US20130168057A1

US20130168057A1 - Modular heat shield and heat spreader

Info

Publication number: US20130168057A1
Application number: US13/672,514
Authority: US
Inventors: Tadej Semenic; Bing-Chung Chen; Qingjun Cai; Ya-Chi CHEN; Kyle D. Gould
Original assignee: Teledyne Scientific and Imaging LLC
Current assignee: Teledyne Scientific and Imaging LLC
Priority date: 2011-12-30
Filing date: 2012-11-08
Publication date: 2013-07-04

Abstract

A modular heat shield and heat spreader (“MHS”) includes top and bottom panels, and a plurality of thermally conductive pillars located between the panels and which support the top panel. A continuous pool of liquid between the panels surrounds some portion of the pillars. Heat to which the top panel is exposed is conducted through the top panel and at least some of the pillars. The heat changes the phase of some of the liquid to a vapor, which spreads the heat to an area larger than that of the heat source and thereby dissipates the heat away from the source at a lower heat flux than that associated with the flux from the source. The MHS preferably includes wicking material on some of the pillars and on the underside of the top panel, such that the wicking material is saturated with the liquid and heated by the conducted heat.

Description

RELATED APPLICATIONS

This application claims the benefit of provisional patent application No. 61/581,930 to T. Semenic et al., filed on Dec. 30, 2011.

GOVERNMENT RIGHTS

This invention was made with Government support under Office of Naval Research contract N00014-10-C-252 awarded by the United States Department of Defense. The Government has certain rights in this invention.

BACKGROUND OF THE INVENTION

1. Field of the Invention
This invention relates generally to heat shields, and more particularly to heat shields and heat spreaders capable of protecting surfaces from high temperature heat sources or lowering the temperature of heat generating components attached to a heat spreader by effectively spreading the heat over a large area.
2. Description of the Related Art
In many applications, high temperature and large area heat sources impinge on structural surfaces and cause thermal damage. As a result, frequent structural repairs may be required that are not only costly, but may also cause down time. An example is thermal damage to a flight deck caused by high temperature exhaust plumes from an aircraft. Temperatures on the top surface of a flight deck can in some cases approach or exceed annealing temperatures, and thus cause permanent thermal deformation. This problem may result in limiting the frequency with which aircraft operations can be practiced on the flight deck, to allow sufficient cooling time. The flight deck is also likely to require more frequent repairs than the rest of the ship, and any non-skid coating applied to the flight deck is likely to delaminate after being repeatedly heated with high temperature exhaust plumes.
In other applications, components that generate significant amounts of waste heat, such as the semiconductor dies of power electronics, are attached to heat sinks that spread and transfer the heat to a heat transfer medium. However, in many cases, such heat sinks are unable to sufficiently cool the electronics components, compromising the performance, lifetime, and reliability of those components.
One approach to handling large area heat sources of this kind is a heat spreader made from large plates with embedded heat pipes; an example is shown in FIG. 1. The heat spreader 10 includes a number of parallel heat pipes 12 embedded in an aluminum plate 14, a number of which can be placed adjacent to each other to increase the effective thermal conductivity of the heat spreader. Heat from a heat source 16 is conveyed away (18) from the source via the heat pipes on which the heat impinges.
However, heat pipes of this sort have a number of drawbacks. For example, heat can only propagate in one direction, making the heat spreader inefficient. Thermal resistance between adjacent plates is high, which can result in large temperature differences across the gaps 20 between the plates and inefficient heat transfer. In addition, only a small number of the heat pipes 22 in adjacent plates are heated, resulting in high heat fluxes into heat pipe evaporators, which can cause the evaporators to dry out. Also, plates with embedded heat pipes are not scalable, and are not suitable for large area heat shields and heat spreaders.

