Abstract:
The invention is for an apparatus and method for removal of waste heat from heat-generating components including high-power solid-state analog electronics such as being developed for hybrid-electric vehicles, solid-state digital electronics, light-emitting diodes for solid-state lighting, semiconductor laser diodes, photo-voltaic cells, anodes for x-ray tubes, and solids-state laser crystals. Liquid coolant is flowed in one or more closed channels having a substantially constant radius of curvature. Suitable coolants include electrically conductive liquids (including liquid metals) and ferrofluids. The former may be flowed by magneto-hydrodynamic effect or by electromagnetic induction. The latter may be flowed by magnetic forces. Alternatively, an arbitrary liquid coolant may be used and flowed by an impeller operated by electromagnetic induction or by magnetic forces. The coolant may be flowed at very high velocity to produce very high heat transfer rates and allow for heat removal at very high flux.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority from U.S. provisional patent application U.S. Ser. No. 61/000,855, filed on Oct. 29, 2007; and U.S. provisional patent application U.S. Ser. No. 61/191,304, filed on Sep. 8, 2008. 
     
    
     FIELD OF THE INVENTION 
       [0002]    This invention relates generally to heat removal from heat-generating components and more specifically to heat removal at high heat flux. 
       BACKGROUND OF THE INVENTION 
       [0003]    The subject invention is an apparatus and method for removal of waste heat from heat-generating components including analog solid-state electronics, digital solid-state electronics, semiconductor laser diodes, light emitting diodes, photo-voltaic cells, vacuum electronics, and solid-state laser crystals. 
         [0004]    There are many devices generating waste heat as a byproduct of their normal operations. These include analog solid-state electronic components, digital solid-state electronic components, semiconductor laser diodes, light emitting diodes for solid-state lighting, solid-state laser components, laser crystals, vacuum electronic components, and photovoltaic cells. Waste heat must be efficiently removed from such components to prevent overheating and consequential loss of efficiency, malfunction, or even catastrophic failure. Methods for waste heat management may include conductive heat transfer, convective heat transfer, and radiative heat transfer, or various combinations thereof. For example, waste heat removed from heat generating components may be transferred to a heat sink by a flowing heat transfer fluid. Such a heat transfer fluid is also known as a coolant. 
         [0005]    Cooling requirements for the new generation of heat-generating components (HGC) are very challenging for thermal management technologies of prior art. For example, an ongoing miniaturization of semiconductor digital and analog electronic devices requires removal of heat at ever increasing fluxes now on the order of several hundreds of watts per square centimeter. Traditional heat sinks and heat spreaders have large thermal resistance contributing to elevated junction temperatures and thus reducing device reliability. As a result, removal of heat often becomes the limiting factor and a barrier to further performance enhancements. More specifically, a new generation of high-power semiconductors for hybrid electric vehicles and future plug-in hybrid electric vehicles requires improved thermal management to boost heat transfer rates, eliminate hot spots, and reduce volume, while allowing for higher current density. 
         [0006]    High-brightness light emitting diodes (LED) being developed for solid-state lighting for general illumination in commercial and household applications also require improved thermal management. These new light sources are becoming of increased importance as they offer up to 75% savings in electric power consumption over conventional lighting systems. Waste heat must be effectively removed from the LED chip to reduce junction temperature, thereby prolonging LED life and making LED cost effective over traditional lighting sources. 
         [0007]    Another class of electronic components requiring improved cooling are semiconductor-based high-power laser diodes used for direct material processing and pumping of solid-state lasers. Generation of optical output from laser diodes is accompanied by production of large amount of waste heat that must be removed at a flux on the order of several hundreds of watts per square centimeter. In addition, the temperature of high-power laser diodes must be controlled within a narrow range to avoid undesirable shifts in output wavelength. 
         [0008]    Photovoltaic cells (solar electric cells and thermo-photovoltaic cells) are becoming increasingly important for generation of electricity. Such cells may be used with concentrators to increase power generation per unit area of the cell and thus reduce initial installation cost. This approach requires removal of waste heat at increased flux. Similarly, high-performance crystals used in solid-state lasers generate waste heat that may require removal at fluxes in the neighborhood of thousand watts per square centimeter. 
         [0009]    Anodes in x-ray tubes are subjected to very high thermal loading. Rotating anodes are frequently used to spread the heat to avoid overheating. Such rotating anodes inside a vacuum enclosure are impractical for use in a new generation of x-ray tubes for use in compact and portable devices in medical and security applications. A compact and lightweight heat transfer component having no moving parts inside the vacuum is very desirable. 
         [0010]    Current approaches for removal of waste heat for at high fluxes include 1) spreading of heat with elements having high thermal conductivity and/or 2) forced convection cooling using liquid coolants. However, even with heat spreading materials having extremely high thermal conductivity such as diamond films and certain graphite fibers, a significant thermal gradient is required to conduct large amount of heat even over short distances. In addition, passive heat spreaders are not conducive to temperature control of the HGC. Forced convection methods for removal of waste heat at high fluxes may use microchannel heat exchangers or impingement jets to obtain desirable heat transfer coefficient with conventional coolants such as water, alcohol, or ethylene glycol. Liquid metal coolants have been also considered to attain target heat transfer coefficient. Known forced convection systems have many components, are bulky, heavy, and have geometries that require the coolant to make complex directional changes while traversing the coolant loop. Such directional changes are a potential source of increased flow turbulence causing higher pressure drop in the loop and, therefore, necessitate higher pumping power. 
