BE1030508B1 - INTERMEDIATE RING ROTOR - Google Patents

Abstract

Method of manufacturing a turbomachine rotor (12) comprising: producing a groove or a notch on each blade (40) of an annular row of blades (40), at an intermediate radial position of the blades (40); positioning an intermediate ring (42) comprising housings into which the grooves or notches engage; and fixing, preferably by welding, the intermediate ring (42) to the blades (40). The invention also relates to a rotor (12) obtained by this method and a turbomachine provided with such a rotor (12).

Description

Description

INTERMEDIATE RING ROTOR

Domain

The invention relates to the manufacture of a rotor for a multi-flow turbomachine.

Prior art

Multi-flow turbomachines have the characteristic of using different annular air flows, coaxial with each other, each flow meeting or not various components of the turbomachine (compressor, combustion chamber, turbine, rectifier blades, etc.). ).

It is known to use a moving compressor wheel common to two adjacent flows separated from a wall. As such, the wheel includes an intermediate ring which keeps the two flows sealed from each other. To assemble this intermediate ring, supports are provided on each of the blades and angular sections of the ring are riveted to each of the supports.

On the one hand, such an assembly is complex. It requires significant assembly time and requires the use of specific tools.

On the other hand, such a moving wheel is only suitable for a design where two flows are already separated from each other upstream of the wheel. The geometry of the intermediate ring is not suitable for an area of the turbomachine where a single flow upstream of the wheel is split into several flows downstream of the wheel, because the geometry known from the state of the art would generate in such a situation significant aerodynamic losses and therefore lower engine efficiency.

Such flow separation zones are found in particular at a separation nozzle downstream of a fan.

Summary of the invention

Technical problem

The invention aims to resolve the drawbacks noted in the state of the art. In particular, the invention aims to propose a rotor design and its manufacturing method, which allow easier manufacturing and which make the rotor capable of occupying an area of the turbomachine where an air flow is split without affecting the efficiency of the turbomachine.

Technical solution

The invention relates to a method of manufacturing a turbomachine compressor rotor comprising: producing a groove or a notch on each blade of an annular row of blades, at an intermediate radial position of the blades ; positioning an intermediate ring comprising housings in which the grooves or notches engage, the intermediate ring comprising a leading edge, a trailing edge, an internal radial surface for guiding the flow and a radial surface external flow guide, said surfaces extending from the leading edge to the trailing edge; and the fixing, preferably by welding, of the intermediate ring to the blades.

The groove or notch provided on each blade allows the ring to be pre-maintained during its fixation. They thus make assembly easier and potentially more precise. The shape and position of the groove or notch can be such that the intermediate ring is positioned so as to pre-guide the flow in an advantageous direction for its separation.

In fact, the intermediate ring subdivides the flow which is upstream of the rotor and pre-guides it towards the two (or more) flows which are downstream of the rotor. It thus improves the efficiency of the turbomachine. It thus subdivides the inter-blade space into two channels in which the particles from the upstream flow move towards one or the other of the downstream flows. The ring is “intermediate” in the sense that it is neither placed at the feet nor at the heads of the blades, but in a radially intermediate position between the feet and the heads of the blades. When an upstream flow is split into more than two downstream flows, several respective intermediate rings can be provided.

According to an advantageous embodiment of the invention, the intermediate ring is formed of several ring sections and the step of positioning the ring comprises positioning all the ring sections. The sections allow easier handling and therefore simplified assembly. They also allow the simultaneous installation of several parts of the ring by different operators.

According to an advantageous embodiment of the invention, the fixing step is at least partially carried out before all of the sections have been placed in position. This overlap of the two steps saves manufacturing time. It is understood that alternatively, the ring sections are all positioned before the fixing step (for example by welding) begins.

According to an advantageous embodiment of the invention, positioning the ring comprises the radial fitting of ring sections onto the blades. The camber of the blades and the curvature of the ring sections (which, together, form a ring) act as a key when inserting the ring sections radially around the blades, further offering a gain in efficiency when the making.

According to an advantageous embodiment of the invention, positioning the ring comprises the simultaneous axial and circumferential sliding of ring sections. Thus, ring sections can be brought parallel to the propeller described by the blades. For example, two hoops, one or both of which include notches corresponding to the profile of the blades, can be brought from upstream and downstream, in particular simultaneously, then fixed to one another.

