Prediction of blade-passing frequency noise in a low specific speed centrifugal blower

The blade-passing frequency noise of a low specific speed centrifugal blower is discussed by separating the intensity of the noise source and the response of the transmission passage. The frequency-response function in the transmission passage has previously been evaluated by the authors using a plane wave model, and the results have indicated various effects of the blower speed, blade number and inlet duct length. In the present study, the effective location of the noise source is determined by analyzing the cross correlation between measured data of the blower suction noise and pressure fluctuations on the scroll surface. Then, the surface density distribution of a dipole source is determined from the pressure fluctuation which is expressed by the quasi-steady dynamic pressure of the traveling blade wake. By integration of the surface density spectrum of the noise source over the effective source area, the free-field noise level is obtained. The blower suction noise level is finally predicted by operating the frequency-response function. The results agree well with the measured data, and the effect of the distance between the impeller and the scroll cut off is accounted for satisfactorily.