Improved shroud for a counter-rotating turbine impeller

Abstract

A contra-rotating turbine (C) for a turbomachine (10), comprising: an inner rotor having an inner drum (50a) to which a plurality of inner impellers (22) are attached, the inner impellers (22) being rotatably supported by a first shaft (26); an outer rotor including an outer drum (50b) to which a plurality of outer impellers (20) are fastened, the outer impellers (20) being rotatably supported by a second shaft (24) coaxial with the first shaft (26), the outer rotor including a downstream impeller (60) having a plurality of downstream moving blades (61) extending between an outer shroud (62) and an inner shroud (63), an upstream end of the outer shroud (62) being attached downstream of the outer drum (50b), the inner shroud (63) being attached to the second shaft (24), and at least one of the inner shroud (63) and the outer shroud (62) including at least one flexible transition portion (620) configured to allow elastic deformation of the shroud in a radial direction.

Description

Field of the Invention

[0001] The present invention relates to the field of turbomachines. More precisely, the present invention relates to a contra-rotating turbine for a turbomachine and to a turbomachine comprising such a turbine. Background of the Invention

[0002] From upstream to downstream in the direction of the air flow, an aircraft turbomachine generally comprises a fan, a low-pressure compressor, a high-pressure compressor, a combustion chamber, a high-pressure turbine and a low-pressure turbine. The rotor of the low-pressure compressor is driven by the rotor of the low-pressure turbine and the rotor of the high-pressure compressor is driven by the rotor of the high-pressure turbine.

[0003] In order to improve the efficiency of the engine, an aircraft turbomachine may be equipped with a contra-rotating turbine in place of the low-pressure turbine. The contra-rotating turbine comprises: an inner rotor, called the fast rotor, which is connected to a first turbine shaft and is configured to rotate in a first direction of rotation; and an outer rotor, called the slow rotor, which is connected to a second turbine shaft and is configured to rotate coaxially with the inner rotor and in a second direction of rotation opposite to the first direction of rotation. The blades of the first rotor are staggered axially with the blades of the second rotor. The blades of the inner rotor are attached to a disk or an inner drum integral with the first turbine shaft and rotating in the first direction of rotation, and the blades of the outer rotor are attached to a disk or an outer drum integral with the second turbine shaft and rotating in the second direction of rotation.

[0004] In a known manner, the connection between the outer drum of the outer rotor and the second turbine shaft is achieved by means of a downstream impeller attached downstream of the outer drum. More precisely, the downstream impeller comprises: an outer radial shroud attached axially downstream of the outer drum to the outer drum; and an inner radial shroud, or inner collar or disk, which is integral with the second turbine shaft in particular by means of a rear support shaft extending between the inner shroud and the second turbine shaft. The blades extend radially between the inner shroud and the outer shroud and allow the latter to be held together, the rotation of one driving the rotation of the other. Thus, when the blades of the outer rotor are driven to rotate in the second direction of rotation, the rotation of the outer drum drives the rotation of the outer shroud. This rotational movement is transmitted to the inner shroud by means of the blades of the downstream shroud, allowing the second turbine shaft to rotate.

[0005] The inner shroud and the outer shroud, and the blades extending between the inner shroud and the outer shroud, form a single identical part, so that the downstream impeller is manufactured in one piece. However, the blades of the downstream impeller are subjected to the temperature of the hot air flow in which they are immersed. Thus, these blades expand more than the inner shroud and the outer shroud, which are subjected to a lower temperature. The blades are deformed by the high temperature of this flow, which in turn generates high stresses on the outer shroud and the inner shroud, called the "punching" phenomenon. In addition, the centrifugal force exacerbates these stresses. These different stresses can affect the reliability and the lifespan of the blades of the downstream impeller and, by extension, the reliability and the lifespan of the turbine.

