Patent Publication Number: US-6655148-B2

Title: Fixing metal caps onto walls of a CMC combustion chamber in a turbomachine

Description:
FIELD OF THE INVENTION 
     The present invention relates to the specific field of turbomachines and it relates more particularly to the problem posed by assembling a metal end-wall of a combustion chamber in a turbomachine to the side walls of said chamber, when said side walls are made of a composite material of the ceramic matrix composite (CMC) type. 
     PRIOR ART 
     Conventionally, in a turbojet or a turboprop, the high pressure turbine, in particular its inlet nozzle (HPT nozzle), the injection system, the combustion chamber, and the casing (also called the shell) of said chamber are all made out of metal materials. However under certain particular conditions of use implementing particularly high combustion temperatures, a chamber made entirely of metal turns out to be completely unsuitable from a thermal point of view and it is necessary to make use of a chamber that is based on high temperature composite materials of the CMC type. However, since those materials are very costly and are unable to withstand strong mechanical stresses, they are generally restricted to being used for the composite chamber itself and more particularly to its axially-extending side walls only, with the high pressure turbine inlet nozzle, the injection system, and the casing then still being made more conventionally out of metal materials. Unfortunately, metals and composites have coefficients of thermal expansion that are very different. This gives rise to particularly awkward problems, specifically with connecting together the composite material side walls and the metal end-wall of the combustion chamber. 
     OBJECT AND BRIEF SUMMARY OF THE INVENTION 
     The present invention mitigates those drawbacks by proposing a mounting for the metal end-wall with the ability to accommodate the displacements induced by the different coefficients of expansion of the metal end-wall and of the composite side walls of the combustion chamber. An object of the invention is thus to provide a mounting that has good dynamic behavior and good sealing. 
     These objects are achieved by an annular combustion chamber including outer and inner axially-extending side walls of composite material and an end wall of metal material, said end wall being held in position on said outer and inner side walls by fixing means, wherein said fixing means pass through annular cavities that are designed to receive cylindrical end portions of said outer and inner side walls, and that are created between peripheral edges of said end wall and facing portions folded downstream, a determined amount of clearance J being provided between said peripheral edges and the facing faces of said outer and inner side walls in such a manner as to allow expansion to take place freely, in operation, in a radial direction between said end wall and said side walls. 
     With this system of fixing merely by means of bolts and sliding mounts, the expansion of the metal end-wall is accommodated without deteriorating the composite material walls. 
     The fixing means are constituted by a plurality of bolts, preferably captive-nut bolts. 
     The outer and inner side walls are advantageously provided with a plurality of holes designed to co-operate with said fixing means once said fixing means are mounted on said end wall. 
     In a preferred embodiment, the end wall can further comprise means to ensure sealing between said end wall and said side walls. The sealing means include a “spring blade” type circular gasket mounted in a circular groove of said metal end-wall and designed to bear on said facing side wall of the facing combustion chamber. In its downstream portion, said “spring blade” circular gasket preferably includes a rim designed to bear in toroidal manner on said facing side wall of the combustion chamber. Said circular sealing gasket should be divided into sectors, and held against said side wall by means of a resilient element fixed on said metal end-wall. The resilient element is constituted by a blade spring. 
     In an advantageous embodiment, the end wall can also integrate inner and outer caps of metal material which extend its peripheral edges upstream and provide better control over dynamic behavior. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The characteristics and advantages of the present invention appear better from the following description made by way of non-limiting indication and with reference to the accompanying drawings, in which: 
     FIG. 1 is a diagrammatic axial half-section of an injection portion of a turbomachine incorporating a first embodiment of an assembly of the invention; 
     FIG. 1 a  shows a detail of the assembly of FIG. 1; 
     FIG. 2 is a diagrammatic axial half-section of an injection portion of a turbomachine incorporating a second embodiment of an assembly of the invention; and 
     FIG. 2 a  shows a detail of the assembly of FIG.  2 . 
    
