FORM 2
THE PATENTS ACT, 1970
(39 of 1970)
& The Patent Rules, 2003
COMPLETE SPECIFICATION
1.TITLE OF THE INVENTION:
LABYRINTH SEAL ABRADABLE STRUCTURE, NOTABLY FOR AIRCRAFT
TURBINE
2. APPLICANT:
Name: SAFRAN AIRCRAFT ENGINES
Nationality: France
Address: 2 boulevard du Général Martial Valin, 75015 PARIS, France.
3. PREAMBLE TO THE DESCRIPTION:
The following specification particularly describes the invention and the manner in which it
is to be performed :
2
DESCRIPTION
The subject of the invention is a labyrinth seal abradable 5 structure,
notably for an aircraft turbine.
Labyrinth seals are disposed between two parts in relative rotation and
are characterised by facing surfaces, separated from each other by a radial clearance and
which tolerate a leakage flow therebetween, while limiting it by means of a sinuous shape
10 and therefore high pressure drops which they impose thereon. The use of labyrinth seals is
frequent in the turbomachines between the rotor and the stator.
It is sought to reduce the leakage rates through such seals to improve the
efficiency of the turbomachine. In aeronautics, recourse is very often made to seal
constructions where one of the facing parts is provided with a structure called abradable
15 structure, having the property of low resistance to friction and wear, and which often has
a honeycomb structure. The other portion of the seal, disposed on the other part comprises
wipers, that is to say circular or conical projecting ridges, the free end of which is directed
towards the abradable structure. This construction allows reducing the clearances between
the two portions of the labyrinth seal, since the wipers can enter the abradable structure
20 without destroying the seal, when differential thermal or mechanical expansions (due to
overheating or to centrifugal forces), and occurring in circumstances such as transitional
regimes temporarily bringing the abradable structure and the ridges of the wipers into
contact. An example of a labyrinth seal, mentioned here because it has superficial
resemblances to the invention, is the subject of document FR-3028882-A. The abradable
25 structure in this document is designed to have a higher resistance to impacts, by avoiding
tearing off large surfaces therefrom, and the abradable structure is fixed to its substrate by
a connecting structure composed of honeycombs, whose cells are occupied by a portion of
the abradable structure, which is here a continuous and homogeneous layer. It is specified
(page 4, lines 4 to 8) that the connection structure should not be subjected to the wear of
3
the wipers. The connection structure has denser partitions, and smaller cells, at a region
located further downstream, in order to better anchor the abradable structure to a place
where it can be subjected to significant forces at certain restarts of the engine, as will be
developed below; but this disposition of the connection structure is opposite to that found
in some embodiments of the invention, which makes it less effective in avoiding 5 wiper lock
situations in the abradable structure during the different engine operating phases ("rotor
lock" situation).
An example of a very common type of labyrinth seal is shown in Figure 1,
which illustrates a fragment of a turbomachine turbine. The rotor 1 comprises at this
10 location a stage of movable vanes 2, joined together at their tip end, which is radially
external, by a root 3 here carrying two conical wipers 4 and 5, protruding radially outwards.
The turbomachine also comprises a stator 6 comprising a casing 7 provided with two
circular ribs 8 and 9 protruding radially inwards and carrying ends of a sealing ring 10
(normally divided into angular sectors on the circumference) connected to the ribs 8 and 9
15 by well-known hook adjustments 11 and 12. The sealing ring 10 comprises a rigid substrate
13, carrying the hooks, and an abradable layer 14 soldered on the substrate 13 and directed
towards the wipers 4 and 5.
The gas leakage flow around the rotor 1 has its flow rate reduced by the
section restrictions imposed thereon to bypass the wipers 4 and 5, the free ends of which
20 are in the immediate proximity of the abradable structure 14. The permeability of the
labyrinth seal comprising the wipers 4 and 5 and the abradable structure 14 depends above
all on the clearances 15 and 16 separating the abradable structure 14 from the tip of the
wipers 4 and 5. These clearances 15 and 16 have been cold adjusted by preliminary settings,
and they are adjusted during the operation of the turbomachine thanks to a refreshing of
25 the casing 7 operated by fresh air, drawn off at the compressor portion of the
turbomachine and which is blown on the external face of the casing 7 by circular ramps 17
and 18 provided with orifices 19 and 20.
