R.Manasseh Fluid Dyn Papers
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Li Chen, Yuguo Li and Richard Manasseh
Advanced Fluid Dynamics Laboratory, CSIRO BCE
PO Box 56, Highett, VIC 3190, Melbourne, Australia
Third International Conference on Multiphase Flow, ICMF'98
Lyon, France, June 8-12, 1998, Paper 626
The dynamics of bubble coalescence plays an important role in many engineering processes. For example, in mixing, bubbles or drops can generate large changes in interfacial areas through the action of vorticity via stretching, tearing and folding which facilitates the mixing processes. A good understanding of the fundamental mechanism of multiple bubble coalescence can be crucial in maintaining the dispersion process.
Discontinuous fluid properties in a flow system can produce a complex flow structure with rich physical length scales, which presents both computational and experimental challenges. Numerically, a robust algorithm for solving multi-phase flows with an accurate representation of interfaces is required to accommodate the complex topological changes in bubble coalescence.
In this paper, the dynamics of bubble coalescence is studied using our robust VOF method associated with a semi-implicit algorithm for Navier-Stokes equations. The effects of liquid viscosity and surface tension on bubble coalescence, for which Reynolds number ranges from 10 to 100 and Bond number ranges from 5 to 50, are investigated. It is shown that the numerical model used in this study can accurately capture the complex topological changes during the coalescence. The predicted behaviour of air bubble coalescence in glycerin liquid is in good agreement with our experimental observation and an error of 20% in the estimation of the rise velocity with reference to the leading bubble centre is obtained.
It is found that the interaction between the leading and following bubbles depends mainly on the liquid viscosity. The higher the liquid viscosity, the easier the bubbles interact. Therefore, bubble coalescence is more likely for high viscosity. On the other hand, for low viscosity, the liquid jet behind the leading bubble becomes stronger which prevent the bubble interaction. A postponed or non-coalescence is obtained. It confirms that the leading bubble travels with a constant velocity until it merges with the following bubble but the following bubble slightly accelerates due to the wake of the leading bubble. Regarding the surface tension effect, high surface tension results in a weak liquid jet, and high surface tension force prevents the surface from stretching. Therefore a late coalescence occurs.
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