WO1992013339A1

WO1992013339A1 - Noise attenuation panel

Info

Publication number: WO1992013339A1
Application number: PCT/GB1992/000121
Authority: WO
Inventors: Robert Samuel Wilson
Original assignee: Short Brothers Plc
Priority date: 1991-01-22
Filing date: 1992-01-21
Publication date: 1992-08-06
Also published as: GB9101353D0; EP0568570A1; CA2098099A1; GB2252077B; AU1170892A; GB2252077A; JPH06504631A; GB9201246D0

Abstract

A noise attenuation panel (10) for an aero engine comprises a cellular component (12) having an open-celled structure and presenting a front face for exposure to sound to be attenuated. The component has a frontal region which includes the front face and in which the cell structure provides intercommunicating cells of a predetermined first size and density effective to offer low resistance to the passage of gaseous flow across the front face and to prevent normal ingress of liquids into the component through the front face and a base region in which the cell structure provides intercommunicating cells of a predetermined second size and density effective for sound energy absorption. The frontal region is provided by a facing sheet formed from a porous permeable thermoplastics material produced by powder sintering the material and the base region is formed from an open celled plastics foam. Alternatively, the component is wholly in the form of a plastics foam or is made wholly from a porous permeable thermoplastics material produced by powder sintering the material.

Description

Noise Attenuation Panel

The present invention relates to noise attenuation panels and is particularly, although not exclusively, concerned with noise attenuation panels for use in the attenuation of noise in aero engines.

In patent application publication GB-A-2223448 there is disclosed a noise attenuation panel having a backing component part, a facing component part and a cellular component part having a multiplicity of open-ended juxtaposed cells, the backing component part extending across the ends of the cells of the cellular component part at the rear thereof and the facing component part extending across the ends of the cells of the cellular component part at the front thereof.

In one embodiment of the invention described in GB-A- 2223448, the cellular component part comprises a single cellular element having wall portions which extend across the element from the front face thereof to the rear face thereof and which provide bounding faces for an array of open-ended juxtaposed cells which extend from the front face of the cellular component part to the rear face thereof.

In another embodiment of the invention described in GB-A- 2223448, the cellular component part is sub-divided into a front cellular element and a rear cellular element positioned to the rear of the front cellular element with a septum element extending between the two cellular elements. Each cellular element has wall portions which extend across the element from the front face thereof to the rear face thereof and which provide bounding surfaces for an array of open-ended juxtaposed cells extending from the front to the rear of the element.

In the panel disclosed in GB-A-2223448 use is made of a facing component part for the panel which comprises or includes an outer facing sheet which is made of a porous permeable thermoplastics material which is preferably produced by powder sintering a thermoplastics material. The outer facing sheet produced as proposed in GB-A-2223448 is of an open-celled structure, the cell size and cell density being such as to make it sufficiently permeable for transmission of sound energy which is then absorbed by the cellular component part at the rear of the facing sheet.

The panel disclosed in GB-A-2223448 and formed with an outer facing sheet as described has been found to be successful particularly for use in aero engine environments in positions in which the outer facing sheet provides a surface subjected to the passage of gaseous flow across it, as for example the intake airflow across a nacelle internal surface. It, however, requires the use of complex and costly manufacturing techniques, particularly in the production of panels of complex shape of single and double curvature.

It is one object of the present invention to provide a noise attenuation panel which can be produced readily and inexpensively, which conforms, where required, to a complex shape and which is particularly well adapted for use in aero engine environments.

According to a first aspect of the present invention, there is provided a noise attenuation panel comprising a cellular component having an open-celled structure and a front face for exposure to sound to be attenuated, characterised in that the component has a frontal region which includes the front face and in which the cell structure provides intercommunicating cells of a predetermined first size and at a predetermined first density effective to offer low resistance to the passage of gaseous flow across the front face and to prevent normal ingress of liquids into the component through the front face and a base region beneath the frontal region in which the cell structure provides intercommunicating cells of a predetermined second size and at a predetermined second density effective for sound energy absorption.

