CA1108061A - Absorption of acoustic energy from a fluid - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

A device for absorbing acoustic energy from a fluid stream passing over a surface on which there is a multiplicity of fibers or filaments operatively attached to the surface and extending into the fluid stream, the device may be employed as a silencer in an automobile in which case the surface may be the interior surface of a conduit for association with an exhaust pipe. Automobile and marine mufflers or silencers employing the invention are effective in reducing or eliminating objectionable noise with only nominal back pressure.

Description

This invention relates to the selective absorption of acoustic energy from kinetic energy under conditions of relative motion between a surface and a fluid, More particularly the invention provides means for absorbing acoustic energy from a fluid stream, such as a gas, which is a device including a surface past which surface the fluid stream can flow, preferably with minimal or no loss of kinetic energy due to surface resistance. ~he surface is pro-vided with a multiplicity of fibers or filaments of a relatively 1~ small diameter, each fiber or filament being operatively attached, either directly or indirectly, to the surface with the ~ibers or filament ends extending freely into the fluid stream.
In accordance with one aspect of the invention there is provided a method for absorbing acoustic energy from a fluid stream which comprises providing a surface past which the fluid stream can flow, the surface having a multiplicity of fibrous or filamentous members operatively attached to and extending from the surface, flowing the fluid s-tream past ~0 the surface, and allowing the members to extend freely into the 1uid stream parallel to the direction of flow of the fluid stream and absorb acoustic energy.
In another aspect of the invention there is provided a device for absorbing acoustic energy from a fluid stream which device comprises a surface past which the stream can flow, the surface having a multiplicity of fibrous or fila-mentous members operatively attached to and extending from the surface, the members being adapted to extend generally parallel to and in the direction of fluid flow to provide an unobstructed fluid 10w passage.
Particularly, the fibrous or filamentous members G~ may comprise fibers or filaments in which each fiber or fila-- 1 - ~

ment may be operatively attached to the surface at one end, the other end extending freely into the fluid stream.
The fibrous ox filamentous members suitably define a generally regular geometrical surface providing an unob-structed fluid flow passa~e.
In particular the free ends of the fibers or fila-ments may define such a surface.
In a particular embodiment the surface is an interior sur~ace of a conduit.
In another aspect of the invention there is provided a vehicle exhaust system comprising a device of the invention as a silencer. Particularly there is provided in a vehicle exhaust system comprislng a silencer, the improvement wherein the silencer comprises a device of the invention in which the support surface comprises an exhaust pipe, the fibers or fila-ments extending inwardly of the pipe.
Thus in one embodiment of the method aspec-t, the invention provides for a method of absorbing acoustic energy

2~ from a moving fluid stream, usually a gas, by passing the ~luid stream over and in contact with at least one surface carrying a multiplicity of fibers or filaments or a relatively small diameter, as described above. The moving fluid stream impinges upon the free ends of the fibers or filaments and the fibers absorb acoustic energy.
The invention is particularly useful, for example, in providing an exhaust system, or portion of the exhaust system of any desired shape or configuration, ~or an internal combustion engine for automotive purposes, such as a motor vehicle, marine craft or aircraft, or as a replacement for a conventional automotive muffler.

The device may also be useful as an integument applied to an external or internal wall face in order to absorb acoustic radiation from the solid.
According to a feature of the invention the fibrous or filamentous members comprise fibers or filaments which are pre~erably ~lexible, and according to a further ~eature free ends of the fibers or filaments usually extend parallel to and trailing in the fluid with respect to the direction of fluid or surface movement.
The fibers or filaments may be of inorganic, metal-organic or organic material provided they have physical and chemical properties appropriate for their integrity and survival for an acceptable period of use in the environment in which they are placed.
Thus, for example, for endurance in an engine exhaust of a marine craft where the exhaust gas is cooled, such as by water injection, the fibers or filaments may be of organic origin. By contrast in the dry and uncooled conditions of the engine exhaust of an automobile, aircraft or the like, the fibers or filaments should preferably be of inorganic material having a refractoriness and insolubility which are appropriate, and in this case could be of siliceous, ceramic, carbon or similar material. Conversely they may be metallic, or a mixture of any number of them. Selection of an appropriate fiber may be determined through preliminary experimentation by one skilled in the art, It is preferred that the average dia-meter of the fibers or filarnents employed will be in th~ range oE about 1 to about 50 microns in size.
The configuration of the fibers may be such tha-t they are suficiently close together so that their free ends define a substantially regular geometrical surface or fairly defined plane beyond which there is no obstruction, or relatively little obstruction, to the passage of the fluid.
Furthermore, since this surface is flexible and of low ~a reflectivity, the acoustic energy present in the fluid is readily transmitted throu~h it and absorbed by the mass of fibers between it and the outer casing. The fibers at least initially extend normal to the surface to which their fixed ends are attached, although over a period of use may become bent or curved in the direction of the fluid flow.
One embodiment of the invention will now be described, by way of example, with reference to the accompanying drawing which is an axial cross-section through a conduit lined with an investment o~ fibers.
~0 T~le interior wall of conduit 10 is provided with an investnlent of flexible uni-directional closely spaced fibers 11 analogous to certain types of animate fur, the majority of which fibers after rising from the internal wall 10 to which they are operatively attached, at least initially assume positions parallel or substantially so, to the direction of fluid flow.
When the construction of the invention is used in an automobile enyine exhaust system, it has been found that the acoustic energy present in the gas can be absorbed to a very high degree without incurring any substantial decrease in the kinetic energy of the gas.

