Visualization study of a high-temperature oscillating heat pipe using X-ray imaging

In this study, an apparatus is constructed to perform visualization experiments on a high-temperature oscillating heat pipe (HTOHP) based on X-ray imaging. Experiments are carried out on an HTOHP with a sodium–potassium alloy as the working fluid at 24 heating powers and a cooling temperature of 188 °C. The HTOHP starts up at 250 W. The working fluid transitions into an unstable unidirectional flow state at 400 W accompanied by intermittent stopover phenomena, which substantially deteriorates the thermal performance of the HTOHP. At a heating power of 750 W, stopover phenomena cease to occur, and the working fluid transitions into a stable unidirectional flow state. Visualization images are used to describe the working fluid flow states and elucidate the mechanism for the variation in HTOHP wall temperatures under different operating conditions. An analysis of the thermal performance of the HTOHP shows that as the heating power increases, the average thermal resistance decreases by 94 %, from 3.759 °C/W at 200 W to 0.208 °C/W at 1350 W mainly because of variations in the flow state of the working fluid. Importantly, the thermal resistance decreases from 0.314 °C/W at 750 W to 0.208 °C/W at 1350 W, where this decrease is only 3.08 % of that from 200 W to 750 W because the working fluid transitions into a unidirectional flow state at 750 W, that remains stable until the end of the experiment. The flow state of the working fluid remains unchanged beyond 750 W, with the velocity emerging as the primary influencing factor on the thermal performance. As the heating power increases, the velocity of the working fluid increases from 0.081 m/s at 350 W to 0.431 m/s at 1350 W at a relatively uniform rate.