Comparative analysis of fire plume behaviors between stationary and moving trains in a tunnel

When a train catches fire in a tunnel, in order to facilitate the evacuation and rescue of passengers, the train should continue to operate until it exits the tunnel and arrives the next station or the emergency rescue station. To deeply understand the evolution process of moving train plumes in tunnels and provide theoretical guidance for the operation safety of railway tunnels, this study conducted moving model experiments to compare and analyze the differences of fire plumes between stationary and moving trains in a tunnel.

Based on the existing moving model test platform, a 1:10 train-tunnel model was designed by using the Froude similarity criterion. The head of the train model adopted a streamlined design to avoid the influence of the vortex caused by boundary layer separation on the experimental results. The front surface of the tunnel was designed with fire-resistant transparent tempered glass to facilitate the observation of the train plume behavior. Thermocouple array was used to measure the temperature distribution at the top of the train and under the tunnel ceiling, and the total and radiative heat flux at the top of the train were measured by heat flux gauges. Based on biharmonic spline interpolation algorithm, a Matlab script was written to reconstruct the discrete temperature values on the top of the train, and the two-dimensional temperature distribution contour on the top of the train was obtained.

The results show that, unlike stationary train fires, the fire plume of the moving train moves forward by sweeping the roof of the tunnel. This paper defines this type of flow as "ceiling sweep" for the first time and divides its evolution into three stages: (1) Rise stage. The train fire plume rises and inclines to the upstream of the fire source under the coupling of inertia and viscous forces; (2) Contact stage. Depending on the heat release rate, the train speed and the tunnel height, this stage includes direct flame contact with the ceiling and smoke plume contact with the ceiling. (3) Sweep stage. After contacting the tunnel, the fire plume expands below the ceiling. At the same time, due to the relative motion between the train and the tunnel, the fire plume will sweep the tunnel ceiling and move forward. This aforementioned flow pattern is defined as "ceiling sweep" in this paper. Due to heat accumulation and thermal feedback of the tunnel, the maximum temperature, maximum total heat flux and maximum radiative heat flux at the top of the train under the action of the moving train fire plume in the tunnel are greater than that under the open line scenario. The maximum temperature under the roof of the tunnel is significantly decreases, and the longitudinal temperature presents an asymmetric distribution of higher upstream and lower downstream of the fire source.

The above results show that the threat of the fire plume of moving trains to the train body increases, but the threat to the tunnel decreases. This study can provide reference for the design of flame-retardant materials and emergency operation strategy of trains.