Abstract:
Embodiments of the invention relate generally to rotary machines and, more particularly, to the control of wheel space purge air in gas turbines. In one embodiment, the invention provides a turbine bucket comprising: a platform portion; an airfoil extending radially outward from the platform portion; a shank portion extending radially inward from the platform portion; an angel wing extending axially from a face of the shank portion; and a plurality of voids disposed along a length of the angel wing, each of the plurality of voids extending radially through the angel wing.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    Embodiments of the invention relate generally to rotary machines and, more particularly, to the control of wheel space purge air in gas turbines. 
         [0002]    As is known in the art, gas 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 to turn the rotor as the combustion gasses flow therethrough. 
         [0003]    Axial/radial openings at the interface between rotating buckets and stationary nozzles can allow hot combustion gasses to exit the hot gas path and radially enter the intervening wheelspace between bucket rows. To limit such incursion of hot gasses, the bucket structures typically employ axially-projecting angel wings, which cooperate with discourager members extending axially from an adjacent stator or nozzle. These angel wings and discourager members overlap but do not touch, and serve to restrict incursion of hot gasses into the wheelspace. 
         [0004]    In addition, 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. Angel wing seals therefore are further designed to restrict escape of purge air into the hot gas flowpath. 
         [0005]    Nevertheless, most gas turbines exhibit a significant amount of purge air escape into the hot gas flowpath. For example, this purge air escape may be between 0.1% and 3.0% at the first and second stage wheelspaces. The consequent mixing of cooler purge air with the hot gas flowpath results in large mixing losses, due not only to the differences in temperature but also to the differences in flow direction or swirl of the purge air and hot gasses. 
       BRIEF DESCRIPTION OF THE INVENTION 
       [0006]    In one embodiment, the invention provides a turbine bucket comprising: a platform portion; an airfoil extending radially outward from the platform portion; a shank portion extending radially inward from the platform portion; an angel wing extending axially from a face of the shank portion; and a plurality of voids disposed along a length of the angel wing, each of the plurality of voids extending radially through the angel wing. 
         [0007]    In another embodiment, the invention provides a gas turbine comprising: a diffuser; and a last stage turbine bucket adjacent the diffuser, the last stage turbine bucket including: an airfoil extending radially outward from a platform portion; a shank portion extending radially inward from the platform portion; and an angel wing extending axially from a face of the shank portion, the angel wing including a plurality of voids disposed along a length of the angel wing, each of the plurality of voids extending radially through the angel wing. 
         [0008]    In yet another embodiment, the invention provides a turbine bucket comprising: angel wing; and a plurality of voids along a length of the angel wing, wherein each of the plurality of voids includes a concave face extending radially through the angel wing and angled with respect to both a longitudinal axis of the turbine bucket and a direction of rotation of the turbine bucket. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]    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: 
           [0010]      FIG. 1  shows a schematic cross-sectional view of a portion of a known turbine; 
           [0011]      FIG. 2  shows a perspective view of a known turbine bucket; 
           [0012]      FIG. 3  shows a perspective view of a portion of a turbine bucket according to an embodiment of the invention; 
           [0013]      FIG. 4  shows an axially-inwardly looking view of a portion of the turbine bucket of  FIG. 3 ; 
           [0014]      FIG. 5  shows a radially-downward looking view of a portion of the turbine bucket of  FIG. 3 ; 
           [0015]      FIG. 6  shows a schematic view of purge air flow in a known turbine bucket; 
           [0016]      FIG. 7  shows a schematic view of purge air flow in a turbine bucket according to an embodiment of the invention; 
           [0017]      FIG. 8  shows a schematic view of a last stage turbine bucket and diffuser according to an embodiment of the invention; 
           [0018]      FIG. 9  shows a graph of swirl spike profiles at a diffuser inlet plane for known turbines and turbines according to embodiments of the invention; and 
           [0019]      FIG. 10  shows a graph of total pressure spike profiles at a diffuser inlet plane for known turbines and turbines according to embodiments of the invention. 
       
    
    
       [0020]    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 
       [0021]    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 . 
         [0022]    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. 
         [0023]    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 each of 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 . 
         [0024]    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. 
         [0025]      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 . 
         [0026]      FIG. 3  shows a perspective view of a portion of a turbine bucket  40  according to an embodiment of the invention. As can be seen in  FIG. 3 , a plurality of voids  110  extend radially through angel wing  70 . As shown in  FIG. 3 , the plurality of voids  110  is disposed axially inwardly along angel wing  70 , closer to face  62  than angel wing rim  74 . Each of the plurality of voids  110  is shown in  FIG. 4  having a rectangular cross-sectional shape (i.e., a rectangular shape looking radially inward), although this is neither necessary nor essential. As will be recognized by one skilled in the art, any number of cross-sectional shapes may be employed and are within the scope of the invention. 
         [0027]    As shown in  FIG. 3 , the plurality of voids  110  is substantially evenly disposed along a length of angel wing  70 . It is noted, however, that this is neither necessary nor essential. According to other embodiments of the invention, the plurality of voids  110  may be unevenly disposed along the length of angel wing  70 , such that voids are more numerous at one end of angel wing  70  than the other end, are more numerous toward a middle portion of angel wing  70 , or any other configuration. 
