EP2800878A1

EP2800878A1 - Cellular acoustic structure for a turbojet engine and turbojet engine incorporating at least one such structure

Info

Publication number: EP2800878A1
Application number: EP12819120.2A
Authority: EP
Inventors: Xavier Cazuc; Jean-Philippe Joret
Original assignee: Aircelle SA
Current assignee: Safran Nacelles SAS
Priority date: 2012-01-04
Filing date: 2012-12-28
Publication date: 2014-11-12
Also published as: RU2014131244A; FR2985287A1; BR112014013202A2; US9670878B2; US20140341744A1; CA2857171A1; CN104114817A; FR2985287B1; WO2013102724A1

Abstract

The cellular acoustic structure, in particular for a turbojet engine, comprises a closed wall comprising at least two faces and is filled with a plurality of cells. It comprises means (31, 40, 41; 52, 53; 61, 64, 65, 67) for combining the various cells on at least one portion of acoustic path (42; 56; 65, 66) in such a way as to take advantage of the complete thickness of the structure for acoustic filtering.

Description

Cellular acoustic structure for turbojet and turbojet incorporating at least one such structure

The present invention relates to a cellular acoustic structure for a turbojet engine in particular. It also relates to a rboreactor incorporating at least one such structure.

The turbofan engines have a first envelope and a second envelope, one inside the other and imitating respectively a cold pressurized flow which is established between the first and second envelopes and a hot flow which is establishes inside the second envelope. The cold flow is most often generated by a fan disposed at the reactor inlet. The flow of hot air is composed of a part of the cold air that has passed through the fan and the combustion gases of at least one combustion chamber disposed inside the second envelope and which cause a turbine whose shaft drives the fan.

In propulsion mode, the two flows are grouped at the outlet of the nozzle.

In reverse thrust mode, by a mechanism that is not implemented in the context of the present invention, a more or less high fraction of the cold flow is returned upstream of the turbojet engine, thus applying a thrust in the opposite direction of the reactor advance.

As a result, the cold flow has turbulence that it is important not to increase or even reduce on the one hand and that the propagation of acoustic noise that results must be filtered as much as possible.

In the state of the art, it is known to make mechanical structures such as panels or sleepers, in the form of cellular composites, each cell behaving substantially as a Helmholtz resonator.

The invention relates to the improvement of the acoustic absorption characteristics of the structures which are in the air flow of an aircraft nacelle, such as the radial spreaders, but still others.

Classically, such structures are formed by honeycomb panels ("n ids d 'abeil") recovered from an aerodynamic outside pe pierced with holes for forming Helmholtz resonators, having an acoustic attenuation effect. By nature, these structures must have a small thickness, in order to limit the aerodynamic impact on the flow of air.

Thus, when seeking to improve the acoustic performance of these structures, it is not possible to increase the depth of the cells without deteriorating the aerodynamic performance.

The present invention is an improvement of this type of structure, and fo u rn it for this purpose alveolar acoustic structure for turbojet engine in particular, comprising:

a closed wall comprising at least two faces comprising acoustically transparent zones, this wall being filled by a plurality of cells, and

acoustic reflection means disposed inside said closed wall so that the acoustic path of the aerial sound vibrations passing through said acoustically transparent zones, penetrating inside said cells and reflecting on said means of acoustic reflection, present in at least some of said cells a deep eu r perieueu re at the dem-thickness of the ite structure.

According to other optional features, taken alone or in combination:

the cellular acoustic structure comprises two levels of cellular material separated by a medial porous septum and an outer skin alternating between acoustically transparent zones and acoustically reflective zones, arranged in such a way that an acoustically reflective zone on one side of the outer skin opposes an acoustically transparent area on the opposite side of the outer skin;

the cellular acoustic structure comprises a single level of cellular material and an outer skin alternating acoustically transparent zones and acoustically reflective zones, arranged in such a way that an acoustically reflective zone on a first face opposes an acoustically reflective zone; acoustically transparent zone on a second face of the outer skin; the cellular acoustic structure comprises two levels of cellular material separated by a medial porous septum, an acoustically transparent outer skin and a plate comprising a plurality of acoustically reflective inclined faces, connected by edges so that the acoustic paths made through the two Alveolar material levels and porous septum are of varying heights along the section of the structure.

The invention also relates to a turbojet engine incorporating at least one such cellular acoustic structure.

According to other optional features, taken alone or in combination:

the acoustic honeycomb structure is provided with an aerodynamic profile for producing a radial separator between a first envelope and a second envelope limiting a cold flow;

the acoustic honeycomb structure is provided with an aerodynamic profile for producing at least one flow straightening blade for a cold flow generation turbine propelled between a first envelope and a second envelope.

