Influence of ultrasound on heat transfer of copper tubes with different surface characteristics in sub-cooled boiling

The heat transfer enhancement due to ultrasonic vibration on horizontal copper tubes with different surface characteristics (smooth tube, screwed tube and finned tube) in a sub-cooled boiling regime is reported. The experiments are performed in degassed water at atmospheric pressure. A two-dimensional numerical model is established to reveal the acoustical pressure distribution around the tested tube. The experimental results show that the heat transfer enhancement is higher at frequency of 21 kHz than 45 kHz and that the ultrasonic power of 90 W results in a better cooling effect than that of 30 W. The heat transfer enhancement induced by ultrasound for the tubes with surface structures is more effective than that for the smooth tube. For the screwed and finned tube, the variational trend of heat transfer coefficient presents a "wave" character. Ultrasonic waves can reduce the differences in the heat transfer enhancement induced by the heating surface characteristics. In addition, the acoustical pressure distribution around the tested tube is figured out and the bubble behavior is imaged to interpret the mechanism of ultrasound on heat transfer of tubes with different surface characteristics. (C) 2015 Elsevier Ltd. All rights reserved.