Numerical Modelling of Dissolved Air Flotation

Licentiate thesis, 2009

Dissolved Air Flotation (DAF), a well-established treatment method for water containing e.g. dissolved organic matter and Cryptosporidium, has previously been examined experimentally. However, most measuring techniques still suffer from disturbances from air bubbles and intrusion of the measuring equipment into the flow. Consequently, researchers have turned to Computational Fluid Dynamics (CFD) and numerical models of flotation tanks have been developed with the aim of increasing knowledge and efficiency of the process. In this licentiate thesis, one- and two-phase, steady-state numerical models of a down scaled DAF tank are evaluated. All modelling choices are made with a compromise struck between computational cost and the required accuracy of the flow. The geometry of the tank is modelled in GAMBIT and the flow simulated in the CFD software FLUENT 6.3. The turbulence is modelled using the standard k-epsilon model with standard wall functions in the one-phase model and using the realizable k-epsilon model with non-equilibrium wall functions for the two-phase model. The multiphase flow of air and water is solved in the Eulerian-Lagrangian frame of reference. The simulated flow is compared to experimental measurements for validation. The option of making a two- or a three-dimensional model is discussed further in the study and parameters influencing the modelling, such as the outlet geometry, the inlet flow and the air bubble size, are also examined. A two-dimensional model requires adjustments in the geometry and in the parameters governing the flow since the models do not take into account the three-dimensional effects. Simulations show that a two-dimensional, one-phase model can capture the flow in the separation zone reasonably well, but that a three-dimensional model is required if the flow in the contact zone is to be studied. Although a stratified flow shows both steadiness in time and two-dimensional nature of the flow, a two-dimensional, two-phase model should be used with caution. A three-dimensional, two-phase model would reflect the flow in a DAF tank more truthfully, but requires more computer capacity and results indicate that a transient solver is required.