Abstract:
A turbomachine module, comprising a mobile wheel rotatably mounted inside a housing of the module and surrounded by a sectorised sealing ring ( 18 ) that comprises an annular row of sectors, each ring sector comprising at least one circumferential hook that is configured to engage with an annular hooking rail of the housing, the module further comprising a sectorised protective annular shim ( 50 ) that is interposed between the hooks of the ring sectors and the rail of the housing and that comprises an annular row of sectors of shim, characterised in that the edges ( 60 ) of the circumferential ends of the sectors of shim are not aligned with the edges ( 58 ) of the circumferential ends of the ring sectors along the longitudinal axis of the module.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a turbine engine module, which may be a turbine or form part of a turbine for example. 
       PRIOR ART 
       [0002]    The prior art comprises in particular the documents WO-A1-98/53228, EP-A2-2 612 998 and EP-A2-2 508 715. 
         [0003]    A turbine engine turbine comprises one or more stages, each comprising a nozzle formed by an annular row of fixed blades carried by a casing of the turbine, and an impeller rotatably mounted in general downstream of the nozzle. The impeller is surrounded by a sealing ring that is sectorised and formed by sectors that are arranged circumferentially end to end and are attached to the turbine casing. 
         [0004]    Each ring sector in general comprises a circumferentially oriented metal plate that carries a block of abradable material fixed to the inner surface of the plate. This block is for example of the honeycomb type and is intended to wear by friction on external annular wipers of the blades of the impeller, in order to form a labyrinth seal and to minimise the radial clearances between the impeller and the ring sectors. 
         [0005]    Each ring sector comprises, at its upstream and downstream ends, means for attaching to the casing. Each ring sector may comprise, at its upstream end, a circumferential hook that defines an annular groove in which there both an annular rail of the casing and a downstream circumferential hook of the nozzle, located upstream, are engaged. The downstream circumferential hook of the nozzle is kept radially clamped against the casing rail by means of the upstream circumferential hook of the ring, which comprises two coaxial annular walls which extend one inside the other and inside the hook of the nozzle and outside the casing rail, respectively. This makes it possible to participate in the radial holding of the nozzle relative to the casing. The nozzle can be held circumferentially or tangentially by means of an anti-rotation pin that is carried by the casing and is engaged in a recess in the nozzle. Said nozzle is generally axially held in the downstream direction by an annular split ring that is mounted in an annular groove in the aforementioned casing rail which emerges radially towards the outside. The downstream circumferential hook of the nozzle is in axial abutment in the downstream direction on this ring, which is held radially in the groove in the casing rail by the internal walls of the hooks of the ring sectors which extend radially inside the ring. In a variant, the axial stop function of this ring may be provided directly by the casing rail. 
         [0006]    It is known to use an annular foil for protecting the casing rail, in particular against wear and high temperatures. This foil is sectorised and comprises an annular row of foil sectors arranged circumferentially end to end. Said foil has a generally U-shaped or C-shaped cross section and comprises two coaxial annular walls, inner and outer, respectively, that are interconnected by a middle bottom wall. 
         [0007]    The opening of the hooks of the ring sectors is oriented axially upstream and receives the foil sectors, which are designed so that their walls line those of the hooks of the ring sectors. The inner walls of the foil sectors are intended to extend over the radially outer faces of the inner walls of the hooks of the ring sectors, the outer walls of the foil sectors are intended to extend over the radially inner faces of the outer walls of the hooks of the ring sectors, and the bottom walls of the foil sectors are intended to extend over the upstream radial faces of the bottom walls of the hooks of the ring sectors. 
         [0008]    In the position of mounting the ring sectors on the casing rail, the inner walls of the foil sectors are interposed between the inner walls of the hooks of the ring sectors and the hooks of the nozzle, or even the annular ring, the outer walls of the foil sectors are interposed between the outer walls of the hooks of the ring sectors and the casing rail, and the bottom walls of the foil sectors are interposed between the bottom walls of the hooks of the ring sectors and the casing rail. 
