Abstract:
A turbine bucket comprising an airfoil portion and a shank portion with a substantially horizontal platform radially therebetween, the shank portion having a forward shank wall and an aft shank wall; at least the aft shank wall convexly curved from one side of the bucket to an opposite side of the bucket.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates generally to turbine blades and, specifically, to a design modification in a turbine blade aft shank wall for stress reduction. 
     When a bucket in a power systems turbine or a blade in an aircraft engine requires a hollow core for cooling air, the proximity of the core cooling cavities to the forward and rear shank walls (coverplates) can be problematic. For example, the cooling cavity closest to the aft shank wall may create a barely sufficient, thin metal ligament between the aft wall and the cooling cavity, causing a high stress location and potentially low part life. Additionally, this situation may preclude the shank wall from undercutting the airfoil trailing edge, a desired condition for airfoil root trailing edge stress relief. In other words, the desire to have the trailing edge overhang the aft shank wall often competes with the requirement to maintain adequate wall thickness between the core cavity and the aft shank wall. 
     Reduction of stress, particularly in the airfoil root trailing edge, requires both thermal management of the part to reduce thermal stress, and a geometry designed to relieve the loading of the airfoil trailing edge. 
     In a related matter, sealing of the turbine bucket or blade is crucial, as parasitic leaks degrade engine performance. Seal pins are typically used to reduce leaks between adjacent buckets. It is therefore also imperative that the ends of adjacent shanks join relatively closely from the concave sides to the convex sides of the buckets. 
     BRIEF DESCRIPTION OF THE INVENTION 
     This invention relieves stress at the root trailing edge of a rotating turbine airfoil while maintaining adequate wall thickness between core cavities and the aft shank wall. The invention also maintains mating side surfaces for sealing and leak reduction. More specifically, the invention utilizes a convexly curved aft shank wall that provides additional material between the aft shank wall and the nearest cooling cavity, and that also enables increased overhang of the trailing edge of the airfoil. The aft wall is curved from one side of the bucket to the other, so as to allow adjacent edges to align and thus facilitate sealing between adjacent buckets. 
     It will be appreciated that for some bucket applications, it may be desirable to provide a convexly curved surface on the forward shank wall as well. 
     In one aspect, therefore, the invention relates to a turbine bucket comprising an airfoil portion and a shank portion with a substantially horizontal platform radially therebetween, the shank portion having a forward shank wall and an aft shank wall; at least the aft shank wall convexly curved from one side of the bucket to an opposite side of the bucket. 
     In another aspect, the invention relates to a turbine bucket comprising an airfoil portion and a shank portion with a substantially horizontal platform radially therebetween, the shank portion having a forward shank wall and an aft shank wall; the aft shank wall convexly curved from one side of the bucket to an opposite side of the bucket; wherein the internal cooling circuit includes a plurality of cavities in the shank portion and the airfoil portion and wherein one of the cavities lies adjacent the aft shank wall; and further wherein the airfoil portion includes a leading edge and a trailing edge, the trailing edge overhanging the aft shank wall. 
     The invention will now be described in detail in connection with the drawings identified below. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of a conventional turbine bucket; 
     FIG. 2 is an enlarged partial perspective of the bucket shown in FIG. 1, sectioned at a location of minimum wall thickness between an aft core cavity and the aft shank wall of the bucket; 
     FIG. 3 is a simplified partial section, in plan, of a pair of conventional buckets shown in side-by-side relationship as installed on a turbine wheel; 
     FIG. 4 is a view similar to FIG. 3, but sectioned through the airfoil portions of the buckets; 
     FIG. 5 is a partial section in plan of two adjacent buckets, with curved aft shank walls in accordance with an exemplary embodiment of this invention; and 
     FIG. 6 is a view similar to FIG. 5 but sectioned through the airfoil portions of the buckets. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 illustrates a conventional turbine bucket  10  that includes generally a shank portion  12 , an attachment portion  14 , a platform portion  16  and an airfoil portion (or simply, “airfoil”  18 ). In terms of a bucket as mounted on a turbine rotor wheel, the platform portion  16  is radially between the shank portion  12  and the airfoil portion  18 , with the airfoil portion  18  joined to the upper or radially outer surface of the platform. So called “angel wings” or sealing flanges  20 ,  22  project axially away from the forward and aft walls  24 ,  26 , respectively, of the shank portion  12 . The attachment portion  14  is shown to be of the generally “axial entry” type and includes a “firtree”  28 , (the attachment portion could also be in the form of a dovetail or other surface locking configuration) designed to mate with a complementary-shaped groove (not shown) on the periphery of the turbine rotor wheel. It will be understood that the invention described herein is not limited by the configuration of the attachment portion  14 . The airfoil portion  18  has a leading edge  30  and a trailing edge  32 , with a concave surface  34  on the pressure side of the airfoil portion. 
     As best seen in FIG. 2, the bucket  10  is formed with an internal cooling circuit that includes cooling passages or cavities  36  in the shank portion  12  that communicate with larger inlet and outlet manifolds (not shown) in the shank portion attachment  14  and that extend upwardly into the airfoil portion  18 . Of particular concern here is the cavity  38  that is located adjacent the aft wall  26 . FIGS. 2 and 3 illustrate the close proximity of the cavity  38  to the aft shank wall  26  with a relatively thin ligament  48  therebetween. FIG. 3 also illustrates the manner in which adjacent buckets  10  and  110  are closely aligned when mounted on a turbine wheel, with respective corner edges  40 ,  42  of bucket  10  closely aligned with corner edges  140 ,  142  of bucket  110 . The space between side surfaces  44  and  144  may be on the order of about 0.040 inch, and as mentioned above, seal pins (not shown) are typically located within this space to prevent leakage between the adjacent buckets. Specifically, there are two seal pins, one oriented horizontally as mentioned above, and the other oriented radially, mating between radial surfaces adjacent to edges  42  and  142  of FIG.  3 . 
     FIG. 4 is similar to FIG. 3 but also shows the airfoil portions  18 ,  118  of the respective buckets  10 ,  110 . Of note in this Figure is the extent of overhang of the trailing edge  32  relative to the aft shank wall  26 . It will be appreciated that simply extending the aft wall to increase the width of ligament  48  between cavity  38  and the aft shank wall  26  would impair the ability to have an acceptable overhang. 
     Turning now to FIG. 5, the bucket  210 , otherwise similar to buckets  10  and  110 , is formed with forward and aft shank walls  224 ,  226 . In accordance with this invention, the aft shank wall  226  is convexly curved from side to side, i.e., such that the convex surface projects axially relative to the turbine rotor centerline indicated at  46 . This effectively thickens the ligament  248  between the closest cooling cavity  238  and the aft shank wall  226 . In addition, as made apparent in FIG. 6, the trailing edge  232  is afforded considerably more overhang vis-a-vis the aft shank wall  226  than in the prior design (see FIG.  4 ). At the same time, the corner edges  240 ,  242  of the bucket  210  may still correctly align with facing corner edges  340 ,  342  of adjacent bucket  310 . 
     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.