Abstract:
An evaporative compact high intensity cooler (ECHIC) for transferring heat from a heat source along a heat conduction surface of the heat source with a two-phase coolant, comprises a flow passage labyrinth of flow passages with short conduction paths interrupted by coolant columns that all radiate from at least one coolant supply passage and offer the coolant expanding volume as the coolant evaporates due to absorbing heat within the flow passages from the heat conduction surface to maintain nearly isobaric conditions for the coolant to maintain relatively constant temperature throughout the ECHIC as it absorbs heat from the heat source and limit boundary layer formation within the flow passages to improve heat transfer.

Description:
FIELD OF THE INVENTION 
       [0001]    The invention relates to heat sinks and coolers, and more particularly to heat sinks and coolers for limited space applications that require a high degree of heat transfer. 
       BACKGROUND OF THE INVENTION 
       [0002]    Very high heat dissipations occur in high power electronic devices such as high-energy laser and the high power microwave devices. Surface heat fluxes for laser diodes are approximately 100 to 500 w/cm 2 . Microwave interaction and collector cavity heat fluxes can reach 1000 to 2000 w/cm 2  respectively. High power electronics such as converters, inverters and motor drives typically have devices that generate heat fluxes of 10 to 40 w/cm 2  at the device level and even higher at the die level. High power CPU packages for high performance computers will dissipate as much as 100 w/cm 2  over a footprint of a few square centimetres. All of these devices must operate within acceptable temperature ranges regardless of their heat dissipation. Such devices also demand good surface isothermality for optimum device performance. Where the ultimate heat sink is at a temperature level that is too high for direct or cascaded loop cooling, rejection of device waste heat to the heat sink requires thermal pumping to a higher temperature level. An efficient refrigeration system is a vapour cycle wherein an evaporator absorbs heat at a lower temperature and a loop condenser rejects it at a higher temperature. Vapour cycle systems offer two advantages in thermal control at the evaporator. First, pressure level may control temperature due to the pressure-temperature relationship of the saturated vapour. Second, vapour cycle systems exhibit better heat source isothermality than single-phase systems because the coolant changes temperature with heat addition in such systems, such as with the compact high intensity cooler (CHIC) described in U.S. Pat. No. 6,167,952 to Downing. Given these and other advantages of two-phase heat absorption, a high performance evaporator must be capable of accepting high heat fluxes and providing nearly constant temperature heat rejection over the device footprint. Additionally, the thermal resistance of the device should be small, thereby reducing the required lift of the refrigeration system. The evaporator should be capable of evaporating the coolant to high outlet qualities without high pressure drops that would penalise the cycle. Vapour specific volumes are in the range of 140 to 1000 times larger than their liquids. To manage flow velocities and thereby pressure losses during the large changes in volumetric flow a flow structure with and expanding flow area is required. Flow velocities should remain large enough to maintain shear control, however. These conditions serve to maintain annular flow and wet wall conditions capable of withstanding high heat fluxes to high outlet qualities. 
         [0003]    Some high energy systems that have short duty cycles make expendable coolants an attractive solution to energy management. In this case, stored liquid coolant may evaporate in the evaporator and then vent in an open cycle arrangement. The desired features of an evaporator for this open cycle system are identical to the closed cycle heat absorber. 
       SUMMARY OF THE INVENTION 
       [0004]    The invention generally comprises an evaporative compact high intensity cooler (ECHIC) for transferring heat from a heat source along a heat conduction surface of the heat source with a two-phase coolant, comprising: a flow passage labyrinth of flow passages with short conduction paths interrupted by coolant columns that all radiate from at least one coolant supply passage and offer the coolant expanding volume as the coolant evaporates due to absorbing heat within the flow passages from the heat conduction surface to maintain nearly isobaric conditions for the coolant to maintain relatively constant temperature throughout the ECHIC as it absorbs heat from the heat source and limit boundary layer formation within the flow passages to improve heat transfer. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0005]      FIG. 1  is a side view of an Evaporative Compact High Intensity Cooler (ECHIC) according to a possible embodiment of the invention that transfers heat from a heat source along a heat source conduction surface. 
           [0006]      FIG. 2  is a top view of the ECHIC according to the possible embodiment of the invention shown in  FIG. 1 . 
           [0007]      FIG. 3  is a top view of a target layer flow passage lamination within the ECHIC that covers a substantial portion of the conduction surface of the heat source shown in  FIGS. 1 and 2 . 
