Abstract:
A cover of a centrifugal compressor intended to be rigidly connected to a turbomachine housing, which includes a plurality of ports, is provided. The cover further includes a fastening device for fastening the cover to the housing. A portion of the fastening device is located upstream relative to the ports and can be accessed by a fastening tool through at least one of the ports of the cover. A turbomachine uses this cover to form a sealed space, in particular with a view to collecting air.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to the design of the cover of a centrifugal compressor in a turbine engine. It relates more particularly to the technique for the assembly thereof in the turbine engine in order to maintain the sealing of the space which is intended for the gases taken off and is defined by said cover, together with the casing of the turbine engine, at the periphery of the compressor. 
         [0002]    The function of the centrifugal compressor in a turbine engine is that of injecting the high-pressure air supply into the combustion chamber. For this purpose, the rotor has a flared shape, taking air from the output of an axial compressor in order to direct said air radially into the recovery duct travelling to the combustion chamber. The cover forms the stator, that is the outer wall of the duct for the flow of air around the rotor. 
         [0003]    It is known to take off air at the centrifugal compressor in order to use this high-pressure air as a source of power for turbine engine or of the aircraft equipment. Openings formed in the cover make it possible to take off said air in the duct of the compressor in order to convey it into the circuit which is to distribute it to the equipment which use it. In order fully benefit from this offtake, it is important for the circuit to be sealed. In order to achieve this, the cover is brought into sealed contact with parts of the casing around the offtake openings in order to create a recovery space at the input of the circuit which distributes the high-pressure air. 
         [0004]    However, the cover of a compressor of this kind is a part which is difficult to install. This is because said cover must ensure that there is as little clearance as possible from the blades of the rotor in order to optimise the efficiency of the compressor. It must also ensure the continuity of the duct upstream and downstream. Conversely, the mechanical and thermal stresses related to the operation of the turbine cause significant deformation of all the parts in the region in which the cover is located, in particular of the rotor blades. 
         [0005]    It is fairly common to fix the cover to the casing by means of an annular flange which is positioned downstream on said cover, in the region having the largest radius. However, the offtake openings are generally located in a region having an intermediate radius, further upstream on the cover. In this case, bolting said flange to the casing makes it possible to easily ensure the sealing of the recovery space, downstream of the offtake openings. The problem is therefore that of ensuring the sealing at the joint between the cover and another part of the casing, dosing the recovery space upstream of the offtake openings. In order to achieve this result, the sealing must be ensured by joints between the part and the cover, as is described in U.S. Pat. No. 7,824,151, or by means of simple centring, as is proposed in EP2206882. These solutions limit the leaks, but do not ensure perfect sealing, in particular due to deformation of the parts between the different operating speeds of the turbine engine. This results in a loss of pressure, and therefore a reduction in performance, both for the compressor and for the system for taking off the high-pressure air. 
         [0006]    Some designs of compressor covers, such as those described in US2011/0002774 and FR2931521, use a flange which is positioned upstream on the cover of the compressor and is bolted to the casing of the turbine engine in order to fix said cover. In this case, bolting of the flange ensures sealing of the space upstream of the cover. It is even possible to configure, as is the case in FR2931521, a structure which downstream, closes the space which surrounds the cover of the compressor. 
         [0007]    However, when a flange is bolted on the upstream portion of the cover, in particular in order to ensure sealing in this region, it creates a problem of accessibility to the fixing means during assembly. This is because, either, a part of the structure or a flange blocks access to said means when the cover is positioned for assembly or, at best, the space located directly behind the fixing holes is restricted by the shape of the cover itself. In this case, it is necessary to design the structure of the turbine engine such that the fixing screws of the flange of the cover can be accessed from the cavity downstream of the axial compressor, located in front of the centrifugal compressor. 
         [0008]    The object of the present invention is that of proposing a simple solution for easily fixing the cover, by the upstream edge thereof, to the casing of the turbine engine using a clamping means, and which can be adapted, without significant modification, to a design in which the casing of the turbine engine does not permit access to this portion of the cover during assembly. 
