Inner shell sector for aircraft turbomachine

Abstract

An inner shell section (1) configured to internally delimit a secondary flow (41) of an aircraft turbomachine (50), the inner shell section (1) comprising a downstream end (2) comprising an upstream longitudinal portion (3) and a downstream longitudinal portion (4) which are connected by a median portion (5) and configured to extend respectively upstream and internally to an inner nacelle casing (31), the inner shell section (1) comprising an inner layer and an outer layer of composite material, the median portion (5) of the downstream end (2) comprising at least a volume of intumescent material sandwiched between the inner and outer layers of composite material so as to form a local overthickness in the longitudinal direction. Abstract figure: Figure 4

Description

Title of the invention: Inner shell sector for aircraft turbomachine technical field

[0001] The present invention relates to the field of aircraft turbomachine housings and more specifically targets an inner shell sector.

[0002] In a known manner, with reference to [Fig. 1], an aircraft turbomachine 500, with longitudinal axis X, comprises from upstream to downstream a fan 200, a low-pressure compressor 220, a high-pressure compressor 230, a combustion chamber 240, a high-pressure turbine 250 and a low-pressure turbine 260. The compressors 220, 230, the combustion chamber 240 and the turbines 250, 260 together define a primary airflow channel 400, delimited externally by a central casing 270. The aircraft turbomachine 500 also comprises a secondary airflow channel 410 which extends externally around the primary channel 400 and is surrounded by a nacelle 300.

[0003] As is known and illustrated in [Fig. 1], the aircraft turbomachine 500 comprises an intermediate casing 190 including an inner ring 100 and an outer ring 180 which respectively delimit the secondary flow 410 internally and externally. The inner ring 100 is fixed upstream to a hub 160 of the intermediate casing 190. The inner ring 100 and the outer ring 180 are connected by arms 170 extending radially in the secondary flow 410, downstream of a row of fixed blades 330 (outlet guide vanes (OGVs)). The outer ring 180 extends in the downstream extension of the fan casing 210 and in the upstream extension of an outer fan duct 320.

[0004] In a known manner and as illustrated in Figures 1 and 2, the inner fan 100 extends in the upstream extension of an inner fan duct 310, which, together with the central housing 270, delimits an inter-vein compartment 280 extending radially between the primary vein 400 and the secondary vein 410. More specifically, with reference to [Fig. 2], the inner fan 100 comprises a downstream end 120 in the form of a step configured to be overlapped by an upstream end of the inner fan duct 310 and fixed thereto by fixing screws inserted radially into recesses 130. Such an assembly is known as a swaging.

[0005] In practice, the inner ferrule 100 is formed by several angular inner ferrule sectors 110, connected together by joining plates 140 using screws radially inserted mounting brackets in housings 130. The inner ferrule sectors 110 are typically in the form of one-piece titanium parts. Titanium advantageously offers high mechanical strength and thermal resistance to deformation, but also a high density, resulting in heavy inner ferrule sectors 110. This increases the weight of the aircraft turbomachine 500 and therefore its energy consumption in flight.

[0006] To reduce the mass of an inner ferrule sector, variable-thickness titanium inner ferrule sectors are known, with the thickness reduced in areas subjected to little stress and greater in areas subjected to the most stress, for example, at the mounting housings. However, such a mass reduction proves insufficient.

[0007] The invention thus aims to reduce the mass of an inner shell sector for an aircraft turbomachine while maintaining satisfactory mechanical and thermal resistance to deformation. PRESENTATION OF THE INVENTION

[0008] The invention relates to an inner shell sector configured for mounting in an aircraft turbomachine, the inner shell being a part of revolution defined with respect to a longitudinal axis oriented upstream to downstream, configured to internally delimit a secondary flow of the aircraft turbomachine, the inner shell sector comprising a downstream end comprising: • an upstream longitudinal portion located at a first radial distance from the longitudinal axis and configured to extend in the upstream continuation of an inner nacelle casing, • a downstream longitudinal portion located at a second radial distance from the longitudinal axis, less than the first radial distance, and configured to extend internally into the inner nacelle casing, and • a median portion connecting the upstream longitudinal portion and the downstream longitudinal portion.

