Study of the evolution mechanism of multi-window carriage fire under longitudinal ventilation in a metro tunnel

The fire evolution and ejected flame characteristics of a metro train under longitudinal wind were investigated by numerical simulation and scale-model experiment. The combustion heat release rate (HRR), geometric characteristics of the external flame, and temperature distribution beneath the tunnel ceiling were studied under a total of 159 working conditions. The results show the influence of longitudinal ventilation in the metro tunnel on the airflow of upstream and downstream windows. The airflow enters the carriage from downstream windows and exits through upstream windows. The flow rate of windows is positively correlated with ventilation velocity. Parameters such as fire load, ventilation factor and longitudinal ventilation determine the heat release rate of fire combustion in the enclosed space. When the fire load is less than 1500 times the ventilation factor, longitudinal ventilation will reduce the combustion heat release rate; otherwise, the longitudinal ventilation will promote combustion reaction. Longitudinal ventilation affects the external flame size by changing the window airflow and air entrainment. In addition, the longitudinal inertial force increases the flame deflection angle and horizontal length. The temperature beneath the tunnel ceiling decreases significantly under longitudinal ventilation. Therefore, in a metro tunnel with longitudinal ventilation, passengers should be protected from high temperature when passing by the fire carriage during evacuation.