Void fraction variations in a fractal-like branching channel network Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/5t34sm656

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  • Two-phase flow in a fractal-like branching microchannel heat sink may have enhanced heat transfer capabilities over single phase flow in the same branching channel network. In order to exploit this potential, a complete understanding of flow boiling in this geometry is required. The fractal-like geometry is similar to that of the human circulatory system; a larger diameter channel, or artery, branches into two channels, or arteries, of smaller diameter. An experimental study of flow boiling in a fractal-like branching channel heat sink was performed. The fractal-like network, fabricated for use in this study, utilized a circular configuration, in which fluid entered the center of the device and flowed radially outward. The resultant, bifurcating pattern is perfectly symmetrical, and has four different branching levels. The channels of the branches range in hydraulic diameter from 218 micron at the inlet to 120 micron at the periphery of the heat sink. High-speed, high-resolution imaging was used to visualize flow regimes and quantify void fraction variations in the channels. Regional and local changes in void fraction were analyzed for the effects of this novel geometry on flow boiling characteristics. Of particular interest was the interaction between different branching levels and the impact of bifurcations on vapor flow. Global measurements of pressure, temperature, flow rate, and power input were also made. Operating conditions included an inlet temperature of 88°C, inlet mass flow rates between 45 and 65 g/min, and two power levels of 61 W and 66 W. Flow regimes observed during qualitative analysis of a base case, defined as q = 66 W, ṁ = 45 g/min, and Tᵢ = 88°C, included single phase, bubbly, and slug flow. Quantitative analysis of the base case indicate that the levels of the fractallike device interact; vapor in a downstream channel affects the flow behavior in an associated upstream channel. In addition, bifurcations associated with the fractallike geometry affects flow characteristics. Bifurcations split downstream vapor flow and redirected vapor flowing upstream. Comparison of base case results with two other cases indicate increased flow rate, and decreased power, independently t produced void fraction variations that followed the same trends as those found in the base case but with different magnitudes and frequencies.
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