Abstract:
A gas turbine ring segment ( 10 ) for use in gas turbine engines made from a ceramic matrix composite (CMC) material is disclosed. The ring segment includes a stacked multiplicity of CMC thin-sheet lamellae ( 25   a,    25   b ) each comprising a peripheral surface collectively defining a cross-section profile of the ring segment. The lamellae collectively define a channel ( 11 ) formed in the center thereof for receiving a bow-tie member ( 27 ). The bow-tie member is disposed in the channel for holding together the stacked lamellae in a through thickness direction, and the in-plane strength of the bow-tie member is perpendicular to the in-plane strength of the lamellae. A stem portion ( 33 ) of the assembly may be further secured with a wrap ( 38 ) of CMC ribbon.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims benefit under 35 USC 119(e)(1) of the 21 Sep. 2007 filing date of U.S. provisional application 60/974,148, incorporated by reference herein. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention generally relates to ring segments as may be used in gas turbine engines, and more particularly to components of such ring segments made from a ceramic matrix composite (CMC) material. 
       BACKGROUND OF THE INVENTION 
       [0003]    As those skilled in the art are aware, the maximum power output of a combustion turbine is achieved by heating the gas flowing through the combustion section to as high a temperature as is feasible. The hot gas, however, heats the various turbine components, such as the combustor, transition ducts, vanes and ring segments, which it passes when flowing through the turbine. 
         [0004]    Accordingly, the ability to increase the combustion firing temperature is limited by the ability of the turbine components to withstand increased temperatures. Consequently, various cooling methods have been developed to cool turbine hot parts. These methods include open-loop air cooling techniques and closed-loop cooling systems. Both techniques, however, require significant design complexity, have considerable installation and operating costs and often carry attendant losses in turbine efficiency. 
         [0005]    In addition, various ceramic insulation materials have been developed to improve the resistance of turbine critical components to increased temperatures. Thermal Barrier Coatings (TBC&#39;s) are commonly used to protect critical components from elevated temperatures to which the components are exposed. 
         [0006]    The first stage of turbine vanes direct the combustion exhaust gases to the airfoil portions of the first row of rotating turbine blades and their corresponding ring segments. A ring segment is a stationary gas turbine component, located between the stationary vane segments at the tip of a rotating blade or airfoil. These ring segments are subjected to high velocity, high temperature gases under high pressure conditions. In addition, they are complex parts with large surface areas and, therefore, are difficult to cool to acceptable temperatures. Conventional state-of-the-art first row turbine vanes and ring segments may be fabricated from single crystal super-alloy castings, may include intricate cooling passages, and may be protected with thermal barrier coatings. Ceramic matrix composites (CMC) have higher temperature capabilities than metal alloys. By utilizing such materials, cooling air can be reduced, which has a direct impact on engine performance, emissions control, and operating economics. 
         [0007]    One of the limitations of CMC materials, whether oxide or non-oxide based, is that their strength properties are not uniform in all directions (e.g., the inter-laminar tensile strength is less than 5 percent of the in-plane strength). Anisotropic shrinkage of matrix fibers results in de-lamination defects in small radius corners and tightly curved sections, further reducing the already low inter-laminar properties. Thus, the use of CMC materials for gas turbine components has been limited. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]    The invention is explained in the following description in view of the drawings that show: 
           [0009]      FIG. 1  is a cut-away perspective view of a coolant plenum structure including a portion of a ring segment in accordance with the present invention. 
           [0010]      FIG. 2  is a perspective view of the stacked lamellae bowtie ring segment in accordance with the present invention. 
           [0011]      FIG. 3  is an exploded view of the stacked lamellae bowtie ring segment in accordance with the present invention. 
           [0012]      FIG. 4  is a top view of the stacked lamellae bowtie ring segment in accordance with the present invention, taken along the line  4 - 4  of  FIG. 5 . 
