Abstract:
A heat pipe with electrical pumping. The heat pipe includes a condenser to condense vapor into liquid droplets and an evaporator for the liquid-vapor conversion. Furthermore, the heat pipe includes liquid conduits for carrying the liquid droplets, where every liquid conduit includes electrodes for moving the liquid droplets along the liquid conduits. Additionally, the heat pipe includes vapor conduits for carrying the vapor. After the liquid is condensed and droplets are formed, they are electrically pumped towards the evaporator by sequentially actuating a series of electrodes in the liquid conduits. By implementing electrical pumping instead of wick-based pumping, the heat transport capacity over long distances is greatly increased. Additionally, since the electrical force is greater than gravity, it is possible to develop orientation independent long heat pipes. Other benefits include planar form factors, noiselessness, high reliability due to the absence of moving mechanical parts and ultralow power consumption.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to U.S. Provisional Patent Application Ser. No. 62/002,241, “Heat Pipes with Active Electrical Pumping,” filed May 23, 2014 as well as claims priority to U.S. Provisional Patent Application Ser. No. 61/968,744, “Electrical Pumping-Based Active Heat Pipes,” filed Mar. 21, 2014, which are both incorporated by reference herein in their entirety. 
     
    
     TECHNICAL FIELD 
       [0002]    The present invention relates generally to heat pipes, and more particularly to heat pipes with electrical pumping thereby allowing higher capacity, longer and thin, planar form factor heat pipes to be developed. 
       BACKGROUND 
       [0003]    Heat pipes are heat transfer devices to move heat from a hot source (referred to as an “evaporator”) to a cold sink (referred to as a “condenser”).  FIG. 1  illustrates the cross section of a conventional heat pipe  100 . Referring to  FIG. 1 , heat flow occurs via evaporation of the heat pipe working fluid at the evaporator end  101 ; the heat absorbed is used to convert liquid into vapor (see arrows near evaporator end  101 ). This hot vapor flows through the pipe center (see arrows in middle of heat pipe  100 ) and condenses at the condenser (cold) end  102  where it rejects the heat (see arrows near condenser end  102 ). A wick structure  103  lining the inside of heat pipe  100  pulls the condensed liquid back to evaporator  101  (see arrows in wick structure  103  directed to evaporator  101 ) to complete the closed cycle. Heat pipe  100  utilizes capillary action in an internal wick structure  103  to drive liquid circulation. As a result, the length of conventional heat pipes is limited by the capillary pressure generated in the heat pipe wick beyond which the wick cannot provide sufficient liquid to the evaporator. Consequently, conventional heat pipes cannot transport heat over very long distances (at high enough heat flow rates) since the wick pressure is not sufficient to pump liquid condensate back to the evaporator. Furthermore, conventional heat pipe architectures do not easily lend themselves to very thin and slender shapes which may expand the potential applications of heat pipes. 
       BRIEF SUMMARY 
       [0004]    In one embodiment of the present invention, a heat pipe comprises a condenser region configured to condense vapor and split the liquid into liquid droplets. The heat pipe further comprises a plurality of liquid conduits for carrying the liquid droplets, where each of the plurality of liquid conduits comprises a first plurality of underlying electrodes. The heat pipe additionally comprises a plurality of vapor conduits for carrying the vapor. In addition, the heat pipe comprises an evaporator region connected to the condenser region via the plurality of liquid conduits and vapor conduits, where the evaporator region is configured to convert the liquid droplets into the vapor. The liquid droplets condensed by the condenser region are electrically pumped towards the evaporator region by sequentially actuating a series of the first plurality of underlying electrodes. 
