This paper describes the recent investigations and results on performance and NPSH-instabilities of a mixed-flow impeller. Several techniques (experimental and numerical) were combined to explain the specific phenomena in the behaviour of impeller momentum-transfer (energy-transfer) under non-cavitating and cavitating conditions. Subject of the investigations was a shrouded impeller of medium specific speed (nq = 61) which shows a performance instability (hysteresis) described by two stable operating points and a NPSH3%-peak at the same particular flow rate. Blade pressure measurements in the rotating system as well as velocity measurements downstream of the impeller in combination with CFD-results were used to explain the differences in momentum-transfer leading to the unstable performance behaviour.

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