Numerical Investigations on the Start-Up Characteristics and Heat Transfer Performance of a Single-Loop Oscillating Heat Pipe With Different Surface Wettable Vertical Tubes

The heat transfer performance of an oscillating heat pipe (OHP) was significantly enhanced by changing the inner wall surface wettability and filling ratios. Among the various wettability types, a hybrid surface, where the evaporator and condenser have different wettability, has shown enhancement of heat transfer performance significantly. The effects of different surface wettable vertical tubes are yet to be investigated. Therefore, in this study, a single-loop OHP with three different surface wettable vertical tubes, namely, alternate wettability OHP (AWOHP) were numerically investigated at different filling ratios of 30-90%. The different wettability rates of two vertical tubes caused an uneven distribution of slugs and plugs, which generated an unbalanced pressure difference between the tubes, thereby reducing the start-up time by 60-79%. The bulk mean temperature of the left and right adiabatic tubes was investigated to understand the nature of fluid flow. For filling ratios (FRs) of 30-70%, the right tube temperature was always higher than the left tube in AWOHP 3 (hydrophilic left tube, super-hydrophobic right tube), indicating unidirectional circulatory flow, with an occasional change in direction at 70% FR. At lower FR of 30%, AWOHP 2 (super-hydrophilic left tube, super-hydrophobic right tube) exhibited the highest heat transfer performance. In contrast, at higher FRs of 50-90%, AWOHP 3 exhibited the highest heat transfer performance with conductance (G) of 1075 W/m2 K at an FR of 70%, 55% higher than conventional oscillating heat pipe (COHP).