Experimental investigation and modeling of local mass transfer rates in pure and contaminated Taylor Flows
Sven Kastens, M.Sc. (2nd Project period),
Dipl.-Ing. Christoph Meyer (1st Project period)
In nature and most applications for Chemical, Bio-chemical, Medical and Environmental Engineering, the transport of molecules from one phase into another phase (gas/liquid or liquid/liquid) plays a dominant role to achieve high yields and purities. For a process optimization, a deep understanding of local transport processes at the fluidic interphases is essential. In Priority Programme SPP1506 "transport processes at fluid interfaces" of the DFG, these transport processes are investigated under defined conditions in sequential gas / liquid flows, so-called Slug - or Taylor Flows (Fig. 1. left), and at single Taylor Bubbles (Fig.1, right) rising in vertical capillaries.
Taylor Flows owing to their defined bubble shape, interfacial and hydrodynamic adjustability for the very study of the coupling of hydrodynamics and mass transport.
Experimental Setup "Taylor Flows"
Fig.2: Left: scheme experimental setup for µPIV Measurements in the slug between two bubbles. Right: Post Processing of the raw image to flow structure and the velocity field in the slug as result.
Fig.3: Left: image of experimental setup for µPIV Measurements in the lliquid film. Right: Comparison of simulation and experiments of the liquid velocity in the film.
Interface of Taylor bubbles is mobile and “crawls” along the channel wall
No-slip condition at the channel wall and the large viscosity ratio - low liquid velocities in the film.
Using micro particle image velocimetry velocity profiles and velocity fields can be measured and visualized by vector fields. This is illustrated in the local flow between bubbles in a Taylor flow (see Figure 2, right).
The analyses of the direct and indirect influence of surfactants (ionic and non-ionic surfactants) on the transport processes in such systems is of great interest, because even slight contamination may have already an influence on the bubble shape and the flow around them and can influence processes massively (see Fig. 4, compare left and right). Moreover, in most industrial processes, small contamination of fluids can not be avoided or are wanted, that the transfer of results from model processes to real industrial processes are challenging. Therefore, the deep investigation of the effect by surfactants can optimize the economic design of multiphase reactors.
Fig.4: Left: bubble shape and velocity field around the bubble rear in a pure system. Right : bubble shape and velocity field around the bubble rear in a contaminated system. Changes of the Flow field are marked with a red circle.
2nd Project period, Taylor Bubble
For detailed and time-dependent analysis of local transport processes at fluidic interfaces single Taylor Bubbles are examined in rising vertical channels. Those Taylor bubbles show the unusual behavior of a volume independent rise velocity and can be kept in the observation area by a constant counter-current flow (see Set-up in Figure 5 left). This enables to investigate the global and local transport processes, for example during the dissolution process of CO2 bubbles (section) channels with different cross-sectional geometries (see Figure 5, right).
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Fig. 5: Left: Schematic experimental Set-up for investigations at single Taylor Bubbles. Right: Single Taylor Bubbles of the same gas volume in different channel sizes with circular and square cross section.
