Soundproofing panel and method for its manufacture

Description

Technical Field

[0001] The present invention relates to an acoustic attenuation panel intended to absorb the sound energy generated in particular by aircraft turbomachines such as an aircraft turbojet or turboprop. Prior art

[0002] An aircraft is powered by one or more propulsion units, each comprising a turbojet / turboprop engine housed in a tubular nacelle. Each propulsion unit is attached to the aircraft by a mast generally located under a wing or at the fuselage.

[0003] A nacelle generally has a structure comprising an upstream section forming an air inlet upstream of the engine, a middle section intended to surround a fan of the turbojet engine, a downstream section which can house thrust reverser means and intended to surround the combustion chamber of the turbojet engine, and is generally terminated by an ejection nozzle whose outlet is located downstream of the turbojet engine.

[0004] The air intake serves to optimize the air capture required to supply the turbojet fan throughout the flight envelope and to channel the air towards the fan.

[0005] The air inlet mainly comprises an air inlet lip forming a leading edge, attached to an annular structure.

[0006] The annular structure comprises an external fairing ensuring the external aerodynamic continuity of the nacelle and an internal fairing ensuring the internal aerodynamic continuity of the nacelle, in particular with the external fan casing at the mid-section. The air inlet lip ensures the upstream junction between these two fairings.

[0007] The internal air intake fairing is exposed to a significant airflow and is located close to the fan blades. It therefore contributes to the transmission of noise from the turbojet engine to the outside of the aircraft.

[0008] Also, it is known from the prior art to equip the internal fairing of the air intake of the nacelle with an acoustic panel in order to attenuate the transmission of noise generated by the turbojet.

[0009] Typically, the acoustic panel comprises a perforated acoustic structure and a honeycomb-type alveolar structure which is assembled in particular by gluing onto the perforated acoustic structure.

[0010] The honeycomb structure comprises a plurality of acoustic cells, forming Helmholtz resonators or quarter-wave resonators, which are separated from each other by peripheral partitions.

[0011] The perforated acoustic structure is facing the noise emission area, so that the acoustic waves can penetrate through the openings of the perforated acoustic structure into the acoustic cells. The acoustic energy is dissipated by visco-thermal effect in these perforations.

[0012] The honeycomb structure of the acoustic panel can consist of a single layer of cells, or two layers separated by a micro-perforated septum to improve the acoustic performance of the panel, in particular the width of the absorbed frequency band.

[0013] A disadvantage of these types of panels is that the perforated acoustic structure has a strong acoustic non-linearity which means that its surface impedance varies significantly with the acoustic level and the speed of the grazing flow, particularly due to the interaction between the perforations and the air flow.

[0014] A solution of the prior art consists of applying to the perforated acoustic structure a porous layer by gluing, acoustically equivalent to perforations of a diameter smaller than the perforations of the perforated acoustic structure, reducing the interaction with the air flow.

[0015] Although this solution reduces acoustic non-linearity, such a structure has the major disadvantage of additional assembly, which is penalizing in terms of manufacturing time and cost. In addition, poor assembly can lead to risks of acoustic inhomogeneity. Finally, there is also a risk of corrosion of the porous layer exposed to the air flow if it is made of metallic material, imposing constraints on the choice of materials.

[0016] Another solution is to provide an acoustic attenuation panel formed of two superimposed honeycomb structures, separated by a porous layer.

[0017] However, this type of panel makes the manufacturing process longer and more expensive, and introduces risks of acoustic inhomogeneities caused by possible misalignments of the honeycombs inducing poor aerodynamic performance. US 2018 / 162542 A1 discloses an acoustic attenuation panel. Statement of the invention

[0018] The invention aims to overcome at least one of the aforementioned drawbacks and relates, according to a first aspect, to an acoustic attenuation panel according to claim 1.

[0019] In other words, the porous acoustic layer formed from the second material is attached to a surface of the perforated acoustic structure by entanglement of the first material in the second material and / or of the second material in the first material, i.e. the porous acoustic layer is attached to a surface of the perforated acoustic structure by entanglement of the first material and the second material with each other.

