Patent Publication Number: US-2007119568-A1

Title: System and method of enhanced boiling heat transfer using pin fins

Description:
TECHNICAL FIELD OF THE INVENTION  
      This invention relates generally to the field of cooling systems and, more particularly, to a system and method for enhanced boiling heat transfer using pin fins.  
     BACKGROUND OF THE INVENTION  
      A variety of different types of structures can generate heat or thermal energy in operation. To prevent such structures from over heating, a variety of different types of cooling systems may be utilized to dissipate the thermal energy. To facilitate the dissipation of such thermal energy in such cooling systems, a variety of different types of coolants may be utilized.  
     SUMMARY OF THE INVENTION  
      According to one embodiment of the invention, a cooling system for a heat-generating structure comprises a channel having an inlet and an exit and a plurality of pin fins extending at least partially across the channel. The inlet is operable to receive a fluid coolant into the channel substantially in the form of a liquid. The exit is operable to dispense of the fluid coolant out of the channel at least partially in the form of a vapor. The plurality of pin fins are operable to receive thermal energy from the heat generating structure and transfer at least a portion of the thermal energy to the fluid coolant. The thermal energy from the heat-generating structure causes at least a portion of the fluid coolant substantially in the form of a liquid to boil and vaporize in the channel upon contact with the plurality of pin fins.  
      Certain embodiments of the invention may provide numerous technical advantages. For example, a technical advantage of one embodiment may include the capability to enhance heat transfer in a cross-flowing coolant stream. Other technical advantages of other embodiments may include the capability to utilize pin fin configurations to alter the heat transfer phenomenology and thereby enhance heat transfer.  
      Although specific advantages have been enumerated above, various embodiments may include all, some, or none of the enumerated advantages. Additionally, other technical advantages may become readily apparent to one of ordinary skill in the art after review of the following figures and description.  
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      For a more complete understanding of example embodiments of the present invention and its advantages, reference is now made to the following description, taken in conjunction with the accompanying drawings, in which:  
       FIG. 1  is a block diagram of an embodiment of a cooling system that may be utilized in conjunction with other embodiments;  
       FIG. 2A  is an isolated perspective view of a pin fin configuration, according to an embodiment of the invention;  
       FIG. 2B  is a side cross-sectional view of a pin fin configuration, according to an embodiment of the invention;  
       FIG. 3A  shows pin fin configurations, according to embodiments of the invention;  
       FIG. 3B  shows a graph comparing performance of the pin fin configurations; and  
       FIGS. 4A, 4B , and  4 C show pin fin configurations, according to embodiments of the invention.  
    
    
     DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS OF THE INVENTION  
      It should be understood at the outset that although example embodiments of the present invention are illustrated below, the present invention may be implemented using any number of techniques, whether currently known or in existence. The present invention should in no way be limited to the example embodiments, drawings, and techniques illustrated below, including the embodiments and implementation illustrated and described herein. Additionally, the drawings are not necessarily drawn to scale.  
      In the transfer of a heat or thermal energy from a structure to a cross-flowing coolant stream, conventional heat transfer configurations utilize straight or wavy fin stock to enhance heat transfer. To further enhance heat transfer, such cross-flowing coolant streams can be replaced with either jet impingement or spray cooling. Although jet impingement and spray cooling offer improved performance, they are more complex and may not be able to be used due to packaging limitations. Accordingly, teaching of some embodiments of the invention recognize pin fins configurations that can be utilized in cross-flowing coolant streams to alter the heat transfer phenomenology and thereby enhance heat transfer.  
       FIG. 1  is a block diagram of an embodiment of a cooling system  10  that may be utilized in conjunction with other embodiments disclosed herein, namely pin fin embodiments described with reference to  FIGS. 2A-4C . Although the details of one cooling system will be described below, it should be expressly understood that other cooling systems may be used in conjunction with embodiments of the invention.  
