Abstract
Shapes and rise velocities of single air bubbles rising through stagnant water confined inside an annular channel were investigated by means of experiments and numerical simulations. Fast video imaging and image processing were used for the experiments, whilst the numerical simulations were carried out using the volume of fluid method and the open-source package OpenFOAM. The confinement of the annular channel did not affect the qualitative behavior of the bubbles, which exhibited a wobbling rise dynamic similar to that observed in bubbles rising through unconfined liquids. The effect of the confinement on the shape and rise velocity was evident; the bubbles were less deformed and rose slower in comparison with bubbles rising through unconfined liquids. The present data and numerical simulations, as well as the data collected from the literature for use here, indicate that the size, shape, and rise velocity of single bubbles are closely linked together, and prediction methods that fail to recognize this perform poorly. This study and the limited evidence documented in the literature indicate that the confinement effects observed in non-circular channels of complex shape are more complicated than those observed with circular tubes, and much less well understood.
Highlights
The confinement of the present annular channel did not affect the qualitative behavior of the bubbles, which exhibited a wobbling rise dynamic similar to that observed with bubbles rising through unconfined liquids
The effect of the confinement was evident on the shape and rise velocity; the bubbles were less deformed and rose more slowly in comparison with bubbles rising through unconfined liquids; The present results are in fair agreement with previous observations by Tomiyama et al [15]
This indicates that confinement effects with non-circular channels of complex shape could be more complicated than those observed with circular tubes, and both the size of the channel cross-section and its shape may affect the dynamics of the bubbles; results generated with comparatively small channels may not extrapolate to channel sizes of industrial relevance
Summary
The dynamics of single bubbles rising through stagnant liquids is of pivotal importance for the fundamental understanding of two-phase bubbly flows, which are relevant in a number of applications including gas–liquid reactors, bubbly columns, nuclear reactors, heat exchangers, and environmental flows. When bubbles rise in bounded liquids their dynamics are affected by the walls of the container. Available experimental studies of single bubbles rising through stagnant liquids were performed in containers of finite dimensions, typically vertical tanks of either circular or square cross-sections. It is currently accepted that, if the dimensions of the horizontal cross-section of the container are much larger than the size of the bubble (indicatively, 10–20 times or more), wall effects on the bubble dynamics are small or absent. The bubble can be considered unconfined, and the observed bubble dynamics can be regarded as representative of the free bubble rise through a stagnant unconfined liquid
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