         [0009]    The foil sectors are made from sheet metal and make it possible to prevent any direct contact between the hooks of the ring sectors and the casing rail, which makes it possible both to protect said rail against wear by friction and to protect it thermally from the ring, which may be very hot in operation because of its proximity to the combustion gases flowing in the turbine duct. 
         [0010]    Because of the sectorisation of the ring, the longitudinal edges of the circumferential ends of two adjacent sectors of the ring face one another and are separated from one another by a circumferential clearance through which the hot gases of the duct can pass. These hot gases have a tendency to heat the casing, which is detrimental for several reasons. One of the reasons is that heating of the casing would cause expansion and deformation thereof, which would risk altering the radial clearances between the movable impeller and the ring, and therefore reducing the performance of the turbine. A known solution to this problem consists in interposing sealing tongues between the ring sectors, which tongues are housed in grooves in the aforementioned longitudinal edges of said ring sectors. 
         [0011]    However, because of the sectorisation of the foil, the longitudinal edges of the circumferential ends of two adjacent foil sectors face one another and are separated from one another by a circumferential clearance. In the prior art, the circumferential clearances between the foil sectors are aligned axially with the circumferential clearances between the ring sectors, and in particular with the circumferential clearances between the hooks of the ring sectors in the region of which it is not possible to mount tongues of the aforementioned type, in particular for reasons of space requirement. Hot gases can thus pass through the circumferential clearances between the hooks of the ring sectors and between the foil sectors and heat the casing rail, which risks reducing its service life. 
         [0012]    The object of the present invention is in particular to provide a simple, effective and economical solution to this requirement by improving, in particular, the thermal protection of the casing rail in the aforementioned case. 
       DISCLOSURE OF THE INVENTION 
       [0013]    The present invention thus proposes a turbine engine module comprising a movable impeller that is rotatably mounted inside a casing of the module and is surrounded by a sectorised sealing ring that comprises an annular row of ring sectors arranged such that circumferential end edges of two adjacent sectors substantially face one another, each ring sector comprising at least one circumferential hook that is designed to cooperate with an annular attachment rail of the casing, the module further comprising an annular sectorised protective foil that is interposed between the hooks of the ring sectors and the casing rail and comprises an annular row of foil sectors arranged such that circumferential end edges of two adjacent sectors substantially face one another, characterised in that the number of ring sectors is equal to the number of foil sectors, and in that the foil sectors comprise positioning means and/or rotational locking means designed so that the edges of the circumferential ends of the foil sectors are not aligned with the edges of the circumferential ends of the ring sectors along the longitudinal axis of the module. 
         [0014]    The invention makes it possible to better protect the casing rail, since the gases that would be liable to pass between the edges of the circumferential ends of the ring sectors would then be blocked by the foil sectors (because of the angular offset thereof relative to the ring sectors) and would not reach as far as the casing rail. 
         [0015]    The module according to the invention may comprise one or more of the following features, taken in isolation or in combination with one another:
       the number of ring sectors is equal to the number of foil sectors;   the ring sectors are arranged so as to be staggered with respect to the foil sectors;   the hooks of the ring sectors have a generally U-shaped or C-shaped cross section, the opening of which is oriented axially, and each comprise a middle bottom wall that connects two coaxial annular walls that are radially inner and outer, respectively;   the foil sectors have a generally U-shaped or C-shaped cross section, the opening of which is oriented axially, and each comprise a middle bottom wall that connects two coaxial annular walls, radially inner and outer, respectively, the foil sectors being engaged in the openings of the hooks of the ring sectors and mounted on the casing rail so that the inner walls of the foil sectors are interposed between an inner face of the casing rail and the outer walls of the hooks of the ring sectors, so that the bottom walls of the foil sectors are interposed between a substantially radial face of the casing rail and the bottom walls of the hooks of the ring sectors, and so that the outer walls of the foil sectors are interposed between an outer face of the casing rail and the outer walls of the hooks of the ring sectors;   the inner walls of the hooks of the ring sectors have a radius of curvature different from that of the casing rail so as to be mounted in a radially pre-stressed manner on the rail,   the inner walls of the foil sectors comprise radial recesses that emerge on free circumferential edges of the foil sectors and are substantially aligned axially with the edges of the circumferential ends of the hooks of the ring sectors; these recesses form positioning means within the meaning of the invention;   said recesses are each generally V-shaped and are substantially formed at the centre of the inner walls of the foil sectors;   the circumferential ends of the inner walls of the hooks of the ring sectors are in radial abutment on the inner walls of the foil sectors, substantially in line with the bottoms of the recesses;   the inner or outer walls of the hooks of the ring sectors comprise radial recesses substantially at the centre thereof, and the inner or outer walls of the foil sectors comprise either radial end recesses that are substantially aligned radially with the aforementioned recesses of the hooks of the ring sectors, or foldable radial lugs that are designed so as to be folded and engaged in the aforementioned recesses of the hooks of the ring sectors; these recesses and/or lugs form means for rotational locking (about the longitudinal axis of the module) within the meaning of the invention; and   the module is a turbine.       
