Study of Rotational Noise Reduction for Axial Flow Fan : Analysis and Estimation of Secondary Fan-Noise Component

Abstract

Rotational noise is one of the main sources of noise emitted by a fan. This type of noise has discrete frequency components, and is so severe that many methods have been developed for its reduction. There have been few studies on rotational noise, however, to clarify the generation mechanism and determine concrete methods for noise reduction. In the present study the spacings of the blades were changed and comparisons were made with a fan having equally spaced blades. The discrete frequency components were found to be suppressed in the fan with unequally spaced blades and the noise tended to become white noise if the blade spacing and blade setting angles were properly chosen. The instantaneous pressure change on the shroud due to blade passing was examined since it is closely related to the discrete frequency components of noise. By decomposing the measured pressure change in terms of a Fourier series the characteristics of rotational noise were analyzed and compared with simulation results obtained using a pressure wave model. The prediction of the rotational noise emitted by axial fans with unequally spaced blades is presented and found to agree well with the experimental results.