         [0011]    In summary, prior art does not teach a heat transfer device capable of removing heat at very high fluxes that is also compact, lightweight, self contained, capable of accurate temperature control, has a low thermal resistance, and requires very little power to operate. It is against this background that the significant improvements and advancements of the present invention have taken place. 
       SUMMARY OF THE INVENTION 
       [0012]    The present invention provides a heat transfer device (HTD) wherein a coolant flows in a closed channel with a substantially constant radius of curvature. This arrangement offers low resistance to flow, which allows to flow the coolant at very high velocities and thus enables heat transfer at very high rate while requiring relatively low power to operate. HTD of the subject invention may be used to cool HGC requiring removal of waste heat at very high heat flux. Such HGC may include solid-state electronic chips, semiconductor laser diodes, light emitting diodes for solid-state lighting, solid-state laser components, laser crystals, optical components, vacuum electronic components, and photovoltaic cells. Heat removed by HTD from HGC may be transferred to a heat sink or environment at a reduced heat flux. For example, HTD may transfer acquired heat to a structure, heat pipe, secondary liquid coolant, phase change material (PCM), gaseous coolant, or ambient air. 
         [0013]    In one preferred embodiment of the present invention, the HTD comprises a body having a first surface, a second surface, and a closed flow channel. The first surface is adapted for receiving heat from a heat generating component and the second surface is adapted for transferring heat to a heat sink. The flow channel has a substantially constant radius of curvature in the flow direction. An electrically conductive liquid coolant is flowed inside the flow channel by means of a magneto-hydrodynamic (MHD) effect (MHD drive). 
         [0014]    In another preferred embodiment of the present invention, electrically conductive liquid or ferrofluid coolant may be used and flowed by the means of a moving magnetic field. Moving magnetic field induces eddy currents in the electrically conductive coolant that, in turn, provide force coupling to the coolant (inductive drive). Alternatively, moving magnetic field directly couples into the ferrofluid (magnetic drive). Suitable moving field may be generated by a rotating magnet. 
         [0015]    In yet another preferred embodiment of the present invention, the moving (rotating or traveling magnetic) magnetic field may be generated by stationary electromagnets operated by alternate current in an appropriate poly-phase relationship. In a still another embodiment of the present invention, the coolant is an arbitrary liquid flowed in a closed channel with a substantially constant radius of curvature. The coolant flow is induced by a rotating impeller (impeller drive) spun by a flow of secondary coolant, mechanical means, moving magnetic field, or by electromagnetic induction. 
         [0016]    Accordingly, it is an object of the present invention to provide a heat transfer device (HTD) for removing waste heat from HGC. The HTD of the present invention is simple, compact, lightweight, self-contained, can be made of materials with a coefficient of thermal expansion (CTE) matched to that of the HGC, requires relatively little power to operate, and it is suitable for large volume production. 
         [0017]    It is another object of the invention to provide means for cooling HGC. 
         [0018]    It is still another object of the invention to provide means for temperature control of HGC. 
         [0019]    It is yet another object of the invention to cool a semiconductor electronic components. 
         [0020]    It is yet further object of the invention to cool semiconductor laser diodes. 
         [0021]    It is a further object of the invention to cool LED for solid-state lighting. 
         [0022]    It is still further object of the invention to cool computer chips. 
         [0023]    It is an additional object of the invention to cool photovoltaic cells. 
         [0024]    These and other objects of the present invention will become apparent upon a reading of the following specification and claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0025]      FIG. 1A  is a side cross-sectional view of a heat transfer device (HTD) in accordance with one embodiment of the subject invention using a magneto-hydrodynamic drive. 
           [0026]      FIG. 1B  is a cross-sectional view of an HTD in a plane transverse to coolant flow in accordance with one embodiment of the subject invention using a magneto-hydrodynamic drive. 
           [0027]      FIG. 2A  is an enlarged view of portion  2 A of the HTD of  FIG. 1A . 
           [0028]      FIG. 2B  is an enlarged view of portion  2 B of the HTD of  FIG. 1B . 
           [0029]      FIG. 3  is an enlarged view of alternative portion  2 B of the HTD of  FIG. 1B  showing a flow channels with surface extensions. 
           [0030]      FIG. 4  is an enlarged view of another alternative portion  2 B of the HTD of  FIG. 1B  showing multiple flow channels arranges side-by-side. 
           [0031]      FIG. 5  is an enlarged view of portion  2 A of the HTD of  FIG. 1A  showing a mounting of a laser diode array HGC. 
           [0032]      FIG. 6  is an enlarged view of portion  2 A of the HTD of  FIG. 1A  showing a mounting of a laser diode bar HGC. 
