Turbojet afterbody comprising an internal sleeve and a perforated anti-squeal plate running along this sleeve

Abstract

The invention relates to a turbojet afterbody (1), comprising a casing (13) extending around a longitudinal axis (AX) and a thermal protection jacket (16) coaxial with the casing (13) and which extends inside this casing (13) to delimit a post-combustion chamber (12), characterized in that it comprises a cylindrical anti-squeal element which extends coaxially with the jacket (16) along a radially internal face of this jacket (16) and at a distance from this radially internal face. Figure for the abstract: Figure 2

Description

Title of the invention: Turbojet afterbody comprising an internal sleeve and a perforated anti-squeal plate running along this sleeve Technical field

[0001] The invention applies to the field of military aeronautical propulsion, and more particularly to turbojets equipped with an afterburner chamber. STATE OF THE PRIOR ART

[0002] In a turbojet, air is admitted into an inlet sleeve to pass through a low-pressure compressor before splitting into a central primary flow and a secondary flow surrounding the primary flow.

[0003] The primary flow is then compressed in a high-pressure compressor before arriving in a combustion chamber, after which it is expanded in a high-pressure turbine and then in a low-pressure turbine before being discharged to the rear. The secondary flow is propelled to the rear without being compressed.

[0004] In the case of an afterburner turbojet, an afterbody comprising an afterburner chamber extends downstream of the low-pressure turbine. This afterburner chamber collects the primary and secondary flows to inject fuel into it to generate additional combustion making it possible to temporarily increase the total thrust.

[0005] In this post-combustion chamber, the fuel is introduced by injectors, downstream of which radially extending flame-holder arms and spark plugs are provided to ignite the injected fuel.

[0006] Such an engine can thus operate in so-called dry mode in which the post-combustion is not activated, or in post-combustion mode in which the post-combustion chamber is supplied with fuel.

[0007] The post-combustion chamber comprises a substantially cylindrical casing equipped with an internal jacket, and a portion of the secondary flow circulates between the jacket and the casing to cool it in order to limit its heating.

[0008] In post-combustion mode, combustion instabilities can excite acoustic modes of vibration by unsteady release of heat. This can cause natural modes of vibration of the post-combustion chamber to resonate, which can damage the liner, or even destroy it.

[0009] These natural modes of vibration which are located in the high frequency domain are generally designated by the term “screech” in English.

[0010] One solution may be to equip the upstream portion of the liner with a Helmholtz resonator type arrangement to dampen these vibration modes in order to protect this liner. In this case, a toroidal compartment or box is formed on the external face of the liner in its upstream region, and holes opening into this compartment are formed through the liner. The radial thickness of the compartment and the diameter of the holes are then determined as a function of the frequencies of the vibration modes to be absorbed.

[0011] The implementation of such a box, however, introduces significant complexity since it requires its integration into the restricted space extending between the jacket and the casing which surrounds it and which is itself subject to severe thermal constraints.

[0012] In practice, it is possible to do without such a box by only forming holes passing through the jacket in its upstream region, these holes having diameters determined on the basis of the frequencies to be absorbed. Such a solution is similar to a Helmholtz resonator type arrangement, so that in practice it allows the vibration modes to be absorbed.

[0013] However, with such a solution, a significant portion of the secondary flow admitted into the space extending between the jacket and the casing is evacuated towards the post-combustion chamber, through the holes provided to absorb the natural vibration modes, which penalizes the cooling of the casing and / or the consumption of the engine.

[0014] In this context, the invention aims to provide a solution to avoid the appearance of the screech phenomenon when the afterburner of the turbojet is activated, without penalizing the cooling of the afterburner casing or the consumption of the engine. Statement of the invention

[0015] For this purpose, the invention relates to a turbojet afterbody, comprising a casing extending around a longitudinal axis and a thermal protection jacket coaxial with the casing and which extends inside this casing to delimit a post-combustion chamber, characterized in that it comprises an anti-squealing element which is a perforated revolution sheet extending coaxially with the jacket along a radially internal face of this jacket and at a distance from this radially internal face while being located at an upstream portion of this jacket.

[0016] The invention allows for very simple integration of an anti-squeal device. This device can be a perforated cylindrical sheet, it does not disturb the flow and introduces almost no pressure loss into this flow. The invention also makes it possible to increase the flow rate in the annular space surrounding the jacket of cooling, to possibly improve the cooling of a nozzle located downstream of the rear casing.

[0017] The invention also relates to a rear body thus defined, in which the revolution sheet is carried by bridges fixed to the jacket and located at the upstream end of this revolution sheet.

[0018] The invention also relates to a rear body thus defined, in which the revolution sheet is carried by bridges fixed to the jacket and located at the downstream end of this revolution sheet.

