04. Boiling and Multi-Phase Flow

The air lubrication technique is known as a frictional drag reduction approach for ships. This approach can reduce friction drag remarkably; however, it requires large amount of energy to supply an air layer, air films, or air bubbles continuously. This reduces appreciably the net energy-saving efficiency. Recently, hydrophobic surfaces (i.e., HPO surfaces) have attracted the attention of researchers as a new approach of reducing friction drag. This approach does not require a continuous air supply because the air can remain at the surface. With the aim of keeping air on an HPO plate at high Reynolds numbers, this study focuses on HPO objects on the plate. Under the Reynolds number of Re = 3000−10000, we visualize and measure air films between HPO objects with heights of 0.2, 0.5, 1.0 mm in horizontal channel flows. When three pairs of HPO objects with 1 mm interval are placed separately in the spanwise direction on the HPO surface, the air film in each pair is observed at all Reynolds numbers. In particular, HPO objects with a height of 0.5 mm exhibit the highest air-film entrapping probability. On the other hand, HPO objects with a height of 0.2 mm exhibit the lowest air-film entrapping probability. This is because the injection of air into the space between such low HPO objects are quite difficult even if the Reynolds number is very low. In addition, at Re = 10000, the air-film entrapping probability for HPO objects with a height of 0.5 mm decreases with time due to a rupture of the air film at the downstream part and drops to 20 % after 30 minutes.