Bubbly flows are ubiquitously found in natural systems and are widely used in (bio)-chemical and energy-producing processes. A variety of design options, ease of maintenance, and a large operability window make bubble columns a commonplace across industries, ranging from oil processing to biotechnology and electrolyzers. Despite their wide applicability and advantages, the complexity of designing and optimizing largescale bubble columns arises from inherent multiphysics - multiscale phenomena. The design and scale-up methodologies, from laboratory to large-scale, require a detailed understanding of the interplay between the different physical phenomena and processes, for a variety of fluid compositions and operation conditions, at various scales: from single bubbles to dense bubbly flows in industrial situations. For example: (i) the interplay between rheology, hydrodynamics and interfacial phenomena for a single bubble; (ii) the interplay between the collective dynamics of the bubbles inside the column, the bubble size distribution and the bubble generation process....@en

In this work, the rise characteristics of a single H₂ bubble, in the ellipsoidal regime, in (i) water, (ii) single electrolyte (2 M, 4.5 M NaCl) solution and (iii) various concentrations of electrolyte mixture (up to 6.4 M of 1:5 weight fraction NaCl-NaClO₃), have been studied, at temperatures up to 80°C. Our results show that both individual and collective effects of the temperature and the electrolyte concentration on the rise velocity and the bubble shape are purely dependent on the changes in liquid properties (density, viscosity, and surface tension); the bubble motion can be described by known non-dimensional correlations for clean bubble rise in pure fluids.

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In this work, the effect of an electrolyte (up to 2 M of NaCl dissolved in water) on a homogeneous dense bubbly flow, in an airlift bubble column, is studied using nonintrusive techniques. X-ray and high-speed imaging are used to investigate the bubble size distribution, the local and the global gas-fraction profiles. The major effect of the electrolyte is the bubble size distribution at the fine-pore sparger, which is a consequence of the bubble coalescence inhibition promoted by the electrolyte. The bubble plume widening, the increase in overall gas fraction, and the onset of bubble recirculation in the column can all be explained by the bubble size reduction at the fine-pore spargers. As a result of the bubble size reduction, the overall role of the electrolyte is in a reduction of the driving force for the liquid recirculation. Furthermore, an accumulation of the small bubbles causes a layer of foam at the free surface, which is dynamic in nature and induces additional bubble recirculation.

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