[0019] The invention also relates to a rear body thus defined, in which the sleeve comprises a cylindrical portion at the level of the bridges for holding the revolution sheet metal.

[0020] The invention also relates to a rear body thus defined, in which the revolution sheet is cylindrical.

[0021] The invention also relates to a rear body thus defined, in which the revolution sheet is fixed by riveting.

[0022] The invention also relates to a turbojet engine comprising an afterbody thus defined.

[0023] The invention also relates to a turbojet engine thus defined, comprising an exhaust casing carrying a confluence sheet, and in which the perforated revolution sheet extends in the extension of the confluence sheet.

[0024] The invention also relates to a turbojet engine thus defined, in which the perforated revolution sheet is formed by a downstream portion of the confluence sheet. Brief description of the drawings

[0025] [Fig.l] is a schematic longitudinal sectional view of a turbojet engine according to the invention;

[0026] [Fig. 2] is a partial and schematic longitudinal sectional view of the rear- body of the turbojet according to the invention;

[0027] [Fig. 3] is a partial longitudinal sectional view of the rear body of the turbojet according to the invention.

[0028] DETAILED DESCRIPTION OF PARTICULAR EMBODIMENTS

[0029] In [Fig.l], the turbojet 1 according to the invention has a general shape of revolution extending around a longitudinal axis AX, it comprises at its upstream AM an inlet sleeve through which the air is admitted to pass through a low pressure compressor 2 before being split into a central primary flow F1 and a secondary flow F2 surrounding the primary flow.

[0030] The primary flow F1 is then compressed in a high pressure compressor 3 before arriving in a combustion chamber 4, after which it is expanded through a high pressure turbine 6 and a low pressure turbine 7.

[0031] The secondary flow is propelled directly towards the rear by the low pressure compressor 2 in a vein delimited externally by a main casing 8.

[0032] The primary flow F1 and secondary flow F2 then pass through an exhaust casing 9 located downstream of the low-pressure turbine, which is extended by a confluence plate 11 extending downstream of this exhaust casing.

[0033] This turbojet is equipped downstream of the turbine 7 with an afterburner chamber 12 delimited by a rear casing 13, in which the secondary flow F2 joins the primary flow F1 to form the propulsion flow Fp. The two flows join downstream of the confluence plate 11 which extends the separation between the primary flow F1 and the secondary flow F2.

[0034] After passing through the low pressure turbine 7, the primary flow F1 thus circulates between the confluence sheet 11 and an outlet cone 14 surrounded by this sheet 11 to reach the post-combustion chamber 12.

[0035] The wall of the rear casing 13 is lined at the level of the chamber 12 by an internal jacket 16 delimiting with it an annular space 17. A part of the secondary flow is directed into this annular space 17 to ventilate it in order to limit heating of the casing 13.

[0036] The afterburner chamber corresponds to the rear body of the turbojet engine which may also include a nozzle comprising a convergent portion and a divergent portion which then extend in the extension of the afterburner chamber.

[0037] Injectors not shown located downstream of the low pressure turbine 7 are provided to introduce additional fuel in order to generate additional combustion in the chamber 12 when the engine is in post-combustion mode. In addition, flame-holder arms 18 located downstream of the confluence plate 11 ignite the fuel introduced by the injectors, when the engine is operating in post-combustion mode.

[0038] According to the invention, a cylindrical anti-squeal element is provided which extends coaxially with the liner along a radially internal face of this liner.

[0039] As can be seen more clearly in [Fig. 2], this anti-squealing element is a perforated revolution sheet 19 which is cylindrical and coaxial with the axis AX. This revolution sheet 19 extends radially at a distance from the internal face of the jacket 16. It extends longitudinally from the upstream end of this jacket 16 over a length of the order of a third or half the length of this jacket 16.

[0040] This revolution sheet 19 thus delimits with the jacket 16 which surrounds it a revolution space 21 which is open towards the upstream AM and towards the downstream AV instead of being closed. A part of the secondary flow F2 coming from the engine thus circulates through this revolution space 21 by entering through its upstream end 22 and exiting through its downstream end 23. Another part of the secondary flow F2 circulates in a similar manner in the annular space 17.

[0041] As visible in [Fig.2], this revolution sheet 19 has through holes 24 which are regularly distributed over the majority of its surface, so that it constitutes with the revolution space 21 which it delimits a set of Helmholtz resonators.

[0042] The holes 24 and the radial thickness of the revolution space 21 are dimensioned according to the range of squeal frequencies to be absorbed. This range of frequencies which is determined by the geometry of the engine and by its operating speed can be determined from tests carried out on the engine and / or by numerical simulation of its operation.

