Merging collision of a drops stream

In this study, a collision of stream water drops aligned in a vertical direction was examined experimentally using a freefalling drop apparatus that provided two gaseous conditions, namely no flow (quiescent air, v_g = 0 m/s) and convective air flow (v_g = 0.3 and 0.5 m/s).The initial drop diameter (d_i) was kept at 900 μm. Drops generated for examination were considered to be two drops, three drops and a drop stream. Two initial dimensionless drop spacings, S_i = 5 and 10, were selected in the experiment. The Reynolds number of drops depended on the drop velocity, drop diameter and v_g in the free-falling drop process. The drop wake effect was affected by the initial drop spacing (S_i), Reynolds number (Re), and number of drops in the free-falling drop process. The effect of S_i was more dominant than that of the Re and the number of drops. In the downward movement process of drops with a given S_i, the first merging of the first two drops was observed at first and then resulted in a large drop; thereafter this large drop was caught by the last drop showing the second merging. The first merged collision occurred either along the same axis or in an off-center situation, but the second merged collision occurred only in the off-center situation. The results showed that the merged drop position shifted to the downstream region with increases in v_g or S_i and that variations in the merged position shifted from linear to exponential with increases in the number of drops under the same d_i and v_g conditions.