The disposition of the labyrinth seal in cruising speed of the turbomachine
is typically that shown in Figure 2, wherein the free ends of the wipers 4 and 5 penetrate
30 slightly inside the original surface 21 of the abradable structure 14, after having dug
4
notches 22 and 23 by local wear of the abradable structure 14 during transient states of
the turbomachine. The clearances 15 and 16 in the seal are then much reduced, and the
path of the leakage flow is sinuous and irregular, which ensures a significant pressure drop
of the leakage flow, and a bearable loss of efficiency of the turbomachine.
A delicate situation nevertheless arises when the engine 5 was turned off
in flight and then re-ignited. The differential thermal expansions are modified, the stator 6
then cooling more than the rotor 1. This results in both a greater contraction of the stator
6 and an axial displacement of the rotor 1 (displacement downstream thereof, in the case
of a turbine), which leads the wipers 4 and 5 to penetrate deeper into the abradable layer
10 14 to form new notches 24 and 25 located next to the previous ones and which are deeper,
thanks to the rotation of the windmilling rotor 1 that remains. But when the engine is reignited,
the heating first benefits the rotor 1, which expands the wipers 4 and 5, pushes
them even more significantly into the new notches 24 and 25 and risks leading to a locking
of the rotor 1, proscribed as very dangerous for the safety of the aircraft.
15 Reference is made to Figure 3 to develop this problem of possible locking
of the rotor 1 by an abradable structure 14 that is too hard. A conventional structure of the
abradable 14 is composed of metal strips folded by a passage in a matrix, and several strips
of this species are then assembled and welded by welding points 27 to form the
conventional hexagonal structure of honeycombs. However, when the strips 26 are
20 soldered on the substrate 13, it has been observed that the soldering material rises by
capillary action between the contiguous faces of the strips 26, possibly as far as the free
end of the outer layer of the abradable structure 14 intended for be disposed facing the
wipers, which gives hard sections 28 between the faces in contact with the strips 26, which
increase the resistance of the abradable structure 14 to wear, and all the more when they
25 are oriented in the direction of the movement of penetration of the wipers 4 and 5 into the
abradable structure 14. These portions constitute the portions of the abradable structure
14 which are the most resistant to wear and the most likely to be responsible for locking
the rotor 1.
Efforts have therefore been made to reduce this risk of damage, and a
30 new and improved design of the abradable structure has been proposed.
5
According to the invention, the recommended abradable structure
comprises, in an outer layer provided with partitions delimiting cells, at least three regions
juxtaposed to succeed one another in a direction of the axis and having wear resistances
of progressive values in the direction of the axis.
The regions (which generally correspond to annular 5 bands surrounding
the axis) which are the most resistant to wear will be disposed in front of the wipers of the
other portion of the seal under the conditions of cruising speed, and the regions which are
the less resistant will be placed at the places where the wipers will be likely to come during
events which could predict a locking of the rotor, that is to say, in the example represented
10 above, at the location of the new notches 24 and 25.
This structure of the invention is at least on the outer layer of the
abradable structure, that is to say which comprises the free surface directed towards the
wipers or more generally the structures capable of wearing it, unlike FR-3028882-A; the
free surface may, as the case may be, be at an internal radial or external radial position of
15 the abradable structure.
And since there are three or more regions, with progressive wear
resistances along the axis, that is to say with intermediate values for the intermediate
regions along the axis, an abradability adapted to different engine operating phases is
provided, depending on more or less large axial and radial expansion, in more or less
20 marked transient states.
The preceding explanation can indeed be generalised to different
durations of shutdown of the engine in flight, which then produce different differential
expansions from one situation to another, the values of the expansions in the axial direction
being however correlated with the values in radial direction. Then, the resistance of the
25 abradable structure, and the permeability of the seal to leakage rates can be reduced, only
to the necessary degree, therefore losing little permeability compared to a homogeneous
abradable structure but subjected to the risk of rotor locking.
Now here are some features of preferred embodiments of the invention,
and an explanation of their corresponding advantages. The partitions of each region can
30 first of all advantageously join junctions which connect only three of the cells to each other
6
and are therefore points of intersection for three of the partitions, or for three portions,
which may be in extension, of the partitions. Thus, this avoids, with the junctions of a larger
number of partitions to each other, the formation of hard points corresponding to a greater
local rigidity of the abradable structure, with an increased risk of rotor locking.
