Patent Publication Number: US-2023156960-A1

Title: Systems and methods for thermal management of high-capacity devices in immersion-cooled datacenters

Description:
BACKGROUND 
     Background and Relevant Art 
     Computing devices can generate a large amount of heat during use. The computing components can be susceptible to damage from the heat and commonly require cooling systems to maintain the component temperatures in a safe range during heavy processing or usage loads. Different computing components produce different amounts of thermal energy and require different amounts of thermal management. Conventional thermal management systems cool the entire device uniformly, which may insufficiently cool the high-capacity components and/or waste energy cooling components that may not need the thermal management. 
     BRIEF SUMMARY 
     In some embodiments, a method of thermal management of a computing device includes immersing a first electronic component of the computing device in a first working fluid contained in a first volume, immersing a second electronic component of the computing device in a second working fluid contained in a second volume, and changing a pressure in the first volume to alter a boiling temperature of the first working fluid in the first volume. 
     In some embodiments, a system for immersion cooling of a server computer includes an immersion tank, a server computer positioned in the immersion tank, a component tank positioned on a high-capacity component of the server computer, a first working fluid in the component tank, and a second working fluid in the immersion tank. The first working fluid contacts the high-capacity component in the component tank, and the second working fluid contacts a heat-generating component of the server computer outside of the component tank. 
     In some embodiments, a system for immersion cooling of a server computer includes an immersion tank, a first server computer positioned in the immersion tank, a second server computer positioned in the immersion tank, and a liquid working fluid in the immersion tank. The first server computer includes at least one first heat-generating component and at least one first high-capacity component, a first component tank positioned around the at least one first high-capacity component, and a first component working fluid positioned in the first component tank and contacting the at least one first high-capacity component. The first component working fluid has a first component fluid pressure. The second server computer includes at least one second heat-generating component and at least one second high-capacity component, a second component tank positioned around the at least one second high-capacity component, and a second component working fluid positioned in the second component tank and contacting the at least one second high-capacity component. The second component working fluid has a second component fluid pressure. The liquid working fluid positioned in the immersion tank contacts the first heat-generating components of the first server computer and the second heat-generating components of the second server computer. The liquid working fluid in the immersion tank has a fluid pressure different than the first component fluid pressure or the second component fluid pressure. 
     This Summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This Summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used as an aid in determining the scope of the claimed subject matter. 
     Additional features and advantages will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by the practice of the teachings herein. Features and advantages of the disclosure may be realized and obtained by means of the instruments and combinations particularly pointed out in the appended claims. Features of the present disclosure will become more fully apparent from the following description and appended claims or may be learned by the practice of the disclosure as set forth hereinafter. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In order to describe the manner in which the above-recited and other features of the disclosure can be obtained, a more particular description will be rendered by reference to specific embodiments thereof which are illustrated in the appended drawings. For better understanding, the like elements have been designated by like reference numbers throughout the various accompanying figures. While some of the drawings may be schematic or exaggerated representations of concepts, at least some of the drawings may be drawn to scale. Understanding that the drawings depict some example embodiments, the embodiments will be described and explained with additional specificity and detail through the use of the accompanying drawings in which: 
         FIG.  1    is a side schematic representation of an immersion cooling system, according to at least one embodiment of the present disclosure; 
         FIG.  2    is a side schematic representation of an immersion cooling system with an external condenser, according to at least one embodiment of the present disclosure; 
         FIG.  3 - 1    is a side schematic representation of a single-phase immersion cooling system with an on-component cooling loop with two-phase immersion cooling, according to at least one embodiment of the present disclosure; 
         FIG.  3 - 2    is a flowchart illustrating an embodiment of a method of thermal management using the immersion cooling system of  FIG.  3 - 1   , according to at least one embodiment of the present disclosure; 
         FIG.  4    is a side schematic representation of a single-phase immersion cooling system with an on-component cooling loop with single-phase immersion cooling, according to at least one embodiment of the present disclosure; 
         FIG.  5    is a side schematic representation of a two-phase immersion cooling system with an on-component cooling loop with single-phase immersion cooling, according to at least one embodiment of the present disclosure; 
         FIG.  6    is a side schematic representation of a two-phase immersion cooling system with an on-component cooling loop with single-phase immersion cooling that shared a working fluid with the immersion chamber, according to at least one embodiment of the present disclosure; 
         FIG.  7    is a side schematic representation of a two-phase immersion cooling system with an on-component cooling loop with two-phase immersion cooling and a controller that adjusts the fluid pressure in the on-component cooling loop, according to at least one embodiment of the present disclosure; 
         FIG.  8    is a side schematic representation of a server rack with a two-phase immersion cooling system with an on-component cooling loop for each server blade that is connected to a shared manifold, according to at least one embodiment of the present disclosure; and 
         FIG.  9    is a side schematic representation of a server rack with a two-phase immersion cooling system with a self-contained on-component cooling loop for each server blade, according to at least one embodiment of the present disclosure; 
     
    
    
     DETAILED DESCRIPTION 
     The present disclosure relates generally to systems and methods for thermal management of electronic devices or other heat-generating components Immersion chambers surround the heat-generating components in a liquid working fluid, which conducts heat from the heat-generating components to cool the heat-generating components. As the working fluid absorbs heat from the heat-generating components, the temperature of the working fluid increases. In some embodiments, the hot working fluid can be circulated through the thermal management system to cool the working fluid and/or replace the working fluid with cool working fluid. In some embodiments, the working fluid vaporizes, introducing vapor into the liquid of the working fluid which rises out of the liquid phase, carrying thermal energy away from the heat-generating components in the gas phase via the latent heat of boiling. 
     In large-scale computing centers, such as cloud-computing centers, data processing centers, data storage centers, or other computing facilities, immersion cooling systems provide an efficient method of thermal management for many computing components under a variety of operating loads. In some embodiments, an immersion cooling system includes a working fluid in an immersion chamber and a heat exchanger to cool the liquid phase and/or a condenser to extract heat from the vapor phase of the working fluid. The heat exchanger may include a condenser that condenses the vapor phase of the working fluid into a liquid phase and returns the liquid working fluid to the immersion chamber. In some embodiments, the liquid working fluid absorbs heat from the heat-generating components, and one or more fluid conduits direct the hot liquid working fluid outside of the immersion chamber to a radiator, heat exchanger, or region of lower temperature to cool the liquid working fluid. 
     Whether the immersion cooling system is a two-phase cooling system (wherein the working fluid vaporizes and condenses in a cycle) or a one-phase cooling system (wherein the working fluid remains in a single phase in a cycle), the heat transported from the heat-generating components outside of the immersion chamber is further exchanged with an ambient fluid to exhaust the heat from the system. 
     A conventional immersion cooling system  100 , shown in  FIG.  1   , includes an immersion tank  102  containing an immersion chamber  104  and a condenser  106  in the immersion chamber  104 . The immersion chamber  104  contains a working fluid that has a liquid working fluid  108  and a vapor working fluid  110  portion. The liquid working fluid  108  creates an immersion bath  112  in which a plurality of heat-generating components  114  are positioned to heat the liquid working fluid  108  on supports  116 . 
     Referring now to  FIG.  2   , an immersion cooling system  200  according to the present disclosure includes an immersion tank  202  defining an immersion chamber  204  with a working fluid positioned therein. The working fluid transitions between a liquid working fluid  208  phase and a vapor working fluid  210  phase to remove heat from hot or heat-generating components  214  in the immersion chamber  204 . The liquid working fluid  208  more efficiency receives heat from the heat-generating components  214  and, upon transition to the vapor working fluid  210 , the vapor working fluid  210  can be removed from the immersion tank  202 , cooled and condensed by the condenser  206  (or other heat exchanger) to extract the heat from the working fluid, and the liquid working fluid  208  can be returned to the liquid immersion bath  212 . 
     In some embodiments, the immersion bath  212  of the liquid working fluid  208  has a plurality of heat-generating components  214  positioned in the liquid working fluid  208 . The liquid working fluid  208  surrounds at least a portion of the heat-generating components  214  and other objects or parts attached to the heat-generating components  214 . In some embodiments, the heat-generating components  214  are positioned in the liquid working fluid  208  on one or more supports  216 . The support  216  may support one or more heat-generating components  214  in the liquid working fluid  208  and allow the working fluid to move around the heat-generating components  214 . In some embodiments, the support  216  is thermally conductive to conduct heat from the heat-generating components  214 . The support(s)  216  may increase the effective surface area from which the liquid working fluid  208  may remove heat through convective cooling. 
