Patent Publication Number: US-2022228502-A1

Title: Dynamic seal for a turbomachine comprising a multi-layer abradable part

Description:
TECHNICAL FIELD 
     The invention relates to the field of turbomachines, in particular for an aircraft, and more specifically to dynamic seals implemented in turbines or compressors of such turbomachines. The invention applies to any type of turbomachine, such as a turbojet or turboprop engine. 
     STATE OF PRIOR ART 
     A dynamic seal optimises the performance of a turbomachine, typically by reducing leakage of pressurised gases. 
       FIG. 2  shows an aircraft turbomachine turbine  90  of prior art. Conventionally, this turbine  90  comprises a number of stages for recovering some of the combustion energy to rotate a rotor of the turbine. Each stage comprises a stationary vane  91 , belonging to a stator of the turbine  90 , and a movable vane  92  constituting part of the rotor. The stationary vane  91  is radially inwardly delimited by an inner annular wall  93 . A dynamic seal  94  is typically provided under the radially inner face of this annular wall  93  to limit gas circulation radially inwardly of it. 
     The seal  94  in  FIG. 2  comprises an abradable wear part  95 , integral with the stator of the turbine  90 , and a pair of strip seals  96  integral with the rotor. The strip seals  96  are arranged to interact with the wear part  95  so that, at least during a running-in phase of the turbomachine, when the rotor and therefore the strip seals  96  are rotatably driven, contact of the strip seals  96  with the wear part  95  tends to wear the latter. 
     Typically, the wear part  95  consists of a honeycomb structure, which promotes its abradability. This structure generally forms cells, the depth of which defines the thickness of the wear part  95 . 
     One drawback of this type of dynamic seal is especially that it leads to residual gas leakage, which can typically bypass the strip seals by passing inside the cells, thus limiting effectiveness of the seal. 
     It is a purpose of the invention to improve effectiveness of such a dynamic seal by reducing the residual leakage produced at the contact zone between the strip seal(s) and the wear part. 
     Another purpose of the invention is to provide a dynamic seal the wear part of which has good abradability properties while allowing it to be manufactured at reduced costs. 
     DISCLOSURE OF THE INVENTION 
     To this end, one object of the invention is a dynamic seal for an aircraft turbomachine, comprising a stationary part provided with at least one abradable wear part and a part rotatably movable about a central axis, the movable part comprising at least one strip seal arranged to interact with the at least one wear part during rotation of the movable part about the central axis. 
     According to the invention, the at least one wear part includes a structure forming cavities arranged in one or more series so that, in each series, the cavities of that series are superimposed radially with respect to the central axis, said structure being shaped at least to limit gas circulation between each pair of radially adjacent cavities, each cavity of the at least one wear part constituting a channel extending circumferentially with respect to the central axis over the whole circumferential dimension of the wear part. 
     The phrase “[. . . ] shaped at least to limit [. . . ]” means that said structure of the at least one wear part is shaped:
         either to limit gas circulation between each pair of radially adjacent cavities, for example by an air flow cross-section restriction,   or to prevent any gas circulation between each pair of radially adjacent cavities, for example by plugging one of these cavities with respect to the other.       

     Such a radial superimposition of cavities makes it possible to reduce the effective volume of a cavity in which the strip seal engages during rotation of the movable part of the seal, regardless of the thickness of the wear part. This is because, for example, when the wear part is faintly worn, this cavity is radially adjacent to another cavity from which it is separated by part of the structure of the wear part which limits or prevents gas circulation from one of these cavities to the other. This makes it possible to reduce residual leakage at the contact zone between the strip seal and the wear part, since gases likely to bypass the strip seal by passing through such a cavity have a reduced volume relative to a structure which would not radially delimit several cavities. 
     The invention therefore makes it possible to produce a wear part comprising several layers of cavities so as to reduce residual leakage, regardless of the wear level of the wear part. 