It is recommended, to simplify the design and manufacture 5 of the device,
that the cells be in section of polygons which have the same area, which contributes
notably in the homogeneity of the rigidity of the abradable structure.
The polygons or more generally the cells of the different regions can then
be of identical shape, but with either different resistances, or different dimensions, and
10 with sections of different areas. Here again, the design and manufacture of the abradable
structure is facilitated. Besides hexagonal honeycomb cells conventional in the art, it will
be appreciated according to the invention to also be able to select rectangular cells, square
or not, which also simplify the manufacture, while allowing easy junctions between the
different regions of the abradable structure.
15 The rectangular cells are particularly suitable for a preferred embodiment
of the invention, according to which the partitions are all oriented at a non-zero angle with
an angular direction perpendicular to the axis, so as to guarantee the absence of
displacement of a wiper along the entire length of a partition and thus reducing the
corresponding risks of rotor locking. The partitions of the cells can then be oriented at an
20 angle comprised between 30 and 60° relative to the direction of the axis.
However, this disposition is not essential, and the rectangular cells can be
composed of partitions oriented in the direction of the axis and partitions oriented in an
angular direction perpendicular to the axis.
Some other dispositions allow improving the cohesion of the abradable
25 structure. Some of the partitions may thus be main rectilinear partitions which extend
through several of the cells, and others of the partitions are subdivision partitions which
extend only between two consecutive main partitions. This particular disposition is
particularly adapted for an embodiment described below, by allowing a staggered
disposition of the cells between successive bands, which meets the criterion stated above,
7
that the junctions between partitions are all advantageously surrounded by only three of
the cells.
Some of the partitions can also be main rectilinear partitions, which
extend through several of the regions, thus improving the cohesion of the assembly.
The partitions can generally have a thickness comprised 5 between 0.1 mm
and 0.3 mm.
An advantageous manufacturing method is an additive manufacturing
technique, therefore without welding or similar reinforcement between portions of the
abradable structure which would be manufactured separately and which would then have
10 to be assembled, since additive manufacturing can on the contrary give a single-piece
abradable structure. Thus, this also avoids any local hard point of the abradable structure
and the corresponding increase in the locking risk.
Another aspect of the invention is a turbomachine turbine comprising a
labyrinth seal, this seal comprising an abradable structure in accordance with the above,
15 which is annular and preferably sectored, the regions being formed by annular bands of
the abradable structure.
The labyrinth seal may then comprise wipers facing exclusively some of
said annular bands of the labyrinth seal abradable structure, which are more resistant to
wear, in cruising speed, others of said annular bands, but less resistant to wear, being
20 located downstream of the previous ones.
The invention will now be described in more detail by means of the
following figures, appended in an illustrative and non-limiting manner:
- Figure 1, already described, shows in cross-section a known labyrinth
seal;
25 - Figure 2, also described, a usual pattern of wear of the abradable
structure;
- Figure 3, also described, shows the structure of a conventional
abradable;
- Figure 4 illustrates a first embodiment of the invention;
30 - Figures 5 and 6 illustrate a second embodiment of the invention;
8
- Figure 7, a third embodiment of the invention;
- Figure 8, the implementation of an embodiment of the invention in a
labyrinth seal.
A first embodiment of the invention appears in Figure 4. The abradable
structure, henceforth 29, is composed as above of a cellular structure, 5 but which here is
formed of rectilinear partitions 30, which are intersecting and delimiting rectangular cells,
here square cells. The partitions 30 are advantageously all oriented obliquely to the axis X
of the turbomachine, and more favourably between 30 and 60°, notably at 45° to this axis.
In other words, none of the partitions 30 is directed in the angular direction T of the
10 turbomachine perpendicular to the axis X and which is the direction of penetration of the
wipers 4 and 5 into the abradable structure 29. Main partitions 31 of great length, and
subdivision partitions 32 parallel to the preceding ones and extending therebetween, but
only over reduced lengths and therefore along only a portion of these main partitions 31,
can be distinguished among the partitions 30. As a result, the cells have variable areas: one
15 can recognise, a band of small cells 33, a band of medium cells 34, four times larger than
the previous ones, and a band of large cells 35, sixteen times larger than the first ones,
succeeding each other along the axis X. The abradable structure 29 has a single-piece
structure, produced by an additive technique, and the thicknesses of the partitions 30 are
here uniform, of the order of 0.1 mm to 0.3 mm (thin thicknesses being preferred) for
20 typical abradable building materials.
