3.4.2 Trajectory
Figure 15 shows bubble trajectory with different diameters. When the
diameter of bubble is greater than 1000 μm, the trajectory is spirally
rising, and the residence time is short. While the trajectory is almost
straight up at a diameter less than 1000 μm, and the residence time is
longer. The bubbles show morphological changes with different
oscillation amplitudes during the ascent. Under the action of buoyancy,
the bubbles begin to gradually accelerate to the final velocity state.
Affected by resistance and buoyancy, there is a large pressure
difference between the upper and lower parts of the
bubbles.31 The bubbles are deformed by the pressure
difference. The larger the bubble diameter, the more obvious the
deformation. This reveals that the smaller diameter bubbles are more
stable. As the diameter of the bubble grows, the effect of liquid
shear-induced lift is more obvious, and the lateral movement of the
bubble is more significant, so that the movement trajectory of the
bubble is a spiral upward.28,32 As the diameter of the
bubble decreases, the response of the bubble to the velocity gradient in
the channel gradually decreases, and the upward trajectory is hardly
affected. Figure 16 indicates the rising velocity of bubbles varies with
different diameters. The velocity of the bubbles with a diameter of 1.3
mm fluctuates between 0.15 and 0.27 m/s. The inflection point of the
velocity variation corresponds to the turning point when the bubbles
spiral upward, and the rising velocity of the bubbles tends to decrease
in general. The velocity of the bubble with a diameter of 0.56mm
fluctuates in the range of 0.04-0.12 m/s, which corresponds to a nearly
straight line of its upward trajectory. The results demonstrate that
bubbles with small diameters are more stable with slow rising velocity,
and the residence time in the liquid phase is longer, which is
consistent with the rising trajectory of the previous bubbles.