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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]

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]