US20100132924A1 - Cooling device for electronic components - Google Patents
Cooling device for electronic components Download PDFInfo
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- US20100132924A1 US20100132924A1 US12/597,774 US59777408A US2010132924A1 US 20100132924 A1 US20100132924 A1 US 20100132924A1 US 59777408 A US59777408 A US 59777408A US 2010132924 A1 US2010132924 A1 US 2010132924A1
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- cooling device
- fins
- condenser
- evaporator section
- working fluid
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
- H01L23/00—Details of semiconductor or other solid state devices
- H01L23/34—Arrangements for cooling, heating, ventilating or temperature compensation ; Temperature sensing arrangements
- H01L23/42—Fillings or auxiliary members in containers or encapsulations selected or arranged to facilitate heating or cooling
- H01L23/427—Cooling by change of state, e.g. use of heat pipes
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
- H01L23/00—Details of semiconductor or other solid state devices
- H01L23/34—Arrangements for cooling, heating, ventilating or temperature compensation ; Temperature sensing arrangements
- H01L23/46—Arrangements for cooling, heating, ventilating or temperature compensation ; Temperature sensing arrangements involving the transfer of heat by flowing fluids
- H01L23/467—Arrangements for cooling, heating, ventilating or temperature compensation ; Temperature sensing arrangements involving the transfer of heat by flowing fluids by flowing gases, e.g. air
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
- H01L2924/00—Indexing scheme for arrangements or methods for connecting or disconnecting semiconductor or solid-state bodies as covered by H01L24/00
- H01L2924/0001—Technical content checked by a classifier
- H01L2924/0002—Not covered by any one of groups H01L24/00, H01L24/00 and H01L2224/00
Definitions
- the present invention relates to cooling devices for electronic components.
- the invention provides a cooling device comprising a chamber having an evaporator section and a condenser section, the condenser section extending from a periphery of the evaporator section.
- a compact cooling device is thus achieved with the back-to-back configuration of the evaporator and condenser sections.
- the compact arrangement of the cooling device results in almost zero pressure differential between the evaporator and condenser sections (and hence, a uniform temperature distribution in the chamber) for fluid flow within the chamber. This reduces the overall thermal resistance of the cooling device.
- a working fluid may be arranged to absorb and transfer heat energy.
- the working fluid may have a latent heat of vaporisation of greater than or equal to about 1500 joules per gram (J/g).
- the working fluid may include a plurality of nanoparticles, the nanoparticles having a thermal conductivity of between about 200 Watt per metre Kelvin (W/m ⁇ K) to about 400 W/m ⁇ K.
- the condenser section may comprise a plurality of condenser fins extending from the periphery of the evaporator section.
- a plurality of convective fins may extend from surfaces of the condenser fins. This facilitates heat transfer from the condenser fins.
- the convective fins may extend between adjacent ones of the condenser fins. This enhances the compactness of the cooling device.
- a fan may be arranged to direct a flow of air at the convective fins. This enhances convective heat transfer from the convective fins and thus improves the heat rejection rate.
- An air guide may be arranged to direct the flow of air from the fan to the convective fins. This enhances convective heat transfer from the convective fins.
- the convective fins may be integrally moulded to the surfaces of the condenser fins. This reduces thermal contact resistance and thus increases thermal conductivity between the convective fins and the condenser fins.
- a capillary element may be arranged to facilitate return of condensate from the condenser section to the evaporator section. This facilitates return of the condensate to the evaporator section regardless of the orientation of the cooling device.
- a base portion of the condenser section may be at an angle to a base portion of the evaporator section. This facilitates return of the condensate to the evaporator section by gravitational means.
- the evaporator section may have a capillary structure.
- the capillary structure enhances boiling in the evaporator section as it creates a micro-flow situation within the evaporator section by drawing heated working fluid away from the base portion of the evaporator section and fresh working fluid in the form of condensate from the condenser fins to the evaporator section. The degree of superheat (i.e. the difference between the temperature of the base portion of the evaporator section and the saturation temperature of the working fluid) is thus reduced.
- the capillary structure may comprise an open cell foam metal.
- the open cell foam metal may be bonded to a base portion of the evaporator section by diffused bonding or brazing to reduce thermal contact resistance and thus improve thermal conductivity between the base portion of the evaporator section and the capillary structure.
- the capillary structure may have a pore density of from about 8 to about 52 pores per centimetre (ppc) and more particularly, a pore density of about 16 ppc.
- FIG. 1A is an exploded top perspective view of a cooling device in accordance with one embodiment of the present invention.
- FIG. 1B is an exploded bottom perspective view of the cooling device of FIG. 1A ;
- FIG. 2A is an assembled top perspective view of the cooling device of FIG. 1A ;
- FIG. 2B is an assembled bottom perspective view of the cooling device of FIG. 1B ;
- FIG. 3 is a schematic cross-sectional view of the cooling device taken along section lines A-B and B-C in FIG. 2B ;
- FIG. 4 is a graph showing the transient performance of a cooling device with convective fin thickness of 0.25 millimetres (mm).
- a cooling device 10 for an electronic component such as, for example, a microprocessor or central processing unit (CPU) of a computer will now be described below with reference to FIGS. 1A , 1 B, 2 A, 2 B and 3 .
- an electronic component such as, for example, a microprocessor or central processing unit (CPU) of a computer
- the cooling device 10 comprises a fan 12 mounted to an air guide 14 , a hermetically sealed chamber 16 having a plurality of convective fins 18 and a sleeve 20 , and an attachment member 22 in the form of a leg support.
- the hermetically sealed chamber 16 is provided with a working fluid 24 to absorb and transfer heat energy.
- the fan 12 is arranged to direct a flow of air at the convective fins 18 and the air guide 14 is arranged to direct the flow of air from the fan 12 to the convective fins 18 .
- the fan 12 is mounted to the air guide 14 by a plurality of beams 26 extending from the stator 28 of the fan 12 to the air guide 14 , the fan 12 being positioned such that a separation is maintained between the rotor 30 of the fan 12 and the hermetically sealed chamber 16 .