[0006] Therefore, it is necessary to modify the contra-rotating turbine structure to at least partially alleviate the above-mentioned drawbacks. SUMMARY OF THE INVENTION

[0007] The present invention relates to a contra-rotating turbine for a turbine engine, the contra-rotating turbine extending around a rotation axis and comprising:

[0008] - an inner rotor configured to rotate around the rotation axis and comprising an inner drum to which a plurality of inner impellers are attached, each inner impeller comprising inner moving blades and being rotatably supported by a first shaft,

[0009] - an outer rotor configured to rotate around the rotation axis in a direction opposite to the rotation direction of the inner rotor and comprising an outer drum to which a plurality of outer impellers are attached, each outer impeller comprising inner moving blades and being rotatably supported by a second shaft coaxial with the first shaft, the outer rotor comprising a downstream impeller having a plurality of downstream moving blades extending between an outer shroud and an inner shroud, an upstream end of the outer shroud being attached downstream of the outer drum, and the inner shroud being attached to the second shaft,

[0010] At least one of the inner shroud and the outer shroud comprises at least one flexible transition portion configured to allow elastic deformation of the shroud in the radial direction.

[0011] In the present invention, the terms "inner" and "outer" and the terms "inner side" and "outer side" and their derivatives are considered in the radial direction of the turbine. Similarly, the terms "upstream" and "downstream" are considered in the direction of the air flow along the rotation axis in the turbine engine.

[0012] The first shaft and the second shaft may be tubular and coaxial and extend along the rotation axis. The inner impellers of the inner rotor are staggered in the axial direction with the outer impellers of the outer rotor. The downstream impeller is attached to the outer drum downstream of the latter and rotates therewith. The downstream impeller allows connection between the outer drum and the second shaft and thus transmits the torque of the outer impellers to the second shaft.

[0013] The elevated temperature to which the downstream impeller is subjected in the hot air flow causes these blades to expand and thus causes their extensions to expand in the radial direction. However, the presence of the flexible transition portion allows absorption of this deformation of the blades. More precisely, the deformation of the blades, in particular their extensions, causes elastic deformation of the shroud by means of the flexible transition portion. In the case where cooling of the flow causes retraction of the downstream moving blades, the shroud elastically returns to its initial shape. Thus, the flexible transition portion allows absorption of the stress generated by the expansion of the downstream moving blades and avoids punching of the inner shroud and / or the outer shroud, while still allowing transmission of the torque generated by the blades of the outer rotor. This allows limitation or even elimination of the deterioration of the inner shroud and / or the outer shroud caused by the expansion of the downstream moving blades, thereby improving the reliability and life of the turbine.

[0014] In some embodiments, the outer shroud is attached to a platform at the radially outer end of the downstream moving blade, and a flexible transition portion is attached between the downstream end of the outer drum and the platform, such that displacement of the platform in the radial direction causes elastic deformation of the flexible transition portion.

[0015] Preferably, each blade extends between two separate platforms. Alternatively, the same platform can be attached to the ends of several blades simultaneously, thus forming a blade segment. The platform is attached to the radially outer end of the downstream moving blade, thus forming a single identical component therewith. The platform and the blade can in particular be manufactured as a single piece. The platform allows the hot air flow to be defined.

[0016] The torque generated by the rotation of the outer drum is transmitted to the downstream moving blade by means of the flexible transition portion and the platform. Thus, the flexible transmission portion is configured to be elastically deformable in the radial direction while having sufficient stiffness in the circumferential direction to be able to transmit the torque of the outer drum to the downstream moving blade. This configuration can be similarly applied to the outer shroud.

[0017] In some embodiments, the flexible transition portion has the shape of a plate which is attached to the outer drum at its upstream end and to the platform at its downstream end, and the flexible transition portion is radially spaced from the platform between the upstream end and the downstream end.

[0018] The flexible transition portion and the platform form two plates attached to each other at their downstream ends. Thus, in addition to the attachment between the flexible transition portion and the platform, the latter are radially spaced from each other. Thus, in the case where the radial expansion of the blade causes the platform to shift outward in the radial direction, the platform approaches the flexible transition portion by reducing the space between the platform and the flexible transition portion, which is also attached to the outer drum at its upstream end. In other words, the flexible transition portion acts like a spring attached between the outer drum and the platform, and the elastic deformation of the flexible transition portion allows the radial expansion stress of the downstream moving blade to be absorbed.

