High-Speed Flow Visualization and IR Imaging of Pool Boiling on Surfaces Having Differing Dynamic Wettabilities

Nicholas ToanNang Vu, Purdue University

Abstract

Boiling is used in a wide variety of industries, including electronics cooling, distillation, and power generation. Fundamental studies on the boiling process are needed for effective implementation. Key performance characteristics of boiling are the heat transfer coefficient, which determines the amount of heat flux that can be dissipated for a given superheat, and critical heat flux(CHF), the failure point that occurs when vapor blankets the surface. The wettability of a surface is one of the key parameters that affects boiling behavior. Wetting surfaces(e.g., hydrophilic surfaces), typically characterized by a static contact angle below 90°,have better critical heat flux due to effective rewetting, but compromised heat transfer coefficients due to increased waiting times between nucleation of each bubble. Meanwhile, nonwetting surfaces (e.g., hydrophobic surfaces), characterized by static contact angles greater than 90°, have better heat transfer coefficients due to improved nucleation characteristic, but reach critical heat flux early due to surface dry out. However, recent studies have shown that the static contact angle alone offers and incomplete, and sometimes inaccurate, description of this behavior, which is instead governed entirely by the dynamic wettability. Specifically, the receding contact angle impacts the size and contact area of bubbles forming on a surface during boiling, while the advancing contact angle determines how the bubble departs. With this more complete set of wettability descriptors, three characteristic wetting regimes define the boiling behavior: hygrophilic surfaces having advancing and receding contact angles both under 90°; hygrophobic surfaces having both these dynamic contact angles over 90°;and ambiphilic surfaces having a receding contact angle less than 90°, but an advancing contact angle greater than 90°.The goal of this thesis is to experimentally characterize and compare the behavior of boiling surfaces in each of these regimes, observe the contact line behavior, and explain the mechanisms for their differences in performance. Using high-speed infrared (IR) thermography of the surface along with side-view highspeed visualization, the key boiling characteristics of each wettability regime were studied in a pool boiling facility. The infrared camera allowed for mapping of the surface temperatures as viewed from below, revealing nucleation phenomenon near the surface that are not possible to ascertain by the traditional high-speed visualization. Departing bubbles are observed to grow at a constant receding angle, pin at the maximum contact diameter, and depart the surface as the liquidvapor interface moved at the advancing contact angle. For the surfaces having advancing contact angles > 90°, a pinched-off remnant of the bubble remains on the surface and promotes immediate growth of a subsequent bubble, without any waiting period required for renucleation. The boiling curves calculated from averaging the surface temperatures recorded using the IR camera indicated that the ambiphilic surface offered the lowest surface temperatures of all wettability regimes, without having the drawback of a premature CHF. The first-of-their-kind IR visualizations of the surface temperatures during boiling from ambiphilic surfaces revealed that this was attributed to an order of magnitude increase in the nucleation site density, short ebullition and waiting times, and very small contact diameters for individual bubbles. The favorable nucleation characteristics allow for efficient heat transfer during bubble formation, while the pinned contact line (and thereby small contact diameter) prevents vapor spreading over the surface and critical heat flux.

Degree

M.Sc.

Advisors

Weibel, Purdue University.

Subject Area

Condensed matter physics|Energy|Marketing|Materials science|Physics|Thermodynamics

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