Abstract:
A cooling circuit for a turbine bucket having a shank, a platform and an airfoil. The cooling circuit includes a first cooling passage extending from an inlet located at a radially inward end of the shank and adapted to communicate with a turbine wheel-space, the first cooling passage, in use, supplying cooling air to a serpentine cooling circuit extending within and across at least one region of the platform. The serpentine cooling circuit connects with a separate internal cooling circuit in the airfoil, such that the cooling air used to cool the platform is re-used in the airfoil cooling circuit.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The present invention relates generally to gas turbine buckets or blades and particularly relates to cooling a so-called platform portion interposed between the bucket airfoil and the bucket shank. 
         [0002]    Over the years, gas turbines have trended toward increased inlet firing temperatures to improve output and engine efficiencies. As hot gas path temperatures have increased, however, bucket platforms have increasingly exhibited distress including oxidation, creep and low-cycle fatigue cracking, spallation and in some cases, platform liberation. With the advent of closed circuit steam cooling in, for example, the buckets and nozzles in the first two stages of industrial gas turbines, inlet profiles have become such that the bucket platforms are exposed to temperatures close to peak inlet temperatures for the blade row. The problem is particularly acute at the leading edge fillet where the airfoil joins the platform at the forward portion of the pressure side of the airfoil. 
         [0003]    Accordingly, it would be beneficial if more effective cooling arrangements can be designed to cool the platform areas of buckets used particularly in the first and second stages of the turbine. 
       SUMMARY OF THE INVENTION 
       [0004]    In a first exemplary but nonlimiting embodiment, the present invention relates to a cooling circuit for a turbine bucket having a shank portion, a platform portion and an airfoil portion, the cooling circuit comprising a first cooling passage extending from a cooling air inlet located at a radially inward end of said shank portion so as to communicate with a turbine wheelspace when in use, said first cooling passage connecting to a second cooling passage extending within and across at least one region of said platform, said second cooling passage connecting with a third cooling passage extending radially outwardly in said airfoil portion, said third cooling passage terminating at one or more cooling air outlets located at a radially outward end of said airfoil portion. 
         [0005]    In another exemplary but nonlimiting embodiment, the invention relates to a cooling circuit for a turbine bucket having a shank, a platform and an airfoil, the cooling circuit comprising: a first cooling passage extending from an inlet located at a radially inward end of the shank and adapted to communicate with a turbine wheel-space, the first cooling passage, in use, supplying cooling air to a serpentine cooling circuit extending within and across at least one region of the platform, said serpentine cooling circuit connecting with a separate internal cooling circuit passage proximate a trailing edge of the airfoil, such that the cooling air used to cool the platform is re-used in the airfoil cooling circuit; wherein the platform includes a first region on a pressure side of the airfoil portion and a second region on a suction side of the airfoil portion, the at least one region comprising the first region on the pressure side of the airfoil. 
         [0006]    In still another exemplary but nonlimiting embodiment, the invention provides a method of cooling a gas turbine bucket platform comprising: extracting compressor cooling air from a wheel space area between blade wheels mounted on a turbine rotor; feeding extracted compressor cooling air from a radially oriented passage along a leading edge of a shank portion of the bucket to a serpentine cooling passage formed in the platform; dumping the extracted compressor cooling air into an internal cooling circuit in the bucket airfoil; and exhausting the extracted compressor cooling air along a trailing edge of the bucket airfoil. 
         [0007]    The invention will now be described in detail in connection with the drawings identified below. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]      FIG. 1  is a side elevation, partly in section, of a turbine bucket in accordance with a first exemplary but nonlimiting embodiment of the invention; 
           [0009]      FIG. 2  is a side elevation, partly in section, showing an alternative cooling air inlet configuration; 
           [0010]      FIG. 3  is a top plan view in schematic form showing a serpentine platform cooling circuit in accordance with the first exemplary embodiment of the invention; 
           [0011]      FIG. 4  is a top plan view in schematic form illustrating an alternative serpentine cooling circuit in accordance with another exemplary but nonlimiting embodiment of the invention; and 
           [0012]      FIG. 5  is a top plan view in schematic form illustrating a serpentine cooling circuit in accordance with another exemplary but nonlimiting embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0013]    In general terms, the present invention relates to a turbine bucket platform cooling arrangement where a portion of the compressor-extracted air that is used to cool the wheel space areas between the rotor wheels is fed to the bucket platform through a passage on the lower outlet side of the bucket shank portion. This passage will feed the extracted air radially outwardly to the platform where it will turn substantially 90 degrees and follow a serpentine passage along and around the “inner portion” of the platform, i.e., that portion on the pressure side of the bucket airfoil. The extracted cooling air will then dump into one of the radially-extending internal core cooling passages of the bucket airfoil to be used for airfoil cooling. 
