Visualization experiment on the evolution of vapor bubbles in pool boiling heat transfer enhancement of the smooth and porous surfaces using ultrasonic waves

Abstract

The effects of ultrasonic waves on the nucleate boiling heat transfer and the bubble behavior of smooth and porous surfaces are investigated. The formation and evolution of bubbles in a boiling liquid is one of the crucial factors that determine the heat transfer of a surface. We exploit ultrasonic waves to enhance boiling heat transfer by altering the formation and evolution of the bubbles. The pool boiling curves and the corresponding bubble dynamics of smooth and sintered porous surfaces with ultrasonic waves are investigated based on visualization experiments. The results show that the boiling heat transfer of the two surfaces is enhanced up to different degrees by ultrasonic waves. The heat transfer coefficients (HTCs) of smooth and porous surfaces were increased by 23.7% and 30.9%, respectively, with ultrasonic waves. The bubble departure diameters for the porous surface were much lower than that for the smooth surface. Because of that, bubbles on the porous surface were more susceptible to acoustic streaming and tended to decrease departure diameters. Furthermore, additional vaporization cores were activated during alternating positive and negative acoustic pressure. However, the enhancement of boiling heat transfer by ultrasonic waves was restricted with the increase of heat flux.