Abstract:
A cooling system and method that significantly improves spray evaporative cooling by using arrays of slot or plane sprays to create coverage of the entire heated surface to be cooled without allowing interaction between plumes that are spraying from the nozzles. The sprays are directed at an angle to the surface to take advantage of the high droplet momentum possessed by the spray to direct a flow of coolant fluid across the surface toward desired draining points, thereby enabling drainage regardless of the orientation of the unit.

Description:
[0001]     This invention was made with United States government support awarded by the following agencies: NSF 0134510. The United States has certain rights in this invention. 
     
    
     FIELD OF THE INVENTION  
       [0002]     This invention pertains generally to spray cooling or spray evaporative cooling systems and methods, and more particularly to such systems and methods as used to cool electronic circuitry in high-performance computers and similar systems.  
       BACKGROUND OF THE INVENTION  
       [0003]     Removal of heat from electronic circuitry has become one of the limiting factors in the design and performance of most computer systems and many other electronic devices. Power dissipation of a computer increases approximately as the operating frequency squared. Thus, doubling the clock speed, roughly doubling computer system performance, will require nearly four times the power dissipation. Without further innovation in the area of heat removal the development of a next generation computer design will be hampered.  
         [0004]     One of very few practical ways to remove heat generated in the processing modules of very high-speed high-performance computers (supercomputers) is by spraying a thin layer of liquid directly onto the computer chips. Heat is then transported from the surface by heating the flowing liquid and by boiling off some fraction of the liquid (two phase cooling). This method of heat removal is known as spray cooling, or spray evaporative cooling, and is a very efficient method of removing very high heat fluxes from small surfaces. Thus, spray evaporative cooling is growing in prominence and application in the cooling of electronics and laser diodes because of the need for high heat transfer in a small area in such applications.  
         [0005]     The physical mechanisms of spray evaporative cooling are not completely understood. However, current research based on single nozzle spray systems suggests that less than 50% of the heat is removed through nucleate boiling, similar to boiling a pot of water on a stove. More than 50% of the heat is removed either directly by heating the flowing liquid film or by evaporation at the surface of the film. It also has been shown that the heat flux increases both with the area of the heated surface directly covered by the spray and by the flux of spray droplets impinging on the surface.  
         [0006]     Current spray cooling systems and methods operate by spraying a dielectric fluid, e.g., Fluorinert 72 (FC-72), at a normal or other angle directly onto computer chips, from either above or below the chips, using a cone shaped spray. The objective is to create a thin film of cooling fluid on the surface to be cooled to remove heat through single phase and two phase convection. While such systems are adequate for most current computer systems and other electronic applications, the ability of such systems to remove heat has been pushed to its limits. For example, in such systems multiple nozzles often are used to direct multiple cone shaped cooling fluid sprays onto a surface to be cooled to increase the total spray droplet flux impinging on the surface. However, in such systems the plumes of fluid that are sprayed from the nozzles tend to interact to create pockets of relatively low cooling fluid momentum, where the fluid tends to build up and where boiling often occurs. During the cooling process bubbles are generated that cause greatly enhanced mixing so that the liquid surface temperature is raised to the point that a large amount of evaporation occurs there. Stagnation regions will stop the mechanical mixing, leading to bubble growth and dry out at the surface being cooled. Thus, the interaction of the sprays leads to inefficient cooling and liquid build-ups on the heated surface, creating regions of poor heat transfer and, therefore, non-uniform heat transfer across the surface of the chip. In the regions of poor heat transfer, the surface temperatures can rise significantly above the average surface temperature, causing the surface temperature to “run away”, leading to catastrophic failure.  
         [0007]     For optimal operation, it also is imperative that the coolant be removed from the spray region quickly and efficiently to prevent the buildup of warm liquid. Failures also can occur if the coolant liquid is not efficiently removed from the surface being cooled. Current spray cooling systems are limited in the ability to drain spray coolant after impacting a heated surface. In particular, in many such systems changing the orientation of the system even slightly will affect coolant drainage patterns, thereby adversely affecting the cooling ability of the system. This limits the application of current spray cooling systems in many applications where the system to be cooled is portable and thus subject to changes in orientation.  
         [0008]     Thus, current spray cooling systems suffer from various limitations including low critical heat flux (burnout) conditions, non-uniform heat transfer over large areas (area coverage limitations), and orientation constraints due to draining patterns.  
