| With the enlargement turbofan’s diameter and thrust, the aeroacoustic noise of turbofanis becoming more and more intense, so we must reduce the turbofan noise to satisfy theairworthiness certification of civil aircraft. There are lots of rotor/stator or stator/rotorinteractions in multi-stage turbomachinery, and the interaction flow generates significantaeroacoustic noise, which is the main noise source in turbomachinery.The rotor/stator or stator/rotor interaction noise is the main noise source ofturbomachinery. The reduction of the rotor-stator interaction noise in turbomachinery hasbeen researched thoroughly. Adjusting the number of rotor and stator blade to satisfy theTyler-sofrin cut-off theorem, laying acoustic liner at the duct, and adopting skew/sweepblade or increasing the axial gap of rotor-stator are the traditional methods to decrease theturbofan noise. Nowadays, the technique of changing the geometry and the relative positionof the blade are widely applied to reduce the turbofan noise. Although the rotor/stator orstator/rotor interaction noise is reduced by increasing the axial gap of rotor/stator blade row,the length of the aeroengine will also be increased, and the thrust-weight ratio will decreaseas well. In this paper, on the basis of the fixed axial gap of rotor/stator blade row, throughmatching the phase of the unsteady pressure fluctuation in radial direction, the interactionnoise will reduced by the blade of stator leaned in circumferential direction.The research object of the present study is a1.5stage low speed axial compressor, andstator of the compressor has adjustable lean angle blades. Through experiments, CFDsimulations and noise prediction, the impact of blade lean angle to the internal flowproperty and aeroacoustic noise were uncovered. The noise reduction mechanism of leanblade was studied in detail, and the influence of blade lean angle to the phase of rotor’swake was also discussed carefully. Finally, the method of noise prediction based on the duct mode was employed to reveal the impact of the lean blade. Detailed contents are listed asfollows:1. The test rig of the compressor was established to measure the aerodynamicperformance at different stator blade lean angles. And the aerodynamic performance curvesof the compressor with stator blade angles of-25°,0°and25°were obtained. Theaeroacoustic duct mode has been measured by rotating casing using dynamic pressuresensor, and the stator/rotor interaction mode of first order harmonic has been captured.2. The internal flow characteristics of different stator blade lean angle were revealedby unsteady CFD, and the results were validated by dynamic pressure sensors fixed atshroud. The development of the rotor’s wakes at stator’s passage, the flow property of theboundary layer at stator blade, and the phase of rotor’s wake were employed to reveal theimpact of the stator’s blade lean angle. Additionally, the disturbance coefficient andunsteady force of the blade were employed to analyze the interaction noise source.3. The noise measurements were carried out at the outside of the lab to get the noisedistribution characteristics at the compressor inlet and the interface of rotor with stator. Thetonal noise, the broadband noise, and the ratio of tonal noise were employed to illustrate thenoise control effect. The noise measurement show that the stator with positive lean bladereduced noise better than the negative lean, and the blade lean angle should exceed10°toobtain better noise reduction.4. By adopting the unsteady CFD results, the noise prediction model was establishedby Green duct function and the Helmholtz function, which based on the Tyler-sofrin ductmode and strip theory. The cut on/off condition of this compressor was studied thoroughly,both on the stator/rotor and rotor/stator interaction. The noise prediction result shows thatthe stator with positive lean blade has better noise reduction effort than the negative lean. |
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