Low Cost Boiling Coolers Utilizing Liquid Boiliπi

⁷ IeId of invention

This invention relates to design and building of low cost boiling coolers that utilize nucleate boiling heat transfer for a plurality of cooling applications.

Background of Invention

The ever-increasing heat-flux from electronic devices has driven people to seek more effective and inexpensive cooling technologies. Liquid boiling, rather than single-phase heat transfer or two-phase cooling based on vaporization (such as thermosyphon in heat pipes), is becoming an attractive option because it produces much higher heat transfer coefficients and can yield a far more uniform temperature distribution across the surface of a device and/or an array of devices. In addition, many boiling enhancement schemes are developed to overcome poor properties, such as highly wetting, low contact angle, and low specific heat, of some most suitable dielectric liquid coolants for cooling electronic devices. One promising boiling enhancement method is to use a microporous coating that provides a significant enhancement of nucleate boiling heat transfer and critical heat flux while reducing incipient hysteresis. Cooling modules taking advantage of these technologies make boiling to vaporization of liquid a primary means of spreading heat

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rather than conduction or convection used in conventional heat sinks. This can icac! to at least partial replacement or even complete elimination of the critical use of metal materials as cooler's body frame or simplification in module design without the complicated radiator, greatly reducing its manufacture cost. One current electronic cooling apparatus employs an air-conditioning manifold to distribute chilled air to the heating electronic assembly through a plurality of orifices or vortex tubes within the enclosure of the apparatus. However, power consuming air circulating system and its complicated structure make such apparatus expensive, bulky, and not flexible for various electronic system designs. Another cooling apparatus for electronic apparatus utilizes combination of liquid cooling and convection. But the liquid cooling is limited to use much less efficient pump-driven liquid circulating and may need a help of an electric fan for forced air convection to meet the required thermal load. The boiling cooler described in this invention overcomes many drawbacks of those conventional electronic cooling apparatus in terms of zero-power consumption, high heat- transfer efficiency, low cost, flexibility in physical shapes and ability to miniaturize, making it very suitable for a plurality of cooling applications including electronic component/system cooling.

Summary of Invention

The boiling cooler that utilizes boiling two-phase cooling in accordance with the

current invention provides a low-cost solution with great structural and materia! flexibility for a plurality of cooling applications. In one aspect of the invention, the boiling cooler comprises a vessel that is partially filled with a liquid coolant, enclosed by a body-shell mainly made of non-metal materials with extra open spaces within the vessel for liquid vapor to spread heat around and an extended surface area for additional cooling by natural convection or, if necessary, by forced-air convection. A part of the body-shell is thermally conductive and used to couple the heat-generating component so that the neat flux is transferred to the liquid and nucleate boiling is induced on the surface where a plurality of boiling enhancement treatments (such as mechanical roughening or microporous coating) are applied. This thermally conductive side may be a part of the body surface itself of the heat-generating device component, making an integrated device cooler. In another embodiment of the invention, the plastic body- shell of the vessel can be easily molded to form a plurality of desired shapes, complex or asymmetric, which would otherwise be impossible or too expensive to make with metal, hi yet another embodiment of the invention, for some device/system with bigger thermal load the cooler can have a molded vessel molded by baked copper powder with extruded shapes or fins, which provides much better thermal conductivity than those modules using all-plastic materials but still costs less than those using all-machined metals, An alternative structure of the cooler can have plastic end-caps or top/bottom for the vessel with metal body-shell.

In another embodiment of the invention, a low cost liquid coolant such as water can be

used.

B ^r i ef Description of Drawings FIG. IA illustrates a cross-sectional view, as a preferred embodiment of this

invention, of a microporous boiling enhancement coating coupled to top surface of a thermally- conductive side within the boiling cooler vessel.

FIG. IB is a SEM image of the coating structure comprising particles of sizes of

30-50μm. FIG. 2 illustrates a cross-sectional view of a boiling cooler as an enclosed vessel comprising a base chamber and one (or more) upper chamber with extended plates and extruded fins in combination with a boiling enhancement coating at the bottom of the base chamber with a liquid coolant partially filling the chamber

FIG. 3 is a cross-sectional view showing schematically a boiling cooler with a vessel comprising a relatively complex, asymmetric shell shape and extended surface area, according to another embodiment of the invention.

FIG. 4 is a cross-sectional view showing schematically a boiling cooler with a vessel partially wrapping around a heating electronic component whose part of thermally conductive body-shell, is also a part of the body-shell of the cooler vessel. The microporous coating is applied on said thermally conductive surface, which is at least

partially submerged in the liquid in the \ cssel. to enhance the nucleate boiling heat

transfer.

