Abstract:
In a turbine bucket having an airfoil portion and a root portion, with a substantially planar platform at an interface between the airfoil portion and root portion, a platform cooling arrangement including at least one bore in the root portion and at least one impingement cooling tube seated in the bore, the tube extending beyond the bore with an outlet in close proximity to a targeted area on an underside of the platform.

Description:
This application is a continuation of Ser. No. 09/404,589 filed Sep. 24, 1999. 
    
    
     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 the cooling of turbine buckets and, specifically, to the cooling of the platform region of the bucket, at the base of the airfoil. 
     BRIEF SUMMARY OF THE INVENTION 
     Stage one and stage two buckets in gas turbines experience high temperatures and stress, creating low cycle fatigue life concerns, particularly at the leading edge fillet where the airfoil joins the platform at the forward portion of the pressure side of the airfoil. 
     In aircraft engines, the bucket platforms are cooled by air entering the cavity below the platform, but this arrangement is not completely satisfactory, particularly in connection with the leading edge fillet. 
     This invention provides a unique solution to the above problem by directing cooling air through a tube, the forward end of which is located closely adjacent the underside of the platform, below the forward portion of the pressure side of the airfoil, so as to effectively impingement cool the platform in the localized region below the leading edge fillet. 
     In addition, film cooling holes are formed in the platform itself so that air introduced into the cavity by the impingement cooling tube can escape the cavity and flow along the upper surface of the platform along the contour of the pressure side of the airfoil, in a direction toward the trailing edge of the airfoil. 
     In its broader aspects, the invention relates to a turbine bucket having an airfoil portion and a root portion, with a substantially planar platform at an interface between the airfoil portion and root portion, a platform cooling arrangement comprising at least one bore in the root portion and at least one impingement cooling tube seated in the bore, the tube extending beyond the bore with an outlet in close proximity to a targeted area on an underside of the platform. 
     In another aspect, the invention relates to a method of cooling a leading edge fillet region of a turbine bucket having an airfoil portion and a root portion, the airfoil portion joined to a platform extending over the root portion, comprising: a) providing a through bore in a leading side of a cover plate in the root portion, the bore communicating with a recess in the root portion underlying the airfoil portion; b) inserting a tube in the bore, with a tip of the tube extending beyond the bore and in close proximity to a targeted area on an underside of the platform; c) supplying cooling air to the recess via the tube such that the targeted area is impingement cooled by the cooling air passing through the tube. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a partial side cross section of a bucket in accordance with the invention; 
     FIG. 2 is a partial end view of a lower dovetail portion (of the leading side edge) of the bucket not entirely shown in FIG. 1; and 
     FIG. 3 is a top plan view of the bucket in accordance with the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     With reference to FIGS. 1-3, a turbine bucket  10  includes an airfoil portion  12  extending vertically upwardly from a horizontal, substantially planar platform  14 . The airfoil portion has a leading edge  15  and a trailing edge  17 . Below the platform  14 , there are two pair of so-called “angel wings”  16 ,  18  extending in opposite directions from the leading and trailing sides  20 ,  22  of the cover plate  24  in the root portion of the bucket. The platform  14  is joined with the cover plate  24 . Below the root portion, there is a dovetail  26  by which the bucket is secured to a turbine wheel (in a preferred embodiment, the stage  1  or stage  2  wheels of a gas turbine). 
     Cooling passages (not shown) extend in radially outward and inward directions in a serpentine path through the airfoil portion, the path having passages in the root portion communicating with supply and return passages  28 ,  30  in the dovetail. The internal airfoil cooling circuit forms no part of the invention, however, and is therefore not shown in detail. Of significance here is the substantially hollow space or recess  32  in the root portion, above the dovetail  26  but below the platform  14 , defined further by the sides  20 ,  22  of the cover plate. This recess is generally on the pressure side  34  of the airfoil, and it will be appreciated from FIG. 3 that a relatively large portion of the platform  14  on the pressure side  34  of the airfoil overlies the hollow space or recess  32 . 
     An angled hole  36  is drilled through the leading side  20  of the cover plate  24  from the lower end of the root portion, on the external side thereof, extending upwardly (i.e., substantially radially) and opening into the recess  32 . Due to the angled orientation, the hole  36  has an exit profile on the interior of the root wall  38  which is elliptical in shape as shown in FIG.  2 . An impingement cooling tube  40  is pressed into the hole  36  from under the root portion, so that an outlet at the upper end or tip  42  of the tube is located close to the underside of the platform, and close to a targeted area along the leading edge fillet  44  where the airfoil joins the platform. 
     The lower end of the tube  40  is flared at  46  to limit the extent of insertion, and the tube may be fixed in place by a spot weld at  47  to prevent the tube from sliding back through the hole in a direction opposite the assembly direction. Preferably, the tube end  42  is within about 0.5″ from the underside of the platform. 
     Since the lower edge of the cover plate  24  will seat on the wheel surface, thus blocking the hole  36 , a second hole  48  is drilled from the leading side  20  of the cover plate  24  so as to intersect with the hole  36 , just below the lower end of the tube  40 . In this way, rotor or wheel space purge air can enter the tube  40  via hole  36 . 
     In addition, an array of film cooling holes  50  extend through the platform  14  from the recess  32  to a location on the external side of the platform. 
     With the above arrangement, purge air from wheel space below the bucket will enter the hole  48  and flow through the tube  40  so as to impinge directly on the underside of the platform  14 , below but proximate the leading edge fillet  44 . The underside of the platform  14  is thus impingement cooled in the leading edge fillet area to reduce stress and improve low cycle fatigue margins. Of course, the enhancement of heat transfer in this specific region continues along the platform and at least partially under the airfoil itself. 
     Another feature of the invention lies in the combination of the above described impingement cooling with film cooling along the upper surface of the platform: This is achieved by utilizing the array of film cooling holes  50  in the platform, allowing the cooling air in the hollow space  32  to exit along the top surface of the platform. The film cooling holes  50  are directionally oriented so that the cooling air merges with the gas path air and flows along the pressure side of the airfoil toward the trailing edge  17 . The flowpath of the film cooling air is such that it can carry over to the suction side of the platform of the adjacent bucket, increasing the overall effectiveness of the system. 
     To further enhance heat transfer, the underside of the platform  14  can be provided with any suitable rough coat, thus increasing the surface area available for heat transfer. In addition, one or more additional impingement cooling tubes may be used in conjunction with the tube  40  to enhance heat transfer at targeted locations along the underside of the platform. 
     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.