For downhole coaxial heat exchanger (DCHE), the working fluid is injected from the annulus and produced from the central insulated tubing. There have been many studies on the heat extraction performance of DCHE. However, to the best of our knowledge, most previous numerical models did not consider the fluid flow in the reservoir, which has a significant effect on DCHE performance. Thus, an unsteady-state heat transfer model considering flow in reservoir is presented. The effects of the key factors are studied. The field test is conducted to verify the feasibility of DCHE system. The simulation results depict that the increase of flow velocity in reservoir will increase the outlet temperature and thermal power. As the mass flow increases, the thermal power increases. As the inlet temperature decrease, the outlet temperature decreases and the thermal power increases. The simulation data are in good agreement with the test data, and the error is less than 12%. The findings can offer guidance for optimal design of DCHE.
Keywords geothermal energy, DCHE, unsteady-state heat transfer numerical model, field test