Patent Publication Number: US-2016237853-A1

Title: Turbine exhaust case with coated cooling holes

Description:
BACKGROUND 
     Turbine exhaust case assemblies include inner and outer rings that are spaced from one another by a plurality of radially extending struts. These struts are fixedly attached at both their inner and outer ends to the inner and outer rings. The outer ring defines the radially outer surface of the engine gas flow path downstream of the last stage of turbine blades of a gas turbine engine. 
     Due to the high temperature of the exhaust flow path, thermal protection systems are used to prevent damage to the turbine exhaust case. Thermal barrier coatings can be applied to the exhaust case. Furthermore, cooling air may be used to effect effusion cooling and/or impingement cooling. Common exhaust cases are formed of a metal alloy, then coated with a coating such as a thermal barrier coating. Effusion cooling holes are then formed in the case, often by laser ablation or electrical discharge machining Cooling air, such as bypass air, may then be routed from radially outside the turbine exhaust case through the effusion cooling holes and into the exhaust flow path, preferably forming an effusion film of cool bypass air along the surface of the case that would otherwise be exposed to hot core flow. 
     Cooling air may be routed into the exhaust flow path for other purposes than for the protection of the turbine exhaust case. For example, diffusion cooling holes may also be present in the turbine exhaust case that promote mixing of cooling air with exhaust gases to modify engine acoustics, exhaust temperature, or promote combustion in an augmentor or afterburner. Effusion cooling holes are distinct from diffusion cooling holes in that effusion air is routed along the surface of the turbine exhaust case, rather than into the exhaust gas flow. By arranging several effusion cooling holes together, an effusion cooling film is generated that protects the TEC from damage. In order to maximize the efficacy of the effusion film, effusion cooling hole spacing and orientation are selected based on expected exhaust gas temperature and velocity at each location. 
     SUMMARY 
     An effusion-cooled component includes a base portion defining at least one oversized cooling hole preform. A coating is disposed on the base portion and at least partially covers the oversized cooling hole preform to define a cooling hole. The component may be repaired by removing the coating from a component, visually inspecting the resultant base, and re-coating the base with a coating that oversprays to at least partially fill the oversized cooling preforms to define a plurality of cooling holes. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of a turbine exhaust case. 
         FIGS. 2A-2D  are cross-sectional views of two cooling holes undergoing construction, coating, wear, and un-coating, respectively. 
     
    
    
