Turbomachine multi-annular exhaust duct with acoustic lining

The present invention relates to an exhaust device for a turbomachine such as an auxiliary power unit, a turbine generator, a fan, a charge compressor, a turboprop or a turbine engine capable of driving for example a rotor shaft. helicopter, the exhaust device being treated so that its acoustic signature is reduced.

It is known that the turbomachines are equipped with an exhaust device acting as a diffuser capable of slowing down the exhaust flow. Such an exhaust device generally comprises an outer casing of symmetrical shape with respect to an axis. The exact shape of the exhaust system as well as its length along the axis of the exhaust flow can be determined so as to ensure a given performance taking into account in particular the static pressure at the inlet and the outlet of the device exhaust.

It has already been provided, in exhaust systems for turbomachines, to have inside the casing one or more internal separations defining several substantially concentric channels for the flow of the exhaust stream. The multichannel exhaust device thus produced has, at equal performance, a reduced axial size.

It is also known that it is appropriate in many applications to minimize the noise generated by the turbomachines. For this purpose, an acoustic treatment of the exhaust system could be envisaged. So far, however, the decrease in acoustic signature was considered insufficient. US-A-4 109 750 discloses a sound damping device for a turbomachine by means of the use of an acoustic material which can be applied to the walls of an internal separation. But this application is at the expense of aerodynamic performance.

The present invention aims to improve the reduction of the acoustic signature of a turbomachine while allowing the obtaining of aerodynamic performance data of an exhaust device. More specifically, the present invention makes it possible to improve both the aerodynamic performances and the acoustic performances of an exhaust device of a turbomachine, and in particular of a helicopter rotor drive turbine engine.

For this purpose, the turbomachine diffusion exhaust device according to the invention, which is of the multichannel type with an annular inlet, comprises an outer shell of generally symmetrical shape with respect to an axis and one or more internal separations defining several channels substantially. concentric for the flow of the exhaust stream. According to the present invention, an absorbent coating capable of absorbing a portion of the acoustic energy generated by the flow is provided on at least one surface of an internal separation and / or on the inner surface of the outer casing . The thickness of the absorbent coating is adapted along the length of the exhaust device to optimize the flow in the different channels, maintaining progressive diffusion and avoiding detachments in the vicinity of the immobile surfaces.

Thanks to this arrangement, the acoustic signature of the turbomachine is greatly reduced since it becomes possible to benefit from the increase of acoustically treated surface due to the presence of the internal separations.

The absorbent coating is chosen not only in terms of its own absorbency, characterized by its absorption coefficient as a function of frequency, but also as a function of the characteristics of the aerodynamic flow (exhaust gas temperature and engine speed). 'flow).

In order to optimize the aerodynamic performance, the area of ​​the different channels may advantageously vary along the length of the axis in order to simultaneously optimize the flow velocity and the thickness of the absorbent coating, ie the acoustic performance.

The absorbent coating is disposed on the surface of the inner partition and / or the outer casing facing the flow.

The absorbent coating comprises an acoustic material which may be of the porous type or of the single or multilayer resonator type.

In the case of a porous material, it may consist of agglomerated fibers (felt, glass wool or rock) or an interstitial network made by assembling hollow microspheres. In the case of such a porous material, the absorbent coating further comprises an acoustically transparent wall having a role of mechanical retention of the porous material. This wall may or may not be integral with the porous material.

When a central body is provided in the axis of the device over a major part of its length, said central body may also have on its surface an absorbent coating as defined above, which further increases the acoustically treated surface.

Similarly, if the outlet of the exhaust stream is by a deflection portion of the flow with respect to the inlet axis, said deflection portion, of any shape, may have on its inner surface an absorbent coating .

The present invention will be better understood from the study of some particular embodiments taken as non-limiting examples and illustrated by the appended drawings, in which:<ul id="ul0001" list-style="none" compact="compact"><li>Figure 1 is a schematic sectional view taken along the axis of the flow, an embodiment of a turbomachine exhaust device according to the invention;</li><li>Fig. 2 is a sectional cutaway view showing on a larger scale a first alternative absorbent coating installed in the embodiment illustrated in Fig. 1;</li><li>Figure 3 is a sectional view with severally similar to Figure 2, an alternative absorbent coating;</li><li>Figure 4 is a view similar to Figures 2 and 3, showing another alternative absorbent coating;</li><li>Figure 5 is a sectional view along VV of Figure 4;</li><li>Figure 6 is a partial sectional view with tearing of a modification of the variant illustrated in Figure 4;</li><li>Figure 7 is a schematic sectional view similar to Figure 1 of another embodiment of the invention; and</li><li>Figure 8 is a diagram illustrating the aerodynamic performance of an exhaust system.</li></ul>

