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Apparatus and methods according to the present invention utilize micropumps that are capable of generating high pressure and flow without moving mechanical parts and the associated generation of unacceptable electrical and acoustic noise, as well as the associated reduction in reliability. These mic

Apparatus and methods according to the present invention utilize micropumps that are capable of generating high pressure and flow without moving mechanical parts and the associated generation of unacceptable electrical and acoustic noise, as well as the associated reduction in reliability. These micropumps are fabricated with materials and structures that improve performance, efficiency, and reduce weight and manufacturing cost relative to presently available micropumps. These micropumps also can allow for recapture of evolved gases and deposited materials, which may provide for long-term closed-loop operation. Apparatus and methods according to the present invention also allow active regulation of the temperature of the device through electrical control of the flow through the pump and can utilize multiple cooling loops to allow independent regulation of the spatial and temporal characteristics of the device temperature profiles. Novel enclosed microchannel structures are also described.

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What is claimed is: 1. A closed-loop fluid cooling system for a heat-generating device comprising: a powered pump for pumping the fluid through the closed-loop system; a temperature control circuit capable of regulating a device temperature by adjusting a fluid flow rate; a first heat exchanger inc

What is claimed is: 1. A closed-loop fluid cooling system for a heat-generating device comprising: a powered pump for pumping the fluid through the closed-loop system; a temperature control circuit capable of regulating a device temperature by adjusting a fluid flow rate; a first heat exchanger including an enclosed microchannel and coupled to the heat-generating device for transferring heat to the fluid, the fluid entering the heat exchanger in a liquid state and exiting in a liquid-vapor state, wherein the microchannel has a cross section, defined by a width and a height, that varies along a length thereof to reduce a temperature variation along the length when compared to a microchannel having a constant cross section; a second heat exchanger coupled to ambient for transferring heat from the fluid to an external environment; and wherein the static pressure is below the ambient pressure in at least one location in the closed-loop. 2. The closed-loop fluid cooling system of claim 1 wherein the temperature of the heat-generating device is maintained below 90�� C. 3. The closed-loop fluid cooling system of claim 1 wherein the temperature of the heat-generating device is maintained below 80�� C. 4. The closed-loop fluid cooling system of claim 1 wherein the temperature of the heat-generating device at a location where vapor is generated is maintained in the range between 90�� C. and 50�� C. 5. A closed-loop cooling system for a heat-generating device, comprising: a powered pump; a substrate including at least a portion of a microchannel enclosed therein, the microchannel having a varying cross-sectional dimension between an inlet and an outlet, the substrate disposed on the heat-generating device to transfer thermal energy from the heat-generating device to the substrate, and the further transfer of thermal energy to a fluid disposed within the microchannel; and wherein the microchannel is configured to provide flow of the fluid therethrough, wherein the microchannel cross section, defined by a width and a height, varies along a length of the microchannel so as to reduce an overall pressure drop when compared to the pressure drop that occurs with a microchannel having fixed dimensions. 6. The closed-loop cooling system for a heat-generating device of claim 5 further comprising a heat exchanger to provide the flow of fluid therethrough and the transfer of thermal energy from the heat exchanger to the surroundings. 7. The closed-loop cooling system for a heat-generating device of claim 5 wherein the microchannel is configured to reduce flow resistance through the microchannel below the flow resistance that is provided by a microchannel having fixed dimensions. 8. The closed-loop cooling system for a heat-generating device of claim 5 wherein the microchannel is configured to provide a lower average wall temperature than can be obtained with a microchannel having fixed dimensions. 9. The closed-loop cooling system for a heat-generating device of claim 5 wherein the microchannel has a width that varies between an inlet and an outlet of the micro channel. 10. The closed-loop cooling system for a heat-generating device of claim 5 wherein the pressure at an exit from the substrate is in a range from atmospheric pressure to 0.1 pound per square inch. 11. The closed-loop cooling system for a heat-generating device of claim 5 wherein the pressure at an exit from the substrate is selected to generate a specific liquid-vapor transition temperature and a specific heat-generating device temperature. 12. The closed-loop cooling system for a heat-generating device of claim 5 wherein the fluid is at least one of a de-ionized water, an aqueous buffer solution and an organic liquid. 13. The closed-loop cooling system for a heat-generating device of claim 6 wherein the flow rate is greater than 1 ml/min. 14. A method for transferring heat from a heat-generating device to a heat exchanger in a closed-loop cooling system including a powered pump, the heat exchanger including an enclosed microchannel having a varying cross-section that can transfer a fluid therethrough, the fluid entering the microchannel in a liquid state and exiting in a liquid-vapor state, the method comprising the steps of: determining a plurality of dimensions of the microchannel such that the fluid exits at sub-atmospheric pressure and sufficient thermal energy is transferred to the fluid to maintain the temperature of the heat-generating device below an operational limit; wherein the step of determining the plurality of dimensions includes selecting a cross section, defined by a width and a height, that varies with position along a length of the microchannel; and coupling the microchannel with the determined dimensions to the heat-generating device. 