According to another alternative, the intermediate ring is in one piece and it is put into position by sliding simultaneously axially and circumferentially.

According to an advantageous embodiment of the invention, the method further comprises a step of positioning an outer ring and a step of fixing, preferably by welding, the outer ring to the outer radial ends of the blades. This ring helps prevent leakage vortices between the intrados and extrados.

According to an advantageous embodiment of the invention, the method further comprises positioning and fixing, preferably by welding, auxiliary blades extending radially internally and/or externally from the intermediate ring, and/or extending radially inwardly from the outer ring. These auxiliary blades make it possible to act independently on one of the two downstream flows, thus optimizing the efficiency of the turbomachine by compressing or accelerating one of the flows more than the other. The auxiliary vanes can be moved into position and attached before or after the rings (or sections of rings) are/have been attached to the vanes. According to an advantageous embodiment of the invention, the auxiliary blades have a radial height of at least 15% of the height of the blades.

According to an advantageous embodiment of the invention, the intermediate ring and/or the outer ring is/are essentially formed of composite material, and is/are optionally provided with metal inserts intended to be welded to the blades. This weight gain for a rotating element further allows for increased efficiency.

Alternatively, all rotor elements are metallic.

According to an advantageous embodiment of the invention, the blades are encircled by the intermediate ring. Thus, the ring is axially longer than the blades and it encapsulates them.

According to an advantageous embodiment of the invention, the intermediate ring and/or the outer ring presents irregularities on its radially outer surface(s) and/or on its surface(s). s) radially interior(s), the irregularities being in particular of the three-dimensional contouring type. These irregularities are bumps and/or hollows, the height or depth of which are of a very small order of magnitude compared to the dimensions of the ring. These irregularities further improve the guidance of the flow and therefore the efficiency of the turbomachine.

The invention also relates to a turbomachine rotor obtained by a manufacturing process conforming to one of the embodiments explained above. The groove, the notch and the fixing, in particular by a weld bead, are observable on the rotor once manufactured and distinguish it from a rotor obtained by a different process.

The same goes for the ring sections. Micrographs of the rotor or sections of it allow it to be distinguished from another rotor if necessary.

The invention also relates to a turbomachine comprising: a non-ducted propeller propelling a tertiary flow; a rotor downstream of the propeller, the rotor comprising an annular row of rotor blades; a separation nozzle downstream of the rotor to separate the flow propelled by the rotor into a primary flow and a secondary flow; and a compressor compressing the primary flow; the turbomachine being remarkable in that the rotor is obtained by a manufacturing process conforming to one of the embodiments explained above and the trailing edge of the intermediate ring is arranged at a radial position close to that of the nozzle of seperation.

The separation nozzle may have a circular edge forming an upstream end, rounded or not. From the circular edge extends an external surface, for internal guidance of the secondary flow, and an internal surface, for external guidance of the primary flow. 5 The flow is thus pre-guided by the intermediate ring before being split into two flows. This pre-guiding reduces the risk of turbulence or separation of the flow and allows an improvement in the efficiency of the turbomachine.

According to an advantageous embodiment of the invention, in the vicinity of the separation nozzle, the primary flow and the secondary flow flow in two respective main flow directions, respectively defining a primary angle and a secondary angle, relative to the axis of rotation of the rotor, and the trailing edge of the intermediate ring guides the inlet flow at a trailing angle which is between the primary angle and the secondary angle. A compromise is thus found to limit losses when the two flows downstream of the nozzle are very divergent.

This orientation of the intermediate ring also offers flexibility for the design of the veins downstream of the rotor, notably allowing a steep swan neck at the outlet of a compressor, and therefore a high compression ratio and better efficiency.

According to an advantageous embodiment of the invention, the transverse profile of the ring has a camber and/or a point of inflection. Alternatively, the profile may be straight, presenting a constant angle from the entry angle at the leading edge to the exit angle at the trailing edge. Depending on the geometry of the primary and secondary flow veins, one or the other design is more or less advantageous. In particular, a camber allows a greater volume of air to be conducted towards one or other of the downstream flows.

According to an advantageous embodiment of the invention, the rotor comprises a plurality of rings, at least two of which differ in their profiles, their radial position, their circumferential length, their thickness, their shape and/or their method of assembly.

Advantages of the invention

The invention is particularly advantageous in that it makes it possible to propose a rotor adapted to an area of the turbomachine where a flow is split into two flows (or more) while limiting the effects on the efficiency of the engine and offering less complex manufacturing. than known rotors.

