Study On Characteristics Of Smoke Movement And Smoke Exhaustion In The Subway Tunnel With Metro Train

The subway transportation system has the advantages of not occupying ground space,big passenger capacity,high speed,energy-saving and environmental protection.It has become the public transportation system that many significant cities choose for relevant development.However,fire safety has always been an essential issue that has attracted widespread attention to the subway transportation system.This thesis mainly focused on the fire scenario of a subway train parked in a subway tunnel when a fire occurs and the propagation characteristics of the smoke in the subway tunnel under the influence of a subway train.The contents mainly include the smoke temperature distribution under the tunnel ceiling,the smoke back-layering length,the thermal environment of the downstream tunnel,together with the influences of subway trains on them.In addition,the thesis also analyzed the effects of several common ventilations and smoke exhaustion modes in the subway tunnel and explored their feasibility.The main conclusions are listed as follows:1.The effects of a subway train carriage on the tunnel ceiling smoke temperature were investigated numerically when the train was stopped in a longitudinal ventilated tunnel fire.The numerical results showed that the existence of subway trains leads to the ceiling temperatures no longer being symmetrically distributed in the upstream and downstream tunnel with the centralized fire source.And the ceiling smoke temperature of the fire in the upstream tunnel decays to room temperature faster than those of the downstream.It was indicated that subway trains could effectively block heat from spreading upstream,restraining the fire from spreading upstream.A dimensionless analysis of the smoke temperature under the tunnel ceiling in the downstream tunnel showed that its distribution along the longitudinal direction of the tunnel presents an exponential attenuation law and the attenuation index increases with the increase of the longitudinal ventilation velocity.The influence of the subway train length on the temperature distribution was relatively small.2.For the thermal environment downstream of the fire source,the presence of subway trains strengthens the inter-mixing of longitudinal ventilation and fire plume.The intensity of the inter-mixing effect increases with the increase of longitudinal ventilation velocity.The smoke concentration in the downstream space of the fire source increases,and the visible distance decreases rapidly at the height of 2 m.Under the same longitudinal ventilation velocity,the presence of subway trains also raises the temperature significantly in the downstream space.Under the dilution effect of a large amount of ventilation,the concentration of toxic gas CO showed a lower concentration than when there was no subway train.3.The influences of the subway train length on the smoke back-layering length in the upstream tunnel were studied based on a reduced-scale subway section tunnel model.The experimental data analysis found that the suppression effect of subway train length on the hot smoke back-flowing process shows a solid relationship with the condition of whether the hot smoke can pass the subway train to back flow in the upstream tunnel.When the length of hot smoke back-layering is longer than the length of a subway train,the smoke back-layering length rapidly decreases with the increase of the train length.Under such conditions,the suppression effect is obvious.However,when the hot smoke back-layering length is less than the length of the subway train,the smoke back-layering length almost does not change with the subway train length,and the suppression effect is relatively weak under this condition.Considering the influences of subway trains in the interval tunnel,a dimensionless smoke back-layering length prediction model was also developed.4.Through a comparative analysis on the effect of six different forms of ventilation and smoke exhaust methods by numerical simulation,it was found that the smoke control effect of pure longitudinal ventilation is relatively poor because it does not exhaust the fire smoke but blows it to the downstream tunnel.This shows an obvious adverse influence on personnel evacuation,firefighting and rescue.For the fully transverse ventilation,semi-transverse ventilation style combining specific point type smoke exhaust and the longitudinal ventilation combining points type smoke exhaust,their smoke control effects are very good.While for the combination of longitudinal ventilation and smoke extraction in a single point,the smoke control effect is not as good as those of the above several ways.They are still better than those of the traditional longitudinal ventilation.5.With different smoke exhaust methods,the impacts of subway trains are almost the same.All of them reduce the effective ventilation area in the tunnel through the blocking effect to increase the longitudinal air flow speed in the blocked area of the tunnel.Still,the effects are different for smoke control and exhaustion.In terms of the simple longitudinal ventilation and the combination of longitudinal ventilation and single-point smoke exhaust,subway trains are not conducive to the control of smoke.It is beneficial for the hot smoke control in full-horizontal,semi-horizontal and specific multi-point smoke exhaust model.In the combination of longitudinal ventilation and multi-point smoke exhaust,the role of subway trains is also related to the opening position of the smoke exhaust vent.Combining the fixed-type and portable type devices greatly reduces the temperature downstream of the fire source,and the addition of the mobile fan further reduces the temperature.Increasing the air volume of the mobile fan,the difference in the smoke exhaustion effect under different mobile fan inclination angles gradually appears,and the smoke exhaustion effect is gradually enhanced.When applying the mobile fans in series to exhaust the smoke,increasing the air volume of mobile fans showed a more significant impact on reducing the temperature in the tunnel,compared to increasing the number of mobile fans.