Improved variable-timing stator and method using such a stator

Abstract

Improved Variable-Pitch Stator and Method Using Such a Stator: Turbomachine stator (10) extending about a central axis (X) and comprising an inner wall (11) and an outer wall (12) defining an annular groove (V), a plurality of variable-pitch blades (13), each blade (13) extending radially in the annular groove between a base (132) movably fixed to the outer wall (12), and a free end (134), the inner wall (11) comprising a main profile (110) and at least one circular platform (20) arranged opposite the free end (134) of one of the blades, the platform (20) comprising a main surface (210) and at least one longitudinal groove (22), the platform (20) being rotatable relative to the main profile (110) between a first position in which a main axis (P) of the groove (22) is substantially parallel to the central axis (X),and at least one second position in which the principal axis (P) is transverse to the central axis (X). Figure for the abbreviation: Fig. 5.

Description

Title of the invention: Improved variable timing stator and method using such a stator Technical field

[0001] The present disclosure relates to the field of variable-pitch stators in shrouded aeronautical turbomachines, in particular compressor inlet steering wheel stators, but not only. Prior art

[0002] In a known manner, a turbomachine stator comprises fixed blades extending radially between an inner wall (the hub) and an outer wall (the casing). The inner wall and the outer wall delimit between them an annular flow path for the gases from upstream to downstream of the engine. These stator blades can be variable-pitch, allowing each of them to pivot about a radial axis to modify their angle of incidence. Such a stator with variable-pitch blades can, for example, be an inlet guide wheel (RDE) of high- or low-pressure compressors.

[0003] These variable-pitch vanes are, for example, arranged on rotating platforms with a pivot on the casing and make it possible to direct the flow at the compressor inlet so that it is aligned with the leading edge of the moving wheel located downstream. The opposite end of the vanes is a free end which is not in contact with the wall of the hub. These variable-pitch stator vanes are therefore cantilevered.

[0004] Variable pitch blades are generally movable between an aligned position and a stalled position. In the aligned position, the angle of incidence, which can be defined as the angle between the blade chord and the engine axis, is zero or small, for example less than 20°. This position allows the blades to be aligned with the upstream flow. In the stalled position, the angle of incidence is high, for example greater than 40°. This position allows the flow arriving at the downstream moving wheel to be reoriented. However, changing the angle of incidence of the variable pitch blades has consequences on the topology of the flow, in particular involving secondary flows that generate pressure distortion. These unwanted secondary flows must be managed.

[0005] In the aligned position in particular, the pressure differential between the intrados and the extrados is low and little flow crosses the space existing between the free end of the blade and the surface of the hub, which generates few vortices, and therefore few pressure losses. However, the free ends of the blades, close to the wall of the hub, can undergo a flow of the corner separation type, often caused by a through flow, which is a flow close to the hub wall starting at the intrados leading edge of the blade and impacting further downstream towards the trailing edge of the adjacent blade, on the extrados side. A corner separation can generate a vortex on the extrados face close to the trailing edge of the blade, which can generate pressure losses.

[0006] In the wedged position, the pressure gradient between the intrados and the extrados is high. This can create a transverse flow at the level of the spacing between the free end of the blade and the wall of the hub, which can generate clearance flows, and in particular a vortex called a clearance vortex, and therefore pressure distortions likely to impact the downstream moving wheel and promote the triggering of the pumping phenomenon.

[0007] To manage these secondary flows, it is known to use surface treatment type technologies, consisting of local modifications in the wall of the hub or the casing delimiting the air flow path. For example, it is possible to arrange fins on the wall of the hub between two adjacent variable pitch blades, in order to limit the consequences linked to the passage flow and to corner separation when the blades are in the aligned position. Document FR3106614 A1 describes an example of a fin for managing corner separations. These fins nevertheless have a negative impact on the performance of the stator when the blades are in the stalled position.

[0008] Furthermore, grooves may be made in the hub wall to limit the consequences of the appearance of clearance vortices and pressure distortions when the blades are in the wedged position. However, although they are useful for large wedge angles, such grooves are less useful when the blades are in the aligned position, and are even likely to degrade the performance of the stator, by generating unwanted vortices.

[0009] In other words, although these technologies present advantages for certain types of flow, they involve a degradation of the stator performance for other types of flow, and are therefore difficult to integrate.