     According to a first alternative embodiment, said structure of the at least one wear part may form, between each pair of radially adjacent cavities, a solid wall preventing any gas circulation from one of these cavities to the other. 
     According to a second alternative embodiment, said structure of the at least one wear part may form, between each pair of radially adjacent cavities, an obstacle providing an opening between these cavities so as to limit gas circulation from one of these cavities to the other. 
     Indeed, it is not essential to completely isolate two radially adjacent cavities to significantly reduce residual leakage. A cross-section restriction leaving an opening between two radially adjacent cavities may turn out to be sufficient to prevent some or all of the gases bypassing the strip seal from passing through such an opening, depending of course on the dimensions of such an opening relative to the dimensions of the cavities. Such an opening may typically be used to discharge powder accumulated in a cavity upon manufacturing the wear part. 
     Preferably, the at least one wear part may include several series of cavities extending circumferentially with respect to the central axis. 
     The stationary part of the dynamic seal may be provided with a single wear part forming a ring centred on said central axis. 
     In this case, said circumferential dimension of this single wear part is 360°. 
     Alternatively, the stationary part may be provided with several wear parts circumferentially arranged end to end so as to form together a ring centred on said central axis. 
     In this case, said circumferential dimension of each of the wear parts is less than 360°. 
     The invention also relates to a turbine or compressor comprising such a dynamic seal, as well as a turbomachine equipped with such a turbine and/or compressor and more generally with such a dynamic seal. 
     Another object of the invention is a method for manufacturing such a dynamic seal. 
     Preferably, this method may comprise a step of additively manufacturing the at least one wear part. 
     Additive manufacturing makes it possible to obtain a wear part with good structural qualities and abradability at reduced costs. 
     The abradability of a wear part thus manufactured is improved in comparison with a wear part manufactured by a conventional method. In prior art, an abradable honeycomb-type wear part is typically manufactured by welding embossed metal sheets, with the welds tending to harden the part and thus reduce its ability to wear. 
     In one embodiment, this method may comprise a step of additively manufacturing, as a single piece, the at least one wear part and a support element for the stationary part of the dynamic seal. 
     The one-piece manufacture of the wear part and its support element makes it possible to achieve good abradability properties over the whole thickness of the wear part, in comparison with assembly by soldering, which tends to harden the wear part, especially because of the solder wicking. 
     Further advantages and characteristics of the invention will become apparent from the following detailed, non-limiting description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The following detailed description refers to the appended drawings in which: 
         FIG. 1  is a schematic axial cross-section view of an aircraft propulsion assembly comprising a turbomachine of the turbofan engine type; 
         FIG. 2  (already described) is a partial schematic axial cross-section half-view of a low-pressure turbine of a turbomachine, comprising a dynamic seal of prior art; 
         FIG. 3  is a partial schematic view of a dynamic seal according to the invention; 
         FIG. 4  is a partial schematic perspective view of a dynamic seal wear part according to a first embodiment; 
         FIG. 5  is a partial schematic axial cross-section view of a dynamic seal wear part according to a second embodiment; 
         FIG. 6  is a partial schematic view of the wear part of  FIG. 5 , showing the hexagonal shape of the cavities; 
         FIG. 7  is a partial schematic perspective view of a dynamic seal wear part according to a third embodiment in accordance with the invention; 
         FIG. 8  is a partial schematic axial cross-section view of the wear part of  FIG. 7 ; 
         FIG. 9  is a partial schematic perspective view of a dynamic seal wear part according to a fourth embodiment in accordance with the invention; 
         FIG. 10  is a partial schematic axial cross-section view of the wear part of  FIG. 9 ; 
         FIG. 11  is a partial schematic perspective view of a dynamic seal wear part according to a fifth embodiment in accordance with the invention; 
         FIG. 12  is a partial schematic axial cross-section view of the wear part of  FIG. 11 ; 
         FIG. 13  is a partial schematic perspective view of a dynamic seal wear part according to a sixth embodiment in accordance with the invention; 
         FIG. 14  is a partial schematic axial cross-section view of the wear part of  FIG. 13 . 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     The invention relates to a turbomachine, for example for an aircraft propulsion assembly  1  (not represented) as represented in  FIG. 1 . In this example, the turbomachine  10 , which is housed in a nacelle  11  of the propulsion assembly  1 , is a turbofan engine well known in the aeronautical field. Of course, the invention is not limited to such a turbomachine and can be applied to any type of turbomachine, such as a turboprop engine for example. 