An additional advantage of the presence of an intermediate ring is the protection of the downstream veins in the event of ingestion of foreign objects into the engine.

Pre-guiding the flows also allows greater freedom in the trajectory of the downstream veins and in particular a steeper swan neck, because the flow entering the swan neck can be pre-guided and present less risk of separation .

Then, the secondary flow (respectively primary) can be “energized”, that is to say accelerated and/or compressed more than the primary flow (resp. secondary).

Description of the designs

Figure 1 is a sectional view of a turbomachine;

Figure 2 illustrates the separation of a flow;

Figure 3 represents an isometric view of a rotor according to the invention;

Figures 4A to 4C show examples of blades;

Figures SA and 5B illustrate examples of ring sections;

Figures GA and GB show examples of hoops and rings;

Figure 7 is an isometric view of a rotor according to the invention;

Figures 8A and 8B represent an isometric view of a rotor according to the invention.

Description of an embodiment

In the description which follows, the terms “internal” and “external” refer to positioning relative to the axis of rotation of a turbomachine. The axial direction corresponds to the direction along the axis of rotation of the turbomachine. The radial direction is perpendicular to the axis of rotation. Upstream and downstream must be considered with reference to the direction of flow in the turbomachine.

The figures show the elements schematically and are not represented to scale. In particular, certain dimensions are enlarged to facilitate reading of the figures.

Figure 1 shows a schematic sectional view of a turbomachine 1. An interior casing 2 guides a primary flow F1 which successively travels through compressors 4 (low and high pressure), a combustion chamber 6 and turbines 8 (high and low pressure), before escaping through a nozzle 10. The energy from the combustion drives the turbines 8 in rotation around the axis X. The turbines 8 drive the compressors 4, directly via transmission shafts, or indirectly by means of reducers.

The turbines 8 also rotate a rotor 12 which sets in motion an input flow (F1+F2) which separates into a primary flow F1 directed towards a compressor 4 and a secondary flow F2. In the example shown in Figure 1, a non-ducted propeller 14 propels a tertiary flow F3 in addition to propelling the inlet flow (F1+F2).

A fairing 16 and a nacelle 18 delimit a passage 19 which is traversed by the secondary flow F2.

Structural arms 20 take up the forces between the nacelle 18 and the casing 4.

An annular row of stator vanes 22 (“outlet guide vanes”, OGV) can be arranged downstream of the rotor 12 to straighten the flow F2.

The rotor 12 and the propeller 14 can rotate in opposite directions to each other via a gear reducer (not shown). This gearbox can also greatly reduce the rotation speed (between the turbines and the rotor/propeller).

Figure 2 schematically shows the separation of a flow into two annular flows.

An input stream F is split into a primary stream F1 and a secondary stream F2. The primary and secondary flows F1 and secondary flows F2 flow respectively in an annular vein 32, 34. These flows can correspond to the primary and secondary flows of Figure 1 or to other flows separated from each other in the turbomachine.

The separation of the flows is carried out by a separation nozzle 36 of circular shape around the axis of the turbomachine X. The separation nozzle 36 may have an upstream circular edge which may be a sharp or rounded edge. It is arranged directly upstream of a surface 34.1 which internally guides the secondary flow

F2 and a surface 32.1 which externally guides the primary flow F1. These two surfaces 32.1, 34.1 can be approximately truncated cones in the vicinity of the nozzle 36. The flows F1 and F2 have a main direction of flow which is oriented according to the angles a, and à, in the vicinity of the nozzle 36. angle a, can be between -60° and +15°. The angle a» can be between -15° and +60°, while naturally being greater than 04.

The separation nozzle 36 or its circular edge defines a radius R up to the axis X.

Directly upstream of the nozzle 36 is a rotating assembly in the form of a rotor 12 equipped with an annular row of blades 40 extending from an internal disk 41. The blades 40 are fixed to the internal disk 41 by welding, or the blades 40 are manufactured simultaneously with the disc 41, by machining the same stock or by additive manufacturing. Any type of welding can be considered (electron beam welding, orbital friction, linear friction, TIG).

Alternatively, the blades 40 can be fixed to the disc 41 by dovetails or by any other removable means.