           [0033]      FIG. 7  is an enlarged view of portion  2 A of the HTD of  FIG. 1A  showing a mounting of a light emitting diode HGC. 
           [0034]      FIG. 8  is an enlarged view of portion  2 A of the HTD of  FIG. 1A  showing a mounting of a solid-state laser crystal HGC. 
           [0035]      FIG. 9  shows an alternative HTD body having internal passages for a Secondary coolant. 
           [0036]      FIG. 10  shows another alternative HTD body having external fins for heat transfer to gaseous coolant or ambient air. 
           [0037]      FIG. 11A  is a side cross-sectional view of an HTD in accordance with another embodiment of the subject invention wherein coolant flow is induced by a rotating magnetic field produced by a rotating magnet. 
           [0038]      FIG. 11B  is a side cross-sectional view of an HTD in a plane transverse to coolant flow in accordance with another embodiment of the subject invention wherein coolant flow is induced by a rotating magnetic field produced by a rotating magnet. 
           [0039]      FIG. 12A  is a side cross-sectional view of an HTD in accordance with a yet another embodiment of the subject invention wherein coolant flow is induced by a rotating magnetic field produced by stationary electromagnets. 
           [0040]      FIG. 12B  is a side cross-sectional view of an HTD in a plane transverse to the flow loop in accordance with yet another embodiment of the subject invention wherein coolant flow is induced by a rotating magnetic field produced by stationary electromagnets. 
           [0041]      FIG. 13  shows a suitable connection of electromagnets to a single phase alternating current supply. 
           [0042]      FIG. 14  shows a variant to the HTD in accordance with a yet another embodiment of the subject invention wherein the electromagnets are arranged to generate translating magnetic field. 
           [0043]      FIG. 15A  is a side cross-sectional view of an HTD in accordance with still another embodiment of the subject invention using an impeller. 
           [0044]      FIG. 15B  is a side cross-sectional view of an HTD in a plane transverse to coolant flow in accordance with still another embodiment of the subject invention using an impeller. 
           [0045]      FIG. 16A  is a plan view of an HTD in accordance with a further embodiment of the subject invention having a planar flow loop. 
           [0046]      FIG. 16B  is a side cross-sectional view of an HTD in accordance with further embodiment of the subject invention having a planar flow loop. 
           [0047]      FIG. 17A  is a plan view of an HTD in accordance with a still further embodiment of the subject invention having a planar flow loop with an impeller. 
           [0048]      FIG. 17B  is a side cross-sectional view of an HTD in accordance with still further embodiment of the subject invention having a planar flow loop with an impeller. 
           [0049]      FIG. 18  is a plan view of an alternative impeller of the HTD of  FIG. 17A . 
           [0050]      FIG. 19A  is a side cross-sectional view of an HTD in accordance with a yet further embodiment of the subject invention having an elongated flow loop. 
           [0051]      FIG. 19B  is a face view of an HTD in accordance with yet further embodiment of the subject invention having an elongated flow loop. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0052]    Selected embodiments of the present invention will now be explained with reference to drawings. In the drawings, identical components are provided with identical reference symbols in one or more of the figures. It will be apparent to those skilled in the art from this disclosure that the following descriptions of the embodiments of the present invention are merely exemplary in nature and are in no way intended to limit the invention, its application, or uses. 
         [0053]    Referring now to  FIGS. 1A and 1B , there is shown a heat transfer device (HTD)  100  in accordance with one preferred embodiment of the subject invention. HTD  100  comprises a body  102 , magnets  128   a  and  128   b,  electrodes  130   a  and  130   b,  and electric conductors  126   a  and  126   b.  The body  102  further comprises a first surface  106  adapted for receiving heat from a heat generating component (HGC), a second surface  108  adapted for rejecting heat to a heat sink, and a flow channel  104 . The body  102  is preferably made of material having high thermal conductivity. Preferably, such a material may also have a low electrical conductivity or such a material may be dielectric. Suitable materials for construction of the body  102  may include silicon, berylia, and silicon carbide. A heat generating component (HGC)  114  may be also attached to the first surface  106  and arranged to be in a good thermal contact therewith. HGC  114  may be, but it is not limited to a solid-state electronic chip, semiconductor laser diode, light emitting diodes (LED), solid-state laser crystal, optical component, x-ray tube anode, or a photovoltaic cell. If desired, the body  102  may be made from material having a coefficient of thermal expansion (CTE) matched to the CTE of the HGC  114 . The second surface  108  is arranged to be in a good thermal communication with a heat sink. Suitable heat sinks include a structure, heat pipe, secondary liquid coolant, phase change material (PCM), gaseous coolant, or ambient air. Fluid used as a heat sink may employ natural convection or forced convection to remove heat from the second surface  108 . The second surface  108  may also include surface extensions such as fins or ribs to enhance heat transfer therefrom. 
