Effect of Length Ratios on Heat Transfer Characteristics of Closed-Loop Oscillating Heat Pipe with Non-Uniform Diameter

Abstract

Problem statement: Closed-loop oscillating heat pipe is a high performance heat exchanger especially as this heat exchanger has one direction flow of working fluid inside tube. Approach: This research studies the effect of length ratios on heat transfer characteristic of Closed Loop Oscillating Heat Pipe with Non-Uniform Diameter (CLOHP/NUD). The 2.03 and 1.06 mm inner diameter of capillary tube were alternated connection and bent into 16 turns and both ends were connected to form of loop. Length ratio of 3, 1, 0.33 and 0.2 were studied. Evaporator, adiabatic and condenser length were 100 mm. R123, ethanol and water were used as the working fluid and filling ratio 50% by total inner volume of CLOHP/NUD was used. The evaporator and condenser temperatures were controlled at 100 and 20 °C CLOHP/NUD operated at vertical plane. Results: It was found that, the CLOHP/NUD transferred higher heat than the conventional Closed Loop Oscillating Heat Pipe (CLOHP) with the same heat transfer area because the working fluid flowed in only one direction. Working fluid moved to condenser section in larger inner diameter and returned to evaporator section in smaller inner diameter. The heat flux of the CLOHP/NUD with R123 as working fluid increased from 15.49-20.85 kW m^-2 when length ratio decreased from 1-0.2, respectively. This is due to the head loss from the sudden enlargement resulted in well circulation of working fluid. Furthermore, higher heat flux was obtained by using water as working fluid. Conclusion: The heat transfer performance of CLOHP/NUD can be improved if one directional circulation of working fluid can be induced.