Abstract:
A component formed at least in part by a CMC material and equipped with an integrally-formed surface feature, such as an airflow enhancement feature in the form of a turbulator or flow guide. The CMC material comprises multiple sets of tows woven together to form a preform that is infiltrated with a matrix material. The surface feature is integrally defined at a surface of the cooling passage by an insert member disposed between adjacent tows of at least one of the tow sets. The insert member has a cross-sectional size larger than the adjacent tows, forming a protrusion in the preform that defines the surface feature in the infiltrated, consolidated and cured CMC material.

Description:
BACKGROUND OF INVENTION 
     1. Field of Invention 
     The present invention generally relates to air-cooled components, such as combustor liners for gas turbine engines. More particularly, this invention is directed to a process for incorporating surface features along the airflow passages of a component, such as airflow enhancement features to improve the cooling efficiency of the component. 
     2. Description of the Related Art 
     Higher operating temperatures for gas turbine engines are continuously sought in order to increase their efficiency. However, as operating temperatures increase, the high temperature properties of the engine components must correspondingly increase. While significant advances have been achieved through formulation of iron, nickel and cobalt-base superalloys, the high temperature properties of such alloys are often insufficient to withstand long exposures to operating temperatures within the turbine, combustor and augmentor sections of some high-performance gas turbine engines. As a result, internal cooling of components such as combustion liners, blades (buckets) and nozzles (vanes) is often employed, alone or in combination with a thermal barrier coating (TBC) system that thermally protects their exterior surfaces. Effective internal cooling often requires a complex cooling scheme in which air is forced through passages within the component and then discharged through cooling holes at the component surface. 
     The performance of a turbine component is directly related to the ability to provide a generally uniform surface temperature with a limited amount of cooling air. To promote uniform convective cooling of the component interior, it is conventional to cast airflow enhancement features, such as turbulators (trip strips) and flow guides, on the surfaces of the component that define the cooling passages. The size, shape and placement of the airflow enhancement features affect the amount and distribution of air flow through the cooling circuit and across the external surfaces downstream of the cooling holes, and as such can be effective in significantly reducing the service temperature of the component. 
     Ceramic matrix composite (CMC) materials have been considered for combustor liners and other high-temperature components. Continuous fiber-reinforced CMC materials are typically woven from tows (bundles of individual filaments) using conventional textile weave patterns, in which two or more sets of tows are woven, with the individual tows of each set passing over and under transverse tows of the other set or sets. As with air-cooled components formed of metal alloys, it is desirable to incorporate airflow enhancement features in air-cooled CMC components. However, because CMC materials exhibit relatively poor interlaminar tension and shear strengths, airflow enhancement features and other surface features cannot be reliably attached using secondary attachment manufacturing procedures if the component is intended for use in the high thermal strain environment of a gas turbine engine. Moreover, because of tow size and weave limitations, it is difficult to weave small geometry turbulators and flow guides (typically projecting from the surrounding surface a distance of about 0.3 to about 2.0 mm) as integral features of a CMC component. Consequently, while airflow enhancement features of the type used with air-cooled metal components can generally be incorporated in the metal casting process so as to be integral with the primary component, attempts to design integral turbulators, flow guides and other surface features in CMC materials have proven problematic. Faithfully reproducing turbulators and other extremely small-scale, detail geometric features in continuous fiber-reinforced CMC materials is particularly difficult. 
     In view of the above, while CMC materials offer the capability of significantly increasing the maximum operating temperatures sustainable by turbine and other high-temperature components, it would be desirable to incorporate airflow enhancement features in air-cooled CMC components in order to further extend component life and increase engine efficiency. 
     SUMMARY OF INVENTION 
     According to the present invention, there is provided an air-cooled component formed at least in part by a CMC material, and having at least one cooling passage equipped with an integrally-formed surface feature, such as an airflow enhancement feature. The CMC material comprises at least first and second sets of tows woven together to form a preform that is infiltrated with a matrix material. The tows within each set are side-by-side to each other, but transverse to tows of the other set, with tows of each set passing over and under transverse tows of the other. The surface feature is integrally defined at a surface of the cooling passage by an insert member disposed between adjacent tows of at least the first set of tows. In the method of forming the integral surface feature, the insert member is placed between the adjacent tows of the first set of tows during the weaving process, preferably when forming the outermost layer (lamina) of the preform. The insert member has a cross-sectional size larger than the adjacent tows, thereby forming a protrusion in the preform and, after infiltration, consolidation and curing, the surface feature in the surface of the CMC material. The surface feature projects into the cooling passage relative to the immediately surrounding surface region of the passage surface. 
