The Study of Downhole Flow Field Numerical Simulation in Geothermal Closed-loop System Considering Phase Change of Working Fluids

Abstract

As a resource-rich, low-cost and widely distributed energy, geothermal energy is easy to exploit and can be directly used, so it can serve as an important supplement for traditional fossil fuels, which is cleaner in terms of CO2 emission and air pollution. The Downhole Coaxial Heat Exchanger (DCHE) geothermal system, which injects and heats working fluids through the annulus and produces them through the insulated inner pipe, is suitable for the development of medium-deep geothermal resources. Firstly, a DCHE two-dimensional axisymmetric unsteady-state flow and heat transfer model for a 10-meter interval was established, and CO2 was selected as the working fluid for circulation and heat transfer. In consideration of the pressure and temperature variations including phase change, the datasets of several real thermophysical parameters were established for the working fluid. Next, the dynamic analyses were made in terms of the flow field, the temperature field and the volume fraction of liquid phase. Then, the outlet temperature and the thermal power of the working fluid were chosen as the criterion parameters to evaluate the heat extraction performance of the working fluid, and the simulation results were compared with the situation with water as the working fluid. Finally, a three-dimensional unsteady-state flow and heat transfer model which includes the DCHE and the neighboring formation was established to verify the comparison results. Furthermore, the inlet temperature and volume flow of working fluids had also been studied. The simulation results show that considering the outlet temperature and the thermal power of the system, high-pressure CO2 has better heat extraction performance compared with water. The higher the volume flow rate, the higher the priority of CO2 as a working fluid.