[0010] There is therefore a need for a system making it possible to at least partially overcome the aforementioned drawbacks, and making it possible to improve the performance of the pitched stator regardless of the position of the blades, aligned or pitched, without degrading its performance when they change position. Statement of the invention

[0011] The present disclosure relates to a turbomachine stator extending around a central axis and comprising: - an internal wall and an external wall delimiting an annular vein, - a plurality of variable-pitch blades, each blade extending radially in the annular vein between a base movably fixed to one of the inner wall or the outer wall, and a free end, - the other of the internal wall or the external wall comprising a main profile and at least one circular platform arranged opposite the free end of one of the blades, the platform comprising a main surface and at least one longitudinal groove, the platform being movable in rotation relative to the main profile between a first position in which a main axis of the groove is substantially parallel to the central axis, and at least a second position in which the main axis is transverse to the central axis.

[0012] In the present disclosure, the terms “axial direction”, “radial direction” and their derivatives are defined relative to the central axis of the stator, which is also the central axis of the turbomachine, around which the moving parts (the rotors) thereof rotate. Preferably, the stator is generally axisymmetric around the central axis. Similarly, the terms “upstream” and “downstream” are understood relative to the normal flow direction of the gases in a turbomachine, from the inlet to the outlet nozzle, passing successively through the compressors, the combustion chamber and the turbines.

[0013] It is understood that the stator vanes are fixed to one or the other of the radially inner wall and the radially outer wall, in a movably manner by being configured to pivot relative to this wall about a radial axis. The end of the vanes opposite the end which is fixed to one of the inner or outer wall, is a free end, which is not in contact with the other of the inner or outer wall. A space therefore exists between this wall and the free end.

[0014] The platform is a portion of the inner or outer wall, this portion being movable relative to the rest of the inner or outer wall. The platform, which is circular, is in particular capable of pivoting about a radial axis passing through the center of the platform. It is thus understood that the platform, in particular its main surface, is in line with the main profile of the inner or outer wall, the main profile comprising the surface portion of the inner or outer wall other than the platform, delimiting the annular air flow vein.

[0015] The main surface of the platform is in other words the reference surface of the platform, in the absence of grooves / recesses or projections / protrusions. It is thus understood that the longitudinal groove is a portion of the platform which is hollowed out relative to its main surface, forming a slot comprising a first dimension, its length, greater than a second dimension, its width. The main axis of the groove is an axis parallel to its length.

[0016] In the first position, the main axis of the groove is substantially parallel to the central axis, for example parallel to the central axis to within plus or minus 2°. thus understands that in the first position, the groove extends mainly in the axial direction of the turbomachine. In the second position, the main axis is transverse to the central axis, forming for example an angle greater than 45° with respect to the latter.

[0017] It is thus possible to modify the orientation of the groove relative to the central axis and therefore relative to the flow of incident gases, by pivoting the platform between the first and second positions. The longitudinal groove thus has a more or less significant influence on the flow depending on its orientation, which can thus be adapted according to the flight conditions and the pitch angle of the stator blades. In particular, when the blades are in the pitched position, the groove in the second position makes it possible to limit the consequences of the appearance of clearance vortices and pressure distortions. Conversely, when the blades are in the aligned position, the groove can be arranged in the first position, in which it is aligned with the main flow and does not disturb the latter or only slightly. This device thus makes it possible to improve the performance of the stator in the pitched position, without degrading it in the aligned position.The overall performance of the turbomachine is therefore improved.

[0018] In some embodiments, in the second position, the main axis of the groove is perpendicular to the central axis. This makes it possible to maximize the effect of mitigating the consequences linked to the appearance of clearance vortices and pressure distortions by creating a hollow channel between the intrados and the extrados of the stator blades.

[0019] In some embodiments, the stator is configured such that when a pitch angle between a chord of the blade at the free end and the central axis is between 0 and 20°, the platform is disposed in the first position, and when the pitch angle is between 40 and 80°, the platform is disposed in the second position.

[0020] In other words, an aligned position of the blades corresponds to a situation in which the pitch angle is between 0 and 20°, preferably less than 10°, and a wedged position of the blades corresponds to a situation in which the pitch angle is between 40 and 80°, preferably equal to 60°. The mobile platform thus makes it possible to adapt the orientation of the groove and its influence on the flow, efficiently depending on the pitch angle of the blades.

