ROTATING BLADE FLOW INSTABILITY AS A SOURCE OF NOISE IN AXIAL TURBOMACHINES

An experimental study is presented to investigate the aeroacoustic gen eration mechanism of the tip clearance noise in axial turbomachines. I n addition to the increased broadband levels reported in the literatur e when the tip clearance is enlarged, significant level increases were observed within narrow frequency bands below the blade passing freque ncy. Measurements of the pressure fluctuations at the casing wall just upstream of the entrance plane of the impeller and on the rotating bl ades reveal that the tip clearance noise is associated with a rotating blade flow instability at the blade tip which in turn is only present under reversed flow conditions in the tip clearance gap. The rotating instability is interpreted as a rotating source or vortex mechanism w hich moves relative to the blade row at a fraction of the impeller sha ft speed, similar to the cell(s) of rotating stall. A model for the ge neration of the narrow-band tip clearance noise is presented. (C) 1997 Academic Press Limited.