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 db=0.55…0.85mm 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.
Highlights
The mass transfer from a gaseous, dispersed phase into the surrounding liquid phase is a major task within the chemical industry as well as in bio, food- or environmental engineering
We investigated the same ratio of convective to reactive timescale Da, meaning that we decreased the reaction rate corresponding to the decrease in the rise velocity
We investigated the reactive mass transfer from small rising oxygen bubbles by means of experiments and numerical simulations
Summary
The mass transfer from a gaseous, dispersed phase into the surrounding liquid phase is a major task within the chemical industry as well as in bio-, food- or environmental engineering. Semi-empirical correlations for the Sherwood number (Sh) typically have the form of Eq (1) below. The idea behind the equation is as follows: the transport of a passive chemical species around a fluid particle rising in a stagnant liquid may be characterized by the Reynolds (Re 1⁄4 dbUb=m) and Schmidt (Sc 1⁄4 m=D) number, where db is the bubble diameter, Ub is the rise velocity, m is the liquid side kinematic viscosity, and D is the molecular diffusivity of the dissolved gas. 0, the Sherwood number of a spherical particle should approach Sh 1⁄4 2 and not zero. This theoretical considerations presumably led Brauer (1971) to suggest the empirically corrected equation
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