Experimental study on the thermal performance of vertical closed-loop oscillating heat pipes and correlation modeling

Experimental studies were performed to investigate the thermal performance of three closed-loop oscillating heat pipes (CLOHPs) operating at the vertical bottom heat mode with heating power input in a range of 15–127 W. The tested CLOHPs are all made from copper capillary tubes with inner diameters (IDs) of 1.2, 2, and 2.4 mm. Two working fluids, pure water and ethanol, were used with filling ratios of 40%, 50%, and 60% by volume. The evaporator of each CLOHP was electrically heated with alterable lengths, while the condenser was liquid cooled with a constant length. Experimental results show that the thermal performance of the CLOHPs depends on the conjugation effects of working fluid, filling ratio, inner diameter, evaporator length, and heating power input. The 2 mm ID and 2.4 mm ID CLOHPs had better thermal performance when charged with water as compared with ethanol, while ethanol was preferred for the 1.2 mm ID CLOHP. The thermal performance of these CLOHPs was enhanced at the relatively lower filling ratios (40% and 50%). An optimum evaporator length corresponding to the lowest thermal resistance was proved. Finally, an empirical correlation based on 510 sets of available experimental data both from the present study and other literatures was proposed to predict the thermal performance of vertical CLOHPs. The proposed correlation agreed with the experimental data within a deviation of approximately ±40%.