Rise of Taylor bubbles through power law fluids - Analytical modelling and numerical simulation

An attempt has been made to explore the dynamics of Taylor bubbles rising through circular tubes filled with stagnant power law fluids using CFD and a semi-analytical technique. While the freeware OpenFOAM was used for CFD simulation, the semi-analytical model was developed considering viscous film drainage around the axisymmetric bubble and was closed using a single tuning coefficient from experimental data. Bubble rise velocity derived from both the techniques exhibit good agreement with experiments. Though the velocity field around a bubble rising through a power law fluid does not show any remarkable difference with that observed for bubbles through Newtonian liquids, the rise velocity is noted to depend strongly on both K and n of the power law relationship. Nevertheless, the influence of n is much greater as is evident from the change of nose shape with its variation. Effect of such variation in nose shape is also reflected in the predicted rise velocity. However, as observed in case of Newtonian fluids, for a given n and K, the rise velocity of Taylor bubble asymptotically reaches a constant value as the bubble volume is increased. It is also seen that surface tension does not have any significant effect on the nose shape and rise velocity while it modifies the tail shape. Finally, it has been noted that the increase in rise velocity with tube diameter depicts a non linear trend for non-Newtonian fluids while it is linear for their Newtonian counterpart. (C) 2019 Elsevier Ltd. All rights reserved.