Other features and advantages of the present invention will emerge in the light of the description which follows and the examination of the appended figures in which:

FIG. 1 is a perspective view of a turbojet of the type of that of the invention,

FIG. 2 is a schematic sectional view of an air vein separation structure according to the state of the art,

FIG. 3 is a schematic sectional view of a first embodiment of a cellular acoustic structure for a turbojet engine according to the invention,

FIG. 4 is a diagrammatic sectional view of a second embodiment of a cellular acoustic structure for a turbojet engine according to the invention, and FIG. 5 is a diagrammatic sectional view of a third embodiment of a cellular acoustic structure for a turbojet engine according to the invention, and

FIG. 6 is a graph showing the comparative acoustical efficiency of the three embodiments.

In Figure 1, there is shown a perspective view of a turbojet incorporating an air stream separation structure. The turbojet engine 1 comprises a first envelope 3 and a second envelope 5 which respectively limit a cold flow 2 which is established between the first and second envelopes, and a hot flow 4 inside the second envelope 5. The direct trihedron ( xyz) 1 5 of the drawing of Figure 1 is directed downstream of the turbojet, with the y-axis according to the orientation of the thrust of the turbojet engine.

As is known, the cold flow is composed of the air sucked by a blower (not shown) upstream of the reactor 1. The fan is itself driven by the shaft of a turbine (not shown) disposed downstream of the turbojet engine and inside the second shell 5. To drive the turbine, a fraction of the cold air flow is taken for a combustion chamber (not shown), disposed in relation to the second casing 5 and whose combustion gases drive the turbine before being ejected by a nozzle (not shown) downstream, where they mix with the cold propulsive flow.

In this arrangement, the first 3 and second 5 envelopes have aerodynamic structures that can generate strong acoustic noises. As is known, the arrangement of cellular acoustic structures, here aerodynamically profiled, is known in the state of the art. In this state of the art, four structures are available according to two different kinds:

- two bifurcations, called "6H" (for "6 hours") 9 and "12H" (for "12 hours") 7, directed along the z axis of the direct trihedron

15, vertically to the drawing; and

- Two radial splitters, called "beam splitters" 1 1 and 13 directed along the x axis of the direct triad 15, horizontally to the drawing.

The two radial dividers 1 1 and 13 are each made of a cellular acoustic structure which, in addition, is formed according to a profile. In this case, it was assumed that the flow of refrigerant flow was at that level of the turbojet engine.

FIG. 2 shows a schematic sectional view of a cellular acoustic structure whose aerodynamic profile makes it possible to use it as a beam splitter structure.

The cellular acoustic structure of the state of the art comprises a closed wall 20 or skin, having two opposite faces 20A and 20B in one section. The skin is acoustically transparent, that is to say that it is likely to pass the aerial sound vibrations in both directions.

This skin can be made from several glass plies (typically 2 or 3), polymerized and microporous (with diameter holes ranging from 0.2 to 0.5 mm in diameter, typically).

Preferably, the skin is acoustically transparent because it is, over its entire surface opposite faces 20A and 20B, pierced with holes of porosity determined according to the constraints of mechanical strength and transmission constraints of the acoustic energy incident to the inside the cellular acoustic structure.

The enclosed volume between the two faces 20A and 20B is filled by a cellular material in the middle of which is mounted a mass central plate 21 which tends to reflect the sound waves that try to cross it. The central plate 21 separates the cells in a first level of a cellular material such as the cell 26 which opens on the face 20A and the cells of a second level of a cellular material such as the cell 27 which opens on the face 20B.

Each cell is composed, in the cross-section of the section shown in FIG. 2, by two lateral mass walls 23 and 24. The opening of the cell 22 directed towards the top of FIG. 2 is closed off by the central mass plate 21. , while the opening of the cell 22 directed downwards of FIG. 2 is closed off by the face 20B of the acoustically transparent skin 20. The sound waves reflected by the central mass plate 21 are strongly attenuated by the walls lateral masses 23 and 24 of the cell 22, and only a fraction of the incident acoustic energy is reflected out of the cellular acoustic structure. The set is known as an acoustic filtering cell with a single degree of freedom "SDOF". To increase the acoustic filtering gain of such a cellular acoustic structure, it is necessary to increase its thickness, or distance between the two faces 20A and 20B of the skin 20. However, when this cellular acoustic structure is profiled according to aerodynamic characteristics as for to achieve a radial splitter for turbojet engine, it is not possible to increase this beam to improve its acoustic performance.

In Figure 3, there is shown a schematic sectional view of a first embodiment of a cellular acoustic structure for turbojet according to the invention.

