Abstract:
A method for fabricating a turbine casing including a plurality of turbine shroud assemblies is provided. The method includes providing a base casing having a forward mounting flange and an aft mounting flange and at least one channel defined therebetween, machining a rim on the base casing proximate the at least one channel, and coupling a ring member to the base casing with an interference fit such that the rim is at least partially received within a groove formed within the ring member.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     This invention relates generally to gas turbine engines, and more specifically to turbine casings used with gas turbine engines.  
         [0002]     Gas turbine engines generally include, in serial flow arrangement, a high pressure compressor for compressing air flowing through the engine, a combustor in which fuel is mixed with the compressed air and ignited to form a high energy gas stream, and a high pressure turbine. The high pressure compressor, combustor and high pressure turbine are sometimes collectively referred to as the core engine. Such gas turbine engines also may include a low pressure compressor, or booster, for supplying compressed air to the high pressure compressor.  
         [0003]     At least some known turbines include a rotor assembly including a plurality of rows of rotor blades. Each rotor blade extends radially outward from a blade platform to a tip. A plurality of shrouds couple together to form a flow path casing that extends substantially circumferentially around the rotor assembly, such that a tip clearance is defined between each respective rotor blade tip and the casing. The tip clearance is designed to be a minimum, while still being sized large enough to facilitate rub-free engine operation through a range of available engine operating conditions.  
         [0004]     During operation, turbine performance may be influenced by the tip clearance between turbine blade tips and the shroud. Specifically, as the clearance increases, leakage across the rotor blade tips may adversely limit the performance of the turbine assembly. To facilitate maintaining blade tip clearance at least some known shroud designs attempt to match the rate of thermal expansion of the stator case to the rate of thermal expansion of the turbine rotor assembly by supplying a variable amount of cooling fan air to the casing flanges. Cooling the flanges facilitates controlling thermal movement to facilitate eliminating rocking of the shrouds. The mass at the flange also pushes the casing downward to facilitate maintaining blade tip clearances.  
         [0005]     To facilitate the controlling of thermal movement and the maintaining of blade tip clearances, casing members include a pseudo flange which adds structural integrity to the shroud casing.  
         [0006]     In some instances, the pseudo flange is hourglass-shaped with a large mass of material formed at its outer diameter and a thin mid section. However, fabricating such pseudo flanges may be both expensive and time consuming.  
       BRIEF DESCRIPTION OF THE INVENTION  
       [0007]     In one aspect, a method for fabricating a turbine casing including a plurality of turbine shroud assemblies is provided. The method includes providing a base casing having a forward mounting flange and an aft mounting flange and at least one channel defined therebetween, machining a rim on the base casing proximate the at least one channel, and coupling a ring member to the base casing with an interference fit, such that the rim is at least partially received within a groove formed within the ring member.  
         [0008]     In another aspect, an engine casing assembly for a gas turbine engine is provided. The assembly includes a base casing that includes a forward flange, an aft flange, and a body extending therebetween. The body includes at least one channel defined therein. An annular ring member is coupled to the base casing. The ring member is configured to thermally expand at a rate that is substantially identical to a rate of thermal expansion of the forward and aft flanges.  
         [0009]     In another aspect, a gas turbine engine is provided. The engine includes a turbine section including a turbine, and an outer casing assembly circumscribing the turbine. The casing assembly includes a base casing including a forward flange, an aft flange, and a body extending therebetween. The body includes at least one channel defined therein. The casing assembly further includes an annular ring member coupled to the base casing. The ring member is configured to thermally expand at a rate that is substantially identical to a rate of thermal expansion of the forward and aft flanges. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]      FIG. 1  is a schematic illustration of a gas turbine engine;  
         [0011]      FIG. 2  is a schematic illustration of a portion of a high pressure turbine shown in  FIG. 1 ; and  
         [0012]      FIG. 3  is an enlarged cross sectional view of a portion of the high pressure turbine shown in  FIG. 2 . 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0013]      FIG. 1  is a schematic illustration of a gas turbine engine  10  including a low pressure compressor  12 , a high pressure compressor  14 , and a combustor assembly  16 . Engine  10  also includes a high pressure turbine  18 , and a low pressure turbine  20  arranged in a serial, axial flow relationship. Compressor  12  and turbine  20  are coupled by a first shaft  24 , and compressor  14  and turbine  18  are coupled by a second shaft  26 . In one embodiment, engine  10  is an GE90 engine commercially available from General Electric Company, Cincinnati, Ohio.  
         [0014]     In operation, air flows through low pressure compressor  12  from an upstream side  11  of engine  10  and compressed air is supplied from low pressure compressor  12  to high pressure compressor  14 . Compressed air is then delivered to combustor assembly  16  where it is mixed with fuel and ignited. The combustion gases are channeled from combustor  16  to drive turbines  18  and  20 .  
         [0015]      FIG. 2  is a schematic illustration of a portion of high-pressure turbine  18 .  FIG. 3  is an enlarged cross sectional view of a portion of high pressure turbine  18 . Turbine  18  includes a plurality of stages  30 , each of which includes a row of turbine blades  32  and a row of stator vanes  34 . Turbine blades  32  are supported by rotor disks (not shown), that are coupled to rotor shaft  26 . Stator casing  36  extends circumferentially around turbine blades  32  and stator vanes  34 , such that vanes  34  are supported by casing  36 .  
