Abstract:
Embodiments of the invention relate generally to rotary machines and, more particularly, to the cooling of at least portions of a turbine bucket. In one embodiment, the invention provides a method of cooling at least a portion of a turbine bucket, the method comprising: during operation of a turbine, altering a swirl velocity of purge air beneath a platform lip extending axially from the platform, wherein altering the swirl velocity of the purge air includes interrupting a flow of the purge air with a plurality of voids disposed along a length of an angel wing extending axially from a face of a shank portion of the turbine bucket.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation-in-part of co-pending U.S. patent application Ser. No. 14/603,316 filed 22 Jan. 2015, which is incorporated herein as though fully set forth. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    Embodiments of the invention relate generally to rotary machines and, more particularly, to the cooling of at least portions of a turbine bucket. 
         [0003]    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. 
         [0004]    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. 
         [0005]    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. 
         [0006]    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. 
         [0007]    In addition, the mixing of purge air and the hot gas flow results in a more chaotic flow of gasses across the platform of the turbine bucket. This increase in chaotic gas flow results in unequal heating of the platform during operation of the turbine, with attendant increases in thermal stresses to the platform and a resultant shortening of the working life of the turbine bucket. 
       BRIEF DESCRIPTION OF THE INVENTION 
       [0008]    In one embodiment, the invention provides a method of cooling at least a portion of a turbine bucket, the method comprising: during operation of a turbine, altering a swirl velocity of purge air beneath a platform lip extending axially from the platform, wherein altering the swirl velocity of the purge air includes interrupting a flow of the purge air with a plurality of voids disposed along a length of an angel wing extending axially from a face of a shank portion 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 an axially-facing view of a portion of a turbine bucket suitable for use according to an embodiment of the invention; 
           [0013]      FIG. 4  shows a schematic view of a turbulator suitable for use according to various embodiments of the invention; 
           [0014]      FIG. 5  shows a perspective view of the operational heating of a known turbine bucket; 
           [0015]      FIG. 6  shows a perspective view of the operational heating of a turbine bucket according to embodiments of the invention; 
           [0016]      FIGS. 7-10  show schematic views of turbulators suitable for use according to various embodiments of the invention; 
           [0017]      FIG. 11  shows an axially-facing view of a portion of a turbine bucket suitable for use according to another embodiment of the invention; 
           [0018]      FIGS. 12 and 13  show perspective views of portions of turbine buckets suitable for use according to still other embodiments of the invention; 
           [0019]      FIG. 14  shows a schematic view of purge air flow in relation to a typical turbine bucket; 
           [0020]      FIG. 15  shows a schematic view of purge air flow in relation to a turbine bucket according to an embodiment of the invention; 
           [0021]      FIG. 16  shows a cross-sectional side view of a portion of a turbine bucket suitable for use according to an embodiment of the invention; 
           [0022]      FIG. 17  shows a perspective view of the portion of the turbine bucket of  FIG. 16 ; 
           [0023]      FIG. 18  shows a perspective view of a portion of a turbine bucket suitable for use according to another embodiment of the invention; 
           [0024]      FIG. 19  shows a perspective view of a portion of a turbine bucket suitable for use according to yet another embodiment of the invention; 
           [0025]      FIGS. 20-26  show perspective views of turbine buckets suitable for use according to still other embodiments of the invention; 
           [0026]      FIG. 27  shows a perspective view of a portion of a turbine bucket suitable for use according to an embodiment of the invention; 
           [0027]      FIG. 28  shows a radially inward view of a portion of the turbine bucket of  FIG. 27 ; 
           [0028]      FIG. 29  shows a perspective view of a portion of a turbine bucket suitable for use according to another embodiment of the invention; 
           [0029]      FIG. 30  shows a perspective view of a portion of a turbine bucket suitable for use according to yet another embodiment of the invention; 
           [0030]      FIG. 31  shows a cross-sectional side view of the turbine bucket of  FIG. 30 ; 
           [0031]      FIG. 32  shows a perspective view of a portion of a turbine bucket according to an embodiment of the invention; 
           [0032]      FIG. 33  shows an axially-inwardly looking view of a portion of the turbine bucket of  FIG. 32 ; 
           [0033]      FIG. 34  shows a radially-downward looking view of a portion of the turbine bucket of  FIG. 32 ; 
           [0034]    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 between the drawings. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0035]    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 . 
