Experimental investigation of water droplet binary collisions and description of outcomes with a symmetric Weber number

The outcomes of binary collisions of water droplets are investigated experimentally for the range of parameters of a spray system used during a severe nuclear reactor accident. Droplets diameters range from 220 to 450 μm with a diameter ratio between 0.5 and 1 and impact velocities between 3 and 10 m s−1. Values of the Weber number based on the small droplet size are between 20 and 280. For droplets with a Weber number up to 120, results in a map of impact parameters versus Weber number show the various regimes, namely, stretching separation, coalescence and reflexive separation. For a higher Weber number, between 120 and 280, the only observed outcomes are the two separation regimes. It is shown that results obtained for unequal droplet sizes match on a single map in terms of the impact parameter and of a new “symmetric” Weber number based on the sum of kinetic energies in the frame of the center of mass and on the sum of surface energies of the two droplets. Ashgriz and Poo [“Coalescence and separation ...