Diffusion cone for the rear section of a jet engine with a flame-holding ring on the rear edge

Description

TECHNICAL FIELD

[0001] The present invention relates to the field of turbojet engines for aircraft propulsion and comprising an afterburner. More particularly, it relates to a diffuser cone for the rear section of a turbojet engine, a rear section of a turbojet engine comprising such a diffuser cone, and a turbojet engine comprising the latter. PREVIOUS STATE OF THE ART

[0002] Known types of turbojet engines, particularly those intended for supersonic flight, include in their rear section, also called the rear body, an afterburner channel, also called a "reheat channel", in which the gas flow from the turbine can undergo further combustion thanks to the oxygen remaining within it, before expanding in an ejection nozzle.

[0003] This channel is internally delimited by a turbine outlet cone, called a diffusion cone, which generally has the additional function of limiting harmful vibrational phenomena known as "screech" in Anglo-Saxon terminology, by means of series of through or non-through holes, called "anti-screech holes", made in the downstream part of the diffusion cone and / or by means of one or more boxes, called "anti-screech boxes", attached to the internal surface of the diffusion cone.

[0004] Arm-shaped fuel injection devices are generally arranged at the inlet of the combustion channel, for example around the diffusion cone, to diffuse fuel downstream towards a flame-catching device intended to stabilize the flame within the afterburner channel.

[0005] Such a flame-arresting device typically comprises an annular row of radially extending flame-arresting arms, designed to promote radial flame propagation along each arm, and a flame-arresting ring connecting these arms in pairs to promote circumferential flame propagation from arm to arm. This flame-arresting ring generally has a C- or V-shaped cross-section, defining two rigidly connected branches on the upstream side of the turbojet engine, and having free ends downstream that, during operation, promote the formation of gas flow recirculation zones, contributing to flame stabilization. A diffusion cone according to the prior art is known from US 5,367,874.

[0006] The design of such a device involves making a compromise between the device's ability to best catch and stabilize the flame, and the pressure drop induced by the obstruction of the afterburner channel by the device.

[0007] Furthermore, such a device has a limited ability to fuel the core of the afterburner channel and therefore to allow satisfactory combustion in this area during afterburner operation, which in some cases results in a thermal profile at the nozzle outlet with a hollow in its center, which can impair the thrust of the turbojet.

[0008] In this context, there is a need for an improved rear section of the turbojet engine. DESCRIPTION OF THE INVENTION

[0009] The invention proposes for this purpose a diffusion cone intended to internally delimit the inlet of an afterburner channel within a rear part of a turbojet engine, comprising an annular wall extending from a base and converging towards an opposite side where said annular wall forms a truncated apex of the diffusion cone defining an annular trailing edge of the latter, and comprising a flame-catching ring having a cross-section comprising an external branch and an internal branch rigidly connected to each other on the side of the base and such that the external branch extends around the internal branch, whereby the external and internal branches delimit between themselves an internal space of the flame-catching ring opening on the opposite side to the base of the diffusion cone, one of the external and internal branches being constituted by said annular trailing edge,and the flame-holding ring having at least one fuel inlet arranged for the admission of fuel into the internal space of the flame-holding ring.

[0010] Such a flame-catching ring helps to ensure and control the propagation of the flame to the heart of the afterburner channel.

[0011] According to other advantageous aspects of the invention, the connecting device has one or more of the following characteristics, taken individually or in all technically possible combinations: the outer branch is said branch consisting of said annular trailing edge; the diffusion cone further includes an annular thermal protection screen which is arranged between the outer and inner branches so as to partially obstruct an opening through which the internal space of the flame-catching ring opens on the side opposite the base of the diffusion cone, and shaped to delimit, along each of the outer and inner branches, a corresponding outlet passage through which the internal space of the flame-catching ring opens on the side opposite the base of the diffusion cone; the fuel inlet is arranged on the side of the base of the diffusion cone.

