Abstract:
An apparatus for maintaining a temperature differential between a component and a source of heat is described. The apparatus includes a micro-truss structure having a plurality of nodes and members which define a first surface and a second surface. The second surface is operable for attachment to the component. The apparatus further includes a skin material attached to the first surface of the micro-truss structure such that the skin material is operable for placement between the heat source and the micro-truss structure. The skin material defines at least a portion of a fluid flow path through the micro-truss structure. A skin material is not utilized with certain configurations of the micro-truss structure.

Description:
BACKGROUND 
       [0001]    The field of the invention relates generally to cooling of structures, and more specifically, to methods and apparatus for a micro-truss based structural insulation layer. 
         [0002]    Multiple solutions have been utilized in thermal protection of structures. Many of these solutions include low density core materials as a part of the structure, which allow air to pass through while also providing an insulation factor. These core materials include one or more of carbon foam, silicon carbide foam, alumina tile, and slotted honeycomb. Other core materials may be known. 
         [0003]    Ceramic foams have been used for thermal protection systems and heat exchanger applications. However, due to their random foam cell orientation, they are not as mechanically efficient as is desired. Also, the random foam cell orientation results in some degree of difficulty, when attempting to pass forced air through the foam. In addition, the random reticulated foam also provides limited design variables (primarily foam cell size) for optimizing these foam structures from a thermal-mechanical performance perspective. 
         [0004]    One solution incorporates a ceramic thermal protection system, in which the ceramic is porous, allowing cooling air to pass therethrough. However, this porous ceramic has many of the same features as does the reticulated foam. Specifically, the randomness of the individual cells results in inefficient air passage through the ceramic. 
       BRIEF DESCRIPTION 
       [0005]    In one aspect, an apparatus for maintaining a temperature differential between a component and a source of heat is provided. The apparatus includes a micro-truss structure having a plurality of nodes and members which define a first surface and a second surface. The second surface is operable for attachment to the component. The apparatus further includes a skin material attached to the first surface of the micro-truss structure such that the skin material is operable for placement between the heat source and the micro-truss structure. The skin material defines at least a portion of a fluid flow path through the micro-truss structure. 
         [0006]    In another aspect, a structure for protecting a surface from heat fluctuations emanating from a heat source is provided. The structure includes a micro-truss structure having a plurality of hollow members intersecting at nodes. The hollow members define a first surface and a second surface and a plurality of spaces therebetween. The second surface is configured for placement proximate the surface that is to be protected from the heat source, while the hollow members and nodes are configured such that a fluid flow may be directed therethrough. The structure further includes an insulating material filling the spaces defined by the hollow members and the nodes of the micro-truss structure. 
         [0007]    In still another aspect, a method for insulating a surface from a source of heat that is proximate the surface is provided. The method includes attaching a micro-truss structure to the surface, the micro-truss structure between the surface and the source of heat, and associating a fluid flow with the micro-truss structure such that operation of the fluid flow removes heat from an area associated with the micro-truss structure. 
         [0008]    The features, functions, and advantages that have been discussed can be achieved independently in various embodiments of the present invention or may be combined in yet other embodiments further details of which can be seen with reference to the following description and drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]      FIG. 1  is a cross sectional view of a micro-truss based actively cooled insulation layer that includes an impermeable skin. 
           [0010]      FIG. 2  is a cross sectional view of a micro-truss based actively cooled insulation layer that includes a porous skin. 
           [0011]      FIG. 3  is a cross sectional view of a micro-truss based actively cooled insulation layer that includes directional cooling holes incorporated into a skin. 
           [0012]      FIG. 4  is a cross sectional view of a micro-truss based actively cooled insulation layer where cooling air is directed through hollow truss members. 
           [0013]      FIG. 5  is an illustration of a micro-truss structure. 
           [0014]      FIG. 6  is an illustration of a micro-truss structure that includes hollow truss members. 
           [0015]      FIG. 7  is a close up view of a hollow truss member. 
       
    
    
     DETAILED DESCRIPTION 
       [0016]    The described embodiments relate to a thermal insulation structural element having a truss structure therein. In various embodiments, the truss structure includes a plurality of members extending from a node and attached to a skin surface. In certain embodiments, the truss structure and its members are ceramic. In certain embodiments, the truss members are hollow. With regard to both hollow and non-hollow truss embodiments, an overall structure may include a skin and one surface of the truss structure attached to the skin. An opposite surface of the truss structure is attached to a surface that is to be protected from heat flux. With the truss structure between the skin and the surface, a fluid flow path is formed that allows for a less constricted air flow across the truss structure. 
