Generalizing the effects of the baffling structures on the buoyancy-induced turbulence in secondary settling tanks with eleven different geometries using CFD models

Abstract

Computational fluid dynamics (CFD) models are applied to understand the effects of baffling structures on the buoyancy-induced turbulence flow in secondary settling tanks (SSTs). Eleven SST geometries, including four circular SSTs and seven rectangular SSTs, are evaluated in this study. The main research tool is a Fluent-based two-dimensional SST model. The model is verified with observations from literature. The validated model is then applied to compare the predictions of the effluent suspended solids (ESS) by the buoyancy-decoupled and buoyancy-coupled turbulence models on each SST geometry. Next, the effects of the SST baffling structures on the turbulence properties are compared and quantified. Additionally, stress tests are simulated on the SSTs, to predict their performances using both the buoyancy-decoupled and buoyancy-coupled turbulence models.The results for the original flow conditions show that the effects of the SST baffling structures on the buoyancy-induced turbulence can be divided into three zones: Zones A, where the baffling structures have negligible damping effects, and only the buoyancy-coupled turbulence model provides accurate predictions; Zone B, where the baffling structures partially dissipate the effect of buoyancy on turbulence, and the buoyancy-coupled model continues to provide accurate predictions but the buoyancy-decoupled turbulence model only provides qualitative but similar predictions; and Zone C, where the baffling structures fully dissipate the effect of buoyancy on turbulence, and both models provide similar, accurate predictions. Two indicators, the densimetric Froude number and the differences in turbulent kinetic energy can be used to predict the need for buoyancy coupling.