[0020] Thus, thanks to the acoustic attenuation panel according to the invention in which the honeycomb structure and the perforated acoustic structure are formed in a single piece and in which the connection between the perforated acoustic structure and the porous layer is obtained by interlocking their material, the linearity of the panels is improved so that the aerodynamic performances are optimized. The panels are robust against bad weather and simpler to manufacture.

[0021] According to other characteristics of the invention, the acoustic attenuation panel of the invention comprises one or more of the following optional characteristics considered alone or in all possible combinations. The single piece formed from the honeycomb structure and the perforated acoustic structure, and the acoustically porous layer are made of identical materials. The single piece formed from the honeycomb structure and the perforated acoustic structure, and the acoustically porous layer are made of different materials. Perforations of the perforated acoustic structure are spaced apart from each other by a distance that increases in an axial direction A. The perforations of the perforated acoustic structure have a dimension S that increases in an axial direction A. The perforations of the perforated acoustic structure have a substantially rectangular shape. The perforations of the perforated acoustic structure represent at least 20% of the surface area of ​​said structure, preferably at least 40% of the surface area of ​​said structure, even more preferably at least 60% of the surface area of ​​said structure.

[0022] The porous acoustic layer has holes of dimensions smaller than the dimensions S of the perforations of the perforated acoustic structure. The porous acoustic layer has holes of increasing size along the axial direction A. The porous acoustic layer has a woven architecture comprising interlacings of elementary threads or fiber strands, the threads or fiber strands of the porous acoustic layer are partially bonded together at the crossing levels and / or along edge segments with each other, that is to say that the threads or fiber strands are partially bonded together by parallel adjacent parts.

[0023] The perforated acoustic structure and the peripheral partitions have a thickness of between 0.5 and 5 millimeters, preferably the perforated acoustic structure and the peripheral partitions have a thickness of 0.5 to 2 millimeters.

[0024] The sound attenuation panel comprises a removable cover, arranged on one side of the honeycomb structure opposite the perforated acoustic structure. The perforated acoustic structure may be made of plastic material, for example thermoplastic material or thermosetting material such as polyurethane, silicone, or epoxy, composite material with a thermoplastic or thermosetting matrix reinforced with short fibers or long fibers such as polyester fibers, polyethylene fibers, glass fibers, carbon fibers, Kevlar fibers, synthetic fibers, or even metal alloys. The porous acoustic layer may be made of plastic material comprising for example plastic threads, composite material comprising for example composite threads such as fibers coated with a thermoplastic or thermosetting matrix, or even metal material comprising for example metal threads of light alloys or ferrous alloys.

[0025] According to another aspect, the invention relates to an air inlet of a nacelle, arranged upstream of a fan, the air inlet comprising an inner face oriented opposite the fan, said inner face receiving at least one acoustic attenuation panel as previously described.

[0026] According to another aspect, the invention relates to a method of manufacturing an acoustic attenuation panel as described above by means of a molding tool comprising at least one mold and one counter-mold, the method comprising the steps of: Pinching the porous layer between said at least one mold and the counter mold, Injection of material intended to form a perforated acoustic structure and a honeycomb structure in one piece into the molding tool, Entanglement of the material intended to form a perforated acoustic structure and a honeycomb structure with the material of the porous layer, Removal of the formed acoustic attenuation panel from the molding tool.

[0027] According to other characteristics of the invention, the method comprises one or more of the following optional characteristics considered alone or in all possible combinations. -said at least one mold has a shape intended to form an acoustic cell of the honeycomb structure. -said at least one mold has at least one punch intended to form a perforation in the perforated acoustic structure. - the surfaces of the punches and the counter mold are coated with a flexible material such as silicone or fluorocarbon. - the surfaces of the punches and the counter mold are made of a rigid material.

[0028] Other characteristics and advantages of the invention will appear on reading the non-limiting description which follows and the appended figures which schematically illustrate the acoustic attenuation panel according to the invention. Brief description of the drawings

[0029] [ Fig. 1 ] There figure 1 is a partial cross-sectional view of an acoustic attenuation panel according to the invention [ Fig. 2 ] There figure 2 an enlarged view of the cross-sectional view of the sound attenuation panel. Fig. 3 ] There figure 3 is a perspective view of the acoustic attenuation panel according to one embodiment of the invention. Fig. 4 ] There figure 4 is a partial perspective view of the acoustic attenuation panel according to one embodiment of the invention. Fig. 5 ] There figure 5 is a partial perspective view of the acoustic attenuation panel according to one embodiment of the invention. Fig. 6 ] There figure 6 is a schematic representation of a method of manufacturing an acoustic panel according to the invention. Fig. 7 ] There figure 7 is an illustration of a propulsion assembly comprising a nacelle whose air intake receives an acoustic attenuation panel according to the invention. Description of the embodiments

[0030] For the sake of simplification, identical elements are identified by identical reference signs throughout the figures.

