Finite Element Modelling On Coupling Of Fluid Flow And Heat Transfer In Fractured Rocks

With the depletion of traditional fossil energy, nuclear energy has been used in more and more occasions. The use of nuclear energy will inevitably produce nuclear waste. Currently, the only way to deal with nuclear waste is to build disposal repository to store it. On the background of the disposal of nuclear waste, this paper uses the method of finite element method to analyze the relationship between fluid flow and temperature fields in single fractured rock models and regular multi-fractured rock model, and analyze the effect of crack opening, water flow velocity, rock thermal conductivity, heat resource temperature and other factors on the distribution of the temperature field in fractured rock. The conclusions of this paper are as follows:(1) Through building a single rock fracture model, the temperature control equations of the fluid flow and fractured rocks are deduced. Based on it, the analytical solution is obtained, which is compared with the results of the finite element calculation of COMSOL. The results show that, the simulation results based on variation method are in good agreement with the analytical solution, and the process for the analytical solution of ignoring some minor conditions will lead to deviation.(2) In the model of regular multi-fractured rock, control equations of fluid flowing and heat transferring are deduced, which will be discrete, and finite element calculation will be carried out. Compare the results with the experimental results by research group before and the simulating results by finite difference software FLAC. It can be seen that the results of numerical simulation for temperature field is slightly higher than the experimental results. This is due to the adiabatic boundary condition is hard to realize in reality, so the inevitable dissipation of heat reduces the temperature of the model. Although COMSOL, the finite element software will take longer in the calculation process, its adaptability of irregular mesh region and the function for the multi physics field coupling analysis is better than finite difference method.(3) The two important factors which affect the temperature is the opening width of cracks and fluid flow velocity. The wider cracks, and the bigger velocity of the fluid flow, will make more water to enter the fractured rock to transfer heat, which will reduce the rock temperature and temperature increasing of water. Similarly, the difference of heat resource temperature can bring change to the value of the entire temperature field in fracture rock, and heat source temperature has a great influence on the outflow temperature of fluid flow. The increase in thermal conductivity of rock will make more heat transfer take the way of thermal conduction, and the temperature gradient of rock and water also become larger, but its degree of influence is less than the crack opening and the velocity of flow.(4) The fracture which is closest to the heat source usually controls the entire distribution of temperature field in fractured rock. The heat transfer from the heat source to the distance will be obstructed by fracture flow. The distant crack and fracture between two cracks also have an impact on the entire distribution of temperature field in fractured rock. On the one hand, the distant crack will affect the adjacent rock temperature. On the other hand the distant crack mainly controls the temperature of more distant rock and the boundary. The fracture between the two cracks provides the way to fluid flow, which makes more influence for a single fracture on the temperature distribution.