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 the root portion, a platform cooling arrangement including a cavity extending along the forward portion of the platform, and at least one inlet bore extending from a source of cooling medium to the cavity, and at least one outlet opening for expelling cooling medium from the cavity.

Description:
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 leading edge of the bucket. 
   BRIEF DESCRIPTION OF THE INVENTION 
   Over the years, gas turbine firing temperatures have been increasing in order to improve turbine efficiency and output. As firing temperatures increase, bucket platforms, which in the past have been un-cooled, exhibit distress, such as oxidation, low cycle fatigue and creep. Film cooling has been used more recently to help cool the platforms, but film cooling is generally limited to the aft portions of the platform where the gas path flow has been accelerated sufficiently to drop the static pressure to a level where there is sufficient supply pressure to have positive film flow without hot gas ingestion. Platform leading edges are in a region where there is insufficient pressure to utilize film cooling but is also a region where there is distress due to high temperatures. 
   The present invention provides a unique solution to the above problem by actively cooling the bucket platform leading edge such that the bucket meets life requirements while minimizing the impact on engine performance. Active cooling is provided by directing cooling media to a cavity extending along the platform leading edge. Thus, the invention may be embodied 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 the root portion, a platform cooling arrangement including a cavity extending along the forward portion of the platform, at least one inlet bore extending from a source of cooling medium to said cavity and at least one outlet opening for expelling cooling medium from said cavity. 
   The invention may also be embodied in a method of cooling a leading edge of a turbine bucket having an airfoil portion and a root portion, said airfoil portion being joined to a platform extending over said root portion, comprising: forming a cavity to extend along and adjacent at least a portion of said leading edge; flowing a cooling medium from a source of cooling medium through at least one inlet bore to said cavity; and expelling cooling medium from said cavity through said at least one outlet opening. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic, partial side cross-section of a bucket in an example embodiment of the invention; 
       FIG. 2  is a top plan view of the bucket of  FIG. 1 ; 
       FIG. 3  is a schematic, partial side cross-section of a bucket according to another example embodiment of the invention; and 
       FIG. 4  is a top plan view of the bucket of  FIG. 3 . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The leading edges of bucket platforms have begun to exhibit distress such as oxidation, low cycle fatigue and creep as firing temperatures have increased. There is insufficient cooling pressure ratio to film cool the bucket platform leading edge. Therefore, in an example embodiment of the invention, active cooling is provided to eliminate oxidation, low cycle fatigue and creep distress on the bucket platform leading edge. The cooling medium flow is fed through a cast cavity, machined cavity or a drilled hole which runs along the forward portion of the bucket platform. 
   As an example embodiment,  FIGS. 1 and 2  illustrate a turbine bucket  2  having an airfoil portion  4  and a root portion  6  with a substantially planar platform  8  at an interface between the airfoil portion and the root portion. A cooling media, such as cooling steam, is supplied from the bucket cooling circuit (schematically shown at  15 ) or platform cooling circuit (schematically shown at  14 ) to a forward cavity  12  that has been cast, machined or drilled in the forward portion of the bucket platform. Examples of cooling circuits that may serve as a source for the cooling medium in the example embodiment of  FIGS. 1-2  include the cooling circuits disclosed in U.S. Pat. Nos. 6,422,817, 6,390,774 and 5,536,143 the disclosures of which are incorporated herein by this reference. The coolant is supplied to the forward cavity through one or more passages or bores  16  or  17  connecting this cavity  12  to the airfoil steam circuit  15  or the pressure side platform cooling circuit  14 , as schematically illustrated. In this example embodiment, the high velocity steam directed to the forward cavity  12  generates high heat transfer and convection cooling. Cooling may be enhanced with bumps, dimples (hereinafter generically referred to as turbulators) in passages(s)  16 ,  17  or cavity  12  to further augment convection cooling. These turbulators are schematically illustrated in  FIG. 2  with hatch marks in cavity  12  and passages  16 ,  17 . 
   After the steam has been used to convectively cool the platform leading edge  10 , the steam is expelled through at least one opening. In the illustrated embodiment, the exit openings  18  are defined on the bucket slash face at each longitudinal end of the cooling cavity  12 . The expelled steam impinges on the adjacent bucket slash face, thereby cooling the adjacent bucket slash face as well. The coolant steam then purges the gap between the buckets, reducing the amount of hot gas path air entering the gap between buckets. This is possible with steam due to the steam pressure being much greater than the gas path pressure. 
   Another example embodiment of the invention is illustrated in  FIGS. 3 and 4 . As in the embodiment of  FIGS. 1 and 2 , a cast cavity, machined cavity or a drilled hole is defined to run along the forward portion  10  of the bucket platform  8  thereby defining a forward cavity  112 . In this example embodiment, compressor discharge air is fed via a hole or holes  116  drilled or otherwise formed to extend from the bucket shank pocket  114  to supply the cavity  112 . U.S. Pat. No. 6,431,833, the disclosure of which is incorporated herein by this reference, discloses the supply of cooling air to the shank pocket. The high velocity air through the forward cavity  112  generates high heat transfer and convection cooling. As in the  FIG. 1-2  embodiment, heat transfer can be further enhanced with turbulators, to augment the convection cooling. These turbulators are schematically illustrated in  FIG. 4  with hatch marks in cavity  112  and passage  116 . 
   After the air has been used to convectively cool the platform leading edge, the air exits via at least one exit opening. Opening may be provided at the longitudinal end(s) of the cavity. In addition or in the alternative, the exit opening(s) may include film holes  118  that extend through the platform to the suction side of the airfoil  4 , where the gas path static pressure is low enough to drive flow through the circuit. These film holes cool the leading edge suction side portion of the platform  8 . The air that exits the film holes  118  generates a layer of cool air which further insulates the platform  8  suction side from the hot gas path air. The platform gas path could also be coated with TBC, thermal barrier coating, applied in order to further reduce the heat flux into 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.