      The cooling system  10  of  FIG. 1  is shown cooling a structure  12  that is exposed to or generates thermal energy. The structure  12  may be any of variety of structure, including, but not limited to, electronic components and circuits. Because the structure  12  can vary greatly, the details of structure  12  are not illustrated and described. The cooling system  10  of  FIG. 1  includes channels  23  and  24 , pump  46 , inlet orifices  47  and  48 , a condenser heat exchanger  41 , an expansion reservoir  42 , and a pressure controller  51 .  
      The structure  12  may be arranged and designed to conduct heat or thermal energy to the channels  23 ,  24 . To receive this thermal energy or heat, the channels  23 ,  24  may be disposed on an edge of the structure  12  or may extend through portions of the structure  12 , for example, through a thermal plane of structure  12 . In particular embodiments, the channels  23 ,  24  may extend up to the components of the structure  12 , directly receiving thermal energy from the components. Although two channels  23 ,  24  are shown in the cooling system  10  of  FIG. 1 , one channel or more than two channels may be used to cool the structure  12  in other cooling systems.  
      In operation, a fluid coolant flows through each of the channels  23 ,  24 . As discussed later, this fluid coolant may be a two-phase fluid coolant, which enters inlet conduits  25  of channels  23 ,  24  in liquid form. Absorption of heat from the structure  12  causes part or all of the liquid coolant to boil and vaporize such that some or all of the fluid coolant leaves the exit conduits  27  of channels  23 ,  24  in a vapor phase. To facilitate such absorption or transfer of thermal energy, the channels  23 ,  24  may be lined with pin fins or other similar devices which, among other things, increase surface contact between the fluid coolant and walls of the channels  23 ,  24 . Further details of the pin fin embodiments are described below with reference to  FIG. 2A-4C . Additionally, in particular embodiments, the fluid coolant may be forced or sprayed into the channels  23 ,  24  to ensure fluid contact between the fluid coolant and the walls of the channels  23 ,  24 .  
      The fluid coolant departs the exit conduits  27  and flows through the condenser heat exchanger  41 , the expansion reservoir  42 , a pump  46 , and a respective one of two orifices  47  and  48 , in order to again to reach the inlet conduits  25  of the channels  23 ,  24 . The pump  46  may cause the fluid coolant to circulate around the loop shown in  FIG. 1 . In particular embodiments, the pump  46  may use magnetic drives so there are no shaft seals that can wear or leak with time.  
      The orifices  47  and  48  in particular embodiments may facilitate proper partitioning of the fluid coolant among the respective channels  23 ,  24 , and may also help to create a large pressure drop between the output of the pump  46  and the channels  23 ,  24  in which the fluid coolant vaporizes. The orifices  47  and  48  may have the same size, or may have different sizes in order to partition the coolant in a proportional manner which facilitates a desired cooling profile.  
      A flow  56  of fluid (either gas or liquid) may be forced to flow through the condenser heat exchanger  41 , for example by a fan (not shown) or other suitable device. In particular embodiments, the flow  56  of fluid may be ambient fluid. The condenser heat exchanger  41  transfers heat from the fluid coolant to the flow  56  of ambient fluid, thereby causing any portion of the fluid coolant which is in the vapor phase to condense back into a liquid phase. In particular embodiments, a liquid bypass  49  may be provided for liquid fluid coolant that either may have exited the channels  23 ,  24  or that may have condensed from vapor fluid coolant during travel to the condenser heat exchanger  41 .  
      The liquid fluid coolant exiting the condenser heat exchanger  41  may be supplied to the expansion reservoir  42 . Since fluids typically take up more volume in their vapor phase than in their liquid phase, the expansion reservoir  42  may be provided in order to take up the volume of liquid fluid coolant that is displaced when some or all of the coolant in the system changes from its liquid phase to its vapor phase. The amount of the fluid coolant which is in its vapor phase can vary over time, due in part to the fact that the amount of heat or thermal energy being produced by the structure  12  will vary over time, as the structure  12  system operates in various operational modes.  
