Heat transfer mechanism and emergency operating speed of moving train fires in a metro tunnel

The primary strategy for a burning train in metro tunnels is to drive it out of the tunnel, or to the station or safe area ahead. However, the heat transfer mechanism and the emergency operating speed of moving train fires in tunnels have not been addressed. In this study, a 1:10 train-tunnel model was designed to conduct a series of moving model experiments. The results show that with the increasing train speed in tunnels, the flame height decreases, the flame width and length first increase and then decrease, and the flame tilt angle increases. The maximum temperature at the train top increases by 81.7 % when the train speed increases from 2 to 6 m/s. A piecewise model is proposed to predict the maximum temperature at the train top. The maximum temperature rise at the tunnel ceiling is 17.4 K and exponentially decreases with the train speed, posing a limited threat to the tunnel structure. The convective heat flux received by the train accounts for 82.1 %-94.4 %. The heat transfer between the train, tunnel and flame is codetermined by the train-tunnel aerodynamic effect and fuel-rich zone. The emergency operating speed of the train is suggested to be 22.8 km/h when the heat release rate is 7.5 MW. This study innovatively reveals the combustion behavior of moving train fires in a metro tunnel and proposes guidelines for determining the emergency operating speed of burning trains. The results can provide references for the fire safety design and running capacity improvement of metro trains.