WO1994028619A1 - Ducted axial fan - Google Patents

Ducted axial fan Download PDF

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Publication number
WO1994028619A1
WO1994028619A1 PCT/US1994/005270 US9405270W WO9428619A1 WO 1994028619 A1 WO1994028619 A1 WO 1994028619A1 US 9405270 W US9405270 W US 9405270W WO 9428619 A1 WO9428619 A1 WO 9428619A1
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WO
WIPO (PCT)
Prior art keywords
fan
duct
speakers
noise
actuator
Prior art date
Application number
PCT/US1994/005270
Other languages
French (fr)
Inventor
Jeffrey N. Denenberg
Original Assignee
Noise Cancellation Technologies, Inc.
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Noise Cancellation Technologies, Inc. filed Critical Noise Cancellation Technologies, Inc.
Publication of WO1994028619A1 publication Critical patent/WO1994028619A1/en

Links

Classifications

    • FMECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F04POSITIVE - DISPLACEMENT MACHINES FOR LIQUIDS; PUMPS FOR LIQUIDS OR ELASTIC FLUIDS
    • F04DNON-POSITIVE-DISPLACEMENT PUMPS
    • F04D29/00Details, component parts, or accessories
    • F04D29/66Combating cavitation, whirls, noise, vibration or the like; Balancing
    • F04D29/661Combating cavitation, whirls, noise, vibration or the like; Balancing especially adapted for elastic fluid pumps
    • F04D29/663Sound attenuation
    • F04D29/665Sound attenuation by means of resonance chambers or interference
    • FMECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F05INDEXING SCHEMES RELATING TO ENGINES OR PUMPS IN VARIOUS SUBCLASSES OF CLASSES F01-F04
    • F05BINDEXING SCHEME RELATING TO WIND, SPRING, WEIGHT, INERTIA OR LIKE MOTORS, TO MACHINES OR ENGINES FOR LIQUIDS COVERED BY SUBCLASSES F03B, F03D AND F03G
    • F05B2260/00Function
    • F05B2260/96Preventing, counteracting or reducing vibration or noise
    • F05B2260/962Preventing, counteracting or reducing vibration or noise by means creating "anti-noise"
    • GPHYSICS
    • G10MUSICAL INSTRUMENTS; ACOUSTICS
    • G10KSOUND-PRODUCING DEVICES; METHODS OR DEVICES FOR PROTECTING AGAINST, OR FOR DAMPING, NOISE OR OTHER ACOUSTIC WAVES IN GENERAL; ACOUSTICS NOT OTHERWISE PROVIDED FOR
    • G10K2210/00Details of active noise control [ANC] covered by G10K11/178 but not provided for in any of its subgroups
    • G10K2210/10Applications
    • G10K2210/112Ducts

Definitions

  • This invention relates to a ducted axial fan.
  • These fans are known to generate tonal noise at harmonics of the rotation rate times the number of blades in the fan as well as some random noise from air turbulence. It is also well documented that most of the noise is generated at the tips of the blades and that the tonal components increase rapidly in intensit when the fan must work against back pressure.
  • the instant invention solves the problems inherent in the situation where the diamete of the fan is large when compared to a wavelength of the tonal noise from the blade tips. This occurs whenever the fan is large, rotating at high speed and/or has a high number of blades.
  • Figure 1 is a perspective view of a general configuration of a typical ducted axial fan
  • Figure 2 is a perspective view of the ducted axial fan comprising the instant invention.
  • This invention recognizes that the predominant perceived tonal noise from a ducted axial fan is the secondary acoustical wave generated when the rotating pressure wave produced by the fan hits physical supporting members near the fan. Most of the work to date in active control of fan noise cancels this secondary acoustical wave. It has proven difficult to accomplish this cancellation when the dimensions of the fan and/or duct are large (more than VAX) compared to the wavelength ( ⁇ ) of the noise due to the complexity of dealing with the multiple propagation modes that the acoustical wave can use to travel dow the duct.
  • the primary pressure wave is different on each side (inlet/outlet) of the axial fan. O both sides it is a maximum at the blade tips (mostly due to the higher speed of the blades at the tips) and is almost zero at the axis of the fan.
  • One solution would then be to position a set of speakers around the duct at or near the plane of the fan and operate a multiple interacting algorithm (MIS ACT) to cancel the noise.
  • the required number of speakers is determined by the complexity of the pressure waveform around the circumference of the duct but will be a minimum of two per fan blade for smaller fans and more for fans with larger diameters.
  • Figure 1 shows an axial four-bladed fan 10 adapted to rotate within duct 11.
  • the tips 12 of blades 13 of fan 10 generate tonal noise at harmonics of the rotation rate times th number of blades in the fan as well as random noise from air turbulence.
  • Figure 2 shows a diagram of the physical actuator system.
  • the fan 20 having blade tips 25 is adapted to rotate within duct 21, microphones 22, 23 are located downstream and upstream, respectively and a series of actuators, e.g., speakers 24, are located around the periphery of duct 21.
  • a second set of actuators 26 are located around the duct periphery of du ⁇ t 10. It should be noted that all the speakers are equally spaced around the duct.
  • the frequency limits are not as severe as the limits in matching acoustical modes. Since some noise is also generated along the length of the blades, this approach may not achieve perfect cancellation at higher frequencies, but it should generally do a good job.
  • the generation of the rotating sound field is a straight forward addition to a MISACT controller.
  • the present MISACT system generates an image of the required anti- noise output wave form and stores it in memory. It then reads this memory in a rotating cycle, synchronous with the noise cycle. All that is needed here is to read the output wave form with N different pointers (N being the number of speaker pairs per fan blade) that are equally spaced around the anti-noise cycle. The resulting 2*N output signals are then each amplified and distributed to a number of speakers equal to the number of fan blades.
  • the update algorithm can be slowed down to maintain stability in the presence of the non-linear relationship between the generated anti-noise waveform and the residual noise sensed by the microphone on each sid of the form.