[0021] In some embodiments, the inner wall comprises the platform and, in the first position, the major surface of the platform is flush with the major profile of the inner wall.

[0022] By "flush", we understand that the main surface of the platform is in continuity with the main profile of the internal wall, without positive or negative difference in level, that is to say without steps and without recess. Taking into account the annular shape of the stator, the radially inner wall of the stator, delimiting the annular vein, has a curved shape. It is understood that the main surface of the platform therefore has the same curved shape, and consequently the same radius of curvature, as the main profile of the inner wall. Thus, in the first position, the main surface of the platform is integrated and merged with the main profile of the inner wall.

[0023] This configuration makes it possible to limit the impact of the mobile platform on the incident flow when the blades are in the aligned position, the groove being furthermore aligned with the central axis and said flow.

[0024] In some embodiments, the platform comprises at least one longitudinal fin projecting from the main surface, extending between a first rim of the platform and a lateral side of the at least one groove, and having an elongated shape in the direction of the main axis of said groove.

[0025] Unlike the groove forming a recess relative to the main surface of the platform, the fin forms a protuberance relative to said main surface. It is understood that the fin extends mainly longitudinally, that is to say along an axis substantially parallel to the main axis of the groove, being arranged on one side thereof.

[0026] Consequently, when the platform is in the first position, corresponding to a situation in which the blades are in an aligned position, the fin is located between two adjacent blades, and extends mainly axially along the central axis. This fin constitutes a “wall” blocking the transverse flows (the passing flows) which connect the intrados of a blade to the extrados of the adjacent blade, making it possible to limit the negative consequences linked to corner separation when the blades are in an aligned position.

[0027] When the platform is pivoted to the second position, the fin also pivots and is no longer between the two adjacent blades, but upstream or downstream of a blade, depending on the direction of rotation of the platform. Given the convex shape of the inner wall and the platform, the height at which the fin projects from the main profile of the inner wall is thus reduced when moving to the second position. It is thus possible to improve the performance of the stator when the blades are in the aligned position thanks to the presence of the fin between the adjacent blades, while limiting the impact of this fin when the blades are in the wedged position.

[0028] In some embodiments, the fin comprises an inclined plane extending from a base of the main surface to an apex, the base of the inclined plane being adjacent to the lateral side of the groove and the apex being adjacent to the first rim of the platform, such that in the first position, the apex projects relative to the main profile of the inner wall, and in the second position, the summit is flush with the said main profile.

[0029] It is understood that the base of the inclined plane is located at the main surface of the platform, close to the groove, the inclined plane rising relative to the main surface away from the groove, up to the top. In the first position, the fin projects from both the main surface of the platform and the main profile of the inner wall, thus limiting the negative consequences linked to corner separation when the blades are in the aligned position.

[0030] In the second position, the top of the inclined plane is flush with the main profile of the inner wall. Therefore, the fin projects from the main surface of the platform but is arranged below the level of the surface of the main profile of the inner wall. This limits its impact on the flow when the blades are in the stalled position.

[0031] Furthermore, the inclined plane constitutes a gentle slope, connecting the main profile of the internal wall to the main surface of the platform. Thus, when in the second position, the fin is on the downstream part of the platform in the direction of gas flow, the inclined plane constitutes an upward slope which connects to the main profile of the internal wall, which makes it possible to limit the upward step effects and therefore to limit the losses on the downstream part of the blade.

[0032] In certain embodiments, in the second position, a second rim of the platform disposed at one end of the platform diametrically opposite the first rim, is recessed relative to said main profile.

[0033] It is understood that the main surface comprises the second rim, which is flush with the main profile of the inner wall when the platform is in the first position, and which is recessed relative to said main profile when the platform is in the second position. In other words, the recess is generated by the difference in height between the main profile of the inner wall and the second rim, creating a step. The convex shape of the inner wall and the platform thus makes it possible, when the platform rotates, to create this recess and thus provide an additional groove, in addition to the longitudinal groove machined in the platform.

[0034] This surprising effect is all the more advantageous when, in the second position of the platform, the second rim is arranged on an upstream side of the platform. Indeed, the additional groove thus created is arranged at the level of the leading edge of the blades in the wedged position, making it possible to limit the dynamics of creation of clearance vortices close to the leading edge of the blades, and thus to further improve the performance of the stator.

