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With the highly integrated development of electronic components, higher requirements are put forward for heat transfer components, there is an urgent need for high-performance heat transfer components. Oscillating heat pipe (OHP) is novel types of heat pipe with excellent heat transfer capability. However, the heat transfer limit hinders their operation and application. This paper proposes an OHP under trans-critical conditions with the working fluid of R218 and explores its operating characteristics and heat transfer capability. Visualization experiments and pressure monitoring were conducted to analyze the state changes within the OHP under trans-critical conditions. After evaporation section temperature surpassing the critical temperature of R218, the pressure increases with the temperature, once the pressure reached near the critical pressure, OHP can transcend the limitation of the critical temperature and realize efficient oscillating operation under trans-critical conditions. With stable operation, the working fluid velocity can reach up to 1.49 m/s, and the OHP under trans-critical conditions presented excellent heat transfer performance with the thermal resistance of 0.22 °C/W. This paper proves the feasibility of the OHP under trans-critical conditions and provides a basis for further research. [ABSTRACT FROM AUTHOR]

• 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]

• A liquid metal high temperature OHP (LMHOHP) was first developed. • The LMHOHP that can function at a temperature of over 500 °C. • The LMHOHP can transport heat of 3169 W and achieve a thermal resistance of 0.08 °C/W. This paper introduces the development of a liquid metal high-temperature oscillating heat pipe (LMHOHP) that can function under high-temperature conditions (over 500 °C). The LMHOHP has six turns with a total height of 310 mm. The sodium-potassium alloy (potassium 78%) was used as the working fluid, and the 310S stainless steel tube with an inner diameter of 6 mm as the shell tube. The LMHOHP heat transfer performance was investigated experimentally with a filling ratio of 45% at different input powers. Experimental results show that the LMHOHP can effectively generate steady-state oscillating motions at a working temperature of over 500 °C. When the steady-state oscillating motions exist, the temperature difference and thermal resistance between the evaporator and condenser were reduced significantly. At a heat input of 3169 W, the LMHOHP presents a minimum thermal resistance of 0.08 °C/W. This LMHOHP provides a new approach to next-generation high-temperature cooling devices. [ABSTRACT FROM AUTHOR]