# Study on Fire Smoke Distribution Characteristics in Mountain Spiral Highway Tunnels
## 4 Discussion
Furthermore, there has been partial confirmation of the descent of the smoke. However, further verification is required for the section on the speed of smoke dispersion, particularly in determining the numerical relationship between slope and altitude. This is necessary due to the predominant focus of existing experimental studies on high-temperature smoke dispersion [38]. In current numerical simulations, the heat release rate of the fire source is consistently assumed to be a super-fast fire, and the heat release rate of the fire source exhibits a discernible trend of gradual increase. In addition, numerical simulations of tunnel fires are predicated on the following assumptions: (1) The air temperature in the tunnel is assumed to be uniform prior to the fire. (2) The average velocity and temperature of airflow at the entrance remain constant before the occurrence of a tunnel fire. (3) Ventilation airflow and smoke generated by the fire are treated as ideal gases. (4) No chemical reactions are considered to take place in the smoke as it traverses through the tunnel. However, these assumptions may be influenced in practical scenarios. Additionally, there are limitations in the selection of simulation. In normal circumstances, a certain longitudinal ventilation velocity must be present in the tunnel under the combined action of natural and mechanical ventilation. However, this aspect has not been addressed in the current study. Additionally, for identical fire source conditions such as a fully loaded truck, it should be noted that at high altitudes, both the heat release rate and maximum heat release rate will decrease, thereby impacting factors such as the highest temperature, smoke release, and incomplete combustion degree. Nevertheless, these considerations have not been incorporated into this article. Moreover, variations in air density, humidity levels, and oxygen content at different tunnel locations are also expected to influence the outcomes. Despite its preliminary character, this study clearly indicates the characteristics of smoke dispersion and the impact between the main influencing factors in spiral highway tunnels. One important future direction is to explore the quantitative relationships between different variables. For instance, while it has been observed that smoke propagation accelerates both upstream and downstream of a fire source with an increase in altitude, the extent of this acceleration requires further elucidation. Additionally, there is a need to clarify the relationship between this acceleration and the tunnel slope gradient. Another important future direction is studying temperature distribution and CO propagation at the characteristic height in a spiral highway tunnel as well as their impact on personnel evacuation. This study reveals that for tunnels with a curvature radius of over 600 m, there exists an independent effect on smoke diffusion, relatively impervious to variations in the altitude and slope gradient of the tunnel. Nevertheless, the effect of the altitude and chimney effect caused by the slope gradient would affect each other. The main function of chimney effects is to accelerate the uphill dispersion of smoke from the downhill area. The effect of altitude is mainly to enhance the acceleration of smoke uphill but weaken its inhibitory effect downhill. The research results can provide certain support for further research on fire smoke diffusion in spiral highway tunnels. The main point is that the curvature effect is independent and will not be affected by the altitude and slope nor will it affect the effects of the altitude and slope. This point can have a certain impact on the design of simulations in subsequent studies.