Abstract:
The annular combustion chamber having ceramic matrix composite material walls is mounted inside a metal casing by linking members fastened to the chamber by brazing. The linking members comprise a plurality of inner linking tabs and a plurality of outer linking tabs connecting the chamber to the inner and outer metal shrouds of the casing, each linking tab has a first portion fastened to the outside surface of a wall of the combustion chamber by brazing, the first portions of the linking tabs being spaced apart from one another circumferentially so that the brazed connections between the chamber and the linking members occupy a set of limited zones that are spaced apart from one another.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     The present invention relates to mounting a combustion chamber having a wall made of ceramic matrix composite (CMC) material inside a metal casing, in a gas turbine. The field of application of the invention is more particularly that of industrial gas turbines and of turbojets or turboprops for airplanes.  
         [0002]     It is common practice for a gas turbine combustion chamber to be made of metal and to be mounted or secured inside a metal casing by linking members, ferrules or tabs, that are made of metal. Using a metal for the wall of the chamber is appropriate so long as it is possible to ensure effective cooling of said wall. However, there is a need to increase temperatures within the combustion chamber in order to increase the efficiency of the gas turbine and reduce polluting emissions. The use of metals for combustion chamber walls can then become inappropriate, even when implementing cooling as effectively as possible. Proposals have therefore been made for the walls of combustion chambers to be made out of ceramic matrix composite materials, such as composite materials having a silicon carbide (SiC) matrix and presenting good strength at high temperatures.  
         [0003]     A problem which then arises is that of connecting the CMC combustion chamber to the metal casing, because of the differences between their coefficients of thermal expansion.  
         [0004]     Document FR 2 825 783 proposes connecting the inner and outer annular walls of a CMC combustion chamber of a gas turbine to inner and outer metal shrouds of a metal casing by means of elastically-deformable metal linking tongues. Those metal tongues are secured at one end to a metal ferrule fastened to the inner or outer metal shroud, and at an opposite end to a CMC ferrule that is brazed onto the outside face of an inner or outer wall of the combustion chamber.  
         [0005]     Accommodating the differential changes in dimensions between the combustion chamber and the metal casing is thus made possible by the flexible linking tongues having CMC-on-CMC connections at the combustion chamber end and metal-on-metal connections at the casing end. However, the brazed connection between the CMC ferrule and the annular wall of the combustion chamber leads to real difficulties. An effective brazed connection requires the spacing between the surfaces that are to be brazed together to be well controlled in order to guarantee a uniform thickness of brazing material and in order to avoid harmful discontinuities in the brazing. Unfortunately, given the processes whereby CMC parts are manufactured, the dimensional tolerances thereof are greater than is the case for metal parts. It is therefore very difficult to guarantee uniform spacing between two complete annular surfaces that are to be connected together by brazing.  
       OBJECT AND BRIEF SUMMARY OF THE INVENTION  
       [0006]     An object of the invention is to provide a combustion chamber having a CMC wall in a metal casing while avoiding the above problem.  
         [0007]     This object is achieved by a gas turbine of the type having an annular combustion chamber with walls made of ceramic matrix composite material mounted inside a metal casing by linking members fastened to the chamber by brazing and connecting the chamber to inner and outer metal shrouds of the casing, in which gas turbine, according to the invention, the linking members comprise a plurality of inner linking tabs and a plurality of outer linking tabs which connect the combustion chamber to the inner and outer metal shrouds respectively, each linking tab having a first portion fastened to the outside surface of a wall of the combustion chamber by brazing, the first portions of said linking tabs being spaced apart from one another circumferentially so that the brazed connections between the chamber and the linking members are provided via a set of limited zones that are spaced apart from one another.  
         [0008]     By limiting the dimensions of the zones of brazing, it is possible to make it easier to control the spacings between the surface portions to be brazed together, and thus avoid irregularities in brazing thickness. It is thus possible to obtain effective bonding by brazing.  
         [0009]     Advantageously, the first portions of the inner linking tabs and of the outer linking tabs are integral with continuous inner and outer end ferrules respectively, defining bearing surfaces for annular sealing gaskets between the combustion chamber and a high pressure turbine nozzle situated immediately downstream from the chamber.  
         [0010]     Also advantageously, the inner and outer end ferrules are made of ceramic matrix composite material and are made as a single piece together with the inner or outer linking tabs respectively.  
