## 3 Analysis of the EV Thermal Runaway Fire Scenarios
### Analysis of the Combustion Process of an EV Fire  
The heat release rate (HRR) is defined as the amount of heat released by the combustion of a material per unit of time. The HRR curve of the combustion process can be divided into three stages, according to the fire process. In the initial growth stage, where the HRR of the fire source is proportional to the square of time [2,14], the HRR curve generally conforms to \(Q=at^{2}\), where \(Q\) is the HRR of the fire source, kW; \(\alpha\) is fire development coefficient, kW/s\({}^{2}\); and \(t\) is the fire development time, s. In the full development stage, the HRR remained unchanged. In the attenuation stage, the decline in the HRR appeared to be linear. In this study, the entire combustion process of an EV fire caused by the thermal runaway of the power battery was divided into two parts: (i) the thermal runaway fire in the power battery [11] and (ii) the vehicle combustion stage after the flame spread from the power battery to other parts of the vehicle [11,17]. (1) Analysis of the thermal runaway process of the power battery  
The trigger factors for the battery thermal runaway can be divided into mechanical, thermal and electrical abuse [32]. During the operation of EV batteries, the thermal runaway may occur because of uncertain risk disturbances. In addition, battery aging may occur owing to long-term operation, which also reduces the risk resistance of the battery system. When the thermal runaway spreads in the battery pack, a large amount of heat and toxic, combustible gases are generated. Finally, when the temperature reaches the ignition point, a fire appears. A more serious case is when the pressure reaches the limiting value of the pressure relief valve. Part of the premixed jet flame is sprayed a considerable distance outward from the bottom of the vehicle. (2) Analysis of the combustion process of EVs  
The entire vehicle combustion stage of EVs is similar to that of the ICEV. The process of an EV fire spread, caused by the thermal runaway of the power battery, is determined according to the data from previous experimental studies on vehicle fires and EV fire accident cases [2,15]. When the thermal runaway of the power battery ignites, heat is transferred to the other combustible parts of the vehicle (i.e., plastic, leather, textile fiber, \begin{table}
\begin{tabular}{c c c} \hline \hline
**Flame Surface** & **Size Parameters** & **m** \\ \hline \#tr (flame surface in the thermal runaway stage) & Flame length (y direction) & 1 \\  & Flame width (x direction) & 2 \\ \#1 & Flame height (z direction) H\({}_{\rm f}\) & 2.8 \\  & Flame length (y direction) W\({}_{\rm f}\) & 1 \\  & Flame width (x direction) D\({}_{\rm f}\) & 1.85 \\ \#2 & Height from the bottom of the flame to the ground H\({}_{0}\) & 0.681 \\  & Flame height (z direction) H\({}_{\rm f2}\) & 0.941 \\  & Flame length (y direction) W\({}_{\rm f2}\) & 3.047 \\  & Flame width (x direction) D\({}_{\rm f}\) & - \\ \#3 & Height from the bottom of the flame to the ground H\({}_{03}\) & 1.622 \\  & Flame height (z direction) H\({}_{\rm f3}\) & 1.178 \\  & Flame length (y direction) W\({}_{\rm f2}\) & - \\  & Flame width (x direction) D\({}_{\rm f}\) & - \\ \#4 & Height from the bottom of the flame to the ground H\({}_{04}\) & 1.04 \\  & Flame height (z direction) H\({}_{\rm f4}\) & 0.528 \\  & Flame length (y direction) W\({}_{\rm f2}\) & - \\  & Flame width (x direction) D\({}_{\rm f}\) & - \\ \#5 & Height from the bottom of the flame to the ground H\({}_{05}\) & 1.622 \\  & Flame height (z direction) H\({}_{\rm f5}\) & - \\  & Flame length (y direction) W\({}_{\rm f2}\) & - \\  & Flame width (x direction) D\({}_{\rm f}\) & - \\ \#f (flame surface at the front of the vehicle) & Flame height (z direction) H\({}_{\rm f}\) & 2.8 \\  & Flame width (x direction) D\({}_{\rm f}\) & 1.85 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Size parameters of the SFM. etc.). When the heat received by them exceeds their combustion threshold, they ignite and generate a flame.