Abstract:
This invention enables mechanical design of flexible and elastic heat pipes. It provides structural solution that reduces weight/performance ratio for traditional heat pipes and prevents freeze damages.

Description:
FIELD OF THE INVENTION  
       [0001]     This invention relates to a field of structural design of heat pipes. It provides functional solution that reduces weight/performance ratio for traditional heat pipes and improves performance of flexible heat pipe designs.  
       PRIOR ART  
       [0002]     Applications of heat pipes often become impractical due to complexity associated with fitting these components into tightly packed spaces or because of mobility restrictions they could cause. Another implication is that systems can be utilized at temperatures below freezing point of selected liquid. Liquid crystallization might severely damage the shell of a heat pipe. It also disables pipe functions until the liquid melts. Prior art (U.S. Pat. No. 4,194,559) exploit compromise solutions that prevent freeze damage of the pipe in expense of added free volume that increases total volume and weight of a solution.  
         [0003]     Prior art (U.S. Pat. Nos. 4,279,294, 5,647,429, and 6,595,270) include attempts to utilize polymer materials such as an aromatic polyamide fiber of extremely high tensile strength (Kevlar fiber), polytetrafluoroethylene, nylon, or polyimides as a construction material for outer shell of a flexible heat pipe. All of them nevertheless require rigid geometry such as thick round walls or imbedded spacers to prevent collapse of the pipes. Invention U.S. Pat. No. 4,279,294 uses low boiling point liquid and utilizes underground installation to prevent explosion of the pipe. Devices of the other two inventions fail to account for high air permeability of polymers that explains impractically short service life of the inventions.  
         [0004]     One prior art invention (U.S. Pat. No. 6,446,706) provides highest performance among all competitors. It has form factor of a tape with high unidimensional flexibility. Nevertheless design is subject to freeze damage and its specific heat transfer performance is reduced by presence of one or two layers of vapor transmitting spacer material.  
         [0005]     Another invention employs an array of rigid machined capillars sealed between solid plates forming a flat heat pipe. While this design provides very high performance it is inherently solid and relies on load bearing shell. This design fails when the pipe must operate at positive internal pressure. 
     
    
     DETAILED DESCRIPTION  
       [0006]     This invention overcomes cited technical challenges utilizing a soft shell and a load bearing wick structure in a heat pipe design. Unlike competitive technologies it provides convenient topological solution for compact designs, confined spaces, and flexible solutions.  
         [0000]     Wick  
         [0007]      FIG. 1  shows schematic the structure of the wick. Preferred embodiment of the invention uses a wick  1  is formed by braided fiber. The yarns are braided to form plaits along preferred direction of the pipe. Each yarn could be formed by braided fiber or tows. 6-tow maypole braided construction is a good example for one of many possible yarn designs.  
         [0008]     The total wick structure accounts for small (capillary  3 ) and large (passages  2 ) gaps. The capillary are formed by fiber-fiber gaps and inner-yarn tubes, Inner yarn tube is formed when small number of carriers ( 4 - 8 ) are used in rotary or maypole braiding. The passages are formed by yarn-yarn and braid-braid spacing. This construction has high mechanical stability and yet flexible. Its capillary channels are suitable for efficient transport of a liquid, and intrinsic passages allow for vapor transport and efficient liquid-vapor interaction.  
         [0009]     Plaiting ensures anisotropic capillary properties that increase liquid transport rates. The braids are shaped to allow free passages  2  for vapor in spaces between the yarns. At the same time each plait performs capillary functions by transporting liquid through channels formed by adjacent yarns  3  and inside the yarn. Twisted braiding increases capillary efficiency since adjacent twists of braids create shortcuts that significantly reduce effective capillary length.  
