Experimental and numerical simulation research on heat transfer performance of coaxial casing heat exchanger in 3500m-deep geothermal well in Weihe Basin

The deep coaxial borehole heat exchanger (DCBHE), as an important part of the deep-seated ground heat exchanger system, undertakes the important task of heat exchange with the underground geotechnical body to provide heating for the building. In this paper, through field test, numerical simulation and sensitivity analysis, the influence of thermal insulation performance of inner tube, heat preservation method of inner tube and diameter ratio of inner tube to outer tube on heat extraction capacity of deep coaxial borehole heat exchanger is studied. Field tests in intermittent operation mode (9.5 h operation and 14.5 h shutdown) show that the short-term single-well heat extraction capacity of the deep coaxial borehole heat exchanger using the vacuum gel steel tube with a thermal conductivity of 0.002 W/(m·K) as the inner tube is 574.96 W/m. The simulation results show that the diameter ratio of the inner and outer tubes has little effect on the heat extraction capacity, and the diameter ratio of the inner and outer tubes of 0.65 has the smallest resistance loss along the way. For heat exchanger with depth of 3500 m, the location of severe thermal interference between the inner and outer tubes is mainly located in the upper part of the heat exchanger near the surface, especially in the 1400 m to 2100 m section. The research results can provide a reference for the optimization design of medium-deep coaxial heat exchange buried pipe system and the promotion of medium-deep geothermal energy utilization.