Setting Up a Numerical Model of a DAF Tank: Turbulence, Geometry, and Bubble Size
Publication: Journal of Environmental Engineering
Volume 136, Issue 12
Abstract
This paper discusses the modeling framework and identifies a number of parameters relevant when setting up a computational fluid dynamics simulation of a dissolved air flotation (DAF) tank. The selection of a turbulence model, the choice between performing two-dimensional (2D) or three-dimensional (3D) simulations, the effects of the design of the flow geometry and the influence of the size of the air bubbles are addressed in the paper. The two-phase flow of air and water is solved in the Eulerian-Lagrangian frame of reference. The realizable model with nonequilibrium wall functions is suggested as a compromise between a need to effectively resolve the flow and the cost of the simulations. There is a discussion on the conditions for which the steady-state simulations are appropriate. We demonstrate that a steady 2D model can simulate a stratified flow pattern. Our results show that 2D models require adjustments in geometry (e.g., substitution of the outlet pipes to an outlet distributed over the total width of the tank) and in the parameters governing the flow in order to account for the true 3D nature of some of the flow patterns. In addition, we show that the bubble size has a larger influence on the flow in the separation zone than in the contact zone.
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Acknowledgments
The writers would like to thank Måns Lundh for providing all experimental data used in this paper. Financial support from the Swedish Research Council for Environment, Agricultural Sciences and Spatial Planning (FORMAS), the Swedish Water and Wastewater Association through the Dricks project, the TECHNEAU project funded by the European Commission and, finally, the European Cooperation in Science and Technology (COST) are gratefully acknowledged.
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Information & Authors
Information
Published In
Journal of Environmental Engineering
Volume 136 • Issue 12 • December 2010
Pages: 1424 - 1434
Copyright
© 2010 ASCE.
History
Received: Nov 9, 2009
Accepted: Apr 25, 2010
Published online: May 27, 2010
Published in print: Dec 2010
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