Abstract: To address the limited effectiveness of ventilation-based cooling during high geothermal tunnel construction, the author investigates the cooling mechanism and parameter optimization strategy under the combined operation of mechanical refrigeration and tunnel ventilation systems. A case study is conducted on a representative high-temperature tunnel project, and the influence of supply air temperature and airflow rate on the environmental temperature at the tunnel face is analyzed based on field measurements and numerical simulations. First, environmental temperature and humidity are monitored at the tunnel face and multiple cross-sections, capturing the temperature distribution and cooling magnitude before and after the operation of the refrigeration units. Then, a multi-physics numerical simulation framework based on a transient flow-thermal coupling model is established to systematically evaluate the temperature response under different cooling parameter combinations. Quantitative relationships between the tunnel face temperature and the supply air velocity and temperature are subsequently derived. Findings are as follows: (1) The new mechanical refrigeration units considerably improves the thermal environment of the tunnel, with the temperature in the tunnel face area dropping from 38.6 ℃ to 29.5 ℃ (a decrease of 23.6％), and the overall average temperature of the tunnel decreasing by 7. 0 ℃ (a decrease of 18.4%). (2) The output air volume (V) of the refrigeration unit decreases in an upward-concave quadratic function relationship with the temperature (T_a) of the tunnel face, while the output air temperature (T) of the refrigeration unit is linearly negatively correlated with T_a. (3) Based on the temperature control requirements of high-temperature tunnels, a parameter combination of the output air volume and air temperature of the refrigeration unit is proposed, which can stably control the tunnel face temperature to≤28 ℃, that is, when V=10 m³/s, T≤14.7 ℃; when T=15 ℃, Ｖ≥11.4 m³/s. Taking into account the cooling effect and energy efficiency, the optimal operating parameters of the refrigeration unit are determined to be T=18.8 ℃ and Ｖ=30 m³/s.

Key words: high geothermal tunnel, mechanical refrigeration, ventilation heat transfer, cooling parameters