Effects of ultrasonic waves on subcooled pool boiling on a small plain heating surface

Experiments were conducted to investigate the effects of ultrasonic waves on subcooled pool boiling on a small plain heating surface. Ultrasonic waves with different amplitudes (0-10 mu m) were applied for both nucleate boiling and transition boiling modes with the liquid subcooling and heat flux covering 15-60 K and 0.15-8 MW/m(2) , respectively. The results show that the acoustic streaming induced by high amplitude ultrasound has an obvious enhancement effect on nucleate boiling at low heat flux, but the augmentation will be restricted with increase in heat flux and decrease in ultrasonic amplitude. In transition boiling region, the ultrasound makes different impact. At liquid subcooling of 15 K under which microbubble emission boiling (MEB) would not occur normally, the strong acoustic streaming triggers MEB, thereby heat transfer is significantly enhanced. At subcooling of 21-40 K, the transition boiling is in the mode of stable MEB, the ultrasound barely affects the heat transfer and bubble behaviors. With the subcooling increasing to 50-60 K, the heat transfer of MEB is enhanced by the 10 mu m amplitude ultrasound but deteriorated by the 5 um amplitude ultrasound. The opposite results depend on the different effects arising from the ultrasound on the collapse and oscillation of the vapor film on the heating surface. (C)2019 Elsevier Ltd. All rights reserved.