JP2013170578A - Low-ductility turbine shroud - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a shroud segment for a gas turbine engine.SOLUTION: In a shroud segment for a gas turbine engine which is constructed from a composite material including a reinforcing fibers embedded in a matrix, and has a cross sectional shape defined by an opposed forward wall and oft wall, and an opposed inner wall and outer wall extending between opposed first and second end surfaces wherein the inner wall defines an arcuate inner flow path surface, a composite fillet is disposed at a junction between first and second ones of the walls, the composite fillet includes first and second portions, and the second portion has a concave curvature extending into the first one of the walls.

Description

  The present invention relates generally to gas turbine engines, and more particularly to shrouds formed from low ductility materials in the turbine section of such engines.

  A typical gas turbine engine includes a turbomachine core having a high pressure compressor, a combustor, and a high pressure turbine in a DC relationship. The core can operate in a known manner to generate a primary gas stream. High pressure turbines (also called gas generator turbines) include one or more rotors that extract energy from a primary gas stream. Each rotor comprises an annular row of blades or buckets carried by a rotating disk. The flow path through the rotor is partially defined by the shroud. The shroud is a fixed structure that circumscribes the tip of the blade or bucket. These parts operate in very hot environments and must be cooled by air flow to ensure proper service life. Usually, air used for cooling is extracted (bleeded) from a compressor. The use of bleed air should generally be minimized because it adversely affects fuel consumption rate (SFC).

  Instead of metal shroud structures, it has been proposed to use materials with better high temperature performance such as ceramic matrix composites (CMC). Such materials have unique mechanical properties that must be considered during the design and application of articles such as shroud segments. For example, CMC materials have relatively low tensile ductility or less strain to failure than metal materials. CMC also has a coefficient of thermal expansion (CTE) in the range of about 1.5-5 microinches / inch / degree F, which is very different from commercially available metal alloys used as metal shroud supports. Such metal alloys typically have a CTE in the range of about 7-10 microinches / inch / degree F.

  The CMC material is composed of a laminate of matrix material and reinforcing fibers and is at least partially orthotropic. The matrix direction or non-primary fiber direction, also referred to herein as the interlayer, is usually weaker (ie, 1/10 or less) than the fiber direction of the composite system and can be a limiting design factor.

  The shroud structure is subject to interlaminar tensile stress applied to the wall-to-wall joint and must be supported on a weak matrix material. Such an interlaminar tensile stress can be a limited stress position of the shroud design.

  Therefore, there is a need for a composite shroud structure with low interlayer stress.

US Pat. No. 7,968,217

  This need is addressed by the present invention which provides a shroud segment configured to minimize interlaminar stress.

  According to one aspect of the present invention, an inner wall facing the front wall and the rear wall facing each other, which is made of a composite material including reinforcing fibers embedded in a matrix and extends between the first and second end faces facing each other, and A shroud segment for a gas turbine engine having a cross-sectional shape defined by an outer wall, wherein the inner wall defines an arcuate inner flow path surface, and a composite fillet between the first and second walls of the walls A shroud segment is provided having a concave surface disposed in the portion, the composite fillet including first and second portions, the second portion having a concave surface extending into the first wall of the walls.

According to another aspect of the present invention, an annular front metal hanger; and an opposing front wall comprised of a composite material comprising reinforcing fibers embedded in a matrix and extending between opposing first and second end faces; A shroud segment disposed inside a hanger having a cross-sectional shape defined by an inner wall and an outer wall facing the rear wall, the inner wall defining an arcuate inner flow path surface, and the composite fillet being out of the walls A shroud segment disposed at a junction between the first and second walls, wherein the composite fillet includes first and second portions, and the second portion has a concave surface extending into the first wall of the walls. Holding mechanically connected to a hanger that engages the shroud segment, which holds the shroud segment against the hanger while allowing radial movement of the shroud segment; By reference to the following description in conjunction with the accompanying drawings comprising a vessel, it is possible to optimally understanding of the present invention.

1 is a schematic cross-sectional view of a portion of a turbine section of a gas turbine engine that incorporates a shroud mounting device constructed in accordance with aspects of the present invention. It is a schematic perspective view of the shroud segment shown in FIG. FIG. 3 is a bottom view of the shroud segment of FIG. 2. FIG. 4 is a partially enlarged view of FIG. 3. It is a cross-sectional front elevation view of a part of the turbine section shown in FIG. FIG. 2 is a partial cross-sectional view of the shroud segment shown in FIG. 1. FIG. 2 is a cross-sectional view of a portion of another shroud segment shown in FIG. FIG. 8 is a partial cross-sectional view of the shroud segment shown in FIG. 7.

