Parametric Study Towards the Optimal Design of Capillary Heat Exchangers for Metro Shield Energy Tunnels

The prolonged operation of semi-enclosed subway tunnels results in waste heat accumulation, energy wastage, and potential safety hazards. Integrating capillary heat exchangers (CHEs) with tunnel linings to form an energy tunnel is an effective approach to addressing these challenges. However, the current design methodologies for energy tunnels are still non-systematic, and the studies on the design optimization of CHE are still inadequate. To address these gaps, the shield energy tunnel demonstration project in Qingdao was selected as the research object, and a three-dimensional fluid–thermal coupled numerical model was established using the COMSOL simulation platform. A total of 48 CHE design configurations were analyzed to evaluate the heat transfer performance and temperature uniformity of the energy segment. The results indicate that the capillary bundle geometry and the installation position of the CHE are the dominant factors. Specifically, the I-shaped CHE achieves the highest heat transfer capacity, while the W-shaped CHE provides the best temperature uniformity. Installing the CHE within the inner arc protective layer, as opposed to the outer arc protective layer, significantly enhances the heat flux per unit area and improves temperature field uniformity. In addition, the reverse-return connection offers only a marginal advantage over the direct-return configuration, and the capillary group spacing exerts a relatively minor influence on both the heat transfer rate and temperature uniformity of the energy segment. This study provides a reference for the design and engineering application of subway shield energy tunnels.