Abstract:
In a turbine bucket having an airfoil portion and a root portion with a platform at an interface between the airfoil portion and the root portion, a platform cooling arrangement including: a cooling passage defined in the platform to extend along at least a portion of a concave, pressure side of the airfoil portion, at least one cooling medium inlet to said cooling passage extending from an airfoil cooling medium cavity in a vicinity of an axial center of the airfoil portion, and at least one outlet opening for expelling cooling medium from said cooling passage.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     The present invention relates to a novel cooling system for increasing the useful life of a turbine bucket.  
         [0002]     A gas turbine has (i) a compressor section for producing compressed air, (ii) a combustion section for heating a first portion of said compressed air, thereby producing a hot compressed gas, and (iii) a turbine section having a rotor disposed therein for expanding the hot compressed gas. The rotor is comprised of a plurality of circumferentially disposed turbine buckets.  
         [0003]     Referring to  FIG. 1 , each turbine bucket  10  is comprised of an airfoil portion  12  having a suction surface and a pressure surface; and a root portion  14  having structure  18  to affixing the blade to the rotor shaft, a platform  16  from which said airfoil extends, and a shank portion  20 .  
         [0004]     The platforms are employed on turbine buckets to form the inner flow path boundary through the hot gas path section of the gas turbine. Design conditions, that is gas path temperatures and mechanical loads, often create considerable difficulty to have bucket platforms last the desired amount of time in the engine. In this regard, the loading created by gas turbine buckets create highly stressed regions of the bucket platform that, when coupled with the elevated temperatures, may fail prior to the desired design life.  
         [0005]     A variety of previous platform cooling designs have been used or disclosed. Referring to  FIG. 2 , one previous platform cooling design was based on utilizing the cavity  122  formed by adjacent bucket shanks  120  and platforms  116  as an integral part of the cooling circuit. This type of design extracts air from one of the buckets internal cooling passages and uses it to pressurize the cavity  122  formed by the adjacent bucket shanks  120  and platforms  116  described above. Once pressurized, this cavity can then supply cooling to almost any location on the platform. Impingement cooling is often incorporated in this type of design to enhance heat transfer. The cooling air may exit the cavity through film cooling holes in the platform or through axial cooling holes which then direct the air out of the shank cavity. This design, however, has several disadvantages. First, the cooling circuit is not self contained in one part and is only formed once at least two buckets  110  are assembled in close proximity. This adds a great degree of difficulty to pre-installation flow testing. A second disadvantage is the integrity of the cavity  122  formed between adjacent buckets  110  is dependent on how well the perimeter of the cavity is sealed. Inadequate sealing may result in inadequate platform cooling and wasted cooling air.  
         [0006]     Another prior art design is disclosed in FIGS. 1(a) and 5(a) of U.S. Pat. No. 6,190,130. This design uses a cooling circuit that is contained fully within a single bucket. With this design, cooling air is extracted from an airfoil leading edge cooling passage and directed aft through the platform. The cooling air exits through exit holes in the aft portion of the bucket platform or into the slash-face cavity between adjacent bucket platforms. This design has an advantage over that described above and depicted in  FIG. 2  in that it is not affected by variations in assembly conditions. However, as illustrated therein, only a single circuit is provided on each side of the airfoil and, thus, there is the disadvantage of having limited control the amount of cooling air used at different locations in the platform. This design also has the disadvantage of restricting the cooling air supply to the leading edge cavity.  
         [0007]     Yet another prior art cooling circuit configuration is disclosed in FIG. 3(a) of U.S. Pat. No. 6,190,130 and also in U.S. Pat. No. 5,639,216. This design also uses a cooling circuit fully contained within a single bucket, but it is supplied by air from underneath the platform, i.e. shank pocket cavity or forward wheel space (disc cavity).  
       BRIEF DESCRIPTION OF THE INVENTION  
       [0008]     The invention proposes a platform geometry designed to reduce both stress and temperature in the bucket platform.  
         [0009]     Thus, the invention may be embodied in a turbine bucket having an airfoil portion, a root portion with a platform at an interface between the airfoil portion and the root portion, and a platform cooling arrangement including: a cooling passage defined in the platform to extend along at least a portion of a concave, pressure side of the airfoil portion, at least one cooling medium inlet to said cooling passage extending from an airfoil cooling medium cavity in a vicinity of an axial center of the airfoil portion, and at least one outlet opening for expelling cooling medium from said cooling passage.  
