Abstract:
A method facilitates assembling a stator assembly for a turbine engine. The method comprises positioning a shroud fabricated from a ceramic matrix composite material adjacent a metallic stator block, and coupling the shroud to the stator block using a coupling arrangement such that a predetermined radial clearance is defined between the shroud and a rotor assembly coupled radially inward thereof.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     This application relates generally to turbine engines and, more particularly, to methods and apparatus for assembling turbine engine components that are fabricated from ceramic matrix composite materials.  
         [0002]     Turbine engines include at least one stator assembly and at least one rotor assembly. At least some known rotor assemblies include at least one row of circumferentially-spaced rotor blades. The blades extend radially outward from a platform to a tip. A plurality of static shrouds coupled to a stator block abut together to form flowpath casing that extends circumferentially around the rotor blade assembly, such that a radial tip clearance is defined between each respective rotor blade tip and the casing or shroud. The tip clearance is tailored to be a minimum, yet is sized large enough to facilitate rub-free engine operation through the range of available engine operating conditions.  
         [0003]     During operation, tip leakage across the rotor blade tips may limit the performance and stability of the rotor assembly. However, during operation, because the shrouds may be subjected to higher operating temperatures than the stator block, the shrouds may thermally expand at a different rate than the stator block or the fastener assemblies used to couple the shrouds to the stator block. More specifically, the differential thermal expansion may undesirably cause increased tip leakage as the operating temperature within the engine is increased. In addition, over time, the heat transfer from the shrouds and the differential thermal expansion may also cause premature failure of the fastener assemblies.  
         [0004]     Accordingly, to facilitate reducing tip leakage caused by the differential thermal expansion, at least some known engines supply increased cooling flow past the shrouds and fastener assemblies. However, excessive cooling flow may adversely affect engine performance. To facilitate increasing the operating temperature of the engine, and thus facilitate improving engine performance, other known stator assemblies have included shrouds fabricated from stronger or higher temperature capability materials. However, although such materials should enable the shrouds to be exposed to higher operating temperatures, the operation of the engine may still be limited by the increased thermal differential expansion rates between the shrouds and the stator block through the fastener assemblies.  
       BRIEF DESCRIPTION OF THE INVENTION  
       [0005]     In one aspect, a method for assembling a stator assembly for a turbine engine is provided. The method comprises positioning a shroud fabricated from a ceramic matrix composite material adjacent to a metallic stator block, and coupling the shroud to the stator block using a coupling arrangement such that a predetermined radial clearance is defined between the shroud and a rotor assembly coupled radially inward thereof.  
         [0006]     In another aspect, a stator assembly for a turbine engine is provided. The stator assembly includes a stator block including at least one fastener opening, a coupling arrangement, and a shroud coupled to the stator block by the coupling arrangement. The shroud includes at least one fastener opening. The coupling arrangement includes at least one fastener extending through the shroud at least one fastener opening and at least one fastener opening through the stator block. The fastener includes an external surface coated with at least one of a wear coating and a thermal barrier coating.  
         [0007]     In a further aspect, a turbine engine is provided. The turbine engine includes a rotor assembly, and a stator assembly that includes a stator block, at least one fastener, and a shroud. The shroud is coupled to the stator block by the at least one fastener such that a radial clearance is defined between at least a portion of the rotor assembly and the shroud. The at least one fastener includes an external surface coated with at least one of a wear coating and a thermal barrier coating.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]      FIG. 1  is a schematic illustration of an exemplary gas turbine engine;  
         [0009]      FIG. 2  is an enlarged side view of an exemplary fastener that may be used with a turbine engine, such as the gas turbine engine shown in  FIG. 1 ;  
         [0010]      FIG. 3  is a cross-sectional view of the fastener shown in  FIG. 2 ;  
         [0011]      FIG. 4  is an enlarged cross-sectional view of a portion of a stator assembly that may be used with a turbine engine, such as the gas turbine engine shown in  FIG. 1 , and including the fastener shown in  FIG. 2 ; and  
         [0012]      FIG. 5  is an enlarged cross-sectional schematic view of a portion of the stator assembly shown in  FIG. 4 . 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0013]      FIG. 1  is a schematic illustration of an exemplary gas turbine engine  10  coupled to an electric generator  16 . In the exemplary embodiment, gas turbine system  10  includes a compressor  12 , a turbine  14 , and generator  16  arranged in a single monolithic rotor or shaft  18 . In an alternative embodiment, shaft  18  is segmented into a plurality of shaft segments, wherein each shaft segment is coupled to an adjacent shaft segment to form shaft  18 . Compressor  12  supplies compressed air to a combustor  20  wherein the air is mixed with fuel supplied via a stream  22 . In one embodiment, engine  10  is a  6 FA+e gas turbine engine commercially available from General Electric Company, Greenville, S.C.  
