# Study on Fire Smoke Distribution Characteristics in Mountain Spiral Highway Tunnels
## 3 Results
### Impact at Characteristic Height  
In order to further explore the influence of smoke diffusion characteristics in the spiral tunnel, Figure 7 shows the temperature distribution in the tunnel centerline at the characteristic height at 360 s.  
It should be emphasized that the diffusion characteristics of high-temperature flue gas and all flue gas are not consistent. The high temperature smoke mainly concentrated in the core area of the fire source. As the distance from the fire source increases, the smoke temperature at the characteristic height decreases rapidly. Based on the comparative results presented in Figure 7, it is evident that variations in tunnel curvature radius have no impact on the maximum temperature or the extent of the high-temperature zone. This finding aligns with the analysis of tunnel fire behavior in relation to the tunnel curvature radius discussed earlier. Specifically, it indicates that the influence of the tunnel curvature radius on smoke dispersion and temperature distribution characteristics during a tunnel fire remains largely unaffected by changes in altitude and slope. The existing studies [18] have already conducted a thorough analysis of the curvature radius, so this paper will not discuss it further. As altitude increases, the highest temperature at the characteristic height decreases. However, this decreasing trend is mitigated by the increase in slope. In other words, at the same altitude, an increase in slope leads to a higher temperature peak at the human eye height. Nevertheless, the location of the temperature peak at the characteristic height  
Figure 7: Distribution of temperature of centerline at characteristic height at 360 s.  
remains largely unaffected by slope variations. This observation may be attributed to relatively minor changes in slope encountered in practical engineering projects. With increasing slope, there is a significant expansion of high-temperature regions at characteristic heights. This primarily manifests as an upward trend in temperature with increasing slope over equivalent distances from the fire source. At a 20% slope, the highest temperature at 500 m elevation has already reverted to its initial value. Conversely, under a 25% slope condition, temperatures continue to rise at 500 m from the fire source and notably more so under a 30% incline. As discussed in Section 3.2, while curvature exerts an independent influence on smoke diffusion, it nonetheless plays a significant role. Specifically, the unique curved geometry of the tunnel partially inhibits the dispersion of high-temperature smoke generated directly from the fire source. In the absence of longitudinal ventilation, this high-temperature smoke tends to concentrate more in the vicinity of the fire source's central region. Figure 7 illustrates that under identical slope and elevation conditions, an increase in curvature radius results in a reduction of overall temperature near the fire source--excluding peak values--and an extension of the high-temperature zone. Concurrently, as slope increases, high-temperature smoke is no longer concentrated directly above the fire source but shifts towards higher slopes. Consequently, within a tunnel exhibiting a slope of 20%, the distance between secondary temperature peaks and primary peaks measures approximately 20 m. However, when this slope escalates to 30%, this distance expands to about 50 m. It is noteworthy that current research on tunnel slopes indicates that with increasing gradient angles, high-temperature smoke diffuses more rapidly uphill--thereby reflecting a decrease in peak temperatures. Nevertheless, considering national codes and practical engineering, actual slope gradients do not exert substantial direct effects on temperature peaks. As previously indicated, the primary factor influencing temperature distribution is the dispersion of high-temperature flue gases. However, different principles govern the dispersion of various flue gases.