Drag Reduction Effects of Micro-Bubbles in Straight and Helical Pipes

Abstract

Experimental investigations were conducted to study the effect of micro-bubbles injected into water flowing through straight and helical pipes with different curvature ( 0.025, 0.05 and 0.1). The micro-bubbles were produced using two hydrocyclone micro-bubble generators positioned face to face with opposite direction of swirl flow. The mean diameter of the micro-bubbles generated was about 60μ m, while the maximum diameter observed was 174μ m, with air fraction, which will be defined in Eq.(4), ranging from 0.21% to 0.44%. The effect of the Reynolds number on the drag reduction is also presented. The experimental results show that, the micro-bubble has an effect of drag reduction on both straight and helical pipes, though the drag reduction in a straight pipe is much higher than that in a helical pipe, because of the secondary flow in a helical pipe. It is found that the drag reduction increases with the decrease of curvature and the increase of air fraction. The maximum value of drag reduction ratio was 51% in case of a straight pipe, while that for a helical pipe was 16%.

Keywords

• pipe flow
• secondary flow
• pressure drop
• drag reduction
• micro-bubble
• curved pipe
• void fraction