Two-dimensional Particle Image Velocimetry (PIV) measurements have been conducted in a radial diffuser pump under different operating points, in order to investigate the flow rate effects on impeller-diffuser interactions. The measurements are conducted at mid-span for different relative positions between the impeller and the diffuser. The measuring region covers a complete impeller channel and also a complete diffuser channel. The analysis of the experimental results shows that the flow rate has striking influences both on the phase-averaged velocity field and on the turbulence kinetic energy field. The influence on the impeller flow is bigger than that on the diffuser flow. Flow separation is observed on the impeller suction side when reducing the flow rate to 0.5Q_(des), and the periodicity between impeller channels disappears. The smallest turbulence level is found to occur at Q_(des) among all four flow rates.
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