Jiaxuan Ma

Date of Award

Spring 2021

Document Type

Open Access Dissertation

Department

Mechanical Engineering

First Advisor

Chen Li

Abstract

Flow boiling in microchannels using dielectric fluids is one of the most desirable cooling solutions for high power electronics. Primary two-flow patterns, including bubbly flow, slug flow, and annular flow, have been well established in microchannels. However, it is challenging to promote flow boiling performance, particularly critical heat flux (CHF), due to their unfavorable thermophysical properties. Considering these situations, flow boiling in parallel and isolated microchannels have been extensively studied. In this dissertation, a novel concept that has five parallel microchannels (W=200 µm, H=250 µm, L=10 mm) are interconnected by micro-slots (20 µm wide and 250 µm deep) starting from the beginning section, and the middle section to the channel outlet have been proposed. The visualization study shows that these micro-slots designed as artificial nucleation sites can enable high-frequency nucleate boiling by drastically reducing the bubble waiting time and remaining the micro-slots entirely fully activated simultaneously. More importantly, such rapid switch on-off of uniquely coordinated nucleate boiling in the adjacent channels creates a highly desirable periodic rewetting mechanism to delay CHF conditions and enhance heat transfer rates substantially. Flow boiling in this innovative microchannel configuration has been systematically characterized with mass flux ranging from 462 kg/m2∙s to 1617 kg/m2∙s. Compared to plain-wall microchannels with inlet restrictors (IRs), the flow boiling heat transfer coefficient (HTC) has a significant enhancement primarily owing to the enhanced latent heat transfer, including nucleate boiling and thin-film evaporation. Moreover, CHF is substantially enhanced by ~76% at a mass flux of 1155 kg/m2s owing to the rapid and periodic rewetting enabled by these micro-slots. Such drastic enhancements have been achieved without compromising the two-phase pressure drop. Based on the experimental studies of the novel design, a theoretical model and bubble dynamic studies are conducted to investigate the enhanced mechanism of flow boiling in the microchannel. The bubble dynamics have been systematically characterized in terms of bubble growth rates, bubble departure diameter, and bubble departure frequency, and thus nucleate boiling enhancement has been explained. The real-time wall temperature for the present design fluctuates periodically and more stable compared to the chaotic fluctuation for the plain wall microchannel, which shows that micro slots can effectively manage the boiling instability. Furthermore, non-dimensional fitted correlations have been obtained to predict the bubble departure frequency and bubble departure diameter. The models and bubble dynamic studies provide insights into the enhanced HTC mechanism.

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© 2021, Jiaxuan Ma

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