Abstract:
A heat exchanger is provided for transferring heat to a working fluid. The heat exchanger comprises a housing having a plurality of grooves formed in a surface of the housing. The grooves have a first end and a second end, and define fluid flow channels. Each channel has a fluid flow inlet and a fluid flow outlet. The fluid flow inlets of an alternating first set of channels are adjacent to the first end of the grooves, and the fluid flow inlets of a second set of alternating channels are adjacent to the second end of the grooves. The first set of channels and the second set of channels are arranged such that fluid in immediately adjacent channels flows in opposite directions.

Description:
BACKGROUND 
   This invention relates generally to heat exchangers, and more particularly to counter flow microchannel heat exchangers. 
   There are many industrial devices and processes wherein a component has to be maintained at a precise and uniform temperature. Examples of such devices and processes include optical devices and components, such as precision telescopes, solid-state lasers, and semiconductor laser diodes; wafer processing equipment in the semiconductor industry; and bio-processing containers in the pharmaceutical industry. 
   A suitable heat exchanger for these applications can be either of the microchannel type or the impingement type. Microchannel heat exchangers typically use unidirectional liquid coolant flow in a single layer of channels. While a microchannel heat exchanger is conducive to maintaining a very uniform temperature in a component in a direction perpendicular to the coolant flow, the lateral temperature parallel to the direction of coolant flow exhibits an increase as the liquid coolant receives heat. The temperature rise can be limited by increasing the coolant flow rate, but this results in a high pressure drop and poor coolant utilization. A 2-layer, 2-pass microchannel heat exchanger is described in U.S. Pat. No. 5,005,640, the contents of which are hereby incorporated by reference in their entirety. The 2-pass heat exchanger improves lateral temperature uniformity and coolant utilization. However, to achieve the second pass, the direction of coolant flow is reversed, which leads to a very high pressure drop. 
   Impingement type heat exchangers can provide uniform cooling, but exhibit very high pressure drop and poor coolant utilization. 
   For the foregoing reasons, there is a need for a microchannel heat exchanger which can provide substantially uniform cooling over a large area. The new microchannel heat exchanger should also handle high heat flux with a low pressure drop. 
   SUMMARY 
   According to the present invention, a heat exchanger is provided for transferring heat to a working fluid. The heat exchanger comprises a housing having a plurality of grooves formed in a surface of the housing. The grooves have a first end and a second end, and define fluid flow channels. Each channel has a fluid flow inlet and a fluid flow outlet. The fluid flow inlets of an alternating first set of channels are adjacent to the first end of the grooves, and the fluid flow inlets of a second set of alternating channels are adjacent to the second end of the grooves. The first set of channels and the second set of channels are arranged such that fluid in immediately adjacent channels flows in opposite directions. 
   Also according to the present invention, a system is provided for controlling the temperature of a heat source. The system comprises a heat generating component having a surface and a heat exchanger having a surface adapted for thermal communication with the surface of the heat generating component. The heat exchanger includes a housing having a plurality of grooves formed in a surface of the housing. The grooves have a first end and a second end, and define fluid flow channels. Each channel has a fluid flow inlet and a fluid flow outlet. The fluid flow inlets of an alternating first set of channels are adjacent to the first end of the grooves, and the fluid flow inlets of a second set of alternating channels are adjacent to the second end of the grooves. The first set of channels and the second set of channels are arranged such that a working fluid in immediately adjacent channels flows in opposite directions. 
   Further according to the present invention, a method is provided for controlling temperature of a heat source having a surface. The method comprises the steps of providing a heat exchanger having a surface adapted for thermal communication with a surface of the heat source. The heat exchanger includes a housing having a plurality of grooves formed in a surface of the housing. The grooves have a first end and a second end, and define fluid flow channels. Each channel has a fluid flow inlet and a fluid flow outlet. The fluid flow inlets of an alternating first set of channels are adjacent to the first end of the grooves, and the fluid flow inlets of a second set of alternating channels are adjacent to the second end of the grooves. The method further comprises the steps of providing a working fluid, and supplying the working fluid to the channels such that the working fluid in immediately adjacent channels flows in opposite directions for transferring heat from the heat source to the working fluid. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more complete understanding of the present invention, reference should now be had to the embodiments shown in the accompanying drawings and described below. In the drawings: 
       FIG. 1  is a perspective view of an embodiment of a microchannel heat exchanger according to the present invention. 
       FIG. 2  is a close up cross-section view of an upper peripheral portion of the heat exchanger of  FIG. 1  showing a supply manifold and a return manifold. 
       FIG. 3  is a close up perspective view of a portion of the upper surface of the heat exchanger of  FIG. 1  showing an open microchannel array. 
       FIG. 4  is a cross-section view taken along line  4 - 4  of  FIG. 1 . 
       FIG. 5  is a cross-section view taken along line  5 - 5  of  FIG. 1 . 
       FIG. 6  is a graph showing the temperature rise in a cooled component as a function of position downstream from the supply manifold in a prior art unidirectional flow microchannel heat exchanger. 
       FIG. 7  is a graph showing the temperature rise in a cooled component as a function of position downstream from the supply manifold in a counter-flow microchannel heat exchanger according to the present invention. 
   

