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

Abstract

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 ReD = 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, L1, 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, L1 = L2 = 0, and a bend radius of rb≥ 2dh 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 L1 = 20dh, a secondary length of L2 = 0 and a bend radius of rb = 2.5dh.