A population balance approach for polydispersed bubbly flows considering size dependent bubble forces

A generalized inhomogeneous Multiple Size Group (MUSIG) Model based on the Eulerian modeling framework was developed in close cooperation of ANSYS-CFX and Forschungszentrum Dresden-Rossendorf and implemented into CFX-10. By simulating a poly-dispersed gas-liquid two-phase flow, the mass exchanged between bubble size classes by bubble coalescence and bubble fragmentation as well as the momentum exchange due to bubble size dependent bubble forces have to be considered. Particularly the lift force has been proven to play an important role for establishing a certain bubble size distribution dependent flow regime.
The derived model has been validated against experimental data from the TOPFLOW test facility at the Forschungszentrum Dresden-Rossendorf (FZD). The wire-mesh technology measuring local gas volume fractions, bubble size distributions and velocities of gas and liquid phases was applied. Numerous tests investigating air-water flow and steam-water flow at saturation conditions in vertical pipes having a length up to 8 m and a diameter up to 200 mm were performed and used for model validation. Furthermore in order to check the model framework for a more complex flow situation, further experiments on the flow field around a half moon shaped asymmetric obstacle were performed and the flow conditions were simulated by applying the inhomogeneous MUSIG model in direct comparison.
The paper describes the main concepts of the CFD model approach and presents model validation and application cases. The inhomogeneous MUSIG model approach was shown to be able to describe bubbly flows with higher gas content. Particularly the separation phenomenon of small and large bubbles, which was proven to be a key phenomenon for the establishment of the corresponding flow regime, is well described. Weaknesses in this approach can be attributed to the characterization of bubble coalescence and bubble fragmentation, which must be further investigated.