Patent Publication Number: US-7908867-B2

Title: Wavy CMC wall hybrid ceramic apparatus

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to a hybrid apparatus including a wavy wall of ceramic matrix composites (CMCs) bonded to a ceramic insulating layer, the wall having specified wave parameters in various embodiments. The hybrid apparatus may be a component of a gas turbine engine, such as a duct-like component wherein the ceramic insulating layer defines a hot gas passage. 
     BACKGROUND OF THE INVENTION 
     Engine components that are exposed to the hot combustion gas flow of modern combustion turbines are required to operate at ever-increasing temperatures as engine efficiency requirements continue to advance. Ceramics typically have higher heat tolerance and lower thermal conductivities than metals. For this reason, ceramics have been used both as structural materials in place of metallic materials and as coatings for both metal and ceramic structures. Ceramic matrix composite (CMC) wall structures with ceramic insulation outer coatings, such as described in commonly assigned U.S. Pat. No. 6,197,424, have been developed to provide components with the high temperature stability of ceramics without the brittleness of monolithic ceramics. 
     Further as to the relatively lower thermal conductivity of CMCs, it is known to use radiation cooling, such as described in commonly assigned U.S. Pat. No. 6,767,659, and/or convective cooling or impingement cooling on back surfaces of component walls. However, backside cooling efficiency is reduced by the low thermal conductivity of ceramic material and by the fact that the wall thickness of a CMC structure, to achieve a desired strength, may be thicker than an equivalent metal structure. U.S. Pat. No. 5,687,572 teaches a backside impingement-cooled cylindrical ceramic liner of a combustor attached by pins to an outer metal shell. This reference cites thicknesses expected to withstand particular loads, discusses that thinner liners have lower thermal stresses, and refers to an analysis of buckling. It does not deviate from a uniform cylindrical configuration of the ceramic liner. 
     More generally, the issues related to strength properties per unit weight or thickness and to the cooling of structures made with CMCs are of particular concern for gas turbine engine components that are exposed to or are near the hot combustion gas path. As one approach to address these issues, a CMC lamellate wall structure with a high temperature ceramic insulation coating, commonly referred to as friable grade insulation (FGI), is disclosed in commonly assigned U.S. Pat. No. 6,197,424. Current materials of this type provide strength and temperature stability to temperatures approaching 1700° C. Also, the commonly assigned U.S. Pat. No. 6,709,230 describes cooling channels in a ceramic core of a gas turbine vane behind an outer CMC airfoil shell, and commonly assigned U.S. Pat. No. 6,746,755 uses ceramic matrix composite cooling tubes between CMC face sheets to form a CMC wall structure with internal cooling channels. 
     Notwithstanding these advances, further improvements in the design of hybrid CMC/ceramic insulating layer apparatuses are desired to support further applications of such structures in gas turbine engines, particularly in those engines in which an increase in the firing temperatures is expected and/or greater loads are imposed on the transition. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention is explained in the following description in view of the drawings that show: 
         FIG. 1  is a schematic depiction showing the major components of a modern gas turbine engine. 
         FIG. 2A  is a perspective view of a typical transition of a gas turbine engine. 
         FIG. 2B  provides a cross-sectional view of the transition of  FIG. 2A  taken along line B-B of  FIG. 2A , showing features of the present invention. 
         FIG. 3  is a linearized depiction of the portion of the CMC wall shown in the box of  FIG. 2B , provided to describe parameters of the waves of the CMC wall. 
         FIG. 4  is a graph showing the relationship between the y/t ratio and stiffness and strength ratios for two data sets. 
         FIG. 5A  is a perspective view of a ring segment of the present invention. 
         FIG. 5B  is a cross-sectional view of a ring segment of the present invention. 
         FIG. 5C  is a partial cut-away view of a portion of the ring segment of  FIG. 5B , taken along line C-C of  FIG. 5B . 
