3.4.1 Coalescence
The change in bubble size distribution may be related to the change of
bubble coalescence and fragmentation. Therefore, the investigation of
bubble coalescence is of great significance for the stable rising of
bubbles. Figure 14 shows typical coalescence process of two bubbles. In
that case a bubble accelerated to impact a previous bubble, two bubbles
will experience (1) bubble collision ; (2) formation and drainage of
liquid film between bubbles ; (3) rupture of the liquid film until the
three stages of coalescence.29 Due to the fast
velocity of the large bubble below, it catches up with a neighboring
small bubble above at 2 ms, while the two bubbles deform (gourd shape)
at 4 ms. A large bubble catches up with a neighboring small bubble above
at 2 ms due to the fast velocity of large bubble below, and the two
bubbles deform to a gourd shape at 4 ms. The liquid between the two
bubbles is squeezed out and formed a thin liquid film. The two bubbles
break the liquid film limit at 7ms to complete the coalescing. The
coalesced large bubbles are continuously deformed under the action of
surface tension and inertial force.30