SUMMARY OF THE INVENTION

A modular heat shield and heat spreader is presented which addresses the challenges noted above.
The present modular heat shield and heat spreader (“MHS”) includes a top panel, a bottom panel, and a plurality of thermally conductive pillars located between the top and bottom panels such that the pillars support the top panel. There is preferably a continuous pool of liquid between the top and bottom panels, such that at least a portion of at least some of the pillars is surrounded by the liquid. When so arranged, heat from a heat source to which the top panel is exposed is conducted through the top panel and at least some of the pillars. The heat changes the phase of at least some of the liquid to a vapor, and the vapor spreads the heat to an area larger than that of the heat source and thereby dissipates the heat away from the source at a lower heat flux than that associated with the heat flux from the source.
The MHS preferably includes wicking material on at least some of the pillars and on the underside of at least a portion of the top panel, such that at least some of the wicking material is saturated with the liquid and heated by the conducted heat. Wicking material may also be formed into columns adjacent to at least some of the pillars such that liquid can flow from the continuous pool to the top panel through the wicking material columns.
The MHS is preferably made from modules, each of which is made with support pillars between a top and bottom panel. To form a large area heat spreader, multiple modules are joined together at the module edges and may share a common continuous pool of liquid, and vapor can transfer and dissipate heat away from the modules nearest the heat source towards other ones of the modules. The top and bottom panels of each module are preferably arranged such that each module is hermetically sealed. The modules making up the MHS can have different sizes and shapes.
These and other features, aspects, and advantages of the present invention will become better understood with reference to the following description and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view of a known heat spreader with embedded heat pipes.

FIGS. 2 a and 2 b are plan and sectional views, respectively, of a MHS in accordance with the present invention.

FIG. 3 is a sectional view of one possible embodiment of a MHS module in accordance with the present invention.

FIG. 4 is a sectional view of another possible embodiment of a MHS module in accordance with the present invention.

FIG. 5 is a sectional view of another possible embodiment of a MHS module in accordance with the present invention.

FIG. 6 is a perspective view of one possible embodiment of a large area MHS made from multiple MHS modules.

FIG. 7 is a cutaway view of an integral pressure relief valve as might be used with a MHS module in accordance with the present invention.

FIG. 8 is a sectional view of another possible embodiment of a MHS module in accordance with the present invention.

FIGS. 9 a, 9 b and 9 c are perspective, sectional and plan views, respectively, of a cylindrical MHS in accordance with the present invention.

FIGS. 10 a and 10 b are perspective and close-up views of a ramp assembly as might be used with a MHS module in accordance with the present invention.

