Abstract:
A turbine bucket includes an airfoil portion having leading and trailing edges; at least one radially extending cooling passage within the airfoil portion, the airfoil portion joined to a platform at a radially inner end of the airfoil portion; a dovetail mounting portion enclosing a cooling medium supply passage; and, a crossover passage in fluid communication with the cooling medium supply passage and with at least one radially extending cooling passage, the crossover passage having a portion extending along and substantially parallel to an underside surface of the platform.

Description:
This invention was made with Government support under Contract No. DE-FC21-95MC31176 awarded by the Department of Energy. The Government has certain rights in this invention. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates to a closed loop, convection cooled gas turbine bucket and to a method for cooling the platform and airfoil fillet region of the bucket. 
     The technology of gas turbine bucket design is continually improving. Current state-of-the-art designs employ advanced closed loop cooling systems, higher firing temperatures and new materials to achieve higher thermal efficiency. Coincident with these advances, there is an ever increasing need to design components to avoid crack initiation and subsequent coolant loss due to low cycle fatigue. 
     Low cycle fatigue (LCF) is a failure mechanism common to all gas turbine buckets. It is defined as damage incurred by the cyclic reversed plastic flow of metal in a component exposed to fewer than 10,000 load cycles. Low cycle fatigue stress is a function of both the stress within the section as well as the temperature. The stress may come from mechanical loads such as pressure, gas bending, or centrifugal force, or the stress may be thermally induced, created by the difference in metal temperatures between various regions and the geometric constraints between these regions. Minimizing thermal gradients within a structure is key to reducing LCF damage. 
     In advanced gas turbine cooled bucket designs, particularly those with thermal barrier coatings, the airfoil bulk temperature tends to run cooler than the platform at the base of the airfoil, creating a thermal stress in the platform and airfoil fillet region on the pressure side of the airfoil (where the airfoil portion joins the platform). Adequate cooling of this region is necessary to reduce the stress and to improve the low cycle fatigue life. 
     During the production of the present bucket casting, the crossover core that generates the hollow cavity through which coolant is delivered to the machined trailing edge holes is locked into the shell system at the root of the bucket. The crossover core is also held by the shell at two mid-span locations (reference crossover core supports denoted in FIG.  1 ), and again at another location near the top of the crossover core. 
     It is critical to control the location of the top of the core since it is this location that forms a “target” for drilling the trailing edge cooling holes in the airfoil portion of the bucket. These machined trailing edge cooling holes must intersect the top of this core in order for coolant to flow through these holes and provide cooling to the airfoil trailing edge. One of the root causes of poor position control is inherent in the design. Specifically, since there is a difference in thermal expansion between the ceramic shell and ceramic core used in the casting process, and due to the relatively long length of the crossover core (approximately 12 inches) the crossover core is “pulled” by its root end where it is locked in the shell. Attempts to lock this design at the tip have failed due to the fragility of the core. 
     BRIEF SUMMARY OF THE INVENTION 
     This invention seeks to improve the low cycle fatigue capability of turbine buckets through use of an improved cooling system that is also more producible and cost effective. The design and manufacturing improvements are summarized below. 
     In terms of design, the crossover passage is opened to the cooling passage in the shank portion of the bucket at a location close to the underside of the platform, and then runs along the underside of the platform towards the trailing edge of the airfoil. This arrangement cools both the platform and the airfoil fillet region. For a second stage bucket, the flow direction can run from the aft portion of the bucket toward the leading edge where the flow enters a radially extending cooling passage in the airfoil portion of the bucket. 
     This design change means that the total height of the core used in the manufacture of the bucket can be shortened to reduce the amount of thermal mismatch. The redesigned crossover core can be locked in the shell at the forward or radially outer core end, thus eliminating the prior core end location problem. Since the crossover core will bump against the main body core, there is also no concern with respect to relative radial movement of the two cores. The crossover core will be allowed to float at the aft or radially inner core location. Since the core will be completely encapsulated by shell, however, and in close proximity to the platform, it is anticipated that relative movement between the core and the platform will be reduced, and thus dimensional control improved. A further benefit of this design will be lighter weight, chiefly due to the reduced size of the central rib in the shank portion of the bucket. 
