Abstract
An experimental investigation was made to determine the effectiveness of unequally spaced blades and blade/vane interaction techniques in reducing the pure-tone aerodynamic noise generated by vane-axial fans. Balanced mirror-image and other modulated blade patterns were investigated for 13, 19, 20, and 21 blades in combination with nine equally spaced downstream stator vanes. In-duct discrete-frequency noise analyses were obtained. Blade-passing frequency and its harmonics were modulated and reduced 10–20 dB, using unequally spaced blades. Large spacing deviation had the effect of spreading the sound energy to more harmonics of rotational frequency. In some instances, significant downstream stator vane and upstream strut frequencies were observed. Predictable pure-tone multiples of rotational frequency were also detected. Elimination of inlet strut frequency and large reductions at the harmonics of blade-passing frequency were obtained upon replacement of the inlet bell and stationary nose cone and eight support struts with an inlet bell and spinning nose cone. Unequally spacing the blades of the fan had little effect on the ability of the fan to move air.
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