Multiple Thermal Circuit Heat Spreader
A heat spreader has more than one thermal circuit to give better performance over a wider range of heat input regimes. Different working fluids may be used in the different thermal circuits. The thermal circuits may extend in three dimensions to improve the density of the channels in limited space.
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The present invention relates generally to a heat spreader using a plurality of thermal circuits (each thermal circuit sometimes referred to elsewhere as an oscillating heat pipe, loop-type heat pipe, or pulsating heat pipes) which can have many shapes and sizes and be made of a variety of materials and working fluids in order to handle a wide range of heat flux and total heat load and be able to operate in different orientations all while maintaining a minimal temperature difference between the area(s) of the spreader that receive heat and the areas of the spreader that reject heat.BACKGROUND OF THE INVENTION
First proposed by Akachi (see, e.g., U.S. Pat. No. 4,921,041), an oscillating heat pipe (OHP) utilizes the oscillating motion of liquid slugs and vapor plugs enclosed in a series of meandering tubes to efficiently transfer heat from the heat source at the oscillating heat pipe's evaporator section to the heat sink at the oscillating heat pipe's condenser section. If the tubing has a small enough diameter, the surface tension forces of the working fluid overcome gravitational forces and distinct liquid plugs and vapor slugs form throughout the tubing volume. When heat is added to the evaporator section, the liquid plugs and vapor slugs inside the tubing at the evaporator section receive the heat, causing them to increase in temperature and pressure and expand. Portions of the liquid slugs in the evaporator section evaporate and expand even further. The pressure difference between the evaporator section and condenser section drives the liquid slugs and vapor plugs toward the condenser section where heat is transferred to the heat sink. When the working fluid travels to the condenser section, the liquid slugs and vapor plugs lose temperature and pressure and portions of the vapor plugs condense. Thus, both sensible and latent heat is transferred out of the thermal circuit to the heat sink at the condenser section. This causes contraction of the liquid plugs and vapor slugs inside the tubing at the condenser section. In addition, the liquid slugs and vapor plugs previously occupying the space in the condenser section are forced by the incoming flow to travel toward the evaporator section, where they receive heat from the heat source and restart the cycle. In this way, heat is transferred from the evaporator section to the condenser section using both convective heat transfer (liquid and vapor flows) and phase change heat transfer (evaporation and condensation). Lastly, because the oscillating heat pipe has no internal wick structure, the fabrication cost can be very low and when the oscillating motion starts, no capillary limitation exists in an oscillating heat pipe. Experimental results show that it is not susceptible to dry out at high thermal power densities as opposed to vapor chambers and traditional heat pipes.
There are generally two types of oscillating heat pipes by its tubing characteristics: tubular oscillating heat pipe and flat plate oscillating heat pipe (FP-OHP). The latter has advantages in the cooling of electric devices as shown by Akachi (see, e.g., U.S. Pat. No. 5,737,840). The flat plate oscillating heat pipe has channels engraved on a metallic plate, which design is more suitable to its applications on electronic devices than the tubular oscillating heat pipes. Previous research revealed that many factors would affect the performance of flat plate oscillating heat pipe, such as meandering turn number, working fluid, charging ratio, and filling ratio. For example, Borgmeyer and Ma in their article Experimental Investigations of Oscillating Motions in a Flat Plate Pulsating Heat Pipe (J. Thermophysics and Heat Transfer, pp. 405-409, vol. 21, no. 2 April-June 2007) describe successfully sealing a copper flat plate oscillating heat pipe with square internal channels of 1.59 mm hydraulic diameter. The Borgmeyer and Ma article is incorporated herein by reference. The oscillating motion of liquid plugs in an oscillating heat pipe has been observed to be dependent on the working orientation. Khandekar et al. (Thermofluid Dynamic Study of Flat-Plate Closed-Loop Pulsating Heat Pipes, Microscale Thermophysical Eng'g 3:303-317, 2002) conducted the experiments on aluminum 6 turn flat plate oscillating heat pipes, all sealed with transparent glass and charged with ethanol and water, and