Patent Publication Number: US-2012024500-A1

Title: Thermosyphon for cooling electronic components

Description:
TECHNICAL FIELD 
     The present invention relates to thermosyphons and more particularly to thermosyphons for cooling electronic components such as, for example, central processing units (CPUs), graphics processing units (GPUs) and concentrating solar cells. 
     BACKGROUND OF THE INVENTION 
     Thermal management is an important aspect in the design of electronic packaging. Proper thermal management of electronic devices ensures that operating temperatures remain within a reliable operating range. Operating at temperatures beyond the set boundary is undesirable as it leads to lower device performance, an increased probability of failure and a reduced lifespan. 
     With the introduction of multi-core processors and high-power electronics, component heat fluxes have risen to new higher levels and there is concern that current heat management technologies will not be able to cope with future heat load requirements. Thus, there is a need for a cooling device that is capable of dissipating waste heat generated by electronic components effectively. 
     SUMMARY OF THE INVENTION 
     Accordingly, in a first aspect, the present invention provides a thermosyphon including an evaporator section, a condenser section coupled to the evaporator section, and a condensate guide lining an inner portion of the evaporator section and inner surfaces of the condenser section. The condensate guide defines a vapour core in the evaporator and condenser sections and is configured to return condensate to the evaporator section regardless of an orientation of the thermosyphon. Advantageously, this allows operation of the thermosyphon at various physical orientations with minimal or no performance degradation. 
     Preferably, the condensate guide includes a plurality of pores, the pores of the condensate guide being sized to allow vapour to pass through and prevent condensate flow through. Advantageously, this aids in returning the condensate to the evaporator section. 
     A boiling enhancement structure may be coupled to the evaporator section. Advantageously, the boiling enhancement structure enhances nucleate boiling at the evaporator section and thereby increases the boiling heat transfer coefficient. 
     The boiling enhancement structure may include a plurality of pin fins. Preferably, a separation between adjacent ones of the pin fins is less than a bubble characteristic length of a working fluid in the evaporator section. Advantageously, the bubble confinement effect enhances nucleate boiling of the working fluid and consequently increases heat transfer away from the heat source. 
     In a preferred embodiment, the boiling enhancement structure is configured to draw the condensate back to the evaporator section. Advantageously, this enhances the heat transfer process. 
     Preferably, the boiling enhancement structure is integrally formed with a heat receiving portion of the evaporator section. Advantageously, this reduces the heat transfer resistance. 
     In yet another preferred embodiment, a thermal interface material is coupled to the heat receiving portion of the evaporator section. Advantageously, this further reduces the heat transfer resistance. 
     Preferably, a working fluid is provided in the evaporator section in an amount sufficient to submerge the boiling enhancement structure. Advantageously, this maximizes the boiling heat transfer. The working fluid is preferably in a saturated state. 
     One of a plurality of grooves and a plurality of knurls may be formed on the inner surfaces of the condenser section for condensation enhancement. 
     In one embodiment, a port is provided for charging the evaporation section with a working fluid and for deaerating the thermosyphon. 
     In a second aspect, the present invention provides a thermosyphon including an evaporator section, a condenser section coupled to the evaporator section, and a boiling enhancement structure coupled to the evaporator section. The boiling enhancement structure includes a plurality of pin fins. Advantageously, the boiling enhancement structure enhances nucleate boiling at the evaporator section and increases both the boiling heat transfer coefficient and critical heat flux. 
     Other aspects and advantages of the invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which: 
         FIG. 1  is an enlarged cross-sectional view of a thermosyphon in accordance with one embodiment of the present invention; and 
         FIG. 2  is an enlarged perspective view of a boiling enhancement structure for the thermosyphon of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     The detailed description set forth below in connection with the appended drawings is intended as a description of a presently preferred embodiment of the invention, and is not intended to represent the only form in which the present invention may be practiced. It is to be understood that the same or equivalent functions may be accomplished by different embodiments that are intended to be encompassed within the scope of the invention. 
     Referring now to  FIG. 1 , a thermosyphon  10  for cooling electronic components such as, for example, central processing units (CPUs) and graphics processing units (GPUs) is shown. The thermosyphon  10  includes an evaporator section  12 , a condenser section  14  coupled to the evaporator section  12 , and a condensate guide  16  lining an inner portion of the evaporator section  12  and inner surfaces of the condenser section  14 . A vapour core  18  is defined in the evaporator and condenser sections  12  and  14  by the condensate guide  16 . A boiling enhancement structure  20  is coupled to the evaporator section  12 . A working fluid  22  is provided in the evaporator section  12  in an amount sufficient to submerge the boiling enhancement structure  20 . The boiling enhancement structure  20  is integrally formed with a heat receiving portion  24  of the evaporator section  12 . A thermal interface material  26  is coupled to the heat receiving portion  24  of the evaporator section  12 . A port  28  is provided for deaerating the thermosyphon  10  and for charging the evaporation section  12  with the working fluid  22 . A plurality of fins  30  is coupled to the condenser section  14 . 