         FIG. 6A  is a perspective view of a combustor liner of the present invention. 
         FIG. 6B  is a partial cut-away view of a portion of the ring segment of  FIG. 6A , taken along line B-B of  FIG. 6A . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present inventors have appreciated that uses of ceramic matrix composites (CMCs) in gas turbine engine components exposed to high temperatures must take into account their relatively low thermal conductivity as well as difficulties related to the fabrication of intricate cooling passages, such as may be needed in part to overcome the relatively low thermal conductivity. Rather than solely utilizing more traditional approaches, such as developing specific cooling passage technologies for CMCs (some of which novel approaches are referred to herein), the present inventors conceived of forming and using hybrid apparatuses comprising a relatively thin and wavy CMC wall with a ceramic insulating material on one side, the latter suitable for direct exposure to a hot gas passage of a gas turbine engine or other exposure to elevated temperatures, while maintaining the other uninsulated side with an exposed wavy form providing an increased surface area for cooling, in such a way as to increase stiffness and strength along desired axes, while also achieving a desired thermal transfer across, and cooling of, the thin and wavy CMC wall. 
     This approach, which involves imparting a designed waviness to the CMC thin wall, overcomes the relatively low thermal conductivity of CMCs yet provides a structure of sufficient stiffness and strength in one or more desired axes. In some embodiments the hybrid wavy CMC wall/ceramic insulating structure may form only part of a component or apparatus, and in other embodiments an entire component may be formed of the inventive structure. When utilized in gas turbine engine components, the ceramic insulating layer may comprise a wearable or abradable insulation, and/or it may define an insulated hot gas flow passage. 
     Features of the invention may be appreciated by reference to the appended figures and table, which are meant to be exemplary and not limiting. Prior to presentation of specific embodiments of the invention, however, a discussion is provided of a common arrangement of elements of a prior art gas turbine engine into which may be provided embodiments of the present invention. 
       FIG. 1  provides a schematic cross-sectional depiction of a prior art gas turbine engine  100  such as may be improved with various embodiments of the present invention. The gas turbine engine  100  comprises a compressor  102 , a combustor  107 , and a turbine  110 . During operation, in axial flow series, compressor  102  takes in air and provides compressed air to a diffuser  104 , which passes the compressed air to a plenum  106  through which the compressed air passes to the combustor  107 , which mixes the compressed air with fuel in a burner. Combustion occurs in a combustion chamber  108  downstream of the combustor  107 . Further downstream combusted gases are passed via a transition  114  to the turbine  110 , where the energy of combustion is extracted as shaft power. A shaft  112  is shown connecting the turbine to drive the compressor  102 , and may also be connected to an electrical generator (not shown). 
     As may be appreciated, a transition such as the transition  114  of  FIG. 1  is exposed to structural and thermal challenges based on its position immediately downstream of the combustor  107  and the desire to operate turbines at the highest feasible temperature range.  FIG. 2A  provides a perspective view of a transition  200  of a gas turbine engine (such as shown in  FIG. 1 ) however comprising features in accordance with the present invention. Transition  200  has an upstream end  202 , a downstream end  204 , and an outer surface  210  that may be exposed to flows of fluid, such as compressed air from a compressor, while such fluid is en route to a combustor intake. The flow of such fluid may make the transition suitable for backside cooling under appropriate circumstances. At the upstream end  202  is an inlet flange  203  that connects to the combustion chamber, and at the downstream end  204  is an exit flange  205  that connects to the turbine. 
