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This paper presents and discusses the results of an experimental program that has been made on an air test rig of a radial flow pump. The tested impeller is the so-called SHF impeller. Many experimental data have already been produced (tests in air and in water) on that geometry and these results are still used as databases for the validation of CFD codes. For the present study, an air test rig has been chosen for optical access facilities and measurements were realized with a vaneless diffuser. The 2D Particle Image Velocimetry technique has been used and measurements of flow velocities have been made simultaneously in the outer part of the impeller and in the vaneless diffuser. Measurements have been realized in five planes, in the hub to shroud direction, for various relative flow rates (design and off-design operating conditions). First, the paper focus on the evolutions of the phase averaged velocity charts in the impeller and the diffuser. Limitations of the phase averaging technique clearly appear in the very low partial flow rates and this will be related to previous pressure measurements analysis establishing the occurrence of rotating stall within the impeller for such operating conditions. The paper also proposes an analysis of the rates of fluctuations of the velocity charts and the evolutions in the various measuring planes as the relative flow rate becomes lower.

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