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This device then provides an exhaust which can be totally silent with minimal back pressure, or, at high velocities of flow, pressure of such low value that the engine maintains higher efficiency than is normally the case.
~ conventional automobile muf-fler reduces the noise made by the auto engine using a series of baffle plates, packings and walls inside of the muffler. In reducing noise a substantial amount of back pressure is created which d~cr~ases the efficiency of the engine. By reducing back p~essure, the overall operating efficiency and economy of the engine are improved.
While the theory of operation is not fully elaborated, a reasonable explanation appears to be that alternating com-pression/depression waves of acoustic energy of both longitudinal and transverse propagation are absorbed by reason of the multiplicity of phase changes and this energy is transformed into heat. Also a high viscosity is provided in the closely spaced fibers by the very great number of air columns of minute diameters which interspace the fibers, such columns ~0 being a factor in the acoustic energy absorption.
To allow the exhaust gases to flow without resistance the minimum cross-sectional area normally rQquired is main-tained as an empty space 12 in the fiber field, and the annualar space between the empty space and the roots at or near the outer casing is occupied by the fibers as described.
The fibers or filaments may be mechanically or adhesively attached to the conduit at their roots or the fibers or fîlaments may be secured to a backing layer fi~ed to the internal wall of the conduit.
When so attached an adhesive will be selected to be compatible with the fibers and capable of maintaining its integrity during conditions of operation.

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~ ccording to their composition they may, for example, be deposited electrochemically, cataphoretically or by precipitation directly on the conduit surface or on a support which is attached to the conduit surface. They may be supported, or additionally supported at intervals by a solid keeper which exerts a retaining pressure on the investment from the fluid face, such retainer having a small cross-sectional dimension. An example of this is an arrangement of small diameter rods or a splral of rod or wire so introduced 1~ that the rcd or wire becomes substantially buried in the investment.
The keeper may even be, for example, a yauze or mesh havin~ a very high percentage of open area. Clearly, the greater the total area of such solid and rigid keeper material which is not well buried into the investment, the less the acoustic absorbent efficiency of the investment will be.
In the event the keeper or retainer is inadvertently exposed to the sound waves present in the fluid, it is possible to arrange, in the case of the spiral rod, for example, that the pitch, or wave length is an aliquot of the total length of uninterrupted pipe section. An aliquot part could conceivably give rise to harmonics of certain frequencies.
According to the various embodiments of the devices the fibers or filaments may extend perpendicularly from the internal wall of the conduit and remain so over the whole of their length. With this arrangement, in use, the fluid flow may cause the fibers or filaments to bend over at some distance from their roots. So as to provide the optirnum or most economical use of materials, a ratio will be established involving several factors, including the amount of incident energy and the statistical data relating to the fibers or filaments, population `:

per unit area, density or specific gravity, Youn~'s modulus, diameter, and length, particularly that part of the fiber or filament investment which is parallel to the direction of fluid flow, the effective thickness or depth from the roots when in use, environment humidity, and the length of axial path invested with the fibers or filaments.
It is thus possible to provide a gas passage which has a relatively smooth and flexible face having low acoustic reflectivity, but high transmission, we have, according to an-other aspect of our invention, found that the acoustic energy present in a flowing yaseous stream can be absorbed effectively without incurring any substantial decrease in the kinetic energy of the gas by providing an investment to the interior wall of a muffler or exhaust system silencer casin~ which comprises a sound absorbing material of a mass of fibers, either oriented or randomly orien-ted as in the case of a felt.
The fibers which extend to the surface, being fle~ible, are readily bent over by the gas stream to become trailing, as pre~iously described.
The ~nvention is further described and illustrated by the ollowing examples of the use of various embodiments. In the testing reported relative aspects of approximate noise levels, temperatures and bac~ pressures were measured on a comparative basis, absolute values not being required TWo types of embodiment of the invention were tested, one in a wet (marine) environment and the other in a dry environment.

