10403 Effect of Ultrasonic Wave on Microbubble Emission Boiling

In highly-subcooled boiling, microbubble emitted from coalescing bubbles formed on the heating surface and the heat flux increases higher than the ordinary critical heat flux. This boiling regime is called microbubble emission boiling (MEB). In this study, the effect of ultrasonic wave on the MEB have been investigated in subcooled quasi-pool boiling condition for an upward flat copper heating surface of 10 mm in diameter. The liquid subcooling is kept at 15, 20, 25, 30K and the liquid is the aqueous solution of ethanol of 0, 10, 50 wt%. The high speed video imaging system is employed to observe the bubble behaviors on the heating surface. In the case of liquid subcooling of 20K without ultrasonic wave, no MEB occurs and it turns to film boiling immediately even if MEB occurred. On the other hand, in the case liquid subcooling of 20K and 25K with ultrasonic wave, the large coalescing bubbles are collapsed to many microbubbles and the heat flux increases surprisingly higher than the critical heat flux. Therefore, Heat transfer enhancement is remarkably introduced in the subcooled pool boiling with ultrasonic wave at liquid subcooling of 20K.