Clearance Effects on the Evolution of the Flow in the Vaneless Diffuser of a Centrifugal Compressor at Part Load Condition

This work reports on flow measurements taken within the vaneless diffuser of a scaled-up model of a small-scale, highly loaded unshrouded compressor with large relative tip clearance. The aims are to describe and to analyze the influence of the clearance flow on the flow structure at the impeller exit in part load operation. The kind of compressor described herein is widely used in distributed power applications and automotive turbocharging. It demands further enhancement of the operation range, as well as a high head rise and an improved efficiency. Therefore, the understanding of flow features and their interaction is crucial. The interaction and mixing of the flow pattern downstream of the impeller are shown using spatially and temporally resolved 3D-velocity data. The measurements have been obtained by using a 3D laser Doppler anemometry system throughout the vaneless parallel wall diffuser. This unique data set provides insight into the development of the flow within the diffuser and allows conclusions on the mixing and migration of the three-dimensional pattern. The flow structure in part load condition is strongly affected by the flow across the large relative tip gap. Due to the large relative tip clearance, a low momentum zone is formed as an additional pattern at the shroud. This clearance flow is highly vortical and interacts with the channel wake structure but remains stable throughout the vaneless diffuser. At the pressure side hub corner, a jet structure is formed, which interacts rapidly with the blade wake. This flow behavior does not comply with the classical jet-wake pattern. It is proposed that in a centrifugal compressor with large relative tip clearance, a modified flow model that includes tip leakage is more appropriate to describe the flow structure at part load condition.