  Throughout the drawings, the same reference numerals denote the same elements. FIG. 1 shows a small portion of a turbine that is part of a known type of gas turbine engine. The function of the turbine is to extract energy from hot pressurized combustion gases from an upstream combustor (not shown) and convert this energy into mechanical work in a known manner. The turbine drives an upstream compressor (not shown) through a shaft and supplies pressurized air to the combustor.

  The principles described herein are equally applicable to turbofan engines, turbojet engines, and turboshaft engines, and turbine engines for use in other vehicles or stationary applications. Further, using a turbine shroud as an example, the principles of the present invention are applicable to low ductility flow path components that are at least partially exposed to the primary combustion gas flow path of a gas turbine engine.

  The turbine includes a fixed nozzle 10. The fixed nozzle 10 is a single structure or an assembled structure, and includes a plurality of airfoil-shaped fixed turbine blades 12 circumscribed by an annular outer belt 14. The outer belt 14 defines the outer radial boundary of the gas flow through the turbine nozzle 10. The outer belt 14 may be a continuous annular element or may be segmented.

  Downstream of the nozzle 10 is a rotor disk (not shown) that rotates about the central axis of the engine and carries a row of airfoil-like turbine blades 16. A shroud comprising a plurality of arcuate shroud segments 18 is arranged to define an outer radial flow path boundary for hot gas flow flowing through the turbine blade 16 by closely surrounding the turbine blade 16. The

  Downstream of the turbine blade 16 is a downstream fixed nozzle 17. The downstream fixed nozzle 17 has a single structure or an assembled structure, and includes a plurality of airfoil-shaped fixed turbine blades 19 circumscribed by the annular outer belt 21. The outer belt 21 defines the outer radial boundary of the gas flow through the turbine nozzle 17. The outer belt 21 may be a continuous annular element or may be segmented.

  As shown in FIG. 2, each shroud segment 18 has a generally hollow cross-sectional shape defined by opposing inner and outer walls 20 and 22 and front and rear walls 24 and 26. Rounded, pointed or angular transitions can be used at the intersections of the walls. A shroud cavity 28 is defined in the walls 20, 22, 24, 26. The transition wall 29 extends at an angle between the front wall 24 and the outer wall 22 and is arranged at an acute angle with respect to the central longitudinal axis of the engine when viewed in cross section. An axially elongated attachment hole 27 passes through the outer wall 22, the transition wall 29, and the front wall 24. The inner wall 20 defines an arcuate radially inner flow surface 30. The inner wall 20 extends axially forward past the front wall 24 to define a front flange or protrusion 32 and extends axially rearward past the rear wall 26 to define a rear flange or protrusion 34. The channel surface 30 follows an arc in an elevational view (eg, backward as viewed from the front or vice versa).

  The shroud segment 18 is constructed from a known type of ceramic matrix composite (CMC) material. In general, commercially available CMC materials include ceramic type fibers, such as SiC, whose form is coated with a compatible material such as boron nitride (BN). The fibers are supported on a ceramic type matrix, one form of which is silicon carbide (SiC). Typically, the room temperature tensile ductility of CMC type materials is about 1% or less, and CMC type materials are used herein to define and imply low tensile ductility materials. In general, the room temperature tensile ductility of CMC type materials ranges from about 0.4 to about 0.7%. This is compared to a metal having a room temperature tensile ductility of at least about 5%, such as in the range of about 5 to about 15%. The shroud segment 18 can also be constructed from other low ductility, high temperature performance materials.

  CMC materials are at least somewhat orthotropic, ie, the tensile strength of the material in a direction parallel to the length of the fiber (“fiber direction”) is perpendicular (“matrix”, “interlayer”, or “secondary” Or “tertiary” fiber direction). Physical properties such as modulus and Poisson's ratio also differ between the fiber direction and the matrix direction.

