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The objective of this paper is to investigate startup conditions of a closed loop oscillating heat pipe (CLOHP). The CLOHP are made of a long copper capillary tube with an inner diameter of 2.03 mm. The number of turns was 4, 5 and 7. The ends of the tube are connected to complete the loop. The evaporator, adiabatic and condenser sections are all an equal length of 50 mm. R123, acetone and water were used as the working fluids with a 50% filling ratio. The CLOHP was operated horizontally and the condenser temperature was controlled at 25 ∘C. A low-voltage high-current heater was used as the heat source and the cooling medium was a solution of water and ethylene-glycol with a 1:1 mixing volume ratio. The number of turns is 7, changing the working fluid between R123, acetone and water, it was found that the heat transfer rate increases from 5.72 W using R123 as working fluid, to 7.35 W and 14.71 W using acetone and water as working fluids, respectively. The frequency of the working fluid decreased from 0.94 cycle/s using R123 as working fluid, to 0.65 cycle/s and 0.19 cycle/s using acetone and water as working fluids, respectively. The amplitude increased from 0.36 °C using R123, to 0.55 °C and 3.62 °C using acetone and water, respectively. In the case of using water as working fluids, changing the number of turn from 4 to 5 and 7 turns, it was found that the heat transfer rate increased from 11.43 W using 4 turns, to 12.25 W and 14.71 W using 5 and 7 turns, respectively. The frequency did not change switching to 5 turns but did change from 0.18 cycle/s to be 0.19 cycle/s when using 7 turns. The amplitude increased from 2.35 °C using 4 turns, to 3.24 °C and 3.62 °C using 5 and 7 turns, respectively. [ABSTRACT FROM AUTHOR]

In recent times pulsating heat pipes turn into a pleasing field of study for researchers as useful devices to enhance and intensify heat transfer. Experimental data under various operating conditions for a better theoretical understanding are needed. This paper presents an experimental study on a closed loop pulsating heat pipe made of copper with 14 U-turns. The experiments were carried out for different working fluids (acetone, distilled water, ethanol and methanol) and their mixing, heat input, filling ratio and evacuation levels to optimize operational conditions. Thermal resistance at steady state specified for performance evaluation and showed better results for acetone. The mentioned working fluids in filling ratio of 60 % have better operation and acetone has the best. As well as experiments in different evacuation levels showed, more evacuation lead to more performance. [ABSTRACT FROM AUTHOR]