[0009] The invention is remarkable in that the inner ferrule sector comprises at least one inner layer of composite material and at least one outer layer of composite material, the composite material being in the form of a plurality of reinforcing fibers in a matrix, the middle portion of the downstream end comprising at least a volume of intumescent material sandwiched between the inner layer of composite material and the outer layer of composite material so as to form a local overthickness in the longitudinal direction.

[0010] The use of composite material advantageously allows for a reduced mass in the inner ferrule sector. The intumescent material, for its part, allows for Reducing the porosity of the composite material in the midsection results in an inner shell section with excellent mechanical strength. The initially malleable layers of composite material require curing together by radial compression at a heating temperature. However, radial compression is not effective in compressing the midsection, especially when it extends radially over a distance greater than twice the thickness of the longitudinal portion. The volume of intumescent material, thanks to its heat-expanding properties, further compresses the composite material in the midsection, locally reducing porosity. Furthermore, the intumescent material allows for the creation of an additional thickness more easily and quickly than using pre-cut layers of composite material.Such an arrangement with an intumescent film allows local variations in thickness at the downstream end of the composite sector, resulting in an internal step profile that differs from the external step profile; typically, an angular (abrupt) external profile and a regular, progressive internal profile well suited to joint sealing.

[0011] According to a preferred aspect of the invention, the inner ferrule sector comprises a plurality of layers of composite material, each extending uniformly throughout the entire downstream end. The use of intumescent material eliminates the need to locally cut and layer composite material to define the required thickness, which is a time-consuming process.

[0012] According to one aspect of the invention, the volume of intumescent material in the middle portion has a longitudinal thickness in a direction parallel to the longitudinal axis that decreases radially from the outside to the inside, preferably from a maximum value to a minimum value at least half the maximum. Preferably, the maximum value of the longitudinal thickness is at least four times greater than the minimum value. Preferably, the maximum value of the longitudinal thickness is at most ten times greater than the minimum value. The intumescent material is advantageously located in the middle portion so as to increase the longitudinal component of the middle portion and reduce its radial component. The middle portion thus forms a gentler radial slope, which allows for better compression of the composite material.

[0013] According to one aspect of the invention, the middle portion comprises an outer wall transverse to the longitudinal axis. This provides a stop and cooperation with the upstream end of the inner nacelle housing.

[0014] According to one aspect of the invention, the middle portion comprises at least a first angular slice and a second angular slice juxtaposed, the volume of intumescent material extending only into the first angular slice. This forms a longitudinal fixing band, preferably on the angular edge of the inner ferrule sector to ensure its fixing to a neighboring inner ferrule sector.

[0015] According to one aspect of the invention, the middle portion has an inner surface forming, at the first angular slice, a longitudinal protrusion relative to the second angular slice. This longitudinal protrusion increases the longitudinal component of the middle portion and reduces its radial component. The middle portion thus has a gentler radial slope, which allows for better compression of the composite material. Advantageously, no protrusion is formed on the outer surface to allow for interaction with the inner housing of the nacelle.

[0016] According to one aspect of the invention, the volume of intumescent material extends radially throughout the middle portion, so as to connect the upstream longitudinal portion and the downstream longitudinal portion and form a gentle radial slope ensuring good radial compression.

[0017] According to one aspect of the invention, the middle portion comprises at least one separation film separating the volume of intumescent material from the inner layer of composite material and the outer layer of composite material. Preferably, the separation film is airtight. This prevents material displacement during compression and heating, and thus improves the mechanical strength of the final part.

[0018] According to one aspect of the invention, the middle portion extends radially over a distance at least twice the radial thickness of the upstream longitudinal portion, preferably at least four times greater. According to a preferred aspect, the middle portion extends radially over a distance greater than the radial thickness of the downstream longitudinal portion, preferably at least three times greater. Such a middle portion makes the use of intumescent material to reinforce compression all the more necessary.

