Noise source analysis and control for small axial-flow fans

Abstract

Fan noise is always an important topic in applications ranging from small cooling fans for computers, transport vehicles and machinery, and building ventilation fans, to large compressors and turbo-engine fans in aircraft. When air flow with higher pressure or larger flow rate is needed in some operating conditions, two fans in series or a contra-rotating fan is often adopted. The purpose of this project is to conduct the noise source analysis for two identical small axial-flow cooling fans and a newly designed small axial-flow contra-rotating fan (both about 120 mm in diameter), and to explore some passive methods to reduce the more complex radiated noise, compared with that encountered in a conventional single fan.

For two fans in series, a technique of synchronous averaging with time-base stretching is used to decompose the raw sound signals into various noise source components. Acoustic directivity measurements are conducted for two operating conditions: free space and its usual working condition; each condition with two different configurations: free (unobstructed) inlet and distorted inlet by a flat plate covering part of inlet area. For both operating conditions, the rotary noise from the downstream fan dominates over that from the upstream fan for unobstructed inlet while the dominance by the downstream fan is reversed with distorted inlet. The total noise, total broadband noise and rotary noise radiated by the upstream fan all increase gradually with decreased plate-fan spacing. However, the rotary noise radiated by downstream fan is hardly affected by the obstacle plate, whatever the plate-fan spacing is. The flow straightener between the two fans contributes to considerable improvement of aerodynamic performance for free inlet case, while it has a limited role in remedying the severe inlet distortion when placed in front of the upstream fan.

From the derived formula of acoustic spectrum from rotor blade forces resulting from the viscous and potential wakes of the upstream counter-rotating blade row impinging in the downstream one, two obvious mechanisms for interaction noise reduction can be easily found, which are either to increase the order of the Bessel function of the first kind, or to reduce the interaction fluctuating forces which can be realized by reducing the magnitude of the wake centerline velocity deficit or broaden the wake width. Several passive methods for reducing the aerodynamic interaction noise of a newly designed small axial-flow contra-rotating fan are explored. These methods include: (1) Perforated trailing edge for the upstream rotor and perforated leading edge for the downstream rotor; (2) Slitted trailing edge serration for the upstream rotor; (3) Chordwise perforated leading edge of the downstream rotor. Up to 5–7 dB overall noise reduction at all directions is achieved through the first two methods compared under the same aerodynamic output. Only 1.3 dB of acoustic benefit is achieved by the third method, but it hardly causes any adverse impact on the aerodynamic performance. The findings are useful and helpful in designing a quiet coaxial fan (two/multiple-stage or contra-rotating) and adopting an appropriate approach for noise mitigation.