Abstract

A ducted axial fan for large diameter ducts (11) which includes equidistantly spaced sensors (22, 23) upstream and downstream of an axial fan and spaced actuators (24, 26) located around the periphery of said duct to cancel tonal noise caused by the air turbulence generated by the rotation of the fan.

Description

DUCTED AXIAL FAN
This invention relates to a ducted axial fan. These fans are known to generate tonal noise at harmonics of the rotation rate times the number of blades in the fan as well as some random noise from air turbulence. It is also well documented that most of the noise is generated at the tips of the blades and that the tonal components increase rapidly in intensit when the fan must work against back pressure.
Prior efforts to solve this problem through active cancellation have been limited to cases where the diameter of the duct is small and its length long with respect to a wavelengt of the tonal noise. This allows for effective coupling of the anti-noise from a small number of speakers in the duct with the non-rotating noise field downstream in the duct.
The instant invention solves the problems inherent in the situation where the diamete of the fan is large when compared to a wavelength of the tonal noise from the blade tips. This occurs whenever the fan is large, rotating at high speed and/or has a high number of blades.
Objects of the Invention
Accordingly, it is an object of this invention to improve upon the prior art in active axial fan noise cancellation to handle cases where the diameter of the fan is large compared to a wavelength of the tonal noise from the blade tips.
This and other objects will become apparent when reference is had to the accompanying drawings in which:
Figure 1 is a perspective view of a general configuration of a typical ducted axial fan, and Figure 2 is a perspective view of the ducted axial fan comprising the instant invention.
Description of the Invention
This invention recognizes that the predominant perceived tonal noise from a ducted axial fan is the secondary acoustical wave generated when the rotating pressure wave produced by the fan hits physical supporting members near the fan. Most of the work to date in active control of fan noise cancels this secondary acoustical wave. It has proven difficult to accomplish this cancellation when the dimensions of the fan and/or duct are large (more than VAX) compared to the wavelength (λ) of the noise due to the complexity of dealing with the multiple propagation modes that the acoustical wave can use to travel dow the duct.
The primary pressure wave is different on each side (inlet/outlet) of the axial fan. O both sides it is a maximum at the blade tips (mostly due to the higher speed of the blades at the tips) and is almost zero at the axis of the fan. One solution would then be to position a set of speakers around the duct at or near the plane of the fan and operate a multiple interacting algorithm (MIS ACT) to cancel the noise. The required number of speakers is determined by the complexity of the pressure waveform around the circumference of the duct but will be a minimum of two per fan blade for smaller fans and more for fans with larger diameters.
Figure 1 shows an axial four-bladed fan 10 adapted to rotate within duct 11. The tips 12 of blades 13 of fan 10 generate tonal noise at harmonics of the rotation rate times th number of blades in the fan as well as random noise from air turbulence.
In general, the propagating pressure wave is different on either side of the fan. This will require twice as many speakers and that they be in pairs, on either side of the fan and double the number of cancellation channels. Figure 2 shows a diagram of the physical actuator system.
In Figure 2, the fan 20 having blade tips 25 is adapted to rotate within duct 21, microphones 22, 23 are located downstream and upstream, respectively and a series of actuators, e.g., speakers 24, are located around the periphery of duct 21. In cases where th pressure waves are different on opposite sides of the fan, a second set of actuators 26 are located around the duct periphery of duςt 10. It should be noted that all the speakers are equally spaced around the duct.
Since the noise sources (fan tips) 25 are close to the anti-noise speakers, the frequency limits are not as severe as the limits in matching acoustical modes. Since some noise is also generated along the length of the blades, this approach may not achieve perfect cancellation at higher frequencies, but it should generally do a good job.
To control the speakers, one can employ a system as shown and described in U.S. Patent No. 5,091,953, hereby incorporated by reference herein. This system is known as a MIS ACT (Multiple Interacting Sensors and Actuators) system.
One problem with a direct application of MISACT to this problem is the complexity and speed of the calculations required to implement that solution to this problem. Recognizing that the rotating pressure wave has a slowly changing (almost unchanging) shape, an alternate solution is feasible. Therefore an anti-noise generating element is used which has one channel of active control (two channel MISACT for bi-directional cancellation) to determine the shape of the required anti-pressure wave and then output a replicated (by the number (N) of fan blades) version of this shape rotating around the set of speakers in sync with the fan rotation. A bi-directional system requires only a two channel MISACT controller with an added function to do the synchronous time to spacial transformation. The MISACT controller will need to have a number of D/A output channe (and amplifiers) equal to the number of speakers per fan blade. It will only require two A/ input channels (assuming no serious propagation mode problems at the microphones).
The generation of the rotating sound field is a straight forward addition to a MISACT controller. The present MISACT system generates an image of the required anti- noise output wave form and stores it in memory. It then reads this memory in a rotating cycle, synchronous with the noise cycle. All that is needed here is to read the output wave form with N different pointers (N being the number of speaker pairs per fan blade) that are equally spaced around the anti-noise cycle. The resulting 2*N output signals are then each amplified and distributed to a number of speakers equal to the number of fan blades. Since the anti-noise output waveform is slowly varying, the update algorithm can be slowed down to maintain stability in the presence of the non-linear relationship between the generated anti-noise waveform and the residual noise sensed by the microphone on each sid of the form.
Having described the invention, attention is directed to the appended claims.