[0043] The dimensioning of the holes 24 (their diameter) and their number, as well as the diameter of the sheet 19 (which conditions the radial thickness of the space 21) are determined based on the dimensioning of the Helmholtz resonators, and by additionally carrying out tests or numerical simulations to adjust these values.

[0044] As visible in [Fig.2], the revolution sheet 19 advantageously extends in the extension of the confluence sheet 11, so as not to disturb the flow of the flow F2 entering and circulating in the rear body. The revolution sheet 19 then has the same diameter as the confluence sheet 11.

[0045] This revolution plate 19 is held by bridges 26 which are carried by the jacket 16, as visible in [Fig. 3]. More particularly, the rear casing 13 is terminated at its upstream end by a flange 27, by means of which it is secured to the main casing 8 by bolting.

[0046] This rear casing 13 has its internal face equipped with bridges 28 by means of which it carries the liner 16, each bridge 28 having a cross section having a shape corresponding to that of the letter I. Each bridge 28 comprises a radially external sole fixed to the internal face of the casing 13, and this radially external sole is connected by a core extending radially to an internal sole receiving the external face of the liner 16.

[0047] Similarly, the jacket 16 has its internal face equipped with bridges 26 by means of which it carries the revolution plate 19. Each bridge 26 has a cross-section having a shape corresponding to that of the letter I, which comprises a radially external sole fixed to the internal face of the jacket 16, this sole radially external being connected by a core extending radially to an internal flange receiving the external face of the revolution sheet 19.

[0048] The revolution sheet 19 is fixed to the internal sole of the bridges 26 for example by riveting, the bridges 26 also being able to be fixed by their radially external soles to the jacket 16 by riveting.

[0049] The bridges 26 and 28 are regularly distributed around the circumference of the upstream portion of the jacket 16.

[0050] As visible in [Fig. 3], the jacket 16 comprises a cylindrical upstream portion 29 by which it is fixed to the bridges 26 and 28, and which is extended by a portion comprising undulations 31 provided to generate turbulence in the flow of the stream crossing the annular space 17, so as to increase the thermal exchanges with this cooling stream.

[0051] The revolution sheet 19 comprises an upstream part 32 which is devoid of holes 24, extended by a downstream part in which the holes 24 are formed which are uniformly distributed over this downstream part.

[0052] In the example of the figures, the revolution sheet 19 is fixed at its upstream end to the jacket 16 by bridges 26, but it can also, alternatively or additionally, be fixed at its downstream end to the jacket 16, by bridges, according to an arrangement of the same type as that of the bridges 26.

[0053] In the example of the figures, the sheet 19 has a cylindrical shape, but it can have a different shape, such as for example a substantially conical shape which flares out to give the space of revolution 21 a smaller radial thickness at its downstream end than at its upstream end, so as to dampen different frequency ranges at its upstream end and at its downstream end.

[0054] Generally speaking, the invention allows a very simple integration of an anti-squeal device into a turbojet afterbody. This device being in the form of a cylindrical sheet extending inside the post-combustion chamber, it does not disturb the flow in the latter and has a negligible cross-section (limited to the thickness of the revolution sheet), so that it introduces almost no pressure loss into this flow.

[0055] The very low additional mass represented by such a revolution sheet constitutes an additional advantage for its integration. The placement of this revolution sheet in the extension of the confluence sheet makes it possible to further reduce the very small disturbances that it can introduce.

[0056] The invention thus makes it possible to increase the flow rate in the annular space surrounding the cooling jacket, to improve this cooling. It thereby makes it possible to increase the cooling flow available to cool a nozzle located downstream of the post-combustion chamber.

Claims

Claims

1. Turbojet (1) comprising an exhaust casing (9) carrying a confluence plate (11), and an afterbody comprising a casing (13) extending around a longitudinal axis (AX) and a thermal protection jacket (16) coaxial with the casing (13) and which extends inside this casing (13) to delimit a postcombustion chamber (12), an anti-squeal element which is a perforated revolution plate (19) extending coaxially with the jacket (16) along a radially internal face of this jacket (16) and at a distance from this radially internal face, characterized in that the perforated revolution plate (19) is formed by a downstream portion of the confluence plate (11).

2. Turbojet according to claim 1, in which the revolution plate (19) is carried by bridges (26) fixed to the jacket (16) and located at the upstream end of this revolution plate (19).

3. Turbojet according to claim 1, in which the revolution plate (19) is carried by bridges fixed to the jacket (16) and located at the downstream end of this revolution plate (19).

4. Turbojet according to claim 2 or 3, in which the sleeve (16) comprises a cylindrical portion at the level of the bridges (26) for holding the revolution sheet metal (19).

5. Turbojet according to claim 2 or 3, in which the revolution sheet (19) is cylindrical.

6. Turbojet according to claim 2, in which the revolution sheet (19) is fixed by riveting.

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