In order to understand the flow pattern of the diffuser internal flow field and improve the diffuser hydraulic design method, a special test rig was designed and installed to investigate the flow fields in an inward radial diffuser by using the particle image velocimetry(PIV)technique. The test model is one stage of a deep-well multistage centrifugal pump, two water lubricated plain bearings were used to support the pump shaft, and a mirror placed in 45° was used to reflect the flow field of diffu-ser. The velocity flow fields under a variety of operational points were obtained by using the phase averaged method. The results show that the flow in the diffuser is uniform and stable at the design flow rate. At the larger flow rates, the flow is blocked by the finite throat area and a considerable shock loss is generated at the diffuser inlet. At the lower flow rates, flow separation and vortices appear, and the vortex intensity is strengthened with reducing flow rate. Meanwhile, the position of vortex core is mo-ving from the location nearby diffuser suction side to the middle of diffuser. Large hydraulic losses produce at the diffuser inlet, and the diffuser inlet setting angle have a significant effect on the pump performance. The flow area in the middle of diffuser passage needs to be further adjusted to improve the flow pattern under lower flow conditions.
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