found that flat plate oscillating heat pipe with larger rectangular cross section and filling ratio below 0.3 is less dependent on gravity. The lowest thermal resistance of 1 K/W was achieved with 2.2×2.0 mm2 rectangular channel. The Khandekar et al. article is incorporated herein by reference. In the research of Thompson el al. reported in Experimental Investigation of Miniature Three-Dimensional Flat-Plate Oscillating Heat Pipe (J. Heat Transfer, vol. 131, issue 4 043210, 9 pgs., April 2009), a three dimensional flat plate oscillating heat pipe design was proposed, in which the channel density over unit heating area is dramatically increased. The hydraulic diameter of the channel was 0.762 mm, and charged with acetone at 0.8 volume fraction (i.e. 80% of channel's volume filled with acetone fluid). A much lower thermal resistance of 0.07° C./W was achieved and the heat flux was up to 20 W/cm2. The Thompson et al. article is incorporated herein by reference. Other pertinent work was reported by Cheng et al. in An Investigation of Flat-Plate Oscillating Heat Pipes (J. Electron. Packaging, vol. 132, issue 4, 041009, 6 pgs., December 2010). The Cheng et al. article is incorporated herein by reference.
Using this heat transfer mechanism, a thermal circuit utilizes both convective as well as phase change heat transfer to move thermal energy from the heat source at the heat spreader's evaporator section(s) to the heat sink at condenser section(s). Importantly, such a heat transfer mechanism requires some minimum amount of heat load (start up power) at the evaporator section of the heat spreader to activate the flow of liquid slugs and vapor plugs within a thermal circuit due to inertia, gravity and frictional forces between the working fluid and the thermal circuit walls. At the other extreme, at some higher heat load (critical power), the heat flux between the heat spreader's walls at the evaporator section and the working fluid within a thermal circuit is so great that a vapor phase remains constant in the area of the thermal circuit nearest the evaporator section of the heat spreader, and as a result the mass and heat transfer mechanism described above ceases to function at such critical power. It has been shown that thermal circuits charged with some working fluids (e.g., acetone) have relatively low “start up power” but also have relatively low “critical power” which makes such thermal circuit useful in low heat load applications but not useful in higher heat load applications. By contrast, thermal circuits charged with other working fluids (e.g., water) have relatively high required “start-up power” and relatively high “critical power” which make them useful for applications requiring high heat flux heat transfer but less useful if lesser amounts of heat need to be dissipated.
Oscillating heat pipes have generally higher critical powers than alternative heat spreading technologies such as heat pipes and vapor chambers. Also, oscillating heat pipes are less affected by gravity than traditional heat pipes and to have the ability to transport heat greater distances with less heat transfer rate degradation. Unfortunately, traditional single-loop oscillating heat pipes suffer from the following undesirable attributes that have prevented more widespread application: A) unpredictable temperature spikes in their evaporator section(s)s; B) limited operating power ranges (i.e. an oscillating heat pipe heat spreader designed for minimal thermal resistance at a relatively low thermal input power has an undesirably high thermal resistance when relatively high thermal input power is applied; and conversely an oscillating heat pipe heat spreader designed for minimal thermal resistance at a relatively high thermal input power has an undesirably high thermal resistance when relatively low thermal input power is applied to it); and C) generally lower overall heat transport capability (e.g. higher thermal resistance at given thermal input powers) than is desired by end users of heat spreading technologies. These undesirable attributes are considered at present to be inherent to the traditional oscillating heat pipe design and their applicability to heat spreaders.SUMMARY
A heat spreading device for transferring heat from a heat source to a heat sink constructed according to the principles of the present invention generally comprises a first conduit extending in a loop and having a length between the heat source and the heat sink. A second conduit proximate the first conduit and in thermal communication with the first conduit over at least some of the length of the first conduit.
Other objects and features of the present invention will be in part apparent and in part pointed out hereinafter.
