Abstract:
A multi-pieces gas turbine engine combustor heat shield is fabricated from sheet metal. The fabrication involves: sheet metal forming a base sheet having opposed hot and cold facing sides, providing cold side details separately from the base sheet, and mounting the cold side details to the cold facing side of the base sheet. The cold side details may be brazed to the base sheet.

Description:
TECHNICAL FIELD 
       [0001]    The application relates generally to gas turbine engines and, more particularly, to combustor heat shields. 
       BACKGROUND OF THE ART 
       [0002]    Combustor heat shields are typically cast from high temperature resistant materials. The casting process is known to be relatively expensive. Furthermore, the casting materials used for combustor heat shield applications may require pack coating for improved oxidation resistance, which adds to the manufacturing costs. 
         [0003]    Also, such heat shields are typically produced as a one piece casting, with studs and rails in place. When effusion holes are drilled into the heat shields, special care must be taken to avoid striking the rails and other heat shield details. 
         [0004]    Accordingly, it is desirable to have an alternate heat shield construction. 
       SUMMARY 
       [0005]    In one aspect, there is provided a method of manufacturing a combustor heat shield adapted to be mounted to a combustor shell of a gas turbine engine, the method comprising: sheet metal forming a base sheet having opposed hot and cold facing sides, the base sheet being configured to be mounted to an inner surface of the combustor shell; providing cold side details separately from said base sheet, and mounting the cold side details to said cold facing side of the base sheet, the cold side details including rails and studs separately formed and joined to the cold facing side of the base sheet. 
         [0006]    In a second aspect, there is provided a gas turbine engine combustor heat shield assembly adapted to be mounted to an inner surface of a combustor shell, the combustor heat shield assembly comprising: an arcuate sheet metal base having opposed cold and hot facing sides, the cold facing side being configured to be mounted in opposed facing relationship with the inner surface of the combustor shell, separate sheet metal rails brazed or welded to the cold facing side of the sheet metal base, the sheet metal base and the sheet metal rails being made from different gauges of sheet metal material and/or from different sheet metal materials, and studs separately joined to the sheet metal base and projecting from the cold facing side of the sheet metal base. 
         [0007]    In a third aspect, there is provided a method of fabricating a combustor heat shield adapted to be mounted to an inner surface of a combustor shell, comprising separately forming hot and cold side combustor heat shield details, the hot side detail including an arcuate base sheet configured to cover a segment of a full circumference of the combustor shell, the cold side details including at least a rail configured to define a perimeter along a cold facing side of the arcuate base sheet, the base sheet and the rail being respectively stamped and cut from different gauge of sheet metal material and/or different sheet metal materials; brazing the rail to the cold facing side of the base sheet; and welding studs to the cold facing side of the base sheet within the perimeter defined by the rail. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0008]    Reference is now made to the accompanying figures, in which: 
           [0009]      FIG. 1  is a schematic cross-sectional view of a turbofan gas turbine engine; 
           [0010]      FIG. 2  is an isometric back view of a fabricated heat shield, the effusion holes defined in the base sheet of the heat shield being omitted; 
           [0011]      FIG. 3  is an exploded isometric back view illustrating the separate parts forming the heat shield shown in  FIG. 2 , and 
           [0012]      FIG. 4  is an isometric view illustrating the mounting of fabricated heat shields to the shell of the gas turbine engine combustor. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0013]      FIG. 1  illustrates a turbofan gas turbine engine  10  of a type preferably provided for use in subsonic flight, generally comprising in serial flow communication a fan  12  through which ambient air is propelled, a multistage compressor  14  for pressurizing the air, a combustor  16  in which the compressed air is mixed with fuel and ignited for generating an annular stream of hot combustion gases, and a turbine section  18  for extracting energy from the combustion gases. 