     The turbomachine  10  has a central longitudinal axis A 1  about which its various components extend, in this case, from upstream to downstream of the turbomachine  10 , a fan  2 , a low-pressure compressor  3 , a high-pressure compressor  4 , a combustion chamber  5 , a high-pressure turbine  6  and a low-pressure turbine  7 . The compressors  3  and  4 , the combustion chamber  5  and the turbines  6  and  7  form a gas generator. 
     Conventionally, during operation of such a turbofan engine  10 , an air flow  8  enters the propulsion assembly  1  through an air intake upstream of the nacelle  11 , passes through the fan  2  and then splits into a central primary flux  8 A and a secondary flux  8 B. The primary flux  8 A flows in a main stream  9 A for circulating gases passing through the compressors  3  and  4 , the combustion chamber  5  and the turbines  6  and  7 . The secondary flux  8 B in turn flows in a secondary stream  9 B surrounding the gas generator of the turbojet engine  10  and radially outwardly delimited by the nacelle  11 . 
     Throughout this description, the terms “upstream” and “downstream” are defined in relation to a main direction D 1  of gas flow through the propulsion assembly  1  along the axial direction X. The terms “inner” and “outer” refer respectively to a relative proximity, and a relative distance, of an element with respect to the central axis A 1 . The axial direction X is a direction parallel to the longitudinal central axis A 1  of the turbomachine  10 ; the radial direction R is, at any point, a direction orthogonal to and passing through the central axis A 1 ; and the circumferential or tangential direction C is, at any point, a direction orthogonal to the radial direction R and to the central axis A 1 . 
       FIG. 2 , already described, represents in more details part of a turbine  90  of prior art. Such a turbine typically constitutes the low pressure turbine  6  of a turbojet engine of a propulsion assembly of the type represented in  FIG. 1 . 
     The invention is more specifically concerned with a dynamic seal, which may especially replace the dynamic seals  94 ,  97  and/or  98  of the turbine  90  of  FIG. 2 . 
     A dynamic seal  20  is partially represented in  FIG. 3 . This seal  20  consists of a stationary part  21  and a movable part  22 . 
     The stationary part  21  of the seal  20  is to be connected to a turbomachine stator, for example to the stator of the compressor  3  or  4 , or of the turbine  6  or  7  of the turbomachine  10 . With reference to the known configuration of  FIG. 2 , the stationary part  21  of the seal  20  can thus be fixed to the radially inner face of the inner annular wall  93  of the stationary vane  91  of the turbine  90 . 
     More precisely, the stationary part  21  of the seal  20  comprises a wear part  23  and a support element  24  for holding the wear part  23  fixedly relative to the movable part  22 . Still referring to the known configuration of  FIG. 2 , attaching the stationary part  21  of the seal  20  to the inner annular wall  93  may here be achieved by attaching the support element  24  to said radially inner face of this wall  93 . 
     In this example, the wear part  23  is located radially inwardly of the support element  24 . 
     The movable part  22  of the seal  20  is to be connected to a turbomachine rotor, for example to the rotor of the compressor  3  or  4 , or of the turbine  6  or  7  of the turbomachine  10 . With reference to the known configuration of  FIG. 2 , the movable part  22  of the seal  20  may be connected to the annular flange  99  connecting the movable vanes  92 . The movable part  22  of the seal  20  can thus be secured to the rotor, rotating about the central axis A 1  of the turbomachine  10 . 
