ACOUSTIC PANEL FOR AN AIRCRAFT TURBOMACHINE

Abstract

The invention relates to an acoustic panel (17) for an aircraft turbomachine (1), the acoustic panel (17) having a sandwich structure comprising: - a honeycomb structure (18) comprising a plurality of acoustic cells (18a), - a perforated acoustic structure (19), and - a porous acoustic structure (20) attached to the perforated acoustic structure (19), the perforated acoustic structure (19) being arranged between the honeycomb structure (18) and the porous acoustic structure (20), characterized in that the porous acoustic structure (20) is multi-layered and comprises: - a first woven textile layer (21), and - a second woven textile layer (22) connected to the first layer (21), the second layer (22) being arranged between the first layer (21) and the perforated acoustic structure (19). Abstract figure: Figure 2

Description

Title of the invention: ACOUSTIC PANEL FOR AN AIRCRAFT TURBOMACHINE Technical field of the invention

[0001] The invention relates to the field of acoustic panels for aircraft turbomachines.

[0002] The invention relates in particular to the field of acoustic panels having a sandwich structure. Technical background

[0003] An aircraft turbomachine typically has a longitudinal axis. It comprises, for example, from upstream to downstream in the direction of gas flow along the longitudinal axis, a fan, a low-pressure compressor, a high-pressure compressor, a combustion chamber, a high-pressure turbine, a low-pressure turbine and a gas exhaust nozzle.

[0004] The blower allows the suction of an air flow dividing into a primary flow and a secondary flow. The primary flow passes through a primary vein of the turbomachine while the secondary flow is directed towards a secondary vein surrounding the primary vein.

[0005] The primary flow is compressed within the compressors. The compressed air is then mixed with a fuel and burned within the combustion chamber. The gases resulting from the combustion pass through the turbines and then escape through the nozzle, the cross-section of which allows the acceleration of these gases to generate propulsion.

[0006] The fan typically comprises a movable disc rotating about the longitudinal axis and blades mounted on the disc. The blades are surrounded by a fan casing centered on the longitudinal axis and intended to retain the blades in the event of damage, for example, to the blades.

[0007] The fan casing is typically surrounded by a nacelle which protects the fan. Such a fan is called ducted as opposed to unducted fans whose blades are not surrounded by a casing.

[0008] The turbomachine further comprises an intermediate casing located downstream of the fan casing. The intermediate casing typically comprises an annular inner shell centered on the longitudinal axis and an annular outer shell arranged coaxially around the inner shell, the outer and inner shells being connected by radial arms. The outer shell defines with the inner shell a portion of the secondary vein.

[0009] Turbomachines are a significant source of noise pollution and there is a strong demand to reduce this type of pollution. To this end, it has been proposed to equip certain components of the turbomachine such as the fan and intermediate casings and / or the nacelle with acoustic panels in order to reduce the noise generated by the turbomachines.

[0010] An acoustic panel has a sandwich structure and typically comprises a honeycomb structure comprising a plurality of acoustic cells forming Helmholtz resonators. The acoustic panel further comprises a perforated acoustic structure allowing sound waves to propagate in the core. The acoustic energy is dissipated by viscous effects in the perforated acoustic structure and the height of the core makes it possible to adjust the attenuated frequency range. The acoustic panel may further comprise a porous acoustic structure such that the perforated acoustic structure is located between the honeycomb structure and the porous acoustic structure. In this case, it is the porous structure which dissipates most of the acoustic energy. The porous acoustic structure typically has openings smaller than the diameter of the perforations of the perforated acoustic structure.Indeed, the perforated acoustic structure is located inside the honeycomb structure and is in contact with the secondary airflow. However, the interactions between the secondary airflow and the perforations tend to generate turbulence promoting aerodynamic drag, thus reducing the aerodynamic performance of the turbomachine. The porous acoustic structure makes it possible to reduce these interactions with the secondary airflow while ensuring the absorption of sound waves. However, this solution presents challenges related in particular to the assembly of the acoustic panel. Indeed, the addition of the porous acoustic structure requires, among other things, an additional assembly step of the porous acoustic structure.

