The effect of bubble on pressure drop reduction in helical coil

In the present paper, the effect of bubbles on the pressure drop and drag reduction of air-water two phase flows in vertical helical coils was experimentally investigated. The curvature ratio of the studied coils was 0.06 and 0.095. The ranges of the investigated Reynolds numbers and void fractions were 8000-50,000 and 0-0.09, respectively. The effect of the drag reduction will decrease with the increase of Reynolds number and the amount of drag reduction increases with increasing void fraction. It was found that for helical coil, the drag reduction increases with the decrease of curvature because of the secondary flow increases with the increase of curvature. The average diameter of injected bubbles was 270 μm; and as the Reynolds number increased, the diameters of injected bubbles diminished. The experimental results indicate that in this range of void fraction, the maximum reduction of friction drag is 25%, which occurs at low Reynolds numbers. Also, in void fraction 0.01 the drag reduction was 9% which was observed in Reynolds of 11,000.