| With the development of infrastructure construction in China,the total length of railway tunnels is increasing,and the number of extra-long railway tunnels is becoming larger and larger,especially in the western mountainous areas of China.Generally,extra-long railway tunnels need to build shafts for tunnel construction,operation ventilation,disaster prevention and rescue.However,the flow of smoke in the tunnel is more complicated when a fire occurs due to the influence of the shaft on the air flow,and improper control of the smoke often causes heavy casualties.Reasonable smoke control technology is conducive to the rapid evacuation of personnel and guarantees personnel life safety.Therefore,studying the piston wind flow mechanism under the influence of shafts in extra-long railway tunnels and the characteristics of flow field,temperature field distribution and smoke diffusion law during the process of the burning train entering the rescue station is conducive to the formulation of smoke control schemes,which is of great significance to the disaster prevention of extra-long railway tunnels.In this paper,theoretical analysis,model test and numerical simulation are used to deeply study the piston wind flow mechanism and fire smoke characteristics under the influence of shafts in a railway tunnel with intent to provide technical support for disaster prevention of extra-long railway tunnel.The research results obtained are as follows:(1)The extra-long railway tunnel is divided into several sections by shafts,when the train runs in different tunnel sections,the air flow in the entire tunnel and shaft will change.Based on the continuity equation and Bernoulli equation,the unsteady flow mathematical model of piston wind in an extra-long railway tunnel was established,and the formulas for calculating the piston wind in different tunnel sections were deduced when the train was traveling in the extra-long tunnel with shafts.(2)Based on the similarity theory,a double-shaft tunnel test device with a geometric scale of 1 / 50 for piston wind characteristics was built,and an experimental study on the characteristics of the piston wind flow in an extra-long railway tunnel was carried out.The variation law of the air flow field and pressure field in the double shaft tunnel was obtained,and the correctness of the theoretical calculation formula of piston wind was verified.The results show that when a train travels in an extra-long railway tunnel with shafts,the piston wind has the highest speed and the fastest growth rate in the tunnel section where the train is located.The results also show that when the train travels from upstream to downstream of the shaft,the pressure in the upstream tunnel decreases and that in the downstream tunnel increases,the air in the shaft changes from exhaust to inlet,and the flow relationship of the air between the shaft and the upstream and downstream tunnels also changes from diversion to confluence.(3)Using the dynamic grid technology of Fluent 17.0,the piston wind flow characteristics during the train traveling in the double-shaft extra-long railway tunnel were simulated,and the change of the flow field in the tunnel was analyzed when the relative position of the train and the shaft was different.According to the simulation results,the theoretical calculation formula of piston wind was revised and parameter sensitivity analysis was carried out.The results show that factors such as train speed,train length,tunnel length,blockage ratio,shaft height and the area ratio of shaft to main tunnel all have a significant impact on the piston wind.The greater the train speed,train length and blockage ratio,the more momentum the train transmits to the air in the tunnel,and the stronger the piston wind.The longer the tunnel,the greater the resistance required to overcome by air flow and the weaker the piston wind,however,the change of the air resistance caused by the shaft height is small,and its influence on the piston wind is basically negligible.Since the resistance required to overcome when the air flows through the shaft is smaller than when it flows through the main tunnel,the larger the area ratio of the shaft to the main tunnel,the more air flow through the shaft,the greater the air volume of the main tunnel,and the higher the piston wind speed.(4)Based on the similarity theory,a double-shaft railway tunnel test device with a geometric scale of 1 / 50 for the smoke flow characteristics in a rescue station was built.An experimental study on the smoke flow law under the influence of shafts when a burning train stopped at the rescue station was carried out.The results show that the piston wind occupies the main position during the train running stage and the air flow in the tunnel is mainly driven by the train movement,which is not greatly affected by the fire source;after the train stops,the piston wind gradually attenuates to zero,the hot smoke at the fire source rises,and the cold air in each part of the tunnel flows to the fire;at the same time,due to the chimney effect of the shaft,the high-temperature smoke from the fire flows to the nearest shaft to form a reverse air flow,resulting in complex changes in the flow field in the tunnel.(5)Based on the dynamic grid technology,a numerical simulation study on the smoke flow characteristics under the influence of shafts when a burning train entered the rescue station with a fire power of 15 MW was carried out.The results show that the fire has a greater impact on the flow field in the tunnel after the train stops.At this time,the piston wind gradually attenuates.Under the influence of the shaft chimney effect,part of the smoke flows back to the upstream of the fire source and exits the tunnel through the shaft,and part of the smoke invades into the cross passage and enters the parallel heading.The temperature of the area where the smoke flows is high.When the fire source position of the train is different,the smoke spreading area will change accordingly,and the high-temperature area will also change.Personnel cannot escape from the cross passage with high smoke concentration and temperature,effective smoke control measures must be carried out to ensure the safety of personnel.(6)Using the combination of model test and numerical simulation,the smoke control effects of different schemes were compared,and a smoke control strategy for rescue stations in a double-shaft extra-long railway tunnel was proposed.The results show that compared to the vertical smoke control by shafts in the rescue station,the semi-transverse smoke control scheme that sets smoke exhaust-duct on the top of the rescue station is safer,and a reasonable plan for opening the connecting duct can improve the efficiency of smoke exhaust. |
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