Effect of Inclination Angle on Performance Limit of a Closed-End Oscillating Heat Pipe

This study presents the effect of inclination angle on the performance limit of a Closed-End Oscillating Heat Pipe (CEOHP). The study was divided into 2 corresponding parts. The first part was visual study. The CEOHP was made of Pyrex glass tube with the inner diameter of 1.0 mm, evaporator length of 50 mm and had 10 meandering turns. The lengths of the condenser, adiabatic and evaporator sections were equal. The working fluid was R123 with filling ratio at 50% of total volume of the tube. It was found from the experiment that the performance limit at the horizontal orientation was the initial dry-out because the insufficient condensed liquid film was supplied the evaporator section. At the range of inclination angle of 5-90°, the cause of performance limit was dry-out due to the flooding phenomena at the entrance of the evaporator section. The second part was quantitative study. The CEOHPs were made of copper tubes with 0.66, 1.06 and 2.03 mm inner diameters. The evaporator lengths of CEOHPs were 50, 100 and 150 mm and the number of meandering turns was 10. For each CEOHP, the lengths of evaporator, adiabatic and condenser sections were equal. The working fluids were R123, ethanol and water at a filling ratio of 50% of total volume of the tube. The experiments were conducted at the inclination angles of 0-90° with 10° increments. With the controlled vapor temperature of 60±5°C, it was found from the quantitative results that the critical heat flux decreases as the evaporator length increases and the critical heat flux increases with an increase in the inner diameter for all inclination angles. Moreover, this study establishes the correlation to predict the ratio of the critical heat flux at the inclination angle range of 10-90° and that at vertical position. The Standard Deviation (STD) is ±13.8%.