Study On Characteristics Of Piston Wind And Its Effect On The Smoke Movement In Metro Tunnels

The metro system is advantageous in carriage capacity,speed,safety,punctuality,environmental protection,energy and land resource saving,etc.It is also very effective in promoting the urban development by mitigating the shortage of land resources and traffic congestion.Accompanied with the convenience in transportation,its environmental problems and safety hazards cannot be ignored.The piston wind induced by metro trains running in the tunnel has important influence on the tunnel ventilation.Systematic analysis of the piston wind evolution is of great significance to achieve the better usage use of piston wind.In addition,metro fires have become one of the major problems that threaten public safety due to the relatively sealing environment.When a metro train stops in a tunnel due to the fire,the influence of piston wind and train blockage on the fire smoke control cannot be ignored.The present thesis carried out a systematic study focusing on the characteristics of piston wind in the long metro tunnel without fires.Further,the influence of the piston wind on the smoke flow is revealed.Through theoretical analysis,evolution of piston wind in long metro tunnels with intermediate shafts was investigated.Influence of shaft on the piston wind characteristics was analyzed.Relationship among the piston wind speed,ventilation rate and shaft parameters is explored.Theoretical model of piston wind in long metro tunnel without intermediate shafts was then established,considering the single and double train tracking modes.The mono-factor analysis was utilized to study how the influencing factor like train departure interval,train speed,train length,tunnel length,blocking ratio,and wall friction coefficient affects the piston wind under the single or double train tracking mode.Thereafter,variation law of the piston wind speed,as well as the relationship among the maximum piston wind speed,variables related to the train and tunnel are discussed.The research shows that in a long metro tunnel the intermediate shaft has significant influence on the piston wind.The maximum speed of piston wind and ventilation rate in the long metro tunnel with the intermediate shaft are larger than those without the intermediate shaft.The piston wind characteristics in the tunnel,the airflow characteristics in the air shaft and the piston ventilation rate are closely to the shaft location and its cross-sectional area.However,shaft height was found to have no obvious effect on the piston wind.The piston wind speed in the long metro tunnel without intermediate shaft was found to periodically change accordingly with the train departure interval.When the parameters of the train and the tunnel are the same,the maximum stable piston wind speed caused by double train is 1.24 times that of a single train,and linearly increases with the train speed and length,follows an increasing power function with the blocking ratio,and decreasing power function with the tunnel length and frictional resistance coefficient.Based on the aforementioned research outcomes,a prediction model of the maximum stable piston wind speed was proposed.Theoretical analysis and numerical simulation calculation methods are conducted to study the effect of piston wind on the smoke spread in case of a fire.Based on the theoretical analysis results of piston wind in long metro tunnels without intermediate shaft,theoretical models of piston wind in both the single and double train tracking modes are established.Also,the dynamic change law of piston wind speed during tunnel fire is analyzed.The equivalent longitudinal wind speed was introduced to explain the interaction action between longitudinal ventilation and piston wind.Effect of dynamic piston wind on the spread of fire smoke was realized by using numerical simulation method.Furthermore,in the critical state wherein smoke has been effectively controlled,the critical driving force for eliminating the smoke backflow was studied.The pressure balance in the critical state is analyzed by using the one-dimensional network model.The critical wind speed and critical driving force under the influence of the train blocking effect and the fire source throttling effect are analyzed by numerical simulation.Prediction model of the critical driving force was then proposed.The research shows that the development law of the piston wind and its influence on the smoke flow under the fire condition exhibit very limited difference in the single and double train tracking modes.However,in the double train tracking mode,speed of the initial piston wind is found to decrease faster.When the direction of the piston wind is the same with the longitudinal ventilation,smoke control by the longitudinal ventilation is enhanced.On the contrary,longitudinal velocity calculated by the existing prediction model cannot effectively control the smoke in the piston wind action stage due to the opposite direction of piston wind.In such condition,longitudinal ventilation should be designed as the equivalent longitudinal critical wind speed.In the critical state of effective smoke control,the throttling effect of the fire will be enhanced by the blockage effect,resulting in the significant increase of the critical driving force and decrease of critical velocity.Furthermore,blockage effect is found to contribute more in affecting the driving force compared to the heat release rate.