         [0005]    The foregoing has outlined rather generally the features and technical advantages of one or more embodiments of the present invention in order that the detailed description of the present invention that follows may be better understood. Additional features and advantages of the present invention will be described hereinafter which may form the subject of the claims of the present invention. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0006]    A better understanding of the present invention can be obtained when the following detailed description is considered in conjunction with the following drawings, in which: 
           [0007]      FIG. 1  illustrates the cross section of a conventional heat pipe; 
           [0008]      FIG. 2  illustrates the cross section of a device for electrowetting-based liquid pumping in accordance with an embodiment of the present invention; 
           [0009]      FIG. 3A  illustrates an isometric view of an electrowetting heat pipe in accordance with an embodiment of the present invention; 
           [0010]      FIG. 3B  illustrates a top view of the electrowetting heat pipe in accordance with an embodiment of the present invention; and 
           [0011]      FIG. 4  is a graph of the heat dissipation capacity versus length for both conventional heat pipes and the electrowetting heat pipes of the present invention in accordance with an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0012]    As stated in the Background section, heat pipes are heat transfer devices to move heat from a hot source (referred to as an “evaporator”) to a cold sink (referred to as a “condenser”). In conventional heat pipes, heat flow occurs via evaporation of the heat pipe working fluid at the evaporator end; the heat absorbed is used to convert liquid into vapor. This hot vapor flows through the pipe center and condenses at the condenser (cold) end where it rejects the heat. A wick structure lining the inside of the heat pipe pulls the condensed liquid back to the evaporator to complete the closed cycle. Conventional heat pipes utilize capillary action in an internal wick structure to drive liquid circulation. As a result, the length of conventional heat pipes is limited by the capillary pressure generated in the wick, beyond which the wick structure cannot provide liquid to the evaporator. That is, conventional heat pipes cannot transport heat over very long distances (at high enough heat flow rates) since the wick pressure is not sufficient to pump liquid condensate back to the evaporator. Furthermore, conventional heat pipe architectures do not easily lend themselves to very thin and slender shapes which may expand the potential applications of heat pipes. 
         [0013]    The principles of the present invention provide a means for developing a longer and thinner heat pipe (e.g., more than six feet in length as well as less than 2 millimeters in thickness) than conventional heat pipes by transporting liquid (as droplets) using electrical pumping as opposed to the use of a wick as discussed below in connection with  FIGS. 2, 3A-3B and 4 .  FIG. 2  illustrates the cross section of a device for electrowetting-based liquid pumping.  FIG. 3A  illustrates an isometric view of an electrowetting heat pipe.  FIG. 3B  illustrates a top view of the electrowetting heat pipe.  FIG. 4  is a graph of the heat dissipation capacity versus length for both conventional heat pipes and the electrowetting heat pipes of the present invention. 
         [0014]    As discussed above, conventional heat pipes can be made longer and thinner (e.g., more than six feet in length as well as less than 2 millimeters in thickness) than existing heat pipes via the use of transporting liquid (as droplets) by electrical pumping as opposed to the use of a wick structure. The device for achieving droplet motion using electrowetting is shown in  FIG. 2 . 
         [0015]    Referring to  FIG. 2 ,  FIG. 2  illustrates the cross section of a device  200  for electrowetting-based liquid pumping in accordance with an embodiment of the present invention. Device  200  includes two flat plates  201 A- 201 B separated by a fixed spacing  202 , where a liquid droplet is transported. Plates  201 A- 201 B may collectively or individually be referred to as plates  201  or plate  201 , respectively. Control electrodes  203 A- 203 B on lower plate  201 B can be independently actuated. Control electrodes  203 A- 203 B may collectively or individually be referred to as electrodes  203  or electrode  203 , respectively. While  FIG. 2  illustrates two electrodes, device  200  includes numerous electrodes  203 , such as along an entire length of a channel (see discussion of liquid conduit  303  of electrowetting heat pipe  300  further below), where a series of the electrodes are selectively actuated so as to propel a liquid droplet along the length of the channel as discussed further below. 
         [0016]    As illustrated in  FIG. 2 , upper plate  201 A is covered by a single electrode  204  which is grounded. Both plates  201 A- 201 B are covered by a dielectric layer  205 A- 205 B, respectively, and a thin hydrophobic layer  206 A- 206 B, respectively, to provide low friction. 
         [0017]    Dielectric layers  205 A- 205 B may collectively or individually be referred to as dielectric layers  205  or dielectric layer  205 , respectively. In one embodiment, dielectric layer  205  can range in thickness from nanometers to microns depending on the application. Furthermore, dielectric layer  205  can be smooth or textured (rough). Additionally, the surface energy of dielectric layer  205  will be low which will make dielectric layer  205  superhydrophobic. In one embodiment, dielectric layer  205  is made of a single material. In one embodiment, dielectric layer  205  is composed of multiple materials. In one embodiment, dielectric layer  205  is multilayered. Furthermore, dielectric layer  205  may be a polymer, oxide, ceramic or another type of insulating material. 