FIGS. 11 a, 11 b, and 11 c are sectional views of different types of joints that might be used between MHS modules in accordance with the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The basic principles of the present modular heat shield and heat spreader (MHS) are illustrated in the plan and sectional views shown in FIGS. 2 a and 2 b, respectively. The MHS 30 is typically made up of a number of individual MHS modules 32, which may be different shapes and sizes that are assembled over a surface 34 which needs to be shielded from one or more high temperature, large area heat sources. Each module includes a top panel 36 and a bottom panel 38; the top and bottom panels are joined together at joints 40 to form a watertight enclosure which is preferably hermetically sealed.
A plurality of thermally conductive pillars 42 are located between the top and bottom panels such that they support top panel 36. A wicking material 44 is preferably (though not necessarily) on at least some of the pillars and on the underside of at least a portion of top panel 36; additional wicking material 45 may be formed into columns adjacent to at least some of the pillars. A continuous pool of liquid 46 is located between the top and bottom panels such that at least a portion of at least some of pillars 42 is surrounded by the liquid and such that at least some of the wicking material (if present) is saturated with the liquid.
In operation, heat from a heat source 50 to which top panel 36 is exposed is conducted 52 through the top panel, the liquid saturated wicking material 44 and 45 (if present), and at least some of thermally conductive pillars 42. The heat changes the phase of at least some of the liquid 46 in the pool and in the voids of the wicking material to a vapor 54, which spreads the heat to an area larger than the area of the heat source and thereby dissipates heat away from the heat source at a lower heat flux than that associated with the heat flux from the heat source. The vapor condenses outside the heat impingement zone and rejects the heat to the ambient air, while the condensate drips into the liquid pool 46.
The MHS may include through-holes 56 to accommodate surface protrusions or to attach the MHS to the surface being shielded. The wicking material 44 and 45, which may be as simple as a porous coating, serves to enhance boiling and evaporation of liquid 46, and provides capillary liquid transport between the pool and the underside of top panel 36.
The MHS components are preferably mechanically strong and corrosion resistant. A surface 34 protected by the present MHS 30 will be cooler than it would be otherwise, and thus may prevent thermal deformations that would otherwise occur. Some of the key benefits of the MHS are: (1) a modular design which is scalable to any size and can accommodate surface protrusions, (2) low weight as a result of a large vapor space inside the MHS, (3) high lateral thermal conductivity as a result of using vapor 54 to transfer and spread the heat, (4) resistance to mechanical impacts as a result of using mechanically strong panels and structural supporting elements, and (5) durability and long life as a result of using corrosion resistant and compatible materials to construct the MHS.
The most basic MHS design, illustrated in FIG. 3, includes no wicking material—heat is conducted to liquid pool 46 via the supporting elements 42 (ribs or pillars) only. In operation, heat conducts through top panel 36 and pillars 42 and boils the working fluid 46. Heat 50 is absorbed by the latent heat of vaporization and is removed with vapor 54.
As shown in FIG. 4, the pillars 42 could also include a porous coating or wicking material 60. This provides for more effective boiling that results in a lower top panel surface temperature. Porous coating 60 provides artificial nucleation sites that result in smaller vapor bubbles and higher bubble departure frequency than would uncoated (smooth) supporting elements. The porous coating 60 also serves to increase the heat transfer surface area.
There may also be a porous coating or wicking material on the underside of some or all of the top panel 36, as was shown in FIG. 2 b. In this design, capillary forces transport the liquid through pores of the wicking material from the continuous liquid pool 46 in the area beneath the heat impingement zone. Heat from heat source 50 conducts through top panel 36 and the liquid saturated wicking material 44 and evaporates the working fluid from the top surface of the wick. This design is expected to result in a lower top panel surface temperature than the designs shown in FIGS. 3 and 4.
Any of the MHS designs could include compressible pads 70 on the bottoms of pillars 42, as shown in FIG. 5. Pads 70 compress during mechanical impacts and absorb some of the mechanical loading. The pads could be made of a low thermal conductivity material such as silicone rubber, so as to minimize heat conduction into the surface 34 being shielded. An MHS might also include a conformable foam 72 affixed to the underside of bottom panels 38 that serves to conform the bottom of the MHS to minor protrusions 74 from shielded surface 34. Foam 72 can also function to minimize heat conduction from the MHS into shielded surface 34, to absorb mechanical impacts by compression of the foam, to act as an electrical insulator between the MHS and shielded surface 34, and to provide a seal between the MHS and the shielded surface.
As noted above, the MHS is typically formed from multiple modules, with the top and bottom panels of each module arranged such that each module is hermetically sealed. This is further illustrated in FIG. 6, in which individual modules 80 are assembled into a large area MHS 82; the modules may be affixed to an optional mounting frame 84. The large area MHS may be used to shield a surface from a single heat source such as an aircraft exhaust plume, or act as a heat spreader and an effective heat transfer device that can remove waste heat from a single or multiple heat sources 86 such as electronic devices. Each module may include heat sink fins 88 affixed to the bottom panel for improved heat dissipation to the environment. Each module is preferably charged with an amount of working fluid that is equal to or larger than the volume of all the voids in the wicking material.
At elevated pressures, a top and/or bottom panel may start to deform or bulge if the two panels are only joined at the periphery. This can be avoided if at least some of the pillars are bonded in some fashion to the top and bottom panels by, for example, providing additional fixing points for some of the pillars. One method might be to make some of the pillars with a larger diameter so that a through-hole can be formed down the center of the pillars, and then using bolts to bolt the top panels to the bottom panels. An o-ring or a gasket could be used to seal around the pillars with through-holes. In an alternative design, some or all of the pillars could be brazed at the tips to hold both panels together.
The present MHS can be used in numerous applications. As noted above, the surface to be shielded may be a landing spot on an amphibious ship or an aircraft carrier, such that the MHS shields the flight deck from airplane exhaust plumes. One or more MHS modules might also be used to dissipate heat from one or more electronic components or devices. Another possible application is to use an MHS as described herein to shield a surface from concentrated directed energy devices such as high power lasers. Many other possible applications are envisioned.