         [0009]     What is desired, therefore, is an improved evaporative spray cooling system and method in which a coolant is provided onto a surface to be cooled so as to maintain a uniform thin layer of coolant thereon, avoiding the creation of areas of interaction between coolant sprays from different spray nozzles that can result in coolant build up, poor heat transfer, and possible circuit failure. What also is desired is an improved evaporative spray cooling system and method in which the uniform thin layer of coolant and adequate coolant drainage are maintained despite variations in orientation of the system being cooled thereby.  
       SUMMARY OF THE INVENTION  
       [0010]     The present invention provides a spray cooling system and method that significantly improves spray evaporative cooling by creating a directed momentum flow of cooling fluid across a surface to be cooled. In accordance with the present invention, a spray of cooling fluid is directed directly onto the surface of a work piece to be cooled at an angle with respect to the work piece surface so as to create a flow of cooling fluid in a substantially single direction along the work piece surface. The spray of cooling fluid preferably may be delivered via a plurality of generally fan shaped sprays. The sprays are positioned and aligned to create cooling fluid coverage of the entire heated surface to be cooled without allowing interaction between the spray plumes in a manner that may cause areas of cooling fluid stagnation on the surface.  
         [0011]     A full coverage spray and drainage system in accordance with the present invention may be implemented in an otherwise conventional spray cooling system including a reservoir of an appropriate cooling fluid (e.g., Fluorinert-72 for the cooling of electronic circuitry, preferably saturated with a non-condensable inert gas, such as nitrogen), a pump for delivering the cooling fluid under pressure from the reservoir to a spray chamber to be sprayed therein from nozzles onto the work piece to be cooled, and appropriate filtering, metering, and control systems. Cooling fluid is returned from the spray chamber to the coolant reservoir via a drainage point or points in the spray chamber. In accordance with the present invention, the drainage point or points in the spray chamber may be positioned with respect to the coolant spray such that the flow of cooling fluid directed in a substantially single direction along the work piece surface also is directed toward the drainage point or points. Thus, the cooling fluid momentum directs the fluid toward the drainage point, thereby assuring proper drainage of the cooling fluid despite changes in the orientation of the cooling system.  
         [0012]     The spray nozzles used in accordance with the present invention preferably provide generally fan shaped sprays directed at an angle toward the work piece surface. In the longer dimension thereof, the fan shaped sprays preferably correspond to the width (in the direction perpendicular to the fluid flow) of the work piece surface to be cooled. Preferably a sufficient plurality of such fan shaped sprays are employed in a spaced apart relation along the length (in the direction of the fluid flow) of the work piece surface to be cooled such that a continuous flow of a thin film of cooling fluid in a substantially single direction is created across the work piece surface. Preferably the sprays are far enough apart so as not to interact with each other (in the direction of the fluid flow) either at the surface of the work piece or before contacting the work piece surface. Thus, full coverage of the work piece surface to be cooled by flowing cooling fluid is achieved, while the creation of stagnation areas resulting from interaction of the spray plumes is avoided.  
         [0013]     The spray nozzles to be used in accordance with the present invention may be formed as slots formed at an angle in the wall of a small tube. A small orifice is formed in the tube wall within of each such slot to connect the angled slot in fluid communication with the tube bore. When pressurized cooling fluid is provided to the tube bore, from either one or both ends of the tube, the fluid is expelled in a fan shaped spray pattern at an angle from the slots formed in the tube. An array may be formed of a plurality of such tubes, with such slot nozzles formed therein, positioned side by side such that the fan shaped sprays provided by the tubes in the array extend across the width of a chip or other surface to be cooled. The length of the tubes in the array may be selected to correspond to the length of the chip or other surface to be cooled. Cooling fluid may be provided under pressure to either or both ends of the tubes in the array via appropriate manifolds.  
         [0014]     In an alternative embodiment, the spray nozzles to be used in accordance with the present invention may be formed as a plurality of parallel slot shaped nozzles formed at an angle through a spray plate. The longer dimension of each such slot shaped nozzle preferably corresponds approximately to a width dimension of the surface to be cooled. Each such nozzle generates a substantially fan shaped spray of cooling fluid when cooling fluid under pressure is provided thereto. Alternatively, a spray nozzle to be used in accordance with the present invention may be formed using linear arrays of small nozzle orifices formed at an angle through a spray plate. Each such linear array may generate a fan shaped spray similar to that generated by a slot shaped nozzle. An appropriate dispersion or swirling structure preferably is provided adjacent to such slot shaped or linear array spray nozzles, to assure that a spray of cooling fluid droplets, rather than a continuous jet flow of cooling fluid, is developed from the nozzles.  
