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

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.Comment: 14 pages, 9 figure

Three-dimensional computational fluid dynamics simulations of interfacial flows with surfactants

Thin film flows are at the nexus of a large number of industrial applications that include manufacturing of fast-moving consumer goods, enhanced oil recovery, electronics, coating processes, and many more. The multiphase flow dynamics of such systems are affected by a myriad of physics (e.g., capillarity, complex rheology, heat and mass transfer, phase change, gravity, intermolecular and electro-magnetic interactions, and others). The physical complexity of thin film flow has excited the scientific community for decades. Over the years, many experimental efforts in the field have unravelled a multitude of challenges associated with the multi-scale nature of the physical phenomenon. Most attempts fall short of scrutinising these problems fully. Additionally, the complex topological structures of these flows are often influenced by naturally occurring, or deliberately placed surface-active species, resulting in the creation of surface tension gradients that drive the formation of Marangoni stresses. This work identifies the need to develop high-fidelity numerical models to study the influence of surfactants on the non-linear, three-dimensional physics of four industrially-relevant scenarios where thin films govern the dynamics. This work uses a new massively-parallel solver for the simulation of three-dimensional thin film flows (Shin et al., 2018). The numerical technique adopts a hybrid front-tracking/level-set approach, making it extremely advantageous in the study of interfacial phenomenon in the presence of surfactants. Numerical models are developed for the study of elongated bubbles propagating through liquid capillaries. The presence of inertia in these systems is responsible for the formation of complex spatio-temporal undulating structures near the bubble tail. This work elucidates, for the first time, the effect of Marangoni stresses on the dynamics of these oscillations for a wide range of flow and surfactant-related parameters. Additionally, the work investigates the effect of surfactant addition on the thin film region of these bubbles and the development of %critical flow vortical structures. The numerical approach is also extended to study the effect of Marangoni stresses on the three-dimensional wave dynamics of falling liquid films. Finally, the work concludes with the study of surfactant addition on the interaction of drops with thin liquid layers by looking at low-speed coalescence events first before arriving at high speed impacts. Escape from pinchoff was observed for all surfactant-laden coalescence systems, where a non-monotonic response was observed in relation to the vertical stretching and neck radius of the drop, driven by higher Marangoni stresses observed at the mid-range of the tested Marangoni parameters. Finally, surfactants were observed to significantly affect the vertical evolution of crowns in drop impact events, in addition to suppressing the breakup of the ejecta sheet for high Weber number impacts.Open Acces

Dynamics of retracting surfactant-laden ligaments at intermediateOhnesorge number

International audienceThe dynamics of ligaments retracting under the action of surface tension occurs in a multitude of natural and industrial applications, such as inkjet printing and atomisation. We perform fully three-dimensional, two-phase direct numerical simulations of the retraction dynamics with soluble surfactants. A full parametric study is performed using a hybrid interface-tracking/level-set method, which is utilised to treat the interface; this method is capable of capturing faithfully the topological transitions that are a feature of the flow over a certain range of ligament aspect ratios and Ohnesorge numbers. Our results demonstrate the delicate interplay between capillarity, modulated by the presence of surfactants, surfactant-induced Marangoni stresses, inertial and viscous effects. Particular attention is paid to the formation of vortices, which accompany the retraction process, and the influence of surfactant on the vortex dynamics

A numerical investigation of three-dimensional falling liquid films

AbstractIn this article, we present a full three-dimensional numerical study of thin liquid films falling on a vertical surface, by solving the full three-dimensional Navier–Stokes equations with a hybrid front-tracking/level-set method for tracking the interface. General falling film flow applications span across many types of process industries but also occur in a multitude of natural and environmental applications such as ice sheets, glaciology and even volcanic lava flows. In this study, we propose three configurations of falling films. Two of them, with small and moderate Reynolds number, are set to mimic pulsed and forced falling film types inside a minimum periodic domain, able to cover entirely the temporal evolution of a single wave. The latest example, corresponding to a high Reynolds number, is initialised with a flat interface without any specific perturbations. For the first time, this study highlights the natural transition from a non-deformed interface to its first streamwise disturbance (two-dimensional wavy flow), and then a second spanwise wave disturbance (three-dimensional wavy flow).</jats:p