         [0016]     Casing  36  includes a base case segment  38 . Case segment  38  includes a forward mounting hook  40  and an intermediate mounting hook  41 . Mounting hooks  40  and  41  define a shroud channel  52  in case segment  38 . A forward shroud assembly  42  in shroud channel  52  is coupled to mounting hooks  40  and  41 . Case segment  38  also includes an aft mounting hook  50  that is coupled to an adjacent downstream shroud assembly  43 . Each shroud assembly  42  and  43  includes a shroud  44  and  45  that are each radially outward of turbine blade tips  46  such that a tip clearance  48  is defined between shrouds  44  and  45  and turbine blade tips  46 .  
         [0017]     Case segment  38  also includes a forward mounting flange  54  and an aft mounting flange  56  for coupling case segment  38  substantially axially within engine  10 . Forward mounting hook  40  extends radially inward from forward mounting flange  54 , and aft mounting hook  50  extends radially inward of aft mounting flange  56 . A mounting hook  51  is coupled between mounting flange  56  of case segment  38  and a mounting flange  58  extending from an adjacent case segment  59 . Thus, shroud assembly mounting hooks  50  and  51  are both positioned at case segment mounting flanges, specifically, mounting flange  56  and mounting flange  58 .  
         [0018]     A pseudo flange assembly  60  extends from case segment  38  radially opposite intermediate mounting hook  41 . Pseudo flange  60  includes a rim  62  and a ring  64  that is coupled to an outer diameter of rim  62 . More specifically, rim  62  has a radius R 1  measured with respect to an engine center line  66  that is slightly larger than one of a radius R 2  of forward case segment mounting flange  54  and a radius R 3  of aft mounting flange  56 . Rim  62  is defined within base casing  38  radially opposite intermediate mounting hook  41  of shroud assembly  42 . In one embodiment, rim  62  is formed via a machining process. In the exemplary embodiment, rim  62  has straight parallel sides  68 ,  70  to facilitate the machining. However, in alternative embodiments, rim sides  68 ,  70  are non-parallel.  
         [0019]     Ring  64  has a width W 1  that is greater than a width W 2  of rim  62  and includes a groove  72  defined therein. Grove  72  is sized to receive at least a portion of an outer periphery of rim  62 . Ring  64  also includes a lip  74  that circumscribes each side  76 ,  78  of groove  72  to facilitate inhibiting axial movement between ring  64  and rim  62 . In one embodiment, ring  64  is coupled to rim  62  with a shrink fit engagement. Ring  64  is separately machined and can be fabricated in any geometric shape. Ring  64  can also be fabricated from a material different from the case material as long as ring  64  is sized such that the thermal characteristics of ring  64  and rim  62  in combination can be matched to the thermal characteristics of the case segment mounting flanges  54  and  56 .  
         [0020]     Pseudo flange  60  is formed by machining ring  62  into base case segment  38  at the location of intermediate mounting hook  41  of shroud assembly  42 . For ease of machining, rim  62  is machined with generally straight parallel sides. Rim  62  is machined with a radius R 1  slightly larger than one of radius R 2  of forward mounting flange  54  and radius R 3  of aft mounting flange  56  such that rim  62  will have a diameter (not shown) that is also slightly larger than one of a diameter (not shown) of forward mounting flange  54  and a diameter (not shown) of aft mounting flange  56 . Ring  64  is machined with a groove  72  sized to receive the outer periphery of rim  62 . Ring  64  includes a lip  74  on each side of groove  72  to inhibit any axial movement of ring  64  with respect to rim  62 . After fabrication, ring.  64  is heated so that it expands sufficiently to pass over one of forward mounting flange  54  and aft mounting flange  56  so that it can be fitted on rim  62 . A shrink fit is created as ring  64  cools.  
         [0021]     In operation, turbine performance is influenced by tip clearance  48 , and as such, it is desired to maintain tip clearance  48  to a designed minimum distance while preventing blade tips  46  from contacting shrouds  44  and  45 . In order to optimize and maintain tip clearance  48 , it is desired to substantially match the thermal growth of the turbine casing  36 , including case segment  38 , to that of the rotor disks (not shown) and turbine blades  32 . Pseudo flange assembly  60  is provided on base case segment  38  so that thermal growth characteristics of case segment  38  at mounting hooks  40  and  41  for shroud assembly  42  can be matched with the thermal characteristics of forward and rearward case mounting flanges  54  and  56 , respectively, so that turbine blade tip to shroud clearance  48  is facilitated to be maintained.  
         [0022]     In one embodiment, the thermal expansion matching is facilitated by cooling the casing flanges, including flanges  54  and  56 , and pseudo flange assembly  60  with a variable amount of cooling air. In one embodiment, the cooling air is compressor discharge air. The matching of the thermal behavior of pseudo flange assembly  60  to casing flanges  54  and  56  facilitates the avoidance of any rocking of shroud assembly  42  which facilitates preventing contact between shroud assembly  42  and turbine blades  32 .  
         [0023]     The above-described pseudo flange provides a cost-effective flange that can be used for matching thermal growth characteristics in a case segment so that turbine blade tip to shroud clearances may be maintained. The pseudo flange is of a simplified design that also allows for simplifying the design of bleed ports in the area of the pseudo flange. The pseudo flange also provides for the use of a ring of a different material than that of the casing which may provide a better thermal match due to differing coefficients of thermal expansion between the ring material and the case material.  
         [0024]     Exemplary embodiments of turbine casing shrouds are described above in detail. Each shroud casing assembly is not limited to the specific embodiments described herein, but rather each component may be utilized independently and separately from other components described herein. Each component can also be used in combination with other turbine casing shroud assemblies.  
         [0025]     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.