Unstable Characteristics And Hydraulic Vibration Study Of Pelton Turbine

The Pelton turbine is the main type of machine for developing high-head hydropower resources,It offers benefits such as a wide range of high-efficiency areas and simple structures for overflow components.However,during operation,the flow in a Pelton turbine exhibits significant characteristics,including varying scales and strong transients,resulting in serious interference between the jet and the bucket.This poses certain difficulties in accurately evaluating the performance of the unit.Additionally,the bucket of a Pelton turbine is subjected to intense alternating pressure loads,which can easily lead to fatigue failure problems and threaten the safe and stable operation of the unit.In this paper,we systematically investigated the unstable characteristics and hydraulic vibration of a specific six-nozzle Pelton turbine.We analyzed the two-phase flow characteristics of the turbine under different operating conditions and examined the impact of these characteristics on its dynamic behavior.The main findings and conclusions are presented below:(1)Based on the unsteady numerical simulation of two-phase flow in an Pelton turbine,the energy loss and internal flow characteristics of the feedwater mechanism were analyzed.The results showed that the energy loss in the nozzle of the feed water mechanism was much greater than that in the casing.Increasing the opening of the nozzle reduced the outlet resistance and energy loss of the nozzle,but increased the energy loss in the casing.The phenomenon of low pressure and low velocity occurred in the casing due to the effect of the Dean vortex,which continuously developed about the middle plane and finally entered the free jet.The jet accelerated as it left the nozzle,causing changes in the inner shape.The rib plates and outlet contraction segment in the nozzle could produce more vortex pairs,but the rib plates could reduce vortex intensity.The outlet contraction segment of the nozzle had high velocity and pressure gradients,and the pressure on the needle surface decreased first and then increased,coinciding with the high pressure region where the "velocity deficit" at the needle tip occurred.(2)Study the power output characteristics of a single bucket and the mechanism by which the speed and opening affect the power output characteristics of a single bucket.This study explores the output characteristics of a single bucket in a Pelton turbine during its three operational stages:torque rise,high-torque,and torque decrease.The rotational speed has a significant impact on the interaction time between the water film and the bucket,as well as the extent of water film extension,leading to both phase differences and numerical differences in the bucket output.Our results show that the opening only causes phase differences during the torque decrease stage,while numerical differences exist in the other stages.The splitter blade of the bucket cuts off the jet,causing the jet to twist and deform.This deformation results in the jet diameter increasing significantly,which in turn strengthens the backflow effect on the bucket surface.(3)Using one-way fluid-structure interaction numerical calculation of the Francis turbine runner,we analyzed the deformation,equivalent stress variation rules,and vibration characteristics of the bucket under the action of alternating fluid loads.Our results indicate that the material of the turbine wheel meets the yield strength requirements under both static and dynamic stresses.The deformation of the bucket is concentrated at the front end,with the maximum deformation occurring near the gap on the left and right sides of the outflow edge.The displacement at the front decreases and then increases from the splitter blade to the outlet edge,while the displacement at the back decreases continuously from the middle to the outlet edge.The equivalent stress is concentrated at the back half of the splitter blade,the outlet edge,the working surface at the bottom of the bucket,the gap at the back of the bucket,and the root of the bucket.The maximum amplitude of the equivalent stress is generated at 6 times the rotational frequency.The material at the root of the bucket splitter blade is most susceptible to high-cycle fatigue,providing important references for engineering applications and optimization design.Additionally,the low-order natural frequency of the turbine wheel is much higher than the frequency of high-amplitude stress vibration,making resonance almost impossible.