Analysis And Research Of Francis Turbine Vibration Caused By Runner Inter-blade Vortices

Inter-blade vortices are a cavitation phenomenon produced after Francis turbine deviates from the optimum operating condition,in addition to the vortex rope in draft tube and Karman vortex train,it also can cause vibration of the turbine.When Francis turbine deviates from the optimum operating condition,water vortices are generated behind the runner blade inlet edge near the crown,and form inter-blade vortices in the flow channel.Inter-blade vortices which develop to a certain stage can cause the hydraulic vibration of turbine.When the vibration exceeds the allowable value,especially units are in the long-term state of vibration,even the resonance happens about the turbine components,they will be a serious threat to the security and stable operation of hydropower station electromechanical devices.The turbine runner is a device which converts hydroenergy into rotational mechanical energy,and is the core component of the turbine.The water flows out from the spiral case and stay vanes,flows around guide vanes and runner blades,forms the complicated flow pattern changes in the flow channel,and results in the applied moment,which achieves the energy conversion.According to the action principle of water and runner,the magnitude and direction of flow velocity near the runner inlet have important impacts on the change of the runner inner flow field.In the paper,in view of this situation that abnormal vibration appears in Francis turbines in a hydropower station of Southwest China at the higher head than the rated head and full load,the following studies are carried out:(1)The demonstration about that inter-blade vortices cause the vibration: Using characteristic curve analytical method,based on the operation data and field test data of plant,some vibration operating points selected are marked on the turbine performance curve,it is obtained that all the characteristic points of vibration condition are inside the inter-blade vortex area of runner performance curve;Based on the ANSYS software,the CFD numerical simulation comparative analysis about the flow of fluid in runner flow channel at the vibration operating conditions and normal operating conditions,shows that the obvious inter-blade vortices appear between runner blades at the vibration operating conditions,but at the normal operating conditions vortices are less.These research work demonstrate that the turbine vibration of this hydropower station is caused by inter-blade vortices,and the vortices are high head inter-blade vortices which are generated behind the suction side of blade.(2)The mitigation study of inter-blade vortices: This part of the study chooses the research scheme of modifying runner blade head profile to mitigate inter-blade vortices at the turbine output of 20.619 MW and the net head of 62.59 m.Based on the runner inlet velocitytriangle,the blade head is cutted for two times,the position of the blade inlet edge is adjusted,water angle of attack of the blade inlet is reduced.CFD numerical simulation about the flow field before and after the modification of blade head is done,and monitoring points are set up behind the suction side of blade,through comparative analysis to streamline charts,cloud pictures and pressure,velocity,turbulent kinetic energy of monitoring points,it is proved that the scheme improves the vortices behind the suction side of runner blade,mitigates or partially eliminates inter-blade vortices.Through the above work,it is systematically demonstrated that runner inter-blade vortices are the root cause of the turbine vibration,and this method of the blade head modification can mitigate inter-blade vortices.The study has certain reference value to solve vibration problems about the turbine of hydropower station,also does an effective exploration to mitigate inter-blade vortices in turbines of the hydropower station of Southwest China,provides a research method.