A NUMERICAL-MODEL OF FLOW IN CIRCULAR SEDIMENTATION TANKS

Single-phase flow characteristics of two circular sedimentation tanks were investigated numerically using a computational fluid dynamics pro gram called CFDS-FLOW3D and the k-epsilon turbulence model. Results we re compared with salt tracer experiments for a scale-model of a primar y clarifier and a full-scale humus tank. The centre-fed circular clari fiers were modelled in two dimensions, viz. radius and depth. The inle t flow boundary was modelled both as a vertical inlet pipe (not attemp ted before) and a horizontal inlet flow between the inlet pipe and the water surface. A time-stepping algorithm for flow rate was used to mo del the diurnal variation in flow to the full-scale tank. The residenc e time distributions (RTD) for both computations agreed favourably wit h the tracer studies. Results indicated that the model of a vertical i nlet instead of a horizontal inlet did not improve the correlation wit h experimental data for the pilot-scale tank. Modelling the diurnal va riation in flow to the full-scale tank significantly improved the corr elation with experimental data.