Contact

Dr. Stephan Boden
HZDR Innovation GmbH
s.bodenAthzdr.de
Phone: +49 351 260 3773

Dr. Markus Schubert
Experimental Thermal Fluid Dynamics
m.schubertAthzdr.de
Phone: +49 351 260 2627
Fax: +49 351 260 2383

Taylor bubbles in small hydraulic channels

Micro- and mini-structured devices are of particular interest for reaction as well as heat and mass transfer in the process engineering area. For two-phase operation, the Taylor flow is a preferable flow regime, where high gas-liquid mass transfer and well defined residence times can be achieved. The Taylor bubble flow is characterized by an intensified gas-liquid contact in the liquid film surrounding the Taylor bubbles and by an enhanced mixing in the liquid slugs downstream the Taylor bubbles.

In the framework of the German Research Foundation (DFG) funded Priority Programme DFG-SPP 1506 “Transport processes at Fluidic Interfaces” the “Taylor Flow” was defined as a benchmark for the validation of mathematical models and numerical methods. In the works done at HZDR the integral and local mass transfer for Taylor bubbles with different equivalent bubble diameters ranging from 0.5 mm to 10 mm in channels with different shapes and roughness was measured. In particular, the effect of surfactants on mass transfer is clarified and explained by analyzing interface surfactant concentration and surface tension locally.

To disclose the three-dimensional shape of Taylor bubbles in small channels here at HZDR enhanced X-ray radioscopic and X-ray tomographic measurement techniques were further developed and qualified. The experimental studies were conducted utilizing an X-ray microfocus imaging device. Single Taylor bubbles were immobilized at a fixed vertical position via countercurrent liquid flow in channels with hydraulic diameters of 6 mm. Three-dimensional tomographic images were captured at an effective spatial resolution of 27 µm. Thereby, the characteristic non-symmetric Taylor bubble shape in square channels was disclosed for the first time, which was not accessible before using optical methods.



Utilizing these results, an X-ray radioscopic measurement technique was qualified for the measurement and evaluation of the dynamic mass transfer of Taylor bubbles considering also the effect of surfactants. Therefore, the dissolution rate of single immobilized CO2 Taylor bubbles into water was unceasingly monitored using microfocus X-ray radiography and tomography techniques. The liquid-side mass transfer coefficient is calculated by measuring the change in the size of the bubble at constant pressure and correlated in terms of a modified Sherwood number.



Further, experiments at the synchrotron radiation source ANKA (KIT Karlsruhe) enabled exact measurements of the Taylor bubble shape in channels with different cross-sections. At an effective spatial resolution of 5.6 µm and at an exposure times of only 1/36,000 seconds with corresponding high frame rates, even smallest morphological details such as the interface curvature at bubble front and rear were resolved. Parametric experimental studies were conducted for different channel cross-sections (circular, square) and superficial velocities (Ub = 20-320 mm/s) in aqueous glycerol solutions (Ca = 0.01-0.16). The resulting data are provided for the validation of numerical flow simulation tools (“Interface Capturing and Tracking”-methods, “Sharp Interface and Diffuse Interface”-methods).



Additionally, the behaviour of Taylor bubbles in mechanically agitated channels was investigated. Bubble size and dissolution rate were determined from microfocus X-ray radiographs and the liquid-side mass transfer coefficient was calculated from the shrinking rate. The rise velocity of the bubbles and the surface wave motion were analyzed using a videometric technique. The comparison of the results for the stationary and the oscillating channel showed that mechanical vibration of the channel is able to enhance the mass transfer coefficient from gas to the liquid phase by 80 % to 186 %, depending on frequency and amplitude of the vibration, which is mainly attributed to the development of propelling surface waves and to the increase of the liquid film flow rate. Furthermore, analyzing the surface wave motion of the bubbles revealed that the enlargement of the contact area between the phases and the increased mixing enhance the mass transfer additionally up to 30 % compared to non-agitated bubbles of similar Péclet number.



Cooperation

  • TU Darmstadt,
  • RWTH Aachen,
  • KIT Karlsruhe,
  • TU Hamburg-Harburg,
  • FAU Erlangen-Nürnberg,
  • OvGU Magdeburg,
  • TU Dresden,
  • ANKA synchrotron radiation source - KIT Karlsruhe

Funding

Deutsche Forschungsgemeinschaft (DFG, HA3088/7–1,  HA3088/7-2)

References


Contact

Dr. Stephan Boden
HZDR Innovation GmbH
s.bodenAthzdr.de
Phone: +49 351 260 3773

Dr. Markus Schubert
Experimental Thermal Fluid Dynamics
m.schubertAthzdr.de
Phone: +49 351 260 2627
Fax: +49 351 260 2383