Experimental study on heat transfer performance of double U-shaped buried tube heat exchanger

Double U-shaped buried tube heat exchanger (DUBTHE) are prone to pipe crossing and thermal short-circuiting in the practical applications, resulting in the reduced heat transfer performance and directly affecting the operational efficiency of shallow geothermal heat pump system. Based on the initial line thermal average temperature, thermal conductivity theory and slope method, and taking a shallow geothermal heat pump project in Xi'an as the engineering background. Then, this study investigate the effects of initial average temperature, thermal conductivity and volumetric heat capacity of rock-soil, and tube clamp spacing on the heat transfer performance of DUBTHE through field geotechnical thermal response testing and different temperature measurement methods. The results show that the initial average temperature of rock-soil measured by the multi-point temperature measurement cable is 17.08℃, which is closer to the actual formation temperature. Besides, the comprehensive thermal conductivity and volumetric heat capacity of rock-soil in the formation are 1.65 W/(m·K) and 2.81×10⁶ J/(m³·K), respectively. In addition, the heat exchange per linear meter of DN25 DUBTHE in summer and winter increases with the decreasing tube clamp spacing, with the acceleration increasing firstly and then decreasing with the decreasing tube clamp spacing. Definitely, the heat exchange per linear meter of DN25 DUBTHE in summer is increased by 21.03%, 19.48%, 15.16% and 3.92% respectively compared with that without tube clamp, and that in winter is increased by 20.83%, 19.48%, 14.94% and 3.79%, respectively, in case that the tube clamp spacing is 1 2, 3, and 4 m. DN25 DUBTHE with a tube clamp spacing of 2 m or 3 m is the optimal arrangement in the project. Therefore, the results could provide experience and data support for the optimal design of shallow ground source heat pump systems in Guanzhong area.