## 3 Results
### Comparison of the Smoke Distribution in Different Sides of the Tunnel
#### 3.2.3 CO Concentration  
The variation in the longitudinal CO concentration at the characteristic height of the critical velocity group and the general velocity group in the 360 s tunnel is shown in Figures 9 and 10. It is observed that tunnels with curved structures exhibit a more pronounced increase in CO concentration within the core area of the fire source. This suggests that compared with straight tunnels, the incomplete combustion of combustible materials is exacerbated in curved tunnels during fire events. Toxic gases like CO, predominantly found in fire smoke, pose significant risks to personal evacuation. Incomplete combustion occurs at various stages of a fire. However, it takes some time for CO concentration to reach a hazardous level.  
Figure 8: Variation in longitudinal visibility 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.  
Figure 9: Variation in longitudinal CO concentration at the characteristic height of the critical velocity group. (**a**) Comparison of CO concentration on different sides of the tunnel for Group A. (**b**) Comparison of CO concentration on different sides of the tunnel for Group B.  
As shown in Figure 9, the longitudinal ventilation in the critical velocity group effectively suppresses smoke back-layering. However, because of the unique tunnel structure, high-temperature smoke does not directly disperse outward along the tunnel but instead transports between the lateral walls. The longitudinal diffusion of CO is generally consistent with temperature and visibility trends. Except for the fire source core area, CO concentration remains below hazardous levels. Nevertheless, a comparison between Group A and Group B reveals that as the radius of curvature increases, both sides of the tunnel and its centerline experience decreased CO concentrations. Furthermore, the longitudinal distribution of CO concentration on the outer and inner sides gradually approaches that observed in straight tunnels. However, higher CO concentrations persist at the centerline compared with those found in straight tunnels.  
However, when the longitudinal ventilation velocity is below the critical velocity, the longitudinal distribution pattern of CO concentration in curved tunnels differs significantly from that observed in straight tunnels, as shown in Figure 10. The CO concentration in curved tunnels with different radii of curvature at the outer side and the centerline of the tunnel upstream of the fire source remains consistent with that in straight tunnels, indicating a low level. However, because of structural influences on airflow patterns, there is a notably higher CO concentration on the inner side of curved tunnels. In terms of core areas near the fire source, CO concentrations are generally comparable between curved and straight tunnels. Nevertheless, within a range of 100-350 m away from the fire source, both the inner and outer sides exhibit higher CO concentrations compared with those found in straight tunnels. However, CO concentration levels remain similar to those observed along the centerline in straight tunnels. Beyond 350 m from the fire source, there is a significant increase in CO concentration along the centerline of curved tunnels relative to that seen in straight tunnels. The downward trend observed for CO concentrations on both the inner and outer sides gradually causes them to become lower than those measured within straight tunnels.  
The tunnel set in the simulations comprises an arched ceiling, through which CO is transported along with the flue gas. Subsequently, CO ascends to the tunnel ceiling before descending towards both sides. The spatial distribution of CO is deemed to be influenced by the radius of curvature of the tunnel.