[0011] To overcome this drawback, document WO-A1-2021 / 084206 proposes forming the honeycomb structure and the perforated acoustic structure in a single piece and connecting this piece to the porous acoustic structure by interweaving their respective materials, for example during a step of molding the acoustic panel. Such an acoustic panel thus has the advantage of being able to be manufactured in a single piece.

[0012] Although it has many advantages, this solution is not entirely satisfactory. Indeed, the porous acoustic structure generally has a small thickness so as not to penalize the mass of the acoustic panel. However, the thinner the porous acoustic structure, the more difficult it is to ensure cohesion between the porous acoustic structure and the assembly formed by the honeycomb structure and the perforated acoustic structure. Since the mechanical resistance of such an acoustic panel is low, it is difficult to guarantee the integrity of the acoustic panel.

[0013] There is therefore a need to improve the mechanical strength of acoustic panels for aircraft turbomachines, while minimizing the impact on the aerodynamic performance of the turbomachine. Summary of the invention

[0014] To this end, the invention proposes an acoustic panel for an aircraft turbomachine, the acoustic panel having a sandwich structure comprising:

[0015] - a honeycomb structure comprising a plurality of acoustic cells,

[0016] - a perforated acoustic structure, and

[0017] - a porous acoustic structure attached to the perforated acoustic structure, the perforated acoustic structure being arranged between the honeycomb structure and the porous acoustic structure.

[0018] The acoustic panel is remarkable in that the porous acoustic structure is multi-layered and comprises:

[0019] - a first woven textile layer, and

[0020] - a second woven textile layer connected to the first layer, the second layer being arranged between the first layer and the perforated acoustic structure.

[0021] According to the invention, the porous acoustic structure is multi-layered and comprises first and second woven textile layers.

[0022] The woven textile layers guarantee the porosity of the structure and therefore the penetration of acoustic waves into the alveolar structure.

[0023] Furthermore, the second woven textile layer provides the connection between the perforated acoustic structure and the porous acoustic structure while the first woven textile layer provides the masking of the second woven textile layer to minimize disturbances to the flow of the air flow in the turbomachine and guarantee the aerodynamic performance of the turbomachine.

[0024] Thanks to the combination of these two layers, the mechanical resistance of the acoustic panel is improved. Also, the impact on the aerodynamic performance of the turbomachine is minimized.

[0025] Finally, the combination of these two textile layers makes it possible to increase the thickness of the porous acoustic structure, which makes it easier to handle and assemble.

[0026] The invention may comprise one or more of the following features, taken in isolation from one another or in combination with one another:

[0027] - the first and second layers each comprise yarns having respectively different first and second diameters, the first diameter being smaller than the second diameter,

[0028] - the first diameter of each of the wires of the first layer is between 0.01 mm and 0.2 mm, in particular between 0.01 mm and 0.15 mm,

[0029] - the second diameter of each of the wires of the second layer is between 0.01 mm and 1 mm, in particular between 0.03 mm and 0.5 mm,

[0030] - the porous acoustic structure comprises connecting wires which intersect wires of the first and second layers for connecting the first and second layers together,

[0031] - the threads of the second layer comprise weft threads and at least one weft thread warp, at least two of the weft threads forming the connecting threads,

[0032] - the connecting wires have a diameter smaller than the diameter(s) of the wires of the first and second layers,

[0033] - the first and second layers are connected by welding or gluing,

[0034] - the first and second layers respectively have first and second openings, the first openings being smaller than the second openings,

[0035] - the second openings have a size between 0.1 mm and 2 mm, preferably between 0.1 mm and 1 mm,

[0036] - the first layer has a roughness Ra of less than 20 pm, advantageously less than 10 pm, preferably less than 5 pm,

[0037] - the porous acoustic structure is fixed to the perforated acoustic structure by entanglement of the second layer in the perforated acoustic structure,

[0038] - the first and second layers comprise a polymeric, metallic material or ceramic. Brief description of the figures

[0039] Other characteristics and advantages will emerge from the following description of non-limiting embodiments of the invention with reference to the appended drawings in which:

[0040] [Fig.l] is a schematic representation in longitudinal section of a half aircraft turbomachine,

[0041] [Fig.2] is a perspective representation of an acoustic panel according to the invention,