     Given combustion dynamics, aerodynamic pressure forces, and associated vibrations imparted to transition  200 , as well as thermally induced stresses, there is a need for stiffness along a flow-based axis, shown by axis line  206 . Considering the temperature tolerance of CMCs and the desired operating temperature range of gas turbine engines, there also is a need to deal with insulation of the hot gas path  250  (see  FIG. 2B ) defined by the transition  200  in a way that provides a desired operating temperature for the CMC material. 
     In view of these considerations and criteria, the transition  200  is an apparatus that comprises a hybrid ceramic structure  207 , also referred to as a sub-combination, comprising, as shown in  FIG. 2B , a wavy CMC wall bonded to a ceramic insulating layer. At its forward and rearward ends, the hybrid ceramic structure  207  joins, respectively, the inlet flange  203  and the exit flange  205 . 
     Certain features of the hybrid ceramic structure  207  are better viewed in  FIG. 2B , which is a cross section view taken along line B-B of  FIG. 2A . As may be so viewed, a wavy CMC wall  214  is designed and constructed so as to provide a desired stiffness and strength, leading to a desired robustness, while also being sufficiently thin so as to benefit from external backside cooling during operation. In particular, a given degree of stiffness may be obtained with the present invention by using a thinner CMC wall than would otherwise be necessary with a non-wavy CMC wall of the prior art. The CMC wall  214  comprises waves  216 , parameters of which are described in greater detail below, which are manifested exteriorly by a first wavy surface  217  and a second wavy surface  218  (the “backside” surface). While not meant to be limiting, the waves  216  may be substantially aligned with a flow-based axis (i.e.,  206 ) extending through the hot gas passage, so as to provide a desired resistance to bending along the length of the transition  200 . Two-dimensional and/or three-dimensional weaves of CMC fibers may be utilized in various embodiments to form the CMC wall  214 , with combinations of such weaves utilized to provide a desired performance for a particular embodiment. Bonded to the first wavy surface  217  of the CMC wall  214  is a ceramic insulating layer  230  that has a distal surface  242  that is smooth (not wavy) due to a varying thickness of the layer  230  and defines a hot gas passage  250 . Thus, the present invention provides for improved stiffness/strength for a given thickness of CMC wall, or conversely, a thinner wall for a required stiffness/strength parameter; it presents an increased surface area on its non-insulated back side for improved radiant, convective or impingement cooling; and it also provides a desired non-wavy surface to the hot combustion gas flow. 
     In view of the previously noted development of CMC components for turbines and other high temperature applications, and also recognizing that adding corrugations to metal turbine components are known (for example, see U.S. Pat. Nos. 5,970,715, 5,279,127, and 5,181,379), the latter having the corrugations of the structural metal directly along the hot gas passage (and aligned transversely to the flow-based axis), it is appreciated that in various embodiments of the present invention the waves of the second wavy surface are exposed for an effective backside cooling, whereas the waves of the first wavy surface do not define the shape of the distal surface that defines the hot gas passage or is otherwise closer to a source of high temperature. This arrangement of elements is effective to provide a strong yet thin CMC wall insulated from extreme temperature and capable of a desired thermal conductivity for cooling. 
     In various embodiments the ceramic insulating layer is of a wearable type, such as those described in commonly assigned U.S. Pat. Nos. 6,013,592, 6,197,424, 6,235,370, and 6,287,511, which are incorporated by reference herein as to such teachings. In various embodiments, the ceramic insulating layer comprises a ceramic insulating material that is non-reinforced and has a heterogeneous microstructure. 
     Construction of apparatuses of the present invention may be accomplished by any methods known to those skilled in the art. Examples of construction methods, and of particular ceramic materials, are provided in the immediately above-cited patents and also in commonly assigned U.S. Pat. Nos. 6,733,907 and 7,093,359, which are incorporated by reference herein as to such teachings. Further to construction approaches, the hybrid ceramic structure may be manufactured in numerous ways that include, but are not limited to, the following four examples: 
