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Thesis - University Access Only
Master of Science (MS)
As microprocessor systems continue their ever-onward march of logarithmic package size reduction, thermal management of the waste heat is becoming increasingly problematic. Traditional forced-air cooling systems cannot keep up with the required heat rejection rates. Pulsating heat pipes (PHPs) are passive two-phase heat transfer systems which have shown the potential to meet and exceed this requirement. PHPs can facilitate the rejection of high heat fluxes in a small and cost elective package. The primary factors which influence the operation of a PHP were investigated in this study. PHP design takes advantage of capillary-scale behavior of the working fluid to avoid the traditional wicking or reliance on gravity of conventional heat pipes. A 10-turn apparatus was constructed out of 1.6 mm ( 1/16 in) inner diameter copper tubing. Relatively small areas of the pipe were used for the evaporator and condenser sections, with a long adiabatic region in the middle, similar to the type of dimensions one might find in portable electronics. An electric resistance heater was fitted to the evaporator section, and the condenser section was immersed in a temperature-controlled water bath. Sections of the adiabatic region were replaced with transparent tubing to allow for visual inspection of PHP operation and optical measurement of flow properties. This type of apparatus is capable of testing many levels of operation in a variety of configurations. The variables of interest include: (a) heat input, which, controls the operation and presence of pulsation of the PHP, (b) heat input rate, which was found to affect the transition of PHP operation into steady state (c) condenser temperature, which affects thermal resistance of the apparatus, and (d) charging ratio, which controls how the PHP behaves at different heat transfer rates. This apparatus was observed to function between charging ratios of 0.55 and 0.90 when using input powers between 0 and 60 watts. Pulsation was observed to happen for brief periods in the range immediately below 0.55, but was not sustainable enough to reach pseudo steady state operation. The ideal range of input power at a charging ratio of 0.57 was observed to be between 24 - 34 watts. This power range yielded ideal thermal resistance (1.6 - 2.2 K/W ) and evaporator temperatures (65°c - 71°c). 32 W was selected to be the constant input power to be used in the condenser temperature and charging ratio experiments. It was observed that there exists an ideal charging ratio of approximately 0.75, which yielded the lowest thermal resistance (1.05 K/ W at TC= 20°c). It was also observed that this ideal charging ratio did not change significantly with changing condenser temperatures. A correlation between condenser temperature and thermal resistance was observed: increasing condenser temperature increases evaporator temperature and decreases thermal resistance. At a charging ratio of 0.8, increasing the condenser temperature by 5°c corresponded to an increase in evaporator temperature of 6°c and an decrease in thermal resistance of 0.06 K/W . Two types of start-up regimes were observed outside of those described by Xu et al.  (explained in Section 1.1.6). The "quick" start-up, wherein pulsation occurred almost immediately after heat application and at very low evaporator temperatures, was observed to occur more often with immediate application of heat to the evaporator. When the heat was applied slowly, the "slow" start-up occured, wherein pulsation did not occur until a large temperature difference (80 - 100°c) was created between the evaporator and condenser.Optimizing the charging ratio and condenser temperature can allow for a significant improvement in thermal performance of a given PHP. Additionally, many types of operation occur within a given PHP geometry, so it is very important to be able to predict PHP behavior during both start-up and steady state when designing an application.
Library of Congress Subject Headings
Includes bibliographical references.
Number of Pages
South Dakota State University
In Copyright - Educational Use Permitted
Versteeg, John, "Thermal Resistance and Operation of a Pulsating Heat Pipe: A Parametric Study" (2013). Electronic Theses and Dissertations. 1690.