         [0054]    Referring now to  FIGS. 2A and 2B , the HGC  114  may be thermally coupled to the first surface  106  with a suitable joining material  120 . Preferably, joining material  120  has a good thermal conductivity. Suitable joining materials include solder, thermally conductive paste, epoxy, liquid metals, and adhesive. Alternatively, HGC  114  may be diffusion bonded onto surface  106 . The flow channel  104  comprises an outer surface  110  and an inner surface  112 . Each of the surfaces  110  and  112  may have a width “W” and they may be separated from each other by a distance “H”. Each of the surfaces  110  and  112  preferably has a constant radius of curvature “R” and “R-H”, respectively. For example, surfaces  110  and  112  may each be cylindrical and mutually concentric, thereby giving the flow channel  104  a general shape of a torus having a rectangular cross-section of width “W” and height “H”. Because the channel forms a closed loop, it may be also referred to in this disclosure as the “closed flow channel.” Preferred range for the width “W” is 0.1 to 20 millimeters, but dimensions outside this range may be also practiced. Preferred range for the radius of curvature “R” is 5 to 25 millimeters, but dimensions outside this range may be also practiced. Preferably, the distance “H” is chosen so that the channel  104  has a hydraulic diameter (=2 WH/(W+H)) about one to three millimeters, and most preferably about ten to micrometers to one millimeter. In addition, surfaces  110  and  112  should be made very smooth. Preferably, surfaces  110  and  112  are finished to surface roughness of less than 8 micrometers root-mean-square value, and most preferably to surface roughness of less than 1 micrometer root-mean-square value. Surfaces of the flow channel  104  may also have a coating to protect them from corrosion. The first surface  106  may be separated from the outer surface  110  by a distance “S” ( FIG. 2B ). Preferred range for the distance “S” is 0.1 to 1 millimeter, but dimensions outside this range may be also practiced. 
         [0055]    The flow channel  104  contains a suitable electrically conductive liquid coolant  116 . Preferably, the flow channel  104  is not entirely filled with the liquid coolant and at least some void space free of liquid coolant is provided inside the channel to allow for thermal expansion of the coolant. Preferably, the liquid coolant  116  has a good thermal conductivity, low viscosity, and low freezing point. Suitable liquid coolants  116  include selected liquid metals. For the purposes of this disclosure, the term “liquid metal” shall mean suitable metals (and their suitable alloys) that are in a liquid (molten) state at their operating temperature. Liquid metals have a comparably good thermal conductivity while being also electrically conductive and, in some cases have a relatively low viscosity. Examples of suitable liquid metals include mercury, gallium, indium, bismuth, and sodium. Examples of suitable liquid eutectic metal alloys include Indalloy 51 and Indalloy 60 (manufactured by Indium Corporation in Utica, N.Y.), and galinstan (obtainable from Geratherm Medical AG in Geschwenda, Germany). Galinstan is a nontoxic eutectic alloy of 68.5% by weight of gallium, 21.5% by weight of indium and 10% by weight of tin, having a melting point around minus 19 degrees Centigrade. It is important that electrodes  130   a  and  130   b  ( FIG. 1B ), and surfaces of the flow channel  104  are made of materials compatible with the coolant  116 . In particular, it is well know that gallium and its alloys severely corrode many metals. Prior art indicates that certain refractory metals such as tantalum and tungsten may be stable in gallium. See, for example, “Effects of Gallium on Materials at Elevated Temperatures,” by W. D. Wilkinson, Argonne National Laboratory Report ANL-5027 (August 1953). To protect against corrosion, surfaces of the flow channel  104  may be coated with suitable protective film. Prior art indicates that TiN and certain organic coatings may be stable in gallium. If a protective coating is additionally dielectric, the body  102  may be constructed from electrically conductive materials. 
         [0056]    The outer surface  110  may also include extensions  118  to increase the contact area between the surface  110  and liquid coolant  116  ( FIG. 3 ). Suitable form of surface extension  118  includes fins and ribs. Alternatively, multiple flow channels  104   a - 104   e  may be employed ( FIG. 4 ). In some variants of the invention, a portion of the HGC  114  may form a portion of the outer surface  110  of the flow channel  104 .  FIG. 5  shows a mounting of HGC  114 ′, which is an array of semiconductor laser diodes (or laser diode bars)  150  imbedded in a substrate  148  and producing optical output  152 . Suitable array of semiconductor laser diode bars imbedded in a substrate known as “silver bullet laser diode assembly submodule” and as “golden bullet laser diode assembly submodule” may be obtained from Northrop-Grumman Cutting Edge Optronics in St. Charles, MO.  FIG. 6  shows a mounting of HGC  114 ″, which is a laser diode bar producing optical output  152 . Suitable laser diode bar known as “unmounted laser diode bar” may be obtained from Northrop-Grumman Cutting Edge Optronics in St. Charles, Mo.  FIG. 7  shows a mounting of HGC  114 ″′, which is a high-power light emitting diode producing optical output  153 . Suitable high-power light emitting diode known as “Luxeon® K2” may be obtained from Philips Lumileds Lighting Company, Sun Valley, Calif..  FIG. 8  shows a mounting of HGC  114   iv , which is a solid-state laser crystal receiving optical pump radiation  151  and amplifying a laser beam  155 . Suitable solid-state laser crystal may be in the form of a thin disk laser as, for example, described by Kafka et al., in the U.S. Pat. No. 7,003,011. 
