Thermal performance and flow characterization of oscillating heat pipe under ultrasonic field

Oscillating Heat Pipes (OHPs) are passive, wickless two-phase thermal devices which can be integrated into Ultrasonic Machining Systems (UMS) to stabilize operating temperatures, prevent performance degradation due to excessive heat, and improve both system stability and machining outcomes. Ultrasonic vibrations influence the heat transfer characteristics of OHPs, but the effects are not yet fully understood. This paper experimentally examines how an ultrasonic field affects the thermal performance of OHPs, focusing on temperature distribution and flow pattern changes to understand the heat transfer mechanisms. The study shows that the ultrasonic field can enhance the heat transfer coefficient of OHPs by almost 20%. This enhancement decreases as the operating temperature rises, with the greatest improvement at 70 °C. Further analysis reveals that ultrasonic field promotes a periodic steady-state operation inside the OHPs and characteristic periodic temperature signals. Additionally, ultrasonic cavitation enhances bubble formation and the phase change heat transfer within the OHPs, facilitating the development of flow pattern and promoting unidirectional circulation, both of which improve heat transfer. By integrating OHPs into UMS, operating temperatures can be better stabilized, leading to enhanced vibration stability in the UMS and improved surface quality of the workpiece.