## 2 Simulation Setup
### Simplified Model and Configurations  
Modeling: In this paper, the selected building is a 21-storey office building located in the Cross-border E-Commerce Comprehensive Experimental Zone (Hefei, China). Figure 1 shows a model of the whole building (Figure 1A) based on the Computer Aided Design (CAD) of the building (Figure 1B). However, to save computational cost and enhance the simulation accuracy, a simplified model was applied in this paper, presented in Figure 2, simulating the stairwell (as well as compartment) of a 21-story high rise building with a height of 89.8 m. The building parameters of the stairwell are presented in Table 1. Each floor was of a different height, ranging from 3.9-8.0 m, and the floor slabs were 0.2 m thick. The cross-sectional dimensions of the stairwell and compartment were 6.75 m \(\times\)  
Figure 1: The picture of: (**A**) the whole building model; (**B**) Computer Aided Design (CAD) of the building. 2.9 m and 4.2 m \(\times\) 2.9 m, respectively. The dimensions of the side door and front door were 1.4 m \(\times\) 2.1 m and 1.4 m \(\times\) 2.1 m, respectively. These doors were all located at the middle of the wall, and the coordinates in the y direction of the front door ranged from 2.65-2.95 m.  
Thereafter, the mesh size was reduced to further refine the mesh. According to the _FDS user Guide_, the characteristic diameter of the fire source D* could be used to conduct a grid independence study, as shown in Equation (1):  
\[D^{*}=(\frac{Q}{\rho_{0}\cdot\ c_{0}\cdot\ T_{0}\sqrt{g}})^{2/5} \tag{1}\]  
where \(Q\) is the HRR of the fire source, kW; \(\rho_{0}\) is the air density, kg/m\({}^{3}\); \(c_{0}\) is the air specific heat, kJ/(kg.\({}^{\circ}\)C); \(T_{0}\) is the ambient temperature, K; and \(g\) is the acceleration of gravity, m/s\({}^{2}\). According to the _FDS user Guide_, the ratio of \(D^{*}/\delta\) (\(\delta\) is the side length of the mesh), which implies good computational accuracy, should be in the range of 8-12. Considering the fire HRR of 3 MW as an example, the most suitable mesh length is in the range of 0.12-0.18. At last the one type of grid (0.2 m \(\times\) 0.2 m \(\times\) 0.2 m) was chosen after preforming a grid independence. The total number of grids is 55 \(\times\) 20 \(\times\) 452 = 4.972 \(\times\) 10\({}^{5}\). Besides, for the stability of the numerical transmission, the grid was configured as only one set of grids rather than several sets. In addition, the lining material of the stairwell construction was specified as \"CONCRETE\", and its density, specific heat, and conductivity were 2200 kg/m\({}^{3}\), 0.88 kJ/(kg K), and 1.2 W/(m K), respectively [24; 25; 26]. The ambient temperature was 20 \({}^{\circ}\)C, and the area of the fire source was 0.5 m\({}^{2}\). Detailed information was listed in Table 2. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Floor** & **Room Height (m)** & \
\begin{tabular}{c} **Size of Stairwell (m)** \\ **(Length \(\times\) Width \(\times\) Height)** \\ \end{tabular} &
\begin{tabular}{c} **Size of Compartment Room (m)** \\ **(Length \(\times\) Width \(\times\) Height)** \\ \end{tabular} \\ \hline
1 & 6.0 & 6.75 \(\times\) 2.9 \(\times\) 6 & 4.2 \(\times\) 2.9 \(\times\) 6 \\ \hline
2–3 & 4.5 & 6.75 \(\times\) 2.9 \(\times\) 4.5 & 4.2 \(\times\) 2.9 \(\times\) 4.5 \\ \hline
4–20 & 3.9 & 6.75 \(\times\) 2.9 \(\times\) 3.9 & 4.2 \(\times\) 2.9 \(\times\) 3.9 \\ \hline
21 & 8.0 & 6.75 \(\times\) 2.9 \(\times\) 8 & 4.2 \(\times\) 2.9 \(\times\) 8 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Building parameters of simplified model.