Abstract:
A heat pipe utilizing extended membrane instead of traditional wick structure provides dramatic increase in both flux densities and transport distances both in horizontal and vertical directions.

Description:
RELATED APPLICATIONS  
       [0001]     This application is a continuation-in-partof each of: 
    (1) U.S. patent application Ser. No.: 11/308438, filed Mar. 24, 2006, entitled “Heat conductive textile and method producing thereof”, hereby incorporated by reference     (2) U.S. patent application Ser. No.: 11/308107, filed Mar. 7, 2006, entitled “Tunable heat regulating textile”, hereby incorporated by reference     (3) U.S. patent application Ser. No.: 11/307359, filed Feb. 2, 2006, entitled “Stretchable and transformable planar heat pipe for apparel and footwear, and production method thereof”, hereby incorporated by reference     (4) U.S. patent application Ser. No.: 11/307,292, filed Jan. 31, 2006, entitled “High throughput technology for heat pipe production”, hereby incorporated by reference     (5) U.S. patent application Ser. No.: 11/307,125, filed Jan. 24, 2006, entitled “Integral fastener heat pipe”, hereby incorporated by reference (6) U.S. patent application Ser. No.: 11/307,051, filed Jan. 20, 2006, entitled “Process of manufacturing of spongy heat pipes”, hereby incorporated by reference     (7) U.S. patent application Ser. No. 11/306,530, filed Dec. 30, 2005, entitled “Heat pipes utilizing load bearing wicks”, hereby incorporated by reference     (8) U.S. patent application Ser. No. 11/306,529, filed Dec. 30, 2005, entitled “Perforated heat pipes”, hereby incorporated by reference     (9) U.S. patent application Ser. No. 11/306,527, filed Dec. 30, 2005, entitled “Heat pipes with self assembled compositions”, hereby incorporated by reference   
 
     
    
