Nucleate boiling surfaces for cooling and gas generation

Details Nucleate boiling surfaces for cooling and gas generation Nucleate boiling surfaces for cooling and gas generation

1995-06-07

2002-04-16

Ford, John K. (Department: 3743)

Heat exchange

Structural installation

C165S133000, C165S104210, C165S911000

Reexamination Certificate

active

06371199

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to methods and systems for cooling surfaces and generating gases by nucleate boiling, and more particularly, to photo etched microconfigured surfaces disposed in contact with refrigerants.
BACKGROUND OF THE INVENTION
Various liquid emersion cooling systems have been provided in the past decade for cooling semi-conductors so as to retain their reliability, longevity, and speed. Boiling enhancement studies have often focused on increasing the overall active surface area by roughing, coating the surface in a random fashion, or by creating a known number of artificial nucleation sites.
Early heat transfer surfaces relied on the assumption that a rougher surface would have more locations that promote stable nucleation than a smoother surface. Although random roughness is easily created on most surfaces, a consistent performance over time required for use in electronic packages is not generally guaranteed by mere roughness alone. Moreover, such surfaces often contain voids containing trapped air which are likely to inhibit or cause irregular boiling.
Higher heat fluxes have been achieved using flow boiling or conduction cooling of silicon substrates by dielectric liquids. Although high heat fluxes are obtainable with such techniques, considerable plumbing and pumping power is required which may hinder the use of such systems in practical applications.
More recently, research has focused on studying arrays of artificially produced sites of known geometry and spacing. See U.S. Pat. No. 4,050,507 to Chu et al., herein incorporated by reference. Chu describes a method for customizing the heat transfer from walls of an electronic device, such as a chip or wafer, by laser-drilling apertures having a narrow opening which tapers to a larger oblong-shaped cavity. This nucleation cavity geometry relies upon the fact that the narrow opening prevents complete flooding of the cavity, so that vapors are trapped in the nucleation sites. Accordingly, it is stated that nucleate boiling can be initiated at approximately the same temperature each time without temperature overshoot.
Recent studies have indicated, however, that when dielectric coolants are employed, such as FREON®, these otherwise stable vapor-filled cavities become more readily flooded, since these fluids, unlike water, have a very small contact angle with the substrate surface, i.e., less than 5°. Chu attempts to get around this problem by artificially creating vapor bubbles in the cavities with a nucleation heater, but this adaptation requires additional space and expense, much like the earlier flow boiling techniques, and may not uniformly provide sufficient vapor to all sites to prevent flooding.
Accordingly, a need exists for nucleate boiling surfaces for cooling densely populated microelectronics circuits which are easy to make and implement. There is also a need for a boiling technique for removing large heat fluxes even at small superheats. There is a further need to provide a more efficient and compact system for generating gases from liquids, such as cryogenic liquids employed in industrial and medical applications.
SUMMARY OF THE INVENTION
This invention provides heat transfer and gas generation systems and methods of cooling surfaces and heating fluids which employ nucleate boiling. In the methods of this invention, a surface is prepared to obtain a predetermined minimum surface density of discrete nucleation sites having a conical cross-section tapering to at least a minimum predetermined depth. The surface is then immersed in a refrigerant having a preselected boiling point so that the nucleation sites become substantially flooded by the refrigerant. Finally, the surfaces are permitted to heat up to a temperature of at least the preselected boiling point, whereupon nucleate boiling initiates in the refrigerant without a temperature overshoot on the initial ascent.
This invention provides repeatable performance in water, FC-72, and liquid nitrogen. The spaced apart conical nucleation sites of this invention permit a large number of active nucleate boiling sites on the surface. These sites can be spaced randomly, uniformly, selectively, or in groups to permit uniform or tailored cooling of the surface. At greater heat fluxes, the active sites increase in number and become more closely spaced, yet efficient cooling or gas generation is maintained. Temperature hysteresis was found to depend on the spacing of the conical sites, and temperature overshoot, as well as the reversal of trend, was shown to be substantially eliminated by the surfaces of this invention.
Boiling heat transfer performance of regular microconfigured silicon surfaces in saturated water, FC-72, and in liquid nitrogen were conducted for testing this invention. The microconfigured surfaces were photo etched with inverted square pyramids 10 &mgr;m on a side and 7.1 &mgr;m deep. The inverted pyramids were repeated on 20, 40, and 60 &mgr;m centers. One set of experiments in FC-72 was also conducted with a microconfigured surface photo etched with 9.4 &mgr;m hexagonal dimples, 3.3 &mgr;m deep on 18.8 &mgr;m centers.
The number of nucleation sites in the microconfigured surfaces of this invention can be very large and all the features can include exactly the same geometry. This promotes repeatability of heat transfer performance, as well as accurate determinations of the transfer rates without statistical guesswork employed by the prior art. A characteristic dimension of the nucleation sites of this invention, the cavity diameter, is believed to be the size scale of a trapped vapor embryo in a cavity. This contributes to the improved heat transfer properties, despite the fact that the preferred refrigerants and microgeometries of this invention fail to satisfy the classical criteria for a stable vapor trapping conical cavity.
This invention is suitably applied to electronics cooling applications where the heat generated by densely packed chips is often detrimental to efficient operation of the circuitry. Computer components could be equipped with surfaces, such as on one side of the chip or on the outside surface of the encapsulation or package which contains the chip, containing selected arrays of microconfigured nucleation sites having different geometries and/or different spacings to provide uniform cooling of the component despite the non-uniform nature of the heat generated. The outside surface of these individual encapsulations or the exposed side of the chip could then be immersed in a suitable cooling module containing a sufficient quantity of refrigerant to effect cooling. Upon operating the computer, the component would heat up and nucleate boiling would occur in either a saturated or subcooled refrigerant. Bubbles would rise and would begin to dissipate their trapped heat into the surrounding liquid, resulting in a very efficient heat transfer from the component into the liquid. Since temperature overshoot is eliminated by this invention, the cooling of electronic components can be accomplished without the thermal shock normally associated with boiling mechanisms. This will enhance the reliability and extend the useful life of electronic components.
More efficient gas generating systems are also made possible by this invention, since the large heat transfer fluxes produced with the disclosed microconfigured surfaces can be harnessed to provide faster gas production with a smaller surface area. Such systems would be ideally suited for generating gas from cryogenic liquids, such as oxygen, nitrogen, and helium. The microconfigured surfaces of this invention could be heated above the vaporization temperature of these cryogenic fluids so as to rapidly produce a gaseous phase upon contact with the liquid. The resulting gas in pure or mixed form could be used in a variety of commercial applications, including, for example, respirators and industrial gas delivery systems.


REFERENCES:
patent: 3301314 (1967-01-01), Gaertner
patent: 3392781 (1968-07-01), Zuber et al.
patent: 3454081 (1969-07-01), Kun et al.
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