         [0010]     The invention may also be embodied in a method of cooling a platform of a turbine bucket having an airfoil portion and a root portion, said airfoil portion being joined to the platform and the platform extending over said root portion, comprising: providing a cooling passage at least a portion of a concave, pressure side of the airfoil portion; flowing a cooling medium through a bore from a cooling medium cavity in a vicinity of an axial center of the airfoil portion to said cooling passage; and expelling cooling medium from said cooling passage through at least one outlet opening. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]     These and other objects and advantages of this invention, will be more completely understood and appreciated by careful study of the following more detailed description of the presently preferred exemplary embodiments of the invention taken in conjunction with the accompanying drawings, in which:  
         [0012]      FIG. 1  is a schematic perspective view of a turbine bucket and platform;  
         [0013]      FIG. 2  is a schematic illustration of a prior art cooling circuit using a cavity between adjacent bucket shanks;  
         [0014]      FIG. 3  is a top plan view of a bucket as an example embodiment of the invention;  
         [0015]      FIG. 4  is a schematic cross-sectional view of a conventional platform structure;  
         [0016]      FIG. 5  is a schematic cross-sectional view of a platform design according to an example embodiment of the invention;  
         [0017]      FIG. 6  is a top plan view of a bucket according to a modification of the embodiment of  FIG. 3 ;  
         [0018]      FIG. 7  is a top plan view of a bucket according to a another example embodiment of the invention;  
         [0019]      FIG. 8  is a top plan view of a bucket according to a modification of the embodiment of  FIG. 7 ;  
         [0020]      FIG. 9  is a top plan view of a bucket according to a further example embodiment of the invention;  
         [0021]      FIG. 10  is a top plan view of a bucket according to a modification of the embodiment of  FIG. 9 ; and  
         [0022]      FIG. 11  is a top plan view of a bucket according to a yet another example embodiment of the invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0023]     According to an example embodiment of the invention, one or more preferential cooling passages are defined through the bucket platform on the concave or pressure side of the airfoil as schematically illustrated in  FIGS. 3, 6 ,  7 ,  8 ,  9 ,  10  and  11 . These cooling passages are supplied with a cooling medium, such as air, from the airfoil cooling circuit, more specifically from a vicinity of an axial center or mid-section of the respective airfoil. In the illustrated examples, where plural cooling passages are provided, each is supplied with air from a respective airfoil cooling circuit cavity or passage.  
         [0024]     The cooling passages are respectively sized and shaped to accomplish at least two goals. First, the passages are defined to allow for a preferential cooling of the platform. Preferential cooling allows the correct amount of cooling to be performed at various locations on the platform.  
         [0025]     Referring by way of example to  FIG. 3 , it can be seen that in this example embodiment, two passages  224 ,  226  are defined on the concave or pressure side  228  of the airfoil  212 . The first cooling passage  224  is in flow communication with a cooling circuit cavity or passage  230  of the airfoil  212  in a vicinity of an axial center or midpoint of the airfoil and is disposed to define a flow passage for cooling air that extends along a first, serpentine path  232  towards a leading edge  234  of the platform  216 , then extends along a part circumferential path  236  towards the slash-face  238  on the pressure side of the airfoil, and then finally extends along a substantially straight side cooling path  240  extending generally parallel to the slash-face  238  towards the trailing edge of the platform  216 . In the illustrated example embodiment, the first cooling passage  224  terminates axially in a plurality of film cooling holes  242  to discharge the cooling medium, such as air, onto the flow path surface of the platform, providing even further cooling benefit.  
         [0026]     In the embodiment of  FIG. 3 , a second cooling passage  226  is also provided on the concave, pressure side  228  of the airfoil  212  and is disposed to be in flow communication with a cooling air cavity  244 , again in the vicinity of the axial center or midpoint of the airfoil  212 . The second cooling passage  226  extends along a serpentine path  246  towards the aft or trailing edge of the platform  216 . In the illustrated example embodiment, the second cooling flow passage also terminates axially in a plurality of film cooling holes  248 . The serpentine paths  232 ,  246  in this example embodiment each include a plurality of part circumferential portions interconnected with part axial portions for distributing cooling medium through the platform for preferential cooling purposes.  
