Abstract:
A panel assembly for exchanging heat with an ambient environment maintains minimal temperature differential by virtue of operation as a heat pipe apparatus. Panels of a composite material having excellent structural strength and structural stiffness but comparatively modest thermal conductivity are machined as mirror images of one another. Two orthogonal arrays of parallel channels are machined in the faces of two panels, each intersection of channels forming and bounded by pedestals having a lower, broader base with a narrower upper portion extending from a shoulder of the base portion of the pedestals. The pedestals, in turn, form the bounds of the channels, each having a deeper and a narrower aspect extending along the bases of all the pedestals. Channels have a broader aspect extending along near the tops of the pedestals.

Description:
RELATED APPLICATIONS 
       [0001]    This application claims the benefit of co-pending U.S. Provisional Patent Application Ser. No. 61/042,205, filed on Apr. 3, 2008 and incorporated herein by reference. This application is also a continuation-in-part of co-pending U.S. patent application Ser. No. 11/743,555, filed on May 2, 2007, also incorporated herein by reference, which claims the benefit of U.S. Provisional Patent Application Ser. No. 60/861,583 filed Nov. 29, 2006 and the benefit of U.S. Provisional Patent Application Ser. No. 60/836,901 filed Aug. 9, 2006. 
     
    
     BACKGROUND  
       [0002]    1. The Field of the Invention 
         [0003]    This invention relates to heat transfer and, more particularly, to novel systems and methods for two-phase heat and mass transport. 
         [0004]    2. The Background Art 
         [0005]    Heat transfer is fundamental to many processes. Engines releasing energy from fuel must routinely reject heat. Meanwhile, electrical and electronic devices that consume electricity likewise require cooling. One particularly demanding environment occurs with spacecraft. Spacecraft contain many electronic instruments. Weight is at a premium, as is space. Meanwhile, the loads required of any structure during launch may be substantial. Thus, the combination of light weight and high strength is difficult to achieve. 
         [0006]    In heat exchange, metals have comparatively high thermal conductivities. By contrast, composite materials such as bonded composites, fiber-reinforced layers, and the like typically have comparatively poor thermal conductivity. 
         [0007]    What is needed is an apparatus and method for heat exchange from a satellite to the surrounding space environment. It would be an advance in the art to provide heat exchangers having comparatively light weight, high structural strength, high structural rigidity and stiffness, and yet having comparatively superior heat transfer capacity. 
       BRIEF SUMMARY OF THE INVENTION 
       [0008]    In accordance with the foregoing, an apparatus and method in accordance with the invention may rely on a composite panel formed of fibers in a resin or other matrix. Fibers may be glass, carbon, or other suitable material. Likewise, resins surrounding the fibers may be selected from various hydrocarbons, carbon, and the like. 
         [0009]    In certain embodiments of an apparatus in accordance with the invention, panels may be formed of structurally stiff, structurally strong composite materials. Panels may be machined to excavate channels within the panel. Channels may run at right angles to one another forming residual pedestals between channel intersections. Moreover, channels may be machined at distinct widths and depths. For example, a first, deep channel may be machined at a comparatively narrower width. Thereafter, a comparatively shallower and wider channel may be excavated along the path of the first channel. Thus, a channel may have two depths, each having a corresponding width. For example, the deeper portion of a channel, may have a narrower width while the shallower portion of a channel may be relieved, e.g. have a wider width. 
         [0010]    In such a channel, liquids may transport along the narrower portions of channels, requiring less volume for passage. Meanwhile, vapors may more readily pass in the larger (wider) channel portion where a greater volume is required according the laws of fluid mechanics and boundary layer theory. 
         [0011]    In a method and apparatus in accordance with the invention, two panels may be formed to constitute mirror images of one another. Accordingly, the tops of the pedestals thereof may be layered with a suitable bonding material compatible with the material of the panels as well as with the working fluid that will be used as a heat and mass transport medium therein. The individual pedestals of a pair of panels may then be placed in intimate contact with one another, such that the bonding layer on the top of each pedestal bonds each of the two respective panels together at substantially each and every pedestal. 
