## 3 Analysis of Simulation Results
### Temperature Field Distribution of Different Lining Insulation Materials
#### 3.2.1 Temperature Distribution  
As shown in Figure 13, the numerical simulation results demonstrate the temperature distribution at various measurement points within the tunnel for different lining materials, as well as the temperature variation over time for each material at the same location (see Figure 14). From Figure 13, it can be observed that, during a fire caused by an electric freight train in the tunnel, the temperature of the lining materials directly above the fire source is relatively low, with an average temperature below 200 \({}^{\circ}\)C. This may be related to the ventilation in the tunnel, as the airflow carries the heat generated by the fire downstream, reducing the heat above the fire source. As the distance from the fire source increases, the temperature of the lining materials also increases. From the figure, it is evident that the temperature rises the fastest at 15 m from the fire source but drops rapidly around 600 s. At the 10 m position, the temperature remains lower than that at the 15 m point before 600 s, and after reaching its peak at 600 s, the temperature remains relatively stable. This phenomenon may be caused by some of the heat at the 15 m position being carried away by the airflow, while at the 10 m position, heat continues to be transferred downstream, causing  
Figure 11: CO\({}_{2}\) concentration in the tunnel over time.  
Figure 12: Soot concentration in the tunnel over time.  
a significant amount of heat to accumulate there, resulting in a higher temperature at the 10 m position compared to the 15 m position. Although there are some differences in the peak temperatures at different measurement points, the overall temperature distribution trend and the duration of the peak temperatures show a high degree of consistency.  
Figure 13: Variation of fire temperature fields for different lining materials at various measurement points.  
In a tunnel fire, the heat transfer at the lining surface mainly relies on convection and radiation from hot air, while heat transfer within the lining occurs through conduction. By comparing the temperature distribution at the interface between various insulation materials and the concrete lining in Figure 14, it is observed that, except for magnesium oxide (MgO) and SiO\({}_{2}\) gel insulation materials, the peak temperatures in tunnels with fireproof coatings and calcium silicate materials are higher than those in tunnel models without any fire protection measures (Condition 4). This phenomenon may be related to the physical properties of the materials, with thermal conductivity and emissivity being the most influential factors. As shown in Table 2, calcium silicate has the lowest thermal conductivity and the highest emissivity, meaning that under the same conditions, it transfers more heat to the reinforced concrete of the tunnel interior. This could exacerbate the risk of concrete spalling, leading to a thinning of the tunnel lining structure. On the other hand, SiO\({}_{2}\) gel, with its higher thermal conductivity, excellent high-temperature stability, and chemical inertness, can more effectively insulate and reduce the heat transfer to the tunnel's internal structure, thereby lowering the risk of damage to the tunnel structure.