Experimental investigation on pressure fluctuation characteristics of a mixed-flow pump as turbine at turbine and runaway conditions

Abstract

The pump as turbine (PAT) converts water energy into electricity energy, which greatly accelerates the construction of new power system. However, in the actual use of pump as turbine, its complex internal flow usually causes high-amplitude pressure pulsation, which seriously affects the operation efficiency of the PAT. The investigation on pressure-pulsation characteristics of a mixed-flow pump at turbine and runaway conditions is carried out in an open test rig, and the frequency domain characteristics of the pressure pulsation are analyzed based on FFT (Fourier Frequency Transform). The results show that, at the turbine mode, when the flow rate is greater than 0.8Q, the pressure fluctuation amplitude is almost three times that under small flow condition. Furthermore, when the flow rate is greater than 0.9Q, the shaft frequency and blade passing frequency occupy the dominant and secondary frequency positions in terms of the pressure fluctuation at the inlet of the volute, respectively. Moreover, when the flow rate is greater than 0.9Q, the pressure amplitude of the high-frequency component in the second section of the volute is very high, and the pulsation amplitude under a high flow rate condition is almost three to four times that under a low flow rate condition. The above phenomena show that the mixed-flow PAT is more suitable for operation under the flow rate conditions of 0.7Q and 0.8Q. In addition, at the runaway condition, when the head increases to 5.8 m, the pulsation amplitude at 7 times the shaft frequency increases significantly, which is almost 3.7 times that at the first two head conditions. The pressure pulsation which ranges from the double blade passing frequency to the triple blade passing frequency is significantly intensified, and the amplitude of the pressure pulsation at 7 times the shaft frequency is almost 3.7 times that at 3.75 m and 4.9 m.