Effective viscosity of bubbly two-phase flow is experimentally investigated by means of the falling sphere method. The terminal falling velocity of the sphere is measured by image processing to calculate the relative viscosity of the two-phase flow to the single-phase flow. The measurement results show that the effective viscosity is reduced for a range from 0 to 2% of void fraction as the shearing Weber number increases. This fact implies that the reduction of the effective viscosity is governed by the deformation of the bubbles, and the mechanism is explained by the interruption of the shear stress transfer in the two-phase medium.

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