Smoke Movement And Mechanical Smoke Exhaust Mode Of Subway Platform Fire

At present, the construction of metro rail transit has entered a period of high-speeddevelopment.With the increasing of the subway lines, there are more and more transferstations, space staggered subway station has been widely applied for its desirablefeatures of simple in structure and convenience of exchange. Because of the specialstructure and the large passenger flow of the transfer station, it is easy to causetremendous life damage and property losses in the event of fire. Statistics indicates thatthe main cause of the casualties in the subway fire is the smoke,which inhaling toomuch will cause people in fire unable to escape and result in the suffocation deaths.Therefore, it is of important significance to study the smoke flow regularity in subwaystation and smoke exhaust mode of the space staggered subway station.A local model of the subway platform with a ratio of1:10was put up on thebasis of existing models in laboratory and experiments were conducted to study the theeffects of fire intensities and mechanical extraction volume on the temperature andheight of the smoke in different fire positions. By taking Administrative Center stationof Xian metro line2as the research object, full-scale numerical simulation werecarried out by use of the Fire Dynamics Simulator to investigate the smoke movementregulation and the distribution of temperature, concentration of CO, smoke density andvisibility in different smoke control zone in space staggered subway station, inaddition, fire spread characteristic with different smoke exhaust mode, the optimumschemes of smoke control and extraction system is chosen according to the judgement criterion for the security of the life and safety of persons.According to similarity theory,1:10parameter ratios and scale of subway platformare deduced and experimental model with a ratio of1:10is established, based on whicha lot of experimental investigations are carried out, the results show:(1) When a fire occurs at the end of subway platform,only the temperature nearthe fire origin at the closed end is higher than that of the open end, while temperature atother regions are almost consistent in two ends.(2) A lot of high-temperature smoke will be produced when the subway platform ison fire. The smoke show significant temperature stratification, the temperature of toplocated smoke is higher than that of the smoke below, based on situation of the firesource burning, the variation of temperature with time can be divided into threestages: a) growing phase during the fire source is burning; b) stable burningphase; c) the fire source burning weakening Phase.(3) The increase of the mechanical exhaust volume can not only reduce thetemperature inside the station effectively, but also slow down the the descent of thesmoke layer. When the smoke exhaust volume increases to a certain degree, the smokecan be maintained at a certain height and a dynamic equilibrium will be formed.By use of the Fire Dynamics Simulator software, a full-scale numerical model ofthe space staggered subway station was established, through visualization analysis ofvarious fire conditions, the following conclusions can be obtained:(1) When the smoke curtain is applied, it will form a smoke reservoir in the smokecontrol zone where the fire happens，which will on the one hand delay smoke spread,but on the other hand lead to a increase of the smoke temperature at the ceiling inthat smoke control zone.(2) When the fire breaks out at the smoke control zone in atrium of the islandplatform, the dome has great significance for controllingthe velocity of smoke spread, itcan delay the diffusion of the smoke to other smoke control zones, and the time that thesmoke spread out the dome increases exponentially with the height of dome.moreover, the smoke can’t diffuse to underground three-layered side-platform, whichwill provide a new way for personnel evacuation.(3) When the end of the underground two-layered island platform is on fire, use Platform screen doors for auxiliary smoke exhaust and close thermal exhaust port atrail base, only by opening thermal exhaust port at rail top for smoke exhaust can keepthe smoke within the smoke control zone and ensure environmental condition within2msafety height can meet personnel Safety Standards in360s after the fire take place.(4) When the fire occurs at underground three-layered side-platform, thebuoyancy-driven smoke moved to ceiling and then diffuse in vertical derection alongthe ceiling, once the smoke hit the side wall backflow will be fomed, when smoke layerheight will descend and it will desend below the safety height at240s after the firehappens.(5) Temperature at the ceiling will decrease with its height above the fire sourceincrease and it obey exponential damping law, smoke show no vertical temperaturestratification at the end far away from the fire source.(6) When the side-platform catches fire, both the temperature and concentration ofco at the stairs’ access will increase as the height become higher and higher, but theincreasing rules of them are different, the temperature increases exponentially with theheight while the concentration of co increases linearly with the hight.(7) Using the Platform screen doors as auxiliary smoke exhaust can keep the smokewithin the underground three-layered Side-Platform, while there will be dead angles atthe two ends of the station, where the visibility will be less than10m.(8) Even the platform screen doors are opened for smoke exhaust, it will also existdead space in the situation of reverse buoyancy wall jet for the platform is long-narrowand the height is lower, by opening the fire doors at two ends of the platform and usingtunnel to exhaust smoke, the problem of dead angle can be avoided effectively,meanwhile the visibility at two ends of the platform will thereby increase.The research in this paper on the one hand provide reference for related research onsubway fire, on the other hand, it will also provide a certain amount of theoretical andexperimental basis for the smoke control design of space staggered subway station andthe safety evacuation of crowds.