           [0013]      FIG. 5  is a cross-sectional view of the stacked lamellae bowtie ring segment in accordance with the present invention, taken along the line  5 - 5  of  FIG. 4 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0014]    The present invention is a ceramic matrix composite (CMC) ring segment utilizing a series of stacked and bonded flat CMC lamellae. The CMC material may be any such material known in the art. One example of a commercially available oxide fiber/oxide matrix CMC material is a Nextel 720 fiber/alumina matrix composite available from COI Ceramics, Inc. of San Diego, Calif. The individual stacked lamellae are machined to the desired shape then bound together, and held in place with a bowtie shaped plate of CMC material oriented to carry the inter-laminar loads of the stacked lamellae assembly. The structure of the present invention takes advantage of the strengths of the CMC two-dimensional lamella materials while overcoming their fundamental weakness, that is, low inter-laminar strength, by incorporating another plate oriented with a strong axis in the inter-laminar direction of the stacked assembly. Advantages of this design include ease of manufacture, repeatability, design robustness and flexibility. 
         [0015]    Referring now to the drawings and to  FIG. 1  in particular, a cut-away perspective view of a portion of a coolant plenum structure including a ring segment  10  in accordance with one embodiment of the present invention is shown. The ring segment  10  is constructed of CMC material. The ring segment is held in place by a pair of isolation rings  12  and  13 , which are manufactured of a metal alloy as may be known in the art. The isolation ring  12  is upstream relative to a flow of working gases  15  moving through a chamber  14  of the turbine structure, whereas isolation ring  13  is downstream relative to the working gas movement. The turbine blades (not shown) rotate in the space immediately below the ring segment within the chamber  14 . 
         [0016]    A seal  16  is disposed over the ceramic ring segment  10  between the isolation rings  12  and  13 . The seal  16  and walls  17  of the ring segment  10  create a plenum  18 , which conducts a coolant for the structure. The coolant is directed into the plenum  18  through one or more openings  20  formed in the seal assembly stack  16 . The coolant is typically at a pressure substantially higher than that of the working gas  15 , and passes through a small crevice  21  formed between the bottom of the assembly stack  16  and the top ledges of the ring segment  10 , which movement is denoted by arrows  22 . The coolant then passes through small orifices  23  formed in each of the isolation rings  12  and  13  and on to the working gas chamber  14 . 
         [0017]    With reference now to  FIG. 2 , a perspective view of the stacked lamellae bowtie ring segment  10  of  FIG. 1  is shown. As stated hereinabove, the ring segment is made of CMC material and comprises several individual parts. First, there is the main structure  25 , which is formed of a plurality of individual flat CMC lamellae bonded together (as will be shown in the exploded view of  FIG. 3 ). The strongest plane of the CMC lamellae (i.e. plane of orientation of the reinforcing fibers of the 2-D fiber weave) is oriented in the plane of the lamellae and perpendicular to a longitudinal axis of the structure, as denoted by an arrow  26 . Second, the individual lamellae are held together by a bowtie plate  27  and by wraps of CMC ribbons  28 , both having their strongest planes (i.e. reinforcing fiber orientation) parallel to the longitudinal axis of the structure and perpendicular to the strong plane of the CMC lamellae (arrow  26 ). The bow-tie member  27  forms a double wedge that mechanically constrains the lamellae from separating when it is inserted into a cooperatively shaped double wedge channel  11  defined in the stacked assembly by channels  27   a ,  27   b , . . . formed in the perimeter shape of the respective lamellae. Thus, each lamella may have a slightly different shape than its adjacent lamellae such that the assembly defines a double wedge shaped channel  11  into which the bow-tie member  27  can be lowered, as illustrated in  FIG. 3 . A top plate  29  is inserted over the bowtie  27  by sliding it into slots  30  to hold the bow-tie member  27  in the channel  11 . the top plate  29  may also be a CMC member and the strong plane of the top plate may be parallel to the longitudinal axis of structure and perpendicular to the strong plane of the lamellae (arrow  26 ). 
         [0018]    Once the individual lamellae are bound together to form the ring segment  10 , the bottom surface  31  may be ground down to form an arc approximating the travel of the tips of the turbine rotor blades (not illustrated) in the chamber  14 . Moreover, the surface may be left irregular—that is, it is not ground smooth, in order to receive a coating  32  of an abradable ceramic material, which is well known in the art. Abradable materials are used for high temperature insulation. Abradability is usually achieved by altering the density of the material. During operation of the turbine, rotation of the blades causes them to approach the abradable coating  32 , and when heated, the blades expand slightly and the tips then contact the coating  32  and carve grooves in the coating without contacting the structural CMC portion of ring segment  10 . These grooves provide a seal for the turbine blades. 