[0019] In some embodiments, the first attachment between the downstream end of the flexible transition portion and the downstream end of the platform is radially offset towards the inside of the turbine relative to the second attachment between the upstream end of the flexible transition portion and the outer drum.

[0020] This type of configuration allows the space between the platform and the flexible transition portion to be emphasized, thus allowing the ability of the flexible transition portion to absorb the expansion of the downstream moving blade to be increased.

[0021] In some embodiments, the flexible transition portion and the platform are two different components, and the downstream end of the platform is attached to the downstream end of the flexible transition portion by means of a coupling member.

[0022] The flexible transition portion may in particular include radial flanges at each of its axial ends, an upstream radial flange attached to the outer drum, and a downstream radial flange attached to the downstream end of the platform by means of a coupling member.

[0023] In some embodiments, the coupling member includes a bolt connection portion.

[0024] The advantage of this connection means is that it is easy to implement and allows effective attachment of the two components. In the case of blade deformation, it allows the flexible transition portion to elastically deform, and the deformation force passes through the bolt connection portion.

[0025] In some embodiments, the flexible transition portion and the platform form a single identical component.

[0026] This configuration also allows effective transmission of the deformation force originating from the blade, causing elastic deformation of the flexible transition portion. This configuration also allows limitation of the number of components of the device, thereby simplifying the structure and manufacture of the turbine.

[0027] In some embodiments, the flexible transition portion includes at least one inclined portion inclined towards the center of the turbine from its upstream end to its downstream end, and at least one straight portion substantially parallel to the rotation axis.

[0028] The presence of the inclined portion promotes radial deformation of the flexible transition portion. Therefore, the bending portion formed between the inclined portion and the straight portion substantially parallel to the rotation axis allows improvement of the elasticity of the flexible transition portion in the radial direction.

[0029] In some embodiments, the flexible transition portion includes at least one radial folding portion. The radial folding portion is a part of the flexible transition portion and includes, from upstream to downstream: a first wall that extends radially towards the outside of the turbine; a second wall that bends towards the inside of the turbine and extends from one end of the first wall; and a third wall that extends radially towards the inside of the turbine from one end of the second wall.

[0030] The presence of the radial folding portion allows improvement of the flexibility of the flexible transition portion in the axial direction. This flexibility allows absorption of dynamic changes, especially vibrations, extreme loads, etc.

[0031] In some embodiments, the outer shroud includes a plurality of shroud segments that are circumferentially arranged end to end. Each shroud segment is attached to at least one downstream moving blade and includes a platform and a flexible transition portion.

[0032] The present invention also relates to a turbomachine that includes a contra-rotating turbine according to any one of the foregoing embodiments. Description of the Drawings

[0033] The present invention and its advantages will be better understood by reading the following detailed description of different embodiments of the present invention given by way of non-limiting examples. The description refers to the attached pages of the drawings, in which:

[0034] Figure 1 Figure 1 ​​Shows an overall view of the operating principle of a turbomachine with a contra-rotating fan,

[0035] Figure 2 Figure 2 Shows a perspective view of an outer shroud section according to a first embodiment of the present disclosure,

[0036] Figure 3 Figure 3 Shows Figure 2 a side view of the outer shroud section of,

[0037] Figure 4 Figure 4 Shows a perspective view of an outer shroud section according to a second embodiment of the present disclosure,

[0038] Figure 5 Figure 5 Shows a side view of the outer shroud section according to a third embodiment of the present disclosure. DETAILED DESCRIPTION

[0039] Referring to Figure 1 , a turbomachine 10 with a contra-rotating fan includes a longitudinal axis X-X. In the direction of the flow of the gas in the turbomachine (as indicated by the solid arrows), from upstream to downstream, the turbomachine 10 essentially includes three parts: an upstream module A (or fan section), an intermediate module B (or high-pressure rotor), and a downstream module C (or low-pressure turbine section). In addition, the terms "inner" or "outer" and their derivatives refer to the radial direction of the turbomachine, which is perpendicular to the axis X-X.

[0040] The three parts A, B, and C of the turbomachine are modular, i.e., they each form a single component and can be replaced by separating them from the other components of the turbomachine.