A MHS module might optionally include an integral pressure relief valve (IPRV) to exhaust air from the reservoir containing the pool of liquid when triggered by excessive internal vapor pressure or by the increase in air pressure due to heating. A cutaway view of one possible IPRV embodiment is shown in FIG. 7. The IPRV 100 consists of a valve body 102, the central portion of which includes a piston 104, a spring 106, an O-ring 108 and an exhaust port 110, a liquid barrier 112 and a porous barrier 114. The spring constant is selected based on the required cracking and reseal pressure. When the internal pressure within the MHS is sufficiently high, the IPRV opens and air 116 and vapor 118 start to flow toward piston 104. Both air and vapor have to pass through porous barrier 114. While flowing through the porous barrier, some of the vapor will condense. The porous barrier thickness and pore size are preferably optimized such that most of the vapor condenses within the porous barrier and returns to pool 120, with mostly just air 116 passing through the porous barrier. The latent heat of condensation is conducted through the porous barrier into the liquid pool 120 or top panel 36. Air then flows around liquid barrier 112, through the holes in piston 104 and out of the MHS via exhaust port 110. As soon as the pressure inside the MHS module drops to the reseal pressure, spring 106 pushes piston 104 against the IPRV body and seals the MHS. O-ring 108 is used as a seal between piston 104 and the IPRV body.
IPRV 100 could be set to any pressure, depending on the application. An IPRV that is set to a lower pressure will open soon after heating begins and the air in the liquid reservoir expands. On the other hand, an IPRV that is set to a higher pressure will remain closed during normal operation. The IPRV should be set to open only when the MHS internal pressure exceeds normal operating pressure, so as to minimize the loss of liquid from pool 120. It is expected that an MHS module with an IPRV that has a high pressure set point will remain closed most of the time, and will thus lose less working fluid during operation. Providing a relief valve in this way is desirable, as it is expected that an MHS with less air will cool faster and may result in a lower top surface temperature.
A wicking material might also be affixed to the topside of at least a portion of the bottom panel, thereby enabling liquid to be transported against gravity when the MHS is tilted. This is illustrated in FIG. 8. Connected open cell foam or wick 130 can be included on the bottom panels 38 to transfer the working fluid 46 from one side of the reservoir to the other side against gravity, using the bottom panel wick's capillary pumping.
Applications that use a MHS to protect surfaces from high temperature heat sources will in general require a flat MHS design. However; some applications require a cylindrical heat shield that can be placed around—and thereby intercept radiation from—the heat source(s). One possible embodiment of such a cylindrical MHS is shown in the perspective, sectional and plan views depicted in FIGS. 9 a, 9 b and 9 c, respectively. Here, a top panel 140 and a bottom panel 142 form a cylindrical enclosure 144, with support pillars 146 located between the top and bottom panels. The cylindrical enclosure is deployed vertically, such that an enclosure is formed which surrounds a heat source 148—with top panel 140 located nearest the source, the pool of liquid 150 located between the top and bottom panels at the bottom of the enclosure, and a wicking material 152 on at least some of the pillars and on the inside of at least a portion of the top panel such that liquid from the liquid pool is transported up the sides of the enclosure against gravity via the wicking material. As the heat source is surrounded, the heat source can be pointing in any direction.
For applications where the heat source is at or below the top level of the liquid within the cylindrical MHS assembly, the wicking material is not required. The cylindrical MHS could be built from any number of cylinders that are placed on top of each other. Seals 154 are used to seal the gaps between the cylinders. Flexible wick structures such as foams or screens are preferably placed close to the seals to provide bridges for the working fluid between adjacent modules. The gap between the top and bottom panels is used to transport vapor and to return condensed liquid back to the liquid pool. The heat is absorbed by evaporation or boiling, and the resulting vapor spreads the heat to the entire cylindrical MHS assembly. The vapor condenses outside the heat zone and rejects the heat to the environment. The outside of bottom panels 142 could also include fins for more effective heat dissipation to the environment. A cylindrical MHS with high temperature working fluids such as liquid metals could enable shielding of very high temperature heat sources (e.g. plasmas) that would not be possible with conventional solid heat shields.
The present MHS may also include a ramp coupled to at least one module to provide better access to the MHS top surface; this is illustrated in FIG. 10 a, with a close-up view of the ramp/surface interface shown in FIG. 10 b. The ramp 160 could be made of individual panels and attached to some or all or some of the peripheral MHS modules. The ramp panels could be welded 162 to the shielded surface 34 to prevent any fluids from wicking underneath the MHS and causing corrosion of the shielded surface. As noted above, there may be a conformable foam 72 affixed to the underside of the MHS bottom panel, which may also extend to the underside of ramp 160, that serves to conform the bottom of the ramp to minor protrusions 74 from shielded surface 34. An electrical insulator layer 164 might also be installed between the outer edge of the ramp and a layer 166 made from the same material as the shielded surface.
One of the key advantages of the present MHS is that it can be assembled where it is required, from any number of modules. The joints between the panels could be rigid (e.g., welded, brazed, soldered, or glued seals), flexible (e.g., gasket or o-ring seals), or a combination of both. A MHS with seals of the first type is shown in FIG. 11 a. Two modules 170, 172 are shown. In this example, the joints 174 between the top panel 36 and bottom panel 38 of each module are formed by a method such as welding, brazing or soldering.
Flexible seals are illustrated in FIG. 11 b. Again, two modules 180, 182 are shown. Here, the top panels 184, 186 and the bottom panels 188, 190 should have overlapping lips 192, 194 on their edges, with gaskets 196 fitting in-between the lips.
For the two modules 200, 202 shown in FIG. 11 c, the top panels 204, 206 are joined using a gasket 208, while bottom panels 210, 212 are joined with a rigid joint 214. An advantage of using a rigid joint for the bottom panels and a flexible joint for the top panels is that the rigid joint on the bottom will result in better sealing of the reservoir, while the flexible joint on the top makes it easy to change top panels if they are damaged. Bottom panels are less likely to get damaged.
As noted above, the present MHS has numerous applications. Examples include shielding from high energy sources (e.g. hot exhaust plumes, directed energy weapons or high temperature plasmas) and spreading and transferring heat from electronics devices (e.g. cooling semiconductor dies, cooling laser diodes, or cooling concentrated photovoltaic cells).
The MHS modules and wicking material are preferably made from lightweight materials such as aluminum or magnesium. The working fluid preferably includes a corrosion inhibitor that promotes passivation of the wicking material during operation and prevents generation of non-condensable gas within the enclosure.
The embodiments of the invention described herein are exemplary and numerous modifications, variations and rearrangements can be readily envisioned to achieve substantially equivalent results, all of which are intended to be embraced within the spirit and scope of the invention as defined in the appended claims.

