Experimental and numerical investigation of reactive species transport around a small rising bubble

Abstract

In this article, we present experimental and numerical techniques to investigate the transfer, transport, and reaction of a chemical species in the vicinity of rising bubbles. In the experiment, single oxygen bubbles of diameter d b = 0.55 . . . 0.85 mm are released into a measurement cell filled with tap water. The oxygen dissolves and reacts with sulfite to sulfate. Laser-induced fluorescence is used to visualize the oxygen concentration in the bubble wake from which the global mass transfer coefficient can be calculated. The ruthenium-based fluorescent dye seems to be surface active, such that the rise velocity is reduced by up to 50 % compared to the experiment without fluorescent dye and a recirculation zone forms in the bubble wake. To access the local mass transfer at the interface, we perform complementary numerical simulations. Since the fluorescence tracer is essential for the experimental method, the effect of surface contamination is also considered in the simulation. We employ several improvements in the experimental and numerical procedures which allow for a quantitative comparison (locally and globally). Rise velocity and mass transfer coefficient agree within a few percents between experiment, simulation and literature results. Because the fluorescence tracer is frequently used in mass transfer experiments, we discuss its potential surface activity.

Determination of Interfacial Concentration of a Contaminated Droplet from Shape Oscillation Damping

“…In particular, the Marangoni stresses at a drop surface that result from buoyancy-induced motion strongly reduce the settling velocity as compared to clean conditions [3][4][5][6][7]. The presence of adsorbed contaminants leads generally to discrepancy between experimental observations and simulation predictions [8,9], making difficult the establishment of reliable predictive laws. However, measuring minute interfacial concentrations of a droplet is challenging and beyond the reach of previous experimental techniques.…”

Data‐Driven Subgrid‐Scale Modeling for Convection‐Dominated Concentration Boundary Layers

“…A single adjustable parameter assures that the cell average of the analytical solution is in agreement with cell‐centered values coming from the FV discretization. This local adjustment is very accurate and robust .…”

“…For the SGS modeling of reactive‐species boundary layers, the substitute problem needs to be modified in several aspects: (1) Along a planar interface, the boundary layer of the transfer species reaches a steady state when diffusion and reaction are in equilibrium. Therefore, it becomes more difficult and sometimes impossible to adjust the profile to a more complex setting such as a rising bubble . To overcome this limitation, convection in the direction normal to the interface must be included.…”

“…Reactive‐species boundary layers, however, are more complicated because several chemical species may interact in a reaction. Additionally, the asymptotic behavior of the reactive planar 2D problem mentioned before is not sufficient to explain transport mechanisms encountered at a rising bubble .…”

“…They reported maximum errors of 4 % for the integral mass transfer and a wide range of Sc numbers ( Sc = 10 2 to 10 4 ). The advantage of such SGS models is not only the improved accuracy but also the ability to investigate coupled phenomena, e.g., the influence of surface contamination on hydrodynamics and mass transfer , , whereby the correct ratio of different timescales is essential.…”