         [0011]     The inner and outer end ferrules may be connected by brazing to the outside surfaces respectively of the inner and outer walls of the combustion chamber, the brazing being performed along continuous circumferential zones, in order to provide sealing between the inner and outer ferrules and the inner and outer walls of the chamber.  
         [0012]     Since the mechanical connection is implemented via the brazing between the linking tabs and the walls of the combustion chamber, the brazing of the end ferrules on the walls of the chamber serves merely to provide circumferential sealing. It can therefore be performed over a narrow width, which is therefore easier to control, than would be possible if it were also to provide the mechanical connection.  
         [0013]     In known manner, the inner and outer walls of the combustion chamber present a plurality of perforations allowing a cooling flow around the combustion chamber in the spaces between the chamber and the metal casing to maintain a protective film on the inside surface of the chamber walls. Since the brazing zones between the linking tabs and the walls of the combustion chamber are spaced apart from one another, they leave between them zones in which the multiple perforations through the chamber walls remain unaffected.  
         [0014]     Nevertheless, perforations can also advantageously be made through the brazed zones of the linking members (CMC linking tabs and/or CMC end ferrules) and the walls of the combustion chamber so as to avoid the inside surface of the chamber walls presenting any zones that are not fed by perforations.  
         [0015]     In an embodiment, each linking tab of ceramic matrix composite material has a second end portion fastened to the metal casing.  
         [0016]     In another embodiment, the inner and outer linking tabs of ceramic matrix composite material are connected to the metal casing by respective inner and outer flexible metal linking parts. Under such circumstances, and advantageously, the inner and outer linking parts comprise inner and outer metal linking tabs each having a first end portion connected to a second end portion of a linking tab made of ceramic matrix composite material. The inner and outer metal linking tabs can then have second end portions that are secured to the inner and outer metal ferrules that are integral respectively with the inner and outer metal ferrules that are themselves secured to the inner and outer metal shrouds. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]     The invention will be better understood on reading the following description given by way of non-limiting indication and with reference to the accompanying drawings, in which:  
         [0018]      FIG. 1  is a fragmentary axial half-section view of a gas turbine showing an embodiment of the invention;  
         [0019]      FIGS. 2 and 3  are fragmentary perspective views showing the linking members between the chamber and the casing and showing how they are connected by brazing to the walls of the combustion chamber in the embodiment of  FIG. 1 ;  
         [0020]      FIG. 4  is a fragmentary axial half-section view of a gas turbine showing another embodiment of the invention; and  
         [0021]      FIGS. 5 and 6  are fragmentary perspective views showing the linking members between the chamber and the casing and showing their brazed connections with the walls of the combustion chamber in the embodiment of  FIG. 4 . 
     
    
     DETAILED DESCRIPTION OF EMBODIMENTS  
       [0022]      FIG. 1  is an axial half-section of a portion of a gas turbine comprising an annular combustion chamber  10 , a high pressure turbine nozzle  20  disposed immediately downstream from the combustion chamber  10 , a metal casing comprising inner and outer metal shrouds  30  and  40 , and inner and outer linking tabs  50  and  60  holding the chamber  10  inside the metal casing. Below, the terms “upstream” and “downstream” are used relative to the flow direction (arrow F) of the gas stream coming from the chamber  10 .  
         [0023]     The combustion chamber  10  is defined by an inner annular wall  12  and an outer annular wall  13  sharing a common axis  11 , and by an end wall  14  secured to the walls  12  and  13 . In well-known manner, the end wall  14  presents openings  14   a  that are distributed around the axis  11  to house injectors for injecting fuel and oxidizer into the chamber  10 . The walls  12  and  13  of the chamber  10  are made of CMC, e.g. a composite material having an SiC matrix, and optionally the wall  14  is made of the same material.  
         [0024]     The HP turbine nozzle  20 , which constitutes the inlet stage of the turbine, has a plurality of stationary vanes angularly distributed around the axis  11 . The vanes comprise airfoils  21  whose ends are secured to inner and outer platforms  22  and  23  in the form of juxtaposed ring sectors. Each corresponding pair of platforms  22 ,  23  can be associated with one or more airfoils  21 . The inside faces of the platforms  22  and  23  define the boundaries of the flow path within the nozzle for the gas stream coming from the combustion chamber.  