         [0015]     By maintaining a thin layer flow of coolant in a substantially single direction over an entire heated surface to be cooled, thereby preventing the build up of liquid anywhere on the surface, a full coverage spray and drainage system in accordance with the present invention will provide significantly enhanced spray cooling performance for the next generation of computers, as well as for a wide range of other applications, and will remove system orientation constraints. Further objects, features, and advantages of the present invention will be apparent from the following detailed description taken in consideration with the accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]      FIG. 1  is a schematic block diagram of an exemplary system for spraying a coolant fluid onto a work piece in which a full coverage spray and drainage system in accordance with the present invention may be implemented.  
         [0017]      FIG. 2  is a detailed schematic illustration of an exemplary array of slot shaped coolant spray nozzles as employed in an exemplary full coverage spray and drainage system in accordance with the present invention, showing the orientation thereof with respect to a work piece being cooled by substantially fan shaped sprays of coolant from the nozzles.  
         [0018]      FIG. 3  is a view of the substantially fan shaped spray generated by the nozzles illustrated in  FIG. 2 , as seen from the direction  3  of  FIG. 2 .  
         [0019]      FIG. 4  is a perspective view of an exemplary spray plate having slots formed therethrough at an angle to form nozzles for generating substantially fan shaped sprays of coolant fluid for use in accordance with the present invention.  
         [0020]      FIG. 5  is a plan view of an alternative exemplary embodiment of a spray plate having linear arrays of nozzle orifices formed therethough at an angle such that each linear array of nozzle orifices generates a substantially fan shaped spray of coolant fluid for use in accordance with the present invention.  
         [0021]      FIG. 6  is a perspective view of the exemplary spray plate of  FIG. 4  as mounted on a spray plate holder.  
         [0022]      FIG. 7  is a perspective view of the exemplary spray plate and holder of  FIG. 6  with a cover piece attached thereto, the cover piece having an orifice formed therein for providing cooling fluid under pressure to the spray plate.  
         [0023]      FIG. 8  is a schematic cross section view of an exemplary structure for dispersing or swirling pressurized cooling fluid before it is provided to a spray nozzle to assure that a spray of cooling fluid droplets, rather than a jet stream, is developed from the nozzle.  
         [0024]      FIG. 9  is a detailed schematic illustration, in side view and enlarged partial cross section, of an exemplary and preferred structure for generating fan shaped sprays of cooling fluid from angled slots formed in a tube, as may be employed to cool a work piece in a full coverage spray and drainage system in accordance with the present invention.  
         [0025]      FIG. 10  is a plan view of an array of tubes with angled slots formed therein as illustrated in  FIG. 9  as may be used to generate fan shaped sprays of cooling fluid to provide full coverage across of a surface to be cooled in a full coverage spray and drainage system in accordance with the present invention, including manifolds for providing cooling fluid under pressure to the array of tubes.  
         [0026]      FIG. 11  is a cross-section illustration of another exemplary structure that may be used to form fan shaped sprays of cooling fluid to provide full coverage across a surface to be cooled in a full coverage spray and drainage system in accordance with the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0027]     The present invention will be described in detail herein with reference to the use thereof for the cooling of electronic circuits and electronic devices, particularly microelectronic circuitry as employed in computer systems, such as high-speed, high-performance computer systems (supercomputers). However, it should be understood that the present invention also may be applied in other applications, such as in the biomedical industry and for the cooling of laser arrays and high powered weaponry, etc. Spray evaporative cooling in accordance with the present invention may be applied to cryogenic devices, as an aid to laser-assisted surgical procedures, etc. It should also be understood that various cooling fluids other than those described herein, and combinations thereof, may be used in a full coverage spray and drainage system in accordance with the present invention.  
         [0028]     A full coverage spray and drainage system and method in accordance with the present invention may be implemented in an otherwise conventional system  10  for providing sprayed coolant onto a work piece to be cooled. An exemplary system  10  of this type is illustrated schematically in  FIG. 1 , and will be described in detail with reference thereto. The work piece to be cooled may, for example, be a microelectronic circuit chip in a supercomputer, or any other electronic circuit or device, such as a diode laser package, or other work piece or surface for which the improved heat flux removal provided by a system and method in accordance with the present invention is desired or required.  
         [0029]     The liquid coolant to be employed is stored in a coolant reservoir  12 . An appropriate conventional reservoir  12  may be selected for the particular coolant to be employed. An exemplary coolant fluid that may be employed for cooling electronic circuitry is Fluorinert-72 (FC-72), commonly available from 3M Corporation, although other similar or different dielectric or other coolant fluids, or combinations thereof, may be employed, depending upon the particular application. The coolant fluid preferably is degassed and filtered before being placed in the reservoir  12  of the fully evacuated system  10 . The system  10  preferably may be pressurized, e.g., to 1 atmosphere, with nitrogen gas, which causes partial fluid saturation. It has been found that using nitrogen-saturated FC-72 enhances cooling performance. (Nitrogen is remarkably soluble in FC-72.)  
