Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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TWO-PART SHROUD OR SHROUD SECTION FOR A STATOR STAGE WITH
VANES OF AN AXIAL COMPRESSOR
Technical field
[0001] The invention relates to a shroud for a stator with vanes of an axial
compressor, more
particularly to an internal shroud of a stator, more particularly still, to an
internal shroud made of
composite material. Such compressors are typically present in turbojet
engines, jet prop engines,
and gas generators.
Prior art
[0002] An axial compressor typically comprises a series of compression stages,
each being
constituted of a circumferential arrangement of vanes mounted on their bases
on a rotor. A stator
serving as a casing surrounds the rotor and the blades. The ends of the rotor
blades move in the
vicinity of the internal surface of the stator. The fluid, typically air, is
thus displaced and
compressed according to an annular jet, concentric to the rotation axis of the
rotor. The rotor
thus comprises several rows of circumferential blades, spaced apart from one
another. Rows of
stationary vanes are mounted on the stator between the rows of rotor blades
for the purpose of
rectifying the airflow between two stages of the compressor. These stator
vanes conventionally
comprise an internal shroud at their ends for the purpose of delimiting the
internal envelope of
the fluid stream. This shroud conventionally has an annular shape with an
outer surface that is
shaped specifically for the defining of the flow. The outer surface comprises
a series of
apertures or holes adapted to receive the inner ends of the vanes which are
fixed at their outer
ends or bases to the stator. This shroud also ensures that the vanes are
affixed to one another in
the area of their inner ends. The internal surface of the shroud is lined with
a friable material or
else more conventionally called "abradable" from the English term. This layer
of abradable
material is adapted to cooperate by friction with one or several
circumferential ribs on the rotor
in order to make it somewhat leak-proof. These ribs are more conventionally
called knife edges.
[0003] The patent document EP 1 213 484 Al of the same applicant as this
application discloses
an internal shroud and an external shroud of a compressor stator stage, these
shrouds being
conventionally made of metal. This document relates to a device for connecting
the vanes to the
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shrouds by means of a band which is inserted in apertures made at each end of
the vanes passing
through the holes of the shrouds, the band serving as a locking key with
respect to the outer and
inner surfaces of the external and internal shrouds, respectively.
[0004] As a function of the dimensions and materials used and also to
facilitate the assembly, it
can be interesting to segment the shroud into several sections. Indeed, in the
case of a shroud
made of composite material, for example, it can be difficult to inject the
resin of the composite
material over lengthy pieces, which means that it is interesting to segment
the shroud so as to
reduce the length of the sections. Segmenting the shroud can also prove to be
interesting to
compensate for the differential expansions in the area of the vanes themselves
and of parts of the
stator supporting the vanes. The aerodynamic performance of a segmented
composite shroud is
diminished for the following reasons:
[0005] The method of manufacture by plastic or resin injection requires the
use of a mold to give
the final shape. The general shape of a section is an arc of circle whereas
the unmolding
direction on a surface of the section is axial and not radial. The shroud
holes have main axes
which correspond to radii starting from the rotation axis of the rotor; these
axes are therefore
inclined with respect to the unmolding direction. Consequently, the holes have
an unnecessarily
flared section toward the unmolding direction. The adjustment between the
holes of the shroud
and the vanes is unnecessarily large and unfavorable from an aerodynamic
standpoint. It is
indeed desirable for the shroud surface in contact with the fluid stream,
particularly in the area of
the junction with the vanes, to be as continuous as possible.
[0006] Furthermore, independent of this requirement related to the unmolding,
the holes of a
segmented composite shroud must be provided to be slightly larger than the
section of the vanes
which go through them because of the assembly process. Indeed, once a first
section is in place,
it needs to be pulled up along the vanes in order to leave minimum room for
the adjacent end of
the section to be set in place next, so the first section can then be pulled
down toward its final
position. The clearance increase between the holes and the vanes, in
particular in the area of the
outer surface, is detrimental to the aerodynamics of the stator stage.
The object of the invention is to provide a shroud that overcomes at least one
of the
aforementioned drawbacks, more particularly to provide a shroud that is easy
to mount and has
good aerodynamic characteristics.
