On the design of compact hydraulic pipe flocculators using CFD-PBE

Designing a compact hydraulic pipe flocculator is a common challenge for various water purification and wastewater processes where space is limited. Various geometrical parameters are analysed to identify the most important parameters when designing an efficient system for a Reynolds number of Re D = 20, 000. A coupled CFD-PBE model is applied to a total of 123 geometrical configurations to simulate particle aggregation and breakage due to the local velocity gradients in the configurations. The shear present in the 90∘ pipe bends is the dominating factor in the final aggregate size and therefore the most important geometrical factor is the bend radius. Secondly, it is observed that the primary length, L 1, has the second-largest impact as a linearly increasing particle diameter is observed along the straight pipe. Helically coiled geometrical configurations with no straight sections, L 1 = L 2 = 0, and a bend radius of r b≥ 2d h result in large particles as a constant but moderate cross-sectionally averaged turbulent energy dissipation is observed throughout the pipe. The largest volume-averaged particle size is observed for a configuration with a primary length of L 1 = 20d h, a secondary length of L 2 = 0 and a bend radius of r b = 2.5d h.