Patent Publication Number: US-2023132688-A1

Title: Gravity independent liquid cooling for electronics

Description:
STATEMENT OF FEDERALLY SPONSORED RESEARCH AND DEVELOPMENT: 
     This patent application is based on research sponsored, in part, by the National Aeronautics and Space Administration under agreement number NNX16AT09G and 80NSSC18K1295. The Government has certain rights in the invention. 
    
    
     RELATED APPLICATIONS 
     This patent application claims the benefit under 35 U.S.C. § 119(e) of U.S. Provisional Patent App. No. 63/272,874, filed Oct. 28, 2021, entitled “GRAVITY INDEPENDENT LIQUID COOLING FOR ELECTRONICS,” incorporated herein by reference in entirety. 
    
    
     BACKGROUND 
     Modern electronic devices generate substantial heat due to a concentration of many electrical components in an integrated circuit of a relatively small size. As modem semiconductor technology improves to result in increasing circuit density, the number of heat-producing circuit elements occupy a smaller footprint. An electron flow through a circuit generates heat, and therefore an increase in circuit density produces a corresponding increase in the heat generated. Modern semiconductors, for example, require fluidic cooling, either by forced air fans, or by liquid based cooling, which is becoming more prevalent due to the increased heat transfer capabilities of liquid. 
     SUMMARY 
     A combined electrohydrodynamic (EHD) and dielectrophoretic (DEP) cooling approach for a processor or similar electronics in a zero gravity environment is beneficial in space exploration. An EHD pumping mechanism is defined by an array of alternating, polarized electrodes surrounding a heat sink coupled to the processor for heat exchange, such as a thermally conductive layer or coating. The array may be circular, rectangular, or any suitable geometry, generally guided by a shape of the heat sink/processor. Cooling fluid is drawn or pumped towards the heat sink by EHD electrodes, and a dielectrophoretic (DEP) electrode disposed above the center of the heat sink extracts the generated vapor bubbles away from the heated surface with a diverging electrical field. One configuration calls for a radial arrangement of EHD electrodes drawing the cooling fluid towards a centrally located heat sink. An alternate configuration may employ an EHD pumping surface having a linear arrangement of EHD electrodes for directing the cooling fluid towards an edge of the EHD pumping surface. Specifically, the dielectrophoretic (DEP) force/mechanism is used to remove the vapor bubbles, and in a zero gravity environment, overcomes the absence of gravity, thus making pool boiling feasible for transport and removal of gaseous bubbles. 
     Configurations herein are based, in part, on the observation that zero gravity environments, such as space vehicles, satellites and related equipment often employ substantial computing hardware. Unfortunately, conventional approaches suffer from the shortcoming that environmental controls such as HVAC systems and liquid cooling approaches operate differently in zero gravity environments and may have inconsistent power expectations where power efficiency is paramount. With conventional methods such as liquid phase cooling, the capability of removing heat in the absence of gravity at high heat flux levels diminishes. Thus, it is not a viable solution to rely on liquid cooling in the absence of gravity at high heat flux levels. In contrast to alternate approaches, where EHD pumped fluids are directed by gravitational forces to separate gaseous bubbles resulting from boiling, a zero gravity environment allows no such considerations. Accordingly, configurations herein present a system for cooling of electronics/computers at especially high heat flux levels in the absence of gravity with an electric field, thereby making pool boiling of cooling liquid feasible in space for cooling of electronics/computers. It should be further noted that the DEP generated forces need not be coupled with a heat flux/boiling arrangement in order to effect DEP forces, however. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing and other objects, features and advantages of the invention will be apparent from the following description of particular embodiments of the invention, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention. 
         FIG.  1    is a schematic flow of combined EHD/DEP zero gravity liquid cooling for electronics as disclosed herein; 
         FIG.  2    shows a side view of an electrohydrodynamic cooling device as in  FIG.  1   ; 
         FIG.  3    shows a plan view of the device  100  of  FIG.  1    illustrating the EHD pumping surface; and 
         FIG.  4    shows an alternate cutaway view of the cooling device as in  FIGS.  1 - 3   . 
     
    
    