[0042] [Fig.3] is a schematic cross-sectional representation of the porous acoustic structure equipping the acoustic panel of [Fig.2],

[0043] [Fig.4] is a schematic representation of the first layer of the porous acoustic structure of [Fig.3],

[0044] [Fig.5] is a schematic representation of the second layer of the porous acoustic structure of [Fig.3],

[0045] [Fig.6] is a perspective representation of the porous acoustic layer in which the first and second layers are connected according to a first embodiment of the invention,

[0046] [Fig.7] is a sectional representation along a plane parallel to the X direction, of the porous acoustic structure in which the first and second layers are connected according to another embodiment of the invention,

[0047] [Fig.8] is a perspective view of a portion of the porous acoustic layer connected to the perforated acoustic structure. Detailed description of the invention

[0048] An example of a turbomachine 1 for an aircraft is shown in [Fig.l]. The turbomachine 1 extends around and along a longitudinal axis A.

[0049] In the present application, the terms “upstream” and “downstream” are defined in relation to the direction of circulation of the gases in the turbomachine 1 along the longitudinal axis A.

[0050] The terms “axial”, “axially”, “radial”, “radially” are defined with respect to the longitudinal axis A.

[0051] The terms “internal”, “interior”, “internally”, “external”, “exterior”, “externally”, are defined in relation to the distance from the longitudinal axis A along an axis Z perpendicular to the longitudinal axis A.

[0052] The turbomachine 1 is preferably a turbojet, for example a twin-spool, twin-flow turbojet. It comprises, from upstream to downstream, a fan 2, at least one compressor such as a low-pressure compressor 3 and a high-pressure compressor 4, a combustion chamber 5, at least one turbine such as a high-pressure turbine 6 and a low-pressure turbine 7, and a gas exhaust nozzle.

[0053] The low and high pressure compressors 3, 4 and the high and low pressure turbines 6, 7 each comprise at least one rotor. The rotor of the low pressure compressor 3 is connected to the rotor of the low pressure turbine 7 by a low pressure shaft 8 and the rotor of the high pressure compressor 4 is connected to the rotor of the high pressure turbine 6 by a high pressure shaft 9. The high pressure shaft 9 is arranged coaxially around the low pressure shaft 8. The low and high pressure shafts 8, 9 are centered on the longitudinal axis A.

[0054] The fan 2 comprises a disk movable in rotation about the longitudinal axis A and blades 10 extending radially from the disk. The fan 2 further comprises a fan shaft (not shown) connected to the low pressure shaft 8 via a speed reducer for example.

[0055] The blower 2 allows the suction of an air flow F dividing into a primary air flow Fl and a secondary air flow F2. The primary air flow Fl passes through a primary vein vl of the turbomachine 1 and the secondary flow F2 flows into a secondary vein v2 of the turbomachine 1. The secondary vein v2 surrounds the primary vein vl.

[0056] The primary flow F1 is compressed within the low pressure compressor 3 then the high pressure compressor 4. The compressed air is then mixed with a fuel and burned within the combustion chamber 5. The gases formed by the combustion pass through the high pressure and low pressure turbines 6, 7. The gases finally escape through the nozzle, the cross-section of which allows the acceleration of these gases to generate propulsion.

[0057] The fan 2 is of the ducted type. The turbomachine 1 thus further comprises a fan casing 11. The fan casing 11 is annular and centered on the longitudinal axis A. It is arranged around the blades 10. The fan casing 11 forms a portion of the secondary vein v2.

[0058] The turbomachine 1 further comprises an intermediate casing 12. The intermediate casing 12 is arranged downstream of the fan casing 11. It is connected to the fan casing 11 for example by flanges.

[0059] The intermediate casing 12 is centered on the longitudinal axis A and comprises an inner shell 13 and an outer shell 14 connected by arms 15. The outer shell 14 is annular and centered on the longitudinal axis A. It is arranged coaxially around the inner shell 13. The outer shell 14 delimits with the inner shell 13 a portion of the secondary vein v2.

[0060] The turbomachine 1 further comprises a nacelle 16. The nacelle 16 is arranged around the fan and intermediate casings 11, 12.

