On the modelling of bubbly flow in vertical pipes

To qualify CFD codes for two-phase flows, they have to be equipped with constitutive models standing for the interaction between the gaseous and the liquid phases. In case of bubbly flow this particularly concerns the forces acting on the bubbles and bubble coalescence and break-up. Applying a two fluid approach, besides the drag forces describing the momentum exchange in flow direction, the non-drag forces acting perpendicular to the flow direction play an important role for the development of the flow structure. Gas–liquid flow in vertical pipes is a very good object for studying the corresponding phenomena. Here, the bubbles move under clear boundary conditions, resulting in a shear field of nearly constant structure where the bubbles rise for a comparatively long time. The evolution of the flow within the pipe depends on a very complex interaction between bubble forces and bubble coalescence and break-up, e.g. the lift-force, which strongly influences the radial distribution of the bubbles, changes its sign depending on the bubble diameter. The consequence is the radial separation of small and large bubbles. Neglecting this phenomenon, models are not able to describe the correct flow structure. Extensive experiments measuring the radial gas volume fraction distribution, the bubble size distribution and the radial residence of bubbles dependent on their size were determined for different distances from the gas injection. Basing on these experiments the applicability and the limits for the simulation of bubble flow with current CFD-codes are demonstrated, using the simulation of vertical pipe flow with CFX-4 as an example. Using a simplified model focusing particularly on the radial phenomena described above, parametric studies were conducted. They give an indication for necessary improvements of the codes. Finally a possible way for the improvement of the CFD-codes is shown.