         [0025]     The inner metal shroud  30  is made of two portions  31  and  32  that are united by bolting together respective inwardly-directed flanges  31   a  and  32   a.  Similarly, the outer metal shroud  40  comprises two portions  41  and  42  that are united by bolting together respective outwardly-directed flanges  41   a  and  42   a.  The space  33  between the inner wall  12  of the chamber  10  and the inner shroud  30 , and the space  43  between the outer wall  13  of the chamber  10  and the outer shroud  40  convey a secondary stream of cooling air (arrows f) flowing around the chamber  10 .  
         [0026]     The nozzle  20  is mounted by a mechanical connection by bolting  25  between a radial flange  24  subdivided into sectors and secured to the inner platforms  22 , and a radial flange  34  at the downstream end of the inner shroud  30 . An annular sealing gasket  36 , e.g. of the “omega” type closes the downstream end of the space  33  in leaktight manner. The gasket  36  is housed in a housing formed in the upstream surface of the flange  34  and presses against the downstream surface of the flange  24 . The space  43  is closed in leaktight manner at its downstream end by a sealing gasket  46 , e.g. of the strip type. The gasket  46  is held by pins  46   a  in an annular housing  26   a  in an annular flange  26  that is subdivided into sectors and that is integral with the outer platforms  23 . The gasket  46  presses against a rib  44   a  formed on the upstream face of a radial flange  44  integral with the casing  40 .  
         [0027]     In the embodiment of FIGS.  1  to  3 , the linking tabs  50  and  60  are made of CMC, and preferably out of the same material as the walls  12  and  13  of the chamber  10 .  
         [0028]     Each linking tab  50  has an end portion  51  connected by bolts to the inner metal shroud  30 . On its inside surface, the shroud carries threaded rods  37  passing through holes  51   a  formed in the end portions  51  of the linking tabs  50  and having nuts  38  engaged thereon. Similarly, each linking tab  60  has an end portion  61  bolted to the outer metal shroud  40 . On its inside surface, this shroud carries threaded rods  47  that pass through holes  61   a  formed in the end portions  61  of the linking tabs  60  and having nuts  48  engaged thereon.  
         [0029]     The linking tabs  50  present end portions  52  that are connected to the outside surface of the inner wall of the chamber  10  by being brazed thereto in the vicinity of the downstream end of the chamber. The end portions  52  of the linking tabs  50  are integral with an inner ferrule  54 . The ferrule  54  has an upstream annular portion  54   a  which is brazed to the outside surface of the wall  12  of the chamber, and a downstream portion  54   b  which is connected to the upstream portion  54   a  while making an obtuse angle relative thereto. At its downstream end, the ferrule  54  bears against an annular sealing gasket  38 , e.g. of the strip type. The gasket  38  is held by pins  38   a  in an annular housing  28   a  of a flange  28  that is subdivided into sectors and that is integral with the platforms  22  in the vicinity of their upstream ends.  
         [0030]     Similarly, the linking tabs  60  present upstream portions  62  which are connected to the outside surface of the outer wall  13  of the chamber  10  by being brazed thereto in the vicinity of the downstream end of the chamber. The end portions  62  of the linking tabs are integral with an outer ferrule  64 . The ferrule  64  has an upstream annular portion  64   a  which is connected to the outside surface of the wall  13  of the chamber  10  by brazing, and a downstream portion  64   b  which is connected to the upstream portion  64   a,  while making an obtuse angle relative thereto. At its downstream end, the ferrule  64  bears against an annular sealing gasket  48 , e.g. of the strip type. The gasket  48  is held by pins  48   a  in an annular housing  49   a  of a flange  29  that is subdivided into sectors and that is integral with the platforms  23  in the vicinity of their upstream ends.  
         [0031]     The linking tabs  50  and the ferrule  54  are advantageously made as a single piece, as are the linking tabs  60  and the ferrule  64 . Along their portions extending through the spaces  33  and  43 , the linking tabs  50  and  60  are curved or folded in shape so as to present the flexibility necessary for accommodating differential dimensional variations between the walls of the chamber that are made of CMC and the shrouds  30  and  40  that are made of metal.  
         [0032]     The combustion chamber is held essentially by the brazing at the end portions  52  and  62  of the linking tabs  50  and  60 . Compared with continuous circumferential brazing, the brazing zones  53  and  63  are limited, such that it is possible to control the spacing between the surfaces that are to be brazed together without excessive difficulty.  
