Date of Award

December 2015

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Mechanical and Aerospace Engineering

Advisor(s)

Shalabh C. Maroo

Keywords

Boiling Heat Transfer, Critical Heat Flux, Microlayer Evaporation, Three-Phase Contact Line

Subject Categories

Engineering

Abstract

Boiling, a dynamic and multiscale process, is widely used in industrial applications as it can transfer a large amount of heat over a small surface area. It has been studied for over five decades; however, a comprehensive understanding of the process is still lacking. The bubble ebullition cycle (nucleation, growth and departure) happens over a very short time-span (milliseconds) making it challenging to study the near-surface interfacial characteristics of a single bubble, which involves a microlayer. The microlayer is a thin film present at the bubble base and varies from nano-scale to micro-scale in thickness. The dynamics of the microlayer dictates bubble growth and departure, making it of significant importance in understanding the fundamental behavior of the boiling phenomenon, and is the focus of this work.

Firstly, a new mechanism of boiling enhancement based on the additional evaporation of the thin film is proposed and validated by fabricating micro-ridges on a surface and testing its boiling performance. A critical height of the ridges is found to exist, determined by the film thickness, below which no enhancement is observed while above which similar enhancements are achieved regardless of the ridge height. An analytical model is developed to determine the critical height from the experimental results.

Secondly, the effect of ridge spacing on boiling enhancement is investigated. A ~120% enhancement in the critical heat flux is attained with only 18% increase in surface area due to the presence of ridges. The new enhancement mechanism is determined to be the early evaporation of microlayer, which leads to an increase in the bubble growth rate and departure frequency. Three enhancement regions are mapped based on ridge spacing and height: full enhancement region, partial enhancement region, and no enhancement region. The mechanism of early evaporation of microlayer is further verified by comparison of the bubble growth rate of a laser-created vapor bubble on a ridge-structured surface and on a plain surface.

Next, in-situ imaging of microlayer and contact line region is performed in a steady-state vapor bubble created by laser heating. The in-situ measurement of contact angle of vapor bubbles is conducted. For the laser power studied, the contact line readily forms in regular DI water which contains dissolved air, while in degassed water, the microlayer covers the entire bubble base. The vapor bubble contact angle is found to resemble a drop contact angle on the same surface if the three-phase contact line forms; otherwise it is dependent on the curvature of the microlayer and the bubble, and decreases with increasing heating power. The overall heat transfer coefficient and width of the evaporating region in the microlayer are estimated using experimental data and finite-element-method based numerical simulations, thus defining an upper limit to the heat transfer coefficient possible in nucleate boiling and thin-film evaporation.

Finally, the contact line and microlayer behavior during bubble formation, growth, and movement is investigated. During bubble formation, the microlayer initially covers the entire bubble base, decreases in thickness as the bubble grows, and eventually forms the three-phase contact line. The surface wettability strongly affects contact line motion. On hydrophobic Trichlorosilane (FOTS) surface, the bubble remains adhered to the surface and does not follow the laser movement; while on hydrophilic SiO2 surface, the bubble moves smoothly following the laser. This difference in behavior is determined to originate from the change in direction of the liquid-vapor surface tension force, which results in a negative net force on FOTS surface inhibiting the bubble from following the laser, while it results in a positive net force on SiO2 surface causing the bubble to move on the surface.

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Open Access

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