Abstract:
A turbine shroud apparatus is provided for a gas turbine engine having a central axis. The apparatus includes: (a) an annular support member; (b) a turbine shroud disposed in the support member, the shroud being a continuous ring comprising a low-ductility material and having opposed flowpath and back surfaces, and opposed forward and aft ends; and (c) a spring mounted between the support member and the shroud and arranged to resiliently urge the shroud to a concentric position within the structural member.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates generally to gas turbine engines, and more particularly to apparatus and methods for mounting shrouds made of a low-ductility material in the turbine sections of such engines. 
     A typical gas turbine engine includes a turbomachinery core having a high pressure compressor, a combustor, and a high pressure turbine in serial flow relationship. The core is operable in a known manner to generate a primary gas flow. The high pressure turbine (also referred to as a gas generator turbine) includes one or more rotors which extract energy from the primary gas flow. Each rotor comprises an annular array of blades or buckets carried by a rotating disk. The flowpath through the rotor is defined in part by a shroud, which is a stationary structure which circumscribes the tips of the blades or buckets. These components operate in an extremely high temperature environment, and must be cooled by air flow to ensure adequate service life. Typically, the air used for cooling is extracted (bled) from the compressor. Bleed air usage negatively impacts specific fuel consumption (“SFC”) and is should generally be minimized. 
     It has been proposed to replace metallic shroud structures with materials having better high-temperature capabilities, such as ceramic matrix composites (CMCs). These materials have unique mechanical properties that must be considered during design and application of an article such as a shroud segment. For example, CMC materials have relatively low tensile ductility or low strain to failure when compared with metallic materials. Also, CMCs have a coefficient of thermal expansion (CTE) in the range of about 1.5-5 microinch/inch/degree F., significantly different from commercial metal alloys used as supports for metallic shrouds. Such metal alloys typically have a CTE in the range of about 7-10 microinch/inch/degree F. Therefore, if a CMC type of shroud is restrained by a metallic support during operation, forces can be developed in the CMC type shroud sufficient to cause failure. 
     Given the difference in thermal expansion coefficients between the CMC shroud and surrounding metal structures it is not possible to hold the shroud to the engine using mechanical fasteners such as bolts or C-clips. Additionally, any type of rigid mechanical connection would induce very high stresses into the shroud and impact turbine clearance control. 
     BRIEF SUMMARY OF THE INVENTION 
     These and other shortcomings of the prior art are addressed by the present invention, which provides a turbine shroud mounting assembly that supports a turbine shroud while permitting thermal growth. 
     According to one aspect of the invention, a turbine shroud apparatus is provided for a gas turbine engine having a central axis. The apparatus includes: (a) an annular support member; (b) a turbine shroud disposed in the support member, the shroud being a continuous ring comprising a low-ductility material and having opposed flowpath and back surfaces, and opposed forward and aft ends; and (c) a spring mounted between the support member and the shroud and arranged to resiliently urge the shroud to a concentric position within the structural member. 
     According to another aspect of the invention, a turbine shroud apparatus for a gas turbine engine having a central axis is provided, including: (a) an annular support member including a plurality of hanger tabs extending radially inward from an inner surface thereof; (b) a mounting block extending radially inward from the inner surface of the support member near each hanger tab; (c) a turbine shroud disposed in the support member, the turbine shroud being a continuous ring comprising a low-ductility material and having opposed flowpath and back surfaces, and opposed forward and aft ends, the back surface having a plurality of longitudinally-extending ribs extending radially therefrom, each rib disposed between one of the hanger tabs and the neighboring mounting block; and (c) a spring disposed between each of the mounting blocks and the associated rib, the springs urging each of the ribs in a tangential direction relative to the central axis, so as to bear against its respective hanger tab. 