As illustrated in FIG. 1, the turbomachine exhaust device according to the present invention, referenced 1 as a whole, is mounted on the outlet end 2 of a turbine engine not shown in the figure. The exhaust device 1 generally comprises an outer casing 3 which has, in the example illustrated, a generally frustoconical divergent shape. Inside the envelope 3, there is mounted a separation 5, which has, in the example shown, also a generally frustoconical divergent shape. The casing 3 has a first part 3a on the inlet side 4, corresponding to the output of the turbine engine, with a first cone angle and a second part 3b on the output side 6 with another more closed cone angle.

The separation 5 defines a first central channel 7 for the flow of the exhaust stream and a second annular channel 8 between the partition 5 and the outer shell 3, the annular channel 8 being concentric with the central channel 7.

Maintaining the separation 5 inside the casing 3 is made in the vicinity of the inlet 4 by three radial support arms 9 advantageously arranged at 120 ° from each other, said support arms 9 being fixed, on one side on an inner tube 10 of the inner cone 11 and on the other hand on a return 3c of the outer casing 3. The separation 5 is extended on the inlet side 4 by a conical connecting piece 12, which is fixed on the support arms 9. The top of the cone of the connecting piece which constitutes the leading edge of the separation 5, is in the plane of the inlet 4, corresponding to the output of the turbine engine.

On the output side 6, the separation 5 is maintained by means of support arms 13 fixed on both the separation 5 and the outer casing 3 and also arranged at 120 ° from each other.

According to the present invention, an absorbent coating 14 is provided on the surface of the inner partition 5.

In the embodiment illustrated in Figure 1, the absorbent coating 14 is disposed on the outer surface of the partition 5, while a similar absorbent coating 15 is provided on the inner surface of the partition 5. This absorbent coating is capable of to absorb a portion of the acoustic energy generated by the flow symbolized in Figure 1 by the arrows 16. The thickness of the coatings 14 and 15 is increasing from the connecting piece 12 to the outlet 6.

In the embodiment illustrated in Figure 1, the inner surface of the outer casing 3 also has an absorbent coating 17 of structure similar to that of the absorbent coatings 14 and 15. The absorbent coating 17 defines the outer surface of the annular channel 8 The envelope 3 thus has a recess 3d, corresponding to the thickness of the coating 17 in the vicinity of the inlet 4 and directed outwards.

The various absorbent coatings 14, 15 and 17 may have different absorption characteristics along the length of the exhaust device 1. Likewise, the thickness of each of the absorbent coatings 14, 15 and 17 may vary along the length of the exhaust device. Thus, the thickness of the absorbent coatings 14 and 15 increases from the side of the inlet 4 to the side of the outlet 6. The thickness of the absorbent coating 17 is substantially constant over the entire length of the exhaust device 1 in the embodiment illustrated in FIG. As a result, the passage section useful for the flow of the exhaust gas stream flowing through the annular channel 8 on the one hand and the central channel 7 on the other hand, can be adapted to optimize the flow whose diffusion remains advantageously progressive and does not take off in the vicinity of immobile surfaces.

In the variant illustrated in Figure 1, the passage section of the central channel 7 is substantially constant, while the passage section of the annular channel 8 increases from the inlet to the outlet 6 through the increase of diameter of the channel 8.

In any case, the geometrical shape of the internal separation must be adapted to the thickness of the absorbent coatings 14, 15 and 17 so that the useful flow section of the gas flow allows the best possible energy recovery by maintaining the pressure the lowest possible at the entrance of the exhaust, that is to say at the output of the turbine engine by an appropriate slowdown effect of the gas flow in the exhaust system.

FIG. 8 illustrates the aerodynamic performance of an exhaust device such as that illustrated in FIG. 1. The graph of FIG. 8 has the ratio ΔS = S as ordinate._s/ S_e between the usable area S_s for the passage of the gas stream at the outlet of a section of the exhaust device and the useful passage surface S_e from the entrance of another section. On the abscissa, the values ​​ℓ / Δr are taken where ℓ is the axial length between the two considered sections of the exhaust device and Δr is the difference between the external radius and the internal radius of the inlet annular section. The total length of the escapement illustrated in FIG. 1 is L, the inlet vein height being ΔR.

