Heat transfer performance analysis of front-end capillary heat exchanger of a subway source heat pump system

During the operation of subway systems, a significant amount of heat is stored annually in the rock surrounding the tunnel, which results in underground thermal pollution. A subway source heat pump system with a front-end capillary heat exchanger (CHE) is considered an effective technology to address this problem. However, it has not yet been employed in systematic design and operation methods. This study numerically simulates and analyses the heat transfer performance of a tunnel lining CHE for an entire year under the typical conditions of an actual project. The results are expected to provide theoretical guidance for CHE design and operation. Moreover, the proposed model was verified by comparing the simulation results with those of field experiments. The simulation results indicated that both the temperature and heat flux of the CHE and tunnel surrounding rock experienced periodic fluctuations during the heating and cooling seasons and exhibited a strong heat recovery ability in the transitional seasons. The temperature of the surrounding rock (Tmid) gradually decreased with increasing depth of the surrounding rock, and the change in Tmid at greater depths (≥15 m) was less than 0.1 and 0.13 °C during the heating and cooling seasons, respectively. However, phase delays and amplitude attenuation phenomena occurred during the harmonic temperature transfer process. The phase delay of the tunnel wall temperature (Tw) valley and peak during the heating and cooling seasons were 13 and 2.2 h from the CHE wall temperature (TC) valley and peak, respectively. It was suggested that the efficient operation of the CHE should harmonise the two temperature waves of the tunnel wall and CHE. Furthermore, the average heat flux values at the lining-CHE outer wall contact surface during the CHE operation were −16 and 22 W during the heating and cooling seasons, respectively. In addition, the CHE exhibited abnormal heat fluxes during the heating and cooling seasons along the CHE pipe (10 m), indicating that the design and operation of the CHEs must consider the influence of the pipe length. •A 3D numerical heat transfer model was built based on an actual project.•Heat transfer performance of capillary heat exchanger under period boundary conditions was analysed.•Phase delay and attenuation characteristics of temperature and heat flux harmonics were analysed.