Abstract:
An inner diameter shroud for receiving an inner diameter base portion of a rotatable vane in a gas turbine engine has a single piece channel and a core. The channel has a leading edge wall, an inner diameter wall, a trailing edge wall, a radial outer surface, and at least two axial projections. The axial projections prevent radial movement of the core. The core has an outer radial surface that generally aligns with the radial outer surface of the channel. The core is movable in the channel in a circumferential direction and is configured to rotatably retain the inner diameter base portion of the rotatable vane.

Description:
BACKGROUND 
     The present invention relates to a gas turbine engine shroud, and more particularly to an inner diameter shroud that has a single exterior channel and a lightweight core. 
     In the high pressure compressor section of a gas turbine engine, the inner diameter shroud protects the radially innermost portion of the vanes from contact with the rotors  12 , and creates a seal between the rotors and the vanes. Typically, the inner diameter shroud is a clam shell assembly comprised of two shroud segments, a clamping bolt, and a clamping nut. The bolt fastens to the nut through the two shroud segments. Turbine engine inner shroud average diameters typically range from 18 to 30 inches (475 mm to 760 mm) in diameter. This diameter, coupled with dynamic loading and temperatures experienced by the shroud during operation of the turbine engine, require the use of at least a #10 bolt (0.190 inches, 4.83 mm, in diameter) in the conventional clam shell assembly. The #10 bolt prevents scalability of the shroud assembly because the shroud must be a certain size to accommodate the bolt head, corresponding nut and assembly tool clearance. Thus, the radial height, a measure of the inner shroud&#39;s leading edge profile, typically approaches 1 inch (25.4 mm) with the conventional clam shell shroud. The excessive radial height of the clam shell configured shroud diminishes the compressor efficiency, increases the weight of the shroud, and potentially negatively impacts the weight-to-thrust performance ratio of the turbine engine. 
     SUMMARY 
     An inner diameter shroud for receiving an inner diameter base portion of a rotatable vane in a gas turbine engine has a single piece channel and a core. The channel has a leading edge wall, an inner diameter wall, a trailing edge wall, a radial outer surface, and at least two axial projections. The axial projections prevent radial movement of the core. The core has an outer radial surface that generally aligns with the radial outer surface of the channel. The core is movable in the channel in a circumferential direction and is configured to rotatably retain the inner diameter base portion of the rotatable vane. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a partial sectional view of a compressor section for a gas turbine engine. 
         FIG. 2  is a sectional view of a shroud assembly according to an embodiment of the present invention bisecting a vane. 
         FIG. 3  is a sectional view of the shroud assembly of  FIG. 2  bisecting a dowel pin. 
         FIG. 4  is an exploded end view of the shroud assembly of  FIG. 2  showing a core containing a vane and a channel with an inner air seal removed. 
         FIG. 5A  is an exploded outer diameter view of the core of  FIG. 4 . 
         FIG. 5B  is an exploded inner diameter view of the core of  FIG. 4 . 
         FIG. 6  is an exploded sectional inner diameter view of the shroud assembly core with a composite bearing according to another embodiment of the present invention. 
         FIG. 7  is a sectional view of a shroud assembly according to another embodiment of the present invention bisecting a dowel pin. 
         FIG. 8  is an exploded end view of the shroud assembly of  FIG. 7  showing a core containing a vane and a channel with an inner air seal removed. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  is a partial sectional view of a compressor section for a gas turbine engine  10  that includes a rotor  12 , a case  14 , a variable inlet guide vane  16 , a first stage rotor blade  18 , a first stage variable vane  20 , a second stage rotor blade  22 , a second stage variable vane  24 , a third stage rotor blade  26 , and a third stage variable vane  28 . Each of the vanes  16 ,  20 ,  24 ,  28  includes an outer diameter trunnion  30 , an inner diameter base portion  32 , an inner diameter shroud  34 . The inner diameter shroud  34  includes radially inward facing inner diameter air seal  36 . Connected to each outer diameter trunnion  30  is a vane positioning mechanism that includes a fastener  38 , an actuating arm  40 , and a unison ring  42 . The rotor  12  includes knife edge seals  44  positioned opposite each of the inner diameter air seals  36  to create a leakage restriction. 
