The maximum ceiling temperature rise during moving train fire in subway tunnel: the influence of longitudinal ventilation

Longitudinal ventilation systems are extensively found in subway tunneling projects due to their low cost and superior smoke extraction capabilities. However, when a moving train in a tunnel is on fire, its fire characteristics, especially the temperature distribution characteristics, will be complicated by the simultaneous effects of longitudinal ventilation and piston wind. A series of scaled-down experiments were carried out to examine the ceiling temperature rise characteristics in this case. The characteristics of the tunnel flow field under the combined influence of piston wind and longitudinal ventilation are revealed, and it is discovered that the ventilation wind direction and wind velocity are the main factors controlling the dimensionless maximum airflow velocity. Ventilation velocity, fire power, and vehicle speed impact the variation in ceiling temperature rise by altering flame geometry, smoke flow, and heat accumulation time. Under the downwind condition, the maximum ceiling temperature rise for some scenarios increases and then decreases with increasing ventilation velocity. Under the upwind condition, owing to the significant overlapping enhancement effect of the annular gap flow field, the ceiling temperature rise value decreases monotonically with the increase of ventilation wind velocity and vehicle speed, and it is always lower than the safe value. It is recommended that the daily longitudinal ventilation wind velocity is less than 5.4 m/s, and the wind velocity can be increased in the later phase of the fire to optimize the safety evacuation and rescue conditions. A prediction model of ceiling temperature rise based on the dimensionless theorem is established by introducing the convective heat transfer intensity factor and the barrier effect factor. The cross-experiment results show that its prediction accuracy is high. The research results are helpful for monitoring and controlling tunnel fires and providing guidance for evacuation and rescue.