Flow structures in bubbly flows with a tailored bubble size with and without liquid co-flow

The aim of this thesis is to provide experimental data on bubbly flows for validation of Computational Fluid Dynamics simulations. Therefore, a new experimental facility was developed to study the interaction of two vertical, initially separated, parallel bubbly flows in a rectangular channel of 0.40×0.20×2.63 m (W×D×H). The superficial gas and liquid velocities for each inlet were independently controlled and a large variety of unique a-symmetric bubble column configurations was investigated. Two multi-needle gas spargers, each comprising 14×14 needles in a square perforated grid, produced uniform large (4-8 mm) bubbles. Preparatory experiments in a downscaled setup, to study bubble formation rates and spreading rates of a single bubble train, yielded a novel correlation for the bubble diameter as a function of the gas flow rate, liquid co-flow velocity and needle diameter. Operating regimes were identified for which bubbles were formed individually. Bubble formation rates for the multi-needle gas sparger, determined with high-speed imaging and a novel acoustic technique, agreed very well with those for a single needle bubbling under similar conditions. Void fraction measurements in the large test-setup resulted in a new correlation for the gas hold-up. Lateral profiles of the bubble velocity were measured by using optical fibres and compared with parcel velocities obtained by using Bubble Image Velocimetry (BIV). Unique data on chord length distributions was obtained as the bubble size was uniform and accurately described by the developed correlation. The effects of a-symmetric gas sparging and (non-)uniform co-flow on the flow patterns were illustrated by contour plots of the bubble velocity magnitude and root-mean-square velocity fluctuations. An operating map was constructed to predict a-symmetric operating conditions for which both inlets yield equally sized bubbles and equal gas fractions, such that flow patterns appeared without buoyancy driven flow structures showing up. Vortex roll-up and circulatory motions were observed under certain conditions.