   DESCRIPTION 
   As used herein, the term “microchannel” refers to a channel having a maximum depth of up to about 10 mm, a maximum width of up to about 2 mm, and any length. 
   Certain terminology is used herein for convenience only and is not to be taken as a limitation on the invention. For example, words such as “upper,” “lower,” “left,” “right,” “horizontal,” “vertical,” “upward,” and “downward” merely describe the configuration shown in the FIGs. Indeed, the components may be oriented in any direction and the terminology, therefore, should be understood as encompassing such variations unless specified otherwise. 
   Referring now to the drawings, wherein like reference numerals designate corresponding or similar elements throughout the several views, a counter flow microchannel heat exchanger according to the present invention is shown in  FIG. 1  and generally designated at  20 . The heat exchanger  20  comprises a housing  22  having a single layer of a plurality of parallel microchannels  24 . As will be described below, the heat exchanger  20  is designed such that a fluid coolant flows through adjacent alternating microchannels in opposite directions. This counter-flow configuration reduces the lateral temperature variation as compared to a unidirectional flow heat exchanger, while maintaining low pressure drop and high coolant utilization. 
   The housing  22  of the heat exchanger  20  comprises two separate portions, a base portion  26  and a surface portion  28 . The surface portion  28  of the housing  22  has a plurality of slots which define the microchannels  24 . The housing  22  shown in the FIGs. is generally cylindrical. A cylindrically-shaped housing  22  represents a compact design and minimizes coolant flow thereby reducing power requirements for a liquid coolant pump. However, it is understood that the housing  22  of the heat exchanger  20  can be any shape, including rectilinear. Opposed holes  30  are formed in the housing  22  of the heat exchanger  20  for receiving pins on the component to be cooled (not shown) in order to provide proper angular alignment of the housing  22  relative to the component. 
   The base portion  26  and the surface portion  28  of the heat exchanger  20  are preferably formed from single crystal silicon and bonded together to form an integral unit. The heat exchanger  20  may also be constructed of a material comprising a metal (e.g, aluminum, nickel, copper, stainless steel or other steel alloys), ceramics, glass, graphite, single crystal diamond, polycrystalline diamond, a polymer (e.g., a thermoset resin), or a combination thereof. These materials possess thermal conductivities that are sufficient to provide the necessary requirements for overall heat transfer coefficients. It is understood that the scope of the invention is not intended to be limited by the materials listed here, but may be carried out using any material which allows the construction and operation of the heat exchanger described herein. 
   The microchannels  24  are defined by the walls of the slots extending from the surface portion  28  of the housing  22 . The number of microchannels  24  may be any desired number, for example, two, three, four, five, six, eight, tens, hundreds, thousands, tens of thousands, hundreds of thousands, millions, etc. The microchannels  24  may have a cross-section having any shape, for example, a square, a rectangle or a circle. Each of the microchannels  24  may have an internal width ranging from about 50 μm up to about 2 mm. As shown in  FIG. 1 , the microchannel array  24  is circular, and the microchannels extend in parallel substantially across the surface portion  28  of the housing  22 . In this configuration, the depth of the microchannels  24  varies in order to match flow impedance and thus achieve the same heat transfer conditions in spite of the different microchannel lengths. Alternatively, the microchannel array  24  may be rectangular, square, polygonal, or any other suitable shape. The microchannels  24  can be straight or curved, and the depth of the microchannels can be constant or variable. 
   A suitable supply manifold  32  provides for the flow of the fluid coolant into the microchannels  24 . A suitable return manifold  34  provides for the coolant return. In the embodiment of the present invention shown in the FIGs., the supply manifold  32  and the return manifold  34  are each a pair of radially opposed crescent-shaped openings formed in the housing  22 . As seen in  FIGS. 1 and 2 , each of the supply manifold  32  openings penetrates the surface portion  28  of the housing  22  and extends nearly one half of the circumference of the housing  22 . The supply manifold  32  openings open onto the ends of the microchannels  24 . Each of the opposed supply manifold  32  openings communicates with alternate microchannels  24 , whereby one supply manifold  32  opening passes fluid coolant to alternating microchannels  24  extending in one direction, and the other supply manifold  32  passes fluid coolant to the adjacent alternating microchannels  24  extending in the other direction. As shown in  FIG. 3 , inlets  36  to the corresponding return manifold  34  are formed in the bottom of alternating slots at the opposite end of the microchannels  24  from the supply manifold  32 . 
