Characteristics of the Vortex and Wake at the Rear of a Moving Droplet

Abstract

Literature relating to the characteristics of the separated “vortex” region and wake at the rear of a moving droplet has been “reviewed. The significance of this region to single drop transfer and to the axial mixing of the continuous phase in spray contractors — has been examined. Investigations into the flow patterns associated with moving droplets in eleven “uncontaminated” liquid-liquid systems, and within the range 20<(Re)<700, have indicated a critical (Re) above which the droplet exhibits a developed internal circulation. This phenomenon has been related to the effects of trace surfactant material. Below this critical (Re) the vortex volume increases with increasing (Re), a variation which may be expressed, when droplet distortion is slight, as:  (6V/πD3) = (Re) | 0.00495 + 0.0000153(Re) | = 0.085  Droplet distortion leads to an increase in the vortex dimensions above this relationship and may give rise to instability if extreme. Beyond the critical (Re) the development of internal circulation is accompanied by a decrease in the relative vortex size with increasing (Re) and a reduction in drag. The recurrence of critical (Re) values within the range 200<(Re)<210 suggested that the criterion for the development of internal circulation is a characteristic of the external flow pattern within this range. The development of internal circulation in “uncontaminated” droplets and in droplets emerging from a zone of surfactant contamination have been discussed in relation to a mechanism for augmented surfactant desorption. A model for “erosion” of the leading edge of a stagnant cap of surfactant by proximity to the boundary layer separation point has been examined though not satisfactorily proven. A variable height transfer column has allowed the estimation of the equilibrium vortex concentration for a limited number of droplets in binary systems. The concentration reduces with increasing (Re), droplet oscillation enhancing this reduction.