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Summary of the invention
[0007] The invention consists of a shroud or shroud section made of composite
material for a
stator stage with vanes of an axial compressor, the shroud or shroud section
being adapted to
receive an end of the vanes, comprising: at least one first curved element
with at least one first
notch corresponding to a first portion of the contour of the end of the vanes;
at least one second
curved element having a generally similar shape as the first curved element
with at least a second
notch corresponding to a second portion of the contour of the end of the
vanes, the first and
second notch or notches forming one or several pair(s) of notches; the first
and second curved
elements being adapted to be arranged axially against one another so as to
confine the profile of
the end of the vanes in each pair of first and second notches. The second
portion of the contour
of the end of the vanes preferably corresponds to the rest of the contour
which is not concerned
with the first portion, so that the first and second notches surround the
entirety of the contour of
the vanes.
[0008] Such a construction has numerous advantages, mostly from the standpoint
of an ease of
assembly on the vanes and from the standpoint of aerodynamics of the flow of
the fluid stream
into the stator.
[0009] Indeed, this modular construction with the pairs of notches forming the
apertures of the
shroud enables a substantially easier assembly in comparison with a
conventional segmented
shroud, each section of which being required to be carefully placed in front
of the series of vanes
to which it must be mounted and must then be pulled up along the vanes in the
area of one lateral
end in order to leave enough room for the adjacent end of the next section.
[0010] Furthermore, this modular construction makes it possible to use another
way of
unmolding the segments made of composite material by getting rid of the
unavoidable
requirement with the segments of a conventional shroud of having to unmold in
a direction
forming an angle with the main directions of some apertures thus formed. The
apertures of each
segment of a conventional shroud are indeed different from one another in
order to take into
account the projection of the vane according to the direction of assembly.
These differences lead
to assembly adjustments which are too great, having a detrimental effect on
the aerodynamics of
the flow.
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[0011] Each curved element forming the shroud comprises at least two,
preferably three, even
more preferably at least four, notches.
[0012] According to an advantageous embodiment of the invention, each of the
first and second
curved elements comprises an assembly edge in a plane, perpendicular to the
main axis of the
compressor, the assembly edges being adapted to come into contact with one
another during the
assembly of the two curved elements, preferably, by becoming closer together
in a direction
corresponding approximately to the flow direction.
[0013] According to another advantageous embodiment of the invention, the
first and/or second
portion of the contour of the end of the vanes correspond(s) to approximately
half of the profile
in the direction of the flow. This measure allows the apertures to take on the
shape of the barrel-
shaped vanes.
[0014] According to yet another advantageous embodiment of the invention, the
first curved
element comprises an assembly edge adapted to cooperate by insertion with an
assembly edge of
the second segment during the assembly of the two curved elements, preferably
by becoming
closer in a direction corresponding approximately to the direction of the
flow.
[0015] According to yet another advantageous embodiment of the invention, the
assembly edge
of one of the first and second curved elements comprises a groove according to
the curved
direction and the assembly edge of the other of the first and second curved
elements comprises a
projecting portion, the groove and the projecting portion being interrupted by
the respective
notch or notches of the first and second curved elements.
[0016] According to yet another advantageous embodiment of the invention, the
shroud is an
internal shroud with a U-shaped section with the opening of the U being
directed toward the
center of the shroud so as to form a recess adapted to receive some abradable
material and where
the limit between the first and second curved elements is located
approximately toward the
center of the U.
[0017] According to another advantageous embodiment of the invention, each of
the first and
second curved elements comprises means for mutual fastening, these means being
preferably of
the clip-type.
[0018] According to another advantageous embodiment of the invention, the
mutual fastening
means are such that they cooperate by elastic engagement of surfaces with
positive contact when
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the first and second curved elements move closer together in a direction
corresponding
approximately to the flow direction.
[0019] According to another advantageous embodiment of the invention, the
mutual fastening
means comprise several elements projecting over one of the first and second
curved elements in
a direction corresponding approximately to the direction for moving the first
and second curved
elements closer together.
