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Research Papers

Self-Propelled Sliding Bubble Motion Induced by Surface Microstructure in Pool Boiling of a Dielectric Fluid Under Microgravity

[+] Author and Article Information
Naveenan Thiagarajan

Electronics Cooling Laboratory,
GE Global Research,
Niskayuna, NY 12309
e-mail: naveenan.thiagarajan@ge.com

Sushil H. Bhavnani

Professor
Department of Mechanical Engineering,
Auburn University,
Auburn, AL 36849
e-mail: bhavnsh@auburn.edu

Vinod Narayanan

Associate Professor
School of Mechanical Industrial
and Manufacturing Engineering,
Oregon State University,
Corvallis, OR 96331
e-mail: vinod.narayanan@oregonstate.edu

Contributed by the Electronic and Photonic Packaging Division of ASME for publication in the JOURNAL OF ELECTRONIC PACKAGING. Manuscript received August 22, 2014; final manuscript received November 23, 2014; published online January 19, 2015. Assoc. Editor: Pradip Dutta.

J. Electron. Packag 137(2), 021009 (Jun 01, 2015) (8 pages) Paper No: EP-14-1073; doi: 10.1115/1.4029246 History: Received August 22, 2014; Revised November 23, 2014; Online January 19, 2015

This paper reports bubble dynamics observed during pool boiling over microstructures with an asymmetric saw-tooth cross section, under reduced gravity. The periodic saw-toothed ratchets etched on a silicon surface include fabricated vapor bubble nucleation sites only on the shallow slope. Reduced gravity pool boiling experiments were conducted aboard a Boeing 727 aircraft carrying out parabolic maneuvers. The fluid used was FC-72, a highly wetting dielectric fluid used as a coolant for electronics. Under microgravity, it was observed that the bubble diameters were six times larger than in terrestrial gravity. Also, self-propelled sliding bubble motion along the surface of the 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 is 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, which resulted in the sliding bubble motion, has the potential to alleviate dry-out caused due to stagnant vapor bubbles over heat sources under microgravity when the buoyancy forces are negligible compared to the surface tension forces.

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References

Figures

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Fig. 1

Experimental setup used for the reduced gravity experiments

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Fig. 2

Accelerometer data of gravity profile along the three axes—gx, gy, and gz. The nose of the aircraft was oriented toward the y-axis. x is the horizontal lateral axis and z is the vertical axis.

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Fig. 3

Bubble dynamics in FC-72 under μg. (a) Sliding motion of vapor bubbles at a velocity of ≈10 mm/s, across the saw-teeth (left to right in images) at q″ = 0.5 W/cm2 and ΔTsub = 23.6 °C. Sliding motion was observed at all tested conditions. (b) Sliding motion of vapor bubbles (left to right in the images) at q″ = 1.4 W/cm2 and ΔTsub = 22.8 °C.

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Fig. 4

Bubble dynamics in FC-72 under 1.8 g. Bubble departure diameters are very small compared to those at 1 g (≈0.25 D1g). Bubbles were observed to grow and depart at an angle normal to the shallow slope of the saw-teeth, a phenomenon that was previously demonstrated under 1 g.

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Fig. 5

(a) Average sliding velocity of bubbles under reduced gravity. (b) Effect of bubble coalescence on sliding velocity. Uncertainty in bubble velocity is ±0.5%.

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Fig. 6

Schematic diagram of bubble motion over a saw-tooth in FC-72 under μg (not to scale). The arrows marked between the saw-tooth and the vapor bubble indicate the direction of forces acting on the bubble due to pressure differences.

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Fig. 7

(a) Liquid–vapor interface of a sliding vapor bubble in FC-72 under microgravity at 0.45 W/cm2. The estimated liquid film thickness using image processing techniques is ≈20–75 μm (b) Schematic diagram of the liquid-vapor interface.

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Fig. 8

Estimated bubble sliding velocities for liquid film thickness varying from 0 to 25 μm. The experimental bubble velocity of 27.4 mm/s is predicted closely for a H value of 24 μm.

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Fig. 9

Boiling curve with FC-72 under μg at ΔTsub = 21.5 °C. Uncertainties in q″ and (Tw − Tsat) are ±1% and ±0.3%, respectively.

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