Evolution of unsteady vortex structures and rotating stall cells in a centrifugal compressor with vaneless diffuser

Abstract

The rotating stall in a centrifugal compressor with a vaneless diffuser is investigated using the IDDES (Improved Delayed Detached Eddy Simulation) method. The flow fields from design to off-design conditions are assessed for comparison. The characteristic frequency of the rotating stall is accurately predicted, illustrated by both the pressure spectra and the velocity signals. This low-frequency peak corresponds to the stall cells, induced by different vortex structures in the impeller passages and the diffuser. Compared with the flow fields of the rotating instability and the near design conditions, a unique flow structure of the rotating stall condition is the longitudinal vortices upstream of the impeller inlet, connecting to the tornado-type vortices in one or two consecutive passages. In the downstream passages, the large-scale vortices on the shroud side cause the blockage. The deformed vortices stretching along streamlines towards the outlet are dominant in the diffuser. Through the high fidelity numerical method, physical mechanisms are analysed in detail to give a new perspective of the rotating stall in a centrifugal compressor.