Study And Control Of The Vortex Rope In Draft Tube Of Francis Turbine

The high speed development of hydropower promotes the hydropower unit towards tosafety, high efficiency and large capacity. It is bound to put forward higher requirements forhydroturbine, acting as the core power equipment of hydropower. At present, pressurefluctuation caused by vortex rope in the draft tube is a common phenomenon. Therefore, thestudy and control of stability of the vortex rope in turbine draft tube have become a focus of thecurrent study.Firstly, a90°bend is regard as research model for deep research of the flow structure andcharacteristics in the draft tube. The internal flow field of bend is simulated by usingthree-dimensional incompressible Navier-Strokes equations, and the equations are coupled withRNG turbulence model for turbulent flow as work foundation. Then the study on the stability ofthe internal flow in draft tube of Francis turbine begins. The main research contents and resultsof this paper are as follows:(1) The phenomenon of Dean vortex splitting is found in the study of the internal flow in90°bend with square cross-section. For laminar flow, the Dean vortices on the cross-section ofelbow split when the Reynolds number is larger than338. While the Dean vortices neversplit when the Reynolds number is smaller than338. For turbulent flow, the Dean vorticeson the cross-section are always two.(2) The impact factors of the Dean vortices’ splitting is studied further. It found that theReynolds number of the coming flow, the shape of the cross-section and curvature haveinfluence on the phenomenon of the Dean vortex splitting.(3) Unsteady numerical simulations are carried out to study the internal flow of Francis turbinedraft tube on five different tangential velocity entrance conditions. When the location of themaximum of tangential velocity moves from the centre to the wall on the cross section, theamplitude of the pressure fluctuation decreases, the proportion of the quasi free vortex onvelocity profile increases, the stability of vortex rope is better.(4) The effect of pier spacing and angle on the hydraulic stability of draft tube is studied. Itfound that the performance of draft tube changes with the different piers, recoverycoefficient and relative hydraulic coefficient, while, there is no change in stability of turbinedraft tube.