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

TL;DR

This study uses advanced numerical simulations to analyze how a viscous fluid in a stirred tank transitions to aeration, revealing complex vortex structures and flow history effects that influence bubble entrainment and interface dynamics.

Contribution

It introduces a high-fidelity hybrid front-tracking/level-set numerical method to accurately capture complex interfacial phenomena and flow transitions in stirred tank aeration processes.

Findings

Identification of primary and secondary vortical structures.

Observation of the transition to aeration at higher rotation rates.

Flow history significantly affects the aeration process.

Abstract

We consider the mixing of a viscous fluid by the rotation of a pitched blade turbine inside an open, cylindrical tank, with air as the lighter fluid above. To examine the flow and interfacial dynamics, we utilise 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, three-dimensional direct numerical simulations, 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.