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Abstract
The effect of baffle spacing on the hydrodynamics and transport characteristics in a scaled variable-baffle-spacing contact tank is investigated by employing large-eddy simulation. Simulated solute transport is quantitatively validated by comparing simulated and measured residence time distribution curves, for which good agreement is found. The flow in serpentine disinfection tanks is dominated by a path of high streamwise velocities, a so-called short-circuiting path, which is almost constant in width regardless of the baffle spacing. In addition, large-scale turbulent vortices are shed from baffle edges and their size and dynamics depend on the spacing between subsequent baffles. The transport in serpentine-flow contact tanks is advection dominated and transport occurs mainly along the short-circuiting path. Wider compartments feature large recirculation zones with instantaneous vortices entraining solutes into these zones and thereby increasing residence times. Contact tanks with wide compartments suffer from severe short-circuiting and internal recirculation. This is directly reflected in hydraulic efficiency indicators.
Acknowledgements
The research is partially funded by the US Environmental Protection Agency (Cooperative Agreement Nos. CR-83327701 and 834118). Provision of experimental data by Dooil Kim of Dankook University is gratefully acknowledged.
Notation
| Acompartment | = | cross-sectional area at the inlet of the tank |
| B | = | contactor width |
| C | = | filtered tracer concentration |
| C0 | = | total tracer released |
| D | = | molecular diffusivity |
| Δt | = | time step |
| Dt | = | turbulent eddy diffusivity |
| E | = | non-dimensional tracer concentration |
| F | = | cumulative non-dimensional tracer concentration |
| H | = | water level |
| k | = | turbulent kinetic energy |
| νsgs | = | SGS viscosity |
| Q | = | discharge |
| = | Reynolds number | |
| = | molecular Schmidt number | |
| = | turbulent Schmidt number | |
| T10 | = | time when 10% of the released tracer has passed through the tank |
| T90 | = | time when 90% of the released tracer has passed through the tank |
| tHRT | = | theoretical hydraulic residence time |
| ⟨ v ⟩ | = | time-averaged velocity in the y-direction |
| Vabs | = | absolute velocity |
| ⟨ w ⟩ | = | time averaged velocity in the z-direction |
| W | = | baffle spacing |
| win | = | inlet bulk velocity |
| We | = | average width of the short-circuiting path |
| x, y, and z | = | Cartesian coordinates |
| ωy | = | instantaneous spanwise vorticity |