At least one intermediate ring 42 extends circumferentially over 360° encircling the disc 41. The ring 42 can be axially confined to the blades as drawn in Figure 2 or alternatively encircle each blade 40 on the upstream and/or downstream side (see figure 3). Figure 2 highlights the fact that the intermediate ring 42 comprises a leading edge 42.1 and a trailing edge 42.2. The latter is located at a radial height R (+/- 10%) which can in particular be between 30% and 70% of the radial height of the blades 40. The leading and trailing edges of the intermediate ring are circular around the axis of rotation of the rotor 12.

The ring 42 may be metallic or made of composite material. The ring 42 can be fixed by pins, by screwed elements, or welded to the blades 40. If necessary, the welding of a composite ring is made possible by the integration of metal inserts in the composite ring.

The intermediate ring 42 has an internal radial surface 42.3 and an external radial surface 42.4, each extending from the leading edge 42.1 to the trailing edge 42.2.

The radial thickness of the ring can vary from upstream to downstream. The surfaces 42.3, 42.4 can be provided with irregularities helping to guide the flow (“contouring”) and thus present local thickness variations.

The profile of the ring 42 may also have a camber and/or a point of inflection. The intermediate ring 42 may have an upstream guiding angle at the leading edge which is different from the downstream angle at the trailing edge.

Alternatively, the flow guidance angle can be constant from upstream to downstream.

Figure 2 highlights the leakage angle a which corresponds to the orientation of the inlet flow F in the vicinity of the trailing edge 42.2 of the ring 42. The ring 42 is designed so that this angle is between to, and 02.

Figure 3 shows an isometric view of the rotor 12 with its blades 40 extending from the disk 41, and its intermediate ring 42.

Figures 4A to 4C show different examples of blades 40.

Figure 4A represents a blade 40 with its leading edge 40.1, its trailing edge 40.2 and its intrados 40.3. A notch 40.5 is provided to receive the ring 42. In the example illustrated, the notch 40.5 is arranged on the leading edge but it can be arranged at other locations on the blade 40.

Figure 4B shows a variant with a groove 40.6. In this example, the groove 40.6 extends axially over the entire blade 40 as well as on both sides (extrados and intrados). A shorter groove extending only on one side of the blade 40 is also possible. The groove can be rectilinear, parallel or not to the axis of the turbomachine. It can alternatively be curved and follow the camber of the ring 42.

Figure 4C shows another alternative with a notch on the intrados 40.3, creating a shoulder at the junction between an external part of the intrados 40.31, thinner, and an internal part of the intrados 40.32, thicker.

The notch or groove allows the ring 42 to be fixed by means of a pin, screw elements, or by means of a point or a weld bead.

In one embodiment, the ring 42 is formed of several ring sections which, when assembled together, form the ring 42. Figure 5A shows a ring section 44. The leading and trailing edge 44.1 44.2 of each section form part of the leading edge 42.1 and trailing edge 42.2 of the ring. Edges 144.3 and 44.4 are intended to abut other similar edges of adjacent sections. A weld bead can be used to secure the ring sections

44 to each other. The edges 44.3 and 44.4 can be rectilinear, parallel to the axis X or not, or be curved and follow the curvature of an intrados or an extrados.

The upper surface 44.5 forms part of the external surface 42.4 of the ring 42.

A housing 44.6 is arranged in section 44. This housing has a closed contour of dimension slightly greater than the profile of the blade 40, considered in the radial position intended to receive the section 44. This section 44 can be fitted around the blade 40 by a substantially radial movement (within the curvature of the blade 40).

The clearance between the housing 44.6 and the blade 40 can be filled by a seal or by a weld bead, in order to provide continuity of material between the blades 40 and the ring 42 once the assembly is assembled.

In an alternative not illustrated, the housing 44.6 is made of two half-housings at each circumferential end (on the edges 44.3 and 44.4) and the section 44 is thus inserted radially in the circumferential space between the blades.

Figure 5B shows a section 44 in an alternative embodiment with an opening housing 44.6. This design makes it possible to insert section 44 around blade 40 by a substantially helical movement (combining axial translation and circumferential rotation) from downstream of blade 40.

Housing 44.6 can open onto the leading edge of ring 42.

Alternatively, a section 44 (not shown, presenting an opening housing and complementary to that of Figure 5B) can be brought in the same way from upstream.