[0012] The invention also relates to a rear part of a turbojet engine, comprising an afterburner channel, an afterburner channel housing surrounding said channel, a diffusion cone of the type described above internally delimiting said channel, and a fuel injection system configured to inject fuel into the internal space of the flame-catching ring through said fuel inlet.

[0013] In preferred embodiments of the invention, the rear part of the turbojet further comprises an annular row of flame-catching arms having radially internal ends arranged axially opposite the flame-catching ring.

[0014] In preferred embodiments of the invention, the flame-holding arms extend freely relative to each other from their radially external ends.

[0015] In preferred embodiments of the invention, the rear part of the turbojet further includes an air circulation system configured to supply the internal space of the flame-holding ring with air from a secondary turbojet flow.

[0016] The invention also relates to an aircraft turbojet engine, comprising a diffusion cone of the type described above or a rear part of the type described above. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] The invention will be better understood, and other details, advantages, and features thereof will become apparent from the following description, given by way of non-limiting example and with reference to the accompanying drawings, in which: [ Fig. 1 ] is a schematic half-view in axial section of a turbojet engine including an afterburner duct; [ Fig. 2 ] is a schematic half-view in axial section of a rear part of a turbojet engine of a known type; [ Fig. 3 ] is a schematic half-view in axial section of a rear part of the turbojet engine of the figure 1 , comprising a diffusion cone according to a preferred embodiment of the invention; [ Fig. 4 ] is a larger-scale view of a part of the figure 3 .

[0018] Throughout these figures, identical references may designate identical or analogous elements. DETAILED EXPLANATION OF PREFERRED METHODS OF IMPLEMENTATION

[0019] There figure 1 This illustrates a turbojet engine 10, for example a twin-spool, twin-flow engine, intended for example for the propulsion of an aircraft capable of supersonic flight, and therefore intended in particular for installation in the fuselage of such an aircraft. The invention is of course applicable to other types of turbojet engines.

[0020] Throughout this description, the axial direction X is the direction of the longitudinal axis 11 of the turbojet engine. Unless otherwise specified, the radial direction R is at every point a direction orthogonal to and passing through the longitudinal axis 11, and the circumferential direction C (sometimes called the azimuthal or ortho-radial direction) is at every point a direction orthogonal to the radial direction R and to the longitudinal axis 11. The terms "upstream" and "downstream" are defined with reference to a general direction D of the gas flow in the turbojet engine 10.

[0021] By way of illustration, such a turbojet 10 comprises, from upstream to downstream, an air inlet 12, a low-pressure compressor 14, a high-pressure compressor 16, a combustion chamber 18, a high-pressure turbine 20, a low-pressure turbine 22, an afterburner duct 24, and a variable-geometry nozzle 26, for example of the convergent-divergent type. These components of the turbojet are all centered along its longitudinal axis 11.

[0022] A post-combustion channel housing 28 surrounds said channel.

[0023] In addition, a flow separation ferrule 30, commonly referred to as a "confluence", extends downstream from a rear TRF casing of the low-pressure turbine 22, inside the casing 28, concentrically to the latter, so as to externally delimit the inlet of the afterburner channel 24 and to delimit, with the casing 28, an annular channel 32 forming the downstream end of the secondary SF stream.

[0024] Finally, a diffusion cone 33 extends downstream in line with a hub of the rear TRF casing of the low-pressure turbine 22, so as to internally delimit the inlet of the afterburner channel 24.

[0025] As is well known, the high-pressure compressor 16, the combustion chamber 18, and the high-pressure turbines 20 and low-pressure turbines 22 define a primary flow PF. This is surrounded by a secondary flow SF of the turbomachine which extends from upstream to downstream, from an outlet of the low-pressure compressor 14 to a downstream end of the flow separation shell 30, and which therefore includes the aforementioned annular channel 32.

