Experimental observations related to the lateral lift force in poly-dispersed bubbly flows

The influence of the lateral lift force on trajectories of single bubbles under idealized conditions was investigated in the past by means of Direct Numerical Simulations (DNS) and by well defined experiments. However, contradictory discussions can be found in literature on the meaning of this force for poly-dispersed flows, i.e. flow with medium or high gas volume fraction. Usually such flows are turbulent and bubble-bubble interaction, i.e. bubble coalescence and breakup play an important role. For this reason it is not self-evident, that the correlation for the lift force coefficient obtained by Tomiyama can be applied for such flows.
The separation of small and large bubbles in poly-dispersed flows was clearly shown in experiments on vertical pipe flow basing on wire-mesh sensor measurements which allow the measurement of bubble size distributions. The critical diameter at which the lift force changes its sign predicted by the Tomiyama correlations (5.8 mm for air-water flow at ambient conditions) seems to fit in general well with the transition between wall and core peaks in the radial gas volume fraction profiles decomposed according to the bubble size. This was also confirmed for steam-water flows at a pressure of 6.5 MPa for which the critical diameter is at about 3.5 mm according to the correlation.
However in case of void fraction larger than about 5 % the effect of the lift force is superposed by dynamic effects as bubble coalescence and breakup and radial migration of the bubbles. For this reason it is not possible to conclude on the critical diameter directly from the shape of the radial gas volume fraction profiles. Also, it was argued, that the wall peak of small bubbles can be caused by large bubbles moving fast upwards in the pipe centre and pushing the small bubbles towards the wall. For this reason in the presented work the distribution of bubbles in dependence on radial position and on bubble sizes is investigated in detail for developing flows. New, high-quality data on air-water as well as on steam-water flows in a DN200 pipe are used for these investigations. The validity of the Tomiyama correlation can be confirmed for a wide range of flow rates. Finally the consequences of the lateral lift force on the flow bubble columns, on the stability of homogeneous bubbly flows and on bubble plumes produced by bubble entrainment from impinging jets are discussed.