[0041] In a manner known per se, the high-pressure body B includes a gas generator for generating combustion gases. The gas generator includes a compressor 12, a combustion chamber 14, and a high-pressure turbine 16.

[0042] The air compressed by the compressor 12 is mixed with fuel in the combustion chamber 14 and then burned there. The resulting combustion gases drive the moving blades of the high-pressure turbine 16, and the high-pressure turbine 16 in turn partially drives the compressor 12 by means of a high-pressure shaft 18. The circulation of the combustion gases in the turbomachine 10 takes place axially from upstream to downstream.

[0043] The fan section A is located upstream of the turbomachine 10. A shroud 28 surrounds the fan section A in an annular manner. The shroud 28 is supported by arms 30 that extend radially inwardly towards the turbomachine.

[0044] ​​​​​​​​The fan section A includes a first row of fan blades 32 mounted on the upstream fan shaft 34, and the upstream fan shaft 34 is coupled to the upstream end of the first low-pressure shaft 24.

[0045] The fan section A also includes a second row of fan blades 36 which are axially spaced downstream from the first row of fan blades 32 and are mounted on the downstream fan shaft 38, and the downstream fan shaft 38 is coupled to the upstream end of the second low-pressure shaft 26. However, the invention is not limited to an architecture with a two-stage fan and can be applied to other types of architectures.

[0046] Thus, the first row of fan blades 32 and the second row of fan blades 36 rotate in opposite directions, which are illustrated by the corresponding arrows F1 and F2 by way of example. Therefore, such a configuration with contra-rotating fans enables the turbomachine to have high efficiency at a relatively low specific fuel consumption.

[0047] The fan blades 32, 36 extend radially from the upstream fan shaft 34 and the downstream fan shaft 38, practically up to the shroud 28. They are arranged in the air circulation passage, supplying the main flow 40 and the secondary bypass flow 42, and the main flow 40 is directed to the compressor 12 of the high-pressure rotor B.

[0048] At the upstream end of the first low-pressure shaft 24, the first low-pressure shaft 24 rotatably supports the second low-pressure shaft 26 by means of a first antifriction bearing 44 and a second antifriction bearing 46 arranged downstream of the first antifriction bearing 44.

[0049] The first antifriction bearing 44 is of the ball type for resisting axial loads, while the second antifriction bearing 46 is of the roller type for resisting the radial loads of the turbomachine.

[0050] The low-pressure turbine section C includes a first annular rotor or outer rotor. The first rotor includes rows of inner impellers, and the inner impellers include outer turbine blades 20 that extend radially inwards and are axially spaced from each other.

[0051] The low-pressure turbine section C also includes a second annular rotor or inner rotor. The second rotor includes rows of inner impellers, and the inner impellers include inner turbine blades 22 that extend radially outwards and are axially spaced from each other. The turbine blades 20 of the first rotor and the turbine blades 22 of the second rotor are arranged alternately with respect to each other such that the first rotor and the second rotor are interlocked with each other.

[0052] The outer impellers of the first rotor are rotatably supported by the first low-pressure shaft 24. Similarly, the other impellers of the second rotor are rotatably supported by the second low-pressure shaft 26 coaxially arranged around the first shaft 24. The low-pressure shafts 24, 26 extend axially from upstream to downstream in the turbomachine.

[0053] The low-pressure turbine section C has combustion gases originating from the high-pressure rotor B, and these combustion gases pass through the low-pressure turbine section C. Thus, these combustion gases drive the turbine blades 20 of the first rotor and the turbine blades 22 of the second rotor to rotate in opposite directions. Consequently, the first low-pressure shaft 24 and the second low-pressure shaft 24 also rotate in opposite directions.

[0054] In addition, the inner impeller 22 includes a root attached to the inner drum 50a and extends radially outward from this root until the radially outer end. The outer moving blade 20 includes an outer portion attached to the outer drum 50b and extends radially inward from this base to the radially inner end.

[0055] The continuing part of this specification describes the means for attaching the downstream moving blades with reference to the low-pressure turbine C of the turbine engine 10. However, this attachment method is not limited to this low-pressure turbine and can be applied to other elements of the turbine engine, such as the high-pressure turbine.

