Evaluation of Large Eddy Simulation and RANS for Determining Hydraulic Performance of Disinfection Systems for Water Treatment

Reynolds-Averaged Navier–Stokes (RANS) simulation has been demonstrated to be a powerful and efficient approach for conducting numerical assessment of the hydraulic performance of disinfection systems for water treatment at a much lower cost than physical experiments. Recently, large eddy simulation (LES) has been introduced for the first time as a potentially more accurate alternative to RANS for predicting hydraulic performance of disinfection systems such as baffled contactors (Kim et al., 2010, “Large Eddy Simulation of Flow and Tracer Transport in Multichamber Ozone Contactors,” J. Environ. Eng., 136, pp. 22–31). This gives rise to the need to carefully assess RANS and LES in order to understand under which flow characteristics LES should be recommended instead of the less computationally intensive RANS for predicting hydraulic performance of a disinfection system. To that extent, this manuscript presents results from RANS and LES simulations of flow and tracer transport in a laboratory-scale column contactor and a laboratory-scale baffled contactor. Flow fields, residence time distributions, and characteristic residence times are analyzed. LES is shown to be a more reliable strategy than RANS in simulating tracer transport in column contactors due to its ability to better predict the spatial transition to turbulence characterizing the flow. However, in baffled contactors where such transition does not occur and the flow is characterized by a quasi-steady short circuiting jet and dead zones, RANS performs on par with LES.