Claims

We claim:

1. A modular heat shield and heat spreader (MHS), comprising:

a top panel;

a bottom panel;

a plurality of thermally conductive pillars located between said top and bottom panels such that said pillars support said top panel; and

a continuous pool of liquid between said top and bottom panels such that at least a portion of at least some of said pillars is surrounded by said liquid;

such that heat from a heat source to which said top panel is exposed is conducted through said top panel and at least some of said pillars and said heat changes the phase of at least some of said liquid to a vapor, said vapor spreading said heat to an area larger than the area of the heat source and thereby dissipating said heat away from said heat source at a lower heat flux than that associated with said heat flux from said heat source.

2. The MHS of claim 1, further comprising wicking material on at least some of said pillars and on the underside of at least a portion of said top panel, such that at least some of said wicking material is saturated with said liquid and heat from said heat source is conducted through at least some of said liquid saturated wicking material.

3. The MHS of claim 2, wherein additional wicking material is formed into columns adjacent to at least some of said pillars which connect said top and bottom panels such that the liquid can flow from said continuous pool of liquid to the top panel through said wicking material columns.

4. The MHS of claim 2, further comprising a wicking material on the topside of at least a portion of said bottom panel, such that at least some of said liquid is transported against gravity via said wicking material when the heat shield and heat spreader is tilted.

5. The MHS of claim 1, arranged such that said liquid pool includes only as much liquid as needed to fill the voids between said wicking material.

6. The MHS of claim 1, wherein said top and bottom panels and said pillars form a single module, said MHS comprised of a plurality of said modules joined together at their edges such said modules share a common continuous pool of liquid and said vapor can dissipate heat away from the modules nearest said heat source towards other ones of said modules.

