Analysis of heat transfer characteristics of capillary heat exchanger in shield tunnel under typical working conditions

With the extended operation of subway systems, waste heat generated by train operations and lighting systems accumulates within tunnels, leading to significant thermal pollution. Compared to traditional buried pipes, capillary tubes are smaller in size and offer superior heat transfer performance, making them a promising candidate for mitigating waste heat accumulation in subway tunnels. However, existing research primarily focuses on tunnels utilizing conventional buried pipes, with a notable lack of studies on the application of capillary networks as front-end heat exchangers in tunnels. To address this research gap, this study proposes a novel design methodology for energy tube units based on empirical data from Qingdao's metro system and investigates the heat transfer performance of Capillary Heat Exchangers (CHEs) within these units. The research reveals significant variations in CHE heat transfer characteristics under different operational conditions. Temperature emerges as a critical factor influencing CHE performance: reducing inlet water temperature enhances heat transfer efficiency during heating seasons; while increasing it is beneficial in cooling seasons. Additionally, increasing flow velocity enhances CHE unit-area heat transfer, with lower velocities showing more pronounced improvements, albeit caution is warranted against excessive flow rates due to potential energy wastage. Thus, pump adjustments are recommended to achieve optimal flow velocities. Variations in branch pipe spacing and length directly impact CHE heat transfer area, necessitating thoughtful configuration in engineering applications to maximize heat transfer efficiency. This study provides valuable insights into the design and operational strategies of metro energy tunnels.