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Journal of Flow Visualization and Image Processing
BUBBLE DEPARTURE FROM METAL-GRAPHITE COMPOSITE SURFACES AND ITS EFFECTS ON POOL BOILING HEAT TRANSFER
David F. Chao
Fluid Physics and Transport Branch, Ohio Aerospace Institute at NASA Glenn Research Center, Cleveland, OH 44135, USA
John M. Sankovic
National Aeronautics and Space Administration, John H. Glenn Research Center, Cleveland, Ohio, U.S.A.
Brian J. Motil
NASA Glenn Research Center, Cleveland, OH 44135, USA
Department of Mechanical Engineering and Applied Mechanics University of Michigan, Ann Arbor, Michigan 48109-2125, U.S.A.
Pearlica Technologies, Inc.
The formation and growth processes of a bubble in the vicinity of graphite micro-fiber tips on metal-graphite composite boiling surfaces and their effects on boiling behavior are investigated. It is discovered that a large number of micro bubbles are formed first at the micro scratches and cavities on the metal matrix in pool boiling. By virtue of the non-wetting property of graphite, once the growing micro bubbles touch the graphite tips, the micro bubbles are sucked by the tips and merged into larger micro bubbles sitting on the end of the tips. The micro bubbles grow rapidly and coalesce to form macro bubbles, each spanning several tips. The necking process of a detaching macro bubble is analyzed. It is revealed that a liquid jet is produced by sudden break-off of the bubble throat. The composite surfaces not only have higher temperatures in micro- and macrolayers but also make higher frequency of the bubble departure, which increase the average heat fluxes in both the bubble growth stage and in the bubble departure period. Based on these analyses, the enhancement mechanism of pool boiling heat transfer on composite surfaces is clearly revealed.
MOTS CLÉS: micro bubbles, macro bubbles, macro bubble departure, bubble necking, liquid jet, boiling heat transfer enhancement
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