     DETAILED DESCRIPTION 
     In order to provide an effusion cooling film in a turbine exhaust case, cooling holes are defined in the case to permit the passage of relatively cooler bypass air through the case. Portions of the turbine exhaust case are made by creating a base portion with oversized cooling hole preforms, then coating the base—including cladding the oversized cooling hole preforms—with a coating. The deposition of the coating in the oversized cooling hole preforms creates cooling holes of the desired size. A turbine exhaust case made in this manner may be repaired by removing the coating and re-applying a new coating without having to create new cooling holes in the case. 
       FIG. 1  is a perspective view of turbine exhaust case (TEC)  10 . TEC  10  includes outer ring  12  and inner ring  14 , which are connected to one another by a plurality of struts  16 . TEC  10  defines the radially outer extent of core flow C. Outer ring  12  defines a plurality of cooling hole structures  18 . As used in this application, the terms “cooling hole” and “effusion hole” are interchangeable. It will be understood by a person of ordinary skill in the art that the cooling holes described herein could be used in a combustor, turbine exhaust case, or any other part that benefits from effusion cooling. 
     TEC  10  is a component of a gas turbine engine. During normal operation, core flow C passes through a combustor (not shown), is routed through a turbine section (not shown), and then passes through TEC  10 . The TEC can be fabricated using several methods. One such method is Laser Powder Deposition (LPD). In this method, either a portion of the TEC or the entire TEC can be built layer by layer which allows for various features to be included therein. Alternatively, TEC can be manufactured using casting or molding. 
     Core flow C passes through the region between outer ring  12  and inner ring  14 . Core flow C may be sufficiently hot to cause damage to outer ring  12 , inner ring  14 , or struts  16 . In the embodiment shown in  FIG. 1 , outer ring  12  is protected from such damage by at least two mechanisms: a thermal barrier coating, and an effusion cooling film. Effusion cooling is accomplished by routing cooling air, for example bypass air, through effusion cooling hole structures  18  to form an effusion film (e.g. effusion film E of  FIG. 2 ). Effusion cooling hole structures  18  are only shown in outer ring  12  of TEC  10 . However, in alternative embodiments, effusion cooling hole structures  18  may also be present in struts  16  and/or inner ring  14 . 
       FIGS. 2A-2D  are cross-sectional views of two cooling hole structures  18  within outer ring  12  of  FIG. 1 , taken along line  2 - 2 . In particular,  FIG. 2A  illustrates base  20  including two oversized cooling hole preforms  24 .  FIG. 2B  illustrates deposition of coating  22 , including within oversized cooling hole preforms  24  to provide cooling holes  26  having a desired size.  FIG. 2C  illustrates the base  20  and coating  22  of  FIGS. 2A-2B  after use, such that coating  22  is in need of refurbishment or repair.  FIG. 2D  illustrates base  20  after coating  22  has been removed as part of a refurbishment or repair process. 
       FIG. 2A  is a cross-sectional view of base  20  including two oversized cooling hole preforms  24 . In this embodiment, oversized cooling hole preforms  24  are designed as straight, slanted holes in base  20 . Oversized cooling hole preforms  24  are larger than a desired final cooling hole size. 
     In some embodiments, base  20  may include a bond coat (not shown). Such bond coats may be used to promote adhesion between base  20  and an adjacent material, such as coating  22  ( FIGS. 2B-2C ). 
     Cooling hole structures  18  are defined by base  20  and coating  22 . Base  20  is manufactured to define oversized cooling hole preforms  24 . Base  20  may be made by additive manufacturing, such as direct metal laser sintering, laser powder deposition, or other additive methods. Oversized cooling hole preforms  24  can be included in base  20  as it is built. Alternatively, base  20  may be made by casting or molding, then oversized cooling hole preforms  24  may be created by electro-discharge machining (EDM), laser ablation, or other known subtractive manufacturing techniques. 
     Additive manufacturing may be used to define oversized cooling hole preforms  24  that have complex geometries not easily generated using laser ablation of EDM. Such geometries include tapered cooling holes, lobed cooling holes, or groups of cooling holes with non-uniform angles. In addition, additive manufacturing can easily form oversized cooling hole preforms  24  that are large enough that they would be expensive and/or time consuming to create using traditional subtractive manufacturing mechanisms. Various additive manufacturing mechanisms may be used, including direct metal laser sintering, laser powder deposition, selective laser sintering, and electron beam melting, among others. Additive manufacturing can be used to build up layers of a meltable, sinterable, or polymerizable material into a complex, multilayered structure. 
       FIG. 2B  is a cross-sectional view of base  20 , as well as coating  22 , deposited to form cooling hole structures  18 . In particular,  FIG. 2B  illustrates two cooling hole structures  18 , configured to direct bypass air B to an effusion film E to protect surface  28  from damage from core flow C. 
     Coating  22  is applied after base  20 —including oversized cooling hole preforms  24 —is completely formed, as described with respect to  FIG. 2A . Coating  22  oversprays to at least partially fill oversized cooling hole preforms  24 . Effusion holes  26  are defined and shaped as a result of the underlying structure of oversized cooling hole preforms  24 . Due to coating  22  overspraying onto the edges of base  20  that define oversized cooling hole preforms  24 , cooling holes  26  may be completely, or at least partially, clad with coating  22 . The thickness of such cladding is a function of the angle at which the coating is applied, as well as the thickness of coating  22 . 
     Components with clad cooling holes prevent exposure of base  20  to external elements, ranging from thermal energy to radar waves. For example, coating  22  ( FIGS. 2B-2C ) may be of a material that does not have a high reflectance, as compared to the metal alloys typically used to form base  20 . Devices that rely on reflected or radiated waves may not identify aircraft incorporating clad cooling holes as easily as those without. 