- the air guide 14 is spaced from the outermost convective fin 18 to allow airflow through.
- the hermetically sealed chamber 16 is mounted to the air guide 14 via a clipping mechanism 32 .
- the hermetically sealed chamber 16 comprises an evaporator section 34 and a condenser section comprising a plurality of condenser fins 36 extending from a periphery of the evaporator section 34 .
- a base portion of the condenser section is slanted at an angle ⁇ to a base portion of the evaporator section 34 . This facilitates return of condensate 38 from the condenser fins 36 to the evaporator section 34 by gravitational means.
- the base portion of the evaporator section 34 in contact with the electronic component may be polished to reduce thermal contact resistance and thus improve heat transmission from the electronic component to the evaporator section 34 .
- the evaporator section 34 is provided with a capillary structure 40 .
- the capillary structure 40 may, for example, be provided in the form of an open cell foam metal made of copper; aluminium or other highly thermally conductive material and may be bonded to the base portion of the evaporator section 34 by, for example, diffused bonding or brazing to reduce thermal contact resistance and thus improve thermal conductivity between the base portion of the evaporator section 34 and the capillary structure 40 .
- the capillary structure 40 may have a pore density of from about 8 to about 52 pores per centimetre (ppc) (equivalent to from about 20 to about 130 pores per inch (ppi)) and more particularly, a pore density of about 16 ppc.
- each of the condenser fins 36 is provided with a capillary element 42 such as, for example, a cotton thread or braided copper wires to facilitate return of the condensate 38 from the condenser section to the evaporator section 34 at various orientations of the cooling device 10 .
- a small groove may be provided on the slanted bottom of the condenser fin 36 for receiving the capillary element 42 in the form of a capillary pumping thread.
- the working fluid 24 in the hermetically sealed chamber 16 may be a single or multi component condensable fluid.
- the working fluid 24 include, but are not limited to, water, dielectric fluids such as, for example, hydrofluoroethane (HFE) or fluorocarbon (FC) in the refrigerant series, or any other fluid or combination of fluids with a high latent heat of evaporation (more particularly, a latent heat of vaporisation of greater than or equal to about 1500 joules per gram (J/g)) and which can permeate the foam metal material in the evaporator section 34 .
- the working fluid 24 may be provided with a plurality of nanoparticles to augment the effective thermal conductivity of the working fluid 24 .
- the nanoparticles may have a thermal conductivity of between about 200 Watt per metre Kelvin (W/m ⁇ K) to about 400 W/m ⁇ K.
- An optimum amount of working fluid 24 is provided in the hermetically sealed chamber 16 such that the capillary structure 40 is always in contact with and saturated with the working fluid 24 to prevent drying out and to ensure wetting of the capillary structure 40 throughout the operation of the cooling device 10 .
- the optimum amount of working fluid 24 in the hermetically sealed chamber 16 is determined based on the amount of heat generated by the electronic component. A larger quantity of working fluid 24 is required in instances where more heat is generated to prevent dry out.
- the working fluid 24 in the hermetically sealed chamber 16 may be in its liquid-vapour saturated state to facilitate conversion of the working fluid 24 into its vapour form as this enhances heat dissipation by the cooling device 10 .
- the hermetically sealed chamber 16 may be in a pressurized or vacuum state depending on the working fluid 24 employed. In other words, the pressure in the hermetically sealed chamber 16 is dependent on the saturation properties of the working fluid 24 .
- the hermetically sealed chamber 16 may be in a vacuum state when water is employed as the working fluid 24 .
- the convective fins 18 extend from surfaces of the condenser fins 36 and between adjacent ones of the condenser fins 36 .
- the convective fins 18 may be integrally moulded to the surfaces of the condenser fins 36 to reduce thermal contact resistance and thus increase thermal conductivity between the convective fins 18 and the condenser fins 36 .
- the hermetically sealed chamber 16 and the convective fins 18 may be made from a single piece of material such as, for example, copper, aluminium or other highly thermally conductive material that is compatible with the working fluid 24 , via a moulding or machining process.
- the sleeve 20 is provided with threaded holes 44 to receive screws (not shown) for securing the attachment member 22 to the evaporator section 34 .
- the attachment member 22 provides an interface between the hermetically sealed chamber 16 and the electronic component. As can be seen from FIGS. 1B and 2B , the attachment member 22 is provided with first threaded holes 46 corresponding to the threaded holes 44 on the sleeve 20 and second threaded holes 48 for attachment, for example, to a motherboard of a computer.
- the attachment member 22 is secured to the sleeve 20 by passing screws (not shown) through respective pairs of the threaded holes 44 on the sleeve 20 and the first threaded holes 46 on the attachment member 22 , and fastening the screws.
- the attachment member 22 is similarly secured, for example, to the motherboard of the computer.
- the leg support 22 may be of any configuration depending on the layout of the surface to which the cooling device 10 is to be secured.
- FIG. 3 shows a schematic cross-sectional view of the cooling device 10 taken along section lines A-B and B-C in FIG. 2B . Accordingly, the left side of FIG. 3 shows the air flow through the convective fins 18 of the cooling device 10 and the right side illustrates circulation of the working fluid 24 within the hermetically sealed chamber 16 .
- an electronic component When in use, an electronic component generates and dissipates heat.
- the heat from the electronic component is taken in at the base portion of the evaporator section 34 and raises the temperature of the base portion of the evaporator section 34 .
- Boiling of the working fluid 24 inside the evaporator section 34 of the hermetically sealed chamber 16 occurs as heat is taken from the base portion of the evaporator section 34 by the working fluid 24 as latent heat of vaporization.
- the capillary structure 40 in the evaporator section 34 enhances boiling of the working fluid 24 as it creates a micro-flow situation within the evaporator section 34 by drawing the heated working fluid 24 away from the base portion of the evaporator section 34 and fresh working fluid 24 in the form of condensate 38 from the condenser fins 36 to the evaporator section 34 (see arrow 50 in FIG. 3 ).
