Saturated boiling heat transfer under ultrasound.

It has been reported that the benefit of ultrasonic enhancement on saturated boiling heat transfer is limited. This paper studies the synergistic effects of tube structure and ultrasound on saturated boiling heat transfer. Three tubes are analyzed: a smooth tube, a two-dimensional finned tube and a three-dimensional finned tube. It is found the ultrasound enhancement ratios of the three tube types decrease with increasing ultrasonic power and are smaller under a 45 kHz ultrasonic frequency than a 21 kHz ultrasonic frequency. Ultrasound hinders the saturated boiling heat transfer of the smooth tube and the two-dimensional finned tube under nearly all the experimental conditions in this paper. When the ultrasonic frequency is 21 kHz, the ultrasound enhancement ratio of the three-dimensional finned tube is below zero at low heat flux but above zero at large heat flux. The three-dimensional structure of the tube contributes to the improved heat transfer behavior in two ways. First, the interface interaction between wall and bubbles is smaller. Second, the ultrasound propagating along the intersected grooves produces a vortex flow of the liquid, and the resulting stronger stretching and contracting of the bubbles promote heat transfer performance. [ABSTRACT FROM AUTHOR]