  The flow path surface 30 of the shroud segment 18 can incorporate a layer of environmental coating (EBC). This environmentally resistant coating can be a known type of anti-friction material suitable for use with wear and / or CMC materials. This layer may be referred to as the “friction coating” indicated by 38. As used herein, the term “wear” refers to the turbine blade tip while the friction coating 38 contacts the tip of the turbine blade 16 as the turbine blade 16 rotates at high speed within the shroud segment 18. This means that the friction coating 38 can be worn, polished or eroded with little or no damage to the surface. This wearability may be due to the material composition of the friction coating 38, its physical configuration, or a combination thereof. The friction coating 38 can include a ceramic layer such as yttria stabilized zirconia or barium strontium aluminosilicate. An exemplary composition and method suitable for forming the friction coating 38, incorporated herein by reference, is described in US Pat. No. 7,749,565 (Johnson et al.).

  3 and 4 show the friction coating 38 in more detail. In the illustrated example, the friction coating 38 is patterned. The pattern reduces the surface area that is exposed and contacts the tips of the turbine blades 16, thus increasing the wear of the friction coating. Specifically, a plurality of parallel grooves 39 are formed in the friction coating 38. Since there is the groove 39, a shape including alternating peaks 41 and valleys 43 is given to the surface. The grooves 39 extend in the front-rear direction as a whole, and each groove 39 has a front end portion 45, a central portion 47, and a rear end portion 49. In the plan view, the groove 39 may be curved. For example, as shown in FIG. 3, each groove 39 is curved so that the central portion 47 is displaced laterally or tangentially with respect to the front end portion 45 and the rear end portion 49.

  The shroud segment 18 includes opposing end faces 42 (also commonly referred to as “slash” faces). The end face 42 lies in a plane called the “radial plane” parallel to the central axis of the engine and may be slightly offset from the radial plane or oriented at an acute angle with respect to such a radial plane. Also good. When assembled into a complete ring, an end gap is created between the end faces 42 of adjacent shroud segments 18. One or more seals (not shown) can be provided on the end face 42. A similar seal is commonly known as a “spline seal” and takes the form of a thin piece of metal or other suitable material that is inserted into the slot in the end face 42. The spline seal is passed into the gap between the shroud segments 18.

  FIG. 6 shows the internal configuration of the shroud segment 18 in more detail. There is a concave fillet 19 between the inner wall 22 and the rear wall 26. This fillet 19 represents a joint at each of the four intersections where two of the four walls meet each other. In operation, this type of configuration must be carried by a weaker matrix material because it experiences peak interlaminar tensile stress below the material plane near the fillet 19 location. This can be a limited stress position in the design of the shroud segment 18.

  FIG. 7 shows another shroud segment 118. The basic configuration is similar to shroud segment 18, but shroud segment 118 is configured to reduce the interlaminar stress of the composite material. The shroud segment 118 has a generally hollow cross-sectional shape defined by opposing inner and outer walls 120 and 122 and front and rear walls 124 and 126. A shroud cavity 128 is defined in the walls 120, 122, 124, 126. There is a composite fillet 119 between the inner wall 122 and the rear wall 126. This fillet 119 represents a joint at each of the four intersections where two of the four walls meet each other.

  As best seen in FIG. 8, the composite fillet 119 includes a first portion 119 </ b> A having a surface disposed at an acute angle with respect to the inner surface of the rear wall 126 and the inner surface of the inner wall 120. The surface of the first portion 119A is entirely flat. First portion 119A represents the portion of material added to the nominal thickness of rear wall 126, as indicated by the location of dashed line 130. The composite fillet 119 includes a second portion 119B that is a concave surface having a radius R. The first end 132 of the second portion 119B is aligned with the first portion 119A, and the second end 134 of the second portion 119B is aligned with the inner surface of the inner wall 120 for transition. As indicated by the location of dashed line 136, second portion 119B represents the nominal thickness of rear wall 126 minus material. The composite fillet 119, particularly the second portion 119B, can be “undercut” or “thinned” preceding or adjacent to the concentrated interlayer stress region.

  At the junction of the first portion 119A and the inner surface of the rear wall 126 is a first transition surface 138, shown as a gentle concave surface. Other configurations that yield similar results have a straight or spline shape.

  A second transition portion 140 shown as a gentle convex surface is disposed at the joint between the second portion 119B and the inner surface of the inner wall 120. Other configurations that yield similar results have a straight or spline shape.

