Contamination effects on the lift force of ellipsoidal air bubbles rising in saline water solutions

The lift force is known to strongly influence the lateral bubble distribution in bubbly flows and is therefore an important force that has to be modeled in corresponding CFD simulations. For ellipsoidal bubbles, which are mostly present in industrial cases, the strength as well as the direction in which the lift force acts is determined by the bubble deformation. The bubble deformation however, can strongly be reduced when surface-active contaminations like salts are present in the liquid bulk, which implies a change of the lift force by contaminations too. In the present work, lift coefficients for single bubbles rising in aqueous NaCl solutions were determined to investigate the influence of such an inorganic surfactant on the lift force. For this purpose, a recently developed method by Ziegenhein et al. [Int. J. Multiphase Flow, Vol. 108, 11-24 (2018)] is used, which is capable to measure the lift force in low viscous liquids. Besides the lift force, the bubble shape and slip velocity were studied in detail to connect the results to known contamination effects, which showed different behavior in dependence on the salt concentration. The results reveal that the contamination level plays an important role on changes of the lift force in comparison to clean bubbles. Up to a concentration of 1.0 mol/l the salt has only a weak effect on the lift force of larger bubbles. The lift coefficients of smaller bubbles however, clearly show significant changes, which were also reflected in a change of the bubble shape. However, some findings could only be connected to the slip velocity, which implies a connection of the lift force to more than just the shape.