     In view of the above, the present invention entails integrally forming one or more surface features, particularly airflow enhancement features such as turbulators and flow guides, by strategically placing insert members in the CMC preform during the initial preforming step of the CMC process. The insert member is able to create a functional turbulator or flow guide in the form of a permanent integral surface feature after the woven tows are fully processed, including infiltration with a suitable matrix material, densification and consolidation, and curing of the matrix material to form the CMC. As a result of being integrally formed, the surface feature exhibits better structural integrity as compared to a surface feature added to a CMC by a secondary attachment technique. The manner in which the surface feature is an integral feature retained by the woven fiber network provides a load shielding mechanism, capable of keeping interlaminar tension and shear stresses on the surface feature well within the structural capabilities of the CMC material. 
     Other objects and advantages of this invention will be better appreciated from the following detailed description. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     FIG. 1 is a cross-sectional representation of a CMC combustor liner having a cooling passage equipped with integral turbulators in accordance with this invention. 
     FIGS. 2 and 3 are cross-sectional representations of wall portions of preforms for the liner, in which surface features are formed by a stuffer tow and an insert, respectively, in the preform architecture in accordance with two embodiments of this invention. 
    
    
     DETAILED DESCRIPTION 
     The present invention will be described in reference to a combustor liner  10 , a portion of which is represented in cross-section in FIG. 1, though the invention is equally applicable to airfoil components such as a turbine blades and vanes. While particularly useful for forming airflow enhancement features, such as turbulators and flow guides for air-cooled components that operate within a thermally hostile environment, the invention is generally applicable to a variety of CMC components in which a small-scale surface feature is desired. In addition, while CMC materials are of particular interest, the invention is applicable to any continuous fiber-reinforced composite material, including polymer matrix and bismalimide matrix materials. 
     As represented in FIG. 1, the liner  10  has a cooling passage  12  defined by a surface  14 , and a trailing edge  16  near which a number of turbulators  18  are formed. The turbulators  18  are shown as being disposed transverse to the airflow direction through the passage  12 , as indicated by the arrow in FIG.  1 . However, it is foreseeable that the turbulators  18  could be oriented perpendicular or parallel to the airflow direction (to serve as flow guides), may be continuous or discontinuous (interrupted), and may be V-shaped or have another nonlinear shape. According to known practice, the turbulators  18  are intended to disrupt laminar airflow over the surface  14  in order to promote convection heat transfer from the liner  14  to the air. For this purpose, the turbulators  18  preferably project at least 0.30 mm from the surface  14 , with a suitable height being about 0.50 to about 2.0 mm above the surface  14 . 
     The liner  10  is formed of a continuous fiber-reinforced CMC material, such as silicon carbide, silicon nitride or silicon fibers in a silicon carbide, silicon nitride and/or silicon-containing matrix material. The surface  14  of the liner  10  may be protected by a thermal barrier coating (TBC) or an environmental barrier coating (EBC), such as a thermally-insulating ceramic layer adhered to the surface  14  with a bond coat (not shown). Two embodiments of the invention are represented in FIGS. 2 and 3, which depict woven architectures of preforms  28  for the CMC material prior to infiltration by the matrix material, and two types of inserts  24  and  26  suitable for forming the turbulators  18  of FIG.  1 . In each of FIGS. 2 and 3, the architectures of the preforms  28  comprise multiple layers (laminae), each containing sets of woven tows  20  and  22 . The tows  20 / 22  within each set are generally oriented side-by-side and parallel to each other, and transverse to the tows  20 / 22  of the other set, e.g., the tows  20  seen in cross-section in FIGS. 2 and 3 are perpendicular to the tows  22  seen lengthwise. The tows  20  and  22  within a given lamina can be seen to pass over and under each other. While the tows  20 / 22  are shown as passing over and under individual transverse tows  20 / 22 , it is foreseeable that each tow  20 / 22  could pass over one or more transverse tows  20 / 22 , and then under one or more transverse tows  20 / 22 , in accordance with other known weave patterns. 