     FIELD OF THE INVENTION  
       [0010]     Present invention relates to advanced heat pipes, planar and capillary heat pipes, and in particular to flexible and elastic heat pipes. Advanced materials embedding said heat pipes such as textiles, films, tows, yarns and threads are also closely covered by the field of present invention.  
       BACKGROUND OF THE INVENTION  
       [0011]     Heat pipes as proposed by Gorge M. Grover U.S. Pat. No 3,229,759 (1966) as a devices utilizing mass transfer of phase changing chemicals face common balance equation. This equation includes capillary forces acting against hydrodynamic resistance of liquid and vapor components of the designs. The only exceptions from this common case are gravity assisted heat pipes that can operate without any dependency on capillary forces.  
         [0012]     Efficiency of capillary force increases with decrease of capillary dimensions. Wick structures of various type has been used in the industry to counteract effects of gravity and transport liquids along the heat pipe back to heat source. The only exceptions from this design to date are capillary heat pipes such as ones proposed by Akachi U.S. Pat. Nos. 4,921,041 (1990) and 5,219,020 (1993). Yet in all cases decrease of capillary dimensions causes increase in hydrodynamic resistance of transported liquids. This resistance significantly reduces heat transferring capacity of the pipes.  
       SUMMARY OF THE INVENTION  
       [0013]     This invention utilizes concept of membrane  2  to replace bulky wick structure and decouple capillary action from hydrodynamic effects. As depictured on  FIG. 1  membrane  2  permeable to vapors of phase changing refrigerant liquid is extended along dominant heat transfer directions of the heat pipe assembly  1 . The membrane separates volume  3  occupied by the liquid from volume  4  occupied by the vapors.  
         [0014]     Position of membrane  2  is mechanically secured within the body of heat pipe  1  so forces of capillary pressure are transferred from membrane  2  to remaining liquid  5 . Since volume  3  of the liquid is free from mechanical obstacles the effect of viscose friction of liquid  5  is drastically diminished. Capillary pressure forces the motion of liquid  5  back to the heat source region.  
         [0015]     Because reduction of pore size in membrane  2  does not affect viscose friction of liquid  5 , invented heat pipe  1  is capable to transfer liquid  5  at substantially higher rates than any other heat pipe design invented to date. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0016]     In order for heat pipe  1  to operate, specific energy of liquid  5  present in volume  3  and vapor  6  present in volume  4  should be significantly lower than for case when liquid  5  is present in volume  4  and vapor is present in volume  3 . This condition can be easily accommodated by supplying structure of heat pipe  1  wherein surface properties of materials interfacing volume  3  are different from surface properties of materials interfacing volume  4 .  
         [0017]     In fact it is often sufficient than only part of surfaces have such distinct properties. In one example this can be achieved when liquid  5  is water and membrane is hydrophobic and remaining walls of volume  4  are hydrophobic while remaining walls of volume  3  are hydrophilic. In another example, same can be achieved when one side of membrane  2  is hydrophobic and faces volume  4 , while another side of membrane  2  is hydrophilic and faces volume  3 , while remaining walls of volumes  3  and  4  have identical properties. Yet another example, both walls and side of membrane  2  facing volume  3  are hydrophilic, while both walls and side of membrane  2  facing volume  4  are hydrophobic.  
         [0018]     The second essential demand is mechanical stability of the membrane  2 . Because membrane  2  essentially separates two volumes with distinct properties it is capable of performing mechanical work resulting in flow of liquid  5 . But in order for this to happen, membrane  2  must be immobilized. This immobilization can be achieved by various ways one of possible examples is depictured on  FIG. 2 . The opposite walls of heat pipe  1  are joined in plurality of places forming reliable bonds  7 . Each bond may have through hole connecting opposite sided of heat pipe assembly.  
         [0019]     Example of  FIG. 2  utilizes design of co-pending patent application Ser. No. 11/306,529. This design attributes to network like appearance of heat pipe  1 , and makes it essentially planar. Each joint  7  stabilizes not only mechanical position of membrane  2  but also blocks damaging effect of pressure difference between inner volume of heat pipe  1  and ambient volume.  FIG. 2  shows joint  7  interconnecting opposite sides of the heat pipe, alternative design is shown on  FIG. 3  that does not have aforementioned hole. Both side walls and membrane  2  are merged together in each joint  7 .  
         [0020]     In another example heat pipe has essentially string-like shape with cross-section as illustrated on  FIG. 4 (A-D). Membrane  2  may deform from plane shape under influence of internal or external pressure.  FIG. 4 (B) depictures such case. Deformed membrane creates liquid channel  3  with variable area. This area may change dynamically when conditions such as heat source location, orientation, temperature, or heat flux change. As shown on  FIG. 4C , membrane  2  may perform additional mechanical function such as constraint stabilizing overall shape of the heat pipe. When internal pressure exceeds ambient one the heat pipe of shape C can not exist without membrane  2  constraining opposite walls from deforming outward. Shown shape is advantageous as it provides larger area of liquid channel  3  and lesser hydrodynamic resistance.  FIG. 4D  shows design suitable for internal pressure lesser that ambient one. Contracting force acting on external walls of heat pipe  1  is counter balanced by membrane tension.  
