Computational fluid dynamics characterization of a novel mixed cell raceway design

Computational fluid dynamics (CFD) analysis was performed on a new type of mixed cell raceway (MCR) that incorporates longitudinal plug flow using inlet and outlet weirs for the primary fraction of the total flow. As opposed to conventional MCR's wherein vortices are entirely characterized by the boundary conditions at inlet nozzles and outlet center drains in the center of each cell, the new MCR design can develop a wider variety of fluid behaviors due to the additional boundary conditions at the inlet and outlet walls where the weirs are placed. In this study, we investigated how the primary longitudinal flow would affect vortex formations in the cells by designing three different MCR models and simulating three major cases for each model. Through this process, performances of two numerical CFD models (transition k-kl-ω vs. k-ε) were compared, along with two vortex quantification methods (Q-criterion vs. a proposed method). We found that the k-kl-ω CFD model more accurately predicted vortex formation than the k-ε model. The three MCR models differed only by weir geometry or drain size, in order to see their individual influence on cell vortex formation. Each case had its own unique weir flow rate and center drain loading rate values that combined to a total flow rate resulting in 15-min hydraulic retention time (HRT) for the MCR. The ratio of (expressed as percentage) of center drain loading rate to total flow rate (R = 7.5%, 12.5%, and 20.1%.) was defined to establish a relationship between R and vortex strength or size. Simulations demonstrated that inlet weir aspect ratio impacted cell vortex formation and strength. Unlike weir geometry effects, the drain size had non-significant impacts on fluid behavior other than the velocity very near the drains. While R did have positive correlations with vortex strength, vortex size, and self-cleaning performance, an R of 20.1% was sufficient to create uninterrupted vortex formations. Too low of a center drain rate or R value can result in lack of any meaningful cell vortex formation which then obviates any self-cleaning action in an MCR. Our key finding through extensive computational analysis was that an R value of 20% was required in order to maintain effective vortex formation. Expressed more explicitly, this can be described as maintaining a center drain loading rate of 0.010094 m3/s per cell (160 gpm), which correspond to unit loading rates of 16.3 lpm/m2 per cell (0.40 gpm/ft2 per cell).