Experiments and simulation on bubbly flow in a complex 3D flow field


Experiments and simulation on bubbly flow in a complex 3D flow field

Lucas, D.; Prasser, H.-M.; Krepper, E.; Beyer, M.

A new approach for the modelling of poly-dispersed bubbly flows – the so-called Inhomogeneous Multi Bubble Size Group (MUSIG) model was developed jointly by FZD and ANSYS. It was implemented into the CFX code and is now available starting with code version 10. This model enables the consideration of a number bubble classes in the mass balance as well as in the momentum balance. Usually only few classes are required for the momentum balance to reflect the bubble size dependent de-mixing of small and large bubbles caused by the inversion of the sign of the lift force with increasing bubble diameter. The model was developed and first tested using data for vertical pipe flow. To show the indepen¬dency of the model on geometry and to test it for more complex flow situations, now experiments were conducted with a pronounced three-dimensional character of the flow fields. A diaphragm which blocks about one half of the pipe was placed into the vertical DN200 test sections available at the TOPFLOW facility of FZD. Main feature is the trans¬location of the diaphragm to scan the 3D void field with a stationary wire-mesh sensor. Be¬sides the measurement of time-averaged void fraction fields, a novel data evaluation method was developed to extract estimated liquid velocity profiles from the wire-mesh sensor data. Simulations were done using CFX-10 and applying the Inhomogeneous MUSIG model. In general the complex flow including a recirculation zone behind the obstacle (where bubble coalescence is observed) and a pronounced jet of high liquid velocity at the unobstructed side (were bubble break-up dominates) are reflected well in the simulations. There is a strong interaction between size dependent bubble migration in the complex liquid velocity field (mainly caused by the lift force) and local bubble coalescence and break-up. The data clearly demonstrate the necessity to consider the bubble size dependent bubble migration for an adequate simulation of bubbly flows in complex geometries. Some weaknesses of the simulations are connected with shortcomings in the modelling of bubble coalescence and break-up.

Keywords: bubbly flow; 3D; flow field; CFD

  • Lecture (Conference)
    45th European Two-Phase Flow Group Meeting, 22.-25.05.2007, Toulouse, France

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Publ.-Id: 10045