       FIG. 1  shows the compressor section for gas turbine engine  10  with a rotor  12  carrying a plurality of stages of rotor blades  18 ,  22 ,  26 . The rotor  12  acts dynamically on air flow entering the compressor section. The rotor  12  includes an arcuate array of knife edge seals  44  that act with the inner diameter air seals  36  to cut off secondary flow around the rotor  12 . Thus, the base of the rotor blades  18 ,  22 ,  26  and the inner diameter shrouds  34  define an inner diameter flow path  46 , which axially directs compressed air flow through the compressor section. 
     In  FIG. 1 , the case  14  defines an outer diameter flow path  48  for the air flow in the compressor section. The case  14  uses fasteners  38  to interconnect with the outer diameter trunnion  30  on the vane stages  16 ,  20 ,  24 ,  28 . The vane stages  16 ,  20 ,  24 ,  28  are stationary but act on the air flow by directing flow incidence impinging on subsequent rotating blades in the compressor section. The vane stages  16 ,  20 ,  24 ,  28  direct the flow incidence simultaneously via the unison ring  42 . The unison ring  42  interconnects with the actuating arm  40 , which is engaged to the interconnecting surface of the trunnion  30 . The fastener  38  secures the vane arm  40 , which pivots the vane stages  16 ,  20 ,  24 ,  28  about the axes of the outer diameter trunnions  30 . The vanes  16 ,  20 ,  24 ,  28  also pivot about an axes of the inner diameter base portions  32  within the inner diameter shrouds  34 . This allows the inner diameter shrouds  34  and the inner diameter air seals  36  to remain stationary during the pivoting of the vane stages  16 ,  20 ,  24 ,  28 . The stationary inner diameter shrouds  34  and the inner diameter air seals  36 , along with the dynamic rotor  12 , define the inner diameter flow path  46 . Compression cavities  47  adjacent the leading and trailing edge of the inner diameter shrouds  34  create a clearance between the shrouds  34  and air seals  36 , and the rotor  12  and rotor blades  18 ,  22 ,  26 . 
       FIGS. 2 and 3  show sectional views of inner diameter shroud  34 . The shroud  34  is arcuate in shape and includes various components in addition to the inner diameter air seal  36 . These components include a channel  50 , a core  52 , and a dowel pin  54 . The core  52  further includes a leading segment  56  and a trailing segment  58 . The vanes  16 ,  20 ,  24 ,  28  (for convenience  28  will be used in  FIGS. 2 through 8 ) and the inner diameter base portion  32  are illustrated in  FIG. 2 . The inner diameter base portion  32  includes an inner diameter platform  60 , an inner diameter trunnion  62 , and a trunnion flange  64 . 
       FIGS. 2 and 3  show a cross section of the channel  50 . The channel  50  is formed of a single piece metal alloy. In one embodiment of the channel  50 , the metal alloy is 410 stainless steel. The channel  50  is arcuately bowed, and several channel  50  segments may be circumferentially aligned and interconnected around the inner diameter of the compressor section. In one embodiment of the channel  50 , each channel  50  segment extends through an arc of substantially 90 degrees in one embodiment. Once interconnected, the channel  50  segments may be less than about 14 inches (355 mm) in diameter. The channel  50  envelops most of the core  52  and the other components of the shroud  34 . The channel  50  has an external surface(s) that interfaces with the inner diameter flow path  46 . In  FIGS. 2 and 3 , an external surface of the channel  50  has the inner air seal  36  mechanically bonded to it by welding, brazing or other bonding means. The inner air seal  36  forms a seal between the channel  50  and the knife edge seals  44 . In one embodiment, the inner air seal  36  is a conventional honeycomb nickel alloy seal. 
