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.