Towards a robust CFD modelling approach for reliable hydrodynamics and mass transfer predictions in aerobic stirred fermenters

A strategy based on the Reynolds averaged two-fluid model is proved to be effective for the simulation of a turbulent gas-liquid stirred tank in the whole range of possible flow regimes, namely complete recirculation, loading, and flooding. The modelling approach is validated through the simulation of a laboratory scale tank stirred with a Rushton turbine, with superficial gas velocities ranging from 1.5 mm/s to 4.4 mm/s and specific power consumption from 39 W/m3 to 837 W/m3, since such a system in this range of operative conditions is well-characterized and several well-established correlations are available for validation purposes. The gas segregation due to the formation of aerated cavities was accounted for in the determination of the specific interfacial area available for the mass transfer and the numerical predictions of the kLa better agree with the available correlations, especially in the loading regime when clinging and ‘3–3’ cavities develop, with respect to the traditional specific interfacial area formulation that does not consider the phase segregation. Finally, starting from grid independent results, the effect of computational grid coarsening is quantified, obtaining guidelines for the affordable simulation of bioreactors of large scale.