04. Boiling and Multi-Phase Flow

Bubble injection is widely observed in a variety of industries, such as the mechanical, chemical, shipping, and medical industries. For example, the injection of bubbles in the vicinity of a flat plate can reduce remarkably a friction drag on the plate. However, when we apply the bubble injection to real ships, the following issues may occur depending on the shape and speed of ships: (1) injected bubbles separate from the ship’s bottom, and (2) moving bubbles interfere with the ship's screws. These issues could potentially lead to a significant decrease in the total frictional drag reduction and the net power-saving. We therefore need to control the behavior and distribution of bubbles moving along the ship's bottom. This study investigates the cross-flow transport of air bubbles in horizontal turbulent channel flows using multiple hydrophobic strips on a hydrophilic surface. Specifically, we use 1−3 hydrophobic strips whilst maintaining a constant hydrophobic strip area. The Reynolds number ranges from 3000 to 8000, and the angles of the hydrophobic strips relative to the streamwise direction are set at 15, and 30˚. For the measurement of the bubble distribution, one monochrome high-speed camera and two LED sources are set under the channel. Measurements reveal that at both angles, the use of 3 hydrophobic strips exhibits the highest bubble transport performance, especially at higher Reynolds numbers. This is because the detachment of bubbles from the strips is suppressed by the increase in strip edges (i.e., the hydrophobic–hydrophilic boundaries) that can keep entrapping bubbles remarkably.