Impact of thermal insulation duct on cooling effect of high-ground-temperature tunnels during construction ventilation

Forced ventilation is a common means to solve the problem of high ambient temperature during the construction of high-ground-temperature tunnels. In forced ventilation, it is very important to prevent the fresh cold airflow from being heated by the high-ground-temperature environment when it reaches the working surface so as to ensure the cooling effect. Based on heat transfer theory, a temperature numerical calculation model is proposed for forced ventilation in high-ground-temperature tunnels that considers the convection heat transfer between the airflow inside the duct, and the airflow inside the tunnel and the surrounding rock, combined with the influence of the duct leakage rate. This model compensates for the lack of considering the surrounding rock temperature and the duct leakage rate in relevant studies. Using the 7# cross tunnel of an actual tunnel as a case study, the effects of ventilation volume, construction distance and thermal insulation duct on the airflow temperature inside the duct and tunnel were investigated. According to the field test results, the developed numerical calculation model is in good agreement with field test data, with a maximum difference of 2 °C between the numerical calculation results and field test data. The thermal insulation duct has good thermal insulation performance. Under different construction distances, the thermal insulation duct can reduce the airflow temperature at the outlet of the duct by 6–14 °C, up to as much as 28–40%. Simply increasing the ventilation volume has no significant effect on preventing the airflow temperature rise inside the duct. At the same construction distance, although the airflow temperature at the duct outlet gradually decreases with the increase in the ventilation volume, the decrease is small: For every 10 m 3 s −1 increase in the ventilation volume, the temperature is only reduced by 1 °C. For a tunnel with short construction distance and low surrounding rock temperature, the requirements can be met without thermal insulation treatment of the duct, but for a long-distance tunnel, the thermal insulation duct is an effective and economical auxiliary cooling means.