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Oscillating heat pipes (OHPs) have initiation difficulties under reverse-gravity conditions, severely limiting applications. This study introduces a method employing thermal-driven OHP for anti-gravity operation. The experimental results demonstrate that, with the assistance of an additional heat source, the three-dimensional OHP can started and operated efficiently and stably under reverse-gravity conditions. The maximum heat transfer power is increased from 200 W to 500 W, while the thermal resistance at 200 W is reduced from 0.733 °C/W to 0.21 °C/W, representing a reduction of 71.35%. In auxiliary mode, the thermal resistance decreases as the heat source power increases. Larger assisted heat power results in the OHP in efficient oscillations and enables operation at higher powers. However, increasing the power of the auxiliary heat source also raises the temperatures at the evaporator and condenser sections. Overall, the OHP assisted by the heat source performs well in anti-gravity operations. The difference in evaporator section temperature and thermal resistance at different auxiliary heat source powers under reverse-gravity conditions compared to the gravity environment are all less than 5% at 500 W power. The minimum auxiliary heat source for reverse gravity operation increases exponentially with the heat source power. For 500 W power, the minimum auxiliary heat source power is 130 W. • A method of thermal-drive assisted oscillating heat pipe for anti-gravity operation is proposed. • The influence of auxiliary heat source on the anti-gravity operation performance of oscillating heat pipes is investigated. • Minimum auxiliary heat sources are determined at different heat transfer loads for the anti-gravity operation. [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]