         [0014]     In operation, air flows through compressor  12  and compressed air is supplied to combustor  20 . Combustion gases  28  from combustor  20  propels turbines  14 . Turbine  14  rotates shaft  18 , compressor  12 , and electric generator  16  about a longitudinal axis  30 .  
         [0015]      FIG. 2  is an enlarged side view of an exemplary fastener  50  that may be used with a turbine engine, such as engine  10  (shown in  FIG. 1 ).  FIG. 3  is a cross-sectional view of fastener  50 . In the exemplary embodiment, fastener  50  is a pin, and includes an integrally-formed head portion  52 , a nose portion  54 , and a barrel or shank portion  56  extending therebetween. In the exemplary embodiment, head portion  52  is threaded and has a diameter D 1  that is larger than a diameter D 2  of barrel portion  56 . More specifically, in the exemplary embodiment, head portion  52  is formed with a plurality of threads  60  extending outwardly from an external surface  62  of fastener  50 . Threads  60  enable fastener  50  to be secured within a threaded opening (not shown in  FIGS. 2 and 3 ). In an alternative embodiment, head portion  52  does not include any threads  60 , but rather barrel portion  56  is threaded.  
         [0016]     In the exemplary embodiment barrel portion diameter D 2  is substantially constant between head and nose portions  52  and  54 , respectively. Moreover, in the exemplary embodiment, barrel portion  56  is un-threaded such that fastener external surface  62  is substantially smooth across portion  56 . In an alternative embodiment, at least a portion of barrel portion  56  is threaded. In another alternative embodiment, barrel portion diameter D 2  is not constant across barrel portion  56 . In the exemplary embodiment, barrel portion diameter D 2  is between approximately 0.25 inches and 0.3125 inches.  
         [0017]     Nose portion  54  is gradually tapered inward from barrel portion  56  such that a diameter D 3  at an inner end  64  of fastener  50  is smaller than barrel portion diameter D 2 . Moreover, in the exemplary embodiment, nose portion  54  curves inwardly such that portion  54  has a bullnose-shaped cross-sectional profile.  
         [0018]     A sealing flange  70  extends radially outward from barrel portion  56  such that a pair of opposed faces  72  and  74  are defined. In the exemplary embodiment, faces  72  and  74  are substantially parallel and each is substantially perpendicular to a centerline axis of symmetry  76  extending through fastener  50 . Moreover, in the exemplary embodiment, sealing flange  70  is formed integrally with fastener  50 . In an alternative embodiment, fastener  50  does not include a sealing flange  70 .  
         [0019]     Sealing flange  70  is spaced a distance d 4  from head portion  52  such that an annulus  80  is defined between sealing flange  70  and head portion  52 . In the exemplary embodiment, annulus  80  has an external diameter D 5  that is smaller than barrel portion diameter D 2  and is substantially constant therethrough.  
         [0020]     A cooling passageway  90  is defined within fastener  50  and extends through barrel and nose portions  56  and  54 , respectively. Cooling passageway  90  has a diameter D 6  measured with respect to an inner surface  92  of fastener  50 . In the exemplary embodiment, diameter D 6  is substantially constant along a length L 1  of passageway  90 .  
