Flynn PhD Thesis Defense, Tuesday July 1, 3PM

Roger Flynn rflynn at
Mon Jun 30 11:36:28 PDT 2008

PhD Thesis Oral Examination

"Flow Boiling Instabilities in Microchannels"

Candidate: Roger D. Flynn

Advisor: Prof. Ken Goodson


Date: Tuesday, July 1

Time: 3:00PM, refreshments beforehand

Room CISX-101 (Auditorium)




There is a growing demand for compact high heat flux cooling in a number of
components like microprocessors, LEDs and laser diodes.  Microchannel heat
sinks are a natural solution, leveraging microfabrication to thin convection
boundary layers for enhanced heat transfer in a package comparable in size
to the cooled components.  Liquid flow microchannel heat sinks have recently
been commercialized, cooling 250 W/cm2, but much lower flow rates and
pumping powers are achievable with flow boiling which utilizes the fluid's
latent heat of vaporization.  However, boiling produces instabilities which
must be better understood and controlled before implementation is practical.
Instabilities have been well documented, particularly in nuclear reactor
design, but confined bubble growth and short length scales in microchannels
lead to a significantly different balance of forces which govern

This work describes a unique set of experiments which enable decoupling and
characterization of thermal and hydrodynamic microchannel flow
instabilities.  Modeling and analysis are developed around data for a single
channel and then applied to two parallel channels.  The dual channel system
exhibits the same parallel channel instabilities observed in massive
parallel channels, but with fewer coupled channel interactions.  Thermal and
hydrodynamic instabilities are further deconvolved by MEMS fabricated dual
channels with and without lateral heat conduction between channels.
Thermally isolated channels exhibit the worst case of hydrodynamic
instability, leading to premature dryout, while thermally connected channels
redistribute heat to stabilize flow boiling.  Appropriate scaling for each
mechanism gives guidelines for design of a microchannel heat sink with
stable flow boiling, fit for the most demanding high heat flux and space
constrained applications.


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