[0019] According to one aspect of the invention, the inner layer of composite material and the outer layer of composite material have, in the middle portion, a porosity of less than 2%, preferably less than 1% and preferably less than 0.7%. Such a level of porosity ensures good mechanical strength.

[0020] According to one aspect of the invention, the volume of intumescent material in the middle portion is designated the first volume of intumescent material, the downstream longitudinal portion comprising at least a second volume of intumescent material sandwiched between the inner layer of composite material and the outer layer of composite material so as to form a local overthickness in the radial direction, the second volume of intumescent material extending in the longitudinal continuation of the first volume of intumescent material. The first and second volumes of intumescent material together form a longitudinal fixation band of high mechanical strength.

[0021] According to a preferred aspect, the upstream longitudinal portion comprises at least a third volume of intumescent material sandwiched between the inner and outer layers of composite material so as to form a local thickness in the radial direction, the third volume of intumescent material extending in the longitudinal continuation of the first volume of intumescent material. The first and third volumes of intumescent material together form a longitudinal fastening band with high mechanical strength.

[0022] The invention relates to an inner shell sector as described above before compression and heating, in which the intumescent material is adapted to expand under the effect of heat, preferably at a predetermined temperature above 115°C, the predetermined temperature being preferably below 180°C. Before compression and heating, the composite material and the intumescent material are in a malleable state.

[0023] The invention also relates to an inner shell sector as described above, after compression and heating, in which the intumescent material is in an expanded state. After compression and heating, the composite material and the intumescent material are in a hardened state.

[0024] The invention also relates to an inner shell for an aircraft turbomachine comprising a plurality of inner shell sectors as described above, the inner shell being a part of revolution defined with respect to a longitudinal axis oriented upstream to downstream configured to internally delimit a secondary flow of the aircraft turbomachine.

[0025] The invention also relates to an intermediate casing for an aircraft turbomachine comprising an inner shell as described above, the intermediate casing comprising a hub, on which the inner shell is fixed, and an outer shell, which extends opposite the inner shell and is configured to externally delimit a secondary flow of the aircraft turbomachine. The intermediate casing preferably comprises at least one arm connecting the inner and outer shells.

[0026] The invention also relates to an aircraft turbomachine comprising a primary flow and a secondary flow extending externally around the primary flow, the aircraft turbomachine comprising an intermediate casing as described above in which the inner and outer shells respectively delimit the secondary flow internally and externally. Preferably, the outer shell extends in the downstream extension of a fan casing. PRESENTATION OF THE FIGURES

[0027] The invention will be better understood upon reading the following description, given by way of example, and referring to the following figures, given by way of non-limiting examples, in which identical references are given to similar objects.

[0028] Fig. 1 is a schematic representation in longitudinal half-section of an aircraft turbomachine according to the prior art.

[0029] The [Fig.2] is a schematic perspective representation from downstream of an inner ferrule sector according to the prior art.

[0030] The [Fig.3] is a schematic representation in longitudinal half-section of an aircraft turbomachine comprising inner shell sectors according to one embodiment of the invention.

[0031] The [Fig.4] is a schematic perspective representation from downstream of an inner ferrule sector according to one embodiment of the invention.

[0032] The [Fig.5] is a schematic perspective representation from downstream of the downstream end of the inner ferrule sector of the [Fig.4].

[0033] The [Fig.6] is a schematic radial cross-sectional representation of the first angular slice of the downstream end of the [Fig.5].

[0034] [Fig.7] is a schematic perspective representation from upstream of the downstream end of [Fig.5].

[0035] The [Fig.8] is a schematic radial cross-sectional representation of the first angular slice of the downstream end according to another embodiment comprising a separation film.