[0020] According to another advantageous embodiment of the invention, the
mutual fastening
means are arranged on the respective assembly edges of the first and second
curved elements.
[0021] According to another advantageous embodiment of the invention, each end
of the shroud
section comprises a section whose shape corresponds to that of the end of the
vanes so that the
junction between two adjacent sections corresponds to the shape of the vane.
[0022] According to another advantageous embodiment of the invention, each end
of the shroud
section comprises means for connecting to an adjacent section, these
connection means having a
positive contact, preferably on both sides of the junction on each of the
first and second curved
elements.
[0023] According to another advantageous embodiment of the invention, the
first and second
curved elements are each ring-shaped, preferably closed.
[0024] According to another advantageous embodiment of the invention, one of
the first and
second curved elements is ring-shaped, preferably closed, and comprises
several second curved
elements corresponding to sections of the ring and adapted to be arranged
successively against
the ring-shaped curved element so as to confine, over the entire perimeter of
the ring, the shape
of the vane roots in each pair of first and second notches.
[0025] The invention also comprises a shroud for a stator stage with vanes of
an axial
compressor comprising several shroud sections such as described hereinabove.
Brief description of the drawings
[0026] Figure 1 is a partial, cross-sectional view of an axial compressor
with, among other
things, a shroud according to the invention.
[0027] Figure 2 is a perspective view of a shroud section according to the
invention.
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[0028] Figure 3 is a perspective view according to another angle of the shroud
section according
to figure 1 mounted on a row of vanes.
[0029] Figure 4 is a bottom, perspective view of the shroud section of figure
3.
Description of the embodiments
[0030] Figure 1 shows a cross-sectional view of part of an axial compressor,
typically of a jet
engine or turbine engine. The compressor comprises a rotor 5 rotating about
the axis 2. The
rotor comprises a series of blades 6 fixed onto its circumference,
corresponding to a stator stage.
The flow direction of the fluid to be compressed is shown by the arrow. The
casing 3, or the
stator, of the compressor comprises a series of vanes 4 fixed at their bases.
This series of vanes
constitutes a stator of the fluid stream located between two blade rows of the
mobile wheel, the
upstream row not being represented. A row of blades of the mobile wheel and
the guide vane
downstream constitute a stage of the compressor. The inner ends of the vanes 4
are connected to
a shroud 8. The shroud has the general shape of a circular ring following the
shape of the rotor
5. The shroud 8 thus delimits the lower or internal part of the annular fluid
stream passing
through the stator. It is kept in place by the vanes and makes the rotor 5
leak-proof. The shroud
comprises on its inner surface a layer of abradable material 10. The rotor 5
comprises two
circumferential ribs, more commonly called knife edges 9, cooperating with the
layer of friable
material 10. The abradable material 10 is applied and then machined so as to
present a sealing
surface 11 cooperating with the knife edges. This material has friction
properties with the metal
of the knife edges and the ability to disintegrate into fine dust in case of
contact with the knife
edges when the rotor is in rotation. The knife edges 9 and the surface 11 made
of friable material
thus constitute a labyrinth seal.
[0031 ] The shroud 8 is constituted of a series of sections shaped in an arc-
of-circle such as
shown in figure 2. The section shown in figure 2 is constituted of two
elements in an arc-of-
circle 12 and 14, each forming about a half, in a direction corresponding to
that of the axis of the
machine, of the shroud section along to a median arc-of-circle.
[0032] The curved element 12 comprises a series of notches 16 adapted to
follow the shape of
the corresponding end of the vanes. The notches 16 are, in principle,
identical, taking into
account the fact that all the vanes of a stator stage are generally identical
and uniformly
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distributed on the circumference of the shroud. The notches 16 are such that
they correspond to
about half of the profile of the vanes, the half being in a general direction
corresponding to the
flow direction of the fluid, or to the longitudinal direction of the section
of a vane. The curved
element 12 comprises an outer surface 22 of revolution, corresponding
generally to a segment or
piece of a cylinder, that is, a surface directed toward the outside of the
circle or ring which forms
the complete shroud. This surface is the surface delimiting the inner envelope
of the fluid stream
passing through the stator. It is delimited on one side by a rounded edge 26
and on the other side
by an edge 27 crossing through the notches 16 and adapted to come into contact
with a
corresponding edge 29 of the other curved element 14. This contact edge 27
comprises a groove
28 extending in parallel to the outer surface 22 and below this surface. The
groove 28, as the
edge 27, is interrupted by the notches 16.