         [0030]     An appropriate pump  14  is employed to pump the cooling fluid through the system  10 . For example, for FC-72,a magnetically coupled gear pump or a similar pump may be employed.  
         [0031]     Appropriate filters and absorbers preferably are employed to remove any particles  16  or foreign gases  18 , including water vapor, from the coolant fluid. An absorber  20  preferably also is employed to remove perfluoroisobutylene (PFIB) from the coolant fluid. PFIB is a harmful compound that can result from Fluorinert breakdown at extremely high heat levels.  
         [0032]     The coolant fluid flow preferably is passed through a conventional appropriate flow meter  22  that measures, e.g., the volumetric flow rate of the coolant. Rate of flow data from the flow meter  22  may be provided in a conventional manner to a system controller  24 . Additional system operation information that preferably may be provided to the controller  24  may include system pressure and coolant fluid temperature. System pressure information may be provided to the controller  24  via a conventional and appropriate pressure sensor  26  mounted in the system, e.g., in the coolant fluid reservoir  12 . Coolant fluid temperature information may be provided by one or more conventional and appropriate temperature sensors  28 . Preferably at least one temperature sensor  28  is positioned to measure the temperature of the coolant fluid flow just before the coolant fluid enters the spray chamber  30 , wherein the coolant fluid is sprayed onto the work piece to be cooled. For example, as illustrated in  FIG. 1 , a temperature sensor  28  may be implemented as part of, or separate from, the coolant fluid flow meter  22 .  
         [0033]     Based on coolant fluid flow rate, pressure, and temperature feedback information, as well as any other or different feedback information as may be provided, such as the measured temperature of the work piece to be cooled, the controller  24  may control operation of the pump  14  to provide coolant fluid at the desired pressure from the reservoir  12  to the spray chamber  30  to be sprayed on a work piece in a manner in accordance with the present invention to cool the work piece therein. The controller  24  may be implemented in a conventional manner using conventional digital and/or analog circuitry to control the providing of coolant to the spray chamber  30  to provide the desired cooling for a work piece therein. Note that for application in a computer or other similar system, the controller  24  may be implemented, at least in part, in microelectronic circuitry located in the spray chamber  30  that is cooled by the system  10  under control of the controller  24 .  
         [0034]     The spray chamber  30  designates the portion of the system  10  wherein the coolant fluid is sprayed onto a work piece to be cooled. For computer system applications, for example, the spray chamber  30  is a section of the computer system containing electronic circuitry to be cooled by the coolant fluid. However, the spray chamber  30  may be any closed or open area containing a work piece to be cooled by a coolant fluid in accordance with the present invention.  
         [0035]     Coolant fluid sprayed in the spray chamber  30  is drained therefrom and returned to the coolant reservoir  12  via a heat exchanger  32 . Any conventional and appropriate heat exchanger  32  device, system, or structure may be employed for this purpose.  
         [0036]     An exemplary method and structure in accordance with the present invention for spraying coolant fluid onto a work piece to be cooled to achieve improved removal of high heat flux therefrom will be described now in detail with reference to  FIGS. 2 and 3 . The method and structure to be described may take place and be positioned in the spray chamber  30  portion of the exemplary system  10  described above with reference to  FIG. 1 . The coolant fluid illustrated in, and described with reference to,  FIGS. 2 and 3 , thus may be provided to the structure described with reference to  FIGS. 2 and 3  by the system  10  and method described above with reference to  FIG. 1 .  
         [0037]     A work piece  40  to be cooled may be, for example, an integrated circuit chip or other electronic or non-electronic device or structure. The work piece  40 , as illustrated, is relatively thin in thickness. To cool the work piece  40 , e.g., during operation thereof, a coolant fluid  42  is sprayed on at least one of the relatively flat and broad larger substantially two-dimensional surfaces of the work piece  40 . For purposes of this example, the broad surface of the work piece  40  onto which the coolant fluid  42  is sprayed will be referred to as the top surface  44  thereof. Note that for an integrated circuit chip work piece  40  the surface  44  thereof to be cooled may be a surface of the chip itself, a surface of packaging containing the chip, or the surface of a heat spreader or other structure mounted on the chip or chip package. The top surface  44  of the work piece  40  has two elongated dimensions which will be referred to, for example, as the length of the work piece (extending along the page in  FIG. 2 ) and the width of the work piece (extending into the page in  FIG. 2 ). Of course these terms are interchangeable, and it should be understood that the term “width” as used herein, including in the claims, may refer to any dimension of an integrated circuit chip or other work piece  40  or surface or area to be cooled in accordance with the present invention.  