-An engine of 1100 cc displacement was randomly selected for convenience as one commonly used for road vehicles and also for small marine craft. A common practice in marine engineering is to cool the exhaust gas, as ~y water injection, and then discharge the cooled gas from the craft through an essentially unsilenced pipe or conduit. This is in contrast to an automobile exhaust system in which a muffler, silencer or the like is employed.
The dry gas temperature of the above engine emergin~
from the manifold was in the range of 500-720C, according to the gas velocity. Water was injected into the engine exhaust gas system at a rate of 2-2.5 gallons/minute or 9-11 liters of water/minute. A few inches downstream from the water injection point, the station being as usual convenlently close to the engine, the temperature of the engine exhaust gas mixed with water had dropped to 40-60C~
The lower temperature level is well within the temperature tolerance of many organic filaments. In these tests an 8~/o chlorofibrous material, having a monofilament diameter in the region of 8 microns, derived from polyvinyl-chloride, mixed with an acrylonitrile was used for the e~haust pipe investment~ The overall appearance was similar to the investment described in relation to the drawing. The 2~ investment was a typical artificial fur and had a polyester backing with a polyacrylate resinous reinforcement, for all o which insolubility in boiling sea water is claimed by the ma~ers (Borg Textiles Limited), and was secured to the internal part of the exhaust conduit.
For the water cooled run, the invested pipe was 11 ft or 330 cm long, and the filaments had a dormant dep-th of 3/4 inch or 20 mm.
The performance of the invested exhaust was compared with that of a normal water injected commercial system of equal bore, but having, as is also common, one silencer of the reaction type in the line. This routine system is also a standard installation in naval craft.

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sefore running the invested example precautions were taken, after injection, to separate out the water again so that the ~as, having been cooled, was now dry or at least relatively dry. It was observed that the drier the filaments of the investment, the higher the acoustic absorption, There are many convenient methods of effecting separation of the injected water from the cooled exhaust gases, of which one or a combination thereof may be employed. Examples include the centriEugal effect of a pipe bend, making use of the traditional riser on board the craft, a short section containing trans-verse angled louvers, or if considerable length is available, separation by gravity into a collector trough or pipe.
During each test, the engine was run at engine speeds of 2300, 3500 and 5500 rpm. The length of the exhaust pipe was 120 inches measured from the engine manifold. The traditional (unmodified) system produced noise levels rising to about 65dBA and back pressures of 3, 10 and 18 inches of water (75, 250 and 460 mm). With further engine acceleration in excess of 5500 rpm back pressure level rose steeply to 30 ~0 inches of water ~760 mm).
The invested plain pipe of the present invention with no muffler was then compared. At 5S00 rpm and above the effluent was silent and later the pipe had to be shortened to 9 ~t. (270 cm) before exhaust became audible. At the quoted rpm periods the back pressure figures were 0 inches, 0.5-1 inch and 2 inches (0, 12, 25 and 50 mm), respectively.
Zero readings reflected the small inertia of the manometer and the above figures are corrected readings, meaning that the back pressure introduced b~ the bent downtake pipe arrangement of the manifold, which was a common factor through-o~t all observations was deducted.