         [0036]    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. 
         [0037]    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 . 
         [0038]    As shown in  FIG. 1 , nozzle surface  30  and discourager member  32  each serves to restrict the escape of purge air and the incursion of hot gasses. In other embodiments of the invention, a separate discourager member, similar to discourager member  32 , may be provided between angel wing seal  70  and nozzle surface  30  to provide such function. 
         [0039]    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. 
         [0040]      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 . 
         [0041]      FIG. 3  shows a schematic view of bucket  40  looking axially toward face  62 . As can be seen, bucket  40  includes a plurality of turbulators  110 , which, as described in greater detail below, may extend axially outward from face  62  and/or radially inward from a radially inner surface  46  of platform lip  44 . As will also be described in greater detail below, turbulators may be of any number of shapes and orientations. 
         [0042]    For example,  FIG. 4  shows a detailed view of lip with turbulators  110 , which comprise a first concave face  114  opening toward an intended direction of rotation R of bucket  40  ( FIG. 3 ), a second convex face  116  opposite first concave face  114 , and a radially inner face  118  between first and second concave faces  114 ,  116 . These faces  112 ,  114 ,  118  form a body  112  of each turbulator  110 . In the embodiment of  FIG. 4 , each turbulator  110  forms a rib-like member extending radially inward from radially inner surface  46  of platform lip  44 . In other embodiments of the invention, turbulators may be separated from radially inner surface  46  of platform lip  44  and extend axially outward from face  62  ( FIG. 3 ). In other embodiments the turbulators may be attached to either or both of the radially inner surface  46  of platform lip  44  or face  62  of shank  60 . In either case, one or more turbulator  110  may be axially angled, such that, for example, first concave face  114  extends from face  62  at an angle, positive or negative, relative to a longitudinal axis of the turbine. Embodiments of the invention employing axially angled turbulators typically include one or more turbulators which, when installed, are angled ±70 degrees relative to the longitudinal axis of the turbine. 
         [0043]    Turbulators  110  draw in purge air and increase its swirl velocity. Generally, a circumferential velocity of purge air coming out of the wheel space cavity is 0.2-0.4 times the local circumferential speed of an adjacent rotor surface. Turbulators according to embodiments of the invention increase this by 0.9-1.1 times by imparting a force onto the purge flow passing through it. This results in a small loss of torque, but regains a much larger favorable torque force when this flow goes through the main bucket  40  and a net gain in efficiency of approximately 0.5% at the turbine stage. This gain is a consequence of both the increased purge air circumferential swirl velocity, which produces a curtaining effect against the ingestion of hot gasses into the wheel space cavity, described further below, as well as a change in a circumferential angle of the purge air onboarding onto the main flow path of the turbine. This change in circumferential angle results in the purge air being better aligned with the hot gas flow, resulting in significantly reduced mixing losses when purge air escapes from wheelspace  26  ( FIG. 1 ) to hot gas flowpath  28  ( FIG. 1 ). 
         [0044]    This better alignment of purge air and hot gas flow reduces the flow instability of a flow shear layer and the alternating pockets of low- and high-pressure circumferentially across the opening of wheelspace  26 . This results in a reduction of hot gas ingestion and a more even distribution of the film of cold purge air onboarding to the main flowpath  28  across platform  42  ( FIG. 1 ). This film forms a shield between the hot gasses and the metal surface of platform  42 . This reduces “hot spots” across platform  42 . Such a reduction of hot spots may include a reduction in hot spot size, number, temperature, or all three. As will be explained in greater detail below, this reduction results in a decrease in the overall temperature of platform  42 , thereby cooling platform  42 , platform lip  44 , shank face  62 , and airfoil  50 , and produces a more uniform heating of platform  42 . This in turn reduces thermal gradient induced stresses, increasing life of the component and reducing cooling requirements of platform  42  during operation. 