         [0057]    Referring now again to  FIGS. 1A and 1B , the magnets  128   a  and  128   b  are arranged to generate magnetic field that traverses the flow channel  104  in the proximity of electrodes  130   a  and  130   b  in a substantially radial direction. Double arrow  160  indicates preferred directions of the magnetic field. Magnets  128   a  and  128   b  are preferably permanent magnets, and most preferably rare earth permanent magnets. Alternatively, magnets  128   a  and  128   b  may be formed as electromagnets. As a yet another alternative, magnets  128   a  and  128   b  may be pole extensions of a single magnet. Electrodes  130   a  and  130   b  are in electrical contact with the liquid coolant  116  and are arranged so that electric current may be passed through the coolant  116  in the region between the magnets  128   a  and  128   b  in a direction generally orthogonal to magnetic field direction. Electrodes  130   a  and  130   b  may be connected to external source of direct electric current via electric conductors  126   a  and  126   b  respectively. The HTD  100  may further include a magnetic shield (not shown) to prevent adverse effect of magnetic field generated by magnets  128   a  and  128   b  on HGC  114  and/or nearly components. 
         [0058]    In operation, electric current is passed though the liquid coolant  116  between electrodes  130   a  and  130   b.  Because at least a portion of the coolant  116  is immersed in magnetic field orthogonal to the electric current flowing though the coolant  116 , a magneto-hydrodynamic (MHD) effect causes the coolant  116  to flow in the direction indicated by the arrow  122  in  FIG. 1A  and the arrows  124  in  FIG. 2A . As a result, flow of coolant  116  forms a closed flow loop. Because the closed flow loop has a substantially constant radius of curvature and the walls of the flow channel  104  are smooth, the flow of coolant  116  encounters relatively little resistance. As a result, very high flow velocities of coolant  116  can be sustained with a relatively small amount of motive power. 
         [0059]    The HGC  114  is operated and its waste heat is allowed to transfer through the first surface  106  into the body  102  and conducted to the outer surface  110  of the flow channel  104 . The second surface  108  is maintained at a temperature substantially below the temperature of the HGC  114 . Liquid coolant  116  flowing at high velocity enables a very high heat transfer coefficient on the surface  110 . Heat is transferred from the surface  110  into the liquid coolant  116 , transported by the coolant  116 , and deposited into other parts of the body  102 . Heat deposited into other parts of the body  102  is conducted to the second surface  108  and transported therefrom to a suitable heat sink. Using the above process, HTD  100  removes heat from the HGC  114  and transfers it to a heat sink or environment.  FIG. 9  shows an HTD body  102 ′ having a second surface  108 ′ formed as internal passages for flowing secondary liquid or gaseous coolant.  FIG. 10  shows an HTD body  102 ″ having a second surface  108 ″ formed as external fins for transferring heat to gaseous coolant or ambient air. 
         [0060]    Temperature of HGC  114  may be controlled by controlling the flow velocity of the coolant  116 . The latter can be accomplished by controlling the current drawn through the coolant  116  via electrodes  130   a  and  130   b.  For example, by drawing more current through the coolant  116 , the coolant flow velocity may be increased, and the HGC waste heat may be removed at a lower temperature differential between the HGC and the heat sink. Conversely, by drawing less current through the coolant  116 , the coolant velocity may be decreased, and the HGC waste heat may be removed at a higher temperature differential between the HGC and the heat sink. Thus, by drawing more current through the coolant  116 , the temperature of the HGC  114  may decreased, and by drawing less current through the coolant  116 , the temperature of the HGC  114  may be increased. An automatic closed-loop temperature control of HGC  114  can be realized by sensing HGC temperature (for example, with a thermocouple) and using this information to appropriately control the current drawn through the coolant  116 . In particular, if the HGC  114  is an LED, its temperature may be inferred from the output light spectrum. A means for sensing the LED light spectrum may be provided for this purpose. If the HGC  114  is a semiconductor laser diode, its temperature may be inferred from the output light center wavelength. A means for sensing the semiconductor laser diode output light center wavelength may be provided for this purpose. Alternatively, HGC temperature may be determined from certain current and/or voltages sensed in the HGC. If the coolant used in the HTD is susceptible to freezing (solidifying) due to ambient conditions during inactivity, the HTD may be equipped with an electric heater to warm the coolant up to at least its melting point. 