         [0014]    The combustor  16  is housed in a plenum  17  supplied with compressed air from compressor  14 . The combustor  16  typically comprises a combustion shell  20  defining a combustion chamber  21  and a plurality of fuel nozzles (only two being shown at  22 ), which are typically equally circumferentially distributed on the dome end panel of the combustor shell  20  in order to permit a substantially uniform temperature distribution in the combustion chamber  21  to be maintained. The combustion shell  20  is typically made out from sheet metal. In use, fuel provided by a fuel manifold (not shown) is atomized by the fuel nozzles  22  into the combustion chamber  21  for ignition therein, and the expanding gases caused by the fuel ignition drive the turbine  18  in a manner well known in the art. 
         [0015]    Annular rows of circumferentially segmented heat shields  28  are mounted to the inner surface of the combustor shell  20  to thermally shield the same. Each row of heat shields  28  may cover the full circumference of the combustor shell  20 . As shown in  FIG. 1 , some of the heat shields  28  may be mounted to the dome panel of the combustor shell  20  and others to the axially projecting portions of the combustor shell  20 . Depending on the intended application, the heat shields  28  may fully cover the inner surface of the combustor shell  20  from the dome end to the opposed discharged end of the combustor. Alternatively, the heat shields may be only provided on specific portions, such as the dome end wall, of the shell  20 . 
         [0016]    The heat shields  28  have cold side surfaces or back surfaces which are spaced from the inner surface of the combustor shell  20  to define a back cooling space  30  such that cooling air may circulate therethrough to cool the heat shields  28 . Holes are typically defined in the shell  20  to allow cooling air to flow from the plenum  17  to the back cooling space  30  between the heat shields  28  and the combustor shell  20 . 
         [0017]      FIGS. 2 and 3  show an exemplary construction of one of the combustor heat shields  28 . As will be seen hereinafter, the heat shield  28  may be formed from separate sheet metal parts brazed or welded together. Such a sheet metal construction allows reducing the manufacturing costs when compared to conventional one-piece heat shield castings. For instance, the heat shield  28  may comprise a sheet metal base plate  32  having opposed cold and hot facing sides  34  and  36 , and cold side details formed separately from the base sheet  32  and then mounted to the cold facing side  34  of the base sheet  32 . The cold side details may include heat exchange promoting structures, rails, bosses, divider walls, ribs, pin fins etc. According to the illustrated embodiment, the cold side details include a rail arrangement  38  and threaded studs  40 . 
         [0018]    The base sheet  32  may be stamped or cut from a wide variety of high temperature resistant sheet metal materials. For illustration purposes only, the base sheet  32  may be made from a 0.035 inches thick sheet of Haynes 188 material. Materials such as Inco625 or SST347 could be used as well if the thermal and vibratory stresses and creep resistance are acceptable for the intended application. A thermal barrier coating, such as a ceramic coating (TBC), may be applied to the hot facing side  36  of the base sheet  32 . Holes, such as effusion holes  33  (see  FIG. 4 ) and dilution holes, may be drilled, such as by laser drilling, into the base sheet  32 . The effusion holes  33  allow the cooling air to flow from the back cooling space  30  to the front or hot facing side  36  of the heat shields  28 . 
         [0019]    The rail arrangement  38  may be made from a different sheet metal material than the base sheet  32 . According to one embodiment, the rail arrangement  38  is cut in a 0.065 thick sheet of Inco625. However, it is understood that other high temperature resistant materials could be used as well. For instance, the rail arrangement  38  could be made from a sheet of SST3 material. The material may be cut and bent to the required shape. According to the illustrated embodiment, the rail arrangement  38  comprises a peripheral rail  38   a  defining an enclosed perimeter generally corresponding and adapted to be received within the surface area of the cold facing side of the base sheet  32 . The exemplary rail arrangement  38  also comprises a central rail segment  38   b  extending perpendicularly between the longitudinal sides of the peripheral rail  38   a . The central rail segment  38   b  divides the cold side surface of the base sheet  32  which is enclosed by the peripheral rail  38   a  into two equal cold side sections. Bosses or similar projecting structures  38   c  may also integrally extend inwardly from the peripheral rail  38   a , as for instance from the bottom longitudinal rail segment thereof. 