     In some embodiments, the heat-generating components  214  include electronic or computing components or power supplies. In some embodiments, the heat-generating components  214  include computer devices, such as individual personal computer or server blade computers. In some embodiments, one or more of the heat-generating components  214  includes a heat sink or other device attached to the heat-generating component  214  to conduct away thermal energy and effectively increase the surface area of the heat-generating component  214 . In some embodiments, the heat-generating components  214  include an electric motor. 
     As described, conversion of the liquid working fluid  208  to a vapor phase requires the input of thermal energy to overcome the latent heat of vaporization and may be an effective mechanism to increase the thermal capacity of the working fluid and remove heat from the heat-generating components. Because the vapor working fluid  210  rises in the liquid working fluid  208 , the vapor working fluid  210  can be extracted from the immersion chamber  204  in an upper vapor region of the chamber. A condenser  206  cools part of the vapor working fluid  210  back into a liquid working fluid  208 , removing thermal energy from the system and reintroducing the working fluid into the immersion bath  212  of the liquid working fluid  208 . The condenser  206  radiates or otherwise dumps the thermal energy from the working fluid into the ambient environment or into a conduit to carry the thermal energy away from the cooling system. 
     In conventional immersion cooling systems, a liquid-cooled condenser is integrated into the immersion tank and/or the chamber to efficiency remove the thermal energy from the working fluid. In some embodiments according to the present disclosure, an immersion cooling system  200  for thermal management of computing devices allows at least one immersion tank  202  and/or chamber  204  to be connected to and in fluid communication with an external condenser  206 . In some embodiments, an immersion cooling system includes a vapor return line  218  that connects the immersion tank  202  to the condenser  206  and allows vapor working fluid  210  to enter the condenser  206  from the immersion tank  202  and/or chamber  204  and a liquid return line  220  that connects the immersion tank  202  to the condenser  206  and allows liquid working fluid  208  to return to the immersion tank  202  and/or chamber  204 . 
     The vapor return line  218  may be colder than the boiling temperature of the working fluid. In some embodiments, a portion of the vapor working fluid  210  condenses in the vapor return line  218 . The vapor return line  218  can, in some embodiments, be oriented at an angle such that the vapor return line  218  is non-perpendicular to the direction of gravity. The condensed working fluid can then drain either back to the immersion tank  202  or forward to the condenser  206  depending on the direction of the vapor return line  218  slope. In some embodiments, the vapor return line  218  includes a liquid collection line or valve, like a bleeder valve, that allows the collection and/or return of the condensed working fluid to the immersion tank  202  or condenser  206 . 
     In some examples, an immersion cooling system  200  includes an air-cooled condenser  206 . An air-cooled condenser  206  may require fans or pumps to force ambient air over one or more heat pipes or fins to conduct heat from the condenser to the air. 
       FIG.  3 - 1    is a schematic representation of an immersion cooling system  300  with two different volumes of working fluid for cooling different components of a computing device. In some embodiments, a server computer  324  or other computing device is positioned inside an immersion tank  302  for cooling. The immersion tank  302  houses a working fluid that cools the server computer  324  by absorbing heat from the components of the server computer  324 . The liquid working fluid  308  may vaporize into vapor working fluid  310  which rises in the tank  302  toward a condenser  306 . 
     In some embodiments, the immersion cooling system  300  includes on-component cooling that includes a separate volume of working fluid to cool the high-capacity components  326  of the server computer  324 , such as a CPU, GPU, or other components that generate large amounts of heat. Other heat-generating components  314  on the server computer  324  may be cooled by the liquid working fluid  308  in the immersion tank  302 , generally. 
     The immersion cooling system  300  includes a vapor return line  318  to the condenser  306  and a liquid return line  320  from the condenser  306  to the immersion chamber  304  of the immersion tank  302 . The condenser  306  rejects heat to the ambient atmosphere to remove heat from the system and return the cooled liquid working fluid  308  to the immersion chamber  304  to continue cooling the heat-generating components  314 . A tank pump  328  may be positioned in the vapor return line  318  or liquid return line  320  to circulate the working fluid. 
     The on-component cooling volume includes a component tank  330  positioned on the high-capacity component  326  to hold a liquid component working fluid  332  around and/or in contact with a surface of the high-capacity component  326 . The high-capacity component  326  may heat or vaporize the liquid component working fluid  332  and convert the liquid component working fluid  332  to vapor component working fluid  334 . The liquid component working fluid  332  may be delivered to the component tank  330  by a component liquid return line  336  and the vapor component working fluid  334  carries away heat from the high-capacity component  326  through a component vapor return line  338 . The component vapor return line  338  directs the vapor component working fluid  334  to a component condenser  340  that condenses the vapor component working fluid  334  back to a liquid component working fluid  332  and rejects the heat to the ambient atmosphere. In at least one example, the working fluid in the immersion tank  302  and the component working fluid in the component cooling loop are the same working fluid. Using the same working fluid may simplify maintenance by only requiring the stocking and maintenance of a single fluid and/or simplify repairs by ensuring that leaks of a first working fluid in one volume do not contaminate the second working fluid in the other volume. However, using a single working fluid results in a working fluid with a single boiling temperature, despite the components of the server computer  324  (e.g., the heat-generating components  314  and high-capacity components  326 ) operating at different temperatures. 
     Because the working fluid can carry away heat most efficiently by absorbing heat through the latent heat of boiling, the working fluid may work most efficiently when the boiling temperature of the working fluid is at or near the operating temperature of the components being cooled. In other examples, it may be desirable to prevent boiling, as the liquid working fluid  308  may expand by 100 times or more upon vaporization into vapor working fluid  310 . Unintended or undesirable vaporization may cause an increase in pressure that may damage the immersion cooling system  300 . Rapid vaporization can cause a dryout effect when the surface of a heat-generating component  314  vaporizes the liquid cooling fluid  308  so rapidly that an insufficient mass of liquid working fluid  308  contacts the surface of the heat-generating component  314 , which compromises the convective cooling on the surface of the heat-generating component  314  and allows the temperature thereof to increase too quickly. 
     Additionally, working fluid is recycled through the volumes of the thermal management system, and, in some embodiments, the working fluid is a dielectric fluid or other fluid that is expensive. A thermal management system that uses less working fluid and/or uses the working fluid more efficiently allows for cost savings in the working fluid. In some embodiments, the working fluid is relatively dense and containing large volumes of the working fluid requires a strong container. Building and maintaining containers for large volumes and/or masses of working fluid can increase construction costs and container weight, which limits transport and maintenance of the containers. 
     In a conventional immersion tank, the liquid pressure increases as depth of the immersion bath increases. In conventional tanks and fluids, a depth of 1 meter results in a 2.3 pounds per square inch (PSI) increase. The increased pressure results in an increase in the boiling point for the working fluid and a resulting temperature increase of the components adjacent the working fluid at the bottom of the immersion bath. When separate immersion chambers are placed around heat-generating components, and/or the boards are oriented horizontally, the columnar pressure of the fluid around the component is reduced and produces lower operating temperatures for the component. In at least one example, a working fluid exhibits a 4° C. decrease in temperature relative to a component at a depth of 1 meter in a conventional immersion tank. Defining separate volumes can allow for the customized control of boiling temperatures for different components of the server computer and/or for server computers at different locations inside an immersion tank or server rack. 
     As the working fluid carries away thermal energy through latent heat of boiling, managing the boiling temperature of the working fluid is beneficial for the performance of the immersion cooling system. In some embodiments, the liquid working fluid receives heat in a cooling volume of working fluid immediately surrounding the heat-generating components. The cooling volume is the region of the working fluid (including both liquid and vapor phases) that is immediately surrounding the heat-generating components and is responsible for the convective cooling of the heat-generating components. In some embodiments, the cooling volume is the volume of working fluid within 5 millimeters (mm) of the heat-generating components. 
     The working fluid has a boiling temperature below a critical temperature at which the heat-generating components experience thermal damage. For example, the heat-generating components may be computing components that experience damage above 100° Celsius (C). In some embodiments, the boiling temperature of the working fluid is less than a critical temperature of the heat-generating components. In some embodiments, the boiling temperature of the working fluid is less about 90° C. at 1 atmosphere of pressure. In some embodiments, the boiling temperature of the working fluid is less about 80° C. at 1 atmosphere of pressure. In some embodiments, the boiling temperature of the working fluid is less about 70° C. at 1 atmosphere of pressure. In some embodiments, the boiling temperature of the working fluid is less about 60° C. at 1 atmosphere of pressure. In some embodiments, the boiling temperature of the working fluid is at least about 35° C. at 1 atmosphere of pressure. In some embodiments, the working fluid includes water. 