     The thermosyphon  10  is hermetically-sealed and is configured to receive heat from a heat source (not shown). The heat source may have a high heat flux and examples of the heat source include, but are not limited to, central processing units (CPUs), graphics processing units (GPUs) and concentrating solar cells. 
     In the embodiment shown, the heat receiving portion  24  of the evaporator section  12  includes a base plate  32 . The base plate  32  may be mounted or attached to the heat source. The base plate  32  is preferably fabricated from a thermally conductive material such as, for example, aluminium, copper, silver or graphite. 
     The condenser section  14  is connected to and in fluid communication with the evaporator section  12 . In the embodiment shown, the condenser section  14  includes a tube  34  and a top cover  36 , the top cover  36  sealing one end of the tube  34 . To ensure that the thermosyphon  10  is hermetically sealed, the tube  34  and the top cover  36  are bonded via a bonding process such as welding, soldering or diffusion. The tube  34  is preferably made of a thermally conductive material such as, for example, aluminium, copper, silver or graphite. 
     In the embodiment shown, the condenser section  14  is provided with an external means of heat exchange in the form of the cooling fins  30  extending from the tube  34  of the condenser section  14 . The fins  30  are attached to the tube  34  with a degree of interference in order to have proper contact and thereby avoid the presence of gaps that could deteriorate the heat transfer performance of the fins  30 . 
     The fins  30  are preferably made of a thermally conductive material such as, for example, copper or aluminium. Although the use of air-cooled fins is described in the present embodiment, it should be appreciated by those of ordinary skill in the art that the present invention is not limited by the cooling method employed to cool the condenser section  14 . In alternative embodiments, the condenser section  14  may be cooled by other well known methods of cooling such as, for example, evaporative cooling, liquid cooling, spray cooling and impinging jet. 
     Further, for condensation enhancement, an inner surface of the tube  34  of the condenser section  14  may be formed with a plurality of grooves or a knurled surface. 
     The condensate guide  16  is configured to return condensate to the evaporator section  12  regardless of an orientation of the thermosyphon  10 . The condensate guide  16  is porous and the pores of the condensate guide  16  are sized to allow vapour to pass through and prevent condensate flow through. More particularly, the pores of the condensate guide  16  are designed small enough such that liquid is held by surface tension against liquid flow. The condensate guide  16  lines the boiling enhancement structure  20 , the inner walls of the tube  34  and an inner surface of the top cover  36 . Accordingly, when vapour condensation occurs in the condenser section  14 , the working fluid  22  in vapour form is allowed to pass through the condensate guide  16  but the condensate is prevented from returning to the vapour core  18 , and the condensate guide  16  guides the flow of the condensate back to the evaporator section  12 . Orientation independence of the thermosyphon  10  is thus achieved with the condensate guide  16 . The condensate guide  16  may be made from a perforated sheet, a metallic wire mesh for structural integrity, or other porous medium. In one embodiment, the pores of the condensate guide  16  have a diameter of between about 0.1 millimetre (mm) and about 2 mm. 
     The vapour core  18  serves as a conduit for vapour generated from the evaporator section  12  to flow into the condenser section  14  and is therefore designed in a manner such that vapour flow is not constricted so as to prevent pressure build up in the evaporator section  12 . 
     The boiling enhancement structure  20  is employed within the evaporator section  12  and forms a part of the internal surface of the evaporator section  12 . 
     Referring now to  FIG. 2 , the boiling enhancement structure  20  of the thermosyphon  10  of  FIG. 1  is shown. In the embodiment shown, the boiling enhancement structure  20  comprises a plurality of pin fins  38  integrally formed or mounted on an interior surface of the base plate  32 . As can be seen from  FIG. 2 , a circular groove  40  is formed in the base plate  32  for receiving the tube  34  of the condenser section  14 . 
     The boiling enhancement structure  20  improves the boiling heat transfer coefficient by increasing the number of nucleation sites and the heat transfer surface area. Additionally, the boiling enhancement structure  20  also improves the critical heat flux by effectively minimizing the build-up of vapour film in the evaporator section  12  which causes dry out. 
     In a preferred embodiment, a separation between adjacent ones of the pin fins  38  is less than a bubble characteristic length of the working fluid  22  in the evaporator section  12 . Advantageously, it has been shown through experimentation that the bubble confinement effect enhances nucleate boiling of the working fluid  22  and consequently increases heat transfer away from the heat source. The bubble characteristic length of the working fluid  22  may be computed with the following equation: 
     