7. The MHS of claim 6, wherein the top and bottom panels of each of said modules is arranged such that each module is hermetically sealed.

8. The MHS of claim 6, wherein the modules making up said MHS have different sizes and shapes.

9. The MHS of claim 1, further comprising a surface to be shielded, said MHS deployed atop said surface to be shielded.

10. The MHS of claim 9, wherein said surface to be shielded is a landing spot on an amphibious ship or an aircraft carrier, such that said MHS shields the flight deck from airplane exhaust plumes.

11. The MHS of claim 9, wherein said MHS is used to shield said surface from concentrated directed energy devices such as high power lasers.

12. The MHS of claim 1, wherein the top panel of said MHS is used as a mounting surface for one or more electronic components.

13. The MHS of claim 1, wherein said MHS includes heat sink fins on said bottom panel for improved heat dissipation to the environment.

14. The MHS of claim 1, further comprising a conformable layer on the bottom side of said bottom panel.

15. The MHS of claim 1, further comprising a pressure relief valve arranged to exhaust air or air and vapor from said MHS when the air and vapor pressure between said top and bottom panels exceeds a predetermined threshold.

16. The MHS of claim 1, further comprising a ramp coupled to at least one MHS module.

17. The MHS of claim 1, further comprising compressible pads located between said bottom plate and the bottoms of said pillars.

18. The MHS of claim 1, wherein said top and bottom panel form a cylindrical shell which is deployed vertically such that it forms a cylindrical enclosure which surrounds said heat source with said top panel nearest said heat source, said pool of liquid located at the bottom of said enclosure, further comprising a wicking material on at least some of said pillars and on the inside of at least a portion of said top panel such that liquid from said liquid pool is transported up the sides of said enclosure via said wicking material.

19. A modular heat shield and heat spreader (MHS), comprising:

a plurality of modules, each of which comprises:

a top panel;

a bottom panel;

wicking material on at least some of said pillars and on the underside of at least a portion of said top panel;

said modules joined together at their edges to form a MHS capable of being deployed atop a surface to be shielded; and

a common continuous pool of liquid between the top and bottom panels of said modules such that at least a portion of at least some of said pillars is surrounded by said liquid and at least some of said wicking material is saturated with said liquid;

such that heat from a heat source to which one or more of said top panels is exposed is conducted through the top panel and said liquid saturated wicking material, said liquid present in the voids of said wicking material changing to a vapor state when sufficiently heated such that said vapor dissipates said heat away from the modules nearest said heat source towards other ones of said modules where said vapor condenses and said condensate returns to said common continuous pool of liquid.

20. The MHS of claim 19, wherein said modules are joined together so as to form a watertight seal along each module-to-module junction.

21. A modular heat shield and heat spreader (MHS), comprising:

a plurality of modules, each of which comprises:

a top panel;

a bottom panel;

a plurality of pillars located between said top and bottom panels such that said pillars support said top panel, said top and bottom panels joined together to form a hermetically sealed enclosure; and

wicking material attached to said top and bottom panels and also formed into wick columns located between the pillars so that the wick columns connect the top panel wicking material and the bottom panel wicking material;

at least one of said modules including one or more areas to which heat generating electronic devices are to be attached and including heat sink fins which are attached to the opposite side of the module,

each of said modules charged with an amount of working fluid that is equal to or larger than the volume of all the voids in the wicking material;

a plurality of said modules of the same or different size placed next to each other to form a MHS;

such that heat from said heat generating electronic devices attached to one side of the MHS is conducted through the top panel and the wicking material and said liquid present in said voids such that said heat changes the phase of said working fluid to a vapor state, said vapor spreading the heat to the entire module, said heat sink fins dissipating the heat to the environment and thereby changing said vapor to a liquid which is wicked back to the top panel wicking material via said wick columns.

22. The heat shield of claim 21, wherein at least some of the pillars located between the top and the bottom panels are bonded to both the top and the bottom panels to enable the MHS to operate at elevated pressures and prevent mechanical deformation of MHS panels.

23. The heat shield of claim 21, wherein said modules and wicking material comprise aluminum or magnesium and said working fluid includes a corrosion inhibitor that promotes passivation of the wicking material during operation and prevents generation of non-condensable gas within the module.