A measurement study was performed to obtain a full-field quantitative description of a three-dimensional, two-phase bubbly flow. Particle image velocimetry (PIV), a whole-field, non-invasive velocity measurement technique, was utilized. PIV is capable of producing an instantaneous velocity map of steady-state and transient flows of a fluid seeded with microscopically small neutral density particles.The objective of this investigation was to study the turbulence structure in a co-current bubbly flow. The obtained information will help to determine parameters needed for two-phase flow modelling. The study investigated the influence of bubbles on the surrounding flow field (bubble/flow interaction). A stereoscopic reconstruction technique was used to obtain three-dimensional velocity vector data from the recorded planar images. Radial distributions of volume-averaged turbulence intensities and Reynolds stresses were calculated. The volume-averaged turbulent kinetic energy within the measurement zone due to a rising bubble is presented. * This paper was originally invited for the PIV special issue which appeared in volume 8, issue 12 (December 1997) of Measurement Science and Technology.

Particle tracking velocimetry has been used to measure the velocity fields of both continuous phase and dispersed microbubble phase, in a turbulent boundary layer, of a channel flow. Hydrogen and oxygen microbubbles were generated by electrolysis. The average size of the microbubbles was 15μm in radius. Drag reductions up to 40% were obtained, when the accumulation of microbubbles took place in a critical zone within the buffer layer. It is confirmed that a combination of concentration and distribution of microbubbles in the boundary layer can achieve high drag reduction values. Microbubble distribution across the boundary layer and their influence on the profile of the components of the liquid mean velocity vector are presented. The spanwise component of the mean vorticity field was inferred from the measured velocity fields. A decrease in the magnitude of the vorticity is found, leading to an increase of the viscous sublayer thickness. This behavior is similar to the observation of drag reduction by polymer and surfactant injection into liquid flows. The results obtained indicate that drag reduction by microbubble injection is not a simple consequence of density effects, but is an active and dynamic interaction between the turbulence structure in the buffer zone and the distribution of the microbubbles.

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