LAUSR

Abstract Research works over the years have identified a recirculating bubble locating at impeller shroud in centrifugal compressor operating under off-design conditions. However, its roles on compressor performance have been traditionally oversimplified. In this paper, the characteristics of the recirculating bubble and the full extent of its impact on compressor performance are analyzed through a well-validated three-dimensional numerical simulation. The simulation results show that the bubble is initiated within the impeller passage and stretches gradually in both streamwise and spanwise directions with decreasing flowrates. During this process, it continues to reshape the effective area and flow conditions at the impeller inlet through its effects of blockage, preheating, and preswirl. The evolution of the recirculating bubble can be divided into two phases depending on whether the center of the recirculating bubble passes beyond the impeller inlet. In the initial phase, the blockage rapidly increases and helps to alleviate the impeller incidence and the relevant losses of the active flow. Meanwhile, the negative effects of preheating and preswirl remain relatively low. In the secondary phase, the upstream propagation of the recirculating bubble speeds up and reinforces its negative effects of preheating and preswirl. The fact that the positive and negative effects are not synchronous implies different roles that the bubble plays in the two phases. A model is established to characterize the effects of the recirculating bubble. A combination of the model and a single-zone meanline method is then employed to quantitatively assess the roles of the recirculating bubble by comparing the compressor performance with and without the bubble. The results show that in the initial phase, the recirculating bubble acted as a positive role in suppressing the incidence loss, the tip clearance loss, and the blade loading loss, leading to higher compressor efficiency. However, in the secondary phase, the negative effects overwhelmed the positive effects, which deteriorated the efficiency. The implications of the two-phase evolution of the recirculating bubble on centrifugal compressor design and flow control are also discussed.