[0033] The curved element 14 opposite the curved element 12 is generally
similar. It comprises
a series of notches 18 corresponding to the notches 16 of the opposite curved
element 12 and
adapted to form, when the two curved elements are arranged in an adjacent and
corresponding
manner, a series of apertures or openings which follow the shape of the vanes.
The shape of the
notches 18 thus corresponds to the rest of the section of the vanes, that is,
to the portion of the
contour of the vane which is not covered by the notch 16 of the opposite
curved element 12. As
a result, the notches 18 can have a slightly or even substantially different
profile than those of the
opposite notches 16, as a function of the section of the end of the vane to be
surrounded. Similar
to the opposite curved element 12, the curved element 14 comprises an outer
surface 22,
generally cylindrical or ring-shaped, that is, a surface directed toward the
outside of the circle or
ring which forms the complete shroud. This surface is the surface delimiting
the inner envelope
of the fluid stream passing through the stator. It is delimited on one side by
a rounded edge 24
and on the other side by an edge 29 passing through the notches 18 and adapted
to come into
contact with the corresponding edge 27 of the opposite curved element 12. This
contact edge 29
comprises a tongue 30 or lug extending in parallel to the outer surface 22 and
at a lower level of
this surface. The tongue 30, as the edge 29, is interrupted by the notches 18.
[0034] The curved element 14 also comprises in the area of its contact edge
29, at a level that is
lower than that of the outer surface 22, a series of elastic hooks generally
directed toward the
opposite curved element 12 during the assembly and adapted to cooperate by
flexion followed by
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a movement for returning to a position close to the initial position with
corresponding housings
provided in the area of the contact edge 27 of the opposite curved element 12.
These hooks are
arranged by pairs with a generally U-shaped profile directed toward the
outside and where the
ends of the U-shaped legs each comprise a boss forming the hook as such with a
surface inclined
with respect to the direction for inserting the hooks in their housings 21 of
the opposite curved
elements and with a surface that is generally perpendicular to this direction
and being able to
come into positive contact with a corresponding surface of the housings. The
bosses in question
are arranged at each end of the two U-shaped legs and so as to be laterally
directed toward the
outside of the U.
[0035] The shroud section is provided to be mounted by assembling the two
elements about the
vanes. In practice, the two elements 12 and 14 are arranged in front of one
another, similar to the
drawing of figure 2, by first engaging each of the elements with the row of
vanes which is
adapted to be fixed to this shroud section. Once each element is engaged with
the row of vanes,
that is when the notches of each element cooperate, at least partially, with
the vanes, moving the
two elements closer together is very easy since it is guided, at least in a
plane generally at a
tangent to the outer surface 22 of the shroud, by the vanes. Putting the
respective contact edges
27 and 29 in correspondence can require one or more movements in a direction
that is
perpendicular to the plane at a tangent to the outer surface 22 of the shroud.
The tongue 30 of
the element 14 is arranged facing the groove 28 of the opposite element 12 so
as to ensure an
efficient guiding during the assembly. The mutual fastening of the two
elements is carried out
by applying a force for moving the contact edges closer together so as to make
the resilient
hooks penetrate in the respective housings up to the point where the parts
with the positive
contact surfaces override the corresponding surfaces of the housings. The legs
can then return to
a position close to that before insertion and where the positive contact
surfaces of the hooks
mesh with the corresponding surfaces of the housings.
[0036] Alternatively, the shroud section can be assembled before the vanes are
mounted. The
elements 12 and 14 are thus assembled by a similar movement to that described
hereinabove, but
without the presence of the vanes. The section is then mounted pre-assembled
on the vanes.