FIG. 8 also shows a plurality of curves representative of the static pressure recovery coefficient C_p or :<maths id="math0001" num=""><math display="block"><mrow><msub><mrow><mtext>C</mtext></mrow><mrow><mtext>p</mtext></mrow></msub><mtext> = - </mtext><mfrac><mrow><mtext>static outlet pressure - static inlet pressure </mtext></mrow><mrow><msub><mrow><mtext>(total pressure - static pressure)</mtext></mrow><mrow><mtext>Entrance</mtext></mrow></msub></mrow></mfrac></mrow></math><img file="EP1010884A1_D0001.tif" /></maths>

For a given speed of the flow of the gas stream, it is then possible to define an optimal performance line joining the different minima of the curves representative of the static pressure recovery coefficient C_p. This line is referenced 31 in FIG. 8. It makes it possible to determine for an exhaust device of length L and of the height of the inlet vein ΔR, the output section providing the maximum performance of the exhaust.

The optimization of this exhaust device is performed iteratively on intermediate sections. Section S_{i + 1} indicated in Figure 1 is determined according to section S_i, from the distance ℓ_i between the input sections i and output i + 1 and the value Δr_i.

Moreover, the presence of the separation 5, of zero initial thickness and of evolutionary thickness over the length of the escapement, makes it possible to divide the height of the vein at the inlet (ΔR becomes ΔR *), and therefore in accordance with curve 31 (point P *), to increase the output section (ΔS becomes ΔS *) with all other parameters unchanged.

The separation 5 provided with its coatings 14 and 15 therefore, at the same axial size, to improve the aerodynamic performance of the exhaust system. This optimization is independent of the thickness evolution of the separation 5 provided with the coatings 14 and 15 freely chosen for acoustic optimization.

The separation 5 provided with its coatings 14 and 15, therefore allows a combined improvement of aerodynamic and acoustic performance.

With reference to FIG. 2, it appears that in a first variant, the absorbent coating 14, 15, 17 used in the present invention each comprises a sound absorption layer 14a, 15a or 17a and a wall 14b, 15b or 17b acoustically transparent. The acoustically transparent wall may be for example a metal wall provided with a plurality of through holes, schematically shown in the figure in the form of a dashed line. The acoustic absorption layer 14a, 15a or 17a is constituted in the variant shown in FIG. 2 in the form of a fibrous material which dissipates the energy of the sound waves by viscous friction of the gases in the pores of the material. In each of the absorbent coatings 14, 15 and 17, the fibrous material is thus sandwiched between a rigid solid wall such as the wall of the inner partition 5 or the wall of the outer shell 3 on the one hand, and the acoustically transparent wall 14b, 15b or 17b. The acoustically transparent wall 14b, 15b or 17b is disposed on the side of the flow symbolized by the arrows 16 in the figure. The fibrous material 14a, 15a or 17a may advantageously consist of rock wool, glass wool, ceramic fibers or a similar product capable of withstanding the high temperatures prevailing in the exhaust stream.

During the operation of the turbomachine, the exhaust flow flows in the channels 7 and 8. The acoustic treatment obtained is effective over a wide range of frequencies due to the very nature of the sound absorption layer 14a, 15a or 17a constituted by the fibrous material.

In the embodiment illustrated in FIG. on which like parts bear the same references, the acoustic absorption layers 14a, 15a and 17a are constituted by a bed of perforated spheres 19 made of refractory material and accumulated in bulk or in several superimposed layers delimiting the absorbent coating, that is to say in particular between the outer wall 3 and the acoustically transparent wall 17b, the wall of the internal partition 5 and the two acoustically transparent walls 14b and 15b.

In the embodiment illustrated in FIGS. 4 and 5, the absorbent coating 14, 15 and 17 are constituted by panels 14a, 15a, 17a having a resonator structure shown here as Helmholtz resonators. The cavities or open cells 20 have the shape of hexagonal base cylinders as can be seen in the section of Figure 5, thus forming a honeycomb structure. The axis of the cells 20 is, in this example and for illustrative purposes, substantially perpendicular to the direction of the flow. On one side, this structure is welded to a rigid solid wall such as that of the outer casing 3 or the internal separation 5, thus defining a closed bottom for the different cavities or cells 20. On the other side, is disposed an acoustically resistive porous wall 14c, 15c or 17c.

The wall 14c, 15c or 17c generally comprises a relatively thin layer of a material capable of dissipating the energy of sound waves by viscous friction of the gases in the pores of the material.

As a variant, at least one of the walls 14c, 15c or 17c can be replaced by a simple perforated plate.