         [0018]    Furthermore, hydrophobic layers  206 A- 206 B may collectively or individually be referred to as hydrophobic layers  205  or hydrophobic layer  205 , respectively. While  FIG. 2  illustrates the use of hydrophobic layers  206 , the principles of the present invention are not to be limited in scope to the required use of hydrophobic layers  206 . In one embodiment, hydrophobic layers  206  are superhydrophobic layers. 
         [0019]    The size of a droplet  207  is chosen to be bigger than the electrode pitch (pitch of electrodes  203 A- 203 B) so that it overlaps more than one electrode  203 . When electrode  203 B is energized, droplet  207  moves to the right and comes to equilibrium at the center of electrode  203 B. By pulsing voltages along an array of discrete electrodes  203 , it is possible to keep droplet  207  moving continuously. 
         [0020]    The present invention combines the use of electrowetting-based pumping in a heat pipe architecture to arrive at what is referred to herein as the “electrowetting heat pipe” (EHP).  FIG. 3A  illustrates the isometric view of an electrowetting heat pipe  300  in accordance with an embodiment of the present invention.  FIG. 3B  illustrates a top view of electrowetting heat pipe  300  in accordance with an embodiment of the present invention. 
         [0021]    Referring to  FIGS. 3A-3B  in conjunction with  FIG. 2 , electrowetting heat pipe  300  includes an evaporator region  301  and a condenser region  302  at the ends along with separate liquid conduits  303 A- 303 E and vapor conduits  304 A- 304 D for connecting evaporator region  301  with condenser region  302 . Evaporator region  301  is designed to convert liquid drops into vapor; whereas, condenser region  302  is designed to condense vapor into liquid droplets. In one embodiment, evaporator region  301  is textured (roughened) and hydrophilic to promote liquid spreading and evaporation. In one embodiment, condenser region  302  is superhydrophobic and textured (roughened) to promote efficient condensation. Liquid conduits  303 A- 303 E may collectively or individually be referred to as liquid conduits  303  or liquid conduit  303 , respectively. Vapor conduits  304 A- 304 D may collectively or individually be referred to as vapor conduits  304  or vapor conduit  304 , respectively. While  FIG. 3A  illustrates two liquid conduits  303  and two vapor conduits  304  and  FIG. 3B  illustrates five liquid conduits  303  and four vapor conduits  304 , electrowetting heat pipe  300  may include any number of liquid conduits  303  and vapor conduit  304 . Liquid conduits  303  are designed to carry the liquid droplets; whereas, vapor conduits  304  are designed to carry the vapor. 
         [0022]    The heat load in evaporator  301  is absorbed by liquid evaporation. The vapor is directed through the vapor-conduit channels  304  to the opposite end  302 , where heat release occurs due to condensation. In one embodiment, the vapor and liquid flows occur in different channels (vapor flows in vapor conduits  304  and liquid flows in liquid conduits  303 ) to prevent entrainment of the liquid into the vapor stream which would reduce heat transfer efficiencies. At condenser  302 , discrete liquid droplets are “electrically” pinched from the condensed pool (see sections  305 A- 305 B) and pumped towards evaporator  301  in liquid conduits  303  by sequentially actuating a series of underlying electrodes, such as electrodes  203  shown in device  200  of  FIG. 2 . Sections  305 A- 305 B may collectively or individually be referred to as sections  305  or section  305 , respectively. While  FIG. 3A  illustrates two sections  305 , electrowetting heat pipe  300  may include any number of sections  305  comprising the condensed pool. Furthermore, sections  305  may have one or multiple electrodes to create droplets from the liquid pool on condenser  302 . 