     1. The ceramic insulating layer can be cast first and machined on the outside to have a wavy surface that matches the first wavy surface. Then ceramic fabric can be laid up on that wavy surface and processed into the wavy CMC wall with the appropriate matrix, etc. 
     2. The CMC can be laid up in a mold to a desired specific shape. After it is fully fired, the ceramic insulating layer can be cast inside it, along the first wavy surface. 
     3. The CMC can be fiber wound as a cylinder and then formed into a wavy structure. The ceramic insulating layer can then be cast on the CMC. 
     4. The ceramic fiber can be woven as a three-dimensional structure, processed into a CMC structure having the desired thin waves, and the ceramic insulating layer can be cast inside the CMC thereafter. 
     Construction methods may include steps for joining this hybrid ceramic structure with other sub-components of a single apparatus, for example in the case of a transition, there may be steps to join the hybrid ceramic structure with the inlet and outlet flanges. 
     It is noted that transitions made according to the present invention may have a dampening effect on the vibrations driven by combustion dynamics, in terms of damping, transfer, direct damage, or any combination of these. Simple panel or membrane modes of vibration will result in complex stress states by virtue of the anisotropic CMC material oriented in a non-planar, wavy configuration. In-plane shear is induced by simple bending, in addition to interlaminar shear—both of which are known to contribute significantly to damping in composites. 
     More particularly as to certain embodiments of the present invention, the present inventors have determined that a hybrid ceramic apparatus comprising a relatively thin and wavy CMC wall having wave peaks and troughs arranged so as to provide a desired resistance to bending, and a ceramic insulating layer bonded to one surface of the CMC wall, provides a particularly stiff and strong, relatively low weight, and relatively low cost hybrid ceramic apparatus when the wave characteristics and CMC thickness fall within defined ranges. Advantageously, such apparatuses comprising hybrid ceramic structures conforming to the parameter ranges also provide unexpectedly favorable heat management characteristics. 
     These ranges may be understood by reference both to Table 1 and  FIG. 3 .  FIG. 3  is an enlarged and linearized representation of the boxed section of  FIG. 2B  that provides greater details of the wavy CMC wall  214 , and a corresponding portion of the associated ceramic insulating later  230 . Wavy CMC wall  214  has a thickness  222 , an amplitude  224 , and a period or wavelength  226  each of which falls within specific ranges described below. Also, a wave height,  228 , is shown to be equal to the amplitude  224  less the thickness  222 . The parameters x and y also are shown in  FIG. 3  and are evaluated in Table 1, below. The parameter x is equal to one-fourth of the period or wavelength  226  (also referred to as pitch by some in the art), and the parameter y equals one-half of the amplitude  224  (also referred to in some embodiments as depth by some in the art). 
     Table 1 demonstrates the derivation of desired ranges of parameters for a relatively thin and wavy CMC wall used in various embodiments of hybrid ceramic structures of the present invention. Hypothetical examples of wavy CMC walls in Table 1 are defined by parameters described in relation to  FIG. 3 . These examples are divided into two groups: a first group for which x/y=1 and having y/t varying from 1 to 2; and a second group for which x/y=5 and having y/t also varying from 1 to 2; Using a known formula to calculate the second moment of area, designated I, a value for the second moment of area is determined for each member of the three groups. The second moment of area is a measure that indicates resistance to bending along an axis substantially parallel with the waves so formed. Each such value, designated as I Corr (for corrugated), is then compared to a calculated value for a second moment of area for a flat wall having the same CMC wall thickness. This is shown as I Flat. Such comparisons are shown in the column identified as Moment Ratio. A Strength Ratio, designated as the moment, σ, of a flat object divided by a corrugated object, σ flat /σ corr , is calculated based on the following formula: 
     