   The microchannel heat exchanger  20  of the present invention can be used with either open channels or closed channels. In the open channel configuration, shown in  FIGS. 1-3 , the heat generating component (not shown) is positioned against the upper surface  28  of the housing  22  and is in direct contact with the fluid coolant. In the closed channel configuration, shown schematically in  FIGS. 4 and 5 , a wall  38  defines the upper surface of the heat exchanger  20 . The wall  38  seals in the fluid coolant by closing the top of the microchannels  24  and forms an outside surface of the heat exchanger  20 . The use of open microchannels versus closed microchannels depends upon the heat generating component to be cooled. While the wall  38  between the fluid coolant and the heat generating component can be made very small, heat transfer will nevertheless depend upon conduction through the boundary layers between the heat exchanger  20  and the heat generating component. If the contact heat transfer coefficients are low, heat exchange is inefficient. A much higher heat flux is possible with open channels because the component to be cooled is in direct contact with the fluid coolant. 
   A suitable fluid coolant for use according to the present invention is deionized water. It is understood that the coolant may be any fluid, gas or liquid, for use in a heat exchanger, and is not limited to water or other liquid coolants. Other suitable coolants include alcohol, liquid propane, antifreeze, gaseous or liquid nitrogen, freons, air, and mixtures thereof. Preferably, the coolant has low viscosity. 
   Operation of the heat exchanger  20  according to the present invention is shown in the schematic cross-sectional views of the housing  22  shown in  FIGS. 4 and 5 , which depict microchannels  24   a ,  24   b  having opposite fluid flow directions. The arrows denote the direction of fluid flow. Referring to  FIG. 4 , fluid coolant is pumped into the supply manifold  32  as indicated by arrow  40 . Fluid passes from the supply manifold  32  through the supply manifold opening from which the fluid coolant enters the microchannel  24   a . Fluid flows across the plane of the heat exchanger  20  via the microchannel  24   a  as indicated by arrow  42 . Fluid falls through the inlet opening  36  of the return manifold  34  at the end of the microchannel  24   a  and through the return manifold  34  as indicated by arrow  44 . The walls of the slots define a closed end ( 45 ) of the microchannels adjacent the inlet openings ( 36 ) of the return manifold ( 34 ) ( FIG. 3 ). 
   Referring to  FIG. 5 , fluid coolant is pumped into the supply manifold  32  as indicated by arrow  46 . Fluid passes from the supply manifold  32  through the supply manifold opening from which the fluid coolant enters the microchannel  24   b . Fluid flows across the plane of the heat exchanger  20  via the microchannel  24   b  as indicated by arrow  48 , which is in a direction opposite to the direction indicated by arrow  42 . Fluid falls through the inlet opening  36  of the return manifold  34  at the end of the microchannel  24   b  and through the return manifold  34  as indicated by arrow  50 . Although it is not shown, the supply manifold  32  and the return manifold  34  transition into a round cross-section and continue in a downward direction as seen in the FIGs. Once the fluid enters the return manifold  34 , the ΔP is low because the cross-section of the flow member is large. The fluid coolant then returns to the pump where the cycle starts again. 
   The heat exchanger  20  according to the present invention may be used with any heat generating component. The heat exchanger  20  is particularly suitable for use with optical components. In this application, the upper surface portion  28  of the heat exchanger  20  is formed to be optically flat. This feature allows the heat exchanger  20  to seal against an optically flat heat generating component upon contact, which is sufficient to provide a fluid tight seal. As seen in  FIG. 2 , an o-ring  52  may be provided in a circumferential groove in the surface portion  28  of the housing  22  to provide a fluid tight seal. A seal may also be accomplished for other applications by soldering or other means. 
   The counter-flow microchannel heat exchanger  20  according to the present invention has many advantages, including reducing the temperature variation provided by a unidirectional flow heat exchanger by a factor of about 5, while maintaining low pressure drop and low fluid coolant utilization. By flowing fluid coolant in opposite directions in adjacent microchannels, the increase in coolant temperature in a direction parallel to the coolant flow is minimized. The heat exchanger can also provide substantially uniform cooling over a large area, typically about 100 cm 2  to about 1000 cm 2 , and can handle high heat flux (10-1000 W/cm 2 ) with a low pressure drop. 
   Example 
   Table 1 lists parameters of an exemplary unidirectional microchannel heat exchanger and an exemplary counter-flow open microchannel heat exchanger according to the present invention. 
   