     The channel  50  envelopes, protects and therefore minimizes exposed surfaces of components  56  and  58  from particle ingested abrasion along inner diameter flow path. Because the channel  50  envelops most of the core  52  and the other components of the shroud assembly  34 , the channel  50  captivates the other components should they wear or break due to extreme operating conditions. Thus, the worn component pieces do not enter the flow path to damage components of the gas turbine engine  10  downstream of the shroud  34 . The single piece channel  50  eliminates the need for fasteners to retain the core  52  and vane  28  in the shroud  34 . Thus, the radial height profile of the shroud  34  may be reduced. This reduction increases compression efficiency and decreases the size and overall weight of shroud assembly  34 , improving turbine engine  10  performance. 
       FIGS. 2 and 3  also show a cross section of the core  52 . The core  52  is a lightweight material, and may be comprised of either a metallic or a non-metallic. For example, a metallic such as AMS 4132 aluminum, or non-metallic such as graphite or a composite matrix comprised of random fibers, laminates or particulates may be used in embodiments of the invention. The core  52  is sacrificial and disposable and may be replaced after a certain number of engine cycles. The core  52  surrounds and is retained axially, circumferentially, and radially by the base portion  32  of the vane  28 . The core  52  interfaces with and is retained by the channel  50  in multiple directions including both the radial and axial directions. A surface (or multiple surfaces if the core  52  is split) of the core  52  interfaces with the inner diameter flow path  46  around the base portion  32  of the vane  28 . The surface(s) of the core  52  may substantially align with an inner exterior surface(s) of the channel  50  to define the inner diameter flow path  46  annulus for the compressor section of the gas turbine engine  10 . 
     In  FIGS. 2 through 8 , the core  52  may be split into the leading segment  56  and the trailing segment  58  along a plane defined by an actuation axes of the inner diameter base portion  32  of the vane  28 . This split allows each portion  56 ,  58  to symmetrically surround half of the base portion  32 . The portions  56 ,  58  are split to ease assembly and repair of the shroud  34 . In other embodiments of the core, the core may not be split into portions or may be split into portions that are not separated along a plane defined by the actuation axes of the base portion  32 . 
       FIG. 2  is a sectional view bisecting the inner diameter base portion  32  of the vane  28 . The vane  28  and base portion  32  may be comprised of any metallic alloy such as PWA 1224 titanium alloy. The vane  28  interconnects with the base portion  32 . The base portion  32  includes the inner diameter platform  60 , which interfaces with the leading segment  56  and the trailing segment  58  of the core  52 . The exterior portion of the inner diameter platform  60  has a fillet  65  for aerodynamically interconnecting the inner diameter platform  60  with the vane  28 . The exterior portion of the inner diameter platform  60  may substantially align with the exterior surfaces of the leading segment  56  and the trailing segment  58  of the core  52  to create an aerodynamic profile along the inner diameter flow path  46 . 
     The inner diameter platform  60  interconnects with the inner diameter trunnion  62 , which interfaces with and circumferentially retains (in addition to the dowel pin(s)  54 ) the leading segment  56  and the trailing segment  58 . The inner diameter trunnion  62  allows the vane  28  to pivot about an axis defined by the trunnion  62 , while the shroud  34  remains stationary. The inner diameter trunnion  62  interconnects and symmetrically aligns with the trunnion flange  64 . The trunnion flange  64  may interface with the channel  50 . The trunnion flange  64  interfaces with the leading segment  56  and the trailing segment  58 . 
       FIG. 3  is a sectional view bisecting the dowel pin  54 . The pins  54  may be made of a metallic or a non-metallic material. The pins  54  may be of any shape, length or thickness; the shape, length and thickness may vary as dictated by the operating conditions of the turbine engine  10 . The pins  54  fit into a bore to interconnect the leading segment  56  with the trailing segment  58 . The pins  54  may also be used to align the leading segment  56  with the trailing segment  58  during assembly of the core  52 . The pins  54  may be selectively placed in the core  52 . If a greater vane  28  and shroud  34  stiffness is required for a particular application, the pins  54  may be placed between each base portion  32 . Alternatively, a fastener or some other means of interconnecting the leading segment  56  and the trailing segment  58  may be used in lieu of the pins  54 . 
