Study On Natural Ventilation Smoke Gas Characeristics And Vertical Shaft Length Of Subway Tunnel Fires

With the rapid development of urban subway construction in China,in order to improve the urban traffic conditions,subway tunnels are continuously built,and more and more tunnels adopt natural ventilation technology.This ventilation method can not only save the ventilation and ventilation energy of the tunnels in the normal operation of subway trains,but also saves investment in ventilation equipment.However,when a fire breaks out in a tunnel,it is especially important to take effective measures to ensure the safe evacuation of personnel.In this thesis,the model test and theoretical analysis method are used to study the fire smoke characteristics and smoke control in natural ventilation subway tunnel.The research results can be used to design smoke control system in the subway tunnel and tunnel disaster prevention and rescue strategy.Firstly,by comparing the tunnel fire similar simulation method,the Froude simulation method is determined as the basis of the tunnel model design.By considering the test site,operability and other factors,the scale ratio of the comprehensive literature is used to determine the scale ratio of the experimental design for the tunnel fire model,which is 1:20.And an experimental platform for the top natural ventilation shaft subway tunnel model was built.Due to the uncertainty of the tunnel fire,the location of different longitudinal fires in the natural ventilation tunnel was studied,the most unfavorable fire source location was determined,and the longitudinal smoke temperature rise distribution calculation formula under the top wall of the longitudinal fire source location was obtained.Next,in the case of the most worst fire source position,the model test study was carried out on the effects of heat source heat release rate(HRR),shaft lateral position,shaft aspect ratio,shaft spacing,and shaft size,and the longitudinal smoke temperature of the tunnel top wall was measured.The influence of the variation of the shaft parameters on the smoke gas temperature of the top wall of the natural ventilation shaft subway tunnel is analyzed.The results show that:(1)With the spread of smoke gas,the temperature of smoke gas at the top of the tunnel gradually decreases and increases with the increase of HRR.Due to the smoke exhausting function of the shaft,the vertical smoke gas temperature of the top wall of the tunnel drops sharply at the shaft.And the top wall temperature of the fire source section is much higher than the temperature of the non-fire source section.(2)The lateral position of the shaft,the aspect ratio of the shaft,the spacing of the shaft and the size of the shaft have no effect on the longitudinal smoke gas temperature in the fire source section,but will affect the smoke gas temperature of the non-fire source section,which means that the shaft is located in the middle of the tunnel.The longer the shaft spacing,the smaller the shaft aspect ratio.The longer or higher the shaft,the lower the temperature of the smoke gas in the non-fire source section.The exhaust smoke temperature in the shaft decreases with the increase of the shaft spacing,and increases first and then decreases with the increase of the shaft length,and increases slightly with the increase of the shaft height.Finally,preventing the smoke gas from passing through the shaft and the incoming air,and diffusion and sedimentation are more serious the non-fire source portion of the smoke gas.In addition,the smoke gas from the non-fire source section is returned from the bottom of the tunnel through the shaft to the fire section,causing the smoke in the fire source section to sink heavily.Under the condition that the width of the shaft is equal to the width of the tunnel,the high-temperature smoke gas generated by the fire can be drained from the shaft in time to ensure that the smoke gas does not cross the shaft,controlling the smoke gas between the two shafts,obtaining the critical shaft length.And the influence of factors such as HRR and shaft spacing on the critical shaft length is analyzed.And based on the mass flow of smoke gas plume,the mass flow of shaft exhaust and the theoretical model of the control body,a theoretical model for predicting the critical shaft length of natural ventilation subway tunnel fire is established.