Corresponding reference characters indicate corresponding parts throughout the several views of the drawings.DETAILED DESCRIPTION OF THE DRAWINGS
A conventional oscillating heat pipe OHP1 having a single loop L or channel forming a single thermal circuit is schematically illustrated in
Referring now to
It will be understood that although two thermal circuits 4, 6 are illustrated in the heat spreader of
Referring now to
Two three-dimensional thermal circuits 20 and 22 engraved on the both sides of two piece base 26 of a heat spreader 28 are shown in
The multi-loop or multiple thermal circuit design may use thermal circuits with different working fluids and/or different geometries and/or channel patterns through the heat spreader. Referring again to
The thermal circuit's shape is not limited to being square in cross section. Circular, triangular, rectangular, T-type, and any other cross sectional geometry may be used such as is suitable for a given set of design parameters and issues (e.g., channel density, heat transfer performance, heat spreader size, manufacturability, etc.). When a thermal circuit heat spreader includes multiple thermal circuits (or multiple loops), the channel shape for each loop can be different from others as shown in
The internal surfaces of the thermal circuit may be smooth, rough, treated to be hydrophobic with the working fluid, and/or treated to be hydrophilic with the working fluid. For example, copper surface pre-treated by oxidization has shown to enhance heat transfer performance of certain thermal circuits charged with water as working fluid. Further, hydrophilic surface treatments on metals (e.g. microgrooves or chemical coatings) have proven to increase the contact angle of working fluid and in doing so increase the evaporative heat transfer rate between such surface and the working fluid. Also, the inner channel surfaces may be manipulated to be hydrophobic, hydrophilic, or hydrophobic in one area (e.g. condenser) and hydrophilic in another area (e.g. evaporator) using a variety of techniques including surface coatings, laser formed micro/nano structures, and controlled chemical reactions, etc.
The shape of the multiple thermal circuit heat spreader may be flat plate, cylindrical, or any other geometry. The evaporator section(s) may be at the center of the heat spreader, on one or more edges, in one or more corners, or any other location. The orientation of the heat spreader (i.e. the location of the heat source(s) relative to the heat sink(s) in a gravitational field) may be with heat source(s) on top of, to the side of, below, or in any relative position to the heat sink(s).
The multiple thermal circuit heat spreader may be made from any shell material, including non-metals with relatively low thermal conductivities because material conductivity is a relatively small contributor to the heat spreader's overall heat transport capability if the wall thickness of the material is relatively small. For example and without limitation, at 100 microns of wall thickness the impact of the wall's thermal conductivity will contribute little to overall thermal resistance of the heat spreader. The multiple thermal circuit heat spreader shell and internal tubing may be manufactured in a variety of processes, including but not limited to: brazing, stamping, photo-chemical etching, hot forging, cold forging, mechanical engraving, welding, water-jet cutting, laser etching, or any other positive or negative fabricating process of embedding and sealing thermal circuits in shell material.
The multiple thermal circuit design may also use a combination of loops where the combination of thermal circuits is such that they increase or decrease the heat transfer between the thermal circuits. For example, in
The heat spreaders are comprised of more than one thermal circuit, and each thermal circuit may meander back and forth between the heat spreader's evaporator section(s) and condenser section(s), or it may only traverse one Section (e.g., the middle thermal circuit 94 of
Conventional oscillating heat pipes have the advantage of being able to transport heat greater distances between their evaporator and condenser sections than traditional heat pipes or vapor chambers. They also have the advantage of being less affected by gravity. However, comparing with other types of heat pipes, such as loop heat pipe and vapor chamber, prior oscillating heat pipes have suffered from high startup power, high thermal resistances, and sharp temperature spikes at the evaporator section(s) which have prevented them from finding much commercial acceptance. At least some embodiments of the invention disclosed herein resolve these issues by incorporating a plurality of fluidly independent but thermally communicating thermal circuits on a single heat spreading device. By using multiple thermal circuits on the same heat spreader (where thermal circuits may be of the same or different sizes, patterns, hydraulic diameters and/or working fluids) the invented heat spreader is able to transfer heat efficiently at both lower start-up powers and at higher critical powers. Further, it has been empirically observed that by having thermal circuits in thermal communication with at least one other fluidly independent thermal circuit the likelihood of either thermal circuit ceasing to function temporarily (which is a cause of unpredictable temperature spikes in the evaporator section(s) of single loop oscillating heat pipes) is exponentially reduced thus creating an overall lower thermal resistance at any single power input level. Finally, by utilizing three-dimensional turns the heat spreader can transfer greater amounts of thermal energy with lower overall thermal resistance at specified areas for receiving and rejecting heat.