     
       
             
             
             
           
             
             
             
             
           
         
             
                 
               TABLE 1 
             
             
                 
                 
             
             
                 
               HEX10A 
               HEX10A 
             
             
                 
               Parallel 
               Counter 
             
             
                 
               flow 
               flow 
             
             
                 
                 
             
           
           
             
                 
             
           
        
         
             
                 
               Channel width [μm] 
               610 
               610 
             
             
                 
               Land width [μm] 
               406 
               406 
             
             
                 
               Channel depth [μm] 
               1525 
               1525 
             
             
                 
               Water film coef. [w/cm 2 - 
               3.3 
               3.3 
             
             
                 
               K] 
             
             
                 
               Contact film coef. 
               1.9 
               1.9 
             
             
                 
               [w/cm 2 -K] 
             
             
                 
               Channel water flow rate 
               5.5 
               5.5 
             
             
                 
               [gm/s] 
             
             
                 
               Channel water ΔT [° K] 
               3.35 
               3.35 
             
             
                 
               Channel ΔP [psid] 
               15 psid 
               15 psid 
             
             
                 
               Model ΔT(max) [K] 
               107.0 
               105.6 
             
             
                 
               ΔOPD [μm] due to water 
               0.22 (~1/5 λ) 
               0.022 (~1/48 λ) 
             
             
                 
               temperature rise 
             
             
                 
                 
             
           
        
       
     
   
   The results of a computer simulation of the two heat exchangers used to cool an optical component, a second surface mirror, are shown in  FIGS. 6 and 7 . The counter-flow open microchannel heat exchanger according to the present invention reduced the optical path difference (OPD) in the optical component from 0.22 um in the unidirectional microchannel heat exchanger to 0.022 um. 
   Although the present invention has been shown and described in considerable detail with respect to only a few exemplary embodiments thereof, it should be understood by those skilled in the art that I do not intend to limit the invention to the embodiments since various modifications, omissions and additions may be made to the disclosed embodiments without materially departing from the novel teachings and advantages of the invention, particularly in light of the foregoing teachings. Accordingly, I intend to cover all such modifications, omission, additions and equivalents as may be included within the spirit and scope of the invention as defined by the following claims. In the claims, means-plus-function clauses are intended to cover the structures described herein as performing the recited function and not only structural equivalents but also equivalent structures. Thus, although a nail and a screw may not be structural equivalents in that a nail employs a cylindrical surface to secure wooden parts together, whereas a screw employs a helical surface, in the environment of fastening wooden parts, a nail and a screw may be equivalent structures.