 
         [0026]    The present invention also relates to a turbine engine, comprising at least one module as described above. 
         [0027]    Finally, the present invention relates to a sectorised annular protective foil for a module as described above, comprising an annular row of foil sectors, in which each foil sector has a generally U-shaped or C-shaped cross section, the opening of which is oriented axially, and comprises a middle bottom wall that connects two coaxial annular walls, radially inner and outer, respectively, said inner walls comprising radial recesses substantially at the centre thereof which emerge on free circumferential edges of the sectors. 
     
    
     
       DESCRIPTION OF THE FIGURES 
         [0028]    The invention will be understood better and other details, features and advantages of the invention will emerge from reading the following description given by way of non-limitative example and with reference to the accompanying drawings, in which: 
           [0029]      FIG. 1  is a schematic partial half-view in axial section of a turbine engine turbine; 
           [0030]      FIG. 2  is a larger-scale schematic view of part of  FIG. 1  and shows a sealing ring and an annular foil of the turbine; 
           [0031]      FIG. 3  is a schematic partial plan view of the sealing ring and of the annular foil of a turbine according to the prior art; 
           [0032]      FIG. 4  is a schematic partial plan view of the sealing ring and of the annular foil of a turbine according to the invention; 
           [0033]      FIG. 5  is a schematic view in cross section along the line V-V in  FIG. 2 ; 
           [0034]      FIGS. 6 and 7  are schematic perspective views of a ring sector and of a foil sector, according to an embodiment of the invention; 
           [0035]      FIG. 8  is a highly schematic partial view from below of the foil of  FIGS. 6 and 7 ; and 
           [0036]      FIGS. 9 and 10  are schematic views, in perspective and in axial section, respectively, of a variant of a ring sector and of a foil sector. 
       
    
    
     DETAILED DESCRIPTION 
       [0037]    Reference is made first of all to  FIGS. 1 and 2 , which show a turbine  10 , in this case low pressure, of a turbine engine such as an aeroplane turbojet engine or turboprop engine, said turbine comprising a plurality of stages (only one of which is shown here) each comprising a nozzle  12  formed by an annular row of fixed blades carried by a casing  14  of the turbine, and an impeller  16  mounted downstream of the nozzle  12  and rotating in a ring  18  attached to the casing  14 . 
         [0038]    The ring  18  is sectorised and formed by a plurality of sectors that are carried circumferentially end to end by the casing  14  of the turbine. 
         [0039]    Each ring sector  18  comprises a frustoconical wall  20  and a block  22  of abradable material that is fixed by brazing and/or welding to the radially inner surface of the wall  20 , this block  22  being of the honeycomb type and being intended to wear by friction on outer annular wipers  24  of the blades of the impeller  16  in order to minimise the radial clearances between the impeller and the ring sectors  18 . 
         [0040]    Each ring sector  18  comprises, at its upstream end, a circumferential hook  32  having a C-shaped or U-shaped cross section, the opening of which emerges in the upstream direction, and which is engaged, at one end, axially from the downstream direction on a cylindrical hook  34  oriented in the downstream direction of the nozzle  12  located upstream of the ring sectors  18 , and, at the other end on a cylindrical rail  36  of the casing  14  to which said nozzle is attached. 