Detailed Description of Preferred Embodiment(s)

The current invention presents a boiling cooler with a vessel in a simplified design using inexpensive non-metal material or low cost liquid coolant in combination with boiling enhancement vessel surface treatment comprising mechanical roughening, sintering, and/or microporous coating. A preferred embodiment of the invention uses a Thermally-Conductive Microporous Coating (TCMC) developed by You and Kim (2005), described in co-pending U.S. Patent Application Serial No. 1 1/272,332. This coating technique combines the advantages of a mixture batch type and thermally- conductive microporous structures. The microporous surface is created using panicles of various sizes comprising any metal which can be bonded by the soldering process including nickel, copper, aluminum, silver, iron, brass, and various alloys in conjunction with a thermally conductive binder. The coating 40 is applied on the surface of a substrate 30 while mixed wirh a solvent. The solvent is vaporized after the application prior to heating the surface sufficiently to melt the binder to bind the particles, FlG. I A shows a cross-sectional view of the coating structure full of cavities and particles formed on top of the substrate plate

The mixture batch type application is an inexpensive and easy process, not requiring extremely high operating temperatures. The coating surface created by this process is insensitive to its thickness due to high thermal conductivity of the binder.

Therefore, large size cavities can be constructed in the microporous structures for some poorly wetting but potentially low cost fluids, such as water, without causing serious degradation of boiling enhancement. This makes the boiling cooler keep its high cooling efficiency for various types of liquid coolants simply by adjusting the size of

metal particles to allow the size range of porous cavities formed fit well with the surface tension of the selected liquid to optimize boiling heat transfer performance. FIG. IB shows a SEM picture of a coating surface containing nickel particle of sizes around 30- 50μm using -100+325 mesh nickel powder mixed with solder pastes. The solder pastes

were clearly seen as a binder between nickel particles and resultantly produce numerous

microporous cavities, The coating with particles in such a size range of 30-50μm has

been shown to provide supeπor boiling heat transfer performance for water as the liquid coolant.

A boiling cooler according to a preferred embodiment of present invention, is illustrated in Figure 2 as an enclosed vessel comprising a base chamber 120 with a thermally conductive side 130, and one or more upper chambers 110 according to one embodiment of the invention. The Ihermally-Conductive Microporous Coating (TCMC) 140 is applied to the surface of conductive side 130 within the base chamber 120 with which a heating electronic component S 00 is coupled from outside the base chamber i 20.

The liquid coolant 150 partially fills the base chamber 120, at least partially covering the

I CMC 140 surface area so that the heat flux conducted from the heating clement/device

100 can induce the nucleate boiling of the liquid 150 at the microporous surface of

TCMC 140. In this boiling cooler, the nucleate boiling heat transfer is significantly augmented by the TCMC 140 and becomes a dominant way to spread heat into the liquid

150. Conduction in this case becomes less important so that the whole body-shell of the cooling vessel, excepting the thermal conductive side 130, can be made of non-metal such as plastic material. Vapor coming out of the liquid boiling is held within the open space 160 of the upper chamber 1 10 which has one/or more extended plates 180 and multiple extruded fins 170 attached for achieving maximum heat exchange through natural or forced- air convection..

Advantage of the liquid boiling cooler in this invention includes that it does not need a radiator or other complicated heat exchanger within the apparatus, making it very suitable to be miniaturized for electronics cooling applications. As shown in Fig. 2, the boiling cooler can have a height of h and/or a lateral dimension of Z, less than or equal to

300mm. Other than the round shape, the cooler in Fig. 2 can also be elongated

(perpendicular to the section shown in the figure) which requires two end-caps to seal the vessel, As a trade-off for fully using plastic material, one may still use typical metal such as aluminum or copper for the major portion of the vessel including the side to contact the heat-generating device and all extruded fin structure for convection heat

exchange to take advantage of its high thermal conductivity, but the end-caps can be made of plastic to reduce manufacture cost.

Advantage of the boiling cooler in combination with the TCMC 140 is that the

major portion of body-shell of the cooling vessel can be made of non-metal material to save cost. The heat transfer in this boiling cooler is basically taking place within the cooler vessel by TCMC enhanced liquid boiling and additional vapor heat-spreading throughout the open space 160 of the cooling vessel. Therefore, it is not critical for the boiling cooler to have highly conductive metal body-shell as in many conventional coolers including heat sinks for various heating electronics element/device. Highly thermally conductive materia!, usually metal such as copper or aluminum, must be used for the body-shells of those conventional coolers because conducting heat through the shell to the surface, then cooling by using forced air convection, is their prominent way of cooling. In one embodiment, as shown in Fig. 2, the major portion of body-shells of the

vessel chambers 110 and 120 including those extruded fins 170 and extended plate 180 can be made of non-metal material comprising plastic, vinyl, or paper, which is much less expensive than any metal. Not only the material cost is lower, capability of plastic molding for those extruded fin structure also reduces the manufacturing cost comparing to processing metal. In addition, these non-metal body-shells can also be electrically insulating which provides an important advantage over the conventional cooler with electrically conducting metal shells for certain electronics cooling applications.

