Under construction: Finish date: Sunday 5/18/2008
CFD Simulation Scientific Paper (By: Jorge Rodriguez, Yong Sheng Khoo)
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Height: 1 m
Clearance: 0.15 m
Baffle width: 0.1 m
Velocity inlet: 0.1 m/s
With these parameters, the mesh be built.
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At this stage, the Standard k-ε turbulence model was set up. Water was defined as the working material from the FLUENT database. The discretization method for the momentum, turbulent kinetic energy, and turbulent dissipation rate were set to the 'Second Order Upwind' scheme to obtain a 'Second Order Accurate' solution. The boundary conditions were set according to the values shown in the table below1. The solution was obtained by iterating until the residuals converged to 10e-6. Results were then analyzed and plotted.
TABLE with BCS TABLE 1
Table 1. Boundary Conditions
Boundary Conditions | |
|---|---|
Velocity Inlet | 0.1 m/s |
Pressure Outlet | 0 Pa |
2.5 Mesh Sensitivity Analysis
The effect of the number of mesh elements on the result was carried out. Coarse, medium and fine meshes were created and the pressure coefficient drop was compared. This analysis will provide confidence on the accuracy of certain mesh. The pressure coefficient drop should not be sensitive to mesh density. Table 2 below shows the summary Table 2 below shows the summary of the 3 meshes created to perform this analysis. Please refer back to figure 2 for corresponding meshing parameters.
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At the later stage of project, the effect of geometry parameters on the results was analyzed. Different clearance heights were used for analyzing pressure drops and maximum velocities. A parameterization technique was used to automatically create a mesh given the parameters of the geometry. Using this method, the clearance height, baffle width and baffle length were easily adjusted. The Gambit journal file is included in the Appendix A.
2.8 Comparing Turbulence Model
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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)
The velocity vector plot shown above depicts the water velocity throughout the flocculator. As can be seen, there is high velocity at the outer side of the turn and recirculation at near the inner side center of the turnwall. Furthermore, there is a region of stagnant water at the bottom of the flocculator.
Figure 6. Contours of Stream Function (Click on figure for original size)
The contours of stream function shown above tell us how particles of fluid travel in the flocculator. There is an enclosed streamline at the inner side of the turn. This means there is recirculating fluid 'trapped' in that region.
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Figure 7 shows that most of the pressure coefficient drop occurs around the bend. The pressure coefficient drop is about 3.7 75 across the bend. This is in excellent agreement with literature estimates.
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The effect of the clearance height on the pressure coefficient drop was also analyzed. It can be seen that the pressure coefficient drop is independent of the change in clearance height after the clearance height is greater than a critical value. Figure 13 shows that after a critical value of 1, the pressure coefficient drop is constant. This phenomena can be explained by looking at figure 14 below. Figure 14 shows the turbulent dissipation rate for clearance heights of 0.1 m and 01.15 5 m. It can be observed that the length of high dissipation rate is equal for both reactors. This means that 0.1 m is the clearance height after which a stagnant fluid starts to form at the bottom. A similar argument can be made for the values of maximum velocity. A clearance height less than 0.1 m results in a higher pressure coefficient drop as it creates an 'unnatural' constriction to the flow increasing frictional losses. It is therefore recommended for the design team that the clearance height be at least the same as the baffle width. The correlation between pressure coefficient drop and maximum velocity should also be noted.
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Pressure coefficient drop was least in the K-ε model and most in the K-ω model.
Figure 15. Contours of Velocity Magnitude for Different Turbulence Models
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However, as it can be observed from the transparent demo plant, the recirculation area is only the length of one or two baffle widths. This is contrary to the excessively large blue/red regions predicted by the k-e ε realizable, and k-w models. Therefore it was concluded that the standard k-ε model best simulates the turn.
Figure 16. Contours of Stream Function for Different Turbulence Models
Figure 16 clearly shows the recirculation region for the three different turbulence models. Since standard K-ε model best represent the flow features seen in the demo plant, it is concluded that standard K-e model best simulates simulate the turn.
4. Conclusions
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