     The design concept may also be implemented as a post cast fabrication rather than cast. In any event, the manufacturing process employed to produce the new bucket platform cooling circuit is not regarded as part of the invention per se. 
     The internal heat transfer coefficients of the new crossover passage design may be optimized either through tuning the cross sectional area or wetted perimeter, thus controlling flow velocity and heat transfer coefficient. Further, the passage may be locally turbulated to increase the local heat transfer coefficients without unnecessarily increasing pressure loss and heat pickup throughout the passage. 
     Alternative designs within the scope of the invention permit the cooling of virtually any region of the platform by simply re-routing the crossover passage along the underside of the platform. It is also contemplated that cooling steam be metered into the trailing edge holes by the cooling holes themselves. In applications where there are no trailing edge holes to meter the cooling flow, the amount of flow that would bypass the main cooling circuit would be too great given the size limitations that would be placed on the minimum cross sectional area of the crossover passage in order to achieve adequate core producibility. Accordingly, for such applications, a separate means for metering the flow into the trailing edge holes is provided. 
     In its broader aspects, therefore, the present invention relates to a closed circuit steam cooling arrangement in a gas turbine bucket having an airfoil portion joined to a platform along a fillet region and where a steam cooling supply passage is adapted to supply cooling steam to the airfoil portion of the bucket, and which also includes a crossover passage extending adjacent and substantially parallel to the platform. 
     In another aspect, the invention relates to a turbine bucket comprising an airfoil portion having leading and trailing edges; at least one radially extending cooling passage within the airfoil portion, the airfoil portion joined to a platform at a radially inner end of the airfoil portion; a dovetail mounting portion enclosing a cooling medium supply passage; and, a crossover passage in fluid communication with the cooling medium supply passage and with at least one radially extending cooling passage, the crossover passage having a portion extending along and substantially parallel to an underside surface of the platform. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a partially cut away perspective view of a prior gas turbine bucket; 
     FIG. 2 is a partial plan view of the bucket shown in FIG. 1; 
     FIG. 3 is a partial side elevation of a gas turbine bucket in accordance with this invention, illustrating part of an internal cooling circuit; 
     FIG. 4 is a plan view of a gas turbine bucket in accordance with the invention, with the airfoil tip cap removed; 
     FIG. 5 is a partial section of the gas turbine bucket shown in FIG. 4, taken at a location in radial proximity to the bucket platform; 
     FIG. 6 is a cross section of the gas turbine bucket shown in FIG. 4, taken along the line  6 — 6 ; 
     FIG. 7 is an enlarged detail taken from FIG. 6, but with a modified metering plug; 
     FIG. 8 is a partial perspective view of an alternative embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 illustrates a prior bucket trailing edge cooling circuit, which is part of a closed loop, serpentine circuit extending radially within the bucket. Only part of the bucket cooling circuit is shown. The bucket  10  includes an airfoil  12  having a leading edge  14  and a trailing edge  16 . The airfoil is joined to a horizontal platform  18  along an airfoil fillet  19 . So called “angel wings”  20 ,  22  and  24 ,  26  extend laterally away from the respective front and rear sides of the shank portion  27  of the bucket, and a dovetail portion  28  is employed to mount the bucket on a turbine wheel (not shown) in a conventional manner. 
     Trailing edge cooling holes  30 ,  32  (see also FIG. 2) extend internally along and adjacent the trailing edge  14  of the airfoil, while an internal crossover passage  34  extends from the lower end of holes  30 ,  32  to a coolant supply passage  36  in the dovetail portion of the bucket. Cooling steam (or other medium) will flow via passages  36  and  34  into the trailing edge cooling holes  30 ,  32 . The cooling steam reverses direction (indicated by a flow arrow at the tip of the airfoil) and travels radially inwardly via a passage (not shown) flowing eventually into the cooling steam return passage  38 . 
