This is a collaborative research and educational program between investigators at Oregon State University and Auburn University in the general area of phase-change heat transfer enhancement. The research goal is to characterize the effect of a passively imposed asymmetric force on a bubble during the boiling process. Such asymmetry is created by the use of a surface with repeated ratchet structures and pyramidal reentrant cavities located on one of its sides. The hypothesis is that with several ratchets, this local asymmetric motion can be translated to a net fluid pumping. Educational activities aim to provide a synergy between research and education, as well as between several universities. The main objectives of the project are to: (1) Determine, using a suite of non-intrusive imaging methods, time-varying bubble diameters and frequencies, liquid and bubble velocities, and temporally- and spatially- resolved surface temperatures, (2) Refine preliminary models based on information obtained from the above imaging experiments, (3) Validate the model against global measurements of a large-array of ratchets, for which boiling curves will be established, and (4) Integrate research findings into a new module in an existing multi-University Electronics Thermal Management course. Intellectual Merit: The enhancement technique can be applied to heat transfer from adversely oriented surfaces and in space thermal management. The proposed methods of experimentation present a detailed estimation of flow and temperature field in boiling flows that will be useful to modellers. The proposed analyses will highlight forces of importance by performing experiments under different orientations and subcooling levels, and with two fluids of contrasting properties. Passive methods of heat transfer enhancement are important in a broader context of energy efficiency. Broader Impact: Educational activities will include integration of the project results in the form of a module into an existing, past-NSF funded Electronics Thermal Management live-internet multi-campus course and will incorporate Oregon State University as a new partner. At the end of year 2, a DC-9 microgravity flight experiment is planned as a part of a concurrent grant from NASA. Several senior undergraduate students will take part in the design and eventual testing of this experiment. An AT&T-Minority-Engineering-Program student at Auburn University will also be working closely with OSU undergraduate students. One promising high-school student, through the Oregon-state-wide Apprenticeship in Science and Engineering Program, will be involved in experiment design and testing.

Project Report

Developments in the field of electronics fabrication have led to significant miniaturization of devices. However, these developments have also created a huge technological challenge stemming from the need to dissipate the resulting high heat densities in the electronic packages. A particularly effective cooling technique involves immersing the electronics in a dielectric fluid with a low boiling point, like in the early supercomputers. The heat from the electronics converts the liquid to vapor providing very high heat removal rates. These rates are increased even further if the liquid is flowing. The study investigates an innovative concept wherein a heat sink with microscopic surface features in silicon is used to create a self-propelled flow of a dielectric liquid, resulting in a pump-less, compact, and self-regulating flow loop. The system proposed will also be applicable to thermal management of space electronics, where power is a precious commodity. This report presents the work performed under a collaborative project between Oregon State University and Auburn University researchers in the area of passive fluid motion using micro-structured surfaces during boiling. The project was co-funded by the Fluid Physics program of NASA. The goal of the project was to determine whether passive fluid motion could be obtained using asymmetry in surface texture. The asymmetry was created by the use of a surface with repeated 30-60 degree mm-sized saw-teeth with nucleation sites preferentially located on the 30 degree slope. The hypothesis that force asymmetry created due to the asymmetric growth and departure of the bubbles would impart a net lateral (along the surface component) motion of liquid was verified. This is shown schematically in Figure 1. A silicon heat sink with an asymmetric saw-tooth cross-sectioned surface structure was fabricated using a combination of gray-scale lithography, deep reactive ion etching and wet etching techniques. Figure 2 shows details of some of the test surfaces fabricated for this study. The asymmetric nucleation, growth and departure of bubbles led to an angular momentum imparted to the liquid, thereby resulting in a net lateral flow. The study investigated the ability of surface structure to propel the liquid in its immediate vicinity under a variety of test conditions including microgravity. Under regular terrestrial gravity conditions, bubbles generated at the microscopic cavities, grew and departed perpendicular to the shallow slope of each saw-tooth. As shown in Figure 3, hundreds of such bubbles acting in tandem imparted a lateral momentum to the liquid resulting in liquid velocities up to 25 mm/s along the surface. Reduced gravity pool boiling experiments were conducted aboard a Boeing 727 aircraft (Zero-g Inc.) carrying out parabolic maneuvers to achieve reduced gravity. Bubble diameters were observed to be six times larger than in terrestrial gravity in the dielectric fluid FC-72. As shown in Figure 4, self-propelled sliding bubble motion along the surface of the large array saw teeth was observed in reduced gravity. The velocity of the sliding bubbles across the saw teeth, following lateral departure from the cavities, was measured to be as high as 27.4 mm/s. A model for the sliding bubble motion was proposed by attributing it to the force due to pressure differences that arise in the liquid film between the vapor bubble and the saw-toothed heated surface. The pressure difference is due to difference in the radius of curvature of the interface between the crest and trough of the saw teeth. The surface modification technique demonstrated has the potential to alleviate problems caused due to vapor blanketing of heat sources due to imbalances between buoyancy and surface tension forces in microgravity.

Project Start
Project End
Budget Start
2009-06-01
Budget End
2013-05-31
Support Year
Fiscal Year
2008
Total Cost
$86,157
Indirect Cost
Name
Auburn University
Department
Type
DUNS #
City
Auburn
State
AL
Country
United States
Zip Code
36849