Investigating the effects of perforated inlet baffles in a circular sedimentation tank using Computational fluid dynamics

Abstract

A full-scale three-dimensional numerical study on the effects of using perforated inlet baffles in a circular sedimentation tank was performed using a numerical model that was validated with experimental data. A total of 14 cases were investigated in this study — 7 geometries and two influent concentration groups. A two-phase steady-state simulation was performed for each case using the Eulerian multiphase model coupled with the k-ω SST turbulence model. The simulation results are then analysed both qualitatively and quantitatively. For the quantitative analysis, the weighted total settling efficiency is calculated after calculating the settling efficiency for each of the three particle sizes using the sediment concentration at the outlet weir surface. Significant improvement in the settling efficiency was observed due to the introduction of the perforated inlet baffles. An average improvement of 20.7 % was achieved for the lower influent concentration group, while an average improvement of 9.6 % was achieved for the higher influent concentration group. For the qualitative analysis, three performance indicators were examined — volume fraction of sediments along a vertical line near the outlet weir, the contour of turbulence kinetic energy in the inlet region along with the contour of velocity magnitude, and three-dimensionally rendered volume fraction distribution of clarified water/sediments in the whole geometry. The best-performing sedimentation tank was found to be the one that is equipped with a perforated stilling well and a perforated McKinney baffle. A good agreement is observed between the findings from the qualitative analysis and the quantitative analysis.