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Research Papers

Experimental Aeroacoustic Studies on Improved Tip Configurations for Passive Control of Noise Signatures in Low-Speed Axial Fans

[+] Author and Article Information
Stefano Bianchi, Alessandro Corsini, Franco Rispoli

Dipartimento di Meccanica e Aeronautica, Sapienza University of Rome, Via Eudossiana, Rome 18 I-00184, Italy

Anthony G. Sheard

 Fläkt Woods Limited, Axial Way, Colchester CO4 5AR, UK

J. Vib. Acoust 131(6), 061007 (Nov 18, 2009) (10 pages) doi:10.1115/1.4000462 History: Received March 18, 2008; Revised July 21, 2009; Published November 18, 2009; Online November 18, 2009

This paper presents the findings of an investigation on the use of several blade-tip configurations (modified by the addition of various end plates at the blade tip) for passive noise control in industrial fans. Utilizing an experimental technique developed to investigate noise sources along the radius of the blades, together with cross-correlation and coherence analyses of the near field and far field, the modified blade-tip configurations are shown to reduce the rotor-only aeroacoustic signature of the fan as a direct consequence of changes induced in tip-leakage flow behavior. These changes in the nature of flow mechanisms in the region of the blade tip are correlated with the spanwise noise sources, and their role in the creation of overall acoustic emissions is thus clarified. The tip-leakage flow structures are analyzed to identify their contribution to overall noise and interaction with other noise sources. Coherence spectra are also analyzed to investigate the relevance of the noise sources. The cross-correlations reveal distinctive acoustic signatures that are described in detail. The methodology has been demonstrated to be effective in identifying (i) the blade-tip configuration with the best acoustic performance, and (ii) other significant noise sources along the blade span. The modified tip configurations are shown to have a significant effect on the multiple vortex behavior of leakage flow, especially with respect to the near-wall fluid-flow paths on both blade surfaces. The reduction in fan acoustic emissions is assessed and correlated with the control of tip-leakage flows achieved by the modified blade tips.

FIGURES IN THIS ARTICLE
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Copyright © 2009 by American Society of Mechanical Engineers
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References

Figures

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Figure 1

Test fan rotor blades and tip end-plates (not-to-scale) (16)

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Figure 2

Specific noise level (Ks) in fan operating range (dashed lines: datum fan; solid lines: TF fan; line-symbols: TFvte fan; dash-dotted blue line: TF_step fan; and dash-dotted blue line-symbols: TFvte_step fan) (19)

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Figure 3

Static pressure rise and ηtot efficiency curves (dashed lines: datum fan; solid lines: TF fan; line-symbols: TFvte fan; dash-dotted line: TF_step fan; dash-dotted line-symbols: TFvte_step fan)

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Figure 4

Test rig setup (not-to-scale)

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Figure 5

Test rig setup for near-field noise measurements

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Figure 6

Sound-pressure level (Lp), narrow-band spectra up to 1 kHz, datum fan, 28 deg, operating point D

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Figure 7

Datum fan: spanwise distribution of outlet coherence spectra for operating point P

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Figure 8

Sound-pressure level (Lp) narrowband spectra up to 1 kHz, datum fan, 28 deg, operating point D

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Figure 9

Spanwise maps of coherence values between the near-field and far-field microphones at operating point D: (a) datum, (b) TF, (c) TFvte, (d) TF_step, and (e) TFvte_step

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Figure 10

Outlet coherence spectra at the blade tip up to 1 kHz: (a) operating point L; (b) operating point D; (c) operating point P (dashed lines: datum; solid lines: TF; line-circle: TFvte; line-empty square: TF_step; and line-filled square: TFvte_step)

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Figure 11

Tip noise comparisons for narrowband and broadband sound pressure level (Lp) cross-spectra at 28 deg: ((a) and (b)) operating point D; ((c) and (d)) operating point L; and ((e) and (f)) operating point P

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Figure 12

Tip noise comparisons for sound-pressure level (Lp) cross-spectra in near-design operation at: (a) 16 deg pitch angle, (b) 24 deg pitch angle, (c) 28 deg pitch angle

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