Abstract:
A transition duct for a gas turbine includes a tubular body having a forward end and an aft end; a plurality of cooling channels formed on an exterior surface of the tubular body at the aft end; a closure band surrounding the aft end, covering at least a portion of the cooling channels; and a seal attached to the closure band, surrounding the aft end of the tubular body.

Description:
[0001]    This invention relates to gas turbine combustor technology generally, and to an apparatus and related method for cooling the aft end of a transition pieces or duct that extends between a combustor and the first stage of the turbine. 
       BACKGROUND OF THE INVENTION 
       [0002]    Typically, transition ducts have an aft frame which is attached, or integrated into, the aft end of the duct, facilitating attachment of the duct to the inlet of the turbine first stage. The aft frame is often cooled by means of controlled seal leakage and small cooling holes that allow compressor discharge air to pass through the frame. It has proven difficult, however, to cool the aft end of transition ducts which do not have an aft frame integrally formed with, or attached to the duct body. In accordance with exemplary but nonlimiting implementation of this invention, forced convection and potentially impingement cooling are used as a means to directly cool a transition duct which does not have an aft frame structure. 
         [0003]    Accordingly, in one aspect, the present invention relates to a transition duct for a gas turbine comprising: a tubular body having a forward end and an aft end; a plurality of cooling channels formed on an exterior surface of the tubular body at the aft end; a closure band surrounding the aft end, covering at least a portion of the plurality of cooling channels; and a seal attached to the closure band, surrounding the aft end of the tubular body. 
         [0004]    In another aspect, the present invention relates to a method of providing cooling air to an aft end of a gas turbine transition duct comprising: forming plural open cooling channels on an exterior surface of the transition duct at the aft end thereof, the plural cooling channels extending from an aft edge of the duct in an upstream direction; closing at least a portion of the plural open cooling channels with a peripheral closure band to thereby form cooling passageways; and incorporating a seal into the closure band. 
         [0005]    The invention will now be described in greater detail in connection with the drawings identified below. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0006]      FIG. 1  is a partial aft end perspective view of a turbine transition duct with cooling channels formed therein; and 
           [0007]      FIG. 2  is a perspective view similar to  FIG. 1  but with a band enclosing portions of said cooling channels, and with a seal attached to the band. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0008]    In a typical can-annular combustor configuration in a gas turbine, an array of combustors surrounding the turbine rotor supply hot combustion gases to the turbine first stage via a corresponding array of transition ducts that extend between the combustors and the first stage inlets. With reference to  FIG. 1 , one such transition duct  10  connects at a forward end to a combustor liner (not shown). The aft end  12  of the transition duct in the exemplary embodiment has no integral or attached aft frame surrounding the outlet  14 , thus making it difficult to adequately cool the aft end. The aft end  12  is received within a bracket (not shown) fixed to first stage turbine nozzle and formed with a correspondingly-shaped aperture. In this kind of arrangement, cooling techniques commonly employed to cool the aft end of a transition piece that does utilize an aft frame (which provides a ready vehicle for incorporating cooling geometry), are not available. 
         [0009]    Accordingly, in one nonlimiting implementation, an array of cooling channels or grooves  16  are formed on the exterior surface of the aft end  12  of the transition duct  10 . The cooling channels  16  provide cooling air outlets  18  at the aft edge  20  of the duct  10 , extending toward the opposite end of the duct. The channels terminate at respective tapered inlets  22 , the axial location of which may be varied as dictated by combustor and duct design, cooling requirements, etc. 
         [0010]    The cooling channels  16  may be provided on one, all or any combination of the exterior top surface  24 , side surfaces  26 ,  28 , and bottom surface  30  of the duct, and the number of channels or grooves in each of those surfaces may also vary as desired. The channels  16  may be formed by means of any acceptable manufacturing process, e.g., milling, casting, laser etching, drop forging, etc.), and may be of any suitable cross-sectional shape including rectangular as shown in FIGS.  1  and  2 , but also including semi-circular, oval, V-shaped etc. 
         [0011]    The channels  16  are substantially closed at the top by a metal wrap or closure band  32  ( FIG. 2 ) that surrounds the transition duct, thus forming closed-periphery passageways having substantially rectangular-shaped cross sections. The band  32  extends axially from the aft edge  20  to the tapered inlets  22 , leaving the latter exposed for facilitating entry of air into the channels. The band  32  may be fastened to the duct by any suitable process including bolting or welding. 
         [0012]    The interior surfaces of the cooling channels may also be formed or provided with any of several known heat transfer enhancement mechanisms applied to one, all, or any combination of bounding walls of the cooling channels. Such surface enhancements include turbulators, fins, dimples, cross-hatch grooves, sand-dune shapes, chevrons or any combination thereof. The arrangement and number of such enhancements may be varied as desired among the various channels. Cooling air may be delivered to the channels  16  in any number of ways. For example, the channels  16  may be exposed, via inlets  22 , at their upstream ends to compressor discharge flow, or they may be fed directly from a separate inlet or manifold. Alternatively, or additionally, the cooling channels  16  may be fed from any number of cooling apertures  36  (three shown in  FIG. 2 ) provided in the band  32 . For example, one or more cooling apertures could be provided in overlying relationship with any one or more of the channels  16 . 
         [0013]    It is also a feature of the exemplary embodiment to combine a seal  36  with the closure band  32 . The seal  36  is shown schematically in  FIG. 2  to include a pair of brush seal bands  38 ,  40  but the seal may also be composed of may any of a variety of other conventional seals such as leaf seals, cloth seals, rope seals hula seals and the like. As noted above, the aft end of the transition piece will be received within a bracket assembly that is correspondingly-shaped aperture in a fixed to the stage  1  nozzle of the turbine. By incorporating a seal into the wrap or closure band  32 , air in the compressor discharge chamber will be prevented from leaking into the cavity between the bracket and the turbine first stage inlet. 
         [0014]    Note that the above-described aft end cooling arrangement can be used with or without conventional impingement cooling sleeves that are used to impingement cool areas of the duct upstream of the aft end. 
         [0015]    While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.