Interaction of three aligned drops in flows

In this study, the collision of three water drops aligned in the vertical direction was examined experimentally using a free-falling drop apparatus under three conditions, namely no flow (quiescent air, v_g = 0 m/s), convective air flow (v_g = 0.3 - 1 m/s), and hot flow (v_g = 2.5 m/s). Three initial drop diameters (d_i ≈ 500, 700, and 900 μm) and three initial dimensionless drop spacings (S_i ≈ 2.5, 5 and 10) were considered in the experiment. In the downward movement of three aligned drops with given values of d_i and S_i, the first two drops first merge into a large drop, which then merges with the remaining drop. The results show that for all cases, the leading merged position increases linearly with the initial gas velocity because of a rear-ending process along the same axis, except for S_i ≈10 with d_i ≈ 900 μm. For these values, the leading merged position increased exponentially with the initial gas velocity. Further, the final merged position increased exponentially with the initial gas velocity for S_i ≈ 2.5 and 5 with d_i ≈ 500, 700, and 900 μm. The final merged occurred in hot flow for S_i ≈ 2.5 with d_i ≈ 500 and 700 μm and for S_i ≈ 5 with d_i ≈ 500 μm. It was found that the position of the merging drop collisions which resulted in merged shifted downstream with larger initial drop diameter, larger initial drop spacing, and higher initial gas velocity due to the weakened wake effect. The leading merged collision included along the same axis condition and under the off-center condition, but the final merged collision occurred only under the off-center condition. The oscillation period of the leading merged increased with increasing initial drop diameter, but the final merged did not exhibit a oscillate cycle.