Experimental study of single and two-phase heat transfer in mini-channel

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Title: Experimental study of single and two-phase heat transfer in mini-channel
Author: Tian, Shurong
Abstract: Flow boiling in mini-channels has been the most recent trend in high performance cooling techniques, in spite of an increased pressure drop. An experimental study of pressure and heat transfer characteristics of water in mini-channel evaporators was carried out in this study. The test section consisted of 6, U-shape, parallel, mini-channels. The mini-channels were formed by brazing an etched, flat, stainless steel plate. The hydraulic diameter of a single mini-channel was 0.725 mm. Distilled water, used as the fluid medium, flowed in the mini-channels; while the heating oil flowed on the outside of mini-channel. The inlet temperature of the water was maintained at room temperature and the outlet pressure was ambient pressure. The mass flux was in the range of 87.36 kg/m~ s to 656.75 kg/m2 In order to enhance the heat transfer of the oil side, offset-strip fins were applied on the stainless steel surface of the oil side channel. Parafherm heat transfer oil, the fluid medium, was used on the oil side. The inlet temperature of the oil channel was varied through a range of 100C to 275C. The heat transfer and pressure drop results were obtained as a function of water flow rate and inlet oil temperature. The results indicate that the heat transfer coefficient on the water side was a function of the water mass flow rate, the outlet steam quality, and the oil inlet temperature. The pressure drop of the water side increased with decreasing oil inlet temperature and increasing water mass flow rate in the single-phase flow region. In the two-phase flow region, the pressure drop behavior was more complex since a higher water mass flow resulted in a lower average quality in the evaporator. The vapor generation phenomena associated with boiling and the resultant two-phase flow gives the pressure drop and heat transfer 2-10 fold. The study of two-phase heat transfer visualization gives more detail of the mechanism of two-phase flow and heat transfer. The capillary phenomena play a very important role in the small size channel. In this study, four predominant flow patterns, isolated bubbles, confined bubbles, slug flow and annular flow were observed. As nucleating bubbles were generated and being confined, slug flow easily formed at lower wall temperatures in the mini-channel and resulted in a higher heat transfer coefficient.
Record URI: http://hdl.handle.net/1850/14296
Date: 2001-10

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