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 platform lip extending axially from the platform portion; and a plurality of voids disposed along a surface of the platform lip.

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 at the first and second stage wheelspaces may be between 0.1% and 3.0%. 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 platform lip extending axially from the platform portion; and a plurality of voids disposed along a surface of the platform lip. 
         [0007]    In another embodiment, the invention provides a turbine bucket comprising: a platform portion; an airfoil extending radially outward from the platform portion; a platform lip extending axially from the platform portion; and a plurality of voids disposed along a surface of the platform lip, each of the plurality of voids extending radially through a body of the platform lip. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]    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: 
           [0009]      FIG. 1  shows a schematic cross-sectional view of a portion of a known turbine; 
           [0010]      FIG. 2  shows a perspective view of a known turbine bucket; 
           [0011]      FIG. 3  shows a cross-sectional side view of a portion of a turbine bucket according to an embodiment of the invention; 
           [0012]      FIG. 4  shows a perspective view of the portion of the turbine bucket of  FIG. 3 ; 
           [0013]      FIG. 5  shows a perspective view of a portion of a turbine bucket according to another embodiment of the invention; 
           [0014]      FIG. 6  shows a perspective view of a portion of a turbine bucket according to yet another embodiment of the invention; 
           [0015]      FIGS. 7-13  show perspective views of turbine buckets according to still other embodiments of the invention; 
           [0016]      FIG. 14  shows a schematic view of purge air flow in relation to a typical turbine bucket; 
           [0017]      FIG. 15  shows a schematic view of purge air flow in relation to a turbine bucket according to an embodiment of the invention; 
           [0018]      FIG. 16  shows a schematic view of a last stage turbine bucket and diffuser according to an embodiment of the invention; 
           [0019]      FIG. 17  shows a graph of swirl spike profiles at a diffuser inlet plane for known turbines and turbines according to embodiments of the invention; 
           [0020]      FIG. 18  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 
           [0021]      FIG. 19  shows a schematic cross-sectional side view of a steam turbine bucket according to an embodiment of the invention. 
       
    
    
       [0022]    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 
       [0023]    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 . 
         [0024]    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. 
         [0025]    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 . 
         [0026]    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. 
         [0027]      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 . 
         [0028]      FIG. 3  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. 3 , a distal end  48  of platform lip  44  is angled radially outward toward airfoil  50 . 
         [0029]      FIG. 4  shows a perspective view of the bucket  40  of  FIG. 3 . A plurality of voids  110  are provided along distal end  48  of platform lip  44 . As shown in  FIG. 4 , 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. 
         [0030]    For example,  FIG. 5  shows a perspective view of a bucket  40  according to another embodiment of the invention. Here, platform lip  44  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  44  in an arcuate path such that remaining portions of platform lip  44  adjacent voids  210  include an arcuate face  45 . 
         [0031]    The embodiment of the invention shown in  FIG. 6  shows a perspective view of bucket  40 . Here, platform lip  44  includes an angled distal end  48 , as in  FIGS. 3 and 4 . However, voids  310  are formed in a body  46  of platform lip  44  rather than at its distal end  48 . As noted above, voids  310  may take any number of shapes, including, for example, rectangular, trapezoidal, rhomboid, arcuate, etc. 
         [0032]      FIGS. 7-9  show perspective views of other embodiments of the invention. In  FIG. 7 , voids  410  are elliptical in shape and angled with respect to a radial axis of bucket  40 . 
         [0033]    In  FIG. 8 , elliptical voids  510  of differing sizes are employed with void size increasing along platform lip  44  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  44  and angel wing  70  will generally be more pronounced adjacent the larger voids. This 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. 
         [0034]    In  FIG. 9 , elliptical voids  510  of differing size are employed with void size decreasing along platform lip  44  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. 
         [0035]      FIGS. 10-13  show perspective views of turbine buckets  40  in accordance with various embodiments of the invention. In each of the embodiments in  FIGS. 10-13 , voids are disposed unevenly along platform lip  44 . 
         [0036]    In  FIG. 10 , a plurality of substantially rectangular voids  610  are disposed along platform lip  44  nearer the convex leading face than the concave trailing face of airfoil  50 . 
         [0037]    In  FIG. 11 , the area of void concentration is opposite that in  FIG. 10 , with the plurality of substantially rectangular voids  610  disposed along platform lip  44  nearer the concave trailing face than the convex leading face of airfoil  50 . 
         [0038]      FIGS. 12 and 13  show embodiments similar to those in  FIGS. 10 and 11 , respectively, in which voids  710  are rhomboid in shape rather than substantially rectangular. The use of rhomboid voids  710  may be employed, for example, to direct purge air toward either convex leading face or concave trailing face of airfoil  50 . 
         [0039]      FIG. 14  shows a schematic view of purge air flow in a typical turbine bucket. Purge air  80  is shown concentrated and having a higher swirl velocity in area  82 , with a significant amount of escaping purge air  84  entering hot gas flowpath  28 . The concentration of purge air  80  having a higher swirl velocity in area  82 , closer to face  62 , allows for incursion of hot gas  95  into wheelspace  26 . 
         [0040]    In contrast,  FIG. 15  shows the effect of voids  110  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  80  is concentrated and exhibits a higher swirl velocity is distanced further from face  62  and toward a distal end of platform lip  44 , as compared to  FIG. 14 . This, in effect, produces a curtaining effect, restricting incursion of hot gas  95  from hot gas flowpath  28  while at the same time reducing the quantity of escaping purge air from wheelspace  26  into hot gas flowpath  28 . 
         [0041]    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 reduces the escape of purge air into hot gas flowpath  28 , changes in swirl angle 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. 
         [0042]    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. 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. 
         [0043]    While reference above is made to the ability of platform lip 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 platform lip 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 platform lip voids are employed in the last stage bucket (LSB). 
         [0044]    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 platform lip 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. Platform lip 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%. 
         [0045]      FIG. 16  shows a schematic view of a LSB  40  adjacent diffuser  850 . Hot gas  195  enters diffuser  850  at diffuser inlet plane  860  and passes toward struts  870 . Platform lip 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  870 . At the same time, such platform lip voids increase the P T  spike. 
         [0046]      FIG. 17  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 platform lip voids according to embodiments of the invention. Profile B represents a swirl spike profile for a turbine having a platform lip known in the art. Profile A exhibits a marked decrease in swirl spike at a radially inward position of the diffuser inlet plane. 
         [0047]      FIG. 18  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 platform lip voids according to embodiments of the invention. Profile B represents a P T  spike profile for a turbine having a platform lip known in the art. Profile A exhibits an increase in P T  spike at a radially inward position of the diffuser inlet plane. 
         [0048]    The principle of operation of the voids described above may also be applied to the operation of steam turbines. For example,  FIG. 19  shows a schematic cross-sectional view of a steam turbine bucket  940  having an airfoil  950  and a shank  960  affixed to a disk  990 . A magnified view is provided of platform lip  944 , along which voids  910  (shown in phantom) may be deployed similarly to the voids shown in  FIGS. 3-5,12, and 13  above. 
         [0049]    Steam turbines employing embodiments of the invention such as those described herein will typically realize improvements in efficiency of between 0.1% and 0.5%, depending, for example, on the leakage flow and the stage at which the features are employed. 
         [0050]    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. 
         [0051]    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.