Abstract:
A heat pipe system for conducting thermal energy. The heat pipe system includes a sealed tube having along its length a reservoir region, an evaporator region, and a condenser region, the tube having a first end and a second end and an inside wall. The system also includes a wick disposed adjacent the inside wall of the tube, the wick including a first portion at the first end of the tube and a second portion adjacent the first portion, wherein the first portion of the wick is thicker than the second portion of the wick, and wherein the second portion of the wick does not extend to the second end of the tube. The system also includes a working fluid contained within the tube.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to U.S. Provisional Patent Application No. 61/548,262 filed Oct. 18, 2011, the content of which is incorporated herein by reference in its entirety. 
     
    
     BACKGROUND 
       [0002]    Heat pipes are passive devices used to draw heat from one location and dissipate the heat at a different location, and can take a number of different shapes and forms, including thermosyphons. Heat pipes may be used in a variety of applications, including, for example, drawing heat from electronics components. Heat pipes contain a working fluid and typically a wick on the inside wall of the pipe. In some applications, however, excess fluid may build up in certain areas of the heat pipe and form pools that are not absorbed by the wick. If the heat pipe is subjected to extreme conditions such as subfreezing temperatures, this excess working fluid (e.g. water) may undergo cycles of freezing and thawing that can damage the wick and/or the heat pipe itself. 
       SUMMARY 
       [0003]    Current heat pipes typically operate with an oversupply of working fluid, leaving them with pooled liquid when idle, and thus are susceptible to freeze/thaw damage. Heat pipes that not oversupplied with working fluid typically have a uniform layer of wick, resulting in a uniformly thicker wick throughout the heat pipe with a higher ΔT wick . Some heat pipes have distinct wick regions for reservoirs and heat input zones. However, in many cases, the condensate does not inherently flow over the heat input zone. Instead, replenishment of the wick in the heat input zone depends on capillary action to draw liquid from a reservoir, leaving it more susceptible to dry-out, which is generally an undesirable condition in such cases. In cases where there is a very large ratio of condenser area to heat input area (as in a space radiator), or where gravity is less strong to pull condensate back to the evaporator (e.g., in space or on the moon), addressing the simultaneous challenges of freeze/thaw and low ΔT wick  is particularly difficult. The hybrid wick according to various embodiments of the present invention is particularly advantageous for such applications. 
         [0004]    Accordingly, some embodiments of the invention provide a heat pipe with a hybrid wick which is thicker at one end of the heat pipe (in a reservoir region) so as to hold all or substantially all of the fluid in the condensed state when the pipe is idle. The hybrid wick can also include a thin portion adjacent to the thick portion, wherein the thin portion corresponds to an evaporator region of the heat pipe to which a first heat source is applied. The opposing end of the heat pipe, corresponding to a condenser portion in which fluid condenses to dissipate heat absorbed in the evaporator region, does not have any wick material. In some embodiments, the reservoir region has a second heat source applied to it to promote drying of the thicker portion of the wick in operation of the heat pipe. 
         [0005]    In some embodiments, the present invention provides a heat pipe system for conducting thermal energy. The heat pipe system includes a sealed tube having along its length a reservoir region, an evaporator region, and a condenser region, the tube having a first end and a second end and an inside wall. The system also includes a wick disposed adjacent the inside wall of the tube, the wick including a first portion at the first end of the tube and a second portion adjacent the first portion, wherein the first portion of the wick is thicker than the second portion of the wick, and wherein the second portion of the wick does not extend to the second end of the tube. The system also includes a working fluid contained within the tube. 
         [0006]    Some embodiments of the present invention provide a heat pipe system for conducting thermal energy. The heat pipe system includes a sealed tube having along its length a reservoir region, an evaporator region, and a condenser region, the tube having a first end, a second end, and an inside wall extending between the first and second ends. The heat pipe system also includes a wick disposed adjacent the inside wall of the tube, the wick including a first portion at the first end of the tube and a second portion adjacent the first portion and thinner than the first portion; and a quantity of working fluid contained within the tube. The heat pipe system has a first state in which the wick holds substantially the entire quantity of working fluid, and a second state in which heat is supplied to the evaporator region, in which the wick holds a portion of the quantity of working fluid, and in which a first part of a remainder of the working fluid has been heated to a vapor form, and in which a second part of the remainder of the working fluid is in condensed form on the inside wall of the tube in the condenser region of the tube. 
