Compact turbojet exhaust housing

Abstract

A turbomachine exhaust casing (1) extending around an axis (X) and comprising an inner ring (5) and an outer ring (7) centered on the axis (X), the outer ring (7) being radially further from the axis (X) than the inner ring (5), the inner ring (5) and the outer ring (7) each being annular and defining between them an airflow channel (3), the inner ring (5) and the outer ring (7) each comprising an annular channel surface, the exhaust casing being characterized in that at least one of the channel surfaces extends around the axis (X) passing through a first point (P1) at a first distance (D1) from the axis (X) and a second point (P2) circumferentially offset from the first point (D1) and which is located at a second distance (D2) from the axis (X), the first distance (D1) being different of the second distance (D2). Figure for the abbreviation: Fig. 2

Description

Title of the invention: Turbojet exhaust casing with reduced bulk FIELD OF THE INVENTION

[0001] The invention relates to turbomachines, and more particularly to turbomachine casings and in particular to turbojet exhaust casings. STATE OF THE ART

[0002] A turbojet engine generally comprises a fan, a body comprising downstream thereof, the downstream being defined relative to the general direction of flow of the gases in the turbojet engine, an exhaust casing which surrounds downstream of the body, the gas exhaust zone.

[0003] The exhaust casing is a stator part in the turbomachine, it is also known by the English designation “Turbine Rear Frame” abbreviated to TRF.

[0004] With the aim of generally reducing the size of the parts of a turbojet engine, there is a need to reduce the size of the exhaust casing. In particular, it is sought to reduce the radial size, that is to say the size in a direction transverse to the axis of the turbojet engine which corresponds to the vertical direction when the turbojet engine is mounted on an aircraft. In particular, it is sought to reduce the length to be provided under the wing to place the turbojet engine there compared to the prior art. Statement of the invention

[0005] An aim of the invention is to propose an exhaust casing which has a reduced size compared to the prior art.

[0006] The aim is achieved in the context of the present invention by means of a turbomachine exhaust casing extending around an axis, the exhaust casing comprising an inner shroud and an outer shroud centered on the axis, the outer shroud being radially further from the axis with respect to the inner shroud, the inner shroud and the outer shroud each being annular and defining between them an air flow vein surrounding the axis, the inner shroud and the outer shroud each comprising an annular vein surface which delimits the air flow vein, the exhaust casing being characterized in that at least one of the vein surfaces extends around the axis passing through a first point at a first distance from the axis and a second point circumferentially offset from the first point and which is located at a second distance from the axis, the first distance being different from the second distance.

[0007] The casing according to the invention has among the inner ferrule and the outer ferrule, a ferrule which is not circular. This degree of freedom makes it possible to adjust the shape of the exhaust casing according to the space constraints and to solve the problem mentioned above.

[0008] Such a system / method is advantageously and optionally supplemented by the following different characteristics taken alone or in combination:

[0009] - a ratio of a difference between the first distance and the second distance on the first distance is greater than or equal to 5% and less than or equal to 40%;

[0010] - the vein surface passing through the first point and the second point comprises in besides a plurality of other points, the first point, the second point and the other points being located on an ellipse centered on the axis;

[0011] - the vein surface passing through the first point and the second point presents at at least in part a polygonal shape, the plurality of points forming vertices of the polygonal shape;

[0012] - the internal ferrule has an elliptical shape or a shape all around the axis polygonal defined by vertices located on an ellipse, the ellipse being defined by an inner ferrule ellipse ratio calculated between a first inner ferrule major axis and a first inner ferrule minor axis, the outer ferrule having all around the axis a shape of an ellipse or a polygonal shape defined by vertices located on an ellipse, the ellipse being defined by an outer ferrule ellipse ratio calculated between a second outer ferrule major axis and a second outer ferrule minor axis, the inner ferrule ellipse ratio and the outer ferrule ellipse ratio each being between 1.05 and 1.40;

[0013] - the external shell ellipse ratio is between 85% and 97% of the ratio internal ferrule ellipse;