     In the example shown in  FIG. 3 , the movable part  22  of the seal  20  comprises two annular strip seals  25  arranged to interact with the wear part  23  during rotation of the movable part  22  about the central axis A 1 . 
     Each strip seal  25  comprises a machining end  26  arranged facing and at a short distance from the wear part  23 , in order to limit as much as possible the gas flow between the stationary part  21  and the movable part  22  of the seal  20 , in the manner of a sealing labyrinth. 
     In a manner known per se, the strip seals  25  and the wear part  23  are made of respective materials allowing the strip seals  25  to machine the wear part  23  with their machining end  26  during rotation of the rotor. In other words, the wear part  23  is abradable. 
     The invention is more specifically characterised by the structure of the wear part  23 , several embodiments of which are illustrated in  FIGS. 7 to 14 . 
       FIGS. 4 to 6  show other embodiments of a wear part  23  which are not part of the invention. 
     In each of the embodiments in  FIGS. 4 to 14 , the wear part  23  has a structure forming multiple layers of cavities. 
     More specifically, with reference to the embodiment in  FIG. 4 , the wear part  23  comprises several layers of cavities spaced apart along the radial direction R. 
     In this example, for each layer, the cavities extend both axially, that is along the axial direction X, and circumferentially, that is along the circumferential direction C. 
     In other words, the cavities of the wear part  23  are arranged in several series so that, in each series, the cavities of that series are superimposed radially with respect to the central axis A 1 . 
     The wear part  23  is shaped to limit or prevent gas circulation between each pair of radially adjacent cavities, that is to limit or prevent gas circulation from a first cavity belonging to one of the layers to a radially adjacent second cavity, that is belonging to a layer adjacent to the layer including the first cavity. 
     In the embodiments in  FIGS. 4 to 6 , each layer has a honeycomb structure, the wear part  23  thus forming a structure including a superimposition of honeycomb-type cell cores. The cavities of each series respectively consist of respective cells of the cell cores. 
     In the example in  FIG. 4 , the wear part  23  comprises five cores A 41 -A 45  separated in twos by walls  231 . The walls  231  comprise holes  232  each forming an opening between two radially adjacent cavities. The holes  232  have a diamond-shaped cross-section in this example and are for discharging powder (see below). The holes  232  are sized to allow such powder discharge while limiting the flow rate of gas that may pass from one cavity to the other. Of course, depending on the manufacturing method used, such holes  232  may be unnecessary so that, in embodiments not represented, the walls  231  may be solid and completely plugs the cavities with respect to each other. 
     In the example in  FIGS. 5 and 6 , the wear part  23  comprises three cores A 51 -A 53  separated in twos by walls  231 . In this example, the walls  231  comprise hexagonal cross-section holes  232  each forming an opening between two radially adjacent cavities. The function of these holes is similar to that of the holes in the embodiment of  FIG. 4  (see above). 
     In the embodiments in  FIGS. 7 to 14  in accordance with the invention, each cavity forms a channel extending circumferentially with respect to the central axis A 1 , over the whole circumferential dimension of the wear part  23 . 
     In the embodiment in  FIGS. 7 and 8 , the wear part  23  comprises circumferential bars  233 . Each bar  233  comprises a central portion  234  and radially spaced apart branches  235  so as to define cavities C 1 -C 4  open onto each other. The branches  235  form air flow cross-section restrictions between radially adjacent cavities (see  FIG. 8 ). In the example illustrated, each branch  235  is fir tree shaped. 
     In other words, the branches  235  form an obstacle between each pair of radially adjacent cavities, providing an opening between these cavities so as limit the gas circulation from one of these cavities to the other. 