[0031] It should be noted that in the description and in the claims, the terms upstream and downstream must be understood in relation to the circulation of the air flow inside the propulsion unit formed by the nacelle and the turbojet, that is to say from left to right with reference to the figure 7 .

[0032] There figure 1 is an enlarged cross-sectional view of an acoustic attenuation panel ( figure 3 ) according to the invention.

[0033] The acoustic panel 1 comprises a honeycomb structure 10, a perforated acoustic structure 20 and a porous acoustic layer 30 attached to the perforated acoustic structure 20.

[0034] Thus the cellular structure 10 of the acoustic panel according to the invention has a free face and a face provided with a perforated acoustic structure 20 and a porous acoustic layer 30.

[0035] The honeycomb structure 10 and the perforated acoustic structure 20 are formed in a single piece, that is to say they are made in a single piece.

[0036] This eliminates the risks of erosion, abrasion, detachment or delamination of prior art acoustic panels in which the two structures are connected to each other, for example by gluing.

[0037] The honeycomb structure 10 comprises a plurality of acoustic cells 100 which are separated from each other by peripheral partitions 102.

[0038] In this example, each cell has a square shape with sides of 5 centimeters.

[0039] In an alternative embodiment, each cell may have a rectangular shape as illustrated in figure 3 .

[0040] Acoustic cells are not limited to the shapes previously described and can have other polygonal shapes such as a triangle, a pentagon, a hexagon etc.

[0041] Each peripheral partition 102 extends from the perforated acoustic structure 200 at an angle of 90°. In other words, each peripheral partition extends substantially perpendicular to the perforated acoustic structure 200.

[0042] These partitions 102 may include protrusions or threads (not shown) intended to allow the acoustic attenuation panel to be fixed in a nacelle ( figure7 ). In preferred embodiments, the acoustic panel is intended to be fixed or glued to a duct of the nacelle to provide the aerodynamic surface and acoustic attenuator functions of this nacelle duct. In the case of fixing, the surface between the panel and the nacelle may have a clearance. To this end, an arrangement is provided in the acoustic attenuation panel 1 to support a sealing element such as a seal.

[0043] Preferably, the perforated acoustic structure 20 and the peripheral partitions 102 have a thickness of between 0.5 and 5 millimeters. For example, the perforated acoustic structure 20 has a thickness of between 0.5 and 2 millimeters, and the peripheral partitions 102 have a thickness of between 1 and 5 millimeters, even more preferably between 1 and 3 millimeters.

[0044] This relatively thin thickness is particularly preferred in acoustic treatments.

[0045] The perforated acoustic structure 20 comprises a plurality of perforations 200 regularly formed in the perforated acoustic structure 20.

[0046] In an embodiment not shown, the perforations 200 can be spaced from each other by a distance which varies along the axial direction A.

[0047] The term axial direction should be understood in relation to the circulation of the air flow as represented by arrow A of the figure 1 .

[0048] As illustrated by the figure 3 , each perforation 200 of the perforated acoustic structure has a substantially rectangular shape with rounded tops. In the present example, each acoustic cell 100 comprises four perforations 200.

[0049] The invention is not limited to this type of shape, in fact the shape of the perforations 200 can be variable and correspond to fractal shapes for example as illustrated by figure 4 .

[0050] In an alternative embodiment, the perforations 200 may have different shapes as illustrated in figure 5 wherein at least one perforation 200 is circular in shape and at least one perforation is trapezoidal in shape.

[0051] In the same way, the dimension S of the perforations 200 can vary and be, for example, increasing along the axial direction A. This evolving distribution also makes it possible to provide larger dimensions S near the blower and smaller at the distal end of the latter to provide acoustic treatment which is proportional to the sound level.

