Abstract:
A turbine flowpath structure includes a first generally U-shaped flowpath channel having a first leg having a first inwardly facing side shaped as a turbine blade suction-side airfoil surface, a second leg having a second inwardly facing side shaped as a turbine blade pressure-side airfoil surface, and a web connecting the first leg and the second leg, wherein the web has an inwardly facing inner flowpath surface. There is typically provided a spar that engages at least one of the first leg and the second leg. In one application, there is a plurality of the generally U-shaped flowpath channels arranged around the periphery of a turbine disk, and a plurality of spars positioned between each pair of adjacent generally U-shaped flowpath channels to anchor them to the turbine disk.

Description:
[0001]     This invention relates to gas turbine engines and, more particularly, to the gas flowpath structure of the turbine.  
       BACKGROUND OF THE INVENTION  
       [0002]     In an aircraft gas turbine (jet) engine, air is drawn into the front of the engine, compressed by a shaft-mounted compressor, and mixed with fuel. The mixture is combusted, and the resulting hot combustion gases are passed through a turbine mounted on the same shaft. The flow of gas turns the turbine by contacting an airfoil portion of the turbine blade that is positioned in the gas flowpath, which turns the shaft and provides power to the compressor. The hot exhaust gases flow from the back of the engine, driving it and the aircraft forward. There may additionally be a bypass fan that forces air around the center core of the engine, driven by a shaft extending from the turbine section.  
         [0003]     The efficiency of the gas turbine engine increases with increasing temperature of the hot combustion gases, and there is therefore an incentive to operate the engine at higher combustion-gas temperatures. However, the ability to increase the combustion-gas temperature is limited by the permissible maximum operating temperatures of the components that are subjected to the highest temperatures.  
         [0004]     One of the most demanding applications in the gas turbine engine is the turbine blades, upon which the hot combustion gases impinge, and which are also under high loads. Many techniques have been used to increase the operating temperatures of the gas turbine blades, including the use of improved metallic materials, improved manufacturing techniques, and insulating coatings. The turbine blades may be hollow, so that cooling air may be forced through the hollow turbine blades to openings from which the cooling air is expelled.  
         [0005]     The use of ceramic gas turbine blades has been discussed and evaluated, but at this time ceramic gas turbine blades have not yet entered service. Some ceramic materials are operable to higher temperatures than are the best available metallic alloys. However, ceramic materials also tend to be of low ductilities and thence low fracture toughnesses, which may lead to premature failure of the ceramic materials in service. One possible solution is to use ceramic matrix composite (CMC) materials in which a ceramic or metallic fiber is embedded in a ceramic matrix. An example is silicon carbide fibers embedded in a silicon carbide matrix. Such CMC materials have better fracture toughnesses than do the monolithic ceramic materials.  
         [0006]     On the other hand, the most promising of the CMC materials must be cooled, even though they are ceramics, because their maximum service temperatures in the gas turbine application are near to or less than the combustion-gas temperature. The cooling may be accomplished in essentially the same manner that conventional metallic turbine blade materials are cooled, with a flow of bleed compressor air. There have been techniques proposed to manufacture cooled turbine blades from CMC materials. The proposed techniques are complex and expensive, and have limited success.  
         [0007]     There is therefore a need for an improved approach to the manufacture of cooled gas turbine blades from CMC materials and other types of materials, particularly low-ductility materials. The present invention fulfills this need, and further provides related advantages.  
       SUMMARY OF THE INVENTION  
       [0008]     The present approach provides a turbine flowpath structure that is particularly suited to the use of cooled ceramic materials of construction, such as ceramic-matrix composite materials. All of the surfaces of the turbine flowpath structure are readily accessible for fabrication and machining, including the surfaces facing away from the flowpath. The fabrication of inaccessible cooling passages in the turbine flowpath structure is not required.  
         [0009]     A turbine flowpath structure comprises a first generally U-shaped flowpath channel including a first leg having a first inwardly facing side shaped as a turbine blade suction-side airfoil surface, a second leg having a second inwardly facing side shaped as a turbine blade pressure-side airfoil surface, and a web connecting the first leg and the second leg. The web has an inwardly facing inner flowpath surface.  
         [0010]     In the preferred construction, the turbine flowpath structure further includes a spar that engages at least one of the first leg and the second leg. The spar is hollow and has a cooling hole through a wall thereof. Preferably, the spar is spaced apart from the engaged one of the first leg and the second leg to define a cooling passage between the spar and the engaged one of the first leg and the second leg. The spar may engage at least one of the first leg and the second leg at a location at an end thereof remote from the web, or at a location intermediate between the web and an end thereof remote from the web.  
