Numerical Calculation And Performance Improve-ment Of Cavitation Flow In Francis Turbine Runner

If the pressure in a turbine is reduced to a certain degree,the gas nuclei depositing in the undissolved gas,sediment and other impurities,as well as in the cracks on the surface of sediment will form into bubbles,which will cause cavitation and erosion,and lead to the surface damage of flow components,the noise and vibration of unit,the damage of structural components,the significant reducing of efficiency,and the inability of working normally.Due to the complicity of cavitation flow in the turbine,the problem of turbine cavitation has always been an important and difficult research topic.By taking the Francis turbine as the research object,the geometrical physical model of the whole flow passage of the turbine was established by UG,Calculating and analyzing the cavitation characteristics in the turbine with the flow in the runner as the emphasis.The main research works were as follows:First,the single-phase steady numerical calculation of the whole flow passage of the hydraulic turbine is carried out,and the influence of the speed and the opening on the flow field of the turbine,especially the runner region,is discussed.The results showed that:when under a same speed,a larger opening will leads to a smoother streamline regional runner,and then the blade edge tail low-pressure area gradually extended;when in the same opening,the streamline in the unit area in a small opening with a low unit speed is more disorder than with a high unit speed,and the large opening is the opposite;no matter whether in a same speed,the low pressure area in small opening is more likely to appear at the blade edge near the crown position,and on the contrary,in large opening is more likely to appear at the blade edge near the ring position.Secondly,the numerical calculation is carried out under the steady and unsteady cavitation of the whole flow of the turbine.The cavitation performance and the pressure pulsation of the wheel area are analyzed.The results show that there is a 'big difference in the same blade cavitation of the large opening and small opening;under the large opening,the cavitation is mainly concentrated in the blade suction surface near the side of the lower ring;but under the small opening,the cavitation occurs more concentrated near the crown position.The high-frequency pressure pulsation is mainly caused by the dynamic and static interference between the movable guide vane and the rotating wheel.The high frequency pressure fluctuation is mainly caused by the static and dynamic interference between the moving guide vanes and the runner,the high frequency pressure pulsation amplitude is related to the distance between the movable guide vane and the monitor point on the surface of the rotor blade.The amplitude of the blade surface decreases from the direction of the water inlet edge of the blade,which proved that the pressure in the low pressure zone tends to be stable and more favorable to the occurrence of the primary cavitation in the outlet position of the suction surface.Finally,two methods for improving cavitation performance are proposed,namely,additional blade and perforated blade,and making the numerical calculation and comparative the analysis of the corresponding schemes.The results showed that the two methods can improve cavitation performance effectively,It shows that the two methods should be practical value for us.However,this study is only a preliminary study,especially for opening holes to reduce cavitation.This scheme is still relatively general,and further studies are needed to prove the further research.