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Research Papers

Axial Fan Blade Tone Cancellation Using Optimally Tuned Quarter Wavelength Resonators

[+] Author and Article Information
Lee Gorny

Department of Mechanical and Nuclear Engineering, The Pennsylvania State University, 157 Hammond Building, State College, PA 16802ljg138@psu.edu

Gary H. Koopmann

Center of Acoustics and Vibrations, The Pennsylvania State University, 157 Hammond Building, State College, PA 16802ghk1@psu.edu

J. Vib. Acoust 131(2), 021002 (Feb 13, 2009) (13 pages) doi:10.1115/1.2980369 History: Received August 29, 2006; Revised January 15, 2008; Published February 13, 2009

Fan noise challenges noise control engineers in developing products ranging in scale from small ventilation systems to large turbomachines. The dominant noise source in many axial fans is the tonal noise due to rotor/stator interactions at the fundamental blade passing frequency. Flow-excited resonators have been used in the past for minimizing blade tone sound pressure levels (SPLs) generated by centrifugal fans through means of secondary source cancellation. The focus of this research is to extend that cancellation method to axial fans by attaching flow-driven quarter wavelength resonators fitted with optimal mouth perforations around the perimeter of the fan’s shroud. A ducted-fan test facility was developed to measure upstream and downstream noise radiated from a test fan. Resonators were mounted at specific locations around the fan’s shroud to obtain reductions in blade tone SPLs in both flow directions. They were driven into resonance via the unsteady pressure from the passing blades. An analytical model using transmission line theory was developed and validated experimentally to characterize the resonator’s behavior under various flow conditions and mouth geometries. This model was used to predict the resonator’s potential for reducing in-duct blade tones for specific flows and mouth perforation patterns. In a series of experiments to obtain the optimal resonator mouth perforations, it was observed that upstream and downstream SPL attenuations require different placement of the resonator mouth relative to the blade of the fan. With a single tuned resonator it was demonstrated that the fundamental blade tone SPLs can be reduced by as much as 20 dB in either the upstream or the downstream duct but not in both directions simultaneously. This behavior results when combining the resonator’s monopolelike sound field with the dipolelike sound field of the fan’s blades. Further studies are underway to extend the above method to higher pressure fans operating at speeds that generate higher order duct modes.

Copyright © 2009 by American Society of Mechanical Engineers
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References

Figures

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Figure 1

Flow driven quarter wavelength resonator (13)

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Figure 2

Perforated resonator end in baffle, with perforated resonator end patterns (numbered in order of decreasing opening size)

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Figure 3

Comparison of closed end resonator response sound pressure level for different opening patterns (length=135 mm test)

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Figure 4

Comparison of closed end resonator response phase for different opening patterns (length=135 mm test)

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Figure 5

Comparison of analytical transmission line theory resonator model to measured results for the pressure at the closed resonator end (for pattern 6 resonator of 215 mm length)

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Figure 6

Comparison of analytical transmission line theory resonator model to measured results for the pressure at the closed resonator end (for pattern 6 resonator) (with and without end pattern weighting factor)

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Figure 7

Superposition of fan and resonator sources

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Figure 8

Diagram of fan facility with mounted fan (relevant dimensions in millimeters)

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Figure 9

Outer fan shroud and inner motor mount with relevant dimensions (in millimeters)

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Figure 10

Fan curve profile of fan efficiency at different loading conditions for radiator fan

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Figure 11

Baseline acoustic spectra for axial radiator fan upstream and downstream directions

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Figure 12

Profile of fundamental SPL variation in upstream and downstream measurements for fundamental BPF level as loading condition is modified (BPF maintained at constant 360 Hz)

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Figure 13

Shroud pressure measurement probe configuration (figure on the left shows microphone placement; figure on the right shows probe hole numbering and labels)

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Figure 14

Blade passage frequency level and the level of each harmonic from 12 velocity loading condition measurements

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Figure 15

Fundamental blade passage frequency SPL magnitude incident on fan shroud for three loading conditions (fundamental BPF=346 Hz)

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Figure 16

Fundamental blade passage frequency phase measurements of incident pressure on fan shroud for the three loading conditions (fundamental BPF=346 Hz)

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Figure 17

Phase measurements of shroud pressure for fundamental BPF and its harmonics for 12 maximum velocity loading case

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Figure 18

Model 1 of determining resonator driving pressure for open end of the resonator

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Figure 19

Model 2 of determining resonator driving pressure at the open end including the phase delays between adjacent holes

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Figure 20

SPL measured at closed end of the tuned resonator with comparison to analytically predicted level using calculation Models 1 and 2 for driving pressure approximation

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Figure 21

SPL measured at the closed end of the tuned resonator with comparison to analytically predicted level using Models 1 and 2 for driving pressure approximation

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Figure 22

Measurements of downstream SPL for no resonator and fully open perforated resonator tuned to reduce upstream and downstream noise (12 maximum velocity fan condition)

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Figure 23

Measurements of upstream SPL for no resonator and fully open perforated resonator tuned to reduce upstream and downstream noise (12 maximum velocity fan condition)

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Figure 24

Mouth perforated patterns tested in experimental noise reduction model

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Figure 25

Optimal downstream noise reduction using pattern 7 resonator compared with initial fan noise spectrum

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Figure 26

Blade tone SPL reductions with optimized resonator mouth perforations

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Figure 27

Phase diagram of the downstream blade tone SPL with and without a tuned resonator with resonator response (measured and calculated)

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