[0026] Thus, during operation, air F1, which enters through the air inlet 12 and is compressed by the low-pressure compressor 14, is then divided into a primary flow F2, which circulates in the primary stream PF, and a secondary flow F3, which circulates in the secondary stream SF. The primary flow F2 is then further compressed in the high-pressure compressor 16, then mixed with fuel and ignited in the combustion chamber 18, before undergoing expansion in the high-pressure turbine 20 and then in the low-pressure turbine 22.

[0027] The combustion gases constituting the primary flow F2 at the outlet of the turbine and the secondary flow F3 mix within the afterburner channel 24 from the downstream end of the flow separation shell 30, and thus constitute an exhaust gas flow F4 which continues its circulation in the afterburner channel 24 and then escapes from the turbojet 10 through the outlet delimited by the nozzle 26.

[0028] In afterburning operation, for example to propel an aircraft to supersonic speeds, fuel is injected into the gas stream within the afterburner channel 24, and the resulting mixture is ignited within this channel to generate additional thrust.

[0029] For this purpose, fuel injection devices 34 in the general shape of arms are arranged at the inlet of the afterburner channel 24 to vaporize fuel downstream towards a flame-catching device 36 intended to promote flame stabilization.

[0030] There figure 2illustrates on a larger scale the rear part of such a turbojet engine, in a known configuration. This figure shows in particular the flame-gripping device 36, which comprises an annular row of flame-gripping arms 38 extending radially with respect to the longitudinal axis 11 of the turbojet engine, and a flame-gripping ring 42 centered on the axis 11 and connecting the flame-gripping arms 38 in pairs.

[0031] The flame-catching arms 38 are designed to promote radial flame propagation along each of the arms, while the flame-catching ring 42 is designed to promote circumferential flame propagation from arm to arm.

[0032] The flame-catching arms 38 are each connected to the afterburner channel housing 28, from which these arms extend radially inwards.

[0033] In the example shown on the figure 2The flame-catching ring 42 is arranged at the downstream end of the annular space 32 forming the downstream end of the secondary SF vein. In other known configurations, the flame-catching ring 42 is arranged inside the flow-separating ferrule 30.

[0034] The flame-holding arms 38 generally include means for circulating relatively cool air from the secondary stream SF within them to provide thermal protection for the flame-holding arms. For example, each of these arms includes a thermal protection device 39 capable of channeling the cool air from the secondary stream. In some cases, such as the illustrated example, the flame-holding arms 38 also include fuel injection means 41.

[0035] The flame-catching ring 42 has a concave cross-section facing downstream, generally C-shaped or V-shaped, thus defining two branches 42A and 42B rigidly connected to each other on the upstream side and having free ends downstream which, during operation, promote the formation of gas flow recirculation zones, contributing to flame stabilization. The flame-catching ring 42 generally also includes means for circulating fresh air from the secondary stream SF within it, and in some cases, such as the illustrated example, means for injecting fuel 43. The flame-catching ring 42 typically includes a thermal protection screen 45 arranged between the branches 42A and 42B to channel the fresh air from the secondary stream and thus protect the ring 42 on the downstream side from the surrounding hot gases and the thermal radiation of the flame.

[0036] As for the diffusion cone 33, it is mainly composed of an annular wall converging downstream, and has at its upstream end a base 50, for example provided with a radial flange 52, fixed to a downstream end ferrule 54 of the TRF rear housing, which ferrule 54 extends radially inwards from a downstream end of the hub 56 of the rear housing. On the downstream side, the diffusion cone 33 has a truncated apex defining an annular trailing edge 58.

[0037] In addition to its function of internally conforming the inlet of the afterburner channel 24 to the extension of the primary PF stream, the diffusion cone 33 can also be designed to limit harmful vibrational phenomena known as "screech," which involve high-frequency resonances favored by the presence of a flame-catching device. To limit such phenomena, the diffusion cone 33 is, for example, provided with through or blind holes 60, called "anti-screech holes," made in the downstream part of the annular wall of the diffusion cone, and by means of a box 62, called an "anti-screech box," attached to an internal surface of the diffusion cone 33, for example, on the internal surface of a downstream end transverse wall 64 of the diffusion cone 33, beyond which the annular trailing edge 58 is formed projecting downstream and radially inwards.The downstream end transverse wall 64 is typically also provided with anti-screech holes 60 opposite the wall constituting the anti-screech box 62.

