Study On Flow Field Simulation And Performance Improvement Of The Medium-High Specific Speed Francis Turbine

The comprehensive performances, including the efficiency, cavitation and stability are the emphases of research of the hydraulic machinery. Accurate simulation of the flow field in the hydraulic machinery will help us to analyze performance of the different components with less expense. So it is becoming an essential tool for researching the energy performance and the hydraulic stability of the Francis turbine. And until now, people have been looking for good methods which can work well in a wide operation range so that can predict the hydraulic machinery comprehensive performances include the efficiency, cavitation and the stability.At first of the paper, some theoretical issues about the computational fluid dynamics (CFD) of the hydraulic machinery are introduced, and of the relative merits of the turbulence models is the emphasis to discuss. Then the CFD simulations for a medium-high specific speed Francis turbine will be done in different operations by the method of partly coupling. Internal flow of each part of the Francis turbine is analyzed in detail and different flow field feature is found to connect with different operations. And the specific energy performance of each part of the Francis turbine is investigated in detail too. Further the performance of the cavitation of the runner is investigated to improve the cavitation capability. Then the modification of the runner vane to improve the cavitation capability will be presented. After one year spot running, the performance of the cavitation of the original and modified runner vane blade will be compared. The specific energy efficiency of a Francis turbine can be predicted by the 3-D numerical analysis of the main flow through the turbine from the spiral case inlet to the draft tube outlet. And there are many papers that compute the energy efficiency of the main flow through the Francis turbines with the computational fluid dynamics (CFD) analysis. In order to predict the discharge efficiency and the power efficiency of the Francis turbine, it is necessary to perform the 3-D numerical analysis of the leakage flow through the clearances around the runner, under the inflow and outflow conditions for the clearances defined by the main flow analysis. Moreover in the paper, a lump-sum analysis of the combined leakage-main flow is prerequisite to investigate the interaction between the leakage flow and the main flow in the Francis turbine. The aim the paper is to clarify the effect of leakage flows along the runner band and crown on the main flow through the medium-high specific speed Francis turbine. Firstly, the paper will focuses on the numerical analysis of leakage flow through the clearance around the runner band and runner crown to evaluate the discharge and power efficiencies. Then the interaction of the main flow and the leakage flow will be investigated. And the interaction of the flow outflow from the band clearance and flow in the draft tube will be the emphases to investigate.As the medium-high specific speed Francis turbine, the pressure pulsations caused by the draft tube vortex rope is the most important unsteady hydraulic factor. The phenomenon known as a draft tube surge or a draft tube vortex rope is being much discussed. Almost every people familiar with the water turbines have had the opportunity to meet this exciting and physically not sufficiently explained phenomenon, which can arise at partial load operation of Francis turbines. The theory of form of the helical vortex in the draft tube is various. In the paper, the shear layer vortex between the dead-water core and the main flow in the draft tube will be analyzed, and it will be presented to explain the form of the cork-screw vortex in the draft tube at first time. Then the CFD simulation will be validated the theory of form of the cork-screw vortex in the draft tube. And CFD simulation of draft tube in different operations will be investigated the movement rule of the draft tube cork-screw vortex. And the pressure pulsations caused by it will be investigated in different ope...