Relationship between Sound and Heat Transfer on Microbubble Emission Boiling

Microbubble emission boiling (MEB) demonstrates characteristic aspects such as emitted microbubbles, high heat flux and loud sound. To clarify them, observation with a high-speed camera and measurements of sound pressure level with a microphone are performed in this experiment with subcooled water flowing horizontally along the surface. It is clarified that MEB has louder sound compared with nucleate boiling, and two peaks in the frequency distribution that steeply increase as heat flux increases are recognized. The level of each peak depends on liquid subcooling during MEB. Observation shows that bubbles in MEB do not depart from the surface, but collapse onto the surface. Collapse and life cycles of bubbles, obtained by counting the frames of high-speed films, are processed through a 1/3 octave window, and frequency distributions taken. The results are that collapse and sound frequency distributions show similar tendencies. Since the sound pressure level strongly increases as heat flux increases, it is supposed that bubble dynamics, especially in the collapse process, is closely related with the mechanism of heat transfer in MEB.