[0061] In order to reduce the noise pollution generated by the turbomachine 1, the turbomachine 1 comprises at least one and advantageously acoustic panels 17. Each acoustic panel 17 is advantageously capable of absorbing acoustic energy over a frequency range between 300 Hz and 3000 Hz.

[0062] Each acoustic panel 17 extends over an angular sector or has an annular shape centered on the longitudinal axis A. Each acoustic panel 17 can be attached and fixed inside the fan casing 11 and / or inside the external shroud 14 of the intermediate casing 12 and / or inside the nacelle 16.

[0063] With reference to [Fig. 2], each acoustic panel 17 has a sandwich structure. Each acoustic panel 17 comprises a honeycomb structure 18, a perforated acoustic structure 19 and a porous acoustic structure 20. The perforated acoustic structure 19 is located between the honeycomb structure 18 and the porous acoustic structure 20. When the acoustic panel 17 is mounted in the turbomachine 1, the porous acoustic structure 20 faces the air stream in which the acoustic waves propagate, and the honeycomb structure 18 is located towards the supporting structure of the nacelle, in the opposite direction of the air stream.

[0064] The honeycomb structure 18 comprises acoustic cells 18a. The acoustic cells 18a form Helmholtz resonators. The acoustic cells 18a are delimited from each other by peripheral walls 18b. Each acoustic cell 18 has, for example, a polygonal cross-section, for example square, rectangular or even hexagonal. The cross-section of the acoustic cells 18a may differ from one acoustic cell 18a to another. Each acoustic cell 18 is hollow and has an internal cavity 18c. The internal cavities 18c are delimited by the peripheral walls 18b.

[0065] The peripheral walls 18b extend from the perforated acoustic structure 19.

[0066] The honeycomb structure 18 may also include additions such as fixing systems, or any other characteristic.

[0067] Advantageously, the alveolar structure 18 has a thickness of between 1 mm and 5 mm, in particular between 1 mm and 3 mm.

[0068] The cellular structure 18 comprises, for example, a metallic material such as aluminum, in particular an aluminum alloy chosen from the 6000 series or a polymeric material chosen, for example, from thermoplastics or composites.

[0069] The perforated acoustic structure 19 is located between the porous acoustic structure 20 and the alveolar structure 18. The perforated acoustic structure 19 comprises perforations 19a. Preferably, the perforations 19a are regularly distributed in the perforated acoustic structure 19. The perforations 19a communicate with the cavities 18c of the acoustic cells 18a. Preferably, a group of four perforations 19a communicate with a cavity 18c of an acoustic cell 18a. The perforations 19a have, for example, a substantially polygonal cross-section, for example square and / or rectangular and / or hexagonal and / or circular. The perforations 19a have a dimension, for example, greater than or equal to 1 mm, in particular greater than or equal to 2 mm.

[0070] Advantageously, the perforated acoustic structure 19 has a thickness less than the thickness of the cellular structure 18. The thickness of the perforated acoustic structure 19 is preferably between 0.5 mm and 3 mm.

[0071] The perforated acoustic structure 19 comprises a material identical to or different from the material of the honeycomb structure 18.

[0072] According to an advantageous embodiment, the perforated acoustic structure 19 and the alveolar structure 18 form a monolithic part.

[0073] According to the invention, the porous acoustic structure 20 is multi-layered and is in the form of a lattice or a mesh.

[0074] Advantageously, the porous acoustic structure 20 has a surface mass of between 40 gsm and 1500 gsm, in particular between 100 gsm and 1100 gsm.

[0075] Advantageously, the porous acoustic structure 20 has a thickness e of between 0.5 mm and 2 mm.

[0076] With reference to [Fig. 3], the porous acoustic structure 20 comprises a first layer 21 and a second layer 22.

[0077] The first layer 21 is arranged inside the second layer 22. Advantageously, the first layer 21 is the innermost layer of the acoustic panel 17 when it is mounted in the turbomachine 1. Thus, the first layer 21 is in contact with the secondary air flow F2.

[0078] Advantageously, the first layer 21 has a first thickness el of between 0.1 mm and 0.5 mm.

[0079] The first layer 21 is a woven textile layer. With reference to [Fig.4], the first layer 21 comprises yarns 23. Each yarn 23 comprises an assembly of fibers or filaments.

