Assessing Baffle Minor Loss for the Design of Hydraulic Flocculators

Flocculation is a key step in treating drinking water for pathogen removal. Baffled hydraulic flocculators (BHFs) have many advantages, including plug-flow reactor kinetics, ease of construction, and operation without moving parts. Optimal design for BHFs requires the appropriate velocity gradient, which depends primarily on the minor losses associated with the 180 degree redirection of flow around the ends of the baffles. While previous design guidelines have estimated these minor loss coefficients in the range of 2.5–4.0, and some work has been done to identify key geometric parameters impacting minor loss coefficients, no physics-based mathematical model has been presented for estimating minor loss coefficients a priori as a function of these geometric parameters. A dimensionally homogeneous physics-based model describing the relationship between the flow expansion and flow constriction length scales is derived in terms of baffle geometry and tuned to computational fluid dynamics (CFD) simulation results, using the realizable k-epsilon model and the Reynolds stress model on periodic domains. These results are then compared with baffle minor loss coefficient estimates presented in the literature, as well as with several plant-scale BHF minor loss measurements made in Central America. Across CFD simulations, hydraulic height to baffle spacing ratios varying from 2 to 10 corresponded to minor loss coefficients ranging from 9.29 to 2.55. The minor loss coefficients associated with height-to-spacing ratios less than six tended to be larger than the minor loss coefficients associated with fully expanded flow presented in the literature, with the largest minor losses occurring at the smallest height-to-spacing ratios. The mathematical model derived in this study is expected to better inform the initial design of BHFs to reduce the need for post-construction adjustments to BHFs.