Abstract:
A cooling system for a heat-generating device includes: coolant fluid; an evaporator for holding the coolant fluid and for heating the coolant fluid; said evaporator in close proximity to the heat-generating device for removing unwanted heat. The cooling system also includes a plurality of tubes for providing a flow path for the coolant fluid and gases produced by the evaporator; a heat exchanger through which the tubes pass for cooling the coolant fluid. The heat exchanger includes: a reservoir, a coolant, and a heating element for heating the gas so that it expands and pushes cool coolant fluid back to the evaporator. The heating element may be located inside the reservoir.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     Not applicable. 
     STATEMENT REGARDING FEDERALLY SPONSORED-RESEARCH OR DEVELOPMENT 
     Not applicable. 
     INCORPORATION BY REFERENCE OF MATERIAL SUBMITTED ON A COMPACT DISC 
     Not Applicable. 
     FIELD OF THE INVENTION 
     The invention disclosed broadly relates to the field of cooling devices for microelectronics and more specifically relates to passive cooling heat sinks. 
     BACKGROUND OF THE INVENTION 
     Electronic chips such as microprocessor chips generate much heat. As processing power goes up, the chips produce more heat which could damage the electronic circuits in the chip; therefore it is important to cool the chips. Many cooling methods have been developed for directly cooling hot chips, heat sinks being one such method. Generally, heat sinks fall into two categories: active heat sinks and passive heat sinks. Active heat sinks cool a chip using a fan or other active devices to move heat away from the chip. Passive heat sinks perform the cooling function without a fan, instead relying on ambient conditions provided by design to cool the chip. 
     Active heat sinks, because they employ a fan or other mechanism, require that some energy be expended in order to cool the chip. Additionally, the introduction of a moving part (the fan) to the cooling device increases the possible failure mechanisms. Some passive heat sinks address these problems by using a cooling fluid to cool the hot chips rather than a fan. The use of fluid is not without its problems. In one of the latest developments in passive heat sink technology, the Heatlane™ heat pipe device (Heatlane is a trademark of TS Heatronics Co., Ltd.) relies on an unstable, oscillatory exchange of fluid and gas back and forth within the evaporation section of the device. This is problematic because hot fluid and gas might return into the evaporation section. Also, with the Heatlane™, energy needs to be expended to cool the chip by way of a fan blowing air through the air heat exchanger. Heat pipes use a wick to return the fluid to the evaporation section. This technology has reached its limits in modern systems—multiple heat pipes need to be used on single chips because not enough coolant is available in a single pipe. 
     Therefore, there is a need for a better passive cooling method and apparatus for chips that generate more heat. 
     SUMMARY OF THE INVENTION 
     Briefly, according to an embodiment of the invention, a cooling system for a heat-generating device includes: coolant fluid; an evaporator for holding the coolant fluid and for heating the coolant fluid; said evaporator in close proximity to the heat-generating device for removing unwanted heat. The cooling system also includes a plurality of tubes for providing a flow path for the coolant fluid and gases produced by the evaporator; a heat exchanger through which the tubes pass for cooling the coolant fluid and gas. The heat exchanger includes: a reservoir, a coolant, and a heating element for heating the gas so that it expands and pushes coolant fluid back to the evaporator. The heating element may be located inside the reservoir. 
     Additionally, the cooling system according to an embodiment of the invention may include check valves, gas traps, and a coolant return tube. The cooling system may also include a baffle disposed within the evaporator far from the tubes. 
