Experimental Study On Hydraulic Characteristics Of Swirl Shaft Spillway With Different Inlet Shapes

Water conservancy and hydropower projects have been developed vigorously and rapidly in China.A great number of large and super hydropower stations are mostly built or to be built which located in deep mountain valleys,with narrow river valleys and large flood discharge gaps.In order to ensure the efficient and stable operation of water conservancy projects,it is necessary to fully consider the problem of water discharge safety,so the energy dissipation requirements for water discharge structures are relatively important.As a new type of internal energy dissipator,the swirling shaft spillway has the advantages of good hydraulic characteristics,high energy dissipation rate,and flexible layout.The body structure of the swirling shaft spillway directly affects the flow pattern,energy dissipation effect,and the safety of the side wall.Based on the hydraulic model test and the controlled variable method,the different connecting body types of the diversion channel and the vortex chamber,the change in the depth of the stilling well,and the influence of the flow change on the pressure characteristics,flow pattern,and velocity distribution of the vortex chamber and the still well are studied systematically.The main research content and results are as follows:The flow patterns for different flow rates and different angles of the baffle sill,and the changes of the water surface lines are analyzed.The angle of the baffle sill at the entrance of the vortex chamber and the flow affect each other,and the flow patterns that may appear in the vortex chamber are divided into four situations:Jet type,mixed type,spiral flow,submerged type.The water flow in the shaft section can be divided into three areas from top to bottom,namely the cavity area,the water cushion area and the hydraulic jump area.With the increase of the flow rate,the depth of pure water(with no air bubbles)at the bottom of the stilling well gradually decreases.Under working conditions,the entire stilling well is a two-phase flow of water and gas,which is like a milky white foam.The water surface line of the elliptical arc segment increases with the increase of the angle of the baffle sill,and the water surface line in the vortex chamber is smoother and more stable than that of the non-baffle sill.The flow velocity of the elliptical arc decreases with the increase of the angle of the baffle sill.The flow velocity of different layers inside the water flow has a shearing effect.The larger the angle of the baffle sill is,The more obvious the shearing effect.The pressure characteristics(both the time-average pressure and the fluctuating pressure)of the flow on the elliptical arc section,on the side wall of the vortex chamber and on the bottom plate of the stilling well are analyzed.There is no negative pressure in the vortex chamber and shaft.The time-averaged pressure of the elliptical arc and the vortex chamber decreases with the elevation of the measuring point.At the same measuring point,the time-averaged pressure increases with the increase of the flow rate.On the bottom plate of the stilling well,in the working condition of small flow rate in the test,the time-averaged pressure of the bottom of the stilling well is relatively stable,and its fluctuation is small.The bottom of the stilling well is pure water,and the pressure at this time tends to the hydrostatic pressure,which varies with the depth of the well.The time-averaged pressure near the edge of the bottom of the stilling well is stronger than that on the central area.As the depth of the energy-dissipating well further increases,the impact on the bottom pressure of the stilling well decreases.The elliptical arc segment and the pulsating pressure variance of the side wall of the vortex chamber increase with the increase of the angle of the baffle sill.The fluctuating pressure of the bottom of the stilling well increases with the depth of the stilling well.The peak frequency of the fluctuating pressure spectrum first rises and then decreases.The dominant frequency gradually becomes prominent,and the fluctuating energy tends to increase slightly.For the35 cm well depth,the scale of the vortex is larger in the area close to the bottom of the energy dissipation well.The water flow characteristics in the energy dissipation well tend to be uniform,and the energy of the pressure fluctuation is mainly concentrated in the low frequency region.The influences of different connection sizes between the diversion channel and the vortex chamber at different flow rates are analyzed,and the influence of different energy dissipation well depths on the energy dissipation rate of the swirl shaft spillway is studied.The calculation results show that the energy dissipation rates of different connection sizes decrease at first and then increase slightly with the increase of the flow rate.The change trend is basically the same.With the increase in the angle of the baffle sill under the same flow rate,the energy dissipation rate increases gradually.With the well depth increases,the energy dissipation rate first increases and then stabilizes.It indicates that the increase of the well depth will increase the energy dissipation water volume.But after the well depth increasing to a certain level,the energy dissipation rate slows down significantly and gradually stabilizes.It shows that it is not the correct that the deeper the stilling well,the higher the energy dissipation rate.The reasonable depth of the stilling well should be used.