Temperature field and anti-freezing system for cold-region tunnels through rock with high geotemperatures

Abstract

The tunnels of the Sichuan-Tibet railway reach altitudes of above 4200 m and have very deep burial depths; thus, serious freeze and heat damage will be encountered during the long-term usage of these tunnels. Taking the Zilashan Tunnel as an example, the dynamic change in the temperature field during tunnel operation was studied by using a transient heat transfer model that coupled convection and conduction. The results show that a high geotemperature fails to directly eliminate freeze damage of the tunnel entrance. The temperature of the tunnel section with a low original geotemperature changes periodically, and the train-induced wind dramatically affects the temperature field. After long-term operation, the surrounding rock a certain distance from the tunnel wall in the radial direction maintains the original high geotemperature, serving as a stable heat source. To prevent and control the freeze damage of tunnels and utilize the high geotemperatures, an efficient and energy-saving anti-freezing system was proposed using an air-source heat pump in a heat extraction tunnel. The calculation method of the anti-freezing system was proposed, and the effect of the length of the extraction tunnel on the air temperature and coefficient of performance (COP) was also studied. The total heat consumption required for freeze damage prevention was obtained. Considering the energy loss of the transmission system, the maximum heating load of Zilashan Tunnel’s heating system was determined to be 37.23 kW. In addition, it is suggested that the length of the extraction tunnel is 60 m, and the resulting COP of the air-source heat exchanger is 4.75. These research results can support anti-freezing design for cold-region tunnels through rock with high geotemperatures.