Please use this identifier to cite or link to this item: http://hdl.handle.net/1783.1/7042

Acoustically driven micro thermal bubble dynamics in a mini/microspace

Authors Qu, Xiaopeng
Issue Date 2010
Summary Micro scale thermal bubble dynamics is an important research topic and it has been studied extensively in the past two decades because of its wide applications in MEMS and microfluid devices. However, most conventional investigations have been focused on thermal bubble dynamics in normal conditions, such as a micro thermal bubble induced by a micro heater in an open space or micro boiling in a microchannel. Due to the complexity of the processes and lack of measurement techniques, micro thermal bubble dynamics is not well understood. The motivation of this thesis work is to study micro thermal bubble dynamics and interactions with and without an acoustic field in a mini/micro space. First of all, to quantitatively measure transient temperature behaviors surrounding a thermal bubble, a novel micro temperature sensor array in a microchannel was designed, fabricated and characterized. This polysilicon based micro temperature sensor array can also be used as a micro heater array in bubble dynamic experiments. Then, single and multi thermal bubble dynamics under a downward-facing surface were studied. Different numbers of micro heaters were applied to generate different numbers of micro thermal bubbles. Several different types of bubble dynamic phenomena, such as bubble sweeping, bubble departure/retraction, and multi bubble interaction/coalescence, have been investigated utilizing a high speed photography system. The effects of Marangoni force, buoyancy force and drag force on the bubble dynamic phenomena have been studied utilizing experimental data. Furthermore, the dynamics of micro thermal bubbles existing in an acoustic field were studied. The acoustic field inside a mini/micro chamber was generated by a piezoelectric plate which was attached on the top of the chamber wall. Compared with micro thermal bubble dynamics in normal conditions, several different bubble dynamic phenomena in an acoustic condition have been found, such as bubble departure and attraction around the heater, bubble oscillating in the liquid volume, etc. By theoretical analysis, the main mechanism of bubble movements is attributed to the balance between the Marangoni force and acoustic force. All these bubble dynamic phenomena improve the liquid convective flow and enhance the heat and mass transfer. Hence, this investigation about acoustic thermal bubble dynamics may find some potential applications in micro fluid devices for different functions, such as heat/mass transfer enhancement, micro electronic cooling, micro heater protection, etc. Temperature measurement in both normal conditions and acoustic conditions confirmed that the heat transfer was enhanced by the acoustic field. Vapor bubble dynamics in a microspace under the effect of an acoustic field was investigated not only for theoretical understanding of this complex dynamic phenomenon but also for the development of new microfluidic devices, such as micro mixers. Through theoretical analysis, the complex bubble dynamic process in two conditions can be both roughly divided into four steps: (1) bubble generation, (2) satellite bubble movement, (3) bubble evolution and (4) bubble shrinkage/removal. The effects of acoustic vibration on all these four steps were found to be significant. A prototype of an acoustic-thermal-bubble based microfluid mixer has been successfully developed and tested. Finally, the effects of acoustic frequency, acoustic power and mass flow rate on bubble incipience in flow boiling in a microchannel were investigated. The experimental results are instructive for design of a novel thermal management device. Keywords: Micro Thermal Bubble Dynamics; MEMS; Microfluid; Acoustic Vibration; Thermal Management; Microchannel; Boiling Heat Transfer.
Note Thesis (Ph.D.)--Hong Kong University of Science and Technology, 2010
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Language English
Format Thesis
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