     In some embodiments, the working fluid includes glycol. In some embodiments, the working fluid includes a combination of water and glycol. In some embodiments, the working fluid is an aqueous solution. In some embodiments, the working fluid is an electronic liquid, such as FC-72 available from 3M, or similar non-conductive fluids. In some embodiments, the heat-generating components, supports, or other elements of the immersion cooling system positioned in the working fluid have nucleation sites on a surface thereof that promote the nucleation of vapor bubbles of the working fluid at or below the boiling temperature of the working fluid. 
     Pumps, valves, inlet sizes, outlet sizes, and conduit diameters may all affect the fluid pressure inside a given volume in the immersion cooling system  300 . For example, a flowrate of the tank pump  328  may be increased to force liquid working fluid  308  through the liquid return line  320  and pressurize the immersion chamber  304  of the immersion tank  302 . In other examples, a vapor return line valve  344  in the vapor return line  318  may be opened or closed to alter the fluid pressure in the immersion chamber  304  by allowing more or less vapor working fluid  310  to exit the immersion tank  302 . 
     In some embodiments, a fluid pressure in the on-component cooling loop may be changed by a component fluid pump  342  in communication with the component condenser  340 . For example, a flowrate of the component fluid pump  342  may be increased to force liquid component working fluid  332  through the component liquid return line  336  and pressurize the liquid component working fluid  332  of the component tank  330 . Increasing the pressure of a working fluid increases the boiling temperature of the working fluid in that volume. Similarly, decreasing the pressure of the working fluid decreases the boiling temperature of the working fluid. By altering the pressure in a volume of working fluid, the boiling temperature can be customized to the heat of the components being cooled. 
     For example,  FIG.  3 - 2    is a flowchart illustrating an embodiment of a method  346  of thermal management using two volumes of working fluid, such as illustrated in  FIG.  3 - 1    and elsewhere in the present disclosure. The method  346  includes immersing a first electronic component (e.g., high-capacity component  326 ) of the computing device (e.g., server computer  324 ) in a first working fluid at  348  (e.g., liquid component working fluid  332 ) and immersing a second electronic component (e.g., heat-generating component  314 ) of the computing device (e.g., server computer  324 ) in a second working fluid (e.g., liquid working fluid  308 ) at  350 . 
     The method further includes changing a pressure of the first volume (e.g., the fluid pressure in the component tank  330 ) of the first working fluid in the first volume at  352 . As described herein, the pressure can be changed through the use of pumps, valves, inlet and/or outlet sizes to control the flux of fluid in and/or out of a portion of the volume. In at least one example, a reservoir may store working fluid and a pump (e.g., the component fluid pump  342 ) may selectively introduce additional working fluid from the reservoir or remove working fluid to the reservoir to alter the pressure. 
       FIG.  4    is a schematic representation of an embodiment of an immersion cooling system  400  with two different volumes of single-phase working fluid for cooling different components of a computing device. The immersion cooling system  400  includes an immersion tank  402  with a server computer  424  (or other electronic device) immersed in a liquid working fluid  408 . In the example of a single-phase immersion cooling system  400 , the liquid working fluid  408  is not intended to boil. The liquid working fluid  408  may absorb heat from heat-generating components  414  and then may be circulated through a heat exchanger  406  to cool the liquid working fluid  408  before being returned to the immersion chamber  404 . In some embodiments, there is some gas in the immersion chamber  404 , while in other embodiments, the entire immersion chamber  404 , heat exchanger  406  and associated conduits are filled with liquid working fluid  408 . Vaporization of the liquid working fluid  408  may suddenly increase pressure and damage the immersion cooling system  400 . 
     High-capacity components  426  may be cooled by a second volume of working fluid that is configured to handle higher temperatures and/or pressures than the immersion tank  402 . For example, the high capacity components  426  may have on-component cooling by a liquid working fluid  432  that is pressurized to remain in a liquid phase above the boiling temperature at atmospheric pressure. For example, the liquid component working fluid  432  may be the same liquid working fluid  408  as in the immersion tank  402 , but the high-capacity components  426  may operate at a temperature that would boil the liquid working fluid  408  in the immersion tank  402 . By increasing the pressure in the volume around the high-capacity components  426 , for example, with a component fluid pump  442  and/or valves, the liquid component working fluid  432  may have a higher boiling temperature. The on-component cooling loop may therefore also use a heat exchanger  440  instead of a condenser. In some embodiments or applications, it may be preferable to use a heat exchanger rather than a condenser, for example, due to space constraints, power constraints, or geometry limitations. 
       FIG.  5    is a schematic representation of an immersion cooling system  500  with a combination of single phase and two-phase cooling for cooling different components of a computing device. As described herein, the working fluid used in immersion cooling systems may be expensive or the working fluid may have negative environmental effects that make highly efficient use of the working fluid desirable. For example, a two-phase cooling system may spray a liquid working fluid onto the heat generating components in droplets to absorb heat from the heat-generating components. The droplets may vaporize into vapor working fluid, carrying away heat from the heat-generating components. A condenser  506  may subsequently condense the vapor working fluid  510  to liquid working fluid  508 , which may drip back down through a headspace  556  of the tank  502  to a reservoir  554  of liquid working fluid  508  at the bottom of the tank  502 . 
     In some embodiments, a nozzle pump  558  supplies liquid working fluid  508  to spray nozzles  560 . The spray nozzle  560  sprays the liquid working fluid  508  to create a plurality of droplets  562 . In addition to the condenser  506  condensing the vapor working fluid  510 , the plurality of droplets  562  has an increased surface area to cool and condense the vapor working fluid  510 , and the droplets  562  may provide a site onto which the vapor working fluid  510  may condense, further lessening the energy required to condense the vapor working fluid  510 . Similar to a cold plate or cold surface in a conventional condenser, the droplets may be the subcooled surface that allow condensation upon the droplets themselves. 
     In some embodiments, the spray nozzles  560  are located on a sidewall of the immersion tank  502 . In some embodiments, the spray nozzles  560  are located on top wall or lid of the immersion tank  502 . In some embodiments, the spray nozzles  560  are located on both the sidewall and the top wall or lid of the immersion tank  502 . In some embodiments, the spray nozzles  560  are positioned in the immersion bath or reservoir  554  of the liquid working fluid  508  to spray the droplets  562  upward into the headspace  556  to deliver the droplets  562  to the heat-generating components  514 . 
     The spray nozzles  560  may be adjustable to vary the size of the droplets  562  depending upon desired flowrate through the nozzles  560  and the desired droplet size. In some embodiments, the spray nozzles  560  create a plurality of droplets  562  with an average droplet diameter of less than 1 millimeter. In some embodiments, the spray nozzles  560  create a plurality of droplets  562  with an average droplet diameter of less than 0.5 millimeters. In some embodiments, the spray nozzles  560  create a plurality of droplets  562  with an average droplet diameter of less than 0.25 millimeters. In some embodiments, the spray nozzles  560  create a plurality of droplets  562  with an average droplet diameter of less than 0.1 millimeters. In some embodiments, the spray nozzle  560  sprays the subcooled working fluid onto another component, such as a fan, which further disrupts the surface tension and creates the droplets  562  and/or more droplets. 
     While sprayed droplets  562  may efficiently cool the heat-generating components  514  using a comparatively small mass of liquid working fluid  508 , the overall cooling capacity of the sprayed droplets  562  may be insufficient for the high-capacity components  526  of the server computer  524 . In the illustrated embodiment of  FIG.  5   , the high-capacity component  526  is surrounded by a component tank  530 . The component tank  530  contains a pressurized volume of the liquid working fluid  508  that allows for a single-phase immersion cooling loop for the high-capacity components  526 . Immersing the high-capacity component  526  in a component liquid working fluid  532  may allow for greater convective cooling and higher thermal management capacity. A component heat exchanger  540  may then reject the waste heat to the environment. 