[0052] Preferably, the size of the perforations is at least 1 millimeter, preferably at least 2 millimeters.

[0053] The acoustic panel 1 has a porous acoustic layer 30. By porous layer is meant a layer having at least one hole perforated by various mechanical means or a hole formed by the porosity of the material of the porous acoustic layer 30.

[0054] The porous acoustic layer 30 is arranged on the surface of the perforated acoustic structure 20 opposite the face of said structure 20 from which the peripheral partitions 102 extend. The porous acoustic layer 30 extends parallel to the perforated acoustic structure 20

[0055] The porous acoustic layer 30 is rigidly attached to the perforated acoustic structure 20 by an entanglement of at least one of the materials with the second material. figure 2 illustrates in more detail the attachment between the porous acoustic layer 30 and the perforated acoustic structure 20. The porous acoustic layer 30 is attached to the perforated acoustic structure 20 by an entanglement of the respective materials of the porous acoustic layer 30 and the perforated acoustic structure.

[0056] Entanglement means a diffusion of material into another material, a penetration of material to create bridges of material between the components.

[0057] This entanglement is obtained during the manufacture of the acoustic attenuation panel which will be detailed in figure 2 .

[0058] It is thus possible to avoid the prior art bonding techniques which cause problems of strength and detachment over time, particularly in the case of porous layers of materials liable to corrode.

[0059] The porous acoustic layer 30 has holes 32 of dimensions smaller than the dimensions S of the perforations 200 of the perforated acoustic structure 20.

[0060] In an embodiment not shown, the dimension of the holes 32 of the porous acoustic layer 30 can vary and be, for example, increasing along the axial direction A.

[0061] According to one embodiment of the invention, the alveolar structure 10, the perforated acoustic structure 20 and the porous acoustic layer 30 are made of identical materials.

[0062] According to a first example, the honeycomb structure 10, the perforated acoustic structure 20 and the porous acoustic layer 30 are made of metallic material, preferably aluminum, even more preferably, an aluminum alloy chosen from the 6000 series.

[0063] The porous acoustic layer 30 may be a micro-perforated metal sheet, for example using a micro-drilling technique carried out by laser so as to form a layer similar to a grid.

[0064] In a variant, the porous acoustic layer 30 may be a very fine mesh made of aluminum alloy from the same series or from a compatible alloy series such as an alloy chosen from the 5000 series.

[0065] According to a second example, the honeycomb structure 10, the perforated acoustic structure 20 and the porous acoustic layer 30 are made of composite material, preferably in a material formed from a thermoplastic or thermosetting matrix.

[0066] According to another embodiment of the invention, the honeycomb structure 10, the perforated acoustic structure 20 and the porous acoustic layer 30 are made of different materials. For example, the honeycomb structure 10 and the perforated acoustic structure 20 are made of composite materials and the porous acoustic layer 30 is made of metallic material.

[0067] In an embodiment not shown, the acoustic attenuation panel 1 may comprise a removable cover (not shown), arranged on the face of the honeycomb structure opposite the face comprising the perforated acoustic structure 20.

[0068] There figure 6 is an illustration of a method of manufacturing the acoustic attenuation panel 1 according to one embodiment of the invention.

[0069] In the present example, the acoustic attenuation panel is produced using a molding tool 4 comprising a plurality of molds 40 and a counter mold 400.

[0070] In the present example, each mold 40 has a shape intended to form an acoustic cell 100. Each mold comprises a plurality of punches 42 complementary to the perforations 200, intended to form an opening in the perforated acoustic structure 20. In the present example, a mold comprises four punches.

[0071] In a first embodiment, a porous layer 30 is placed between the plurality of molds 40 and the counter-mold 400. As such, the molding tool 4 has a space located between the molds 40 and the counter-mold 400 corresponding to the thickness of the porous layer 30.

[0072] The porous layer 30 is pinched between the molds 40 and the counter-mold 400, for example by moving the mold 40 or moving the counter-mold 400.

[0073] The material used to produce the perforated acoustic structure 20 and the honeycomb structure 10 in one piece is injected into the molding tool 4 so that it infiltrates the parts of the mold located between two adjacent punches and fuses with the material of the porous layer or coats the material therewith.