         [0045]      FIGS. 5 and 6  show perspective views of a bucket  40  during operation with and without, respectively, the turbulators according to embodiments of the invention. In  FIGS. 5 and 6 , the airfoil  50  and platform  42  are shown separately, merely for purposes of simplicity and explanation. In  FIG. 5 , a plurality of hot spots  43 A,  43 B,  43 C,  43 D can be seen along platform  42 , a consequence of chaotic or unreduced mixing of purge air and hot gas flow, as is typical of known devices and methods. Similar hot spots  53 A,  53 B,  53 C can be seen along airfoil  50 , generally extending upward from platform  42  to about 20% of the overall length of airfoil  50 . These hot spots  43 A,  43 B,  43 C,  43 D,  53 A,  53 B,  53 C can reach temperatures in excess of 1700° F. and can cover a majority of the surface area of platform  42  and the proximal 20% of airfoil  50 . What is more, the temperature differential between these hot spots  43 A,  43 B,  43 C,  43 D,  53 A,  53 B,  53 C and other portions of platform  42  and airfoil  50  can be more than 600° F. In  FIG. 6 , a reduction in mixing of purge air and hot gas flow, according to embodiments of the invention, has resulted in a more even distribution of the film of cold purge gasses across platform  42 , resulting in a more even cooling  45  of platform  42  and a more even cooling  55  of airfoil  50 . Although temperature differences may still be observed across platform  42  and the proximal portion of airfoil  50 , a larger portion of the surface area of platform  42  and airfoil  50  has a lower temperature and the temperature differential across these surfaces is significantly reduced. In some cases, the lowest recorded temperature was reduced from about 1400° F. ( FIG. 5 ) to about 1300° F. ( FIG. 6 ) and the highest recorded temperature reduced from about 2000° F. ( FIG. 5 ) to about 1800° F. ( FIG. 6 ). Some degree of improved cooling was also observed on platform lip  44  and shank face  62 . 
         [0046]    What is more, because larger portions of these surfaces were subjected to lower temperatures, the average temperature to which the overall surfaces were subjected, was reduced. This more even heating  45 ,  55  of platform  42  and airfoil  50 , respectively, reduces thermal stresses to which these components are subjected, thereby extending its working life. 
         [0047]    The concave turbulators in  FIG. 4  are but one embodiment capable of reducing the mixing losses of purge air and hot gas flow.  FIGS. 7-10 , for example, show turbulators having different configurations. In  FIG. 7 , first and second faces  214 ,  216  are substantially straight and radially inner face  218  is substantially perpendicular to both first and second faces  214 ,  216 , such that body  212  is substantially rectangular in cross-section. In other embodiments the rectaungular projections may be angled to the radial or axial plane. In  FIG. 8 , each of first and second faces  314 ,  316  are substantially straight but radially non-perpendicularly angled, such that body  312  has a substantially trapezoidal cross-sectional shape, with the wider dimension disposed radially inward. In  FIG. 9 , on the other hand, first and second faces  414 ,  416  are radially non-perpendicularly angled such that body  412  has a substantially trapezoidal cross-sectional shape, with the narrower dimension disposed radially inward. In  FIG. 10 , each turbulator  510  is formed by the intersection of radially inner surface  518  and at least one adjacent arcuate face  514 ,  516  disposed on either side of radially inner surface  518 . End faces  515 ,  517  are substantially straight and extend radially from platform lip  44 , thereby enclosing the plurality of turbulators  510 . 
         [0048]    As noted above, turbulators according to embodiments of the invention may extend axially outward from face  62  and/or radially inward from a radially inner surface  46  of platform lip  44 . Where turbulators extend axially outward from face  62 , improvements in turbine efficiency are higher the nearer the turbulators are to the radially inner surface  46  of platform lip  44 . That is, as turbulators are moved radially inward and away from inner surface  46  of platform lip  44 , gains in efficiency are reduced. As will be described in greater detail below with respect to  FIGS. 14 and 15 , this effect is attributable to the combined ability of platform lip  44  and the turbulators to throw the purge air with the greatest velocity axially away from the shank face  62 , which generates a curtaining effect against the hot gas ingestion into the wheel space cavity, which reduces the incursion of hot gas into wheelspace  26  ( FIG. 1 ). Increasing the space between the turbulators and the platform lip  44  steadily reduces this curtaining effect induced. 
         [0049]      FIG. 11  shows a view of a portion of bucket  40  looking axially toward face  62 . As can be seen in  FIG. 11 , each of the plurality of turbulators  110  is axially angled, such that at least first concave face  614  of each turbulator  110  is not normal to face  62 . As noted above, such an embodiment may result in a change in the swirl angle of the purge air. 
