SOUND INSULATION ARRANGEMENT FOR AN AUXILIARY POWER UNIT WITH A CENTRIFUGAL COMPRESSOR

Description

Technical field

[0001] The invention relates to an air inlet casing of an auxiliary power unit, and relates more particularly to the acoustic insulation of such casings. State of the prior art

[0002] An aircraft generally comprises a powertrain formed by a plurality of turbomachines intended to provide the thrust necessary to set the aircraft in motion.

[0003] Each turbomachine is housed in a nacelle and includes compressors, combustion chambers, turbines and a fan positioned upstream of an air inlet of the turbomachine.

[0004] In order to supply power to various electrical, mechanical, pneumatic or hydraulic systems of the aircraft and thus reduce the load on the turbomachines, the aircraft includes an auxiliary power unit (Auxiliary Power Unit) generally located at the rear of the fuselage and more particularly in the tail cone of the aircraft.

[0005] However, the noise produced by the auxiliary power unit during operation is annoying for residents near airports.

[0006] To this end, such auxiliary power units must meet acoustic standards which impose a noise level threshold that the air inlets must not exceed.

[0007] To reduce the noise generated by the air intake, various solutions are then proposed by aircraft manufacturers such as the insertion of mechanical parts with an acoustic insulation coating, called honeycomb, in the air intake.

[0008] Such an acoustic covering generally comprises a honeycomb structure and an air-permeable layer intended to be in contact with the air flow circulating inside the air inlet.

[0009] The international application, bearing the reference WO202058651, describes curved honeycomb acoustic panels located inside the nacelle of a turbomachine and / or on the nacelle of said turbomachine, thus making it possible to reduce the noise pollution from the fan.

[0010] This solution can also be integrated into the air intake of the auxiliary power unit, but as the unit is constantly subjected to vibrations generated by its own engine, the acoustic coating suffers degradation as it is used.

[0011] Similarly, during the take-off or landing phases of the aircraft, the auxiliary power unit is likely to ingest foreign bodies of varying sizes (birds, ice, etc.).

[0012] These incidents can generate violent shocks which can partially or completely alter the acoustic covering and degrade its performance.

[0013] The noise generated by the auxiliary power unit then exceeds the regulatory threshold and must therefore be changed quickly so that the aircraft can be returned to service.

[0014] Installing a new acoustic panel then requires replacing the entire air intake casing, which can be tedious and slow to implement. Document FR3055612A1 describes another prior art acoustic insulation assembly.

[0015] The aim of the invention is therefore to propose a solution for soundproofing the air intake casing which is easy to access and simple to replace if necessary during maintenance. Statement of the invention

[0016] In view of the above, the subject of the invention is an acoustic insulation assembly for an auxiliary power unit with a centrifugal compressor, comprising an annular-shaped air inlet casing and comprising a plurality of reinforcing arms connecting its internal walls.

[0017] The assembly comprises a plurality of removable mechanical sound insulation parts, each mechanical part being annular in shape, having a fixing face formed by a radial surface intended to be positioned in contact with an inner radial surface of the casing, and an opposite face intended to be opposite the interior of the casing and comprising fixing means intended to cooperate with the casing so that the mechanical part and the flow surface of the casing, delimited by a pair of reinforcing arms, are superimposed.

[0018] The mechanical part being annular in shape, it is inserted in a removable manner between two reinforcement arms of the casing.

[0019] The mechanical part is then placed on the flow surface of the casing, which considerably reduces the noise generated by the air intake.

[0020] Thus, when the mechanical part is damaged in such a way as to degrade its sound insulation performance, it is possible to replace it with another mechanical sound insulation part without having to completely dismantle the casing and replace it with another.

[0021] The number of mechanical parts is equal to the number of reinforcement arms of the casing.

[0022] As a result, the duration of maintenance operations intended for this purpose is significantly reduced.

[0023] Advantageously, the fixing means comprise at least two first housings hollowed out in a radially outer end surface of the mechanical part and at least one second housing hollowed out in a radially inner end surface of the mechanical part, the first two housings each being capable of receiving a clamping screw and the second housing being capable of receiving a centering pin located on a radially inner end surface of the casing.

[0024] Thus, on the one hand, the mechanical part is held in position via one of the centering pins of the casing capable of being inserted into the second housing of the fixing means and, on the other hand, by the first two housings of the means of fixing the mechanical part and two of the housings of the casing hollowed out in its radially outer end surface.

[0025] The two housings of the casing and the first two housings of the means of fixing the mechanical part then each receive a clamping screw.

[0026] The fixing means thus arranged facilitate the disassembly of the mechanical part.