         [0023]    The liquid that may be transported herein via liquid conduits  303  may include water, organic solvents, liquid metals, etc. In one embodiment, the liquid flow in liquid conduits  303  is achieved by device  200  which forms liquid conduit channel  303 . By implementing electrowetting for achieving droplet motion as opposed to the use of a wick structure, the length of a heat pipe may be increased (e.g., more than six feet in length), since the length restrictions due to fundamental wick limitations are no longer valid. Additionally, since the electrowetting force is greater than 10× the force of gravity, it is possible to transport fluid uphill over long distances, which has distinct advantages. Furthermore, by implementing electrowetting for achieving droplet motion as opposed to the use of a wick structure, the thickness of the heat pipe may be decreased (less than 2 millimeters in thickness) in comparison to conventional heat pipes. In one embodiment, electrowetting heat pipe  300  may be less than 2 millimeters in thickness by having 1.5 millimeter tall channels  303 ,  304  for droplet and vapor flow, respectively, and a 0.25 millimeter thick top and bottom plates  201 A- 201 B, respectively. 
         [0024]    At evaporator  301 , the droplets are spread using electrowetting across sections  306 A- 306 B, such as described above in connection with  FIG. 2 , to form a thin film which evaporates and continues the cycle. In one embodiment, the droplets are spread using electrowetting by having device  200  of  FIG. 2  form sections  306 A- 306 B. Sections  306 A- 306 B may collectively or individually be referred to as sections  306  or section  306 , respectively. While  FIG. 3A  illustrates two sections  306 , electrowetting heat pipe  300  may include any number of sections  306  for electrowetting-based droplet spreading and evaporation. By using electrowetting-based spreading in evaporator  301 , the evaporation area is increased and evaporation is enhanced. 
         [0025]    In another embodiment of the present invention, liquid droplet pumping occurs on superhydrophobic textured surfaces or oil-infused textured surfaces of liquid conduits  303 , which act as a lubricant and reduce the electrical voltage needed for liquid pumping. The electrical voltage may be a DC voltage (positive or negative) or an AC voltage. Furthermore, the electrical voltage may be any other complex electrical waveform. Additionally, the electrical voltage can be supplied to individual electrodes (e.g., electrode  203 A) by a network of electrical bus bars and interconnects on the heat pipe bottom plate  201 B. 
         [0026]    In another embodiment of the present invention, electrowetting heat pipe  300  does not contain any walls  307  to separate the liquid and vapor conduit channels  303 ,  304 , respectively, as illustrated in  FIG. 3A . Liquid and vapor flows will form as discussed above using the electrowetting principles of the present invention but without any walls  307  to separate the liquid and vapor regions. In other words, droplets will flow in liquid conduits  303  towards evaporator  301  and vapor will flow in vapor conduits  304  towards condenser  302  as discussed above but without the existence of walls  307  to separate these liquid and vapor regions. 
         [0027]    As discussed above, by replacing the wick structure of the conventional heat pipe with electrical pumping, the capillary limit becomes irrelevant. As a result, longer heat pipes can be developed.  FIG. 4  is a graph  400  illustrating the heat dissipation capacity (Watts, W)  401  versus length (centimeters, cm)  402  for both conventional heat pipes (see dashed line  403 ) and the electrowetting heat pipes of the present invention (see dashed line  404 ) in accordance with an embodiment of the present invention. 
         [0028]    Referring to  FIG. 4 ,  FIG. 4  illustrates that the heat dissipation capacity of conventional heat pipes decreases as the length increases (see dashed line  403 ). This sharp performance degradation is due to the limitation of the wick structure (see wick structure  103  of  FIG. 1 ) in transporting heat over long distances. On the other hand, the EHP heat dissipation capacity (see dashed line  404 ) does not depend on the heat pipe length, but on the condenser and the evaporator performance.  FIG. 4  shows that &gt;10× heat dissipation capacity enhancement is possible for long heat pipes by utilizing the electrowetting-based pumping of the present invention to beat the capillary limit. Such a heat pipe may be packaged using existing packaging materials and processes. 
         [0029]    The descriptions of the various embodiments of the present invention have been presented for purposes of illustration, but are not intended to be exhaustive or limited to the embodiments disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the described embodiments. The terminology used herein was chosen to best explain the principles of the embodiments, the practical application or technical improvement over technologies found in the marketplace, or to enable others of ordinary skill in the art to understand the embodiments disclosed herein.