         [0027]     In this regard, as will be understood, by selecting a cooling air supply passage diameter and dimensions of the respective flow passages, differential mass flows and velocities can be achieved for preferential cooling of the respective portions of the platform.  
         [0028]     Referring to  FIGS. 4 and 5 , in an example embodiment of the invention, in addition to providing first and second passages for preferential cooling of the platform, the platform is configured so as to have a high stiffness to weight ratio. In this regard, referring to  FIG. 4 , a conventional platform  116  having for example a “L” shaped cross-section requires a large thickness to be stiff about the bending axis. In an example embodiment of the invention, as illustrated in  FIG. 5 , the paths  232 ,  246 ,  240  of the cooling passages  224 ,  226  are defined by casting the platform so as to define grooves on the radially inner surface of the platform  216  and providing a bottom plate  250 , to define a bottom of the respective cooling passages  224 ,  226  and complete the platform structure  216 . The resulting “box” section is inherently stiffer than a conventional “L” section, whereas the weight is minimized by the material omitted to define the internal passages. Thus, in addition to the increased cooling effect as mentioned above, the stiffness and thus strength of the platform is increased while minimizing the weight thereof. Furthermore, the platform structure is simplified and production of passages having a desired configuration is facilitated.  
         [0029]     Another example embodiment of the invention is illustrated in  FIG. 6 . As illustrated therein, the first and second cooling passages generally correspond to those as illustrated in  FIG. 3  except that the first cooling passage  224  in this embodiment has exit holes  252  to the slash-face  238 . Providing exit holes in the slash-face provides additional cooling and increases the part&#39;s ability to resist hot gas ingestion. In the illustrated example, the slash-face exit holes  252  are provided in lieu of film cooling holes  242 , although is it to be understood that a combination of slash-face exit holes and film cooling holes could be provided.  
         [0030]     A further example embodiment of the invention is illustrated in  FIG. 7 . It can be seen that in this example embodiment, two passages  324 ,  326  are defined on the concave or pressure side  328  of the airfoil  312 . The first cooling passage  324  is in flow communication with a cooling circuit cavity or passage  330  of the airfoil  312  in a vicinity of an axial center or midpoint of the airfoil and is disposed to define a flow passage for cooling air that extends along a first, part circumferential path  336  towards slash-face  338  on the pressure side of the airfoil and then extends along a substantially straight side cooling path  340  extending generally parallel to the slash-face  338  towards the leading edge  334  of the platform  316 . In the illustrated example embodiment, a plurality of film cooling holes  342  are defined to discharge the cooling medium, such as air, from the first cooling passage  324  onto the flow path surface of the platform, providing even further cooling benefit.  
         [0031]     In the embodiment of  FIG. 7 , a second cooling passage  326  is also provided on the concave, pressure side  328  of the airfoil  312  and is disposed to be in flow communication with a cooling air cavity or passage  344 , again in the vicinity of the axial center or midpoint of the airfoil  312 . The second cooling passage  326  is a substantial mirror image of the first cooling passage  324 , having a first, part circumferential path  337  towards slash-face  338  and having a substantially straight side cooling path  341  extending generally parallel to the slash-face  338  towards the trailing end of the platform  316 . In the illustrated example embodiment, the second cooling flow passage also terminates in a plurality of film cooling holes  348 . Again, as will be understood, by selecting a cooling air supply passage diameter and dimensions of the respective flow passages, differential mass flows and velocities can be achieved for preferential cooling of the respective portions of the platform.  
         [0032]     Yet another example embodiment of the invention is illustrated in  FIG. 8 . In this embodiment the first and second cooling passages generally correspond to those as illustrated in  FIG. 7  except that the cooling passages in this embodiment have exit holes  352 ,  353  to the slash-face  338 . Providing exit holes in the slash-face provides additional cooling and increases the part&#39;s ability to resist hot gas ingestion. In the illustrated example, the slash-face exit holes  352 ,  353  are provided in lieu of film cooling holes  342 ,  348  although is it to be understood that a combination of slash-face exit holes and film cooling holes could be provided.  
