The geometry of the region between the impeller tip and diffuser throat is accepted as being very influential, little understood and crucial to the performance of centrifugal compressors. This paper presents an experimental investigation of the flow in this region where the diffusing section of the compressor is of the 'pipe diffuser' configuration. The results are presented with an emphasis on designtype correlations. The principal features under investigation were the size of the diffuser throat and the influence of the number of diffuser passages.

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This paper describes the flow analysis in a centrifugal compressor stage using a threedimensional computational fluid dynamics (CFD) algorithm. The flow unsteadiness arising from the interaction between the impeller and the diffuser has also been analysed using an algorithm suitable for equal or multiple numbers of rotor and diffuser blades. The multi-block, structured grid CFD code TASCflow which provides an approximate solution to the Reynolds-averaged Navier-Stokes equations was used as a basis and algorithm development was undertaken to provide the required capability of modelling the unsteady interactions of the impeller and the diffuser.The centrifugal compressor stage under investigation consists of 28 radial blades (of which 14 are splitter blades) in the impeller and 27 vanes in the diffuser. The presented results and analysis are for off-design flow conditions where experimental results were available for comparison. The results obtained for the steady-state model show a reasonable agreement with the measurements. In general the unsteady flow field obtained shows a reasonable agreement with experimental data and exhibits considerable differences when compared with the steady-state results.