Research On The Hydraulic Excitation Characteristics Of The Axial-flow Pump Unit With Two-way Channel

The large-scale vertical axial-flow pump device is widely used in the pumping station projects of maj or water conservancy projects and plays a very important role in the national economy.The vibration of axial flow pump device will affect the reliability of pump station operation,and even lead to major safety accidents.Axial-flow pump device is a complex hydro electromechanical coupling system,and its excitation mechanism is not clear.At present,researchers at home and abroad have studied and analyzed the vibration characteristics of axial flow pump device at various technical levels such as vibration source identification,internal flow field,fluid structure coupling and rotor dynamics.The conclusion is that the vibration of axial flow pump device is the result of the joint action of mechanical force,electromagnetic force and fluid force,in which fluid force accounts for a large proportion;the impeller area in the axial-flow pump device is the core area of fluid excitation.The excitation force mainly comes from the pressure pulsation caused by many factors,such as cavitation,vortex band,non-uniform incoming flow,dynamic and static interference and tip clearance leakage vortex.The current research still has the following problems:most of the existing research is limited to the axial flow pump section,and there is little research on the overall stability of the device;the correlation mechanism between flow induced pressure fluctuation and vibration is not clear enough;there are few studies on the correlation between hydraulic excitation test measurement and numerical simulation of prototype/model pump device.In view of the shortcomings of the existing research,this paper studies the overall hydraulic excitation characteristics of the two-way channel vertical axial-flow pump unit by combining theoretical analysis,numerical simulation,experimental analysis and field test.It is found that the excitation force mainly comes from the uneven flow field at the inlet of the impeller and the uneven flow field at the outlet bell nozzle affected by the blind end.The vibration displacement distribution and the axis trajectory of the rotor system are similar to the shape of the fluid force at the corresponding position.From the spectrum analysis,it can be seen that both of them are mainly three times the conversion frequency.The following research results are mainly formed:Based on the airfoil lift resistance theory,the expression of blade water force on flow and blade placement angle is deduced.Through theoretical analysis,it is concluded that a single blade will have obvious harmonic pulse force several times the frequency of the guide blade due to the interference of it.However,when the number of blades and guide vanes are prime numbers to each other,this component of the resultant force on all blades will be eliminated.Through the numerical simulation of the internal flow field of the model pump device,the expression of water force on the blade obtained from the theoretical analysis and the frequency component of water force under dynamic and static interference are verified,that is,the dominant frequency is f3.It is found that the vortex between guide vanes is the main factor causing the pressure fluctuation near the outlet of the blade pressure surface.Through model test and field test,it is concluded that the pressure fluctuation amplitude at the measuring points of impeller inlet and outlet bell mouth is large.The numerical simulation shows that the pressure pulsation at the measuring point at the inlet of the impeller is caused by the rotation of the positive pressure zone between the blades and the negative pressure zone under the suction surface of the blades with the rotation of the blades,and the pressure pulsation at the outlet horn is caused by the uneven flow field caused by the separation of the blind end of the outlet channel.Through the fluid structure coupling numerical simulation,it is concluded that the water force in the outlet channel contributes the most to the vibration at the measuring points,accounting for more than 90%.The wall amplitude of the outlet horn pipe measured in the field is large,and it decreases with the increase of flow,just like the wall pressure fluctuation amplitude obtained by numerical simulation.Through the model test,it is found that the flow enhances the vibration of horizontal measuring points f0?f3 pulsation component,which suppresses the high-frequency component of horizontal measuring point and the amplitude of axial measuring point.Through the numerical simulation of internal flow field,it is found that the pressure on the surface of guide vane and blade and their junction with hub is the largest,and the spatial pressure difference near the outlet horn nozzle is large.The standard deviation of pressure fluctuation of each flow passage component is the largest under small flow,and the maximum value appears at the inlet of blade and guide blade.Based on Newton's second law and Lagrange's law,the blade tip particle vibration equation,thrust bearing axial vibration equation,rotor system torsional vibration equation and transverse bending coupling vibration equation under fluid force are established respectively,and the stability criteria of the above vibration equations are obtained by Lyapunov stability criterion.It provides a basis for the selection of damping coefficient in fluid structure coupling numerical simulation.Through theoretical analysis and numerical simulation,it is confirmed that the impeller will be eccentric under the unbalanced radial fluid force,resulting in the eccentricity of other axes.Each axis whirls about the eccentric point,and the whirl trajectory is similar.The eccentricity and whirl radius at the coupling are large.The blade frequency and its frequency doubling components in the axial whirl signal of the coupling obtained from field test account for more than 40%,indicating that the fluid excitation accounts for a large proportion...