         [0038]     The work piece  40  may be mounted for operation in a spray chamber  30  in a conventional manner. It should be understood, for example that many integrated circuit chips and other electronic devices may be mounted in a spray chamber  30  for operation. Each such chip or other device or work piece may be cooled by a method and system in accordance with the present invention as described herein.  
         [0039]     Within the spray chamber  30  there is provided at least one drain point  46 , illustrated schematically in  FIG. 2 . The work piece  40 , and particularly the top surface  44  thereof to be cooled, is in a certain position with respect to the drain point  46 . As will be discussed in more detail below, in accordance with the present invention, coolant fluid  42  is sprayed onto the work piece  40  to cool the work piece  40  such that the coolant fluid flows across the top surface  44  of the work piece  40  in a substantially single direction  47 . The drain point  46  may, therefore, preferably be positioned with respect to the work piece  40  such that the flow of the cooling fluid  42  from the work piece surface  44  is directed  47  toward the drain point  46  to be returned therefrom from the spray chamber  30  to the coolant fluid reservoir  12 . By using the directed momentum flow of the cooling fluid  42  to direct the cooling fluid  42  toward the drain point  46 , proper drainage of the cooling fluid  42  can be achieved despite changes in orientation of the cooling system  10 .  
         [0040]     In accordance with the present invention, a coolant fluid  42  is sprayed directly onto the surface  44  of the work piece  40  to be cooled to create a flow of cooling fluid  42  in a substantially single direction  47  along the work piece surface  44 . This directed momentum flow  47  of cooling fluid  42  across the surface  44  of the work piece  40  is achieved by spraying the cooling fluid  42  onto the surface  44  at an angle with respect thereto via one or more spray nozzles  48 .  
         [0041]     In accordance with the present invention, the spray nozzles  48  are positioned to spray the coolant fluid  42  directly onto the surface  44  of the work piece  40  to be cooled, rather than transversely across or indirectly onto the surface  44  of the work piece  40 . The nozzle openings preferably are positioned with respect to the surface  44  of the work piece  40  such that lines extending perpendicularly from the surface  44  of the work-piece  40  will intersect the nozzle openings. In other words, the nozzles  48  preferably are located directly “above” the work piece  40  to be cooled. (Although it should be understood that the combination of the work piece  40  and nozzles  48  may be oriented together in space vertically side by side or in some other orientation.) The distance between the spray nozzle  48  openings and the surface  44  of the work piece  40  to be cooled preferably may be in the range of 3-5 mm, although other distances therebetween also may be employed in accordance with the present invention.  
         [0042]     In accordance with the present invention, the spray nozzles  48  preferably generate a substantially fan shaped spray  42  of cooling fluid. That is, as illustrated in  FIGS. 2 and 3 , a spray shape that is significantly longer in a first direction than in a second, perpendicular, direction. This is in contrast to the generally cone shaped spray provided in conventional spray cooling systems. For example, such a substantially fan shaped spray may be generated using slot shaped nozzles  48 . Preferably, the slot shaped spray nozzles  48  may be, in their longer dimension, approximately as wide as the chip or other work piece  40  to be cooled, such that the fan of coolant spray  42  provided by the nozzle  48  extends across the surface  44  of the chip in one direction. (Full coverage across the width of the work piece surface  44  may be achieved using a smaller slot providing a greater fan-out of spray in the longer dimension, although this is not preferred.) Thus, for example, a slot shaped spray nozzle  48  having a nozzle orifice width in the shorter dimension of significantly less than 100 microns, and preferably about 4 microns, and a nozzle orifice width in the other dimension of 15-25 mm may be used to spray coolant onto the surface of a conventional integrated circuit chip that is 15-25 mm across, such that the spray provided thereby extends across the surface  44  of the chip  40 . (A very narrow orifice width preferably is used both to limit the spread of the spray in the dimension along the length of the work piece  40  being cooled as well as to keep the total orifice size (width times length) on the same order of magnitude as that of conventional cone shaped spray nozzles with circular orifices typically 150-200 microns in diameter. This allows coolant fluid pressures and flow rates per nozzle similar to those used in conventional spray cooling systems to be used in a spray cooling system and method in accordance with the present invention.)  
