Spread Characteristics Study On The Fire Smoke Back-layering Under The Synergy Between Ceiling Exhaust And Longitudinal Ventilation In The Tunnel

Along with the growth of the vehicle holdings in China,the traffic flow in tunnel is also increasing.In order to alleviate traffic pressure,a large number of road tunnel are built in recent years.Although tunnel fire rare occurs,it would lead to catastrophic results once it happens and gets out of control.It is the high temperature and poisonous smoke due to incomplete combustion cause the most death in tunnel fire,according to statistics.Thus,it is significant to figure out the how to control the fire smoke in tunnel.To stop the fire smoke from propagating and to protect the occupants downstream from the fire,the approach that combines the longitudinal ventilation and ceiling extraction is applied in many tunnels now in China.The propagation of fire smoke is much complicated than that under the sole longitudinal ventilation,because of being affected by forces both in vertical and horizontal.The back-layering length of the fire smoke than becomes an important factor when determines the ventilation system.In other words,it gives both economic and safety parameters(e.g.the interval of the exhaust outlet)of ventilation system with considering the back-layering length for expected fire load.However,it is rarely found the work about the back-layering length of the tunnel fire under the combination of longitudinal ventilation and ceiling extraction.Thus,this thesis focuses on this problem.It intends to describe the spreading progress of fire smoke mathematically,so that some methods could be proposed to support the practical tunnel designing.Firstly,a global prediction model of the back-layering length in the tunnel fire under the combination ventilation system is deduced.It is based on the process of fire smoke spreading,considering the fundamental principles of mass and energy conservation.It is noteworthy that this theoretical model applies to the tunnel fire either under the longitudinal ventilation acts alone or under the longitudinal ventilation operates with the ceiling extraction(the exhaust outlet locates upstream,downstream and above the fire source,respectively).Secondly,serials of reduced-scale(the scale is 1/3)experiments are conducted;the results shows that the ratio of the mass flow rate of upstream smoke and total mass flow rate of smoke above the fire source changes with the longitudinal velocity: the ratio decreases with increasing of longitudinal velocity once the velocity reaches certain value.Thirdly,the model is verified by comparing to the available experimental data and predictions by previous models,after introducing the correlation obtained in the reduced-scale model experiments.Finally,a series of predictions of back-layering length are calculated with current model,to demonstrate the effects of factors e.g.longitudinal velocity,exhaust velocity and the position of exhaust outlet,on the back-layering length.The results shows that the back-layering length drops when the longitudinal velocity grows;the ceiling extraction does reduce the back-layering length but it depends on the position of the exhaust outlet to the fire source;and a calculation for the exhaust outlet interval of an existed tunnel is performed with considering the basis.