         [0021]     Cooling passageway  90  extends from an inlet  94  to a discharge outlet  96 . Inlet  94  extends generally radially from fastener external surface  62  to passageway  90  and enables cooling fluid to be supplied to fastener passageway  90  from a cooling circuit (not shown in  FIGS. 2 and 3 ) when fastener  50  is secured within the threaded opening. More specifically, in the exemplary embodiment, inlet  92  is defined within annulus  80 . Outlet  96  extends substantially axially from fastener external surface  62  to passageway  90  and enables cooling fluid to be discharged from fastener passageway  90  when fastener  50  is secured within the threaded opening. More specifically, in the exemplary embodiment, outlet  96  is substantially concentrically aligned with respect to fastener  50 , and extends axially inward from fastener end  64 .  
         [0022]     Fastener external surface  62  is coated with a wear coating and/or a thermal barrier coating (TBC) that facilitates improving the wear characteristics of fastener  50  and/or thermally insulates fastener  50 , as described in more detail below. For example, in one embodiment, fastener  50  is fabricated from a metallic alloy material, such as L605, commercially available from Haynes International, Inc., Kokomo, Ind. More specifically, fasteners  50  are fabricated from metallic materials which facilitate fasteners  50  operating with a desired fracture toughness, and a demonstrated reliability.  
         [0023]     Moreover, in at least some embodiments, the coating also facilitates reducing oxidation of fastener  50 . For example, in the exemplary embodiment, fastener  50  is coated with a wear or thermally insulating bond coat, such as a NiCrAlY, and is then further coated with an external oxidation resistive coating, such as Deloro-Stellite&#39;s Tribaloy T-800. The gradual transition of nose portion  54  facilitates enhancing the coating adhesion to fastener  50 , as more radical transitions may result in loss of coating during, or shortly after, the coating process. Accordingly, as described in more detail below, the fastener coating enables fastener  50  to be utilized in increased stress environments and/or in increased operating temperatures, without requiring that fasteners  50  be fabricated from more expensive or brittle materials that are more temperature or wear resistive.  
         [0024]      FIG. 4  is an enlarged cross-sectional view of a portion of a stator assembly  100  that may be used with a turbine engine, such as gas turbine engine  10  (shown in  FIG. 1 ).  FIG. 5  is an enlarged cross-sectional schematic view of a portion of stator assembly  100 . Specifically, stator assembly  100  includes a stator block  102  that forms a portion of a casing within engine  10 , and a shroud  104 . In one embodiment, stator casing  100  extends circumferentially around a rotor assembly, such as turbine  14 .  
         [0025]     In the exemplary embodiment, stator block  102  is fabricated from a metallic material and is formed with a plurality of leading edge fastener openings  110 , a plurality of trailing edge fastener openings  112 , and a shroud slot  114 . Fastener openings  110  are circumferentially-spaced across a leading edge side  116  of stator block  102 , and openings  112  are circumferentially-spaced across a trailing edge side  118  of stator block  102 . Openings  110  and  112  are each sized to receive a fastener  50  therein to enable shroud  104  to be coupled to stator block  102 , as described in more detail below.  
         [0026]     In the exemplary embodiment, fasteners  50  include a plurality of pins  120  and a plurality of bolts  122 . Pins and bolts  120  and  122 , respectively, are substantially similar and each includes a wear or thermally insulating coating, internal cooling passageway  90 , and head, nose, and barrel portions  52 ,  54 , and  56 , respectively. Unlike pins  120 , threads  60  are not formed within bolt head portion  52 , but rather instead each barrel portion  56  is threaded. In addition, bolt barrel portion  56  is stepped such that at least one segment  124  of barrel portion  56  has an external diameter D 8  that is sized differently than the remaining barrel portion diameter D 2 . For example, in the exemplary embodiment, barrel portion diameter D 8  is larger than barrel portion diameter D 2 .  
