Session: 07-08: Thermal-Hydraulics General Studies and Analyses - I

Paper Number: 64697

Start Time: August 6, 2021, 03:15 PM

64697 - General Discussion on Terminal Velocity for Rising Single Bubble 

For a single bubble rising in infinite stagnant liquid, the terminal velocity is a significant parameter, since it is often used as a reference index to judge whether the liquid medium is clean or not. f the terminal velocity does exist, the question comes to where to capture the terminal velocity. 
Some researchers just presented the terminal velocity, but without explanation of position to measure the terminal velocity and the method adopted to calculate the terminal velocity[1][2]. Most of experiments captured the local average velocity as the terminal velocity at a distance far (relative to the bubble diameter) above the generation point, like a blind feeling an elephant, since it was firmly believed that as long as the distance above the generation point is far enough, the bubble would definitely reach the terminal velocity.
For small bubbles and bubbles in surface tension dominant regime (D: 0.048-5 mm), distances from 3.5 cm to 1.2 m above the generation point were adopted to measure the terminal velocity[3][4] [5] [6] [7] [8].
Despite the fact that a significant number of researches on single bubble rising have been carried out, even for bubbles with the same size, different distances were adopted by different researchers to measure the terminal velocity. Furthermore, different methods were adopted to evaluate the local velocity and the terminal velocity. Consequently, the distance for a bubble to arrive at the terminal velocity is not completely clear yet and the terminal velocities are not concordant. What’s more, distinct from solid particle, bubble is soft matter, which always adjusts its shape according to motion, which make the determination of the terminal velocity much more difficult. Further investigations are needed. 
 

Presenting Author: Qinghua Wang kyoto university

Authors:

Qinghua Wang kyoto university
Takehiko Yokomine Kyoto Univeristy
Zensaku Kawara kyoto university
Tomoaki Kunugi Zhejiang university