[0074] Thus the porous acoustic layer 30 is fixed to the perforated acoustic structure 20 at the level of the parts P surrounding the perforations 200.

[0075] The materials are then hardened and the acoustic panel thus created is demolded, for example by removing the molds 40 and then removing the acoustic panel created from the counter mold 400.

[0076] In a first variant, the surfaces of the punches 42 and the counter-mold 400 are coated with a flexible material such as silicone or fluorocarbon so that they deform to prevent migration of material into the perforations 200 being formed. In addition, this soft coating prevents the porous acoustic layer from being deformed during the pinching step.

[0077] In another variant, the surfaces of the punches 42 and the counter mold 400 may be made of rigid material, the pinching of the porous layer causes it to deform by work hardening. In a particular solution, the pinching is slightly greater in the areas where the perforations 200 are formed to prevent material migration.

[0078] In a second embodiment, the perforated acoustic structure 20 and the honeycomb structure 10 formed in a single piece are produced during a first operation using the molds 40 and the counter mold 400 to form the acoustic cells 100 and an opening of the perforated acoustic structure. The single piece is hardened and then demolded.

[0079] The porous acoustic layer 30 is placed against the perforated acoustic structure 20.

[0080] The connection between the porous acoustic layer 30 and the perforated acoustic structure 20 is obtained by melting or partial dissolution of either the material of the single piece formed of the perforated acoustic structure 20 and the honeycomb structure 10, or of the porous layer 30, or of both, so that the materials become entangled with each other. The melting can be achieved by ultrasound, induction, Joule effect or by heating the molds 40 or the counter mold 400.

[0081] In one embodiment, the perforated acoustic structure 20 has irregularities, for example a micro-uneven surface, i.e. one which is not smooth, in order to improve the anchoring of the material of the porous layer 30 in the perforated acoustic structure 20.

[0082] The newly formed sound attenuating panel is cooled so as to form a mechanical and / or molecular bond between the panel components.

[0083] Thanks to these manufacturing methods, it is possible to reliably and effectively interpose the porous acoustic layer 30 on the perforated acoustic structure 20. The production of the perforations 200 of the acoustic panel 1 is optimized and the dimension S of the perforations 200 is controlled.

[0084] The acoustic attenuation panel 1 may be formed in a single, circular, 360° piece corresponding to the dimensions of the nacelle or be sectorized into a plurality of assemblable panels, for example into twenty-four panels having a deformation at 15°. As such, the counter mold 400 may have a concave or convex shape of the order of 15° so that the panel 1 has a desired degree of deformation. The arrangement in assemblable acoustic panels facilitates the replacement of a damaged panel in the nacelle.

[0085] The production of acoustic attenuation panels is not limited to the manufacturing processes described above and can, for example, be carried out by additive manufacturing without using molding tools.

[0086] This embodiment has the advantage of optimizing the design of the acoustic attenuation panel, in particular the perforated acoustic structure in which perforations of complex shape can be made. Furthermore, the material surrounding the perforations can have a variable thickness or width, thus making it possible to provide acoustic structures with multiple stiffness scales.

[0087] There figure 7 is an illustration of a propulsion assembly 5 extending along a longitudinal axis X comprising a short nacelle 50 and a turbojet 500. The nacelle 50 has a structure comprising an upstream section forming an air inlet 52, a middle section 54 comprising fan cowls intended to surround a fan 510 of the turbojet 500, a downstream section 56 comprising a thrust reverser and intended to surround the combustion chamber of the turbojet and an ejection nozzle 58.

[0088] The air inlet 52 comprises an inner face 53 oriented opposite the blower 510, said inner face receives at least one acoustic attenuation panel 1 according to the examples which have just been described.

[0089] As illustrated by the figure 7 , other components of the nacelle such as the thrust reverser for example, can receive the acoustic attenuation panel 1 according to the examples which have just been described.

[0090] The acoustic attenuation panel can be used in several nacelle installations such as an air intake, an extended air intake, at the level of a fan casing or even a thrust reverser.

[0091] Obviously, the invention is not limited to the examples which have just been described and numerous adjustments can be made to these examples without departing from the scope of the invention as defined by the appended claims. In particular, the various characteristics, forms, variants and embodiments of the invention can be associated with each other in various combinations to the extent that they are not incompatible or exclusive of each other.

