Study On Local Loss Characteristics And Ventilation Control Mode Of Tunnel With Shaft Bifurcation

China’s urban traffic is seriously overloaded and land resources are tight.Tunnel construction has become an important measure to alleviate traffic pressure.In order to increase the working face during long tunnel construction,the shaft can be used as a ventilation and sewage channel after the tunnel is connected.In addition,in order to connect the surrounding traffic,urban tunnels often set inlet and outlet ramps along the line,so as to form a bifurcation structure to realize the multi-level diversion of traffic.As the tunnel is located in the center of the city,it faces more severe environmental constraints than the highway tunnel.It is necessary to simultaneously consider the requirements of the environment in and around the tunnel.Therefore,it is necessary to finely control the air flow of each branch of the shaft and tunnel to realize the fixedpoint and quantitative discharge of pollutants.In this paper,the ventilation and sewage system of tunnel with shaft bifurcation is studied:(1)By building a 1 / 12.7-scale parallel bifurcation tunnel ventilation test platform that simultaneously meets the similar resistance and pressure,combined with CFD simulation,the flow characteristics and local resistance coefficient of the parallel diversion tunnel are studied,and the influence of diversion structure,deceleration lane and other tunnel bifurcation form characteristics on the local resistance coefficient is further analyzed.The results show that the local resistance coefficient of main line and ramp decreases first and then increases with the increase of diversion ratio.Compared with the directional diversion tunnel,the local resistance coefficient of the main line is not much different in the parallel diversion tunnel.When the diversion ratio is small,the local resistance of the ramp of the parallel diversion tunnel is larger,and the opposite is true when the diversion ratio is large.Adding deceleration lanes will reduce the local resistance coefficient of the ramp when the diversion ratio is small,but will increase the local resistance coefficient of the main line.When the diversion ratio is large,the opposite is true.(2)A three-dimensional CFD model of parallel diverging tunnel is established to study the flow characteristics and local resistance coefficient of parallel converging tunnel.The results show that the local resistance coefficient of the main line decreases and the local resistance coefficient of the ramp increases with the increase of the merging ratio in the parallel merging tunnel.The mechanism of local loss in parallel bifurcated tunnel is explored,and the prediction formula of local loss coefficient in parallel bifurcated tunnel is established.(3)CFD simulation is used to analyze the variation of flow field in shaft confluence tunnel.The research shows that in the shaft confluence tunnel,the local resistance coefficient from the two main tunnels to the shaft decreases with the increase of confluence ratio.Compared with the square shaft,shortening the transverse or longitudinal length of the shaft will increase the local resistance coefficient.When the shaft aspect ratio is 1,with the increase of shaft area,the local resistance coefficient from the two main tunnels to the shaft first decreases and then increases.Explore the mechanism of local loss in shaft confluence,and establish the prediction formula of the relationship between the size,confluence ratio and local loss coefficient.Finally,the influence of vehicle motion on the flow field of shaft tunnel is analyzed.(4)Relying on the river crossing tunnel project of Gen Shandong Road,combined with the prediction formula of local loss coefficient proposed in(1)and(2),and based on the pressure balance formula,the optimal operation ventilation control scheme of the tunnel under different speed conditions and meeting the requirements of internal and external environmental standards and minimum energy consumption is calculated.