         [0017]    One purpose of the described structures is to maintain a thermal differential (ΔT) between a surface and an incident heat flux. An ability to adjust the flow of cooling air through the structure of the micro-truss enables control of the surface temperature. Several advantages of such a micro-truss structure include a variety of material options, such as ceramics and metals, a potential for net shape fabrication, no additional machining operations for cooling air flow channels, and the micro-truss architecture is capable of providing additional structural functionality. 
         [0018]    One identified application for the below described embodiments, is in the environment associated with an aircraft exhaust nozzle. However, other applications that require surface temperature control are certainly contemplated. 
         [0019]    More specifically, the truss structure relates to embodiments of a micro-truss that are attached to a surface requiring protection from a high heat flux source. Referring to  FIG. 1 , a skin material  10  is attached to a micro-truss structure  12  along a first surface  16  of the micro-truss structure  12 . A second surface  18  of micro-truss structure  12  is attached, using an attachment  20 , such that the second surface  18  of micro-truss structure  12  is adjacent a surface  30  of a device, or substructure  32 , that is to be protected from heat flux  40 . In the illustrated embodiment, the surface  30  of the substructure  32  is protected from the high heat flux  40  by convective cooling that is provided by cooling air  50  passing through the micro-truss structure  12 . One purpose of the skin  10  is to enclose an interior region  60  of the micro-truss structure  12  to allow for the flow of cooling air  50 . 
         [0020]    As described elsewhere herein, micro-truss structure  12  may be fabricated from a polymer, a metal (or alloy), or from a ceramic material. For temperatures exceeding approximately 200 degrees Celsius, micro-truss materials must be converted to either a metal or a ceramic. One preferred embodiment utilizes a ceramic micro-truss. Silicon carbide and alumina are two examples of such a ceramic, though there are others. The reasons are many, and include: because ceramic materials are generally lower density than metals, because ceramic materials are generally more thermally stable in higher temperature environments, and because ceramic materials generally have a lower thermal conductivity, which inhibits the conduction of heat through the truss members to the surface that requires protection from the heat flux. 
         [0021]    In the case of the impervious skin material  10 , incident thermal energy conducts through the material from which the members of micro-truss structure  12  are fabricated towards the surface  30  requiring protection from the high heat flux  40 . Cooling air  50  is directed through the micro-truss structure, providing a convective cooling mechanism to maintain a desired ΔT. One embodiment of an impervious skin material is a ceramic fiber reinforced ceramic matrix composite (CMC). 
         [0022]    For the impervious skin material  10 , the temperature of the cooling air  50  directed through the micro-truss structure  12  will increase as the cooling air  50  removes heat from the individual members of micro-truss structure  12 . This phenomenon reduces the efficiency of the cooling air  50  as the effective path length through the micro-truss structure increases, due to a decreasing temperature differential between the cooling air  50  and the skin material(s)  10 . Limitations on the cooling air flow rate will ultimately determine if this cooling mechanism is sufficient to maintain a safe ΔT for the required temperature conditions in a specific application. 
         [0023]    As shown in  FIG. 1  and in subsequent figures, the micro-truss structure  12  is attached to the surface  30  requiring protection from the high heat flux  40 . Bonding or mechanical attachment approaches may be utilized. In one preferred embodiment, the micro-truss structure  12  is attached to the surface  30  with a high temperature silicone adhesive, which provides an efficient strain relief layer. If a lower thermal gradient were expected at the bonding surface, other commercially available bonding approaches could be utilized. 
         [0024]    As is the case with other embodiments described herein, a temperature differential between the skin material  10  and the surface  30  is controlled/maintained by passing the cooling air  50  through the natural flow channels of the structure associated with micro-truss structure  12 . In addition, and as shown in  FIG. 2 , a skin material  100  may be porous, enabling cooling air to flow from the interior region  60  of the micro-truss structure  12 , through a porous skin material  100 , and onto the high heat flux  40 , providing a transpiration mechanism. In the illustrated embodiment, the surface  30  of the substructure  32  is protected from the high heat flux  40  by convective cooling of the micro-truss structure  12  and transpiration cooling at the surface  102  of skin  100 . 
         [0025]    As one described embodiment, transpiration cooling can be achieved by utilizing a porous skin material  100  that will enable the cooling air  50  to “transpire” from the interior region  60  of the micro-truss structure  12  towards the direction of the incident heat flux  40 . This active cooling mechanism reduces the skin temperature for a given heat flux (compared to an impervious skin material with a similar thermal conductivity), thus reducing the amount of heat conducted through the truss members. Examples of porous skin materials  100  include sintered particles and/or fibers that create an open porosity of &gt;10%. In the case of a porous ceramic skin material, the particles and/or fibers may be comprised of oxide or non-oxide constituents. 