[0038] The diffusion cone is generally ventilated by means of a flow of fresh air taken from the secondary vein SF, corresponding to arrows 66 of the figure 2 . This airflow is for example evacuated through all or part of the anti-screech holes 60. This does not of course prevent another part of the airflow taken from the secondary vein from being evacuated into the primary vein PF through openings 68 provided in radial arms of the rear crankcase TRF.

[0039] However, in some cases it is desirable to improve the attachment and stabilization of the flame at the core 24A of the afterburner channel 24.

[0040] US4798048 proposes a diffusion cone incorporating a double-walled annular recess located in an axially mid-region of the cone, between its base and downstream tip, to generate a flow recirculation zone. Assuming this device achieves such an advantage, the two objectives mentioned above would be resolved, at least to some extent. However, this proposal has the drawback of requiring a complete redesign of the cone's configuration, particularly the design of the means intended to limit vibrational "screech" phenomena, if applicable.

[0041] Furthermore, it is generally desirable to improve the flame carburization at the core 24A of the afterburner channel, which is not possible with the diffusion cone of the aforementioned document.

[0042] The present invention proposes a solution to these problems, consisting of providing a flame-catching ring at the downstream end of the diffusion cone 33, taking advantage of the annular trailing edge 58 of the cone to form a branch of the flame-catching ring, and providing the flame-catching ring with a fuel inlet to supply the flame-catching ring with fuel, as will become clearer in what follows.

[0043] Such a flame-catching ring improves flame attachment and stabilization at the heart of the afterburner channel without altering the overall configuration of the diffusion cone, and also improves flame carburization at the heart of the afterburner channel.

[0044] More specifically, with reference to figures 3 And 4, a diffusion cone 33 according to a preferred embodiment of the invention comprises a flame-catching ring 70 with a cross-section comprising an external branch 70A and an internal branch 70B rigidly connected to each other on the side of the base 50 of the cone, and such that the external branch 70A extends around the internal branch 70B.

[0045] In this way, the external branches 70A and internal branches 70B delimit between themselves an internal space 72 of the flame-catching ring 70, which opens on the opposite side to the base 50 of the cone, that is to say in the direction of downstream.

[0046] As indicated above, one of the outer branches 70A and inner branches 70B is formed by the annular trailing edge 58. The term "trailing edge" refers to the convergent downstream end of the diffusion cone 33, regardless of whether this end is formed as a single piece with the rest of the cone or is made of a part or assembly attached to the downstream transverse wall 64 of the cone, and regardless of whether this end is formed of a single wall or several superimposed walls. In the preferred example shown, the diffusion cone is formed of a single piece extending from the base 50 to and including the trailing edge 58.

[0047] Furthermore, in the preferred embodiment, it is the outer branch 70A which is constituted by the annular trailing edge 58 of the cone, which allows the flame-catching ring to be brought as close as possible to the heart of the afterburner channel.

[0048] The two branches 70A, 70B are, for example, connected to the downstream end transverse wall 64 of the cone at a distance from each other, so that the transverse wall 64 forms a bottom 74 of the internal space 72 of the flame-catching ring 70. The two branches 70A, 70B and the bottom 74 thus define an overall C-shaped section for the flame-catching ring 70. Alternatively, the two branches can be connected directly to each other and thus define an overall V-shaped section for the flame-catching ring.

[0049] In the illustrated example, the two branches 70A and 70B diverge from each other downstream. Thus, the inner branch 70B exhibits a higher downstream convergence rate than the outer branch 70A.

