Abstract:
Embodiments of the invention relate generally to rotary machines and, more particularly, to the reducing mixing of packing leakage and the main flow of hot gas or steam in gas and steam turbines, respectively. In one embodiment, the invention provides a turbine bucket comprising: a platform portion; an airfoil extending radially outward from the platform portion; and at least one recess extending radially inward into the platform portion, the at least one recess being disposed at an angle relative to a leading edge of the platform portion.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    Embodiments of the invention relate generally to rotary machines and, more particularly, to the reducing mixing of packing leakage and the main flow of hot gas or steam in gas and steam turbines, respectively. 
         [0002]    As is known in the art, turbines employ rows of buckets on the wheels/disks of a rotor assembly, which alternate with rows of stationary vanes on a stator or nozzle assembly. These alternating rows extend axially along the rotor and stator and allow combustion gasses or steam to turn the rotor as the combustion gasses or steam flow therethrough. 
         [0003]    Axial/radial openings at the interface between rotating buckets and stationary nozzles can allow hot combustion gasses or steam to exit the main flow and radially enter the intervening wheelspace between bucket rows. In gas turbines, cooling air or “purge air” is often introduced into the wheelspace between bucket rows. This purge air serves to cool components and spaces within the wheelspaces and other regions radially inward from the buckets as well as providing a counter flow of cooling air to further restrict incursion of hot gasses into the wheelspace. Nevertheless, incursion of combustion gasses or steam into the wheelspaces between bucket rows contributes to decreased turbine efficiency of between about 1% and about 1.5%. 
       BRIEF DESCRIPTION OF THE INVENTION 
       [0004]    In one embodiment, the invention provides a turbine bucket comprising: a platform portion; an airfoil extending radially outward from the platform portion; and at least one recess extending radially inward into the platform portion, the at least one recess being disposed at an angle relative to a leading edge of the platform portion. 
         [0005]    In another embodiment, the invention provides a turbine comprising: a first turbine bucket including: a first platform portion; a first airfoil extending radially outward from the first platform portion; and at least one recess extending radially inward into the first platform portion, the at least one recess being disposed at an angle relative to a leading edge of the first platform portion; and a second turbine bucket including: a second platform portion; a second airfoil extending radially outward from the second platform portion; and at least one recess extending radially inward into the first platform portion, the at least one recess being disposed at an angle relative to a leading edge of the second platform portion. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]    These and other features of this invention will be more readily understood from the following detailed description of the various aspects of the invention taken in conjunction with the accompanying drawings that depict various embodiments of the invention, in which: 
           [0007]      FIG. 1  shows a schematic cross-sectional view of a portion of a known gas turbine; 
           [0008]      FIG. 2  shows a perspective view of the gas turbine of  FIG. 1 ; 
           [0009]      FIG. 3  shows a perspective view of a pair of turbine buckets according to an embodiment of the invention; 
           [0010]      FIG. 4  shows a radially-inward looking schematic view of turbine buckets according to an embodiment of the invention; 
           [0011]      FIG. 5  shows the turbine buckets of  FIG. 4  in relation to hot gas flow; and 
           [0012]      FIG. 6  shows a schematic view of a steam turbine bucket according to en embodiment of the invention. 
       
    
    
       [0013]    It is noted that the drawings of the invention are not to scale. The drawings are intended to depict only typical aspects of the invention, and therefore should not be considered as limiting the scope of the invention. In the drawings, like numbering represents like elements among the drawings. 
       DETAILED DESCRIPTION OF THE INVENTION 
       [0014]    Turning now to the drawings,  FIG. 1  shows a schematic cross-sectional view of a portion of a gas turbine  10  including a bucket  40  disposed between a first stage nozzle  20  and a second stage nozzle  22 . Bucket  40  extends radially outward from an axially extending rotor (not shown), as will be recognized by one skilled in the art. Bucket  40  comprises a substantially planar platform  42 , an airfoil extending radially outward from platform  42 , and a shank portion  60  extending radially inward from platform  42 . 
         [0015]    Shank portion  60  includes a pair of angel wing seals  70 ,  72  extending axially outward toward first stage nozzle  20  and an angel wing seal  74  extending axially outward toward second stage nozzle  22 . It should be understood that differing numbers and arrangements of angel wing seals are possible and within the scope of the invention. The number and arrangement of angel wing seals described herein are provided merely for purposes of illustration. 
         [0016]    As can be seen in  FIG. 1 , nozzle surface  30  and discourager member  32  extend axially from first stage nozzle  20  and are disposed radially outward from angel wing seals  70  and  72 , respectively. As such, nozzle surface  30  overlaps but does not contact angel wing seal  70  and discourager member  32  overlaps but does not contact angel wing seal  72 . A similar arrangement is shown with respect to discourager member  32  of second stage nozzle  22  and angel wing seal  74 . In the arrangement shown in  FIG. 1 , during operation of the turbine, a quantity of purge air may be disposed between, for example, nozzle surface  30 , angel wing seal  70 , and platform lip  44 , thereby restricting both escape of purge air into hot gas flowpath  28  and incursion of hot gasses from hot gas flowpath  28  into wheelspace  26 . 
         [0017]    While  FIG. 1  shows bucket  40  disposed between first stage nozzle  20  and second stage nozzle  22 , such that bucket  40  represents a first stage bucket, this is merely for purposes of illustration and explanation. The principles and embodiments of the invention described herein may be applied to a bucket of any stage in the turbine with the expectation of achieving similar results. 
