Abstract: China has gradually developed into a country with the largest number of tunnels and underground projects in the world, and the number and length of its long tunnels rank the top in the world. According to statistics, 85% of the deaths in fires are caused by hot smoke. At present, the smoke exhaust systems widely used in the tunnel include longitudinal smoke exhaust system, centralized smoke exhaust system and horizontal smoke exhaust system. However, the combinations of vertical shaft exhaust and longitudinal ventilation are widely used for long tunnels in China. Therefore, it is of great significance to study the smoke flow characteristics and temperature distribution of tunnel fires under the combined effects of longitudinal ventilation and vertical shaft natural smoke exhaust. A 1:10 reduced scale shaft tunnel model is established, and the main tunnel is 16.5 m in length, 1.3 m in width and 0.65 m in height. The vertical shaft is connected with the main tunnel through the transverse smoke exhaust with the dimensions of 1.2 m long, 0.6 m wide and 0.4 m high, which is set in the middle of the side of the main tunnel. The vertical shaft with a cross section of a quarter circle with a radius of 0.6 m is 4.6 m in height. A series of pool fire experiments are carried out in the 1:10 reduced scale shaft tunnel model. The variables of 2 pool sizes ( 20 cm×20 cm、23 cm×23 cm ) and 2 fire source locations (position A and B: the distances between the center line of fire source and the center line of transverse smoke exhaust passage are 0.375 m and 1.375 m, respectively ) , and 7 longitudinal ventilation velocities (0, 0.18, 0.27, 0.35, 0.44, 0.52, 0.69 m/s) are considered. The temperature distribution and smoke back-layering length under different conditions are quantitatively analyzed. The results showed: for no longitudinal ventilation conditions, the flame is perpendicular to the tunnel floor and presents an axisymmetric shape. Whereas for longitudinal ventilation conditions, the flame shifts to the downstream, and the larger the longitudinal ventilation velocity, the more obvious the flame shifts to the downstream. When the longitudinal ventilation is 0 m/s, the smoke temperature distribution on the upstream and downstream of the fire source is not symmetrical due to the existence of the vertical shaft, and the smoke temperature on downstream of the fire source (shaft side) decreases faster, which is obviously different from that in single tunnel fires. With the ventilation velocity increases, the back-layering length and smoke temperature distribution decrease, while the maximum temperature drift length shows an increasing trend. When the dimensionless longitudinal ventilation velocity v′＜0.19, the maximum temperature rise of the main tunnel △Tmax is proportional to Q2/3/Hef 5/3,while the maximum temperature rise of the main tunnel △Tmax is proportional to

Key words: vertical shaft tunnel, longitudinal ventilation, vertical shaft natural smoke extraction, smoke characteristics, prediction model