[0035] In some embodiments, in the second position, third and fourth flanges of the platform, located on one side of the longitudinal ends of the groove, protrude relative to the main profile.

[0036] Given the convex shape of the internal wall and the platform, the third and fourth edges of the platform located on the side of the longitudinal ends of the groove, that is to say the edges located at the upstream and downstream ends of the platform along the central axis are flush with the platform in the first position, and are raised relative to the main profile of said platform in the second position, after a rotation (for example of 90°) of the latter.

[0037] The presence of these projections makes it possible, in addition to the longitudinal groove oriented transversely relative to the central axis, to contribute to reducing the dynamics of the clearance vortices, and therefore the size of these vortices and their negative effect on the performance and operability of the stator.

[0038] In some embodiments, the stator comprises at least three longitudinal grooves parallel to each other.

[0039] It is thus understood that the main axes of the three grooves are parallel to each other. Similarly, the main axis of each of the grooves is substantially parallel to the central axis in the first position, and transverse, preferably perpendicular, to the central axis in the second position. The presence of at least three grooves makes it possible to limit even more effectively the consequences of the appearance of clearance vortices and pressure distortions, and thus to further improve the performance of the stator in the wedged position, without degrading it in the aligned position.

[0040] In some embodiments, the stator is an inlet steer wheel of a turbomachine compressor, the inner wall is a hub and the outer wall is an outer casing.

[0041] The present disclosure also relates to a turbomachine comprising a stator according to any one of the preceding embodiments.

[0042] The present disclosure also relates to a method of modifying the pitch of the blades of a variable pitch stator according to any one of the preceding embodiments, the method comprising detecting the pitch angle of the blades and, when the blades are in an aligned position, positioning the platform in the first position and when the blades are in a pitched position, positioning the platform in the second position. Brief description of the drawings

[0043] The invention and its advantages will be better understood upon reading the detailed description given below of different embodiments of the invention given as non-limiting examples. This description refers to the appended pages of figures, in which:

[0044] [Fig-1] [Fig.l] represents a schematic view in longitudinal section of a tur bomachine,

[0045] [Fig.2] [Fig.2] represents a partial view in a half section plane parallel to the central axis, of a variable-pitch stator according to one embodiment,

[0046] [Fig.3] [Fig.3] represents a radial section of a variable pitch stator blade, at its free end, in an aligned position (left) and in a pitched position (right),

[0047] [Fig.4] [Fig.4] is a top view of a moving platform of the stator of [Fig.2],

[0048] [Fig.5] [Fig.5] is a perspective view of the platform in the first position, in a bird's eye view from upstream,

[0049] [Fig.6] [Fig.6] represents the platform of [Fig.5] in a front view from upstream,

[0050] [Fig.7] [Fig.7] represents the platform of [Fig.5] in a side view,

[0051] [Fig.8] [Fig.8] is a perspective view of the platform in the second position, in a bird's eye view from upstream,

[0052] [Fig.9] [Fig.9] represents the platform of [Fig.8] in a front view from upstream,

[0053] [Fig. 10] [Fig. 10] represents the platform of [Fig.8] in a side view. Description of the embodiments

[0054] An embodiment of the present disclosure will be presented with reference to Figures 1 to 10.

[0055] The terms “upstream” and “downstream” are subsequently defined with respect to the direction of flow of gases through a turbomachine, indicated by the arrow F in [Fig. 1] and the following. Furthermore, the terms “internal” and “external” are considered in a radial direction R perpendicular to the central axis X of the turbomachine. Thus, the external wall 12 of the stator 10, for example, is further from the central axis X than its internal wall 11 in the radial direction R.

[0056] [Fig. 1] illustrates a dual-flow turbomachine 100 comprising, in a known manner from upstream to downstream, successively at least one fan S, at least one low-pressure compressor stage 1, high-pressure compressor stage 2, a combustion chamber 3, at least one high-pressure turbine stage 4 and low-pressure turbine stage 5. The rotors of the compressors 1, 2, the turbines 4, 5 and the fan S rotate around the central axis X of the turbomachine 100.

[0057] The stator 10 considered in the following example is an inlet guide wheel of the high-pressure compressor 2, arranged upstream of the latter and making it possible to orient the flow F entering this compressor, in particular to align it with the leading edge of the mobile wheel of the first stage of the high-pressure compressor 2. This example is however not limiting, the invention also applying to any stator including variable pitch blades.