         [0018]      FIG. 2  shows a perspective view of a portion of bucket  40 . As can be seen, airfoil  50  includes a leading edge  52  and a trailing edge  54 . Shank portion  60  includes a face  62  nearer leading edge  52  than trailing edge  54 , disposed between angel wing  70  and platform lip  44 . 
         [0019]      FIG. 3  shows a perspective view of a pair of buckets  140 ,  240  according to an embodiment of the invention. Here, bucket  140  includes a pair of recesses  192 ,  194  along platform  142  adjacent leading edge  152  of airfoil  150 . Specifically, platform  142  includes an upstream recess  192  and a downstream recess  194 . Platform  242  includes a downstream recess  294  along platform  242  adjacent leading edge  252  of airfoil  250  and upstream recess  192  of bucket  140 . 
         [0020]    Recesses  192 ,  194 ,  294  may be machined into platforms  142 ,  242  according to any known or later-developed method. Alternatively, recesses  192 ,  194 ,  294  may be cast as part of platforms  142 ,  242 . 
         [0021]      FIG. 4  shows a radially-inward looking schematic view of three buckets  140 ,  240 ,  340  according to an embodiment of the invention. As in  FIG. 3 , upstream recess  192 , extends from leading edge  146  to upstream edge  145  of platform  142 . Upstream recess  192  is adjacent downstream recess  294 , which extends from leading edge  246  to downstream edge  247  of platform  242 . Similarly, upstream recess  292  extends from leading edge  246  to upstream edge  245  of platform  242 . Upstream recess  292  is adjacent downstream recess  394 , which extends from leading edge  346  to downstream edge  347  of platform  342 . 
         [0022]      FIG. 5  shows a radially-inward looking schematic view of buckets  140 ,  240 ,  340  with respect to the flow of hot gas  280 ,  380 . Recesses  192 ,  294 ,  292 ,  394  alter the flow of hot gas  280 ,  380 . Specifically, recesses  192 ,  294 ,  292 ,  394  act to alter a swirl of hot gas  280 ,  380 , which is directed around a leading face  253 ,  353  of airfoils  250 ,  350 , respectively. Directing hot gas  280  around leading face  253  of airfoil  250  reduces incursion of hot gas  280  between platforms  142  and  242  and into wheelspace  26  ( FIG. 1 ). The reduction in incursion of hot gas  280  into wheelspace  26  improves turbine efficiency. Typically, turbine efficiency is improved by up to about 0.08% where recesses according to embodiments of the invention are employed in high-pressure and/or intermediate-pressure stages of a gas turbine. 
         [0023]    The extent to which the swirl of hot gas  280 ,  380  is altered depends, for example, on the depth to which recesses  192 ,  294 ,  292 ,  394  extend radially inward into platforms  142 ,  242 ,  342 . Typically, recesses  192 ,  294 ,  292 ,  394  extend radially inward into platforms  142 ,  242 ,  342  to a depth up to about 100 mil (i.e., about 0.1 inch), e.g., to a depth between about 10 mil and about 100 mil, or between about 20 mil and about 90 mil, or between about 30 mil and about 80 mil, or between about 40 mil and about 70 mil, or between about 50 mil and about 60 mil. 
         [0024]    Similarly, the extent to which the swirl of hot gas  280 ,  380  is altered depends on the angles at which recesses  192 ,  294 ,  292 ,  394  are disposed relative to platform leading edges  146 ,  246 ,  346 . Upstream recesses  192 ,  292 ,  392  are typically angled between about 45° and about 80° relative to platform leading edges  146 ,  246 ,  346 . Downstream recesses  194 ,  294 ,  394  are typically angled between about 90° and about 120° relative to platform leading edges  146 ,  246 ,  346 . As described herein and as shown in  FIGS. 3-5 , the angles of recesses  192 ,  294 ,  292 ,  394  are angled as measured from upstream edge  145 ,  245 ,  345 . 
         [0025]    The principle of operation of the platform recesses described above with respect to the operation of gas turbines may is also applicable to the operation of steam turbines. For example,  FIG. 6  shows a schematic side view of a steam turbine bucket  440  according to an embodiment of the invention. Magnified views A and B show radially-inward looking views of platform  442  adjacent, respectively, upstream edge  445  and downstream edge  447 . In magnified view A, upstream recess  492  is shown angled at angle α relative to leading edge  446 . In magnified view B, downstream recess  492  is shown angled at angle β relative to leading edge  446 . 
         [0026]    As noted above with respect to  FIGS. 3-5 , upstream recess  492  and downstream recess  494  extend radially inward into platform  442  to a depth up to about 100 mil, e.g., to a depth between about 10 mil and about 100 mil, or between about 20 mil and about 90 mil, or between about 30 mil and about 80 mil, or between about 40 mil and about 70 mil, or between about 50 mil and about 60 mil. Increases in the efficiencies of steam turbines employing platform recesses according to embodiments of the invention are similar to those described above with respect to gas turbines. Typically, increases in efficiency of up to about 0.08% are observed. 
         [0027]    As used herein, the singular forms “a,” “an,” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” and/or “comprising,” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. 
         [0028]    This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any related or incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal language of the claims.