Contact

Dr. Eckhard Krepper
Computational Fluid Dynamics
e.krepperAthzdr.de
Phone: +49 351 260 - 2067
Fax: +49 351 260 - 12067

Dr. Dirk Lucas
Head Computational Fluid Dynamics
d.lucasAthzdr.de
Phone: +49 351 260 - 2047
Fax: +49 351 260 - 12047

Concept of a generalized CFD-Model for Multiphase Flows


Numerical simulations of multiphase flows in industrial processes require the understanding of many different flow regimes. The scales of interest range from homogeneous-disperse bubble columns in chemical engineering applications to strictly separated gas structures during the fluid transport in long pipelines. For the modelling of such large systems the Euler-Euler method has turned out to be the most efficient approach enabling the use of coarse computational grids. A huge number of closure models has been developed to describe specific flow regimes. However, in many practical flow applications separated and polydispersed flow regimes occur simultaneously and show strong interactions (see Fig. 1). An example is the impingement of a liquid jet on a free surface when continuous gas above the water level is entrained and breaks up into lots of bubbles with different sizes. The inverse mass transfer is for instance the formation of large continuous Taylor bubbles out of a dispersed bubbly flow in a vertical pipe showing slug flow regime. Such flow situations turn out to be multi-scale problems regarding the interfacial structures. They play a major role in many industrial processes with high gas volume fractions forming heterogeneous bubbly flows. A new generalized concept was development for the description of such complex flow regime transitions.

Currently the concept extends the inhomogeneous MUltiple SIze Group (MUSIG)-Model by adding a continuous gas phase resolving its gas-liquid interface within the computational grid. By including appropriate models mass transfers between polydispersed and continuous gas phases are possible, including the appearance and evanescence of a particular phase. First simulations performed with the CFD-code CFX demonstrate the principle of the new concept.


Click on the picture for full view.

Fig. 1:
Multi-scale flow problems regarding the gas-liquid interfacial structures

Fig. 2:
Comparison of experimental and computational results for the first seconds after impingement of the liquid jet on the free surface

Fig. 3:
Bubble size distribution after reaching a quasi-steady state in the simulation

The simulation of the air entrainment process accompanying an impinging jet is used to demonstrate the breakup of a continuous gas phase and the associated appearance of polydispersed gas underneath the free surface. After the jet impingement mass is transferred from the free surface region into the different bubble size groups forming a characteristic bubble plume. The entire process of impingement, penetration and rise of entrained gas structures can be reproduced using the new approach as shown in Fig. 2. The bubble size distribution within the dispersed bubble plume shown in Fig. 3 agrees well with experimental data.


Acknowledgement

This work is carried out in the frame of a current research project funded by the German Federal Ministry of Economics and Technology, project number 150 1411.

Literature



Contact

Dr. Eckhard Krepper
Computational Fluid Dynamics
e.krepperAthzdr.de
Phone: +49 351 260 - 2067
Fax: +49 351 260 - 12067

Dr. Dirk Lucas
Head Computational Fluid Dynamics
d.lucasAthzdr.de
Phone: +49 351 260 - 2047
Fax: +49 351 260 - 12047