         [0033]     A further example embodiment of the invention is illustrated in  FIG. 9 . It can be seen that in this example embodiment, two passages  424 ,  426  are defined on the concave or pressure side  428  of the airfoil  412 . The first cooling passage  424  is in flow communication with a cooling circuit cavity or passage  430  of the airfoil  412  in a vicinity of an axial center or midpoint of the airfoil and is disposed to define a flow passage for cooling air that extends along a first, part circumferential path  436  towards slash-face  438  on the pressure side of the airfoil and then extends along a substantially straight side cooling path  440  extending generally parallel to the slash-face  438  towards the leading edge  434  of the platform  416 . The flow passage for the cooling air then hooks back towards and along a part of the airfoil  412 . In the illustrated example embodiment, a plurality of film cooling holes  442  are defined to discharge the cooling medium, such as air, from the first cooling passage  324  onto the flow path surface of the platform, providing even further cooling benefit.  
         [0034]     In the embodiment of  FIG. 9 , a second cooling passage  426  is also provided on the concave, pressure side  428  of the airfoil  412  and is disposed to be in flow communication with a cooling air cavity or passage  444 , again in the vicinity of the axial center or midpoint of the airfoil  412 . The second cooling passage  426  is a substantial mirror image of the first cooling passage  424 , having a first, part circumferential path  437  extending towards slash-face  438  and having a substantially straight side cooling path  441  extending generally parallel to the slash-face  438  towards the trailing end of the platform  416 . The second cooling passage then hooks back towards and along a part of the airfoil  412 . In the illustrated example embodiment, the second cooling flow passage also terminates in a plurality of film cooling holes  448 . Again, as will be understood, by selecting a cooling air supply passage diameter and dimensions of the respective flow passages, differential mass flows and velocities can be achieved for preferential cooling of the respective portions of the platform.  
         [0035]     Yet another example embodiment of the invention is illustrated in  FIG. 10 . In this embodiment the first and second cooling passages generally correspond to those as illustrated in  FIG. 9  except that the cooling passages in this embodiment have exit holes  452 ,  453  to the slash-face  438 . Providing exit holes in the slash-face provides additional cooling and increases the part&#39;s ability to resist hot gas ingestion. In the illustrated example, the slash-face exit holes  452 ,  453  are provided in lieu of film cooling holes  442 ,  448 , although is it to be understood that a combination of slash-face exit holes and film cooling holes could be provided.  
         [0036]     Yet a further example embodiment of the invention is illustrated in  FIG. 11 . It can be seen that in this example embodiment, two passages  524 ,  526  are defined on the concave or pressure side  528  of the airfoil  512 . The first cooling passage  524  is in flow communication with a cooling circuit cavity or passage  530  of the airfoil  412  in a vicinity of an axial center or midpoint of the airfoil and is disposed to define a flow passage for cooling air that extends along a first, part circumferential main supply path  536  to the slash-face  538  on the pressure side of the airfoil. In the illustrated example embodiment, the main supply passage  536  terminates at a metering hole  542  the slash face  538  to control the mass flow level. Further cooling benefit is provided by cooling holes or passages  552  that extend through platform  516 , diagonally from the main supply passage  536  of the first cooling passage  524  to the slash face  538 . While two cooling holes  552  are illustrated in  FIG. 11 , it is to be understood that more or fewer such branch passages could be provided for preferentially cooling the platform.  
         [0037]     In the embodiment of  FIG. 11 , a second cooling passage  526  is also provided on the concave, pressure side  528  of the airfoil  512  and is disposed to be in flow communication with a cooling air source  544 , again in the vicinity of the axial center or midpoint of the airfoil  512 . The second cooling passage  526  is a substantial mirror image of the first cooling passage  524 , having a first, part circumferential main supply path  537  extending towards slash-face  538 . In the illustrated example embodiment, the second cooling flow passage also terminates in a metering hole  548  at the slash face  538 . Further, additional cooling benefit is provided by cooling holes or passages  553  that extend diagonally from the main supply passage  537  to the slash face  538 . Again, as will be understood, by selecting a cooling air supply passage diameter and dimensions of the respective flow passages, differential mass flows and velocities can be achieved for preferential cooling of the respective portions of the platform.  
         [0038]     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.