     In some embodiments, the component working fluid and the liquid working fluid in the immersion tank at large are the same working fluid. The high-capacity components may be cooled by a pressurized volume of liquid working fluid to maintain the liquid working fluid in a liquid phase while the liquid working fluid in the rest of the immersion chamber and/or headspace may be allowed to vaporize. 
       FIG.  6    is a schematic representation of an immersion cooling system  600  that shares working fluid between different volumes for cooling different components of a computing device. The high-capacity components  626  are cooled using forced convective liquid immersion cooling in the liquid working fluid  608 , while the other heat-generating components  614  are cooled using two-phase cooling through sprayed droplets  662 . 
     Similar to the embodiment described in relation to  FIG.  5   , the droplets  652  of liquid working fluid  608  may efficiently cool the heat-generating components  614  but have insufficient thermal management capacity to cool the high-capacity components  626 . In some embodiments, the high-capacity components  626  are cooled with shared liquid working fluid  608  from the immersion chamber  604 . The high-capacity components  626  may be cooled using convective cooling from immersion in flowing liquid working fluid  608 . In some embodiments, the liquid working fluid  608  proximate the high-capacity component  626  is pressurized above the immersion chamber  604  at large to increase the boiling temperature of the liquid working fluid  608  and maintain the liquid working fluid  608  in the liquid phase. In some embodiments, the liquid working fluid  608  proximate the high-capacity component  626  is pressurized above the immersion chamber  604  at large to increase the boiling temperature of the liquid working fluid  608  and keep the boiling temperature within a threshold range of the temperature of the high-capacity component  626 . 
     For example, a condenser  606  located in the headspace  656  of the immersion chamber  604  may condense the vapor working fluid  610  to liquid working fluid  608 . The liquid working fluid  608  may be directed or pumped through a component liquid return line  636  toward the high-capacity component  626 . The liquid working fluid  608  in the component liquid return line  636  may flow to and past the high-capacity component  626 , absorbing heat from the high-capacity component  626 . The liquid working fluid  608  pumped into the component liquid return line  636  may increase the pressure therein and proximate the high-capacity component  626  as the liquid working fluid  608  is restricted from exiting the fluid conduit (e.g., the volume proximate the high-capacity component  626 ) by an exit valve  664  or small area outlet that limits flowrates therethrough. When the liquid working fluid  608  exits through the exit valve  664 , the hot liquid working fluid  608  may vaporize in the immersion chamber  604  and introduce vapor working fluid  610  into the immersion chamber  604 . If the high-capacity component  626  does not heat the liquid working fluid  608  beyond the boiling temperature at the fluid pressure in the immersion chamber  604  at large, the liquid working fluid  608  will remain in the liquid phase and fall to the reservoir  654  below to be circulated through the spray nozzles  660 . 
     As described in relation to  FIG.  6   , in some embodiments, the volume proximate the high-capacity components may be pressurized to maintain the boiling temperature of the working fluid at or near the operating temperature of the high-capacity components.  FIG.  7    is a schematic representation of an immersion cooling system  700  with two-phase cooling for cooling different components of a computing device. The immersion cooling system  700  may cool the heat-generating components  714  of the server computer  724  with two-phase immersion cooling, while the high-capacity component(s)  726  have on-component cooling with a different volume of working fluid. 
     As described herein, most efficient two-phase cooling occurs when the liquid working fluid absorbs heat from the component(s) and boils. The working fluid can absorb heat in the form of the latent heat of boiling to expand without increasing in temperature. The working fluid can, therefore, vaporize and the vapor working fluid can form bubbles which rise out of the liquid working fluid without the liquid working fluid around the component heating beyond the boiling temperature. However, if the component operating temperature is significantly above the boiling temperature, dryout can occur with insufficient thermal mass in contact with the surface of the component. In some embodiments, it may be beneficial to adjust the working fluid boiling temperature to be below the operating temperature, but within a threshold value of the operating temperature. 
     In some embodiments, the target boiling temperature is in a range including a lower value of less than 5° C. less than the operating temperature of the high-capacity component. In some embodiments, the target boiling temperature is in a range that includes a lower value of less than 3° C. less than the operating temperature of the high-capacity component. In some embodiments, the target boiling temperature is in a range that includes a lower value of less than 1° C. less than the operating temperature of the high-capacity component. For example, it may be beneficial to adjust the fluid pressure in the volume to change the boiling temperature to 1° C. less than the current operating temperature of the high-capacity component. 
     In some embodiments, the target boiling temperature is greater than the operating temperature of the high-capacity component to prevent the working fluid from boiling. In some embodiments, the target boiling temperature is in a range including an upper value of less than 5° C. greater than the operating temperature of the high-capacity component. In some embodiments, the target boiling temperature is in a range including an upper value of less than 3° C. greater than the operating temperature of the high-capacity component. In some embodiments, the target boiling temperature is in a range including an upper value of less than 1° C. greater than the operating temperature of the high-capacity component. 
       FIG.  7    illustrates an embodiment of an immersion cooling system  700  with a plurality of pressure control mechanisms, such as pumps, valves, or other adjustable portions of the on-component cooling loop that are in communication with a controller  766 . The controller  766  receives measurements from one or more sensors  768 , such as a pressure sensor or temperature sensor, to determine whether and when to alter the fluid pressure proximate the high-capacity component  726 . 
     In some embodiments, the sensor  768  is a temperature sensor that measures an operating temperature of the high-capacity component  726 . In other embodiments, the sensor  768  measures an electrical power draw of the high-capacity component  726 , which may be used to calculate an operating temperature of the high-capacity component  726 . The controller  766  can, based at least partially on the component operating temperature, determine a target boiling temperature for the working fluid proximate the high-capacity component  726 . In some embodiments, the sensor  768  includes a pressure sensor that measures the fluid pressure proximate the high-capacity component  726  and reports the fluid pressure to the controller  766 . 
     The controller  766  may be in data communication with a component condenser  740 , a fluid pump  742 , and/or one or more valves  770 ,  772  to control relative flow rates through portions of the on-component cooling loop. For example, the controller  766  may reduce flow through an outlet valve  770  out of the portion of the volume proximate the high-capacity component  726  to increase the fluid pressure proximate the high-capacity component  726 . In some examples, the controller  766  may reduce flow through an inlet valve  772  into the portion of the volume proximate the high-capacity component  726  to decrease the fluid pressure proximate the high-capacity component  726 . In some examples, the controller  766  may increase flow through the fluid pump  742  toward the portion of the volume proximate the high-capacity component  726  to increase the fluid pressure proximate the high-capacity component  726 . In some examples, the controller  766  may increase condensation rates of the component condenser  740  to decrease the fluid pressure throughout the on-component cooling loop. In some embodiments, the controller  766  may perform a combination of the above examples or other procedures to adjust the fluid pressure proximate the high-capacity component  726 . For example, as described in relation to  FIG.  3 - 2   , a reservoir may store working fluid and a pump (e.g., the component fluid pump  742 ) may selectively introduce additional working fluid from the reservoir or remove working fluid to the reservoir to alter the pressure. 
     While examples and embodiments described herein refer to a single computer in an immersion tank, it should be understood that any example or embodiment described herein may be applicable to a rack or a tank with a plurality of computing devices or blades positioned therein.  FIG.  8    is a schematic representation of a server rack  874  including a plurality of blades of server computers  824  with different volumes for cooling different components of server computers  824  positioned on the blades. A plurality of server computers  824  have on-component cooling loops that connect to an inlet manifold  876  and an outlet manifold  878  of the rack  874 . The first volume of working fluid is the on-component cooling loop that includes component tanks  830 - 1 ,  830 - 2  around the high-capacity components  826  on the server computer  824  of the blade. Fluid conduits connect the component tanks  830 - 1 ,  830 - 2  to the shared inlet manifold  876  and the shared outlet manifold  878  used by the plurality of server blades. The component vapor working fluid  834  is circulated by a component fluid pump  842  through a component heat exchanger  840  or condenser to cool the component working fluid before returning the component liquid working fluid  832  to the inlet manifold  876 . 
     In some embodiments, the second volume of working fluid is the immersion chamber  804  into which the server blades are positioned in the rack  874 . The liquid working fluid  808  is sprayed into the immersion chamber  804  in droplets  862  before heat-generating components of the server computers  824  vaporize the liquid working fluid  808  into vapor working fluid  810 . The vapor working fluid  810  is removed to an external heat exchanger  806  or condenser. The condensed liquid working fluid  808  is returned by a fluid pump  828 . The first volume and second volume are maintained at different temperatures to adjust the boiling temperatures of the working fluids. 
