Optimization Of Hydraulic Performance For 1000MW Scale Francis Turbine

Hydraulic stability towards the giant hydropower stations has become an issue of interest in the hydropower industry due to tremendous development of hydropower in the past 20 years,such as the Three Gorges,Xiangjiaba and Xiluodu station.The stable and secure operation of the giant hydro-turbine not only has a heavy bearing on the economic benefits but also plays a dominant role to ensure the national strategy security.This paper aims at the hydraulic research and development of the Baihetan 1000MW giant Francis turbine,the three key issues affecting the stability of Francis turbines,including blade channel vortex,cavitation,and upper part load pressure fluctuation,have been deeply and systematically studied.The main innovative results obtained are as follows:(1)The influence of flow angle at the leading edge of runner blade,the runner blade profile,the guide vane profile and the tail water pressure on the blade channel vortex are investigated.The results show that the shock at the leading edge of the runner blade is not the only factor that induces the blade channel vortex.The formation mechanism of blade channel vortex is attributed to the breaking of the continuous pressure gradient at part load condition that forces the flow spin to a certain extent along the main flow direction.Additionally,the investigation found that the blade channel vortex is induced at the leading edge of the runner blade towards the highhead hydro-turbine,while it is inside of the runner for the low-head hydro-turbine,and the vortex structure moves towards the leading edge of the runner blade with the decrease of the flow rate.Furthermore,created the blade channel vortex suppression technology by compressing the runner flow passage,bionic streamline design,controlling the blade distortion and increasing the guide blade thickness-length ratio,which effectively suppresses and delays the initiation and development of the blade channel vortex.(2)The influence of the shape of the leading edge,the profile lines and the spatial morphology of the runner blade on cavitation characteristics is investigated,the findings show that main mechanism of cavitation formation towards the cavitation induced by flow separation and the blade cavitation are both attributed to the low local pressure on the blade surface.A new cavitation classification method including the cloud cavitation,beam cavitation and airfoil cavitation is proposed based on the cavitation characteristics and its incipient location.Furthermore,a bionic fish head airfoil design,improved ridge line distribution law,and optimized blade curvature are established to suppress airfoil cavitation,which increases the minimum pressure in the cavitation zone and significantly improves the cavitation performance of the turbine.(3)The motion characteristic of the draft tube vortex is studied,and it is found that the unstable vortex in the transition zone is directly related to the formation of upper part load pressure fluctuation.Based on the amplitude-frequency characteristics of pressure fluctuations,upper part load pressure fluctuations are divided into two types:primary pressure fluctuation and induced pressure fluctuation.Established a hydraulic design method to control the flow rate distribution at the runner outlet to eliminate the primary high part-load pressure fluctuation,and put forward a series of effective suppression of induced induction by installing a long drain cone,optimizing the draft tube profile,increasing the tail water pressure and replenishing air.The problem of upper load pressure fluctuation is solved.(4)A Francis model hydro-turbine with capacity of 1000MW is successfully developed,the main indicators for the model turbine are as follows:the peak hydraulic efficiency is 95.02%,the incipient line of blade channel vortex is plotted on the left of the 60%rated output line,the cavitation lines of separated flow at the leading edge of the runner blade are all outside the range of operation head,at the same time,the turbine has a large cavitation remainder and the safety remainder for incipient cavitation is greater than 1.4,the amplitudes of pressure fluctuations in the vaneless space is 3%max calculated by the ratio of the amplitude to the test head,accordingly,amplitude in the draft tube is 5%max.More importantly,upper part load pressure fluctuation and abnormal frequency are effectively eliminated.The developed turbine has been successfully applied to Baihetan hydropower station.