Patent Publication Number: US-8967973-B2

Title: Turbine bucket platform shaping for gas temperature control and related method

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates generally to rotary machines and, more particularly, to the control of forward wheel space cavity purge flow and combustion gas flow at the leading angel wing seals on a gas turbine bucket. 
     A typical turbine engine includes a compressor for compressing air that is mixed with fuel. The fuel-air mixture is ignited in a combustor to generate hot, pressurized combustion gases in the range of about 1100° C. to 2000° C. that expand through a turbine nozzle, which directs the flow to high and low-pressure turbine stages thus providing additional rotational energy to, for example, drive a power-producing generator. 
     More specifically, thermal energy produced within the combustor is converted into mechanical energy within the turbine by impinging the hot combustion gases onto one or more bladed rotor assemblies. Each rotor assembly usually includes at least one row of circumferentially-spaced rotor blades or buckets. Each bucket includes a radially outwardly extending airfoil having a pressure side and a suction side. Each bucket also includes a dovetail that extends radially inward from a shank extending between the platform and the dovetail. The dovetail is used to mount the bucket to a rotor disk or wheel. 
     As known in the art, the rotor assembly can be considered as a portion of a stator-rotor assembly. The rows of buckets on the wheels or disks of the rotor assembly and the rows of stator vanes on the stator or nozzle assembly extend alternately across an axially oriented flowpath for the combustion gases. The jets of hot combustion gas leaving the vanes of the stator or nozzle act upon the buckets, and cause the turbine wheel (and rotor) to rotate in a speed range of about 3000-15,000 rpm, depending on the type of engine. 
     As depicted in the figures described below, an axial/radial opening at the interface between the stationary nozzle and the rotatable buckets at each stage can allow hot combustion gas to exit the hot gas path and enter the cooler wheelspace of the turbine engine located radially inward of the buckets. In order to limit this leakage of hot gas, the blade structure typically includes axially projecting angel wing seals. According to a typical design, the angel wings cooperate with projecting segments or “discouragers” which extend from the adjacent stator or nozzle element. The angel wings and the discouragers overlap (or nearly overlap), but do not touch each other, thus restricting gas flow. The effectiveness of the labyrinth seal formed by these cooperating features is critical for limiting the undesirable ingestion of hot gas into the wheelspace radially inward of the angel wing seals. 
     As alluded to above, the leakage of the hot gas into the wheelspace by this pathway is disadvantageous for a number of reasons. First, the loss of hot gas from the working gas stream causes a resultant loss in efficiency and thus output. Second, ingestion of the hot gas into turbine wheelspaces and other cavities can damage components which are not designed for extended exposure to such temperatures. 
     One well-known technique for reducing the leakage of hot gas from the working gas stream involves the use of cooling air, i.e., “purge air”, as described in U.S. Pat. No. 5,224,822 (Lenehan et al). In a typical design, the air can be diverted or “bled” from the compressor, and used as high-pressure cooling air for the turbine cooling circuit. Thus, the cooling air is part of a secondary flow circuit which can be directed generally through the wheelspace cavities and other inboard rotor regions. This cooling air can serve an additional, specific function when it is directed from the wheel-space region into one of the angel wing gaps described previously. The resultant counter-flow of cooling air into the gap provides an additional barrier to the undesirable flow of hot gas through the gap and into the wheelspace region. 
     While cooling air from the secondary flow circuit is very beneficial for the reasons discussed above, there are drawbacks associated with its use as well. For example, the extraction of air from the compressor for high pressure cooling and cavity purge air consumes work from the turbine, and can be quite costly in terms of engine performance. Moreover, in some engine configurations, the compressor system may fail to provide purge air at a sufficient pressure during at least some engine power settings. Thus, hot gases may still be ingested into the wheelspace cavities. 
     Angel wings as noted above, are employed to establish seals upstream and downstream sides of a row of buckets and adjacent stationary nozzles. Specifically, the angel wing seals are intended the prevent the hot combustion gases from entering the cooler wheelspace cavities radially inward of the angel wing seals and, at the same time, prevent or minimize the egress of cooling air in the wheelspace cavities to the hot gas stream. Thus, with respect to the angel wing seal interface, there is a continuous effort to understand the flow patterns of both the hot combustion gas stream and the wheelspace cooling or purge air. In addition, there is concern for the gap between the platforms of adjacent buckets, another potential avenue for hot combustion gas ingress. 