         [0021]     There are many ways to produce the heat pipes of the present invention, some were disclosed in co-pending patent application Ser. Nos. 11/308438, 11/307359, 11/307,292, 11/307,051, 11/306,529. One experienced in related art understands that alternative techniques are equally suitable. As an example, heat pipe  1  can be produced by joining film  8  of a material one surface of which repels refrigerant liquid  5 , a membrane film  2  one side of which repels refrigerant liquid  5  and faces film  8 , another side of membrane  2  has high affinity to liquid  5  and pores of membrane  2  are permeable to vapors  6 , another film  9  one side of which has high affinity to liquid  5  faces membrane  2 . All three layers joined to form sealed cavity with liquid  2  trapped inside.  
         [0022]     Material of the walls in this example can be selected from plurality of readily available sheet or film materials. As example PTFE-aluminum laminate can be used to make both walls, wherein one has aluminum inner surface  9  and another has PTFE inner surface  8 . Membrane  2  composed of PTFE membrane sintered with Nylon membrane. Currently, commercially available PTFE membranes have pore sizes up to 50 nm. Nylon membranes are broadly available for range of pore sizes starting from 30 nm. In this example 50 nm PTFE membrane was sintered with 30 nm Nylon membrane. Refrigerant liquid selected to be water. In order to maintain operating range for the pipe in +1 to +100° C. heat pipes of prior art utilize spacers or round construction. Present example uses linear heat pipe with profile shown on  FIG. 5D . Inner volume of heat pipe  1  is evacuated in this example. Membrane  2  prevents profile from collapse without any spacers.  
         [0023]     Contact angle of water on PTFE is nearly 180°, and 40° on Nylon.  FIG. 5  depictures schematic of capillary forces distribution. When gravity is applied along membrane  2  capillary forces of membrane segment  8  create additional pressure at the bottom of the pipe, while capillary forces of membrane segment  9  create additional suction at the top of the pipe. Considering that pore size is 50 nm on PTFE membrane and 30 nm on Nylon membrane, maximal differential pressure created by capillary forces exceeds 67 atm. This means that this heat pipe is able to operate against gravity providing height drop in excess of 676 m. In order to estimate heat transfer capacity of this pipe design we must assume profile dimensions. Smaller dimensions allow wall material to withstand higher pressure forces at lesser wall thickness.  
         [0024]     In present example let&#39;s assume that profile has circular form and inner diameter of 200 microns. To withstand backpressure membrane should have thickness of 32 microns. Walls should contain at least 20 micron aluminum foil. To estimate hydrodynamic resistance Poiseuille equation can be used. For heat pipe 676 m long positioned horizontally at 20° C. throughput of transferred heat will be 178 mW. This value will linearly drop to zero when heat surface elevation reaches 676.8 m, this is equivalent to 5.7 KW/cm2 of axial heat flux.  
         [0025]     However suction forces created by upper meniscus may create instability in liquid volume, this is true especially when contaminants present are in it. That is why practical applications should use smaller height drop of approximately 297 m. In this case selected length of heat pipe  1  is 297 m and the heat source is vertically positioned above the heat sink. Axial heat flux will be 227 mW, which is equivalent to 7.2 KW/cm2 of axial heat flux. For horizontally positioned heat pipe axial heat flux increases to 404 mW, this is equivalent to 12.7 KW/cm2 of axial heat flux.  
         [0026]     These values exceed heat transfer performance of any water based prior art heat pipe by more than an order of magnitude. The closest performance competitor in this case is Mercury heat pipe, nevertheless, it is only true for horizontal placement, as to the vertical (against gravity) scenario invented heat pipe  1  outperforms all existing heat pipes by more than an order of magnitude.  
         [0027]     Yet alternative design of heat pipe  1  is depictured on  FIG. 6 . Membrane in this case appears as tubular profile  2  disposed through volume of heat pipe  1 .  FIG. 6  shows network-like shape of both membrane  2  and heat pipe  1  itself. It is obvious that same relative positioning of membrane  2  and the shell of heat pipe  1  can be created in simpler designs such as one of depictured on  FIG. 7  linear heat pipes. Membrane  2  is mechanically secured with respect to the shell, which prevents it from motions along dominant heat transfer directions.  
         [0028]     Because membrane has tubular form with circular profile it may provide significantly higher pressure at lesser material thickness. In addition complete volume  4  become enclosed by membrane  2 , this allows to reduce number of distinct surfaces from four to three. Inner surface  8  of membrane  2  must repel liquid  5 , while outer surface  9  and inner surface of volume  3  should have high affinity to liquid  5 . It is also possible to omit surface  9 , this, however, will slightly diminish performance of heat pipe  1 .  
         [0029]     As an example of design shown on  FIG. 7 , let&#39;s consider copper heat pipe with outer diameter of 0.125″ and PTFE membrane  2  with pore size of 50 nm. Note that layer  9  is missing in this design. Pipe inner diameter is 0.061″. To find out how stable operations of invented design are, let&#39;s consider that volume  4  also contains droplets of liquid of size r in vicinity of condensing area. These droplets must be unstable and should disappear during heat transfer operation of heat pipe  1 . To demonstrate this it is sufficient to show that vapor pressure in vicinity of these droplets is less than vapor pressure next to meniscus inside the membrane. Due to heat transfer temperature of meniscus inside membrane is lower than one of the droplets. The same is true for saturation pressure  
           P   3   sat     =       P   4   sat     ⁢     exp   ⁡     (       q   R     ⁢     (       1     T   4       -     1     T   3         )       )           ,       
 
