# Study on Fire Smoke Distribution Characteristics in Mountain Spiral Highway Tunnels
## 3 Results
### Smoke Diffusion in the One-Dimensional Longitudinal Diffusion Stage  
Figure 6 shows the smoke distribution along the centerline of the tunnel of all simulations at different time of simulation. Smoke emanates from the fire source and disperses to various locations over time. The distinct colors illustrate the positions to which the smoke has diffused at this particular moment. As the length of back-layering is not the main research aim, the upstream of the fire source is only chosen at 200 m for a better comparison. With no longitudinal ventilation considered in the simulations, the distribution of fire smoke should be symmetrical along both sides of the fire source. However, the chimney effect caused by the slope leads the fire smoke to shift to the uphill side. As the slope gradient increased, the fire smoke shifted more to the uphill side. For example, comparing Test 1 and Test 2, the upstream fire smoke extended to a distance of 400 m at 300 s in Test 1, while it had already surpassed this range and reached approximately 450 m in Test 2 at the same time. Meanwhile, comparing the results with the same radius of curvature  
Figure 4: The development process of the fire smoke layer in the tunnel. (**a**) The side view. (**b**) The top view. Figure 5: The smoke state of Test 2, Test 5, and Test 8 at 60 s. (**a**) The top view of Test 2; (**b**) the side view of Test 2; (**c**) the top view of Test 5; (**d**) the side view of Test 5; (**e**) the top view of Test 8; and (**f**) the side view of Test 8.  
and altitude, it was observed that the thickness of the smoke inversion layer decreased longitudinally upstream of the fire source in the tunnel. However, as the altitude increases, the thickness of the smoke layer upstream of the fire source also proportionally increases. This is primarily attributed to the reduced buoyancy force of smoke in high-altitude areas owing to its decreased air density. At the same time, a marked increase in the rate of smoke dispersion was observed uphill of the fire, as altitude increased. This means the increase in altitude amplifies the effect of the chimney effect. Meanwhile, Figure 6 illustrates the variations in smoke descent throughout the combustion process. It is evident that during the initial and stable stages of the fire, the stratified structure of the smoke layer near the fire source area remains relatively intact, with a minimal thickness. Across all experiments, the thickness of the smoke layer within 50 m of the fire source was consistently less than 3 m within 180 s. As the fire continued to develop, the smoke layer experienced significant descent with a rapid sinking speed. Across all tests, within the area beyond a distance of 50 m from the fire source, it took approximately 210 s for the smoke thickness to decrease to 4 m. However, within a 50 m distance of the fire source, the increase in altitude significantly accelerated the descent of smoke. In simulations for Test 28-Test 45, which are at altitudes of 4000 m and 5000 m, the descent of smoke exceeded 4 m within 240 s of the fire source. In other simulations, this was observed within 300-360 s.  
Figure 6: Cont. Figure 6: Smoke diffusion characteristics at the centerline of the tunnel of different times. (**a**) Tunnels with a radius of curvature of 600 m and a slope gradient of 20% at different altitudes. (**b**) Tunnels with a radius of curvature of 600 m and a slope gradient of 25% at different altitudes. (**c**) Tunnels with a radius of curvature of 600 m and a slope gradient of 30% at different altitudes. (**d**) Tunnels with a radius of curvature of 800 m and a slope gradient of 20% at different altitudes. (**e**) Tunnels with a radius of curvature of 800 m and a slope gradient of 25% at different altitudes. (**f**) Tunnels with a radius of curvature of 800 m and a slope gradient of 30% at different altitudes. (**g**) Tunnels with a radius of curvature of 1000 m and a slope gradient of 20% at different altitudes.