[0050] The flame-catching ring 70 has one or more fuel inlets 75, for example formed in the bottom 74 of the internal space 72, for the admission of fuel into the internal space 72, as will become clearer in what follows.

[0051] Furthermore, an annular thermal protection screen 76 is advantageously arranged between the outer branches 70A and the inner branch 70B so as to partially obstruct an opening 78 through which the internal space 72 opens on the side opposite the base 50 of the diffusion cone (i.e., on the downstream side). Such an annular screen 76 is, for example, concave so as to delineate, along each of the branches 70A and 70B, a corresponding outlet passage 78A, 78B of the internal space 72. The outlet passages 78A, 78B thus constitute the unobstructed portion of the aforementioned opening 78 of the internal space 72.

[0052] The rear section of the turbojet engine includes a fuel injection system, comprising one or more fuel lines 80, optionally distributed around the longitudinal axis 11, for injecting fuel into the internal space 72 of the flame-gripping ring 70 via the fuel inlet(s) 75, and allowing the ejection of said fuel into the afterburner channel 24 through the opening 78, in this case through the outlet passages 78A, 78B. It should be noted that such a fuel inlet 75 may, for example, consist of a simple orifice allowing the passage of a corresponding fuel line 80. Such a fuel line may, for example, have an end terminated by a multi-perforated wall to facilitate the diffusion of fuel into the internal space 72.

[0053] The fuel line(s) run, for example, within the TRF turbine rear casing, to a suitable fuel supply device. One or more openings are provided through the downstream end ferrule 54 of the TRF rear casing to allow the passage of the fuel line(s).

[0054] Furthermore, the rear section of the turbojet advantageously includes air circulation means configured to supply the internal space 72 of the flame-gripping ring 70 with air 66 from the secondary flow F3. These means include one or more air passage orifices 82 provided in the bottom 74 of the internal space 72. One or more of the fuel inlets 75 may constitute such air passage orifices 82, for example, due to a clearance provided between the corresponding fuel line 80 and the edge of the fuel inlet 75, as shown in the figures 3 And 4 .

[0055] In the illustrated embodiment, the rear part of the turbojet engine comprises an annular row of flame-gripping arms 38 having radially internal ends 38A arranged axially opposite the flame-gripping ring 70 so as to be located within gas recirculation zones 90 induced by the branches 70A and 70B of the flame-gripping ring, as more clearly shown in the figure 4 .

[0056] Furthermore, the rear section of the turbojet is advantageously devoid of flame-holding rings connecting the arms in pairs, unlike what was described above with reference to the figure 2 In other words, the flame-holding arms 38 of the figure 3extend freely from each other from their radially external ends 38B. Indeed, the flame-catching ring 70 of the diffusion cone 33 ensures the circumferential propagation of the flame between the flame-catching arms 38.

[0057] The diffusion cone 33 according to the preferred embodiment of the invention advantageously comprises anti-screech holes 60 and / or one or more anti-screech boxes 62, which are, for example, similar to those of the diffusion cone of the figure 2The diffusion cone 33 advantageously includes an anti-screech box 62 attached to the inner surface of the annular wall of the diffusion cone 33, opposite the anti-screech holes 60 formed in this wall. This anti-screech box 62 is preferably closed, as is the anti-screech box 62 attached to the transverse wall 64, so as to prevent air circulation through the anti-screech holes 60 and thus reserve more air from the secondary stream for the flame-catching ring 70.

[0058] In operation, the fresh air 66 from the secondary vein and conveyed by the aforementioned air circulation means enters the internal space 72 of the flame-catching ring 70 and thus thermally protects the flame-catching ring, both against the surrounding hot gas flow and against the radiation from the flames.

[0059] This fresh air is expelled from the internal space 72 through outlet passages 78A and 78B, and thus enters the heart of the afterburner channel. This fresh air improves the mixing of the gases from the primary and secondary flows during dry operation (i.e., without afterburner), thereby improving the turbojet's performance in terms of thrust.

