CFD Modelling of Bubble-Bubble Coalescence

The content of this report summarizes the outcome of the CFD Modelling of
Bubble-bubble Coalescence project. The main project objective is to investigate the
dynamics of coalescence process of two co-axial bubbles within liquid phase under
laminar flow condition. Computational Fluid Dynamics (CFD) software has been used in
this project to simulate the coalescence process which focuses on 3-dimensional
simulation. In many engineering process, it is founded that a lack of understanding in
bubble coalescence mechanism will complicate the study ofdispersion process and mass
transfer mechanisms to optimize equipment design. Thus it is important to understand
the bubble coalescence behaviours that can lead to the change of bubble size
distributions in controlling mass transfer between gas and liquid. This project deals with
multiphase flow which is gas-liquid flow. The modelling approach selected is Volumeof-
Fluid method (VOF) which commonly used to analyse the dynamic and deformation
of the liquid-gas interface. The main tool required in this project is FLUENT which is
CFD software and other software have been used are GAMBIT and AutoCAD.. Based
on the numerical result, the analysis was done to visualize coalescence mechanism
which can be described into three consecutive steps; (1) collision of bubbles, (2)
trapping and thinning of a thin liquid film and (3) film rupture. This agrees with the
description given by Oolman, T. O. and Blanch, H. W., 1986. Futhermore, the effect of
surface tension on the coalescence have been studied as one of the objectives. From the
result, a high surface tension is observed to produce a weak liquid jet behind the bubbles
and the resultant high surface tension force prohibits the surface stretching. These all
cause a late coalescence to occur. In addition, from the result generated by CFD, the
bubble trajectories can be plotted accurately and such an information should be helpful
in hydrodynamics modelling of bubbly flows. In conclusion, the mechanism of
coalescence can be investigated using CFD software and the project objectives are
satisfied.