CFD Simulation Scientific Paper (By:Jorge Rodriguez, Yong Sheng Khoo)

MSWord version

Title: CFD Analysis of a Flocculation Tank for Sustainable Drinking Water Treatment

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An analytical estimate of the energy dissipation rate in the dissipation cell can be obtained. Use the following equations:

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AGUACLARA:GequalrootepsilonovernuAGUACLARA: Gequalrootepsilonovernu

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epsilon epsilonCpVsquaredover2theta CpVsquaredover2thetaepsilonCpVsquaredover2theta

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AGUACLARA:epsiloncellAGUACLARA: epsiloncell

Using the equation above to estimate the energy dissipation rate in the expansion zone we obtain an average value of ??. This is based on the assumption that the length of the dissipation region is approximately twice (Π _cell = 2) the distance between the baffles.

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Figure 10. Wall Yplus

Calculate the value of Gθ for each cell in the computational grid and sum this up over the domain containing the dissipation zone to get a more accurate measure of the actual Gθ for a 180 degree bend. Compare that with the estimate that the AguaClara team is currently using.

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Gthetacell Gthetacell

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Figure 10. Wall Yplus

Figure 10 shows that the y+ values. According to FLUENT documentation "the mesh should be made either coarse Figure 10 shows that the y+ values. 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 (y+ = 5~30)". Since the y+ from the model was consistently less than five (in the viscous sublayer) the turbulence near the walls was resolved properly.

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The effect of the Reynolds number on the pressure coefficient drop was analyzed. This was done by changing the inlet velocity which initially produces a Reynolds number of 10,000. From figure 12, it can be seen that the value of the pressure coefficient drop has a small change when compared to big changes in Reynolds number. In other words, the pressure coefficient drop is not sensitive to the Reynolds number at the inlet. This implies that the design of the flocculator should not be altered by the inlet flow rate. This is to say that one flocculator design can be used for different flow rates. You are right that the pressure coefficient is insensitive to Reynolds number. But that does not mean that a single flocculator can handle a wide range of flows. The energy dissipation rate is still proportional to the velocity cubed.

Figure 13. Clearance Height Effect on Pressure Coefficient Drop and Maximum Velocity

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 phenomenon can be explained by looking at figure 14 below. Figure 14 shows the turbulent dissipation rate for clearance heights of 0.1 m 1b and 1.5 m5b. It can be observed that the length of high maximum 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 bottomA recirculation zone begins to form at the bottom of the reactor when the clearance height is larger than 1b. A similar argument can be made for the values of maximum velocity. A clearance height less than 0.1 m 1b results in a higher pressure coefficient drop as it creates an 'unnatural' additional constriction to in the flow increasing frictional expansion losses. It is therefore recommended for the design team that the clearance height be at least the same as the baffle widthspacing. The correlation between pressure coefficient drop and maximum velocity should also be noted. Image Removed
Figure 14. Comparison of Turbulent Dissipation Rate for Clearance height of 0.1 m and 0.15 m Add the theoretical connection using the pressure drop in an expansion. The conclusion that there is no need to make the clearance height larger than 1b is noteworthy. This will make it possible to handle higher flow rates with the same channel width since we are dealing with a constraint that requires neighboring baffles to overlap in the center of the reactor to prevent short circuiting.

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Figure 14. Comparison of Turbulent Dissipation Rate for Clearance height of 1b and 1.5b

Figure 14 above further validates that results Figure 14 above further validates that results are not sensitive to the change in clearance height. Contours of turbulence dissipation rate for clearance heights of 0.1 m and 0.15 m 1b and 1.5b were compared. These results show that the region of active turbulent dissipation is the same for both reactors, about two times the length of baffle spacing.

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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 simulate the turn.

4. Conclusions

1. An area of recirculation occurs near the center wall immediately after the turn

2. Pressure coefficient drop over one baffle turn is 3.75

3. After a clearance height of one baffle width or greater, the pressure coefficient drop and the maximum velocity becomes constant

4. Most of the energy dissipation occurs in the region after the turn over a distance of about two baffle widths

5. The pressure coefficient drop is insensitive to the Reynolds number for a large range of inlet velocities

6. A mesh with 20,000 mesh elements is sufficient to obtain accurate results

7. Different turbulence model resulted in fairly different results

8. The standard k-e model best simulates the turn

6. Future Research

1. Measure Gθ value from FLUENT

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I am uncomfortable with the method of selecting the turbulence model. The demonstration plant is in the laminar flow region and thus it may not be a good reference point for full scale turbulent reactors. The difference between the various models is quite significant especially when you consider that we are most interested in Gθ. The models that predict a large wake behind the expansion will also predict lower average values of the energy dissipation rate and that will correlate with a larger Gθ. Thus we need a better basis for choosing the model that we will use for further research. I suggest further research to determine which models do a better job of modeling the evolution of a turbulent jet. The region where the models diverge significantly is in their predictions about the length of the zone influenced by the jet.

4. Conclusions

1. An area of recirculation occurs near the center wall immediately after the turn
2. Pressure coefficient drop over one baffle turn is 3.75
3. After a clearance height of one baffle spacing or greater, the pressure coefficient drop and the maximum velocity becomes constant
4. Most of the energy dissipation occurs in the region after the turn over a distance of about two baffle spacings
5. The pressure coefficient drop is insensitive to the Reynolds number for a large range of inlet velocities
6. A mesh with 20,000 mesh elements is sufficient to obtain accurate results
7. Different turbulence model resulted in fairly different results
8. The standard k-e model best simulates the turn

6. Future Research

1. Measure the Gθ value from FLUENT
2. Analyze the region of very high energy dissipation along the wall near the contraction and explore methods to reduce the energy dissipation in this zone
3. Devise improved methods of creating more uniform energy dissipation in the reactor to enhance flocculation efficiency
4. Analyze the effect of changing the ratio of b/h all the way to the extreme where the baffles don't overlap in the center of the reactor. This is important to learn what will happen as we design for larger flow rates. Larger flows require the spacing between baffles to increase. If we hold the reactor depth constant that will increase the ratio of b/h.
5. Better understanding of different turbulence models

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1. Model droplet collision - breakup

7. Acknowledgements

During the course of this project, we received invaluable advice and direction from Prof. Monroe Weber-Shirk, whose leadership of the AguaClara team project spurred this research and technical investigation. All questions we had were directed towards Dr. Rajesh Bhaskaran, his expertise and academic guidance kept us on schedule and on task. We would also like to thank Prof. Brian Kirby for elucidating some technical aspects we encountered. For their help, we are grateful.

References

On the collision of drops in turbulent clouds
P. G. Saffman and J. S. Turner
, Journal of Fluid Mechanics Digital Archive, Volume 1, Issue 01, May 1956, pp 16-30
doi: 10.1017/S0022112056000020, Published online by Cambridge University Press 28 Mar 2006

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