| Abstract | A review of the existing literature on the subject of centrifugal fan and blower noise studies is presented in this report to establish further areas of research needed to aid in the development of a quiet blower. Noise measurements on a wide variety of blowers used in the laboratory, ranging from 1/3 to 700 horsepower are described, with the object of identifying important frequency components from various types of blowers.
The blade passing frequency tone and its harmonics are shown to be produced by the interaction of the flow issuing from the blade exit with the cut off edge formed by the junction of the blower casing and its exhaust duct. The clearance distance between the cut off edge and the impeller blade tip is recognized to be an important parameter in reducing these tones. The random noise is generated by the unsteady flow processes within the impeller. The empirical equations developed to predict the sound power generated have been shown to give a rough estimation of the broadband noise but cannot estimate the tones with adequate accuracy. Attempts have been made to establish fan noise laws based on similarity principles to predict the noise from a group of similar machines based on detailed measurements on a single model fan. The influence of Mach number, Reynolds number, Strouhal number and flow coefficients is discussed.
The blower casing and ducted environment are shown to exert a powerful influence on noise characteristics. The effects of the casing and ducted environment on blower noise have been discussed on the basis of the theory of black boxes as used in communication engineering, and the lumped impedance model. While these methods help the understanding of the acoustical behaviour of the fan system, they fail to describe adequately the effects of the casing and ducted environment, especially at high frequencies. Acoustical resonance effects in small blower casings are discussed.
Among the various blowers tested, the prominent noise component appeared to be the tone at the blade passing frequency. There were two exceptions, in that the noise characteristic of a large 700 horsepower blower was predominantly broadband, but a small 1/3 horsepower blower indicated the absence of the blade passing frequency tones and the presence of a tone due to the struts supporting the blades. |
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