A heat pipe is a passive thermal unit that utilizes a phase change mechanism to dissipate the heat from the hot spot of a device to the outside environment. Evaporation of the liquid at the hot end of the heat pipe absorbs the heat. The vapor then travels to the cold end and condenses back to the liquid phase. The condensation of the vapor releases the latent heat, and heat sinks at the cold end draw the heat out of the heat pipe system. Fluid mechanics, however, allows the cooling process to be continuous in the heat pipe. The surface tension of the liquid creates the capillary pressure that recycles the condensate from the cold end to the hot end. Wick structures are often used on the internal solid surface of the container to improve the efficiency of capillarity. Therefore, the heat transfer cycle can be repeated indefinitely without any external mechanical pumping systems.The engineering of an efficient heat pipe requires an understanding of processes at both the macroscopic and microscopic scales of operation. Among the different factors that can affect a heat pipe’s performance, the physical properties of working fluid in a heat pipe system play a critical role in the phase change capability, heat transfer, and fluid flow behavior. The Constrained Vapor Bubble (CVB) experiment used a transparent and wickless fused silica cuvette to serve as the heat pipe. Thus, the behavior of the working fluid could be analyzed through optical measurements.
In the first section of the study, we examined how varying condenser temperatures affect heat transfer and fluid flow behavior using pentane as the working fluid. The system's performance declined as the condenser temperature dropped. The behavior seen while employing a mixture of 94 vol% pentane and 6 vol% iso-hexane was the opposite of this performance decline when using the pure working fluid. A rise in the apparent strength of Marangoni flows at the heater end of the system was the cause of the performance drop as the condenser temperature was lowered. For experiments where we held the condenser temperature constant while increasing the heater power and for experiments where we held the heater power constant while decreasing the condenser temperature, a fin-type heat transfer model was fit to the experimental data and used to extract an evaporator heat transfer coefficient. One Nusselt number vs. Marangoni number curve was found to represent all the results. In this formulation, it was discovered that the Nusselt number decreased as the Marangoni number was raised to the third power.
In the second section of the study, we analyzed more than 100 nucleation events in a wickless heat pipe under microgravity. The experiment ran for 20 hours with pentane as the working fluid. Peak pressures and vapor temperatures in the device were momentarily increased by bubble nucleation at the heater end. The original vapor bubble collapsed due to increasing pressure at the time of nucleation, and the heater wall temperature considerably dropped due to enhanced evaporation. Heat transfer coefficients near the heater end of the system were determined using a thermal model that was created using the measured temperatures and pressures. Peak heat transfer coefficients during the nucleation event exceeded steady-state values by a factor of three. A linear correlation between the Nusselt number and the Ohnesorge number was found that explains all the heat transfer coefficient data from the nucleation events.
The third section of the study focuses on the microscopic perspective of the heat pipe system. The local heat flux in the wickless heat pipe is governed by the shape of the microscale meniscus. Thus, we employed an interferometry method to calculate the thickness profile of the corner meniscus. This process is quick and simple and does not direct interaction with the liquid surface. Thin Film Interference generates fringe patterns at the meniscus region. Five different image analysis methods are explained to demonstrate the estimation of meniscus thickness from the interferometry images collected in two experiments.
In the last section of the study, we analyzed and questioned the interfacial ideality of the pentane and iso-hexane liquid mixture. The ideal mixture assumes linear behavior of the surface tension and Hamaker constant. These physical characteristics, however, might not always follow a linear relationship with changes in the volume fraction. The non-ideality of the binary mixture was investigated using five different compositions. Through the isothermal experiments employing the CVB system on Earth, the retarded Hamaker constants and surface tension of the liquid mixtures were evaluated.
The experiments, discoveries, and analyses presented in this thesis provide an in-depth inspection of the wickless heat pipe operation under the microgravitational environment. In addition, the non-ideal behavior of the pentane and iso-hexane mixture was revealed using the interferometry images of the corner meniscus and multiple processing tools. In the future, two newly designed CVB apparatuses, linear and looped systems, with pentane/iso-hexane mixture as the working fluid, will be studied under non-isothermal conditions. These experiments will further investigate the impact of the non-linear interfacial behavior of the liquid mixture on the heat transfer and fluid mechanics of the wickless heat pipe. ;
December2022; School of Engineering
Dept. of Chemical and Biological Engineering;
Rensselaer Polytechnic Institute, Troy, NY
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