CFD modelling of helically coiled tube flocculators for velocity gradient assessment

Helical tubes are used as reactors in applications such as food processing and water and wastewater treatment. In water and wastewater treatment plants, helically coiled tube flocculators (HCTFs) provide efficiency gains over the more traditionally used baffled tanks. Their superior performance has been credited to more favourable velocity gradients (G) but detailed fluid dynamics information on the response of such reactors to varying design and operational conditions is still lacking. In this study, three-dimensional computational fluid dynamics (CFD) simulations were conducted to address this shortcoming. A validated CFD model of HCTFs was applied to assess the impact of varying reactor diameter and operating flow rate on the distributions of G, axial velocity and secondary flow structures. The developed flow region of the reactor was characterised for the occurrence and corresponding response of two cross-section zones, which govern the reactor efficiency. An equation is proposed associating G with a normalised parameter involving the reactor torsion, curvature and Reynolds number, which can be used to support the rational design, optimisation and operation control of HCTFs.

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