The present work reports an experimental study of single Taylor bubbles rising in vertical tubes filled with water-glycerin mixtures by using the pulse-echo ultrasonic technique. A 2m long acrylic tube with inner diameter of 24 mm was used in the experiments. Initially, the tube was sealed at the ends and filled partially with the liquid mixtures to leave an air pocket of length L_0 at the top end. A Taylor bubble was formed by the inversion of the tube. The rising bubbles were detected by ultrasonic transducers located at the upper part of the tube. The velocity, the length and the profile of the bubbles and the thickness of the liquid film around them were obtained from the ultrasonic signals processing. The liquid film thickness in the vertical tube was also determined by a graphic method that relates the bubble length L_b with the initial length of the air pocket L_0. It was observed that the bubble velocity decreased with increasing viscosity, while the film thickness increased. It was shown that the liquid film thickness determined by the graphic method fitted well the higher viscosities data, but overestimated the lower viscosities data. Additionally, the results indicated that some correlations developed to estimate the thickness of liquid films falling down inside/outside of tubes and down a plane surface could be applied to estimate the thickness of liquid films falling around Taylor bubbles in an Inverse Viscosity Number (N_f) range different to those considered in the literature.
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