CFD simulations of the transition between non-aerated and aerated conditions in uncovered unbaffled stirred tanks

• Use of Volume of Fluid (VOF) method to model free surface dynamics in UUSTs. • Comprehensive assessment of suitable turbulence models for unbaffled stirred tank analysis. • Study of effects of impeller speed on vortex shape and aeration at different viscosities. • Prediction of Power Number (Np) and vortex shape in non-aerated and aerated conditions. • Prediction of cavities near the impeller blades in super-critical regime. The transition between non-aerated and aerated regimes in uncovered unbaffled stirred tanks (UUSTs) was investigated using computational fluid dynamics (CFD). The Volume of Fluid (VOF) method was employed to model the free surface dynamics under various operational conditions. The simulations were able to predict the peculiar experimentally observed behavior of UUSTs and revealed that at velocities below a critical threshold ( N_ < _N crit ) , the system remains non-aerated, while exceeding this threshold ( N_ ≥ _N crit ) induces bubble ingestion, leading to significant changes in power consumption and flow patterns. The CFD simulations accurately predicted the behaviour of the Power Number ( Np ) as well as the vortex shape inside the tank both in subcritical and supercritical regimes and showed good agreement with original experimental data and correlations from the literature. Additionally, the modeling of the aerated regime successfully predicted the vortex shape, the bubble dispersion within the tank, and the cavities formed behind the blades.