Experimental study of flow boiling heat transfer and critical heat flux in microchannels

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Title: Experimental study of flow boiling heat transfer and critical heat flux in microchannels
Author: Kuan, Wai Keat
Abstract: Advancements in microprocessors and other high power electronics have resulted in increased heat dissipation from those devices. In addition, to reduce cost, the functionality of microprocessor per unit area has been increasing. The increase in functionality accompanied by reduction in chip size has caused its thermal management to be challenging. In order to dissipate the increase in heat generation, the size of conventional fin-type heat sinks has to be increased. As a result, the performance of these high heat flux generating electronics is often limited by the available cooling technology and space to accommodate the larger conventional air-cooled heat sinks. One way to enhance heat transfer from electronics without sacrificing their performance is the use of heat sink with many microchannels and liquid passing through it. The present work is aimed toward understanding the flow boiling stability and critical heat flux (CHF) with water and R-123 in microchannel passages. Experimental data and theoretical model to predict the heat transfer and CHF are the focus of this work. The experimental test section has six parallel microchannels with each having a cross sectional area of 1054 x 157 um2 or 1054 x 197 um2. The effect of flow instabilities in microchannels is investigated using flow restrictors at the inlet of each microchannel to stabilize the flow boiling process and avoid the backflow phenomena. This technique resulted in successfully stabilizing the flow boiling process as seen through a high-speed camera. The present CHF result is found to correlate to mean absolute error (MAE) of 24.1% with a macro-scale empirical equation by Katto et al. [34]. A theoretical analysis of flow boiling phenomena revealed that the ratio of evaporation momentum to surface tension forces is an important parameter. For the first time, a theoretical CHF model is proposed using these underlying forces to represent CHF mechanism in microchannels, and its correlation agrees with the experiemental data with overall MAE of 12.3%. CHF is found to increase with increasing mass flux, and it leads to a change in the flow pattern from a wetted film flow to liquid streams that flow in the core of the flow.
Record URI: http://hdl.handle.net/1850/1887
Date: 2006-05-30

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