A theoretical and experimental study has been conducted on the mass oscillation instability in hydraulic systems with entrapped gas pockets and pumps with positive slope in the head curve. The theoretical study was composed of an analysis of the critical conditions for the instability to develop, followed by the numerical resolution of the fundamental equations that govern the phenomenon, assuming unsteady one-dimensional flow, in order to simulate the limit cycle oscillations of the unstable system. Additionally a series of laboratory tests was conducted on a conventional centrifugal pump, with variation of the initial volume of an entrapped air pocket in the circuit. As expected from the predictions of the theoretical model, instability was found to developed with pressure amplitude oscillations and frequency dependent on the amount of entrapped air.

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