Minguan Yang

Vibration Characteristics Induced by Cavitation in a Centrifugal Pump with Slope Volute

Cavitation is one of the instability sources in centrifugal pump, which would cause some unexpected results. The goal of this paper was to analyze the influence of cavitation process on different frequency bands in a centrifugal pump with slope volute. And special attention was paid to low frequency signals, which were often filtered in the reported researches. Results show that at noncavitation condition, vibration level is closely related to flow structure interior pump. At partial flow rates, especially low flow rates, vibration level increases rapidly with the onset of rotating stall. At cavitation condition, it is proved that cavitation process has a significant impact on low frequency signals. With cavitation number decreasing, vibration level first rises to a local maximum, then it drops to a local minimum, and finally it rises again. At different flow rates, vibration trends in variable frequency bands differ obviously. Critical point inferred from vibration level is much larger than that from 3% head drop, which indicates that cavitation occurs much earlier than that reflected in head curve. Also, it is noted that high frequency signals almost increase simultaneously with cavitation occurring, which can be used to detect cavitation in centrifugal pump.

Effect of geometrical parameters on submerged cavitation jet discharged from profiled central-body nozzle

The flow characteristics of cavitation jets are essential issues among relevant studies. The physical properties of the jet are largely determined by the geometrical parameters of the nozzle. The structure and cavitation jets characteristics of the angular-nozzle and the self-resonating cavitation nozzle have been extensively studied, but little research is conducted in the central-body cavitation nozzle mainly because of its hard processing and the cavitation jet effect not satisfactory. In this paper, a novel central-body nozzle (a non-plunger central-body nozzle with square outlet) is studied to solve above problems. Submerged jets discharged from the novel central-body nozzle are simulated, employing the full cavitation model. The impact of nozzle configuration on jet properties is analyzed. The analysis results indicate that when central-body relative diameter keeps constant, there is an optimal contraction degree of nozzle’s outlet, which can induce intense cavitation in the jet. The central-body relative diameter also affects jet profiles. In the case of large central-body relative diameter, most of the bubbles settle in the jet core. On the contrary, a smaller relative diameter makes bubbles concentrate in the interface between the jet and its surrounding fluid. Moreover, the shorter outlet part allows the cavitation zone further extend in both the axial and racial directions. The research results further consummate the study on the central-body nozzles and the correlation between cavitation jet and the structure, and elementarily reveal the mechanism of cavitation jet produced in a non-plunger novel central-body nozzle and the effect of the structure parameters on the cavitation jet, moreover, provide the theoretical basis for the optimal design of the nozzle.

Unsteady Phenomena Induced Pressure Pulsation and Radial Load in a Centrifugal Pump With Slope Volute

Vibration in centrifugal pump still remains a tough problem to solve. Many studied have been done to find the relationship between hydraulic design and vibration. Both experiments and numerical simulation methods have been proposed to make clear the influence of pump parameters related to vibration. It is evident that unsteady phenomena in centrifugal pump are the main factors causing vibration. Also motor-stator interaction of impeller and volute keeps predominant role considering hydraulic factors.Till now almost all reported researches consider pump geometry parameters, but the shape of volute is rarely proposed. To reduce interaction between impeller and volute tongue, a special volute with slope diffuser section is put forward in this paper. The relative position of volute tongue changes compared to the conventional spiral volute. Thus the effect of flow field striking with volute tongue can be receded effectively, and vibration will be reduced.As the developing of computer technology, it is possible for us to achieve the unsteady flow field inner pump by using numerical simulation methods. A commercial software Fluent was adopted to analyze pressure pulsation and radial load of model pump. Slant angle and clearance rate were optimized considering pressure magnitude. It is observed that pressure amplitude at blade passing frequency achieve the lowest level at slant angle 15°. Pressure magnitude decreases significantly with the increasing of clearance, but limited to radial size of volute, it is suggested to be in the range of 0.134∼0.250.Several monitor points are selected along the volute to have overall understanding of pressure pulsation characteristics. Pressure pulsation at blade passing frequency keeps the predominant in motor-stator interaction. Amplitude of both pressure and radial load increase rapidly when pump operating at off designed conditions.Copyright

Experimental Investigation on Cavitating Flow Induced Vibration Characteristics of a Low Specific Speed Centrifugal Pump

Cavitating flow developing in the blade channels is detrimental to the stable operation of centrifugal pumps, so it is essential to detect cavitation and avoid the unexpected results. The present paper concentrates on cavitation induced vibration characteristics, and special attention is laid on vibration energy in low frequency band, 10–500 Hz. The correlation between cavitating evolution and the corresponding vibration energy in 10–500 Hz frequency band is discussed through visualization analysis. Results show that the varying trend of vibration energy in low frequency band is unique compared with the high frequency band. With cavitation number decreasing, vibration energy reaches a local maximum at a cavitation number much larger than the 3% head drop point; after that it decreases. The varying trend is closely associated with the corresponding cavitation status. With cavitation number decreasing, cavitation could be divided into four stages. The decreasing of vibration energy, in particular cavitation number range, is caused by the partial compressible cavitation structure. From cavitation induced vibration characteristics, vibration energy rises much earlier than the usual 3% head drop criterion, and it indicates that cavitation could be detected in advance and effectively by means of cavitation induced vibration characteristics.

