Contact

Prof. Dr. Uwe Hampel
Head Experimental Thermal Fluid Dynamics
u.hampel@hzdr.de
Phone: +49 351 260 - 2772
Fax: 12772, 2383

X-ray tomography of Taylor bubbles in capillary two-phase flow

The project „X-ray tomography of Taylor bubbles in capillary two-phase flow” is conducted within the framework of the Priority Programme SPP 1506 “Transport processes at fluidic interfaces” funded by the German Research Foundation (DFG). Therein the guiding measure “Taylor flow” defines a systematic Taylor-bubble experiment, whose data is commonly used as a benchmark case for validation of mathematical models and numerical methods. Within this Priority Programme there is a notable cooperation between experimental groups (HZDR, TU Hamburg-Harburg) and numerical groups (KIT Karlsruhe, TU Darmstadt, RWTH Aachen, TU Dresden, OvGU Magdeburg, FAU Erlangen-Nürnberg).

New developments in microreactors, compact heat exchangers, microcondensers and fuel cells require precise knowledge of fluid dynamics in channels with hydraulic diameters in the millimeter and sub-millimeter range. Especially in microreactor applications, Taylor bubble flow is a desired operational state due to the frequent change of efficient gas-liquid contacting in the film around the bubbles and the enhanced turbulent mixing in the liquid slugs behind the bubbles. Knowledge of the flow topology and precise data of the liquid film thickness, bubble shape and liquid velocity profiles around bubbles on the microscopic scale is a decisive input for the development and validation of heat and mass transfer models as well as interface-resolving CFD codes. Within the frame of this project the development of an X-ray tomography method is aimed at, which can disclose the three-dimensional shape of Taylor bubbles in capillary two-phase flow.

An experimental two-phase flow setup is realized for the investigation of upward moving Taylor bubbles in capillary channels. The hydraulic diameter of the channels with quadratic and circular cross-section is dh = 2 mm. The bubble velocity is up to Ub = 20 … 200 mm/s. High-speed and high-resolution synchrotron X-ray visualization experiments are performed at a synchrotron beamline with a resolution of 5.6 µm pixel spacing within a maximum field of view of 5.7 x 5.7 mm² and a frame repetition rate of up to 36,000 frames per second. This enables the recording of the Taylor bubbles without motion artifacts. With a dedicated bubble tracking algorithm the signal-to-noise ratio is improved by utilizing redundant data in the recorded image stream. With that the shape of the Taylor bubble’s gas-liquid interface can be captured with an uncertainty below 2.5 µm. The repeatability error between consecutive measurements is also below a few micrometers. The implementation of highly dedicated image processing algorithms further allows an automated discrimination of the gas-liquid interface in the reconstructed images and thus reveals the moving bubble’s interfacial shape and their axial and radial variations. Moreover, tomographic reconstruction is used to measure the three-dimensional ensemble averaged shape for both front and rear tip of the flowing Taylor bubbles in square capillaries.

Fig 1: Experimental setup and principle of X-ray tomographic measurement.

References

  • S. Aland, S. Boden, A. Hahn, F. Klingbeil, M. Weismann, S. Weller.
    Quantitative comparison of Taylor Flow simulations based on sharp- and diffuse-interface models.
    International Journal for Numerical Methods in Fluids, 2013, doi:10.1002/fld.3802.
  • H. Marschall, S. Boden, Ch. Lehrenfeld, C. J. Falconi D., U. Hampel, A. Reusken, M. Wörner, D. Bothe.
    Validation of Interface Capturing and Tracking Techniques with different Surface Tension Treatments against a Taylor Bubble Benchmark Problem,
    2013. Submitted.

Funding

This project is funded by the German Research Foundation (DFG) within the Priority Programme SPP 1506 “Transport Processes at Fluidic Interfaces” (project HA 3088/7-1 in the period 2010-2013).


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Contact

Prof. Dr. Uwe Hampel
Head Experimental Thermal Fluid Dynamics
u.hampel@hzdr.de
Phone: +49 351 260 - 2772
Fax: 12772, 2383