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In general terms the invested exhaust system of the invention produced about one eighth of the back pressure of the standard marine exhaust system, The higher the velocity, the larger the difference in comparative back pressure of the standard e~haust system and that according to the inven~ion.
The dry tests were then done. To the engine was fitted the standard exhaust pipe with silencer made and sold by the engine maker for that engine.
Corrected readings showed 2 - 3 inches water (50 -75 mm) BP at 2000 - 2500 rpm, 3 - ~ inches (75 - 100 mm) at 3300 - 40000, and 10 inches (250 mm) at 5500 rpm. Again the noise level rose to a level of discomfort, as commonly experienced in cities.
The invested pipe, 10 ft. 300 cms. long, with no silencer box was run and produced no audible sound even at 12 inches, or 30 cms from the outlet. Back pressure readings now showed 1, 1.5, and 2.5 inches (25, 37 and 63 mm water column).
This invested pipe was then shortened progressively ~0 until at 24 inches or 60 cms len~th it produced at the outlet an approximately similar noise level as the traditional, except that again the higher fre~uencies have been removed, the re-sulting being easier on the ear. At this much foreshortened length the back pressures were 0, 0.5 - 1, and 2 inches water (1, 12 - 25 and 50 mm).
The indication therefore is that in a ~ry system the invested exhaust pipe produced only about one sixth the back pressure induced by a traditional pipe with its silencer.
It has been found that a gas may be released silently yet at high velocity from the end of a conduit such as a com-pressed air line. Forrelated physical reasons, organ piping is eliminated without reduction of rheological efficiency.

The invention is not to be considered limited in any way to the silencing of an internal combustion engine as there are diverse areas in which the principles set forth above also apply. Other areas in acoustics where our technique may be applied are those where high noise level impulse waves are produced; the absorbent effect of the investment considerably chops down the initial oscilloscope deflection.
Throughout the above discussion, illustrations and examples, the unidirection of the fibers and filaments, or 1~ at least a majority of them, and consequently minimal reduction in kinetic energy, has been the theme~ Paradoxically, the reverse may apply in certain cases. ~n example of this is that in some marine engine exhausts it is not only preferable, but vitally important to prevent any sea water returning under the force of a heavy wave back up to the engine, Clearly, the investment devices and procedures of our invention produce a resistance to such return flowO
The preferred average diameters of the fibers or filaments should be within the range 1 to 50 microns.
Various changes and modifications o~ the invention can be made, and, to the extent that such variations incorporate the spirit of this invention, they are intended to be included within the scope o~ the appended claims, ....

Claims (35)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:-

1. A device for absorbing acoustic energy from a fluid stream which device comprises a surface past which surface the stream can flow, a multiplicity of fibers or filaments operatively attached to said surface and extending generally inwardly from said surface so that the free ends of the fibers or filaments provide an unobstructed fluid flow passage, said free ends being adapted to extend generally parallel to and in the direction of fluid flow.

2. A device as claimed in claim 1, wherein the fibers or filaments have an average diameter of 1 to 50 microns.

3. A device as claimed in claim 1 or 2, in which the fibers or filaments are flexible.

4. A device as claimed in claim 1 or 2, in which the fibers or filaments are sufficiently close together that at the free ends they define a regular geometrical surface beyond which there is no or relatively little obstruction to the passage of the fluid.

5. A device as claimed in claim 1 or 2, in which the fibers extend initially perpendicularly from the surface.

6. A device as claimed in claim 1 or 2, in which the surface is tubular.

7. A device as claimed in claim 1, in which said fibers or filaments have an average diameter 1 to 50 microns and each has an end fixed directly to the said surface and an end adapted to extend freely into the fluid stream.

8. A device as claimed in claim 7, in which the fibers or filaments are flexible.

9. A device as claimed in claim 7 or 8, in which the fibers or filaments are sufficiently close together that at their free ends they provide a fairly defined plane beyond which there is no or relatively little obstruction to the passage of the fluid.

10. A device as claimed in claim 7 or 8, in which the fibers extend initially perpendicularly from the surface.

11. A device as claimed in claim 7 or 8, in which the surface is tubular.

12. A device as claimed in claim 1, in which said fibers ox filaments have an average diameter 1 to 50 microns and each has an end fixed indirectly to the said surface and an end adapted to extend freely into the fluid stream.

13. A device according to claim 7 or 12, in which said multiplicity is of fibers.

14. A device according to claim 7 or 12, in which said multiplicity is of filaments.

15. In a vehicle exhaust system comprising a silencer, the improvement wherein said silencer comprises a device as defined in claim 1, 7 or 12, in which said support surface comprises an exhaust pipe, said fibers or filaments extending inwardly of said pipe.

16. A method for absorbing acoustic energy from a fluid stream which comprises providing a surface past which the fluid stream can flow, said surface having a multiplicity of fibrous or filamentous members operatively attached to and extending from said surface, said members having an average diameter of 1 to 50 microns, flowing the fluid stream past said surface, and allowing the members to extend freely into the fluid stream parallel to the direction of flow of the fluid stream and absorb acoustic energy.

