Research On The Effect Mechanism Of Pump’s Pulsation Characteristics And Cavitation Characteristics On Blade Stress

As the only rotating part of the pump,the appearance of cracks in the impeller will not only cause harm to the operation of the normal blade,but also seriously affect the operation safety of the unit and reduce the economic benefit.The influence of pulsation characteristics and cavitation characteristics on blade stress is studied by numerical calculation and experiment.To explore the crack root of two different pump types.When the impeller is running,it will be affected by various exciting forces.If the frequency of a certain exciting force is close to or equal to the natural vibration frequency of the impeller blade,the blade will produce resonance,and then the impeller blade will vibrate violently.If the blade runs in resonance state for a long time,fatigue damage will be easily caused to the blade.Cavitation is may occur in the process of hydraulic machinery in the operation characteristic of the complex physical process,for the surface of the flow components,especially the impeller parts,easily subjected to steam bubble breaking the destruction of the shock wave,and steam bubble breaking often occur in the process of the water pump cavitation RGL bubble with fluid flow to the process of the high,it is for the operation of the pump and the working life has a certain impact.At the same time,when cavitation occurs in the pump passage,the discontinuous flow causes pressure change,which affects the pressure pulsation characteristics of the pump.In this paper,the influence of these two influencing factors on blade stress is integrated,and the mechanism leading to crack is studied respectively.The research contents and conclusions mainly include two aspects:1.A full flow CFD calculation was carried out for the main feedwater pump model.The calculation results show that there is obvious stress concentration at the interface between the blade outlet and the side cover,which is prone to cracks or fractures,and the impeller side cover is close to the stress concentration.There is a maximum deformation displacement;the results of the transient and modal calculations of the flow field show that the second-order dominant frequency of the impeller’s actual damage position is close to the third-and fourth-order natural frequencies of the impeller.Therefore,the local mode resonance is excited,and the impeller is subjected to a large alternating stress,which causes the fatigue damage of the impeller.2.Aiming at the working conditions of the mixed flow pump and turbine model pumps,three flow conditions of 1.2Qd,1.0Qd,and 0.7Qd were selected to carry out cavitation numerical simulation and experimental research,and the reliability of the numerical calculation was verified by experimental results.The blade stresses in different air chemical conditions were studied through simulation calculations.The research showed that the maximum equivalent stress value of the blades increased with the decrease of the cavitation number and then gradually decreased to the lowest value.When cavitation is severe,the stress decreases instead,because the surface of the blade is covered by a large number of bubbles during severe cavitation,and the effect of bubble collapse on the blade cannot be simulated.Therefore,in this paper,the impact force of the bubble on the blade surface is considered,and the impact of the collapse force on the blade stress is studied and analyzed by applying a large to small bubble force to a region with a large volume fraction of the bubble.The study found that the bubble collapse effect has a significant effect on the blade stress distribution and deformation.With the decrease of collapse force applied,the maximum equivalent stress value of the blade gradually decreases,and its position moves from the blade inlet to the blade outlet.These parts are vulnerable to fracture due to structural reasons.Under the design flow condition: when the collapse force is greater than 0.75 mpa,the maximum equivalent stress value is greater than the yield strength of the material 355 MPa,at which point the impeller blade breaks.The maximum blade deformation occurred in the regions with large bubble volume fraction.Under low flow conditions:when the collapse force is greater than 1MPa,the maximum equivalent stress value is greater than the yield strength of the material 355 MPa,at which point the impeller blade breaks.When the failure force is greater than 0.1mpa,the place with the largest bubble volume fraction corresponds to the place with the largest blade deformation.When the collapse force is less than 0.1mpa,the maximum deformation of the blade does not occur in the region with large bubble volume fraction,but at the outlet of the blade.