Date of Award

1-1-2013

Document Type

Open Access Dissertation

Department

Mechanical Engineering

First Advisor

Chen Li

Abstract

Two-phase transport is widely used in energy conversion and storage, energy efficiency and thermal management. Surface roughness and interfacial wettability are two major impact factors for two-phase transport. Micro/nanostructures play important roles in varying the surface roughness and improving interfacial wettability. In this doctoral study, five types of micro/nanoengineered surfaces were developed to systematically study the impacts of interfacial wettability and flow structures on nucleate boiling and capillary evaporation. These surfaces include: 1) superhydrophilic atomic layer deposition (ALD) coatings; 2) partially hydrophobic and partially hydrophilic composite interfaces; 3) micromembrane-enhanced hybrid wicks; 4) superhydrophilic micromembrane-enhnaced hybrid wicks, and 5) functionalized carbon nanotube coated micromembrane-enhnaced hybrid wicks.

Type 1 and 2 surfaces were developed to investigate the impacts of intrinsic superhydrophilicity and hydrophobic-hydrophilic composite wettability on nucleate boiling. Superhydrophilicity was achieved by depositing nano-thick ALD TiO2 coatings, which were used to enable intrinsically superhydrophilic boiling surfaces on the microscale copper woven meshes. Critical heat flux (CHF) was substantially increased because of the superwetting property and delayed local dryout. Carbon nanotube (CNT) enabled partially hydrophobic and partially hydrophilic interfaces were developed to form ideal cavities for nucleate boiling. The hydrophobic-hydrophilic composite interfaces were synthesized from functionalized multiwall carbon nanotubes (FMWCNTs) by introducing hydrophilic functional groups on the surfaces of pristine MWCNTs. The nanoscale FMWCNTs with heterogeneous wettabilities were coated on the micromeshes to form hierarchical surfaces, which effectively increase the heat transfer coefficient (HTC) and CHF of pool boiling.

To enhance capillary evaporation, micromembrane-enhanced capillary evaporating surfaces, i.e., type 3 surfaces, were developed to separate liquid flows and capillary pressure generation. This new type of surfaces consists of a microchannel array and a micromembrane made from a single layer of micromesh. The capillary evaporation CHF were substantially increased because of the increased capillary pressure provided by micromeshes and the reduced friction drag resulted from microchannels. Based on this newly developed hybrid wick, the effect of interfacial wettability on capillary evaporation was systematically studied. Firstly, superhydrophilic ALD SiO2 was deposited on this type of hybrid wick to create intrinsically superhydrophilic interfaces, i.e., type 4 surfaces, resulting in significantly increased HTC because of the enhanced thin film evaporation on micromeshes. Secondly, CNT-enabled hydrophobic-hydrophilic composite interfaces were deposited on the hybrid wicks to increase the nucleate site density, bubble departure frequency and reduce friction drag. Both nucleate boiling and thin film evaporation were improved, resulting in enhanced HTC and CHF.

In conclusion, the interfacial wettability of micro/nanoengineered surfaces can significantly alter bubble dynamics such as nucleation site density, bubble departure diameter and frequency. Superhydrophilic surface can substantially increase the boiling CHF because of the superwetting property. In addition, more hydrophobic surfaces yield higher HTC, while more hydrophilic surfaces result in higher CHF. The partially hydrophobic and partially hydrophilic surfaces perform better than both superhydrphobic and superhydrophilic surfaces. The separation of liquid flow and capillary pressure generation can be achieved using micromembranes, resulting in dramatically increased CHF. Improved wettability can result in better wettings and enhanced thin film evaporation. Hydrophobic and hydrophilic nanoporous coatings can improve the wetting and reduce the friction, resulting in enhanced HTC and CHF simultaneously.

Rights

© 2013, Xianming Dai

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