As centrifugal compressors find a huge number of applications in industry and in aero-engine, the detailed analysis and comprehension of the impeller - vaned diffuser interaction is of interest to improve the efficiency and the operating range. The paper presents the results of a wide experimental campaign devoted to the understanding of the impeller - diffuser interaction; in particular the paper focuses on the features occurring when the compressor works in off design conditions. Data were taken at 3 operating points (near surge, best efficiency and maximum flow rate point) in the impeller - vaned diffuser gap by a fast response probe; the single stage compressor runs at 12500 RPM giving peripheral Mach number of 0.77. At first, data are reduced to evidence the main impeller flow features which are compared at different flow rates. Furthermore time mean diffuser effects on the impeller are commented and finally the impeller - diffuser interaction is discussed. Results evidence the effect of the diffuser on the impeller, mainly in terms of static pressure and flow velocity, which have a strong dependence on the flow rate.
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