# Study on Fire Smoke Distribution Characteristics in Mountain Spiral Highway Tunnels
## 3 Results
### Smoke Diffusion in the One-Dimensional Longitudinal Diffusion Stage
(**h**) Tunnels with a radius of curvature of 1000 m and a slope gradient of 25% at different altitudes. (**i**) Tunnels with a radius of curvature of 1000 m and a slope gradient of 30% at different altitudes. The above observation indicates a distinct phenomenon of smoke descent during the one-dimensional propagation stage of a tunnel fire, with an accelerated descent speed at higher altitudes. Hence, individuals within the tunnel should promptly initiate evacuation in the event of a fire, particularly in tunnels located at high altitudes. Furthermore, in the initial stage of a fire, as smoke spread vertically, a distinct layer of smoke initially became visible at a distance from the fire source, leading to a significant reduction in visibility within the tunnel. Nevertheless, in areas close to the fire source, the smoke layer appeared thinner and exhibited a more pronounced stratified structure, resulting in improved visibility at the early stage. The chimney effect describes the upward movement of gas resulting from the combined influence of fire, wind pressure, and the expansion force of gas due to heat. This phenomenon leads to the upward dispersion of gas along the vertical shafts of a building [35]. The impact of the chimney effect could be divided into two parts. One is the upstream area of fire source. As analyzed before, the increase in altitude amplifies the chimney effect and results in a faster smoke diffusion. However, the situation becomes different when it comes to areas downhill of the fire source. This distinction becomes more conspicuous as the slope gradient increases, as shown in Figure 6c,f,i. As the altitude increases, the back-layering length and the smoke thickness increase. This phenomenon becomes even more pronounced within 240-360 s. This implies a reduction in the buoyancy responsible for the upward movement of gas in the chimney effect. Overall, the chimney effectively enhances the smoke exhaust capability during tunnel fires as the slope gradient increases, whereas the altitude has the opposite effect on areas uphill and downhill. In areas uphill of the fire source, the increase in altitude accelerates the diffusion velocity of fire smoke, and in areas downhill of the fire source, the increase in altitude reduces the effectiveness of suppressing smoke back-layering, resulting in a longer back-layering length. Moreover, the main effect of high altitudes is low air buoyancy, which leads to an increase in the thickness of the smoke layer. However, when comparing the thickness of the smoke layer in tunnels with the same radius of curvature and altitude under different slope gradient, it could be seen that the thickening of the smoke layer in the high-altitude tunnel is weakened, especially at the front of the smoke. A comparison of Figure 6g,h,i reveals that at 360 s, the smoke layer thickness at a distance of 50 m upstream from the fire source in a tunnel with a slope of 20% exceeds 4.5 m across all altitudes. However, significant variations in smoke layer thickness are observed as the slope increases. Specifically, in Figure 6h, the smoke layer thickness at this same location rises from 3.5 m to 4 m with increasing elevation. Additionally, the maximum diffusion distance of smoke upstream does not reach 200 m but is limited to approximately 180 m. In contrast, Figure 6i demonstrates an even more pronounced effect of the slope; here, the maximum diffusion distance is only between 60 and 70 m, while the smoke layer thickness at a similar position upstream measures merely between 2.5 and 3 m. Thus, it can be concluded that an increase in slope diminishes the impact of elevation on smoke layer thickness by roughly 15%. In brief, an increase in slope mitigates the influence of altitude. As mentioned in Section 3.2, the smoke diffusion in tunnels with different radii of curvature shows that the smoke front protrudes towards the concave wall upstream and towards the convex wall downstream.