         [0011]     The present approach is particularly advantageously applied where the U-shaped flowpath channel comprises a ceramic material. Preferably, the U-shaped flowpath channel comprises a ceramic-matrix-composite material, such as a silicon carbide-silicon carbide composite material. The spar is preferably made of a metallic material, such as a nickel-base superalloy. The spar is not directly exposed to the hot combustion gas and is cooled by the flow of cooling air that passes through it, so that it may be made of a metal.  
         [0012]     The present configuration of the turbine flowpath structure is used to construct a turbine with the U-shaped flowpath channel and spar anchored to a periphery of the turbine disk. For this purpose, there is a second generally U-shaped flowpath channel having the same structure as the first generally U-shaped flowpath channel. A spar is positioned between the first generally U-shaped flowpath channel and the second generally U-shaped flowpath channel. The spar engages both the first leg of the first generally U-shaped flowpath channel and the second leg of the second generally U-shaped flowpath channel. The spar is anchored to a periphery of the turbine disk, thereby holding the first generally U-shaped flowpath channel and the second generally U-shaped flowpath channel to the turbine disk.  
         [0013]     More specifically, a turbine flowpath structure is mounted to a turbine disk. The turbine flowpath structure comprises a plurality of U-shaped flowpath channels, where each U-shaped flowpath channel comprises a ceramic material and has a first leg having a first inwardly facing side shaped as a turbine blade suction-side airfoil surface, a second leg having a second inwardly facing side shaped as a turbine blade pressure-side airfoil surface. A web connects the first leg and the second leg and their inboard ends, and has an inwardly facing inner flowpath surface. A metallic spar is positioned between each adjacent pair of generally U-shaped flowpath channels. An inner end of each spar is anchored to a periphery of the turbine disk so that the spar extends radially outwardly from the periphery of the turbine disk. The spar engages the first leg of one of the adjacent generally U-shaped flowpath channels, and the second leg of the other of the other adjacent generally U-shaped flowpath channel, thereby holding the generally U-shaped flowpath channels to the turbine disk. This configuration is repeated around the entire periphery of the turbine disk and for all of the generally U-shaped flowpath channels and spars.  
         [0014]     The present approach is a significant departure from the usual approach for turbine structures. In the usual approach, each turbine blade is fabricated with an airfoil having a suction side and a pressure side, and a platform to shield the underlying structure from the hot combustion gas. The turbine blade is anchored to the turbine disk. (The turbine blade may be prepared separately from the turbine disk, or integrally with the turbine disk.) If the turbine blade is to be cooled, internal cooling passages are cast and/or machined into the turbine blade. This approach is exceedingly difficult to implement when the turbine blade is to be made from a ceramic material.  
         [0015]     In the present approach, by contrast, the suction side of one turbine blade, the pressure side of the adjacent turbine blade, and the web which provides the connection between the two sides and also the shielding function of the platform are fabricated as a single generally U-shaped flowpath channel. The generally U-shaped flowpath channels are assembled together and anchored to the periphery of the turbine blade by the spars, which provides a passage for cooling air for themselves and for the adjacent generally U-shaped flowpath channels. Each spar and U-shaped flowpath channel are configured with a standoff spacing between them. This design allows complete access during fabrication to all of the surfaces of the generally U-shaped flowpath channel, thereby greatly facilitating the fabrication processing. In service, cooling air is conducted from the turbine disk into the spar, and thence through holes in the sides of the spar. The cooling air impinges on the facing side of the U-shaped flowpath channel, thereby providing cooling of the ceramic material. The result is a cooled ceramic turbine blade structure that is much more readily produced than are conventional designs. The present configuration of the ceramic turbine blade structure is also more resistant to thermal shock than is a conventional design, because the individual legs of the U-shaped flowpath channel are not constrained in the same manner as are the sides of a conventional turbine blade. There are also vibration reduction and frictional damping benefits realized from the present approach.  
         [0016]     Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention. The scope of the invention is not, however, limited to this preferred embodiment. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]      FIG. 1  is a perspective view of a generally U-shaped flowpath channel;  
         [0018]      FIG. 2  is a perspective view of a spar;  
         [0019]      FIG. 3  is a view looking radially inwardly of portions of two generally U-shaped flowpath channels assembled with a spar;  
         [0020]      FIG. 4  is a sectional view of a generally U-shaped flowpath channel and its two adjacent spars anchored to a turbine disk; and  
         [0021]      FIG. 5  is a sectional view similar to that of  FIG. 4 , illustrating a detail with the spar engaged to the generally U-shaped flowpath channel at mid-span. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0022]      FIG. 1  depicts a portion of a turbine flowpath structure  20 . The turbine flowpath structure  20  includes a first generally U-shaped flowpath channel  22  having a first leg  24  with a first inwardly facing side  26  shaped as a turbine blade suction-side airfoil surface, and a first outwardly facing side  28 . The generally U-shaped flowpath channel  22  further includes a second leg  30  having a second inwardly facing side  32  shaped as a turbine blade pressure-side airfoil surface, and a second outwardly facing side  34 . The turbine flowpath structure  20  further includes a web  36  connecting the first leg  24  and the second leg  30  at a location near their inboard ends  37 . The web  36  has an inwardly facing inner flowpath surface  38 . There is a first channel engagement shoulder  40  on the first outwardly facing side  28  of the first leg  24 , and a second channel engagement shoulder  42  on the second outwardly facing side  34  of the second leg  30 . The functions of these engagement shoulders  40  and  42  will be discussed subsequently.  