By "open-celled structure" is meant a cellular structure having a multiplicity of intercommunicating cells obtained by aggregation of particulate material or by the displacement of material by a dispersion or like technique from a body of material in liquid phase followed by a solidifying step.

In an embodiment of the invention hereinafter to be described the cellular structure of the frontal region of the cellular component is obtained by aggregation of particulate material to provide a multiplicity of intercommunicating cells at the predetermined first size and predetermined first density and the' cellular structure of the base region of the cellular component is obtained by the displacement of material by a dispersion or like technique from a body of material in liquid phase followed by a solidifying step to provide a multiplicity of intercommunicating cells at the predetermined second size and the predetermined second density.

In an embodiment of the invention hereinafter described and illustrated the frontal region is provided by a facing cellular component part which comprises or includes an outer facing sheet which is made from a porous permeable thermoplastics material produced by powder sintering the material and the base region of the cellular component is provided by a base component part formed from an open celled rigid or flexible plastics foam.

In an embodiment of the invention hereinafter described but not illustrated the facing cellular component part includes an inner facing sheet which is perforated and which provides structural support for the outer facing sheet. The inner facing sheet is preferably made from an open square weave fabric so woven as to provide apertures constituted by the openings between adjacent warp and weft threads of the fabric. The fabric is preferably so woven as to produce a proportion of open aperture area relative to the total surface area of the sheet of around 30% or substantially 30%.

In an alternative embodiment of the invention hereinafter to be described the cellular structure of the frontal and base regions of the cellular component is obtained by the displacement of material by a dispersion or like technique from a body of material in liquid phase followed by a solidifying step to provide a multiplicity of intercommunicating cells at the predetermined first size and predetermined first density in the frontal region and at the predetermined second size and predetermined second density in the base region. The component is preferably formed from an open-celled rigid or flexible plastics foam.

In yet a further embodiment of the invention hereinafter to be described the cellular structure of the frontal and base regions of the cellular component is obtained by aggregation of particulate material to provide a multiplicity of intercommunicating cells at the predetermined first size and predetermined first density in the frontal region and the predetermined second size and predetermined second density in the base region. The component is preferably made from a porous permeable thermoplastics material produced by powder sintering the thermoplastics material.

In the embodiments of the invention hereinafter to be described a backing component part is secured to the rear face of the cellular component. It is imperforate and made of a non-porous impermeable material.

According to a second aspect of the present invention, there is provided an aero engine having a surface subjected to the passage across it of gaseous flow and a noise attenuation panel according to the first aspect of the invention so positioned that its front face forms the surface or a part of the surface subjected to passage of the gaseous flow across it.

Embodiments of the invention will now be described by way of example with reference to the accompanying drawings in which:-

Fig. 1 is a schematic isometric view from above of a noise attenuation panel according to a first embodiment of the invention;

Fig. 2 is a schematic cross section of an end region of the panel shown in Fig. 1, secured to a supporting channel member; Fig. 3 is a schematic cross section of a noise attenuation panel according to a second embodiment of the invention, and

Fig 4 is a schematic cross-section of an aero engine embodying noise attenuation panels according to the invention.

Referring first to Figs. 1 and 2, the noise attenuation panel 10 comprises a backing sheet 11, a cellular component 12 and a facing sheet 14.

The backing sheet 11 is, as shown in Fig. 2, secured by an epoxy resin adhesive El to the lower face of the cellular component 12. The facing sheet 14 is secured to the upper face of the cellular component 12 by bonding the materials to each other, although an adhesive could if desired be used. The epoxy adhesive El may for example be obtained from Ciba-Geigy Plastics & Additives Company Limited of Cambridge, England. Adhesives and resins need not however be epoxy resin adhesives, but could for example be a phenolic, polyimide or thermoplastics resin.

The facing sheet 14 comprises a sheet of porous permeable thermoplastics material produced by powder sintering the thermoplastics material. Examples of suitable thermoplastics materials include polyether ketone, polyether ether ketone, polyaromatic ketone, polyphenylene sulphide, polyamide-imide and thermoplastic polyimide, pol ether-imide, polyurethane and polyethylene.