         [0050]      FIGS. 12 and 13  show perspective views of portions of turbine buckets according to still other embodiments of the invention. In  FIG. 12 , a plurality of turbulators  710  is formed (e.g., machined, cast, etc.) from additional material extending radially inward from platform lip  44 . Typically, such additional material will be included in platform lip  44  at the time of casting, with subsequent machining of the cast material employed to form turbulators  710 . In other embodiments of the invention, turbulators may be provided in a separate material that is welded, fastened, or otherwise secured to platform lip  44 . Turbulators may contact or be axially spaced from face  62 . In  FIG. 13 , for example, turbulators  810  similarly extend from radially inward from platform lip  44  but are axially spaced from face  62 , which, in the embodiment shown, is curved. These projections of the turbulators may be angled to the radial and/or axial plane. 
         [0051]    Although the turbulators  710 ,  810  shown in  FIGS. 12 and 13 , respectively, are shown having a substantially rectangular cross-sectional shape, this is neither necessary nor essential. Such turbulators, may have any number of cross-sectional shapes, including, for example, those described above with respect to  FIGS. 4 and 7-10 . Similarly, any such turbulators may be axially angled, as described above with respect to  FIG. 11 . 
         [0052]      FIGS. 14 and 15  show, respectively, schematic representations of purge gas flows in a known gas turbine and in a gas turbine including turbulators according to embodiments of the invention. In  FIG. 14 , purge air  80  is shown and has a low axial momentum and the extent of its reaches is confined to area  82 , where it forms a vortex and eventually escapes into the hot gas flowpath  28 . The concentration of purge air  80  thrown out axially from the blade shank surface due to its natural curvature towards area  82 , is only confined to distances closer to face  62 , which allows for incursion of hot gas  95  into wheelspace  26 . 
         [0053]    In contrast,  FIG. 15  shows the effect of turbulators  110 - 810  on purge air  80  according to various embodiments of the invention. As can be seen in  FIG. 15 , the area  83  in which purge air is thrown out with higher axial momentum/velocity is distanced further from face  62 . In addition, this area  83  of purge air has been moved axially away from face  62 , as compared to  FIG. 14 . At the same time, any escaping purge air  85  has been moved away from platform lip  44  ( FIG. 12-13 ) toward nozzle  30 . This, in effect, produces a curtaining effect, restricting incursion of hot gas  95  from hot gas flowpath  28  and eventually escapes from wheelspace  26  into hot gas flowpath  28 . Hence, because of the enhanced curtaining/sealing effectiveness of these embodiments presented here, implementing these could lower the purge flow requirement still retaining same/higher sealing effecting against hot gas ingestion into the wheel-space cavity. 
         [0054]    In addition, as a result of the lower hot gas ingestion, additional components in vicinity of the wheelspace  26 , including nozzle surface  30 , are cooled. Typically, embodiments of the invention have been shown to cool nozzle surface  30  by 100° F. to 400° F. 
         [0055]    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 of purge air into hot gas flowpath  28  reduce the mixing losses attributable to purge air. Further, the curtaining effect induced by turbulators according to the invention reduce or prevent the incursion of hot gas  95  into wheelspace  26 , and prevents heating of wheel space cavity due to less or no hot gas ingestion. Each of these contributes to the increased efficiencies observed. 
         [0056]    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, and has a direct, favorable effect on turbine efficiency. Second, a reduction in the incursion of hot gas  95  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 other system components, such as the compressor from which the purge air is provided. 
         [0057]    The lower temperatures in the bucket platform  42 , the platform lip  44  and the bucket shank face and a more even distribution of the film of cold purge gasses across platform  42  may be achieved according to other embodiments as well. For example,  FIG. 16  shows a cross-sectional side view of a portion of a turbine bucket  40  according to an embodiment of the invention. As can be seen in  FIG. 16 , a distal end  48  of platform lip  44  is angled radially outward toward airfoil  50 . 
         [0058]      FIG. 17  shows a perspective view of the bucket  40  of  FIG. 3 . A plurality of voids  110  are provided along distal end  148  of platform lip  144 . As shown in  FIG. 17 , voids  110  are substantially trapezoidal in shape, although this is neither necessary nor essential. Voids having other shapes may also be employed, including, for example, rectangular, rhomboid, or arcuate shapes. 