     A method according to an embodiment of the invention provides the steps of: heating coolant fluid contained in an evaporator in close proximity to the device; evaporating the coolant fluid to release gases used to pump hot coolant fluid and gases out of the evaporator and into the heat exchanger; transferring heat from the coolant fluid and gases to the heat exchanger in one or more tubes; collecting condensed coolant fluid and cooled gas in the reservoir of the heat exchanger; heating the cooled gas and fluid with the heating element; and returning the condensed coolant fluid back to the evaporator using the generated heated gas. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       To describe the foregoing and other exemplary purposes, aspects, and advantages, the following detailed description of an exemplary embodiment of the invention with reference to the drawings, in which: 
         FIG. 1  shows a cooling system with its heating system in the up cycle, according to an embodiment of the invention; 
         FIG. 2  shows the cooling system with its heating system in the down cycle, according to an embodiment of the invention; 
         FIG. 3  shows a gas trap according to an embodiment of the invention; 
         FIG. 4  shows a cooling system using only one tube, according to an embodiment of the invention; 
         FIG. 5  shows a dual tube design according to an embodiment of the invention; 
         FIG. 6  shows a coaxial return pump design, according to an embodiment of the invention; 
         FIG. 7  shows the cooling system of  FIG. 1  without the check valves and third tube, according to another embodiment of the present invention; and 
         FIG. 8  is a flowchart representing the steps for carrying out the cooling method according to an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     We describe a cooling system for heat-generating devices, such as electronic chips. In particular, a two-phase system in which a coolant is boiled offers an attractive method of moving large amounts of heat away from a hot chip. This provides a novel method of guaranteeing the circulation of the coolant, assuring that the evaporator never boils dry. This is in contrast to heat pipes, which use a wick to return the fluid to the evaporation section. 
     Referring to  FIG. 8 , the process begins at step  805  when a heat-generating device conducts heat through its surface or an interface material to an evaporator filled with a liquid coolant fluid. This direct heating of the coolant in the evaporator causes vaporization of the coolant in the evaporator in step  810 , absorbing heat through the heat of vaporization of the coolant. The hot coolant gases and entrained fluid are rapidly pumped out of the evaporator into a heat exchanger in step  820 . The “pump” effect occurs because the vaporized gas acts as a pump, forcing the hot gas and entrained fluid up through the heat exchanger. The vaporized coolant and hot liquid is forced under pressure to bubble through the coolant in the heat exchanger, transferring its heat to the coolant and to the metal tubes of the heat exchanger. In step  830 , the coolant is cooled, and once cooled, the chilled, condensed coolant and cool gas are collected in a reservoir. 
     Next, in step  840 , when either the coolant level is too low in the evaporator, or the coolant temperature is too high, a heating element is activated in the condensed coolant and gas in the reservoir. This causes gases in the heat exchanger to be generated, heated, and expanded in step  850 . The expanding gases push the condensed coolant which is very rapidly re-injected into the evaporator through a tube. An external sensor can be used to determine when the temperature is too high. This injection of coolant requires very little energy, or heat, added to the system. It returns condensed fluid to where it is needed, right to the surface interface of the heat-generating device, such as a chip. In step  860 , the heating element is quickly deactivated, and the cycle repeats with the hot, gaseous coolant and entrained hot fluid pumped out of the evaporator to the heat exchanger. 
     We now describe in more detail the operation of the cooling system in the “Up” cycle wherein the coolant flows up to the heat exchanger.  FIG. 1  shows a cooling system  100  according to an embodiment of the invention. The cycle begins with the system  100  in a quasi-equilibrium state with a coolant  103  vaporizing in the evaporator  106  and gas being pushed through the coolant  103  trapped in the heat exchanger pipes  102 ,  104 . The heating element  105  is off at this point. 
     The evaporator  106  is positioned in good thermal contact with (in close proximity to) a chip  101  or other heat-generating device. The evaporator  106  may be a chamber with metal heat conductors running through it, or it might be fine metal tubes as in a steam generator. The evaporator  106  should be optimized to boil as much coolant as possible as quickly as possible. Special coatings and structures can be designed, optimized for the coolant, which is understood in the literature of boiling. The evaporator surface might be flat with a special coating, or be further optimized with a finned or pin structure for more surface area within the boiler. 