       FIG. 4  shows an exploded end view of the shroud assembly  34  including the assembled core  52  retaining the vanes  28 , and the channel  50 . In addition to the leading segment  56  and the trailing segment  58 , the core  52  includes a hole  66 , a retention groove  68 , a recessed surface  69 , and an anti-rotation notch  70 . The channel  50  includes an anti-rotation lug  72 , a leading edge surface  74 , a trailing edge surface  76 , a trailing edge lip  78 , and an interior retention railhead  80 . 
     With a split core  52 , the shroud assembly  34  may be assembled by sliding the circumferential arcuate channel  50  segments along the retention groove  68  and the retention track  69  of the core  52 . In the embodiment shown  FIG. 4 , the core  52  may be assembled by aligning the leading segment  56  and the trailing segment  58  around the base portion  32  (shown in  FIG. 2 ) of the vanes  28 . The dowel pins  54  may than be inserted through select thru holes  66  in the leading segment  56  to the depth required to engage both the leading segment  56  and the trailing segment  58 . The hole  66  is radially located along the retention groove  68  on the leading segment  56 . The hole  66  may be between each of the base portions  32  of the vanes  28  or may be selectively arrayed as engine operating criteria dictate. Alternatively, to assemble the core  52  the dowel pins  54  may be placed into or mechanically bonded with select bore holes in the trailing segment  58 . In another embodiment, the dowel pins  54  may also be bonded to the leading segment  56 . In yet another embodiment, the hole  66  may be blind or thru on either segment  56  or  58  or any combination thereof. The hole  66  on the leading segment  56  may then be aligned with and inserted onto the dowel pins  54  to complete assembly of the core  52 . The hole  66  also allows for service access to check wear in the interior of the core  52 . In  FIG. 4 , the assembled core  52  is substantially 60 degrees in circumferential length, and may be abuttably interfaced with additional cores  52  or core portions along the circumferential length of the channel  50 . Cores  52  or core portions of differing degrees of circumferential length may be used in other embodiments, and the core  52  or core portions circumferential length may vary depending on manufacturing and operating criteria. Circumferential movement of the channel  50  may be arrested by an anti-rotation lug  72  contacting the anti-rotation notch  70 . The anti-rotation lug  72  is brazed or mechanically bonded to the trailing edge  78  near the circumferential edges of the channel  50 . In one embodiment, the anti-rotation notch  70  occurs only on the cores  52  interfacing the circumferential edges of the channel  50 . 
     Once the core  52  is assembled the channel  50  is inserted over the core  52 . The channel  50  is movable along the circumferential length of the core  52  until the movement is arrested by an anti-rotation lug  72  contacting the anti-rotation notch  70 . In one embodiment of the invention, the core  52  has a clearance of about 0.003 inch (0.076 mm) between its outer edges and the inner edges of the channel  50 . The core  52  may be comprised of a material that has a greater coefficient of thermal expansion than the channel  50 . The clearance between the channel  50  and the core  52  is reduced to about 0.0 inch (0 mm) at operating conditions. Thus, minimizing relative motion between mated core  52  and channel  50  and efficiency losses due to secondary flow leakage. 
     Once inside the channel  50 , the retention groove  68  on the leading segment  56  interacts with the interior retention railhead  80  to allow slidable circumferential movement of the core  52 . The interior retention railhead  80  retains the leading segment  56  and the trailing edge lip  78  retains the trailing segment  58  from movement into the inner diameter flow path  46  in the radial direction. The interior retention railhead  80  may captivate the lower portion of the leading segment  56  should it wear or break due to extreme operating conditions. The interior retention railhead  80  also allows the base portion  32  to be disposed further forward in the shroud  34  (closer to the leading edge surface  74  of the channel  50 ). This configuration increases compressor efficiency by reducing the leading edge gaps between the vane  28  and the case  14  ( FIG. 1 ) along flow path  48  ( FIG. 1 ) and the vane  28  and the shroud  34  ( FIG. 1 ) along the inner diameter flow path  46 . The forward axis of rotation of the vane  28 , as shown in  FIG. 4 , ensures that the vane  28  will remain open in the event of actuation failure by, for example, the actuating arm  40  ( FIG. 1 ) or the unison ring  42  ( FIG. 1 ). 
