Patent Abstract:
A steam turbine rotor wheel includes a plurality of blades secured about a circumferential periphery of the wheel, each blade comprising a shank portion and an airfoil portion, the airfoil portion having at least one pocket filled with a filler material, wherein at least one edge of the pocket adjacent a leading edge of the blade is formed with an undercut.

Full Description:
BACKGROUND OF THE INVENTION 
   This invention relates generally to steam turbine buckets (or blades) and, more particularly, to the adhesion of filler material in hybrid or composite blades. 
   Steam turbine blades operate in an environment where they are subject to high centrifugal loads and vibratory stresses. Vibratory stresses increase when blade natural frequencies become in resonance. The magnitude of vibratory stresses when a blade vibrates in resonance is proportional to the amount of damping present in the system (damping to a smaller or greater degree is achieved via materials and the aerodynamic and mechanical components), as well as the vibration stimulus level. 
   At the same time, centrifugal loads are a function of the operating speed, the mass of the blade, and the radius from engine centerline where that mass is located. As the mass of the blade increases, the physical area or cross-sectional area must increase at lower radial heights to be able to carry the mass above it without exceeding the allowable stresses for the given material. This increasing section area of the blade at lower spans contributes to excessive flow blockage at the root and thus lower performance. The weight of the blade also contributes to higher disk stresses and thus potentially to reduced reliability. 
   Several prior U.S. patents relate to so-called “hybrid” blade designs where the airfoil portion of the metal blade is formed with one or more pockets filled with a polymer (or polymer/metal, glass or ceramics mix) filler material. These prior patents include U.S. Pat. Nos. 6,287,080; 6,139,278; 6,042,338; 6,039,542; 6,033,186; 5,947,688; 5,931,641 and 5,720,597. See also co-pending commonly owned application Ser. No. 10/249,518, filed Apr. 16, 2003. One area not addressed by the prior work in this area is the problem of achieving more reliable adhesion of the filler within the pocket or pockets formed in the airfoil portion of the blade. 
   More specifically, the large incidence angles of steam flow to the bucket surface could cause the cast polymer filler to delaminate from the pocket formed in the airfoil portion of the blade. In other words, the large angle of incidence of the steam flow to the bucket surface exposes a higher risk of the flow tending to “lift” the filler material off the pocketed surface. 
   BRIEF DESCRIPTION OF THE INVENTION 
   This invention proposes an edge geometry along one or more edges of the pocket formed in the airfoil portion of the blade in order to improve adhesion of the filler at the interface, specifically in the high angle of incidence steam flow field. While this invention utilizes the hybrid blade concept as disclosed, for example, in U.S. Pat. No. 5,931,641, that concept is extended to include optimization of pocket shape within the airfoil portions of the blades in order to improve adhesion of the filler material. 
   In the exemplary embodiment, the marginal area of the pocket, and preferably the marginal edge of the pocket extending along the leading edge of the blade, is formed with an “undercut.” This undercut serves the purpose of not allowing the high angle of incidence steam flow from trying to “lift” the polymer (or polymer/metal mix) filler from the pocket. The undercut thus shields that portion of the filler/bucket interface with the highest angle of incidence to the incoming steam flow. The undercut could also be extended, however, to include the trailing edge or even all edges of the pocket or pockets. 
   Accordingly, in its broader aspects, the invention relates to a steam turbine rotor wheel comprising a plurality of blades secured about a circumferential periphery of the wheel, each blade comprising a shank portion and an airfoil portion, the airfoil portion having at least one pocket filled with a filler material, wherein at least one edge of the pocket adjacent a leading edge of the blade is formed with an undercut. 
   In another aspect, the invention relates to a steam turbine rotor wheel comprising a row of blades secured about a circumferential periphery of the wheel, each blade formed with one or more pockets filled with a filler material and where at least an edge of the pocket adjacent a leading edge of the airfoil incorporates means for enhancing adhesion of the filler material to the blade. 
   In still another aspect, the present invention relates to a turbine blade comprising a shank portion and an airfoil portion, the airfoil portion having at least one pocket filled with a filler material, wherein at least one edge of the pocket adjacent a leading edge of the blade is formed with an undercut. 