         [0059]    For example,  FIG. 18  shows a perspective view of a bucket  40  according to another embodiment of the invention. Here, platform lip  144  extends axially from platform  42  (i.e., a distal end is not angled toward airfoil  50 , as in  FIGS. 3 and 4 ). Voids  210  extend through platform lip  144  in an arcuate path such that remaining portions of platform lip  144  adjacent voids  210  include an arcuate face  145 . 
         [0060]    The embodiment of the invention shown in  FIG. 19  shows a perspective view of bucket  40 . Here, platform lip  144  includes an angled distal end  48 , as in  FIGS. 16 and 17 . However, voids  310  are formed in a body  146  of platform lip  144  rather than at its distal end  148 . As noted above, voids  310  may take any number of shapes, including, for example, rectangular, trapezoidal, rhomboid, arcuate, etc. 
         [0061]      FIGS. 20-22  show perspective views of other embodiments of the invention. In  FIG. 20 , voids  410  are elliptical in shape and angled with respect to a radial axis of bucket  40 . 
         [0062]    In  FIG. 21 , elliptical voids  510  of differing sizes are employed with void size increasing along platform lip  144  from an end nearer the concave trailing face toward the convex leading face of airfoil  50 . In such an embodiment, the effect of voids  510  on purge air between platform lip  144  and angel wing  70  will generally be more pronounced adjacent the larger voids. This may be desirable, for example, where the amount of purge flow passing circumferentially over platform  42  needs to be controlled for various reasons, for example, to make the cooling more uniform by pushing more cold purge flow where a hot spot is expected on platform  42 . 
         [0063]    In  FIG. 22 , elliptical voids  510  of differing size are employed with void size decreasing along platform lip  144  from an end nearer the concave trailing face toward the convex leading face of airfoil  50 . As should be recognized from the discussion above, such an embodiment may be desirable, for example, where a loss of purge air or an incursion of hot gas is greater in the area of the larger voids. 
         [0064]      FIGS. 23-26  show perspective views of turbine buckets  40  in accordance with various embodiments of the invention. In each of the embodiments in  FIGS. 23-26 , voids are disposed unevenly along platform lip  144 . 
         [0065]    In  FIG. 23 , a plurality of substantially rectangular voids  610  are disposed along platform lip  144  nearer the convex leading face than the concave trailing face of airfoil  50 . 
         [0066]    In  FIG. 24 , the area of void concentration is opposite that in  FIG. 23 , with the plurality of substantially rectangular voids  610  disposed along platform lip  144  nearer the concave trailing face than the convex leading face of airfoil  50 . 
         [0067]      FIGS. 25 and 26  show embodiments similar to those in  FIGS. 23 and 24 , respectively, in which voids  710  are notches of material removed from an edge of platform lip  144  ( FIG. 22 ). The use of voids  710  on the edge of platform lip  144  may be employed, for example, to direct purge air toward either convex leading face or concave trailing face of airfoil  50 . 
         [0068]    The more even distribution of the film of cold purge gasses across platform  42  may be achieved according to still other embodiments as well. For example,  FIG. 27  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. 27 , a plurality of voids  910  are disposed along an angel wing rim  174  at a distal end  178  of angel wing  170 . Voids  910  are spaced along angel wing rim  174  such that the remaining portions of angel wing rim  174  form a plurality of column members  175 . As shown in  FIG. 27 , voids  910  are radially angled, i.e., angled with respect to a radial axis (Ar) of turbine bucket  40 , although this is neither necessary nor essential. In other embodiments of the invention, voids may be substantially parallel to a radial axis of the turbine bucket. 
         [0069]    As shown most clearly in  FIG. 28 , a radially-inward looking view of turbine bucket  40 , column members  175  (and correspondingly voids  910 ) include arcuate faces. Specifically, column members  175  include a concave face  175 A (a convex face of void  910 ) and a convex face  175 B (a concave face of void  910 ). As such, void  910  includes a first opening  910 A along an axially inner surface  174 A of angel wing rim  174  disposed laterally to a second opening  910 B along an axially outer surface  174 B of angel wing rim  174 . It should be understood, of course, that column members and voids may have other shapes. For example, column members and voids may include rectangular, trapezoidal, or any other cross-sectional shape. 
