The behaviour of vaned radial diffusers is generally considered to be due to the flow phenomena in the vaneless and the semi-vaned space in the diffuser inlet region. Even considering unsteady aspects, the adjacent diffuser channel is regarded as less important. The flat wedge vaned diffuser of the investigated centrifugal compressor stage allows an independent continuous adjustment of the diffuser vane angle and the radial gap between impeller outlet and diffuser vane inlet, so that information about the importance of these geometric parameters can be obtained.

The time dependent pressure distribution on the diffuser front wall and on suction and pressure surface of the diffuser vanes reveal that in the semi-vaned space mainly the region near the vane suction side is influenced by the unsteady impeller-diffuser-interaction. Downstream in the diffuser channel the unsteadiness does not decay. Here, pressure fluctuations are appearing which are distinctly higher than the pressure fluctuations in the vaneless space.

An estimation of the influence of the unsteadiness on the operating performance of the centrifugal compressor stage is made by measurements at choke and surge limit for different diffuser geometries.

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