- the degree of superheat i.e. the difference between the temperature of the base portion of the evaporator section 34 and the saturation temperature of the working fluid 24 ) is thus reduced.
- Condensation occurs on inner walls 54 of the condenser fins 36 as the inner walls 54 are at a lower temperature than the vaporised working fluid 24 in the condenser fins 36 . Accordingly, the working fluid 24 is collected as condensate 38 on the inner walls 54 of the condenser fins 36 .
- the condensate 38 may be returned to the evaporator section 34 by gravitational means where the condenser fins 36 are at a higher elevation than the evaporator section 34 , by capillary action via the capillary element 42 (particularly when the condenser fins 36 are at a lower elevation than the evaporator section 34 ) or a combination of both.
- Heat in the form of latent heat of condensation transmitted from the working fluid 24 to the condenser fins 36 is conducted to the convective fins 18 which provide a large surface area for heat dissipation. Heat is convected away from the convective fins 18 to ambient space by the flow of air from the fan 12 (see arrows 56 in FIG. 3 ). More particularly, the fan 12 mounted at the top of the main body of the cooling device 10 and located inside the opening end of the air guide 14 forces air to flow through the array of convective fins 18 . The air guide 14 constricts and guides the air flow into the array of convective fins 18 .
- cooling device of the present invention is not limited to a particular orientation. Rather, the cooling device may be used in any orientation as long as the capillary structure is in contact with the bulk working fluid in liquid phase for capillary pumping.
- Convective cooling is provided by an axial flow fan 12 with a diameter of 92 millimetres (mm) and which generates an air flow rate of about 11.80 litre per second (l/s) (equivalent to 25 cubic feet per minute (CFPM)).
- the governing equations for the simulation i.e. conservation equations for energy and mass provided below
- IMSL International Mathematical and Statistical Library
- the steady state performance parameters of the cooling device 10 subjected to 10 watts per square centimetre (W/cm 2 ) of heat flux from a CPU measuring 3 by 3 square centimetre (cm 2 ) at ambient air temperature of 30 degrees Celsius (° C.) are shown in Table 1 below.
- the optimal performance for convective fins 18 of various thicknesses is shown in Table 1.
- Table 1 The optimal performance for convective fins 18 of various thicknesses is shown in Table 1.
- the cooling device 10 with the thinnest convective fins 18 measuring 0.25 mm outperforms all others by maintaining the CPU temperature at a mere 48.1° C. with an overall but optimal thermal resistance of 0.206 K/W as it allows placement of an optimum number of convective fins 18 while allowing an adequate amount air flow from the axial flow fan 12 and thus achieves the highest convective heat transfer.
- convective fin thickness is decreased further, it was observed that the benefits of increasing convective surface area by reducing fin thickness is somewhat offset by a drop in the fin efficiency for the thinner fins. It is concluded therefore that fin strength is also a design consideration.
- FIG. 4 the transient performance of the cooling device 10 with convective fin thickness of 0.25 mm is shown.
- the data for the graph of FIG. 4 was obtained by performing a simulation test on a cooling device 10 with seventeen (17) convective fins 18 , each convective fin having a thickness of 0.25 mm, a height of 30 mm and a length of 40 mm, the cooling device 10 being subjected to varying heat flux loads ranging from 10 W/cm 2 to 25 W/cm 2 from a 9 cm 2 CPU footprint.
- the CPU chip temperature did not exceed 70° C.—the thermal design limit of the CPU—even at a CPU heat flux of 25 W/cm 2 (i.e. a total of 225 watts (W)).
- the next highest thermal resistance in the cooling device 10 is the superheat for boiling.
- the heat transfer coefficient of boiling increases with increasing surface heat flux at the base portion of the evaporator section 34 .
- the corresponding increase in boiling superheat is minimal.
- the heat spreading problem may be alleviated by using an evaporator with a smaller base area.
- the thermal conductivity of the working fluid 24 may be enhanced by adding nanoparticles to the working fluid 24 to reduce the boiling superheat.
- the size of the convective fins 18 may be varied according to the space available to accommodate future higher heat fluxes.
- the energy balance at the base portion of the evaporator section 34 made of copper taking into consideration the heat given out by the CPU chip and the heat extracted by evaporation of the working fluid 24 , may be modelled with the following equation:
- (Mc p ) base represents the thermal capacity of the base portion of the evaporator section 34 ,
- q flux chip represents the heat flux input from the heat source
- a chip represents the footprint area of the heat source
- a evap represents the bottom surface area of the evaporator section 34
- q flux evap represents the heat taken in by the working fluid 24 for evaporation.
- the hermetically sealed chamber 16 includes a porous structure 40 and a saturated working fluid 24 .
- Latent heat is taken in by the working fluid 24 from the superheated surface of the base portion of the evaporator section 34 , and is transported and rejected to the finned condenser section.
- the energy balance of the working fluid 24 may be modelled with the following equation:
- T cond presents the temperature of the condenser section
- the Rohsenow Correlation adjusted for sub-atmospheric conditions as shown in equations (3) and (4) below, is used to predict the heat flux taken in by the working fluid 24 for boiling at a certain superheat at sub-atmospheric conditions in the presence of the porous structure 40 :
- T surface represents the temperature of the base portion of the evaporator section 34
- C Sf an empirical constant related to the fluid-heater surface combination, is 0.0132
- h fg represents the latent heat of evaporation of the working fluid 24
- Pr l represents the Prantl Number
- C pf represents the heat capacity of the working fluid 24 in liquid phase
- ⁇ l represents the viscosity of the working fluid 24 in liquid phase
- ⁇ represents surface tension
- g represents acceleration due to gravity
- ⁇ l represents the density of the working fluid 24 in liquid phase
- ⁇ g represents the density of the working fluid 24 in vapour phase
- P represents the pressure in the evaporator section 34
- P atm represents atmospheric pressure
- a wetted represents the wetted surface area of the porous structure 40
- a base represents the surface area of the base portion of the evaporator section 34 .