     DETAILED DESCRIPTION 
     Cooling of electronics, including computer boards, processors and integrated circuits in low or zero gravity space is a formidable challenge. This is especially true with powerful CPUs where cooling at high capacity is needed. Cooling in the presence of liquid/vapor phase change (such as pool boiling) provides cooling at high heat flux levels. Unfortunately, in the absence of gravity, bubbles generated due to phase change do not leave the heated surface. Thus, the effective cooling techniques, such as pool boiling, becomes impractical. Accordingly, there is a need for a technology that makes pool boiling practical in space by extracting the generated bubbles away from the heated surface (i.e. electronics). 
     Configurations herein differ from conventional approaches because the disclosed approach extracts the generated vapor bubbles away from the heated surface with a diverging electrical field. Specifically, the dielectrophoretic (DEP) force/mechanism is used to transport vapor bubbles away from the heat source. The presence of the DEP mechanism overcomes the absence of gravity, thus, making pool boiling feasible for fluid flow based heat transfer and cooling. 
     The description below presents an example of a fluidic transport system, including a set or array of electrohydrodynamic (EHD) electrodes adapted for disposing a fluid, and a dielectrophoretic (DEP) electrode in proximity to the EHD electrodes. The fluid is a cooling or working fluid in fluidic communication with both the EHD electrodes and DEP electrodes for cooperative transport of the working fluid for thermal transfer away from a heat sink or heat generating component, typically a microprocessor circuit in need of cooling. 
     Various configurations depicting the above features and benefits as disclosed herein are shown and described further below. In a basic configuration, the depictions below illustrate a fluidic transport system including a set of electrohydrodynamic (EHD) electrodes adapted for disposing a fluid, and a dielectrophoretic (DEP) electrode in proximity to the EHD electrodes and adapted to operate on the same fluidic mass. A working fluid is in fluidic communication with both the EHD electrodes and DEP electrodes for cooperative transport of the working fluid, and may be employed for any suitable transport of the working fluid in addition to the heat transfer example disclosed below. 
       FIG.  1    is a schematic flow of combined EHD/DEP zero gravity liquid cooling for electronics as disclosed herein. In a zero gravity environment  10 , a method for cooling electronic circuits includes combining an electrohydrodynamic (EHD)  15  and dielectrophoretic (DEP)  50  fluid transport mechanism in an absence of gravity. The method involves heating a working fluid  30  from a heat source or heat sink  16  thermally coupled to a circuit to be cooled, typically as a film or layer of liquid. As a result of heating, the DEP field imposes a force for disposing bubbles resulting from boiling of the cooling fluid for drawing heat from the heat sink  16  resulting in cooling of the circuit. 
     In operation, a flow of the working fluid  30  is generated towards the heat sink  16  resulting from forces generated from the EHD fluid transport mechanism. This may be in a circumferential, linear or open space arrangement. Once the fluid film, layer or flow is accumulated in thermal communication with the heat sink  16 , and absorbs heat, boiling the fluid. A flow of the working fluid is generated away from the heat sink and between electrodes providing the DEP fluid transport mechanism. In a closed or recirculating vessel, this cycles the working fluid  30  back towards the heat sink in an iterative manner. Condensation of the working fluid may assist in returning gaseous bubbles to a liquid form for cycling the working fluid in an iterative thermal transfer. 
     A particular configuration forms the DEP electrode  50  opposed from the heat sink  16  for imparting dielectrophoretic movement to the bubbles in an absence of gravity induced liquid pressure. In a zero gravity environment, gaseous bubbles do not “rise” based on liquid pressure from gravity. The DEP electrode  50  generates a DEP field for transport of vapor bubbles of the working fluid in an absence of buoyancy from surrounding liquid (working fluid). By forming a EHD electrode  15  adjacent the heat sink, the EHD pumping mechanism is disposed for transporting the working fluid towards the heat sink  16  for boiling into vapor bubbles. In a cycling manner, this alternates the working fluid between a liquid phase and a gaseous phase between the EHD pumping mechanism and the DEP pumping mechanism for continued reheating and cooling of the working fluid for drawing heat off the heat sink. 
     In the example configurations herein EHD, the EHD phenomenon involves the interaction between flow fields and electric fields in a dielectric fluid medium. A general expression of the electric body force in EHD phenomena is given by the following equation: 
     
       
         
           
             
               f 
               e 
             
             = 
             
               ρ 
               e 
             
             E 
             − 
             
               1 
               2 
             
             
               E 
               2 
             
             
               ∇ 
               8 
             
             + 
             
               1 
               2 
             
             ∇ 
             
               
                 
                   E 
                   2 
                 
                 
                   
                     
                       
                         
                           
                             
                               ∂ 
                               8 
                             
                           
                           
                             
                               ∂ 
                               ρ 
                             
                           
                         
                       
                     
                   
                   τ 
                 
                 ρ 
               
             
           
         
       
     
     The first term represents the Coulomb force which acts on free charges within the cooling fluid. The second and third terms represent the translational and distortional responses of polarized charges resulting from the imposed electric field and are known as the dielectrophoretic and electrostriction forces, respectively. EHD conduction pumping is primarily driven by the Coulomb force acting on free space charges which are redistributed to the vicinity of the electrodes. Free charges are formed due to the imbalance in the dissociation and recombination of neutral electrolytic species in the dielectric fluid. Proper asymmetric design of the electrodes generates net axial flow motion, pumping the fluid. EHD conduction pumps may therefore be employed as the sole driving mechanism for small-scale heat transport systems and have a simple electrode design, which allows them to be fabricated in exceedingly compact form (down to microscale). EHD conduction is also an effective technique to pump a thin liquid film. 
     In configurations herein, EHD conduction is combined with an additional mechanism from dielectrophoresis. Dielectrophoresis is a translational motion of neutral matter in a nonuniform electric field provided by the DEP conductors  160 . The nonuniform electric field results in field induced polarization of vapor bubbles or particles in the fluid. Unlike the Coulomb force (which acts on free charges), the DEP force acts on the polarized charges and can be used to influence vapor bubble motion during nucleate boiling. The DEP force acting on a vapor bubble of radius a is given by: 
     
