Investigation of Thermally-Actuated Pumping During Pool Boiling of a Dielectric Liquid on an Asymmetric Microstructured Silicon Heat Sink

Abstract

Developments in the field of electronics fabrication have led to significant miniaturization of devices. Such a reduction in foot-print of the electronic devices coupled with increased capabilities, such as computing power, have offered numerous advantages to human comfort in the form of connectivity and portability. But these developments have also resulted in a bottle-neck which is the demand to dissipate the resulting high heat densities in the electronic packages. With heat dissipation demands exceeding more than 1000 W per sq. cm, traditional air cooling techniques and even evaporative liquid cooling techniques like heat pipes have been pushed to their limits of operation. Future high powered, micro-electronics, thermal management could potentially migrate to liquid cooling involving phase change which is considered as one of the potential solutions for high heat dissipation demands. While a technique such as flow in microchannels with phase-change has proven to meet the demands, it comes with the cost of power required to pump the fluid through narrow passages. With modern electronics getting leaner in power consumption, an ideal cooling technique would be one that while dissipating a large volume of heat, also self-propels the fluid to enhance the heat transfer characteristics. Such a cooling system with a pump-less flow loop will be power-free, compact and self-regulating. The system proposed will also be applicable to thermal management of space electronics, where power is a precious commodity. The study conducted by the author in collaboration with a heat transfer research group from Oregon State University, investigates the liquid self-propulsion effects during pool boiling of a dielectric liquid on asymmetric surfaces. The study describes a novel silicon heat sink with an asymmetric saw-tooth cross-sectioned surface structure, which has the potential to be translated into a liquid propulsion system while dissipating heat efficiently. The heat sink was fabricated using a combination of gray-scale lithography, deep reactive ion etching and wet etching techniques. The novelty of the heat sink lies in the ability to effect lateral motion of bubbles due to nucleation from re-entrant cavities fabricated on the shallow slope of the saw-toothed surface. The asymmetric nucleation, growth and departure of bubbles leads to an angular momentum imparted to the liquid, thereby resulting in a net lateral flow. The study investigates the ability of surface structure to propel the liquid in its immediate vicinity under a variety of test conditions. The tested conditions include heat flux in the range of 0-4 W per sq. cm., liquid subcooling ranging from 0-20 deg-C, and gravity ranging from 0-1.8g. Due to the unique profile of the surface, the bubble characteristics are very different from those reported in the literature for common surfaces and fluids. One of the primary objectives of the study is the characterization of bubble dynamics from such a surface. In the experiments conducted, bubble growth and departure from re-entrant cavities on the asymmetric structures were studied using high speed photography and image processing techniques. The asymmetry in shape of the ratchet and location of re-entrant cavities resulted in nucleation only from the shallow slope of the ratchets. Interestingly, the bubbles were ``light-bulb'' shaped which otherwise would be more circular for a highly wetting fluid such as FC-72. It was observed that the bubble growth and departure were normal to the shallow slope of the ratchet surface structure. Bubble dynamics such as growth rate, bubble departure diameter and frequency were studied as a function of heat flux and subcooling. Asymptotic growth relationships were expressed as D= At and D=βt^m for the inertia and heat transfer controlled regimes respectively. The value of A, varying between 48-181, increased with increasing heat flux and decreasing subcooling. Similarly, in the heat transfer controlled growth regime, the value of β, termed as the growth constant, was observed to increase between 0.25-0.3 with increasing heat flux and decreasing subcooling. Subcooling or heat flux did not affect the value of ‘m’ significantly which varied between 0.20 - 0.25, compared to the value of 0.5 that has been widely reported in the literature. The bubble departure frequency was estimated to be increasing between 0 - 60 with increasing heat flux and decreasing subcooling. However, under the tested conditions bubble departure diameter was not found to be affected significantly with heat flux or diameter. A similar saw-toothed surface was tested at microgravity to understand the effects of gravity on bubble dynamics and self-propelled bubble motion across the surface. Pool boiling experiments were conducted aboard NASA's reduced gravity flight. In FC-72, vapor bubbles six times larger in diameter compared to 1g were observed, due to lack of buoyancy. Interestingly, bubbles were observed to be sliding across the asymmetric surface at velocities as high as 27.4 mm/s. This motion was observed at all tested conditions. Pool boiling on a plain surface would result in stagnant, surface residing vapor bubbles that would affect the heat transfer characteristics adversely causing burn-out of the chip being cooled. The ability to move the bubble along the surface at high velocities prevented any heat transfer deterioration, and actually leads to an enhancement. The sliding motion was attributed to pressure differences in the thin liquid film existing between the saw-toothed surface and the vapor bubble. A model has been proposed based for the sliding velocity of bubbles, which proves that the force due to pressure differences in the liquid film is a potential driving force for the bubbles among other forces.