Application of Ultrasonic Visualization Technique for Visualizing Inner Flow Phenomena of Pulsating Heat Pipe

This study investigates the use of an ultrasonic visualization technique to visualize the working fluid in pulsating heat pipes (PHPs). A flat-plate aluminum alloy PHP used with R1336mzz(Z) hydrofluoroolefins as the working fluid was visualized by measuring the reflected intensity at the boundary between the flow channel wall and its inside. Vapor plugs and liquid slugs were identified by the difference of the reflected intensity. Results showed that when the working fluid in the PHP pulsated with the equivalent thermal conductivity of 2100 W/(mK), the pulsation exhibited low-frequency components equivalent to those reported in previous studies. These findings demonstrate that the ultrasonic visualization technique is useful for visualizing PHPs with multi-layer flow channel structures for measurements over a long period of time.