# Study on Fire Smoke Distribution Characteristics in Mountain Spiral Highway Tunnels
## 4 Discussion  
This study into smoke diffusion in mountain spiral highway tunnel fires revealed distinct patterns of fire characteristics. The spiral highway tunnel is distinct due to various key factors that affect it. The main construction goal of the spiral tunnel is to achieve a height gain by extending length, which means spiral tunnel would be affected by the chimney effect caused by the slope gradient. If a straight highway tunnel was established between the same starting point and the same destination, the slope gradient would be so steep that the climbing capability of a heavy truck will be constrained. Thus, the longer length results in a smaller gradient. However, the longer the tunnel is, the more dangerous it would be in a tunnel fire. In addition, as shown in Figure 1, spiral tunnels are mainly used in mountainous areas. This implies that the characteristics of fire smoke diffusion in the spiral highway tunnel would also be influenced by the altitude, including reduced air buoyancy, decreased air density, and lower oxygen content. Also, as the design pattern of the spiral highway tunnel, the radius of curvature of the tunnel would also have an impact on the fire smoke diffusion. This is mainly shown in the different spread velocity between the inner side and the outer side. The propagation of fire smoke can be categorized into the following two stages: the fire development stage and the one-dimensional longitudinal spreading stage [34]. In addition, the fire development stage could be divided into four stages. During the fire development stage, the fire smoke states generally remain consistent. According to the existing studies [35; 36; 37], the area before the flame spreads into the one-dimensional stage is called the near fire source area. As the radius of curvature of the tunnel is over 600 m, the length of the near fire source area is the same as that in a straight tunnel. The comparison  
Figure 8: Variations of visibility of centerline at characteristic height at 360 s.  
results of Figure 5 show the same conclusion. The radius of curvature in the simulations is not small enough to affect the fire development stage. In the one-dimensional longitudinal spreading stage, the spread of smoke along the tunnel centerline is primarily influenced by changes in altitude and slope gradient. An increase in slope accelerates the spread of smoke on the uphill section while limiting its reach on the downhill section. Moreover, a heightened altitude results in amplified speed and a range of smoke diffusion across both uphill and downhill sections of tunnels with varying slopes. The findings from the investigation into smoke dispersion under the influence of a single factor align with prior research [1; 3; 12; 14]. However, it is important to highlight that our comparison of results with previous studies accounted for the interplay of multiple factors. Furthermore, the selection of the tunnel curvature radius was informed by practical engineering best practices [14]. The study findings also demonstrate the dispersion distance, the thickness of the smoke layer, and the descent of smoke in mountain spiral highway tunnels. The characteristics of smoke dispersion are influenced by a complex interplay of various factors, primarily including dispersion velocity and smoke layer thickness, which exhibit a high degree of consistency with the existing literature [19; 20]. However, variations in the experimental conditions can significantly affect both the generation and dispersion of smoke, particularly regarding the rate of smoke production and environmental pressure. Notably, some studies have focused solely on temperature reductions at elevated altitudes. Furthermore, results may vary based on different construction methodologies for tunnel curvature. The limitations of our study should be acknowledged when interpreting these findings. First, it is noteworthy that there have been specific experimental validations regarding the thickness of the smoke layer [3; 8], ensuring the precision of our findings.