[0027] Preferably, the mechanical part comprises a metal support flange having an active surface provided with a hollowed cavity, an acoustic insulation layer having a honeycomb structure arranged in the cavity, and an air-permeable layer covering the acoustic insulation layer and intended to be in contact with the air flow circulating inside the air inlet.

[0028] Preferably, the sound insulation layer and the air-permeable layer are fixed together by gluing.

[0029] Bonding ensures good distribution of mechanical stress and provides long-term resistance to corrosion.

[0030] Alternatively, the sound insulation layer and the air-permeable layer are fixed together by welding.

[0031] Welding can be used when the air-permeable layer is in the form of woven metal fabrics or perforated sheets, for example.

[0032] Advantageously, the acoustic insulation layer is fixed to the cavity by gluing.

[0033] The glue allows the first layer to adhere to the cavity without applying significant pressure on it.

[0034] Advantageously, the casing includes an air inlet protection grille.

[0035] Such a protective grid prevents the intrusion of a foreign body (FOD for "Foreign Object Debris") into the air intake of the casing and can be fixed to the surface of the casing via the clamping screws used to hold the mechanical part in position.

[0036] The invention also relates to an auxiliary power unit with a centrifugal compressor for an aircraft, comprising an acoustic insulation assembly as defined above.

[0037] The invention also relates to an aircraft which comprises an auxiliary power unit as defined above. Brief description of the drawings

[0038] Other aims, characteristics and advantages of the invention will appear on reading the following description, given solely by way of non-limiting example, and made with reference to the indexed drawings in which: [ Fig 1 ] schematically represents a sectional view of an auxiliary power unit with a centrifugal compressor for an aircraft according to the state of the art; [ Fig 2 ] schematically illustrates a perspective view of the air inlet of the auxiliary power unit according to the prior art; [ Fig 3] schematically illustrates a mechanical acoustic insulation part according to one embodiment of the invention; [ Fig 4 ] illustrates a perspective view of a set of four mechanical acoustic insulation parts according to one embodiment of the invention; [ Fig 5 ] illustrates a perspective view of the means for fixing one of the mechanical parts and the casing according to an embodiment of the invention and, [ Fig 6 ] represents the mechanical parts arranged on the outer radial surface of the air inlet casing according to one embodiment of the invention. Detailed description of the embodiments of the invention

[0039] On the figure 1 an auxiliary power unit 1 with a centrifugal compressor is shown, intended to produce electrical, mechanical, pneumatic or hydraulic energy on board an aircraft.

[0040] Auxiliary power unit 1 is positioned at the rear of the aircraft and more specifically in the tail cone.

[0041] It comprises a compressor 2 capable of receiving ambient air through an air inlet 3 to compress it and then deliver it to a combustion chamber 4.

[0042] The combustion of a fuel, here kerosene, in oxygen from the air is then implemented in said chamber 4 to produce energy, a first part of which is intended to drive the rotation of at least one turbine 5.

[0043] The mechanical energy output from turbine 5 then drives another rotating mechanism such as an alternator, compressor, pump or other receiver.

[0044] The second part of the hot flow is ejected through an ejection nozzle located downstream of auxiliary power unit 1.

[0045] The auxiliary power unit 1 produces noise, and in particular a compressor noise composed in particular of a tonal noise at a given frequency directly linked to the rotor / stator interaction at the compressor. This compressor noise mainly emerges upstream of the auxiliary power unit via the air inlet duct 3. The invention proposes an acoustic treatment of the air inlet duct in order to reduce this noise.

[0046] To do this, it is proposed to insert removable mechanical acoustic insulation parts inside the air inlet 3 illustrated in figure 2 .

[0047] Such an air inlet 3 extends along a longitudinal axis XX' and comprises a conduit 6 intended to convey the ambient air to the compressor 2 via a plenum 7.

[0048] Inside the plenum 7 is arranged a casing 8, of annular shape, which comprises a plurality of reinforcing arms 9, here four, capable of connecting the internal walls of said casing 8.

[0049] It is between each pair of reinforcements 9 that the removable mechanical acoustic insulation parts 10 are positioned on the flow surface of the casing 8.

[0050] “Flow surface” means a surface adjacent to the flow of air in the casing 8.

[0051] Such a mechanical part 10 illustrated in the figure 3 receives acoustic treatment which helps to reduce noise in the air inlet 3.

[0052] More specifically, the mechanical sound insulation part 10 comprises a metal support flange 10A having an active surface provided with a hollowed-out cavity, a sound insulation layer 10B having a honeycomb-like alveolar configuration for example, and a so-called resistive and air-permeable layer 10C.