       
         
           
             
               F 
               
                 D 
                 E 
                 P 
               
             
             = 
             2 
             π 
             
               a 
               3 
             
             
               ε 
               1 
             
             
               
                 
                   
                     
                       ε 
                       2 
                     
                     − 
                     
                       ε 
                       1 
                     
                   
                   
                     
                       ε 
                       2 
                     
                     + 
                     2 
                     
                       ε 
                       1 
                     
                   
                 
               
             
             ∇ 
             
               
                 
                   
                     
                       E 
                       c 
                     
                   
                 
               
               2 
             
           
         
       
     
      In the above equation, particles are repelled from regions of stronger electric fields if their permittivity is less than that of suspension medium, e2&lt;e1. For an example configuration, the liquid medium is the working fluid hydrochlorofluorocarbon (HCFC)-123, although other working fluids responsive to the DEP and EHD forces may be employed, based on available voltage, circuit and ambient temperature ranges, and available volume in which the cooling cycle occurs. The resulting DEP force is proportional to the gradient of the electric field squared. A strong nonuniform electric field results in a DEP force acting on individual vapor bubbles. 
     Introduction of such a system in a zero gravity, artificial pressurized environment, such as space travel, results in particularly beneficial results. Variations in a selected working (cooling) fluid, heating/boiling/vapor pressure of the working fluid, electrode size/voltage and other physical and ambient parameters may be pertinent in a zero gravity configuration. A resulting design at the system level will consider the effects of these controlling parameters along w others on the design, operating conditions etc. of DEP electrode as well as EHD pumping (if needed). 
     Referring to the views of  FIGS.  1 - 3   ,  FIG.  2    shows a side view of an electrohydrodynamic cooling device  100  including a heat sink  116  coupled to an processor  112  of an electronic circuit  110  in a zero-gravity environment  101 . A cooling fluid  130  is in a fluidic coupling with the heat sink  116  for heat transfer. The heat sink  116  is simply a highly thermal conductive material and resistant to a cooling fluid  130 , but may be omitted if the processor  112  is hermetically sealed and thermally consistent to operate in direct communication with the cooling fluid  130 . An electrohydrodynamic (EHD) pumping surface  115  engages the heat sink  116  for transport of the cooling fluid  130 , and a dielectrophoretic (DEP) electrode  150  is in communication with the heat sink  116  by an offset distance for drawing vapor bubbles  152  in boiling cooling fluid away from the heat sink. The heat sink  116  defines a heat source, typically a layer or coating of thermally conductive material for facilitating heat transport. An aluminum or metal plate is often employed, and optionally may have fins for increased surface area. Alternatively, the processor or circuit element itself may define a heat source and the cooling fluid flowing directly over the processor. 
       FIG.  3    shows a plan view of the device  100  of  FIG.  2    illustrating that the EHD pumping surface  115  defines a pumping region around the heat sink  116 , such that the EHD pumping surface  115  directing the cooling fluid  130  towards the heat sink  116 . 
     In the example arrangement, the heat sink  116  is disposed in a substantially centered position in the EHD pumping surface  115 . The pumping surface  115  includes EHD electrodes  114  in the surface positioned to direct the cooling fluid  130  towards the heat sink  116 . The EHD pumping surface  115  includes a plurality of concentric EHD electrodes  114  for directing the cooling fluid  130  towards a center of the EHD pumping surface  115 , powered by a voltage source  117 . A linear or sequential arrangement may also be used for defining a recirculating flow. 
       FIG.  4    shows an alternate cutaway view of the cooling device as in  FIGS.  1 - 3   . Referring to  FIG.  4   , the EHD electrodes  114  may also extend completely over the processor  112 /heat sink  116  assembly, defining a layered structure between the processor  112 , heat sink  116 , vapor region for accommodating bubbles  152  and DEP electrode  150  offset by a distance based on the bubble  152  flow. The DEP electrode  150  is disposed adjacent the heat sink  150  and separated by a pumping gap  132 , such that the pumping gap is based on a type of the cooling fluid  130  and an aggregate volume of bubbles accumulated in a given time. 
     It is generally expected that the DEP electrode  150  has a planar shape and is disposed on a parallel plane from the EHD pumping surface  115  in the offset defined by the pumping gap  132 , and powered by any suitable voltage source  164 . The zero gravity environment  101  ensures that an orientation, such as the vertical arrangement of  FIG.  1   , is generally agnostic to fluid flow due to little or no gravitational influence based on the zero gravity environment. 
     Continuing to refer to  FIG.  3   , the DEP electrode  150  has a plurality of parallel conductors  160 , each having a width and separated by a gap  162  from an adjacent parallel conductor, wherein each gap has a size based on a heat transfer coefficient. Voltage levels and electrode size may be configured based on desired heat and flow rate parameters. 
     While the system and methods defined herein have been particularly shown and described with references to embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the invention encompassed by the appended claims.