         [0061]    Referring now to  FIGS. 11A and 11B , there is shown a heat transfer device (HTD)  200  in accordance with another preferred embodiment of the subject invention. HTD  200  is similar to HTD  100 , except that in HTD  200  the coolant  216  inside the flow channel  204  may be an electrically conductive liquid or a ferrofluid. In addition, the flow of the coolant  216  is caused by a rotating magnetic field. The flow channel  204  in HTD  200  may be of the same construction as the flow channel  104  in HTD  100 . Ferrofluids are composed of nanoscale ferromagnetic particles suspended in a carrier fluid, which may be water, an organic liquid, or other suitable liquid. Certain water-based ferrofluids such as W11 available from FerroTec in Bedford, N.H., are also electrically conductive. Ferrofluids using a liquid metal or liquid metal alloy as a carrier fluid have been reported in prior art; see, for example, an article by J. Popplewell and S. Charles in New Sci. 1980, 97 (1220), 332. The nano-particles are usually magnetite, hematite or some other compound containing iron and are typically on the order of about 10 nanometers in size. This is small enough for thermal agitation to disperse them evenly within a carrier fluid, and for them to contribute to the overall magnetic response of the fluid. The ferromagnetic nano-particles are coated with a surfactant to prevent their agglomeration (due to van der Waals and magnetic forces). Ferrofluids may display paramagnetism, and are often referred as being “superparamagnetic” due to their large magnetic susceptibility. Alternatively, liquid coolant  216  may comprise a liquid having significant paramagnetic, diamagnetic, or ferromagnetic properties. 
         [0062]    The body  202  is similar to body  102  of HTD  100  ( FIG. 1A ) except that it has a round central opening  264 . In addition, the magnets  128   a  and  128   b,  the electrodes  130   a  and  130   b,  and the electric conductors  126   a  and  126   b  ( FIG. 1A ) are omitted. The body  202  further comprises a first surface  206  adapted for receiving heat from HGC  114 , a second surface  208  adapted for rejecting heat. Furthermore, the body  202  may be also constructed from a variety of (preferably non-ferromagnetic) materials preferably having high thermal conductivity. For example, the body  202  may be constructed from copper, copper-tungsten alloy, aluminum, molybdenum, silicon, and silicon carbide. Depending on the choice of coolant  216 , the surfaces of the flow channel  204  may require appropriate protective coating to present corrosion. HTD  200  further comprises a magnet  234  rotatably suspended inside the opening  264  and positioned so that a significant portion of magnetic field lines cross the flow channel  204 . The label “N” designates the north pole of the magnet and the label “S” designates the south pole of the magnet. 
         [0063]    Operation of HTD  200  is similar to the operation of HTD  100  except that the flow of the coolant  216  is caused by different means than flow of the coolant  116  in HTD  100 . In particular, magnet  234  is rotated in the direction of arrow  238  to generate a rotating magnetic field. The magnet  234  may be rotated mechanically by shaft  236  that may be coupled to an external drive such as electric motor. Alternatively, the magnet  234  may be rotated by means of a magnetic coupling to an external rotating ferromagnetic component. As another alternative, the magnet  234  may be rotated by a rotating magnetic field generated by electromagnets. As a yet another alternative, the magnet  234  may be rotated by a turbine operated by a secondary coolant flowing through the central opening  264 . 
         [0064]    If the coolant  216  is an electrically conductive liquid, time varying magnetic field produced by the rotation of the magnet  234  induces eddy currents in the electrically conductive coolant  216 . Such eddy currents, interact with the rotating magnetic field produced by the magnet  234  thereby establishing a force coupling between the rotating magnet  234  and the coolant  216 . As a result, rotating magnet  234  exerts a force onto the coolant  216  causing the coolant  216  to flow inside the flow channel  204  in the direction of the arrow  222  thereby forming a flow loop. Additional information about eddy current devices may be found in “Permanent Magnets in Theory and Practice,” chapter 7.6: Eddy-Current Devices, by Malcolm McCraig, published by Pentech Press, Plymouth, UK, 1977. 
         [0065]    If the coolant  216  is a ferrofluid, magnetic field produced by the rotating magnet  234  directly couples into the coolant  216  and flows it inside the flow channel  104  in the direction of the arrow  222 . Rotational speed of the magnet  234  may used to control the flow velocity of the coolant  216 . Thus, controlling the rotational speed of the magnet  234  allows to control the rate of heat removal from the HGC  114  and thus to control the HGC temperature. 
         [0066]    Referring now to  FIGS. 12A and 12B , there is shown a heat transfer device (HTD)  300  in accordance with yet another preferred embodiment of the subject invention. HTD  300  is essentially the same as HTD  200 , except that in HTD  300  the rotating magnetic field for flowing the liquid coolant  216  is generated by stationary electromagnet coils  332   a,    332   b,  and  332   c,  rather than a rotating magnet  234 . The coils  332   a,    332   b,  and  332  are preferably installed inside the central opening  264  as shown in  FIG. 4A , and supplied with poly-phase alternating electric currents. Phases of the alternating currents supplied to coils  332   a,    332   b,  and  332   c  are set so that the combined magnetic field produced by the coils has a rotating component. For example, the electromagnet coils  332   a,    332   b,  and  332   c  may be connected in a delta or star (Y) configuration as is often practiced in the art of three-phase alternating current systems (see, for example, “Standard Handbook for Electrical Engineers,” D. G. Fink, editor-in-chief, Section 2: Electric and Magnetic Systems, Three-Phase Systems, Tenth Edition, published by McGraw-Hill Book Company, New York, N.Y., 1968) and supplied with an ordinary three-phase alternating current. Rotating magnetic field couples into the coolant in an already described manner and causes the coolant  216  to flow around the closed loop. 