      Turning now in more detail to the fluid coolant, one highly efficient technique for removing heat from a surface is to boil and vaporize a liquid which is in contact with a surface. As the liquid vaporizes in this process, it inherently absorbs heat to effectuate such vaporization. The amount of heat that can be absorbed per unit volume of a liquid is commonly known as the latent heat of vaporization of the liquid. The higher the latent heat of vaporization, the larger the amount of heat that can be absorbed per unit volume of liquid being vaporized.  
      The fluid coolant used in the embodiment of  FIG. 1  may include, but is not limited to mixtures of antifreeze and water. In particular embodiments, the antifreeze may be ethylene glycol, propylene glycol, methanol, or other suitable antifreeze. In other embodiments, the mixture may also include fluoroinert. In particular embodiments, the fluid coolant may absorb a substantial amount of heat as it vaporizes, and thus may have a very high latent heat of vaporization.  
      Water boils at a temperature of approximately 100° C. at an atmospheric pressure of 14.7 pounds per square inch absolute (psia). In particular embodiments, the fluid coolant&#39;s boiling temperature may be reduced to between 55-65° C. by subjecting the fluid coolant to a subambient pressure of about 2-3 psia. Thus, in the cooling system  10  of  FIG. 1 , the orifices  47  and  48  may permit the pressure of the fluid coolant downstream from them to be substantially less than the fluid coolant pressure between the pump  46  and the orifices  47  and  48 , which in this embodiment is shown as approximately 12 psia. The pressure controller  51  maintains the coolant at a pressure of approximately 2-3 psia along the portion of the loop which extends from the orifices  47  and  48  to the pump  46 , in particular through the channels  23  and  24 , the condenser heat exchanger  41 , and the expansion reservoir  42 . In particular embodiments, a metal bellows may be used in the expansion reservoir  42 , connected to the loop using brazed joints. In particular embodiments, the pressure controller  51  may control loop pressure by using a motor driven linear actuator that is part of the metal bellows of the expansion reservoir  42  or by using small gear pump to evacuate the loop to the desired pressure level. The fluid coolant removed may be stored in the metal bellows whose fluid connects are brazed. In other configurations, the pressure controller  51  may utilize other suitable devices capable of controlling pressure.  
      In particular embodiments, the fluid coolant flowing from the pump  46  to the orifices  47  and  48  may have a temperature of approximately 55° C. to 65° C. and a pressure of approximately 12 psia as referenced above. After passing through the orifices  47  and  48 , the fluid coolant may still have a temperature of approximately 55° C. to 65° C., but may also have a lower pressure in the range about 2 psia to 3 psia. Due to this reduced pressure, some or all of the fluid coolant will boil or vaporize as it passes through and absorbs heat from the channels  23  and  24 .  
      After exiting the exits ports  27  of the channels  23 ,  24 , the subambient coolant vapor travels to the condenser heat exchanger  41  where heat or thermal energy can be transferred from the subambient fluid coolant to the flow  56  of fluid. The flow  56  of fluid in particular embodiments may have a temperature of less than 50° C. In other embodiments, the flow  56  may have a temperature of less than 40° C. As heat is removed from the fluid coolant, any portion of the fluid which is in its vapor phase will condense such that substantially all of the fluid coolant will be in liquid form when it exits the condenser heat exchanger  41 . At this point, the fluid coolant may have a temperature of approximately 55° C. to 65° C. and a subambient pressure of approximately 2 psia to 3 psia. The fluid coolant may then flow to pump  46 , which in particular embodiments  46  may increase the pressure of the fluid coolant to a value in the range of approximately 12 psia, as mentioned earlier. Prior to the pump  46 , there may be a fluid connection to an expansion reservoir  42  which, when used in conjunction with the pressure controller  51 , can control the pressure within the cooling loop.  
