Single- and two-phase numerical models of Dissolved Air Flotation: Comparison of 2D and 3D simulations

Abstract

In this work single- and two-phase flow numerical models of a pilot DAF (Dissolved Air Flotation) tank are presented. In the latter framework, the mixture in the tank consists of water and air bubbles. The motion of air bubbles is resolved within the Lagrangian frame of reference. Interaction between bubbles is neglected in the present model. To account for turbulence in the tank, the realizable k– ɛ model is selected as a compromise between a need to correctly resolve the flow and the computational cost of the calculations. To validate the numerical simulations the results are compared with experimental results of an independent research group. The paper discusses the choice between performing two (2D)- or three-dimensional (3D) simulations of a flotation process. Our simulations show that a 3D hydrodynamic model of the flow pattern can well reproduce the single-phase flow in the pilot tank. Despite the major changes in the flow pattern that occur when the second phase (air bubbles) is introduced, the validated single-phase model provides a useful tool in the process of setting up the two-phase model. We show in the paper that a 2D two-phase model cannot resolve successfully the flow in the contact zone, whereas major flow characteristics in the separation zone can, to some extent, be described by a 2D approach (e.g. the stratification of the flow). In addition, two-phase simulations demonstrate that a 3D model is sensitive to the distribution of air and water from the needle valves, which, in turn, influences the stability of the solution obtained.