         [0067]    One skilled in the art can appreciate that there is a variety of electromagnet coil configurations fed by poly-phase alternating currents that can produce a time varying magnetic field with a rotating component (see, for example, “Magnetoelectric Devices, Transducers, Transformers, and Machines,” by Gordon D. Slemon, Chapter 5: Polyphase Machines, published by John Willey &amp; Sons, New York, N.Y., 1966). Electromagnet coils may have ferromagnetic cores such as practiced on electric motors for alternating current. If only a single phase current is available, electromagnet coils  332   a,    332   b,  and  332   c  may be combined with a capacitor  356  as shown, for example, in  FIG. 13  to produce a suitable rotating magnetic field. There is a variety of similar connections practiced in the art of single phase electric motors. Frequency of the alternating currents supplied to the electromagnet coils  332   a,    332   b,  and  332   c  may be used to control the flow velocity of the coolant  216 . Thus, controlling the frequency of the alternating currents allows to control of the rate for heat removal from the HGC  114  and the HGC temperature. Typical range for alternating current frequency is from 1 to 1000 cycles per second. Alternatively, the coolant flow velocity may be controlled by controlling the electric current supplied to the electromagnets. 
         [0068]      FIG. 14  shows an HTD  300 ′ that is a variant to the HTD  300  wherein the electromagnet coils  332   a,    332   b,  and  332   c  are arranged to generate a traveling magnetic field rather than a rotating magnetic field. In particular, the electromagnet coils  332   a,    332   b,  and  332   c  are arranged as often practiced in the art of linear electric motors and supplied with poly-phase alternating current in appropriate phase relationship. The resulting magnetic field is traveling generally in a linear path and it couples into the electrically conductive or ferrofluid coolant in the manner already described in connection with the HTD  300 . It can be appreciated by those skilled in the art that the traveling magnetic field may cause the coolant  216  to flow even if the flow channel  204  may not have a substantially constant radius of curvature. 
         [0069]    Referring now to  FIGS. 15A and 15B , there is shown a heat transfer device (HTD)  400  in accordance with still another preferred embodiment of the subject invention. HTD  400  is similar to HTD  100 , except that in HTD  400  the flow channel  404  is formed by a gap between the outer surface  410  of body  402  and a cylindrical surface  444  of an impeller  440 . The impeller  440 , which may have a shape of a cylinder is a rotatably suspended on bearings  442  and it may be magnetically or inductively coupled to external actuation means. Alternatively, the impeller may be driven by mechanical means. The body  402  further comprises a first surface  406  adapted for receiving heat from a heat generating component (HGC), a second surface  408  adapted for rejecting heat. The flow channel  404  contains a liquid coolant  416 . The coolant  416  preferably has a good thermal conductivity and low viscosity. In operation, external actuation means may be used to spin the impeller  440 . Due to its finite viscosity, at least a portion of the coolant  416  is entrained by the cylindrical surface  444  and travels with it, thereby establishing a flow loop. If desired, the cylindrical surface  444  may have surface extensions (for example, ridges, grooves, or surface irregularities) to better entrain the coolant. Rotational speed of the impeller  440  may be used to control the velocity of the coolant  416 . Thus, controlling the rotational speed of the impeller  440  allows to control the HGC temperature. 
         [0070]    The HTD of the subject invention may be also practiced in a flat package. Referring now to  FIGS. 16A and 16B , there is shown an HTD  500  in accordance with further preferred embodiment of the subject invention comprising a body  576  and a rotating magnet assembly  596 . The body  576 , which is preferably made of material having good thermal conductivity, is a generally flat member comprising a front face  586 , back face  588 , and an annular flow channel  598  therebetween. In one variant of the preferred embodiment, the channel  598  has a thickness in the range from 0.1 to 5 millimeters and an outside diameter in the range from 10 to 100 millimeters. The body is preferably constructed from materials having high thermal conductivity. Either one or both of the faces  586  and  588  may be in a thermal contact with a suitable heat sink. The channel  598  may be substantially filled with liquid coolant  516 . The coolant  516  may be either an electrically conductive liquid and/or a ferrofluid. A heat-generating component (HGC)  114  may be attached to the front face  586  and arranged to be in a good thermal communication therewith. The magnet assembly  596  is rotationally suspended so that its plane of rotation is generally parallel to and in a close proximity to the back face  588 . The magnet assembly  596  may also comprise a permanent magnet  592  and pole extensions  594   a  and  594   b.  Furthermore, the magnet assembly  596  may be affixed to a shaft  577  of an electric motor  574 . A fan  590  may be also affixed to the shaft  577  of the electric motor  574 . 