       
         
           
             
               
                 σ 
                 flat 
               
               
                 σ 
                 corr 
               
             
             = 
             
               
                 
                   I 
                   corr 
                 
                 
                   I 
                   flat 
                 
               
               · 
               
                 
                   y 
                   flat 
                 
                 
                   y 
                   corr 
                 
               
             
           
         
       
     
     where y flat  is half the thickness of the flat object. 
     The comparisons identify and better characterize aspects of the conceived thin and wavy CMC structural wall. The data show the stiffness and strength obtained with thin wavy wall structures of the present invention. An added benefit beyond these properties as to the use of such structures in gas turbines and other devices exposed to high temperatures is the unexpected additional benefit of relatively easy cooling, such as by convection and/or radiation, owing to the relative thinness of the wavy CMC wall and its exposed backside wavy surface (despite the recognized low thermal conductivity of CMC). 
     During the data development and analysis, the present inventors realized that the y/t parameter, which may be conceptualized as a “wave-height-to-thickness ratio,” governs the Moment Ratio. This can be seen by comparing the increase in y/t with the increase in second moment ratio for the two groups. This shows that y/t controls the Moment Ratio whereas neither x/y nor period of wave, reflected in x, controls the Moment Ratio. 
     The data from Table 1 are shown graphically in  FIG. 4 . This shows, first, that regardless whether x/y is one or five, the ratio of bending stiffness of the wavy design to a flat plate is substantially the same at a given y/t value. The strength ratios also are plotted in  FIG. 4 , and present less steep curves that are correlated to y/t. Significantly, strength (or load carrying capability) is also increased, thus enabling achievement of all benefits simultaneously. 
     Based on this, embodiments of the present thin-walled CMC structures have a desired strength/stiffness combination, and additionally provide a good and unexpected advantage: ability to be cooled despite being constructed with a traditionally poor thermal conductor. Embodiments of ceramic hybrid structures including wavy CMC walls conforming to the following parameter ranges are determined to provide a desirable combination of stiffness, strength, and thermal conductivity, particularly for gas turbine structures and components near or defining a hot gas passage in a gas turbine engine. The ranges for the parameters are as follows: 
     t ranges from 1 to 10 millimeters (“mm”); 
     y/t ranges from 0.75 to 3.0; and 
     x/y ranges from 0.5 to 2.0. 
     In that the parameter x is one-fourth of the wave period and y is one-half of the wave amplitude, the latter range may alternatively be defined in terms of a period being between 1 and 4 times the wave amplitude. 
     Also, in various embodiments the height of the wavy CMC wall, which is the wave amplitude (2y) minus the thickness, t, is at least 2 mm. This parameter limit, in combination with the above parameters, has been determined to provide a desired performance for apparatuses of the present invention. It is noted that the height may alternatively be referred to by its relationship to amplitude, namely that it is the amplitude excluding the thickness. 
     While the above ranges in their respective broadest interpretations include their respective endpoints, each of these ranges also is understood to disclose all values therein and all sub-ranges therein, including any sub-range between any two numerical values within the range, including the endpoints. For example, as to the stated range of 0.75 to 3.0 for y/t, this is understood to include the sub-ranges 0.75 to 1.5, 1 to 2, 2 to 3, and other sub-ranges within the stated range of 0.75 to 3.0. 
