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ISBN Print: 978-1-56700-421-2

ISSN: 2377-424X

International Heat Transfer Conference 15
August, 10-15, 2014, Kyoto, Japan

Dynamic Flow Structures in the Wakes of Sliding Bubbles for Convective Heat Transfer Enhancement

Get access DOI: 10.1615/IHTC15.tpf.008720
pages 8431-8441

Abstract

Vapour and bubbles are known to increase heat transfer rates from adjacent heated surfaces, a phenomenon attributed to the interaction between the bubble and the thermal boundary layer. This interaction takes two forms: bluff body forced convection, and the mixing of the bulk fluid via vortex shedding in the bubble wake. Previous studies on sliding bubbles have focused predominately on the heat transfer enhancement at the surface rather than on the underlying wake structures, although these are inextricably linked. A comprehensive understanding of heat transfer due to sliding bubbles requires a better into the wake structures. This study uses 2-component Particle Image Velocimetry in three separate planes to quantify the wake structures of air bubbles at a range of volumes, sliding along a test surface in quiescent water inclined at between 20° and 30° to the horizontal. The PIV data in all three planes infer a wake structure consisting of a set of interconnected, alternately oriented hairpin vortex loops. These are constrained by the sliding surface, and have a shedding frequency linked to the path and shape oscillations of the bubble. For a heated inclined surface, a significant proportion of the heat transfer enhancement is found to be wake-driven, with cooling that lasts an extended time period. This is a consequence of the hairpin vortex structure introducing fluid from the liquid bulk to the surface, and will provide a basis for the future optimisation of multiphase convective heat transfer.
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