## 5 Parametric Study
### Temperature Characteristics of Airflow inside Tunnel under Ventilation Condition
In Case II, the first and third parts are the surfaces made of the heat-insulating material, which is applied on the tunnel lining in the entrance section (K0 + 000-K0 + 700) and exit section (K2 + 343-K2 + 943) respectively. The second part is the concrete surface of the secondary lining on the tunnel vault. In Case I (see Figure 10a), the monthly average temperature of airflow near the surface of the tunnel vault at K0 + 000 starts to increase from \(-8.5\)\({}^{\circ}\)C in January, reaches the highest temperature of 8.2 \({}^{\circ}\)C in June and drops to \(-9.5\)\({}^{\circ}\)C (the lowest temperature) in December. It is found that negative temperatures are observed from January to March and from September to December. The temperature stays above 0 \({}^{\circ}\)C from March to September. At the other three mileages, the temperature drops first in February, increases to the highest temperature in July and drops to the lowest temperature in December. Apparently, the appearance of peak temperature at the entrance (K0 + 000) is one month earlier than inside the tunnel along the longitudinal direction (from K0 + 500 to K1 + 500). Furthermore, it is observed that the monthly average airflow temperature increases with an increase in the mileage, but the increment between different mileages decreases by increasing the mileage. The increment is more significant in cold months (from January to March and from October to December) than in hot months (from April to September). In Case II (see Figure 10b), the temperature variations are similar with Case I. However, the monthly average airflow temperature is much lower than that in Case I in  
Figure 10: Monthly average airflow temperature curves on the tunnel vault surface in the entrance section: (**a**) without heat-insulating layer and (**b**) with heat-insulating layer. cold months (from January to March and from October to December), while the difference is not that significant in hot months (from April to September). Based on the above discussions, it is found that the heat-insulating layer performs different functions (i.e., heat preservation and heat insulation) in different months with the temperature change outside. For example, the monthly average airflow temperature at K0 + 500 is lower in Case II than that in Case I in cold months (from January to March and from October to December). This indicates that the heat-insulating layer prevents the heat transferring from the surrounding rock to the airflow and performs the function of heat preservation in cold months. On the contrary, the monthly average airflow temperature at K0 + 500 is higher in Case II than that in Case I in hot months (from April to September). This means that the heat-insulating layer prevents the heat transferring from airflow to the surrounding rock and performs the function of heat insulation in hot months. According to the calculations, the largest drop in the monthly average airflow temperature at K0 + 500 in Case II compared with the Case I appears in January with a reduction of 3.07 \({}^{\circ}\)C, while the largest rise compared with the Case I appears in June with an increment of 0.48 \({}^{\circ}\)C. Clearly, the effect of heat preservation of the heat-insulating layer is much more significant than its effect of heat insulation.