Measurement of Liquid Film Thickness Between Coalescing Twin Air Bubbles in a Water Pool Using a Modified Laser Extinction Method

Abstract

The objective of this study is to elucidate the mechanisms of bubble coalescence. The coalescence of horizontally contacting twin bubbles is experimentally investigated using an isothermal air–water system. The liquid film thickness formed between the contacting bubbles is measured using an improved laser extinction method. The variation of liquid film thickness between the bubbles at the rupture location and the distribution of the liquid film thickness are evaluated. The experimental parameters are the airflow rate and the measurement position (transmitted laser position). The bubble approach velocity and the time from the start of collision until coalescence were measured. When bubble coalescence occurs rapidly, the liquid film thickness is thinnest near the center, and this position moves toward the periphery from the center with increasing contact duration. The thinnest film thickness appeared just before coalescence and was approximately 1.0 μm. A ring-shaped thinner area in the liquid film emerged and shifted from the center toward the periphery of the liquid film with the increase in the bubble approach velocity and close contact duration. The thinnest liquid film thickness just before the commencement of rupture in the ring-shaped area was approximately 0.3 μm.

NOMENCLATURE

A	=	extinction coefficient, mm−1
d	=	internal diameter of pipe, mm
E	=	detector output, V
I	=	laser intensity, W/m2
Io	=	incident laser intensity, W/m2
IR	=	reference laser intensity, W/m2
IRO	=	reference incident laser intensity, W/m2
L	=	bubble contact length, mm
Q	=	supply airflow rate, mL/s
ReO	=	orifice Reynolds number, dimensionless
s	=	mean square error, μm
sδ	=	mean square error of liquid film thickness, μm
sδR	=	mean square error of corrected liquid film thickness, μm
t	=	time, ms
tc	=	contact duration, ms
Vb	=	bubble approach velocity, mm/s
x	=	horizontal direction, mm
y	=	vertical direction, mm
zp	=	distance between pipe outlets, mm

Greek Symbols

δ	=	liquid film thickness between bubbles, μm
δr	=	rupture liquid film thickness between bubbles, μm
δr min	=	minimum rupture liquid film thickness between bubbles, μm
σ	=	surface tension, N/m
ν	=	kinetic viscosity, m2/s

Additional information

Notes on contributors

Takayuki Morokuma

Takayuki Morokuma is a Ph.D. student in the graduate school of engineering at Yokohama National University, Kanagawa, Japan, under the supervision of Prof. Yoshio Utaka. He received a master's degree of mechanical engineering in 2012 and bachelor's degree in 2010 from Kanagawa University, Kanagawa, Japan. His doctoral research is focused on mechanisms of bubble coalescence and investigating the behavior of liquid film between bubbles.

Yoshio Utaka

Yoshio Utaka is a professor of mechanical engineering at Yokohama National University (YNU), Yokohama, Japan. He received his M.Eng. degree and D.Eng. degree from the University of Tokyo. His research concerns the field of heat transfer with phase change (condensation, boiling, solidification, and melting) and heat and mass transfer in microporous media for PEFC. He has been the head of department, the regent, and the president's aid at YNU and is president of the Heat Transfer Society of Japan.

Masahiro Shoji

Masahiro Shoji is a professor of mechanical engineering and the dean, a director, and a councilor of Kanagawa University in Yokohama, Japan. He is a professor emeritus of the University of Tokyo, from which he received his M.Eng. and D.Eng. degrees. His main research concerns boiling heat transfer and surface-tension-driven phenomena. He is a former president of HTSJ and a member emeritus of HTSJ and JSME. He has received more than 20 awards, including the Nusselt–Reynolds prize of ExFHT.