Effect of laying length on dynamic heat transfer characteristics of capillary heat exchangers in urban subway tunnel lining

Abstract

The subway tunnel is a typical semi-enclosed underground space, which poses challenges in dissipating the waste heat generated by subway operations and may result in thermal environment deterioration of urban underground spaces. However, by incorporating heat exchangers into the tunnel lining, the subway can be transformed into an “energy tunnel” that effectively addresses this issue. Capillary heat exchanger (CHE) is a common type of tunnel lining heat exchanger. Recent studies have investigated its heat transfer properties and influencing factors under the assumption of homogeneity, neglecting the variability of heat transfer properties along the CHE tube. The present study developed a numerical model of tunnel-lined CHE utilizing water as the circulating medium, based on a demonstration project in Qingdao. Subsequently, an analysis was conducted to investigate the evolution of heat transfer properties for different laying lengths. The simulation results indicate that as the laying length increases from 2 to 10 m, the fluctuation range of outlet temperature decreases significantly from 10.6 °C to a mere 0.4 °C. Moreover, there is a noticeable decrease in average heat transfer per unit length from 1.9 W/m to only 0.4 W/m; however, this is accompanied by an impressive increase in average heat transfer efficiency from 28 % up to an impressive level of around 61 %. Based on the aforementioned results, it is recommended that the optimal length range for a tunnel-lined CHE should fall within 4 to 6 m. The findings of this study can serve as a valuable theoretical reference for the practical engineering applications of CHEs in urban tunnel lining.