The cellular acoustic structure of the first embodiment of the invention comprises a closed wall 30 or skin, having two faces 30A and 30B in one section. The skin is composed of an alternation of acoustically transparent zones, that is to say, it is likely to let the aerial sound vibrations pass in both directions, and of mass zones reflecting the aerial sound vibrations in both directions . The two levels 34 and 35 of cells, taken from the alveolar acoustic structure of the state of the art, are separated not by a central mass plate as in the state of the art (FIG. 2), but by a septu m porous 31 likely to isser pass the aerial sound vibrations in both directions.

Thus, with respect to the acoustic path of an incident sound wave substantially normal to the faces 30A and 30B, the alternation of the zones of the beam 30 makes it possible to oppose an acoustically transparent zone such as the zone 33 to a mass zone such as the zone 32, and further, the mass zone 41 to the acoustically transparent zone 40. As a result, an incident sound wave 42 on the face 30B, penetrates inside the first level of cellular material 35, passes through the porous septum 31, passes through the second level 34 of cells, is reflected on the mass zone 41 of the face 30A, crosses again the second level 34 of cells, the porous septum 31, and the first level 35 of cells to come out as a reflected wave 43 strongly weakened.

It will be noted that the beginning and end of alternating opposite zones, such as the acoustically transparent zone 40 and the mass zone 41, are aligned according to the normals 38 and 39 to the planar porous septum 31. The geometric distribution of the alternating areas of the skin 30 is determined as a function of the desired acoustic response of the cellular acoustic structure once in place in the turbojet engine. It is then perfectly determined by construction of the cellular acoustic structure. Likewise, the distribution of the transparent or reflective outer skin acoustic zones is determined with respect to the distribution of the cells in the two levels 34 and 35 of cellular material.

This distribution of means for combining the different cells on at least part of the acoustic path of the first embodiment of the invention makes it possible to use the entire thickness (that is to say, the size of the alveolar acoustic structure) measured in the direction perpendicular to the septum 31, or parallel to the Z direction of FIG. 1 when this structure is a radial splitter or "beam splitter" such as 1 1 or 13) of the cellular acoustic structure, although it is profiled according to aerodynamic criteria which vary the thickness along the section and which prevent increasing this thickness for reasons of acoustic filtering. Since the sound path uses the two levels 34 and 35 of cells, the assembly is qualified as acoustic filtering cell with two degrees of freedom "DDOF".

In Figure 4, there is shown a schematic sectional view of a second embodiment of a cellular acoustic structure for turbojet according to the invention.

The cellular acoustic structure of the second embodiment of the invention comprises a closed wall 50 or skin, having two faces 50A and 50B in the section shown in FIG. 4. The skin is composed of an alternation of acoustically transparent zones, c. that is, capable of passing aerial sound vibrations in both directions, and mass zones reflecting the aerial sound vibrations in both directions, as in the first embodiment of the invention (see FIG. 3).

But, unlike the first embodiment, the cellular acoustic structure of the second embodiment has only one level of cells (or foam material) so that there is no central separation. Thus, between the alternating zones 52 and 53, limited in the section of the structure by the normals at the median plane 54 and 55, it is established a full-thickness acoustic chim 56 of the acoustic structure between the lateral walls 57 and 58. of each cell. The same This kind of acoustic path is repeated according to the alternation of the acoustically transparent zones and the reflective zones of the outer skin 50 from the top (face 50A) towards the bottom (face 50B) or in the opposite direction. The set is qualified as an acoustic filtering cell with a degree of freedom "SDOF".

In Figure 5, there is shown a schematic sectional view of a third embodiment of a cellular acoustic structure for turbojet according to the invention.

The cellular acoustic structure of the third embodiment of the invention comprises a closed wall 60 or skin, having two faces 60A and 60B in a section. The skin is acoustically transparent on at least the two opposite faces 60A and 60B of the outer skin 60, that is to say that it is likely to pass the aerial sound vibrations in both directions. The two cell levels 64 and 65, taken from the alveolar acoustic structure of the state of the art, are separated, not by a central mass plate as in the state of the art (FIG. porous septum 61 likely to pass the aerial sound vibrations in both directions, as in the first embodiment (Figure 3).

To combine the different cells on at least one part of the acoustic path so as to make full use of the thickness of the cellular acoustic structure, the third embodiment comprises a plate 62 comprising a plurality of panels. The edges 70-1 to 70-6 of the plate 62 formed by these sections are integral with the porous outer skin 60, alternately on the faces 60A and 60B. The pitch, or distance, separating two edges is a parameter of the invention.

For the realization of the cellular acoustic structure of the third embodiment of the invention, the panels forming the plate 62 are firstly integrated with the cells 64 and 65, and the assembly thus obtained is then bonded to the faces 60A and 60B of the outer skin. 60.