[0058] The stator 10 comprises a hub 11 (the inner wall) and a casing 12 (the outer wall) delimiting between them an annular vein V in which the gas flow F flows. More precisely, the annular vein V is delimited by a main profile 110 of the hub 11 (the radially outer wall of the hub 11) and a main profile 120 of the casing 12 (the radially inner wall of the casing 12), the surfaces of the profiles 110, 120 being in contact with the gas flow F. The stator 10 further comprises a plurality of variable-pitch vanes 13 (only one is visible in the figures) extending in the vein V and being distributed circumferentially around the central axis X.

[0059] As illustrated in [Fig. 2], representing a partial view of the stator 10 in a half-section plane parallel to the central axis X, each blade 13 is fixed to the casing 12 and is not in contact with the hub 11 (unlike [Fig. 1] representing the reverse configuration). It will be noted that this example is not limiting, the invention also applying to a situation in which the blades 13 are fixed to the hub 11 and are not in contact with the casing 12.

[0060] More specifically, each blade 13 comprises a base 132 at a radially outer end, movably attached to the casing 12. The base 132 is a rotating platform with a pivot to the casing 12, and is capable of pivoting about a radial direction R to adjust the pitch angle of the blade 13. Each blade 13 further comprises a leading edge 136, a trailing edge 138, an extrados 131 and a intrados 133. Furthermore, the radially inner end of the blades 13 is a free end 134. A space E exists between the free end 134 and the surface of the main plane 110 of the hub 11. The thickness of the space E is between 0.1 and 5 mm. It will be noted that the space E is not necessarily constant and can vary between the leading edge 136 and the trailing edge 138, and also depending on the pitch angle of the blade 13.

[0061] [Fig. 3] represents a section of a blade 13, at the free end 134, in a left-aligned position, and in a right-set position. The setting angle [3 is the angle formed between the central axis X and the direction G of the chord of the blade 13 at this section, the chord being the straight line connecting the leading edge 136 and the trailing edge 138. The aligned position of the blade 13 corresponds to a situation in which the setting angle [3 is between 0 and 20°, preferably less than 10°, and the set position corresponds to a situation in which the setting angle [3 is between 40 and 80°, preferably equal to 60°.

[0062] Furthermore, the hub 11 comprises a plurality of platforms 20 (distinct from the platforms constituting the rotating bases 132 of the blades 13), movable relative to the main profile 110 of the hub 11. It will be noted in this regard that the main profile 110 is a fixed portion of the hub 11 and of the stator 10 and represents the major part of the surface of the hub 11 (excluding platforms 20) in contact with the gas flow F. It has a curved shape, in other words convex with respect to the central axis X. This curved shape of the main profile 110 is visible in particular in figures 6 and 9.

[0063] The platforms 20 are rotatable between a first position and at least one second position. The first position of the platforms 20 is shown in Figures 4 to 7, and is adopted when the blades 13 are in the aligned position. The second position of the platforms 20 is shown in Figures 8 to 10, and is adopted when the blades 13 are in the wedged position.

[0064] A platform 20 in the first position will be described with reference to FIGS. 2 and 4 to 7, in which a single platform 20 is visible, it being understood that a platform 20 is arranged opposite each blade 13.

[0065] In [Fig. 2] the platform 20 is shown in section in a simplified manner, without showing any irregularities in its main surface 210, such as grooves or fins described below. The platform 20 is integrated into the wall of the hub 11. In the configuration shown in [Fig. 2], its main surface 210 is flush with the main profile 110 of the hub 11.

[0066] The main surface 210 of the platform 20 corresponds to the upper face of the platform 20 in contact with the gas flow, and to the so-called “regular” portions of the platform 20, not comprising a protuberance such as a fin or a recess such as a groove. The main surface 210 thus has a profile identical to the main profile 110 of the hub 11. In other words, the main surface 210 has a curvature identical to the curvature of the main profile 110 of the hub 11, such that in the first position, the main surface 210 is completely integrated into the main profile 110 and in the continuity of the latter, as shown in [Fig. 6] in particular.

[0067] It is understood that the number of platforms 20 that the hub 11 comprises is equal to the number of blades 13 that the stator 10 comprises, a platform 20 being arranged opposite the free end 134 of each blade 13. The diameter of a platform 20 has a value between 30 and 100% of the length of the chord of the blade 13 at its free end 134. For example, for a chord between 30 mm and 100 mm, the diameter of the platform 20 can be between 30 mm and 70 mm.

