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Numerical Modeling of Bubble Columns Using Experimental Breakup and Coalescence Rates of Bubbles
Azizi, S.; Schubert, M.
The prediction of bubbles size distributions in bubble columns reactors is a great challenge in design and optimization of operating conditions. The implementation of Population Balance Equations (PBE) for bubbly flows into CFD codes allowed better understanding of the hydrodynamic behavior of bubble columns and better quantification of the interfacial area for the estimation of interphase transport phenomena. On the other hand, the complexity of numerical models increased with the introduction of new sub-models for the determination of Bubble Size Distributions (BSD). The formulation of sink and source terms of such PBE is a very controversial issue. These terms depend on assumption on the dominating mechanisms due to turbulence, buoyancy, wake, shear, etc.. However, the unknown physical effects, the variety of constants of Breakup and Coalescence (B&C) kernels as well as their complex coupling with the hydrodynamics of the flow prevent to generalize existing models.
In this work, a new approach will be presented to determine B&C rates in bubble columns using experimental BSD data and hydrodynamic characteristics such as phase fractions at several column heights e.g. obtained by ultrafast X-ray tomography. To calculate the experimental B&C rates, a set of non-linear PBE is solved at each scanning height with the measured hydrodynamic data and statistical equations to consider diameter effect of coalescing bubbles.
The approach was applied to own experimental data and data from literature. Good agreement was found for the numerical simulated BSD and hydrodynamics based on the derived B&C rates compared to experimental data.
For future work, this approach allows to directly validate existing B&C models with different gas and liquid physical properties and operating conditions in order to formulate more generalized B&C models for their implementation in CFD models.
Keywords: Bubble Column; Breakup and Coalescence; Experimental Rates
Annual Meeting of Computational Fluid Dynamics, Mixing processes and rheology, 24.-25.02.2014, Festung Marienberg Würzburg, Germany