      It will be noted that the embodiment of  FIG. 1  may operate without a refrigeration system. In the context of electronic circuitry, such as may be utilized in the structure  12 , the absence of a refrigeration system can result in a significant reduction in the size, weight, and power consumption of the structure provided to cool the circuit components of the structure  12 .  
      Although components of one embodiment of a cooling system  10  have been shown in  FIG. 1 , it should be understood that other embodiments of the cooling system  10  can include more, less, or different component parts. For example, although specific temperatures and pressures have been described for one embodiment of the cooling system, other embodiments of the cooling system  10  may operate at different pressures and temperatures. Additionally, in some embodiments a coolant fill port and/or a coolant bleed port may be utilized with metal-to-metal caps to seal them. Further, in some embodiments, all or a portion of the joints between various components may be brazed, soldered or welded using metal-to-metal seal caps.  
       FIG. 2A  is an isolated perspective view of a pin fin configuration  110 A and  FIG. 2B  is a side cross-sectional view of a pin fin configuration  110 B, according to embodiments of the invention. In particular embodiments, the pin fin configurations  110 A,  110 B may be disposed within the channels  23 ,  24  described with reference to  FIG. 1 . In other embodiments, the pin fin configurations  110 A,  110 B may be disposed in other heat transfer structures. For purposes of illustration, the pin fin configurations  110 A,  110 B will be described as being disposed in a channel operable to receive fluid.  
       FIGS. 2A and 2B  shows a plurality of pin fins  113 ,  115  protruding from channel walls  125  and arranged in the pin fin configuration  110 A,  110 B. Pin fin configuration  110 A shows a staggered arrangement and pin fin configuration  110 B shows an inline arrangement.  FIG. 2B  additionally shows a channel  120  with a fluid flow towards the pin fin configuration  110 B, indicated by arrow  132 , and a fluid flow away from the pin fin configuration  110 B, indicated by arrow  134 . In operation, thermal energy is transferred to the pin fins  113 ,  115  and to a fluid traveling through the channel, for example, channel  120 . In particular embodiments, the pin fin configurations  110 A,  110 B may be utilized to enhance boiling heat transfer. In such embodiments, liquid fluid coolant (e.g., traveling in direction of arrow  132  towards the pin fins  113 ,  115 ) comes in contact with the pin fins  113 ,  115  and is boiled and vaporized. The vaporized fluid coolant (e.g., traveling away from the pin fins in direction of arrow  134 ) inherently contains the thermal energy transferred from the pin fins  113 ,  115  to the fluid coolant during vaporization.  
      As briefly referenced above, teachings of some embodiments of the invention recognize that pin fins configurations, such as pin fin configurations  110 A,  110 B, can be utilized in cross-flowing coolant streams to alter the heat transfer phenomenology and thereby enhance heat transfer. By using pin fin configuration  110 A,  110 B, the cross flowing coolant creates jet-impingement-like flows of coolant that impact the surfaces of the plurality of pin fins  113 ,  115 . As vapor is produced in the transfer of heat to the fluid coolant, the velocity of the coolant increases, which further increases the impacting velocity of the cross flowing coolant on the pin fins  113 ,  115 . The effusing vapor also causes a near chaotic flow of vapor with embedded liquid coolant that impacts the pins fins  113 ,  115 . That is, a situation is created where globs of liquid coolant (e.g., formed from the vaporization of other liquid coolant) are thrown against downstream pin fins  113 ,  115 —creating a spray cooling-like quality. Accordingly, the pin fin configurations  110 A,  110 B allow a cross flowing coolant to be used while taking advantage of the attributes of jet impingement and spray cooling, which are provided by the chaotic cross flowing liquid impacting the pins.  
      The pin fins  113 ,  115  may be made of a variety of materials and may take on a variety of sizes and shapes. In this embodiment, the pin fins are made of a nickel plated copper and vary in size from 0.04 inches high to 0.1675 inches high. The pin fins  113 ,  115  are shown with a columnar shape. In other embodiments, the pin fins  113  may be made of other materials, may have heights less than 0.04 inches, may have heights greater than 0.1675 inches, and may have shapes other than columnar shapes. Additionally, in other embodiments the pin fins  113 ,  115  may be arranged in configurations other than inline or staggered configurations.  