The band of small cells 33 will be placed in front of the wipers 4 and 5
according to their axial location at cruising speed of the engine, which is shown in Figure 8;
while the band of large cells 35 will be placed in front of the positions presumed to have
been reached by the wipers 4 and 5 during the engine re-ignition speeds. The band of
25 medium cells 34 will be intermediate.
The wear resistance of the abradable structure 29 will therefore generally
be determined by the band of small cells 33, while the band of large cells 35 will give a
much lower wear resistance and will be reached by wipers 4 and 5 during the situations
likely to induce locking of the rotor, that is to say at the location of the new notches 24 and
30 25. As a result, the rotor 1 locking risks during restart will be reduced thanks to the locally
9
lower resistance of the abradable structure 29. The use of an additive technique to
manufacture a single-piece structure and form a network of partitions 30, here of uniform
thickness, allows to better control the wear resistance and avoid hard sections due to the
rise of solder joints between the partitions of the abradable structure, the absence of
partitions oriented in the angular direction of the turbomachine 2 (which 5 is perpendicular
to the axis of the turbomachine) having the same effect.
And the medium cell band 34 will give an intermediate, medium wear
resistance and will be reached by the wipers 4 and 5 during situations of stopping the
engine for a shorter duration, at the end of which the thermal expansions will be less
10 significant, as well as the risks of rotor locking at restart. It will therefore be perfectly
relevant that the abradable structure has a local resistance lower than in the band of small
cells 33, but stronger than in the band of large cells 35, so that its overall permeability is
not too reduced.
A second embodiment of the invention is described by means of Figures
15 5 and 6. The abradable structure has the general reference 60 and comprises three
successive bands 61, 62 and 63, which are respectively composed of small cells, medium
cells and large cells, like before. The cells (all referenced by 66) are also rectangular, in
accordance with embodiments considered interesting for the invention. There are two
significant differences compared to the previous design. The first is the orientation of the
20 partitions delimiting the cells 66, which are here all disposed either in the direction of the
axis X of the machine, or in its angular direction T. An orientation of the partitions which
contributes in avoiding possible excessive friction of the wipers on the abradable structure
60, and the corresponding locking risks is gave up, as in the previous embodiment.
This risk is here reduced by an original disposition: the partitions of each
25 of the bands 61, 62 and 63 are composed of main partitions 64, parallel to each other and
here all oriented in the angular direction T, which extend along several of the cells and
optionally over the entire width of the abradable structure 60, that is to say over its
circumference, or a portion of its circumference, depending on whether it is manufactured
in a ring or several annular segments then each extending over a sector of a circle; it is also
30 composed of subdivision partitions 65, which extend between the main partitions 64, and
10
more specifically only between two neighbouring main partitions 64; and the subdivision
partitions 65 on either side of each main partition 64 do not match with each other, which
gives cells 66 disposed with an angular offset, or in a staggered manner, with the cells 66
on the other side of the main partitions 64; and the network of partitions 64 and 65
composing the abradable structure 60 comprises, for each of the bands 5 61, 62 and 63,
junction points 67 between the partitions 64 and 65 which are all surrounded by only three
cells 66, and are located at the intersection of only three portions of the partitions 64 and
65, two belonging to one of the main partitions 64 and the last one to a subdivision partition
65. The junctions 67 thus have a moderate rigidity which reduces the rotor locking risk if
10 the wipers 4 and 5 rub thereon. The main partitions 64 also contribute to the cohesion of
the abradable structure 60, thanks to their large extension. A reverse disposition of the
main partitions 64 and the partitions 65, in the axial X and angular T direction respectively,
would also be possible.
