Different experimental techniques were used to investigate the structure of the rotor tip flow in a highly-loaded single-stage axial-flow pump. Probably the most important result of this investigation is the first-time visualization of a spiral-type vortex breakdown in an axial-flow turbomachine. The occurrence of the breakdown leads to a rapid expansion of the clearance vortex in stream wise, span wise and pitch wise direction and this has a major impact on stall inception. In deep stall the rotor tip flow is dominated by the well-known part load recirculation vortex as well as by a lesser-known cross passage vortex. Furthermore, the paper presents a simple but very effective type of casing treatment, which stabilizes the pump head characteristic completely.

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