[0014] - the vein surface of the inner ferrule comprises a portion of a first ellipse and the vein surface of the outer ferrule comprises a portion of a second ellipse, the first ellipse being different from the second ellipse;

[0015] - the vein surface of the inner ferrule and / or the vein surface of the outer ferrule comprises a first portion which corresponds to a portion of a first ellipse and a second portion which corresponds to a portion of a second ellipse, the first ellipse being different from the second ellipse;

[0016] - the casing comprises a hub defining the internal shell, the exhaust casing comprising a plurality of arms, each arm extending across the flow path from the hub to the outer shell, an angular spacing separating circumferentially adjacent arms of one pair being different from an angular spacing separating circumferentially adjacent arms of another pair;

[0017] - the casing is configured so that each angular section of the flow vein extending from a leading edge of a first arm to a leading edge of a second arm circumferentially adjacent to the first arm defines an area, and for each section a ratio of a difference between the area and a predetermined reference area to the reference area is greater than or equal to 5% and less than or equal to 10%.

[0018] The invention also relates to a turbomachine comprising an exhaust casing as just described. DESCRIPTION OF FIGURES

[0019] Other characteristics and advantages of the invention will emerge from the following description, which is purely illustrative and non-limiting, and must be read in conjunction with the appended drawings in which:

[0020] [Fig.l]

[0021] [Fig.l] schematically represents a turbomachine;

[0022] [Fig.2]

[0023] [Fig.3]

[0024] [Fig.4]

[0025] Figures 2 to 4 schematically represent exhaust casings according to different embodiments of the invention;

[0026] [Fig.5] [Fig.5] is a schematic representation of an ellipse;

[0027] [Fig.6] [Fig.6] is a schematic representation of a detail of a casing exhaust according to one embodiment of the invention. DETAILED DESCRIPTION OF THE INVENTION Turbomachine - General presentation

[0028] With reference to [Fig.l], a turbomachine is shown schematically, more specifically a double-flow axial turbojet 100. The turbojet 100 illustrated extends along an axis A and successively comprises, in the direction of flow of the gases in the turbomachine, a fan 20, a compression section which may comprise a low-pressure compressor 30 and a high-pressure compressor 40, a combustion chamber 50, and a turbine section which may comprise a high-pressure turbine 60 or a low-pressure turbine 70.

[0029] The blower 20 and the low pressure compressor 30 are driven in rotation by the low pressure turbine 70, while the high pressure compressor 40 is driven in rotation by the high pressure turbine 60.

[0030] In operation, the blower 20 generates a secondary flow C which flows through a secondary vein defined by the nacelle 80.

[0031] The blower 20 also generates a primary flow B which flows through a primary vein located radially further inward than the secondary vein. The flow primary B is compressed by the low and high pressure compressors 30 and 40 before feeding a combustion in the combustion chamber 50, the expansion of the combustion gases of which drives the high and low pressure turbines 60, 70. The last part of the turbomachine crossed by the primary flow is the exhaust casing 90.

[0032] The air propelled by the fan 20 and the combustion gases leaving the turbojet 1 through an exhaust nozzle downstream of the turbines 60, 70 exert a reaction thrust on the turbojet 100 and, through it, on a vehicle or machine such as an aircraft (not shown). Exhaust housing

[0033] In relation to figures 2, 3 and 4 an object of the invention is a turbomachine exhaust casing 1 extending around an axis X.

[0034] The X axis is the axis of the turbomachine around which the gases passing through the turbomachine flow from an upstream part of the turbomachine to a downstream part of the turbomachine. The X axis defines a longitudinal direction, radial directions which radiate from the X axis and circumferential or ortho-radial directions. A circumferential direction is associated with a radial direction, the circumferential direction being perpendicular to the radial direction and to the axial direction.

[0035] The exhaust casing 1 comprises an inner shell 5 and an outer shell 7. The outer shell is radially further from the axis X than the inner shell 5. The inner shell 5 and the outer shell 7 extend in the circumferential direction all around the axis X. In other words, each of the inner shell 5 and the outer shell 7 is annular. In particular, each of the inner shell 5 and the outer shell 7 defines a closed curve around the axis. These closed curves are included for example in a plane transverse to the axis X.