         [0012]    In one embodiment of an apparatus and method in accordance with the invention, a panel assembly for exchanging heat with an ambient environment maintains minimal temperature differential by virtue of operation as a heat pipe apparatus. Panels of a composite material having excellent structural strength and structural stiffness but comparatively modest thermal conductivity are machined as mirror images of one another. Two orthogonal arrays of parallel channels are machined in the faces of two panels, each intersection of channels forming and bounded by pedestals having a lower, broader base with a narrower upper portion extending from a shoulder of the base portion of the pedestals. The pedestals, in turn, form the bounds of the channels, each having a deeper narrower aspect extending along the bases of all the pedestals. Channels have a broader aspect extending along near the tops of the pedestals. The tops of the pedestals may be treated with a bonding material and bonded together as panels are placed together as mirror images of one another to make contact at each pedestal. An extremely strong, rigid structure results, capable of surviving launch loads, while still providing excellent heat pipe characteristics. Transport of liquids occurs through the bottoms of narrower portions of channels while vapor returns by way of the broader portions of channels available near the tops of all the pedestals. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0013]    The foregoing features of the present invention will become more fully apparent from the following description and appended claims, taken in conjunction with the accompanying drawings. Understanding that these drawings depict only typical embodiments of the invention and are, therefore, not to be considered limiting of its scope, the invention will be described with additional specificity and detail through use of the accompanying drawings in which: 
           [0014]      FIG. 1  is a perspective view of one embodiment of a closed “panel assembly” or heat exchanger formed of two individual panels (e.g., substrates) machined with channels and pedestals for fluid transport therewithin; 
           [0015]      FIG. 2  is a perspective view of the apparatus of  FIG. 1  illustrating the two mirror-imaged panel units or elements facing one another but separated in an exploded configuration; 
           [0016]      FIG. 3  is a cutaway, perspective view of one embodiment of a single panel element having channels a of single depth and single width; 
           [0017]      FIG. 4  is a perspective view of a portion of a panel unit or element of  FIGS. 1-2  illustrating pedestals and channels of distinct upper and lower dimensions, having two distinct widths for channels and two distinct heights for channels and pedestals; 
           [0018]      FIG. 5  is a perspective view of the apparatus of  FIG. 4 , shaded for easier visualization; 
           [0019]      FIG. 6  is a top plan view of a single, open, panel unit or element for the apparatus of  FIGS. 1-2 ; 
           [0020]      FIG. 7  is a side view of the apparatus of  FIG. 6 ; 
           [0021]      FIG. 8  is an end view of the apparatus of  FIG. 6 ; 
           [0022]      FIG. 9  is a plan view of detail A as designated in  FIG. 6 ; 
           [0023]      FIG. 10  is an end, cross-sectional view of section B-B as identified in  FIG. 9 ; 
           [0024]      FIG. 11  is an edge, cross-sectional view of section C-C as identified in  FIG. 9 ; 
           [0025]      FIG. 12  is a plan view of detail D from the apparatus of  FIG. 6 ; 
           [0026]      FIG. 13  is a cutaway top plan view of a lower half of a panel unit or element of a panel assembly in accordance with the invention of an apparatus in accordance with the invention, having interrupted, individual arrays of pedestals; 
           [0027]      FIG. 14  is a cross-sectional, end view of section A-A of the closed panel assembly from  FIG. 13 ; 
           [0028]      FIG. 15  is an end, cross-sectional view of detail B of the closed panel assembly from  FIG. 14 ; 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0029]    It will be readily understood that the components of the present invention, as generally described and illustrated in the drawings herein, could be arranged and designed in a wide variety of different configurations. Thus, the following more detailed description of the embodiments of the system and method of the present invention, as represented in the drawings, is not intended to limit the scope of the invention, as claimed, but is merely representative of various embodiments of the invention. The illustrated embodiments of the invention will be best understood by reference to the drawings, wherein like parts are designated by like numerals throughout. 