       FIG.  9    is a schematic representation of another server rack  974  with a recirculating cooling volume on the high-capacity components  926  of at least some of the server computers  924 , and the server computers  924  are positioned in an immersion tank  902  for cooling the remaining heat-generating components  914  with a second volume of working fluid. 
     In some embodiments, each blade or server computer  924  includes a self-contained on-component cooling loop with a working fluid therein. The on-component cooling loop includes a component condenser  940  and, optionally, a component fluid pump  942 , to cool the component working fluid and direct the component liquid working fluid  932  toward the component tanks  930 - 1 ,  930 - 2  on the same server computer  924 . The high-capacity components  926 - 1 ,  926 - 2  contained in the component tanks  930 - 1 ,  930 - 2  heat the working fluid and the component vapor working fluid  934  is returned to the component condenser  940 . 
     In some embodiments, the other heat-generating components  914  of the server computers  924  are cooled by the second volume of working fluid, which is maintained at a different pressure than the first volume in the on-component cooling loop of each server computer  924 . In some embodiments, the second volume of working fluid is the immersion chamber  904  into which the server blades are positioned in the rack  974 . The liquid working fluid  908  is sprayed into the immersion chamber  904  in droplets  962  before heat-generating components of the server computers  924  vaporize the liquid working fluid  908  into vapor working fluid  910 . The vapor working fluid  910  may be removed to an external heat exchanger or condenser or condensed in the immersion chamber  904  and allowed to drip down toward the bottom of the immersion chamber  904 . The first volume and second volume are maintained at different temperatures to adjust the boiling temperatures of the working fluids. In some embodiments, each self-contained on-component cooling loop may operate at fluid pressures independent of the fluid pressures of other self-contained on-component cooling loops on other server computers  924  in the server rack  974 . 
     INDUSTRIAL APPLICABILITY 
     The present disclosure relates generally to systems and methods for thermal management of electronic devices or other heat-generating components Immersion chambers surround the heat-generating components in a liquid working fluid, which conducts heat from the heat-generating components to cool the heat-generating components. As the working fluid absorbs heat from the heat-generating components, the temperature of the working fluid increases. In some embodiments, the hot working fluid can be circulated through the thermal management system to cool the working fluid and/or replace the working fluid with cool working fluid. In some embodiments, the working fluid vaporizes, introducing vapor into the liquid of the working fluid which rises out of the liquid phase, carrying thermal energy away from the heat-generating components in the gas phase via the latent heat of boiling. 
     In large-scale computing centers, such as cloud-computing centers, data processing centers, data storage centers, or other computing facilities, immersion cooling systems provide an efficient method of thermal management for many computing components under a variety of operating loads. In some embodiments, an immersion cooling system includes a working fluid in an immersion chamber and a heat exchanger to cool the liquid phase and/or a condenser to extract heat from the vapor phase of the working fluid. The heat exchanger may include a condenser that condenses the vapor phase of the working fluid into a liquid phase and returns the liquid working fluid to the immersion chamber. In some embodiments, the liquid working fluid absorbs heat from the heat-generating components, and one or more fluid conduits direct the hot liquid working fluid outside of the immersion chamber to a radiator, heat exchanger, or region of lower temperature to cool the liquid working fluid. 
     Whether the immersion cooling system is a two-phase cooling system (wherein the working fluid vaporizes and condenses in a cycle) or a one-phase cooling system (wherein the working fluid remains in a single phase in a cycle), the heat transported from the heat-generating components outside of the immersion chamber is further exchanged with an ambient fluid to exhaust the heat from the system. 
     In some embodiments, an immersion cooling system according to the present disclosure includes an immersion tank defining an immersion chamber with a working fluid positioned therein. The working fluid transitions between a liquid working fluid phase and a vapor working fluid phase to remove heat from hot or heat-generating components in the immersion chamber. The liquid working fluid more efficiency receives heat from the heat-generating components and, upon transition to the vapor working fluid, the vapor working fluid can be removed from the immersion tank, cooled and condensed by the condenser (or other heat exchanger) to extract the heat from the working fluid, and the liquid working fluid can be returned to the liquid immersion bath. 
     In some embodiments, the immersion bath of the liquid working fluid has a plurality of heat-generating components positioned in the liquid working fluid. The liquid working fluid surrounds at least a portion of the heat-generating components and other objects or parts attached to the heat-generating components. In some embodiments, the heat-generating components are positioned in the liquid working fluid on one or more supports. The support may support one or more heat-generating components in the liquid working fluid and allow the working fluid to move around the heat-generating components. In some embodiments, the support is thermally conductive to conduct heat from the heat-generating components. The support(s) may increase the effective surface area from which the liquid working fluid may remove heat through convective cooling. 
     In some embodiments, the heat-generating components include electronic or computing components or power supplies. In some embodiments, the heat-generating components include computer devices, such as individual personal computer or server blade computers. In some embodiments, one or more of the heat-generating components includes a heat sink or other device attached to the heat-generating component to conduct away thermal energy and effectively increase the surface area of the heat-generating component. In some embodiments, the heat-generating components include an electric motor. 
     As described, conversion of the liquid working fluid to a vapor phase requires the input of thermal energy to overcome the latent heat of vaporization and may be an effective mechanism to increase the thermal capacity of the working fluid and remove heat from the heat-generating components. Because the vapor working fluid rises in the liquid working fluid, the vapor working fluid can be extracted from the immersion chamber in an upper vapor region of the chamber. A condenser cools part of the vapor working fluid back into a liquid working fluid, removing thermal energy from the system and the working fluid into the immersion bath of the liquid working fluid. The condenser radiates or otherwise dumps the thermal energy from the working fluid into the ambient environment or into a conduit to carry the thermal energy away from the cooling system. 
     In conventional immersion cooling systems, a liquid-cooled condenser is integrated into the immersion tank and/or the chamber to efficiency remove the thermal energy from the working fluid. In some embodiments according to the present disclosure, an immersion cooling system for thermal management of computing devices allows at least one immersion tank and/or chamber to be connected to and in fluid communication with an external condenser. In some embodiments, an immersion cooling system includes a vapor return line that connects the immersion tank to the condenser and allows vapor working fluid to enter the condenser from the immersion tank and/or chamber and a liquid return line that connects the immersion tank to the condenser and allows liquid working fluid to return to the immersion tank and/or chamber. 
     The vapor return line may be colder than the boiling temperature of the working fluid. In some embodiments, a portion of the vapor working fluid condenses in the vapor return line. The vapor return line can, in some embodiments, be oriented at an angle such that the vapor return line is non-perpendicular to the direction of gravity. The condensed working fluid can then drain either back to the immersion tank or forward to the condenser depending on the direction of the vapor return line slope. In some embodiments, the vapor return line includes a liquid collection line or valve, like a bleeder valve, that allows the collection and/or return of the condensed working fluid to the immersion tank or condenser. 
     In some examples, an immersion cooling system includes an air-cooled condenser. An air-cooled condenser may require fans or pumps to force ambient air over one or more heat pipes or fins to conduct heat from the condenser to the air. 
     In some embodiments, a server computer or other computing device is positioned inside an immersion tank for cooling. The immersion tank houses a working fluid that cools the server computer by absorbing heat from the components of the server computer. The liquid working fluid may vaporize into vapor working fluid which rises in the tank toward a condenser. 
     In some embodiments, immersion cooling system includes on-component cooling that includes a separate volume of working fluid to cool the high-capacity components of the server computer, such as a CPU, GPU, or other components that generate large amounts of heat. Other heat-generating components on the server computer may be cooled by the liquid working fluid in the immersion tank, generally. 
     The immersion cooling system includes a vapor return line to the condenser and a liquid return line from the condenser to the immersion chamber of the immersion tank. The condenser rejects heat to the ambient atmosphere to remove heat from the system and return the cooled liquid working fluid to the immersion chamber to continue cooling the heat-generating components. A tank pump may be positioned in the vapor return line or liquid return line to circulate the working fluid. 
