The use of Volume of Fluid technique to analyze multiphase flows: Specific case of bubble rising in still liquids

A numerical study on rising bubbles in still liquids employing the Volume of Fluid (VOF) technique to track the interface is presented here. First, a combination of the correlation provided by Rastello et al. (2011) for the rectilinear motion of a bubble with that given by Clift et al. (1978) in the zig-zag ascension regime, conveniently made dimensionless, is proposed to determine the bubble terminal velocity for a wide range of bubble sizes and fluid properties. Furthermore, the crosspoint of both correlations gives the critical Weber number, Wec=ρlUT2D/σc, at which the transition from a rectilinear to a zig-zag bubble motion takes place in a liquid of a given Morton number, Mo=gμl4/σ3ρl. Concerning the numerical simulations, two different open source solvers have been evaluated, i.e. InterFoam and Gerris Flow Solver, to describe the motion of a stable bubble rising with a rectilinear path by performing two-dimensional axisymmetric simulations. The simulations show the presence of parasitic currents and variable results depending on the mesh resolution in the case of InterFoam, whereas no spurious results are observed in Gerris, which is therefore more suitable for these kinds of flows. Finally, the numerical results indicate that the gas properties hardly affect the bubble terminal velocity and shape, although they show that the flow field inside the bubble is highly affected by the gas density and viscosity. This result can be of relevance in heat and mass transfer processes where the mass diffusion or heat exchange can be enhanced by the convective motion induced inside the bubble.