Thermal performance and parameter study of steel fiber-reinforced concrete segment lining in energy subway tunnels

Abstract

This study proposes an effective approach for improving the energy performance of subway tunnels by using steel fiber-reinforced concrete (SFRC) segments to obtain shallow geothermal energy. Three-dimensional time-dependent thermohydraulic finite element analyses of the subway tunnel lining with reference to the existing Shenzhen subway tunnels were conducted. The width of each pipe segment is 1.5 m and a tunnel lining-ground model with size of 25 m × 25 m × 3 m was studied. The influencing factors including the inlet temperature, flow velocity, operation mode, pipe arrangement, and thermophysical parameters of SFRC were considered in evaluating the heat transfer performance of the tunnel lining. The results confirm that the heat-exchange performance of the SFRC segment is noticeably better than that of the ordinary concrete segment. Also, there is a linear relationship between the thermophysical parameters and the heat exchange per unit length. Heat exchange increased with an increase in flow rate until a certain level and became stabilised. Based on the thermal results and economical considerations, the inlet fluid velocity around 0.6 m/s is recommended. Besides, the heat exchange tube spacing between 0.5 m and 0.7 m can ensure both the heat exchange per unit length of the heat exchange tube and the heat extraction per unit tube sheet inner surface area to have high values at the same time. In terms of heat exchange, the advantages are more obvious for cooling operation than heating operation. The heat exchange per unit segment area of the energy tunnel increased with the inner diameter of the embedded pipes. However, the relationship between the rate of heat exchange increase and the pipe spacing changed with the thermal conductivity of the concrete segment.