     In FIG. 2, multiple “stuffer” tow inserts  24  are shown as being incorporated into the architecture of the preform  28 , while in FIG. 3 monolithic ceramic inserts  26  are shown. Suitable materials for the tow inserts  24  include the same material as the fiber reinforcement (tows  20  and  22 ) of the CMC material, e.g., silicon carbide, silicon nitride or silicon fibers, for thermal compatibility, though it is foreseeable that other materials could be used as long as the chosen material is chemically suitable with the service environment of the liner  10  and compatible with the matrix material of the CMC. Similarly, suitable materials for the inserts  26  include monolithic castings of the same material as the matrix material of the CMC material, e.g., silicon carbide, silicon nitride or silicon-containing materials, though again it is foreseeable that other materials could be used. In each case, the tow inserts  24  and monolithic inserts  26  are used in place of a tow of the first set of tows  20 , and therefore positioned between an adjacent pair of tows  20  so that the tow insert  24  or monolithic insert  26  passes over and under the transverse tows  22  of the second set. 
     As apparent from FIGS. 2 and 3, the diameters of the inserts  24  and  26  are larger than those of the adjacent tows  20 , such that the inserts  24  and  26  define protrusions  30  at the surface of the preform  28 . Following infiltration with the matrix material, consolidation, densification, and then curing to form the liner  10 , the size and shape of the inserts  24  and  26  determine the extent to which the turbulators  18  project above the surrounding surface  14  of the liner  10 . Tows, typically circular in cross-section before compaction, will generally assume an oval shape after compaction. As such, a suitable size for a tow insert  24  is at least 50% larger, preferably about 100% to about 700% larger, than the diameter of the tows  20  and  22 . On the other hand, a precast monolithic insert  26  generally maintains its original height after compaction. Therefore, a suitable size for a monolithic insert  26  is at least 25% larger, preferably about 50% to about 350% larger, than the diameter of the tows  20  and  22 . 
     Preferences can exist for the use of a tow insert  24  or monolithic insert  26  based on the desired characteristics of a particular surface feature. For example, if a continuous surface feature is desired, a tow insert  24  may be more convenient, while a discontinuous surface feature may be more readily formed with a row of spaced-apart monolithic inserts  26 . If a desired surface feature can be formed with either a tow insert  24  or monolithic insert  26 , there may be a preference for using a tow insert  24  because of its greater compliance, allowing for more intimate contact with adjacent tows during processing. Potential benefits of intimate tow contact include lower void content or porosity, corresponding to higher interlaminar strengths and through-thickness thermal conductivity. 
     The tow inserts  24  and monolithic inserts  26  are shown in FIGS. 2 and 3, respectively, as placed between adjacent tows  20  of only the outermost lamina of the architecture. Depending on the relative diameters of the inserts  24  and  26 , it is foreseeable that the inserts  24  and  26  could be incorporated into one or more inner lamina, in addition to or in place of the outermost lamina to provide additional flexibility in the final projected height and shape of the turbulator  18 . Furthermore, though FIGS. 2 and 3 show the tow inserts  24  used separately from the monolithic inserts  26 , it is foreseeable that the inserts  24  and  26  could be used together in a single component. For example, because of the difference in their effect on the final size of the turbulator  18 , it may be,advantageous to use both tow inserts  24  and monolithic inserts  26  to enable the height of the desired surface feature to be fine tuned for a specific application, such as matching specific design, cost or compatibility constraints, or optimizing material, structural or component response. 
     As noted above, following the fabrication of the preform  28  by laying up a desired number of lamina, the preform  28  is infiltrated with the desired matrix material in accordance with any suitable technique, after which the infiltrated preform undergoes consolidation, densification, and curing to form the CMC material. As known in the art, appropriate processing parameters, including curing (firing) temperature, will depend on the particular composition of the CMC material, and therefore will not be discussed here. 
     In view of the above, the process of this invention enables turbulators and other surface features to be selectively formed essentially anywhere in a composite material by strategically placing inserts in the composite preform. Turbulators  18  defined by inserts such as the tow inserts  24  and monolithic inserts  26  described above are permanent integral surface features of the CMC, retained by the woven fiber network of the preform  28  to provide a load shielding mechanism that reduces interlaminar tension and shear stresses on the turbulators  18 . As a result, the turbulators  18  exhibit better structural integrity as compared to turbulators that are added by a secondary attachment technique. 
     While the invention has been described in terms of a preferred embodiment, it is apparent that other forms could be adopted by one skilled in the art. For example, while the term turbulator was used in reference to the Figures, the teachings of the invention are applicable to the fabrication of other surface features in CMC materials. Therefore, the scope of the invention is to be limited only by the following claims.