This paper describes the unsteady flow analysis in a centrifugal compressor stage using a three dimensional CFD algorithm. The flow unsteadiness arising from the interaction between the impeller and the diffuser has been analysed using an algorithm suitable for equal or multiple number of rotor and diffuser blades.

The multi-block, structured grid CFD code TASCflow 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 studied consists of an impeller with splitters and a vaned diffuser. The results presented are for off-design flow conditions where some experimental results were available for comparison. The results obtained for the steady-state model show a good agreement with the measurements.

In general the unsteady flow field obtained show a reasonable agreement with experimental data and demonstrates significant differences when compared to the steady state results especially in terms of the velocity field. A detailed analysis of the unsteady flow field is carried out using Fourier transforms of velocity and pressure at various locations of the flow field and the level of unsteadiness is determined as distributed to various frequencies. The unsteadiness in the impeller passage is much less than in the diffuser where a strong coupling is predicted in the vaneless space.

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