[0080] Each wire 23 advantageously comprises a polymeric material chosen for example from thermoplastics. The thermoplastic material is for example chosen from polyaryletherketones (PAEK) such as a polyetherketone (PEK), a polyetheretherketone (PEEK) or a polyetherketoneketone (or PEKK) or polyacrylonitrile fibers (PAN) such as the HexTow® AS4, AS7 or IM7 fibers marketed by the company Hexcel. According to another example, the wires 23 comprise a metallic material such as aluminum or a ceramic material.

[0081] The wires 23 have a first diameter for example between 0.01 mm and 0.2 mm, in particular between 0.01 mm and 0.15 mm.

[0082] The wires 23 of the first layer 21 are advantageously organized into a plurality of layers 23' of wires 23 superimposed along the radial axis Z to form the first thickness el of the first layer 21. [Fig.4] illustrates a single layer 23' of wires 23.

[0083] The threads 23 are woven. The threads 23 are organized into at least one or more weft threads 23a extending in a first direction X and at least one or more warp threads 23b extending in a second direction Y perpendicular to the first direction X. The weft threads 23a and warp threads 23b intertwine. Preferably, the weft threads 23a and warp threads 23b intertwine in a satin pattern. The weave ratio of the satin pattern is for example 4, 5 or 8. In the first direction X, a weft thread 23a successively overlaps a plurality, in particular at least two, three or even four warp threads 23b then passes under a warp thread 23b and successively overlaps a plurality, in particular at least two, three or even four warp threads 23b. "Overlapping" means the passage of a first wire over a second wire which runs perpendicular to the first wire.

[0084] Weaving according to the satin pattern offers the advantage of minimizing the roughness of the first layer 21. Preferably, the first layer 21 has a roughness Ra less than 20 pm, advantageously less than 10 pm, preferably less than 5 pm. Thanks to such roughness, the flow of the secondary air flow F2 is slightly disturbed and the aerodynamic performance of the turbomachine 1 is improved.

[0085] The first layer 21 further comprises first openings 24. The first openings 24 are located between the wires 23. The first openings 24 advantageously have different sizes so that the acoustic impedance of the first layer 21 varies in the X and Y directions.

[0086] The first layer 21 thus preferably has an acoustic resistance of between 15 Rayls cgs and 120 Rayls cgs.

[0087] The second layer 22 is arranged between the first layer 21 and the perforated acoustic structure 19. The second layer 22 advantageously has a second thickness e2 of between 0.3 mm and 1.5 mm.

[0088] According to the invention, the second layer 22 is a woven textile layer. With reference to [Fig. 5], the second layer 22 comprises threads 25. Each thread 25 may comprise an assembly of fibers or filaments.

[0089] The wires 25 advantageously comprise a polymeric material chosen for example from thermoplastics. The thermoplastic material is for example chosen from polyaryletherketones (PAEK) such as a polyetherketone (PEK), a polyetheretherketone (PEEK) or a polyetherketoneketone (or PEKK) or polyacrylonitrile fibers (PAN) such as the HexTow® AS4, AS7 or IM7 fibers marketed by the company Hexcel. According to another example, the wires 25 comprise a metallic material such as aluminum or a ceramic material. According to a preferred embodiment, the material of the wires 25 of the second layer 22 is identical to the material of the wires 23 of the first layer 21.

[0090] The wires 25 of the second layer 22 are organized into a plurality of layers 25' of wires 25 superimposed along the radial axis Z to form the second thickness e2 of the second layer 22. [Fig.5] illustrates a single layer 25' of wires 25.

[0091] The threads 25 are woven. They comprise one or more weft threads 25a extending in the first direction X and one or more warp threads 25b extending in the second direction Y. The weft threads 25a and warp threads 25b intertwine.

[0092] The weft 23a and warp 23b threads intertwine, for example, in a satin, twill or canvas pattern.

[0093] The wires 25 of the second layer 22 advantageously have a second diameter greater than the first diameter of the wires 23 of the first layer 21. The second diameter is for example between 0.01 mm and 1 mm, in particular between 0.03 mm and 0.5 mm.

