| With the intensification of the contradiction between urbanization and land,the construction of underground transportation systems has continued to develop.The tunnel with vertical shaft makes important contributions to promoting economic development,alleviating urban traffic pressure,and improving transportation capacity,but also brings new challenges to fire prevention.Therefore,it is of great significance to study the smoke flow characteristics and smoke control of tunnel fires to ensure personnel safety.This article takes the fire smoke prevention and control of the tunnel with vertical shaft as the research background,and the the thermal stratification and movement mechanism of fire-induced smoke in tunnel with vertical shaft as the research content.Investigated the structure of domestic highway tunnels and Chongqing Rail Transit Line 6 tunnels.Theoretical analysis,reduced-scale tunnel model experiments,and FDS numerical simulations were used to study the thermal stratification and movement mechanism of fire-induced smoke in tunnel with vertical shaft.The detailed study mainly has the following conclusions:First,a scale-down experiment was conducted to study the thermal stratification of smoke in the tunnel with vertical shaft.The changes of stratification curve,T/Taver=1 and stratification curve were explored.The stratification curve can better describe the steady state of the fire smoke in various areas in the tunnel,that is:the overlap of the stratification curves indicates that the smoke is layered and stable;the non-coincidence of the stratification curves(clockwise rotation)indicates the loss of smoke.Stability,the stratification is damaged.The intersection of T/Taver=1 and the stratification curve can be used to determine the temperature of the smoke layer.That is:the temperature rises faster and the intersection moves to the left;if the temperature decreases faster,the intersection moves to the right.The influence of shaft height on smoke layering in the tunnel was explored.The smoke thermal stratification has the characteristics of segmentation in the shaft ventilation and exhaust mode.From the vicinity of the fire source to the inside of the shaft(0<L/D<0.54),the stratification strength(Is)is stable,and the flue gas thermal stratification is not affected by the change in the height of the shaft;outside the shaft(0.54<L/D<1),the smoke stratification strength increases with the height of the shaft The lowering will seriously affect the thermal stratification of the smoke.It is suggested that the height of the shaft should not be too high in actual engineering design.Second,a full-scale numerical simulation was carried out to study the effect of longitudinal ventilation on the smoke movement of the tunnel with vertical shaft.The plug-holing phenomenon is affected by the longitudinal wind and the height of the shaft,which affects the discharge of smoke from the shaft.When the longitudinal ventilation is relatively small,as the height of the shaft increases,the chimney effect is strengthened,and the plug-holing phenomenon is more obvious.When the longitudinal ventilation is relatively large,smoke can be prevented from passing through,and the exhaust efficiency is increased.The stable and unstable zones of the exhaust gas mass flow of the shaft are divided.When 0.33<v*<0.67,the gas mass flow rate slightly decreases with the increase of the longitudinal ventilation,and remains relatively stable.When v*>0.67,the gas mass flow rate decreases exponentially with the increase of the longitudinal ventilation,showing instability.Suggestions for maximum smoke extraction efficiency.At a certain longitudinal wind speed,the mass flow of exhaust gas from the shaft is relatively stable,and the occurrence of plug-holing penetration can be effectively avoided,so that more smoke flows into the shaft to be quickly discharged.Considering the efficiency of smoke extraction,this paper proposes a dimensionless longitudinal ventilation of 0.67.In addition,based on this research,the higher the shaft height,the more serious the plug-holing penetration phenomenon,and it will also cause a huge waste of shaft space.Therefore,for banning trucks from passing through urban tunnels,and considering both gas mass flow and smoke exhaust efficiency,it is recommended that the shaft height be between 3-4m. |
