Numerical and experimental investigation of the lift force on single bubbles

In recent years CFD has proven itself as a valuable tool for gaining insight in flow phenomena in general and complex multiphase flows arising in process equipment in particular. However for (dispersed) multiphase flows, the reliability of the outcome of these computations depends in a sensitive way on the correctness of the representation of the phase interactions (for instance due to drag and lift forces) which leads to the well-known and difficult closure problem. In this paper we report results of direct numerical simulations supplemented with dedicated experiments to obtain quantitative data for the representation of the lift force. This force is known to be responsible for the segregation of small and large (deformed) bubbles in bubbly flows through pipes and bubble columns. Both numerical simulations using an improved front tracking (FT) model and experiments under well-defined conditions have been performed for air bubbles rising through water/glycerine mixtures, where the bubble diameter, liquid viscosity and linear shear rate were varied. The numerical simulations show a good agreement with the correlation presented by Legendre and Magnaudet (1998) for spherical bubbles at sufficiently high Reynolds numbers. For large deformed bubbles a good agreement with the correlation by Tomiyama et al. (2002) was found over a wide range of liquid viscosities, although the computed lift force was always slightly lower. Therefore a new correlation has been proposed, which combines a fit of the numerical data for deformed bubbles with the correlation by Legendre and Magnaudet (1998) for small bubbles. Finally, it was shown that the shear rate has no significant influence on the drag and lift coefficient. An experimental set-up (similar to the one used by Tomiyama) was constructed using a running belt submerged in a liquid, consisting of a glycerine–water mixture of varying viscosity. PIV measurements have been used to calibrate the linear shear field and to obtain the flow profile around the bubbles. Contrary to the numerical simulations, the experimental data show a very strong influence of the shear rate on the lift force coefficient. This may be attributed to the rigid behaviour of the contaminated bubble surface, which changes the shear stress at the bubble interface