The critical bubble diameter of the lift force in technical and environmental, buoyancy-driven bubbly flows

The lift force as part of the so-called non-drag forces influences distinctly the span-wise gas void fraction in bubbly flows. Towards larger bubble sizes, experiments at single bubbles show that the lift force changes its sign at a critical diameter. This effect would cause a separation of small and large bubbles in bubbly flows when a liquid velocity profile with gradients is present. In the present work, this separation is studied for different bubble columns setups in order to identify such a critical diameter. For all setups, almost the same critical diameter is found. Since the lift force is the only known force that could cause a separation of different bubble sizes, it can be concluded that the found critical diameter is indeed the diameter at which the lift force changes its sign. Therefore, a simple method is obtained with which the sign change of the lift force can be determined under realistic flow conditions.