     Thus, while it has generally been known in related and unrelated arts that corrugation improves rigidity along a particular axis, the present hybrid CMC invention relates to the particular achievement of a desired stiffness and strength, combined with an unexpected benefit of cooling effectiveness through use of a relatively thin wall wavy CMC structure in which the backside wavy surface of the wavy CMC wall is exposed so as to provide for a desired cooling effect. Embodiments of the present hybrid CMC invention comprise a ceramic insulating layer bonded to one side of the wavy, relatively thin CMC wall, insulating the wall from heat on the non-bonded side of the ceramic insulating layer (such as from a hot gas passage), the waviness adding surface area for enhanced bonding between the CMC wall and the ceramic insulating layer, such as enhanced bonding on a macroscopic level, and the other side of the wavy, relatively thin CMC wall having its wavy surface exposed to provide a desired cooling effect. Reference is made to commonly assigned U.S. Pat. No. 6,984,277, which describes one embodiment providing bond enhancement structures formed as waves in an upper surface of a layer of CMC material, that surface contacting a ceramic insulating material that comprises hollow ceramic spheres. The layer of CMC material in that embodiment may comprise rods or cooling passages therein. However, the side opposite the side with the waves in that prior art patent is flat and does not afford the level of thermal conductivity provided by embodiments of the present invention, which have such backside surface having an exposed wavy surface. 
     As to one class of embodiments, one may construct a duct-shaped member comprising a combination of an appropriately wavy thin-walled CMC wall layer bonded to a more internally disposed ceramic insulating layer that defines a path through which flows fluid at an elevated temperature (such as hot combusted gas). This class is exemplified by the transition of  FIGS. 2A and 2B , which may be installed in a suitable gas turbine engine such as that depicted in  FIG. 1 . The wavy hybrid structure  207  joins the inlet flange  203  at a forward end and the exit flange at a rearward end, such as through respective regions in which the duct-like transition wall transitions from the hybrid CMC structure  207  to a more conventional CMC structure (for instance, lacking the waves conforming with the ranges provided above). The inlet flange  203  may be made of a metal alloy or CMC composite or other material but which is not wavy in this embodiment, and the exit flange  205  which may likewise be of a different material and not be formed in accordance with the above teachings for a hybrid ceramic structure. As shown in  FIG. 2A , the exit flange  205  may be formed in a generally rectangular shape, and there is a transition in shapes from the generally cylindrical shape of the main portion of the transition to this generally rectangular shape of the exit flange  205 . In view of the waves, the thickness, and other parameters of the CMC wall varying from the above ranges in such transition regions near the inlet flange  203  and exit flange  205 , supplemental forms of cooling, as are known to those skilled in the art, may be provided in various embodiments for such transition regions. Advantageously, in this embodiment the surface of the ceramic insulating layer that defines the hot gas path is generally tubular in shape and lacks the waves of the thin CMC wall material, thereby ensuring the desired smooth flow of hot gas there through. 
     Another example regarding such duct-shaped components is a ring segment that may form part of a blade ring that surrounds a turbine blade. The role of a blade ring, and the ring segments that form it, is to surround a turbine blade and tightly define the space within which the blade rotates. Aspects of this are taught in co-assigned U.S. Pat. No. 6,758,653, which in incorporated by reference for its teachings of blade rings and their components, and also for its specific teachings of a support member with cooling passages that may be optionally provided in embodiments of the present invention. 
       FIG. 5A  provides a perspective drawing of one embodiment of a ring segment  500  having features of the present invention. The ring segment  500  is comprised of two parallel positioned spaced apart support members  534 , here comprised of CMC, and a hybrid CMC structure  502  comprising a wavy CMC wall  532  and a ceramic insulating layer  538  in close proximity to a blade tip  514 . When the support members are comprised of CMC they may be formed with and thus are integral with the hybrid CMC structure  502  (indicated by dashed lines at junction). The support members  534  may be alternatively be comprised of a metal alloy or other materials, and may comprise, as depicted in  FIG. 5A , one or more grooves  562  that may be provided to relieve hoop stresses that may be imparted during operation. The waves  537  of the wavy CMC wall  532  are observable along a viewable inside wall  536  of one of the support members  534 . As to laterally disposed portions of the waves  537  which lie under the spaced apart support members  534 , heat from these portions is conducted axially outward through the support members  534 , which are exposed to a flow of cooling fluid (not shown). While not meant to be limiting, the waves  537  are substantially aligned with a flow-based axis  506  extending through a hot gas passage  550  defined in part by the ring segment  500 . This orientation of the waves  537  provides a desired resistance to bending from front to back (i.e., upstream to downstream) ends of the ring segment  500 . In various embodiments, the properties of the wavy CMC wall  532  fall within the parameters described above and claimed herein. 
