Liquid-vapour phase change and multiphase flow heat transfer in single micro-channels using pure liquids and nano-fluids
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Date
22/11/2011Author
Wang, Yuan
Metadata
Abstract
Heat management in high thermal-density systems such as CPU chips, nuclear
reactors and compact heat exchangers is confronting rising challenges due to ever more
miniaturized and intensified processes. While searching for heat transfer enhancement,
micro-channel flow boiling and the usage of high thermal potential fluids such as
nanofluids are found to be efficient heat removal approaches. However, the limited
understanding of micro-scale multiphase flows impedes wider applications of these
techniques. In this thesis work, liquid-vapour phase change and multiphase flow heat
transfer in micro-channels were experimentally investigated. Included are studies on the
single phase friction, vapour dynamics, liquid meniscus evaporation, two-phase flow
instabilities and heat transfer. An experimental system was built. Rectangular microchannels
with different hydraulic diameters (571 μm, 762 μm and 1454 μm) and crosssectional
aspect ratios were selected. Transparent heating was utilised by coating the
micro-channels with a layer of tantalum on the outer surfaces. FC-72, n-pentane, ethanol,
and ethanol-based Al2O3 nanofluids were used as working fluids. Pressures and
temperatures at micro-channel inlet and outlet were acquired. Simultaneous visualisation
and thermographic profiles were monitored. Single phase friction of pure liquids and
nanofluids mostly showed good agreement with the conventional theory. The
discrepancies were associated with hydrodynamic developing flow and the early
transition to turbulent flow, but nanoparticle concentration showed minor impact. After
boiling incipient, the single vapour bubble growth and flow regimes were investigated,
exploring the influences of flow and thermal conditions as well as the micro-channel
geometry on vapour dynamics. In addition, liquid meniscus evaporation as the main heat
transfer approach at thin liquid films in micro-channels was studied particularly.
Nanoparticles largely enhanced meniscus stability. Besides, flow instabilities were
analyzed based on the pressure drop and channel surface temperature fluctuations as well
as the synchronous visualization results. Moreover, study on flow boiling heat transfer
was undertaken, the corresponding heat transfer characteristics were presented and the
heat transfer mechanisms were elucidated. Furthermore, ten existing heat transfer
correlations were assessed. A modified heat transfer correlation for high aspect ratio
micro-channel flow boiling was proposed. The crucial role of liquid property and microchannel
aspect-ratio on flow boiling heat transfer was highlighted.