[0036] The inner ferrule 5 and the outer ferrule 7 define between them an air flow vein 3 which surrounds the axis X.

[0037] In particular, this air stream corresponds to the primary stream of the turbomachine, that is to say the air stream which passes through the combustion chamber of the turbomachine.

[0038] The flow vein is delimited on the one hand by an internal vein surface which is included in the internal shell 5 and on the other hand by an external vein surface which is included in the external shell 7.

[0039] One, i.e. at least one, of the inner shell and the outer shell comprises an annular vein surface which delimits the air flow vein. This vein surface extends around the X axis and passes through a first point PI at a first distance DI from the X axis and a second point P2 at a second distance D2 from the X axis, the first distance DI being different from the second distance D2. The second point P2 is circumferentially offset from the first point D1, i.e. the two points PI and P2 are in the same plane perpendicular to the X axis, that is, a plane radial to the X axis so that points PI and P2 correspond to different angular positions around the X axis.

[0040] All the points which form this vein surface are therefore not at the same distance from the X axis.

[0041] At least one of the ferrules among the inner ferrule 5 and the outer ferrule 7 therefore comprises a vein surface which therefore does not have an exactly circular shape. In the example of Figures 2 and 3, it is the inner ferrule 5 which defines a vein surface which passes through the points P1 and P2, the distance D1 being greater than the distance D2.

[0042] The exhaust casing therefore has an air flow stream of which at least one of the stream surfaces is not circular.

[0043] It is not necessary for the exhaust casing to have circular symmetry given the location of this casing downstream of the turbomachine.

[0044] The fact that the casing does not have a circular vein surface creates a degree of freedom. This degree of freedom allows the general shape of the exhaust casing or TRF to be adapted. This degree of freedom allows the size of the casing to be modified in any radial direction.

[0045] In particular, [Fig. 2] illustrates two radial directions H and V perpendicular to each other and perpendicular to the X axis. The direction H is for example intended to be in a horizontal plane when the exhaust casing is mounted in a turbomachine itself mounted on an aircraft. The direction V may be intended to be in a vertical plane in this same situation.

[0046] By reducing the size of the casing in the direction V, the size of the casing along its height can be reduced.

[0047] This allows, for example, more space to be available for positioning a pocket of an arm crossing the vein and thus limiting the protrusion of the pocket into the vein.

[0048] This makes it possible, for example, to locally reduce the diameter of the casing, and consequently the diameter of a nacelle and the height required to install the engine under the wing. There is then a gain in space for the engine installed under the wing.

[0049] Furthermore, this degree of freedom also makes it possible to increase the surface area of ​​the duct in a plane perpendicular to the X axis. With constant operation of the turbomachine upstream of the exhaust casing, the speed of the flow arriving in the casing is reduced by this increase in surface area. The speed of the gas flow or 'Mach level' in the arms crossing the duct is reduced. The increase in surface area, via this reduction in speed, reduces the losses in the exhaust casing but the overall mass increases. These losses increase in fact proportionally to the square of the speed of the gas flow, i.e. an increase in speed produces a increase in structural arm losses equal to the square of the increase in said gas velocity.

[0050] This degree of freedom makes it possible to seek and obtain a better compromise between the aerodynamic performance of the turbomachine and its mass.

[0051] To summarize, the invention makes it possible to no longer be constrained by a cylindrical shape, which makes it possible to increase the surface area available for the gases to flow compared to a circular exhaust casing. This makes it possible to improve the thermodynamic performance of the propulsion unit, and in particular to improve the quantity of kerosene necessary to provide a certain thrust to the engine, a parameter also known by the English expression "Specify Fuel Consumption" abbreviated to SFC. This parameter makes it possible to compare engines with each other regardless of their size or the altitude at which they fly.

[0052] Advantageously, the ratio of a difference between the first distance (D1) and the second distance (D2) to the first distance (D1) is greater than or equal to 5% and less than or equal to 40%.