     The embodiment of  FIG. 5A  is cooled by backside cooling resulting from a flow of cooling fluid provided through pathways known in the art. Alternately, the use of radiation cooling techniques may be used, as noted above. An optional support member comprising cooling passages, as taught in co-assigned U.S. Pat. No. 6,758,653, may also be provided in some embodiments.  FIG. 5B  depicts one such embodiment. One or a plurality of cooling passages  558  may be formed in support member  534 , which in this embodiment extends over the outer surface of wavy wall  532 , to permit a portion of cooling fluid  524  to pass into a gap  544  to provide cooling for wavy CMC wall  532 . Sealing members such as O-ring seal  560  may be provided to direct the flow of the cooling fluid  524 . Cooling fluid can be directed to exit the gap  544  via leakage through seals  560  and/or through circumferential seals between adjacent segments (not shown). Such leakage flows are typically adequate for cooling CMC components. The size of the opening  526 , and cooling passages  558  and the pressure of the cooling fluid  524  may be selected to provide a desired flow rate of cooling fluid  524  through the gap  544 . The temperature of the support member  534 , which in this depicted embodiment is made of metal, is maintained below a desired upper limit as a result of the combination of the insulating action of ceramic insulating layer  538  and wavy CMC wall  532  and the active cooling provided by cooling fluid  526 . The thermal conductivity characteristic of the wavy CMC wall  532 , as well as that of the ceramic insulating layer  538 , which together comprise the hybrid CMC structure  502  (see  FIG. 5C ), is selected to be sufficiently low to maintain the support member  534  below a predetermined temperature during operation of the combustion turbine engine so that it is possible to provide direct contact between the wavy CMC wall  532  and the metal support member  534  without the need for any intervening thermal insulating material. Such contact will occur at least along portions of the mating surfaces of arcuate portions  551 ,  552  and the extending portions  546 ,  548 . 
     Also, in view of the fact that some degree of abrasion is tolerated in an attempt to minimize the amount of combustion gas  516  that passes around blade tip  514  without passing over blade  512 , it is expected that blade tip  514  may on occasion make contact with the ceramic insulating layer  538 , which is abradable. This will thereby impose a mechanical force into wavy CMC wall  532 . From a design perspective, wavy CMC wall  532  must be able to absorb such force without failure. A shroud assembly  530  of  FIG. 5B  accommodates such rubbing forces by allowing such force to be transferred to the metal support member  534 . This is accomplished by controlling the maximum allowable dimension for gap  542  so that when blade tip  514  rubs against the shroud assembly  530 , the wavy CMC wall  532  will deflect to reduce the gap to zero in at least one location opposed the blade  512  and remote from the arcuate portions  551 ,  552  so that the radially inner surface  540  of support member  534  provides support against the radially outer surface  544  of the wavy CMC wall  532 . The support member  534  is designed to provide a predetermined resistance to further deflection of the wavy CMC wall  532  once the gap  542  is reduced to zero, thereby limiting the peak stress in the wavy CMC wall  532 . The maximum dimension of gap  542  is selected to control the level of stress developed in the shroud member  530 , in particular in the arcuate portions  551 ,  552  of CMC member  533 , which comprises the wavy CMC wall  532  as well as the arcuate portions  551 ,  552 , that fit within grooves  554 ,  556  of support member  534 , as the wavy CMC wall  532  deflects during a rubbing event. 
     Further to the features of the present invention,  FIG. 5C  provides a partial cross-section taken along line C-C of  FIG. 5B . In  FIG. 5C , the wave pattern of wavy CMC wall  532  is viewable, as is a cross-sectional view of the hybrid CMC structure  502  that also comprises the ceramic insulating layer  538  that defines hot gas passage  550 . The cooling passages  558  are aligned with the troughs  531  of the waves of wavy CMC wall  532 . In such alignment the cooling passages do not contact the peak points  535  directly upon a deflection of the wavy CMC wall  532 . Alternately, the metal support member  534  may conform to the wavy surface  532  in close proximity to enhance radiative cooling effects. 
