Examining the effect of length/width ratio on the hydro-dynamic behaviour in a DAF system using CFD and ADV techniques

Abstract

Dissolved air flotation (DAF) is a solid–liquid separation system that uses fine bubbles rising from the bottom to remove particles in water. In this study, we investigated the effect of L/W(L; length, W; width) on the hydrodynamic behavior in a DAF system using CFD (computational fluid dynamics) and ADV (acoustic Doppler velocimetry) technique. The factual full-scale DAF system, L/W ratio of 1:1, was selected and various L/W ratio conditions (2:1, 3:1, 4:1 and 5:1) were simulated with CFD. For modelling, 2-phase (gas–liquid) flow equations for the conservation of mass, momentum and turbulence quantities were solved using a Eulerian–Eulerian approach based on the assumption that a very small particle is applied in the DAF system. Also, for verification of CFD simulation results, we measured the actual velocity at some points in the full-scale DAF system with the ADV technique. Both the simulation and the measurement results were in good accordance with each other. We concluded that the L/W ratio and outlet geometry play an important role for flow pattern and fine bubble distribution in the flotation zone. In the ratio of 1:1, the dead zone is less than those in other cases. On the other hand, in the ratio of 5:1, the fine bubbles were more evenly distributed.