           [0008]      FIG. 4  is an “even layer” flow passage lamination that covers the target layer flow passage lamination. 
           [0009]      FIG. 5  is a top view of an “odd layer” flow passage lamination that covers at least one even layer flow passage lamination. 
           [0010]      FIG. 6  is a top view of an “interface layer” flow passage lamination that covers one of the even layer flow passage laminations most remote from the conduction surface of the heat source. 
           [0011]      FIG. 7  is a top view of a “cover layer” flow passage lamination that covers the interface layer flow passage lamination. 
           [0012]      FIG. 8  is a top view of the first even layer flow passage lamination superimposed over the target layer flow passage lamination. 
           [0013]      FIG. 9  is a bottom view of the target flow passage lamination superimposed under the first even layer flow passage lamination. 
           [0014]      FIG. 10  is a top view of the first even layer flow passage lamination superimposed over the first even layer flow passage lamination. 
           [0015]      FIG. 11  is a top view of the interface layer  22  superimposed over the third one of the even layer flow passage laminations  18 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0016]      FIGS. 1 and 2  are side and top views, respectively, of an evaporative compact high intensity cooler (ECHIC)  2  according to a possible embodiment of the invention that attaches to a heat source  4  along a heat source conduction surface  6 . The heat source  4  may comprise any high power device with a level of generated heat flux along the surface  6  that requires heat transfer for suitable operation. The ECHIC  2  comprises a three-dimensional flow passage labyrinth  8  that directs a flow of coolant to and from the surface  6  of the heat source  4 . A housing  10  for the ECHIC  2  may have at least one coolant supply port  12  and at least one coolant exhaust port  14  that each couple to the labyrinth  8  for this purpose. 
         [0017]    The flow passage labyrinth  8  preferably comprises a laminated structure made up of several layers of flow passages.  FIG. 3  is a top view of a target layer flow passage lamination  16  that covers a substantial portion of the conduction surface  6  of the heat source  4 .  FIG. 4  is a top view of an “even layer” flow passage lamination  18  that covers the target layer flow passage lamination  16 .  FIG. 5  is a top view of an “odd layer” flow passage lamination  20  that covers at least one even layer flow passage lamination  16 . An additional even layer flow passage lamination  18  covers the odd layer flow passage lamination  20 . Additional pairs of the odd layer flow passage lamination  20  and the even layer flow passage lamination  18  may cover the additional even layer flow passage lamination  16 .  FIG. 6  is a top view of an “interface layer” flow passage lamination  22  that covers one of the even layer flow passage laminations  16  most remote from the surface  6 .  FIG. 7  is a top view of a “cover layer” flow passage lamination  24  that covers the interface layer flow passage lamination  22 . Fabrication of the laminations  16 ,  18 ,  20 ,  22  and  24  may conveniently comprise photo-etched copper laminations. Other fabrication materials and processes may also be suitable, such as micro electro mechanical systems (MEMS) and integrated circuit (IC) processes, depending upon scale. 
         [0018]      FIG. 1  shows a typical laminated structure for the flow passage labyrinth  8  according to the invention. The target layer flow passage lamination  16  covers a substantial portion of the surface  6  of the heat source  4 . A first one of the even layer flow passage laminations  18  covers the target layer flow passage lamination  16 . A first one of the odd layer flow passages  20  covers the first even layer flow passage lamination  18 . A second one of the even layer flow passage laminations  18  covers the first odd layer flow passage lamination  20 . A second one of the odd layer flow passage laminations  18  covers the first odd layer flow passage lamination  20 . A third one of the even layer flow passage laminations  18  covers the second odd layer flow passage lamination  20 . The interface layer flow passage lamination  22  covers the third even layer flow passage lamination  18 . Finally, the top layer flow passage lamination  24  covers the interface layer flow passage lamination  22 . The flow passage labyrinth  8  may conveniently comprise a stack of diffusion-bonded laminations  16 , 18 ,  20 ,  22  and  24 . Alternatively, bonding of the laminations  16 , 18 ,  20 ,  22  and  24  may be by other processes, such as soldering. Such bonding is desirable to insure heat transfer through the stack of laminations  16 , 18 ,  20 ,  22  and  24 . 