This principle of assembly, just like the previous assembly principle
described in detail in the
previous paragraph, provides the advantage of allowing a shroud with apertures
and a profile that
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is very adjusted to the profile of the vanes to be made, while doing away with
the requirements
caused by the unmolding of a conventional shroud section at the apertures. The
unmolding of a
conventional shroud section is generally done by moving away part of the mold
corresponding to
the outer surface and to the apertures over at least part of the thickness of
the shroud from the
outer surface. The curved nature of the shroud section causes the axes of the
apertures to
correspond to radii of the shroud and can thus form an angle with the
unmolding direction. This
requirement calls for a section of the apertures that flares more than what is
required for the
assembly. However, the previous principle of assembly described in detail in
the previous
paragraph has the added advantage to make it easier to assemble the shroud and
mount it on the
vanes.
[0037] Figure 3 shows the shroud section of figure 2 once assembled and
mounted on a row of
vanes 4. One can see that the outer surface 22 of the shroud 8 remains
continuous and regular in
the junction area. The mutual fastening is ensured by a pair of elastic hooks
20 between each
pair of neighboring vanes. The vanes 4 can be fixed to the shroud by applying
glue or an
elastomer serving as glue ensuring a dual function, namely, that of ensuring a
mechanical
connection and a tight sealing. To do so, the ends of the vanes slightly
project under the
apertures so as to receive the glue or the elastomer.
[0038] It must be noted that other methods for mechanically connecting the
vanes and the shroud
can be provided. Indeed, it is absolutely conceivable to provide for a band to
be placed through
the holes made in the ends of the vanes so as to ensure a mechanical
connection preventing the
vanes from exiting the shroud, such as disclosed in the document EP 1 213 484
Al discussed in
the "Prior art" section. In this case, it would be more practical to provide
for a band to be placed
before the shroud is mounted on the vanes.
[0039] Figure 3 also shows the U-shaped section with the open portion directed
toward the
center of the shroud, forming a recess adapted to receive the abradable
material in the form of a
paste by projection or by any other equivalent or similar method.
[0040] Figure 4 is an enlarged view of a part of the bottom of the shroud
section mounted on the
vanes shown in figure 3. It shows well the precision of the adjustment between
the apertures
formed by the pairs of notches 16 and 18 and the vanes 4. The fact that each
notch of a pair of
notches 16 and 18 forming an aperture surrounds the blade over approximately
half of its length
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(according to the flow direction) makes it possible to precisely adjust the
apertures on tunnel-
section vanes, that is, vanes whose section is substantially thicker in the
middle than on the edges
(leading or trailing).
[0041] The shroud sections such as shown in figures 2, 3, and 4 can have ends
with a profile
corresponding to that of the end of the vanes, so that the junction between
two adjacent sections
corresponds to the profile of the blade (not shown). Preferably, each shroud
section end
comprises means for connecting to an adjacent section, these connection means
being of the
positive contact type, preferably on both sides of the junction on each of the
curved elements
forming a section. This positive contact ideally cooperates with a relative
sliding movement
along vanes between two adjacent sections.
[0042] It must be noted that the shape, number, and position of the elastic
hooks as mutual
fastening means for curved elements are given only by way of non-limiting
example. Indeed,
various alternatives which are similar and/or equivalent to these fastening
means are possible, as
a function of diverse parameters such as the choice of materials,
manufacturing requirements
(molding/unmolding for non-metallic materials), ease of assembly and
disassembly, desired
resistance to disassembly.
[0043] It must also be noted that the shroud can be made of two elements of
the type previously
described, namely the curved elements 12 and 14, each forming a complete ring.
These elements
will preferably form a closed ring. Alternatively, the shroud can be composed
of a first element
forming a complete ring and preferably closed, and a series of curved elements
adapted to be
arranged against the first element and adjacent to one another along the
circumference of the first
element.
[0044] It must also be noted that it is, of course, conceivable to apply the
shroud according to the
invention to a stator stage whose vanes are not necessarily identical and/or
uniformly distributed
over the entire circumference. In this case, the apertures formed by the pairs
of notches will not
be uniformly distributed but rather according to the vanes of the stator.
[0045] It must also be noted that, although the embodiment of the invention
was described in
relation to an internal shroud, the invention is similarly applicable to an
external shroud.
