Conclusions
In summary, the bubble formation process and bubbling regimes for a
submerged capillary nozzle was investigated. When the capillary nozzle
is slightly inclined downward-pointing, continuous microbubbles can be
generated via the continuous partial coalescence of larger bubbles. This
process not only provides a new microbubble generation method but also
has influence on the bubble size distributions in the bubble column.
High-speed visualization shows that the generation of microbubbles is
controlled by two processes: (I) The mother bubble is formed in a very
short time and coalesces with the departed father bubble near the nozzle
orifice. (II) The capillary waves form and converge at the apex of the
mother bubble, leading to the pinch-off of a satellite microbubble. In
previous studies with an
upward-pointing
capillary nozzle, the continuous coalescence of bubbles was available,
but generating microbubbles was still very difficult even under very
wide experimental conditions. Through comparing the formation and
departure process of bubbles from the nozzle with different incline
angle, we find that the angle between the directions of gas injection
and drainage flow inside the coalescing bubbles can be changed from
around 0o to 45-135o to realize
continuous microbubble formation when the capillary nozzle is changed
from upward-pointing direction to the slight inclined downward-pointing
direction. A further analysis indicates that the generation of
microbubbles mainly depends on the rising velocity of father bubble,
growth rate of mother bubble and sizes of the parent bubbles. These
parameters are affected by the gas flow rate and nozzle diameter
directly in present air-water experiments. Therefore, the Bond and Weber
numbers are selected to show their influences, and a scaling law for the
microbubble size and a phase diagram identifying bubble coalescence with
the formation of microbubbles are given accordingly. Our experiments
also suggest that lower Weber and Bond numbers help microbubble
generation.