In this way, as previously, the absorbent coating 14, 15 or 17 allows, during the flow of the exhaust stream, the creation of a movement of the gases through the pores of the wall 14c, 15c or 17c generating a resistive damping by dissipation of sound energy. This damping can be enhanced by judiciously taking a depth for cavities or cells a multiple of a quarter of the wavelength.

It is thus possible to obtain acoustic damping at a determined frequency depending on the depth of the cavities or cells 20. The acoustic damping also depends on other parameters such as the temperature of the gases of the flow as well as the speed of the flow. It will be understood that the structure of the absorbent coating and in particular the thickness of the acoustic absorption layers 14a, 15a or 17a, i.e. the depth of cavities or cells 20 of the cellular structure illustrated in FIGS. 5, can easily be adapted according to the operating parameters of the turbomachine.

FIG. 6 illustrates an example of modification of the cellular structure illustrated in FIG. 4, for the absorbent coating 17.

In this example, a double resonator consisting of two superimposed resonators represented here in the form of Helmholtz resonators is used. A porous wall 23 is inserted between two layers 21 and 22 and cavities or open cells 20. In the example illustrated, the depth of the cavities or cells of the two layers 21 and 22 is identical, but it would be possible instead to provide different depths. The double Helmholtz resonator is associated, on the flow side, with an acoustically resistive porous wall 17c identical to that of the previous embodiment.

The porous walls 17c and 23 may be identical and constitute the resistive walls of the two resonators. The sound waves then penetrate into the first resonator comprising the porous wall 17c and the cavities of the layer 22, then into the second resonator comprising the porous wall 23 and the cavities of the layer 21.

Alternatively, the wall 23 may be simply constituted of a porous material acoustically little or not resistive. The wall 23 may also consist of a simple perforated sheet.

Such a dual resonator absorbent coating of Helmholtz has the advantage of extending the frequency tuning over a wider band than in the case of a single row of resonators.

The embodiment illustrated in FIG. 7 schematically illustrates the case of an escapement 1 comprising a central body 24 extending essentially over the entire length of the escapement 1. The central body has on the side of the inlet 4 a conical shape 24a whose section decreases in the direction of flow. A separation of substantially conical shape 5 and then cylindrical is mounted inside the exhaust 1 between the central body 24 and the outer wall 3. Two substantially annular and concentric flow channels 25 and 26 are thus defined inside the escapement 1 whose general shape is first conical and then approximately cylindrical.

An acoustically absorbing coating is provided on all internal surfaces of the exhaust 1, in contact with the gas flow stream. This coating may be identical to that described with reference to any one of the preceding embodiments. Thus, a coating 17 is provided inside the outer wall 3. Similarly, an acoustically absorbing coating 14, 15 is provided on each side of the solid wall of the internal partition 5. Finally, an acoustically absorbing coating 27 is provided all around the central body 24.

The multichannel turbomachine exhaust according to the present invention is thus treated acoustically and whatever its geometry, the inlet being generally symmetrical with respect to the axis of the flow, the concentric flow channels being able to evolve into a shape any output, either in the motor axis, or outside this axis thus exerting a jet deflection function.

The separation provided inside the exhaust device delimiting the flow channels may comprise an acoustically absorbing coating of progressive or uniform thickness depending on the acoustic characteristics that it is desired to obtain. The vein heights of the different flow channels can be optimized so that the flow rates in each channel are adapted to the acoustically treated surface and that the acoustic performance is the best possible.

Although in the examples illustrated, the outer shell as the internal separation have been provided with an acoustically absorbing coating, it will be understood that the invention also applies to the case where only the internal separation would be provided with such an acoustically coating absorbent. The invention would also apply under the same conditions to a structure in which the internal separation would not be coated with an acoustically absorbing coating, but where this coating would be provided only inside the outer casing of the exhaust .

Moreover, although in the illustrated embodiments, the acoustically absorbent coatings have been of the same structure in a specific embodiment, it will be understood that it would be possible to obtain different characteristics, to provide in the same exhaust different structures of acoustically absorbing coating in different locations, so as to meet specific acoustic treatment conditions.

Thanks to the present invention, it becomes possible to significantly reduce the acoustic signature of the exhaust and therefore of the turbomachine.

Thanks to the existence of internal separation, the acoustic treatment surface can be considerably increased by keeping the same size and aerodynamic performance equal or greater. It is also possible to improve the compactness of the assembly by producing a shorter length exhaust while maintaining an optimal acoustic treatment surface.