       DESCRIPTION OF THE INVENTION 
       [0009]    The invention relates to a cover for a centrifugal compressor which is intended to be fixed to a casing of a turbine engine, comprising an upstream edge and a downstream edge in the flow direction of the gases crossing the compressor, said cover comprising a plurality of openings and fixing means. This cover is characterised in that upstream fixing means are located upstream relative to the openings and can be accessed by a fixing tool through at least one of said openings in the cover. 
         [0010]    The object of facilitating assembly of the cover is achieved by the invention by means of permitting access from downstream to the fixing means when the cover is put in place. Clamping means can have been pre-positioned on the fixing means, and it is then sufficient to pass the fixing tool through the opening providing access to said means in order to fix the cover against the casing by clamping. 
         [0011]    Advantageously this arrangement corresponds to openings intended for air offtake. The cover is thus initially intended to participate in a high-pressure air distribution system by taking off air in the region of the compressor. In this case, there is certainty that said openings are correctly positioned and dimensioned in order not to disrupt the operation of the compressor, and there is also no need to modify the design of the cover in this region. 
         [0012]    Preferably, said upstream fixing means for the cover of the centrifugal compressor comprise an outer flange and fixing holes which pierce said flange, the unit being intended to cooperate with clamping means, of the screw and nut type, which can be actuated by the fixing tool. The holes in the flange are easy to machine and to position on the flange. The screw and nut system, which is widely used, makes it possible to easily fix the cover by clamping said cover against the casing. For example, since each upstream fixing means defines an axis corresponding to the rotation of the movable clamping means for the clamping thereof, said axis passes through at least one of said openings in the cover. In particular, in the case where fixing is achieved by bolting, this makes it possible to use a spindle wrench without a complex mechanism. 
         [0013]    Advantageously, said outer flange forms a wall which completely surrounds said cover. Said flange, which is arranged so as to ensure a sealed circumferential connection, upstream of said openings, to a part of the casing when the fixing means are clamped, makes it possible to prevent air from the compressor, which passes through the openings in the cover, from escaping from the front during operation of the turbine engine. Firstly, this prevents the space which receives the air escaping through the openings in the cover from being brought into communication with the upstream stages of the turbine engine and disrupting the operation thereof. In addition, devices already described in the documents cited above make it possible to ensure sealing of the connection between the cover and the casing in the region of the largest radius thereof, downstream of the openings. Thus, during operation of the turbine engine, the fact that the flow in the compressor is brought into communication with the outside of the cover, via the openings used for allowing the fixing tools to pass through, does not have a detrimental effect on the efficiency of the compressor, since the pressure will equalise. Similarly, in view of an offtake of air for uses in other equipment, the lack of leaks in this region contributes to maintaining an elevated pressure in the air distribution circuit. 
         [0014]    Advantageously, each fixing hole in the flange can receive a damping screw by passing through at least one of said openings in the cover. This makes it possible, for example, to pass the screws through after installing the cover on the casing. In a variant, if the holes in the flange are positioned on studs connected to the casing, the openings make it possible to pass through nuts and to install said nuts on the studs. 
         [0015]    Preferably, said flange extends close to the upstream edge of the cover. 
         [0016]    In a particular embodiment, the cover of the centrifugal compressor further comprises a downstream flange for holding against the casing of the turbine engine, which flange is fastened to the outer wall of the cover between said openings and the downstream edge and forms a sealing means. Using a downstream holding flange, which is arranged so as to ensure a sealed circumferential connection against the casing, makes it possible to easily adapt the cover in order to ensure perfect sealing of the space for receiving the gases taken off, and to permit easy assembly thereof. Advantageously, this flange is bolted onto the casing of the turbine engine. 
         [0017]    The invention also relates to a turbine engine comprising a centrifugal compressor having a cover as described above, the casing of which is arranged so as to form a sealed connection to the upstream flange of the cover when the clamping means are clamped. 
         [0018]    Advantageously, the casing is arranged so as to form, together with said cover, at least one closed space which recovers the air passing through the openings in the cover. 
         [0019]    Advantageously, at least some of the openings in the cover, and said air recovery space, are designed to participate in an air offtake system. 
