Study Of The Influence Of Blade Leading-Edge Geometry On The Inception Cavitation Of Reversible Pump-Turbine

As a key component in pumped storage power stations,the reversible pump-turbines have strict requirements in operation stability and security.Cavitation,which may cause noise,vibration,performance decline and material damage,is a commonly seen phenomenon in pump-turbines.Because of the overlap of the low-pressure impeller side and the local pressure drop on leading-edge,the cavitation performance in pump-turbines in pump mode is extremely strict.Referring the traditional critical cavitation standard may make the inception cavitation to be ignored.Due to the wide discharge range and high-head characteristics,cavitation would happen during operation.To solve the problems above,the influence of the leading-edge geometry on the inception cavitation performance is investigated in this study based on numerical simulation and experiment.A design method for improving the inception cavitation performance of pump-turbine is also established for the cavitation-free operation.Firstly,the hydrofoil past-flow case is used as the simplified flow model of the impeller internal flow to study the leading-edge geometry influences on inception cavitation.The incidence angles of hydrofoil represent the operation conditions.Results show that the leading-edge sudden pressure drop under large incidence angles is caused by the incoming flow's striking and following separation.Thus,the leading-edge geometry directly affects the local pressure drop.Based on the control of the leading-edge arc radius and the thickness distributions,the genetic algorithm is applied to search an optimal hydrofoil profile.The optimal profile is found with larger-leading-edge arc radius and slower thickness-variation.It has a gentler leading-edge pressure drop and later inception cavitation.Analyses show that there are obvious relationships among the incidence angle,leading-edge geometry,pressure gradient,pressure and inception cavitation.A larger leading-edge arc radius and slower thickness-variation will be helpful for widening the leading-edge favorable pressure gradient,lowering the local pressure drop's amplitude and delaying the inception cavitation at large incidence angles.Then,a simpler,rapider and more focused geometric deconstruction method of hydrofoil is established considering the problem of long-procedure and over-abundant parameters of searching optimization.This deconstruction method mainly focuses on the leading-edge arc/ellipse and the following thickness diffusion law.On the basis,a new design method named "Diffusion-angle Integral(DI)design method" which can influence the leading-edge pressure gradient is built.The design parameters include the leading-edge arc/ellipse size,the straighten diffusion angle of thickness and the integral coefficient of thickness diffusion.The thickness distribution of hydrofoil profile can be integrally calculated.After that,the reasonable valuing way of the design parameters are discussed based on the orthogonal test.The hydrofoil is re-designed by the "DI design method" for its large incidence angles.It gets a better inception cavitation performance than the searched-optimal hydrofoil.Finally,investigations on the cavitation performance and the improvement are conducted on a pump-turbine unit.Results show that the cavitation number in operation range may be lower than in inception cavitation number under the off-design conditions of the pump-turbine's pump mode and the cavitation may occur.Hence,the "DI design method" is applied to design the impeller blade for improving its inception cavitation performance under off-design conditions based on the inception cavitation standard.After the improvement,the blade leading-edge changes from arc to ellipse and the thickness increasing rate becomes slower.The improved impeller is found without the leading-edge sudden pressure drop under its off-design conditions.The inception cavitation numbers are lowered.Thus,the inception cavitation performance is enhanced without impacting the initial hydraulic performances.The cavitation-free operation is finally achieved.The research results in this study well clarifies the influence of leading-edge geometric characteristics on the inception cavitation of blade profiles.It shows significant engineering value and scientific meanings for improving the inception cavitation performance of reversible pump-turbines and other bladed hydraulic machineries.