         [0041]    The hook  32  of each ring sector  18  comprises two circumferential walls  38  and  40 , radially outer and radially inner, respectively, that extend in the upstream direction, are interconnected at their upstream ends by a substantially radial middle bottom wall  42 , and extend radially to the outside and to the inside, respectively, of the rail  36 , the inner wall  40  holding the hook  34  of the nozzle radially against the rail  36 . 
         [0042]    The nozzle  12  is held circumferentially by means of an anti-rotation pin  44  that is carried by the casing  14  and is engaged in a recess in the nozzle  12 . Said nozzle is held axially in the downstream direction by an annular split ring  46  that is mounted in an annular groove  48  in the rail  36  which emerges radially towards the inside. The hook  34  of the nozzle  12  is in axial abutment in the downstream direction on the ring  46 , which is held radially in the groove in the casing rail by the inner wall  40  which extends radially inside the ring  46 . In a variant, the axial stop function of the ring  46  can be provided directly by the casing rail  36 . 
         [0043]    The downstream ends of the ring sectors  18  are clamped radially on a cylindrical rail  30  of the casing by the nozzle located downstream of the ring sectors. The ring sectors  18  are in radial abutment towards the outside on a radially inner cylindrical face of the rail  30  of the casing, and towards the inside on a radially outer cylindrical face of a cylindrical rim  28  of the downstream nozzle. 
         [0044]    In order to protect the rail  36  thermally and against wear, it is known to use an annular foil  50  that is sectorised and comprises an annular row of foil sectors arranged circumferentially end to end. Said foil has a generally C-shaped or U-shaped cross section and comprises two coaxial annular walls, inner  52  and outer  54 , respectively, that are interconnected by a middle bottom wall  56 . 
         [0045]    The foil  50  is mounted on the casing rail  36  and on the hook  34  of the nozzle  12  so that the inner walls  52  of the foil sectors  50  are interposed between the inner walls  40  of the hooks  32  of the ring sectors  18  and the hooks  34  of the nozzle  12  and the annular ring  46 , so that the outer walls  54  of the foil sectors are interposed between the outer walls  38  of the hooks  32  of the ring sectors and the casing rail  36 , and so that the bottom walls  56  of the foil sectors are interposed between the bottom walls  42  of the hooks of the ring sectors and the casing rail  36  ( FIG. 2 ). 
         [0046]    The foil sectors  50  are made from sheet metal and make it possible to prevent any direct contact between the hooks  32  of the ring sectors  18  and the casing rail  36 , which makes it possible both to protect said rail against wear by friction and to protect it thermally from the ring, which may be very hot in operation because of its proximity to the combustion gases flowing in the turbine duct. 
         [0047]    As explained above and illustrated by  FIG. 3 , which shows the prior art for the present invention, the longitudinal edges  58  of the circumferential ends of the ring sectors  18  are separated from one another by circumferential clearances through which hot gases of the turbine duct can pass. The longitudinal edges  60  of the circumferential ends of the foil sectors  50  are also separated from one another by circumferential clearances that are aligned axially with the clearances between the ring sectors  18 . The aforementioned hot gases can pass through the circumferential clearances between the hooks  32  of the ring sectors  18  and between the foil sectors  50  and heat the casing rail  36  (arrow  62  in  FIG. 2 ) which risks reducing its service life. This is because the tongues  64  that are mounted between the longitudinal edges  58  of the circumferential ends of the ring sectors  18  do not extend as far as the hooks  32  of the ring sectors  18  and do not prevent the passage of gas in this region. 
         [0048]    The invention makes it possible to overcome this problem by virtue of the angular offset of the longitudinal edges  60  of the circumferential ends of the foil sectors  50  relative to the longitudinal edges  58  of the circumferential ends of the ring sectors  18 .  FIG. 4  shows an embodiment of the invention in which the foil sectors  50  are arranged so as to be staggered relative to the ring sectors  18 . The gases that are liable to pass through the circumferential clearances between the hooks  32  of the ring sectors  18  are then blocked by the foil sectors  50  and do not get as far as the casing rail  36 , which has a better service life. 
