## 2 Simulation Model
### Establishment of Simulation Model of Lithium Battery Pack for Electric Vehicle  
The risk of an electric vehicle fire is affected by the number of lithium batteries inside the vehicle and the size and capacity of the battery pack. With the increase in the number of batteries, the energy storage capacity also increases accordingly, resulting in an increase in the fire risk of electric vehicles when they catch fire. In order to accurately simulate the influence of the thermal runaway of lithium batteries in electric vehicles on tunnel lining material, the battery cell described in Reference [25] was selected as the research object. The single battery is a square soft-package ternary battery of 180 mm \(\times\) 70 mm \(\times\) 205 mm, with a voltage of 3.2 V and a rated capacity of 300 Ah. Figure 2 is the geometric model of the battery core. The detailed thermal physical parameters of the battery are shown in Table 1. The heat release rate of a single battery fire can be found in Figure 3 [25].  
Figure 1: Tunnel simulation model diagram.  
Figure 2: Geometric model of the cell monomer.  
This paper constructs a fire scenario for the battery pack inside an electric truck based on a single lithium-ion battery fire model. Electric trucks generally use a series-parallel configuration for their battery packs. Taking a 550 V, 300 Ah electric truck as an example, if the lithium-ion battery cell from the literature is used, 18 cells are connected in series to form one module and a total of 10 lithium-ion battery packs are required. Since electric trucks have many combustible components, for the sake of simplicity, this study focuses only on the numerical simulation of the lithium-ion battery pack in the electric vehicle. One of the battery packs is set as the fire source, located 20 m to the left side of the tunnel.  
As shown in Figure 3, the peak heat release rate of a single battery fire source is 88.78 kW, which allows the calculation of the maximum heat release rate for the lithium-ion battery pack as 1500 kW. A temperature slice is placed at y = 3 m, and temperature measurement points are set every meter directly above the fire source. Temperature, visibility, CO, and CO\({}_{2}\) measurement points are placed 5 m, 10 m, and 15 m from the fire source center, with additional temperature measurement points placed every 10 m along the downstream of the tunnel to monitor the changes in fire parameters. The layout of the fire source and detector is shown in Figure 4a,b.  
Numerous studies have shown that the fire flames from battery combustion typically appear at the pressure relief valve, and battery jet fires are classified as fast fires. Therefore, the growth time of the fire source follows the \"f2\" fire law, meaning:  
\[Q=\alpha t^{2} \tag{6}\]  
In the equation, Q represents the heat release rate of the fire source in kW, \(\alpha\) = 0.04689, \(\alpha\) represents the fire growth coefficient, and t is the time. The calculated fire growth time is approximately 179 s. The simulation duration is 1500 s.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Argument** & **Positive** & **Negative** & **Electrolyte** & **Diaphragm** \\ \hline Density (kg/m\({}^{3}\)) & 2700 & 8500 & 2600 & 492 \\ Specific Heat Capacity (kj/(kg\(\cdot\)K)) & 900 & 385 & 1100 & 1978 \\ Thermal Conductivity (W/(m\(\cdot\)K)) & 160 & 146 & 21 & 0.334 \\ Heat Absorption Coefficient & 0.8 & 0.8 & 0.9 & 0.8 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Thermophysical Parameters of the lithium battery model [26].  
Figure 3: Heat release rate of battery core [25].