Study On Mechanism Of Hydraulic Loss Variation In Francis Turbine Induced By Cavitation

Efficiency and cavitation are important indicators of turbine performance,and the relationship between them is often determined by model cavitation test,however,cavitation test is not good for revealing the flow mechanism of some special flow phenomena.In this paper,a Francis turbine model is taken as the research object,and cavitation two-phase flow under the same unit speed and different unit flow rates is calculated numerically.Exploring the internal relationship between the evolution of internal vortices and the change in of hydraulic losses during the development of cavitation.The main contents and conclusions are as follows:(1)At the same speed,the inlet massflow rate of volute and the outlet pressure of draft tube are used as orthogonal test variables,and the cavitation characteristic curves of corresponding flow rates are calculated.The results show that from the development stage to the critical stage of cavitation,the variation trend of efficiency on the cavitation characteristic curve is closely related to flow rate,Especially,at low flow rates(guide vane opening α=16°),as the cavitation coefficient gradually decreases,there is a phenomenon of local increase in efficiency on the cavitation characteristic curve.(2)According to the three typical flow rates of α=16°,α=22° and α=32°,the type and distribution position of vortices in the runner are analyzed through flow separation and flow topology.Based on this,an appropriate vortex identification method is selected to display the vortex structure,and the evolution of vortex structure in the runner and draft tube during the development of cavitation is analyzed.The results show that the distribution of the main vortex zone,cavitation zone and low pressure zone in the draft tube is basically the same.When α=16°,the spiral vortex zone gradually expands with the development of cavitation until it reaches the wall of the draft tube at the state of severe cavitation.When cavitation develops to a certain extent,the thin cylindrical vortex zone at α=22° and the spindle cylindrical vortex zone atα=32° will fracture under the axial reverse pressure gradient.When α=16°,cavitation has a significant impact on the morphology and distribution of attached vortices between runner blades,trailing vortices at the outlet of the blades,and vortices near the discharge cone.The trailing vortices at the outlet of the blades and vortices near the discharge cone are mainly generated in the local low pressure area of the crown.Cavitation enhances the reverse pressure gradient downstream of the local low pressure area,causing more singularity points on the flow separation line,resulting in more trailing vortices at the outlet of the blades and formating the cavitation channel vortices in severe cavitation.(3)The pressure drop method is used to verify the hydraulic loss of the whole flow channel calculated by entropy generation method,and the entropy generation variation rule and distribution in the runner and draft tube under three typical flows are analyzed during cavitation development.The results show that,as cavitation gradually intensifies,within a certain range of cavitation,there is a phenomenon of cavitation drag reduction in the cavitation area of the runner;According to the loss proportion and change rule in the runner and draft tube,the reason for different local change rule of cavitation characteristic curve under different flow rates can be explained.In particular,whenα=16°,in the process of cavitation development,the local increase of efficiency is mainly caused by the local decrease of volume entropy production in the vortex precession zone of the draft tube.(4)It is verified that the trend of enstrophy dissipation and dissipation caused by time averaged entropy generation is basically consistent with the cavitation coefficient.The hydraulic losses caused by time averaged entropy generation and vortex are linked through enstrophy and enstrophy dissipation.The evolution of vorticity and enstrophy dissipation during the gradual aggravation of cavitation was analyzed through vorticity transport and enstrophy transport.The results indicate that with the development of cavitation,the baroclinic torque term and dilatation term in the cavitation expansion zone of the runner gradually increase,leading to the increase or generation of vorticity.However,the vector product of vorticity and both of them is negative,resulting in a smaller negative value of the enstrophy baroclinic torque term and enstrophy dilatation term,slowing down the dissipation of enstrophy.This preliminary reveals the mechanism of cavitation drag reduction phenomenon within a certain cavitation range of the runner.At α=16°,with the gradual development of cavitation,the dissipation characteristics of enstrophy in draft tube mainly depend on the change of the enstrophy stretching term,and the local increase in efficiency on the cavitation characteristic curve may be caused by the gradual expansion of the spiral vortex band in the draft tube within a certain cavitation range.