Two stall stages in a centrifugal compressor with a vaneless diffuser

A sufficient understanding of the stall behavior is crucial to improve the stability of a compressor. The stall development process and its sensitive locations in a centrifugal compressor with a volute and a vaneless diffuser are investigated by unsteady simulations and an experiment in this study. During the throttling process, two stall stages are observed: stall I and stall II. During stall I, the mass flow rate has a small fluctuation and a local stall occurs at the impeller inlet. The local stall region is caused by a local spillage flow due to the tip leakage vortex, which results in a static pressure fluctuate at low-frequency and a blade loading reduction in the main blade fore part. During stall II, the mass flow rate fluctuates violently and reduces dramatically, accompanied by a significant reduction in the pressure ratio and efficiency in the experiment. Stall II is the whole-annulus stall caused by the large reverse flow at the impeller inlet region, leading to a large low-frequency signal and a large reduction in the blade loading at the main and splitter blade fore parts. However, stall II also has an effect on the impeller downstream and the diffuser. Due to the influence of the volute, stall I needs to locally extend to a certain size at the impeller leading edge; stall II then appears and causes an abrupt variation in the compressor flow field.