         [0033]     The brazed connections between the portions  54   a,    64   a  of the ferrules  54 ,  64  and respectively the walls  12 ,  13  of the chamber  10  extend continuously in the circumferential direction. These brazed connections serve to provide sealing between the spaces  33 ,  43  and the downstream end of the chamber  10  so as to avoid any uncontrolled injection of cooling flow through the interface between the chamber  10  and the turbine nozzle  20 . Such connections do not need to hold the chamber mechanically, since that function is provided by the brazing at the portions  52 ,  62  of the linking tabs  50 ,  60 . Consequently, the bonding zones  55 ,  65  between the ferrules  54 ,  64  and the walls  12 ,  13  of the chamber  10  can be limited in width, thus also making it very easy to control the spacing between the surfaces to be brazed together. The brazed connections between the ferrules  54 ,  64  and the chamber  10  thus contribute to the stability of the linking tabs  50 ,  60  in the event of an angular displacement.  
         [0034]     Brazing parts made of CMC is a known technique. Both for the connections between the linking tabs  50 ,  60  and the chamber  10  and for the connections between the ferrules  54 ,  64  and the same chamber, it is possible to perform brazing using a material such as “BraSiC” as developed by the French public body “Commissariat à l&#39;Energie Atomique” [Atomic Energy Commissariat] or “Ticusil” from Wesgo Metals, in particular when the brazed parts are made of SiC matrix composite material.  
         [0035]     The walls  12 ,  13  of the chamber  10  may present multiple perforations to allow cooling air to flow from the spaces  33 ,  43  to the inside surfaces of the walls  12 ,  13  in order to maintain a cooling film along said surfaces. The perforations  12   a,    13   a  are shown in part, in  FIGS. 2 and 3  only. The gaps between the brazed zones  53 ,  63  leave portions of the chamber walls where the multiple perforations can be present, thereby improving thermal protection of the walls. If desirable, multiple perforations may also be provided through the brazed end portions  52 ,  62  of the linking tabs  50 ,  60  and the chamber walls  10 , and through the brazed portions between the ferrules  54 ,  64  and the walls of the chamber  10 . These multiple perforations can be made after brazing, e.g. in conventional manner by laser machining. Such perforations  12   b,    12   c,  and  13   b,    13   c  are shown in part, solely in  FIGS. 2 and 3 .  
         [0036]     FIGS.  4  to  6  show an embodiment which differs from that of FIGS.  1  to  3  essentially in that the CMC linking tabs  50 ,  60  have their ends  51 ,  61  connected to the metal shrouds  30 ,  40 , not directly, but via flexible or elastically-deformable metal tabs. Elements that are common to the embodiment of FIGS.  1  to  3  and to the embodiment of FIGS.  4  to  6  are given the same references and are not described again.  
         [0037]     Each metal tab  55  has an end portion  56  connected by bolting ( 57 ) to one end  51  of a corresponding tab  50 , while its other end is integral with an annular metal ferrule  58 . This ferrule constitutes an annular flange  59  that is connected to the shroud  30  by being clamped between the flanges  31   a  and  32   a.    
         [0038]     Each metal tab  65  has an end portion  66  connected by bolting ( 67 ) to one end  61  of a corresponding tab  60  and its other end is integral with an annular metal ferrule  68 . This ferrule has holes  68   a  with threaded rods  45  passing therethrough that are secured to the shroud  40  and that have nuts  46  engaged thereon.  
         [0039]     Naturally, the ferrule  68  could be connected to the shroud  40  in the same manner as the ferrule  58  is connected to the shroud  30 , i.e. by means of a flange clamped between the flanges  41   a  and  42   a.  Conversely, the ferrule  58  could be connected to the shroud  30  by bolting in the same manner as the ferrule  68  is connected to the shroud  40 .  
         [0040]     The metal tabs  55  are advantageously made as a single piece together with the ferrule  58 , and the same applies to the metal tabs  65  and the ferrule  68 .  
         [0041]     The metal tabs  55 ,  65  serve to increase the possibly-insufficient ability of the tabs  50  and  60  made of CMC to deform elastically. In order to present the necessary degree of elastic deformation or flexibility, the tabs  55 ,  65  are curved or folded so as to have a profile that is substantially S-shape (tabs  55 ) or V-shape (tabs  65 ).