In another alternative, the ring sections 44 comprise several housings (see dotted lines in Figure 5B) and each section can thus cover several adjacent blades. The rotor 12 can thus be formed by the assembly of several angular sectors on the disk 41, each sector comprising several blades and one or more corresponding ring sections 44.

Figure GA shows a variant in which two hoops 46, 48 encapsulate the blades 40 from upstream and downstream, potentially with a helical movement, the two hoops 46, 48 forming two axial sections of the ring 42, and comprising respective housings 46.6, 48.6 cooperating with part of the blades and in particular their notches or grooves. The hoop 46 comprises an upstream edge 46.1 which forms the leading edge 42.1 of the ring and the section 48 comprises a downstream edge 48.2 which forms the trailing edge 42.2 of the ring.

Figure 6B shows an alternative where the ring 42 is formed of a single section 50 of which an edge 50.1 corresponds to the leading edge of the ring 42 and an edge 50.2 corresponds to the trailing edge of the ring 42. This one-piece ring 42 includes opening housings 50.6. This is engaged around blades 40 from downstream. Alternatively, it can be brought from upstream.

Figure 7 shows an embodiment where an outer ring 52 is added to the outer ends (heads) of the blades 40. Any means of attachment is possible between the outer ring 52 and the blades 40, such as welding or use fixing platforms.

The outer ring 52 may have irregularities (“contouring”) on the internal surface 52.3 and/or on its external surface 52.4.

Figures 8A and 8B show an embodiment where auxiliary blades 54 are inserted between two circumferentially adjacent blades 40.

In Figure 8A, which does not present an external ring, the auxiliary blades 54 are carried and fixed to the ring 42.

In the example of Figure 8B, the auxiliary blades 54 are in contact with both

Intermediate vane 42 and the outer ring 52. In an alternative, the height of the auxiliary vanes 54 is shorter than the distance between the two rings 42, 52 and the auxiliary vanes 54 are therefore only fixed to one of the two rings 42, 52.

In a variant not illustrated, the intermediate ring 42 is not present and only the outer ring 52 carries the auxiliary blades 54. These extend radially over a distance which corresponds to the radial height of the secondary vein ( 34 in Figure 2).

The auxiliary blades 54 can have a height of at least 15% and in particular approximately a quarter of the height of the blades 40. Alternatively, the auxiliary blades 54 can cover the entire distance between the ring 42 and the disc 41, or the entire distance. height between the ring 42 and the head of the blades 40, or the entire radial distance between the intermediate ring 42 and the external ring 52. As for their axial dimension, the auxiliary blades 54 can extend between 50 and 100 % of the axial length of the blades 40.

In the example illustrated, the auxiliary blades 54 are "one" in number per inter-blade space and extend radially outwardly from the ring 42. It is understood that the number of auxiliary blades carried by the ring 42 and their orientations (exterior and/or interior) can be chosen according to the compression or the speed to be printed independently to the primary or secondary flow. For example, one auxiliary vane 54 may be directed inwards, and two auxiliary vanes 54 may be directed outwards, with the inner vane being circumferentially located midway between the outer vanes.

The invention relates not only to the rotor 12 and the turbomachine 1 but also to the method of manufacturing the rotor 12. This essentially comprises the installation and fixing of the intermediate ring or ring sections to the blades. , in particular the housings of the ring cooperating with a groove or a notch of the blade.

The disc 41 and the blades 40 can be in one piece, made from one piece, or the blades 40 can be attached to the disc 41 and fixed thereon. The different stages of fixing the ring 42 or its sections 44 can be carried out before the blades 40 are fixed to the disk, or after. Alternatively, the assembly can be hybrid, with certain blades being integral with the disc and others being attached to it. Also, fixing the ring or its sections to the blades can be done simultaneously with fixing other blades to the disc. According to another variant, the ring sections 44 can be fixed to the blades 40 (possibly by angular sector of several blades), then the blades and their ring sections can be fixed to the disc 41.

Likewise, making notches and grooves in the blades can be done after fixing the blades to the disc or before. Alternatively, the notches and grooves can be made on some blades while other blades are being attached or while certain ring sections are being attached to some blades.

In the same way, the auxiliary blades 54 can be fixed to the intermediate ring or its sections, and/or to the outer ring, before these rings are fixed to the blades, or after.

The invention is applicable to so-called unducted turbomachines (CROR “Counter-Rotating Open Rotor” or USF “Unducted Single Fan”) which may have an intermediate fan.