         [0019]    Referring to  FIGS. 1 through 7  together, a liquid coolant that is suitable for two-phase cooling enters the coolant supply port  12  in the housing  10 , travels down through a coolant supply passage  26  in the cover layer flow passage lamination  24 , a coolant supply passage  28  in the interface layer flow passage lamination  22 , a coolant supply passage  30  in each even layer flow passage lamination  18 , a coolant supply passage  32  in each odd layer flow passage lamination  20  and a coolant supply passage  34  in the target layer flow passage lamination  16 . Once the coolant reaches the surface  6  of the heat source  4 , it starts to absorb heat and vaporise. It also starts to propagate through a plurality of innermost cooling passages  36  in the target layer flow passage lamination  16  that couple to and propagate radially from the coolant supply passage  34 . 
         [0020]    Since the quality of the coolant starts to increase as the coolant vaporises, the volume of the coolant increases as it propagates through the innermost cooling passages  36 . The cross-sectional area of each innermost cooling passage  36  increases as the coolant propagates through it from the coolant supply passage  34  to an outlet end  38 , thereby tending to increase volume and maintain nearly isobaric conditions as the coolant continues to absorb heat and vaporise. This in turn tends to maintain the coolant at a relatively constant temperature as it propagates through each of the innermost cooling passages  36 . Since the flow path length is very short, boundary layer development is restricted, resulting in a thin boundary layer that improves heat transfer. 
         [0021]    The coolant in each innermost cooling passage  36  then propagates from its outlet end  38  into the first even layer flow passage lamination  18  and starts a new propagation path through a corresponding one of a plurality of innermost cooling passages  40  proximate an inlet end  42 .  FIG. 8  is a top view of the first even layer flow passage lamination  18  superimposed over the target layer flow passage lamination  16 , wherein an overlap between each innermost cooling passage  36  of the target layer flow passage lamination  16  proximate its outlet end  38  and a corresponding one of the innermost cooling passages  40  of the first even layer flow passage lamination proximate its inlet end  42  forms a corresponding one of a plurality of innermost coolant columns  44 . 
         [0022]    As the coolant propagates from the inlet end  42  to an outlet end  46  of each innermost cooling passage  40 , the cross-sectional area of each innermost cooling passage  40  increases to an extent that tends to increase volume and maintain nearly isobaric conditions as the coolant continues to absorb heat and vaporise. This in turn tends to maintain the coolant at a relatively constant temperature as it propagates through each of the innermost cooling passages  40 . Since the flow path length is very short, boundary layer development is restricted, resulting in a thin boundary layer that improves heat transfer. 
         [0023]    The coolant in each innermost cooling passage  40  then propagates from its outlet end  46  back into the target layer flow passage lamination  16  and starts a new propagation path along the surface  6  through a corresponding one of a plurality of intermediate cooling passages  48  proximate an inlet end  50 .  FIG. 9  is a bottom view of the target flow passage lamination  16  superimposed under the first even layer flow passage lamination  18 , wherein an overlap between each innermost cooling passage  40  of the first even layer flow passage lamination  18  proximate its outlet end  46  and a corresponding one of the intermediate cooling passages  48  of the target layer flow passage lamination  16  proximate its inlet end  50  forms a corresponding one of a plurality of primary intermediate coolant columns  52 . 
         [0024]    As the coolant propagates from the inlet end  50  to at least one outlet end  54  of each intermediate cooling passage  48 , the cross-sectional area of each intermediate cooling passage  48  increases to an extent that tends to increase volume and maintain nearly isobaric conditions as the coolant continues to absorb heat and vaporise. This in turn tends to maintain the coolant at a relatively constant temperature as it propagates through each of the intermediate cooling passages  48 . Since the flow path length is very short, boundary layer development is restricted, resulting in a thin boundary layer that improves heat transfer. 
         [0025]    The coolant in each intermediate cooling passage  48  then propagates from each outlet end  54  back into the first even layer flow passage lamination  18  and starts a new propagation path through a corresponding one of a plurality of outer cooling passages  56  proximate an inlet end  58 . An overlap between each intermediate cooling passage  48  of the target layer flow passage lamination  16  proximate each outlet end  54  and each corresponding outer cooling passage  56  of the first even layer flow passage lamination  18  proximate its inlet end  58  forms a corresponding one of a plurality of secondary intermediate coolant columns  60 . 
         [0026]    As the coolant propagates from the inlet end  58  to at least one outlet end  62  of each outer cooling passage  56 , the cross-sectional area of each outer cooling passage  56  increases to an extent that tends to increase volume and maintain nearly isobaric conditions as the coolant continues to absorb heat and vaporise. This in turn tends to maintain the coolant at a relatively constant temperature as it propagates through each of the outer cooling passages  56 . Since the flow path length is very short, boundary layer development is restricted, resulting in a thin boundary layer that improves heat transfer. 
