Study On The Back-layering And Stratification Characteristics Of Smoke Layer Induced By The Carriage Fire In The Tunnel Under Longitudinal Ventilation

In recent years,with the rapid development of highway construction in China,a large number of tunnels have been built.While the tunnel is convenient for people to travel and promotes economic and social development,the tunnel is also accompanied by a large number of tunnel operation safety accidents,especially tunnel fire accidents which are easy to cause group death and group injury.The data show that most of those who died in the tunnel fires were killed by smoke.In the early stage of tunnel fire,how to effectively control the flow of smoke and maintain the stability of the smoke layer are of great significance to the evacuation of people in the tunnel.The “Thirteenth Five-Year Plan for Transportation Technology of the Ministry of Transport” focuses on the development of transportation system security and infrastructure,which are around the objectives of improving safety risk prevention and control,emergency response,and greatly reducing casualties and economic losses.Research on common key technologies in safety,transportation organization safety and emergency support,focusing on breaking key technical bottlenecks such as long tunnel operation safety and rail transit operation safety guarantee,continuously improving the safe production level of transportation and providing technical support for safe transportation.Most of the previous researches on tunnel fires were carried out based on the free fire sources.This paper focuses on the study of smoke flows and stratification stability induced by ejected carriage fire plumes in tunnel fire accidents.In the longitudinal ventilation tunnel,a series of small-scale experiments were carried out to study the critical ejected carriage fire plume under the longitudinal ventilation,the flame shape,the critical velocity when restraining the tunnel smoke reverse flow,the limited velocity,the buoyant flow stratified stability of the smoke in the ceiling and the temperature distribution of the smoke in the tunnel under the coupling of ceiling smoke extraction and longitudinal ventilation.The main conclusions are as follows:In this paper,a series of small-scale model tunnel experiments are carried out to study the critical overflow evolution characteristics of ejected carriage fire plume in the longitudinal ventilation tunnel.The study can be found that,when the carriage opening is against the wind,the critical ejected fire heat release rate in the tunnel is only related to the ventilation factor of the openings and not related to the longitudinal wind speed.Comparing the upper and lower critical ejected fire heat release rate with the theoretical ejected fire heat release rate in the open space,it is found that the critical ejected fire heat release rate from the carriage in the tunnel is larger than the theoretical value of ejected fire heat release rate.When the ejected carriage fire is further developed into continuous combustion outside the carriage,the evolution characteristics of the flame shape along with the longitudinal wind speed are studied,and the prediction model of the flame height and the horizontal length are proposed.The prediction model is in good agreement with the experimental value.The critical velocity and smoke back-layering flow length induced by the ejected carriage fire plume in the tunnel are studied.It is found that when the fire heat release rate is small,the critical velocity increases with the increasing the fire heat release rate.When the fire heat release rate continues to increase to a certain value,the critical velocity no longer changes,which is also related to the results of previous studies.For the smoke back-layering flow length,when the longitudinal wind speed is small,the smoke back-layering flow length is more sensitive to the longitudinal wind speed,and the speed decreases faster as the wind speed increases,and when the smoke back-layering flow length decreases to near the fire source position,it is no longer sensitive to changes in wind speed.The wind speed around this turning point is called the limited velocity.Finally,the critical velocity,smoke back-layering flow length and limited velocity model induced by the ejected carriage fire plume are established.The buoyant smoke stratification characteristics induced by ejected carriage fire plume are studied.The experimental laser sheet light source is used to shoot the smoke layer in the tunnel,then the smoke flow stratification characteristics is captured and analyzed.It also be correlated by using the Fr number based on the velocity shear and temperature vertical temperature distribution.Theoretically,it characterizes the buoyant flow stratification characteristics of smoke layer induced by the ejected carriage fire plume.The evolution characteristics of the temperature distribution of the ceiling smoke under the coupling of longitudinal wind and ceiling smoke extraction are studied.It is found that with the increase of longitudinal wind speed,the upstream smoke temperature of the tunnel ceiling attenuated faster than the downstream,and the ceiling smoke extraction has a greater impact on the upstream smoke temperature of the tunnel.The prediction model of the upstream and downstream smoke temperature decay factors under the coupling of longitudinal wind and ceiling smoke extraction is proposed.