Research Of Internal Unsteady Flow Mechanism In Mixed-flow Pump Under Alford Effect

The mixed-flow pump has the advantages of large flow rate,high efficiency,strong resistance to the cavitation performance,and so on.And,it is widely used in water conservancy,agricultural irrigation,pharmaceutical manufacturing,aerospace,petroleum chemical industry,national defense construction and other fields.So,it plays an important role in national economic construction.Due to factors of the manufacturing error,assembly error,inhomogeneous material and so on,the impeller rotates with eccentricity which makes the non-uniform load and circumferential pressure put on each blade.Thereby,a lateral fluid force that is perpendicular to the impeller eccentric direction would generate,namely the ?Alford force?.The exciting force can reinforce the whirling motion of rotor and accelerate the self-induced vibration of impeller.Therefore,it is meaningful to study the internal unsteady flow characteristic of mixed-flow pump under ?Alford effect? which could improve the operating stability of the mixed-flow pump and increase the efficiency performance of mixed flow pump.This paper was funded by the National Science and Technology Support Project “Stability investigation of large-scale mixed-flow pump under Alford effect”(Project No.51409127).Based on the proportional scaling model of a mixed flow pump with guide vanes,the unsteady flow characteristics of the mixed flow pump during eccentric operation are studied by means of numerical simulation and experimental study.The flow field structure of the eccentric mixed-flow pump with non-uniform tip clearance,the time-domain and frequency-domain of the pressure fluctuation and the influence of eccentricity on the exciting force of radial flow were analyzed.Some relevant factors affecting the radical fluid exciting force are given a further analysis.The main contents and results are as follows:(1)The 2D eccentric straight blade impeller rotating model was established and the dynamic mesh was applied to calculate the internal flow field of the eccentric straight blade impeller with the UDF program which is self-programmed.Meanwhile,based on the dynamic mesh method,the internal flow field of the impeller was simplified to analyze the pressure distribution and velocity distribution on the different cross-section of the eccentric mixed flow pump impeller.The research results showed that the whirling frequency ratio and the eccentricity have a dominant impact on the internal flow field of 2D eccentric straight blade impeller.With the increasing of the whirling frequency ratio,the trend of non-uniform distribution of pressure and velocity tends to increase,more and more secondary flow characteristics such as vortex structures and backflow occurs in the flow channels which is the main reason of the energy loss in the flow field.With the growth of the eccentric distance,the pressure in the smallest tip clearance increase evidently and the low pressure area in the leading edge of the blade suction surface distributes irregularly in the peripheral direction,in which the low velocity fields increases that greatly affecting the stability of the impeller.(2)Based on the Dynamic Sliding Multi-region method,the internal flow fields of the mixed flow pump under the Alford effect were simulated,the performance parameter of the eccentric mixed flow pump under different eccentricity and the pressure and turbulence energy distribution of the blade surface was analyzed in detail.The research results showed that: when the mixed flow pump is eccentric,the pump head lost a little while the efficiency decreases a lot at the design flow rate condition.Compared with the mixed-flow pump that is not eccentric,the efficiency decreases approximately 6.5%.With the increase of eccentricity,the pressure distribution generally changes only a few but the high pressure area in the rim of the blade which is closed to the eccentric position increase a lot.The turbulence energy areas near the blade rime also have an upward trend evidently and the high turbulence energy area which is closed to the eccentric position also increases.With the increase of eccentricity,the scale of the leakage flow vortex structure near the blade rim increases gradually intervening the main flow a lot while the leakage flow in the small tip clearance decreases,but the leakage flow shows a non-uniform trend in the peripheral direction.Overall,the energy loss becomes bigger which affects the efficiency of the pump because of the non-uniform tip clearance of blade.(3)Based on the unsteady calculation of the eccentric mixed flow pump,some key points in the mixed flow pump impeller were monitored during the simulation and the Alford force distribution of the impeller under different eccentricity and the whirling frequency ratio were acquired in this chapter.The research results showed that: Under the different eccentricity condition,from the inlet to the outlet,the frequency domain amplitude of the pressure fluctuation of each monitoring point increases first and then decreases.Moreover,near the impeller,the frequency domain amplitude of the pressure fluctuation is much larger.In the direction of the main flow,the frequency amplitude of the pressure fluctuation of each monitoring point increase rapidly under the different eccentricity condition and the fractional frequency elements also increase.Under different eccentricity conditions,the average Alford force of blade rim differs a little.As the flow rate condition deviates away from the design flow rate condition,under the same whirling frequency ratio,the bigger eccentric impeller undergoes larger Alford force,and the mixed-flow pump operates more unstably.(4)The experimental measurement system of the non-uniform tip clearance mixed-flow pump is established,the external characteristic data was analyzed under different eccentricity condition by the external experiment.The rotor-stator interaction zones between the impeller and guide vane of mixed-flow pump were shot by the DANTEC commercial PIV system to explore the internal flow state of the eccentric mixed flow pump impeller.The results show that: As the eccentricity becomes bigger,the efficiency loss and the downward tendency of the head at the design flow rate are obvious.The non-uniform tip clearance causes the distorted flow in the impeller and the secondary flow increases such as the backflow and the vortex structures.Especially,the flow fields near the chamber wall of the impeller are affected a lot where forms high-level and high-scale vortex structures which causes the energy loss.With the increasing of the eccentricity,the instability of the uniform tip clearance flow field also increases and deteriorated the internal flow in the mixed-flow pump which causes the efficiency loss.With the increasing of the flow rate condition,the position of the vortex structure formed changes from the middle of the cross-section to the hub of the impeller.