         [0043]     Preferably a plurality of spray nozzles  48  for cooling a given work piece  40  are aligned along a single axis extending along the length of the work piece  40 . The spray nozzles  48  preferably are aligned such that the longer dimensions of the nozzle orifices are perpendicular to this axis, and are spaced apart from each other along this axis sufficiently such that the spray of coolant fluid  42  provided by each of the spray nozzles  48  doesn&#39;t interfere with the spray provided by any of the other adjacent spray nozzles  48 . In other words, as illustrated in  FIG. 2 , the substantially fan shaped coolant sprays  42  from adjacent nozzles  48  preferably do not cross or intersect either at the surface  44  to be cooled or before the sprays  42  contact the surface  44 . By preventing interaction of the cooling fluid sprays  42 , areas of fluid flow stagnation on the surface  44  of the work piece  40  being cooled are avoided.  
         [0044]     In accordance with the present invention, the spray nozzles  48  are oriented so as to direct the cooling fluid  42  directly onto the surface  44  of the work piece  40  to be cooled at an angle with respect thereto. For example, an angle of between approximately 30 and 60 degrees with respect to the surface  44  of the work piece  40 , and preferably an angle of approximately 45 degrees, may be employed. All of the nozzles  48  directing fluid at a single work piece  40  preferably are oriented at the same angle with respect thereto, to prevent crossing or other interaction of the sprays  42  provided thereby. Varying angles may be used, however, in some cases, to control the rate of flow of coolant fluid across the surface  44  of the work piece  40 .  
         [0045]     Sufficient nozzles  48 , each configured and oriented as describe above, preferably are provided for each chip or other work piece  40  to be cooled, such that the combined spray of coolant  42  on the surface  44  of the work piece  40  maintains a full coverage thin film  50  of cooling fluid  42  on the surface  44 . (This is achieved even though the entire surface  44  to be cooled is not covered by the areas of the sprays  42  themselves.) Depending on the coolant fluid flow rate, heat flux to be removed, etc., one nozzle every 5 mm or so along the chip or other work piece  40  may be sufficient. For example, for cooling a typical integrated circuit chip, a flow rate of approximately 1 ml/second for each nozzle may be employed. Thus, the total flow rate across the chip or other work piece  40  to be cooled is the flow rate per nozzle multiplied by the number of nozzles employed.  
         [0046]     Coolant fluid  42  sprayed onto the surface  44  of a work piece  40  in a manner in accordance with the present invention as just described creates a thin film  50  of coolant fluid thereon that flows continuously in a substantially single direction  47  across the surface  44 , to cool the work piece  40  by the mechanisms and in the manner discussed above. Such a system and method for spray cooling in accordance with the present invention has several benefits over the round sprays used in conventional spray cooling systems. Since the angled fan-shaped sprays do not overlap, they create a directed uniform flow of fluid across the surface  44  and there are no stagnation points. Multiple sprays along the length of the surface  44  being cooled help break up growing bubbles, suppressing the occurrence of critical heat flux. The angled sprays provide momentum to the fluid flow in a preferred substantially single direction  47 . This does two things. First, it keeps the film  50  thinner and more uniform than it would be from a perpendicularly directed spray. Second, it allows the designer to direct the fluid for better drainage.  
         [0047]     By directing cooling fluid flow in a substantially single direction across work pieces to be cooled, drainage patterns can be designed with some freedom. For instance, a drain trough could be placed around the perimeter of a spray chamber with the sprays directed at multiple work pieces being cooled in the chamber all directed outward. Alternatively, all of the sprays could be directed in one direction across several work pieces in a spray chamber so that the fluid drains to one side. (This results in the added complication that fluid from one side will interact with the other side, leading to non-uniformities in temperatures, which may be desirable in some situations.) With more even fluid momentum coverage, more even temperature distribution on the work pieces being cooled may be achieved. Since cooling fluid momentum and drainage are more controlled, a spray cooling system in accordance with the present invention will be less sensitive to varying orientations.  
         [0048]     A spray nozzle for use in accordance with the present invention may be formed from a spray plate  52 , as illustrated in  FIG. 4 . The spray plate  52  is made of a thin piece of metal or similar material that has a plurality of slots  54  formed therethrough at an angle to form nozzles for generating substantially fan shaped sprays as described above. The nozzle slots  54  may be formed through the spray plate  52  in a conventional manner, such as by etching, cutting, etc.  
