Abstract: Abstract: In order to research smoke movement under water mist firefighting system and longitudinal ventilation, consequently, ensure best ventilation strategy under water mist firefighting system. A serious of numerical simulation model was conducted with fire dynamics simulator, and the change of temperature, carbon monoxide concentration and radiant heat flux under different longitudinal wind speeds were obtained. The results show that: under the fire scale of 30 MW, the smoke layer begins to gradually drop in the spray area 15 m upwind from the fire source, and the smoke layer drops below 2 m; when the fire is extinguished in 300 s, within 150 m upwind, the smoke layer all dropped below 2 m. Therefore, 5 minutes after the fire broke out, the evacuation distance of personnel should be greater than 150 m. Comparing the changes of radiant heat flux before and after the application of water mist under the same ventilation wind speed (1 m/s), it can be concluded that after the water mist fire extinguishing system is turned on for 25 s, the heat radiation intensity at 5 m downstream of the fire source decreases from 6 kW/m2 to 0. It is recommended to keep the ventilation wind speed of 1 m/s in the tunnel as much as possible when turning on the water mist fire extinguishing system.

Key words: Key words: bridge and tunnel engineering, tunnel fire, water mist system, numerical simulation, longitudinal ventilation