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Flocculation is an important process used by AguaClara to treat water. The process involves particle collisions and agglomeration to form flocs. Computational Fluid Dynamics was used to better understand the fluid dynamics in the reactor. The standard kK-e ε model was used for every simulation model. The pressure coefficient drop over one baffle turn is 3.75, which agrees with literature estimates. After a clearance height of one baffle width or greater, the pressure coefficient drop and the maximum velocity become approximately constant. Most of the energy dissipation occurs in the region after the turn over a distance of two baffle widths. An area of flow recirculation occurs near the center wall immediately after the turn. The pressure drop is not sensitive to the Reynolds number for a large range of inlet velocities. Better understanding of the flocculation dynamics will enable optimized particle agglomeration and break-up.
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Figure 3. Mesh of the Model (Click on figure for original size)
Figure 3 shows the overall mesh of the flocculator model. As can be seen, the mesh is fine near the turn and at the wallwalls. The final step at this point was to set up the boundary conditions of the system.
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2.6 Effect of Reynolds Number
The effects of Reynolds number was examined. AguaClara water treatment plant is built in geographically diverse area where the flow rate into the flocculator is different. Hence, it is important to understand the effect of Reynolds number on the resultsSince the inlet flow rate can vary substantially across AguaClara plants the effects of inlet Reynolds number on pressure coefficient drop were also examined.
2.7 Parameterization
At the later stage of project, after the confident on result of the model was built up, the effect of geometry on results was analyzed. Different clearance height was heights were used for analyzing new results as a result of this effectpressure drops and maximum velocities. It would be tedious to individually recreate each geometry and mesh for different clearance height from scratch. For this reason, a parameterization technique was used. The original Gambit journal file was modified to include the variable clearance height. Using this method, changes in corresponding clearance height was plug were plugged into the journal file and run using Gambit to obtain the desired mesh and /geometry. The journal files used for such parameterization file is included in the Appendix.
2.8 Comparing Turbulence Model
Different turbulence model were also employed for comparing the effect of different turbulence model. The pressure coefficient results were compared using Standard K-ε, K-ε Realizable and K-ω turbulence model were used and the results were comparedmodels.
3. Results and Discussions
Some of the important results are The results considered were plots of the velocity vectors, contour of pressure coefficient , contours of pressure coefficient, contours of strain rate and contours of turbulence dissipation rate.
Figure 5. Velocity Vectors (Click on figure for original size)
Velocity vector plot shows the velocity of the fluids throughout the flocculator. As can be seen, there is a region of high velocity at the outer turn and recirculation at the inner turn. At the bottom of the flocculator, there is region of stagnant fluid.
Figure 6. Contours of Stream Function (Click on figure for original size)
Contours of stream function tell us how the particles of fluid travel in the flocculator. As can be seen from figure 6, there is an enclosed streamline at the inner turn. The enclosed streamlines This means there are is recirculating fluid which are 'trapped' in the that region.
Figure 7. Contours of Pressure Coefficient
Figure 7 shows most of the pressure coefficient drop occurs around the bend. There is a pressure coefficient drop of about 3.7 across the bend. (Talk about the experimental result. Literature review data)This is in excellent agreement with literature estimates.
Figure 8. Contours of Strain Rate
Contours of strain rate shows high strain rate right before and after show high velocity gradients around the turn. There are also high strain rate rates in the boundary layer near the wall. The region with high strain rate is the region where the flocculation occurs It is postulated that flocculation is directly proportional to the strain rate.
Figure 9. Contours of Turbulent Disssipation Rate
Contours of turbulent dissipation rate shows about the same show a similar trend as the contours the strain rate right after the turningturn. The region of high highest turbulence dissipation occurs after the turn is about . As can be seen from figure 9, this region is roughly twice the length of baffle spacing (research literature). The high dissipation rate after the turn is because of the expansion of the fluid..
Figure 10. Wall Yplus
Figure 10 shows the yplus at wall was consistently less than 5. FLUENT documentations mentioned that "the mesh should be made either coarse or fine enough to prevent the wall-adjacent cells from being placed in the buffer layer (yplus = 5~30)". Since the yplus from the model was consistenly less than zero and was in the region of viscous sublayer, the turbulence flow near the wall was able to be resolved properly.
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