     For example, it has been determined that even if the angel wing seal is effective and preventing the ingress of hot combustion gases into the wheelspaces, the impingement of combustion gas flow vortices on the surface of the seal and/or on adjacent bucket surfaces may damage and thus shorten the service life of the bucket. Similarly, hot gas ingress into the gaps between platforms of adjacent buckets can lead to thermal degredation of the platform slash face edges and seals located between the buckets. 
     The present invention seeks to provide unique bucket platform geometry to better control the flow of secondary purge air at the angel wing interface and/or in the generally axially-oriented gap between the platform edges or slash faces of adjacent buckets, to thereby also control the flow of combustion gases in a manner that extends the service life of the bucket. 
     BRIEF SUMMARY OF THE INVENTION 
     In one exemplary but nonlimiting embodiment, the invention provides a turbine bucket comprising a radially inner mounting portion; a shank radially outward of the mounting portion; at least one radially outer airfoil having a leading edge and a trailing edge; a substantially planar platform radially between the shank and the at least one radially outer airfoil; at least one axially-extending angel wing seal flange on a leading end of the shank thus forming a circumferentially extending trench cavity along the leading end of the shank, radially between an underside of the platform leading edge and a radially outer side of the angel wing seal flange; and slash faces along opposite, circumferentially-spaced side edges of said platform, at least one of the slash faces having a dog-leg shape, a leading end of one said at least one slash face terminating at a location circumferentially offset from the leading edge of the at least one radially outer airfoil. 
     In another aspect, the invention provides a turbine wheel comprising a plurality of buckets in a circumferential array about the wheel, each bucket comprising a radially inner mounting portion, a shank radially outward of the mounting portion, a radially outer airfoil and a substantially planar platform radially between the shank and the radially outer airfoil; at least one axially-extending angel wing seal flange on a leading end of the shank thus forming a circumferentially extending trench cavity along the leading end of the shank, radially between an underside of the platform leading edge and a radially outer side of the angel wing seal flange; a slash face along opposite, circumferentially-spaced side edges of the platform, at least one of the slash faces having a dog-leg shape, wherein leading ends of the slash faces on adjacent buckets terminate at a location circumferentially offset from the leading edges of the adjacent radially outer airfoils. 
     In still another aspect, the invention provides a method of controlling purge airflow in a radial space between a leading end of a bucket mounted on a rotor wheel and a surface of a stationary nozzle, and wherein the turbine bucket includes a radially inner mounting portion; a shank radially outward of the mounting portion; at least one radially outer airfoil having a leading edge and a trailing edge; a substantially planar platform radially between the shank and the at least one radially outer airfoil; at least one axially-extending angel wing seal flange on a leading end of the shank thus forming a circumferentially extending trench cavity along the leading of the shank, radially between an underside of the platform leading edge and a radially outer side of the angel wing seal flange; and slash faces along opposite, circumferentially-spaced side edges of the platform, the method comprising forming opposed slash faces of adjacent buckets to have a substantial dog-leg shape in a substantially axial direction; and locating leading ends of the opposed slash faces circumferentially between leading edges of the respective radially outer airfoils. 
     The invention will now be described in detail in connection with the drawings identified below. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a is a fragmentary schematic illustration of a cross-section of a portion of a turbine; 
         FIG. 2  is an enlarged perspective view of a turbine blade; and 
         FIG. 3  is a plan view of a turbine bucket pair illustrating a scalloped platform leading edge and a “dog-leg” interface along opposed platform slash faces in accordance with an exemplary but nonlimiting embodiment of the invention; 
         FIG. 4  is a plan view of a turbine bucket pair similar to that shown in  FIG. 3  but wherein the interface between opposed slash-faces is formed by a continuous curve; 
         FIG. 5  is a plan view similar to  FIG. 3  but omitting the scalloped leading edges along the platforms of the bucket pair; and 
         FIG. 6  is a plan view similar to  FIG. 4  but omitting the scalloped leading edges along the platforms of the bucket pair. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  schematically illustrates a section of a gas turbine, generally designated  10 , including a rotor  11  having axially spaced rotor wheels  12  and spacers  14  joined one to the other by a plurality of circumferentially spaced, axially-extending bolts  16 . Turbine  10  includes various stages having nozzles, for example, first-stage nozzles  18  and second-stage nozzles  20  having a plurality of circumferentially-spaced, stationary stator blades. Between the nozzles and rotating with the rotor and rotor wheels  12  are a plurality of rotor blades, e.g., first and second-stage rotor blades or buckets  22  and  24 , respectively. 