         [0030]    Referring to  FIG. 1 , and to  FIGS. 1-5  generally, a system  10  in accordance with the invention may include panels  12 , such as the illustrated panels  12   a,    12   b  bonded to one another to form a closed panel assembly  10  having enclosed channels  14  therebetween. In certain embodiments, a pair of panels  12  may be formed and subsequently bonded together to form a single apparatus  10 . Apertures  13  formed In the apparatus  10  may provide an ability to mount the apparatus  10  to a rack, frame, bracket, or the like for implementation in a particular assembly, such as a spacecraft. The apparatus  10  may be embodied in various sizes. For example, in certain embodiments, the apparatus  10  may extend several feet on an edge. By contrast, in other embodiments, the apparatus  10  may extend mere inches on an edge in order to accomplish a specific purpose. 
         [0031]    Referring to  FIG. 2 , the panels  12   a,    12   b  may each be formed to have an array  15  of pedestals  16 . As illustrated in detail in  FIGS. 4-5 , the pedestals  16  may be arranged in an array  15  extending throughout an interior portion of a panel  12 . Each of the panels  12   a,    12   b  in the illustrated embodiment represents a substantial mirror image of the other. Accordingly, by application of a bond or bonding material to the top of each pedestal  16  in the array  15 , mutual, corresponding arrays  15  in each of the panels  12   a,    12   b  may be bonded together at multiple points. 
         [0032]    A working fluid operating in two phases in the array  15  may be contained in a comparatively small volume between the pedestals  16 , without ballooning or breaking the individual panels  12   a,    12   b.  Large expanses of area exposed to comparatively modest pressures may generate extremely large total forces. Accordingly, maximum stresses at the edges of an apparatus  10  lacking bonding between the pedestals  16  would fail at substantially lower pressures then the apparatus  10 . In the illustrated embodiment, bonding of each of the pedestals  16  to a corresponding pedestal of an opposite panel  12 , provides for well distributed stress in order to reduce stress in the material of the panels  12 . 
         [0033]    Referring to  FIG. 3 , a panel  12 , in general may be configured in several ways. U.S. patent application Ser. No.  11 / 743 , 555  is incorporated herein by reference and details many operational characteristics of panels  12  assembled as various embodiments of an apparatus  10 . 
         [0034]    In general, a panel  12  may be molded, cast, stamped, machined, or otherwise formed to have an array of channels  14  extending in various directions. In the illustrated embodiment, channels  14   a,    14   b  are oriented orthogonally with respect to one another. Typically, the channels  14  may each be of substantially the same dimensions. For example, widths, depths, and the like may be uniform throughout substantially all channels  14 . This is not a necessity, but may be a practical benefit for manufacturing, operation, temperature uniformity, and the like. 
         [0035]    Meanwhile, the excavation or other formation of each of the channels  14  results in the remaining material forming a pedestal  16 . Each of the pedestals  16  thus has outer walls  18  bounding the channels  14 . Likewise a floor  20  of each channel  14  is the closest surface to the outermost (e.g., environmentally exposed) surface of each panel  12 . Accordingly, channels  14  may be formed to create an extremely small distance, selected to be a suitably engineered thickness between an outer surface of the panel  12 , and the floor  20  of the channels  14  interior thereto. Thus, the heat transfer distance through which energy must be conducted may be minimized according to suitable engineering parameters. 
         [0036]    Referring to  FIGS. 4-5 , pedestals  16  may have upper walls  18   a,    18   b  or upper surfaces  18   a,    18   b  extending along a comparatively narrower portion thereof. Similarly, lower walls  18   c,    18   d  or lower surfaces  18   c,    18   d  bound the larger portions of each pedestal  16 . Corresponding channels  14   a,    14   b  are comparatively wider and pass by the upper portions of the pedestals  16 . Similarly, lower, comparatively narrower portions of the channels  14   c,    14   d  pass along the bottom portions of the pedestals  16 . 
