Study On The Smoke Flow Characteristics Of The Shaft’s Natural Ventilation In The Tunnel With One Blocked End

In the planning,design and construction of mountainous cities,due to the limitation of ground space,various forms of blocking tunnels at one end have gradually appeared.When a fire occurs,the smoke from a blocked tunnel at one end is more likely to accumulate and is not easy to discharge.Existing codes do not have clearly regulations on ventilation and smoke exhaust of such blocking tunnels with one closed portal.Therefore,the tunnel with one blocked end lacks reference basis in engineering design,construction,acceptance and operation.Due to the simple and efficient features,natural ventilation and smoke exhaust has been gradually applied in tunnel engineering in recent years.Existing natural ventilation and smoke exhaust in tunnels mainly use three methods: entrances and exits,top openings and shafts.The paper uses small-scale model experiments and FDS(Fire Dynamics Simulation)numerical simulation methods to study the shaft’s natural ventilation smoke flow characteristics and smoke exhaust effects of a tunnel with one blocked end,which can provide a theoretical basis for the design of natural ventilation smoke exhaust in a tunnel blocked at one end.According to Froude’s similar principle,a small-scale tunnel model experiment platform at 1:15 with one blocked end was built.The experimental plan was designed by changing the position of the fire source,the heat release rate of the fire source,and the longitudinal position,cross-sectional diameter and height parameters of the shaft.Then carried out experimental test to study the smoke’s temperature change law in the tunnel ceiling and shaft and the effect of ventilation and smoke exhaust.The research results show that: the temperature of the smoke near the fire source is only related to the heat release rate of the fire source.Compared with the open tunnel at both ends,the smoke’s temperature from the ceiling of the blocked tunnel at one end is higher,and the longitudinal temperature decays more slowly;The tunnel ceiling smoke’s temperature distribution law is different between the zone from the blocked end to the shaft and the zone from the shaft to the open end.Shaft parameters affect the tunnel ceiling smoke’s temperature distribution and the smoke exhaust effect of natural ventilation.Increasing the diameter or height of the shaft can significantly reduce the temperature of the tunnel ceiling far away from the fire source area after crossing the shaft.Under the same heat release rate of the fire source,the maximum temperature rise of the tunnel ceiling smokeis mainly related to the relative position of the fire source and the shaft.When the fire source is located between the blocked end and the shaft,the maximum temperature rise of the tunnel ceiling smoke can be calculated using the Ji prediction model;when the fire source is located between the shaft and the open end,a modified prediction model based on the Alpert model is proposed.to predict the tunnelceiling smoke’s maximum temperature rise.The numerical simulation software FDS is used to establish a full-scale one-end blocked tunnel model with a shaft.By changing the fire source position,the heat release rate of the fire source and the shaft height,a simulation scheme is designed to simulate and analyze the law of smoke flow in the tunnel.It can be effective and natural.Prediction model of critical shaft length for smoke exhaust is proposed.When the fire source is located between the blocked end and the shaft,the smoke discharge of the shaft is significantly greater than that of other fire sources;when the fire source is located between theblocked end and the shaft,the critical shaft length is the largest.When the heat release rate of the fire source is between 2.5MW-7.5MW,the critical shaft length decreases with the increase of the height of the shaft,but it has nothing to do with the heat release rate of the fire source.