Original articles
Experimental study on microbubble ejection method for frictional drag reduction
HIROHARU KATO, KENTO MIURA, HAJIME YAMAGUCHI, and MASARU MIYANAGA
Department of Environmental and Ocean Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan
Abstract: The formation of air bubbles ejected through a single hole in a flat plate was observed in uniform flow of 2-10m/s. It was confirmed that the size of the air bubbles was governed by main flow velocity and air flow rate. According to previous experiments, the size of the bubbles is an important factor in frictional drag reduction by microbubble ejection. Usually bubbles larger than a certain diameter (for example 1 mm) have no effect on frictional drag reduction. Three different methods were proposed and tested to generate smaller bubbles. Among them, a 2D convex (half body of an NACA 64-021 section) with ejection holes at the top was the best and most promising. The diameter of the bubbles became about one-third the size of the reference ejection on a fiat plate.
Moreover, the bubble size did not increase with increasing flow rate. This is a favorable characteristic for practical purposes. The skin friction force was measured directly with a miniature floating element transducer, and decreased drastically by microbubble ejection from the top of the 2D convex shape.
Key words: microbubble, frictional drag, drag reduction, ejection method
Address correspondence to; H. Kato
Received for publication on Feb. 2, 1998; accepted on Sept. 29, 1998
Introduction
The reduction of skin friction by microbubbles is promising for practical uses such as for a ship's hull, because the reduction rate can reach as high as 80% in the best conditions.
The pioneering work was by McCormick and Bhattacharyya.1 They found a reduction of drag in an axisymmetric body by covering the surface with hydrogen bubbles. A group of scientists at Pennsylvania State University made extensive studies in the 1980s.2-7 More recently, Kato et al.8,9 and Guin et al.10 measured the reduction of skin friction directly with a miniature floating element transducer.
Bogdevich et al.11 showed that the reduction rate of skin friction correlated well with the maximum gas concentration in the boundary layer. Madavan et al.3 tried to correlate the volumetric fraction of air Qa/(Qa + Qw) with the reduction rate. They also experimented by changing the pore size of the porous plate through which air was injected. Contrary to their expectations, injection pore size had no major effect on the amount of skin-friction reduction. Guin et al.10 found that the near-wall void fraction tended to collapse the drag-reduction data better than the average void fraction.
Another important factor is the size of the bubbles. When microbubble injection is applied to a low-speed ship's hull model, we often observe an increase of drag. In such cases, the diameter of the bubbles is 2-3mm.
Obviously the bubble size is too large compared with the scale of the boundary layer.
Kato et al. 9 examined the effect of bubble size by changing the main flow velocity. The bubble size decreased according to the increase in the main flow
velocity, resulting in a larger reduction rate of skin friction. Kato et al.8 also changed the bubble size by changing the surface tension. Water with 0.1 % ethanol, whose surface tension is lower than that of pure water, was more effective, although the difference was not large.
Examining this previous work, the authors recognized that it was important to control bubble size, in other words, to generate bubbles smaller than a certain diameter to reduce skin friction. When we apply microbubbles on a relatively slow ship, such as a coaster, we need a device to control the size of the bubbles.
In this paper, we consider the mechanism and physical properties governing the size of bubbles, then propose a few methods to control the size of bubbles. Those