Structure optimization of a three-dimensional coil oscillating heat pipe.

• A guideline to optimize the structure of 3DCOHP was proposed, which can enhance the heat transfer performance significantly. • Reducing the top bend curvature radius of 3DCOHP with asymmetric heating mode can promote the formation of unidirectional flow without check valve. • The existence of unequal flow pressure drop between the push-up and push-down causes the formation of unidirectional flow in 3DCOHP. The effects of the bend curvature radius and tube diameter on the heat transfer performance of the three-dimensional coil oscillating heat pipe (3DCOHP) were experimentally studied in asymmetric heating mode. Three types of copper 3DCOHPs using water as the working fluid were studied in vertical orientation. And the filling ratio of each 3DCOHP was set to 50%. It was found that reducing the top bend curvature radius and increasing the tube diameter can both strengthen the formation of unidirectional flow and significantly improve the heat transfer performance of the 3DCOHP. The heat transfer performance of the OHP with a tube diameter of 1.90 mm increased by up to 34.6% when the heating power was set to 40 W. At the same time, the reason of the unidirectional flow in asymmetric heating mode was theoretically analyzed. It was shown that the calculated pressure drops in liquid slug flowing from the evaporation section to both sides powered by vapor expansion are different, which develops the unidirectional flow. These findings provide a guideline to optimize the structure of OHP to enhance its heat transfer capacity. [ABSTRACT FROM AUTHOR]