         [0037]    By providing substantially increased space or volume near the upper portions of each of the pedestals  16 , substantially increased vapor transport may occur throughout the array  15  of pedestals  16 , with less obstruction. By the same token, the thickness of the material of the panel  12  between the floor  20  of any individual channel  14 , and the opposing outside surface of the panel  12  corresponding thereto may be minimized. 
         [0038]    In certain embodiments, it is contemplated that the thickness of the panel between the floor  20  of a channel  14 , and the outside surface of the panel  12  may be on the order of tens of thousandths of an inch in certain embodiments. Thus, although the thermal conductivity of the composite material forming a particular panel  12  may be comparatively less than the thermal conductivity of most metals suitable for heat transfer, the overall thickness is sufficiently small to render the net thermal resistance tractable tor suitable heat transfer. 
         [0039]    In general, the shoulder  24  of a pedestal may be flat, inclined, or otherwise shaped as suitable to promote heat transfer. In certain embodiments, a filleted shape for the shoulder  24  may be appropriate. Likewise, a trapezoidal or inclined surface may serve well. In one embodiment, the shoulder  24  simply represents a flat surface parallel to the top  20  of the pedestal  16 . 
         [0040]    Defining a longitudinal direction  26 , a lateral direction  28 , and a transverse direction  30 , all mutually orthogonal to one another, one may describe the directions of fluid flow and heat transfer. For example, heat transfer is ultimately to be effected in a transverse direction  30  into and out of panel  12  at selected locations. 
         [0041]    For example, in a spacecraft application, heat may be transferred by radiation into the blackness of space from an outer surface of a panel  12 . Meanwhile, heat may be transported by convection inside the channels  14  by a heat pipe activity. For example, capillary forces (e.g., surface tension forces) may maintain a flow of liquid along the lower, narrower portions of the channels  14  near the floor  20  of each. Meanwhile, in a generally opposite direction, heat and vapors may return back through the upper portions of the channels  14 , such as the paths  14   a,    14   b  illustrated in  FIGS. 4-5 . 
         [0042]    As a practical matter, no overt enforcement of such flow division is needed. Typically, liquids will be pushed out of the free stream of vapor flows, thus relegating liquids to the smaller spaces and paths  14   c,    14   d  by the vapor as a consequence of boundary layer effects. Accordingly, vapor will typically dominate the central flows through the paths  14   a,    14   b  illustrated. 
         [0043]    As a practical matter, the apparatus  10  may be formed of any of several materials, including or excluding metal. Bonding may be done by any suitable method, including spot welding, adhesive bonding, or the like. Likewise, a composite material comprised of a fiber reinforcement in a resin matrix may be bonded by a separate adhesive, or simply by a solvent. For example, in  FIG. 4 , a surface  38  of a layer  40  of an adhesive, resin, or other bonding material  40  may contact a corresponding surface  38  on a pedestal  16  opposite the one illustrated. That corresponding pedestal  16  extends from a corresponding panel  12  placed opposite the panel  12  illustrated in  FIG. 4  (see e.g.,  FIG. 2 ). Thus, the layer  40  of a bonding agent may be selected to be compatible with the panel materials and the operating fluid operating within the channels  14 , in order to provide suitable life for the apparatus  10 . 
         [0044]    In an alternative embodiment, the layer  40  may be replaced with a layer  40  of solvent softening a portion of the top  22  of a pedestal  16 . Accordingly, the pedestals  16  may be placed in a pressurized contact with one another until the solvent has dissipated, leaving the tops  22  of the pedestals  16  bonded to one another. 