 where q—is latent evaporation heat, R—is gas constant. 
 
         [0030]     Changes in surface curvature also contribute to changes in saturation pressure. 
 
 This dependency for large (&gt;100 nm) pore sizes is well modeled as  
             P   surface   saturation     -     P   volume   saturation       =       2   ⁢     ρ   vapor     ⁢   σ       r   ⁡     (       ρ   liquid     -     ρ   vapor       )           ,       
 
 where σ—is surface tension, 
    r—is curvatureradius,     ρ vapor —is vapor density,     ρ liquid —is liquid density.    
 
         [0034]     In order for droplets to evaporate saturation pressure in their vicinity should be higher than one of meniscus inside membrane  2 . This condition is achieved when  
           P   3   surface     =     P   4   surface       ,     
     ⁢     assuming   ⁢           ⁢   that         
           ρ   3   vapor     =           ρ   4   vapor     ⁢         T   4     ⁢     P   3   surface           T   3     ⁢     P   4     surface   ⁢                   ⁢   and   ⁢           ⁢     T   3       -       T   4     ⁢       &lt;&lt;     T   4       ⁢     
     ⇓     
     ⁢     σ   3           ≈     σ   4         ,       ρ   3   liquid     ≈     ρ   4   liquid           
 
 solving equation for r 4   
         r   4     =       2   ⁢     r   3     ⁢     ρ   vapor     ⁢     σ   ⁡     (         exp   ⁡     (       q   R     ⁢     (       1     T   4       -     1     T   3         )       )       ⁢     T   4     ⁢     ρ   vapor       -       T   3     ⁢     ρ   liquid         )                   (       ρ   liquid     -     ρ   vapor       )     ⁢     (         P   4   sat     ⁢     r   3     ⁢     T   4     ⁢     ρ   vapor     ⁢   exp   ⁢     (         2   ⁢   q     R     ⁢     (       1     T   4       -     1     T   3         )       )       +                       P   4   sat     ⁢     r   3     ⁢     T   3     ⁢     ρ   liquid       -     exp   ⁡     (       q   R     ⁢     (       1     T   4       -     1     T   3         )       )                     (         P   4   sat     ⁢       r   3     ⁡     (         T   4     ⁢     ρ   vapor       +       T   3     ⁢     ρ   liquid         )         +     2   ⁢     T   4     ⁢     ρ   vapor     ⁢   σ       )     )                 
 
         [0035]     As temperature difference across membrane increases so stable droplet size. Any droplet of size smaller than r 4  evaporates transferring its mass through membrane  2 . Any droplet of size larger than r 4  does not evaporates but instead continues to grow. It is important to notice that at some critical temperature gradient across membrane  2  r 4  becomes infinitely large indicating that any droplet inside volume  4  is unstable and will be transferred through membrane  2  into volume  3 .  FIG. 8  shows results for r 4  at 293° K. in assumption that pores diameter is 50 nm and liquid  5  is distilled water.  
         [0036]     To achieve stable heat transfer conditions temperature gradient across membrane  2  should exceed value that makes all liquid in volume  4  unstable. Graph depictured on  FIG. 9  is obtained by solving above equation for infinitely large r 4 . Increase of pores size in membrane  2  attributes to reduction of said gradient. Although set criteria are useful for design of heat pipe  1  of the present invention, they are not mandatory. In fact, if during creation process volume  4  received neither contaminants nor liquid  5 , then operations of heat pipe  1  will be stable since condensation of vapor  6  primarily occurs on meniscus within membrane  2  and not in volume  4 . In order for condensation in volume  4  to occur it must contain notable amount of condensation centers that are usually associated with presence of contaminant particles.  
         [0037]     It is obvious to one experienced in the art that all designs disclosed in co-pending patents applications can be trivially adapted to include membrane  2  of present invention. Although examples cited above utilize water as refrigerant fluid  5 , it is equally possible to use medium and high pressure condensed gases that were previously indicated in co-pending applications. Materials of membrane  2  and the shell of heat pipe  1  can be polymers, elastomers, inorganic polymers, and various composites.  
         [0038]     Membrane  2  may include either layer  8  or  9  or both  8  and  9 , wherein each layer can be a distinct material or surface deposited chemical components.  
         [0039]     Heat pipe  1  can be produced as a film similar to one shown on  FIGS. 2 and 3 , or it can be manufactured as a fiber. Wherein in case of fiber each continuous fiber may contain a single heat pipe  1  or plurality of heat pipes  1  sequentially positioned along the fiber in a fashion similar to one disclosed in co-pending U.S. patent application Ser. No. 11/308438.  
         [0040]     Diameter of this fiber can be easily made from 25 to 200 microns that is suitable to replace yarn spools in knitting, weaving, and braiding machines. Use of heat pipes  1  as a yarn allows for creation of advanced textiles, cables and etc. that demonstrate fire retardant and fire protection properties as well as energy transfer and heat management/regulation properties.  
         [0041]     Because heat pipe  1  of the invention provides dramatic efficiency increase over large transfer distances it is suitable for energy conservation designs. One example of which is energy efficient buildings. Modern heat pumps utilize electrical power to transfer heat between a building and a ground mass or water reservoir. Heat pipe  1  is capable of collecting heat from hundreds of meters from bottoms of lakes or from soil. This heat can be transferred without use of additional motors by heat pipe  1  itself to the building to stabilize its walls and/or roof temperature. It is a known fact that underground temperatures only slightly change with seasons. These temperatures are specific to depth and geographical locations. As an example in warm southern Texas, the average reading inside the Caverns of Sonora is 71° F. Use of heat pipes  1  deposited on walls of the building in that geographical location will allow the building to operate with no heating or air conditioning throughout all seasons. This could be achieved as heat pipes  1  in form of a film or textile collects or dissipates heat through large underground area, thus protecting the building from heating and from cooling.