Dynamics of weakly evaporating and non-evaporating drops falling in air

Abstract

The dynamics of evaporating and non-evaporating drops falling in air under the influence of gravity is experimentally and theoretically studied in this work. Water and drops with diameters 2.0 and 2.7 mm and acetone drops 1.8 and 2.0 in diameter were tracked for approximately 2 m, corresponding to a flight-time of almost one second; at the end of the observation region, in most cases, the drops reached velocities ∼80% terminal velocity. Vertical positions of falling drops as a function of time and horizontal positions at different heights were recorded. Initially, the motion of non-evaporating water drops was rectilinear with a remarkable reproducibility in all experimental runs, showing narrow standard deviations in their horizontal position. When the increasing Reynolds numbers reached approximately 700, trajectories of individual drops diverged from each other. This effect may be attributed to the laminar-turbulent transition of the drops wake. It was also observed that the spatial dispersion, along horizontal cuts along their fall, evolved from a narrow to a wider monomodal distribution to bimodal one as the traveled distance increases. The acetone drops evaporated as they fall and although their velocity increased, the diameter reduction prevented the drops from reaching the critical transition Reynolds number, maintaining the dispersion in the horizontal positions relatively narrow. A maximum volume loss due to evaporation of 13% is recorded at the lower end of the observation region. Simplified theories based on the drag coefficients of fixed spheres closely predict the drops vertical positions as a function of time, for both evaporating and non-evaporating drops. Calculations based on heat and mass transfer correlations for fixed spheres show a satisfactory agreement when compared with the experimental observations of the radii of evaporating drops as a function of time.