| The vertical shaft natural smoke extraction has been widely used in shallow buried urban tunnels for its economic,environmental and practical characteristics.The smoke exhaust effect of shallow buried tunnel with vertical shaft is greatly affected by the crosssectional area of vertical shafts,the spacing between vertical shafts,the vertical length of vertical shafts and the grouping of vertical shafts.Taking Wuhan East Lake tunnel as the physical prototype,a full-size tunnel model structure was established by dividing the vertical shaft group,double vertical shafts and single vertical shafts,so as to explore the influence of vertical shaft arrangement on the natural smoke spreading characteristics.The main research work and results were as follows:(1)The reasonable number,length and proper spacing of vertical shafts within the group were determined.Analyzing the thickness of flue gas layer,studied the influence of longitudinal layout of vertical shafts on the natural smoke exhaust effect in shallow buried urban tunnels by numerical simulation.Analyzed the influences of vertical distribution parameters such as vertical shaft length,vertical shaft spacing and number of vertical shafts on natural smoke exhaust characteristics of tunnels under the fixed the total crosssectional area and the total length of vertical shafts.The results show that the shaft in the group will occur plug-holing when the ratio of the shaft length to the shaft group length ls/lg is over 1/6 and the shaft in the group would occur boundary layer separation when ls/lg is over 1/3.A large area of a single vertical shaft is divided into several small areas to form a group of vertical shafts,so as to achieve a better smoke exhaust effect.In addition,when there are 3 shafts in a group of vertical shafts,the length of a single vertical shaft is 2 m and the spacing of vertical shafts in a group of vertical shafts is 6m,the natural smoke exhaust effect of vertical shafts is best one.(2)It is was determined for the relationship between the smoke settling point and the height of the tunnel.The critical spacing between the smoke spreading distance and the shortest vertical shaft was obtained.This paper studied the problem of smoke settling point when the smoke freely spread in the natural smoke exhaust tunnel and obtained the relationship between the smoke settlement position and the tunnel height by numerical simulation.Based on this,changed the vertical shafts spacing.The results show that as the distance between the shafts increases,the length of back-laying smoke flow tunnel decreases first and then increases with the increase of the shaft spacing.It was found the length of back-laying smoke flow was the shortest under this vertical shaft spacing when a vertical shaft was added at the settlement point where the smoke flowed freely.Then,make the critical shaft spacing a point.(3)A model was built for predicting the dimensionless smoke back-layering flow length in the fire source upstream of the natural smoke exhaust tunnel.The formula of smoke back-layering flow in shaft smoke exhaustion was constructed by dimension analysis.Analyzes influence of the heat release rate(HRR),the longitudinal ventilation velocity,the distance between shaft and fire and the width and shaft height on the upstream smoke back-layering flow of fire source quantitatively.The results show that smoke backlayering flow length upstream the fire increases with the HRR and the distance between the shaft and the fire source,but the length tends to be stable when they exceeded certain value which the dimensionless HRR exceeds 0.15 or the spacing between dimensionless shaft and fire source exceeds 13.2.The smoke back-layering flow length decreases with the increase of tunnel longitudinal wind velocity and shaft width and it is also not affected by the change with shaft height. |