15. The method for transferring heat of claim 14 wherein the step of determining the plurality of dimensions includes selecting a varying cross section that minimizes a flow resistance through the microchannel. 16. The method for transferring heat of claim 15 wherein the step of determining the plurality of dimensions includes selecting a width for the microchannel that provides a lower average wall temperature. 17. The method for transferring heat of claim 14 wherein the step of determining the plurality of dimensions includes selecting a variable width for the microchannel, the width being varied between an inlet and an outlet of the microchannel. 18. The method for transferring heat of claim 14 wherein the step of determining the plurality of dimensions includes selecting a cross section that varies with position along the microchannel so as to minimize an overall pressure drop. 19. The method for transferring heat of claim 14 wherein the step of determining the plurality of dimensions includes selecting a cross section that varies with position along the microchannel so as to minimize the temperature variation along the microchannel. 20. The method for transferring heat of claim 14 wherein the step of determining the plurality of dimensions includes selecting cross sections for a plurality of microchannels that vary with position along each microchannel, wherein the cross-sectional variations are different for different microchannels so as to minimize the temperature variation across the heat-generating device where the heat-generating device has a non-uniform spatial distribution of heat. 21. The method for transferring heat of claim 14 wherein the exit pressure from the microchannel is in a range from atmospheric pressure to 0.1 pound per square inch. 22. The method for transferring heat of claim 14 wherein the exit pressure is selected to generate a specific liquid-vapor transition temperature and a specific heat-generating device temperature. 23. The method for transferring heat of claim 14 wherein the fluid is at least one of a de-ionized water, an aqueous buffer solution and an organic liquid. 24. The method for transferring heat of claim 14 wherein the fluid comprises at least 10% acetonitrile by mass. 25. The method for transferring heat of claim 14 wherein the fluid comprises at least 10% methanol by mass. 26. An apparatus for use with a closed-loop cooling system, including a powered pump, that operates using a fluid having both a liquid phase and a liquid-vapor phase, comprising: a heat generating device including a heat generating element and a temperature control circuit, wherein the temperature control circuit is capable of regulating a device temperature by adjusting a fluid flow rate; a substrate physically connected to the heat generating device, the substrate enclosing at least a portion of a microchannel and providing for the transfer of thermal energy to the fluid disposed within the microchannel, the microchannel configured to provide flow of the fluid through the microchannel, wherein the fluid is in the liquid phase when entering and in the liquid-vapor phase when exiting the microchannel. 27. The apparatus for use with a closed-loop cooling system of claim 26 further comprising a heat exchanger configured to provide flow of the fluid therethrough and the transfer of thermal energy out of the fluid. 28. The apparatus for use with a closed-loop cooling system of claim 26 wherein a plurality of dimensions of the microchannel are selected to reduce flow resistance through the microchannel below the flow resistance that is provided by a microchannel having fixed dimensions. 29. The apparatus for use with a closed-loop cooling system of claim 26 wherein a plurality of dimensions of the microchannel are selected to provide a lower average wall temperature than can be obtained with a microchannel having fixed dimensions. 30. The apparatus for use with a closed-loop cooling system of claim 26 wherein a variable width is selected for the microchannel, the width being varied between an inlet and an outlet of the microchannel. 31. The apparatus for use with a closed-loop cooling system of claim 26 wherein a microchannel cross section, defined by a width and a height, varies with position along the microchannel to reduce an overall pressure drop when compared to the pressure drop that occurs with a microchannel having fixed dimensions. 32. The apparatus for use with a closed-loop cooling system of claim 28 wherein a microchannel cross section, defined by a width and a height, varies with position along the microchannel to reduce the temperature variation along the microchannel when compared to a microchannel having fixed dimensions. 33. The apparatus for use with a closed-loop cooling system of claim 26 wherein a microchannel cross section, defined by a width and a height, varies with position along each microchannel, and wherein the cross section variations are different for different microchannels to reduce the temperature variation across the heat-generating device where the heat-generating device has a non-uniform spatial distribution of heat. 34. The apparatus for use with a closed-loop cooling system of claim 26 wherein the exit pressure of the fluid from the substrate is in a range from atmospheric pressure to 0.1 pound per square inch. 35. The apparatus for use with a closed-loop cooling system of claim 26 wherein the exit pressure of the fluid from the substrate is selected to generate a specific liquid-vapor transition temperature and a specific heat generating device temperature. 36. The apparatus for use with a closed-loop cooling system of claim 26 wherein the fluid is at least one of a de-ionized water, an aqueous buffer solution and an organic liquid. 37. The apparatus for use with a closed-loop cooling system of claim 27 wherein the flow rate is greater than 1 ml/min.