         [0026]      FIG. 3  illustrates that the skin material  150  may be fabricated to include a plurality of aligned holes  152  that enable cooling air  50  to flow from the interior region  60  of the micro-truss structure  12 , through the aligned holes  152 , towards the heat source  40  providing a film cooling mechanism. The other aspects of this configuration are as before, specifically, the surface  30  of the substructure  32  is also protected from the high heat flux  40  by convective cooling of the micro-truss structure  12  and by film cooling at the surface of skin  150 . 
         [0027]    In one embodiment, and as illustrated in  FIG. 3 , skin material  150  may include an array of directional cooling holes  152  to accomplish the above mentioned film cooling: In alternative embodiments, the material for skin material may be the impervious skin material  10  described with respect to  FIG. 1 , or may the porous skin material  100  described with respect to  FIG. 2 . In either embodiment, cooling air  50  exits the interior region  60  of the micro-truss structure  12  and forms a protective cooling film adjacent to the surface  154  of the skin material  150 . Similar to transpiration cooling, a cooling air film reduces the surface temperature of the skin material  150 , which is adjacent to the incident heat flux  40 , and thus the amount of heat conducted through the micro-truss members. The array of cooling holes  152  in the skin material  150  can be conventionally drilled or laser machined perpendicular to, or at an angle off the normal of the surface  154 . The architecture of micro-truss structure  12  can be configured such that the cooling holes  152  are located between nodes  160  of the micro-truss structure  12 , enabling a predictable cooling air flow pattern. 
         [0028]      FIG. 4  illustrates another alternative embodiment, where film cooling can be achieved by passing cooling air  50  through hollow members  200  of a micro-truss structure  202  to a surface  210  of a skin material  212 . In this embodiment, the interior  220  of the micro-truss structure  202  can optionally be filled with a highly insulating material  224 , such as an aerogel. The cooling air  230  is directed into the hollow truss members  200  through separate cooling channels  230  formed between the micro-truss structure  202  and the surface  30  of the sub-structure  32  requiring thermal isolation from the high heat flux  40 . The separate cooling channels  230 , in one embodiment, are formed by the placement of a flow channel  240  to the surface  30  of the substructure  32  to be protected from the high heat flux. In this embodiment, a separate skin material, such as skin material  100  or skin material  150 , is optional depending on the air-flow permeability and durability of the insulating material  224  filling the interior  220  of the micro-truss structure  202 . 
         [0029]      FIG. 5  is an illustration of one embodiment of a micro-truss structure  250  which illustrates the channels  252  through which cooling air can flow.  FIG. 6  is a close up illustration of a micro-truss structure  300  that includes hollow truss members  302 .  FIG. 7  is a further close up view of a hollow truss member  302 . 
         [0030]    With regard to dimensions, a total thickness of the actively cooled insulation layer including one of the above described micro-truss structures  12  and  202  is between approximately 0.1 inch and two inches, in a specific embodiment. In one preferred embodiment, the thickness of the micro-truss structure ranges between 0.3 inch and one inch. The skin material ranges from about one percent to about fifty percent of the total thickness. A solid volume fraction, or relative density, of the micro-truss structure ranges between about one percent to about fifty percent. 
         [0031]    In addition to enabling cooling flow through the structure of an actively cooled insulation layer, the micro-truss materials are utilized as a sandwich structure core material that can transfer load between the sub-structure and the skin material. This structural functionality of the micro-truss structures  12  and  202  may reduce parasitic weight of the insulation layer. 
         [0032]    Other embodiments are contemplated that combine one or more of the features described with respect to  FIGS. 1-4 . For example, rather than using insulating material  224 , cooling air could be routed through the hollow truss members  200  and through the interior  220  of the structure, around the micro-truss structure  202  as is described with respect to  FIGS. 1-3 . In addition, the optional skin may be the porous skin material  100  of  FIG. 2  or the skin material  150  of  FIG. 3 , with the holes  152  aligning with the hollow truss members  200 . 
         [0033]    In any of the embodiments, the micro-truss structure can be optimized by changing one or more of a unit cell size, unit cell architecture, truss member diameter, and truss member angle when the micro-truss structure is grown and/or fabricated. 
         [0034]    In one application, the described embodiments may be utilized as part of a thermal protection system for an aircraft. The described embodiments are directed to an integrated thermally resistant structure that uses a truss element to form a composite like sandwich structure to direct heat away from a surface. The truss elements are formed, in one embodiment, using developed processes that result in hollow micro-truss elements. One focus of the present disclosure is to a truss structure where a fluid flow (air) is passed though one or more of a truss structure and hollow truss members to provide cooling for surfaces that need to be protected from large thermal gradients. 
         [0035]    This written description uses examples to disclose various embodiments, which include the best mode, to enable any person skilled in the art to practice those embodiments, including making and using any devices or systems and performing any incorporated methods. The patentable scope is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims.