual saturation increases,the slope of the kr-Sw curve increases significantly,and the permeability decreases significantly.This is because when more voids cannot form a fixed capillary,the capillary flow path is reduced;the kr-Swrelationship of lognormal aperture fractures with different tortuosity fractal dimensions and minimum aperture is quite sensitive to tortuosity fractal dimension and minimum aperture.For the specified minimum opening bmin,the relative permeability decreases with the increase of fractal dimension DT,and for a given DT,the relative permeability decreases significantly with the increase of bmin. |