         [0049]     Alternatively, a spray nozzle for generating a substantially fan shaped spray for use in accordance with the present invention may be formed using a linear array of small nozzle orifices. For example, as illustrated in  FIG. 5 , a spray plate  56  may have formed therethrough a plurality of such linear arrays  58  of small nozzle orifices  60 . The nozzle orifices  60  preferably may be generally circular, or have another appropriate shape. The nozzle orifices  60  are formed at an appropriate angle through the spray plate  56 . Each linear array  58  of nozzle orifices  60  generates a substantially fan shaped spray similar to that generated by the slot shaped nozzles described above.  
         [0050]     For use in a spray cooling system in accordance with the present invention, the spray plate  52  (or  56 ), having the slot shaped  54  (or linear array  58 ) nozzles formed therein, may be mounted on a spray plate holder  62 , as illustrated in  FIG. 6 . The spray plate holder  62  is adapted to hold the spray plate  52  in a conventional manner such that the spray plate nozzles are aligned with an aperture or window (not shown in  FIG. 6 ) in the holder  62  that is positioned adjacent to the work piece to be cooled. A cover plate  64  is mounted on the spray plate holder  62  over the spray plate  52  to form a chamber (not shown) between the spray plate  52  and the cover plate  64 . The cover plate  64  has a cooling fluid inlet port  66  formed therein through which cooling fluid under pressure is admitted into the chamber between the spray plate  52  and the cover plate  64 . This pressurized fluid is ejected through the spray plate nozzles in the desired substantially fan shape as described herein.  
         [0051]     In order to achieve the desired cooling effect, it is important that the cooling fluid spray emitted from the spray nozzles be a spray of cooling fluid droplets, rather than a continuous jet flow of cooling fluid. To ensure the creation of a spray of droplets, the pressurized cooling fluid should be dispersed or swirled before or after being emitted through the spray nozzles. Conventional dispersing or swirling structures, such as a screen or multiple slots, may be used for this purpose. For example, a screen may be used to disperse the cooling fluid either immediately after it is ejected from a spray nozzle or immediately before being provided to the spray nozzle. An exemplary structure  68  for dispersing or swirling cooling fluid is illustrated in cross section in  FIG. 8 . In this structure  68 , cooling fluid  70  is directed under pressure against a curved surface  72  that disperses and swirls the cooling fluid  70  before it is directed through an aperture  74  to the spray nozzles to be sprayed therefrom. Note that any appropriate structure may be employed to ensure that the spray nozzles used in a spray cooling system in accordance with the present invention provides a spray of cooling fluid droplets, rather than a continuous jet stream.  
         [0052]     An alternative and currently preferred spray nozzle for use in accordance with the present invention may be formed from a small tube  80 . For example, as illustrated in  FIG. 9 , conventional metal microbore tubing  80  may be used for this purpose. Such tubing  80  may, for example, typically have an outer diameter on the order of 1/16 inch, with a tube wall thickness of approximately 1/64 inch. (It should be understood that microbore or other tubing having different dimensions also may be used.) In accordance with the present invention, one or more spray nozzles are formed along the length of the tube  80  by cutting (e.g., using a slitting saw), stamping, or otherwise forming one or more slots  82  at an angle into the wall  83  of the tube. (The method used for forming the slots  82  in the tube  80  preferably may form a relatively rough slot well. The resulting rough surfaces in and around the slot well  82  help to increase the production of spray droplets  90  forming the cooling fluid spray  84  formed by the slots  82 .) The slots  82  preferably are formed at a desired angle into the wall  83  of the tube  80  to correspond to the desired angle at which a spray  84  of cooling fluid to be generated from the nozzles formed thereby is to be directed at the surface of a work piece  86  to be cooled. Thus, the slots  82  preferably may be formed at an angle of between 30° and 60° into the wall  83  of the tube  80 . The slots  82  may be formed at a depth through the wall  83  of the tube  80  such that the slots  82  are in fluid communication with the tube bore  88 . Alternatively, and preferably, the slots  82  may be formed partially through the tube wall  83  and a small hole or other aperture drilled, punched, or otherwise formed in the slot  82  to connect the slot  82  in fluid communication with the tube bore  88 . The width of the slot  82  is selected such that, when pressurized cooling fluid is provided to the tube bore  88 , from either or both ends of the tube  80 , a fan shaped spray  84  of cooling fluid is directed from the slot  82  formed in the tube  80  at the desired angle. For example, a slot width of approximately 150 micrometers may be used. A plurality of slots  82  preferably are formed spaced apart along the length of the tube  80  such that the sprays  84  of cooling fluid formed by the slots  82  do not interact before contacting, or at the surface of, the work piece  86  being cooled.  