Claims

1. An acoustic attenuation panel (1) comprising: - an alveolar structure (10) including a plurality of acoustic cells (100) separated from each other by peripheral partitions (102), - a perforated acoustic structure (20), the perforated acoustic structure (20) and the alveolar structure (10) being formed in a single piece in a first material, and - a porous acoustic layer (30) formed of a second material, fastened to a surface of the perforated acoustic structure (20) by entanglement of the first material in the second material and / or the second material in the first material, characterized in that - each perforation (200) of the perforated acoustic structure has a substantially rectangular shape with rounded tops, or - the shape of the perforations (200) corresponds to fractal shapes, or - at least one perforation (200) is circular in shape and at least one perforation is trapezoidal in shape.

2. The acoustic attenuation panel (1) according to the preceding claim, characterized in that the single piece formed of the alveolar structure (10) and of the perforated acoustic structure (20), and the porous acoustic layer (30) are made of identical materials.

3. The acoustic attenuation panel (1) according to claim 1, characterized in that the single piece formed of the alveolar structure (10) and of the perforated acoustic structure (20), and the porous acoustic layer (30) are made of different materials.

4. The acoustic attenuation panel (1) according to any of the preceding claims, characterized in that perforations (200) of the perforated acoustic structure (20) are spaced apart from each other by a distance that evolves in an axial direction A.

5. The acoustic attenuation panel (1) according to any of the preceding claims, characterized in that perforations (200) of the perforated acoustic structure (20) have a dimension S that increases in an axial direction A.

6. The acoustic attenuation panel (1) according to any of the preceding claims, characterized in that perforations (200) of the perforated acoustic structure (20) have a substantially rectangular shape.

7. The acoustic attenuation panel (1) according to any one of the preceding claims, characterized in that the perforations of the perforated acoustic structure represent at least 20% of the surface area of said structure, preferably at least 40% of the surface area of said structure, even more preferably at least 60% of the surface area of said structure.

8. The acoustic attenuation panel (1) according to any one of the preceding claims, characterized in that the porous acoustic layer (30) has holes (32) with dimensions smaller than the dimensions S of the perforations (200) of the perforated acoustic structure (20).

9. The acoustic attenuation panel (1) according to any one of the preceding claims, characterized in that the porous acoustic layer (30) has holes (32) with dimension that increases in an axial direction A.

10. The acoustic attenuation panel (1) according to any one of the preceding claims, wherein the perforated acoustic structure (20) and the peripheral partitions (201) have a thickness comprised between 1 and 5 millimeters, preferably the perforated acoustic structure (20) and the peripheral partitions (201) have a thickness of 1 to 2 millimeters.

11. The acoustic attenuation panel (1) characterized in that the acoustic attenuation panel (1) comprises a removable cover arranged on a face of the alveolar structure (10) opposite the perforated acoustic structure (20).

12. An air inlet (52) of a nacelle (50), disposed upstream of a fan (510), the air inlet (52) including an inner face oriented opposite the fan (510), said inner face receiving at least one acoustic attenuation panel (1) according to any of claims 1 to 11.

13. A method for manufacturing an acoustic attenuation panel according to any of the preceding claims by means of a molding tool (4) comprising at least one mold (40) and a counter-mold (400), the method comprising the steps of: - Placing a porous acoustic layer (30) in a space located between said at least one mold (40) and the counter-mold (400), - Pinching the porous acoustic layer (30) between said at least one mold (40) and the counter-mold (400), - Injecting material intended to form a perforated acoustic structure (20) and a alveolar structure (10) in one piece in the molding tool (4), - Entangling the material intended to form a perforated acoustic structure (20) and a alveolar structure (10) with the material of the porous layer (30), - Removing the formed acoustic attenuation panel (1) from the molding tool (4).

14. The manufacturing method according to claim 13, characterized in that said at least one mold (40) has at least one punch (42) intended to form a perforation (200) in the perforated acoustic structure (20).

15. The manufacturing method according to claim 14, characterized in that surfaces of said at least one punch (42) and of the counter-mold (400) are coated with a flexible material such as silicone or fluorocarbon.

16. The manufacturing method according to any of claims 14 or 15, characterized in that surfaces of the punches (42) and of the counter-mold (400) are made of a rigid material.