     The channel  50  and core  52  fit eliminates the need to use a fastener to retain the core  52  to the channel  50 , as the channel  50  retains the core  52  in multiple directions including the radial and axial directions. By eliminating the need for fasteners, the height of the leading edge surface  74  and the trailing edge surface  76  is reduced. This reduction in height reduces the radial height profile, as the height of the leading edge surface  74  is the radial height profile of the shroud  34 . The height of the leading edge surface  74  may vary by the stage in the compressor section. However, by using the channel  50 , the leading edge surface  74  may be reduced to a range from about 0.250 inch to about 0.330 of an inch (about 6.35 mm to about 8.47 mm) in height when a shroud  34  of less than about 14 inches (355 mm) in diameter is used. This reduction in height minimizes the compression cavities  47 , ( FIG. 1 ) thereby improving the compressor efficiency and decreasing the overall size and weight of shroud  34 . 
       FIGS. 5A and 5B  show exploded views of the core  52  with a vane  28  and dowel pins  54 . In addition to the hole  66  and the retention groove  68 , the leading segment  56  includes a first cylindrical opening  82   a , a first thrust bearing surface  84   a , a journal bearing surface  86   a , a second thrust bearing surface  88   a , and a second cylindrical opening  90   a . The trailing segment  58  includes the anti-rotation notch  70 , a first cylindrical opening  82   b , a first thrust bearing surface  84   b , a journal bearing surface  86   b , a second thrust bearing surface  88   b , and a second cylindrical opening  90   b.    
     The core  52  illustrated in  FIGS. 5A and 5B  is comprised of a composite material and is symmetrically split about the axis of the inner diameter trunnion  62  into the leading segment  56  and the trailing segment  58 ; other embodiments of the invention may include a metallic core  52  or may not be split symmetrically. In  FIG. 5A , the surfaces of the leading segment  56  and the trailing segment  58  interfacing with the inner diameter flow path  46  have symmetrically, circumferentially spaced first cylindrical openings  82   a ,  82   b . The cylindrical openings  82   a ,  82   b  are symmetrically, axially split between the leading segment  56  and the trailing segment  58 . The cylindrical openings  82   a ,  82   b  interface with the side surfaces of inner diameter platform  60  on the vanes  28 . The cylindrical openings  82   a ,  82   b  provide a recess for the inner diameter platform  60 , which allows the external surface of the platform  60  to be aerodynamically aligned with the external surface(s) of the core  52  along the inner diameter flow path  46 . The cylindrical openings  82   a ,  82   b  have tolerances that allow the inner diameter platform  60  to pivot about its axis, which allows the vane  28  to pivot. The cylindrical openings  82   a ,  82   b  also may act as bearings during operation of the turbine engine  10 . 
     In  FIG. 5A , the cylindrical openings  82   a ,  82   b  transition to the first thrust bearing surfaces  84   a ,  84   b . The thrust bearing surfaces  84   a ,  84   b  interface with the inner surface of the inner diameter platform  60 . During operational use of the gas turbine engine  10 , the vanes  28  transmit a thrust force into the first thrust bearing surfaces  84   a ,  84   b  via the inner surface of the inner diameter platform  60 . The composite surfaces  84   a ,  84   b  act as a bearing for this thrust force. 
     The thrust bearing surfaces  84   a ,  84   b  interconnect with the journal bearing surfaces  86   a ,  86   b . The thrust bearing surfaces  84   a ,  84   b  are symmetrically axially split on the leading segment  56  and the trailing segment  58 , and interface around the inner diameter trunnion  62 . The journal bearing surfaces  86   a ,  86   b  may act as a bearing surface for the inner diameter trunnion  62  during operational use. The journal bearing surfaces  86   a ,  86   b  have a tolerance that allows the inner diameter trunnion  62  to pivot around its axis, which allows the vane  28  to pivot. The thrust bearing surfaces  84   a ,  84   b  interconnect with the second thrust bearing surfaces  88   a ,  88   b . The second thrust bearing surfaces  88   a ,  88   b  interface with a surface of the trunnion flange  64 . During operational use of the gas turbine engine  10 , the vanes  28  transmit a thrust force into the second thrust bearing surfaces  88   a ,  88   b  via the surface of the trunnion flange  64 . The composite surfaces  88   a ,  88   b  act as a bearing for this thrust force. 