[0036] It should be noted that the figures set out the invention in detail to implement the invention, said figures being of course able to serve to better define the invention where appropriate. DETAILED DESCRIPTION OF THE INVENTION

[0037] With reference to [Fig. 3] and as described in the preamble, an aircraft turbomachine 50 conventionally extends along a longitudinal axis X oriented from upstream to downstream. Hereafter, the terms "upstream" and "downstream" are defined with respect to the orientation of the longitudinal axis X. The terms "inside" and "outside" are defined with respect to a radial axis extending orthogonally to the longitudinal axis X.

[0038] Still referring to [Fig. 3], the aircraft turbomachine 50 conventionally comprises, from upstream to downstream, a fan 20, a low-pressure compressor 22, a high-pressure compressor 23, a combustion chamber 24, a high-pressure turbine 25, and a low-pressure turbine 26. The compressors 22, 23, the combustion chamber combustion 24 and the turbines 25, 26 together define a primary airflow channel 40, delimited externally by a central casing 27. The aircraft turbine 50 also includes a secondary airflow channel 41 which extends externally around the primary channel 40 and is surrounded by a nacelle 30.

[0039] With reference to [Fig. 3], the aircraft turbomachine 50 comprises an intermediate casing 19 including an inner ring 15 and an outer ring 18 which respectively delimit the secondary flow 41 internally and externally. The inner ring 15 is fixed upstream to a hub 16 of the intermediate casing 19. The inner ring 15 and the outer ring 18 are connected in this example by arms 17 extending radially in the secondary flow 41, downstream of a row of fixed blades 33 (outlet guide vanes (OGVs)). The outer ring 18 extends in the downstream extension of the fan casing 21 and in the upstream extension of an outer fan duct 32.The inner ferrule 15 extends upstream from an inner fan duct 31, which together with the central casing 27 delimits an inter-vein compartment 28 (compartment core) extending radially between the primary vein 40 and the secondary vein 4L.

[0040] As illustrated in Figures 3 and 4, the inner ferrule 15 is a part of revolution defined with respect to the longitudinal axis X in the aircraft turbomachine 50, the longitudinal axis X hereafter referring interchangeably to the axis of the turbomachine 50 and the axis of the inner ferrule 15, which are considered to coincide. The inner ferrule 15 is formed by a set of interconnected inner ferrule sectors 1 distributed angularly around the longitudinal axis X. The inner ferrule sectors 1 preferably extend over a variable angular width.

[0041] According to the invention and as illustrated in [Fig. 4], each inner ferrule sector 1 comprises a downstream end 2 including an upstream longitudinal portion 3 and a downstream longitudinal portion 4 connected by a medial portion 5. The upstream longitudinal portion 3 and the longitudinal portion 4 are located respectively at a first radial distance R3 and a second radial distance R4 from the longitudinal axis X satisfying: R4 > R3 ([Fig. 6]). The upstream longitudinal portion 3 extends in the upstream extension of the inner nacelle housing 31. The downstream longitudinal portion 4 extends internally into the inner nacelle housing 31. Preferably, the medial portion 5 extends radially over a distance H of between 24 mm and 32 mm.

[0042] According to the invention and as illustrated in Figures 4 and 6, the inner ferrule sector 1 comprises at least one inner layer of composite material 9a and at least one outer layer of composite material 9b, the composite material being in the form of a plurality of reinforcing fibers in a matrix, the a median portion 5 of the downstream end 2 comprising at least a volume of intumescent material 10-1 sandwiched between the inner layer of composite material 9a and the outer layer of composite material 9b so as to form a local excess thickness in the longitudinal direction. Preferably, each layer of composite material 9a, 9b has a constant thickness.

[0043] The composite material advantageously has a lower density compared to the titanium traditionally used, which makes it possible to obtain a lower-mass inner ferrule sector 1. The choice of the composite material, usually preferred for aircraft parts of uniform thickness and less exposed to mechanical stresses and thermal deformations, is unprecedented in an inner ferrule sector 1.

