This study attempts to apply a single-probe fiber-optic reflectometer (FOR) to determine the velocities and sizes of oil droplets rising in a static water column. The concept proposed by Chang et al (2003 Rev. Sci. Instrum. 74 3559-65) was employed to derive the oil droplet velocity from a signal acquired using the FOR technique. An oil plume in a vertical water column was set up to verify the applicability in the oil-water mixture flow. A high-speed imaging technique was applied to provide control data sets for quantitative validation. The droplet velocity, residence time, and chord length measurements were validated by comparing with the results from high-speed images using the bubble image velocimetry technique and the image gradient method. To extract the oil-phase residence time, a double-threshold method was applied. It was found that the velocity measured by the FOR probe has an error of approximately 8%, while the measured chord length has an error of 13% and 8% in direct and indirect comparison with images, respectively. Furthermore, to evaluate the applicability of droplet size estimation using the present cleaved-tip single-optical-fiber probe, droplet-probe interaction was studied to examine the lower limit of the measurable range using dominant dimensionless parameters. The lower limit was found as roughly 20 times the diameter of the optical-fiber probe.
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