Influence of two-phase turbulence models on the bubble coalescence and breakup behavior in bubbly pipe flow

In poly-disperse bubbly flows, bubble coalescence and breakup are important local events, which determines the evolution of the flow as well as the transition to slug flow. On the other hand side, the characterization of bubble coalescence and breakup is still one weak point in the CFD simulation of poly-disperse bubbly flows, since the transferability of available models is limited. A generalized model was proposed in our previous work, which takes into account all mechanisms of interest for bubble coalescence and breakup. Turbulence is one major mechanism in a turbulent bubbly flow. In other word, the performance of models for bubble coalescence and breakup relies on the accuracy of turbulence modeling. However, two-phase turbulence modeling is still one open issue.

In the current work, air-water bubbly flow in a vertical pipe (DN200) was investigated. The experimental data were obtained from the TOPFLOW test facility at the Forschungszentrum Dresden-Rossendorf (FZD). The inhomogeneous MUSIG approach in CFD code CFX with a new closure model for bubble coalescence and breakup was used for the simulation. Influence of various two-phase turbulence models was studied. The results of bubble size distribution, radial gas volume fraction, air/water axial velocity, Sauter mean bubble diameter as well as turbulent parameters were compared for each model.
In general, a good agreement with the experimental data has been achieved. The new model for bubble coalescence and breakup was shown as capable of predicting the flow evolution. The influence of two-phase turbulence modeling is considerable. Bubble-induced turbulence can be modeled by additional source term for turbulence transport equations instead of additional viscosity term.