## 3 Results
### Comparison of the Smoke Distribution in Different Sides of the Tunnel
#### 3.2.2 Visibility  
Figure 7 shows the longitudinal variation in the visibility of the critical velocity group at the characteristic height at 360 s. Initially, during the early stages of the fire, smoke primarily accumulates in the tunnel ceiling because of the heat generated by the fire source. This accumulation has minimal impact on visibility at the characteristic height, as there are no significant alterations in temperature or smoke concentration. However, as the fire progresses, smoke diffuses longitudinally throughout the tunnel. The distribution of human visibility corresponds to the longitudinal distribution of temperature and reflects alternating zones of fire smoke caused by the high-temperature smoke and flame between the inner and outer sides of the tunnel.  
Figure 6: Variation in longitudinal temperature at the characteristic height of the general velocity group. (**a**) Comparison of temperature on different sides of the tunnel for Group A. (**b**) Comparison of temperature on different sides of the tunnel for Group B.  
Group B (tests 12, 14, 16, and 18) exhibits a similar trend in visibility distribution compared to that observed in a straight tunnel (test 2). The main difference is that the visibility at 50 m away from the fire source of the straight tunnel remained in the safe range, while group B is at the peak of visibility but still within the hazardous range. For group A (tests 4, 6, 8, and 10), the peak position and peak height of its visibility are significantly different from those of the straight tunnel. However, when the distance from the fire source exceeds 150 m, the visibility is lower than 10 m in all simulations, which means there remains no difference in personal evacuation. However, when the tunnel ventilation velocity is below the critical velocity, as shown in Figure 8, there is a significant disparity in the longitudinal visibility distribution within the general velocity group. Longitudinal ventilation is insufficient to drive the high-temperature smoke in the tunnel under such simulations. Beyond a distance of 50 m away from the fire source, human visibility for all simulations falls into the dangerous range. Noticeable smoke back-layering occurs. In Group A, variations in tunnel curvature greatly influence the impact of smoke back-layering length on human visibility upstream of the fire source. Because of obstruction by the inner sidewall of the tunnel, the smoke temperature decreases more rapidly compared with that on the outer side, resulting in a rapid visibility decrease at the characteristic height. Nevertheless, human visibility remains within safe limits on the outer side of the tunnel; this remains true for Group B as well. Although tests 16 and 18 exhibit similar patterns of smoke and temperature distribution as those observed in a straight tunnel within Group B, there are still notable differences in visibility upstream of the fire source and at the centerline.  
Figure 7: Variation in longitudinal visibility at the characteristic height of the general velocity group. (**a**) Comparison of visibility on different sides of the tunnel for Group A. (**b**) Comparison of visibility on different sides of the tunnel for Group B.