         [0053]     As illustrated in  FIG. 10 , a plurality of nozzle tubes  80  as described above with respect to  FIG. 9  may be formed in an array  92  and used to provide full coverage spray cooling of a work piece in accordance with the present invention. For example, the size of the array  92  of tubes  80  may correspond approximately to the size of the work piece surface to be cooled. Thus, the lengths of the tubes  80  in the array  92  may be selected to correspond to the length of a work piece surface to be cooled, with sufficient tubes  80  placed side-by-side in the array such that the width of the array  92  corresponds to the width of a work piece surface to be cooled. The tubes  80  forming the array  92  are positioned such that the slots  82  formed in adjacent tubes  80  are aligned across the width of the array  92 . Thus, adjacent slots  82  in adjacent tubes  80  across the width of the array  92  form a linear array of nozzle orifices that may be used to produce, in combination, a fan shaped spray of cooling fluid that extends across the width of a work piece surface to be cooled. As discussed above, the slots  82  preferably are formed spaced apart along the length of each tube  80  in the array  92  such that arrays of adjacent slots  82  in adjacent tubes  80  across the width of the tube array  92  are separated from each other by sufficient distance such that the fan shaped sprays of cooling fluid generated thereby do not interact before contacting, or at the surface of, the work piece being cooled. (E.g., the rows of adjacent slots  82  formed in the tubes  80  may be separated by approximately 5mm.)  
         [0054]     Pressurized cooling fluid may be provided to the array  92  of nozzle tubes  80 , at either or both ends thereof, via one or more manifold structures  94 . For example, the manifold structures  94  may be formed of an appropriate metal or plastic material with a slot  96  formed therein for receiving the ends of the tubes  80  forming the array  92 . (For example, for an array  92  formed using microbore tubes  80  having an outer diameter of 1/16 inch, an approximately 5/64 inch wide slot  96  may be formed in the manifold  94 , e.g., using a ball-end mill, to a depth of approximately 4mm, to support the tube array  92  therein.) An appropriate epoxy  98  may be used to seal the ends of the tubes  80  in the slot  96  formed in the manifold  94 . Pressurized cooling fluid thus may be provided to either or both ends of the array  92  of nozzle tubes  80  via the manifolds  94  and the slots  96  formed therein in which the ends of the tubes  80  are mounted and sealed.  
         [0055]     The tubes  80  forming the array  92  of tubes  80  may be positioned closely adjacent to each other, as illustrated in  FIG. 10 . Alternatively, the tubes  80 , or some of the tubes  80 , in the array  92  may be spaced slightly apart (but preferably not so far apart such that a continuous fan shaped spray across the width of the work piece being cooled by the array  92  is not provided at the surface of the work piece). The resulting gap between adjacent tubes  80  may be used conveniently to vent cooling fluid vapor away from the work piece being cooled. Vapor vent tubes, with vent apertures formed therein, may be positioned in the gaps between nozzle tubes  80  in the array  92  to provide this vapor venting function.  
         [0056]     An array of spray nozzles for use in accordance with the present invention alternatively may be formed from an elongated stamped, extruded, or otherwise formed structure  100  have a cross-section as illustrated, generally, for example, in  FIG. 11 . This structure  100  may be formed with a central bore  102 , to which pressurized cooling fluid may be provided. At least one side of the structure  100  is formed to have a series of elongated raised adjacent semi-circular surfaces  104 . (Thus, when viewed from above (in  FIG. 11 ) the structure  100  appears similar to the tube array  92  illustrated in  FIG. 10 .) Slots  106  are cut across this surface  104  of the structure  100  at a desired angle and to an appropriate depth to form spaced apart rows of spray nozzles in fluid communication with the structure bore  102  along the length of the structure  100 . Thus, the structure  100 , when formed in this manner, may be used to generate fan shaped sprays of cooling fluid in a manner similar to that of the tube array  92  structure of  FIG. 10 . The size of the structure  100  preferably may be selected to correspond to the size of the work piece surface to be cooled, to provide full coverage spray cooling of the work piece surface in accordance with the present invention.  
         [0057]     It should be noted that the structures illustrated in  FIGS. 4-11  are appropriate for cooling a single integrated circuit chip or other work piece by spray cooling in accordance with the present invention. It should be understood that multiple spray nozzles for use in accordance with the present invention may be produced in a conventional manner as a laminated structure wherein the various structures described (nozzle openings, dispersion/swirl chamber, fluid input port, etc.) are formed in separate lamination layers that are joined together to form a nozzle structure for spraying multiple work pieces simultaneously.  
         [0058]     It should be understood that the present invention is not limited to the particular exemplary applications and embodiments as illustrated and described herein, but embraces all forms thereof as come within the scope of the following claims.