     For a curved type of duct-like structure, such as the transition and the ring segment described above, it is noted that the ranges above are meant to apply to a linearized modification of the waves as they exist in a curved configuration. A linearization essentially averages out the smaller wave measurements to the interior, and the larger wave measurements (such as peak to peak distance) to the exterior. For example,  FIG. 3  is a linearized depiction of the boxed portion of  FIG. 2B . Accordingly, claiming the parameter ranges on a linearized basis is meant to smooth and standardize the wave deviations due to curving a wave form. 
     Other gas turbine components that could potentially benefit from this invention include combustor liners, interstage turbine ducts, exhaust ducts, afterburner ducts, and exhaust nozzle components including nozzle flaps—virtually any high temperature component having a range of shapes, including flat, relying on backside cooling and having light weight and high stiffness. In some embodiments, such as for these components, the entire structure is comprised of a hybrid wavy wall bonded to ceramic insulation. 
     Also, the use of convective cooling of the wavy backside of the wavy CMC wall is not meant to be limiting. For example, U.S. Pat. No. 6,767,659 teaches coating a backside of a CMC composition with a high temperature emissive material and providing a metal element spaced apart from the CMC composition and defining a gap between the metal element and the ceramic matrix composite, whereby at least a portion of thermal energy exposed to the ceramic insulating material is emitted from the high temperature emissive material to the metal element. A cooling fluid may be made to flow by the backside of the metal element, thereby assisting in the cooling of the CMC composition. Accordingly, the teachings of U.S. Pat. No. 6,767,659 may be combined with the wavy CMC wall hybrid structure by addition of an emissive coating, and may also include a metal element spaced apart from the wavy CMC wall. 
     Further, other forms of cooling may be combined with the wavy CMC wall hybrid structure. Film cooling or effusion cooling through the hybrid CMC wall can also be used with the wavy construction—either separately or in combination with the above cooling techniques. 
       FIG. 6A  is a perspective view of a combustor liner  600  according to the present invention. Combustor liner  600  comprises an outer liner  602  that comprises a wavy CMC wall  603  and a more interiorly disposed ceramic insulating layer  604 . An optional inner liner  608  comprises a wavy CMC wall  609  and a more exteriorly disposed ceramic insulating layer  610 . The ceramic insulating layers  604  and  610  define a hot gas path  650  traveling along a flow-based axis  606 . 
       FIG. 6B  is a partial cut-away view of a portion of the outer liner  602  of  FIG. 6A , taken along line B-B of  FIG. 6A . From this view it is clear that the orientation of the waves  637  are perpendicular to the flow based axis  606  (see  FIG. 6A ). This orientation functions to limit hoop stress. The above discussion and examples are not meant to be limiting as to the geometric form of the waves in embodiments of the present invention. That is, any type of wave geometric form may be utilized, including but not limited to: sinusoidal; rectangular; trapezoidal; and triangular. 
     The present invention may be combined with other approaches to the use of ceramic structures and components for gas turbines and for other devices that are subject to exposure to high temperatures. The ranges of parameters provided above to achieve a stiff (for example, along a desired axis) and strong yet relatively thin and effectively thermally conductive wavy CMC wall may be applied to structures and components that include not only the ceramic insulating layer, but that also may include cooling channels, multiple layerings forming the wavy wall, additional CMC walls, additional ceramic or other core or filler materials, and/or reinforcement pieces. 
     All patents, patent applications, patent publications, and other publications referenced herein are hereby incorporated by reference in this application in order to more fully describe the state of the art to which the present invention pertains, to provide such teachings as are generally known to those skilled in the art, and to provide such teachings as are noted through references herein. 
     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. Moreover, when any range is understood to disclose all values therein and all sub-ranges therein, including any sub-range between any two numerical values within the range, including the endpoints. Accordingly, it is intended that the invention be limited only by the spirit and scope of the appended claims.