Date of Award

1-1-2013

Document Type

Open Access Dissertation

Department

Mechanical Engineering

First Advisor

Chen Li

Abstract

Through the use of latent heat evaporating, flow boiling in microchannels offers new opportunities to enable high efficient heat and mass transport for a wide range of emerging applications such as high power electric/electronic/optical cooling, compact heat exchangers and reactors. However, flow boiling in microchannels is hampered by several severe constraints such as bubble confinement (e.g., slug flow), viscosity and surface tension force-dominated flows, which result in unpredictable flow pattern transitions and tend to induce severe flow boiling instabilities (i.e. low-frequency and large magnitude flows) and suppress evaporation and convection.

In this dissertation, three novel micro/nanoscale thermo-fluidic control methodologies were developed to address these aforementioned constraints faced in flow boiling in microchannels. These include a) "unifying two-phase flow patterns" to radically avoid pattern transitions, b) "nano-tips induced boundary layers" to promote evaporation and advections by reconstructing boundary layers, and c) "high frequency self-sustained two-phase oscillations" to generate strong mixing in the laminar flow. Using superhydrophilic silicon nanowires, the first methodology successfully formulated a new, single and periodic annular flow during the entire flow boiling process, i.e., from the onset of nucleate boiling to the critical heat flux (CHF) conditions by reducing the characteristic bubble size and transforming the direction of the dominant capillary forces from the cross-sectional plane to inner-wall plane. In the second methodology, boundary layers were induced along vertical walls by hydrophilic nanotips using surface tension forces, which is the first time to achieve the design of boundary layers that ultimately govern heat and mass transfer. In the last approach, novel microfluidic transistors were devised to passively introduce and sustain high frequency bubble growth/collapse processes and hence to create strong mixing in microchannels. Compared with the state-of-the- art techniques, by directly targeting on manipulating bubble dynamics and governing forces, consequently, the fluid structures, these three novel principles can enable substantially higher flow boiling performance in terms of heat transfer coefficient, CHF, and flow boiling stabilities. Equally important, pressure drop was also well managed or even greatly reduced. Theoretical study was also conducted to understand the mechanisms and provide insight to new flow boiling phenomena.

Rights

© 2013, Fanghao Yang

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