   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 partially manufactured blade illustrating an unfilled pocket configuration in the airfoil portion of the blade; 
       FIG. 2  is a similar view of the blade in  FIG. 1  but after filler material has been applied over the pockets; 
       FIG. 3  is a partial plan view of another hybrid blade illustrating multiple filled pockets along the airfoil portion of the blade; 
       FIG. 4  is a cross-sectional view of the blade shown in  FIG. 3 ; 
       FIG. 5  is an elevation of a hybrid blade constructed in accordance with the exemplary embodiment of this invention; 
       FIG. 6  is a section taken along the line  6 — 6  in  FIG. 5 ; 
       FIG. 7  is an enlarged detail taken from  FIG. 6 ; 
       FIG. 8  is a partial cross-section of the trailing edge of a hybrid blade with an undercut similar to that shown in  FIG. 7 ; and 
       FIG. 9  is a section taken along the line  9 — 9  of  FIG. 5 , illustrating undercuts on the radially inner and outer edges of the airfoil filler pocket. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   With reference to  FIG. 1 , a steam turbine blade  10  is shown in partially manufactured form. The blade  10  includes a shank portion  12  and an airfoil portion  14 . The airfoil portion is preferably constructed of steel or titanium but other suitable materials include aluminum, cobalt or nickel. Ribs  16 ,  18  are integrally cast with the airfoil portion to form discrete pockets  20 ,  22  and  24 . It will be appreciated, however, that the ribs do not extend flush with the side edges  26 ,  28  of the airfoil portion. The rib height may in fact vary according to specific applications. A polymer based (or polymer/metal, glass or ceramics mix) filler material  30  as described, for example, in U.S. Pat. Nos. 6,287,080 and 5,931,641 is cast-in-place over the pressure side of the airfoil, filling the pockets  20 ,  22  and  24  and covering the ribs to thereby form a smooth face  32  on the pressure side of the bucket, as shown in  FIG. 2 . 
     FIGS. 3 and 4  illustrate another known hybrid blade construction where the blade  34  is formed with a plurality of discrete pockets  36 ,  38 ,  40 , etc. along the pressure side of the airfoil portion  42  of the blade. In this arrangement, filler material  44  ( FIG. 4 ) is cast in each pocket individually, with the filler material flush with the surrounding airfoil surfaces. As a result, each discrete pocket is externally visible.  FIG. 4  also illustrates the conventional practice of forming the pockets  46 ,  48  with side surfaces  50 ,  52  and  54 ,  56  that curve radially outwardly (at an oblique angle to the adjacent airfoil surface) at the interface with the exterior surface of the airfoil portion. 
   Currently, available choices for bonding the filler material  30  or  44  to the metal surface of the airfoil portion include, without limitation, self adhesion, adhesion between the filler material  30  or  44  and the metal surface of the airfoil portion, adhesive bonding (adhesive film or paste), and fusion bonding. As discussed above, however, these adhesion techniques may not be sufficient to prevent delamination of the filler along that part of the filler-blade interface exposed to large angle of incidence steam flow. In accordance with an exemplary embodiment of this invention, and with reference to  FIGS. 5 and 6 , adhesion of the filler is enhanced by the incorporation of an undercut along some or all of the edges of the pocket. Referring initially to  FIG. 5 , the blade  58  is formed with three polymer-filled pockets  60 ,  62  and  64  on the pressure side  66  of the airfoil portion of the blade. Filler material  68  is shown cast-in-place, with the filler material flush with the surrounding airfoil surface. As shown in  FIG. 6 , the pocket  64  is defined by an edge  70  closest to the trailing edge  72  of the bucket that smoothly interfaces with the external surface of the airfoil, in accordance with the prior practice. The pocket edge  74  closest to the leading edge  76 , however, is now formed with an undercut  78  that creates an acute angle α at the interface with the adjacent airfoil surface, as best seen in  FIG. 7 . The undercut itself may be formed of a small or large radius R depending upon the thickness of the airfoil near the leading edge, and the radius is gradually blended into the back wall  80  of the pocket in such a way as to reduce the concentrated stress due to the undercut geometry. It will be understood that the manner of application as well as the composition of the filler material may be in accordance with current practice. 
   It will also be appreciated that the overall configuration of the pocket may vary as desired, and that the invention here relates primarily to the incorporation of an undercut along the marginal edges of the one or more pockets, and especially along the edge closest to (or adjacent to) the leading edge of the bucket where the filler material interfaces with the adjacent external surface on the pressure side of the bucket. The undercut could, however, be extended to include the pocket edge closest to (or adjacent to) the trailing edge of the bucket (see undercut  80  in  FIG. 8 ), or even to include all edges of the one or more pockets (see undercut  82  in  FIG. 9  which extends about the entire periphery of the pocket). As described above, the incorporation of an undercut prevents the steam flow from causing delamination of the pocket fill material at the most vulnerable location, i.e., along the leading edge of the airfoil. 
   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.

Technology Classification (CPC): 5