Start-up and heat transfer characteristics of oscillating heat pipe charged with binary partially miscible organic fluid mixtures

The development of highly-efficient, cost-effective and reliable cooling schemes is still a crucial need for prospective applications in energy- and thermal-related areas, and oscillating heat pipes become a promising option due to their simple structures, high performance and versatilities. In this work, an oscillating heat pipe fabricated by a copper flat-plate evaporator and a capillary copper-tube condenser has been developed, and the experimental study was conducted to investigate the start-up and heat transfer characteristics of the heat pipe device using partially miscible HFE7100-ethanol mixtures at mixing ratios in the range of 1:8 to 8:1 due to the advantage of complementary thermo-physical characteristics and the figure of merit with respect to an OHP. Results demonstrate that the oscillating heat pipe performance was mixing ratio dependent, and there existed optimal mixing ratios of 3.5:1 and 3:1 with respect to the start-up temperature/duration and thermal resistance, respectively. At the optimal mixing ratio of 3.5:1, the time and temperature constants related to the startup performance were reduced by 50.5 s and 5.9 degrees C, respectively, in comparison with that of ethanol. Besides, the thermal resistance of the oscillating heat pipe charged with HFE7100-ethanol mixture was maximally reduced by 35.5% as compared with that of ethanol at an optimal mixing ratio of 3:1. The complementary thermo-physical properties of partially miscible organic fluids as well as the phase transition suppression and easy transition to unidirectional flow circulation account for the performance improvement. Different from single organic mixtures, the binary organic mixtures exhibited distinctive capabilities to improve the oscillating heat pipe performance, and provides a promising candidate for oscillating heat pipe applications.