Quantitative prediction on flame geometry of moving train fires in railway tunnels

• Higher annular gap flow velocity reduces flame height and increases tilt angle. • Flame width and length exhibit two regimes with the increasing train velocity. • Prediction models for flame geometry parameters are established with ± 10% error. The continuous construction of extra-long railway tunnels poses challenges regarding fire safety. Despite existing specifications for the design of safe zones and emergency rescue stations in extra-long tunnels, the burning characteristics of a train fire while in motion toward these designated areas have not been fully addressed. This study investigated the flame morphology characteristics of moving train fires in a tunnel on a moving model rig. A train model and a tunnel model with a scale of 1:10 was designed according to Froude similarity criterion. Seven train velocities and eight heat release rates were considered. The flame was captured by employing a high-speed camera. The results indicate that the flame height increases with the HRR but decreases with the train velocity, while the flame tilt angle exhibits the opposite trend. Based on the HRR, the variation of flame width and length with the train velocity exhibits two regimes, namely decrease regime (1.618≤ Q ̇ ∗ ＜4.006) and increase–decrease regime (4.006≤ Q ̇ ∗ ≤ 7.243). Compared with open track scenarios, the airflow velocity in annular space around the train is larger, leading to a smaller flame height and a larger flame tilt angle with the same heat release rate and train velocity. According to dimensional analysis and experimental data, prediction models for flame geometry parameters are proposed. This study can provide references for the fire safety design and emergency operating strategies for trains and extra-long railway tunnels.