Date of Award

Spring 1-1-2014

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Mechanical Engineering

First Advisor

Ronggui Yang

Second Advisor

Yung-Cheng Lee

Third Advisor

Yifu Ding

Fourth Advisor

Julie Steinbrenner

Fifth Advisor

Kurt Maute

Abstract

As energy consumption drastically increases in the electronics and energy conversion systems, heat dissipation becomes increasingly important for device reliability and system efficiency. Phase change heat transfer, which takes advantage of the latent heat during the phase change process, is the most promising method to efficiently remove high heat fluxes. Nano structures, which provide extended surface areas and enhanced hydrophilicity or hydrophobicity have been widely used in phase change heat transfer enhancement. Copper nanowires are one of the most promising nano structures due to ease of fabrication, controllable structure dimensions, and high thermal conductivity. In this study, the heat transfer performance of pool boiling, dropwise condensation, and evaporation in thermal ground planes (TGP) have been studied experimentally and theoretically on copper nanowire-structured surfaces.

The dropwise condensation heat transfer performance has been greatly enhanced under a subcooling range of ~ 15 K. Delay of Wenzel state droplets, a droplet state that negatively affects heat transfer, was achieved using hydrophobic nanowired surfaces. The solid fraction has been identified as the key factor to delay the occurrence of Wenzel state droplets. Surface energy conservation has been employed to explain the effect of the solid fraction on the coalescence induced jumping behavior and suggest the optimum structure for extended performance enhancement.

A critical heat flux (CHF) of 250 W/cm2has been realized on a hierarchical micropillar-nanowire structured surface under pool boiling. A force analysis on the liquid-vapor interface of a bubble incorporating the effect of liquid spreading has been used to illustrate how the liquid spreading helped to stabilize the bubble interface against the moment resulted from the vaporization process, so as to improve the CHF on structured surfaces.

A 200W/ cm2 maximum heat flux was realized in a TGP equipped with a patterned Cu nanowire array wicking structure. The ratio between the permeability and the effective pore radius of the wicking structures has been obtained and used to predict the maximum heat flux. The well agreement between the experimental and predicted data shows that the superior wicking performance is important for high maximum heat flux and the patterned nanowire array has the potential to further improve the maximum heat flux.

Five video files are provided as supplementary materials to this dissertation. Video1 is the dropwise condensation on smooth surface while Video2, Video3, Video4, and Video5 are dropwise condensation under different subcoolings on nanowired surfaces.

Video1.avi (3301 kB)
Video2.avi (13228 kB)
Video3.avi (6170 kB)
Video4.avi (4859 kB)
Video5.avi (5444 kB)

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