Effect Of Subway Train On The Characteristics Of Back-Flowed Smoke In Longitudinal Ventilated Subway Tunnel

Given the catastrophic results of tunnel fire,it is full of significance to study the tunnel fire,especially the spreading characteristics of smoke gas in the subway tunnel,in terms of protecting passenger's life and reducing the property losses.Based on the summary of previous studies on tunnel fire at home and abroad,this paper investigates the influence of subway train with different length on the characteristics of back-flowed smoke gas in the longitudinal ventilated subway tunnel.The following are the detailed results:1.When the subway train stops in the tunnel due to catching fire,the longitudinal ventilation system will be started.The temperature distribution of back-flowed smoke gas will be greatly affected by the subway train,because the subway train is a big and long obstacle.Through the numerical simulation with FDS software,the influence of subway train with different length on the temperature attenuation of back-flowed smoke under the ceiling is studied.The simulation results show that there is an obvious turning point in the temperature profile of back-flowed smoke when the smoke back-layering length is over subway train length,which means the tendency of temperature distribution is different,because the decay factor of the dimensionless temperature rise of back-flowed smoke is bigger in the train blockage area than in the upstream empty area.On the other hand,the decay factor increases with the higher ventilation velocity,while the difference of this factor among cases with different train is small under the same longitudinal ventilation.2.By setting subway train with different length in the model tunnel,several small scale experiments are carried out to study the influence of subway train on the smoke back-layering length in the tunnel fire.The experimental results show that the subway train makes a great contribution to suppress the spreading of smoke gas,especially with longer length.The smoke back-layering falls into two categories:one is the back-flowed smoke cannot pass over the subway train and the other is the back-flowed smoke passes over the subway train and continue to spread in the upstream empty tunnel.Consequently,the increase rate of dimensionless back-layering length(l*)with the dimensionless variable of fire-ventilation(In(Q*1/3/V*))is different between in the train blockage tunnel and in the upstream empty tunnel.Due to the existence of subway train,the longitudinal ventilation velocity is larger in the blockage tunnel than in the upstream empty tunnel,resulting in the larger resistance in the blockage tunnel,which suppresses the spreading of back-flowed smoke more easily.Based on the theoretical analysis about the relationship between fire heat release rate and back-flowed smoke temperature,a kind of virtual fire source is proposed to describe the smoke back-layering.With this virtual fire source,a prediction model with the factor of subway train length is developed to predict the smoke back-layering length in the tunnel fire with subway train.3.The characteristics of longitudinal ventilation flow field will be affected greatly if the subway train stops in the tunnel and as a consequence,the critical longitudinal ventilation to prevent the smoke back-layering will be changed.A serial of numerical simulations are carried out to investigate the critical ventilation velocity of subway tunnel fire with train which has different length.The simulation results show that the critical ventilation velocity is mainly controlled by the fire heat release rate and blockage ratio.By comparing the characteristics of longitudinal ventilation velocity profile between in the tunnel with and without subway train,the existence of subway train leading to the larger ventilation velocity in the blockage tunnel,making the critical ventilation velocity decreased greatly;However,the subway train is a big and long blockage,the ventilation with higher velocity has become stable before reaching the fire source to suppress the smoke back-layering.That is why the critical ventilation velocity does not change obviously with the subway train length when the length is over 20 m.Based on the theoretical analysis,a prediction model is proposed for the situation where there is a subway train stopping in the subway tunnel.