Study On A Board-coupled Shaft Smoke Extraction Method And Its Influential Mechanisms

With the development of the society,more and more tunnels have been constructed to alleviate the increasingly severe traffic conditions.Due to the long-narrow characteristic of the tunnel buildings,once a fire breaks out,vast of hot and toxic gas will spread along the tunnel ceiling toward both ends of the tunnel.If the smoke cannot be discharged as soon as possible,the smoke will accumulate in the tunnel ceiling,which will seriously affect the fire rescue and evacuation.Therefore,it is necessary to install some smoke prevention and ventilation systems in the tunnel.There are two kinds of ventilation systems commonly used in road tunnels.One is the mechanical ventilation system,and the other is the natural ventilation system that equipped with a roof opening or vertical shaft.The mechanical ventilation system is widely criticized,for the reasons of susceptible to failure,the high installation and maintenance costs,and besides,the smoke layer will be disturbed during the smoke extraction process,making the rescue and evacuation more difficult.Therefore,natural ventilation system have eceived extensive attention from schoars at home and abroad due to its merits of stability,economic and less smoke layer disturbance etc.However.the natural ventilation system with vertical shaft also has disadvantages.The phenomenon of plug-holn and boundary layer separation often occrs during the smoke extraction process.and smoke exhaust efficiency with vertical shaft was found not high,which seriously restrict the development of the nature ventilation system.Therefore the development of a new type of smoke extraction method that can improve the smoke extraction performance with shaft,revealing its influencing mechanisms,and suggesting an optimal design scheme are important both of scientific significances and practical meaningfulness.Based on this,a board-coupled shaft(BCS)method is proposed in this paper.It reveals the effects of the board size.ventilation conditions,heat release rate.and the location of the fire source on the smoke extraction performance with BCS,and gives the corresponding optimization design parameters.The main works being conducted are as following:Firstly,a 1:15 small scale model tunnel was built,and experiments were conducted in the small scale tunnel to study the behavior of fire smoke flow along the tunnel ceiling.Through extensive literature research,it has been found that the types of fuels used in previous researches to study the ceiling jet properties are various,but the effects of fuel combustion characteristics on the smoke flow characteristics have not been considered before.In addition,the height of the ceiling was used as the characteristic height in the previous experimental analysis and model establishment,and the difference in the air supply pattern due to the structural difference between the restricted space and the unconstrained space was not considered.In this paper,the influences of fuel types on the temperature and velocity distribution of smoke flow in tunnel ceiling were studied.The results show that,the temperature distribution along the tunnel ceiling holds little relation to the fuel properties,while the velocity distribution along the tunnel ceiling shows highly dependence on fuel properties.Finally,new correlations to estimate the dimensionless velocity for different fuel types in the one-dimensional flow region along the tunnel ceiling were developed.Secondly,numerical simulations were conducted to study the smoke exhaust characteristics of traditional shaft by Fire Dynamic Simulator(FDS).By changing the value of the heat release rate and the longitudinal ventilation velocity,the effect of the heat release rate and the longitudinal ventilation on the smoke performance of the traditional shaft was investigated.The results show that with the increase of the fire size,the smoke extraction performance of traditional shaft will be improved.In addition.when there is longitudinal ventilation in the tunnel the plug-holing phenomenon w ill be suppressed,but at the same time,the boundary layer separation phenomenon will be strengthened.With the incease of the longitudinal ventilation velocity,the extraction capacity of traditional shaft shows a downward trend as a whole.However,when the longitudinal ventilation velocity is small,the extraction capacity of the traditional shaft is found than that without the longitudinal ventilation.Thirdly,numerical simulations were conducted to study the smoke exhaust characteristics of BCS.The smoke exhaust efficiency of a traditional vertical shaft was found insufficient because of the smoke layer separation and plug-holing phenomenon.Based on this,this paper propose the BCS.By installing a board at a certain distance below the shaft,the influence of plug-holiing on the smoke extraction performance can be restricted or eliminated and the smoke extraction efficiency can be improved.In addition,the effects of the heat release rate and the longitudinal velocity on the smoke extraction performance and the flow fields in the BCS are investigated.The experimental results verify the feasibility and superiority of the BCS.It can be found that the smoke extraction performance of the BCS increases with the increase of the fire size.Besides,the installation position of the board have great influence on the BCS extraction performance,and the BCS can achieve the best extraction performance as hD = 0.45 to the tested cases,which is about 1.55-1.61 times of' the traditional one.In addition,the BCS smoke exhaust capacity decreases as the longitudinal wind increases,when there is longitudinal wind in the tunnel.Fourthly,the effects of the board size,installation height,and the relative position between fire and shaft on the smoke extraction performance of the BCS were studied through the small scale tunnel model.With the assistance of the laser sheet,the flow fields in the tunnel and the shaft were visualised.By analyzing of the experimental results,it can be found that the board size and the board installation position(hD)have a great influence on the exhaust performance of the BCS.In addition,based on relevant theoretical analysis and experiments,the mechanism of the BCS was found out.The results indicate that the smoke extraction process in the BCS is controlled by the stack effect and the block effect.Therefore,when using the BCS,the board size,thickness,and installation position must be carefully selected.In addition,the results also show that with the increase of the distance between the fire and shaft,the smoke extraction performance of the BCS gradually decreases.Therefore,in practical applications.the effective distance of two adjacent shafts must be considered.Finally,based on the relevant assumptions and analysis,an analytical model for predicting the dimensionless volume flow rate of a shaft was established under a general condition that the smoke cannot extract the entirety of the smoke.And based on the relevant experimental data,an empirical model for predicting the dimensionless volume flow rate of a specific tunnel shaft was developed.