Effect of impeller rotation on oil-water emulsion formation in stirred vessels*

The mixing of two immiscible liquids, oil and water, by a pitched blade turbine (PBT) in a cylindrical vessel with various impeller rotating frequencies (f = 5,6,7,8,9,10 Hz) is studied. The vessel is filled with oil in the upper half and water in the lower half, and the PBT is immersed in the water phase with a small clearance from the vessel bottom. A hybrid front-tracking/level-set method is employed to capture the complex flow and interfacial dynamics within such a system. The rotation of the impeller introduces a primary vortex that can be observed in idealised rotational flows as well as in several secondary vortical structures resembling Kelvin-Helmholtz, vortex breakdown, blade tip-vortices, and end-wall corner vortices. These vortical structures lead to the lifting of the water phase around the vessel wall, while the oil in the centre is pulled downwards resulting in a deforming interface of a helical shape with 4 rotating curtains. As the rotation frequency increases, the interface deformation is accelerated, and ligaments are produced which subsequently break up into small droplets. Beyond a critical rotation rate, the system dynamics become extremely complex where the creation of ligaments, and the breakage and coalescence of drops occur simultaneously. The complex dynamics underlying such a mixing system are discussed in detail.