Abstract:
Embodiments of the invention relate generally to rotary machines and, more particularly, to the control of wheel space purge flow 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; a platform lip extending axially from the platform portion, the platform lip disposed radially outward from the at least one angel wing; and a plurality of turbulators disposed along and extending outward from the face of the shank portion between the platform lip and the at least one 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; at least one angel wing extending axially from a face of the shank portion; a platform lip extending axially from the platform portion, the platform lip disposed radially outward from the at least one angel wing; and a plurality of turbulators disposed along and extending outward from the face of the shank portion between the platform lip and the at least one angel wing. 
         [0007]    In another embodiment, the invention provides a turbine bucket comprising: a substantially planar platform portion; an airfoil extending radially outward from the platform portion, the airfoil including a leading edge and a trailing edge; a shank portion extending radially inward from the platform portion; at least one angel wing extending axially from a face of the shank portion; a platform lip extending axially from the platform portion, the platform lip disposed radially outward from the at least one angel wing; and a plurality of turbulators disposed along a radially inner surface of the platform lip. 
         [0008]    In still another embodiment, the invention provides a method of changing a flow of purge air in a wheelspace of a rotating turbine disk, the method comprising: locating at least one angel wing seal on an axially-disposed face of a turbine bucket adjacent the wheelspace; providing a plurality of turbulators between the at least one angel wing seal and a platform lip disposed radially outward from the at least one angel wing and axially from the axially-disposed face of the turbine bucket, whereby the plurality of turbulators changes a swirl velocity of purge air between the platform lip and the at least one angel wing. 
         [0009]    In yet another embodiment, the invention provides a turbine bucket comprising: a substantially planar platform portion; an airfoil extending radially outward from the platform portion; a shank portion extending radially inward from the platform portion; a platform lip extending axially from the platform portion; and a plurality of turbulators disposed along a radially inner surface of the platform lip. 
         [0010]    In still yet another embodiment, the invention provides a turbine disk for securing a plurality of turbine buckets, the turbine disk having an outer radial face into which a plurality of turbulators is formed. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    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: 
           [0012]      FIG. 1  shows a schematic cross-sectional view of a portion of a known turbine; 
           [0013]      FIG. 2  shows a perspective view of a known turbine bucket; 
           [0014]      FIG. 3  shows an axially-facing view of a portion of a turbine bucket according to an embodiment of the invention; 
           [0015]      FIGS. 4-8  show schematic views of turbulators according to various embodiments of the invention; 
           [0016]      FIG. 9  shows an axially-facing view of a portion of a turbine bucket according to another embodiment of the invention; 
           [0017]      FIGS. 10 and 11  show perspective views of portions of turbine buckets according to still other embodiments of the invention; 
           [0018]      FIG. 12  shows a schematic view of purge air flow in relation to a typical turbine bucket; 
           [0019]      FIG. 13  shows a schematic view of purge air flow in relation to a turbine bucket according to an embodiment of the invention; 
           [0020]      FIG. 14  shows a schematic view of a last stage turbine bucket and diffuser according to an embodiment of the invention; 
           [0021]      FIG. 15  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. 16  shows a graph of total pressure spike profiles at a diffuser inlet plane for known turbines and turbines according to embodiments of the invention; 
           [0023]      FIG. 17  shows a schematic cross-sectional view of a portion of a steam turbine bucket according to an embodiment of the invention; and 
           [0024]      FIG. 18  shows a schematic axial view of a portion of the steam turbine bucket of  FIG. 14 . 
       
    
    
       [0025]    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 
       [0026]    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 . 
         [0027]    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. 
         [0028]    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 . 
         [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 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. 
         [0032]    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 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. 
         [0033]    Turbulators  110  draw in purge air and increase its swirl velocity. This results in a small loss of torque, but a net gain in efficiency of approximately 0.5% at the turbine stage. This gain is a consequence of both the increased purge air swirl velocity, which produces a curtaining effect, described further below, as well as a change in swirl angle of the purge air. This change in swirl 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 ). 
         [0034]      FIGS. 5-8  show turbulators having different configurations. In  FIG. 5 , 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  FIG. 6 , 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. 7 , 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. 8 , 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 . 
         [0035]    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. 12 and 13 , this effect is attributable to the combined ability of platform lip  44  and the turbulators to move the area of purge air with the greatest swirl velocity both radially and axially outward, inducing a curtaining effect, 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 the curtaining effect induced. 
         [0036]      FIG. 9  shows a view of a portion of bucket  40  looking axially toward face  62 . As can be seen in  FIG. 9 , 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. 
         [0037]      FIGS. 10 and 11  show perspective views of portions of turbine buckets according to still other embodiments of the invention. In  FIG. 10 , 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. 11 , 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. 
         [0038]    Although the turbulators  710 ,  810  shown in  FIGS. 10 and 11 , 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. 4-8 . Similarly, any such turbulators may be axially angled, as described above with respect to  FIG. 9 . 
         [0039]      FIGS. 12 and 13  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. 12 , 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. 13  shows the effect of turbulators  110 - 810  on purge air  80  according to various embodiments of the invention. As can be seen in  FIG. 13 , the area  83  in which purge air is concentrated and exhibits a higher swirl velocity is distanced further from face  62  and toward a distal end of angel wing seal  70 . In addition, this area  83  of purge air has been moved radially outward and nearer platform lip  44 , as compared to  FIG. 12 . 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  85  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 turbulators 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 other system components, such as the compressor from which the purge air is provided. 
         [0043]    While reference above is made to the ability of turbulators 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 turbulators 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). 
         [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 turbulators 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. Turbulators, 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. 14  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 . Turbulators 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, voids increase the P T  spike. 
         [0046]      FIG. 15  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 turbulators according to embodiments of the invention. Profile B represents a swirl spike profile for a turbine without such turbulators. Profile A exhibits a marked decrease in swirl spike at a radially inward position of the diffuser inlet plane. 
         [0047]      FIG. 16  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 turbulators according to embodiments of the invention. Profile B represents a P T  spike profile for a turbine without such turbulators. 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 turbulators described above may also be applied to the operation of steam turbines. For example,  FIG. 17  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 the area adjacent platform lip, at which turbulators  910  may be disposed.  FIG. 18  shows an axial view of platform lip  944  and a plurality of turbulators  910  extending radially inward from a radially inner surface  946  of platform lip. 
         [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]    In each of the embodiments of the invention described above and shown in the figures, a plurality of substantially uniformly arranged turbulators is shown. This, however, is neither necessary nor essential. It may be desirable, for example, to affect a swirl velocity of purge air differently at different points along a bucket surface. In such a circumstance, the arrangement of the plurality of turbulators may be nonuniform. 
         [0051]    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. 
         [0052]    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.