         [0027]     A sealing face  130  is defined at the intersection created between barrel portion  56  and segment  124 . Accordingly, in the exemplary embodiment, bolts  120  do not include sealing flange  70 , but rather, when bolts  120  are fully secured within openings  112 , sealing flange  70  is secured in sealing contact against stator block  102 , and more specifically, against a sealing boss  132  extending outwardly from stator block  102 . Each sealing boss  132  circumscribes each opening  112 , and extends outwardly from stator block to form a mating surface that receives sealing face  130  in sealing contact.  
         [0028]     Bolt cooling passageway  90  extends between inlet  94  and discharge outlet  96 . However, unlike pins  120 , bolt cooling passageway inlet  94  is defined within bolt barrel portion  56 .  
         [0029]     In the exemplary embodiment, each stator block opening  112  extends radially inward from an external surface  140  of stator block  102  and has a diameter D 10  that is substantially constant therethrough. More specifically, opening  112  has a length L 3  that is longer than a length L 5  of bolt barrel portion  56 . Accordingly, when bolt  120  is threadedly coupled within opening  112 , a hollow space  142  is defined between bolt inner end  64  and a radially inner end  144  of opening  112 .  
         [0030]     Each stator block opening  110  also extends radially inward from stator block external surface  140  and is bifurcated such that a first portion  150  of opening  110  is defined within a radially outer portion  152  of stator block  102  that is adjacent to shroud slot  114 , and a second portion  154  of opening  110  is defined within a radially inner portion  156  of shroud block  102  that is adjacent to shroud slot  114 . In the exemplary embodiment, opening first portion  150  has a diameter D 14  that is slightly larger than pin head diameter D 1 , an opening second portion  154  has a diameter D 16  that is smaller than diameter D 14  and is slightly larger than pin barrel portion diameter D 2 . More specifically, opening first portion  150  extends from external surface  140  to an end wall  160  that defines a portion of shroud slot  114 , and opening second portion  152  extends through end wall  160  and through stator block radially inner portion  156 . Accordingly, when pin  120  is securely coupled within opening  110 , seal flange  70  contacts end wall  160  in sealing contact, and pin barrel portion  56  is inserted through opening portion  150  and at least partially through opening portion  152 . Moreover, when pin  120  is securely coupled within opening  110 , pin head  52  is recessed within opening  110  such that an outer surface  170  of pin head  52  is substantially co-planar with the portion of stator block external surface  140  adjacent to opening  110 .  
         [0031]     Each stator block opening  110  and  112  is coupled in flow communication to a cooling fluid supply source through a cooling circuit  180 . Cooling circuit  180  includes a plurality of supply slots  182  that each supply cooling air into a respective opening  110 , and a plurality of supply slots  184  that each supply cooling air into a respective opening  112 . Cooling circuit  180  also includes a plurality of discharge slots  186  that each route discharged cooling air from a respective opening  112 .  
         [0032]     Shroud  104  includes a plurality of fastener openings  190  which extend from a radially inner side  192  of shroud  104  to a radially outer side  194  of shroud  104 . More specifically, openings  190  include a plurality of fastener pin openings  196  that are sized to receive a portion of a respective pin  120  therethrough, and a plurality of bolt openings  198  that are sized to receive a portion of a respective bolt  122  therethrough. More specifically, openings  196  are sized to receive pin barrel portion  56  therethrough, and openings  198  are sized to receive pin barrel portions  54  and  124  therein such that head portion  52  remains external to opening  198 .  
