## 5 Parametric Study
### Temperature Characteristics of Airflow inside Tunnel under Ventilation Condition  
In order to ensure clear presentation, the airflow temperature fields at the tunnel entrance and the exit with a length of 1000 m are analyzed here instead of the whole length. The average airflow temperature contours in December at the tunnel entrance and the exit for Case I and Case II are shown in Figure 9. In both of the two cases, the airflow temperature increases by increasing the mileage at the tunnel entrance (see Figure 9a,b), while it decreases by increasing the mileage at the tunnel exit (see Figure 9c,d). This is due to the natural ventilation direction from the exit to the entrance in some nights. By comparing Figure 9a,b with Figure 9c,d, it is found that the average airflow temperature in December which is measured at a certain distance from the tunnel entrance is lower than the temperature which is measured at the same distance from the tunnel exit. For example, the airflow temperature at K0 + 600 is mostly below \(-\)2 \({}^{\circ}\)C (see Figure 9a), while the airflow temperature at K2 + 343 is mostly below \(-\)1 \({}^{\circ}\)C (see Figure 9c). This is because the cooling effect from the mechanical ventilation system during the daytime is larger on the entrance section (K0 + 000-K1 + 000) than on the exit section (K1 + 943-K2 + 943). Furthermore, the annual average air temperature outside the entrance (K0 + 000) is lower than that outside the exit (K2 + 943). As shown in Figure 9, the December isotherms of the airflow inside the tunnel are parabolic curves. These curves are dense in the near-wall regions, while sparse in the tunnel center. Furthermore, it is found that the airflow temperature is higher and changes more significantly in the near-wall regions than other regions. In contrast, the airflow temperature is lower and changes more slightly near the tunnel center than other regions. Figure 9: Average airflow temperature contours in December in the tunnel entrance and exit sections. (**a**) Tunnel entrance section without heat-insulating layer; (**b**) Tunnel entrance section with heat-insulating layer covering K0 + 000–K0 + 700; (**c**) Tunnel exit section without heat-insulating layer; and (**d**) Tunnel exit section with heat-insulating layer covering K2 + 343–K2 + 943. By comparing the airflow temperature contours in the entrance section (see Figure 9a,b), and in the exit section (see Figure 9c,d), it can be found that the heat-insulating layer significantly affects the distribution of the monthly average airflow temperature inside the tunnel. After applying the heat-insulating layer, the monthly average airflow temperature grows more slowly by increasing the mileage in the entrance section. On the other hand, it decreases more slowly in the exit section when compared with the case without the heat-insulating layer. For example, the monthly average airflow temperature at the tunnel center at K0 + 820 is observed as 0 \({}^{\circ}\)C in Case I (see Figure 9a), with the temperature at the same position being obtained as equal to \(-1\)\({}^{\circ}\)C in Case II (see Figure 9b). This is due to the fact that the heat-insulating layer significantly reduces the heat transfer between the surrounding rock and the airflow inside the tunnel. The heat-insulating layer maintains the original heat of the surrounding rock and the tunnel lining by taking the advantage of low heat conductivity. Moreover, it prevents the tunnel lining from damages from frost caused by heat loss through the airflow. Figure 10 shows the monthly average temperature curves of airflow near the tunnel vault surface in the entrance section at different mileages (K0 + 000, K0 + 500, K1 + 000 and K1 + 500) under two different heat-insulating conditions (Case I and II). It should be noted that the surface of the tunnel vault along the whole tunnel length consists of three different parts in Case II. In contrast, there is only one surface type in Case I, which is the concrete surface of the secondary lining on the tunnel vault.