[0056] The connection between the outer drum 50b of the outer rotor and the second turbine shaft 24 is accomplished by means of a downstream impeller 60 attached downstream of the outer drum 50b. More precisely, this downstream impeller 60 is the impeller arranged furthest downstream of the outer rotor. This can particularly relate to the 6th stage of a contra-rotating turbine in a known manner. The downstream impeller 60 includes a radially outer shroud 62 and a radially inner shroud 63. The outer shroud 62 is attached axially downstream of the outer drum 50b to the outer drum 50b, and the inner shroud 63 is integral with the second turbine shaft 24 particularly by means of a rear support shaft 25 extending between the inner shroud 63 and the second turbine shaft 24. The radially outer shroud 62 can be attached to the outer drum 50b by means of, for example, bolted connections (not shown). Alternatively, the outer shroud 62 can be arranged in the axial extension of the outer drum 50b and form a single identical piece with the latter.

[0057] The inner shroud 63 and the outer shroud 62 are concentric and axially symmetric about the axis X. Radial arms called downstream moving blades 61 extend radially between the inner shroud 63 and the outer shroud 62 and allow the latter to be held together, such that the rotation of one causes the rotation of the other. Thus, when the blades 20 of the outer rotor are driven to rotate in the second rotation direction, the rotation of the outer drum 50b causes the rotation of the outer shroud 62. This rotational movement is transmitted to the inner shroud 63 by means of the blades 61 of the downstream impeller 60, allowing the second turbine shaft 24 to rotate.

[0058] Reference will be made to Figures 2 to 5 Describe an attachment method between the downstream moving blade 61, the outer shroud 62, and the outer drum 50b. Although the following description refers to the outer shroud 62, the present invention is not limited to the outer shroud 62 and also applies to the inner shroud 63.

[0059] Preferably, the outer shroud 62 includes a plurality of sections that are circumferentially connected end to end with each other to form an annular shroud. For simplicity,Figures 2 to 5 Separate single shroud segments 62 associated with the downstream moving blades 61 are shown. Each outer shroud segment 62 includes a flexible transition portion 620 which is attached to a platform 610 at one end of the downstream moving blade 61.

[0060] The platform 610 is attached to the radially outer end of the downstream moving blade 61 and forms a single unitary piece therewith. When the platforms 610 are assembled circumferentially, the group of platforms 610 defines a hot air flow.

[0061] Reference will be made Figure 2 and 3 to describe a first embodiment of the present invention.

[0062] The platform 610 includes an upstream end 612 and a downstream end 611. The upstream end 612 is a free end, and the downstream end 611 has the shape of a radial flange which extends radially and is fastened to the flexible transition portion 620.

[0063] The flexible transition portion 620 includes a first flange 621 which extends radially at its upstream end. The first flange 621 is attached to the downstream end of the outer drum 50b, in particular to the radial flange thereof, by means of a first bolt connection 71.

[0064] The flexible transition portion 620 includes a second flange 622 which extends radially at its downstream end. The second flange 622 is attached to the downstream end of the platform 610, in particular to the downstream flange 611, by means of a second bolt connection 72. Preferably, the axes of the first bolt connection 71 and the second bolt connection 72 are substantially parallel to the axis of rotation X. The attachment between the downstream flange 611 of the platform 610 and the second flange 622 of the flexible transition portion 620 may include two or more second bolt connections 72. Figure 2 An example is shown where the attachment includes two second bolt connections 72 (the first bolt connection 71 is not shown).

[0065] The flexible transition portion 620 has the shape of a metal plate which includes, for example, a nickel-based alloy. This material can be used for other components of the turbine, so the present invention does not require the use of new materials. The width of the flexible transition portion 620 is preferably equal to the width of the platform 610 to which it is attached. The main plane along which the flexible transition portion 620 extends is substantially parallel to the axis of rotation X. Thus, the flexible transition portion 620 has sufficient stiffness in the circumferential direction to allow the torque originating from the outer drum 50b to be transmitted to the downstream moving blade 61. Conversely, the flexible transition portion 620 is more flexible in the radial direction, such that the forces exerted by the downstream moving blade 61 cause the flexible transition portion 620 to deform.