     Effusion holes  26  include inlet  30  and outlet  32 , which are apertures that allow ingress and egress of bypass air B, respectively, to cooling holes  26 . Inlet  30  and outlet  32  are fluidically connected to permit fluid flow from bypass air B to effusion film E. In alternative embodiments, a bond coating, such as a bimetallic material, may be applied to base  20  prior to coating  22 . 
     Surface  28  of coating  22  is exposed to core flow C. Surface  28  is protected from damage that could be caused by core flow C by effusion film E. Inlet  30  and outlet  32  are configured to optimize effusion film E. By changing the size, shape, or tapering of oversized cooling hole preform  24 , various structures may be generated in surface  28 , inlet  30 , and/or outlet  32  that accelerate fluid flow in a desired direction to promote laminar flow in effusion film E. 
     Effusion film E may be directed in any desired direction to protect a portion of outer ring  12 . In some embodiments, such as the one shown in  FIG. 2B , effusion film E may be directed parallel to core flow C. In other embodiments, effusion film E may be perpendicular, opposite, or any other direction with respect to core flow C. Furthermore, in some embodiments, core flow C may not flow parallel to the plan defined by surface  28 . 
       FIG. 2C  shows the base  20  and coating  22  of  FIGS. 2A and 2B . As shown in  FIG. 2C , coating  22  has been damaged by core flow C of  FIG. 2B , such that surface  28  is uneven. Thus, coating  22  must be refurbished or replaced. 
     Oversized cooling hole preforms  24  allow for a cycle of repair or refurbishment. Prior art components are coated and then cooling holes are manufactured subtractively. Thus, if the coating were to be removed from a prior art component, and the component were then re-coated, the base would be filled by the coating to an unacceptable extent. In order to create acceptable cooling holes, such prior art components would have to be coated and then undergo a second round of subtractive manufacturing. This second round of subtractive manufacturing results in a second set of holes punched through the base. Unlike these prior art components, base  20  of  FIGS. 2A-2D  may be refurbished and recoated many times without the need for additional subtractive manufacturing thereon. 
       FIG. 2D  shows the base  20  of  FIGS. 2A-2C , with coating  22  removed. Removal of coating  22  may be accomplished, for example, by water spraying. Base  20  can be visually inspected to ensure that coating  22  has been removed. Once coating  22  has been removed, base  20  may be re-clad with coating  22 , as previously described with respect to  FIG. 2B , to refurbish cooling hole structures  18 . In this way, coating  22  and cooling holes  26  may be refurbished or repaired without having to manufacture new cooling holes. 
     DISCUSSION OF POSSIBLE EMBODIMENTS 
     The following are non-exclusive descriptions of possible embodiments of the present invention. 
     According to one embodiment, an effusion-cooled component includes a base portion defining an oversized cooling hole preform. The component further includes a coating disposed on the base portion and at least partially covering the oversized cooling hole preform to define a cooling hole.
         The effusion-cooled component of the preceding paragraph can optionally include, additionally and/or alternatively, any one or more of the following features, configurations and/or additional components:   The base may include a high temperature superalloy. The coating may be a thermal barrier coating. The effusion-cooled component may also include a cooling air source on a radially outer side of the outer ring adjacent to the cooling hole at an inlet. The effusion-cooled component may also be arranged adjacent to a core flow on a radially inner side of the outer ring adjacent to the cooling hole at an outlet. The cooling hole may be one of a plurality of cooling holes defined by the effusion-cooled component, wherein the plurality of cooling holes are arranged to provide an effusion film.   In another embodiment, a method of manufacturing an effusion-cooled component includes forming a base defining a plurality of oversized cooling preforms, then coating the base with a coating that oversprays to at least partially fill the oversized cooling preforms to define a plurality of cooling holes.   The method of the preceding paragraph can optionally include, additionally and/or alternatively, any one or more of the following features, steps, configurations and/or additional components:   The method may include applying a bond coating to the base after forming the base and prior to coating the base. Forming the base portion may include additively manufacturing the base portion. Additively manufacturing the base portion may include using laser powder deposition to create a multilayered structure. The method may also include routing a cooling fluid to an inlet of the cooling hole, routing a core flow to an outlet of the cooling hole, and routing the cooling fluid through the cooling hole from the inlet to the outlet to provide an effusion film adjacent to the outlet of the cooling hole.   In another embodiment, a method for repairing a component including a cooling hole includes removing a coating from a component. The component includes a base defining an oversized cooling hole preform, and a coating at least partially covering the base, including at least partially filling the oversized cooling hole preform to define a cooling hole. The method also includes visually inspecting the resultant base, and coating the base with a coating that oversprays to at least partially fill the oversized cooling preforms to define a plurality of cooling holes.   The method of the preceding paragraph can optionally include, additionally and/or alternatively, any one or more of the following features, steps, configurations and/or additional components:   Removing the coating from the component may include using a water jet to remove the coating. Visually inspecting the resultant base may include inspecting the base to ascertain what portion of the coating was removed from the base. Removing the coating from the component may include removing all of the coating from the base.       

     Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.