  The outer shape of the composite fillet 119 is shaped to be compatible with the composite material. The reinforcing fibers in the part generally follow (ie, are parallel to) the boundary surfaces of the inner wall 120, the composite fillet 119, and the rear wall 126. Such surface contours are made so that the fibers do not shrink or wrinkle where the outer tip is located. It should be noted that although the outline of the composite fillet 119 is shown in an exemplary plan cross-sectional view, the actual shape may vary in various parts.

  In the illustrated example, the thickness of the inner wall 120 is minimal at the position of the second portion 119B of the composite fillet 119. The exact shape and dimensions of the composite fillet 119 may be varied to suit a particular application and the particular composite material used.

  In the figure, the composite fillet 119 is disposed between the rear wall 126 and the front wall 120. Note that the same or similar configurations may be implemented at the junction between any or all of the walls 120, 122, 124, 126.

  As shown in FIG. 1, shroud segment 18 is attached to a fixed metal engine structure. In this example, the fixed structure is a part of the turbine case 44. A ring of shroud segments 18 is attached to a row of arcuate shroud hangers 46 by a row of retainers 48 and bolts 50.

  As shown in FIGS. 1 and 5, each hanger 46 includes an annular body 52 that extends substantially in the axial direction. The body 52 is angled so that the front end is radially inward of the rear end. Bolt holes 54 aligned in the radial direction pass through the main body 52 at intervals. An annular front outer leg 56 is disposed at the front end of the body 52. The annular front outer leg portion 56 includes a front hook 58 that extends substantially radially outward of the main body 52 and extends rearward in the axial direction. An annular rear outer leg 60 is disposed at the rear end of the body 52. The annular rear outer leg 60 includes a rear hook 62 that extends substantially radially outward of the main body 52 and extends rearward in the axial direction. An annular front inner leg 64 is disposed at the front end of the body 52. The annular front inner leg portion 64 extends substantially radially inward of the main body 52 and includes a rearward annular front bearing surface 66. An annular rear inner leg 68 is disposed at the rear end of the body 52. The annular rear inner leg 68 extends substantially radially inward of the main body 52 and includes a forward annular rear bearing surface 70. As will be described in more detail below, the rear inner leg 68 is configured to function as a spring element. One or more coolant supply passages 71 are formed in the main body 52, and the coolant supply passage 71 receives coolant (compressor bleed air or the like) from a supply source in the engine and sends the coolant to the inside of the main body 52. Fulfills the function.

  The hanger 46 is installed in the turbine case 44 as follows. The front hook 58 is received by the front rail 72 facing forward in the axial direction of the case 44. The rear hook 62 is received by the rear rail 74 facing forward in the axial direction of the case 44. An anti-rotation pin 76 or other similar anti-rotation configuration is received on the front rail 72 and extends into a mating slot (not shown) in the front hook 58.

  The structure of the cage 48 is shown in more detail in FIG. Each retainer 48 includes a central portion 78 having two laterally extending arms 80. The distal end of each arm 80 includes a concave contact pad 82 that projects radially outward relative to the remainder of the arm 80. The central portion 78 is radially higher than the arm 80 and defines a clamping surface 84. A radially aligned hole 86 passes through the central portion 78. A generally tubular insert 88 is crimped or fixed in hole 86 and includes a threaded fastening hole. If appropriate, the hole 86 may be threaded to eliminate the insertion portion 88.

  The retainer 48 is positioned within the shroud cavity 28 by a central portion 78 and a clamping surface 84 exposed through the mounting hole 27 in the outer wall 22. The retainer 48 is clamped against the boss 90 of the hanger 46 by a bolt 50 or other suitable fastener, and a spring 92 is clamped between the boss 90 and the clamping surface. Each spring 92 includes a central portion having a mounting hole and opposing arms 94 extending in the lateral direction.

  The relative dimensions of the boss 90, the retainer 48, and the shroud segment 18 are selected so that the retainer 48 restricts inward movement of the shroud segment 18 but does not radially clamp the shroud segment 18 relative to the hanger 46. Is done. In other words, the retainer 48 allows a clear clearance for radially outward movement. In operation, a typical gas pressure load in the secondary flow path pushes the shroud segment 18 radially inward against the retainer 48, causing the retainer 48 to bend slightly.

