## 3 Validation of the Numerical Model  
### Validation of Pool Fire Mass Burning Rate  
HRR is the heat release rate, denoted by \(Q\), which is commonly used to refer to the amount of heat released from the fuel per unit of time in W, kW, or MW; the power of the fire source can be calculated using the following equation considering the ideal condition of a fuel efficiency factor of 1 [38]:  
\[Q=A_{P}\times\dot{m}\times\Delta H \tag{1}\]  
In the equation, \(A_{P}\) is the combustion surface area in square meters, m\({}^{2}\); \(\dot{m}\) is the mass burning rate in kilograms per square meter per second, kg/(m\({}^{2}\cdot\)s); \(\Delta H\) is the heat of fuel combustion, which is taken as 48,060 kJ/kg for n-octane. The output from FDS is the HRR value, and through this equation, HRR can be used to calculate \(\dot{m}\).  
The parameters for n-octane in this combustion model are shown in Table 3. The experimental data are derived from the 1.2 m diameter (\(\sim\)1.1 m\({}^{2}\)) n-octane pool fire statistics compiled by [PERSON] [39]. In this model, the pool fire is set as a square liquid pool of 1 m\({}^{2}\). Grid independence calculations were performed using grid sizes of 0.05 m, 0.10 m, and 0.20 m, with results shown in Figure 2. In the stable combustion phase of the flame, the mass burning rate \(\dot{m}\) calculated with the 0.05 m and 0.10 m grids are very close, indicating that further grid refinement is unnecessary.  
Table 4 shows that the 0.05 m grid provides the most accurate prediction of the experimental results, with an error of only 1.5%, while the 0.1 m grid has an error of 10%. However, in practical simulations, the 0.1 m grid could theoretically improve computational efficiency by eight times, and a 10% error is also acceptable. This grid size also complies  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Coating Area** & **Type** & **Density** & **Specific Heat** & **Thermal Conductivity** & **Thickness** \\  & [kg/m\({}^{3}\)] & **Capacity [J/(kg\(\cdot\)\({}^{\circ}\)C)]** & **[W/(m\(\cdot\)\({}^{\circ}\)C)]** & **[mm]** \\ \hline Transformer Surface & Tick Coating & 500 & 1000 & 0.102 & 7 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Thermal insulation coating parameters.  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Parameter** & **Value** & **Unit** \\ \hline Density & 675 & kg/m\({}^{3}\) \\ Specific Heat & 2.24 & kJ/(kg\(\cdot\)K) \\ Thermal Conductivity & 0.14 & W/(m\(\cdot\)K) \\ Evaporation Heat & 317 & kJ/kg \\ Boiling Point & 98.35 & \({}^{\circ}\)C \\ \hline \hline \end{tabular}
\end{table}
Table 3: Heptane key parameter table.  
with the grid selection method recommended by NIST, which states that the grid size should be between 1/4 and 1/16 of the characteristic flame diameter \(D^{*}\)[40]. The calculation of \(D^{*}\) is shown in Equation (2):  
\[D^{*}=\left(\frac{\dot{Q}}{\rho_{\infty}c_{p}T_{\infty}\sqrt{\dot{g}}}\right)^{2 /5} \tag{2}\]  
In the equation, \(\dot{Q}\) is the heat release rate, kW; \(c_{p}\) is the specific heat of air, taken as 1 kJ/(kg\(\cdot\)K); \(T_{\infty}\) is the ambient temperature, taken as 293 K; \(g\) is the gravitational acceleration, taken as 9.81 m/s\({}^{2}\); and \(\rho_{\infty}\) is the air density, taken as 1.2 kg/m\({}^{3}\).  
In the validation experiment, the mass burning rate of 0.067 kg/(m\({}^{2}\cdot\)s) (HRR = 3600 kW) indicates that a grid size between 0.1 m and 0.4 m should be selected, consistent with the grid independence verification results. Therefore, it can be concluded that the predicted results of the mass burning rate in this model are accurate, and the model grid should be selected according to Equation (2).