[0068] A platform 20 is shown in [Fig. 4] in a top view according to its center of rotation, that is to say the radial direction R, the blade 13 being masked. The platform 20 is circular with an axis of rotation coaxial with the radial direction R or parallel to it. In other words, the platform 20 rotates around the same axis of rotation as the blade 13, or an axis parallel but close to the latter.

[0069] To clarify the description, the circumference of the platform 20 is virtually and conventionally divided into four edges, a first edge A, a second rim B diametrically opposite the first rim, a third rim C and a fourth rim D diametrically opposite the third rim C.

[0070] In this example, the platform 20 comprises three longitudinal grooves 22. This example is however not limiting, the platform 20 being able to comprise a greater or lesser number of grooves 22. A groove 22 is a slot hollowed out in the platform 20, which therefore constitutes a recess relative to the main surface 210.

[0071] Each groove 22 extends mainly along a main axis P, which is a longitudinal direction of the groove, between the third rim C and the fourth rim D. Thus, each groove comprises two lateral sides 222 extending longitudinally parallel to the main axis P, and two longitudinal ends 224 extending perpendicular to the lateral sides and having a length less than the latter. The lateral sides 222 may for example extend over a length of between 70 and 90% of the diameter of the platform 20, for example over 50 mm. The width of the longitudinal ends 224 may be between 1.5 and 5 mm. The three grooves 22 are arranged parallel to each other, their respective main axes P being parallel to each other. Preferably, a distance between two adjacent grooves 22 is greater than 1 mm.

[0072] It will be noted that the grooves 22 comprise a bottom 226 which is preferably curved, such that the bottom 226 connects the longitudinal ends 224 continuously. More precisely, the bottom 226 extends from a longitudinal end 224 of the groove, on the side of the third rim C for example, at the same level as the main surface 210 (i.e. at the same height), gradually sinks relative to the level of the main surface 210 to a maximum depth, then gradually rises to the opposite longitudinal end 224 on the side of the fourth rim D. This configuration is clearly visible in [Fig. 8], and makes it possible to disturb the flow of the gas flow F as little as possible when the platform 20 is in the first position. Preferably, the depth of the grooves 22 is such that a depth / width ratio (where the width is the width of the longitudinal ends 224) is between 1 and 2.In other words, the depth can be between 1.5 mm and 10 mm, preferably between 2 mm and 6 mm.

[0073] The platform 20 further comprises a fin 24 projecting from the main surface 210. The fin 24 is arranged between the first edge A of the platform 20 and a lateral side 222 of one of the grooves 22. It has an elongated shape in the direction of the main axis P of said groove 22.

[0074] The fin 24 comprises an inclined plane 242 extending from a base 241 to a top 244. The base 241 is at the same level (i.e., at the same height) as the main surface 210 and extends parallel to the lateral side 222 while being adjacent thereto. The apex 244 is raised relative to the main surface 210, the inclined plane 242 thus forming a slope connecting the main surface 210, at the level of the base 241, to the apex 244.

[0075] An edge 246 of the fin 24 is adjacent to the first rim A, and may even be merged with it, following the curvature of said first rim A of the platform 20. In other words, the edge 246 of the fin 24 has the shape of an arc of a circle with a radius equal to the radius of the platform 20. The edge 246 forms a steep wall making it possible to compensate, in the first position of the platform 20, for the difference in height between the top 244 and the main plane 110 of the hub 11. Furthermore, in this example, the top 244 is a surface connecting the top of the inclined plane 242 and the upper end of the edge 246.Typically, a height h of the fin, corresponding to the maximum height between the top 244 and the main surface 210 (or the main plane 110 when the platform 20 is in the first position), in other words the maximum height of the wall formed by the edge 246, is determined so as to be between 0.5 and 3 times the thickness of the boundary layer upstream of the platform 20, preferably equal to said boundary layer.

[0076] In the first position, the platform 20 is oriented such that the longitudinal grooves 22 are aligned with the central axis X. In other words, the main axis P of the grooves 22 is parallel to the central axis X. The third rim C and the fourth rim D are also aligned with the central axis X and the incident gas flow F. In other words, the fourth rim D is aligned with the third rim C and downstream of it with respect to the central axis X and in the direction of the gas flow F, and the diameter joining the rims C and D is parallel to the central axis X. In this first position, the second, third and fourth rims B, C and D are flush with the main profile 110 of the hub 11, and are therefore arranged in continuity with it.

