Experimental analysis of the pressure fluctuations for a prototype pump-turbine at different operating conditions

Abstract

Abstract The pressure fluctuations are one of the crucial instabilities detrimental to the stable operation of pump turbines. The fluctuations are periodic and stochastic. They naturally exist in all kinds of operating conditions and cannot be eliminated. Many studies have been conducted on the pressure fluctuations for pump-turbines. However, detailed analysis of the characteristics of the prototypes is relatively rare and needs further investigation. In this study, six pressure sensors from the spiral inlet to the draft tube outlet have been mounted on a prototype pump-turbine. The recorded pressure values for these sensors were analysed at four different operating conditions from 25% to 100% of BEP (Best efficiency point) loads. Results show the relative fluctuations of vaneless space is only 2.8% at BEP load, showing the prototype operation is stable at the rated power. However, for off-design conditions, the amplitude of dominant frequencies is greatly enhanced. The amplitude is about 18 times at 25% loads compared to that at the BEP load. In addition, the dominant frequencies in the draft tube inlet are characterized by low-frequency components originating from the vortex rope. The current work provides a preliminary understanding of the instability in a prototype pump-turbine and can be used as a reference to minimize the negative effects of pressure fluctuations.