       FIG. 3A  shows pin fin configurations  110 C,  110 D, according to embodiments of the invention. Pin fin configuration  110 C is an inline configuration and pin fin configuration  110 D is a staggered configuration. The pin configurations  110 C and  110 D may operate in a similar manner, may have similar or different shapes and sizes, and may be made from similar or different materials than the pin fin configurations  110 A,  110 B described with reference to  FIGS. 2A and 2B . The pin fin configurations  110 C,  110 D are shown alongside a conventional fin stock configuration  150 A and a conventional flat heat transfer configuration  150 B.  
       FIG. 3B  shows a graph  190  comparing performance of the pin fin configurations  110 C,  110 D against the conventional fin stock configuration  150 A and the conventional flat heat transfer configuration  150 B. The graph  190  shows measured results of heat flux  170  against temperature rise  160  for a 0.105 inch size  180  for the pin fin configurations  110 C,  110 D and the conventional fin stock configuration  150 A. The graph shows that the amount of wall super heat is significantly less for the pin fin configurations  110 C,  110 D than the conventional fin stock configuration  150 A and the conventional flat heat transfer configuration  150 B. At higher heat fluxes, the flat heat transfer configuration  150 B was not tested because it transitioned to film boiling.  
       FIGS. 4A, 4B , and  4 C show pin fin configurations  110 E,  110 F, and  110 G, according embodiments of the invention. The pin fin configurations  110 E,  110 F, and  110 G of  FIGS. 4A, 4B , and  4 C are intended as illustrating some of the variety of pin fin configurations that may be utilized, according to embodiments of the inventions. Although several are specifically shown, others will become apparent to one of ordinary skill in the art after review of this specification. The pin fin configurations may operate in a similar manner and may be made from similar or different materials that the pin fin configurations  110 A,  110 B described with reference to  FIGS. 2A and 2B . Each of the pin fin configurations  110 E,  110 F, and  110 G of  FIGS. 4A, 4B , and  4 C is shown with a channel  120  with a fluid flow towards the pin fin configurations  110 E,  110 F, and  110 G, indicated by arrow  132 , and a fluid flow away from the pin fin configuration configurations  110 E,  110 F, and  110 G,, indicated by arrow  134 . Each of the pin fin configurations  110 E,  110 F, and  110 G of  FIGS. 4A, 4B , and  4 C is additionally shown with a channel wall  125 .  
       FIG. 4A  shows that pin fins  117  in pin fin configuration  110 E may extend substantially across a channel  120 . In other embodiments, the pin fins  117  may only extend a portion of the distance across a channel. Although the pin fins  117  are show as all the same length, in other embodiments the pin fins  117  may be different lengths.  
       FIG. 4B  shows that pin fins  119  in pin fin configuration  110 F may tilt at an angle. Although the pin fins  119  are shown tilting at a downstream angle downstream from the fluid flow in this embodiment, in other embodiments, the pin fins  119  may tilt at an upstream angle. And, in some embodiments, some of the pin fins  119  may tilt in one direction and other may tilt in a different direction. Similar to that described above, in particular embodiments, the pin fins  119  may also be different lengths.  
       FIG. 4C  shows that pin fins  111 A,  11 B in pin fin configuration  110 G may extend from different channel walls  125 ,  127 . Similar to that described above, in particular embodiments, the pin fins  111 A,  11 B may also be different lengths and tilt in different directions.  
      Although the present invention has been described with several embodiments, a myriad of changes, variations, alterations, transformations, and modifications may be suggested to one skilled in the art, and it is intended that the present invention encompass such changes, variations, alterations, transformation, and modifications as they fall within the scope of the appended claims.