Study On Smoke Movement Under Longitudinal Ventilation In Tunnel Fires

With the acceleration of urbanization and the rapid development of transportation industry in our country,various types of tunnels are constantly emerging.The development of tunnels not only brings convenience to people's lives,but also brings great challenges to fire safety.Although tunnel fire is a small probability event,once a tunnel fire occurs,fire fighting and smoke control will be very difficult,which will easily cause inestimable and irreparable economic losses and casualties.Relevant statistical results show that toxic and harmful smoke is the main cause of death in tunnel fires.Longitudinal ventilation is the most commonly used smoke control mode in the world.Therefore,the study of smoke movenment under longitudinal ventilation is of great significance to the design and control of tunnel ventilation system,fire rescue and personal evacuation.Considering the problems and shortcomings existing in the current researches and based on the main line of the influence of longitudinal ventilation on the smoke movement in tunnel fires,the following aspects from the perspective of different tunnel structure characteristics were studied in this paper:The influences of the longitudinal ventilation on the smoke flow direction in inclined tunnel fire,i.e.the re-direction of the smoke flow,were studied by using small-scale experiments and numerical simualtions.The re-direction of smoke flow in inclined tunnel fires refers to the phenomenon that the smoke flow direction suddenly changes due to the changes of thermal buoyancy or outside pressure or the activation of fans.This poses special risk for fire rescue services fighting fires in tunnels.A one-dimensional model was used to predict the flow velocity in the inclined tunnels,based on two different methods for calculating the mean smoke temperature.Results show that the methd based on the traditional one-dimensional model is very sensitive to the longitudinal ventilation,which could produce larger errors when the ventilation velocity is very low.The smoke flow direction could be well predicted by the improved method.When the ventilation velocity is relatively large and the flow tends to be one dimensional,both methods produce similar results.Further,based on the theoretical model,the influences of important factors on the re-direction of smoke flows were systematically analyzed,and some relevant suggestions on how to prevent Re-d phenomenon by using external positive pressure were given.The important influencing factors include heat release rate,tunnel slope,tunnel length,friction factor,tunnel cross sectional area and fire source location.The influences of longitudinal ventilation and the shaft settings on the characteristics of buoyancy-driven smoke flow in both the tunnel and vertical shafts,such as the smoke extraction rate of the shaft and gas temperature stratification in tunnel,were studied by using small-scale experiments.Considering the smoke layer depth,the pressure condition under the smoke laryer and the pressure loss at the shaft inlet,a theoretical model for the mass flow rate of buoyancy-driven smoke flow in the shaft was developed and validated,and the value of local pressure loss coefficient at shaft inlet was discussed.The results showed that the pressure loss coefficient at shaft inlet may not be fixed value.An average value of 1.0 could be found.This value can provide reference for engineering estimation and design of rectangular natural shaft.More shafts,greater shaft heights and greater shaft cross sectional area can significantly increase the smoke extraction rate,and the total smoke mass flow rate in the shafts increases with the increasing ventilation velocity the increase is relatively slow.Besides,the presence of vertical shaft is beneficial to the smoke stratification and could increase the height of the smoke layer interface,especially for the downstream of the shaft.The smoke stratification and smoke descent in the longitudinal direction in relatively long tunnel fires with and without longitudinal ventilation were studied by using theoretical analysis and numerical simulations.Based on the different assumptions,two theoretical models were developed to predict the smoke depth below ceiling along the tunnel without longitudinal ventilation.When considering the influences of the longitudinal ventilation,some key parameters in the models were modified.A series of numerical simulations of full-scale tunnel fires was conducted to validate and compare with the models developed.The appropriate entrainment coefficient under different ventilation conditions was determined.The results show that the temperature decay along the tunnel,air entrainment at the smoke layer interface and the pressure condition below the smoke layer are the main causes of the smoke descent.There may be obvious inconsistency between the temperature distribution and the smoke particle distribution in the tunnel,and the smoke layer height based on visibility distribution is more conservative.When there is longitudinal ventilation in the tunnel,according to the influence of longitudinal ventialtion on smoke stratification downstream of fire source,it can be divided into three stages or regions,namely,buoyancy dominant region,transition region and ventilation dominant region.Besides,based on the phenomenon of smoke descent in the longitudinal direction,the location where the smoke layer descends to the height that threatens the personnel safety evacuation has been analyzed and discussed in detail.Small-scale experiments were carried out to study the influence of strong longitudinal ventialtion on the smoke movement in a tunnel with relatively large crosssection,and the locations of the plume ceiling impact region,side wall impact region of the smoke and the convergence region of bifurcation smoke flow were analyzed.The results show that the smoke bifurcation flow and a low temperature region below tunnel ceiling will appear,when longitudinal ventilation exceeded a certain value.With the increase of ventilation velocity,the plume ceiling impact region,side wall impact region and the convergence region will continue to migrate downstream and the bifurcation phenomenon will be more obvious.The prediction models of the location with maximum temperature rise and bifurcation low temperature region length were finally proposed.