         [0070]      FIG. 29  shows a perspective view of a portion of a turbine bucket  40  according to another embodiment of the invention. Here, a plurality of dam members  277 , which are adjacent to the radially outer surface of the angel wing seal, extend axially from shank portion  60  to each of the plurality of column members  275 . According to some embodiments, dam members  277  may be angled with respect to a radial axis of turbine bucket  40 , i.e., angled positively or negatively with respect to the direction of rotation of turbine bucket  40 . Similarly, according to some embodiments, dam members  277  may include one or more arcuate faces, as do column members  275 , or may include rectangular, trapezoidal, or any other cross-sectional shape, such as described above. 
         [0071]      FIG. 30  shows a perspective view of a portion of a turbine bucket  40  according to another embodiment of the invention. Here, a continuous angel wing rim  374  extends upward from angel wing seal  370  and a plurality of dam members  377  extend axially from rim  374  toward but not contacting face  62 , leaving a gap  64  adjacent face  62 . 
         [0072]      FIG. 31  shows a cross-sectional side view of turbine bucket  40  of  FIG. 30  with respect to a nozzle surface  130  according to an embodiment of the invention. In  FIG. 31 , nozzle surface  130  comprises or includes a porous or erodible portion along at least a radially inward surface, such that angel wing rim  374  cuts or wears a groove  131  into nozzle surface  130 . The porous or erodible portion of nozzle surface  130  may comprise the material of nozzle surface  130  in a “honey comb” or similar pattern, such that the porous or erodible portion is subject to wear or erosion by angel wing rim  374 . In other embodiments of the invention, the porous or erodible portion of nozzle surface  130  may comprise or include a material that is softer than the other material(s) of nozzle surface  130 , such that the porous or erodible portion is similarly subject to wear or erosion by angel wing rim  374 . 
         [0073]    In operation, purge air  80  passes into groove  131  of nozzle surface  130  and then downward between dam members  377 , toward face  62 . Purge air  80  then flows circumferentially within gap  64 , adjacent face  62 , as turbine bucket  40  rotates, providing increased swirl to purge air  80 . 
         [0074]    As should be apparent from the description above, other modifications to the angel wing may be employed reduce to mixing between purge air and hot gas flow achieve a more even distribution of the hot gas flow across platform  42 . For example,  FIG. 32  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. 32 , a plurality of voids  1110  extend radially through angel wing  470 . As shown in  FIG. 32 , the plurality of voids  1110  is disposed axially inwardly along angel wing  470 , closer to face  62  than angel wing rim  474 . Each of the plurality of voids  1110  is shown in  FIG. 32  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. 
         [0075]    As shown in  FIG. 32 , the plurality of voids  1110  is substantially evenly disposed along a length of angel wing  470 . It is noted, however, that this is neither necessary nor essential. According to other embodiments of the invention, the plurality of voids  1110  may be unevenly disposed along the length of angel wing  470 , such that voids are more numerous at one end of angel wing  470  than the other end, are more numerous toward a middle portion of angel wing  470 , or any other configuration. 
         [0076]      FIG. 33  shows an axially-inwardly looking cross-sectional view of a portion of turbine bucket  40  taken through angel wing  470 . As can be seen in  FIG. 33 , and according to one embodiment of the invention, voids  1110  include a convex face  1112  and a concave face  1114 , forming a curved or arcuate passage through angel wing  470 . That is, voids  1110  follow a path from radially outward opening  1110 A, along convex face  1112  and concave face  1114 , to radially inward opening  1110 B. Radially inward opening  1110 B is thereby disposed closer to end  470 A of angel wing  470  than is radially outward opening  1110 A. 
         [0077]    This curved or arcuate shape of voids  1110  through angel wing  470  increases a swirl velocity of purge air between angel wing  470  and platform lip  44 . As explained above in accordance with other embodiments of the invention, 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 . 
         [0078]      FIG. 34  shows a radially-downward looking view of a portion of turbine bucket  40 . Concave faces  1114  of each void  1110  can be seen. In addition, as shown in  FIG. 32 , concave faces  1114  are axially angled as well. That is, concave faces  1114  are angled with respect to both a longitudinal axis RL and a direction of rotation R of turbine bucket  40 . Thus, the shape of voids  110  as they pass radially outward through angel wing  470  would impart a swirl to the purge gas, directing the purge gas both axially, toward angel wing rim  474  and laterally toward end  470 A of angel wing  470 . 
         [0079]    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.