         [0027]    The coolant in each outer cooling passage  56  then propagates from each outlet end  62  back into the target layer flow passage lamination  18  and starts a new propagation path along the surface  6  through a corresponding one of a plurality of outer cooling passages  64  proximate an inlet end  66 . An overlap between each outer cooling passage  56  of the first even layer flow passage lamination  16  proximate each outlet end  58  and each outer cooling passage  64  of the target layer flow passage lamination  16  proximate its inlet end  66  forms a corresponding one of a plurality of tertiary intermediate coolant columns  68 . 
         [0028]    As the coolant propagates from the inlet end  66  to at least one outlet end  70  of each outer cooling passage  64 , the cross sectional area of each intermediate cooling passage  64  increases to an extent that tends to increase volume and maintain nearly isobaric conditions as the coolant continues to absorb heat and vaporise. This in turn tends to maintain the coolant at a relatively constant temperature as it propagates through each of the outer cooling passages  64 . Since the flow path length is very short, boundary layer development is restricted, resulting in a thin boundary layer that improves heat transfer. The coolant in each outer cooling passage  64  then propagates from each outlet end through a corresponding one of a plurality of coupling passages  72  in the first even layer flow passage lamination  18  to form a corresponding one of a plurality of outer coolant columns  74 . 
         [0029]      FIG. 10  is a top view of the first odd layer flow passage lamination  20  superimposed over the first even layer flow passage lamination  18 . Coolant from each innermost cooling column  44  passes through a corresponding one of a plurality of coupling passages  76  in the first one of the odd layer flow passage laminations  20 . Coolant from each primary intermediate cooling column  52  propagates into a corresponding one of a plurality of inner cooling passages  78  in the first odd layer flow passage lamination  20  proximate an inlet end  80 . As the coolant propagates from the inlet end  80  to at least one outlet end  82  of each inner cooling passage  78 , the cross-sectional area of each inner cooling passage  78  increases to an extent that tends to increase volume and maintain nearly isobaric conditions as the coolant continues to absorb heat and vaporise. This in turn tends to maintain the coolant at a relatively constant temperature as it propagates through each of the inner cooling passages  78 . Since the flow path length is very short, boundary layer development is restricted, resulting in a thin boundary layer that improves heat transfer. The coolant in each inner cooling passage  78  then propagates from each outlet end  82  into a corresponding one of the secondary intermediate coolant columns  60 . 
         [0030]    Coolant from each tertiary intermediate coolant column  68  propagates into a corresponding one of a plurality of outer cooling passages  84  proximate an inlet end  86 . As the coolant propagates from the inlet end  86  to at least one outlet end  88  of each outer cooling passage  84 , the cross-sectional area of each outer cooling passage  84  increases to an extent that tends to increase volume and maintain nearly isobaric conditions as the coolant continues to absorb heat and vaporise. This in turn tends to maintain the coolant at a relatively constant temperature as it propagates through each of the outer cooling passages  84 . Since the flow path length is very short, boundary layer development is restricted, resulting in a thin boundary layer that improves heat transfer. The coolant in each outer cooling passage  84  then propagates from each outlet end  88  into a corresponding one of the outer coolant columns  74 . 
         [0031]    Coolant then flows through passages in the second and third even layer flow passage laminations  18  and the second odd layer flow passage lamination  20  as hereinbefore described, as well as for any additional pairs of the odd layer flow passage lamination  20  and the even layer flow passage lamination  18 . Coolant from each tertiary intermediate cooling column  68  also propagates into a corresponding one of a plurality of cooling passages  90  in the interface layer cooling passage lamination  22  proximate at least one inlet end  92 . 
         [0032]      FIG. 11  is a top view of the interface layer  22  superimposed over the third one of the even layer flow passage laminations  18 . As the coolant propagates from the inlet end  92  to at least one outlet end  94  of each cooling passage  90 , the cross-sectional area of each outer cooling passage  90  increases to an extent that tends to increase volume and maintain nearly isobaric conditions as the coolant continues to absorb heat and vaporise. This in turn tends to maintain the coolant at a relatively constant temperature as it propagates through each of the cooling passages  90 . Since the flow path length is very short, boundary layer development is restricted, resulting in a thin boundary layer that improves heat transfer. The coolant in each cooling passage  90  then propagates from each outlet end  94  into a corresponding one of the outer coolant columns  74 . Finally, coolant in each outer cooling column  74  passes through a corresponding cover layer coupling passage  96  to discharge from the flow passage labyrinth  8 . 
