Patent Publication Number: US-8979481-B2

Title: Turbine bucket angel wing features for forward cavity flow 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. 
     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 may damage the seal and shorten the service life of the bucket. 
     The present invention seeks to provide unique angel wing seal and/or bucket platform geometry to better control the flow of secondary purge air at the angel wing interface to thereby also control the flow of combustion gases at that interface in a manner that extends the service life of the angel wing seal and hence the bucket itself. 
     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, a radially outer airfoil and a substantially planar platform radially between the shank and the airfoil; at least one axially-extending angel wing seal flange on a leading end of the shank forming a circumferentially extending trench cavity along the leading end of the shank, radially between an underside of a platform leading edge and the angel wing seal flange; and a plurality of substantially radially-extending purge air holes formed in the angel wing seal flange, adapted to fluidly connect a turbine rotor wheel space cavity with the trench cavity and thereby supply purge air to the outer surface of the angel wing seal flange. 
     In another aspect, the invention provides a turbine wheel supporting a circumferentially arranged row of buckets, each bucket comprising a radially inner mounting portion, a shank radially outward of the mounting portion, at least one radially outer airfoil and a substantially planar platform radially between the shank and the airfoil; at least one axially-extending angel wing seal flange on a leading end of the shank forming a circumferentially extending trench cavity along the leading end of the shank, radially between an underside of a platform leading edge and the angel wing seal flange; and wherein a plurality of substantially radially-extending purge air holes are formed in said angel wing seal flange, adapted to connect a turbine rotor wheel space cavity with said trench cavity. 
     In still another aspect, method of controlling secondary flow at a radial gap between a rotating turbine disk mounting a plurality of buckets and an adjacent nozzle, the method comprising: locating at least one angel wing seal on a leading edge of each of the plurality of buckets extending axially toward the nozzle to thereby form a barrier between a hot stream of combustion gases on a radially outer side of the angel wing seal and purge air in a wheel space radially inward of the at least one angel wing seal; and providing plural openings in the angel wing seal enabling purge air to flow into an area radially outward of the angel wing seal to thereby prevent the combustion gases from impinging on the angel wing seal flange. 
     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 perspective view of a pair of buckets with leading end angel wing seal flanges in accordance with an exemplary but nonlimiting embodiment of the invention; and 
         FIG. 4  is a partial schematic end view of a bucket with a leading end angel wing seal flange as shown in  FIG. 3  and illustrating purge air combustion gas vortices at the seal flange. 
     
    
    
     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  44 ,  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 platform leading edge  56  on the bucket  22  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 platform leading 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 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 leading edge  28 . 
     As discussed above, the radially outer angel wing seal flange  54  is intended to block or at least substantially inhibit hot combustion gases from entering the wheel space cavity, noting the close proximity between the radially outer seal wing flange  54  and the fixed nozzle surface  58 , best seen in  FIG. 1 . The invention here provides a modification to the radially outer angel wing seal flange  54  that allows purge air from the radially inner turbine wheelspace to prevent the hot combustion gas flow from impinging on the seal flange, thus reducing the flange temperature and extending the service life of the flange and hence the bucket. 
     As best seen in  FIG. 3 , a pair of buckets  64 ,  66  is 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  82  supporting inner and outer angel wing seal flanges  84 ,  86  at the leading end of the bucket, and a dovetail  88 . Similarly, the bucket  66  is formed with a platform  90 , shank  92  supporting angel wing seal flanges  94 ,  96  and a dovetail  98 . Similar angel wing seals are provided on the trailing sides or ends of the buckets but are no of concern here. 
     Recognizing that buckets  64  and  66  are identical, only one need be described below. Accordingly, referencing the bucket  66 , a plurality of purge air holes  100  are drilled or otherwise formed in the angel wing seal flange  94  in the area where the flange  94  is joined to the bucket shank  82 . With reference also to  FIG. 4 , the purge air holes  100  extend angularly through the flange  94  from an inlet  102  on the underside surface  104  of the seal flange  94  to an outlet  106  at the interface between the outer surface of the seal flange  94  and the shank  82 . The location of the outlet  106  is chosen to enhance the natural disk pumping phenomenon described above, fostering a stronger counterclockwise swirl or vortex of cooler purge air flow in the trench cavity  108  formed along the angel wing seal flange  94 . As shown in  FIG. 4 , the resulting purge air vortices  110  are sufficiently strong to push the oppositely swirling hot combustion gas vortices  112  away from the angel wing seal flange  94 . 
     The number of purge air holes  100  per bucket angel wing seal flange may vary, and the pattern of holes  100  may vary as well. For example, a non-uniform pattern may be equally or more effective than a uniform pattern if the locations of the holes  100  are targeted to just those areas along the substantially straight leading edge  114  of the bucket platform adjacent the leading edges  72 ,  76  of the airfoils  68 ,  70  that have been identified as having the highest combustion gas static pressure. In addition, the purge air holes  100  slant toward the shank, but may also slant in a circumferential direction to induce a substantial tangential swirl in the purge air vortices. 
     It will also be appreciated that the incorporation of purge air holes in the leading end angel wing seal flanges is compatible with other angel wing or bucket platform features that are designed to provide secondary flow (purge air flow) control in the forward wheel space cavities of the turbine. 
     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.