Another embodiment of the invention is illustrated in Figure 7. The
15 abradable structure, then referenced by 36, is also composed of partitions 47, here also
intersecting to form cells 37. It is further composed of bands 43, 44, 45, 46, here four in
number, juxtaposed in the direction of the axis X. Here, however, the spacings between the
partitions 39 are identical from one band to another, so that the cells 37 all have the same
dimensions and the same area. The variation in resistance of the abradable structure
20 between the bands 43, 44, 45 and 46 is obtained by embrittlement zones 38, which may
consist, for example, in decreases in thickness of the partitions 47, less resistant materials
or the like. Such embrittlement 38 can be easily obtained in additive manufacturing by
three-dimensional printing, where the process is entirely piloted by a program. The
embrittlement zones 38 are absent from the main band 43, and more and more numerous
25 or dense in the bands 44, 45 and 46. The effect is the same as before: the resistance of the
abradable structure 36 is increasingly lower from the band 43 to the opposite band 46.
Other means of creating the embrittlement zones 38 would consist of
creating notches or perforations through the partitions 47.
The different features of these embodiments can generally be combined
30 with one another.
11
An important aspect of the invention is that the different bands of the
abradable structure 29 or 36 have originally the same height in front of a respective wiper,
which is indicated by the level lines 39, 40 and 41 in Figures 7 and 8. In other words, the
zones of the abradable structure 29 or 36 capable of locking the rotor are not originally cut
off, which would have had the effect of reducing this risk of interference 5 with the wipers 4
and 5, but at the cost of reduced performance during normal service of the turbomachine.
In a general manner, it is sufficient for the structure that has been
described to extend in an outer layer of the abradable structure, limited by a free surface
facing the wipers: such an outer layer is shown in Figure 7, where it bears the reference 55,
10 and where the structural heterogeneities are concentrated; the abradable structure could
also comprise other layers 56, underlying the outer layer 55, constructed with another
structure and optionally homogeneous, without departing from the scope of the invention.
12
WE CLAIM:
1) A labyrinth seal abradable structure (29, 36) disposed around an axis
(X), characterised in that it comprises, in an outer layer (55) provided with partitions (31,
32; 47; 64, 65) delimiting cells, at least three regions (33, 34, 35; 43, 44, 45, 5 46; 61, 62, 63)
juxtaposed to succeed one another in a direction of the axis (X) and having wear resistances
of progressive values in the direction of the axis.
2) The labyrinth seal abradable structure according to claim 1,
characterised in that the partitions of each region are joined to junctions which connect
10 only three cells together.
3) The labyrinth seal abradable structure according to any one of claims
1 or 2, characterised in that, for each of the regions, the cells are in section of the polygons
which have the same area.
4) The labyrinth seal abradable structure according to claim 3,
15 characterised in that the cells have, in cross-section, a different area for each region.
5) The labyrinth seal abradable structure according to any one of claims
1 to 4, characterised in that the cells are rectangular.
6) The labyrinth seal abradable structure according to claim 3,
characterised in that at least one cell of a region has a local embrittlement (38) so as to
20 have a lower wear resistance than the other cells of the region.
7) The labyrinth seal abradable structure according to any one of claims
1 to 6, characterised in that the partitions of the cells are oriented at an angle comprised
between 30 and 60° relative to the direction of the axis (X).
8) The labyrinth seal abradable structure according to any one of claims
25 1 to 7, characterised in that some of the partitions are main rectilinear partitions which
extend through several of the cells, and others of the partitions are subdivision partitions
which extend only between two consecutive main partitions.
9) The labyrinth seal abradable structure according to any one of claims
1 to 8, characterised in that some of the partitions are main rectilinear partitions which
30 extend through several of the regions.
13
10) The labyrinth seal abradable structure according to any one of claims
1 to 9, characterised in that the partitions have a thickness comprised between 0.1 mm and
0.3 mm.
11) The labyrinth seal abradable structure according to any one of the
preceding claims, characterised in that it is manufactured by an additive 5 manufacturing
technique and has a structure formed in a single-piece.
12) A turbomachine turbine comprising a labyrinth seal, characterised in
that it comprises a labyrinth seal abradable structure according to any one of the preceding
claims, which is annular and preferably sectored, said regions being formed by annular
10 bands of the abradable structure labyrinth seal.
13) The turbomachine turbine according to claim 12, characterised in that
the labyrinth seal comprises wipers facing exclusively some of said annular bands of the
labyrinth seal abradable structure, which are more resistant to wear, in cruising speed,
others of said annular bands, but less resistant to wear, being located downstream of the
15 previous ones.