The cellular component 12 is in the form of an open- celled plastics foam which may be manufactured from any suitable thermoplastics material. Examples of suitable thermoplastics materials include polyether ketone, polyether ether ketone, polyaromatic ketone, polyphenylene sulphide, polyamide-imide, thermoplastic polyimide, polyether-imide, polyurethane and polyethylene. It may with advantage be formed from the same thermoplastics material as that from which the facing sheet 14 is formed.

The backing sheet 11 is imperforate and made of a non- porous impermeable material and may be made of any of the following materials:-

(i) A carbon/thermoplastic composite where for example the thermoplastic is polyether ether ketone, the material being automatically tape wound or hand laid.

(ii) A carbon/epoxy resin.

(iii) An aluminium alloy.

The panel 10 is of arcuate form, possibly of double curvature, and is embodied as a structural part of a duct of a nose cowl of a turbofan aero engine, the panel 10 being one of several arcuate panels disposed just upstream of the fan of the engine. It is, of course, of vital importance that the panel does not deteriorate in use and, in particular, that no part of it becomes detached from its supporting structure. The structure will usually include supporting channel members of which only one member 17 is shown in Fig. 2. The panel 10 is secured to the member 17 by bonding the facing sheet 14 to an outer face of a flange 18 of the channel member 17 using carbon to carbon bond 19 and by bonding the backing sheet 11 to the outer face of a flange 20 of the channel member 17 using a carbon to carbon bond 21. The gap between the panel 10 and the base 22 of the channel member 17 may be sealed or closed by use of a mastic 23.

A panel having an outer facing sheet 14 made of a porous thermoplastics material as described with reference to Figs. 1 and 2 has been found to give rise to several advantages over the panels of the prior proposals, including the following:-

(1) The cellular structure of the facing sheet when produced by the powder sintering technique can be made to meet permeability requirements over a wide range. The cellular structure may be made permeable to gaseous flow over a wide range of tightly controlled flow and resistance requirements which will be engine dependent and non permeable to a wide range of liquids and solid contaminants.

(2) The cellular structure of the facing sheet when produced by the powder sintering technique provides a highly complex interference flow path as a result of which the noise attenuation properties are greatly enhanced over other forms of perforate and porous material.

(3) The very smooth surface of the facing sheet when produced by the powder sintering technique has substantial acoustic/air flow advantages over other perforate and porous forms. There is a lower flow resistance to high speed air flow, and therefore the overall aero engine power plant efficiency is improved over that obtained using the previously proposed panels; (4) the sound attenuation is greater and covers a wider frequency range than that of the previously proposed panels;

(5) the thermoplastic component parts do not have the problem of metal galvanic corrosion;

(6) the panel is lighter than previously proposed panels;

(7) there is an improved "blade-off" energy absorption compared with the previously proposed structures; and

(8) there is an improved appearance.

In addition, the panel described with reference to Figs. 1 and 2 has the following further advantages:-

(1) The cellular component 12 provides an uninterrupted layer of intercommunicating cells which provide a well distributed dissipation of sound energy and dampens back cavity resonances which smooths the reactance as a function of frequency and increases high frequency absorption.

(2) At low absorption frequencies, the'speed of sound in the cellular matrix produced by the component 12 is decreased, giving a greater apparent treatment depth and enhancing low frequency absorption. Furthermore, beyond these inherently point reacting arguments, axial and circumferential wave travel in the material can also be used to advantage.

(3) The use of a facing sheet produced by powder sintering a thermoplastics material ensures the formation of a micro-porous facing sheet which prevents normal ingress of liquids which would normally destroy the acoustic effectiveness of the panel and could pose a safety problem.

(4) The thermoplastic foam component 12 can be used to provide structural strength while preserving the desirable characteristics which are feature of it.