         [0023]     The generally U-shaped flowpath channel  22  is preferably a one-piece article made of a channel material of construction. The channel material of construction is preferably a ceramic such as a silicon carbide-silicon carbide ceramic matrix composite (CMC) material. Such CMC materials are known in the art for use in aircraft gas turbine engines, see for example U.S. Pat. No. 6,274,078 and U.S. Pat. No. 6,627,019, whose disclosures are incorporated by reference. However, such materials are not previously known in relation to a structure as discussed herein. The generally U-shaped flowpath channel  22  may be made by layup and subsequent processing as a single piece, followed by machining to define the precise shapes of the sides and surfaces  26 , 28 ,  32 ,  34 , and  38 . As may be seen in  FIG. 1 , these sides and surfaces  26 ,  28 ,  32 ,  34 , and  38  are all readily accessible for machining, coating, and other processing.  
         [0024]     The turbine flowpath structure  20  further includes a spar  44  illustrated in  FIG. 2 . The spar  44  has an elongated body  46  with an integral spar anchor  48  at one end. Preferably, the spar body  46  includes a first side  50  that is generally (but not necessarily exactly) shaped to conform to the first outwardly facing side  28  of the first leg  24  of the generally U-shaped flowpath channel  22 . Preferably, the spar body  46  includes a second side  52  that is generally (but not necessarily exactly) shaped to conform to the second outwardly facing side  34  of the second leg  30  of the generally U-shaped flowpath channel  22 . At a location along the length of the first side  50  of the spar body  46  is a first spar engagement shoulder  54 , and at a location along the length of the length of the second side  52  of the spar body  46  is a second spar engagement shoulder  56 . The locations and shapes of the first spar engagement shoulders  54  and the second spar engagement shoulder  56  are selected to engage the respective first channel engagement shoulder  40  of the first leg  24  and the second channel engagement shoulder  42  of the second leg  30  of the generally U-shaped flowpath channel  22 .  
         [0025]     The spar  44  is preferably made of a metal. The preferred material of construction is a nickel-base superalloy. As used herein, “nickel-base” means that the composition has more nickel present than any other element. The nickel-base superalloys are of a composition that is strengthened by the precipitation of gamma-prime phase or a related phase. A typical nickel-base superalloy has a composition, in weight percent, of from about 4 to about 20 percent cobalt, from about 1 to about 10 percent chromium, from about 5 to about 7 percent aluminum, from 0 to about 2 percent molybdenum, from about 3 to about 8 percent tungsten, from about 4 to about 12 percent tantalum, from 0 to about 2 percent titanium, from 0 to about 8 percent rhenium, from 0 to about 6 percent ruthenium, from 0 to about 1 percent niobium, from 0 to about 0.1 percent carbon, from 0 to about 0.01 percent boron, from 0 to about 0.1 percent yttrium, from 0 to about 1.5 percent hafnium, balance nickel and incidental impurities, although nickel-base superalloys may have compositions outside this range.  
         [0026]     The spar  44  is hollow, as suggested in  FIG. 2  and shown more clearly in  FIGS. 3-5 . Cooling gas, typically compressed air such as compressor bleed air, may enter an interior  58  of the spar  44  through an entry opening  59  ( FIGS. 4-5 ) in the spar anchor  48 . The cooling air flows through the interior  58  of the spar  44  and out through cooling holes  60  in a wall  62  of the spar  44 .  
         [0027]      FIG. 3  is a radially inward view of one spar  44  assembled to portions of a first generally U-shaped flowpath channel  64  and a second generally U-shaped flowpath channel  66 , each of which has the structure described above in relation to the generally U-shaped flowpath channel  22 . The first leg  24  and its first inwardly facing side  26  are supplied from the first generally U-shaped flowpath channel  64 . The second leg  30  and its second inwardly facing side  32  are supplied from the second generally U-shaped flowpath channel  66 .  