         [0007]    In some embodiments, a method of cooling using a heat pipe is provided. The method includes steps of heating a sealed tube at an evaporator region of the sealed tube located along the sealed tube between a condenser region and a reservoir region; evaporating a working fluid in a first wick lining the evaporator portion of the sealed tube; condensing the evaporated working fluid in the condenser region of the sealed tube; moving the condensed working fluid back toward the evaporator portion of the sealed tube; repeating the heating, evaporating, condensing, and moving steps with the condensed working fluid; and maintaining a second wick lining the reservoir region of the sealed tube in a substantially dry condition during the heating, evaporating, condensing, and moving steps, wherein the second wick lining the reservoir region is thicker than the first wick lining the evaporator region. 
         [0008]    Other aspects of the invention will become apparent by consideration of the detailed description and accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]      FIG. 1A  shows a heat pipe having a hybrid wick where the heat pipe is idle, i.e. with no heat applied to the evaporator region of the heat pipe; 
           [0010]      FIG. 1B  shows a shows a heat pipe having a hybrid wick where the heat pipe is active, i.e. with heat being applied to the evaporator region of the heat pipe; and 
           [0011]      FIG. 2  shows a heat pipe having a hybrid wick where the heat pipe is active and is shown with a first heat source thermally coupled to the reservoir region, a second heat source thermally coupled to the evaporator region, and a heat sink coupled to the condenser region. 
       
    
    
     DETAILED DESCRIPTION 
       [0012]    Before any embodiments of the invention are explained in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of components set forth in the following description or illustrated in the following drawings. The invention is capable of other embodiments and of being practiced or of being carried out in various ways. 
         [0013]    In various embodiments, the invention provides a heat pipe  10  with a hybrid wick  20  disposed therein. The heat pipe  10  is generally a sealed tube having along its length a reservoir region  12 , an evaporator region  14 , and a condenser region  16  ( FIG. 1A ). In some embodiments, the heat pipe  10  is made of copper tubing and can be various diameters, ranging from about 0.25 inch to about 0.625 inch, and anywhere from about 3 to about 18 inches in length, although other materials, diameters, and lengths are also possible and are encompassed within the present invention. Furthermore, other pipe cross-sectional shapes (e.g. oval, polygonal, and the like) are also possible. Finally, the heat pipes  10  may be straight or may have one or more bends along their lengths as appropriate for the given application. 
         [0014]    The hybrid wick  20  can be made of various materials, and in some embodiments is made of sintered copper powder. In certain embodiments, the condenser region  16  has a heat sink attached thereto, for example one or more conductive fins attached to the condenser region  16  in a thermally conductive manner. 
         [0015]    The hybrid wick  20  of the illustrated embodiment is disposed adjacent to and in thermal contact with the inside wall of the heat pipe  10  ( FIG. 1A ). Also, the illustrated hybrid wick  20  has a thick portion  22  which corresponds to the reservoir region  12  of the heat pipe  10 , and a thin portion  24  which is thinner than and adjacent to (and generally in capillary contact with) the thick portion  22  of the hybrid wick  20 , and corresponds to the evaporator region  14  of the heat pipe  10 . 
         [0016]    The heat pipe  10  contains a working fluid  30  which is selected so that its evaporation and condensation temperatures are appropriate for the operating temperature range of the particular application. Possible working fluids  30  include water, ammonia, acetone, or methanol. Generally only a small volume of working fluid  30  is added to the heat pipe  10  (e.g. a fraction of a percent of the total volume of the interior of the heat pipe  10 ), and the remaining volume of the heat pipe  10  may be filled with a gas or, more typically, is evacuated so that the interior of the heat pipe  10  contains only the working fluid  30  in either a liquid or vapor form. The interior pressure of the heat pipe  10  may be adjusted when evacuating or adding gas to further adjust the working temperature range of the heat pipe  10 . 