Unsteady Pressure Pulsation and Rotating Stall Characteristics in a Centrifugal Pump with Slope Volute

Unsteady flow structures can lead to severe vibration in centrifugal pump if the eigenfrequency of the rotor is equal to excitation frequency. In order to reduce rotor-stator interaction in centrifugal pump, a special slope volute was proposed. This paper explores the use of numerical simulation method to illustrate unsteady pressure pulsation and rotating stall characteristics under 0.05ΦN–1.4ΦN working conditions. Spectrums of pressure pulsation signals at different flow rates were analyzed. Relative velocity distributions interior blade channels were also studied to clarify correlation between flow structure and pressure spectrum. At high flow rates, predominant components in pressure spectrums always correspond to blade passing frequency (fBPF). With decreasing of flow rate, partial flow separates from suction side of blade at 0.6ΦN, but the separate structure has little impact on pressure spectrum. From 0.8ΦN to 0.6ΦN, peak values in pressure spectrums are still located at fBPF. At rotating stall working conditions, multiple vortex structures exist in impeller, which develop with rotating impeller showing intensive unsteady properties. And partial blade channels are blocked severely. Due to the unsteady stall cell structure, stall frequencies are generated in pressure spectrum, and the excitation frequencies are different at variable flow rates.

The acquisition and measurement of surface waves of high-speed liquid jets

The instability analysis of the liquid jet issuing into ambient air was conducted with an emphasis placed upon the evolution of surface waves of the jet. An experiment was designed to visualize the microscopic morphology on the surface of a liquid jet. A spectral method was proposed to measure wavelength from the obtained jet images. We also discuss key setup parameters that significantly affect the resolution of desired jet features and the accuracy of the spectral measurement. The results show that the liquid jet near the nozzle exit can be divided into a laminar section, a transition section, an instability section, and a turbulence section. Surface wave scales range from 0.06 to 0.11 times of the nozzle diameter with the atomization breakup regime. For the atomization breakup regime, the growth ratio of the surface waves of the instability section is 0.06 which is 1.5 times the value of the second wind-introduced breakup regime and 3 times the value of the first wind-introduced breakup regime.Graphical abstract

Experimental study of jet surface structures and the influence of nozzle configuration

Under the three breakup regimes, the jet surface waves of different nozzles are captured and measured. The nozzles have different length to diameter ratios and contraction angles. The measured wavelengths are compared with the reported conclusions which were obtained by using spatial and temporal linear stability analysis. The results show that the jet wavelengths of different breakup regimes are covered by a single curve when the wavelengths are non-dimensionalized with boundary layer thickness. For the nozzle with equal length and diameter, the entire translation section starts at Re = 3 × 104 and ends at Re = 4.5 × 104. The wavelength non-dimensionalized with boundary layer thickness is independent of nozzle configuration. The ratio of initial wavelength to boundary layer thickness ranges from 2 to 4.

Wavelength measurement of a liquid jet based on spectral analysis of image intensity

The instability analysis of the liquid jet issuing into ambient gas was conducted with the emphasis placed upon the evolution of surface wave on the jet surface. First, an experimental method was developed to visualize the microscopic surface wave on the liquid jet. Camera setting parameters significantly affecting the detection of desired jet features were discussed. Second, a spectral method was applied to process the obtained jet images. The accuracy of this method was validated in several ways. The results show that wavelengths increase monotonically along the streamwise direction and decrease with the increase in Reynolds number which corresponds to the boundary layer momentum thickness at nozzle exit. Various patterns of wave structures on jet surface are revealed. In this article, the pattern transforms from three-dimensional to two-dimensional at Reynolds number of 134.53.

Investigation on Vibration Characteristics in a Centrifugal Pump with Special Slope Volute

Vibration is one of the main problems which should be considered seriously in designing and operating a centrifugal pump. In this paper, a special slope volute was proposed to reduce vibration level of centrifugal pump. The goal of this paper was to analyze vibration characteristics of slope volute pump and provide a new method to reduce vibration energy in centrifugal pump. Experiments were carried out to obtain vibration signals at different flow rates of slope volute pump, which were compared with conventional spiral volute pump. Results show that vibration level is closely related to flow structure interior pump. At low flow rates, especially, vibration energy increases rapidly with the onset of rotating stall. Rotor-stator interaction mechanism changes when adopting slope volute. In slope volute pump, the peaks in power spectra correspond to different harmonics of fBPF at three measuring directions; however, in spiral volute pump, the predominant components always correspond to 2fBPF. From comparison of vibration level in different frequency bands, it is found that vibration level of slope volute pump is smaller than conventional spiral volute pump. So vibration energy of centrifugal pump can be reduced effectively using slope volute.

The Unsteady Flow in a Centrifugal Pump With Special Slope Volute

Conventional single stage centrifugal pumps are mostly designed with a spiral volute. Due to relative movement between impeller and the asymmetric volute the flow at the outlet of the impeller is strongly interacting with the volute flow. The unsteady phenomenon leads to unbalanced radial dynamic forces as well, especially at off-design points, giving rise to pump vibration and hydraulic noises.In order to weaken the unsteady phenomenon, a special kind of slope volute is designed. Different from the spiral one, it keeps the radial size of the volute casing unchanged, but increasing the axial size to make sure the sectional area changing regularly from the tongue to pump outlet. CFD analysis has used to solve the unsteady 3D viscous flow in both conventional and special designed centrifugal pumps with the same impeller for several flow rates.Compared to the spiral volute pump, there’s secondary flow with only one vortex existing in the slope volute. The average pressure and amplitudes of pressure fluctuations keep nearly unchanged along the slope volute wall due to the symmetrical shape in the radial direction. The pressure distributions for both pumps at fBPF are also very different. Interaction of the impeller flow with the tongue is weaken due to the different kind of tongue shape. It has little influence on the interaction flow field in the pump with slope volute. The radial load component always reaches a minimun magnitude at nominal conditions for both pumps. But the traces calculated for the pump with slope vloute are less stretched in the radial direction than those computed for the pump with spiral volute. Results indicate that a lower pulsation of the radial force is expected, and hardly affected by the flow rate for the pump with slope volute.Copyright