Date of Award

2018

Document Type

Open Access Dissertation

Department

Mechanical Engineering

Sub-Department

College of Engineering and Computing

First Advisor

Chen Li

Abstract

Flow boiling in microchannels is one of the most promising cooling techniques for microelectronics. Using latent heat by vaporization can significantly improve heat dissipation of high power density electronic devices. Most of the failure of electronic devices is induced by the occurrence of critical heat flux (CHF), which defines the maximum operating conditions. However, the vigorous rapid generation of vapor through phase change leads to chaotic two-phase flows in microchannels, resulting in flowinstability in terms of severe flow, temperature and pressure drop fluctuations. Particularly, the very well-known bubble confinement exacerbates the two-phase flow instabilities and greatly deteriorates heat transfer performance in terms of CHF and heat transfer coefficient (HTC). On the other hand,the primary two-phase flow patterns, includingbubbly flow, slug flow, and annular flow, are not in controlin conventional microchannels.The regulation of two-phase transport is essential to improve flow boiling in microchannels.

In this dissertation, semi-theoretical and experimental studies are conducted to investigate the enhanced mechanism of microchannel flow boiling.Twonovel methodologies were developed to radically solvethe critical two-phase transportissues inmicrochannel flow boiling,including “two-phase oscillator” and “two-phase separation”. Flow boiling performances are significantly enhanced by promoting nucleate boiling, convection and thin film evaporation through controlling two-phase transport based on the two methodologies.For example, multiple micronozzles are designed to efficiently remove confined bubble and extend mixing in microchannel by creating “two-phase oscillator” to generate high frequency jetting flows and highfrequency two-phase oscillations. Furthermore, a better control of two-phase flow patterns is achieved, i.e., highly desirable annular flow. Novel micropin fin arrays have been developed to achieve “two-phase separation” through rectifying stochastic liquid/vapor interfacesinto on-demand manner. Significantenhancements of flow boiling,and flow stabilities have been demonstrated.HTC model and CHFmodel have been developed to predict the HTC and CHF of flow boiling. HTC modeling provides insights into the enhanced HTC mechanisms

Rights

© 2018, Wenming Li

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