Research On The Correlation Mechanism Between The Flow Field Structure Of Francis Runner Outlet And Pressure Pulsation Characteristics Of Draft Tube

Moreover,the vortex in the draft tube also changes very complicatedly with the influence of the flow pattern.The draft tube vortex is not only one of the important factors for the vibration of the hydraulic unit,but also is considered to be an important factor that seriously affects the working efficiency of the hydraulic turbine.Therefore,it is of great value to study the association relationship between the pressure pulsation characteristics of the draft tube and the runner.In this paper,a high-head Francis turbine is numerically simulated,and the numerical calculations are respectively calculated under the operating conditions of 6,10 and 14 openings.The research content and conclusions of this article are as follows:(1)In the model machine experiment,the pressure pulsation time-domain signal is acquired by installing sensors.The positions of the acquisition signal are from the upper and downstream of the top cover,the upstream and downstream of the draft tube cone section,and the upstream and downstream of the draft tube elbow section.After the CEEMD decomposition of the obtained time-domain signal,the results show that the relative amplitude of the pressure fluctuation in the draft tube shows a decreasing trend as the opening of the guide vane increases.The amplitude of the top cover should be greater than the amplitude of the cone and elbow section of the draft tube,and under the condition of an opening of 6,the amplitude of the amplitude should be significantly greater than the operating conditions of the opening of 10 and the opening of 14;when it is a small opening flow Under working conditions,the main frequency of vibration of the measuring point shows a downward trend as the water head increases;when the opening degree gradually increases,the main frequency of vibration of the measuring point shows an upward trend as the water head increase.(2)With the adjustment of guide vane opening and based on entropy generation theory,the size distribution of hydraulic turbine flow components is understood,the location and magnitude of energy loss of each flow channel unit are analyzed,and the cause of energy loss is studied,relationship between velocity field at runner exit and pressure pulsation distribution in draft tube.The calculation results show that the change of entropy production value is significantly related to the velocity field distribution of the runner outlet and the pressure distribution of the draft tube.By analyzing the variation of the local entropy production rate distribution of the flow field of each component,it can be concluded that the entropy production rate at the cone and elbow is higher than that of other parts of the draft tube,and the entropy production rate is higher the closer to the runner exit.(3)According to the results of numerical simulation,the range of the low velocity zone in the axial velocity field of the runner exit surface decreases with the increase of the guide vane opening,and the range of the vortex generating zone in the straight cone section of the draft tube also decreases,the axial velocity in the other areas of the runner exit surface increases with the increase of the guide vane opening,and the pressure pulsation in the straight cone of the draft tube also increases.In the inner region of draft Tube,the pressure fluctuation of straight cone section is more serious than that of bend section.The pressure at the center of runner exit surface increases with the increase of guide Vane opening.The region with high flow velocity moves from the elbow to the inlet of the draft tube.It can be concluded that the area of the low-velocity zone of the flow field at the runner outlet affects the size of the vortex zone,and the velocity of the rest of the flow field at the runner outlet has a positive correlation with the pressure fluctuation in the draft tube.