Abstract: Tunnels in cold regions are susceptible to early cracking and damage of concrete lining structures, as well as freezing-related issues, when the temperature inside the tunnel falls below 5 ℃. To address this problem, an air curtain insulation system suitable for tunnel construction in cold regions is designed and implemented at the tunnel entrance section. A case study is conducted on the flood and sand discharge tunnel project at a pumped storage power station in Hunyuan county, Shanxi, China. Based on the fluid characteristics of the air curtain and the heat transfer mechanism within the tunnel environment, a calculation model for the trajectory of the air curtain jet is established, and a formula for the blocking efficiency of the air curtain jet is proposed. Additionally, a calculation formula for the mixed air temperature inside the tunnel is developed, and the required quantity of hot air curtain machines is determined using the superposition principle. Finally, the insulation effects, as well as the economic advantages and disadvantages of the cold-proof door, hot air blower, and air curtain insulation system during tunnel construction in cold regions, are compared and analyzed. The findings are as follows: (1) Under an external temperature of －32 ℃, the theoretical average quantity of heating air curtain machines required is 1.67. (2) The insulation measures provided by the cold-proof door and hot air blower fail when the external temperature is －16 ℃ or －32 ℃, whereas the air curtain insulation system maintains the tunnel temperature above 10 ℃ when the external temperature is －26 ℃. (3) Compared with the hot air blower, the cost of the air curtain insulation system is reduced by 25.33% during the construction period.

Key words: tunnels in cold regions, construction period, air curtain, field test, cold-proof and insulation