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 large 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.
The GENTOP concept combines the AIAD model with the inhomogeneous MUltiple SIze Group (iMUSIG)-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. Simulations performed with the CFD-code CFX demonstrate the principle of the new concept.
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.
Furthermore the GENTOP concept is combined with phase transfer models (Fig. 4). For example a wall boiling model was included in the GENTOP framework. A demonstration case shows the appearence of smaller bubbles at the wall and larger gas structures at the center of a verticla pipe (Fig. 5).
|Fig 4. Scheme of the extended GENTOP model including phase transfer|
|Fig. 5 Distribution of the gas volume fraction at 2.0 s (stretched height)|
|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.|
Hänsch, S.; Lucas, D.; Krepper, E.; Höhne, T.
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International Journal of Multiphase Flow 47(2012), 171-182
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