- the Thin Film Condensation Correlation is used to predict the heat transfer coefficient h i of the condenser section as shown in equation (5) below on the assumption that the walls of the condenser section are acting as individual plates:
- k represents the thermal conductivity of the working fluid 24 in liquid phase
- ⁇ represents the viscosity of the working fluid 24 in liquid phase
- T v represents the temperature of the working fluid 24 in vapour phase in the hermetically sealed chamber 16
- L fin represents the length of the condensate 38 flow path.
- Mass balance was performed on the hermetically sealed chamber 16 to account for all the liquid and vapour mass inside the chamber 16 . This provides information for determining the optimum amount of working fluid 24 to fill the hermetically sealed chamber 16 with to prevent drying out and to ensure wetting of the capillary structure 40 throughout the operation of the cooling device 10 .
- the mass balance was modelled with the following equations:
- ⁇ dot over (m) ⁇ evaporation represents the rate of evaporation
- ⁇ dot over (m) ⁇ condensation represents the rate of condensation
- the rate of evaporation ⁇ dot over (m) ⁇ condensation and the rate of condensation ⁇ dot over (m) ⁇ condensation may be determined with the following equations:
- T chamber represents the temperature in the vapour space of the hermetically sealed chamber 16 and A cond represents the surface area of the inner walls 54 of the condenser fins 36 .
- T air represents ambient temperature
- R hs represents the thermal resistance between the convective fins 18 and ambient air.
- the present invention provides a compact cooling device with reduced heat spreading resistance for intense heat emitting surfaces such as, for example, a microprocessor or CPU of a desktop computer, as well as surfaces with high localised heat fluxes.
- the cooling device includes: an evaporator section where working fluid evaporates at its saturation pressure, extracting its latent heat of vaporization from the intense heat surface; a condenser section to reject latent heat from incoming working fluid in vapour phase from the evaporator section; a capillary structure located in the evaporator section to circulate the working fluid in liquid phase from the condenser section back to the evaporator section; an array of convective fins as a means of extending the heat sink surface area; and a fan to blow air through the array of convective fins, thereby enhancing heat rejection to ambient air.
- the evaporator section, the condenser section, the convective fins, and the fan can all be integrated into one compact device.
- the overall size of the cooling device and heat spreading resistance can be significantly reduced, increasing the overall heat transfer capacity of cooling device.
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Abstract
A cooling device (10) including a chamber having an evaporator section (34) and a condenser section, the condenser section extending from a periphery of the evaporator section (34).
Description
- The present invention relates to cooling devices for electronic components.
- Advances in photolithography have enabled dense packing of transistors on a chip. This has in turn resulted in an increase in the amount of heat generated per unit area of the chip, in some instances, exceeding the thermal design limit of conventional cooling devices. There is thus a need for a cooling device that is capable of dissipating the waste heat generated by electronic components effectively.
- In a first aspect, the invention provides a cooling device comprising a chamber having an evaporator section and a condenser section, the condenser section extending from a periphery of the evaporator section.
- A compact cooling device is thus achieved with the back-to-back configuration of the evaporator and condenser sections. The compact arrangement of the cooling device results in almost zero pressure differential between the evaporator and condenser sections (and hence, a uniform temperature distribution in the chamber) for fluid flow within the chamber. This reduces the overall thermal resistance of the cooling device.
- A working fluid may be arranged to absorb and transfer heat energy. The working fluid may have a latent heat of vaporisation of greater than or equal to about 1500 joules per gram (J/g). The working fluid may include a plurality of nanoparticles, the nanoparticles having a thermal conductivity of between about 200 Watt per metre Kelvin (W/m·K) to about 400 W/m·K.
- The condenser section may comprise a plurality of condenser fins extending from the periphery of the evaporator section.
- A plurality of convective fins may extend from surfaces of the condenser fins. This facilitates heat transfer from the condenser fins.
- The convective fins may extend between adjacent ones of the condenser fins. This enhances the compactness of the cooling device.
- A fan may be arranged to direct a flow of air at the convective fins. This enhances convective heat transfer from the convective fins and thus improves the heat rejection rate.
- An air guide may be arranged to direct the flow of air from the fan to the convective fins. This enhances convective heat transfer from the convective fins.
- The convective fins may be integrally moulded to the surfaces of the condenser fins. This reduces thermal contact resistance and thus increases thermal conductivity between the convective fins and the condenser fins.
- A capillary element may be arranged to facilitate return of condensate from the condenser section to the evaporator section. This facilitates return of the condensate to the evaporator section regardless of the orientation of the cooling device.
- A base portion of the condenser section may be at an angle to a base portion of the evaporator section. This facilitates return of the condensate to the evaporator section by gravitational means.
- The evaporator section may have a capillary structure. The capillary structure enhances boiling in the evaporator section as it creates a micro-flow situation within the evaporator section by drawing heated working fluid away from the base portion of the evaporator section and fresh working fluid in the form of condensate from the condenser fins to the evaporator section. The degree of superheat (i.e. the difference between the temperature of the base portion of the evaporator section and the saturation temperature of the working fluid) is thus reduced. The capillary structure may comprise an open cell foam metal. The open cell foam metal may be bonded to a base portion of the evaporator section by diffused bonding or brazing to reduce thermal contact resistance and thus improve thermal conductivity between the base portion of the evaporator section and the capillary structure. The capillary structure may have a pore density of from about 8 to about 52 pores per centimetre (ppc) and more particularly, a pore density of about 16 ppc.
- Other aspects and advantages of the invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.
- Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which:
-
FIG. 1A is an exploded top perspective view of a cooling device in accordance with one embodiment of the present invention; -
FIG. 1B is an exploded bottom perspective view of the cooling device ofFIG. 1A ; -
FIG. 2A is an assembled top perspective view of the cooling device ofFIG. 1A ; -
FIG. 2B is an assembled bottom perspective view of the cooling device ofFIG. 1B ; -
FIG. 3 is a schematic cross-sectional view of the cooling device taken along section lines A-B and B-C inFIG. 2B ; and -
FIG. 4 is a graph showing the transient performance of a cooling device with convective fin thickness of 0.25 millimetres (mm). - The detailed description set forth below in connection with the appended drawings is intended as a description of the presently preferred embodiments of the invention, and is not intended to represent the only form in which the present invention may be practiced. It is to be understood that the same or equivalent functions may be accomplished by different embodiments that are intended to be encompassed within the scope of the invention.
- A
cooling device 10 for an electronic component (not shown) such as, for example, a microprocessor or central processing unit (CPU) of a computer will now be described below with reference toFIGS. 1A , 1B, 2A, 2B and 3. - The
cooling device 10 comprises afan 12 mounted to anair guide 14, a hermetically sealedchamber 16 having a plurality ofconvective fins 18 and asleeve 20, and anattachment member 22 in the form of a leg support. The hermetically sealedchamber 16 is provided with a workingfluid 24 to absorb and transfer heat energy. - The
fan 12 is arranged to direct a flow of air at theconvective fins 18 and theair guide 14 is arranged to direct the flow of air from thefan 12 to theconvective fins 18. In the present embodiment, thefan 12 is mounted to theair guide 14 by a plurality ofbeams 26 extending from thestator 28 of thefan 12 to theair guide 14, thefan 12 being positioned such that a separation is maintained between therotor 30 of thefan 12 and the hermetically sealedchamber 16. As can be seen fromFIGS. 2B and 3 , theair guide 14 is spaced from theoutermost convective fin 18 to allow airflow through. - As shown in
FIG. 3 , the hermetically sealedchamber 16 is mounted to theair guide 14 via aclipping mechanism 32. The hermetically sealedchamber 16 comprises anevaporator section 34 and a condenser section comprising a plurality ofcondenser fins 36 extending from a periphery of theevaporator section 34. In the present embodiment, a base portion of the condenser section is slanted at an angle θ to a base portion of theevaporator section 34. This facilitates return ofcondensate 38 from thecondenser fins 36 to theevaporator section 34 by gravitational means. The base portion of theevaporator section 34 in contact with the electronic component may be polished to reduce thermal contact resistance and thus improve heat transmission from the electronic component to theevaporator section 34. - In the present embodiment, the
evaporator section 34 is provided with acapillary structure 40. Thecapillary structure 40 may, for example, be provided in the form of an open cell foam metal made of copper; aluminium or other highly thermally conductive material and may be bonded to the base portion of theevaporator section 34 by, for example, diffused bonding or brazing to reduce thermal contact resistance and thus improve thermal conductivity between the base portion of theevaporator section 34 and thecapillary structure 40. Thecapillary structure 40 may have a pore density of from about 8 to about 52 pores per centimetre (ppc) (equivalent to from about 20 to about 130 pores per inch (ppi)) and more particularly, a pore density of about 16 ppc. - The
condenser fins 36 are hollow and of a certain thickness with one end open towards theevaporator section 34 to allow workingfluid 24 in the vapour phase to enter from theevaporator section 34 and condense even on its outermost end, thereby spreading heat effectively and achieving a more uniform temperature distribution. In the present embodiment, each of thecondenser fins 36 is provided with acapillary element 42 such as, for example, a cotton thread or braided copper wires to facilitate return of thecondensate 38 from the condenser section to theevaporator section 34 at various orientations of thecooling device 10. A small groove (not shown) may be provided on the slanted bottom of thecondenser fin 36 for receiving thecapillary element 42 in the form of a capillary pumping thread. - The working
fluid 24 in the hermetically sealedchamber 16 may be a single or multi component condensable fluid. Examples of the workingfluid 24 include, but are not limited to, water, dielectric fluids such as, for example, hydrofluoroethane (HFE) or fluorocarbon (FC) in the refrigerant series, or any other fluid or combination of fluids with a high latent heat of evaporation (more particularly, a latent heat of vaporisation of greater than or equal to about 1500 joules per gram (J/g)) and which can permeate the foam metal material in theevaporator section 34. The workingfluid 24 may be provided with a plurality of nanoparticles to augment the effective thermal conductivity of the workingfluid 24. The nanoparticles may have a thermal conductivity of between about 200 Watt per metre Kelvin (W/m·K) to about 400 W/m·K. An optimum amount of workingfluid 24 is provided in the hermetically sealedchamber 16 such that thecapillary structure 40 is always in contact with and saturated with the workingfluid 24 to prevent drying out and to ensure wetting of thecapillary structure 40 throughout the operation of thecooling device 10. The optimum amount of workingfluid 24 in the hermetically sealedchamber 16 is determined based on the amount of heat generated by the electronic component. A larger quantity of workingfluid 24 is required in instances where more heat is generated to prevent dry out. The workingfluid 24 in the hermetically sealedchamber 16 may be in its liquid-vapour saturated state to facilitate conversion of the workingfluid 24 into its vapour form as this enhances heat dissipation by the coolingdevice 10. To achieve the liquid-vapour saturated state, the hermetically sealedchamber 16 may be in a pressurized or vacuum state depending on the workingfluid 24 employed. In other words, the pressure in the hermetically sealedchamber 16 is dependent on the saturation properties of the workingfluid 24. As an example, the hermetically sealedchamber 16 may be in a vacuum state when water is employed as the workingfluid 24. - As can be seen from
FIGS. 1A , 1B, 2A and 2B, theconvective fins 18 extend from surfaces of thecondenser fins 36 and between adjacent ones of thecondenser fins 36. Theconvective fins 18 may be integrally moulded to the surfaces of thecondenser fins 36 to reduce thermal contact resistance and thus increase thermal conductivity between theconvective fins 18 and thecondenser fins 36. In such an embodiment, the hermetically sealedchamber 16 and theconvective fins 18 may be made from a single piece of material such as, for example, copper, aluminium or other highly thermally conductive material that is compatible with the workingfluid 24, via a moulding or machining process. - As shown in
FIG. 1B , thesleeve 20 is provided with threadedholes 44 to receive screws (not shown) for securing theattachment member 22 to theevaporator section 34. - The
