An investigation of small propeller noise with uneven blading

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An investigation of small propeller noise with uneven blading


Author: Chan, Kwok-hung
Title: An investigation of small propeller noise with uneven blading
Degree: M.Sc.
Year: 2000
Subject: Propellers -- Noise
Hong Kong Polytechnic University -- Dissertations
Department: Multi-disciplinary Studies
Dept. of Mechanical Engineering
Pages: iv, 75 leaves : ill. ; 30 cm
Language: English
Abstract: A 20cm-diameter, four-blade propeller is used to investigate noise radiation with various angular spacing between blades. A LabVIEW controlled signal recording and processing system is used to analyze the frequency spectra. Measurements taken include the power consumption, the inlet air flow conditions, dependence of noise on distance, acoustic directivity, and noise spectra. Power consumption and inlet flow condition measurements are essential in establishing the basis of acoustic comparison under similar aerodynamic working conditions. It is found that the propeller power remains fairly constant as the angular spacing changes. Also similar is the inlet flow angle. That means the flow conditions are equivalent when blades are arranged in different angular spacing, and blades work relatively independent of each other. Also, the inlet flow speed measurement shows that the incoming flow speed (at zero 'flight' condition) increases rather linearly with the rotational speed. Therefore the inlet flow angle relative to the rotor remains roughly constant. That means the rotor is able to adjust itself within the range of rotational speed tested. The acoustic studies show that the overall sound pressure level measured in dB decreases with the increase of blade spacing, and it reaches minimum when the blade is evenly spaced. Typically noise radiated by propeller with 30 spacing is 8dB more than that of the evenly spaced, i.e. with 90 spacing. But the quietest spacing changes to 75 at some rotational speed. Spectral analyses reveal that, when the angular spacing is reduced from 90 the acoustic energy concentrated at the blade passing frequency based on four blades is divided between and shifted towards both lower and higher frequencies. This bipolar drift of acoustic energy has the effect of smoothing out the difference of overall noise level when measured with A-weighting. It is beneficial to the propeller rotating at low speed because of low sensitivity of human ear to the lower frequency noise. It is shown that the quietest spacing is 60 and the noise is 1.9dBA lower than the both 30 and 90 spacing. However, we point out that this difference may be enhanced when one considers the far field where high frequency noise is very much damped. The directivity measurement shows the usual pattern for propellers but the details change somewhat with the blade spacing. The mechanism of acoustic self-cancellation is found to be responsible for the difference in noise radiation from propellers with the same aerodynamic power. The fact that such cancellation can be as effective as 8dB means that there is a good potential to reduce noise of small propellers. Finally it is noted that our results contrast sharply with those of others who study noise radiation by unevenly spaced propellers. In their study they found little difference in dB when the angular spacing is changed. This is caused by the fact that the propellers they use are big and noise radiation from each blade has pulse characters. In our case the source is more compact and this provides ample opportunity for noise radiated from one blade to cancel almost completely by noise radiated from its neighbors. The results of this study is more relevant to applications such as noise reduction for small fans used in many electrical appliances.

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