The transition to aeration in turbulent two-phase mixing in stirred vessels

Abstract

We consider the mixing dynamics of an air–liquid system driven by the rotation of a pitched blade turbine (PBT) inside an open, cylindrical tank. To examine the flow and interfacial dynamics, we use a highly parallelised implementation of a hybrid front-tracking/level-set method that employs a domain-decomposition parallelisation strategy. Our numerical technique is designed to capture faithfully complex interfacial deformation, and changes of topology, including interface rupture and dispersed phase coalescence. As shown via transient, a three-dimensional (3-D) LES (large eddy simulation) using a Smagorinsky–Lilly turbulence model, the impeller induces the formation of primary vortices that arise in many idealised rotating flows as well as several secondary vortical structures resembling Kelvin–Helmholtz, vortex breakdown, blade tip vortices and end-wall corner vortices. As the rotation rate increases, a transition to ‘aeration’ is observed when the interface reaches the rotating blades leading to the entrainment of air bubbles into the viscous fluid and the creation of a bubbly, rotating, free surface flow. The mechanisms underlying the aeration transition are probed as are the routes leading to it, which are shown to exhibit a strong dependence on flow history.

Numerical study of oil–water emulsion formation in stirred vessels: effect of impeller speed

“…The water flow is characterised by a high level of turbulence involving the development of a multitude of vortices of varying scales. Similar behaviour in the water phase was demonstrated by Kahouadji et al. (2022) who carried out simulations of air–water mixing in stirred vessels.…”

“…Mass is conserved to within in all of our simulations. Our numerical procedure (Shin et al., 2018) has also been employed to simulate aeration in gas–liquid mixing (Kahouadji et al., 2022), vortex-interface dynamics and dispersion formation in turbulent water jets injected into an oil medium (Constante-Amores, 2021; Constante-Amores et al., 2021b, 2020b), the breakup of ligaments (Constante-Amores et al., 2020a), the coalescence of drops with interfaces (Constante-Amores et al., 2021a), the bursting of bubbles through interfaces (Constante-Amores et al., 2021c) and the dynamics of falling films (Batchvarov et al., 2020). We have also carried out a validation study against the experimental work of Hančil and Rod (1988); the results are in the Supplementary Information.…”