         [0017]    The volume of working fluid  30  in the heat pipe  10  is adjusted so that when the heat pipe  10  is idle, i.e. when no heat source is applied to the evaporator region  14  under normal or intended operating conditions of the heat pipe, all of the working fluid  30  is absorbed to the hybrid wick  20 , and there is no excess fluid pooled in the heat pipe  10  ( FIG. 1A ). Accordingly, if the idle heat pipe  10  with hybrid wick  20  is exposed to low temperatures (e.g. a temperature below the freezing point of the working fluid  30  under the conditions present in the heat pipe  10 ), the working fluid  30  will be contained within the hybrid wick  20  and thus will be less susceptible to freezing. When the heat pipe  10  is active, i.e. when a heat source is applied to the evaporator region  14 , the working fluid  30  in the thin portion  24  of the hybrid wick  20  evaporates, and some or all of the vapor travels to the condenser region  16 . In the condenser region  16 , the evaporated working fluid  30  condenses and forms a film  32  on the inside wall of the heat pipe  10 . Also, after sufficient time in operation, and based upon the selected quantity of working fluid in the heat pipe as described above, working fluid originally in the wick of the reservoir region  12  is drawn up to the evaporator region  14  where it enters the cycle of evaporation and condensation in the evaporator and condenser regions  14 ,  16  (rather than being returned to the reservoir region  12 ). In this manner, the reservoir region  12  dries out, with all or substantially all of the working fluid being utilized in the cooling process of the heat pipe  10 . 
         [0018]    In some applications, the heat pipe  10  with hybrid wick  20  generally is operated in a vertical orientation relative to gravity, i.e. with the condenser region  16  at the top and the reservoir region  12  at the bottom ( FIGS. 1A ,  1 B). When oriented vertically, the film  32  of working fluid  30  on the inside wall in the condenser region  16  of the heat pipe  10  will move by the force of gravity towards the evaporator region  14 , thereby keeping the thin portion  24  of the hybrid wick  20  wetted with working fluid  30  and thereby re-supplying the thin portion  24  with working fluid  30  to promote steady-state heat transfer ( FIG. 1B ). 
         [0019]    When the heat pipe  10  is used in an environment with low or zero gravity (e.g. in a spacecraft), the flow of working fluid  30  would be similar to what is described above, although the rate of flow of working fluid  30  from the wickless condenser region  16  to the thin portion  14  of the hybrid wick  10  might be slower in the absence of gravity or with reduced gravitational force compared to the rate of flow in the presence of Earth&#39;s gravity. For example, the working fluid  30  which condenses on the inside wall of the heat pipe  10  in the condenser region  16  would still form a film  32  in a low- or zero-gravity environment, and the film  32  would spread more or less evenly along the surface of the inside wall of the condenser region  16  of the heat pipe  10 . Thus, the film  32  as it spreads would eventually come into contact with the thin portion  24  of the hybrid wick  20 , at which point the working fluid  30  would be drawn by capillary action into the thin portion  24  of the hybrid wick  20 . 
         [0020]    The thin portion  24  of the hybrid wick  20  is designed to be thin enough such that, in the presence of a high heat flux, there will be a low ΔT wick . In various embodiments, the thin portion  24  of the hybrid wick  20  is sufficiently thin to permit the working fluid  30  to evaporate more rapidly without building up a steep heat gradient, thereby permitting rapid dissipation of the incoming heat flux. 
         [0021]    In use, the evaporator region  14  of the heat pipe  10  is placed in thermal contact with a first heat source  40 , for example an electronics component  50  such as a microprocessor ( FIG. 2 ) to be cooled. The working fluid  30  is evaporated, and vapor  34  moves to the condenser region  16 , which can be in thermal contact with a heat sink (for example, one or more heat-dissipating fins  60 , as shown in  FIG. 2 . The vapor  34  then condenses on the inside surface of the heat pipe  10  in the condenser region  16  to form the film  32 . Optionally, the reservoir region  12  is placed in thermal contact with a second heat source  42  to promote drying of the reservoir region  12 , which in turn puts more of the working fluid  30  in the evaporator region  14  and the condenser region  16  to promote movement of thermal energy. The second heat source  42  may be generated by diverting a fraction of the heat from the evaporator region  14  to the reservoir region  12 . By drying the thick portion  22  of the hybrid wick  10 , this forces most of the working fluid  30  out of the reservoir region  12  of the heat pipe  10  so that the working fluid  30  can cycle between the evaporator region  14  and the condenser region  16  to remove heat from the evaporator region  14 . 
         [0022]    The embodiments described above and illustrated in the figures are presented by way of example only and are not intended as a limitation upon the concepts and principles of the present invention. As such, it will be appreciated by one having ordinary skill in the art that various changes in the elements and their configuration and arrangement are possible without departing from the spirit and scope of the present invention as set forth in the appended claims.