attachment member 22 provides an interface between the hermetically sealedchamber 16 and the electronic component. As can be seen fromFIGS. 1B and 2B , theattachment member 22 is provided with first threadedholes 46 corresponding to the threadedholes 44 on thesleeve 20 and second threadedholes 48 for attachment, for example, to a motherboard of a computer. Theattachment member 22 is secured to thesleeve 20 by passing screws (not shown) through respective pairs of the threadedholes 44 on thesleeve 20 and the first threadedholes 46 on theattachment member 22, and fastening the screws. Theattachment member 22 is similarly secured, for example, to the motherboard of the computer. Theleg support 22 may be of any configuration depending on the layout of the surface to which thecooling device 10 is to be secured. - The operation of the
cooling device 10 will now be described with reference toFIG. 3 which shows a schematic cross-sectional view of thecooling device 10 taken along section lines A-B and B-C inFIG. 2B . Accordingly, the left side ofFIG. 3 shows the air flow through theconvective fins 18 of thecooling device 10 and the right side illustrates circulation of the workingfluid 24 within the hermetically sealedchamber 16. - When in use, an electronic component generates and dissipates heat. The heat from the electronic component is taken in at the base portion of the
evaporator section 34 and raises the temperature of the base portion of theevaporator section 34. - Boiling of the working
fluid 24 inside theevaporator section 34 of the hermetically sealedchamber 16 occurs as heat is taken from the base portion of theevaporator section 34 by the workingfluid 24 as latent heat of vaporization. Thecapillary structure 40 in theevaporator section 34 enhances boiling of the workingfluid 24 as it creates a micro-flow situation within theevaporator section 34 by drawing the heated workingfluid 24 away from the base portion of theevaporator section 34 and fresh workingfluid 24 in the form ofcondensate 38 from thecondenser fins 36 to the evaporator section 34 (seearrow 50 inFIG. 3 ). The degree of superheat (i.e. the difference between the temperature of the base portion of theevaporator section 34 and the saturation temperature of the working fluid 24) is thus reduced. - Evaporation of the working
fluid 24 in thecapillary structure 40 and the escape of vapour bubbles from the liquid bulk in theevaporator section 34 to the liquid-vapour interface result in the creation of a higher pressure region in theevaporator section 34, forcing the vapour in theevaporator section 34 to a relatively lower pressure region in the hollow condenser fins 36 (seearrows 52 inFIG. 3 ). - Condensation occurs on
inner walls 54 of thecondenser fins 36 as theinner walls 54 are at a lower temperature than the vaporised workingfluid 24 in thecondenser fins 36. Accordingly, the workingfluid 24 is collected ascondensate 38 on theinner walls 54 of thecondenser fins 36. Depending on the orientation of thecooling device 10, thecondensate 38 may be returned to theevaporator section 34 by gravitational means where thecondenser fins 36 are at a higher elevation than theevaporator section 34, by capillary action via the capillary element 42 (particularly when thecondenser fins 36 are at a lower elevation than the evaporator section 34) or a combination of both. - Heat in the form of latent heat of condensation transmitted from the working
fluid 24 to thecondenser fins 36 is conducted to theconvective fins 18 which provide a large surface area for heat dissipation. Heat is convected away from theconvective fins 18 to ambient space by the flow of air from the fan 12 (seearrows 56 inFIG. 3 ). More particularly, thefan 12 mounted at the top of the main body of thecooling device 10 and located inside the opening end of theair guide 14 forces air to flow through the array ofconvective fins 18. Theair guide 14 constricts and guides the air flow into the array ofconvective fins 18. - Although illustrated in an upright position in
FIGS. 1A , 1B, 2A, 2B and 3, it should be understood that the cooling device of the present invention is not limited to a particular orientation. Rather, the cooling device may be used in any orientation as long as the capillary structure is in contact with the bulk working fluid in liquid phase for capillary pumping. - A numerical simulation was carried out to gauge the performance of the
cooling device 10 using water as the workingfluid 24 and copper for thechamber 16 and theconvective fins 18. Convective cooling is provided by anaxial flow fan 12 with a diameter of 92 millimetres (mm) and which generates an air flow rate of about 11.80 litre per second (l/s) (equivalent to 25 cubic feet per minute (CFPM)). The governing equations for the simulation (i.e. conservation equations for energy and mass provided below) are solved simultaneously using the International Mathematical and Statistical Library (IMSL) running on a Fortran PowerStation platform where the convergence criterion is a tolerance of 10−9. The steady state performance parameters of thecooling device 10 subjected to 10 watts per square centimetre (W/cm2) of heat flux from a CPU measuring 3 by 3 square centimetre (cm2) at ambient air temperature of 30 degrees Celsius (° C.) are shown in Table 1 below. -
TABLE 1 Thickness No. of CPU Air Flow of Conv. Conv. Temperature Rconv Rtotal Rate Pdrop Fins (mm) Fins (° C.) (K/W) (K/W) (l/s) (Pa) 0.15 18 49.1 0.123 0.218 11.89 24.99 0.25 17 48.1 0.116 0.206 11.94 24.91 0.50 14 48.7 0.123 0.213 10.48 26.71 0.75 12 49.9 0.136 0.226 9.34 28.18 1.00 10 51.2 0.151 0.241 9.25 28.29
wherein Rconv represents the convective thermal resistance between theconvective fins 18 and ambient air measured in kelvin per watt (K/W), Rtotal represents the total thermal resistance of thecooling device 10 and Pdrop represents the pressure drop across theconvective fins 18 and is measured in pascal (Pa). - The optimal performance for
convective fins 18 of various thicknesses is shown in Table 1. As can be seen from Table 1, thecooling device 10 with the thinnestconvective fins 18 measuring 0.25 mm outperforms all others by maintaining the CPU temperature at a mere 48.1° C. with an overall but optimal thermal resistance of 0.206 K/W as it allows placement of an optimum number ofconvective fins 18 while allowing an adequate amount air flow from theaxial flow fan 12 and thus achieves the highest convective heat transfer. However, as convective fin thickness is decreased further, it was observed that the benefits of increasing convective surface area by reducing fin thickness is somewhat offset by a drop in the fin efficiency for the thinner fins. It is concluded therefore that fin strength is also a design consideration. - Referring now to
FIG. 4 , the transient performance of thecooling device 10 with convective fin thickness of 0.25 mm is shown. The data for the graph ofFIG. 4 was obtained by performing a simulation test on acooling device 10 with seventeen (17)convective fins 18, each convective fin having a thickness of 0.25 mm, a height of 30 mm and a length of 40 mm, thecooling device 10 being subjected to varying heat flux loads ranging from 10 W/cm2 to 25 W/cm2 from a 9 cm2 CPU footprint. Temperatures of the CPU, the base portion of theevaporator section 34, the saturated workingfluid 24 in the hermetically sealedchamber 16, and theinner walls 54 of the condenser fins 36 (in particular, at the base portion of the condenser fin array) were measured and plotted against time till a steady state was reached. - In all the simulated cases, the CPU chip temperature did not exceed 70° C.—the thermal design limit of the CPU—even at a CPU heat flux of 25 W/cm2 (i.e. a total of 225 watts (W)).