     The on-component cooling volume includes a component tank positioned on the high-capacity component to hold a liquid component working fluid around and/or in contact with a surface of the high-capacity component. The high-capacity component may heat or vaporize the liquid component working fluid and convert the liquid component working fluid to vapor component working fluid. The liquid component working fluid may be delivered to the component tank by a component liquid return line and the vapor component working fluid carries away heat from the high-capacity component through a component vapor return line. The component vapor return line directs the vapor component working fluid to a component condenser that condenses the vapor component working fluid back to a liquid component working fluid and rejects the heat to the ambient atmosphere. In at least one example, the working fluid in the immersion tank and the component working fluid in the component cooling loop are the same working fluid. Using the same working fluid may simplify maintenance by only requiring the stocking and maintenance of a single fluid and/or simplify repairs by ensuring that leaks of a first working fluid in one volume do not contaminate the second working fluid in the other volume. However, using a single working fluid results in a working fluid with a single boiling temperature, despite the components of the server computer (e.g., the heat-generating components and high-capacity components) operating at different temperatures. 
     Because the working fluid can carry away heat most efficiently by absorbing heat through the latent heat of boiling, the working fluid may work most efficiently when the boiling temperature of the working fluid is at or near the operating temperature of the components being cooled. In other examples, it may be desirable to prevent boiling, as the liquid working fluid may expand by 100 times or more upon vaporization into vapor working fluid. Unintended or undesirable vaporization may cause an increase in pressure that may damage the immersion cooling system. Rapid vaporization can cause a dryout effect when the surface of a heat-generating component vaporizes the liquid cooling fluid so rapidly that an insufficient mass of liquid working fluid contacts the surface of the heat-generating component, which compromises the convective cooling on the surface of the heat-generating component and allows the temperature thereof to increase too quickly. 
     Additionally, working fluid is recycled through the volumes of the thermal management system, and, in some embodiments, the working fluid is a dielectric fluid or other fluid that is expensive. A thermal management system that uses less working fluid and/or uses the working fluid more efficiently allows for cost savings in the working fluid. In some embodiments, the working fluid is relatively dense and containing large volumes of the working fluid requires a strong container. Building and maintaining containers for large volumes and/or masses of working fluid can increase construction costs and container weight, which limits transport and maintenance of the containers. 
     In a conventional immersion tank, the liquid pressure increases as depth of the immersion bath increases. In conventional tanks and fluids, a depth of 1 meter results in a 2.3 pounds per square inch (PSI) increase. The increased pressure results in an increase in the boiling point for the working fluid and a resulting temperature increase of the components adjacent the working fluid at the bottom of the immersion bath. When separate immersion chambers are placed around heat-generating components, and/or the boards are oriented horizontally, the columnar pressure of the fluid around the component is reduced and produces lower operating temperatures for the component. In at least one example, a working fluid exhibits a 4° C. decrease in temperature relative to a component at a depth of 1 meter in a conventional immersion tank. Defining separate volumes can allow for the customized control of boiling temperatures for different components of the server computer and/or for server computers at different locations inside an immersion tank or server rack. 
     As the working fluid carries away thermal energy through latent heat of boiling, managing the boiling temperature of the working fluid is beneficial for the performance of the immersion cooling system. In some embodiments, the liquid working fluid receives heat in a cooling volume of working fluid immediately surrounding the heat-generating components. The cooling volume is the region of the working fluid (including both liquid and vapor phases) that is immediately surrounding the heat-generating components and is responsible for the convective cooling of the heat-generating components. In some embodiments, the cooling volume is the volume of working fluid within 5 millimeters (mm) of the heat-generating components. 
     The working fluid has a boiling temperature below a critical temperature at which the heat-generating components experience thermal damage. For example, the heat-generating components may be computing components that experience damage above 100° Celsius (C). In some embodiments, the boiling temperature of the working fluid is less than a critical temperature of the heat-generating components. In some embodiments, the boiling temperature of the working fluid is less about 90° C. at 1 atmosphere of pressure. In some embodiments, the boiling temperature of the working fluid is less about 80° C. at 1 atmosphere of pressure. In some embodiments, the boiling temperature of the working fluid is less about 70° C. at 1 atmosphere of pressure. In some embodiments, the boiling temperature of the working fluid is less about 60° C. at 1 atmosphere of pressure. In some embodiments, the boiling temperature of the working fluid is at least about 35° C. at 1 atmosphere of pressure. In some embodiments, the working fluid includes water. 
     In some embodiments, the working fluid includes glycol. In some embodiments, the working fluid includes a combination of water and glycol. In some embodiments, the working fluid is an aqueous solution. In some embodiments, the working fluid is an electronic liquid, such as FC-72 available from 3M, or similar non-conductive fluids. In some embodiments, the heat-generating components, supports, or other elements of the immersion cooling system positioned in the working fluid have nucleation sites on a surface thereof that promote the nucleation of vapor bubbles of the working fluid at or below the boiling temperature of the working fluid. 
     Pumps, valves, inlet sizes, outlet sizes, and conduit diameters may all affect the fluid pressure inside a given volume in the immersion cooling system. For example, a flowrate of the tank pump may be increased to force liquid working fluid through the liquid return line and pressurize the immersion chamber of the immersion tank. In other examples, a vapor return line valve in the vapor return line may be opened or closed to alter the fluid pressure in the immersion chamber by allowing more or less vapor working fluid to exit the immersion tank. 
     In some embodiments, a fluid pressure in the on-component cooling loop may be changed by a component fluid pump in communication with the component condenser. For example, a flowrate of the component fluid pump may be increased to force liquid component working fluid through the component liquid return line and pressurize the liquid component working fluid of the component tank. Increasing the pressure of a working fluid increases the boiling temperature of the working fluid in that volume. Similarly, decreasing the pressure of the working fluid decreases the boiling temperature of the working fluid. By altering the pressure in a volume of working fluid, the boiling temperature can be customized to the heat of the components being cooled. 
     In some embodiments, a method of thermal management for server components includes immersing a first electronic component of the computing device in a first working fluid and immersing a second electronic component of the computing device in a second working fluid. 
     The method further includes changing a pressure of the first volume (e.g., the fluid pressure in the component tank) of the first working fluid in the first volume at. As described herein, the pressure can be changed through the use of pumps, valves, inlet and/or outlet sizes to control the flux of fluid in and/or out of a portion of the volume. In at least one example, a reservoir may store working fluid and a pump (e.g., the component fluid pump) may selectively introduce additional working fluid from the reservoir or remove working fluid to the reservoir to alter the pressure. 
     In some embodiments, the immersion cooling system includes an immersion tank with a server computer (or other electronic device) immersed in a liquid working fluid. In the example of a single-phase immersion cooling system, the liquid working fluid is not intended to boil. The liquid working fluid may absorb heat from heat-generating components and then may be circulated through a heat exchanger to cool the liquid working fluid before being returned to the immersion chamber. In some embodiments, there is some gas in the immersion chamber, while in other embodiments, the entire immersion chamber, heat exchanger and associated conduits are filled with liquid working fluid. Vaporization of the liquid working fluid may suddenly increase pressure and damage the immersion cooling system. 
     High-capacity components may be cooled by a second volume of working fluid that is configured to handle higher temperatures and/or pressures than the immersion tank. For example, the high-capacity components may have on-component cooling by a liquid working fluid that is pressurized to remain in a liquid phase above the boiling temperature at atmospheric pressure. For example, the liquid component working fluid may be the same liquid working fluid as in the immersion tank, but the high-capacity components may operate at a temperature that would boil the liquid working fluid in the immersion tank. By increasing the pressure in the volume around the high-capacity components, for example, with a component fluid pump and/or valves, the liquid component working fluid may have a higher boiling temperature. The on-component cooling loop may therefore also use a heat exchanger instead of a condenser. In some embodiments or applications, it may be preferable to use a heat exchanger rather than a condenser, for example, due to space constraints, power constraints, or geometry limitations. 
     As described herein, the working fluid used in immersion cooling systems may be expensive or the working fluid may have negative environmental effects that make highly efficient use of the working fluid desirable. For example, a two-phase cooling system may spray a liquid working fluid onto the heat generating components in droplets to absorb heat from the heat-generating components. The droplets may vaporize into vapor working fluid, carrying away heat from the heat-generating components. A condenser may subsequently condense the vapor working fluid to liquid working fluid, which may drip back down through a headspace of the tank to a reservoir of liquid working fluid at the bottom of the tank. 
     In some embodiments, a nozzle pump supplies liquid working fluid to spray nozzles. The spray nozzle sprays the liquid working fluid to create a plurality of droplets. In addition to the condenser condensing the vapor working fluid, the plurality of droplets has an increased surface area to cool and condense the vapor working fluid, and the droplets may provide a site onto which the vapor working fluid may condense, further lessening the energy required to condense the vapor working fluid. Similar to a cold plate or cold surface in a conventional condenser, the droplets may be the subcooled surface that allow condensation upon the droplets themselves. 