     The second thrust bearing surfaces  88   a ,  88   b  transition to the second cylindrical openings  90   a ,  90   b . The cylindrical openings  90   a ,  90   b  are symmetrically axially split on the leading segment  56  and the trailing segment  58 . The cylindrical openings  90   a ,  90   b  interface with the side surfaces of the trunnion flange  64 . The cylindrical openings  90   a ,  90   b  have a tolerance that allows the trunnion flange  64  to pivot about its axis, which allows the vane  28  to pivot. The cylindrical openings  90   a ,  90   b  may act as bearings during operation of the turbine engine  10 . The cylindrical openings  82   a ,  82   b ,  90   a ,  90   b  allow the trunnion flange  64  to be recessed such that the flange  64  does not make contact with the channel  50 . 
       FIG. 6  shows a split bearing  92  that is application specific. It may be used when the core  52  is comprised of a metallic material such as aluminum or a non-metallic such as graphite composite. The split core bearing  92  is comprised of a composite material, and surrounds and interfaces with the base portion  32  of the vane  28 . The bearing  92  sits between the metallic core  52  and the base portion  32  during operation of the gas turbine engine  10 , and is subject to forces transmitted from the vanes  28  to the base portion  32 . 
     In  FIGS. 7 and 8 , non-offset leading edge vanes  28  are illustrated inserted in another embodiment of the shroud. In this configuration, the leading edge of the vanes  28  nearly aligns with the leading edge surface  74  of the channel  50  when the channel  50  is inserted over the core  52 . The exterior surfaces of the channel  50  and the core  52  act as a seal between the vane  28  and the surfaces to direct the flow along the inner diameter flow path  46 . 
       FIG. 7  also shows a sectional view of another embodiment of the shroud  34  bisecting the dowel pin  54 . The dowel pin  54  has a crown around its center. The crown allows the dowel pin  54  to sit on a counter bore. The counter bore is located on an interior surface both the leading segment  56  and the trailing segment  58 . The pins  54  fit into a bore hole (or thru hole) aligned with the counter bore to interconnect the leading segment  56  with the trailing segment  58 . The bore hole may extend through both the leading segment  56  and the trailing segment  58 . The counter bore provides a stop so the dowel pin  54  does not contact the inner surface of the channel  50  through the bore hole. The pins  54  also may be used to align the leading segment  56  with the trailing segment  58  during assembly of the core  52 . The pins  54  may be selectively placed between the base portions  32  as required by the engine operating criteria. 
       FIG. 8  shows an exploded end view of another embodiment of the shroud  34  including the assembled core  52  retaining vanes  28 , and the channel  50 . In this embodiment, the channel  50  additionally includes a leading edge lip  94 . The core  52  additionally includes a first retention track  96  and a second retention track  98 . 
     The leading edge lip  94 , forms the external surface of the channel  50  adjacent the leading edge of the shroud  34 . The leading edge lip  94  and the trailing edge lip  78  may substantially align with an exterior surface(s) of the core  52  to define the inner diameter flow path  46  annulus for the compressor section of the gas turbine engine  10 . The leading edge lip  94  may act as a seal between the vanes  28  and the shroud  34  to direct the flow of air along the inner diameter flow path  46 . The leading edge lip  94  also protects the leading segment  56  of the core  52  from particle ingested abrasion. 
     The first retention track  96  on the leading segment  56  interacts with the leading edge lip  94 , and the second retention track  98  on the trailing segment  58  interacts with the trailing edge lip  78  to allow slidable circumferential movement of the core  52  in the channel  50 . The leading edge lip  94  retains the leading segment  56  and the trailing edge lip  78  retains the trailing segment  58  from movement into the inner diameter flow path  46  in the radial direction. 
     Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.