         [0033]    The flow passage labyrinth  8  achieves superior heat transfer by optimising the three factors of thermal conductance represented by the relationship G=ηhA. The thermal conductance G is the rate of energy transfer per unit area and temperature difference between the coolant and heat source. The wetted surface area A is the total surface area of each of the sides, top and bottom of each of the linked flow passages within the flow passage labyrinth  8 . The laminated construction of the flow passage labyrinth  8  affords low thermal resistances by combining high heat transfer coefficients, (h), abundant area enhancement (A), and good surface efficiency, η. 
         [0034]    High heat transfer coefficients occur in flows that limit the development of the boundary layer. The boundary layer thickness determines the heat transfer coefficient in that it represents the “conduction thickness” in the fluid layer that insulates the cooler bulk flow from the hot wall. The flow passage labyrinth  8  minimises the boundary layer with flow passages that have direct liquid impingement, short flow path lengths for re-developing flow, and small channel size. Additionally, the laminations of the flow passage labyrinth  8  permit area enhancement ratios, that is, the wetted heat transfer surface area to the cooler footprint area, typically between approximately 10 and 30 to one. Like conventional laminated coolers, the ECHIC  2  for a large-scale heat source  4  may comprise a plurality of flow passage labyrinths  8  ganged together. Ganged flow passage labyrinths  8  may comprise individual units or multiple units fabricated within a single large laminated structure with integral headers. Similarly, the shape of the ECHIC  2  may adapt to the shape of the heat source  4 . For instance, the ECHIC  2  for a heat source  4  that has a generally circular rather than square heat source conduction surface  6  may have a cylindrical instead of a rectangular shape to better conform to the heat source  4 . 
         [0035]    The ECHIC  2  extends the advantages of the bonded laminate technology to two-phase cooling. The bonded lamination of the flow passage labyrinth  8  allows a large wetted surface area for heat transfer to be close to the heat source  4 . Because the surface areas of the coolant flow passages within ECHIC  2  are conductively close to the heat source  4  it is highly efficient in dissipating heat. Two-phase cooling systems greatly reduce coolant flow requirements through utilisation of the latent heat of coolants. The challenge in designing an evaporator for high outlet qualities is accommodating the large volumetric change that occurs during evaporation. The ECHIC  2  provides increasing flow area by using expanding areas in the radial and axially flow directions as the coolant propagates through the coolant flow passages and columns in the flow passage labyrinth  8 . This flow arrangement limits flow velocities whilst still maintaining shear driven flow. 
         [0036]    The ECHIC  2  achieves excellent heat transfer from the heat source  4  because the multiple laminate construction with numerous small linked flow passages provides wetted surface areas many times larger than the base footprint of the ECHIC  2  on the surface  6  of the heat source  4 . This is because the total wetted surface area comprises the total surface area of the sides, top and bottom of each of the linked flow passages. Furthermore, the short flow paths of each flow passage within the flow passage labyrinth  8  restarts boundary layers at the inlet end of each flow passage, thereby limiting the conduction thickness of the boundary layers, resulting in high heat transfer coefficients. Finally, the compact structure of the thin laminations within the flow passage labyrinth  8  inherently has short conduction paths, yielding high heat transfer fin efficiencies. 
         [0037]    The ECHIC  2  is useful in open and closed cooling cycles, as well as in a vapour compression cooling cycle. Due to the large heat of vaporisation, an open cooling cycle may use water as the liquid coolant. The open cooling cycle may simply vent wet steam discharged from the ECHIC  2  or it may separate the water from it and vent the dry steam. The closed cycle may also use water as the liquid coolant wherein it may condense the wet steam discharged from the ECHIC  2  in a condenser and recirculate the condensed water through the ECHIC  2 . In addition to use in such conventional two-phase cooling cycles, the ECHIC  2  may also cool the heat source  4  under standby and low power conditions in a single-phase cooling cycle, such as with water as the liquid coolant. As the heat source  4  increases dissipated heat under normal operating conditions, the liquid coolant, such as water, may then vaporise, switching the ECHIC  2  to an efficient two-phase cooling cycle. 
         [0038]    The described embodiment of the invention is only an illustrative implementation of the invention wherein changes and substitutions of the various parts and arrangement thereof are within the scope of the invention as set forth in the attached claims.