       
         
           
             
               
                 
                   
                     bubble 
                      
                     
                         
                     
                      
                     characteristic 
                      
                     
                         
                     
                      
                     length 
                   
                   = 
                   
                     
                       σ 
                       
                         g 
                          
                         
                           ( 
                           
                             
                               ρ 
                               l 
                             
                             - 
                             
                               ρ 
                               g 
                             
                           
                           ) 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
     where σ represents surface tension, g represents gravitational acceleration, ρ l  represents liquid density, and ρ g  represents vapour density. 
     The boiling enhancement structure  20  is configured to draw the condensate back to the evaporator section  12 . More particularly, the boiling enhancement structure  20  serves as a thermally activated pumping unit that absorbs condensate from the condenser section  12 . 
     The boiling enhancement structure  20  and the base plate  32  may be fabricated from a thermally conductive material such as, for example, aluminium, copper, silver or graphite. The pin fins  38  may be bonded to the base plate  32  via known bonding methods such as, for example, soldering, brazing or diffusion. 
     In the embodiment shown, each of the pin fins  38  has a square profile. In one embodiment, each of the pin fins  38  has a height of between about 2 mm and about 20 mm and a thickness of between about 0.5 mm and about 5 mm. However, it should be understood that the pin fins  38  are not limited to these geometric parameters as optimization of the geometric parameters such as fin profile, fin thickness and fin height is determined based on the thermal properties of the material from which the boiling enhancement structure  20  and the base plate  32  are made and the boiling heat transfer coefficient of the working fluid  22  for a specific geometry. 
     In alternative embodiments, the boiling enhancement structure  20  may be other forms of fins, grooves or an open-cell metal foam. 
     The provision of the groove  40  in the base plate  32  helps to facilitate bonding of the evaporator section  12  to the condenser section  14 . 
     Referring again to  FIG. 1 , the working fluid  22  is preferably in a saturated state. 
     Advantageously, this ensures that the working fluid  22  undergoes phase change instantaneously at any temperature within the component operating range. Examples of the working fluid  22  include, for example, water, a refrigerant or a dielectric fluid. In the embodiment shown, the boiling enhancement structure  20  is fully immersed in the working fluid  22 . Advantageously, this maximizes the boiling heat transfer. 
     The thermal interface material  26  serves to reduce thermal interface resistance between the heat receiving portion  24  and the heat source. 
     The port  28  functions as an evacuation port that is sealed subsequent to liquid charging and deaeration. In the embodiment shown, the port  28  is provided in the form of a tube and is located on the top cover  36 . 
     The operation of the thermosyphon  10  will now be described with reference to  FIG. 1 . 
     In use, an electronic component generates heat. Heat from the electronic component is absorbed by the base plate  32  and spreads from the base plate  32  to the boiling enhancement structure  20  where nucleate boiling of the working fluid  22  occurs and the working fluid  22  changes from a liquid to a vapour. Vapour bubbles are formed on the heated surface of the boiling enhancement structure  20  creating a higher pressure region in the evaporator section  12 . The higher pressure at the evaporator section  12  drives the vapour through the vapour core  18  to the condenser section  14  where pressure is lower. 
     As the walls of the condenser section  14  are at a lower temperature compared to the vapour, the vapour condenses into a liquid condensate on the walls of the condenser section  14  and releases latent heat of vaporization in the process. The heat released from the condensation process is rejected to an external medium via the fins  30  coupled to the condenser section  14 . 
     The liquid condensate is enclosed by the walls of the condenser section  14  and the condensate guide  16  and is forced to flow between the walls of the condenser section  14  and the condensate guide  16  back to the evaporator section  12  by gravity and the capillary force provided by the boiling enhancement structure  20 . 
     As is evident from the foregoing discussion, the present invention provides an orientation-free, two-phase thermosyphon that effectively transfers heat from a heat dissipating component to a colder medium. Advantageously, through the provision of a condensate guide lining an inner portion of the evaporator section and inner surfaces of the condenser section, the thermosyphon of the present invention can be operated at various physical orientations with minimal or no performance degradation. 
     While the preferred embodiments of the invention have been illustrated and described, it will be clear that the invention is not limited to these embodiments only. Numerous modifications, changes, variations, substitutions and equivalents will be apparent to those skilled in the art without departing from the scope of the invention as described in the claims. 
     Further, unless the context dearly requires otherwise, throughout the description and the claims, the words “comprise”, “comprising” and the like are to be construed in an inclusive as opposed to an exclusive or exhaustive sense; that is to say, in the sense of “including, but not limited to”.