[0053] The metal support flange 10A has a fixing face formed by a radial surface intended to be positioned in contact with an interior radial surface (flow surface of the casing 8), and an opposite face intended to be opposite the interior of the casing 8 and which receives the acoustic insulation layer 10B.

[0054] The air-permeable resistive layer 10C is then arranged on the acoustic insulation layer 10B by gluing or by welding when the resistive layer 10C is in the form of braided metal fabrics or in the form of perforated sheets.

[0055] There figure 4illustrates a set of four mechanical acoustic insulation parts 10 of annular shape which is adapted to the shape of the casing 8.

[0056] To hold the mechanical part 10 in position on the flow surface of the casing 8, the metal support flange 10A of the mechanical part 10 comprises fixing means 11.

[0057] The fixing means 11 are intended to cooperate with the casing 8 so that the mechanical part 10 and the flow surface of the casing 8, delimited by a pair of reinforcing arms 9, are superimposed.

[0058] More particularly, the fixing means 11 comprise two first housings 12, hollowed out in a radially outer end surface of the mechanical part 10, as well as a second housing 13, hollowed out in a radially inner end surface of the mechanical part 10.

[0059] More precisely, the housings 12 and 13 are provided in the metal support flange 10A of the mechanical part 10.

[0060] The first two housings 12 are each capable of receiving a clamping screw, while the second housing is capable of receiving a centering pin located on a radially inner end surface of the casing 8.

[0061] So, as illustrated in the Figure 5 , the first two housings 12 each come into contact with two housings 14 hollowed out in a radially outer end surface of the casing 8.

[0062] A first clamping screw can then be inserted into one of the first two housings 12 and into one of the two housings 14 of the casing 8.

[0063] A second clamping screw will then be inserted into the first housing 12 and the housing 14 of the remaining housing 8.

[0064] Likewise, the centering pin 15 of the casing 8 is inserted into the second housing 13 of the mechanical part 10.

[0065] By means of the fixing means 11, each mechanical part 10 can be held in position between a pair of reinforcing arms 9 as illustrated in the figure 6 .

[0066] The mechanical part 10 can then be dismantled without completely removing the casing 8 during maintenance.

[0067] Furthermore, the invention is not limited to these embodiments and implementations but embraces all variants thereof. For example, the mechanical sound insulation parts 10 can be arranged on any annular-shaped casing integrated into any type of device.

Claims

1. An acoustic insulation assembly for an auxiliary power unit (1) with a centrifugal compressor, comprising an annular-shaped casing (8) of an air inlet (3) and comprising a plurality of reinforcing arms (9) connecting its inner walls, the assembly being characterised in that it comprises a plurality of removable acoustic insulation mechanical parts (10), each mechanical part (10) being annular-shaped, having a fastening face formed by a radial surface to be positioned in contact with an internal radial surface of the casing (8), and an opposite face to be facing inside the casing (8) and comprising fastening means (11) to cooperate with the casing (8) such that the mechanical part (10) and the flow surface of the casing (8), delimited by a pair of reinforcing arms (9), are superimposed.

2. The acoustic insulation assembly according to claim 1, wherein the fastening means (11) comprise at least two first housings (12) hollowed out in a radially external end surface of the mechanical part (10) and at least one second housing (13) hollowed out in a radially internal end surface of the mechanical part (10), the first two housings (12) being each capable of receiving a clamping screw and the second housing (13) being capable of receiving a centring pin (15) located on a radially internal end surface of the casing (8).

3. The acoustic insulation assembly according to claim 1 or 2, wherein each mechanical part (10) includes a metal support flange (10A) having an active surface provided with a hollowed out cavity, an acoustic insulation layer (10B) having an honeycomb structure disposed in the cavity, and an air-permeable layer (10C) covering the acoustic insulation layer (10B) and to be in contact with the air flow circulating inside the air inlet (3).

4. The acoustic insulation assembly according to claim 3, wherein the acoustic insulation layer (10B) and the air-permeable layer (10C) are fastened together by bonding.

5. The acoustic insulation assembly according to claim 3, wherein the acoustic insulation layer (10B) and the air-permeable layer (10C) are fastened together by welding.

6. The acoustic insulation assembly according to any of claims 3 to 5, wherein the acoustic insulation layer (10B) is fastened to the cavity by bonding.

7. The acoustic insulation assembly according to claim 6, wherein the casing (8) includes an air inlet protective grid.

8. An auxiliary power unit (1) with a centrifugal compressor for an aircraft, characterised in that it comprises an acoustic insulation assembly according to any of claims 1 to 7.

9. An aircraft characterised in that it comprises an auxiliary power unit (1) according to claim 8.