         [0010]      FIG. 2  show one of possible yarn constructions. This yarn is created by braiding six fibers  4  using a maypole braider. This braid allows for intrinsic channel  5  along the braid axis. The whole yarn works as a capillary size tube with porous walls. The size of the pores  6  is smaller than the size of the channel. When used in conjunction with a liquid that has low surface angle on the material of the fiber, the yarn quickly absorbs the liquid through the walls. When wetted the walls behave as a solid surface. The surface angle of the liquid on this surface is zero degree. This make internal channel an ideal capillary for quick transport of the liquid along the yarn.  
         [0000]     Shell  
         [0011]     Unlike traditional wicks the structure of braided wick easily withstands both positive and negative external and internal pressure. In addition to benefits such as good recovery from ice formation, it enables use of thing foils and flexible films for the shell construction. Use of low pressure refrigerant liquids accounts of negative pressure gradient across the shell. Prior art designs utilize thick wall plates or tubes to counteract this pressure. They also incorporate spacers to prevent collapse of a heat pipe.  
         [0012]     The wick of this invention eliminates the need in all these elements. A thin foil (e.g. 50 gauge aluminum foil) can be uses as a sole constructive element of the pipe shell as shown on  FIG. 3 . The wick  1  forms a sheet with yarns  3  weaved or braided together alike a fabric. Refrigerant liquid is carried mostly by the yarns while vapor is transported through the channels  2  and pores formed by the fabric. Ultra thin metal foil  7  is well supported by the wick.  
         [0013]     The only real limitation for the foil thickness is absence of pinholes as mechanical strength of even 5 micron aluminum is sufficient to resist 1 bar of air pressure when placed on top of the wick of this invention. To eliminate effect of production pinholes a laminate of two layers of capacitor grade aluminum foil is created with total laminate thickness of 12 microns. The laminate  7  under pressure forms corrugated patterns  8  alike well known “diamond plate”. This embossed pattern serves two critical roles for the matter of this invention. First, it forms coplanar mechanical contact  9  with the wick that dramatically increases wick&#39;s mechanical stability. Second, it makes highly efficient thermal contact with surfaces of the yarns. Each yarn walls are saturated with refrigerant liquid that makes said thermal contact highly stable and efficient.  
         [0014]     The load bearing ability of the wick allows for its efficient use with medium and high pressure refrigerant fluids. Prior art designs of heat pipes for high pressure applications accounts for thick walls or round pipe or external constraints (underground installation). The wick of this invention allows to avoid these design solutions as they contribute to higher weight and reduced usability.  
         [0015]     The shell for positive pressure designs can be furnished by organic or inorganic polymers, fiber reinforced materials, or other composites. Dense yarn structure of the wick allows the shell material to be glued or molded into the surface of the wick. Other well known methods of lamination can be used as well. Alternatively layer or layers composing the shell can be interweaved with the wick surface, or sewed with the wick, or any other well known technique of mechanical attachment can be employed to secure the shell on the wick.  
         [0016]     Typical topology of surface for the invented wick has weaves every 1 mm or less. Foil of Aluminum Alloy 2014-T6 100 micron thick will be able to sustain 1700 psi pressure. The wick material for this high pressure scenario can be steel fiber or carbon fiber. Most refrigerant has critical pressure below 1700 psi that allow for use of broader spectrum of wick materials.  
         [0017]     The thin shell design of this invention under positive and negative pressure forms intrinsic corrugations of the surface this makes entire assembly flexible enough to allow in place bending and curling of the pipe. The shape factor for the pipe can be selected as a sheet, tape, sleeve or any others as only one factor defines this shape -the wick cross section.  
       BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]      FIG. 1 . Cross-section of schematic structure of the wick (top), and its appearance from outside (below).  
         [0019]      FIG. 2 . Construction of braided yarn with intrinsic cavity (left), yarn appearance is similar to thread but have braiding (right).  
         [0020]      FIG. 3 . Oriented braided wick constrained between shell layers. Top view (top left), front view (bottom left), side view (center). Details (right) shows deformation of shell surface (top) and persistence of intrinsic cavities (bottom).