     Referring to  FIG. 2 , each bucket (for example, bucket  22  of  FIG. 1 ) includes an airfoil  26  having a leading edge  28  and a trailing edge  30 , mounted on a shank  32  including a platform  34  and a shank pocket  36  having integral cover plates  38 ,  40 . A dovetail  42  is adapted for connection with generally corresponding dovetail slots formed on the rotor wheel  12  ( FIG. 1 ). Bucket  22  is typically integrally cast and includes axially projecting angel wing seals  44 ,  46  and  48 ,  50 . Seals  46 ,  48  and  50  cooperate with lands  52  (see  FIG. 1 ) formed on the adjacent nozzles to limit ingestion of the hot gases flowing through the hot gas path, generally indicated by the arrow  39  ( FIG. 1 ), from flowing into wheel spaces  41 . 
     Of particular concern here is the upper or radially outer angel wing seal  46  on the leading edge end of the bucket. Specifically, the angel wing  46  includes a longitudinal extending wing or seal flange  54  with an upturned edge  55 . The bucket platform leading edge  56  extends axially beyond the cover plate  38 , toward the adjacent nozzle  18 . The upturned edge  55  of seal flange  54  is in close proximity to the surface  58  of the nozzle  18  thus creating a tortuous or serpentine radial gap  60  as defined by the angel wing seal flanges  44 ,  46  and the adjacent nozzle surface  58  where combustion gas and purge air meet (see  FIG. 1 ). In addition, the seal flange  54  upturned edge  55  and the edge  56  of platform  34  form a so-called “trench cavity”  62  where cooler purge air escaping from the wheel space interfaces with the hot combustion gases. As described further below, by maintaining cooler temperatures within the trench cavity  62 , service life of the angel wing seals, and hence the bucket itself, can be extended. 
     In this regard, the rotation of the rotor, rotor wheel and buckets create a natural pumping action of wheel space purge air (secondary flow) in a radially outward direction, thus forming a barrier against the ingress of the higher temperature combustion gases (primary flow). At the same time, CFD analysis has shown that the strength of a so-called “bow wave,” i.e., the higher pressure combustion gases at the leading edge  28  of the bucket airfoil  26 , is significant in terms of controlling primary and secondary flow at the trench cavity. In other words, the higher temperature and pressure combustion gases attempting to pass through the angel wing gap  60  is strongest at the platform edge  56 , adjacent the leading edge  28  of the bucket. As a result, during rotation of the wheel, a circumferentially-undulating pattern of higher pressure combustion gas flow is established about the periphery of the rotor wheel, with peak pressures substantially adjacent each the bucket leading edge  28 . 
     In order to address the bow wave phenomenon, at least to the extent of preventing the hot combustion gases from reaching the angel wing seal flange  54 , the platform leading edge  56  is scalloped in a circumferential direction. 
     More specifically, and as best seen in  FIGS. 3-5 , and  4 , a pair of buckets  64 ,  66  are arranged in side-by-side relationship and include airfoils  68 ,  70  with leading and trailing edges  72 ,  74  and  76 ,  78  respectively. The bucket  64  is also formed with a platform  80 , shank (not shown) supporting inner and outer angel wing seal flanges  84 ,  86  and a dovetail (not shown). Similarly, the bucket  66  is formed with a platform  90 , shank (not shown) supporting angel wing seal flanges  94 ,  96  and a dovetail (not shown). Similar angel wing seals are provided on the trailing sides of the buckets but are no of concern here. 
     While the buckets  64 ,  66  are shown as single airfoil buckets, it will be appreciated that the two airfoils may be formed integrally in one bucket shown as a “doublet”. 
     The platform leading edge  100  of the buckets (for convenience, the leading platform edges of the side-by-side buckets will be referred to in the singular, as the leading platform edge  100 ), in the exemplary but nonlimiting embodiment, is shaped to include an undulating or scalloped configuration defined by a continuous curve that forms substantially axially-oriented projections  102  alternating with recesses  104 . The projections  102  extend in an axially upstream direction, adjacent the bucket leading edges  72 ,  76 , thus blocking the flow of hot combustion gases at the bow wave from entering into the trench cavity  106 . This continuous curve extends along adjacent buckets, bridging the axial gap  107  extending between adjacent, substantially parallel slash faces  108 ,  110  of adjacent buckets. The illustrated embodiment thus includes one projection  102  and one recess  104  per bucket. The projections  102  have an axial length dimension less than a corresponding axial length dimensions of the side-by-side angel wing seal flanges  84 ,  94 . For so-called “doublets”, where each bucket incorporates two airfoils, there would be two projections and two recesses per bucket. 
