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Micro bubble has unique characteristics. One of them is the drag reduction. This study investigates the effect of the pipe diameter on the micro bubble drag reduction. The pressurized dissolution method is applied. The frictional pressure drop is measured in vertical pipes with different diameters. As the flow rate increases, drag reduction decreases. Drag reduction is higher in larger pipe than in smaller pipe for the same Re number. On the other hand, drag reduction is lower in larger pipe than in smaller pipe for the same JL. The lower the void fraction becomes, the higher drag reduction becomes. Pipe diameter has some effects on the drag reduction.
