EXPERIMENTAL-STUDY OF TIP CLEARANCE LOSSES AND NOISE IN AXIAL TURBOMACHINES AND THEIR REDUCTION

An experimental study is described to investigate the negative effects of the tip clearance gap on the aerodynamic and acoustic performance of axial turbomachines. In addition to the increased broadband levels reported in the literature when the tip clearance is enlarged, signifi cant level increases were observed within narrow frequency bands below the blade passing frequency. Measurements of the pressure and velocit y fluctuations in the vicinity of the blade tips reveal that the tip c learance noise is associated with a rotating flow instability at the b lade tip, which in turn is only present under reversed flow conditions in the tip clearance gap. A turbulence generator inserted into the ti p clearance gap is found to be effective in eliminating the tip cleara nce noise and in improving the aerodynamic performance.