[0060] During afterburner operation, the internal space 72 of the flame-catching ring 70 is further supplied with fuel by the fuel line(s) 80. The fuel from this line mixes with fresh air from the secondary stream within the internal space 72 and escapes from it through the outlet passages 78A, 78B. Some of the fuel may also run off in liquid form onto the surfaces delimiting the outlet passages 78A, 78B and be sheared by the surrounding airflow at the outlet of said passages. In all cases, the flame-catching ring 70 thus allows the core 24A of the afterburner channel to be fueled, in particular the gas recirculation zones 90 induced by the branches 70A, 70B of the flame-catching ring 70. The latter thus contributes to a control and circumferential propagation of the flame.

[0061] The ignition of the flame in the heart of the afterburner channel can be achieved by means of a conventional initiator at the level of one or more of the flame-catching arms 38, along which the flame propagates radially inwards until it reaches the heart 24A of the afterburner channel, where the circumferential propagation of the flame is favoured by the recirculation zones 90 and the presence of fuel from the flame-catching ring 70.

Claims

1. Diffusion cone (33) intended to internally delimit the entry of a post-combustion channel within a rear part of a turbojet engine, comprising an annular wall extending from a base (50) converging to an opposite side where said annular wall forms a truncated top of the diffusion cone defining an annular trailing edge (58) thereof, the diffusion cone (33) comprising a flame-holder ring (70) with a section comprising an outer branch (70A) and an inner branch (70B) rigidly connected to each other and such that the outer branch (70A) extends around the inner branch (70B), whereby the outer and inner branches delimit between themselves an inner space (72) of the flame-holder ring, the flame-holder ring comprising at least one fuel inlet (75) arranged for the intake of fuel into the inner space (72) of the flame-holder ring, characterised in that the outer branch (70A) and the inner branch (70B) are rigidly connected to each other on the base side (50), in that the inner space (72) of the flame-holder ring opens on the side opposite to the base (50) of the diffusion cone, and in that one of the outer and inner branches consists of said annular trailing edge (58).

2. Diffusion cone according to claim 1, wherein the outer branch (70A) is said branch consisting of said annular trailing edge (58).

3. Diffusion cone according to claim 1 or 2, further comprising an annular thermal protection screen (76) which is: - arranged between the outer (70A) and inner (70B) branches so as to partially obstruct an opening (78) through which the inner space (72) of the flame-holder ring opens to the side opposite to the base (50) of the diffusion cone, and - shaped to delimit, along each of the outer (70A) and inner (70B) branches, a corresponding outlet passage (78A, 78B) through which the inner space (72) of the flame-holder ring opens on the side opposite to the base (50) of the diffusion cone.

4. Diffusion cone according to any one of claims 1 to 3, wherein said fuel inlet (75) is arranged on the base (50) side of the diffusion cone.

5. Turbojet engine rear part, comprising a post-combustion channel (24), a post-combustion channel housing (28) surrounding said channel, a diffusion cone (33) according to any one of claims 1 to 4 internally delimiting the inlet of said channel, and a fuel injection system configured to inject fuel into the internal space (72) of the flame-holder ring (70) via said fuel inlet (75).

6. Turbojet engine rear part according to claim 5, further comprising an annular row of flame-holder arms (38) having radially inner ends (38A) arranged axially facing the flame-holder ring (70).

7. Turbojet engine rear part according to claim 6, wherein the flame-holder arms (38) extend freely relative to each other from their radially outer ends (38B).

8. Turbojet engine rear part according to any one of claims 5 to 7, further comprising an air circulation system configured to supply the internal space (72) of the flame-holder ring (70) with air from a secondary stream of the turbojet engine (F3).

9. Aircraft turbojet engine, comprising a diffusion cone (33) according to any one of claims 1 to 4 or a rear part according to any one of claims 5 to 8.