[0066] The attachment between the flexible transition portion 620 and the platform 610 results in a space E, which need not be constant, existing between these two components upstream of their attachment in the unconstrained state. The so-called "unconstrained state" means a state in which the flexible transition portion 620 is not subjected to any deformation. Such a configuration is as shown in Figure 2 and 3 . Further, in this example, the flexible transition portion 620 includes a first portion 620a and a second portion 620b. The first portion 620a extends axially from upstream to downstream between the first flange 621 and the second portion 620b and is inclined toward the center of the turbine, i.e., toward the rotational axis X. The second portion 620b extends axially from upstream to downstream between the first portion 620a and the second flange 622 and is substantially parallel to the rotational axis X. Further, the axis of the second bolt connection portion 72 is radially offset relative to the axis of the first bolt connection portion 71 toward the center of the turbine, i.e., toward the rotational axis X. Thus, the space E at the upstream end 612 of the platform 610 is larger.

[0067] In the case where the downstream moving blade 61 expands radially, the platform 610 is displaced radially ( Figure 3 upward in the figure), and its upstream end 612 is free. In contrast, the upstream end of the flexible transition portion 620 is not free but is rigidly attached to the outer drum 50b. Thus, considering the attachment between the platform 610 and the flexible transition portion 620, the displacement of the platform 610 causes the flexible transition portion 620 to elastically deform, and this elastic deformation is facilitated by the structure and configuration of the flexible transition portion 620. The displacement of the platform also causes the space E to decrease. If the temperature in the hot air flow decreases, causing the blade 61 to retract, the flexible transition portion can return to its initial shape. In other words, the flexible transition portion 620 acts as a leaf spring, allowing compensation for and absorption of the deformation of the blade 61 without generating excessive stress due to punching phenomena in the shroud 62, particularly in the platform 610.

[0068] Next, a second embodiment of the present disclosure will be described with reference to Figure 4 .

[0069] According to this embodiment, the flexible transition portion 620 has the same shape as in the first embodiment and particularly includes a first inclined portion 620a and a second straight portion 620b. Different from the first embodiment, the platform 610 and the flexible transition portion 620 are not two different components but together form a single identical piece. Thus, the attachment between these two components at their downstream ends does not include the second bolt connection portion 72 of the first embodiment. On the other hand, the first bolt connection portion 71 remains ( Figure 4 not shown in the figure). Thus, the shroud section 62 is made, for example, by casting or by additive manufacturing to form a single piece with the blade 61.

[0070] Next, a description will be made with reference to Figure 5Describe the third embodiment of the present disclosure.

[0071] According to this embodiment, the method of attaching the flexible transition portion 620 to the outer drum 50b and the platform 610 is the same as that of the first embodiment, specifically by means of the first bolt connection portion 71 and the second bolt connection portion 72. On the other hand, the shape of the flexible transition portion 620 is different from that of the first and second embodiments.

[0072] According to this third embodiment, the flexible transition portion 620 includes two radial folding portions 623. The radial folding portion 623 includes a first wall 623a, a second wall 623b, and a third wall 623c. The first wall 623a extends radially outward from the upstream portion of the flexible transition portion 620. The second wall 623b bends toward the inside of the turbine and extends from one end of the first wall 623a. The third wall 623c extends radially toward the inside of the turbine from one end of the second wall 623b. The two folding portions 623 are joined together by an intermediate wall 624 that bends toward the outside of the turbine. In this example, the flexible transition portion 620 includes two folding portions 623. However, this number is not limited, and the flexible transition portion 620 can include a single radial folding portion 623, or a number greater than two radial folding portions 623. Compared with the structures of the first and second embodiments, this accordion-like structure allows an increase in the axial flexibility of the flexible transition portion 620 while still maintaining its radial flexibility.

