## 1 Introduction  
With the advancement of tunnel excavation technology, there has been a gradual increase in the proportion of curved highway tunnels within highway systems. The unique structure of a curved tunnel introduces greater complexity to smoke flow and temperature variations during fire incidents, depending on the radius of the tunnel turn [1, 2, 3]. Previous research has delineated the movement patterns of fire smoke in linear tunnels and provided formulas to calculate the length of smoke back-layer, critical wind speed, and maximum temperature. However, the diffusion of smoke is subject to various influencing factors such as altitude, slope gradient of the tunnel, and tunnel curvature. Tunnel curvature constitutes the focus of this investigation. Previous research has demonstrated that curved tunnels and straight tunnels exhibit distinct characteristics. [PERSON] et al. studied the smoke control strategy in the straight-line and curved areas of a tunnel through experiments and numerical simulations. Their results pointed out that when the fire source was in the curved area, the temperature near the fire source was significantly lower than that in the straight area and the smoke back-layer length increased [4]. Meanwhile, correction formulas have been proposed for key indicators of curved tunnel fires based on those of straight tunnel fires. [PERSON] constructed numerical models on the fire smoke movement characteristics of a curved tunnel with a radius of 600 m and proposed a calculation method of the resistance along the curve tunnel to clarify the law of the influence of the curved tunnel on the fluid flow [5; 6; 7]. [PERSON] et al. used CFD numerical simulation to study the influence of longitudinal ventilation on smoke back-layering in a two-lane curved tunnel and predicted the location of the maximum ceiling temperature [8]. [PERSON] et al. studied the fire smoke transport characteristics of the Qianhaizi curve highway tunnel to obtain the critical wind speed of the tunnel [9]. [PERSON] conducted numerical simulations of a curved road tunnel fire using Fluent to compare the critical wind velocity at the convex and concave locations of the fire source. They also analyzed the length of the smoke back-layering and concluded that the critical velocity at the convex location was larger [10]. [PERSON] et al. studied the law of smoke spread during fires in inclined and curved tunnels by numerical simulations and experiments and found that the location with the greatest difference in heat transfer rates was the tunnel ceiling under different working conditions [11]. [PERSON] et al. combined numerical simulation with theoretical analysis to determine a fire model suitable for curved tunnels and studied the length of the smoke layer and critical ventilation velocity of curved tunnels [12]. [PERSON] et al. studied the effect of a curved sidewall on the fire shape and maximum temperature beneath the ceiling centerline and established formulas to predict the maximum temperature beneath the ceiling centerline [13]. [PERSON] investigated the temperature distribution in curved tunnels through a numerical simulation method and proposed a new exponential decay model for longitudinal temperature predictions under various tunnel curvature conditions [14]. [PERSON] et al. conducted experiments and numerical simulations to analyze the critical velocity of a curved tunnel under different fire scenarios [15]. Nevertheless, there remains little research directly investigating the impact of fire in curved tunnels on personal evacuation. [PERSON] et al. studied the influence of the curvature radius (>2500 m) on thermal radiation and smoke distribution in a tunnel through numerical simulation [16]. [PERSON] established a numerical model by FDS to compare the smoke diffusion and temperature distribution of a tunnel fire in the straight section and the curved section. They found that the smoke backflow was more serious and the temperature increased more significantly when the fire occurred in the curved section [17]. [PERSON] et al.