Thermal short-circuiting and heat transfer performance of coaxial borehole heat exchanger

The coaxial heat exchanger is a commonly used device for deep ground source heat pump systems. Based on the fluid flow heat transfer equation, the coupled heat transfer model of the coaxial heat exchanger and the surrounding rock mass is established. Taking the first heating period as an example, the influence of the inner tube thermal conductivity and circulating water flow on the heat transfer performance was analyzed, and the heat exchanger efficiency was introduced to evaluate the thermal short-circuiting phenomenon. The following conclusions were obtained: The phenomenon of thermal short circuit becomes obvious with the increase of the thermal conductivity of the inner tube. The thermal short-circuiting causes the temperature difference of the circulating water in the inner and outer tubes to decrease and the heat accumulation in the tube, resulting in a decrease in the power of the heat exchanger. The heat transfer power of the coaxial heat exchanger increases with the increase of circulating water flow. When there is a thermal short-circuiting between the inner and outer tubes, the outlet water temperature increases first and then decreases with the increase of the circulating water flow rate. As the flow rate increases, the heat exchanger efficiency increases.