- The following were observed from the results of the simulation:
- (i) The thermal resistance between the
convective fins 18 and ambient air is highest for all simulated heat flux loads. - (ii) The next highest thermal resistance in the
cooling device 10 is the superheat for boiling. In this regard, it was observed that the heat transfer coefficient of boiling increases with increasing surface heat flux at the base portion of theevaporator section 34. Thus, the corresponding increase in boiling superheat is minimal. - (iii) The temperature differential between the CPU and the base portion of the
evaporator section 34 was observed to increase from a small differential of 2.3° C. to a high of 5.8° C. This indicates that both the heat spreading and interface resistances increase with increasing heat flux loads. - (iv) The temperature differential between the condenser wall and the saturated working
fluid 24 was observed to increase from a small differential of 0.5° C. to 1.5° C. This small increase is acceptable and indicates that condensation is an efficient process for heat rejection. - Observations (i) and (ii) are consistent with existing case studies.
- The heat spreading problem may be alleviated by using an evaporator with a smaller base area. In regard to the boiling resistance, the thermal conductivity of the working
fluid 24 may be enhanced by adding nanoparticles to the workingfluid 24 to reduce the boiling superheat. The size of theconvective fins 18 may be varied according to the space available to accommodate future higher heat fluxes. - The energy balance at the base portion of the
evaporator section 34 made of copper, taking into consideration the heat given out by the CPU chip and the heat extracted by evaporation of the workingfluid 24, may be modelled with the following equation: -
- wherein (Mcp)base represents the thermal capacity of the base portion of the
evaporator section 34, -
- represents the rate of change of the temperature at the base portion of the
evaporator section 34 over time, qflux chip represents the heat flux input from the heat source, Achip represents the footprint area of the heat source, Aevap represents the bottom surface area of theevaporator section 34, and qflux evap represents the heat taken in by the workingfluid 24 for evaporation. - The hermetically sealed
chamber 16 includes aporous structure 40 and a saturated workingfluid 24. Latent heat is taken in by the workingfluid 24 from the superheated surface of the base portion of theevaporator section 34, and is transported and rejected to the finned condenser section. The energy balance of the workingfluid 24 may be modelled with the following equation: -
- wherein (Mcp)eff represents the effective thermal capacity of the working
fluid 24 and is obtained by summing the thermal capacity of the workingfluid 24 in the vapour, liquid and phases (i.e. (Mcp)eff=(MCp)vapor+(MCp)liquid+(MCp)foam), -
- represents the rate of change of the temperature of the vapour space in the
chamber 16 over time, hg and hf represent the respective enthalpies of the workingfluid 24 in vapor and liquid phases at a given temperature, Tevap represents the temperature of thesection 34, Tcond presents the temperature of the condenser section, -
- represents the mass flow rate of the working
fluid 24 in vapour phase and -
- represents the mass flow rate of the
condensate 38. - The Rohsenow Correlation, adjusted for sub-atmospheric conditions as shown in equations (3) and (4) below, is used to predict the heat flux taken in by the working
fluid 24 for boiling at a certain superheat at sub-atmospheric conditions in the presence of the porous structure 40: -
- wherein s is 1, m is 0.293, n is −0.0984, Tsurface represents the temperature of the base portion of the
evaporator section 34, CSf, an empirical constant related to the fluid-heater surface combination, is 0.0132, hfg represents the latent heat of evaporation of the workingfluid 24, Prl represents the Prantl Number, Cpf represents the heat capacity of the workingfluid 24 in liquid phase, μl represents the viscosity of the workingfluid 24 in liquid phase, σ represents surface tension, g represents acceleration due to gravity, ρl represents the density of the workingfluid 24 in liquid phase, ρg represents the density of the workingfluid 24 in vapour phase, P represents the pressure in theevaporator section 34, Patm represents atmospheric pressure, Awetted represents the wetted surface area of theporous structure 40 and Abase represents the surface area of the base portion of theevaporator section 34. - The Thin Film Condensation Correlation is used to predict the heat transfer coefficient hi of the condenser section as shown in equation (5) below on the assumption that the walls of the condenser section are acting as individual plates:
-
- wherein k represents the thermal conductivity of the working
fluid 24 in liquid phase, μ represents the viscosity of the workingfluid 24 in liquid phase, Tv represents the temperature of the workingfluid 24 in vapour phase in the hermetically sealedchamber 16 and Lfin represents the length of thecondensate 38 flow path. - Mass balance was performed on the hermetically sealed
chamber 16 to account for all the liquid and vapour mass inside thechamber 16. This provides information for determining the optimum amount of workingfluid 24 to fill the hermetically sealedchamber 16 with to prevent drying out and to ensure wetting of thecapillary structure 40 throughout the operation of thecooling device 10. The mass balance was modelled with the following equations: -
- wherein
-
- represents the net flow rate of the vapour in the
chamber 16, -
- represents the net flow rate of the liquid in the
chamber 16, in {dot over (m)}evaporation represents the rate of evaporation and {dot over (m)}condensation represents the rate of condensation. - The rate of evaporation {dot over (m)}condensation and the rate of condensation {dot over (m)}condensation may be determined with the following equations:
-
- wherein Qflux evap represents the heat flux at the base portion of the
evaporator section 34 and is equivalent to qflux evap defined above, Tchamber represents the temperature in the vapour space of the hermetically sealedchamber 16 and Acond represents the surface area of theinner walls 54 of thecondenser fins 36. - An energy balance was performed on the
