Impact of Turbulent Mixing on the Performance of a CFD Chloramine Model

Abstract

A study was performed to investigate the impact of turbulent mixing on the performance of a computational fluid dynamics (CFD) chloramine model. Experimental tests were performed in a pilot-scale reactor where the turbulent flow field was altered by the placement and removal of a perforated baffle at the reactor inlet upstream from three ammonia injection mixers (cone-, three-bar-, and T-bar-diffuser). As part of this study, several turbulence models were incorporated into a CFD model to investigate the impact of the selection of turbulence models on the predicted free-chlorine residual. The turbulence models examined in this study included the standard k-ξ, RNG k-ξ, and k-ω models. Furthermore, a sensitivity analysis was performed to investigate the impact of several turbulent mixing time scales [the eddy-dissipation time scale (EDT), Kolmogorov time scale (KOLM), and Corrsin time scale] on the predicted residual free chlorine concentration. All model predictions were compared with experimental free-chlorine residual measurements. Results showed that a baffle, placed upstream from the injection point to reduce the strong inlet jet, has significant impact on the mixing and the formation of chloramine for the T-bar diffuser mixer case. In addition, the CFD chloramine model with the standard k-ξ or RNG k-ξ turbulence model was able to predict the downstream formation of chloramine for a cone-shape diffuser and a three-bar diffuser. Results also showed that the CFD chloramine model with the EDT or KOLM time scale was able to predict the downstream formation of chloramine for the cone-shape diffuser and the three-bar diffuser. However, the CFD chloramine model with different turbulence models constantly overpredicted the residual free chlorine for a T-bar diffuser. In addition, the CFD chloramine model with the Corrsin time scale was found to improve the prediction of the free chlorine spatial distribution for the T-bar diffuser. Finally, CFD chloramine models better predicted the residual free chlorine for all three diffuser mixers with the baffled inlet configuration.