Numerical Simulation Of Hydraulic Turbine And Hydropower Station Tailrace System Based On The Computational Fluid Dynamics

Three dimension turbulent flow simulations are becoming more and more accurate with both computational methods and hardware performances rapid development. In the field of waterpower station optimization design, Computational Fluid Dynamics (CFD) is routinely used today in research and development as well as in design.Recently one of the most important achievements in CFD is simultaneous calculation of the flow in rotating and non-rotating parts. Since very often there are strong interactions between the components (especially between guide vanes, runner and draft tube), it is inevitable to introduce this interaction into the simulation for accurate results. Using coupled analysis of the flow through the whole turbine we can take into account the stator-rotor-draft tube interaction and avoid inaccurate boundary conditions between turbine components.In the paper, the three-dimensional steady turbulent flow in the Shuibuya model and prototype Francis turbines were simulated. At first the flow through the whole turbine was calculated simultaneously. The computational domain included spiral case, stay vanes, guide vanes, runner and draft tube. There were two stage-sliding interfaces between rotating and non-rotating parts. Then the flow through the whole flow passage of waterpower station-from penstock inlet to draft tube or tail channel outlet, was analyzed simultaneously. Thesimulation is based on Navier-Stokes equations, the standard k-ε turbulence model andthe SIMPLEC algorithm, which is applied for the solution of the discrete governing equations. The distribution of velocity and pressure through the flow passage of the Francis turbine was attained. The energy and cavitation property of the model turbine was then predicted. Calculated turbine efficiency was compared with the measured one.Different problems in hydraulic machinery arise from unsteady flow phenomena. In order to get information on this phenomena or solutions to the problems an unsteady flow analysis is necessary. Two major groups of unsteady problems can be distinguished. The first group is flows with an externally forced unsteadiness. This can be caused by unsteady boundary conditions or by changing of the geometry with time. Examples are the closure of a valve, the change of the flow domain in a piston pump, or the rotor-stator interactions. The second group is flows with self-excited unsteadiness, which are e.g. turbulent motion, vortex shedding (Karman vortex street) or unsteady vortex behavior (e. g. vortex rope in a draft tube). Here the unsteadiness is obtained without any change of the boundary conditions or ofthe geometry. There can also occur a combination of both groups (e. g. flow induced vibrations, change of geometry caused by vortex shedding). All these phenomena can take place in a turbine or pump and require different solution procedures. The oscillations in hydraulic machines are becoming more and more important with the increasing of unit power and size, and the oscillation must be analyzed by investigating the unsteady flow field in the hydraulic machine.In this paper different numerical schemes are discussed for the rotor-stator interactions and moving grid. The rotor-stator coupling by application of sliding mesh is shown on the Shuibuya complete Francis model turbine. The PKO (pressure-implicit with splitting of operators) method was used for the pressure velocity coupling. The realized k-e model was used for the turbulence. The instantaneous velocity and pressure distribution through the whole Francis model turbine was attained. Three measuring points were placed at the spiral case inlet center, in front of runner and on the draft tube cone, approximately 0.3 runner diameters upstream of the runner outlet. The pressure pulsations versus time at different positions and its' amplitude spectrums versus frequency were analyzed by FFT (fast Fourier transform) to determine the pressure oscillation frequencies at different points. Although Unsteady simulations have in common a quite large requirement of computational resources, especially for rotor-stator interactions the complete turbine has to be considered and all flow channels in the stator as well as in the rotor have to be included, which leads to many grid nodes and an enormous computational effort, the frequencies and amplitudes of integral quantities (e. g. forces) can be predicted with sufficient accuracy for most of the problems, and on the other side flow phenomena can be predicted and potential of influencing then can be assessed.Large underground stations with long conduits usually construct surge tanks at the downstream of a turbine to accommodate flow fluctuations. In order to obtain an ideal design for the water diversion systems, physical and numerical simulation is necessary. In Longtan waterpower station, three turbine-generator units share one tailrace surge chamber, and there are two different type tailrace systems. One is fork pipe subsequent to the tailrace surge chamber; another is fork pipe under the tailrace surge chamber. In the paper, Flow regime and head loss of two different tailrace systems in hydropower station were investigated.Numerical simulation was based on the RNS equations, closed with the standard k-e turbulence model. Volume-of-fluid (VOF) method and rigid-lid assumption were used to describe the free surface. Computational result shows that the layout pattern of fork tube behind throttled surge chamber is more efficient than the fork tube under surge chamber.