GB1568686A - Absorption of acoustic energy from a fluid - Google Patents

Description

(54) ABSORPTION OF ACOUSTIC ENERGY FROM A FLUID (71) We, CARL MrrHEWS, a British Subject, of 102 Gloucester Road, London London S.W.7, formerly of 21 Upper Cheyne Row, Chelsea, London SW3 5JW, and ELIZABETH DE RECOURT MARTYN, a British Subject, of 37 Stanhope Gardens, London SW7, formerly of 21 Upper Cheyne Row, Chelsea, London SW5 5JW, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement : This invention relates to the selective absorption of acoustic energy from kinetic energy under conditions of relative motion between a surface and a fluid.

The invention provides a device for absorbing acoustic energy which device com- prises a surface and a multiplicity of flexible fibres each fixed directly or indirectly to the surface, the fibres having portions extending freely away from the surface, end portions of the free portions forming a secondary surface, and thereby means to guide a fluid past the secondary surface with acoustic absorption, the arrangement being such that the said end portions will be trailing when fluid flows past the secondary surface.

The invention is particularly useful for example in providing an exhaust system, or part thereof, for an internal combustion engine for automotive purposes, such as a motor vehicle, marine craft or aircraft.

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.

The fibres may be of inorganic, metalorganic or organic material but should have physical and chemical properties appropriate for their survival for a useful period m their environment.

Thus, for example, for endurance in an engine exhaust of a marine craft where the gas is coded, such as by water injection, the fibres may be of organic origin, whereas in the dry and uncooled conditions of the engine exhaust of a motor vehicle or aircraft or similar, the fibres 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.

The fibres may extend initially perpendicularly to the surface.

The fibres are preferably of a diameter between + and 500 microns.

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 of fibres.

The interior wall of conduit 10 is provided with an investment of flexible uni- directional closely spaced fibres 11 analogous to certain types of animate fur, the majority of which fibres after rising from the internal wall 10 to which they are rooted assume positions parallel or substantially so, to the direction of flow.

Where this construction of the invention is used to provide an automotive engine exhaust, or part thereof, 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.

Hence, a totally silent exhaust system can be achieved which produces no measurable back pressure, or, at high velocities of flow, pressure of such low value that the engine maintains higher efficiency than is normally the case.

The alternating compression depression waves of acoustic energy of both longitudinal and transverse propagation are absorbed by reason of the multiplicity of phase changes and transformation to heat, and also to the high viscosity acquired by the very great number of air columns of minute diameters which interspace the filaments.

To allow the exhaust gases to flow without resistance the minimum cross-sectional area normally required is maintained as an empty space 12 in the centre of the fibre field, and the annular space between that and the roots at or near the outer casing is occupied by the fibres as described.

The fibres may be mechanically or adhesively attached to the- conduit at their roots or the fibres may be secured to a backing layer which layer is fixed to the internal wall of the conduit. According to their composition they may, for example, be deposited - electro-chemically, cataphoretically, or by precipitation. Also they maybe 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 spiral of rod or wire so intro- duced that the rod or - wire becomes sub- stantially buried in the investment.~ The keeper may even be, for example, a gauze or mesh having 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 case there should be inadvertently open exposure to the sound waves pre sent -in the fluid, it might be precautionary to arrange, in the case of the spiral rodding, for example, that 'the pitch, or waye length is an aliquant of the total length of uninterrupted pipe section. An aliquot part could conceivably give rise to harmonics at ceitain frequencies.

It is also envisaged that the fibres 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 fibres to bend - over at some distance from their roots: So as to provide the optimum or most economical investment, there will clearly be a ratio involving the -amount of incident energy and the statistical data relating to the fibres, population per unit area, density or specific'gravity, Young's modulus, diameter, and length, particu- larly that part of the fibre investment which is parallel to the direction of fluid - flow, effective thickness or -depth from the roots when so configurated, environment-humi- dity, - and the length of axial path so invested. - - The relative aspects of approximate noise levels temperatures and backs pressures were noted on a strictly comparative basis, absolute: values not being require-d, when two examples of the technique were put to test, one wet and the other dry. - An engine of 1100 cc was chosen, at random, for convenience and being com,-.

monly used for road vehicles and also for sin,, a' ' marine craft.