17. A method according to claim 16, wherein said members comprise flexible fibers.

18. A method according to claim 16, wherein said members comprise flexible filaments.

19. A method according to claim 16, 17 or 18, wherein said surface is defined by the interior of a tubular member.

20. A method of absorbing acoustic energy from a moving fluid stream, comprising flowing a fluid stream past at least one surface carrying a multiplicity of fibers or filaments, said multiplicity extending parallel to and in the direction of flow of the fluid stream and allowing the moving stream to impinge upon free ends of the fibers or filaments extending into the moving stream to absorb acoustic energy, said fibers or filaments extending from said surface so that said free ends provide an unobstructed fluid flow passage.

21. An acoustic energy absorbing device comprising:
a conduit having an interior surface, and a plurality of fibers operatively attached at one end thereof to said interior surface, and extending generally inwardly from the conduit interior so that the the free ends of said fibers define a generally regular geometrical surface providing an unobstructed fluid flow passage, said fiber free ends being adapted to extend generally parallel to and in the direction of fluid flow in said conduit.

22. A device as claimed in claim 21, wherein said fibers have an average diameter of 1 to 50 microns.

23. A device as claimed in claim 21, wherein said conduit has a circular cross-section and wherein said geometrical surface is circular in cross-section and generally concentric with said conduit.

24. A device as claimed in claim 21, further com-prising means for retaining said plurality of fibers in place in operative attachment to said conduit interior surface, said means comprising a spiral coil of rod or wire.

25. A device as claimed in claim 21, further including means for retaining said plurality of fibers in place in operative attachment to said conduit interior surface, said means comprising a wire mesh.

26. A device as claimed in claim 21, wherein said fibers are adhesively attached to said interior surface.

27. An exhaust system for an internal combustion engine comprising:
an exhaust pipe, means operatively attached to said exhaust pipe for effectively absorbing the acoustic energy of fluid flowing through said exhaust pipe while allowing passage of the fluid therethrough such that only negligible back pressure is created, said means comprising a conduit disposed in line with said exhaust pipe, having an interior surface with a plurality of fibers operatively attached to said surface and having the free ends thereof extending into said conduit so that the free ends of said fibers define an unobstructed fluid flow passage of generally the same cross-sectional area as the cross-sectional area of said exhaust pipe, said fiber free ends being adapted to extend generally parallel to and in the direction of fluid flow in said conduit.

28. A silencer for an internal combustion engine comprising:
a silencer, means operatively attached to said silencer for effectively absorbing the acoustic energy of fluid flowing through said silencer while allowing passage of the fluid therethrough such that only negligible back pressure is created, said means comprising a conduit disposed in line with said silencer, having an interior surface with a plurality of fibers operatively attached to said surface and having the free ends thereof extending into said conduit so that the free ends of said fibers define an unobstructed fluid flow passage of generally the same cross-sectional area as the cross-sectional area of said silencer, said fiber free ends being adapted to extend generally parallel to and in the direction of fluid flow in said conduit.

29. An acoustic energy absorbing device comprising:
a conduit having an interior surface, means for absorbing acoustic energy from fluid flowing through said conduit without providing substantial surface resistance to the fluid flow, said means comprising a plurality of flexible fibers, each fiber having a fixed end thereof operatively attached to at least a portion of said conduit interior surface, and having a free end thereof extending inwardly from the conduit interior surface, said fiber free ends being adapted to extend generally parallel to, and in the direction of, the direction of fluid flow in said conduit.

30. A device as claimed in claim 28 or 29, wherein said fibers have an average diameter of 1 to 50 microns.

31. A device as claimed in claim 29, wherein said conduit has a circular cross-section, and the free ends of the fibers define a regular geometrical surface beyond which there is no or relatively little obstruction to the passage of a fluid through the conduit, said geo-metrical surface being circular in cross-section and generally concentric with said conduit.

32. A device as claimed in claim 29, further comprising means for retaining said plurality of fibers in place in operative attachment to said conduit interior surface, said means comprising a spiral coil of rod or wire.

33. A device as claimed in claim 29, further comprising means for retaining said plurality of fibers in place in operative attachment to said conduit interior surface, said means comprising a wire mesh.

34. A device for absorbing acoustic energy from a fluid stream which device comprises a surface past which the stream can flow, said surface having a multiplicity of fibrous or filamentous members operatively attached to and extending from said surface, said members having an average diameter of 1 to 50 microns and being adapted to extend generally parallel to and in the direction of fluid flow to provide an unobstructed fluid flow passage.

35. A device according to claim 34, wherein said surface is defined by the interior of a tubular member.