[0053] In other words, the ratio | D1-D2 | / Dl is between 5% and 40%. The expression | D1-D2 | is the absolute value of the difference between the first distance Dl and the second distance D2.

[0054] This ratio quantifies the deviation of the non-circular vein surface from a vein surface shape that would be circular. The larger the ratio, the greater the deviation. The gain in space in the direction where the vein surface is closer to the axis compared to the circular shape is all the greater as the ratio is large.

[0055] Ideally this ratio is between 5% and 40%. Elliptical vein surface

[0056] In a first embodiment, the non-circular vein surface passes through a plurality of points located on an ellipse centered on the axis. In particular, the plurality of points is located in the same radial plane, i.e. a plane orthogonal to the X axis.

[0057] [Fig.5] shows an illustration of an ellipse referenced 10.

[0058] The ellipse is a plane shape characterized by:

[0059] - a semi-major axis referenced “a” and located along the axis referenced “x”,

[0060] - a semi-minor axis referenced “b” and located along the axis referenced “y”, and

[0061] - a center O at the intersection of the axes referenced x and y, and

[0062] - an “a / b” ratio of the semi-major axis to the semi-minor axis.

[0063] The general equation of such an ellipse in the Cartesian plane (O,x,y) is written (x / a)2+ (y / b)2=l.

[0064] The plurality of points relating to the first embodiment may in particular comprise the points PI and P2 mentioned previously.

[0065] In this case, the ellipse is non-circular. The plurality of points extends circumferentially around the axis or only over an angular sector depending on the needs for adjusting the shape of the exhaust casing.

[0066] The ellipse may have a semi-major axis and a semi-minor axis in the H and V directions mentioned above.

[0067] It should be noted that the first distance DI and the second distance D2 may correspond to the semi-major axis and the semi-minor axis. In this case, when the ratio of the difference between the first distance DI and the second distance D2 to the first distance DI is greater than or equal to 5% and less than or equal to 40%, then the ratio (a / b) of the semi-major axis to the semi-minor axis is greater than or equal to 1.05 and less than or equal to 1.40.

[0068] According to a first variant of this embodiment, the non-circular vein surface has at least in part a polygonal shape, the plurality of points forming vertices of the polygonal shape.

[0069] In the example of [Fig.4], the internal shell 5 has a polygonal shape whose vertices P3, P4 and P5 have been referenced. The polygonal shape extends here all around the X axis. The different vertices of the polygonal shape are located on an ellipse whose semi-major axis is located in the axis referenced H in [Fig.4]. The semi-minor axis is located in the axis referenced V in [Fig.4]. It is not necessary for a vertex of the polygon to be located along the semi-major axis or the semi-minor axis.

[0070] The polygonal shape may have points that are regularly angularly spaced. That is, for each pair of adjacent points, the points of the pair are separated by an angular spacing equal to a predetermined spacing.

[0071] According to a second variant of this embodiment, the non-circular vein surface at least partly follows a shape of the ellipse.

[0072] In other words, on an angular sector corresponding to a non-zero angle, the angular sector having its vertex on the X axis and defined in a plane perpendicular to the X axis, the non-circular vein surface follows the shape of the ellipse.

[0073] In the example of figures 2 and 3, the internal ferrule 5 has an elliptical shape which extends all around the X axis. The angular sector here extends over 360°.

[0074] The semi-major axis of the ellipse is located in the axis referenced H in Figures 2 and 3. The semi-minor axis is located in the axis referenced V in Figures 2 and 3.

[0075] As mentioned previously, at least one of the inner ferrules and the outer ferrule defines a non-circular vein surface in a polygonal shape.

[0076] In the case where these two ferrules each define a polygonal non-circular vein surface, then they can both be implemented according to one of the variants of the first embodiment with an angular sector which extends over 360°.

[0077] In other words, each vein surface can have, all around the X axis, a polygonal shape defined by vertices located on an ellipse (first variant) or an ellipse shape (second variant). The ellipse of the internal surface defined by the internal shell or first ellipse is then of smaller dimensions than the ellipse of the external surface defined by the external shell or second ellipse.