condenser fins 36. Latent heat is transferred to theinner walls 54 of thecondenser fins 36 when the workingfluid 24 condenses on theinner walls 54. The heat transmitted to thecondenser fins 36 is then conducted to theconvective fins 18 where it is transferred to ambient air by forced convection. The energy balance at the condenser section may be modelled with the following equation: -
- wherein (Mcp)cond represents the heat capacity of the condenser section,
-
- represents the rate of change of the temperature of the
inner walls 54 of thecondenser fins 36 over time, Tair represents ambient temperature, and Rhs represents the thermal resistance between theconvective fins 18 and ambient air. - As is evident from the foregoing discussion, the present invention provides a compact cooling device with reduced heat spreading resistance for intense heat emitting surfaces such as, for example, a microprocessor or CPU of a desktop computer, as well as surfaces with high localised heat fluxes. The cooling device includes: an evaporator section where working fluid evaporates at its saturation pressure, extracting its latent heat of vaporization from the intense heat surface; a condenser section to reject latent heat from incoming working fluid in vapour phase from the evaporator section; a capillary structure located in the evaporator section to circulate the working fluid in liquid phase from the condenser section back to the evaporator section; an array of convective fins as a means of extending the heat sink surface area; and a fan to blow air through the array of convective fins, thereby enhancing heat rejection to ambient air. Advantageously, the evaporator section, the condenser section, the convective fins, and the fan can all be integrated into one compact device. By using a two-phase system, the overall size of the cooling device and heat spreading resistance can be significantly reduced, increasing the overall heat transfer capacity of cooling device.
- While the preferred embodiments of the invention have been illustrated and described, it will be clear that the invention is not limited to these embodiments only. Numerous modifications, changes, variations, substitutions and equivalents will be apparent to those skilled in the art without departing from the scope of the invention as described in the claims.
- Further, unless the context dearly requires otherwise, throughout the description and the claims, the words “comprise”, “comprising” and the like are to be construed in an inclusive as opposed to an exclusive or exhaustive sense; that is to say, in the sense of “including, but not limited to”.
Claims (17)
1. A cooling device, comprising: a chamber having an evaporator section and a condenser section, the condenser section extending from a periphery of the evaporator section.
2. The cooling device according to claim 1 , wherein the condenser section comprises a plurality of condenser fins extending from the periphery of the evaporator section.
3. The cooling device according to claim 2 , further comprising a plurality of convective fins extending from surfaces of the condenser fins.
4. The cooling device according to claim 3 , wherein the convective fins extend between adjacent ones of the condenser fins.
5. The cooling device according to claim 4 , further comprising a fan arranged to direct a flow of air at the convective fins.
6. The cooling device according to claim 5 , further comprising an air guide arranged to direct the flow of air from the fan to the convective fins.
7. The cooling device according to claim 3 , wherein the convective fins are integrally moulded to the surfaces of the condenser fins.
8. The cooling device according to claim 1 , further comprising a capillary element arranged to facilitate return of condensate from the condenser section to the evaporator section.
9. The cooling device according to claim 1 , wherein a base portion of the condenser section is at an angle to a base portion of the evaporator section.
10. The cooling device according to claim 1 , wherein the evaporator section has a capillary structure.
11. The cooling device according to claim 10 , wherein the capillary structure comprises an open cell foam metal.
12. The cooling device according to claim 11 , wherein the open cell foam metal is bonded to a base portion of the evaporator section by diffused bonding or brazing.
13. The cooling device according to claim 10 , wherein the capillary structure has a pore density of from about 8 to about 52 pores per centimetre (ppc).
14. The cooling device according to claim 13 , wherein the pore density of the capillary structure is about 16 ppc.
15. The cooling device according to claim 1 , further comprising a working fluid arranged to absorb and transfer heat energy.
16. The cooling device according to claim 15 , wherein the working fluid has a latent heat of vaporisation of greater than or equal to about 1500 joules per gram (J/g).
17. The cooling device according to claim 15 , further comprising a plurality of nanoparticles in working fluid, the nanoparticles having a thermal conductivity of between about 200 Watt per metre Kelvin (W/m·K) to about 400 W/m·K.
Priority Applications (1)
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US12/597,774 US20100132924A1 (en) | 2007-04-27 | 2008-04-24 | Cooling device for electronic components |
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US91441007P | 2007-04-27 | 2007-04-27 | |
PCT/SG2008/000137 WO2008133594A2 (en) | 2007-04-27 | 2008-04-24 | Cooling device for electronic components |
US12/597,774 US20100132924A1 (en) | 2007-04-27 | 2008-04-24 | Cooling device for electronic components |
Publications (1)
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US20100132924A1 true US20100132924A1 (en) | 2010-06-03 |
Family
ID=39926220
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US12/597,774 Abandoned US20100132924A1 (en) | 2007-04-27 | 2008-04-24 | Cooling device for electronic components |
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WO (1) | WO2008133594A2 (en) |
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