     The openings provided between radially adjacent cavities have a dimension D 1  capable of significantly limiting the gas circulation between these cavities, given their own dimensions. Of course, these openings can also be used to discharge powder, in particular when the wear part  23  is annular and manufactured by an additive manufacturing method by powder bed laser melting (see below). 
     In the embodiment of  FIGS. 9 and 10 , the wear part  23  has walls  236  axially delimiting the cavities and walls  237  radially delimiting the cavities. 
     In this example, the walls  236  and  237  are solid and therefore prevent any gas circulation between adjacent cavities. 
     In one embodiment not represented, the walls  237  may comprise holes or openings between radially adjacent cavities for de-powdering (see later). 
     With reference to  FIG. 10 , the cavities of this wear part  23  have a substantially rectangular cross-section, the walls  236  and  237  being substantially straight along the radial R and axial X direction respectively. 
     The embodiment of  FIGS. 11 and 12  is distinguished from that of  FIGS. 9 and 10  by the shape of the walls  237 : each wall  237  radially delimiting two adjacent cavities comprises two parts  2371  and  2372  each extending both along the axial direction X and along the radial direction R, so that the cavities have a “V”-shaped axial cross-section. 
     Of course, the walls  236  and  237  of these different embodiments may have different shapes and hence the cavities of the wear part  23  may have a cross-section of any shape imparting the capacity to perform its function to the wear part  23 . 
     In the embodiment of  FIGS. 13 and 14 , the wear part  23  has walls  238  and  239  that are substantially straight in the plane X-R and perpendicular to each other, each extending along both the axial direction X and along the radial direction R, so that the cavities each have a substantially square cross-section. 
     In this example, most of the cavities are each radially adjacent to at least three other cavities. For example, cavity C 5  is adjacent to cavities C 6 , C 7  and C 8 . Cavities C 5  and C 8  are delimited by a node N 1  formed by the intersection of two walls  238  and  239 . Cavity C 5  is separated from cavity C 6  by a wall  238 , and from cavity C 7  by a wall  239 . 
     The walls  238  and  239  are solid and therefore prevent any gas circulation between adjacent cavities. 
     The axial dimension of the wear part  23  especially depends on the number and dimension of the strip seals  25 . 
     The circumferential dimension of the wear part  23  may vary depending on whether the stationary part  21  is provided with a single or several wear parts  23 . In the first case, the wear part  23  typically forms a ring centred on the central axis A 1 , in which case its circumferential dimension is equal to 360°. In the second case, the stationary part  21  may comprise several wear parts  23  circumferentially arranged end to end so as to together form a ring centred on said central axis A 1 , in which case the circumferential dimension of each of the wear parts  23  is less than 360°. This applies regardless of the embodiment described above. 
     Regarding the manufacture of the wear part  23 , this may be carried out by additive manufacturing, in particular by means of a method of selectively laser melting metal powder layers. 
     After the wear part  23  has been manufactured, the cavities are likely to contain residual powder which has to be discharged before implementing the dynamic seal. 
     The powder can be discharged:
         in the examples of  FIGS. 4 to 6 , through holes  232 ,   in the example of  FIGS. 7 and 8 , through the openings formed by the branches  235  and/or the circumferential ends of the cavities if the wear part  23  does not form a closed ring,   in the examples of  FIGS. 9 to 14 , through the circumferential ends of the cavities if the wear part  23  does not form a closed ring, and/or through openings (not represented) between adjacent cavities.       

     The cavities or openings for powder discharge may be of any shape in cross-section, for example diamond, square, round, triangle, or hexagon, provided that they are small enough to limit gas circulation between radially adjacent cavities. 
     In order to improve abradability of the wear part  23 , both the wear part  23  and the support element  24  of the stationary part  21  may be manufactured as a single piece, using an additive manufacturing method. 
     The examples just described are by no means limiting. 
     By way of example, the walls defining two adjacent cavities may have a thickness in the order of 0.08 mm and the cavities may have a radial dimension of between 0.8 mm and 2 mm.