Abstract:
A transition duct for a gas turbine includes a tubular body having a forward end and an aft end, the aft end surrounded by a frame component; an interior closure band within the aft end, covering interior top, bottom and side wall surfaces of the frame; and a plurality of cooling channels between the frame and the closure band, each having an inlet and an outlet at the forward and aft ends, respectively.

Description:
This invention relates to gas turbine combustor technology generally, and to an apparatus and related method for cooling the aft end frame of a transition piece or duct that extends between a combustor and the first stage of the turbine. 
     BACKGROUND OF THE INVENTION 
     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/or small cooling holes that allow compressor discharge air to pass through the frame. See for example, U.S. Pat. Nos. 6,769,257; 5,414,999; 5,724,816; and 4,652,284. Nevertheless, excessively high temperatures and thermal gradients may be experienced in the vicinity of the transition duct aft end frame. Accordingly, there remains a need for more effective cooling techniques in these areas. 
     BRIEF DESCRIPTION OF THE INVENTION 
     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, the aft end surrounded by a frame component; an interior closure band within the frame covering interior top, bottom and side wall surfaces of the frame; and a plurality of cooling channels between the frame and the closure band, each having an inlet and an outlet at the forward and aft ends, respectively. 
     In another aspect, the invention relates to a method of providing cooling air to an aft end frame of a gas turbine transition duct comprising: forming plural cooling channels between an interior surface of the aft frame and an exterior surface of a closure band located within the aft frame, and attaching the aft frame and the closure band to an aft edge of the transition duct. 
    
    
     
       The invention will now be described in greater detail in connection with the drawings identified below. 
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a partial aft end perspective view of a conventional turbine transition piece fitted with an aft end frame; 
         FIG. 2  is a partial cross section through a conventional transition piece aft end frame, illustrating cooling holes drilled through the frame; 
         FIG. 3  is a partial cross section similar to  FIG. 2  but illustrating a transition piece aft end frame in accordance with an exemplary but nonlimiting embodiment of the invention; and 
         FIGS. 4 and 5  are partial perspective views of a transition piece aft end frame similar to that shown in  FIG. 3 , but with a cooling channel cover band omitted; and 
         FIG. 6  is a partial perspective view of an aft end frame closure band in accordance with another exemplary embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     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 piece or 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 an integral or attached aft end frame  14  surrounding the outlet  16 , thus facilitating attachment to the turbine first stage nozzle (not shown).  FIG. 2  illustrates another known transition piece aft end frame  18  formed with plural cooling holes  20  drilled or otherwise formed in the frame. The frame  18  is welded to the transition duct at  24 . Seal cavities  26 ,  28  are typically provided in the aft frame for receiving seals at the transition duct-turbine nozzle interface. 
     Referring now to  FIGS. 3-5 , in one exemplary but nonlimiting embodiment of this invention, an inner surface  30  of the frame  32  is worked (by milling, casting, laser etching, etc.) to create a plurality of axially-oriented, three-sided, open cooling channels  34  extending from the forward edge  36  of the frame to the rearward or aft edge or face  38 . 
     The cooling channels  34  may be provided on one, all or any combination of the interior top, side and bottom surfaces (generally referred to as the inner surface  30 ) of the aft frame, and the number of channels or grooves  34  in each of those surfaces may also vary as desired. The channels  34  may be of any suitable cross-sectional shape including rectangular as shown in  FIGS. 4-5 , but also including semi-circular, oval, V-shaped etc. In addition, the cross-sectional areas of the various channels in any single frame may be substantially uniform or may vary in any fashion. 
     In this first exemplary embodiment, the three-sided channels  34  are substantially closed by a metal wrapper or closure band  40  ( FIG. 3 ) that forms the inner wall of the channels  34  thus forming closed-periphery passageways  42 . Note that the closure band  40  is exposed to the flow of hot gases through the transition piece. The closure band  40  has an aft outwardly extending flange  44  (the closure band  40  may therefore also be regarded as an “L-bracket”)that engages and is welded or otherwise fixed to the aft edge or face  38  of the frame. Holes or apertures  48  are drilled or otherwise formed in the flange  44  to align with the channels  34  thus providing outlets  50  for the passageways  42 . The forward end  52  of the band  40  extends beyond (i.e., upstream of) the forward edge  36  of the frame, and is welded to the transition piece  54  at  56 . The sloped edge  58  of the frame provides enlarged inlets  60  to the passageways  42 . 
     In another exemplary but nonlimiting embodiment, the cooling channels may be formed by a series of raised ribs which are either integrally formed on, or fastened by any suitable means to the outer surface of the L-bracket such that the channels are closed by the smooth interior frame surface, forming the outer wall of the channels. This arrangement is shown in  FIG. 6  where the exterior surface  62  of the closure band  64  is provided with a plurality of substantially axially-extending ribs  66 , integrally or by attachment, thus forming a plurality of three-sided, open channels  68 . The fourth or open side of the channels is closed by the smooth interior surface of the aft frame, thus forming cooling passageways similar to passageways  42  in  FIGS. 3-5 . As in the earlier-described embodiment, apertures or holes  70  are required to be formed in the vertical stem or flange  72  of the closure band to form the outlets of the passageways. As in the earlier described embodiment, any number of ribs  66  may be formed on any one or all of the top, bottom and side surfaces of the frame. 
     One or more of the bounding walls of the cooling passageways themselves may also be formed or provided with any of several known heat transfer enhancement mechanisms, such as, for example, turbulators, fins, dimples, cross-hatch grooves, chevrons or any combination thereof (see  FIG. 5 ). The arrangement and number of such enhancements may be varied as desired among the various channels. 
     Cooling air may be delivered to the passageways  42  in any number of ways. For example, the passageways may be exposed at their upstream ends (i.e., at their respective inlets) to compressor discharge flow, or they may be fed directly from a separate inlet or manifold. The cooling flow may exit into the hot gas flow from any multiple of outlets in the closure band or L-bracket. 
     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. 
     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.