Abstract:
A turbine assembly includes a shroud for being mounted around a last stage of the low pressure turbine rotor. The shroud includes an arcuate body having spaced-apart forward and aft ends and spaced-apart circumferential ends. Attachments are provided for securing the shroud body to the surrounding engine case. An arcuate extension member protrudes rearward from the aft end of the shroud body. The extension member has an inner surface which defines a portion of the gas flowpath through the engine. Seals may be provided to prevent leakage between adjacent shroud segments. The shroud is especially useful for protecting a downstream turbine frame from debris and thermal stress.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     This invention relates generally to gas turbine engines and more particularly to shroud for a turbine of such engines.  
         [0002]     Gas turbine engines include one or more turbines which extract energy from a gas flow to produce mechanical work. It is common in the prior art for a structural exhaust frame or turbine rear frame (“TRF”) to be disposed behind a low pressure turbine (“LPT”). Because of the configuration typically used, the transition between the structural case of the LPT and the structural case of the TRF must provide an inward-sloping diameter.  
         [0003]     This inward slope from the LPT case to the TRF case creates a radial pocket with a forward axial boundary created by the shroud surrounding the last stage of LPT blades and an aft boundary created by the inward-sloping contour of the TRF case. In the event of a turbine failure in which debris is ejected aftward, this pocket can trap a quantity of the debris and prevent it from being ejected aft out the tailpipe of the engine. Entrapment of debris from a turbine failure in this fashion is highly undesirable, since damage could conceivably sever the engine carcass load path to the aft mount. This, in turn, could lead to engine component liberation and aircraft damage.  
         [0004]     The presence of the inward-sloping frame case directly aft of the LPT exit blade tip seal is also subject to impingement of high-temperature leakage air which bypasses the blade across the tip seal teeth Since this cycle air has had no work extracted by the last LP stage, it will be significantly hotter than the remaining cycle air to which the TRF is exposed. It will also have higher impingement velocity and therefore heat transfer coefficient relative to the remainder of the LPT discharge flow. Locally, this can create adverse thermal gradients and thermal fatigue issues in the TRF case, in addition to overtemperature concerns. The radial transition also requires a length of conical structure aft of the shroud axial position, which adds length and weight to the engine.  
         [0005]     Accordingly, there is a need for a turbine shroud arrangement which protects the TRF case from debris and high temperatures.  
       BRIEF SUMMARY OF THE INVENTION  
       [0006]     The above-mentioned need is met by the present invention, which according to one aspect provides a turbine shroud for a gas turbine engine, including an arcuate body having spaced-apart forward and aft ends and spaced-apart circumferential ends; means for attaching the body to a surrounding case of the engine; and an arcuate extension member protruding rearward from the aft end of the body, the extension member having an inner surface adapted to define a portion of a gas flowpath through the engine.  
         [0007]     According to another aspect of the invention, a turbine assembly includes a rotor which is rotatable about a longitudinal axis, the rotor carrying at least one stage comprising radially-extending turbine blades for interacting with a gas flow through the turbine; an annular first case, the first case having a first section which surrounds the turbine blades, and a second section disposed aft of the first section which defines a portion of a flowpath through the turbine; and a turbine shroud disposed between the first case and the blades. The turbine shroud includes an arcuate body having spaced-apart forward and aft ends and spaced-apart circumferential ends; and an arcuate extension member which protrudes rearward from the aft end of the body and extends from the aft end of the body to the second section of the first case, the extension member having an inner surface adapted to form a portion of a gas flowpath through the turbine. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]     The invention may be best understood by reference to the following description taken in conjunction with the accompanying drawing figures in which:  
         [0009]      FIG. 1  is a schematic cross-sectional view of a prior art turbine section and shroud assembly;  
         [0010]      FIG. 2  is a schematic side view of a portion of a gas turbine engine showing a turbine shroud constructed according to an embodiment of the present invention;  
         [0011]      FIG. 3  is an enlarged view of a portion of the shroud assembly shown in  FIG. 2 ;  
         [0012]      FIG. 4  is a partial perspective view of the shroud assembly shown in  FIG. 2 ;  
         [0013]      FIG. 5  is a view taken along lines  5 - 5  of  FIG. 3 ; and  
         [0014]      FIG. 6  is a view taken along lines  6 - 6  of  FIG. 3 . 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0015]     Referring to the drawings wherein identical reference numerals denote the same elements throughout the various views,  FIG. 1  illustrates a portion of a low-pressure turbine (“LPT”)  10  of a prior art gas turbine engine. The LPT  10  includes alternating stages of turbine blades  12  and turbine nozzles or vanes  14 . The turbine nozzles  14  are attached to an annular LPT case  16  which while the turbine blades  12  are attached to a rotor assembly (not shown) which cooperates with appropriate shafting to turn a compressor or other mechanical load. The aft end  20  of the LPT case  16  carries a circumferentially-extending turbine shroud  22 . The purpose of the turbine shroud  22  is to provide a close-fitting seal between the LPT case  16  and the outer ends of the turbine blades  12 , thus reducing flow losses in the LPT  10 . The shroud  22  illustrated in  FIG. 1  is typical of the prior art and comprises a ring of arcuate segments each including a base  24  that carries an abradable member  26 . The abradable member  26  interacts with radially-outwardly-extending seal teeth  28  of the turbine blades  12  to provide the desired seal.  