[0044] The intumescent material improves the material strength of the inner shell sector 1, namely by ensuring a low level of porosity in the composite material. As will be seen later, the inner shell sector 1 is formed by heating and compressing a series of layers of composite material, with one or more volumes of intumescent material interspersed. The compression is carried out between two molds along the stacking axis of the composite material layers, i.e., radially, which does not allow for sufficient compression at the mid-portion 5. Thanks to its expansion properties upon heating, the intumescent material helps to compress the composite material at the mid-portion 5, in order to obtain the desired level of porosity.

[0045] The volume of intumescent material further advantageously comprises any size and shape, making it easy to obtain the desired thickness. This avoids having to form the thickness from precisely cut layers of composite material, such layers being liable to shift during heating and compression.

[0046] According to a preferred aspect illustrated in Figures 5 to 7, the inner ferrule sector 1 comprises several volumes of intumescent material 10⁻¹, 10⁻², 10⁻³, 10⁻⁴. In this example, the inner ferrule sector 1 comprises: • the volume of intumescent material 10-1 located in the middle portion 5 and hereinafter referred to as “first volume of intumescent material 10-1”, • a second volume of intumescent material 10-2 located in the downstream longitudinal portion 4 in the downstream extension of the first volume of intumescent material 10-1, and • a third volume of intumescent material 10-3 located in the upstream longitudinal portion 3 in the upstream extension of the first volume of intumescent material 10-1.

[0047] The first volume of intumescent material 10-1 is continuous with the second The first volume of intumescent material 10⁻² and the third volume of intumescent material 10⁻³ are arranged to form a longitudinal, raised band extending into a first angular slice T1 of the inner ferrule sector 1, preferably located at an angular edge of the inner ferrule sector 1. This longitudinal raised band allows the inner ferrule sector 1 to be attached to an adjacent inner ferrule sector 1 or to an arm 17, preferably via a connecting plate 14 ([Fig. 4]). The longitudinal raised band includes one or more mounting recesses 13 into which fasteners, such as screws, nails, or rivets, are radially inserted. The inner ferrule sector 1 preferably includes such a longitudinal raised band at both of its angular edges.

[0048] Also according to a preferred aspect illustrated in [Fig. 4], the inner shell sector 1 comprises one or more openings 29 connected to a discharge conduit for an airflow taken upstream of the low-pressure compressor 22, to prevent the occurrence of a low-flow pumping phenomenon. As illustrated in Figures 5 and 7, the inner shell sector 1 preferably comprises a fourth volume of intumescent material 10⁻⁴ forming a thickened transverse band located upstream and downstream of the openings 29 to allow the attachment of the discharge conduits.

[0049] With reference to [Fig. 6], the inner ferrule sector 1 comprises a set of radially stacked layers of composite material 9a, 9b, which preferably extend over the entire surface of the downstream end 2 and preferably over the inner ferrule sector 1. In this example, only two layers 9a, 9b are shown, but the number of layers is in practice greater than eight. Each layer of composite material 9a, 9b preferably has a uniform thickness over its entire surface. The layers 9a, 9b preferably have identical thicknesses. The composite material is in the form of a set of reinforcing fibers, preferably carbon fibers, embedded in a matrix, preferably a polyepoxide, known as "epoxy".The composite material layers 9a, 9b are superimposed and have a malleable structure configured to harden and bond together after heating and pressure to obtain the final shape of the inner ferrule sector 1.

[0050] With further reference to [Fig. 6], the first volume of intumescent material 10-1 is sandwiched between an inner layer 9a and an outer layer 9b of composite material. Preferably, the first volume of intumescent material 10-1 is central, i.e., the inner shell sector 1 comprises substantially the same number of layers on either side of the first volume of intumescent material 10-1. Also preferably, all volumes of intumescent material 10-1, 10-2, 10-3, 10-4 are sandwiched between the inner layer 9a and the outer layer 9b. The volume in The tumescent material is initially malleable and configured to expand under the effect of heat, preferably at a predetermined temperature above 115°C, the predetermined temperature being preferably below 180°C. Once heated, the intumescent material remains in a hardened, expanded state. The intumescent material preferably comprises polyepoxide.

