The local axial relative velocity between the gas and liquid phases in vertical air-water two-phase bubbly, cap-bubbly, and slug flows is investigated at axial measurement locations of z/D = 10 and 32. These measurements are performed in an acrylic vertical pipe with an inner diameter of 50 mm and a height of 3.2 m. The local gas-phase velocity and void fraction measurements are performed using a four-sensor conductivity probe at 14 radial locations. The void fraction profiles are presented to show the flow structure and are used along with visual observation to classify the flow regimes. The liquid-phase velocity measurements are performed using a particle image velocimetry (PIV) system. To separate the liquid-phase information from that of the gas phase an optical phase separation method that uses fluorescent particles and an optical filtration technique is adopted. To remove other noises, such as bubble residuals, background noise, and optically distorted particles from the images that are not removed using the optical phase separation method, an image pre-processing scheme is applied to the raw PIV images. The local axial relative velocity obtained from these local gas- and liquid-phase measurements are important for modeling the momentum transfer between the two phases.

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