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XFLOW Report Summary

Project ID: 307360
Funded under: FP7-IDEAS-ERC
Country: Germany

Final Report Summary - XFLOW (Ultrafast X-Ray Tomography of Turbulent Bubble Flows)

Multiphase reactors are the working horses of the chemical, petrochemical and pharmaceutical industries and most of our daily life products are produced using them. Basically, they contact gaseous and liquid streams to facilitate high transfer rates ensuring high product yield. Prominent examples are disperse gas-liquid flows, for example present in bubble column reactors, waste water treatment units or in distillation columns. However, such apparatuses involve millions of dispersed bubbles, emerging in swarms in a complex manner. The understanding of disperse gas-liquid flows, for example present in bubble columns or above trays in distillation columns, is still fragmentary. It requires a ground-breaking update to provide the fundamentals to improve the apparatuses in order to significantly contribute to savings in energy, greenhouse gases and catalyst materials.

Therefore, a team of engineers and scientists collaborated in the XFLOW project to show for the first time that turbulent bubble swarms can be made completely “transparent” with yet unprecedented spatiotemporal detail applying an advanced imaging technique.

An X-ray tomography system, which is the world-wide fastest imaging technique, was therefore adjusted to visualize gas bubble swarms in liquid columns forming characteristic flow patterns with coherent structures covering a wide range of operating conditions from homogeneous to highly turbulent and chaotic regimes. Proper reconstruction techniques and analysis tools were developed to provide process relevant parameters, such as gas phase distribution, gas velocity profile, bubble size distribution, bubble velocity and gas-liquid interfacial area density. A unique database of characteristic bubbly flow data has been established in the project. As a particular feature, this database includes data describing the liquid phase dynamics obtained from complementary radioactive particle tracking, a second advanced imaging technique, which makes the database unique and the first of its kind.

With these data, the instantaneous spectrum of patterns due to circulation zones was fundamentally deconstructed by recent statistical methods such as chaos analysis and information entropy theory to access hidden information on coherent and fractal structures, on the degree of turbulence and on the present flow regime. Such information is of great importance for design, operation and control of dispersed gas-liquid flows since the degrees of mixing, mass and heat transfer is strongly influenced. In addition, hydrodynamic correlations describing the effects of heat exchanger internals and spargers were derived based on a comprehensive hydrodynamic characterization.

Eventually, all findings were consolidated into Eulerian multi-fluid models for large-scale gas-liquid dispersed flow simulations. For the first time, the modelling of swarm effects such as bubble coalescence and breakage was based on ultrafast high-resolved data gathered at operating conditions that have never been accessible before.

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