The facing sheet 14 is described as being formed by powder sintering of a thermoplastics material. It will however be appreciated that the required porous permeable characteristics of the sheet which provide a low resistance to gaseous flow across it and prevent the normal ingress of liquids may be produced by techniques other than sintering and may for example be produced by lost wax, lost salt or lost starch techniques, if these are regarded as more suitable.

Referring now to Fig. 3, a second embodiment of the invention is illustrated in which a panel 110 comprises a backing sheet 111 and an open-celled cellular component 112. The backing sheet 111 takes the same form as the backing sheet 11 of the panel 10 illustrated in Figs, l and 2 and is joined in the same manner by an adhesive El to the cellular component 112.

The cellular component 112 is in the form of an open- celled plastics foam, and may be manufactured from any of the thermoplastic materials proposed for the cellular component 12 in the embodiment of the invention described with reference to Figs. 1 and 2. It is furthermore manufactured in such a way as to provide a frontal region 140 which includes the front face 141 and in which the cell structure provides cells of a size and at a density effective to offer low resistance to the passage of gaseous flow across the face 141 and to prevent normal ingress of liquids and a base region 120 beneath the frontal region 140 in which the cell structure provides cells of a size and at a density effective for sound energy absorption.

The cellular component 12 in the embodiment of the invention described with reference to Figs. 1 and 2 or the cellular component 112 of Fig. 3 can be produced by any well known technique for the production of a plastics foam of open-cell structure. For example, the foam may be produced by admixing a pore-forming ingredient with a thermoplastic polymer and with a solvent and then removing the solvent and the pore-forming ingredient so as to leave a system of cells intercommunicating with one another at the points of contact between individual cells.

In the embodiment of the invention illustrated in Figs, l and 2, the frontal region is provided by a facing cellular component part in the form of the facing sheet 14. In an alternative embodiment of the invention (not illustrated) the facing cellular component part may comprise an outer facing sheet of the same form as the sheet 14 and an inner facing sheet which is perforated and which provides structural support for the outer facing sheet. The inner facing sheet may be made from an open square weave fabric formed from a carbon fibre/resin matrix composite material, the weave being such as to provide apertures constituted by the openings between adjacent warp and weft threads of the fabric. The fabric is preferably so woven as to produce a proportion of open aperture area relative to the total surface area of the sheet of 30% or substantially 30%. In yet another embodiment of the invention, the cellular component 112 in the embodiment described with reference to Fig 3, which is in the form of an open-celled plastics foam, is replaced by a cellular component having a cellular structure for the frontal and base regions which is obtained by aggregation of particulate material. The component is manufactured in such a way as to provide a frontal region 140 which includes the front face 141 and in which the cell structure provides cells of a size and at a density effective to offer low resistance to the passage of gaseous flow across the face 141 and to prevent normal ingress of liquids and a base region 120 beneath the frontal region 140 in which the cell structure provides cells of a size and at a density effective for sound absorption. The component is preferably made from a porous permeable thermoplastics material produced by powder sintering the thermoplastics material and the thermoplastics materials employed may include any of those hereinbefore proposed for use as the thermoplastics material forming the facing sheet 14 of the embodiment of the invention described with reference to Figs 1 and 2.

Where the cellular component 112 is made of a porous permeable thermoplastics material produced by powder sintering the thermoplastics material, the frontal region 140 will have the same cellular structural form as that of the facing sheet 14 in Figs. 1 and 2 and the panel will have all the advantages hereinbefore set forth for the panel with the facing sheet 14 as described with reference to Figs. 1 and 2.

Referring now to Fig. 4, an aero engine 25 is schematically illustrated and includes a turbofan power unit 26 mounted within a nacelle 27 suspended from a pylon 32. The nacelle 27 includes a nose cowl 28 having an outer wall 29 and an inner wall 30. The inner wall 30 is in part formed by noise attenuation panels P which may take the form of panels 10 or 110 as described and illustrated with reference to Figs. 1 to 3. The panels P are arranged to form part of the inner wall of the nose cowl 28 and serve to reduce noise created by the high speed flow of air passing through the duct 31 into the power unit 26, as well as to reduce noise generated by the fan blades of the unit 26.