  The spring 92 functions to hold the shroud segment 18 radially inward relative to the retainer 48 for an initial grinding process that rounds the ring of the shroud segment 18 during assembly. However, the size of the spring 92 is determined so as not to apply a substantial clamping load to the shroud segment 18.

  In the axial direction, the rear inner leg 68 of the hanger 46 acts as a large cantilever spring to counteract pneumatic loads during operation. Due to this spring action, the front wall 24 of the shroud segment 18 is pressed against the front bearing surface 66 of the front inner leg 64, thereby forming a secure seal between the metal hanger 46 and the CMC shroud segment. Cooling flow leakage is reduced.

  In the installed state, the front protrusion 32 and the rear protrusion 34 are disposed close to each other in the axial direction, or are disposed so as to overlap the front and rear components of the shroud segment 18 in the axial direction. In the illustrated example, there is an overlapping configuration between the rear protrusion 34 and the rear nozzle belt 21, and the front protrusion 32 is disposed close to the front outer belt 14. This configuration minimizes leakage between components and prevents the introduction of hot gas from the primary flow path to the secondary flow path.

  As described above, the mounting hole 27 passes through the outer wall 22, the transition wall 29, and the front wall 24. This allows shroud segment 18 to incorporate a significant amount of open area. There is no air seal between the perimeter of the mounting hole 27 and the hanger 46 and the shroud segment 18 does not function as a plenum by itself. Rather, the shroud segment 18 cooperates with the hanger 46 to form a plenum, as indicated generally by “P” in FIG. Specifically, there is an annular seal contact between the front bearing surface 66 and the front wall 24 of the shroud segment 18. There is also an annular seal contact between the rear bearing surface 70 and the rear wall 26 of the shroud segment 18. Seal contact is ensured by the spring action of the rear inner leg 68 described above. The shroud segment 18 is considered to be “inside” the plenum, and the hanger 46 is considered to be “outside” the plenum.

  A hollow metal impingement baffle 96 is disposed within each shroud segment 18. The impact baffle 96 fits snugly with the retainer 48. A plurality of collision holes 98 are formed in the inner wall of the collision baffle to guide the coolant to the segment 18. The inside of the collision baffle 96 communicates with the coolant supply passage 71 through a transfer passage 73 formed in the cage 48.

  In operation, air flows through the passage 71, the transfer passage 73, the baffle 96, and the collision hole 98 to pressurize the plenum P. Spent cooling air from the plenum P is exhausted through a purge hole 100 formed in the front wall 24 of the shroud segment 18.

  The aforementioned shroud attachment device is effective for attaching a low ductility shroud to a turbine engine without directly applying a tightening load to the turbine engine, and has a number of advantages over the prior art.

  In particular, the tapered edge (or wedge) shape of the shroud front prevents the shroud mounting system from transmitting load from the front of the shroud segment 18 to the turbine case 44 and directly through the shroud segment 18. By redirecting the load around the shroud segment 18, the stress on the shroud segment 18 remains relatively low.

  Further, the protrusions 32, 34 may protect the support structure close to the flow path while preventing the introduction of hot gas using the overlap of the shroud segment 18 and the axially adjacent nozzle. it can. This overlapping configuration improves overall engine performance because less cooling flow is required to purge the cavity from the shroud to the nozzle. Because the shroud material has better high temperature performance and lower stress than adjacent nozzles, the use of protrusions 32, 34 increases the overall turbine life.

  Finally, by incorporating the composite fillet 119, the interlaminar stress at the shroud segment wall intersection can be distributed over a larger region, thus reducing the peak interlaminar tensile stress value. Analysis shows that the configuration can reduce the peak interlaminar tensile stress, for example, about 50%, compared to a configuration without composite fillets and no major change in the main in-plane (or fiber direction) stress. ing.

  A turbine shroud device for a gas turbine engine has been described. While particular embodiments of the present invention have been described, it will be apparent to those skilled in the art that various modifications 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 carrying out the invention are made for illustrative purposes only and are not intended to be limiting.

