The occurrence of meso-scale structures in bubbly flows

Övrigt konferensbidrag, 2018

Bubble columns are used in many industrial and natural processes, ranging from chemical and nuclear reactors to atmosphere-ocean exchange. The current design and prediction of these multiphase systems are still based on global empirical correlations. Furthermore, classic laser-based experimental techniques are not able to measure inside opaque suspensions. One of the issues that are still not sufficiently understood is the appearance of a non-uniform volume fraction field in the form of meso-scale structures. We apply an Eulerian-Eulerian two-fluid model approach to investigate the origin of these characteristics structures. The ability of this numerical framework in detecting the correct dynamics of the unstable modes is still debated1. For this reason, the results are compared with an additional modelling framework like the Eulerian-Lagrangian approach. The primary aim of this comparison is to verify that the observations made using the two-fluid model are not numerical artefacts but real representations of the physical phenomenologies described by the equations. Both approaches show the formation of characteristic meso-scale structures in the steady regime as it is shown in Figure 1 where an instantaneous axial liquid velocity field is plotted for the Eulerian-Eulerian (left panel) and Eulerian-Lagrangian (right panel) simulations. The snapshots show long and elongated streaks along the gravity direction with numerical values of the same order of magnitude between the two numerical approaches. Further statistics will be shown at the Conference presentation. In particular, a good agreement appears in the force balance between the two methodologies supporting a physical nature of the heterogeneities detected with the Eulerian-Eulerian framework.