Effect of gas injection mode and purity of liquid on bubble rising in two-component systems

Gas bubble injection mode has been found by many researcher to be quite affecting the terminal velocity of a gas bubble rising in a liquid column. A direct gas injector, i.e., characterized by a bubble detachment diameter larger than the nozzle diameter, causes an initial distortion of the bubble thus producing an oscillation in the bubble itself. This has a great impact on the terminal rising velocity of the bubble, resulting in high values with respect to a controlled-type injection. None-the-less, the purity level of the liquid in which bubble are injected plays an additional important role in the terminal rising velocity of the bubble. Experiments using pure (distilled) and contaminated water and FC-72 (pure fluid) have been carried out using the direct and the controlled mode bubble injection. The behaviour of the bubble produced with the controlled injection mode in contaminated water is characterized by a nearly rectilinear trajectory, while the bubble exhibits almost a spherical shape. As soon as the bubble diameter is larger than the tube diameter the bubble is distorted at the detachment and starts to oscillate exhibiting a nonlinear trajectory. Its velocity tends to assume higher values, though a large scatter is found. Using distilled water the behaviour of the bubble is quite different, showing large oscillations at the detachment, independent of the injection mode. The terminal velocity is always higher than that obtained for controlled injection mode and contaminated water. Using FC-72 same results as pure water has been found. (C) 2006 Elsevier Inc. All rights reserved.