Award Date

8-1-2018

Degree Type

Thesis

Degree Name

Master of Science in Engineering (MSE)

Department

Mechanical Engineering

First Committee Member

Kwang Kim

Second Committee Member

Brendan O' Toole

Third Committee Member

Hui Zhao

Fourth Committee Member

Jacimaria Batista

Number of Pages

75

Abstract

Nucleate pool boiling is a mode of pool boiling that tends to transfer heat more efficiently than the other modes of pool boiling. Many studies have been done to augment the heat transferred during nucleate pool boiling. Various methods of heat transfer augmentation in

boiling can be performed through active treatments (mechanical aids, surface/fluid vibrations, etc.) or passive treatments (functional coating, surface roughening, etc.). This study focuses primarily on a passive treatment, specifically TiO2 coating, which involves testing a substrate coated with TiO2 using NeverWetTM base coating as a coupling agent and coated using a method outlined by Wu et al. [7]. The results are evaluated by comparing the overall boiling curve and heat transfer coefficient with other baseline samples for water. Anatase TiO2 was chosen as the material for this study because it is naturally hydrophilic, and interestingly, it can become superhydrophilic after being exposed to ultraviolet light at a wavelength of less than 380 nm. The focus, however, is on its physical properties.

A large factor for heat transfer in nucleate pool boiling is the formation of bubbles, bubble release, and the number of active nucleation sites on the sample. Nucleation sites can only be active for a favorable geometry (the cavity radius must be within a specific range) and

must be within an area of influence where the waiting period of the site is less than the rest of the cavities. The water contact angle of the sample and the tilt angle of the sample also influences the formation and detachment of the bubbles on the sample. Based on this information, this study investigates cylindrical samples coated with TiO2. With a lower contact angle, the cylindrical sample yielded more active nucleation sites which resulted in enhancement of heat transfer in nucleate pool boiling conditions. The TiO2 without the coupling agent yielded an 8% improvement in critical heat flux as opposed to the baseline Al 6061 substrate. This enhancement was backed up using image processing to gather bubble departure frequency and bubble diameter data to use on a current vapor-exchange model – this model and the experimental data was compared in this study. The bubble departure frequency was significantly higher in TiO2 coated substrate than the baseline sample, while the bubble diameters, overall, were lower, thus agreeing with the analytical models of the roles of interfacial tension in bubble formation and departure.

Keywords

Boiling; Heat Transfer; Nucleate Pool Boiling; Pool Boiling; Titanium Dioxide

Disciplines

Mechanical Engineering

Language

English

Al 6061 67.mp4 (1801 kB)
Al_Ti 60.mp4 (1434 kB)


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