[0094] According to another example, the weft threads 25a have a diameter less than or equal to the diameter of the threads 23 of the first layer 21 and the warp threads 25b of the second layer 22 have a diameter greater than the diameter of the weft threads 25a of the second layer 22. The diameter of the warp threads 25b is for example between 0.01 mm and 2 mm, in particular between 0.03 mm and 1 mm.

[0095] The second layer 22 further comprises second openings 26. The second openings 26 are located between the wires 25. The second openings 26 advantageously have a size greater than the size of the first openings 24. Preferably, the size of the second openings 26 is between 0.1 mm and 2 mm, in particular between 0.1 mm and 1 mm.

[0096] The second layer 22 thus preferably has an acoustic resistance of between 15 rayls cgs and 120 rayls cgs.

[0097] According to a first embodiment, the first and second layers 21, 22 are connected to each other by weaving.

[0098] With reference to [Fig. 6], the first and second layers 21, 22 are connected to each other by weaving, according to an interlock pattern. According to this embodiment, the porous layer 20 further comprises connecting threads 27 which interlace the threads 23, 25 of the first and second layers 21, 22. Advantageously, the connecting threads 27 have a third diameter smaller than the second diameter of the threads 25 of the second layer 22. Advantageously, the third diameter is smaller than or equal to the first diameter of the threads 23 of the first layer 21. This characteristic makes it possible to reduce the roughness Ra of the first layer 21.

[0099] According to an exemplary embodiment, the connecting threads 27 are constituted by weft threads 25a of the second layer 22 having a diameter less than or equal to the threads 23 of the first layer 21. This characteristic makes it possible to reduce the roughness Ra of the first layer 21.

[0100] According to another exemplary embodiment illustrated in [Fig.6], the connecting wires 27 are additional wires, and therefore distinct from the wires 23, 25 of the first and second layers 21, 22.

[0101] Each additional thread advantageously comprises a polymeric material chosen for example from thermoplastics. The thermoplastic material is for example chosen from polyaryletherketones (PAEK) such as a polyetherketone (PEK), a polyetheretherketone (PEEK) or a polyetherketoneketone (or PEKK) or polyacrylonitrile fibers (PAN) such as the HexTow® AS4, AS7 or IM7 fibers marketed by the company Hexcel. According to another example, the additional threads comprise a metallic material such as aluminum or a ceramic material.

[0102] The connecting threads 27 pass successively and periodically within the first layer 21 and the second layer 22. Thus the first and second layers 21, 22 form a single woven assembly. The connecting threads 27 then disturb the weaving of the first layer 21 even less.

[0103] According to the example of [Fig.6], the first layer 21 is woven according to a satin pattern with a weave ratio of 4. Each weft thread 23a passes successively over three warp threads 23b then under a warp thread 23b.

[0104] The second layer 22 is woven in a plain weave pattern with a weave ratio of 2 to 1. Each weft thread 25a passes successively over two warp threads 25b then under two warp threads 25b and each warp thread 25b passes alternately over and under a weft thread 25a.

[0105] According to this example, the connecting threads 27 extend in the first direction X and pass successively over a warp thread 23b of the first layer 21 and under two warp threads 25b of the second layer 22.

[0106] According to another embodiment illustrated in [Fig.7], the first and second layers 21, 22 are connected to each other by gluing or welding. According to this embodiment, a layer of glue is arranged between the first and second layers 21, 22. The layer of glue can be continuous or discontinuous, that is to say that several points of glue are arranged between the first and second layers 21, 22.

[0107] According to the particular example illustrated in [Fig.7], the first layer 21 is woven in a satin pattern with a weave ratio of 5. The second layer 22 is woven in a plain pattern with a weave ratio of 1.

[0108] The first and second layers 21, 22 are connected by welding or by gluing via glue points.

[0109] According to still other embodiments, the first and second layers 21, 22 are connected to each other by calendering, thermocompression, fusion, stitching, tufting or any other method of joining woven textile layers.

[0110] The porous acoustic structure 20 is fixed to the perforated acoustic structure 19. With reference to [Fig.8], preferably, the porous acoustic structure 20 is fixed to the perforated acoustic structure 19 by entanglement of the second layer 22 in the perforated acoustic structure 19.