Having described the invention in detail, it will be apparent that modifications and variations are possible without departing from the scope of the invention defined in the appended claims.
When introducing elements of the present invention or the preferred embodiments(s) thereof, the articles “a”, “an”, “the” and “said” are intended to mean that there are one or more of the elements. The terms “comprising”, “including” and “having” are intended to be inclusive and mean that there may be additional elements other than the listed elements.
In view of the above, it will be seen that the several objects of the invention are achieved and other advantageous results attained.
As various changes could be made in the above products and methods without departing from the scope of the invention, it is intended that all matter contained in the above description and shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.
1. A heat spreading device for transferring heat from a heat source to a heat sink, the heat spreader device comprising:
- a first conduit extending in a loop and having a length between the heat source and the heat sink;
- a second conduit proximate the first conduit and in thermal communication with the first conduit over at least some of the length of the first conduit.
2. A heat spreading device as set forth in claim 1 wherein the second conduit is in thermal communication with the first conduit over substantially the entire length of the first conduit.
3. A heat spreading device as set forth in claim 1 wherein the second conduit conforms to the first conduit over the entire length of the first conduit.
4. A heat spreading device as set forth in claim 1 wherein the second conduit is in thermal communication with the first conduit over a portion of the first conduit near only one of either the heat source or the heat sink.
5. A heat spreading device as set forth in claim 1 further comprising a working fluid in the first and second conduit.
6. A heat spreading device as set forth in claim 5 wherein at least one of the working fluid in the first conduit and the working fluid in the second conduit includes nanoparticles.
7. A heat spreading device as set forth in claim 5 wherein the working fluid in the first conduit is different from the working fluid in the second conduit.
8. A heat spreading device as set forth in claim 1 wherein the first and second conduits are each formed at least in part out of a single piece of material.
9. A heat spreading device as set forth in claim 1 comprising a first plate and a second plate, portions of the first and second conduits being defined in the material of the first plate, and portions of the first and second conduits being defined in the material of the second plate.
10. A heat spreading device as set forth in claim 1 wherein the first and second conduits follow tortuous paths.
11. A heat spreading device as set forth in claim 9 wherein the first and second conduits each include segments extending in three different dimensions.
12. A heat spreading device as set forth in claim 1 wherein an internal surface of at least a portion of one of the first or second conduits is treated to optimize heat transfer.
13. A heat spreading device as set forth in claim 1 wherein the device is adapted to receive heat from at least two spaced apart heat sources.
14. A heat spreading device as set forth in claim 13 wherein the device is adapted to reject heat to at least two spaced apart heat sinks.
15. A heat spreading device as set forth in claim 1 wherein the device is adapted to reject heat to at least two spaced apart heat sinks.
16. A heat spreading device as set forth in claim 1 wherein the first and second conduits are free of fluid communication with each other.
Filed: Apr 12, 2011
Publication Date: May 30, 2013
Applicants: THERMAVANT TECHNOLOGIES LLC (Columbia, MO), THE CURATORS OF THE UNIVERSITY OF MISSOURI (Columbia, MO)
Inventors: Hongbin Ma (Columbia, MO), Peng Cheng (Columbia, MO), Joseph A. Boswell (San Francisco, CA)
Application Number: 13/640,758
International Classification: F28D 15/00 (20060101);