It is to be emphasised that the panels in Fig. 4 are not employed to reduce air noise by a reduction of the air speed by passage of the air through the panels, but by contrast acoustic attenuation is achieved without affecting the speed of the air which generates the noise, that is to say, the air does not pass through the noise attenuation panels P.

In the aero engine mounting arrangement illustrated in Fig. 4, the power unit is carried by the wing mounted pylon 32. It will however be appreciated that the noise attenuation panels according to the present invention may be equally well be employed for reducing noise in other aero engines installations.

Claims

1. A noise attenuation panel comprising a cellular component having an open-celled structure and a front face for exposure to sound to be attenuated, characterised in that the component has a frontal region which includes the front face and in which the cell structure provides intercommunicating cells of a predetermined first size and at a predetermined first density effective to offer low resistance to the passage of gaseous flow across the front face and to prevent normal ingress of liquids into the component through the front face and a base region beneath the frontal region in which the cell structure provides intercommunicating cells of a predetermined second size and at a predetermined second density effective for sound energy absorption.

2. A panel according to claim 1 characterised in that the cellular structure of the frontal region of the cellular component is obtained by aggregation of particulate material to provide a multiplicity of intercommunicating cells at the predetermined first size and predetermined first density.

3. A panel according to claim 2 Characterised in that the cellular structure of the base region of the cellular component is obtained by the displacement of material by a dispersion or like technique from a body of material in liquid phase followed by a solidifying step to provide a multiplicity of intercommunicating cells at the predetermined second size and the predetermined second density.

4. A panel according to claim 3 characterised in that the frontal region is provided by a facing cellular component part which comprises or includes an outer facing sheet which is made from a porous permeable thermoplastics material produced by powder sintering the material and the base region of the cellular component is provided by a base component part formed from an open celled rigid or flexible plastics foam.

5. A panel according to claim 4 characterised in that the facing cellular component part includes an inner facing sheet which is perforated and which provides structural support for the outer facing sheet.

6. A panel according to claim 5 characterised in that the inner facing sheet is made from an open square weave fabric so woven as to provide apertures constituted by the openings between adjacent warp and weft threads of the fabric.

7. A panel according to claim 6 characterised in that the fabric is so woven as to produce a proportion of open aperture area relative to the total surface area of the sheet of 30% or substantially 30%.

8. A panel according to claim 1 characterised in that the cellular structure of the frontal and base regions of the cellular component is obtained by the displacement of material by a dispersion or like technique from a body of material in liquid phase followed by a solidifying step to provide a multiplicity of intercommunicating cells at the predetermined first size and predetermined first density in the frontal region and at the predetermined second size and predetermined second density in the base region.

9. A panel according to claim 8 characterised in that the component is formed from an open-celled rigid or flexible plastics foam.

10. A panel according to claim 1 characterised in that the cellular structure of the frontal and base regions of the cellular component is obtained by aggregation of particulate material to provide a multiplicity of intercommunicating cells at the predetermined first size and predetermined first density in the frontal region and at the predetermined second size and predetermined second density in the base region.

11. A panel according to claim 10, characterised in that the component is made from a porous permeable thermoplastics material produced by powder sintering the thermoplastics material.

12. A panel according to any of claims 1 to 11 characterised in that a backing component part is secured to the base region of the cellular component at a rear face thereof.

13. A panel according to claim 12 characterised in that the backing sheet is imperforate and made of a non porous impermeable material.

14. An aero engine having a surface subjected to the passage across it of gaseous flow and a noise attenuation panel according to any of claims 1 to 13 so positioned that its front face forms the surface or a part of the surface subjected to passage of the gaseous flow across it.

15. A noise attenuation panel substantially as hereinbefore described with reference to Figs 1 and 2 or Fig 3 of the accompanying drawings.

16. An aero engine substantially as hereinbefore described with reference to Fig 4 of the accompanying drawings.