         [0045]    In certain embodiments, the pedestals  16  may be formed by machining out the channels  14  in a substrate forming the panel  12 . In such an embodiment, the substrate may be formed of a suitable metal, a composite material as described hereinabove, or the like. Likewise, in certain embodiments, a non-reinforced panel  12  may be cast, stamped, molded, or the like in order to form the pedestals  16  and channels  14 . For example, a comparatively thin layer of a polymeric material, or a reinforced polymeric material may be stamped to form pedestals  16  that are effectively hollow along the outer surface thereof. Such pedestals, may still be bonded together by solvents or a layer  40  of suitable adhesive connecting the tops  22  of opposing pedestals on corresponding panels  12   a,    12   b  in an apparatus  10 . 
         [0046]    Due to the relative proximity of each of the tops  22  of adjacent pedestals  16 , the net effective stress imposed by internal pressures of vapors within the channels  14  may still be supported at sufficiently close intervals to use materials of comparatively modest strength. For example, various polymers may have sufficient strength to operate with various commonly available working fluids having suitable boiling points to operate as heat transfer fluids in the apparatus  10 . 
         [0047]    In yet another embodiment, the pedestals  16  may be built up by curing. For example, complex or closed volumes may sometimes be formed by coating a substrate with a resin. Curing a portion of the resin by ultraviolet or other triggering means is followed by removal of all uncured resin. The process may be repeated as desired. Thus, a wall may be built up, and an outer surface may be applied, all for an enclosed volume that might otherwise might not be susceptible to molding. 
         [0048]    In yet another embodiment, one panel  12  may actually have pedestals  16  built up from a metal base. For example, the panel  12  may be formed as an excavated portion in the central region where the array  15  will be located. The panel may thus be surrounded with a boundary wall of integral material with the substrate or base. 
         [0049]    In certain embodiments, an apparatus may be constructed using powdered metal. The metal powder may be distributed in the array portion  15  of a panel  12 . A laser directed to specific locations at regular Intervals along the portion  15  will define an array of spots of heat. At each of the spots of heat, the powdered metal is melted and thus bonded to the substrate panel  12  or a previously bonded location of melted, powdered metal. Thereafter, excess, unmelted powder may be removed. Repeatedly, the panel  12  may be covered with a layer of powdered metal. Repeatedly, in turn, at the same locations, an array of hot spots may be created at regular intervals. Thus, melting additional powdered metal on top of the previous locations of melting builds a column or pedestal at each such location. Channels are left as the powdered metal that was left unmelted is removed. 
         [0050]    By repetition of this process, such pedestals  16  were built up to form the full array  15  of pedestals  16  on panel. The pedestals formed by rastering the laser across the powdered metal at regular locations were built up to a suitable height to operate as pedestals  16 . For example, pedestals  16  operate to maintain open channel spaces, and also to bond to one another in order to bond to panels  12  together at very frequent, regular intervals. 
         [0051]    Rastering a laser across the numerous, built-up pedestals may provide a liquefied surface. Thereupon the panels  12  may be pressed together to bond the metal tops  22  of the pedestals  16  of opposing panels  12  together. The porosity resulting from the irregularities of the melted metals in the pedestals  16  may operate to improve an adherence or capillary adhesion of liquids to the channels  14 . 
         [0052]    Referring to  FIG. 6 , a panel  12  formed of a substrate may be formed to have an array  15  of pedestals  16 . The panel  16  may be provided with apertures  13  to attach an apparatus  10  formed of two panels  12  in a deployable position. 
         [0053]    In the illustrated embodiment, a tool may operate along straight, parallel lines like the cutting element of a milling machine, or like a circular saw. To improve manufacturing speed, a saw-like operation may result in overrun portions  42 ,  44 . Likewise, the overrun portions  42  may provide pedestals that are comparatively long and narrow, rather than having a balanced width-to-length aspect ratio of unity. For example, the overrun portion  42  provides for access by liquid and vapor despite the interference of the apertures  13  and the surrounding material required to support them. 
