A Perforated Baffle Design to Improve Mixing in Contact Tanks

In this study, a perforated baffle design is proposed to improve mixing in contact tanks. Turbulent flow through the perforated baffle is studied at the perforation hole scale. The contribution of jets emerging from the perforations to the mixing process is evaluated in terms of standard mixing indexes for various perforation parameters, such as the solidity ratio and hole diameter. Based on numerical simulation results, the two sets of perforated baffles that yielded the highest performance were manufactured from polycarbonate and tracer studies were conducted on a laboratory model. Comparison of numerical and experimental results demonstrates that the numerical model developed is reliable in simulating the flow through the perforated baffles and the associated mixing level in the contact tank. Numerical simulations indicate that the jet flow structure through the perforated baffle penetrates to the recirculation zones in the neighboring chambers and turns the dead zones into active mixing zones. Furthermore, large scale turbulent eddies shed by the perforations contribute to the mixing process in the chambers of the tank. With the use of the perforated baffle design, it is shown that the hydraulic efficiency of the tank can be improved from average to superior according to the baffling factor, and the associated mixing in the proposed design can be improved by 31% according to the Morrill index.

Effect of Porous Baffles on the Energy Performance of Contact Tanks in Water Treatment

Three-dimensional numerical simulations are performed to evaluate the effect of porous baffles on the efficiency of water treatment contact tanks. A second-order accurate numerical model is employed for the solutions of unsteady flow and tracer transport through the porous baffles. The flow through the porous medium is characterized while using the Darcy-Forchheimer relationship. Large Eddy Simulation (LES) model is used to simulate the instantaneous mixing of the tracer in the chambers of the contact tank. Three different porosities are considered to evaluate the effect of porosity on the hydraulic and mixing efficiencies of the contact tank. Simulated time-averaged flow field shows that porous baffles that are placed at the entrance of each chamber could successfully mitigate short-circuiting and yield plug-flow conditions through the system for low porosities. Flow in the contact tank becomes laminar as the flow velocities decrease due to viscous effects and inertial resistance in the porous zone. For this case, the tracer is transported with bulk flow through the system and leaves the contact tank with a high peak seen in the Residence Time Distribution (RTD) plot. Porous layer increases the hydraulic efficiency of the conventional design from “poor” to “good” according to the baffling factor and increases the overall efficiency from “compromising” to “good” according to the AD index. Comparison of the performance of the porous layer with the previously developed slot-baffle design shows that the slot-baffle design increases the efficiency of the tank with increasing dispersion effects, whereas the porous design increases hydraulic efficiency and reduces the dispersion effects. While the porous design reduces energy efficiency by 33% due to a drastic increase in drag in the flow through porous zone, the slot-baffle design increases the energy efficiency of the conventional design by 67%.