     In some embodiments, the spray nozzles are located on a sidewall of the immersion tank. In some embodiments, the spray nozzles are located on top wall or lid of the immersion tank. In some embodiments, the spray nozzles are located on both the sidewall and the top wall or lid of the immersion tank. In some embodiments, the spray nozzles are positioned in the immersion bath or reservoir of the liquid working fluid to spray the droplets upward into the headspace to deliver the droplets to the heat-generating components. 
     The spray nozzles may be adjustable to vary the size of the droplets depending upon desired flowrate through the nozzles and the desired droplet size. In some embodiments, the spray nozzles create a plurality of droplets with an average droplet diameter of less than 1 millimeter. In some embodiments, the spray nozzles create a plurality of droplets with an average droplet diameter of less than 0.5 millimeters. In some embodiments, the spray nozzles create a plurality of droplets with an average droplet diameter of less than 0.25 millimeters. In some embodiments, the spray nozzles create a plurality of droplets with an average droplet diameter of less than 0.1 millimeters. In some embodiments, the spray nozzle sprays the subcooled working fluid onto another component, such as a fan, which further disrupts the surface tension and creates the droplets and/or more droplets. 
     While sprayed droplets may efficiently cool the heat-generating components using a comparatively small mass of liquid working fluid, the overall cooling capacity of the sprayed droplets may be insufficient for the high-capacity components of the server computer. In some embodiments, the high-capacity component is surrounded by a component tank. The component tank contains a pressurized volume of the liquid working fluid that allows for a single-phase immersion cooling loop for the high-capacity components Immersing the high-capacity component in a component liquid working fluid may allow for greater convective cooling and higher thermal management capacity. A component heat exchanger may then reject the waste heat to the environment. 
     In some embodiments, the component working fluid and the liquid working fluid in the immersion tank at large are the same working fluid. The high-capacity components may be cooled by a pressurized volume of liquid working fluid to maintain the liquid working fluid in a liquid phase while the liquid working fluid in the rest of the immersion chamber and/or headspace may be allowed to vaporize. 
     The high-capacity components are cooled using forced convective liquid immersion cooling in the liquid working fluid, while the other heat-generating components are cooled using two-phase cooling through sprayed droplets. 
     In some embodiments, the droplets of liquid working fluid may efficiently cool the heat-generating components but have insufficient thermal management capacity to cool the high-capacity components. In some embodiments, the high-capacity components are cooled with shared liquid working fluid from the immersion chamber. The high-capacity components may be cooled using convective cooling from immersion in flowing liquid working fluid. In some embodiments, the liquid working fluid proximate the high-capacity component is pressurized above the immersion chamber at large to increase the boiling temperature of the liquid working fluid and maintain the liquid working fluid in the liquid phase. In some embodiments, the liquid working fluid proximate the high-capacity component is pressurized above the immersion chamber at large to increase the boiling temperature of the liquid working fluid and keep the boiling temperature within a threshold range of the temperature of the high-capacity component. 
     For example, a condenser located in the headspace of the immersion chamber may condense the vapor working fluid to liquid working fluid. The liquid working fluid may be directed or pumped through a component liquid return line toward the high-capacity component. The liquid working fluid in the component liquid return line may flow to and past the high-capacity component, absorbing heat from the high-capacity component. The liquid working fluid pumped into the component liquid return line may increase the pressure therein and proximate the high-capacity component as the liquid working fluid is restricted from exiting the fluid conduit (e.g., the volume proximate the high-capacity component) by an exit valve or small area outlet that limits flowrates therethrough. When the liquid working fluid exits through the exit valve, the hot liquid working fluid may vaporize in the immersion chamber and introduce vapor working fluid into the immersion chamber. If the high-capacity component does not heat the liquid working fluid beyond the boiling temperature at the fluid pressure in the immersion chamber at large, the liquid working fluid will remain in the liquid phase and fall to the reservoir below to be circulated through the spray nozzles. 
     In some embodiments, the volume proximate the high-capacity components may be pressurized to maintain the boiling temperature of the working fluid at or near the operating temperature of the high-capacity components. The immersion cooling system may cool the heat-generating components of the server computer with two-phase immersion cooling, while the high-capacity component(s) have on-component cooling with a different volume of working fluid. 
     As described herein, most efficient two-phase cooling occurs when the liquid working fluid absorbs heat from the component(s) and boils. The working fluid can absorb heat in the form of the latent heat of boiling to expand without increasing in temperature. The working fluid can, therefore, vaporize and the vapor working fluid can form bubbles which rise out of the liquid working fluid without the liquid working fluid around the component heating beyond the boiling temperature. However, if the component operating temperature is significantly above the boiling temperature, dryout can occur with insufficient thermal mass in contact with the surface of the component. In some embodiments, it may be beneficial to adjust the working fluid boiling temperature to be below the operating temperature, but within a threshold value of the operating temperature. 
     In some embodiments, the target boiling temperature is in a range including a lower value of less than 5° C. less than the operating temperature of the high-capacity component. In some embodiments, the target boiling temperature is in a range that includes a lower value of less than 3° C. less than the operating temperature of the high-capacity component. In some embodiments, the target boiling temperature is in a range that includes a lower value of less than 1° C. less than the operating temperature of the high-capacity component. For example, it may be beneficial to adjust the fluid pressure in the volume to change the boiling temperature to 1° C. less than the current operating temperature of the high-capacity component. 
     In some embodiments, the target boiling temperature is greater than the operating temperature of the high-capacity component to prevent the working fluid from boiling. In some embodiments, the target boiling temperature is in a range including an upper value of less than 5° C. greater than the operating temperature of the high-capacity component. In some embodiments, the target boiling temperature is in a range including an upper value of less than 3° C. greater than the operating temperature of the high-capacity component. In some embodiments, the target boiling temperature is in a range including an upper value of less than 1° C. greater than the operating temperature of the high-capacity component. 
     In some embodiments, an immersion cooling system includes a plurality of pressure control mechanisms, such as pumps, valves, or other adjustable portions of the on-component cooling loop that are in communication with a controller. The controller receives measurements from one or more sensors, such as a pressure sensor or temperature sensor, to determine whether and when to alter the fluid pressure proximate the high-capacity component. 
     In some embodiments, the sensor is a temperature sensor that measures an operating temperature of the high-capacity component. In other embodiments, the sensor measures an electrical power draw of the high-capacity component, which may be used to calculate an operating temperature of the high-capacity component. The controller can, based at least partially on the component operating temperature, determine a target boiling temperature for the working fluid proximate the high-capacity component. In some embodiments, the sensor includes a pressure sensor that measures the fluid pressure proximate the high-capacity component and reports the fluid pressure to the controller. 
     The controller may be in data communication with a component condenser, a fluid pump, and/or one or more valves to control relative flow rates through portions of the on-component cooling loop. For example, the controller may reduce flow through an outlet valve out of the portion of the volume proximate the high-capacity component to increase the fluid pressure proximate the high-capacity component. In some examples, the controller may reduce flow through an inlet valve into the portion of the volume proximate the high-capacity component to decrease the fluid pressure proximate the high-capacity component. In some examples, the controller may increase flow through the fluid pump toward the portion of the volume proximate the high-capacity component to increase the fluid pressure proximate the high-capacity component. In some examples, the controller may increase condensation rates of the component condenser to decrease the fluid pressure throughout the on-component cooling loop. In some embodiments, the controller may perform a combination of the above examples or other procedures to adjust the fluid pressure proximate the high-capacity component. For example, a reservoir may store working fluid and a pump (e.g., the component fluid pump) may selectively introduce additional working fluid from the reservoir or remove working fluid to the reservoir to alter the pressure. 
     While examples and embodiments described herein refer to a single computer in an immersion tank, it should be understood that any example or embodiment described herein may be applicable to a rack or a tank with a plurality of computing devices or blades positioned therein. In some embodiments, a plurality of server computers each have on-component cooling loops that connect to an inlet manifold and an outlet manifold of the rack. The first volume of working fluid is the on-component cooling loop that includes component tanks around the high-capacity components on the server computer of the blade. Fluid conduits connect the component tanks to the shared inlet manifold and the shared outlet manifold used by the plurality of server blades. The component vapor working fluid is circulated by a component fluid pump through a component heat exchanger or condenser to cool the component working fluid before returning the component liquid working fluid to the inlet manifold. 
