## 2 Modeling and Experimental Validation
### Numerical Method
#### 2.1.3 Initial and Boundary Conditions  
Fluent was used to calculate the smoke flow and CO\({}_{2}\) concentration distribution characteristics of the station hall [28]. This study concentrated on the smoke concentration distribution surrounding the fire source rather than on the precise combustion process at the fire source. Therefore, a volumetric heat source model of 1 m \(\times\) 1 m \(\times\) 0.5 m was used to simulate the fire source combustion [29].  
The location of the fire in the station hall is uncertain, but fires are more likely to occur in regions with dense personnel flow and narrow spaces, such as exits and staircases, which will have more significant consequences [30]. This study focused on the impact of external wind on smoke diffusion. The fire source was placed near the stairs at the A-end to observe the impacts of external wind and the TSE system on smoke.  
Ref. [31] showed that luggage was the primary cause of fires in subway stations, and a constant fire source was utilized with a heat release rate of 2.5 MW. As stated in Appendix A, the outdoor gradient wind speed was achieved by introducing a UDF file into Fluent [32].  
According to the requirements in ref. [33], the passengers on an oncoming train and the waiting personnel on the platform must be evacuated from the platform to reach the safety zone within 6 min after a fire occurs. The initial and boundary conditions in the simulation were obtained based on the field test and the model size, as listed in Table 2. The initial CO\({}_{2}\) concentration was 350 ppm, which corresponded to the general concentration of CO\({}_{2}\) in natural air, and the field test values were also around this value. The initial indoor air temperature, initial wall temperature, wind speed of the stairs leading to the platform, and outside air temperature were measured in our field. The wind speed of the exhaust outlet was calculated by the exhaust air rate and was also checked on the spot.  
According to the parameters of typical meteorological years in ref. [34], the frequency of wind direction and speed in Tianjin, China, were counted in different seasons. Figure 3 displays a rose chart of wind direction over the full year, including summer (June to August), winter (December to February), and transition seasons (March to May and September to November). The dominant wind directions were southeast by south (SSE) in summer, north (N) in winter, and southwest (SW) in the transition season.  
The annual wind speed range was 1.0-5.0 m/s, of which the range of 1.8-2.2 m/s accounted for 45.8%. Therefore, an external wind speed of 2.0 m/s was employed to investigate the effect of different external wind directions on the smoke spread characteristics. To analyze the impact of different external wind speeds, five wind speeds of 1, 2, 3, 4, and 5 m/s were considered under the same wind direction.  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Single Valued Condition** & **Value** \\ \hline Initial CO\({}_{2}\) concentration (ppm) & 350 \\ Initial indoor air temperature (\({}^{\circ}\)C) & 22.3 \\ Initial wall temperature (\({}^{\circ}\)C) & 21.6 \\ Outside air temperature (\({}^{\circ}\)C) & 13.7 \\ Wind speed of stair section (m/s) & 1.57 \\ Wind speed of exhaust outlet (m/s) & 3.77 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Initial and boundary conditions of the simulation.