Numerical Simulation Study On Smoke Exhaust Effect Of Subway Tunnel Under Multi-source Fires

Metro industry has rapidly developed and been widely applied in our country during the past few years.As the subway brings convenience for citizens,it also makes a series of urban security issues.Flood,fire,collapse and other disasters may occur in subway,and the most serious disaster is fire.Toxic smoke is the main cause of death.The subway station is easy to become the target of terrorist attacks because of its dense and relatively closed space.There are many hidden dangers of multi-source fires.Scholars have done a lot of research on the smoke exhaust,but they mainly focused on the single point fire scene.Therefore,this paper focused on the smoke exhaust mode of subway under multi-source firesThis paper took Ningbo subway station as an example and assumeed that there are only two fires at the same time,and utilized the FDS software to simulate the law of smoke movement under muti-source fires in different locations(platformplatform、platform-track area、track area-track area).Taking Ningbo subway station for instance,and assuming that there are only two fires at the same time.This paper studied the influence of the safety evacuation of passengers with the linkage between metro barrier doors and tunnel fans by analyzing the distribution of temperature field,velocity field,visibility,CO concentration and other parameters of the safety level of the evacuation line.Finally,the optimal smoke exhaust mode was determined.In order to determine the optimal smoke-exhaust mode under different fire scenarios,some parameters such as temperature field,velocity field,visibility and CO concentration were selected to analyze the smoke-exhaust effect.The simulation results show that it is best to open shielding doors around fires and use tunnel fans as an auxliary way to exhaust smoke under fires both on platform,open shielding doors around fires and use tunnel fans to exhaust smoke under fires in platform and track area,close shielding doors near non fire side,and use tunnel fans as an auxliary way to exhaust smoke.It can be guaranteed that the temperature is below 60 ℃,the visibility on evacuation path is more than 10 m,the CO concentration is less than 225 ppm,the wind speed at the stairs entrance is more than 1.5m/s with the optimal mechanical smoke-exhaust mode at 360 s after the fire happened.In addition to the fire on the platform and track area,the tunnel will also cause fire due to the cable or train failure.First of all,the single fire scene has been studied,and it is found that the counter current length is inversely proportional to the longitudinal velocity,and decreases when the longitudinal velocity raises.At the same time,we found that the smaller the longitudinal velocity is,the larger the increase of the counter current distance is.The counter current length increases with the increase of fire power,but the increasing rate becomes smaller with the increase of fire power.When the ignition power is more than 12.5MW,the counter current length of flue gas is not obvious as the increase of the power.Finally,the prediction formula of flue gas counter current length of is fitted according to the simulation data in Origin software.The further analysis for the phenomenon of flue gas counter current in multi-source fires is based on the research process for single-source fire.It is found that the larger the distance between double-source fires is,the shorter the counter current length of flue gas is.The relationship between dimensionless flue gas counter current length(d*)and distance influence coefficient(β)is close to power function.It is found that the larger the double-source fires power is,the longer the counter current length of flue gas is.The relationship between proportional coefficient(k)and fire power influence coefficient(α)is close to logarithmic function.The formula for predicting the counter current length of flue gas with double-source fires can be obtained by modifing single-source fire formula with two parameters(β and α).The prediction results are more accurate by comparing the predicted values and the simulated values.