         [0028]     The walls  62  of the spar  44  are provided with outwardly facing standoff spacers  68  that space the spar  44  apart from the facing first leg  24  and second leg  30 , creating cooling passages  70  between the spar  44  and the first leg  24 , and between the spar and the second leg  30 . (Equivalently from a functional standpoint, the standoff spacers  68  may be formed in the first leg  24  and the second leg  30 , although the machining of the metallic spar  44  is easier than the machining of the ceramic generally U-shaped flowpath channel  22 .) The cooling air that flows from the cooling holes  60  of the wall  62  of the spar  44  flows through the cooling passages  70 . The flow of cooling air thus cools the spar  44  as it passes through the interior  58  of the spar  44 , and also cools the first leg  24  and the second leg  30  of each of the generally U-shaped flowpath channels  22  (and  64 ,  66 ). The spar  44  itself is insulated from the hot combustion gases by the first leg  24  and the second leg  30 , so that the metallic spar  44  is never directly contacted by the hot combustion gases. The combination of this isolation of the metallic spar  44  from the hot combustion gases and the interior and exterior flows of cooling air ensure that the spar  44  never is heated to too-high a temperature during service. The ceramic web  36  ( FIG. 1 ) of the generally U-shaped flowpath channel  22  protects the disk  72  from contact by the hot combustion gases that impinge against the pressure side of the generally U-shaped flowpath channel  22 .  
         [0029]      FIG. 4  illustrates the manner in which the generally U-shaped flowpath channels  22  (and  64 ,  66 ) are anchored to a turbine disk  72 . The turbine disk  72  has recesses  74  cut into its periphery  76 . The spar anchor  48  of each spar  44  is received into one of the recesses  74 . Each U-shaped flowpath channel  22  is in turn retained by the two spars  44  on either side of it by engagement between the first spar engagement shoulder  54  and the first channel engagement shoulder  40 , and between the second spar engagement shoulder  56  and the second channel engagement shoulder  42 . The generally U-shaped flowpath channel  22  is not rigidly engaged at both ends, and instead “floats” lengthwise between the two spars  44 . This floating engagement avoids the creation of excessive tensile and bending stresses in the generally U-shaped flowpath channel  22  as the structure is heated and cooled under centrifugal stresses produced as the turbine disk  72  turns.  
         [0030]     The portions of the legs  24 ,  30  of the generally U-shaped flowpath channel  22  that are closer to the web  36  than the location of the engagement shoulders  40 ,  42  are in compression, and the portions further from the web  36  than the engagement shoulders  40 ,  42  are in tension, during service when the turbine disk  72  is rotating rapidly. Ceramic materials are generally stronger in compression than in tension. Nearly the entire lengths of the legs  24 ,  30  are loaded in compression in the embodiment of  FIG. 4 . However, ceramic materials do have some tensile strength, and this capability may be utilized to reduce the weight of the overall turbine rotor system. The magnitude of the tensile stresses in the legs  24 ,  30  may be controlled by the positioning of the engagement between the shoulders  40 ,  54  and  42 ,  56 . As illustrated in  FIG. 5 , the second spar engagement shoulder  56  and the second channel engagement shoulder  42  (and also the first spar engagement shoulder  54  and the first channel engagement shoulder  40 , not shown in  FIG. 5 ), may instead be machined at an intermediate position along the lengths of the spar  44  and the legs  24 ,  30 . Only the inboard portions (i.e., between the engagements  54 ,  56  and the web  36 ) of the legs  24 ,  30  are loaded in compression, while the outboard portions (i.e., the portions further from the web  36  than are the engagements  54 ,  56 ) are in tension. As a result of this alternative configuration, the full centrifugal loads of the channel  22  are not carried through the upper portion of the spar  44 , and the wall thicknesses of the spar  44  in that region may be reduced accordingly. This in turn reduces the weight of the spar  44 , also allowing the weight of the supporting structure of the disk  72  to be reduced. The selection of the exact radial location of the engagement shoulders  40 ,  42  and  54 ,  56  may be optimized according to the material of construction selected for the generally U-shaped flowpath channels  22 . The spar  44  is loaded entirely in tension during service, but it is more suitable for tension loading than the generally U-shaped flowpath channel  22  because the spar  44  is made of metal.  
         [0031]     The alternating pattern of the generally U-shaped flowpath channels  22  and spars  44  as illustrated in  FIGS. 4 and 5  is repeated around the periphery  76  of the turbine disk  72 . The result is a flowpath configuration that has alternating suction sides and pressure sides, as in a conventional gas turbine disk structure, but achieved in a different manner that is more amenable to the use of a cooled ceramic material of construction. In service, the hot combustion gases are directed against the pressure sides of each of the respective generally U-shaped flowpath channels  22 .  
         [0032]     Although a particular embodiment of the invention has been described in detail for purposes of illustration, various modifications and enhancements may be made without departing from the spirit and scope of the invention. Accordingly, the invention is not to be limited except as by the appended claims.