     As the chip  101  heats up, unwanted heat emanating from the chip  101  heats a liquid coolant  103  in the evaporator  106 . The coolant or cooling fluid  103  is preferably chosen to have a high heat of vaporization and a low boiling point. The coolant should also have a high vapor pressure to make the pumping feature work well. And it should boil at a temperature low enough to cool the chip. Examples of the coolant  103  may be one of the following: 1) water at a pressure lower than one atmosphere—although water has a very low vapor pressure and may not work well; 2) butane; 3) any commercial refrigerant; 4) methyl chloride; 5) ammonia at eight (8) bar of pressure; 6) ammonia-hydrogen mix (used in refrigeration); 7) 3M&#39;s HFE-7000, with a boiling point of 34 degrees Celsius; 8) Methanol at 100 mm Hg pressure, boiling at room temperature; or 9) liquid carbon dioxide. It should be noted that other fluids with good vapor pressure, low boiling point, and good environmental properties can and may be used in an embodiment of the invention. 
     As the liquid coolant  103  heats, some of it vaporizes. For example, methanol at a pressure of 100 mm Hg boils immediately at room temperature and builds pressure rapidly. HFE-7000 boils at about 34 degrees Celsius at one atmosphere, cool enough to cool a heated chip  101 . HFE-7000 is not flammable, is not a green house gas, and is non-toxic; therefore, it is well suited for use in this application. The lower density of the heated gases and the pressure exerted by the gases cause it to rise up and additionally entrain some fluid  103  into the heat exchanger  108 . The vaporized gas acts as a pump, forcing the hot gas and the entrained fluid  103  up through the heat exchanger pipes  102  and  104 . Both gas and fluid are simultaneously transferred to the heat exchanger  108  in this way. 
     By moving both hot gas and fluid  103 , large amounts of heat may be transferred more efficiently than by using fluid alone. As the hot gas forces its way through the fluid  103 , bubbling up through the coolant  103  in the heat exchanger pipes  102  and  104 , some fluid is necessarily moved up toward a reservoir in the heat exchanger  108 . This is similar to a coffee percolator with hot gas bubbling through columns of fluid. The pumping of the gas will occur as it moves from the high pressure of the evaporator  106  to the lower pressure in the exchanger  108 . The optimal fill ratio (how much liquid coolant vs. gas) depends upon the system and can be determined by experimentation. Both the pressure differential and the coffee percolator-like “bubble pump” mechanism driven by gravity push hot coolant into the heat exchanger  108 . 
     Rather than two tubes (pipes), however, just one tube could be used. In the alternative, a dozen or more tubes could be employed. The tubes  102  and  104  in the example of  FIG. 1  are preferably thin-walled and of a small diameter, approximately one-eighth of one inch to three-eighths of an inch (⅛″ to ⅜″) in order to conduct heat efficiently. 
     The hot fluid and gas  103  is directed up to the cooler areas of the heat exchanger  108  where it is cooled and condensed Then some of the cooled, condensed fluid  103  collects in the heat exchanger reservoir  110 . The reservoir  110  is a cool chamber or collection of tubes, preferably metal tubes. The chamber  110  remains cool because it is disposed within or in proximity to the heat exchanger  110 , or radiator. Unlike the Heatlane™, the system  100  allows hot bubbles or slugs of gas to be forced through the coolant  103  in the heat exchanger  108 . This hot gas is forced under pressure toward the reservoir  110 , where it can cool and condense on its way by transferring its heat either to the coolant  103  or directly to the tube&#39;s walls in the exchanger  110 . 
     The cooling system  100  requires only one active element—a heating element, wire, or cartridge heater,  105 , used intermittently to return the cooled, condensed fluid  103  back to the evaporator  106 . The fluid  103  can be returned through a third tube  112 . The third tube  112  is a return tube, returning the cooled liquid  103  to the evaporator  106 . If the return tube  112  is long enough and high enough, gravity alone would ensure that cold fluid  103  would return to the evaporator  106 . However, in this embodiment we focus on a design for microprocessors, necessitating a small size. Heating the cooled gas or boiling some coolant causes the gas to expand, thereby moving (injecting) the coolant  103  back to the evaporator  106 . This requires very little energy—a small fraction of the total energy being given off by the chip  101 . Heating the gas quickly in the reservoir  110  does not heat the coolant liquid (anywhere) appreciably. The gas is generated and heated just enough to move the liquid  103  through the return tube  112 . 
