Performance and optimal design parameters of tunnel lining CHEs under typical design conditions

The subway source heat pump system (SSHPS) incorporating a tunnel lining capillary heat exchanger (CHE) is an efficient technology for mitigating thermal degradation in subway tunnels while fully harnessing geothermal energy for heating and cooling purposes. Although extensive research has been conducted on the performance of tunnel lining heat exchangers, a practical engineering design method is still lacking. The objective of this study is to analyze the heat transfer characteristics of tunnel lining CHEs under typical design conditions and subsequently determine the optimal design conditions based on the analysis results. A numerical model of tunnel lining CHEs was developed using ANSYS, based on a demonstration project of SSHPS in Qingdao. Subsequently, the heat transfer performance of CHEs under various design parameters was simulated and analyzed. Finally, a statistical analysis was conducted to examine the impact of different factors on the heat transfer performance of CHEs. The analysis results indicate that the tunnel air temperature has the most significant influence, followed by the CHE inlet temperature parameter and the length of CHE laying pipe. The impact of the CHE inlet flow velocity is minimal. It is recommended that, during the design stage of a CHE heat exchanger, priority should be given to controlling the tunnel air temperature, optimizing the CHE inlet temperature and pipe length, and setting a minimum possible CHE inlet flow velocity. The optimal combination of design parameters for the tunnel lining CHEs were determined as follows: under the cooling mode, a tunnel air temperature of 23 °C, a CHE design inlet temperature of 39 °C, a laying pipe length of 6 m, and an inlet flow velocity of 0.08 m/s; under the heating mode, a tunnel air temperature of 20 °C, a design inlet temperature of 4 °C, a laying pipe length of 6 m, and an identical flow velocity of 0.08 m/s. This study can serve as a valuable reference for tunnel lining CHE design in practical engineering.