[0111] By entanglement is meant an at least partial coating of the wires 25 of the second layer 22 in the material of the perforated acoustic structure 19. Thus, the wires 25 of the second layer 22 are arranged at least partially in the thickness e2 of the perforated acoustic structure 19.

[0112] Thanks to the combination of the first and second layers 21, 22 according to the invention, the mechanical resistance of the acoustic panel 17 is improved. It is noted in particular during the peeling tests that the rupture of the acoustic panel 17 occurs within the porous acoustic layer 20 and not between the porous acoustic layer 20 and the perforated acoustic structure 19.

[0113] Furthermore, the combination of the first and second layers 21, 22 gives the acoustic panel 17 sufficient stiffness, which makes it possible to reduce the thickness of the perforated acoustic layer 19. It is thus possible to reduce the mass of the acoustic panel 17 by 35%.

[0114] The first layer 21 also makes it possible to reduce disturbances to the flow of the secondary air flow F2, thus improving the aerodynamic performance of the turbomachine 1.

Claims

Claims

1. Acoustic panel (17) for an aircraft turbomachine (1), the acoustic panel (17) having a sandwich structure comprising: - a honeycomb structure (18) comprising a plurality of acoustic cells (18a), - a perforated acoustic structure (19), and - a porous acoustic structure (20) attached to the perforated acoustic structure (19), the perforated acoustic structure (19) being arranged between the honeycomb structure (18) and the porous acoustic structure (20), characterized in that the porous acoustic structure (20) is multi-layered and comprises: - a first woven textile layer (21), and - a second woven textile layer (22) connected to the first layer (21), the second layer (22) being arranged between the first layer (21) and the perforated acoustic structure (19), the first and second layers (21, 22) each comprising threads (23, 25) having respectively different first and second diameters,the first diameter being smaller than the second diameter, the threads (23) of the first layer (21) being organized into at least one or more weft threads (23a) extending in a first direction (X) and at least one or more warp threads (23b) extending in a second direction (Y) perpendicular to the first direction (X), the weft (23a) and warp (23b) threads intersecting in a satin pattern, the first layer (21) having a roughness Ra of less than 20 pm.,

2. Acoustic panel according to the preceding claim, characterized in that the first diameter of each of the wires (23) of the first layer (21) is between 0.01 mm and 0.2 mm, in particular between 0.01 mm and 0.15 mm.

3. Acoustic panel according to any one of the preceding claims, characterized in that the second diameter of each of the wires (25) of the second layer (22) is between 0.01 mm and 1 mm, in particular between 0.03 mm and 0.5 mm.

4. An acoustic panel according to any preceding claim, characterized in that the porous acoustic structure (20) comprises connecting wires (27) which interweave wires (23, 25) of the first and second layers (21, 22) to connect the first and second layers (21, 22) together.

5. Acoustic panel according to claims 2 and 4, characterized in that the threads (25) of the second layer (22) comprise weft threads (25) and at least one warp thread (23), at least two of the weft threads (25) forming the connecting threads (27).

6. Acoustic panel according to the preceding claim, characterized in that the connecting wires (27) have a diameter smaller than the diameter(s) of wires (23, 25) of the first and second layers (21, 22).

7. Acoustic panel according to any one of claims 1 to 3, characterized in that the first and second layers (21, 22) are connected by welding or gluing.

8. Acoustic panel according to any one of the preceding claims, characterized in that the first and second layers (21, 22) have first and second openings (24, 26) respectively, the first openings (24) being smaller than the second openings (26).

9. Acoustic panel according to the preceding claim, characterized in that the second openings (26) have a size between 0.1 mm and 2 mm, preferably between 0.1 mm and 1 mm.

10. Acoustic panel according to any one of the preceding claims, characterized in that the first layer (21) has a roughness Ra of less than 10 pm, preferably less than 5 pm.

11. Acoustic panel according to any one of the preceding claims, characterized in that the porous acoustic structure (20) is fixed to the perforated acoustic structure (19) by entanglement of the second layer (22) in the perforated acoustic structure (19).

12. An acoustic panel according to any preceding claim, characterized in that the first and second layers (21, 22) comprise a polymeric, metallic or ceramic material.

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