         [0019]    Referring now to  FIG. 3 , an exploded view of the stacked lamellae bowtie ring segment  10  is shown. It may be appreciated from this exploded view that the main structure  25  is formed of a plurality of similar-shaped lamellae  25   a ,  25   b , . . . , that are bonded together, such as with an adhesive or via a sintering process. The bow-tie structural member  27  is inserted into channel  11 . The bow-tie  27  acts as a wedge for holding the individual lamellae  25   a ,  25   b , . . . together. It is pointed out that the channel  11  is made progressively smaller toward the longitudinal center of the assembly. In this manner the channel is wider toward each end of the ring segment and more narrow toward the center, thereby forming the double wedge shaped channel  11  adapted for receiving the bow-tie member  27 . The assembly and firing sequence for these parts provides a variety of possibilities for achieving favorable shrinkage of the bow-tie member  27  relative to the main structure  25  so that it induces compressive stresses across the stacked lamellae  25 . Alternative materials can be used for the bow-tie member  27 . For example, aluminosilicate matrix can used in cooler regions of the turbine where its superior bond strength and increased shrinkage can be use to advantage. 
         [0020]    The top plate  29  is inserted into the slots  30  and on top of the bow-tie member  27 . The CMC ribbons  28  are wrapped around the structure  25  at a stem  33  thereof. It is pointed out that the stem  33  is made progressively larger in a first half of each of the lamella and then progressively smaller in the second half of each of the lamella. In this manner the stem  33  is most narrow at each end and thickest at the center. Accordingly, a race track shape is formed for receiving the CMC ribbons  28 , as may be seen in the top view of  FIG. 4 . 
         [0021]    The bottom surface  31  of the structure  25  is ground down approximating the arc formed by the rotation of the tip of the turbine blade, and the abradable material layer  32  is deposited onto the ground bottom surface. 
         [0022]    With reference now to  FIG. 4 , a top view of the stacked lamellae bowtie ring segment  10  taken along the line  4 - 4  of  FIG. 5  is shown. The double wedge shape of the bow-tie structural member  27  is shown in dashed line. While the specific embodiment illustrated herein show a “double wedge shape” and “bow-tie” that are formed by generally symmetrical straight lines, it may be appreciated that these terms are meant to be generally descriptive of any such shape effective to constrain the lamellae from separating along the longitudinal axis. Other shapes that may be envisioned under the terms double wedge shape and bow-tie member may have curved lines or a combination of curved and straight lines or non-symmetrical lines, so long as the lamellae are prevented from separating from each other by the shape. It may be appreciated that the bow-tie member  27  functions as a wedge that mechanically constrains and holds together the individual lamellae  25   a ,  25   b , . . . . Also, it may be appreciated from  FIG. 4  that the wrap  28  around the varying width of the stem  33  forms a curved race-track shape that offers several benefits. First, the wrap  28  is not bent around sharp corners, which reduces stress concentrations at the ends. Second, the coolant air is free to move around the ends of the wrap  28 ; and, third the race-track shape helps distribute load during the manufacturing process. 
         [0023]    With reference to  FIG. 5 , a cross-sectional view of the stacked lamellae bowtie ring segment  10 , taken along the line  5 - 5  of  FIG. 4 , is shown. Accordingly, it may be appreciated from the discussion hereinabove that the use of thin-sheet lamellae  25   a ,  25   b , . . . to fabricate the ring segment  10  enhances and simplifies the manufacturing process in that the lamellae are scalable and amenable to automation. Moreover, the thin-sheet lamellae are straight-forward to inspect for critical flaws. The complex outline shapes of the lamellae can be readily cut using programmable lasers or water jet methods. Additionally, it may be appreciated that the bond and inter-laminar weakness of the CMC lamellae stacks are overcome by the CMC bow-tie member  27  and/or wrap  28 . By process sequencing or material selection for the bow-tie member  27  and/or wrap  28 , compressively preloaded assemblies can be achieved in order to further minimize inter-laminar tensile stresses in the stacked lamellae  25 . Finally, the use of the top plate  29 , locked into place by the slots  30 , prevents any buckling of the bow-tie member  27 . 
         [0024]    While various embodiments of the present invention have been shown and described herein, it will be obvious that such embodiments are provided by way of example only. Numerous variations, changes and substitutions may be made without departing from the invention herein. Accordingly, it is intended that the invention be limited only by the spirit and scope of the appended claims.