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TECHNICAL NOTES

A Thermocapillary Mechanism for Lateral Motion of Bubbles on a Heated Surface During Subcooled Nucleate Boiling

[+] Author and Article Information
Paul J. Sides

Department of Chemical Engineering, Carnegie Mellon University, Pittsburgh, PA 15213

J. Heat Transfer 124(6), 1203-1207 (Dec 03, 2002) (5 pages) doi:10.1115/1.1517268 History: Received October 10, 2001; Revised July 11, 2002; Online December 03, 2002
Copyright © 2002 by ASME
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References

Figures

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(a) A bubble in saturated boiling. The bubble is immersed entirely in superheated liquid. The fluid flow is due to the expanding interface because the vapor liquid interface is an isotherm. The tracer bubble does not move toward the other bubble. (b) A bubble of sparingly soluble electrolytically evolved gas near an electrode that is the source of the gas and is itself warm with respect to the electrolyte. The low thermal conductivity of the bubble relative to the thermal conductivity of the liquid supports a temperature gradient at the gas/liquid interface, which causes the liquid to flow away from the electrode and to entrain a tracer bubble. (c) A bubble being generated by vaporization at a heated surface, and consisting primarily of vapor with some amount of sparingly soluble gas immersed in subcooled liquid. Thermocapillary flow ensues because there is a temperature gradient along the surface of the bubble; as in (b) the temperature gradient engenders a surface tension gradient that pumps liquid in the vicinity of each bubble away from the heated surface. Adjacent bubbles entrain each other and consequently move toward each other.
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Experimental observation of the thermocapillary pumping effect. Bubbles move together, apart, and back toward each other as the surface on which they rest is first heated, then cooled, then heated. Air in silicone oil. The bubbles are 1 mm in diameter. See Kasumi et al. 5 for details of the experiments and theory.
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Phenomena occurring during evolution of a bubble from an evaporating liquid containing dissolved noncondensible gases. Heat evaporates the liquid; dissolved gases also enter the vapor phase but accumulate at the top of the bubble as the vapor condenses. The accumulation of gas inside the bubble requires the temperature of the liquid at the top to decrease, which enhances the thermocapillary motion (After Marek and Straub 9).
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A sketch of the simplified geometry for scaling analysis. The bubble is modeled as a block of vapor for use of Cartesian coordinates. The vertical length scale of the block corresponds to a bubble radius a.

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