         [0049]    As can be seen in  FIG. 5 , the walls  38 ,  40  of the ring sectors  18  are “pre-cambered” with respect to the casing rail  36 , that is to say they have radii of curvature greater that are than that of the casing rail  36 , which makes it possible to mount them on the rail so as to be pre-stressed to some degree. Because of this pre-cambering, the ring sector  18  shown in  FIG. 5  has bearing zones C 1 , C 2 , C 3  on the rail  36  that are not very extensive. The middle part of the inner face of the wall  38  of the sector  18  is in abutment at C 1  on the outer face of the rail  36  (by means of the walls  54  of the foil sectors  50 , when used) and the end parts of the outer face of the wall  40  are in abutment at C 1  and C 3  on the inner face of the rail  36  or on the hook  34  of the nozzle  12  and the ring  46 , as in the example shown (by means of the walls  52  of the foil sectors  50 , when used). 
         [0050]    In order not to place excessive stress on the foil sectors  50 , by pinching between the circumferential ends of the ring sectors  18  and the casing rail  36 , the embodiment shown in  FIGS. 6 to 8  proposes a particular shaping of the foil sectors and especially of the inner walls  52  thereof. In the absence of such shaping, the risk would be that of prematurely wearing the foil sectors  50  and creating crack initiation zones in the bearing zones C 1 , C 3 . 
         [0051]    In the example shown, the inner wall  52  of each foil sector  50  comprises a recess  66  substantially at the centre thereof. This recess  66  emerges on the free circumferential edge upstream of the wall  52  and is generally V-shaped here. Each recess  66  has a circumferential extent of between 30 and 60% of the circumferential extent of the foil sector  50  and a longitudinal dimension of between 10 and 50% of the longitudinal dimension of the foil sector  50 . 
         [0052]    As can be seen in  FIG. 8 , in which foil sectors  50  are shown in continuous lines and ring sectors  18  are shown in broken lines, the longitudinal edges  58  of the circumferential ends of the hooks  32  of the ring sectors  18  are located substantially in line with the bottoms  68  of the recesses  66 . These recesses  66  provide the inner walls  52  of the foil sectors  50  with some degree of flexibility. 
         [0053]    According to the invention, the foil sectors  50  may also be equipped with rotational locking means. 
         [0054]    In the example shown in  FIGS. 6 to 8 , these locking means comprise a recess  70  formed at a circumferential end of the inner wall  52  of each foil sector  50 . This recess  70  emerges on the free circumferential edge upstream of the wall  52  as well on the longitudinal edge of the corresponding end of the wall. It has a roughly rectangular shape here. Each recess  70  has a circumferential extent of between 10 and 30% of the circumferential extent of the foil sector  50  and a longitudinal dimension of between 20 and 70% of the longitudinal dimension of the foil sector  50 . 
         [0055]    In the mounting position, the recess  70  of each foil sector  50  is aligned radially with a recess  72  in the inner wall  40  of the hook of the ring sector  18 , which is located substantially at the centre of this wall. The recesses  70 ,  72  are intended to receive a detent (not shown) of the nozzle  12  in order to immobilise the ring sector  18  and the foil sector in rotation with respect to one another as well as with respect to the casing  14 . 
         [0056]      FIGS. 9 and 10  show a variant of the locking means that, here, comprise a foldable lug  74 . In this case, a lug  74  is carried by the outer wall  54  of each foil sector  50 ′. Said lug is located substantially at the centre of the sector  50 ′ and extends, at rest, radially outwards and downstream. Its outer radial end  76  is intended to be deformed and folded radially inwards so as to engage in an outer radial recess  78  in the outer wall  38  of the hook of the ring sector  18 ′. This immobilises the foil sector  50 ′ in rotation with respect to the ring sector  18 ′. In a variant, each foil sector may comprise more than one anti-rotation lug of this type.