[0051] According to a preferred aspect illustrated in [Fig. 8], the inner shell sector 1 further comprises a separating film 12, preferably watertight, extending on either side of the volume of intumescent material 10-1 to separate it from the layers of composite material 9a, 9b. The separating film 12 prevents the intumescent material and the composite material from mixing at the interface during curing and compression. The separating film 12 preferably comprises a composite material, preferably arranged in one or two layers. This ensures better compression of the composite material and helps to reduce the porosity of the composite material.

[0052] Preferably, the inner shell sector 1 is made entirely of composite material, intumescent material, and preferably a separation film 12. Also preferably, the inner shell sector 1 has a one-piece structure for improved mechanical strength. Again preferably, the porosity level of the composite material in the inner shell sector 1 after curing and compression is less than 2%, preferably less than 1%, and preferably less than 0.7%.

[0053] As illustrated in Figures 5 to 7 and described previously, the downstream end 2 comprises an upstream longitudinal portion 3 and a downstream longitudinal portion 4 connected by a median portion 5, together forming a step. The median portion 5 comprises two successive curves, namely a first fold 7 and a second fold 8 radially internal to the first fold 7. In practice, the downstream end 2 forms a step configured to be straddled by an upstream end of the inner nacelle casing 31 and fixed thereto by means of fastening elements such as screws, nails or rivets inserted radially into recesses 13 ([Fig. 4]).

[0054] With reference to [Fig. 6], the upstream longitudinal portion 3 has a radial thickness e3 which is preferably constant along its entire length. Similarly, the downstream longitudinal portion 4 has a radial thickness e4 which is preferably constant along its entire length, and preferably equal to the thickness e3 of the upstream longitudinal portion 3. The median portion 5 extends radially over a distance H satisfying H = R4 - R3, which is preferably at least twice the thickness e3 of the upstream longitudinal portion 3, preferably at least four times greater, and preferably at most ten times greater.

[0055] With reference to Figures 5 and 6, the median portion 5 comprises an outer wall A transverse Pextl, defined between the first fold 7 and the second fold 8, and a longitudinal outer wall Pext2, defined upstream of the first fold 7 and extending in line with the upstream longitudinal portion 3. The first fold 7 is preferably angular at the outer surface Sext, in this example forming a right angle. Similarly, the second fold 8 is preferably angular at the outer surface Sext, in this example forming a right angle. The transverse outer wall Pextl extends in a plane transverse to the longitudinal axis X so as to form a stop for the inner nacelle casing 31.

[0056] With reference to figures 5 to 7, the volume of intumescent material 10-1 has a longitudinal thickness e in a direction parallel to the longitudinal axis X which is decreasing radially from the outside to the inside. The longitudinal thickness e comprises a maximum value emax, defined at the first radial distance R3 and a minimum value emin, defined at the second radial distance R4, the minimum value emin being at least twice less than the maximum value emax, preferably at least two times greater and at most ten times greater.

[0057] With reference to Figures 6 and 7, the first volume of intumescent material 10⁻¹ extends only within a first angular slice T1 of the inner shell sector 1, the inner surface of which Sint forms a longitudinal protrusion 11⁻¹ relative to a neighboring second angular slice T2. In other words, the first volume of intumescent material 10⁻¹ forms an excess thickness visible only from the inner surface Sint. The outer surface Sext is advantageously devoid of a protrusion at the level of the first angular slice TL. According to a preferred aspect, the first fold 7 and the second fold 8 are preferably curved and gradual at the level of the inner surface Sint. According to a preferred aspect illustrated in [Fig. 7], the second, third, and fourth volumes of intumescent material 10⁻², 10⁻³, 10⁻⁴ form a radial protrusion 11⁻², 11⁻³, 11⁻⁴ on the inner surface Sint only.