[0078] Preferably, the main directions of the first ellipse and the second ellipse are chosen to be identical. In other words, each ellipse has a semi-major axis in the same first direction and each ellipse has a semi-minor axis in the same second direction.

[0079] In this situation, the elliptical shapes of the inner ferrule and the outer ferrule are stretched in the same first direction relative to a circular shape, or equivalently compressed in the same second direction relative to a circular shape.

[0080] This situation reinforces the possibilities of adjusting the shape of the exhaust casing so as to limit, for example, the size in the first direction.

[0081] We can define:

[0082] - an internal shell ratio between the first semi-major axis on the first semi- minor axis of the first ellipse, and

[0083] - an external shell ratio between the second semi-major axis on the second semi-minor axis of the second ellipse.

[0084] The internal shell ratio as well as the external shell ratio can be chosen greater than or equal to 1.05 and less than or equal to 1.40.

[0085] Advantageously, when the first ellipse and the second ellipse have a semi-major axis in the same direction, the external shell ratio can be chosen to be greater than or equal to 85% of the internal shell ratio and less than or equal to 97% of the internal shell ratio. In other words, the fraction of the external shell ratio divided by the internal shell ratio is greater than or equal to 85% and less than or equal to 97%.

[0086] In this case, the air flow vein which extends from the inner shell to the outer shell has a vein height which is approximately constant around the X axis.

[0087] The vein height is defined, in the radial direction, for each angular position around the axis, as the distance separating the inner shell from the outer shell in the radial direction corresponding to the angular position.

[0088] A roughly constant vein height around the X axis means that for each angular position the deviation of the vein height from a reference value is less than a certain threshold.

[0089] An interesting case of the second variant corresponds to the case where the vein surface of the internal ferrule 5 comprises a portion of a first ellipse and the vein surface of the external ferrule 7 comprises a portion of a second ellipse, the first ellipse being different from the second ellipse. Vein surface with two concentric ellipses

[0090] As presented previously, there is a first embodiment of the invention according to which the non-circular vein surface passes through a plurality of points located on an ellipse centered on the axis. This first mode can be implemented according to a first “polygonal” variant where the non-circular vein surface follows at least in part a polygonal shape defined by vertices located on an ellipse and a second “continuous” variant where the non-circular vein surface follows at least in part an elliptical shape.

[0091] A second embodiment is now presented depending on the first embodiment, optionally implemented according to the first variant or the second variant, and according to which the plurality of points is a first plurality of points and the ellipse is a first ellipse, the non-circular vein surface which passes through the first point P1 and the second point P2 also passes through a second plurality of points located on a second ellipse centered on the axis, the second ellipse being different from the first ellipse.

[0092] In this second embodiment, another degree of freedom is therefore added, corresponding to the second plurality of points, allowing further adjustment to the shape of the non-circular vein surface. This allows even finer adjustment of the size of the exhaust casing.

[0093] The non-circular vein surface has, for example, two distinct zones, so that:

[0094] - in the first zone the non-circular vein surface passes through the first plurality of points and the shape of the non-circular vein surface in the first area depends on the first plurality of points, and

[0095] - in the second zone the non-circular vein surface passes through the second plurality of points and the shape of the non-circular vein surface in the second area depends on the second plurality of points.

[0096] The adjustment of the shape of the non-circular vein surface may relate in particular to the shape in the first zone independently of the adjustment of the shape in the second zone.

[0097] An interesting case of the second embodiment is the case where the vein surface of the inner ferrule 5 and / or the vein surface of the outer ferrule 7 comprises a first section which corresponds to a section of a first ellipse and a second section which corresponds to a section of a second ellipse, the first ellipse being different from the second ellipse.

[0098] The section of a vein surface is understood here as an angular sector of the surface taken in a radial plane. The ellipses mentioned here are centered on the axis X.

[0099] The two distinct zones can for example be chosen as separated by the horizontal plane H described previously.

[0100] In a plane perpendicular to the X axis, it is possible to define two radial directions H and V perpendicular to each other. The direction H is intended to be in a horizontal plane when the exhaust casing is mounted in a turbomachine itself mounted on an aircraft. The direction V is intended to be in a vertical plane in this same situation.