[0077] Furthermore, the fin 24 is arranged on the side of the blade 13, in particular on the extrados side 131 thereof. In other words, the fin 24 is arranged between the blade 13 visible in the figures, and the intrados of the adjacent blade (not visible). Thus, the edge 246 of the fin 24, at the first rim A, projecting relative to the main profile 110, forms a wall between these two adjacent blades 13, blocking the flow of passage between these adjacent blades, when these are in the aligned position.

[0078] The platform 20 in the second position will then be described with reference to FIGS. 8 to 10.

[0079] The second position is obtained by a rotation of the platform 20, typically 90°, from the first position, such that the first rim A, and therefore the fin 24, are at a downstream end of the platform 20 in the direction of flow of the gas flow F.

[0080] In the second position, the longitudinal grooves 22 are preferably arranged perpendicular to the central axis X. More precisely, the main axis P of the grooves 22 is perpendicular to the central axis X. Preferably, in this position, the main axis P of the grooves 22 form an angle with respect to the direction G of the chord of the blades of between 10° and 50°, typically 30°. In addition, the first rim A and the second rim B are aligned with the central axis X, and the third rim C and the fourth rim D are arranged on either side of the central axis X. In other words, the first rim A is aligned with the second rim B and downstream of it with respect to the central axis X and to the direction of the gas flow F, and the diameter joining the edges C and D is perpendicular to the central axis X.

[0081] Given the curved shape of the main profile 110 and the platform 20, the third edge C and the fourth edge D which were flush with the main profile 110, in the first position of the platform 20, are now raised relative to the main profile 110, in the second position of the platform 20.

[0082] Indeed, taking into account the convex shape of the main profile 110, taking as reference an axis parallel to the central axis X and passing through the center of the platform, a height of the main profile 110 of the hub 11 is maximum along this axis, and decreases as one moves away from this axis, perpendicular to it.

[0083] According to this reference, the region of the main profile 110 arranged in alignment with the edges A and B ([Fig.8]) is higher than the regions of the main profile 110 adjacent to the edges C and D. The edges C and D being at the same height as the main profile 110, in the first position ([Fig.6]), are therefore raised relative to it, in the second position ([Fig.9]).

[0084] As illustrated in [Fig.9], representing the platform 20 in the second position in a front view from the upstream side, the third rim C and the fourth rim D thus form a protuberance 211, 212 respectively, projecting relative to the main profile 110 of the hub 11.

[0085] In the same way, the second rim B, which was flush with the main surface 110 of the hub 11 in the first position, in a region of lower height of said main profile 110 of convex shape according to the reference frame defined above ([Fig. 6] in a front view), is recessed relative to the main profile 110 after rotation of the platform 20 by 90° towards the second position ([Fig. 10] in a side view). Thus, at the level of the second rim B, a step 25 is created between an end 213 of the platform 20 and a contour 113 of the main profile 110 of the hub 11.

[0086] The rotation of the platform 20 from the first position to the second position thus makes it possible to create, in addition to the grooves 22 machined in the platform 20, a fourth groove which was not present in the first position, at the level of the second rim B arranged upstream of the platform 20. This fourth groove is formed by the step 25 which is descending in the direction of flow of the gas flow. F, and is preferably located at 10% (+ / - 5%) of the chord of the blade 13, from the leading edge 136.

[0087] On the other hand, the fin 24 which formed a protuberance relative to the main profile 110 of the hub 11, in the first position ([Fig. 6]), and was arranged between two adjacent blades, now no longer projects relative to said main profile 110, in the second position ([Fig. 10]), and is arranged downstream of the platform 20 relative to the direction of flow of the gas flow F. More precisely, the edge 246 is “retracted” into the hub 11, such that the apex 244 is flush with the main profile 110. Furthermore, the inclined plane 242 constitutes a gentle upward slope from the main surface 210 of the platform 20 towards the main profile 110 of the hub 11.