     It is apparent from FIG. 2 that it is critical to control the location of the top of the core that forms the passage  34  during manufacture, since the trailing edge cooling holes  30 ,  32 . Note also the relatively large distance between the top of the passage  34  and the location of a core support plug  39 , a fact which makes accurate location of the top of the crossover core problematic. 
     Turning now to FIGS. 3-8, the manner in which the present invention alleviates these problems will now be discussed in detail. 
     In FIGS. 3 and 4, similar reference numerals are employed to indicate components corresponding to those in FIGS. 1 and 2, but with the prefix “ 1 ” added. Thus, the bucket  110  includes an airfoil  112  having a leading edge  114  and a trailing edge  116 . The airfoil joins the platform  118  along an airfoil fillet  119 . The bucket  110  also has angel wings  120 ,  122  and  124 ,  126  as well as dovetail portion  128 . Radially extending trailing edge cooling passages, in the form of drilled holes  130 ,  132  extend internally along and adjacent the trailing edge  116 . In this construction, however, the cooling supply passage  136  (see FIG. 6) supplies cooling steam to an enlarged interior chamber  140  which extends radially outwardly to a location generally adjacent the angel wing  120 . A new crossover inlet  142  extends horizontally between the chamber  140  and a new crossover cooling passage  144  which has a radial (or vertical, as viewed in FIGS. 3 and 6) leg  146  and a horizontal leg  148  which extends along the underside of the platform  118  from the forward or leading side of the bucket to the rearward or trailing side of the bucket (as best seen in FIGS. 5) where the passage intersects the trailing edge cooling holes  130 ,  132 . The cooling steam flows radially outwardly along the trailing edge and then reverses direction, flowing radially inwardly, dumping into chamber  150  which, in turn, connects to the cooling return passage  138 . Note that some of the radial passages for the internal bucket cooling circuit are shown in FIG. 4, one such passage indicated at  152 . In FIG. 5, it can be seen how the new core will provide a better target for the drilled trailing edge cooling holes  130 ,  132 . 
     In FIGS. 3-6, it is apparent how the crossover passage  144  follows the contour of the pressure side of the airfoil, along the fillet  119 , thereby providing needed cooling along the underside of the platform  118  as well as along fillet  119 . It is also readily apparent from FIG. 2 that the height of the crossover passage is considerably reduced as compared to the prior arrangement. Crossover passage  144  may be turbulated as shown at  145  in FIG.  4 . 
     FIG. 6 also illustrates the manner in which the airfoil is drilled through the main body so as to connect the passage  146  with the inlet  142  leading to the interior chamber  140 . The hole is then plugged at  154 . 
     FIG. 7 illustrates an alternative arrangement where a plug  156  is inserted into the drilled hole providing the communication between chamber  140  and passage  146  via inlet  142 . Here, the plug  156  is formed with metering holes  158  and  160  which meter air from the chamber  140  into the passage  146 . This arrangement is particularly suitable where there are no trailing edge holes to meter the flow as, for example, in second stage buckets where the cooling flow is toward the leading edge of the bucket, and then in a radially extending passage in the airfoil portion of the bucket. It will be understood that the manner in which the crossover passage are formed and the manner in which access is provided to form interior passages for metering is dependent upon the manufacturing process used to produce the bucket. Where no separate metering mechanism is provided, the trailing edge holes  130 ,  132  are sized to meter the cooling air. 
     FIG. 8 discloses an alternative crossover passage  254  which is of serpentine configuration, permitting more of the platform  218  to be cooled. It will be appreciated that various design configurations can be implemented to cool the platform and/or fillet region as desired. 
     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.