Dissolved Air Flotation

Abstract

One of the well known intermediate stages to treat water and wastewater is Dissolved Air Flotation (DAF). In order to design an efficient DAF unit, it is important to study the system and get familiar with the flow structure in it. Computational Fluid Dynamics (CFD) is used as a tool to study a flotation unit.Most of the previous CFD studies of DAF have been set up in two dimensions in order to reduce the required computational time and space. By modeling only in two dimensions the effects of the flow from the needle valves is excluded. This study aims to consider these effects through three-dimensional modeling. The aim is to study the contact zone considering variables affecting this part of the DAF unit. These variables are number of needle valves, tank geometry, side wall effects and presence of the separation zone.In this study two-and three-dimensional representations of the tank are evaluated. Simulations account for both one-phase and two-phase flow and the flow is assumed to be in steady state. The geometry of the tank is modeled in GAMBIT and exported to FLUENT 12 to perform the flow simulations. The flow in the tank is assumed to be turbulent, which is modeled with the standard and the realizable k-ε models. Standard wall function and non equilibrium wall functions are used for these models respectively. The Lagrangian model is used to simulate the two-phase flow consisting of air bubbles and water. Validation of the results is made by comparing the simulated flow with previously measured flow patterns in a pilot DAF tank.Results show that convergence is very connected to the outlet location and the further away the outlet is located from the contact zone the easier it is to reach a converged solution. The number of needle valves included in the model and the inclusion of the side walls are parameters found to have an effect on the flow structure in contact zone. The two-phase flow result predicts a movement of air bubbles toward the inlet zone a phenomenon not observed in the experimental measurements.