    
     DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT 
     FIGS. 1 and 2 are axial half-section views of an injection portion of a turbomachine comprising: 
     an outer annular shell (or outer casing)  12  having a longitudinal axis  10 ; 
     an inner annular shell (or inner casing)  14  that is coaxial therewith; and 
     an annular space  16  extending between the two shells  12  and  14  for receiving compressed oxidizer, generally air, coming from an upstream compressor (not shown) of the turbomachine via an annular diffuser duct  18  (having a diffuser screen  18   a ) defining a general flow F of gas. 
     In the gas flow direction, this space  16  comprises firstly an injection assembly formed by a plurality of injection systems  20  that are regularly distributed around the duct  18 , each comprising a fuel injection nozzle  22  fixed to the outer annular shell  12  (in order to simplify the drawings, the mixer and the deflector associated with each injection nozzle are omitted), followed by an annular combustion chamber  24  formed by an outer axially-extending side wall  26  and an inner axially-extending side wall  28 , both disposed coaxially about the axis  10  and made of a high temperature composite material of the CMC type or of some other type (e.g. carbon), and a transversely-extending end wall  30  made of a metal material forming the end wall of the combustion chamber, and provided with openings  32  in which a portion of the injection system is fixed, and finally an annular nozzle (not shown) forming an inlet stage of a high pressure turbine. 
     In the two embodiments shown, the outer fairing (or cap)  34  extending the outer wall  26  of the combustion chamber upstream (relative to the flow F), and the inner fairing (or cap)  36  extending the inner wall  28  of the combustion chamber upstream (relative to the flow F) are directly integrated into the end wall  30 , and like said end wall, are thus made of metal material (further simplifying the general shape of the upstream ends of the combustion chamber which can thus be constituted by simple cylindrical portions). Naturally, a configuration with a fairing (a single one-piece cap of toroidal shape) interconnecting the two upstream ends of the side walls of the combustion chamber (and thus provided with openings through which the injection nozzles  22  can pass) can also be envisaged. 
     According to the invention, the metal annular end-wall  30  of the combustion chamber which has a coefficient of thermal expansion that is very different from that of the composite-material outer and inner side walls  26 ,  28  of the combustion chamber is held in position on the upstream cylindrical ends of the side walls by a plurality of fixing means  38 ,  40  regularly distributed around the longitudinal axis  10 . The fixing means pass through annular cavities  42 ,  44  designed to receive the cylindrical end portions of the side walls, and created between peripheral ends of the end wall  30  and facing folded portions  46 ,  48  extending the caps  34 ,  36  downstream. 
     In a first embodiment, shown in general manner in FIG.  1  and in more detail in FIG. 1A, the fixing means  38 ,  40  are formed by a plurality of metal bolts of the captive nut type, i.e. each including a screw  38   a , a nut  38   b , and a holding cage  38   c  fixed on the end wall  30  (advantageously by spot welding) and preventing the nut from rotating. With bolts of this type, tightening (during assembly) is obtained directly, merely by turning the screw without any need to use a special tool (e.g. pliers) to prevent the nut from rotating, said prevention of the nut from rotating being achieved very simply, merely by the cage. In addition, disassembly can also be performed very simply, in the opposite manner, merely by loosening the screw. 
     The amount of clearance J between the inner faces of the outer and inner side walls  26 ,  28  and the facing peripheral edges of the end wall  30  is calculated in such a manner as to allow expansion to take place freely, in operation, between the metal cap and the composite material side walls. The clearance enables expansion of the end wall to be accommodated without deteriorating the composite material side walls which are displaced very little in a radial direction. The dual centering system comprising the outer and the inner walls  26 ,  28  received in the corresponding cavities  42 ,  44  ensures relative sealing of the end wall while also providing axial retention both during assembly (when cold) and in flight at cruising speed (when hot). 
     To facilitate assembly of the end wall on the side walls, said side walls are provided with holes  26   a ,  28   b  designed to receive bushings  38   d ,  40   d  through which the screw shafts of the fixing means  38 ,  40  previously mounted on the end wall pass, the permanent contact of the screws with the cap  34 ,  36  limiting the risks of clamping torque becoming lost during said assembly. The bushings on which the side walls  26 ,  28  slide during expansion of the end wall  30  further enhance centering and support of said side walls. 
     FIG. 2 shows, in general manner, a second embodiment which is shown in detail in FIG. 2 a , and in which a “spring blade” type circular gasket  50 ,  52  is mounted in a circular groove  54 ,  56  formed at the downstream end of the folded portion  46 ,  48  of the outer or inner cap  36  in order to guarantee better sealing between the outer or inner side wall  26 ,  28  and the end wall  30 . In its downstream portion, the sealing gasket includes a rim  58 ,  60  designed to bear in toroidal manner on the facing side wall  26 ,  28  of the combustion chamber. The gasket is pressed against the wall by a resilient element  62 ,  64 , preferably a blade spring, and held in position by a plurality of pegs  66 ,  68  secured to the downstream end of the cap. 
     As above, it is observed that the clearance existing between the peripheral edges of the end wall and the inner faces of the side walls is determined so as to allow expansion of the end wall to be accommodated without deteriorating the composite material side walls. 
     The gasket ensuring sealing with the outer side wall is prestressed when cold, and the gasket ensuring sealing with the inner wall is merely put into contact. When hot, the opposite applies as a result of the expansion differences between the metal end-wall and the inner and outer walls.