IGTC-14 An Acoustic Model for Prediction of Centrifugal Blower Noise Induced by Blade Discharge Flow around the Scroll Cut Off(Session A-6 NOISE AND PRESSURE FLUCTUATION)

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The blade-passing frequency noise of a centrifugal blower should be argued by separating contributions of the noise source and the frequency-response in the transmission passage. The authors have developed a one-dimensional wave model by which the frequency-response characteristics are clearly obtained. The results have indicated effects of the blower speed, blade number and inlet duct length. In the present study, the source strength terms are discussed on the basis of correlation analyses of the acoustic and surface pressure data, and estimated by introducing a quasi-steady model of the blade wake impingement on the scroll surface. The effective location and area of the noise source are determined by analyzing the cross-correlation between measured data of the blower suction noise and pressure fluctuation on the scroll surface. Then, the surface density distribution of a dipole source is determined from pressure fluctuation expressed in terms of a quasisteady dynamic pressure of the traveling blade wake. Finally, the free-field noise level is obtained by integrating the density spectrum of the noise source over the effective source area. The results indicate that the predicted amplitude of the blower suction noise agrees well with the measured data within five decibels. The contribution of the distance between the impeller and the cut off is satisfactorily indicated.
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