Coalescence/breakup characteristics of binary collisions of small water
drops (dS=0.4-1.8 mm diameter) with large drops
(dL=3-3.5 mm diameter) occurring in the absence/presence
of horizontal electric field (EH) = 0, 100, and 300
kVm-1 have been investigated in a small vertical wind
tunnel using a high-speed digital camera. The coalescence efficiency
(EC) of 0.299 observed for average diameters
(dL=3.2 mm, dS=1.2 mm) in
EH = 0 decreased to 0.244/0.211 when EH
is increased to 100/300 kVm-1. The increase in the
electric field reduces the probability of coalescence when Weber number
(We) <1. However, when We ≥ 1, an increase in We restricts the
probability of coalescence. Our data, when plotted in the regime diagram
in the We*- p plane, delineates the collision outcomes in all-electric
field values but does show the overlapping of some
data points in the adjacent categories. After a binary collision, the
relaxation time required for the occurrence of coalescence is higher
than that for the breakup. Further, the relaxation time increases from
the filament to sheet to disk mode of breakup in all-electric field
values. Fragment size distributions after the filament and sheet types
of breakups differ and are differently affected by the applied electric
field. Higher collision kinetic energy (CKE) has a tendency to increase
the number of fragments of the sizes between dL and
dS. It is concluded therefore that, the effect of the
electric field needs to be included in the estimation of drop growth and
precipitation in clouds.