As has been said the standard practice in marine engineering is to cool the ex haust gas, as by water injection.

The dry gas temperature of the above engine - emerging from the manifold was 500 -720 C according to gas velocity. A few inches downstream from the injection point of 2-2z gallons - or -.9-11 litres of water/minute, the station being as, usual conveniently close to the engine the tem perature had dropped: to 40-60 C.

The latter level is well within the tem perature tolerance of some organic fila ments. In these tests-an 80% chlorofibrous material, having a monofilament diameter in the region of 8 mu, derived from poly vinylchloride, mixed with an acrylonitrile was used for the pipe-investment which appeared similar to the investment described in relation to the figure. The investment had a polyester backing with a polyacrylate resinous reinforcement and for all of which insolubility in. boiling sea water. is claimed by the makers.

For the water cooled run, the invested pipe was 11 ft. or 330 cms. long, and the filaments had a dormant depth of - inch or 20 mm.

The performance of the invested exhaust was compared with that of a normal commercial system of equal bore, but having, as is common one silencer - of the reaction type in the line. This routine system is also a. standard installation in naval craft.

Before running the invested example precautions were taken, after injection to separate out the water again so that the gas, having been cooled, was now dry or relatively dry. The drier the filaments the higher the acoustic absorption.

There are many convenient methods of separation, of which one or a combination may be employed, such as the centrifu- gal effect of a pipe - bend, making use of the traditional riser on board, or a short section containing transverse angled louvres, or if considerable length is available, separation by gravity into a collector trough or pipe.

During the test, the engine was run each time at 2300, 3500 and 5500 rpm. The traditional system produced noise levels rising to about 65 .dB,A and back pressures 3, 10, and 18 inches/water (75,250 and 460- mm). - With further acccleration the level rose steeply to 30 ins. water (760 mm.).

The invested-plain pipe with no silencer was then compared. - At 5,50 rpm and above the effluent was silent and later the pipe. had to be shortened to 9 ft.- (270 cms.) before tb'e' threshold of sound was reached.

At. the quoted rpm- periods the back pres sure figures were - 0 in., 0.5-1 ins. and -ins.

(0, 1?, 25 and 50 mm.).

Zero readings reflected the small inertia of the manometer and the above figures are corrected readings, meaning after deduction of the back pressure introduced by the bent downtake pipe arrangement off the manifold, which was a common factor throughout all observations.

In broad terms therefore the invested exhaust produced about one eighth of the back pressure of the traditional exhaust, taken as a mean reading throughout the curves. The higher the velocity, the bigger the difference.

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, H inches (75-100 mm.) at 3 300-4000, and 10 inches (250 mm.) at 5,500 rpm.

Again the noise level rose to a level of discomfort, as commonly experienced in cities.

The invested pipe, 10 ft. 300 emus. long, with no silencer box was run and produced no audible sound even at 12 inches, 30 cms. from the outlet. Back pressure readings now showed 1, 1 5, and 2 5 inches (25, 37 and 63 mm.) water columns This invested pipe was then shortened progressively until at 24 inches, or 60 cms length it produced at the outlet an approximately similar noise level as the traditional silencer, except that again, the higher fre quencies had been removed, the resultant 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 dry 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 compressed air line. For related physical reasons, organ piping is eliminated without reduction of rheological efficiency.

Other areas in acoustics where this technique may be useful are those where high noise level impulse waves are pr duced, because the absorbent effect of the investment is considerably to chop down the initial oscilloscope deflection.

Throughout the whole of the above argument, illustration, and examples, the unidirection of the filaments or majority of them, and consequently minimal reduction in kinetic energy has been the theme.