         [0016]     A stationary frame referred to as a turbine rear frame or “TRF”  30  comprises a central hub (not shown), a plurality of radially-extending arms  34 , one of which is depicted in  FIG. 1 , and an annular TRF case  36  which is attached to the aft end  20  of the LPT case  16  immediately behind the last stage of turbine blades  12 . The TRF case  36  defines the outer flowpath surface directly without any intermediate structure. A result of this construction is that the transition between the LPT case  16  and the TRF case  36  must provide an inward-sloping diameter because of the radial height required for the turbine shroud  22 .  
         [0017]     The inward slope from the LPT case  16  to the TRF case  36  necessarily occurs aft of the turbine shroud exit, creating a radial pocket “P” with a forward axial boundary created by the turbine shroud  22  and an aft boundary created by the inward-extending contour of the TRF case  36 . In the event of a turbine failure in which debris are ejected aftward, this pocket P can trap a quantity of said debris and prevent it from being ejected aft out the tailpipe of the engine. The action of the remaining LPT blades can them serve to rotate this debris and cause further damage to the case in the pocket area. In some situations this condition may have the potential to sever the TRF case  36 .  
         [0018]      FIG. 2  illustrates an exemplary low-pressure turbine (“LPT”)  110  of a gas turbine engine constructed in accordance with an embodiment of the present invention. It should be noted that the present invention could also be implemented with other types of rotating machinery such as a high-pressure turbine or a compressor. The LPT  110  includes alternating stages of turbine blades  102  and turbine nozzles or vanes  104 . The turbine nozzles  104  are attached to an annular LPT case  106  while the turbine blades  102  are attached to a rotor assembly  108  which cooperates with appropriate shafting to turn a compressor or other mechanical load (not shown).  
         [0019]     A stationary turbine rear frame (“TRF”)  109  comprises a hub  112 , a plurality of radially-extending arms  114 , one of which is depicted in  FIG. 2 , and an annular TRF case  116  which is attached to the aft end of the LPT case  106  at a joint  118 . The joint  118 , in this example a bolted flange joint, is made at a location longitudinally forward of the last stage of turbine blades  102 . In this arrangement, there is still a transition from the larger-diameter LPT case  106  to the TRF case  116 , as in the prior art LPT  10 . However, this transition occurs outboard of the turbine shroud  122  (described below) in the TRF  109 , thus eliminating the pocket P of the prior art design. There is, therefore, no geometry which can axially capture rotating debris in the event of a turbine failure and prevent its ejection aft from the engine.  
         [0020]     As shown in  FIG. 3 , the forward end  120  of the TRF case  116  carries a circumferential turbine shroud  122 . The shroud  122  includes a base  124  which spans the chord length of the turbine blades  102 . The base  124  carries a stationary seal member  126  of a known type, for example an abradable honeycomb structure as shown. The seal member  126  interacts with radially-outwardly-extending seal teeth  128  of the turbine blades  102  to provide a seal which minimizes leakage flow past the outer ends of the turbine blades  102 .  
         [0021]     The shroud  122  also includes an aft-protruding extension member  130 . Its inner surface  132  defines a transitional flowpath “F” between the radially outer end of the last stage of turbine blades  102  and the TRF case  116 .  
         [0022]     The construction of the turbine shroud  122  is shown in more detail in  FIGS. 4-6 . The turbine shroud  122  is assembled from a circumferential array of arcuate segments  134 , although it could also be formed as a continuous member. The segments  134  may be made from a high quality superalloy, such as a cobalt or nickel-based superalloy, and may be coated with a corrosion resistant material and/or thermal barrier coating. The base  124  has a forward end  136  and an aft end  138 , and spaced-apart circumferential ends  140  and  142 . A forward rail  144  having a radial leg  146  and an axial leg  148  is disposed at the forward end  136 , and an aft rail  150  is disposed at the aft end  138 . When assembled, the forward and aft rails  144  and  150  engage forward and aft hooks  152  and  154  of the TRF case  116 , respectively (see  FIG. 3 ). A longitudinal seal slot  155  is formed in each of the circumferential ends  140  and  142  of the base  124 . The seal slot  155  accepts an end seal  157  (see  FIG. 5 ) of a known type which reduces leakage between adjacent segments  134 .  
         [0023]     The extension member  130  protrudes from the aft end  138  of the base  124 . In the illustrated example the extension member  130  has a radial portion  130 A, an inwardly-angled, aft-extending portion  130 B, and an outwardly-angled, aft extending portion  130 C. A radially-outwardly extending lip  156  is disposed at its aft end. The exact configuration of the extension member  130  may be changed to suit a particular application.  
         [0024]     Means are provided for sealing the gap between the extension members  130  of adjacent shroud segments  132 . In the illustrated example an extension seal  158  comprises a tab-like seal body which is attached to one of the extension members  130  and protrudes circumferentially to form an overlapping or “ship-lap” seal arrangement with the adjacent extension member  130 .  
         [0025]     In operation, the extension member of the LPT shroud  122  provides protection of the TRF case  116  from impingement from the LPT exit stage blade tip bypass air. Because the turbine shroud  122  is segmented and typically constructed from a higher temperature capable material relative to the TRF  109 , impingement of this bypass air on the segmented shroud extension members  130 , as opposed to the TRF case  116 , is of far less concern from a thermal gradient, thermal-mechanical fatigue, and overtemperature standpoint. The TRF case  116  is further protected from even local impingement of the blade tip bypass air by the extension seals  158  between adjacent extension members  130 . This feature blocks any “line-of-sight” between the tip bypass flow and the TRF case  116 .  
         [0026]     The foregoing has described a turbine shroud and a turbine assembly incorporating the turbine shroud. 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, the invention being defined by the claims.