Low-Speed Fan Noise Attenuation from a Foam-Metal Liner

Abstract

A foam-metal liner for attenuation of fan noise was developed for and tested on a low-speed fan. This type of liner represents a significant advance over traditional liners, due to the possibility of placement in close proximity to the rotor. An advantage of placing treatment in this region is that the acoustic near field is modified, thereby inhibiting the noise-generation mechanism. This can result in higher attenuation levels than could be achieved by liners located in the nacelle inlet. In addition, foam-metal liners could potentially replace the fan rub strip and containment components, ultimately reducing engine components and thus weight, which can result in a systematic increase in noise reduction and engine performance. Foam-metal liners have the potential to reduce fan noise by 4 dB based on this study.

Tonal and Broadband Noise Control of an Axial Flow Fan with Metal Foams: Design and Experimental Validation

“…Especially, the sound-absorption coefficient is usually small at low frequencies because the absolute value of the negative acoustic reactance (imaginary part of the complex acoustic impedance) increases with the decrease of the sound frequency and causes an unsatisfactory sound-absorption performance at low frequencies. This conclusion has been validated by the experimental results given in references [14,15], where the sound-absorption coefficient sharply reduces with the decrease of the sound frequency and noise level is hardly reduced at low frequencies (see Fig. 6 and Fig.…”

“…8 Variation of total pressure rise with volume flow rate Before analyzing the noise reduction mechanism of the acoustic liner, experimental results given in Fig. 8 show that both types of metal foams influence the aerodynamic performance of the fan, which has also been reported in references [14,15]. This drawback widely exists for acoustic absorbers owing to viscous dissipative effects.…”

“…Note that a drawback of the SCMF, similar to MPP absorbers, is the difficulty to achieve a good acoustic absorption performance in a wide frequency range. The experimental results of the acoustic impedance given in references [14,15] for samples of OCMF with PPIs from 40 and 60 have shown a good acoustic absorption performance in wide frequency range, and these samples of OCMF have been successfully applied to control the turbofan noise. Recently Xu and Mao [16,17] employed OCMFs with PPIs of 20, 40, 70 and 80 to reduce a centrifugal fan noise.…”

“…OCMFs have been applied to control aerodynamic noise of airfoil trailing edges [10,11] and high-speed train pantographs [12,13]. Recently, OCMFs have also been used to reduce turbofan [14,15] and centrifugal fan noise [16,17].…”

“…It is worth mentioning that the noise control mechanism of the OCMF used in references [10][11][12][13]16] is completely different from that used in references [14,15] owing to the different values of pores per inch (PPI) of the porous materials. OCMFs with a high PPI, such as PPI≈40, 60 and 80 used in references [14,15], absorb the acoustic energy via viscous and thermal dissipations,…”