[0101] As an option of the second embodiment, the first plurality of points and the second plurality of points are located on two different sides of the horizontal plane H.

[0102] The adjustment of the shape of the non-circular vein surface can then be carried out independently on one side of the horizontal plane and on the other side of the horizontal plane.

[0103] For example, the first ellipse and the second ellipse can be chosen with the same semi-major axis in the horizontal direction and different semi-minor axes but both in the vertical direction.

[0104] It is then possible to choose the first ellipse which governs the shape of the non-circular vein surface above the horizontal plane (or upper part) with a larger semi-minor axis, and the second ellipse which governs the shape of the non-circular vein surface below the horizontal plane (or lower part) with a smaller semi-minor axis.

[0105] Such a configuration allows both room for maneuver for the passages of services in the upper part of the casing (above the horizontal plane relative to the vertical direction) and less space requirement on the lower part side (below the horizontal plane relative to the vertical direction).

[0106] It is possible to implement this option in a first variant according to which the non-circular vein surface has a polygonal shape, the first plurality of reference points and the second plurality of reference points forming vertices of the polygonal shape.

[0107] It is possible to implement this option in a second variant according to which the non-circular vein surface has a shape according to the first ellipse on one side of the horizontal plane and a shape according to the second ellipse on the other side. Arm in the flow vein

[0108] The exhaust casing includes within it the flow stream defined by the inner shell and the outer shell.

[0109] The housing may comprise a hub 2 which defines the internal ferrule.

[0110] The casing comprises a plurality of arms 4, each arm extending across the vein flow from the hub to the outer shell.

[0111] The arms provide mechanical support for the structure of the exhaust casing and the passage of utilities. The arms are aerodynamically profiled so as to orient the direction of the gas flow and limit energy losses linked to the flow of gases.

[0112] Each arm is directed substantially radially in a plane transverse to the X axis. The arm may be slightly inclined relative to a purely radial direction. In any case, it is possible to define an angular distance between two adjacent arms in a plane transverse to the X axis.

[0113] The housing may be configured so that the angular spacing between two adjacent arms varies from one pair of adjacent arms to another.

[0114] In this case, an angular gap separating adjacent arms of one pair is different from an angular gap separating adjacent arms of another pair.

[0115] This degree of freedom allows the arms to be distributed in a non-uniform or irregular manner. By playing on the angular gap separating two adjacent arms, it is possible to optimize the distribution of work per arm, that is to say the distribution of aerodynamic work to the importance of the air flow deflected by each arm.

[0116] This method is advantageous because it allows the distribution to be adjusted so that:

[0117] - the arms of the exhaust casing which generate a lot of losses, such as for example, thick arms or arms with pocketing, correspond to less work, and

[0118] - the arms of the exhaust casing which generate less losses, such as for example thin arms correspond to more important work.

[0119] The work of an arm referenced N depends in particular on the surface area of ​​the angular section of the flow vein which extends from the leading edge of this arm N to the leading edge of the adjacent arm following N+1.

[0120] It may be advantageous for each angular section of the flow vein extending from a leading edge of a first arm to a leading edge of a second arm adjacent to the first arm to define an area, and for each section a ratio of a difference between the area and a predetermined reference area to the reference area is greater than or equal to 5% and preferably less than or equal to 10%.

[0121] In this way, the surface area of ​​the angular sections varies little from one angular section to another around the X axis.

[0122] In relation to [Fig.6], the arms '4N', '4N+1' and '4N+2' extend radially from the inner ferrule 5 to the outer ferrule 7.

[0123] Each arm, such as for example the '4N+1' arm of [Fig.6], has a leading edge 41 on the upstream side and a trailing edge 42 on the downstream side, with reference to the general direction of the flow of gases through the turbomachine.

[0124] Between the arms '4N' and '4N+1' a first angular sector defines a first su- perficie SI.

[0125] Between the arms '4N+1' and '4N+2' a second angular sector defines a second surface area S2.