     
    
     
       DETAILED DESCRIPTION OF THE INVENTION 
         [0020]    The present invention will be more readily understood and other details, features and advantages of the present invention will become clearer upon reading the following description with reference to the accompanying drawings, in which: 
           [0021]      FIG. 1  is an axial section of a first embodiment of a turbine engine compressor comprising a cover according to the invention. 
           [0022]      FIG. 2  is an enlargement of  FIG. 1 , in the upstream fixing region of the cover, showing an exploded view of the screw and nut. 
           [0023]      FIG. 3  is an axial section of the turbine engine, showing the cover of  FIG. 1 , during the assembly phase. 
       
    
    
       [0024]    With reference to  FIG. 1 , the invention relates to the cover  1  of a centrifugal compressor, which forms the radially outer wall of the duct in which the blades  2  of the rotor wheel  3  rotate. The unit has a rotational symmetry about an axis which is not shown but which would be horizontal with respect to  FIG. 1 , and below the parts shown. By rotating about this axis, the rotor sucks air in through its input  4 , which is axially oriented, in order to expel said air at higher pressure through its output  5 , which is radially oriented. In a general manner, said compressor discharges its compressed air into a radial recovery duct  6  which is intended to supply the combustion chamber, which is not shown but is located on the right-hand side with reference to  FIG. 1 , and is supplied upstream by an axial compressor  7 . 
         [0025]    The cover  1  is a part which rotates about the axis of symmetry of the compressor, the shape of which follows, with minimal clearance, the surface swept by the radial end of the blades  2 . In addition, the cover  1  is arranged so that the upstream leading edge  8  thereof ensures continuity in shape with the radially outer wall  9  of the duct of the axial compressor  7 , and so that the downstream trailing edge  10  thereof ensures continuity in shape with the wall  11  of the recovery duct  6 . 
         [0026]    Fixing the cover on the casing of the turbine engine should make it possible to position said cover in a sufficiently precise manner in order to take account of the stresses mentioned above and to also permit the cover to move according to the operation of the turbine engine, in order to follow the deformation of the blades  2  while maintaining an optimal clearance. 
         [0027]    Many types of solutions are proposed in the prior art, in particular fixing the cover by means of two flanges, such as in US2011/0002774 and EP2206882, which have already been cited. With regard to holding the cover, this assembly by means of two flanges overdetermines the fixing points of the cover, which must be taken into account in the type of connections made. However, it is entirely possible to achieve an assembly of this kind, as is shown in the documents cited in the prior art. It is within the scope of the invention to use other combinations between the flanges, by adjusting the resilience of the connections they provide. 
         [0028]    A first embodiment according to the invention uses fixing of this kind, comprising two flanges. An annular flange  12 , fastened downstream in the portion of the cover  1  having the maximum radius relative to the axis of rotation, is bolted to the edge of a circumferential part  13  of the casing. In addition, the cover is also fixed in the upstream portion thereof by a flange  14  to another part  15  of the casing, over the entire circumference thereof. Said flange  14  is a frustoconical part which extends radially outside the cover  1  and separates the upstream edge  8  from the downstream edge  10  over the entire circumference of the cover. Said flange is fixed to the cover  1  very close to the upstream edge  8  of the cover  1  and has a small radial extension. The connection between the cover and the part  15  of the casing is therefore made very close to the upstream edge  8 . Said flange  14 , on account of the position thereof, provides an additional benefit, which is set out below. 
         [0029]    In some turbine engines, the cover also participates in a function of offtake of high-pressure air in the region of the compressor, in order to introduce said air into a distribution circuit towards various equipment inside the aircraft. 
         [0030]    In the example shown, the cover  1  is pierced with a plurality of openings  16 , one of which is shown in the section in  FIG. 1 . These openings  16  are located on a ring about the axis of symmetry, having a radius which is approximately equal to the minimal radius of the cover, on the upstream edge  8 , and increased by a quarter of the difference from the maximum radius, on the downstream edge  10 . Viewed in a different manner, said openings are also located in a region of maximum curvature of the axial profile of the cover  1 . In this way, said openings are located in a region where the cover fulfils a lesser role for containing the flow inside the compressor. Said openings can thus be arranged to have a cross section which is sufficient to allow a fraction of the air circulating in the rotor to pass to the outer side of the cover, without causing significant deterioration in the operation of the compressor. 