[0073] Although the present invention has been described with reference to specific exemplary embodiments, it is obvious that these examples can be modified and changed without departing from the overall scope of the present invention defined by the claims. In particular, the various features of the illustrated / mentioned different embodiments can be combined into additional embodiments. For example, the second and third embodiments can be combined, and the flexible transition portion 620 thus includes at least one radial folding portion 623 and forms a single piece with the platform 610. Therefore, the specification and the drawings should be considered in an illustrative rather than a restrictive sense.

Claims

1. A contra-rotating turbine (C) for a turbomachine (10), the contra-rotating turbine (C) extending about a rotation axis (X) and comprising: - an inner rotor configured to rotate about the rotation axis (X) and comprising an inner drum (50a) to which a plurality of inner impellers (22) are attached, each inner impeller (22) comprising inner moving blades and being rotatably supported by a first shaft (26), - an outer rotor configured to rotate about the rotation axis (X) in a direction opposite to the rotation direction of the inner rotor and comprising an outer drum (50b) to which a plurality of outer impellers (20) are attached, each outer impeller (20) comprising outer moving blades and being rotatably supported by a second shaft (24) coaxial with the first shaft (26), the outer rotor comprising a downstream impeller (60) which is located downstream of the other outer impellers (20) in the direction of flow of the gas in the turbine along the rotation axis (X), the downstream impeller (60) having a plurality of downstream moving blades (61) which extend between an outer shroud (62) and an inner shroud (63), an upstream end of the outer shroud (62) being attached to the outer drum (50b) downstream of the outer drum, and the inner shroud (63) being attached to the second shaft (24), at least one of the inner shroud (63) and the outer shroud (62) comprises at least one flexible transition portion (620) configured to allow elastic deformation of the shroud in the radial direction, the outer shroud (62) being attached to a platform (610) at the radially outer end of the downstream moving blade (61), the flexible transition portion (620) being attached between a downstream end of the outer drum (50b) and the platform (610) such that displacement of the platform (610) in the radial direction causes elastic deformation of the flexible transition portion (620), and wherein an upstream end (612) of the platform (610) is a free end not attached to the outer drum (50b) and the outer shroud (62).

2. The turbine (C) according to claim 1, characterized in that, The flexible transition portion (620) has the shape of a plate which is attached at its upstream end (621) to the outer drum (50b) and at its downstream end (622) to the platform (610), the flexible transition portion (620) being radially spaced from the platform (610) between the upstream end (621) and the downstream end (622).

3. The turbine (C) according to claim 1, characterized in that, A first attachment between the downstream end (622) of the flexible transition portion (620) and the upstream end (611) of the platform (610) is radially offset towards the inside of the turbine relative to a second attachment between the upstream end (621) of the flexible transition portion (620) and the outer drum (50b).

4. The turbine (C) according to claim 1, characterized in that, The flexible transition portion (620) and the platform (610) are two different components, and the downstream end (611) of the platform (610) is attached to the downstream end (622) of the flexible transition portion (620) by means of a coupling member (72).

5. The turbine (C) according to claim 4, characterized in that, The coupling member (72) includes a bolt connection portion.

6. The turbine (C) according to claim 1, characterized in that, The flexible transition portion (620) and the platform (610) form a single identical component.

7. The turbine (C) according to claim 1, characterized in that, The flexible transition portion (620) includes, from its upstream end (621) to its downstream end (622), at least one inclined portion (620a) inclined towards the center of the turbine, and at least one straight portion (620b) substantially parallel to the rotation axis (X).

8. The turbine (C) according to claim 1, characterized in that, The flexible transition portion (620) includes at least one radial folding portion (623), and the radial folding portion (623) is part of the flexible transition portion (620) and includes, from upstream to downstream: a first wall (623a) that extends radially towards the outside of the turbine; a second wall (623b) that bends towards the inside of the turbine and extends from one end of the first wall (623a); and a third wall (623c) that extends radially towards the inside of the turbine from one end of the second wall (623b).

9. The turbine (C) according to claim 1, characterized in that, The outer shroud (62) includes a plurality of shroud segments that are circumferentially arranged end to end, and each shroud segment is attached to at least one downstream moving blade (61) and includes a platform (610) and a flexible transition portion (620).

10. A turbine engine (10) comprising a contra-rotating turbine (C) according to any one of claims 1 to 9.

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