Study On Damage And Control Measures Of Geyser Of Baffle-Drop Shaft In Deep Tunnel System

With the continuous advancement of urbanization,the deep tunnel drainage system based on the concept of sponge city has become an important way to solve the waterlogging disaster and eliminate overflow pollution in big cities.Because of the large drop of the deep tunnel system,it is necessary to use a shaft to transport and dissipate the stormwater.The baffle-drop shaft is widely used because of its strong applicability and remarkable energy dissipation effect.Under normal discharge operation,storm water flows from top to bottom in the shaft.However,under the condition of heavy rainfall,rain water in the deep tunnel system fills rapidly,and in the process,a large amount of air is carried into the main tunnel to form high-pressure trapped air mass.When the trapped air mass reaches the downstream shaft,it is released rapidly and drives the water flow in the shaft to spew out,causing serious geyser damage.When the geyser is formed in the baffle-drop shaft,high-speed fluid may eject from the dry zone and lift the manhole cover,and may also break the baffles in the wet zone,causing structural damage,and seriously threatening the safety of life and property.Therefore,this paper carried out a hydraulic model test with length scale of 1:50 combined with numerical simulation to study and analyz the mechanism and harm of geyser in the baffle-drop shaft,and on this basis,a series of explorations on the control measures of geyser are carried out.The main research results are as follows:(1)By observing the phenomenon of geyser in the test,analyzing the change law of pressure at key positions and studying the void fraction and horizontal pipe diameter,the mechanism of geyser in the baffle-drop shaft is revealed that due to the "continuous" or "staged" release of the high pressure air mass,the fluid in the shaft is highly atomized,and the pressure difference at the bottom of the shaft is huge.Under the combined action of the two,the mixed fluid ejects out of the shaft from the dry zone and violently impacts the baffles in the wet zone.(2)Two different types of damages caused by geyser in the dry and wet zones of the baffle-drop shaft are identified,which are the traffic safety hidden danger caused by flying the manhole cover in the dry zone and the structural damage caused by breaking the baffle in the wet zone.It is pointed out that the key parameters to characterize the two damages are the geyser height and the impact load on baffles.The variation of the two parameters with the initial conditions of geyser such as inlet pressure,inlet volume and initial water depth was analyzed.Analysis shows that the geyser height is positively correlated with the inlet pressure and volume,and decreases first and then increases with the increase of initial water depth.When the initial water depth is about 1/4 of the shaft height,the geyser height will decrease to the lowest.The impact load is also positively correlated with the inlet pressure and volume,and has no obvious correlation with the initial water depth.However,when the free liquid surface in the shaft is " adjacent " to the baffle,the baffle will bear a large impact load.On this basis,the prediction models of the geyser height and the maximum impact load is established respectively.These models can be used to estimate the damage degree of geyser under different initial conditions.(3)On the basis of studying the mechanism and damage of geyser in baffle-drop shaft,the control measures of geyser are studied from the aspects of the contact tube access mode,the connecting area of the dry and wet zone,the orifice plate and the throttled vent pipe.The main conclusions are as follows: When the contact tube is connected to the wet zone,the baffle will have a smaller probability of bearing a great impact load.The impact load on the baffle can be weakened by increasing the connecting area of dry and wet zone,and the geyser height can be effectively reduced by adjusting the connecting area.The setting of orifice plate can greatly reduce the geyser height,but the opening area of orifice plate should not be too small to prevent the structural damage.The optimal setting position of orifice plate is the middle of the shaft.The setting of the throttled vent pipe can effectively inhibit the geyser and reduce the impact load,and the optimal setting position of the throttled vent pipe is about 2/3 of the length of the connecting pipe which is far from the end of the shaft.Furthermore,a comprehensive control scheme for geyser in baffle-drop shaft was proposed: the connecting pipe was connected to the wet zone,and the connecting area of the dry and wet zones was opened at the edge of the middle partition,and the orifice plate and throttled vent pipe were respectively set at 1/2 height of the shaft and 2/3 length of the connecting pipe(far from the end of the shaft).The above research results reveal the dynamic mechanism and define the damage types of geyser in the baffle-drop shaft.Then the corresponding prediction model is established to predict the damage degree under different initial conditions.Furthermore,a variety of control measures and schemes are proposed.The results can provide important technical support and theoretical guidance for the safe operation of the baffle-drop shaft.