     In this embodiment we may have two passively moving parts, the opening and closing of the check valves  114  and  116  which restrict the flow of liquid. In this example the check valves are of the ball-type. The passive check valves  114  and  116  may be of the ball variety shown, but any type will work. The check valves  114 ,  116  improve the efficiency of the device, but the system  100  works without them if the return tube  112  is closed off. This type of valve is passive, analogous to the valves which operate in a heart. An active valve could be used but it would be costly, more subject to failure, and would require a control mechanism. Note that these are an optimization to the system  100  but are not required. The system  100  as presented operates without any moving parts or valves as shown in  FIG. 7 . Check valves are required only for an embodiment employing a return tube. For example, the prototype in  FIG. 4  has no valves. 
     The Heatlane™ device mentioned earlier relies on an unstable, oscillatory exchange of fluid and gas back and forth with the evaporation section of the device. The system  100  is designed so that there is a unidirectional flow of heat from the evaporator  106  to the heat exchanger  108 . The system  100  cools by boiling a liquid which boils at a low temperature. The hot gas generated is uniformly pushed through the heat exchanger to be cooled and condensed. 
     Therefore, unlike the Heatlane™ device, the system  100  uses a nearly unidirectional flow of hot gas and fluid to the colder section of the system  100 . This system  100  does not require gravity to operate, only that the entrances and exits of the reservoirs be placed to make sense with respect to gravity. In fact the one-way check valves  114  and  116  are not required. They are placed in the device for improved efficiency. Refer to  FIG. 7  where a system  700  is shown without the valves  114 ,  116 , and without the third tube. Without the third tube  112  and check valves  114 ,  116 , coolant could be blown back through the heat exchanger  108 , warming up the coldest fluid, and returning much of the liquid coolant  103  back to the evaporator  106 . This would result in an average warmer coolant  103  temperature in the evaporator  106  compared with the embodiment previously discussed, but it would still provide cooling. (An embodiment without the third tube  112  and check valves  114 ,  116  has been demonstrated and measured with methanol and HFE-7000 as examples.) See  FIG. 4  for an illustration of a coolant system with only one tube. 
     This system  100  uses a heater  105  to drive the cycle. The heater  105  can be a coil of tungsten or titanium heating wire, but more likely is a heater cartridge having sufficient surface area and a proper coating to heat a volume of fluid quickly, and is used intermittently in a pulsed fashion to return the cooled, condensed fluid  103  back to the evaporator  106 . The heater  105  can also double as a heat sensor with a built-in thermocouple or fluid detector if a closed loop control is implemented. Alternatively, the heater  105  can be activated periodically, without regard to the status of cooling, or it can be activated by some other external sensor or trigger event. Some examples are: the fluid level in the evaporator falling below a minimum threshold or a temperature sensor reaching a maximum threshold temperature. There are other simple methods for activating the heater  105 . One option is if the chip is a microprocessor, its temperature sensors can detect that the boiler is going dry and request more coolant  103 —just as it requests that the fan controllers spin faster today in personal computers 
     The “Down” cycle of the system  100  is shown in  FIG. 2  wherein cooled, condensed coolant  103  is returned to the evaporator  106 . The heating element  105  is turned on (or is in its intermittent “on” phase). This generates heat which causes the gas to expand and “push” the cooled coolant  103  back to the evaporator  106 . The coolant  103  rapidly flows down the return tube  112  to fill the evaporator  106  again. Immediately, the hot fluid  103  in the evaporator  106  may be pumped to the heat exchanger  110 . Moving hot fluid out of the evaporator enhances the cooling power of evaporation alone. With the return of cool fluid to the evaporator, the total system pressure drops, and coolant  103  begins boiling in the evaporator  106  immediately. It is not intended that the heater  105  do anything other than generate hot gas to force the coolant to return to the evaporator  106 . The check valve  116  is automatically in the closed position, prohibiting the flow of the coolant  103  out of the radiator  104  back into the evaporator  106 . 