         [0071]    In operation, the HGC  114  generates waste heat that is conducted to the front face  586  of the body  596  and, therethrough into the coolant  516 . Electric motor  574  spins the magnet assembly  596 , which generates a rotating magnetic field that penetrates though the back face  588  and interacts with the coolant  516 . If the coolant  516  is electrically conductive, the rotating magnetic field couples to the coolant via eddy currents in a manner already describe in connection with the HTD  200 . If the coolant  516  is a ferrofluid, the rotating magnetic field couples to the coolant magnetically in a manner already describe in connection with the HTD  200 . In either case, rotation of the magnet assembly  596  causes the coolant  516  to flow around the annular flow channel  598  as indicated by the arrow  599 . As a result, waste heat received by the coolant from HGC  514  is transported to other parts of the front face  586  and to the back face  588 , and therefrom to a suitable heat sink. To facilitate improved removal of heat from the back face  588 , fan  590  spun by electric motor may direct ambient air onto the back face  588 . One skilled in the art will recognize that a rotating magnetic field suitable for causing the coolant  516  to flow around the annular flow channel  598  may be also produced by stationary electromagnets supplied with poly-phase alternating currents as already described in connection with the HTC  300 . 
         [0072]    Referring now to  FIGS. 17A and 17B , there is shown a heat transfer device (HTD)  600  in accordance with yet further preferred embodiment of the subject invention. The HTD  600  is essentially the same as the HTD  500 , except that in HTD  600  further comprises an impeller disk  668 . In addition, the flow channel  698  is a disk-like (rather than annular) cavity. Furthermore, the coolant  616  used with HTD  600  may be any suitable liquid coolant. The impeller disk  668  is rotatably suspended inside the flow channel  698  on bearings  684  and substantially immersed in coolant  616 . The impeller disk  668  may be made of an electrically conductive material and/or from a ferromagnetic material. In some variants of this embodiment the impeller disk  668  may have radial slots or perforations  678  such as shown in  FIG. 18  to improve inductive coupling to the rotating magnetic field. The HTD  600  operates similarly to the HTD  500 , except that the rotating magnetic field generated by the magnet assembly  596  couples to the impeller disk  668 . If the impeller disk  668  is made of an electrically conductive material such as copper, the magnetic field may couple into it inductively via eddy current interaction. If the impeller disk  668  is made of ferromagnetic material such as steel, the magnetic field may couple into it magnetically. In either case, rotation of the magnet assembly  596  causes the impeller disk  668  to rotate, which in turn causes the coolant  616  to flow inside the chamber  698  as indicated by arrow  699 . 
         [0073]    Referring now to  FIGS. 19A and 19B , there is shown a heat transfer device (HTD)  700  in accordance with still further preferred embodiment of the subject invention and suitable for cooling semiconductor laser diode bars in densely packed arrays. HTD  700  is similar to HTD  300 ′, except that in HTD  700  the flow channel  704  and the opening  764  are elongated. In particular, the HTD  700  comprises a body  702  having an opening  764 . A plurality of semiconductor laser diode  150  are installed into a substrate  148 , which is attached to the body  702  and in a good communication therewith. The flow channel  704  containing liquid coolant  716  has a generally rectangular configuration, but other suitable configurations may be also practiced. Suitable liquid coolant  716  may be an electrically conductive liquid or a ferrofluid. Coil assemblies  732   a - d  each comprise two coils, one on the outside the body  702  and one inside the opening  764 . Preferably, the coils in each assembly are positioned so that the magnetic field they generate crosses the channel  704  at substantially normal incidence. The coil assemblies  732   a - d  are fed poly-phase alternating currents arranged to produce magnetic field traveling in the direction of arrow  722 , thereby inducing the coolant  716  to flow inside the channel  704  in the same direction. The laser diodes  150  are operated to produce optical output  152  while also generating waste heat. The coolant  716  flowing inside the channel  704  removes waste heat from the laser diodes  150  and transfers it to second surface  708  inside the opening  764 . The opening may contain suitable heat sink such as secondary liquid coolant, gaseous coolant, or phase change material. It can be appreciated that the HTD  700  is conducive to stacking of multiple HTD units vertically and horizontally to produce large arrays that may be required for direct material processing or pumping of solid-state lasers. 
         [0074]    The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” and/or “comprising,” and “includes” and/or “including” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. 
         [0075]    The terms of degree such as “substantially”, “about” and “approximately” as used herein mean a reasonable amount of deviation of the modified term such that the end result is not significantly changed. For example, these terms can be construed as including a deviation of at least ±5% of the modified term if this deviation would not negate the meaning of the word it modifies. 
         [0076]    The term “suitable,” as used herein, means having characteristics that are sufficient to produce a desired result. Suitability for the intended purpose can be determined by one of ordinary skill in the art using only routine experimentation. 
         [0077]    Moreover, terms that are expressed as “means-plus function” in the claims should include any structure that can be utilized to carry out the function of that part of the present invention. In addition, the term “configured” as used herein to describe a component, section or part of a device includes hardware and/or software that is constructed and/or programmed to carry out the desired function. 
         [0078]    Different aspects of the invention may be combined in any suitable way. 
         [0079]    While only selected embodiments have been chosen to illustrate the present invention, it will be apparent to those skilled in the art from this disclosure that various changes and modifications can be made herein without departing from the scope of the present invention as defined in the appended claims. Furthermore, the foregoing description of the embodiments according to the present invention are provided for illustration only, and not for the purpose of limiting the present invention as defined by the appended claims and their equivalents. Thus, the scope of the present invention is not limited to the disclosed embodiments.