     Thus, it will be appreciated that the projections  102  are located as a function of the strongest pitchwise static pressure defined by the combustion gas bow wave. As can be appreciated, the projections  102  prevent the hot combustion gas vortices from directly impinging on the angel wing seal flanges  84 ,  94 , thus reducing temperatures along the seal flanges. The combustion pressures in the alternating recesses  104  circumferentially between the projections  102  are sufficiently offset by the cooler purge air entering the slash face gap  107  from the wheel space. 
       FIGS. 3 and 4  also illustrate an additional platform geometry refinement that further enhances the control of cool purge air flow from the wheelspace cavity. Specifically, the opposed slash faces  108 ,  110  of the adjacent buckets are “dog-leg” shaped as shown in  FIG. 3  or continuous curve-shaped as shown in  FIG. 4 . In this regard, it has been determined that when the slash faces are parallel (as shown by the dashed lines  112 ,  114 , respectively), the aforementioned bow wave pushes hot combustion gas flow into the gap  107  between the slash faces. By changing the shape of the slash face interface to an intersecting-angle or dog-leg shape ( FIG. 3 ) or a continuous curve ( FIG. 4 ), it is possible to locate the entry to the gap  107  within the platform edge recess  104  where the pressure and temperature of the hot gas is reduced as compared to the temperature at the projections  102  corresponding to the bow wave, thus allowing the cooler purge air to effectively combat and prevent combustion gases from entering the gap  107 . 
     In  FIG. 3 , the slash faces  108 ,  110  are each formed by straight sections intersecting approximately midway along the length of the slash faces, at an angle of from about 90° to about 120°. 
     In  FIG. 4 , the opposed slash faces  109 ,  111  are shaped to form opposed continuous curves that generally conform the profiles of the adjacent airfoils  68 ,  70 , with substantially the same effect as the intersecting straight-line interface of  FIG. 3 . Otherwise, for the sake of convenience, the same reference numerals as used in  FIG. 3  are used here to designate corresponding components. 
     In both  FIGS. 3 and 4 , it will be appreciated that by incorporating mated, angled or curved slash faces, it is not possible to load the buckets onto the turbine disk in an axial direction. Loading in a circumferential direction is required, but that loading format is well known in the art. 
       FIGS. 5 and 6  illustrate similar slash-face arrangements but without the scalloped platform leading edge. In these Figs, Reference numerals similar to those used in  FIGS. 3 and 4  (with the prefix “2”) are used to designate corresponding components, and only the differences need be described here. More specifically, the platform edge  200  is straight and devoid of any projections or recesses of the scalloped platform edge shown in  FIGS. 3 and 4 . Nevertheless, the opposed slash faces  208  and  210  remain angled to create a “dog-leg” interface, thereby enabling the gap  207  to be located away or circumferentially offset from the leading edge  272  of the airfoil  268  and the leading edge  276  of the airfoil  270 , and hence circumferentially offset from the higher temperature/pressure bow wave. As a result purge air from the wheelspace is able to effectively combat the ingress of hot combustion gases into the gap  207 . 
     In  FIG. 6 , the opposed slash faces  209 ,  211  are shaped to form opposed continuous curves that generally conform the profiles of the adjacent airfoils  268 ,  270 , with substantially the same effect as the intersecting straight-line interface of  FIG. 5 . Otherwise, the buckets are substantially identical, and the same reference numerals used in  FIG. 5  are used in  FIG. 6  to designate the remaining corresponding components. 
     Accordingly, the relocation of the entry to the slash face gap  107  or  207  to an area circumferentially offset from the bucket airfoil leading edges in  FIGS. 5  and  6  provides the same benefit as described above in connection with  FIGS. 3 and 4  but not to the same degree as in  FIGS. 3 and 4  where the scalloped leading edge provides additional benefits relating to the control of purge air and hot combustion gases at locations of peak static pressure. 
     While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.