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A Microscopic View of Liquid-Liquid Film Flows

Ilja Ausner, Steve Paschke, Jens-Uwe Repke, Andreas Hoffmann, and Guenter Wozny. Technical University of Berlin, Strasse des 17. Juni 135 - Sekr. KWT 9, Berlin, 10623, Germany

Single-liquid film flows are the focus of past and current research and are quite well understood so far. But the flow behavior becomes more complex with the appearance of a second liquid phase as it can be observed e.g. in the distillation of heterogeneous azeotropic mixtures in packed towers also known as three-phase distillation. This changes the liquid flow behavior, which has a strong influence on the separation efficiency of the tower [1], and that makes the modeling of the three-phase distillation process much more difficult. The multiphase flow behavior is a very complex system of rivulet and droplet flow for one liquid phase and film flow for the second liquid phase on structured sheets of the packing, in addition to the vapor phase in counter-current flow.

For a better understanding of this flow behavior, first flow investigations of two immiscible liquids (here water and toluene) over inclined steel plates (50x60mm2) without forced gas or vapor flow were carried out. The multiphase flow was analyzed numerically with CFD as well as experimentally with optical measurement methods like particle tracking velocimetry (PTV) and fluorescence intensity measurement (LIF), which have been adapted to the multiphase flow conditions. Depending on different parameters, e.g. flow rates or contact angles, it was found that both liquids are overlaying each other. While toluene forms a closed film, the water phase flows as rivulets below the toluene film and as droplets above the toluene film. That forms a large interfacial area and leads to strong interactions between both phases, which mainly ends up in deceleration and stabilization of the liquid flow [1].

The modeling of the fluid dynamics requires the knowledge of the mechanism of these liquid interactions. Consequently, the next step is to change the macroscopic view to a microscopic scale. A newly developed micro-particle imaging velocimetry (micro-PIV) method [2] enables the investigation of the velocity field and the liquid flow behavior near the liquid-liquid interface. In this connection, the use of a non-translucent steel plate is a hard restriction to the optical access, which makes the imaging of the flow through the moving free liquid surface necessary.

In the presentation a summary of our comprehensive macroscopic investigations of the multiphase film flow will be given. Then the micro-PIV method will be presented, followed by first results of the microscopic flow investigations on liquid-liquid film flows.

[1] A. Hoffmann, I. Ausner, J.-U. Repke, G. Wozny: detailed investigations of multiphase (gas-liquid and gas-liquid-liquid) flow behaviour on inclined plates. Chem. Eng. Res. Des., 84(A2), 2006, pp. 147-154

[2] I. Ausner, S. Kallweit, G. Wozny: velocity measurements of film flow on inclined steel plates. In: 6th International Symposium on Particle Image Velocimetry (PIV'05), Pasadena, CA, USA, Sept. 21-23, 2005