Turbulence modeling of polydispersed bubbly flow for large scale applications

Aeration of reactors is a wide used concept in biotechnology and chemical engineering to intensify processes. Beside enlarging the interface between liquid and gas phase, enhancement of mixing plays an important role. Turbulence affects these processes strongly. It is a main parameter for mixing and an important parameter for coalescence and break up processes.
Two main turbulence scales can be identified in bubbly flows. The small scale depends on the bubble itself and has a length scale in the magnitude of the bubble diameter. The large scale depends on the reactor geometry and has a length scale in the magnitude of the reactor. To characterize aerated Systems both scales are vital, but the modeling is difficult because of the multiscale problem.
A concept to describe both scales based on Euler-Euler unsteady RANS simulation with a two equation turbulence model is shown. The small scales are modeled with a new bubble induced turbulence model [Rzehak 2013][Ziegenhein 2013] and the large scales are directly computed. The contribution of the modeled and the directly computed turbulence to the total turbulence is shown and discussed on the bases of experimental data.
The aim of this approach is to model a complete reactor. The results can be used to formulate dispersion models for the specific reactor geometry, to support compartment modeling or to simulate different operating modes directly with CFD.

[Rzehak 2013] R. Rzehak and E. Krepper CFD modeling of bubble-induced turbulence, International Journal of Multiphase Flow 2013, 55, 138–155.
[Ziegenhein 2013] T. Ziegenhein, D. Lucas, R. Rzehak, E. Krepper Closure relations for CFD simulation of bubble columns, In proceeding of: International Conference on Multiphase Flow, Jeju, Korea, Volume: 8th