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; at least one angel wing extending axially from a face of the shank portion, the at least one angel wing including an angel wing rim extending radially upward toward the airfoil; and a plurality of voids disposed along the angel wing rim.

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; at least one angel wing extending axially from a face of the shank portion, the at least one angel wing including an angel wing rim extending radially upward toward the airfoil; and a plurality of voids disposed along the angel wing rim. 
         [0007]    In another 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; at least one angel wing extending axially from a face of the shank portion, the at least one angel wing including an angel wing rim extending radially upward toward the airfoil; a plurality of voids disposed along the angel wing rim; and a plurality of dam members extending axially inward from the angel wing rim toward the face of the shank portion. 
         [0008]    In yet another embodiment, the invention provides a gas turbine comprising: at least one turbine bucket extending radially outward from a rotating shaft, the at least one turbine bucket including: an airfoil extending radially outward from a platform; a shank portion extending radially inward from the platform portion; and at least one angel wing seal extending axially from a face of the shank portion, the at least one angel wing seal having an angel wing rim extending radially upward toward the airfoil; and a nozzle surface disposed radially outward from the at least one angel wing seal, the nozzle surface having a radially inwardly facing erodible portion adjacent the angel wing rim, wherein, in operation, the angel wing rim erodes a groove into the erodible portion of the nozzle surface. 
         [0009]    In yet 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 comprising: an airfoil extending radially outward from a platform portion; a shank portion extending radially inward from the platform portion; at least one angel wing extending axially from a face of the shank portion, the at least one angel wing including an angel wing rim extending radially upward toward the airfoil; and a plurality of voids disposed along the angel wing rim. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]    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: 
           [0011]      FIG. 1  shows a schematic cross-sectional view of a portion of a known turbine; 
           [0012]      FIG. 2  shows a perspective view of a known turbine bucket; 
           [0013]      FIG. 3  shows a perspective view of a portion of a turbine bucket according to an embodiment of the invention; 
           [0014]      FIG. 4  shows a radially inward view of a portion of the turbine bucket of  FIG. 3 ; 
           [0015]      FIG. 5  shows a perspective view of a portion of a turbine bucket according to another embodiment of the invention; 
           [0016]      FIG. 6  shows a perspective view of a portion of a turbine bucket according to yet another embodiment of the invention; 
           [0017]      FIG. 7  shows a cross-sectional side view of the turbine bucket of  FIG. 6 ; 
           [0018]      FIG. 8  shows a schematic view of purge air flow in a known turbine bucket; 
           [0019]      FIG. 9  shows a schematic view of purge air flow in a turbine bucket according to an embodiment of the invention; 
           [0020]      FIG. 10  shows a schematic view of a last stage turbine bucket and diffuser according to an embodiment of the invention; 
           [0021]      FIG. 11  shows a graph of swirl spike profiles at a diffuser inlet plane for known turbines and turbines according to embodiments of the invention; 
           [0022]      FIG. 12  shows a graph of total pressure spike profiles at a diffuser inlet plane for known turbines and turbines according to embodiments of the invention; and 
           [0023]      FIG. 13  shows a schematic cross-sectional side view of a steam turbine bucket according to an embodiment of the invention. 
       
    
    
       [0024]    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 
       [0025]    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 . 
         [0026]    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. 
         [0027]    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 . 
         [0028]    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. 
         [0029]    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. 
         [0030]      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 . 
         [0031]      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  are disposed along an angel wing rim  74  at a distal end  78  of angel wing  70 . Voids  110  are spaced along angel wing rim  74  such that the remaining portions of angel wing rim  74  form a plurality of column members  75 . As shown in  FIG. 3 , voids  110  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. 
         [0032]    As shown most clearly in  FIG. 4 , a radially-inward looking view of turbine bucket  40 , column members  75  (and correspondingly voids  110 ) include arcuate faces. Specifically, column members  75  include a concave face  75 A (a convex face of void  110 ) and a convex face  75 B (a concave face of void  110 ). As such, void  110  includes a first opening  110 A along an axially inner surface  74 A of angel wing rim  74  disposed laterally to a second opening  110 B along an axially outer surface  74 B of angel wing rim  74 . 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. 
         [0033]      FIG. 5  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  77  extend axially from shank portion  60  to each of the plurality of column members  75 . According to some embodiments, dam members  77  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  77  may include one or more arcuate faces, as do column members  75 , or may include rectangular, trapezoidal, or any other cross-sectional shape, such as described above. 
         [0034]      FIG. 6  shows a perspective view of a portion of a turbine bucket  140  according to another embodiment of the invention. Here, a continuous angel wing rim  174  extends upward from angel wing seal  170  and a plurality of dam members  177  extend axially from rim  174  toward but not contacting face  162 , leaving a gap  164  adjacent face  162   
         [0035]      FIG. 7  shows a cross-sectional side view of turbine bucket  140  of  FIG. 6  with respect to a nozzle surface  130  according to an embodiment of the invention. In  FIG. 7 , nozzle surface  130  comprises or includes a porous or erodible portion along at least a radially inward surface, such that angel wing rim  174  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  174 . 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  174 . 
         [0036]    In operation, purge air  180  passes into groove  131  of nozzle surface  130  and then downward between dam members  177 , toward face  162 . Purge air  180  then flows circumferentially within gap  164 , adjacent face  162 , as turbine bucket  140  rotates, providing increased swirl to purge air  180 . 
         [0037]      FIG. 8  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. 9  is a schematic view showing the effect of voids  110  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. 8 . This, in effect, produces a curtaining effect at area  87 , 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 . 
         [0038]    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  95  into wheelspace  26 . Each of these contributes to the increased efficiencies observed. 
         [0039]    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  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 the other system components, such as the compressor from which the purge air is provided. 
         [0040]    While reference above is made to the ability of angel wing rim voids 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 angel wing rim 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). 
         [0041]    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 rim 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 rim 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%. 
         [0042]      FIG. 10  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. 
         [0043]      FIG. 11  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 rim 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. 
         [0044]      FIG. 12  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 rim 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. 
         [0045]    The principle of operation of the voids described above may also be applied to the operation of steam turbines. For example,  FIG. 13  shows a schematic cross-sectional view of a steam turbine bucket  240  having an airfoil  250  and a shank  260  affixed to a disk  290 . A magnified view is provided of disk  290 , along which voids  210  (shown in phantom) may be deployed similarly to the voids shown in the figures above. 
         [0046]    Steam turbines employing embodiments of the invention such as those described herein will typically realize improvements in efficiency of between 0.1% and 5%, depending, for example, on the leakage flow and the stage at which the features are employed. 
         [0047]    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. 
         [0048]    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.