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• A 3D-OHP with ammonia and liquid metal was first developed for high-power and high-heat-flux dissipation. • Liquid metal can promote the unidirectional flow in the OHP. • Liquid metal improves the heat absorption and release rate of the working fluid. • The 3D-OHP can effectively handle heat dissipation of 1000 W with local heat flux of 111 W/cm2. With the rapid development of microelectronics technology, the power of some advanced electronic devices has reached 1000 W with heat flux of 100 W/cm2. The problem of high-power and high-heat-flux thermal management has become the bottleneck of the development of advanced electronic devices. In this paper, a three-dimensional oscillating heat pipe (3D-OHP) with 18 channels was developed for this problem. A new hybrid fluid of ammonia and liquid metal was used as a working fluid. The wall temperature and heat transfer performance of the 3D-OHP charged with the hybrid fluid were investigated experimentally. The results indicate that the 3D-OHP can effectively solve the problem of heat dissipation with the heating power of 1000 W and the heat flux of 111.1 W/cm2. Liquid metal can ensure the efficient operation of ammonia OHP under supercritical pipe diameter, and promote the working fluid to form a unidirectional flow. Liquid metal has a great ability to absorb and release heat which significantly enhances the heat transfer performance of the 3D-OHP. When the heat flux reached 111 W/cm2, the 3D-OHP with 2.11% liquid metal had the best heat transfer performance, and the thermal resistance was 0.0351 °C/W. Compared with pure ammonia 3D-OHP, it could reduce the temperature difference of the 3D-OHP by 9.1 °C and enhance the heat transfer performance by 20.57%. The capacity of the ammonia-liquid metal 3D-OHP to deal with high-power and high-heat-flux is higher than the existing studies. [ABSTRACT FROM AUTHOR]

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]

Generally, the oscillating heat pipe (OHP) diameter should be kept below a specific critical size. As a result, the working fluid forms a vapor-liquid slug train under the capillary action, producing a gas-spring-mass mechanical vibration system. Determining the critical diameter is essential for the OHP to form this slug train. A conventional critical diameter model only considers the surface tension and gravity. However, the vapor bubble flow in the liquid slug plays a key role in forming this essential slug train. Even if the hydraulic diameter of an OHP is much larger than the conventional critical diameter, the OHP can still function. The dynamic flow significantly affects the formation of the vapor-liquid slug-train. Therefore, a semi-empirical model was developed considering the effects of the dynamic flow and the heat input, filling ratio, and working fluid. The concept of dynamic critical diameter (DCD) was proposed. The model shows that the OHP DCD increases when the heat input increases for a given evaporator temperature. In addition, the model can predict that the OHP DCD decreases when the evaporator temperature increases. • The concept of dynamic critical diameter (DCD) was proposed. • The mechanism of vapor-liquid plugs formation in OHP with pipe diameters exceeding conventional MCD is revealed. • A semi-empirical DCD model was developed. • The filling ratio should be between the values of 0.24 and 0.75 when the diameter exceeds MCD. [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]