| This work was supported by Jiangsu Provincial Project for Innovative Postgraduate of China (Grant No. CXZZ13-0673) and Project of the University of Padova (Grant No. NCPDA130025/13).For the variable requirements of energy production and consumption, a wide head range is desired for modern pump turbine operation. The reversible pump turbine under pump model could be seen as a centrifugal pump with vaned diffuser. In order to get a wider stable head range, the performance curve instabilities or sudden deviation from the expected smooth performance curves should be avoided.In the present study, the analyzed pump-turbine is a low-pressure stage of a two stages pump-turbine in the pump operating mode. On the performance curve of the test pump, two hump instability regions are detected bellow design flow rate. One is between0.45to0.70Qdes, and the other one is below0.40QDES which is due to the effect of fully developed inlet recirculation. The stable operation range of the pump is restricted by the hump instability between0.45to0.70QDES-The aim of this investigation is to analyze the characteristics instabilities during the hump instability region, study the development of the unsteady phenomena, and find the cause of the hump in instability region which could guide the design and optimize the pump.An Open Turbomachinery Facility (OTF) is improved to conduct the experimental research about the flow instability in a centrifugal pump with vaned diffuser. A program is written to realize the function of pressure data acquisition and post-analysis based on Labview and Matlab. In this study, the characteristics of dynamic pressure signals in frequency domain and time-frequency domain are analyzed combined with the higher order spectral analysis method to diagnose the type of the unsteady structure in centrifugal pump with vaned diffuser.In order to analyze and describe the movement in flow passage between vanes in diffuser and return channel qualitatively, high-speed flow visualization is performed using Photron FASTCAM PCI digital camera.In this research, numerical simulation is carried out both on the flow field and the relative acoustics field. The effect of leakage flow on the fluid flow would not be considered to be negligible in this research. The leakage system is modelled both at the impeller inlet and outlet on the basis of the experimental data. The distribution of the velocity at the boundary of leakage is calculated based on this model and set as a boundary condition in the flow field simulation by ANSYS-CFX. The dipole sources caused by the surface pressure are predicted by a hybrid method through the ANSYS-CFX and LMS Virtual. Lab Acoustics.It is found that the leakage system adopted to model the effect of leakage flow in pump is of a reasonable performance. Thus, the simulation of the flow field in this research could be used as reliable results to help understand the features of the flow in pump under different flow rates. In this study some conclusions from the comparison between experimental and numerical calculation are listed as below.1. Through the above analysis, the unsteady flow under design flow rate, which occurs in return channel, includes several periodic fluctuations. Both the forced fluctuation caused by impeller rotating (such as blade pass frequency) and the unforced fluctuation at St1=0.6625are obvious, and the intensity of these unsteady patterns mainly presents to be strongest near hub.2. Although several periodic unsteadiness are found under full load, based on the acoustic analysis in diffuser and return channel, it is found that the intensity of acoustics pressure at BPF plays a critical role in the contribution of the dipole sources under full load.3. It is indicated that, with the falling flow rate, a disturbance at St2=0.335occurs near the throat of suction side of each impeller blade when the flow rate is below the critical flow rate0.7QDES.This disturbance hardly impairs the head of the impeller, but exacerbates the uneven distribution of velocity in the inlet triangle region of diffuser. It plays a remarkable role in the trend of head losses in diffuser. At the same time, the periodic unforced unsteadiness at St1=0.6625from return channel also plays an important role on the saddle instability of Q-H curve.4. The disturbance at St2=0.335result in a flow separation at the leading edge of vane pressure side, and the disturbance at St1=0.6625from return channel also causes dramatic unsteady flow at the outlet of diffuser. With further reducing of flow rate, these two kinds of unsteady patterns mix together and cause a blockage at the inlet of diffuser near shroud section. At the beginning of Q-H curve instability, the recirculation at inlet of diffuser is the exchange of momentum, which means that the head increases. While with the further development of the mixed flow, the interaction of the fluctuation at St1=0.6625and St2=0.335flow becomes more severe and lead to more energy loss. That is the reason for the pressure amplitude drop at these two frequencies under0.584QDES. By analogy, the energy loss would increase and bring about the drop of head during the fully development of the mixed flow. This can explain why the positive slope of Q-H curve appears around0.6QDES-Meanwhile, the blockage caused by the mixed flow could be the reason for the drop of the mean variation pressure at the outlet of impeller.5. Additionally, a reduction of the pressure amplitude at St2=0.335and St1=0.6625is detected around0.6QDES-With the further falling flow rate, the disturbance of St2=0.335disappears. It is indicated from the numerical results that the reason for the above phenomenon is caused by the blockage at the inlet of the diffuser incept near shroud. This blockage is generated by the mixture of two local flow separations, and the fluctuations are at St2=0.335and Sti=0.6625, respectively. During the further development of blockage, the interaction of the fluctuation at Sti=0.6625and St2=0.335becomes more severe, and results in more energy loss. |
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