Paradoxically, the reverse may apply in certain cases.

An 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 produces a resistance to such return flow.

WHAT WE CLAIM IS:- 1. A device for absorbing acoustic energy which device comprises a surface and a multiplicity of flexible fibres each fixed directly or indirectly to the surface, the fibres having portions extending freely away from the surface, end portions of the free portions forming a secondary surface, and thereby means to guide a fluid past the secondary surface with acoustic absorption, the arrangement being such that the said end portions will be trailing when fluid flows past the secondary surface.

2. A device as claimed in claim 1 comprising a conduit in which the surface is provided on the interior of the conduit.

3. A device as claimed in claim 2 in which the conduit is tubular.

4. A device as claimed in any preceding claim having fibres of a diameter between + and 500 microns.

5. A device as claimed in any preceding claim in which the fibres extend initially perpendicularly to the surface.

6. A vehicle exhaust system comprising a device for absorbing acoustic energy as claimed in any one of the preceding claims.

7. A vehicle exhaust system as claimed in claim 1 substantially as hereinbefore described with reference to any of the fore going examples.

8. A vehicle exhaust system substantially as hereinbefore described with reference to and as shown in the accompanying drawing.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (8)

**WARNING** start of CLMS field may overlap end of DESC **. Zero readings reflected the small inertia of the manometer and the above figures are corrected readings, meaning after deduction of the back pressure introduced by the bent downtake pipe arrangement off the manifold, which was a common factor throughout all observations. In broad terms therefore the invested exhaust produced about one eighth of the back pressure of the traditional exhaust, taken as a mean reading throughout the curves. The higher the velocity, the bigger the difference. 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, H inches (75-100 mm.) at 3 300-4000, and 10 inches (250 mm.) at 5,500 rpm. Again the noise level rose to a level of discomfort, as commonly experienced in cities. The invested pipe, 10 ft. 300 emus. long, with no silencer box was run and produced no audible sound even at 12 inches, 30 cms. from the outlet. Back pressure readings now showed 1, 1 5, and 2 5 inches (25, 37 and 63 mm.) water columns This invested pipe was then shortened progressively until at 24 inches, or 60 cms length it produced at the outlet an approximately similar noise level as the traditional silencer, except that again, the higher fre quencies had been removed, the resultant 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 dry 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 compressed air line. For related physical reasons, organ piping is eliminated without reduction of rheological efficiency. Other areas in acoustics where this technique may be useful are those where high noise level impulse waves are pr duced, because the absorbent effect of the investment is considerably to chop down the initial oscilloscope deflection. Throughout the whole of the above argument, illustration, and examples, the unidirection of the filaments or majority of them, and consequently minimal reduction in kinetic energy has been the theme. Paradoxically, the reverse may apply in certain cases. An 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 produces a resistance to such return flow. WHAT WE CLAIM IS:-

1. A device for absorbing acoustic energy which device comprises a surface and a multiplicity of flexible fibres each fixed directly or indirectly to the surface, the fibres having portions extending freely away from the surface, end portions of the free portions forming a secondary surface, and thereby means to guide a fluid past the secondary surface with acoustic absorption, the arrangement being such that the said end portions will be trailing when fluid flows past the secondary surface.

2. A device as claimed in claim 1 comprising a conduit in which the surface is provided on the interior of the conduit.

3. A device as claimed in claim 2 in which the conduit is tubular.

4. A device as claimed in any preceding claim having fibres of a diameter between + and 500 microns.

5. A device as claimed in any preceding claim in which the fibres extend initially perpendicularly to the surface.

6. A vehicle exhaust system comprising a device for absorbing acoustic energy as claimed in any one of the preceding claims.

7. A vehicle exhaust system as claimed in claim 1 substantially as hereinbefore described with reference to any of the fore going examples.

8. A vehicle exhaust system substantially as hereinbefore described with reference to and as shown in the accompanying drawing.