The heat transfer performance of a compact three-dimensional oscillating heat pipe (3D-OHP) was investigated at vertical

horizontal and side orientation modes. The evaporator section of the 3D-OHP was fabricated by a 40 mm×40 mm×3.5 mm copper plate with nine parallel circular channels inside

and the condenser section was made of copper capillary tubes with inner and outer diameters of 2 and 3 mm

respectively. The capillary tubes were connected with the plate to form the 3D structure. Ethanol and deionized water were used as the working fluid with volumetric filling ratios from 20% to 70%. Two heat blocks were mounted on the copper flat evaporator to provide heating power inputs ranging from around 10 to 100 W. The results show that the 3DOHP could start-up and work well at the filling ratio range of 30% to 70% and had the best heat transfer performance at 30% filling ratio. In addition

the 3D-OHP had the lowest start-up temperature at the vertical orientation and the start-up temperature increased with the increase of filling ratio. The 3D-OHP still functioned at the horizontal orientation but had lower heat transfer performance than at the vertical orientation

and the reduction in filling ratio enhanced the performance. Compared with ethanol

the 3D-OHP charged with water had lower evaporation temperatures

higher evaporator temperature uniformity as well as heat transfer performance.