The constrained vapor bubble heat pipe : interfacial forces at both macroscopic and microscopic scales

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Nguyen, Thao
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Electronic thesis
Chemical engineering
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The heat pipe is one of the most effective and widely-used devices for thermal management. A heat pipe combines the phenomena of thermal conduction, phase change, and capillary flow to transport energy between a heat source(s) and a heat sink(s). Heat pipes operate without any moving parts because fluid circulation is driven by interfacial forces rather than by mechanical pumps. This makes them simple, light, and reliable heat transfer systems for electronics cooling, permafrost isolation, and spacecraft and satellite thermal management. Several million heat pipes are produced every month. Understanding the interfacial phenomena within a heat pipe is key to developing optimal designs for long-term space and terrestrial applications. Thus, the Constrained Vapor Bubble (CVB) heat pipe project was initiated by researchers from NASA and RPI and run on the International Space Station (ISS).
The work in this thesis has presented new, fundamental understanding on the operation of a wickless heat pipe in microgravity and the role of binary fluid mixtures on the heat transfer process at both the macroscale and microscale levels. The thesis also provides a wealth of high-resolution data for theorists to use in developing new models to explain the heat pipe operation, the contact line dynamics, and fluid flow in a wide range of processes on Earth and in microgravity environments.
In most of the theoretical studies, the thin liquid meniscus was assumed to be static or quasi-static although it was shown to oscillate in many cases. Although the role of oscillation was not well studied for the contact line region, oscillation of liquid film in the macro level was shown to play an important role in many systems, especially oscillating heat pipes. Studies by many researchers showed the positive effect of oscillation on the heat transfer process in these systems. Our experimental data showed the increase in liquid meniscus oscillation with increasing heat flux. In Chapter 6, we developed a model to study the effect of oscillation in an evaporating liquid meniscus on the heat transfer efficiency. We found that the coupling of oscillation and short period of condensation in each oscillation cycle helps to improve the overall evaporation heat transfer process. The short period of condensation increases the liquid film thickness, and therefore decreases the disjoining pressure that holds the liquid molecules from evaporating. These short condensation periods followed by fast evaporation created “spikes” in the liquid film thickness over time. These “spikes” were also observed in our experimental data. The finding that condensation helps with overall evaporation heat transfer process may explain why we observed thick, oscillating, condensed liquid films on the wall surfaces at the heated end of a wickless heat pipe previously. Our data also show that the heat transfer efficiency increases with increasing oscillation amplitude. This phenomenon agrees with the results obtained experimentally by our lab and other research groups.
The effect of cooling temperature on heat pipe performance has generally received little consideration. In Chapter 4, we studied the interfacial forces and their effect on overall performance of the CVB heat pipe at different cooling temperatures in the microgravity environment. The heat transfer coefficient of the evaporator section was shown to decrease with increasing cooler temperature. Interestingly, the decreasing trend was not the same across the cooler settings studied in the paper. This trend corresponded with the change in the temperature profile along the cuvette. When the cooling temperature went from 0 to 20 oC, the temperature of the cuvette decreased monotonically from the heater end to the cooler end and the heat transfer coefficient decreased slowly from 456 to 401 (Wm-2K-1) (at the rate of 2.75 Wm-2K-2). However, when the cooling temperature increased from 25 to 35 oC, a minimum point formed in the temperature profile, and the heat transfer coefficient dramatically decreased from 355 to 236 (Wm-2K-1) (at the rate of 11.9 Wm-2K-2). A similar change in decreasing trend was observed in the pressure gradient and liquid velocity profile. The reduced heat pipe performance at high cooling temperatures was consistent with the reduced evaporation which was indicated by the decreasing internal heat transfer and the increasing liquid film thickness along the cuvette as seen in the surveillance images. The result obtained is important for future heat pipe design because we now have a better understanding of the working temperature ranges of these devices.
For the first time, an ideal fluid mixture of 94 vol%-pentane and 6 vol%-isohexane was used as the working fluid in the CVB heat pipe experiment on the International Space Station (ISS). Using a simple heat transfer model developed in our laboratory, an internal heat transfer coefficient in the evaporator section was determined and shown to be almost twice that of the case where pure pentane was used under the same conditions. The Marangoni stress in the mixture was five times lower. Interestingly, reducing the Marangoni stress led to less liquid accumulation near the heater end and surveillance images of the device, taken at the steady state, showed that the bubble gets much closer to the heater end in the mixture case instead of being isolated from the heater by a thick liquid pool as in the pure pentane case. The proximity of the bubble to the heater wall led to more evaporation at the heater end in the mixture case, and therefore a higher heat transfer coefficient. The pressure profile calculated from the Young–Laplace equation supports the observations made from the surveillance images. The obtained results are discussed in detail in Chapter 3.
In the microgravity environment on the ISS, the two main interfacial forces governing fluid flow inside a wickless heat pipe are the capillary and Marangoni forces. The capillary force is defined by the sharp corners of the heat pipe and pumps the liquid from the condenser to the evaporator. The Marangoni force causes liquid to flow in response to a change in its surface tension. Previous studies on wickless heat pipes showed that a temperature induced ‘‘Marangoni flow” prevents liquid from recirculating to the heater end, and therefore reduces the effectiveness of the heat pipe. Recently, several research groups used a water and alcohol mixture, with a low concentration of alcohol, resulting in better performance of the heat pipe. The alcohol/water combinations were peculiar in that for a certain composition range, the surface tension increases with increasing temperature thereby driving liquid toward the hotter end. It was believed that changing the direction of the Marangoni stress or reducing its magnitude by differential evaporation of an ideal binary mixture would also improve the performance of the heat pipe.
The Constrained Vapor Bubble (CVB) is a prototype of a wickless heat pipe that consists of a transparent quartz cuvette with sharp corners partially filled with either pentane (CVB1) or an ideal mixture of pentane and isohexane (CVB2) as the working fluid. Form the engineering point of view, without the complicated wick structure, the heat pipe will have lower maintenance and longer working lifetime. The wickless heat pipe also allows significant weight savings for space flight. From the experimental point of view, the wickless heat pipe with transparent walls allows us to image the two-dimensional profile of the liquid-vapor interface and study the interfacial phenomena within the heat pipe. In the absence of gravity, the interfacial forces control the fluid flow even in regions of large dimensions, and therefore, will be more accurately evaluated. In this dissertation, I studied the interfacial forces and their effect on the fluid dynamics and heat transfer process in a heat pipe at both macroscopic (Chapters 3 and 4) and microscopic (Chapters 5 and 6) scales.
December 2017
School of Engineering
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Rensselaer Polytechnic Institute, Troy, NY
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