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|Title: ||Hydrodynamics, heat transfer and flow boiling instabilities in microchannels|
|Authors: ||Barber, Jacqueline Claire|
|Supervisor(s): ||Sefiane, Khellil|
|Issue Date: ||2010|
|Publisher: ||The University of Edinburgh|
|Abstract: ||Boiling in microchannels is a very efficient mode of heat transfer with high heat and
mass transfer coefficients achieved. Less pumping power is required for two-phase
flows than for single-phase liquid flows to achieve a given heat removal.
Applications include electronics cooling such as cooling microchips in laptop
computers, and process intensification with compact evaporators and heat
Evaporation of the liquid meniscus is the main contributor to the high heat fluxes
achieved due to phase change at thin liquid films in a microchannel. The microscale
hydrodynamic motion at the meniscus and the flow boiling heat transfer mechanisms
in microchannels are not fully understood and are very different from those in
macroscale flows. Flow instability phenomena are noted as the bubble diameter
approaches the channel diameter. These instabilities need to be well understood and
predicted due to their adverse effects on the heat transfer.
A fundamental approach to the study of two-phase flow boiling in microchannels has
been carried out. Simultaneous visualisation and hydrodynamic measurements were
carried out investigating flow boiling instabilities in microchannels using two
different working fluids (n-Pentane and FC-72). Rectangular, borosilicate
microchannels of hydraulic diameter range 700-800 μm were used. The novel
heating method, via electrical resistance through a transparent, metallic deposit on
the microchannel walls, has enabled simultaneous heating and visualisation to be
achieved. Images and video sequences have been recorded with both a high-speed
camera and an IR camera.
Bubble dynamics, bubble confinement and elongated bubble growth have been
shown and correlated to the temporal pressure fluctuations. Both periodic and nonperiodic
instabilities have been observed during flow boiling in the microchannel.
Analysis of the IR images in conjunction with pressure drop readings, have allowed
the correlation of the microchannel pressure drop to the wall temperature profile,
during flow instabilities.
Bubble size is an important parameter when understanding boiling characteristics
and the dynamic bubble phenomena. In this thesis it has been demonstrated that the
flow passage geometry and microchannel confinement effects have a significant
impact on boiling, bubble generation and bubble growth during flow boiling in
flow boiling instabilities
|Appears in Collections:||Engineering thesis and dissertation collection|
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