     Referring now to  FIG. 3  there is shown an illustration of a gas trap  300  that can be used with the system  100 . The gas trap  300  helps stand coolant up in the exchanger tubing, especially if wide diameter tubing is used. The gas trap  300  keeps fluid from flowing back by gravity. It is useful if the tubes are of a large diameter, but it is generally not necessary. Gas pressure will hold the column of fluid up if provided with a trap. Without the trap  300 , gas might fill the tube enough to allow the free fall of fluid back, which is undesirable. Each Up tube  102 ,  104  might have a trap, or all up tubes can be fed from a single large pipe with a trap. 
     Referring to  FIG. 4  there is shown a coolant system  400  using only one tube  440 , according to another embodiment of the present invention. The tube  440  is approximately ⅜″ in diameter and is disposed inside of a Lytron™ heat exchanger  410  and filled with approximately 0.5 L of liquid methanol  420 . Each end of the tube  440  is inserted into boro-silica view tubes  450  of a thicker diameter than the tubing  440 . Each of the two boro-silica tubes  450  are attached to a chamber containing a three-inch long, ⅜ inch diameter stainless heater cartridge  430 . In this demonstration, one heater cartridge simulates the heat of a chip and is run continuously at a few hundred watts of power. The other is pulsed periodically and acts as the return pump. 
       FIG. 5  is a cross-section image of two tubes in a dual tube design  500 . One tube  540  conducts the heated liquid and gas to the exchanger  108  while the other tube  560  (the return tube) returns the cooled liquid and gas to the evaporator  106 . The pulse pump action is the same as described earlier; a heating element in a reservoir, with check valves fore and aft.  FIG. 6  shows co-axial tubing according to another embodiment of the present invention. The tubing of  FIG. 6  has the external appearance of a single tube, but is co-axial as shown. This co-axial tubing implementation may be used in an embodiment such as that shown in  FIG. 1  with the exception being the return tube is co-axial with the “up” tube. This co-axial system may be easier to deploy. 
     Referring to  FIG. 4 , most of the heat is dissipated in the first tube  420  running up into the heat exchanger, especially inside the exchanger  410 . Heat dissipation drops off rapidly in a dry tube as described in “Heat and Mass Transport,” Incroppera and DeWitt Textbook, Wiley, 2002, p 612. Looking at  FIG. 4 , it is best that much of the tubing contain liquid coolant, especially in the left-most tube  420  leading up from the evaporator  106 . The heat transfer is best if bubbles of hot gas rise through the liquid coolant  103 . Heat transfer and dissipation drop off rapidly if the tube becomes dry, containing only gas. In a prototype example as in  FIG. 4 , at less than 150 watts, gravity alone does most of the work and the pump  490  does almost no pumping. At greater than 200 watts, vigorous entrainment of the coolant  103  with rising gas slugs spills and condenses into the return pump reservoir  490 . At greater than 300 watts the system is close to Critical Heat Flux (CHF) and becomes difficult to operate. This system works by “pool boiling,” just as in a tea kettle. The CHF is the point beyond which pool boiling fails to occur, and the chip heat becomes insulated by a layer of gas. A simple example of CHF is that if you turned the heat up high enough under a tea kettle, the bottom will melt out, even though there is liquid in the kettle. The layer of steam between the bottom of the kettle and the water inside insulates the heat source and pool boiling stops. 
     In this methanol test, the pressure will never exceed one atmosphere and will usually remain well below one atmosphere. Within the parameters of the test, the CHF was somewhere around 300 Watts, and cooling was not sustainable. Known methods of improving the boiler could easily raise this maximum flux. 
     A preferred pump system is shown in  FIG. 6 . A coaxial return pump design  600  shows a pulse pump  680  disposed within the return tube  560 . The return tube  560  is placed inside the “up” tube  540 . The pulse pump  680  is identical to the heating element pump previously described, situated to return coolant from the reservoir in the radiator back to the evaporator. The pump  680 , like the pump in  FIG. 5 , is placed where the condensed coolant  103  is within the return tube  560 . This coaxial implementation may be simpler and more efficient than the system of  FIG. 5  and the one-tube system of  FIG. 4 . 
     Therefore, while there have been described what are presently considered to be the preferred embodiments, it will understood by those skilled in the art that other modifications can be made within the spirit of the invention.