     According to another aspect of the invention, a turbine shroud apparatus for a gas turbine engine having a central axis is provided, including: (a) an annular support member; (b) a turbine shroud disposed in the support member, the turbine shroud being a continuous ring comprising a low-ductility material and having opposed flowpath and back surfaces, and opposed forward and aft ends, the back surface having a plurality of longitudinally-extending ribs extending radially therefrom; and (c) a plurality of elongated springs disposed between the support member and the shroud, each spring being oriented in a generally tangential direction relative to the central axis and having a first end secured to the support member and a second end which engages ones of the ribs of the shroud, wherein the springs are collectively arranged to resiliently urge the shroud to a concentric position within the structural member. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention may be best understood by reference to the following description taken in conjunction with the accompanying drawing figures in which: 
         FIG. 1  a schematic cross-sectional view of a turbine shroud and mounting apparatus constructed in accordance with an aspect of the present invention; 
         FIG. 2  is a partial perspective view of the turbine shroud and mounting apparatus shown in  FIG. 1 ; 
         FIG. 3  is a cross-sectional view of an alternative support member; 
         FIG. 4  is a schematic cross-sectional view of a turbine shroud and mounting apparatus constructed in accordance with an alternate aspect of the present invention; 
         FIG. 5  is a partial perspective view of the turbine shroud and mounting apparatus shown in  FIG. 4 ; 
         FIG. 6  is a schematic view from aft looking forward at a turbine shroud and mounting apparatus constructed in accordance with another alternate aspect of the present invention; 
         FIG. 7  is an enlarged view of a portion of  FIG. 6 ; and 
         FIG. 8  is a partial perspective view of the turbine shroud and mounting apparatus shown in  FIG. 6 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to the drawings wherein identical reference numerals denote the same elements throughout the various views,  FIGS. 1 and 2  depict a portion of a high pressure turbine in gas turbine engine. A row of airfoil-shaped turbine blades  10  are carried by a rotating disk (not shown) in a conventional manner. It will be understood that the disk rotates about a longitudinal central axis of the engine. The blades  10  are surrounded by an annular turbine shroud  12  which is supported within the central aperture of an encircling support member. In the illustrated example the support member is an annular “shroud hanger”  14  which is itself supported by a stationary casing (not shown). The shroud hanger  14  may be continuous or segmented. For the purpose of the invention it is not critical whether or not a separate shroud hanger is present, as the shroud  12  may be mounted directly to a casing. 
     The shroud  12  is a one-piece 360° component. It is generally cylindrical and has a radially inner flowpath surface  16  and an a radially outer back surface  18 . The cross-sectional shape of the shroud  12  includes, from front to rear, a first generally cylindrical portion  20 , a raised step  22 , a radially-outwardly-extending flange  24 , and a second generally cylindrical portion  26 . As best seen in  FIG. 2 , one or more longitudinal grooves  28  are formed in the step  22 . 
     The shroud  12  is constructed from a ceramic matrix composite (CMC) material of a known type. Generally, commercially available CMC materials include a ceramic type fiber for example SiC, forms of which are coated with a compliant material such as Boron Nitride (BN). The fibers are carried in a ceramic type matrix, one form of which is SiC. Typically, CMC type materials have a room temperature tensile ductility of no greater than about 1%, herein used to define and mean a low tensile ductility material. Generally CMC type materials have a room temperature tensile ductility in the range of about 0.4 to about 0.7%. This is compared with metals having a room temperature tensile ductility of at least about 5%, for example in the range of about 5 to about 15%. The shroud  12  could also be constructed from other low-ductility, high-temperature-capable materials. 
     The flowpath surface  16  of the shroud  12  is coated with a layer of an abradable material  30  of a known type suitable for use with CMC materials. This layer is sometimes referred to as a “rub coat”. In the illustrated example, the abradable material  30  is about 0.762 mm (0.030 in.) thick. 
     A spring  32  is disposed between the shroud hanger  14  and the shroud  12  and serves to provide a radial centering force on the shroud  12 . In the illustrated example, the spring  32  is a continuous ring with a cylindrical portion  34  and an array of longitudinally-extending spring fingers  36  that press against the first generally cylindrical portion  20  of the shroud  12 , in an inboard direction. 
     The shroud hanger  14  is generally “L” shaped in cross-section and includes an axially-extending body  38  and a radially-inwardly-extending flange  40 . It may be a continuous ring or segmented. The flange  40  bears against the forward edge of the shroud  12  and restrains it from moving axially forward. 