[0088] Thus, in a method for modifying the pitch of the blades 13 of the variable-pitch stator 10, a calculation unit (not shown) controls both the base 132 of the blades and the platforms 20. The platforms 20 may in particular be pivoted by an electric or hydraulic actuator, for example. The method thus comprises the detection of the pitch angle of the blades 13 by the calculation unit. When it is detected that the blades 13 are in the aligned position, the calculation unit positions the platforms 20 in the first position. When it is detected that the blades 13 are in the pitched position, the calculation unit controls the platforms 20 to pivot them and position them in the second position.

[0089] Although the present invention has been described with reference to specific exemplary embodiments, it is obvious that modifications and changes may be made to these examples without departing from the general scope of the invention as defined by the claims. In particular, individual features of the various illustrated / mentioned embodiments may be combined in additional embodiments. Therefore, the description and drawings should be considered in an illustrative rather than restrictive sense.

[0090] It is also obvious that all the characteristics described with reference to a method are transposable, alone or in combination, to a device, and conversely, all the characteristics described with reference to a device are transposable, alone or in combination, to a method.

Claims

Claims

1. A turbomachine stator (10) extending around a central axis (X) and comprising: - an inner wall (11) and an outer wall (12) delimiting an annular vein (V), - a plurality of variable-pitch blades (13), each blade (13) extending radially in the annular vein between a base (132) movably fixed to one of the inner wall or the outer wall (11, 12), and a free end (134), - the other of the inner wall or the outer wall (11, 12) comprising a main profile (110, 120) and at least one circular platform (20) arranged opposite the free end (134) of one of the blades, the platform (20) comprising a main surface (210) and at least one longitudinal groove (22), the platform (20) being movable in rotation relative to the main profile (110, 120) between a first position in which a main axis (P) of the groove (22) is substantially parallel to the central axis (X),and at least a second position in which the main axis (P) is transverse to the central axis (X).,

2. Stator (10) according to claim 1, wherein, in the second position, the main axis (P) of the groove is perpendicular to the central axis (X).

3. Stator (10) according to claim 1 or 2, configured such that, when a pitch angle (|3) between a chord (G) of the blade (13) at the free end (134) and the central axis (X) is between 0 and 20°, the platform (20) is arranged in the first position, and when the pitch angle (|3) is between 40 and 80°, the platform (20) is arranged in the second position.

4. Stator (10) according to any one of claims 1 to 3, wherein the inner wall (11) comprises the platform (20), and wherein, in the first position, the main surface (210) of the platform (20) is flush with the main profile (110) of the inner wall (11).

5. A stator (10) according to any one of claims 1 to 4, wherein the platform (20) comprises at least one fin (24) projecting from the main surface (210), extending between a first rim (A) of the platform (20) and a lateral side (222) of the at least one groove (22), and having an elongated shape in the direction of the axis main (P) of said groove (22).

6. A stator (10) according to claim 4 and claim 5, wherein the fin (24) comprises an inclined plane (242) extending from a base (241) on the main surface (210) to an apex (244), the base (241) of the inclined plane (242) being adjacent to the lateral side (222) of the groove (22) and the apex (244) being adjacent to the first rim (A) of the platform (20), such that in the first position, the apex (244) projects relative to the main profile (110) of the inner wall (11), and in the second position, the apex (244) is flush with said main profile (110).

7. Stator (10) according to claim 5 and any one of claims 4 or 6, wherein, in the second position, a second rim (B) of the platform (20) arranged at one end of the platform diametrically opposite the first rim (A), is recessed relative to said main profile (110).

8. Stator (10) according to claim 4 and any one of claims 5 to 7, wherein, in the second position, third and fourth flanges (C, D) of the platform (20), located on one side of the longitudinal ends (224) of the groove (22), project relative to the main profile (110).

9. Stator (10) any one of claims 1 to 8, comprising at least three longitudinal grooves (22) parallel to each other.

10. Stator (10) according to any one of claims 1 to 9, the stator (10) being an inlet steering wheel of a turbomachine compressor, the inner wall (11) being a hub and the outer wall (12) being an outer casing.

11. A turbomachine (100) comprising a stator (10) according to any one of the preceding claims.

12. A method of modifying the pitch of the blades (13) of a variable pitch stator (10) according to any one of claims 1 to 10, the method comprising detecting the pitch angle (|3) of the blades (13) and, when the blades (13) are in an aligned position, positioning the platform (20) in the first position, and when the blades (13) are in a pitched position, positioning the platform (20) in the second position.

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