         [0020]    The rail arrangement  38  may be brazed to the cold facing side of the base sheet  32  using high temperature braze, such as AMS4777 filler. The braze not only bonds the rail arrangement  30  to the base sheet  32 , but also provides a conductive heat transfer path between them. The braze process is controlled to minimize voids and ensure proper braze coverage/interface between the parts and thus provides an appropriate heat transfer path. According to one embodiment, the braze joint has a braze coverage of at least about 80%. 
         [0021]    The threaded studs  40  may be welded to the cold facing side  34  of the base sheet  32  by capacitor-discharge welding or other suitable weld or braze processes. The studs  40  may have a machined ring or flange  42  at a proximal end portion. As shown in  FIG. 4 , the studs  40  protrude through holes defined in the shell  20 . Self-locking nuts  44  are threadably engaged on the studs  42  from outside of the combustor shell  20  for retaining the heat shields  28  thereon. The flanges  42  on the studs  40  act as a bearing surface for the nuts  44 , and are spaced on stud  40  to have their bearing surface at the same height, or slightly below the height of the rail arrangement  38   
         [0022]    The heat shields  28  may be individually fabricated or, alternatively, a circumferentially continuous heat shield ring may be fabricated and subsequently segmented. For instance, it is herein contemplated to form the base sheet  32  into a one-piece annular member or ring, to mount the cold side details to the one-piece annular member before or after drilling the effusion holes therein and to then cut the one-piece annular member into individual circumferential segments. The thermal barrier coating on the hot facing side of the heat shields may also be applied prior to the one-piece annular member being segmented. This may contribute to generally simplify the manufacturing process of the heat shields, thereby further reducing the manufacturing costs as compared to conventional heat shield castings. 
         [0023]    According to one possible manufacturing process, a base sheet is provided in the form of a one-piece frusto-conical sheet metal member adapted to be mounted to the inner surface of a combustor shell. If a weld is required to form the frusto-conical member, the weld line is positioned to correspond to a location where the conical member will be subsequently cut to form the individual segments. Then, the studs  40  may be friction welded onto the cold facing side of the frusto-conical member. Next, the hot side of the frusto-conical member can be coated with a thermal barrier material. After coating, the effusion holes may be all laser drilled in one set-up on a laser machine. This allows drilling using a drilling-on-the-fly method. Since the rail arrangement is not yet installed on the cold side of the frusto-conical sheet metal member, the position of the effusion holes can be freely selected without the constraints generally imposed by the presence of integrally cast rails. Because the studs do not have load bearing pins around them but only a circular flange, the laser drilling operation may be simplified, as the laser is free to strike the sheet  32  very close to the studs (on conventional cast heat shields with bearing pins strikes are not allowed at the base of the bearing pins because of the risk of the pins being damaged and separating in service). Thereafter, a separately obtained sheet metal rail arrangement may be brazed to the cold facing side of the frusto-conical sheet metal member. Because the frusto-conical member is still a one-piece annular member, it provides increased stiffness to ease tack welding and braze application, while allowing for brazing of all parts in a same operation. After the brazing operation, dilution holes may be laser drilled into the frusto-conical member. Finally, the frusto-conical member with the studs and the rail arrangements mounted thereto can be laser split into individual heat shields. Since the weld line was initially positioned on a split line, to lie between two heat shield segments, it is removed during the segmenting operation. 
         [0024]    The above description is meant to be exemplary only, and one skilled in the art will recognize that changes may be made to the embodiments described without departing from the scope of the invention disclosed. Modifications which fall within the scope of the present invention will be apparent to those skilled in the art, in light of a review of this disclosure, and such modifications are intended to fall within the appended claims.