Laboratory Measurements Of Coalescence Efficiencies for Small Precipitation Drops

Abstract

Coalescence efficiencies were determined for small precipitation drops falling at terminal velocity in air. A jet of water was broken into drops of two sizes using a piezoelectric transducer. Unwanted drops were charged and deflected electrostatically to isolate pairs of uncharged drops with wide vertical separations. Streak photographs of drop trajectories were used to determine interaction geometries and types (i.e., miss, coalescence or bounce). The coalescence efficiency was estimated from the maximum horizontal offset for coalescence and also from the probability for coalescence based on the number of observed coalescences and bounces. Data were obtained for ten pairs of drop sizes with collector drop radii of 128?308 ?m and size ratios between 0.67 and 0.92. The measured coalescence efficiencies generally decreased from 1.0 to 0.39 as the Weber number increased. Although the findings did not agree with measurements from pendant drop experiments, the results were consistent with previous observations for freely falling drops with the same Weber number but much smaller size ratios.