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Proceedings of Symposium on Energy Engineering in the 21<sup>st</sup> Century (SEE2000) Volume I-IV

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Shigefumi Nishio
Key Laboratory of Enhanced Heat Transfe and Energy Conservation, Ministry of Education, School of Chemical and Energy Engineering, South China University of Technology, China; and Institute of Industrial Science and Technology, University of Tokyo, Japan

Hiroaki Tanaka
Department of Mechanical Engineering, The University of Tokyo Bunkyo-ku, Tokyo 113, Japan


The present study is an attempt to attain a good understanding of the effects of liquid's wetting ability and subcooling on bubble behavior of high heat-flux boiling. This was accomplished by observing bubble behavior for boiling of water, ethanol, and R141b in a quasi-two-dimensional space. It was found that, even in the quasi-two-dimensional space employed in the present study, CHF increases for increasing liquid subcooling in a range of low subcooling and the dependence of CHF on subcooling in this range is in reasonable agreement with the correlating equation by Ivey and Morris. If subcooling is increased further, CHF approaches a unique value as Elkassabgi and Lienhard reported. For bubble behavior in saturated boiling, large coalesced bubbles like filmwise bubbles were observed at high heat fluxes even for R141b which is a highly wetting liquid. As for the effect of subcooling on bubble behavior of high heat-flux boiling, the time and spatial mean structure changes from filmwise to discrete bubbles for increasing subcooling. Even at highly subcooled boiling, however, a large coalesced bubble appears intermittently and it triggers CHF. This result indicates that, even in subcooled boiling, the appearance of large coalesced bubble is the key condition for CHF.