Experimental study on the reduction of skin frictional drag in pipe flow by using convex air bubbles

In response to the ever increasing need for efficient management of energy consumption, there have been extensive studies on drag reduction in many types of transport systems. In this paper, we examine the reduction of skin frictional drag in a pipe with an internal surface fabricated with cavity array by using the slip obtained on a convex air bubble array. The bubble formation was observed in a microchannel by using a high-speed CCD camera with respect to time and a micro PIV characterized by measuring velocity distribution around the convex bubble. Also, to investigate the possibility of the drag reduction, the volumetric flow rate and momentum flux were compared with and without the convex air bubble array in the microchannel. The measured momentum flux was rapidly increased around the convex air bubbles, which expected the reduction of skin frictional drag. Also, the slip influence distance was determined for the different bubble heights along the microchannel. The convex air bubble with larger height provides longer slip influence distance. Finally, the cavity array was fabricated on the internal surface of a pipe. The size of the cavity array was designed 100 μm in a rectangle, and they were spaced with 150 μm. The pipe diameter was 28.4 mm, and its length was 500 mm. The pipe was installed into a test rig to evaluate the drag reduction and was experimented in the turbulent flow condition, in which Reynolds number was ranged from 40,000 to 220,000. Maximum drag reduction of 10 % was obtained in the cavity pipe, while that of the smooth pipe was shown <2 %.