An experimental investigation of the flow field around a single Taylor bubble rising in a vertical pipe filled with stagnant water is presented. The Reynolds number of the flow based on the Taylor bubble rise velocity and the pipe diameter is 4350. The velocity field around the bubble was determined by Particle Image Velocimetry (PIV). The mean velocity fields in front of the bubble, in the liquid film, and in the wake region were calculated by ensemble-averaging the instantaneous velocity fields measured around 100 different bubbles. Ensemble-averaged velocities become negligible at 0.5D from the bubble nose and at $12 D from the bubble tail. However, notable instantaneous velocity fluctuations were found to exist up to 50D from the Taylor bubble tail. These residual vortices may influence the shape and the propagation velocity of the trailing bubble even at large separation distances. Ó

The development of gas-liquid slug flow along pipes is governed by the interaction between consecutive elongated bubbles. It is commonly accepted that the trailing bubble's shape and velocity are affected by the flow field in the liquid phase ahead of it. Particle image velocimetry ͑PIV͒ measurements of the velocity field in the wake of an elongated Taylor bubble are performed for different pipe diameters and various Reynolds numbers. Experiments are carried out in both laminar and turbulent background flows. Ensemble-averaged quantities in the frame of reference moving with the Taylor bubble are calculated. Peculiarities regarding the variation of the mean velocity distributions, as well as of the normal and shear Reynolds stresses, with the distance from the Taylor bubble bottom are discussed.

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An experimental approach is developed to carry out PIV measurements and to perform digital processing of the recorded PIV images to measure simultaneously the instantaneous turbulent velocity field and the void fraction distribution in the wake region of a Taylor bubble. Advanced methods of data processing, such as ensemble averaging and Proper Orthogonal Decomposition (POD), are applied. Results of measurements performed in our newly constructed experimental facility are presented. Two liquid flow rates were employed, corresponding to Reynolds numbers of 820 for laminar background flow and 7500 for turbulent flow. The mean characteristics of the velocity field in the wake region are calculated by ensemble-averaging the instantaneous velocity fields measured around 200 different bubbles. An algorithm to estimate the void fraction distribution in the axial pipe crosssection from the recorded PIV images is developed.

Abstract. The development of slug flow along vertical pipes is governed by the interaction between consecutive elongated bubbles. It is generally assumed that the trailing bubble's shape and velocity are affected by the flow field in the liquid phase ahead of it. To examine this assumption, a facility is used that allows controlled injection of pairs of Taylor bubbles into vertical pipes filled with stagnant or flowing liquid. An experimental approach is developed to perform particle image velocimetry measurements of the velocity field in front of the trailing Taylor bubble, simultaneously with video imaging of Taylor bubble pairs, to be able to relate the instantaneous parameters (shape and velocity) of the trailing bubble to the instantaneous velocity distribution in liquid ahead of it. Experiments are performed in pipes of two diameters and for a number of Reynolds numbers based on mean water velocity, corresponding to both laminar and turbulent background flows. A model relating the propagation velocity of the trailing Taylor bubble to the local mean centerline velocity of the leading bubble is suggested and verified experimentally. The effect of the velocity fluctuations in the leading Taylor bubble's wake on the instantaneous propagation velocity of the trailing bubble is studied. 2 SHEMER, GULITSKI and BARNEA