Effects of a slotted diffuser on the aerodynamic performance of a highly loaded centrifugal compressor

Abstract

High-pressure ratio centrifugal compressors usually adopt vaned diffusers to reach high efficiency. Nevertheless, the compressor operating range might be narrow on account of the diffuser stall resulting from the large flow separations in diffuser passages at low flow rates. Flow control techniques, aimed at expanding the compressor operating range, are required to suppress these flow separations. In this paper, the flow control strategy, in terms of the slotted diffuser was used to widen the operating range for a highly loaded centrifugal compressor. The main focus of the research is to address the effects of the slotted diffuser vane on the aerodynamic performance, including the underlay flow physics of the compressor. In the case of the baseline compressor, a large flow separation exists near the pressure surface between the hub and approximately 40% of the span inside diffuser passages over the entire flow range. In addition, flow separation appears at the junction of hub and suction surface at low flow rates, which promotes compressor stall. Therefore, a hub-side slotted diffuser is utilized to mitigate the flow separations inside diffuser passages. Through application of the slotted diffuser, the stall margin is improved a lot without performance decline as a result of the improvement of the flow field within diffuser passages. The flow separation on the suction surface is suppressed by a vortex induced by the leakage flow. Additionally, the flow angle at the diffuser inlet becomes more circumferential owing to the leakage flow passing through the slot; thus, the incidence angle is increased, which results in mitigation of the flow separation at the junction of hub and pressure surface. As a result, a 13.5% stall margin increment is gained in the presence of the slotted diffuser vane.