[0126] These areas SI and S2 are close to a reference area SR so that:

[0127] - the ratio | Sl-SR | / SR is greater than or equal to 5% and preferably less than or equal to 10%, and

[0128] - the ratio | S2-SR | / SR is greater than or equal to 5% and preferably less than or equal to 10%.

[0129] Finally, the invention relates to a turbomachine comprising an exhaust casing such as has just been presented and an aircraft comprising such a turbomachine.

Claims

Claims

1. A turbomachine exhaust casing (1) extending around an axis (X), the exhaust casing (1) comprising an inner shroud (5) and an outer shroud (7) centered on the axis (X), the outer shroud (7) being radially further from the axis (X) than the inner shroud (5), the inner shroud (5) and the outer shroud (7) each being annular and defining between them an air flow duct (3) surrounding the axis (X), the inner shroud (5) and the outer shroud (7) each comprising an annular duct surface which delimits the air flow duct, the exhaust casing being characterized in that at least one of the duct surfaces extends around the axis (X) passing through a first point (PI) at a first distance (Dl) from the axis (X) and a second point (P2) circumferentially offset from the axis (X). first point (Dl) and which is located at a second distance (D2) from the axis (X), the first distance (Dl) being different from the second distance (D2),the inner ferrule (5) having at least in part around the axis (X) a shape of an ellipse or a polygonal shape defined by vertices located on an ellipse, the ellipse being defined by an inner ferrule ellipse ratio calculated between a first major axis of the inner ferrule (5) on a first minor axis of the inner ferrule (5), the outer ferrule (7) having at least in part around the axis (X) a shape of an ellipse or a polygonal shape defined by vertices located on an ellipse, the ellipse being defined by an outer ferrule ellipse ratio calculated between a second major axis of the outer ferrule (7) on a second minor axis of the outer ferrule (7), the inner ferrule ellipse ratio and the outer ferrule ellipse ratio each being between 1.05 and 1.40.,

2. An exhaust housing (1) according to claim 1 wherein a ratio of a difference between the first distance (D1) and the second distance (D2) to the first distance (D1) is greater than or equal to 5% and less than or equal to 40%.

3. Exhaust casing (1) according to claim 1 or 2 wherein the vein surface passing through the first point (P1) and the second point (P2) further comprises a plurality of other points, the first point (P1), the second point (P2) and the other points being located on an ellipse centered on the axis (X).

4. An exhaust casing (1) according to claim 3 wherein the surface of vein passing through the first point (PI) and the second point (P2) has at least in part a polygonal shape, the plurality of points forming vertices of the polygonal shape.

5. An exhaust casing (1) according to claim 4 wherein the outer shell ellipse ratio is between 85% and 97% of the inner shell ellipse ratio.

6. An exhaust casing (1) according to any one of claims 1 to 5, wherein the vein surface of the inner shroud (5) comprises a portion of a first ellipse and the vein surface of the outer shroud (7) comprises a portion of a second ellipse, the first ellipse being different from the second ellipse.

7. Exhaust casing (1) according to any one of the preceding claims wherein the vein surface of the inner ferrule (5) and / or the vein surface of the outer ferrule (7) comprises a first portion which corresponds to a portion of a first ellipse and a second portion which corresponds to a portion of a second ellipse, the first ellipse being different from the second ellipse.

8. An exhaust casing (1) according to one of the preceding claims, comprising a hub (2) defining the inner shroud (5), the exhaust casing comprising a plurality of arms (4), each arm (4) extending through the flow path from the hub (2) to the outer shroud (7), an angular gap separating circumferentially adjacent arms (4) of one pair being different from an angular gap separating circumferentially adjacent arms (4) of another pair.

9. An exhaust casing (1) according to claim 8 configured so that each angular section of the flow stream extending from a leading edge of a first arm (4) to a leading edge of a second arm (4) circumferentially adjacent to the first arm (4) defines a surface area, and for each section a ratio of a difference between the surface area and a predetermined reference surface area to the reference surface area is greater than or equal to 5% and less than or equal to 10%.

10. A turbomachine comprising an exhaust casing (1) according to any one of claims 1 to 9.

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