Experimental characterization of rotating instabilities in a centrifugal pump

The paper presents experimental investigations of large-scale instabilities in a centrifugal pump. Pressure fluctuations were measured with transducers placed flush with the inlet duct and at the impeller discharge. Two impeller rotation speeds and two configurations with a vaneless and a vaned diffuser, were utilized in the study, as well as several angular and radial transducer positions at design and at off design flow rates. The spectra analyses of the pressure signals were carried out both in frequency and time-frequency domains to identified precursors, inception and evolution of the pressure instabilities. Because of the non-linearity of the unsteady phenomena developing in the pump, high-order moment functions were applied to discriminate between non-linearly coupled pulsations and self-excited pulsations and to determine the fraction of the power of each pulsation that was due to the nonlinear interaction of unsteady phenomena. The results highlighted the existence of an asymmetrical rotating pressure structure at the impeller discharge, having a fluid-dynamical origin and propagating both in the radial and circumferential direction, inside and downstream of the impeller.