         [0014]    More specifically, and with reference to  FIG. 1 , a turbine bucket  10  includes an airfoil  12  and a shank  14 , typically formed with so-called angel-wing seals  16 . A relatively flat platform  18  is located radially between the airfoil  12  and the shank  14 . In accordance with an exemplary but nonlimiting embodiment, a cooling air inlet passage  20  is formed (e.g., drilled or cast) in a forward or leading face  22  of the bucket shank  14 . The inlet passage  20  extends radially outwardly to the platform  18  where it turns substantially 90 degrees into a platform cooling circuit generally indicated at  24 . The inlet  26  to the radial passage  20  is radially aligned with the passage  20 . 
         [0015]      FIG. 2  illustrates an alternative arrangement by where the inlet  28  to the passage  20  is formed at an acute angle to the passage, illustrating an alternative manufacturing expedient. The construction is otherwise substantially identical to that shown in  FIG. 1 , and either inlet arrangement may be employed with each of the serpentine cooling circuits described below. 
         [0016]    Turning now to  FIG. 3 , a serpentine cooling circuit  24  for cooling the platform  18  is shown in accordance with one exemplary but nonlimiting embodiment. Note initially that the bucket airfoil  12  has a suction side  30 , a pressure side  32 , a leading edge  34  and a trailing edge  36 . The inlet passage  20  is located along the leading edge of the shank  14 , adjacent the leading edge  34  of the airfoil. The serpentine cooling circuit  24  is formed within the platform  18  (by e.g., casting) so as to provide a first cooling passage section  38  that serves to cool an area proximate the pressure side  32  of the airfoil and including the fillet area where the airfoil  12  is joined to the platform  18 . The cooling flow then reverses through a cooling passage section  40  in a middle region of the platform, and then reverses again in a cooling passage section  42  that runs proximate an edge  44  of the platform. The circuit then turns substantially 90° in a cooling passage section  46  and then dumps the cooling air into a radially extending internal airfoil cooling passage  48  closest to the airfoil trailing edge  36 . The radial cooling passage  48  is part of an internal serpentine cooling circuit in the airfoil  12  which includes a number of radial connected passages  50 ,  52 ,  54 ,  56 ,  58  and  48 . Typically, the coolant flows through the circuit in a direction from the leading edge to the trailing edge, exiting the airfoil through plural passages  60  extending from the radial passage  48  to the trailing edge  36 . 
         [0017]      FIG. 4  shows an alternative serpentine cooling circuit  124  for cooling the platform  18 . Here, the inlet passage  20  remains adjacent the leading edge  34  of the airfoil  12 . A first cooling passage section  62  of the cooling circuit  124  runs along the edge  44  of the platform  18  and then reverses in a cooling passage section  64  along a middle region of the platform before reversing again in a cooling passage section  66  closer to the suction side  32  of the bucket airfoil. The cooling circuit then reverses through a cooling passage section  68  and turns into the middle portion of the airfoil via cooling passage section where it dumps into the radially-extending internal airfoil cooling passage  56 . The internal airfoil cooling circuit remains as described above in connection with  FIG. 3 . To facilitate the manufacturing process, the cooling passage section  70  is more easily formed by initiating a drilling operation from the opposite edge  76  of the platform  18 , forming an extending cooling passage section  72 . To maintain the integrity of the cooling circuit, the extended cooling passage section  72  is plugged at  74 . The otherwise relatively short cooling passage section  72  may provide some additional, albeit minor, cooling to the platform. 
         [0018]      FIG. 5  illustrates a third exemplary but nonlimiting embodiment of a suitable serpentine cooling circuit. This cooling circuit  224  contains the same cooling passage sections  62 ,  64  and  66  as shown in  FIG. 4 . In this embodiment, however, the cooling circuit  224  again dumps into the trailing edge airfoil cavity  48  as in the first described embodiment, via a cooling passage section  78 . The manufacture of cooling passage section  78  is facilitated by drilling an extended passage  80  through the platform, on the suction side  30  of the airfoil  12 , plugged at  82 , similar to the manner in which passage section  72  is plugged at  74  in  FIG. 4 . Because of the length of the extended passage section  80 , some meaningful cooling of the suction side of the platform  18  is provided. 
         [0019]    In each of the above-described embodiments, the serpentine cooling circuit  24 ,  124  and  224  formed in the bucket platform  18  is fed from compressor-extraction air taken in at the lower, leading side of the bucket shank. The cooling air is then routed along the serpentine platform cooling circuit before being dumped into the internal airfoil cooling circuit where the platform cooling air is re-used for cooling the airfoil. The cooling air is then exhausted through the trailing edge of the bucket along with the airfoil cooling circuit air. This arrangement effectively film cools both the forward face of the shank and the platform, while providing additional cooling air to the airfoil. In addition, pulling compressor extraction air directly into the bucket provides air at higher pressure to the problematic platform area which helps reduce the platform temperature and prolong the life of the bucket. This, in turn, results in reduced repair costs over the service life of the bucket. 
         [0020]    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.