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Two-phase flow boiling of working fluid within a closed-loop oscillating heat pipe in the horizontal orientation (HCLOHP) was studied visually. The HCLOHP was made of Pyrex glass tubes with an inside and outside diameter of 2 and 7 mm, respectively. The evaporator, adiabatic and condenser of the HCLOHP were 50 mm in length. The internal flow phenomena was carefully investigated for various number of turns, evaporator temperatures and filling ratios of the two working fluids, i.e. distilled water and absolute ethanol. The HCLOHP was installed on cooling and heating copper plates and the two-phase flow patterns were recorded by digital still and video cameras. The rate of heat transferred to the cooling water at the condenser was evaluated. The fluid motion characteristics may be separated into two main conditions viz., the oscillating slug flow and the standstill condition. The thermal performance improved when the number of turns reached the critical value of 10 turns at which the vapor fraction was small and the time fraction of oscillating flow was long. For both working fluids, the time fraction of oscillating flow was longest for 50% filling ratio, which led to the lowest thermal resistances. The wavy vapor-liquid interface which induced the vapor/liquid slug train formation was only found for water. Nucleate boiling followed by oscillating flow was discovered in the evaporator part only at the 50% and 80% filling ratios of water. At these filling ratios the thermal resistance of water tended to be lower than that of ethanol.
PAPER REVISED: 2017-11-04
PAPER ACCEPTED: 2017-11-09
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THERMAL SCIENCE YEAR 2019, VOLUME 23, ISSUE Issue 2, PAGES [1055 - 1065]
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