Numerical Study On The Heat Transfer Characteristics Of Geothermal Downhole Heat Exchanger With High Aspect Ratio

Under the background of “dual carbon target”,the development of geothermal energy is of great significance.Downhole heat exchanger is a device for extracting heat,not groundwater.It is conducive to promoting the utilization of geothermal energy.In this thesis,the coaxial downhole heat exchanger and the U-tube downhole heat exchanger are selected to be the research objects,and numerical simulation method is used to study the influence of heat transfer performance and resistance loss along the way.The results provide reference for the design and operation of downhole heat exchanger.Firstly,by controlling a single variable,the variation of heat transfer effect of two underground heat exchangers with operation parameters and geothermal reservoir parameters is obtained.The results show that reducing the inlet temperature and improving the mass flow rate can increase the heating power of downhole heat exchanger,but the outlet temperature decreases.The heat transfer efficiency of the two downhole heat exchangers increases with the increase of reservoir thermal conductivity,reservoir temperature(gradient)and rock specific heat.Underground water flow has great influence on heat transfer of coaxial underground heat exchanger.Ignoring the groundwater flow will result in inaccurate evaluation of heat recovery.Well depth,coaxial casing diameter and U-shaped tube diameter are positively correlated with heat exchange effect,while coaxial inner pipe diameter is negatively correlated with heat exchange effect.The heat transfer efficiency of coaxial underground heat exchanger system can be enhanced by cementing with high thermal conductivity and improving the heat insulation performance of inner pipe.The results provide suggestions for the design and operation of downhole heat exchanger.Secondly,the coupling effects of different inner tube thermal conductivity and other operating conditions(such as operating parameters)on the heat transfer process of coaxial underground heat exchanger are studied.Through the analysis of the influence law,the guidance scheme is provided for the selection of the inner tube of coaxial underground heat exchanger under different conditions.Considering the influence of underground water flow and cementing cement,the heat insulation ratio of inner tube of coaxial downhole heat exchanger is 0.93 when it runs for 120 days.Finally,the parametric influence analysis is carried out on the resistance loss along the downhole heat exchanger.The results show that the resistance loss along the path increases with the increase of mass flow of the working fluid and roughness of pipe wall.The increase in pipe diameter reduces the resistance loss along the path of the U-shape pipe.The coaxial downhole heat exchanger has the best ratio of inner and outer pipe diameter to minimize the total resistance loss along the way.