Numerical and experimental investigation of onset of gas entrainment phenomenon

The phenomenon of surface aeration, entrapment of gas through liquid surface, is employed in many industrial processes such as ammonolysis, chlorination, waste water treatment, etc. Although many studies have been performed to establish the operational limits of surface aeration systems like stirred tank, the hydrodynamic conditions that characterize the onset remain elusive. In the present study, the onset of the gas entrainment phenomenon has been studied using experimental techniques such as Ultrasonic Velocity Profiler (UVP), Particle Image Velocimetry (PIV) and High Speed Photography (HSP) in conjunction with Large Eddy Simulations (LES). The LES could predict the experimental data within±15%. LES identified the phenomenon of onset of entrainment as the mechanism by which gas cavities on the surface, formed as a result of surface deformation and entrapped in surface flows, relieve them self of strain and regain force equilibrium over their surface. The effect of surface tension and viscosity of liquid on the onset of gas entrainment was studied by changing the respective values in the simulation. The reduction in either of these properties caused the critical strain rate experienced by cavity before entrainment to decrease. However, onset of gas entrainment was found to be more sensitive to changes in viscosity of the continuous phase as compared to surface tension of gas–liquid system.

► Study of phenomenon of onset of gas entrainment using Large Eddy Simulations (LES) and Volume of Fluid Method (VOF).
► Local hydrodynamic conditions at the point of entrainment were investigated.
► Visualization of the onset of gas entrainment phenomenon using high speed photography.
► Mechanism of entrainment proposed.
► Effects of surface tension and viscosity studied.