## 3 Results and Discussions
### Influence of External Wind  
#### 3.2.1 SSE Wind  
All entrances and exits were on the windward side when the external wind direction was SSE. Figure 8 shows the smoke concentration distribution characteristics with time  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Case** & **External Wind Direction** & **External Wind Speed (m/s)** \\ \hline
0 & No wind & - \\
1 & SSE & 1 \\
2 & SSE & 2 \\
3 & SSE & 3 \\
4 & SSE & 4 \\
5 & SSE & 5 \\
6 & N & 1 \\
7 & N & 2 \\
8 & N & 3 \\
9 & N & 4 \\
10 & N & 5 \\
11 & SW & 1 \\
12 & SW & 2 \\
13 & SW & 3 \\
14 & SW & 4 \\
15 & SW & 5 \\ \hline \hline \end{tabular}
\end{table}
Table 5: Basic simulation conditions.  
Figure 7: Smoke concentration distribution characteristics in the station hall under Case 0.  
and pressure distribution characteristics at 360 s in the station hall under the Case 2 condition. In Figure 8a, the smoke concentration distribution contours indicate the spread of smoke toward B-end overtime. According to Figure 8b, smoke moved sequentially to the monitoring points C\({}_{\text{B10}}\)-C\({}_{\text{B60}}\), and the related concentrations gradually rose over time, but the smoke concentrations at monitoring points C\({}_{\text{A10}}\) and C\({}_{\text{A20}}\) increased and subsequently declined. This was due to the smoke being less impacted by the external wind in the first 90 s, when it expanded to both sides with the fire source as the center at the same time. The smoke spread toward A-end was restricted after the external wind reached a constant wind speed, while the smoke spread to B-end was aided by the SSE wind.  
Figure 8: \(Cont.\)  
A further analysis of the pressure distribution contour is shown in Figure 8c. The external wind entered the station hall through the three entrances and exits, which were determined by the orientation. However, because passageways A and D were on the same side, the positive pressure created by the external wind entering the station hall on this side was larger than the positive pressure created by passageway C. As a result of the pressure gradient, the smoke in the station hall spread to the B-end. When the flow field was steady, the values of wind speeds in passageways A, C, and D were 0.41 m/s, 0.36 m/s, and 0.70 m/s, respectively.  
Under the SSE wind direction, five typical working conditions were simulated, with wind speeds ranging from 1 to 5 m/s. Figure 9 shows the smoke concentration distribution characteristics of the station hall. In Figure 9a, the CO\({}_{2}\) concentration distribution contour displays that the smoke spread to B-end increased with increasing wind speed. The highest CO\({}_{2}\) concentration values along the longitudinal direction of the station hall were extracted, as shown in Figure 9b, and the concentration distribution curves of the station hall were produced under different external wind speeds. The CO\({}_{2}\) concentration was greatest around the fire source. The value of CO\({}_{2}\) concentration progressively fell as the external wind speed rose, and the SSE wind resulted in a peak value closer to the B-end compared to that in Case 0; the concentration of the smoke at the B-end steadily increased at the same time. Therefore, the smoke moved to the B-end due to the SSE wind.  
As shown in Figure 9c, the longitudinal diffusion distance of smoke rose with rising external wind speed, indicating that the high SSE wind speed aggravated the degree of longitudinal spread to the B-end. The greatest longitudinal diffusion distance of 61.3 m was attained with a wind speed of 5 m/s, representing a 67.9% increase over Case 0. Hence, the smoke control capability of the TSE system was reduced for the SSE wind direction, and the wind speed affected the smoke diffusion distance.  
Figure 8: Smoke concentration distribution characteristics with the time and pressure distribution contour in the station hall under Case 2. (**a**) Distribution contours, (**b**) variation at the monitoring points, (**c**) pressure distribution contour at 360 s.  
Figure 9: _Cont_.