## 3 Analysis of Simulation Results
### Lithium Battery Vehicle Tunnel Fire Hazard
#### 3.1.2 Temperature Distribution Temperature Distribution of Fires in Different Vehicles  
When a fire occurs in the tunnel, the smoke carrying a large amount of heat will rise to the ceiling due to buoyancy, causing thermal damage to the lining material and affecting the normal operation of the tunnel. In order to prevent the damage of fire to the tunnel structure, it is necessary to study the temperature change in the tunnel during fire. Thermoouples are set at different positions of the tunnel ceiling to measure the temperature of the tunnel ceiling. The temperature changes in the tunnel under different fire types are shown in Figures 6-8. Figure 6 shows the longitudinal temperature distribution of combustion engine truck fire and electric truck fire in the tunnel at 1500 s. From the simulation results, it can be seen that the plume temperature of the electric truck fire is significantly higher than that of the heavy fuel vehicle, and the maximum temperature of the tunnel ceiling is 1177 \({}^{\circ}\)C, which is 277 \({}^{\circ}\)C higher than that of the heavy fuel vehicle. Figure 5: Heat release rate curve with time. Figure 7 shows the temperature variations at different cross-sectional locations during the fire of different vehicle types at 300 s. In both electric truck fires and heavy-duty truck fires, the temperature downstream of the fire source is generally higher than that upstream. This is because, in a one-way tunnel with a constant ventilation flow rate, most of the heat generated by the fire is carried downstream along the airflow. The upstream area, however, experiences lower temperatures, likely due to smoke backflow. By comparing the temperature variations of the two types of fires at different cross-sections, it is evident that the temperature generated by the electric truck fire is higher than that of the heavy-duty truck fire, indicating that the fire risk associated with electric trucks is greater than that of heavy-duty trucks. Figure 8 shows the simulated temperature distribution at different distances along the longitudinal direction of the tunnel at 1500 s, for a combustion engine truck and an electric truck, respectively. From the simulation results, it can be seen that whether it is a combustion engine truck or an electric truck, the temperature of the tunnel ceiling gradually decreases with the increase of the distance from the fire source, and the maximum temperature of the tunnel ceiling is located 2 m above the fire source. Comparing the two ceiling temperature profiles, it is found that the electric truck fire causes higher ceiling temperature than the fuel vehicle fire. smoke concentration gradually spreads to the left end of the tunnel, resulting in the smoke concentration in the downstream of the fire being higher than that in the upstream of the fire, and the visibility in the tunnel decreases. Therefore, when a fire occurs, it is necessary to focus on evacuating the smoke downstream of the fire to reduce the risk of fire. The temperature, smoke, and gas emissions caused by lithium battery electric vehicle fires are significantly higher than those of traditional fuel vehicle fires. As shown in Figures 10-12, these figures show the variation in gas concentration over time at a distance of 10 m from the tunnel during the fire simulations of a combustion engine truck and  
Figure 7: Temperature distribution contour map for different vehicles. an electric truck. Compared with combustion engine trucks, lithium-ion battery electric vehicles generate more harmful smoke in tunnel fires. Among them, the concentration of CO\({}_{2}\) is the most obvious, and the peak concentration of CO\({}_{2}\) is maintained at about 180,000 ppm. The peak concentration of CO is maintained at about 140,000 ppm, and the peak concentration of flue gas is maintained at about 16,000 ppm. The simulation results show that the range of smoke diffusion near the fire source expands with the increase of the heat release rate of the fire source.