Additional advantage of the boiling cooler using TCMC is that the TCMC boiling enhancement coating can be optimized in terms of cavity-generating particle size using easy process modifications so that it can ensure no degradation in nucleate heal transfer rate and critical heat flux specification for a wide selection of liquid coolant types. This naturally translates to lower cost boiling cooler if the cheap liquid, such as water, instead of specially developed refrigerant or chemical fluid, can be used, As mentioned earlier a

TCMC surface containing nickel particles of sizes around 30-50μm using -100 +-325 mesh

nickel powder mixed with solder pastes has been shown to provide superior boiling heat transfer performance for water as the liquid coolant. Though TCMC boiling enhancement surface treatment is a preferred embodiment of this invention, a plurality of other boiling enhancement technologies such as mechanical roughening, sintering and/or conventional microporous coating shall be utilized to be integrated in the boiling cooler according to current invention.

In yet another embodiment of this cooler in combination with nucleate boiling, the chamber shells including fins can be constructed by utilizing molded and baked copper powder, which provides better thermal conductivity than those modules using ail-plastic materials but still costs less than those using all-machined metals. Similarly, thermally conductive plastic composite material can be another candidate for constructing the boiling cooler according to current invention. For cooling some devices/systems with relatively large thermal load, in addition to the nucleate boiling heat transfer within the

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cooling vessel, conductive body-shell is necessary for more efficient heat exchange with cooler's environment,

In another embodiment, of the invention the body-shell made of plastic can be molded into a relatively complex structure with asymmetric shape or small detailed features, which is usually much more expensive, if not impossible, to be manufactured using metal. FIG. 3 illustrates an example of such a boiling cooler with a vessel 220 enclosed by a body-shell 271 comprising multiple upper chambers 221 and 222 in irregular shapes and different heights, for example, h _\ and Zz ₂ on top of a common base chamber with dimension L _x (dimension along perpendicular direction is not shown) The boiling cooler also comprises a boiling enhancement surface 241 on a thermally conductive side shell 231 (a part of the body-shell 271), partially filled liquid coolant 251. Vapor generated from boiling helps spread heat over all extended space 261 adding extra pathway for cooling through convection. In some cases, the electric circuit module boards and/or system line cards, on which many electric or photonic components/devices

are tightly packed, have very stringent requirements in the mechanical design for the associated or integrated coolers. Using plastic material for the body-shell of the cooler vessel can easily make the cooler in complex shape or specific dimension without worry about driving the cost high. In other system applications forced air cooling may be not available, making the traditional heat sink impossible to handle the ever-increasing heat

flux out of those electronics system. The cooler utilizing boiling enhancement surface according to this invention achieves efficient cooling within the vessel, making the forced

air convection not critical for the system. In addition, natural or forced-air convection can still provide extra heat exchange through the extended exterior surface of the vessel of this boiling cooler.

In yet another embodiment of the invention, the boiling cooler vessel has been transformed to a built-m part of the heat-generating component, in other words, the cooler and the heat-generating component are built together as a coherent or integrated unit. Fig. 4 shows an example of such a boiling cooler whose thermally conductive side 330 essentially is the body-shell (at least covering partial surface) of the heat-generating component 300. Surface boiling enhancement treatment 340 is applied at least partially on side 330 at a surface within the vessel. Other parts of the body-shell 320 can be made of less-expensive materials comprising plastic, vinyl, paper, or molded and baked copper powder, which is sealed with side 330 to form an enclosed vessel holding partially- filled liquid coolant 350. As shown in Fig. 4, the shape of the body-shell 320 is intentionally made to have an extended surface area for extra benefit of convection cooling. Although the shape of the body-shell 320 is relatively irregular it can be done easily and inexpensively because of the material selection such as plastic. The dimensions of /? ₃ and Z ₃ in this case basically depend on that of the heat-generating component 300 itself.

Foregoing described embodiments of the invention are provided as illustrations and descriptions. They are not intended to limit the invention to the precise form described.

In particular, it is contemplated that functional implementation of invention described

herein may be implemented equivalent!}' in hardware, software, firmware, and/or other available functional components or building blocks Other variations and embodiments are possible in light of above teachings, and it is thus intended that the scope of invention not be limited by this Detailed Description, but rather by the following claims.