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Figure 3 shows the mesh of the flocculator model. As can be seen, the mesh is fine finer near the turn and at the walls. The next step was to set up the boundary conditions of the system.
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The effect of the number of mesh elements on the result was also carried out. Coarse, medium and fine meshes were created and the pressure coefficient drop across the turn from each mesh was compared. This analysis will provide confidence on the accuracy of certain mesh. If the changes in mesh elements does not result in a lot of change in The pressure coefficient drop , it is concluded that the mesh elements were refined enough that the truncation and discretization errors can be neglected. Table 2 should not be sensitive to mesh density. Table 2 below shows the summary of the 3 meshes created for mesh sensitivity to perform this analysis. Please refer back to figure 2 for corresponding meshing parameters.
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Mesh | Number of Mesh Elements | Wall Boundary Layer Conditions | Second Edge | Third Edge | Fourth Edge |
|---|---|---|---|---|---|
Coarse 18762 | 18,762 | First row = 0.003 | Interval size = 0.007 | Interval size = 0.003 | Interval size = 0.003 |
Medium 30000 | 30,000 | First row = 0.003 | Interval size = 0.005 | Interval size = 0.002 | Interval size = 0.002 |
Fine 52260 | 52,260 | First row = 0.003 | Interval size = 0.0038 | Interval size = 0.0014 | Interval size = 0.0014 |
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Since the inlet flow rate can vary substantially across AguaClara plants the effects of inlet Reynolds number on pressure coefficient drop were was 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 parameters on the results was analyzed. Different clearance heights were used for analyzing pressure 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 A parameterization technique was used . The original Gambit journal file was modified to include the variable clearance height. to automatically create a mesh given the parameters of the geometry. Using this method, changes in corresponding the clearance height were plugged into the journal file to obtain the desired mesh/geometry. The , baffle width and baffle length were easily adjusted. The Gambit journal file is included in the Appendix.
2.8 Comparing Turbulence Model
The pressure Pressure coefficient results drops were compared using for the Standard K-ε, K-ε Realizable and K-ω turbulence models.
3. Results and
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Discussion
The results considered were plots of the velocity vectors, pressure coefficient contours, contours of strain rate and contours of turbulence dissipation rate.
Figure 5. Velocity Vectors (Click on figure for original size)
Velocity The velocity vector plot shows shown above depicts the water velocity of the fluids throughout the flocculator. As can be seen, there is a region of high velocity at the outer side of the turn and recirculation at the inner side of the turn. At the bottom of the flocculatorFurthermore, there is a region of stagnant fluidwater at the bottom of the flocculator.
Figure 6. Contours of Stream Function (Click on figure for original size)
Contours The contours of stream function shown above tell us how particles of fluid travel in the flocculator. As can be seen from figure 6, there There is an enclosed streamline at the inner side of the turn. This means there is recirculating fluid 'trapped' in that region.
Figure 7. Contours of Pressure Coefficient
Figure 7 shows that most of the pressure coefficient drop occurs around the bend. There is a The pressure coefficient drop of is about 3.7 across the bend. This is in excellent agreement with literature estimates.
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Figure 9. Contours of Turbulent Disssipation Dissipation Rate
Contours of turbulent dissipation rate show a similar trend as the contours the of strain rate right after the turn. The region of highest turbulence dissipation occurs after the turn. As can be seen from figure 9, this region is roughly twice the length of baffle spacing.
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Figure 10. Wall Yplus
Figure 10 shows that the yplus y+ values were consistently less than 5. According to FLUENT documentation "the mesh should be made either coarse or fine enough to prevent the wall-adjacent cells from being placed in the buffer layer (yplus y+ = 5~30)". Since the yplus y+ from the model was consistently less than five (inside the viscous sublayer) the turbulence flow near the walls was resolved properly.
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