Each technical characteristic of each illustrated example is applicable to the other examples. In particular, the number of blades, the number of rings or ring sections, their arrangement, their transverse profile, their length, their camber, their shape, etc., can be taken from an embodiment and be applied to another.

The different types of ring section attachment can be combined on the same rotor. Thus, certain segments of the same ring can be brought axially while others are brought radially around the blades. Also, in variants with several rings, the same blade can have several notches and/or grooves of different types to engage each of the rings.

Claims (13)

Claims 1. Method for manufacturing a rotor (12) of a turbomachine compressor (1) comprising: - producing a groove (40.31, 40.6) or a notch (40.5) on each blade (40) of a annular row of blades (40), at an intermediate radial position of the blades (40); - positioning an intermediate ring (42) comprising housings (44.6, 46.6, 48.6, 50.6) in which the grooves engage (40.31, 40.6) or notches (40.5), the intermediate ring (42) comprising a leading edge (42.1), a trailing edge (42.2), an internal radial surface (42.3) and an outer radial flow guiding surface (42.4), said surfaces (42.3, 42.4) extending from the leading edge (42.1) to the trailing edge (42.2); and - fixing, preferably by welding, the intermediate ring (42) to the blades (40). 2. Method according to claim 1, characterized in that the intermediate ring (42) is formed of several ring sections (44) and the step of positioning the ring (42) comprises positioning of all ring sections (44). 3. Method according to claim 2, characterized in that positioning the ring (42) comprises the radial fitting of ring sections (44) onto the blades (40). 4. Method according to one of claims 2 or 3, characterized in that the positioning of the ring (42) comprises the simultaneous axial and circumferential sliding of ring sections (44). 5. Method according to claim 1, characterized in that the intermediate ring (42) is one piece and it is put into position by sliding simultaneously axially and circumferentially. 6. Method according to one of claims 1 to 5, characterized in that it further comprises a step of positioning an outer ring (52) and a step of fixing, preferably by welding, the ring exterior (52) to the external radial ends of the blades (40). 7. Method according to one of claims 1 to 6, characterized in that it further comprises positioning and fixing, preferably by welding, auxiliary blades (54) extending radially internally and/or externally from the intermediate ring (42), and/or extending radially internally from the outer ring (52). 8. Method according to the preceding claim, characterized in that the auxiliary blades (54) have a radial height of at least 15% of the height of the blades (40). 9. Method according to one of the preceding claims, characterized in that the intermediate ring (42) and/or the outer ring (52) is/are essentially formed of composite material, and is/are optionally provided (s) metal inserts intended to be welded to the blades (40). 10. Method according to one of the preceding claims, characterized in that the blades (40) are encircled by the intermediate ring (42). 11. Method according to one of the preceding claims, characterized in that the intermediate ring (42) and/or the outer ring (52) presents irregularities on its radially outer surface(s). s) (42.4, 52.4) and/or on its radially interior surface(s) (42.3, 52.3), the irregularities being in particular of the three-dimensional contouring type. 12. Rotor (12) of a turbomachine (1) obtained by a manufacturing process according to one of the preceding claims. 13. Turbomachine (1), comprising: - a non-ducted propeller (14) propelling a tertiary flow (F3); - a rotor (12) downstream of the propeller (14), the rotor (12) comprising an annular row of rotor blades (40); - a separation nozzle (36) downstream of the rotor (12) to separate the flow (F) propelled by the rotor (12) into a primary flow (F1) and a secondary flow (F2); And - a compressor (4) compressing the primary flow (F1); the turbomachine (1) being characterized in that the rotor (12) is obtained by a manufacturing process comprising: o producing a groove (40.31, 40.6) or a notch (40.5) on each blade (40) of the annular row of blades (40), at an intermediate radial position of the blades (40); o positioning an intermediate ring (42) comprising housings (44.6, 46.6, 48.6, 50.6) in which the grooves (40.31, 40.6) or notches (40.5) engage, the intermediate ring (42) comprising a leading edge (42.1), a trailing edge (42.2), an internal radial flow guiding surface (42.3) and an external radial flow guiding surface (42.4), said surfaces (42.3, 42.4) 'extending from the leading edge (42.1) to the trailing edge (42.2); and o fixing, preferably by welding, the intermediate ring (42) to the blades (40), the trailing edge (42.2) of the intermediate ring (42) being arranged at a radial position (R) close to that of the separation nozzle (36).