         [0054]    The overrun portions  42 ,  44  do not provide all the advantages of the uniform pedestals  16  of unitary aspect ratio found elsewhere, along the panel  12 . However, they still provide access by fluid to those reaches of the panel  12 . 
         [0055]      FIGS. 7 and 8  illustrated comparative depths of cuts used to form the channels  14  in the panel  12 . The dimensions outlined represent those for one embodiment of a test apparatus manufactured in a laboratory for evaluation of the performance of an apparatus  10  in accordance with the invention. 
         [0056]    Referring to  FIG. 9 , detail A shows the overrun portions  42 ,  44  of cuts forming the various pedestals  16 . In the proximity of the apertures  13 , the overruns  42  provide access by heat transfer fluid to portions of the panel  12  not accessible in both longitudinal and lateral directions, due to the requirement for material integrity in the vicinity of the apertures  13 . 
         [0057]      FIGS. 10-12  illustrate dimensions of channels  14  and pedestals  16  in the embodiment of  FIG. 6  fabricated as a laboratory test apparatus  10 . Notwithstanding the nearly 80 thousandths of an inch that the channels  14  descend into the panel  12 , approximately that much additional material is left as a wall. The material remaining between the floor of each of the channels  14 , and the opposite surface of the panel  12 , may typically be reduced to something on the order of between 10 and 40 thousandths of an inch. Shaping the floor  20  of each channel  14  may provide a balancing of strength and heat transfer. 
         [0058]    Referring to  FIGS. 13-15 , in one embodiment of a panel  12  in accordance with the invention, the array  15  of a panel  12  may actually be provided with periodic barriers  46 . For example, the barriers  46   a,    46   b  may be bonded to corresponding barriers  46   a,    46   b  in an opposing panel  12 . Thus, the array  25  of pedestals  16  may actually constitute various sub-arrays  15  sealed away from one another. In such an embodiment, each individual portion between adjacent barriers  46   a,    46   b  in the panel  12  may operate as a single heat transport device. Thus, in an environment such as high launch loads, the barriers  46   a,    46   b  may resist or limit the ability of fluids to transfer from end of the panel  12  to another under the influence of acceleration forces. 
         [0059]    In general, detail B of  FIG. 15  illustrates a shape of pedestals  16  bonded to one another to form the channels  14 . The portion  14   a  is seen as the narrow portion  14   a  wherein liquid will accumulate to be boiled off eventually by addition of heat into a panel  12 . Meanwhile, returning vapor from a boiling portion of the panel  12  will return along the larger cross-section of channel  14   c  to be condensed at a cooler location on the panel  12  or the panel assembly  10 . 
         [0060]    Typical dimensions for an apparatus  10  may be on the order of about a foot square. Meanwhile, apparatus  10  prototypes of about 200 mils in total thickness may be 100 mils or less in thickness in a deployed apparatus. Likewise, 30 thousandths of an inch may be a suitable wall thickness between a floor  20  and outer surface. That is, the distance between the bottom  20  of the channel  14 , and the opposing outer wall surface of that same panel  12  may typically be separated by as little as 30 mils or less. 
         [0061]    In certain prototypes, an effective thermal conductivity for panel material may still yield an effective heat transport equivalent to a thermal conductivity on the order of thousands of watts per meter per degree kelvin. This is superior to the effectiveness of solid copper as a transport mechanism. Meanwhile, the structural stiffness and strength of a graphite fiber composite material such as a carbon-carbon composite has been found to provide a suitably stiff, strong, effective thermal panel for discharge of heat from a spacecraft. 
         [0062]    The present invention may be embodied in other specific forms without departing from its spirit or essential characteristics. The described embodiments are to be considered in all respects only as illustrative, and not restrictive. The scope of the invention is, therefore, indicated by the appended claims, rather than by the foregoing description. All changes which come within the meaning and range of equivalency of the claims are to be embraced within their scope.