         [0033]     When assembled, shroud  104  is suspended from pins and bolts  120  and  122 , respectively. More specifically, when stator assembly  100  is fully assembled, a downstream side  200  of shroud  104  is coupled to stator block  102  by bolts  122  such that bolts  122  are inserted through shroud openings  198  prior to being threadingly coupled to stator block  102  within block openings  112 . Accordingly, when bolts  122  are secured to block  102 , shroud downstream side  200  is suspended from bolt barrel portion  124  between bolt head portion  52  and stator block external surface  140 . Furthermore, when stator assembly  100  is fully assembled, an upstream side  202  of shroud  104  is coupled to stator block  102  by pins  120  such that shroud  104  is suspended by pin barrel portion  56  within shroud slot  114 . Accordingly, when coupled to stator block  104 , a radial clearance is defined between shroud  104  and rotating members of a rotor assembly, such as  
         [0034]     Shroud  104  is fabricated from a ceramic matrix composite (CMC) material that enables shroud  104  to be exposed to, and to sustain, higher operating temperatures than fasteners  50  or stator block  102 . Accordingly, a rate of thermal expansion for shroud  104  may be different than a rate of thermal expansion of fasteners  50  or stator block  102  during engine operation. The pin and bolt concepts described herein, permit fasteners  50  to accommodate the difference in thermal expansion rates between stator block  102  and shroud  104 . More specifically, because a width W 3  of shroud  104  is smaller than a width W 5  of shroud slot  114 , shroud  104  may slide axially within slot  114  to accommodate differential thermal expansion such that a radial clearance defined between shroud  104  and a rotor assembly, such as turbine  14 . Moreover, the pin and bolt concepts described herein also enable fasteners  50  to operate within the thermal environment sustained by ceramic matrix composites, without melting. Notably, the wear or thermal coating across fasteners  50  facilitates enabling the material used in fabricating fasteners  50  to operate beyond its un-coated melting point. Moreover, because the coating provides both thermal insulation and oxidation resistance, the coating facilitates extending a useful life of fasteners  50 .  
         [0035]     In addition, when fully assembled, cooling fluid is supplied internally to fasteners  50  during engine operation. Specifically, cooling fluid is supplied to stator block openings  110  through supply slots  182 . As the fluid enters openings  110 , annulus  80  is pressurized by the cooling fluid prior to the fluid being channeled into pin cooling passageway  90  through inlet  94 . The cooling fluid flows through pin cooling passageway  90  and is discharged through outlet  96  and flows external to stator block  102 . More specifically, the cooling fluid flowing through pin cooling passageway  90  facilitates maintaining an operating temperature of pin  120  within acceptable limits.  
         [0036]     In addition, cooling fluid is supplied to stator block openings  112  through slots  184 . Fluid supplied through slots  184  is channeled into bolt cooling passageway  90  through inlet  94 . The cooling fluid flows through pin cooling passageway  90  and is discharged through bolt cooling passageway outlet  96  wherein the fluid enters space  142  prior to being discharged externally to stator block  102  through discharge slots  186 . More specifically, the cooling fluid flowing through bolt cooling passageway  90  facilitates maintaining an operating temperature of bolt  122  within acceptable limits.  
         [0037]     The above-described fasteners provide a cost-effective and highly reliable method for coupling a ceramic matrix composite shroud to a metallic stator block. Accordingly, the combination of the ceramic matrix composite shroud and the fasteners described herein, facilitate enabling the turbine to operate at higher temperatures, thus improving thermodynamic efficiency of the turbine. The fasteners described herein accommodate the differential thermal expansion between the shroud and the stator block, while maintaining the radial clearance defined by the shroud. As a result, the fasteners facilitate extending a useful life of the stator assembly and improving the operating efficiency of the gas turbine engine in a cost-effective and reliable manner.  
         [0038]     Exemplary embodiments of stator assemblies and turbine engines are described above in detail. The stator assemblies are not limited to the specific embodiments described herein, but rather, components of each stator assembly may be utilized independently and separately from other components described herein. For example, each stator assembly component can also be used in combination with other turbine engine components, and is not limited to practice with only stator assembly  100  as described herein. Rather, the present invention can be implemented and utilized in connection with many other high temperature attachment configurations.  
         [0039]     While the invention has been described in terms of various specific embodiments, those skilled in the art will recognize that the invention can be practiced with modification within the spirit and scope of the claims.