Claims

1. In a duct having an ingress and egress, said duct having a multi-bladed axially mounted fan means mounted therein, the improvement comprising a first sensor means mounted upstream of said fan means, a second sensor means mounted downstream of said fan means, a series of actuator means mounted around said duct means adjacent said fan means, and control means operatively connected to said actuator means and said first and second sensor means and adapted to directly cancel the tonal noise generated by said axial fan by canceling the pressure waves generated by said fan's rotation.
2. As in claim 1 wherein said actuator means comprise a series of speakers mounted inside the duct means.
3. As in claim 2 wherein said speakers are spaced equidistant from one another.
4. As in claim 1 wherein said actuator means comprises two sets of speakers mounted in said duct, each set mounted adjacent said axially mounted fan so as to be adapted to directly cancel the pressure waves generated by the fan's rotation on either side.
5. As in claim 4 wherein said control means has two channels of active control and adapted to do a synchronous time to spatial transformation.
6. As in claim 1 wherein said control means has one channel of active control and only one sensor means either upstream or downstream.
7. As in claim 6 wherein said actuator means comprises one set of speakers mounted in said duct, said speaker means mounted adjacent said axially mounted fan so as to be adapted to directly cancel the pressure waves generated by the fan's rotation on one side.
8. As in claim 7 wherein said actuator means comprises a series of speakers mounted inside the duct means. As in claim 8 wherein said speakers are spaced equidistant from one another.
PCT/US1994/005270 1993-05-21 1994-05-18 Ducted axial fan WO1994028619A1 (en)

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
US6459893A 1993-05-21 1993-05-21
US08/064,598 1993-05-21

Publications (1)

Publication Number Publication Date
WO1994028619A1 true WO1994028619A1 (en) 1994-12-08

Family

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Family Applications (1)

Application Number Title Priority Date Filing Date
PCT/US1994/005270 WO1994028619A1 (en) 1993-05-21 1994-05-18 Ducted axial fan

Country Status (1)

Country Link
WO (1) WO1994028619A1 (en)

Cited By (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP0786051A2 (en) * 1994-10-11 1997-07-30 Noise Cancellation Technologies, Inc. Ducted axial fan

Citations (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4044203A (en) * 1972-11-24 1977-08-23 National Research Development Corporation Active control of sound waves
JPS6374399A (en) * 1986-09-18 1988-04-04 Toyota Motor Corp Suppressing device for trapped sound of vehicle
JPH0313998A (en) * 1989-06-12 1991-01-22 Hitachi Plant Eng & Constr Co Ltd Electronic sound deadening system
WO1992017936A1 (en) * 1991-04-05 1992-10-15 Applied Acoustic Research Active noise control

Patent Citations (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4044203A (en) * 1972-11-24 1977-08-23 National Research Development Corporation Active control of sound waves
JPS6374399A (en) * 1986-09-18 1988-04-04 Toyota Motor Corp Suppressing device for trapped sound of vehicle
JPH0313998A (en) * 1989-06-12 1991-01-22 Hitachi Plant Eng & Constr Co Ltd Electronic sound deadening system
WO1992017936A1 (en) * 1991-04-05 1992-10-15 Applied Acoustic Research Active noise control

Cited By (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP0786051A2 (en) * 1994-10-11 1997-07-30 Noise Cancellation Technologies, Inc. Ducted axial fan
EP0786051A4 (en) * 1994-10-11 1998-08-05 Noise Cancellation Tech Ducted axial fan

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