Flow boiling and two-phase flow instabilities in silicon microchannel heat sinks for microsystems cooling
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Date
2010Author
Bogojević, Dario
Metadata
Abstract
Flow boiling in microchannels, while very promising as a cooling technology in
electronics thermal management, is still a subject being explored that requires further
investigation. Before applying this technology for high heat flux computer chip
cooling, challenging issues such as fully understanding boiling mechanisms in
confined spaces, extending and stabilising the nucleate boiling regime, suppressing
flow boiling instabilities, maintaining uniform flow distribution among
microchannels, have to be addressed. If flow boiling is to be used as a thermal
management method for high heat flux electronics it is necessary to understand the
behaviour of a non-uniform heat distribution, which is typically the case observed in
a real operating computer chip. In this study, flow boiling of deionised water in a
silicon microchannel heat sink under uniform and non-uniform heating has been
investigated with particular attention to flow boiling instabilities. An experimental
system was designed and constructed to carry out the experimental investigations.
The experimental heat sink consisting of forty parallel rectangular microchannels
with 194 μm hydraulic diameter together with integrated inlet and outlet manifold
was fabricated on a silicon wafer using inductive coupled plasma dry etching, in
conjunction with photolithographic techniques. A design with integrated temperature
sensors made from a thin nickel film allows local temperature measurements with a
much faster response time and smaller thermal resistance as compared to temperature
measurements using thermocouples. The integrated heater was designed to enable
either uniform or non-uniform heating (hotspot investigation) with a low thermal
resistance between the heater and the channels. Numerical simulations for single
phase flow in adiabatic conditions were used to assist the design of the manifold
geometry in the microchannels heat sink. Microfabricated temperature sensors were
used together with simultaneous high speed imaging in order to obtain a better
insight related to temperature fluctuations caused by two-phase flow instabilities
under uniform and non-uniform heating. Two types of two-phase instabilities with
flow reversal were identified and classified into flow stability maps. The effect of
inlet water temperature on flow boiling instabilities was experimentally studied, with
the influence of different subcooling conditions on the magnitude of temperatures as
well as the influence on temperature uniformity over the heat sink being assessed.
The effect of various hotspot locations on flow boiling instabilities has been
investigated, with hotspots located in different positions along the heat sink. Bubble
growth and departure size have been experimentally investigated. The results of this
study demonstrate that bubble growth in microchannels is different from that in
macroscale channels. Furthermore, the effects of bubble dynamics on flow
instabilities and heat transfer coefficient have been investigated and discussed.