The paper presents the results of an experimental investigation on the three-dimensional relative flow at the exit of the backward bladed centrifugal impeller of the high-pressure stage of a two-stage biregulating pump-turbine model, operating at the pump nominal point. Mean velocity, Reynolds stress tensor, and total pressure of the relative flow have been measured with stationary hot-wire probes and fast response miniature pressure transducers, by means of a phase-locked ensemble-average technique. The results, shown in terms of secondary vector plots and contours of mean flow characteristics and Reynolds stress components, give a detailed picture of the flow kinematic structure and of the complex relative total pressure loss and turbulence distributions. In spite of strongly backswept blades, the flow leaving the impeller presents a jet and wake structure and an intense secondary flow activity. Large relative total pressure losses affect the wake and the region where secondary vortices interact. The turbulence data analysis provides information about the effects of the impeller rotation on the turbulence structure and about the mechanisms of the flow mixing process and of the secondary flow decay downstream of the impeller.
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