Noise Reduction of a Turbofan Bleed Valve

The Journal of the Acoustical Society of America

Papamoschou²

2013

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A study has been conducted on the transmission loss of perforated plates at normal incidence. The investigation includes a theoretical analysis of the problem with validation through experimentation. The experiments comprised microphone measurements of transmission loss for 11 perforated plates with variable thickness, hole size, and porosity. The theoretical model is based on planar wave propagation through a single contraction/expansion chamber with modifications to account for hole interaction effects. The resulting formula for transmission loss yields superior predictions over past theories for the range of properties investigated. Deviations between experimental measurements and theoretical predictions of transmission loss are less than about 1.5 dB for dimensionless hole diameter d/λ < 0.5. The accuracy of the model does not show a strong dependence on plate thickness-to-diameter ratio or porosity.

Section: Introductionmentioning

confidence: 99%

High frequency acoustic transmission loss of perforated plates at normal incidence

The Journal of the Acoustical Society of America

Papamoschou²

2013

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“…13, and characterization of their attenuation capabilities provides important design guidance. The present model did not address the effect of flow through the perforation.…”

Section: Discussionmentioning

confidence: 99%

“…An example of its implementation is in a silencer design for pneumatic bleed valves on gas turbines [13]. Figure 1 depicts a simplified drawing of a bleed valve.…”

Section: Introductionmentioning

confidence: 99%

Acoustic Transmission Loss of Perforated Plates

18th AIAA/CEAS Aeroacoustics Conference (33rd AIAA Aeroacoustics Conference)

Papamoschou

2012

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A study has been conducted on the acoustic response of perforated plates at normal incidence. The investigation includes a theoretical analysis of the problem, with validation through experimentation. The acoustic response was quantified through the transmission loss and absorption coefficient of the perforate. The theoretical analysis is based on planar wave propagation through a single contraction/expansion chamber, with modifications to account for hole interaction effects. The resulting formula for transmission loss yields superior predictions over past theories. The theoretical model is validated through rigorous parametric experimentation. Eleven perforated plates with different thickness, hole size, and porosity were tested. Deviations between experiment measurements and theoretical predictions of transmission loss are shown to be less than about one decibel for dimensionless hole diameter d/λ < 0.5. The accuracy of the model does not show a strong dependency on plate thickness or porosity. Nomenclature= acoustic wave number L T = transmission loss l = plate thickness p' = acoustic pressure R π = reflection coefficient R P = perforation resistance R = reflected wave amplitude R m1m2 = cross correlation of microphones 1 and 2 SPL = sound pressure level S = area of unit element containing perforation T = transmitted pressure amplitude t = time u = particle velocity Z = acoustic impedance Z P = perforation impedance Z 0 = characteristic impedance α = absorption coefficient β = perforation porosity γ 2 = coherence ΔL T = difference in modeled and experimental transmission loss ε = perforation thickness correction θ = polar angle λ = acoustic wavelength μ = coefficient of viscosity ρ 0 = density of air τ = time lag

“…Interaction effects were ignored on the basis of low porosity applications. [9][10][11]13,14 A recent study on the design of a pneumatic bleed valve muffler 15 indicated a parameter space (porosity, Mach number, and acoustic frequency) where previous theoretical models lack validation. [7][8][9][10][11][12][13][14] Perforated plates are used in mufflers for bleed valves on gas turbines.…”

Section: Introductionmentioning

confidence: 99%

“…[7][8][9][10][11][12][13][14] Perforated plates are used in mufflers for bleed valves on gas turbines. 15 The bleed valves regulate pressure in the compressor of a turbofan engine by discharging air into the fan duct, preventing compressor surge during transients or throttle back. Without a muffler, the valve exhaust forms a high-speed turbulent jet which contributes significantly to the total aircraft noise.…”

Section: Introductionmentioning

confidence: 99%

Normal incidence acoustic insertion loss of perforated plates with bias flow

The Journal of the Acoustical Society of America

Papamoschou

2015

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The transmission of sound at normal incidence through perforated plates with bias flow is investigated experimentally and theoretically over a large parameter space. A specially designed experimental apparatus enabled the measurement of insertion loss with bias flow Mach number up to 0.25. A theoretical model for insertion loss was constructed based on inviscid, one-dimensional wave propagation with mean flow through a single contraction/expansion chamber. The mass end correction of the contraction is modified for hole interaction effects and mean flow. Hydrodynamic losses are modeled using a vena contracta coefficient dependent on both perforation geometry and Reynolds number. Losses in acoustic energy that occur in the mixing region downstream of the perforations are modeled as fluctuations in entropy. The proposed model was validated experimentally over a range of plate thickness, porosity, and hole size. The experimental results indicate an increase in insertion loss with increasing frequency, followed by saturation and decline as resonant conditions are established in the perforations. The insertion loss at low frequency increases with increasing Mach number through the perforation. The proposed model captures these trends and its predictions are shown to be more accurate than those of past models.