Exploration of Surface Gas Entrainment in Elliptical Unbaffled Stirred Tanks

This study investigates interface instability and surface gas entrainment in elliptical unbaffled stirred tank reactors (STRs), employing visualization experiments and large-eddy simulations coupled with the volume-of-fluid (LES-VOF) method. Compared to conventional cylindrical unbaffled STRs, elliptical unbaffled STRs can achieve higher aeration through unique hydrodynamics, such as dynamic dual vortex cores and asymmetric flow patterns. Experiments show that the critical agitation speed (Nc) for gas entrainment is lower in elliptical versus cylindrical tanks. LES-VOF simulations reveal the presence of a dual gas entrainment mechanism in elliptical unbaffled STRs, as well as pressure heterogeneity driving interface deformation. The Weber number (We = 5.28) confirms that surface turbulent kinetic energy overcomes surface tension during bubble formation. In addition, four machine learning regression models are developed to predict Nc, with the artificial neural network regressor (ANNR) achieving the highest accuracy (test set R2 = 0.95, MAPE = 6.34%). These findings highlight the potential of elliptical unbaffled STRs for industrial applications requiring efficient gas–liquid mixing, such as wastewater treatment.