DESCRIPTION OF SYMBOLS 10 Fixed nozzle 12 Turbine blade 14 Outer belt 16 Turbine blade 17 Downstream fixed nozzle 18, 118 Shroud segment 19 Fixed turbine blade, concave fillet 20, 120 Inner wall 21 Outer belt 22, 122 Outer wall 24, 124 Front wall 26, 126 Rear wall 27 Mounting hole 28, 128 Shroud cavity 29 Transition wall 30 Flow path surface 32, 34 Projection portion 38 Friction coating 39 Groove 41 Mountain 42 End surface 43 Valley 44 Turbine case 45 Front end portion 46 Shroud hanger 47 Center portion 48 Cage 49 Rear end portion 50 Bolt 52 Main body 54 Bolt hole 56 Annular front outer leg 58 Front hook 60 Annular rear outer leg 62 Rear hook 64 Annular front inner leg 66 Annular front bearing surface 68 Annular rear inner leg 70 Annular rear bearing surface 71 Coolant supply passage 72 Previous 73 Transfer path 74 Rear rail 76 Anti-rotation pin 78 Central portion 80 Arm 82 Concave contact pad 84 Tightening surface 86 Hole 88 Insertion portion 90 Boss 92 Spring 94 Arm 96 Collision baffle 98 Collision hole 100 Purge hole 119 Composite fillet 119A First Part 1 119B Second part 130, 136 Dashed line 134 Second end 138 First transition surface 140 Second transition part P Plenum

Claims (20)

Cross-sectional shape defined by an inner wall and an outer wall facing opposite front and rear walls, comprising a composite material comprising reinforcing fibers embedded in a matrix and extending between opposing first and second end faces A shroud segment for a gas turbine engine, wherein the inner wall defines an arcuate internal flow path surface, and a composite fillet is disposed at a joint between the first and second walls of the wall, A shroud segment, wherein the fillet includes first and second portions, and wherein the second portion has a concave surface extending into the first wall of the wall. The shroud segment of claim 1, wherein the thickness of the first wall is minimal within the second portion of the composite fillet. The shroud segment of claim 1, wherein the first portion comprises a surface disposed at an acute angle with respect to the first and second walls. The shroud segment of claim 1, wherein the first portion represents a portion added to a nominal thickness of the second wall. The shroud segment of claim 1, wherein the first wall is the inner wall. The shroud segment of claim 1, wherein the second wall is the rear wall. The shroud segment of claim 1, wherein the composite material comprises a ceramic matrix composite material. An annular metal hanger,
Cross-sectional shape defined by an inner wall and an outer wall facing opposite front and rear walls, comprising a composite material comprising reinforcing fibers embedded in a matrix and extending between opposing first and second end faces A shroud segment disposed on the inside of the hanger, wherein the inner wall defines an arcuate inner flow path surface, and a composite fillet at a junction between the first and second walls of the wall A shroud segment disposed and wherein the composite fillet includes first and second portions, the second portion having a concave surface extending into the first wall of the walls;
For a gas turbine engine comprising: a retainer mechanically coupled to the hanger that engages the shroud segment that retains the shroud segment against the hanger while allowing radial movement of the shroud segment Shroud device. The apparatus of claim 8, wherein the retainer includes a central portion having a pair of opposing arms extending laterally outward. The apparatus of claim 8, wherein the retainer face is clamped against the hanger and the outer wall of the shroud segment is confined between the hanger and a portion of the retainer. 11. A spring is clamped between the hanger and the retainer to resiliently abut the shroud segment so as to press the shroud segment radially inward against the retainer. Equipment. 9. The apparatus of claim 8, wherein the inner wall extends axially forward past the front wall to define a front protrusion, and the inner wall extends axially rearward past the rear wall to define a rear protrusion. . The apparatus of claim 8, wherein the hanger is enclosed and carried by an annular turbine case. The apparatus of claim 13, wherein the hanger comprises axially spaced front and rear hooks received by the front and rear rails of the turbine case, respectively. 9. The apparatus of claim 8, wherein the hanger comprises an annular body having a front end disposed radially inward with respect to a rear end. The apparatus of claim 8, comprising a transition wall disposed between the front wall and the outer wall and extending at an acute angle with respect to the front wall and the outer wall. The apparatus of claim 16, wherein the transition wall extends substantially parallel to the body of the hanger. 9. The apparatus of claim 8, wherein the hanger includes an elastic rearward inner leg that elastically loads the shroud segment axially forward relative to a bearing surface of the front inner leg of the hanger. The apparatus of claim 8, wherein the shroud segment comprises a ceramic matrix composite. The apparatus of claim 8, wherein the annular rings of the shroud segments are arranged in an annular row within the case.