Claims

Demands

1. Inner shell sector (1) configured for mounting in an aircraft turbomachine (50), the inner shell being a part of revolution defined with respect to a longitudinal axis (X) oriented upstream to downstream configured to internally delimit a secondary flow (41) of the aircraft turbomachine (50), the inner shell sector (1) comprising a downstream end (2) comprising: • an upstream longitudinal portion (3) located at a first radial distance (R3) from the longitudinal axis (X) and configured to extend in the upstream continuation of an inner nacelle casing (31), • a downstream longitudinal portion (4) located at a second radial distance (R4) from the longitudinal axis (X) less than the first radial distance (R3) and configured to extend internally into the inner nacelle casing (31), and • a median portion (5) connecting the upstream longitudinal portion (3) and the downstream longitudinal portion (4), • the inner ferrule sector (1) being characterized in that it comprises at least one inner layer of composite material (9a) and at least one outer layer of composite material (9b), the composite material being in the form of a plurality of reinforcing fibers in a matrix, the middle portion (5) of the downstream end (2) comprising at least one volume of intumescent material (10-1) sandwiched between the inner layer of composite material (9a) and the outer layer of composite material (9b) so as to form a local overthickness in the longitudinal direction, the layers of composite material (9a, 9b) and the volume of intumescent material (10-1) being malleable and adapted to harden during a heating and compression step, the volume of intumescent material (10-1) being adapted to expand during the heating and compression step.

2. Inner ferrule sector (1) according to claim 1, wherein the volume of intumescent material (10-1) of the middle portion (5) presents a longitudinal thickness (e) along a direction parallel to the longitudinal axis (X) which is radially decreasing from the outside to the inside, preferably from a maximum value (emax) to a minimum value (emin) at least twice as low.

3. Inner ferrule sector (1) according to any one of claims 1 and 2, wherein the middle portion (5) has a transverse outer wall (Pextl) with respect to the longitudinal axis (X).

4. Inner ferrule sector (1) according to any one of claims 1 to 3, wherein the middle portion (5) comprises at least a first angular slice (T1) and a second angular slice (T2) juxtaposed, the volume of intumescent material (10-1) extending only in the first angular slice (T1).

5. Inner ferrule sector (1) according to claim 4, wherein the middle portion (5) has an inner surface (Sint) forming at the level of the first angular slice (T1) a longitudinal protrusion (11-1) with respect to the second angular slice (T2).

6. Inner ferrule sector (1) according to any one of claims 1 to 5, wherein the volume of intumescent material (10-1) extends radially throughout the middle portion (5).

7. Inner ferrule sector (1) according to any one of claims 1 to 6, wherein the middle portion (5) comprises at least one separation film (12) separating the volume of intumescent material (10-1) from the inner layer of composite material (9a) and the outer layer of composite material (9b).

8. Inner ferrule sector (1) according to any one of claims 1 to 7, wherein the middle portion (5) extends radially over a distance (H) at least twice greater than a radial thickness (e3) of the upstream longitudinal portion (3), preferably at least four times greater.

9. Inner ferrule sector (1) according to any one of claims 1 to 8, wherein the inner layer of composite material (9a) and the outer layer of composite material (9b) have, at the level of the middle portion (5), a porosity of less than 2%, preferably less than 1% and preferably less than 0.7%.

10. Inner ferrule sector (1) according to any one of claims 1 to 9, wherein the volume of intumescent material (10-1) of the middle portion (5) is designated the first volume of intumescent material (10-1), the downstream longitudinal portion (4) comprising at least a second volume of intumescent material (10-2) sandwiched between the inner layer of composite material (9a) and the outer layer of composite material (9b) so as to form a local overthickness in the radial direction, the second volume of intumescent material (10-2) extending in the longitudinal continuation of the first volume of intumescent material (10-1).

11. Inner shell sector (1) obtained by heating and compression of the inner shell sector (1) according to any one of claims 1 to 10, wherein the layers of composite material (9a, 9b) and the volume of intumescent material (10-1) are hardened, the volume of intumescent material (10-1) being in an expanded state.

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