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The extended meniscus and the intermolecular and capillary forces that govern its behavior and connection to change-of-phase heat transfer have been the subject of an increasing body of research over the past 50 years. We have been fortunate to be at the forefront of this effort starting from the development of a capillary feeder, in Earth's gravity, to stabilize film boiling to running a series of transparent heat pipe experiments aboard the International space station hoping to better understand the role of intermolecular forces in microgravity. The use of ellipsometry and interferometry to highlight the location and state of the vapor–liquid interface have been key to these studies and have helped to uncover many new, interesting, and sometimes unexpected phenomena associated with fluid flow and change-of-phase heat transfer.

An experimental study on subcooled flow boiling with engineering fluid HFE-7000 in a microchannel fitted with piranha pin fins (PPFs) is presented. Heat fluxes of up to 735 W/cm2 were achieved and mass fluxes ranged from 618 kg/m2s to 2569 kg/m2 s. It was found that the flow boiling heat transfer was significantly enhanced with PPFs. The heat transfer coefficient with flow boiling was double the corresponding single-phase flow. Correlations for two-phase heat transfer coefficient and pressure drop in the nucleate flow boiling regime were developed based on the boiling, Weber, and Jakob numbers. The onset of nucleate boiling (ONB) and the critical heat flux (CHF) conditions were determined through visualization and was typically initiated from the last row of fins where temperatures were highest and flow rates lowest.

In this paper, we report on the recent development of an advanced microscale heat sink, termed as piranha pin fin (PPF). A 200 μm deep microchannel embedded with PPFs was fabricated and tested. Fluid flow and heat transfer performance were evaluated with HFE7000 as the working fluid. The surface temperature, pressure drop, heat transfer coefficient, and critical heat flux (CHF) conditions were experimentally obtained and discussed. A 676 W/cm2 CHF was achieved based on the heater area and at an inlet mass flux of 2460 kg/m2 s. Microchannels with different PPF configurations were investigated and studied for different flow conditions. It was found that a microchannel with PPFs can dissipate high heat fluxes with reasonable pressure drops. Flow conditions and PPF configuration played important roles for both fluid flow and heat transfer performances. These studies extended knowledge and provided useful reference for further PPF design in microchannel for flow boiling.

Image analyzing interferometry was used to study the spreading characteristics of an evaporating octane meniscus (purity: 99+%) on a quartz surface. The thickness, slope, and curvature profiles in the contact line region of the meniscus were obtained using a microscopic data analysis procedure. The results obtained for the octane were compared to that of pure pentane (purity: >99.8%) under similar operating conditions. Isothermal experimental conditions of the menisci were used for the in situ estimation of the retarded dispersion constant. The experimental results for the pure pentane demonstrate that the disjoining pressure (the intermolecular interactions) in the thin-film region controls the fluid flow. Also, an imbalance between the disjoining pressure in the thin-film region and the capillary pressure in the thicker meniscus region resulted in a creeping evaporating pentane meniscus, which spreads over the solid (quartz) surface. On the contrary, for less pure octane, the intermolecular interactions between octane and quartz had a significantly different contribution for fluid flow, and hence, the octane meniscus of lower purity did not creep over the quartz surface. As a result, we had a stationary, evaporating octane meniscus. Using the experimental data and a simple model for the velocity distribution, we evaluated the Marangoni shear in a portion of the stationary, evaporating octane meniscus. An extremely small change in the concentration due to distillation had a significant effect on fluid flow and microscale heat transfer. Also, it was found that nonidealities in small interfacial systems, i.e., the presence of impurities in the working fluid, can have a significant effect on the thickness of the adsorbed film, the heat flux, the spreading characteristics of an almost pure fluid, and, therefore, the assumptions in modeling.

Using image-analyzing interferometry, the thickness profile, in the range of δ0 (adsorbed thickness)

A Constrained Vapor Bubble Loop Thermosyphon, CVBLT, made of quartz was used to study the stability and oscillations of an evaporating curved wetting film of pentane in a corner. The film thickness profile was measured as a function of heat input, time, and axial position using image analyzing interferometry. The curvatures and apparent contact angles for the evaporating film under various operating conditions were obtained from the measured film thickness profiles. Instability (oscillation) of the liquid film was observed at relatively higher values of the heat input. The behavior of the curvature and the apparent contact angle of an oscillating film with changes in heat input was evaluated. Moving velocities of the oscillating film were calculated from the measured values of the liquid-wall wetted lengths and were found to be directly proportional to the difference between the instantaneous force acting on the curved film and the reference force. Using an augmented Young-Laplace pressure jump model, the effect of the excess free energy at the contact line on the oscillations was demonstrated.

The generic nonisothermal constrained vapor bubble (CVB) is a miniature, closed heat transfer device capable of high thermal conductance that uses interfacial forces to recirculate the condensate on the solid surface constraining the vapor bubble. Herein, for the specific case of a large length-to-width ratio it is equivalent to a wickless heat pipe. Experiments were conducted at various heat loads on a pentane/quartz CVB to measure the fundamental governing parameter fields: temperature, pressure, and liquid film curvature. An “intermediate” section with a large effective axial thermal conductivity was identified wherein the temperature remains nearly constant. A one-dimensional steady-state model of this intermediate section was developed and solved numerically to yield pressure, velocity, and liquid film curvature profiles. The experimentally obtained curvature profiles agree very well with those predicted by the Young-Laplace model. The operating temperature of the CVB was found to be a function of the operating pressure and not a function of the heat load. Due to experimental design limitations, the fundamental operating limits of the CVB were not reached.