Effects Of The Impeller Structure On Pressure Fluctuations In The Double-volute Centrifugal Pump

The impeller assumes a pivotal role in the operation of the pump.It is conceivable that the impeller structure has a significant influence on the flow inside the pump.Exposed to the rotating impeller and intricate geometries of hydraulic components,flow structures in the pump are featured by multiple scales and complex shapes.Flow structures determine to a large extent the pressure distribution in the pump.More importantly,pressure fluctuations in the pump are related to flow structures as well.It has been widely acknowledged that pressure fluctuations incur pump vibration and noise and even give rise to operation instability of the pump.Therefore,studying unsteady flows and pressure fluctuations in the pump with different impeller structures is of great significance.In this dissertation,both numerical and experimental techniques were used to study flows and pressure fluctuations in the double-volute centrifugal pump.Different impeller structures were devised and investigated.Effects of the combination of the impeller and the volute were described and explained.Main work and conclusions are as follows:(1)Flows in the pump with the regular impeller were simulated using the commercial computational fluid dynamics(CFD)code ANSYS CFX.Pressure monitoring points were deployed in the pump to obtain pressure fluctuation signals.Various flow rates were considered.Typical flow structures,pressure fluctuation characteristics in the time and frequency domains were analyzed.The results indicate that the impeller-volute interaction is intensified with the introduction of the double volute.The impeller passage is wide,facilitating the production of adverse flow structures such as the secondary flow and the axial vortex.With decreasing flow rate,local low-pressure areas are witnessed and the influence of the axial vortex is extended to a larger scope.As the flow rate increases,the accordance between the flow direction in the impeller passage and the impeller blade profile is enhanced.(2)Numerical simulation was carried out for the three impeller schemes with different impeller structures.A comparison was implemented between the three schemes from the aspects of flow parameter distributions,dominant frequencies and pressure fluctuation amplitudes.A comprehensive evaluation of the effect of the impeller structure on pump performance,flow characteristics and pressure fluctuations was implemented.The results show that the inlet flow for the impeller is regulated with the scheme of long and short blades.A short distance between the inlets of the short and the long blades results in non-uniform flows near the two inlets.In this case,the production of small-scale flow structures and pressure fluctuations is facilitated.The scheme of staggered blades helps to mitigate pressure fluctuations in the pump.At large stagger angle,the suppression of blade passing frequency is remarkable.Nevertheless,the two impellers are thereby independent and the blade number is doubled.Consequently,pressure fluctuation magnitude corresponding to the doubled blade passing frequency rises.The dividing plate between the two blade layers aggravates the inlet flow quality.The intrusion of two splitter blades between neighboring long blades reinforces the interaction between the impeller and the volute tongue.The number and length of the splitter blades impose a significant effect on predominant pressure fluctuation frequencies.Shorter splitter blades help to mitigate the pressure fluctuation at the pump outlet and to lower the pressure fluctuation amplitudes associated with medium and low frequencies.(3)Experiments were conducted for the two schemes,the scheme with the regular impeller and the scheme with splitter blades.Pressure fluctuation signals were acquired and the pump vibration was measured with the monitored point deployed at the pump base.Fast Fourier transformation(FFT)and RMS energy analysis methods were used to extract further information of pressure fluctuations.The influence of the impeller structure on pressure fluctuations and the pump vibration was described.Experimental results demonstrate that the maximum pressure fluctuation amplitude for the scheme with the regular impeller arises at the tripled blade passing frequency.Near the volute baffle,the RMS energy of pressure fluctuations attains its minimum at the design flow rate.It increases as the flow rate deviates from the design flow rate.The dominant frequency of pressure fluctuations for the scheme with splitter blades is the blade passing frequency;corresponding pressure fluctuation amplitude considerably exceeds its counterparts.This proves that pressure fluctuation energy is more concentrated for the scheme with splitter blades.Difference between the numerical and experimental results is seen.Frequency signals obtained in the experiment are more abundant.With the numerical work,only predominant frequencies related to the blade passing frequency are captured.The participation of splitter blades promotes to some extent the influence of blades on the fluid.The pressure fluctuation energy is concentrated near the blade passing frequency.Moreover,pressure fluctuation amplitudes corresponding to other frequencies are reduced.The vibration acceleration obtained at the pump base indicates that the predominant pressure fluctuation in the pump serves as an important excitation source for the pump vibration.