# Study on Fire Smoke Distribution Characteristics in Mountain Spiral Highway Tunnels
## 1 Introduction  
With the advancement of tunnel construction technology, spiral tunnels have been applied in highway systems to achieve the goal of exchanging length for height. Table 1 is an overview of main existing spiral tunnels. Slope, curvature, and altitude are the three major factors that have a significant impact on the smoke distribution in a tunnel fire. However, the existing studies mainly focus on the effects of one or two of these factors. The research on the impact of a single factor has reached a well-established status at this point. In terms of the influence of altitude, [PERSON] performed several full-scale tunnel fire experiments at an altitude of 4100 m and proved that reduced atmospheric pressure impacts the smoke movement significantly. The heat release rate (HRR), the smoke diffusion, and the temperature distribution were experimentally studied [1]. [PERSON] et al. performed numerical simulations in the high-altitude road tunnel with low ambient atmospheric pressures to explore the characteristics of critical ventilation and smoke back-layering [2]. [PERSON] et al. conducted numerical simulations in a highway tunnel with various ambient pressures and HRRs and found the relationship between low ambient pressure due to lower air density and the distribution of smoke movement and gas temperature [3]. [PERSON] et al. conducted tunnel fire simulations with different altitudes and HRRs and provided an empirical formula used for calculating the distribution of smoke velocity in tunnel fires [4]. [PERSON] et al. established fire dynamics simulator (FDS) models to perform three-dimensional simulation in Yajiageng Tunnel to explore the law of smoke flow at a fire inthe high-altitude two-way highway tunnel and the effect of smoke control under different wind speeds [5]. [PERSON] et al. explored the CO concentration, smoke temperature, and HRR of gasoline and diesel pool fires through carrying out small-scale high-altitude tunnel fire field experiments [6]. [PERSON] et al. studied the influence of altitude on the flue gas velocity and temperature distribution in the cross passage near the accident tunnel, the rescue station, and the middle carriage of the train [7]. In regard to the influence of the slope gradient, [PERSON] et al. performed FDS models in single-hole double-track and single-hole single-track tunnels to simulate fires in tunnels with different slopes. The settling height of the smoke layer, the distribution of CO concentration, temperature, and visibility were explored [8]. [PERSON] et al. performed simulations on the smoke flow behaviors in slope tunnels with natural ventilation, concentrated mainly on the impact of the slope of the tunnel on the distribution of temperature in the shaft and tunnel, tunnel air velocity at the entrance, and smoke back-layering length [9]. [PERSON] et al. experimentally studied the characteristic of smoke diffusion in vaulted tunnels with different slopes to explore the HRR, the smoke back-layering length, and ceiling temperature [10]. [PERSON] et al. conducted simulations to explore the impact of tunnel slope on the smoke back-layering length, including naturally inducing the inlet airflow and inducing resistance against smoke downward [11]. [PERSON] conducted numerical simulations to reveal the impact of longitudinal slope on temperature distribution and smoke diffusion in tunnels with different HRRs [12]. [PERSON] et al. used Computational Fluid Dynamics (CFD) numerical simulations to study smoke back-layering in a two-lane curved tunnel under the influence of longitudinal ventilation and predicted the location of the maximum ceiling temperature [13]. In regard to the influence of the radius of curvature, [PERSON] et al. studied the influence of the curvature radius on smoke diffusion and thermal radiation in a tunnel via numerical simulations [14]. [PERSON] et al. studied the fire smoke diffusion characteristics of the Qianhaizi highway tunnel to explore the critical wind velocity of the tunnel [15].