Fujio Yamamoto

Backlight imaging tomography for gas–liquid two-phase flow in a helically coiled tube

Air–water two-phase flow in a helically coiled tube is investigated using backlight imaging tomography to elucidate the effect of centrifugal acceleration on phase distribution and interfacial structure. Superficial velocities up to 6 m s−1 in 20 mm diameter tube are tested. We focused on a slug flow regime in which centrifugal acceleration dominates the flow. The interfacial structure is visualized in six directions using a set of originally designed mirror-mounted water jackets. A temporal expansion image is made from line-sampled images and is used to reconstruct phase distribution through a linear backward projection algorithm. The present topography measurement showed various new features of gas–liquid two-phase flow in a helically coiled tube, such as a wall-covering effect in the case of high superficial velocity.

Bubble-Driven Convection Around Cylinders Confined in a Channel

This paper is concerned with flow visualization and image measurement of bubbly flows around various shapes of cylinders. A coaxial confined double rectangular chamber is constructed in order to provide a wide two-dimensional uniform bubble distribution upstream. The experiment shows that a wide two-phase convection is induced around the obstacle, though such an effect is not observed in research on the single-phase flow around objects. The spatial scale of the two-phase convection depends sensitively on the shape of the obstacle. Dense arrangement of cylinders is also investigated to find the interaction among the convection. The measurement results of void fraction, bubble velocity and liquid phase flow, which are obtained by image processing including particle tracking velocimetry, explore the detailed mechanism of generating the convection

Frictional Drag Reduction Provided with Small Bubbles in Concentric Rotating Cylinders

The frictional drag reduction provided with small bubbles is investigated experimentally using vertical concentric cylinders. The friction reduction ratio is measured using a torque meter mounted on the inner cylinder for a wide range in Reynolds number from wavy vortex flow (WVF) to turbulent Taylor vortex flow (TTV). The present data show around 36% drag reduction in the case of WVF regime at Re =600 and the reduction maintains until Re =4 000. The friction reduction ratioη defined by the unit void fraction obeyed in a linear relation with inverse values of Froude number, and reached up to 10 in the best case, that implies the highest sensitivity for the drag reduction. The bubble distribution measured in the gap showed a peak near the inner cylinder surface resulting in high local shear stress reduction.