     In some embodiments, the second volume of working fluid is the immersion chamber into which the server blades are positioned in the rack. The liquid working fluid is sprayed into the immersion chamber in droplets before heat-generating components of the server computers vaporize the liquid working fluid into vapor working fluid. The vapor working fluid is removed to an external heat exchanger or condenser. The condensed liquid working fluid is returned by a fluid pump. The first volume and second volume are maintained at different temperatures to adjust the boiling temperatures of the working fluids. 
     In some embodiments, each blade or server computer includes a self-contained on-component cooling loop with a working fluid therein. The on-component cooling loop includes a component condenser and, optionally, a component fluid pump, to cool the component working fluid and direct the component liquid working fluid toward the component tanks on the same server computer. The high-capacity components contained in the component tanks heat the working fluid and the component vapor working fluid is returned to the component condenser. 
     In some embodiments, the other heat-generating components of the server computers are cooled by the second volume of working fluid, which is maintained at a different pressure than the first volume in the on-component cooling loop of each server computer. In some embodiments, the second volume of working fluid is the immersion chamber into which the server blades are positioned in the rack. The liquid working fluid is sprayed into the immersion chamber in droplets before heat-generating components of the server computers vaporize the liquid working fluid into vapor working fluid. The vapor working fluid may be removed to an external heat exchanger or condenser or condensed in the immersion chamber and allowed to drip down toward the bottom of the immersion chamber. The first volume and second volume are maintained at different temperatures to adjust the boiling temperatures of the working fluids. In some embodiments, each self-contained on-component cooling loop may operate at fluid pressures independent of the fluid pressures of other self-contained on-component cooling loops on other server computers in the server rack. 
     The present disclosure relates to systems and methods for cooling heat-generating components of a computer or computing device according to at least the examples provided in the sections below: 
     [A1] In some embodiments, a method of thermal management of a computing device includes immersing a first electronic component of the computing device in a first working fluid contained in a first volume, immersing a second electronic component of the computing device in a second working fluid contained in a second volume, and changing a pressure in the first volume to alter a boiling temperature of the first working fluid in the first volume. 
     [A2] In some embodiments, changing the pressure in the first volume of [A1] includes changing a pump rate of a fluid pump. 
     [A3] In some embodiments, changing the pressure in the first volume of [A1] includes changing a flow rate through at least one valve. 
     [A4] In some embodiments, the method of [A1] further includes measuring a surface temperature of the first electronic component and changing the pressure in the first volume in response to the surface temperature. 
     [A5] In some embodiments, changing a pressure in the first volume to alter a boiling temperature of the first working fluid in the first volume of [A4] includes maintaining the boiling temperature within 5° C. of the surface temperature. 
     [A6] In some embodiments, changing a pressure in the first volume to alter a boiling temperature of the first working fluid in the first volume of [A4] includes maintaining the boiling temperature less than the surface temperature. 
     [A7] In some embodiments, the method of any of [A1] through [A6] further includes cooling the second electronic component with single phase immersion cooling. 
     [A8] In some embodiments, the method of any of [A1] through [A6] further includes cooling the second electronic component with two phase immersion cooling. 
     [B1] In some embodiments, a system for immersion cooling of a server computer includes an immersion tank, a server computer positioned in the immersion tank, a component tank positioned on a high-capacity component of the server computer, a first working fluid in the component tank, and a second working fluid in the immersion tank. The first working fluid contacts the high-capacity component in the component tank, and the second working fluid contacts a heat-generating component of the server computer outside of the component tank. 
     [B2] In some embodiments, the system of [B1] further comprises a component fluid pump configured to flow the first working fluid through the component tank. 
     [B3] In some embodiments, the component fluid pump of [B2] is configured to change the fluid pressure in the component tank. 
     [B4] In some embodiments, the system of [B1] further comprises an inlet valve on a component liquid return line, where the inlet valve adjusts a flowrate of the first working fluid to the component tank. 
     [B5] In some embodiments, the system of [B4] further comprises an outlet valve on a component vapor return line, the inlet valve configured to adjust a flowrate of the first working fluid from the component tank. 
     [B6] In some embodiments, the system of any of [B1] through [B5] further comprises a controller in data communication with one or more pressure control mechanisms configured to alter the fluid pressure in the component tank. 
     [B7] In some embodiments, the system of [B6] further includes a pressure sensor to measure a fluid pressure in the component tank, wherein the pressure sensor is in communication with the controller. 
     [B8] In some embodiments, the system of [B6] further includes a temperature sensor to measure an operating temperature of the component tank, wherein the temperature sensor is in communication with the controller. 
     [B9] In some embodiments, the controller of [B6] is configured to instruct the one or more pressure control mechanisms to change the fluid pressure in the component tank based at least partially upon a measurement from a sensor. 
     [B10] In some embodiments, the system of any of [B1] through [B9] includes at least one spray nozzle. The second working fluid is sprayed on the heat-generating component in droplets from the spray nozzle. 
     [C1] In some embodiments, a system for immersion cooling of a server computer includes an immersion tank, a first server computer positioned in the immersion tank, a second server computer positioned in the immersion tank, and a liquid working fluid in the immersion tank. The first server computer includes at least one first heat-generating component and at least one first high-capacity component, a first component tank positioned around the at least one first high-capacity component, and a first component working fluid positioned in the first component tank and contacting the at least one first high-capacity component. The first component working fluid has a first component fluid pressure. The second server computer includes at least one second heat-generating component and at least one second high-capacity component, a second component tank positioned around the at least one second high-capacity component, and a second component working fluid positioned in the second component tank and contacting the at least one second high-capacity component. The second component working fluid has a second component fluid pressure. The liquid working fluid positioned in the immersion tank contacts the first heat-generating components of the first server computer and the second heat-generating components of the second server computer. The liquid working fluid in the immersion tank has a fluid pressure different than the first component fluid pressure or the second component fluid pressure. 
     [C2] In some embodiments, the system of [C1] includes an inlet manifold and outlet manifold. The first component tank and second component tank are in fluid communication with the inlet manifold and outlet manifold. 
     The articles “a,” “an,” and “the” are intended to mean that there are one or more of the elements in the preceding descriptions. The terms “comprising,” “including,” and “having” are intended to be inclusive and mean that there may be additional elements other than the listed elements. Additionally, it should be understood that references to “one embodiment” or “an embodiment” of the present disclosure are not intended to be interpreted as excluding the existence of additional embodiments that also incorporate the recited features. For example, any element described in relation to an embodiment herein may be combinable with any element of any other embodiment described herein. Numbers, percentages, ratios, or other values stated herein are intended to include that value, and also other values that are “about” or “approximately” the stated value, as would be appreciated by one of ordinary skill in the art encompassed by embodiments of the present disclosure. A stated value should therefore be interpreted broadly enough to encompass values that are at least close enough to the stated value to perform a desired function or achieve a desired result. The stated values include at least the variation to be expected in a suitable manufacturing or production process, and may include values that are within 5%, within 1%, within 0.1%, or within 0.01% of a stated value. 
     A person having ordinary skill in the art should realize in view of the present disclosure that equivalent constructions do not depart from the spirit and scope of the present disclosure, and that various changes, substitutions, and alterations may be made to embodiments disclosed herein without departing from the spirit and scope of the present disclosure. Equivalent constructions, including functional “means-plus-function” clauses are intended to cover the structures described herein as performing the recited function, including both structural equivalents that operate in the same manner, and equivalent structures that provide the same function. It is the express intention of the applicant not to invoke means-plus-function or other functional claiming for any claim except for those in which the words ‘means for’ appear together with an associated function. Each addition, deletion, and modification to the embodiments that falls within the meaning and scope of the claims is to be embraced by the claims. 
     It should be understood that any directions or reference frames in the preceding description are merely relative directions or movements. For example, any references to “front” and “back” or “top” and “bottom” or “left” and “right” are merely descriptive of the relative position or movement of the related elements. 
     The present disclosure may be embodied in other specific forms without departing from its spirit or characteristics. The described embodiments are to be considered as illustrative and not restrictive. The scope of the disclosure is, therefore, indicated by the appended claims rather than by the foregoing description. Changes that come within the meaning and range of equivalency of the claims are to be embraced within their scope.