     A static element  42  is disposed just aft of the shroud  12 . In the illustrated example, the static element  42  is a portion of a second-stage turbine nozzle. The primary function of the static element  42  is not critical to the present invention, which may also be implemented in a single-stage turbine. In any event, the static element  42  includes an axially-forward facing front face  44 . A spring element  46  is disposed between the front face  44  and the shroud  12  and serves to elastically load the shroud  12  against the flange  40  of the shroud hanger  14 . In this particular example, the spring element  46  is an annular “W” seal with a convoluted cross-section. The shroud  12  is free to move against the spring element  46  as it expands and contracts without breakage. 
     One or more anti-rotation pins  48  are carried by the shroud hanger  14 . Three or more equally-spaced anti-rotation pins  48  provide complete centering of the shroud  12 . The outer end of each anti-rotation pin  48  is securely retained in the shroud hanger  14 , for example by interference fit, mechanical fit, or bonding (e.g. welding or brazing). The anti-rotation pins  48  extend radially inward and are received in the grooves  28 . The anti-rotation pins  48  and the grooves  28  are sized to provide a tight fit in a tangential direction in order to provide effective anti-rotation. As used herein the term “tight fit” means that the shroud  12  has the minimum practical clearance in the tangential direction, while also being free to move radially relative to the anti-rotation pin  48 . In the radial direction, the gap between the groove  28  and the end of the anti-rotation pin  48  is sized so that radially outward movement of the shroud  12  will be stopped by the anti-rotation pin  48  before the turbine blade  10  can penetrate the abradable material  30  and contact the CMC portion of the shroud  12 . In other words, the range of motion permitted by the anti-rotation pin  48  is less than the thickness of the abradable material  30 . This configuration prevents severe blade tip damage. 
     As an alternative to the separate anti-rotation pins  48 , anti-rotation may be provided as an integral feature of the shroud hanger  14 . For example,  FIG. 3  illustrates a shroud hanger  14 ′ with an integral pin  48 ′ extending from a radially inner end of a flange  40 ′. The pin  48 ′ is received in a blind slot  28 ′ formed at the forward end of the shroud  12 ′. 
       FIGS. 4 and 5  depict an alternative shroud  112  supported by a support member. In the illustrated example the support member is an annular “shroud hanger”  114  which is itself supported by a stationary casing  116 . For the purpose of the invention it is not critical whether or not a separate shroud hanger  114  is present, as the shroud  114  may be mounted directly to the casing  116 . The shroud hanger  114  includes a plurality of longitudinal hanger tabs  118  extending radially inward, as well as a plurality of spring mounting blocks  120  extending radially inward. Each mounting block  120  is spaced a short distance from one of the hanger tabs  118 . 
     The shroud  112  is a one-piece 360° component constructed from a ceramic matrix composite (CMC) material as described above, and may include an abradable material or “rub coat” as described above (not shown). The shroud  112  is generally cylindrical and has a radially inner flowpath surface  122  and an a radially outer back surface  124 . The cross-sectional shape bounded by the back surface  124  includes, from front to rear, a first generally cylindrical portion  126 , a radially-outwardly-extending flange  128 , and a second generally cylindrical portion  130 . As best seen in  FIG. 5 , one or more longitudinal ribs  132  extend radially outward from the back surface  124 . 
     A spring  134  is disposed between the rib  132  and the mounting block  120  and urges the rib  132  tangentially against the adjacent hanger tab  118 , in the direction of blade rotation. It will be understood that, while the spring  134  is oriented in a tangential direction relative to the shroud  112 , it will oppose radial forces acting on the shroud  112  at a location 90° from the spring  134 . Three or more of these combinations of a rib  132 , hanger tab  118 , spring  134 , and mounting block  120  are provided around the periphery of the shroud  112 . In combination they serve to provide complete radial centering of the shroud  112 , while allowing thermal (diametrical) growth. In the illustrated example, the spring  134  is a compression type spring with a convoluted leaf configuration. A mounting pin  136  secures one end of the spring  134  through the spring  134  and the mounting block  120 . 