The plate 62 may be formed into folds of glass or polymerized carbon.

Unlike the first two embodiments, an incident acoustic path on one side of the face 60A or the face 60B passes through the two-level cellular material and / or its median porous septum 61 on a variable height according to the point d entrance between two edges of the plate 62. Thus, an input acoustic path 65 via the face 60B penetrates into a cavity 63 of the first level of cells 73, passes through the porous septum 61, then a fraction of height of the cell 64 of the second level of cells 72. acoustic path then meets the reflection on the inclined face of the plate 62 to return by the same path.

At the same level of the section of the cellular acoustic structure, but on the side of the face 60A, an incident acoustic path 66 enters the cell 64 of the second level of cells 72 and immediately meets the reflection on the inclined section opposite the plate 62.

In at least some cells, the acoustic path has a depth greater than half the thickness of the structure, and in some of these cells such as those located at the edges 70-1 to 70-6 of the plate 62 , the acoustic path is full thickness in the structure: the assembly is qualified as acoustic filtering cell with two degrees of freedom "DDOF".

In Figure 6, there is shown a graph showing the comparative acoustic efficiency of the three embodiments. The vertical axis carries the power loss gain (in dB) and the horizontal axis carries the frequencies of the acoustic wave in Hertz.

The characterization of the acoustic filtering of the second embodiment (FIG. 4) is represented by the internal curve referenced SDOF, that of the first embodiment (FIG. 3) is represented by the median curve referenced DDOF and that of the third embodiment (FIG. 5) is represented by the outer curve referenced DDOF HVAR. It should be noted that the filtering spectrum is progressively widened between the second, then the first and finally the third embodiment and this widening acts especially towards the high frequencies.

The cellular acoustic structure which has been described in the three embodiments mentioned above is applicable to a radial turbojet separator as it has been exposed by applying to the chosen cellular acoustic structure a suitable aerodynamic profile. A turbojet equipped with such a radial separator (1 1, 13 - Figure 1) has a reduced acoustic emission.

In addition, the cellular acoustic structure that has been described in the three embodiments mentioned above is applicable to other types of parts of a turbojet engine, among which the flow straightening vanes as OGV (Outlet Guide Vane) provided you receive a suitable aerodynamic profile.

Claims

1. Cellular acoustic structure for turbojet engine in particular, the structure being of the type comprising:

a closed wall (30; 50; 60) comprising at least two faces (30A, 30B; 50A, 50B; 60A, 60B) having acoustically transparent zones (33, 40; 52; 60), this wall being filled by a plurality (34, 35; 72, 73) of cells, and

- acoustic reflection means (32, 41; 53; 62) arranged inside the closed wall so that the acoustic chim (42, 43; 56; 65, 66) of the aerial sound vibrations passing through said acoustically transparent zones, penetrating inside said cells and reflecting on said acoustic reflection means, present in at least some of said cells a depth greater than half the thickness of said structure.

2. A honeycomb acoustic structure according to claim 1, characterized in that it comprises two levels of cellular material (34, 35) separated by a porous septum (31) median and an outer skin (30) alternating between acoustically transparent zones (40) and acoustically reflective zones (41), arranged so that an acoustically reflective zone (41) on one face (30A or 30B) of the outer skin opposes an acoustically transparent zone (40) on the opposite face (30B or 30A) of the outer skin (30).

3. Alveolar acoustic structure according to claim 1, characterized in that it comprises a single level of cellular material and an outer skin (50) alternating acoustically transparent zones (52) and acoustically reflective zones (53). , arranged so that an acoustically reflective area (53) on a first face (50A) opposes an acoustically transparent area (52) on a second face (50B) of the outer skin (50).

4. Alveolar acoustic structure according to claim 1, characterized in that it comprises two levels of cellular material (34, 35) separated by a porous septum (61) median, an outer skin (60) acoustically transparent and a plate (62) having a plurality of acoustically reflective inclined faces (67, 68) connected by ridges (70-1, 70-2, ...) so that the acoustic paths made through the two levels (34, 35) of cellular material and the porous septum (61) are of varying depths along the section of the structure.

5. Turbojet engine characterized in that it incorporates at least one acoustic cellular structure according to any one of claims 1 to 4.

6. Turbojet engine according to claim 5, characterized in that the acoustic cellular structure is provided with an aerodynamic profile to produce a radial separator (1 1, 13) between a first envelope (3) and a second envelope (5) limiting a cold flow.

7. Turbojet engine according to claim 5, characterized in that the acoustic cellular structure is provided with an aerodynamic profile for producing at least one flux rectification blade for a cold flow generation turbine propelled between a first envelope (3) and a second envelope (5).