Flynn PhD Thesis Defense, Tuesday July 1, 3PM

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)

http://campus-map.stanford.edu/index.cfm?ID=04-055

 

Abstract

 

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 instabilities.

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.