     The shroud hanger  114  is generally “L” shaped in cross-section and includes an axially-extending body  138  and a radially-inwardly-extending flange  140  (see  FIG. 4 ). It may be a continuous ring or segmented. The flange  140  bears against the forward edge of the shroud  112  and restrains it from moving axially forward. 
     A static element  142  is disposed just aft of the shroud  112 . In the illustrated example, the static element  142  is a portion of a second-stage turbine nozzle. The primary function of the static element  142  is not critical to the present invention, which may also be implemented in a single-stage turbine. In any event, the static element  142  includes an axially-forward facing front face  144 . A spring element  146  is disposed between the front face  144  and the shroud  112  and serves to elastically load the shroud  112  against the flange  140  of the shroud hanger  114 . In this particular example, the spring element  146  is an annular “W” seal with a convoluted cross-section. The shroud  112  is free to move against the spring element  146  as it expands and contracts without breakage. 
       FIGS. 6-8  depict an alternative shroud  212  supported by a support member. In the illustrated example the support member is an annular “shroud hanger”  214  which is itself supported by a stationary casing (not shown). For the purpose of the invention it is not critical whether or not a separate shroud hanger  214  is present, as the shroud  212  may be mounted directly to the casing. 
     The shroud  212  is a one-piece 360° component constructed from a ceramic matrix composite (CMC) material as described above, and may include an abradable material or “rub coat” as described above (not shown). The shroud  212  is generally cylindrical and has a radially inner flowpath surface  216  and an a radially outer back surface  218 . The cross-sectional shape bounded by the back surface  218  includes, from front to rear, a first generally cylindrical portion  220 , a radially-outwardly-extending flange  222 , and a second generally cylindrical portion  224 . One or more longitudinal ribs  226  extend radially outward from the back surface  218 . 
     A plurality of springs  228  are disposed between the shroud  212  and the shroud hanger  214 . In the illustrated example, each spring  228  is a leaf-type spring oriented in a generally tangential direction and has first and second ends  230  and  232 . The first end  230  is secured to the shroud hanger  214 , for example using the illustrated mounting pins  234 . The second end  232  is formed into a C-shape which is clipped over one of the ribs  226  of the shroud  212 . The spring  228  is preloaded in bending, and urges the rib  226  radially inward. Three or more of these combinations of a rib  226  and spring  228  are provided around the periphery of the shroud  212 . Each spring  228  is substantially rigid in the tangential direction, and will oppose radial forces acting on the shroud at a location 90° from the spring  228 . In combination they serve to provide complete radial centering of the shroud  212 , while allowing thermal (diametrical) growth. 
     For purposes of illustration the forward end of the shroud hanger  214  is not shown in  FIG. 8 . However, like the shroud hangers  14  and  114  described above, it is generally “L” shaped in cross-section and includes a radially-inwardly-extending flange which bears against the forward edge of the shroud  212  to restrain the shroud  212  from moving axially forward. 
     A static element  236  including an axially-forward facing front face  238  is disposed just aft of the shroud  212 . A spring element  240  is disposed between the front face  238  and the shroud  212  and serves to elastically load the shroud  212  against the shroud hanger  214 . The shroud  212  is free to move against the spring element  240  as it expands and contracts without breakage. 
     The shroud and mounting apparatus described herein has several advantages over a conventional design. The mounting apparatus supports and center the shroud within the turbine case while allowing for unrestricted radial growth. For example, a single piece, 360 degree CMC turbine shroud ring weighs less (approximately 66% reduction) and utilizes less cooling flow (approximately 50%) compared to prior art shroud designs. In addition to the performance benefit, the associated part count reduction (approximately 80%) improves maintainability of the turbine. 
     The foregoing has described a turbine shroud and mounting apparatus for a gas turbine engine. While specific embodiments of the present invention have been described, it will be apparent to those skilled in the art that various modifications thereto can be made without departing from the spirit and scope of the invention. Accordingly, the foregoing description of the preferred embodiment of the invention and the best mode for practicing the invention are provided for the purpose of illustration only and not for the purpose of limitation.