Abstract: In tunnel fires, cross passages may become pathways for smoke spread after the evacuation function is completed, and subsequent longitudinal ventilation can also affect smoke propagation within the tunnel. Based on the Xuefengshan tunnel fire scenario, this study established an FDS numerical model. A combined fire source consisting of a truck (15 MW) and a car (5 MW) was set using time-sequential ignition to simulate the initial fire conditions and ventilation conditions during the fire decay stage. The influence of parameters such as gas velocity, temperature, and visibility on fire rescue operations was analyzed under these two scenarios. The results indicate that the temperature at the eighth vehicle cross passage near the fire source is higher than that at other cross passages, while the seventeenth pedestrian cross passage on the downwind side of the fire source exhibits the lowest visibility, dropping to 5.83 m at 1 926 s. During the fire decay stage, longitudinal ventilation does not cause significant fluctuations in the fire source but does lead to weakened stratification on the downwind side, resulting in rapid mixing of smoke with air. Overall, during the initial fire stage and the subsequent ventilation stage, the non-accident tunnel and the cross passages on the upwind side of the fire source offer a relatively safe environment for fire rescue operations.

Key words: tunnel, fire, FDS numerical simulation, tunnel cross passages, longitudinal ventilation