Liquid-liquid vortex rings: hydrodynamics and mass transfer

Vortex rings have been formed at a tube (5.06 cm or 6.98 cm internal diameter) in a 20 cm deep layer of kerosene and projected vertically downwards through an interface and into 70 cm depth of water. These kerosene-in-water rings with initial volume in the order of 300 ml have been observed by video camera. The rapid rotation (vorticity) is concentrated in a toroidal region, but the transported volume of kerosene has an oblate spheroidal shape enclosing the toroid. The volume decays exponentially by a wake shedding process and the decay constant is related to density difference and ring velocity by a simple model. The extraction rate of benzoic acid (kerosene to water) has been measured under batch conditions. The drops which detach in the wake are large so the extraction efficiencies are relatively low. Efficiency is increased when the vortex rings have sufficient energy to impact on the base of the tank to form fine droplets.