         [0031]    The air taken off is recovered in a recovery space  17 , which itself is brought into communication with a distribution circuit (not shown). In the example shown in  FIG. 1 , said space  17  is delimited, close to the cover, by a portion of the cover  1 , by means of:
       in the downstream direction, the downstream flange  12  and then an annular part  13  of the casing,   in the upstream direction, the upstream flange  14  then an annular part  15  of the casing.       
 
         [0034]    Thus, as shown in  FIG. 2 , the upstream flange is fixed to the part  15  by means of systems consisting of a screw  18  and a nut  19  which pass through holes  20  and  21 , the flange  14  and the part  15 . Said holes are, of course, positioned so as to be facing one another during assembly of the cover. Said holes, in particular the holes  20  formed in the flange  14 , are circumferentially distributed. The holes  20  which pierce the flange  14  thus form, together with the portion of the flange  14  which surrounds them, as many fixing means fastened to the cover  1 . Said fixing means  20  cooperate with the holes  21  arranged on the edge of the annular part  15  and the bolting means,  18  and  19 , in order to press the flange  14  against the part  15 . The portions in contact with the flange and the edge of the part  15  are arranged, optionally with an interposed adjustment shim  23 , so as to ensure that the connection is sealed with respect to the high-pressure air, by virtue of the pressure applied by the bolting means  18 - 19  when said means are clamped. Similarly, the downstream flange  12  is bolted to the edge of the part  13  of the casing and ensures that the space  17  for receiving the air taken off is sealed on the downstream side of the cover  1 . 
         [0035]    The flange  12  generally extends radially outside the cover, which makes it possible to access, from downstream of said flange, the end thereof bolted to the casing when assembling or disassembling the cover. 
         [0036]    The difficulty in assembling this configuration lies in the fact that, in order to ensure sealing upstream of the cover, the part  15  of the casing approaches the upstream edge  8  of the cover  1  where the radius thereof is the smallest. During assembly in the turbine engine, the cover is installed downstream of the parts of the casing and of the compressor which are already assembled, as is shown in  FIG. 3 . The cover  1 , similarly to the downstream flange  12 , thus blocks access to the fixing means  20  and the bolting means  18 - 19 . In the cover according to the invention, the openings  16  in the cover  1 , which are provided for the offtake of air, have been located opposite the fixing holes  20  in the flange  14 , such that a fixing tool, for example a wrench  22 , can grip the head of the screw  18  (not visible in  FIG. 3 ) by passing through the holes  20  in the flange  14  and the holes  21  in the part  15  of the casing, in order to screw said casing onto the nut  19 . 
         [0037]    In the example shown, the nut  19  is fixed opposite the hole  21  in the part  15  of the casing, on the upstream side thereof, and is prevented from rotating. It is thus possible to install the cover from the rear of the casing, in order to screw the screws  18  there in the nuts  19  through the holes  20  in the flange and the holes  21  in the part  15  of the casing, which holes have been aligned in advance. The offtake opening  16  permits the spindle of a wrench  22 , aligned with the axis of the fixing hole  20 , to pass through, which makes it possible to turn said wrench in order to screw the screw  18 . Moreover, while still meeting the operating criterion of the compressor, this opening  16  is sufficiently large to permit the screw  18  to pass through, so as to introduce said screw into the nut  19  through the holes  20  and  21 , as well as the head of the wrench  22  which cooperates with the head of the screw  18 . 
         [0038]    In a variant, the head of the wrench  22  can be narrower than that of the screw  18 , conversely to what is shown in  FIG. 3 , if said wrench head cooperates with a hollowed pattern in the head of the screw. In this way, the screws  18  can have been pre-positioned on the fixing holes  20  of the upstream flange  14  of the cover before said cover is put into position on the casing. In this case, the cross section of the offtake opening  16  only has to permit a relatively narrow wrench  22  to pass through. 
         [0039]    In another variant, the direction of the placement of the screws  18 , or of the nuts  19 , is reversed. The screw  18 , which is prevented from rotating on the part  15  of the casing, thus forms a stud on which the hole  20  of the flange  14  comes into position, before the nut  19  is screwed.