## 3 Results and Discussions
### Influence of External Wind
#### 3.2.3 SW Wind  
When the external wind direction was SW, the entrances and exits of A and C were on the windward side, while the entrance and exit of D were on the leeward side. Figure 12 displays the characteristics of smoke concentration distribution over time under the Case 12 condition. As shown in Figure 12a, the smoke gradually spreads to both sides. Figure 12b demonstrates that the smoke spread to monitoring points C\({}_{\text{A10}}\) (8 s), C\({}_{\text{A20}}\) (28 s), and C\({}_{\text{A26}}\) (44 s) sooner than it did to C\({}_{\text{B10}}\) (22 s), C\({}_{\text{B20}}\) (60 s), and C\({}_{\text{B30}}\) (96 s), indicating that the smoke tended to diffuse toward the A-end under the influence of SW wind.  
Figure 11: Smoke concentration distribution characteristics in the station hall under different speeds of N wind. (**a**) Distribution contours, (**b**) longitudinal distribution, (**c**) diffusion distance.  
According to Figure 12c, the N wind caused a positive pressure at the entrances and exits A and C. Under the suction of the TSE system, the wind speed of the entrances and exits A and C was 0.25 m/s and 0.39 m/s, respectively. The entrance and exit D on the leeward side showed a negative pressure, with a pressure value lower than that in the station hall, and the corresponding wind speed was \(-\)0.1 m/s. The external wind caused the pressure gradient of the station hall to drop from passageways A and C to passageway D, which also contributed to the smoke's progressive spread to passageway D.  
Figure 13 depicts the characteristics of the smoke concentration distribution in the station hall and passageway D under different SW wind speeds. Figure 13a illustrates how the smoke reached the A-end, as the windward side of the entrance and exit C caused the external wind to enter the station hall. As shown in Figure 13c, the smoke concentration near the fire source was the lowest in Case 0. The peak value of the smoke concentration grew gradually as the external wind speed increased, and it was closer to the A-end compared to elsewhere. This indicated that the N wind intensified smoke diffusion to the A-end. Figure 13d demonstrates that the greatest smoke diffusion distance was 37.0 m under the SW wind, which was similar to Case 0.  
Figure 12: Cont.  
Figure 12: Smoke concentration distribution characteristics with time and pressure distribution contours in station hall under Case 12. (**a**) Distribution contours, (**b**) variation at the monitoring points, (**c**) pressure distribution contour at 360 s.  
Figure 13: _Cont._  
When the wind speed was 5 m/s, the CO\({}_{2}\) concentration in passageway D exceeded 1%. As seen in Figure 13b, the opening of the TSE system direction was unable to completely prevent smoke from spreading to the passageways. Because of the influence of the draught between passageways A and D, the smoke at the A-end was carried to passageway D, which resulted in a drop in the CO\({}_{2}\) concentration at the A-end. From Figure 13d, it can be seen that the smoke diffusion distance of passageway D was 7.28 m at an external wind speed of 5 m/s.  
Figure 13: Smoke concentration distribution characteristics in the station hall and passageway D under different speeds of SW wind. (**a**) Distribution contours of the station hall, (**b**) distribution contours of passageway D, (**c**) longitudinal distribution of the station hall, (**d**) diffusion distance of the station hall and passageway D.