Characterization and development of composite structured hydrophobic surfaces

Chang, Ho Chan

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https://hdl.handle.net/2142/121234 Copy

Description

Title

Characterization and development of composite structured hydrophobic surfaces

Author(s)

Chang, Ho Chan

Issue Date

2023-07-12

Director of Research (if dissertation) or Advisor (if thesis)

Miljkovic, Nenad

Doctoral Committee Chair(s)

Miljkovic, Nenad

Committee Member(s)

• Shao, Chenhui
• Wang, Sophie
• Banerjee, Arijit

Department of Study

Mechanical Sci & Engineering

Discipline

Mechanical Engineering

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

• Heat Transfer
• Dropwise Condensation
• Composite Coating
• Hydrophobic Surface
• Nanostructure
• Microstructure

Abstract

Dropwise condensation of steam on hydrophobic substrates has a 10X higher heat transfer coefficient compared to filmwise condensation. This can potentially increase 2% overall energy efficiency of steam-based power plants. To promote dropwise condensation, low surface energy hydrophobic coatings (polymers) are typically utilized. The low intrinsic thermal conductivity (k < 1 W/(m·K)) of polymers, however, necessitates that the coating be thin (< 1µm) in order to avoid reducing the overall heat exchanger conductance. However, thin polymeric films easily degrade. The two opposing requirements result in the need for optimization between the durability (thick coating) and the heat transfer (thin coating). In this thesis, to enable high thermal conductivity in thicker coatings, metal-polymer structured surfaces are selected. By using porous structure of metals as inter-connected heat-conducting backbones that are filled with hydrophobic materials, polymer as selected, desired coating effective thermal conductivity and surface energy can be achieved. First, heat transfer performance using metal and polymer composite surface was calculated using three-dimensional finite element method simulations. This result was used to predict the heat transfer performance as a function of metal fraction by volume and by surface area for condensation. By coupling the thermal simulation results with a previously verified analytical model for predicting wetting behavior on heterogeneous surfaces, I propose a regime map to predict dropwise-to-filmwise transition. Based on the results, two actual samples are fabricated: sintered copper powders and copper nanowires, both filled with polydimethylsiloxane (PDMS). The thermal and wettability characterizations are done on both samples. Also, the condensation heat transfer performance is analyzed for copper nanowires. These samples demonstrated a comparable overall heat transfer coefficient to conventional polymer coatings, even with a thicker coating (> 10µm), thereby proposing the possible enhanced durability of the coating. These findings enabled the possibility of durable metal-polymer structured surface usage for industrial applications whereas condensation occurs, enhancing the overall efficiency of thermal systems.

Graduation Semester

2023-08

Type of Resource

Thesis

Copyright and License Information

© 2023 HO CHAN CHANG

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