         [0028]      FIG. 4  shows an axially-inwardly looking cross-sectional view of a portion of turbine bucket  40  taken through angel wing  70 . As can be seen in  FIG. 4 , and according to one embodiment of the invention, voids  110  include a convex face  112  and a concave face  114 , forming a curved or arcuate passage through angel wing  70 . That is, voids  110  follow a path from radially outward opening  110 A, along convex face  112  and concave face  114 , to radially inward opening  110 B. Radially inward opening  110 B is thereby disposed closer to end  70 A of angel wing  70  than is radially outward opening  110 A. 
         [0029]    This curved or arcuate shape of voids  110  through angel wing  70  increases a swirl velocity of purge air between angel wing  70  and platform lip  44 . As will be explained in greater detail below, this produces a curtaining effect, restricting incursion of hot gas into wheelspace  26  ( FIG. 1 ) while simultaneously reducing the quantity of purge air escaping from wheelspace  26 . 
         [0030]      FIG. 5  shows a radially-downward looking view of a portion of turbine bucket  40 . Concave faces  114  of each void  110  can be seen. In addition, as shown in  FIG. 4 , concave faces  114  are axially angled as well. That is, concave faces  114  are angled with respect to both a longitudinal axis R L  and a direction of rotation R of turbine bucket  40 . Thus, the shape of voids  110  as they pass radially outward through angel wing  70  would impart a swirl to the purge gas, directing the purge gas both axially, toward angel wing rim  74  and laterally toward end  70 A of angel wing  70 . 
         [0031]      FIG. 6  shows a schematic view of purge air flow in a known turbine bucket. Purge air  80  is shown concentrated and having a higher swirl velocity in area  82 , closer to face  62 . In contrast,  FIG. 7  is a schematic view showing the effect of voids  110  ( FIG. 5 ) on purge air  80  according to various embodiments of the invention. Here, area  83 , in which purge air  80  is concentrated and exhibits a higher swirl velocity is distanced further from face  62 , as compared to  FIG. 6 . This, in effect, produces a curtaining effect at area  83 , restricting incursion of hot gas  95  from hot gas flowpath  28  while at the same time reducing the quantity of purge air  80  escaping from wheelspace  26  into hot gas flowpath  28 . 
         [0032]    The increases in turbine efficiencies achieved using embodiments of the invention can be attributed to a number of factors. First, as noted above, increases in swirl velocity reduce the escape of purge air into hot gas flowpath  28 , increases in swirl reduce the mixing losses attributable to any purge air that does so escape, and the curtaining effect induced by voids according to the invention reduce or prevent the incursion of hot gas into wheelspace  26 . Each of these contributes to the increased efficiencies observed. 
         [0033]    In addition, the overall quantity of purge air needed is reduced for at least two reasons. First, a reduction in escaping purge air necessarily reduces the purge air that must be replaced, which has a direct, favorable effect on turbine efficiency. Second, a reduction in the incursion of hot gas into wheelspace  26  reduces the temperature rise within wheelspace  26  and the attendant need to reduce the temperature through the introduction of additional purge air. Each of these reductions to the total purge air required reduces the demand on the other system components, such as the compressor from which the purge air is provided. 
         [0034]    While reference above is made to the ability of voids in an angel wing to change the swirl velocity of purge air within a wheelspace, and particularly within a wheelspace adjacent early stage turbine buckets, it should be noted that such angel wing voids may be employed on turbine buckets of any stage with similar changes to purge air swirl velocity and angle. In fact, Applicants have noted a very favorable result when angel wing rim voids are employed in the last stage bucket (LSB). 
         [0035]    Spikes in total pressure (P T ) and swirl profiles at the inner radius region of the diffuser inlet are a consequence of a mismatch between the hot gas flow and the swirl of purge air exiting the wheelspace adjacent the LSB. Applicants have found that angel wing voids according to various embodiments of the invention are capable of both increasing P T  spikes at a diffuser inlet close to the inner radius while at the same time decreasing swirl spikes at or near the same location. Each of these improves diffuser performance. Angel wing voids, for example, have been found to change the swirl angle of purge air exiting the LSB wheelspace by 1-3 degrees while also increasing P T  spikes by 15-30%. 
         [0036]      FIG. 8  shows a schematic view of a LSB  140  adjacent diffuser  300 . Hot gas  195  enters diffuser  300  at diffuser inlet plane  310  and passes toward struts  320 . Voids according to embodiments of the invention reduce the swirl mismatch of purge air as it combines with hot gas  195 , preventing separation of hot gas  195  as it enters struts  320 . At the same time, voids increase the P T  spike. 
         [0037]      FIG. 9  shows a graph of swirl spike as a function of diffuser inlet plane height. Profile A represents a swirl spike profile for a turbine having angel wing voids according to embodiments of the invention. Profile B represents a swirl spike profile for a turbine having angel wings known in the art. Profile A exhibits a marked decrease in swirl spike at a radially inward position of the diffuser inlet plane. 
         [0038]      FIG. 10  shows a graph of P T  spike as a function of diffuser inlet plane height. Profile A represents a P T  spike profile for a turbine having angel wing voids according to embodiments of the invention. Profile B represents a P T  spike profile for a turbine having angel wings known in the art. Profile A exhibits an increase in P T  spike at a radially inward position of the diffuser inlet plane. 
         [0039]    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. 
         [0040]    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.