Parameters analysis in 2D - simulation experiments

Objectives and Hypothesis

The preliminary simulations had encouraging outcomes about the dependence of performance parameters on h/b ratio. In further experiments described below, we simulated a series of cases with h/b ratio of 2, 3, 4, 5, 8, 10, 15, 20 and 40 and plotted each performance parameters as a function of h/b ratio. We would expect:

1. K_baffle is a measure of energy loss within each baffle spacing. It will converge as h/b ratio increases, and will also converge in a single baffle for successive turnings. Shultz and Okun has suggested its value is between 3 and 4.
2. Pi_cell will increase as h/b ratio increase, which measure the active volume of flocculation tank. Pi_cell/(h/b) can be a measure of the active fraction of flocculation tank, which is expected to decrease as h/b ratio increases.

Methods and Procedures

h/b ratios of 2, 3, 4, 5, 8, 10, 15, 20 and 40 were tested with 5 baffles. The completed test are summarized in the following table:

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Some of the important parameters are summarized below:

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The various geometry and mesh are created from journal file, by varying the flocculation tank height. All the FLUENT settings including turbulence model, boundary conditions, solutions schemes etc. can be found in a sample of report summary for h/b=2 case.

Results and Discussion

Click here for the results of the series of simulation experiments and parameter analysis, completely summarized in an Excel workbook. Only parts of the results are given graphically below.

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Take the values calculated from the 4th baffle, and plot each parameter as a function of h/b:

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Number of baffles
Note that in the Excel workbook, the performance parameters were calculated for both the 4th and 5th baffles. There are minor differences in numerical values while the general trend stays the same. Particularly, a more pronouncing difference is observed for h/b=4. Thus simulations with more baffles and a sensitivity analysis of the number of baffles could be included in the future work.

K_baffle
As shown in the plots above (and the Excel workbook), the trend of K_baffle values is consistent with the hypothesis. It decreases rapidly for h/b = 2~5 and stabilizes around 3.2. An upward trend appear around the tail of the K_baffle vs h/b curve, which indicates the major loss in addition to minor losses is playing a more significant part in the total energy loss.

Pi_cell
Pi_cell was shown to increase continually as h/b ratio increases in preliminary experiments, which was also reported in Fall 2008 research. This trend is further confirmed with a large h/b ratio of 40. As reported in Fall 2008 research, Pi_cell value keeps increasing because energy is dissipated along the wall as well as around the 180 degree turning region.

Pi_cell/(h/b)
The plot of Pi_cell/(h/b) as a function of h/b is also consistent with the hypothesis. As a measure of active fraction of the total volume of the flocculator, it reaches a maximum of 95% for h/b=4, and keeps decreasing as h/b increases. However, the curved flattens out at the tail. This also suggests although an optimal value exists for h/b, it wouldn't be the most critical constraint for design, since even for an un-pratically tall flocculator of h/b=20, it still has an active volume fraction value around 70%.

Collision potential/(h/b)
The power function fit for collision potential (theta*epsilon^(1/3))/(h/b) was suggested by Dr Monroe Weber-Shirk. It shows a unexpectedly simple relationship with a power of about -0.5 and a coefficient of 0.33. Further investigation is needed to better interpret this relationship.

Conclusion

• The number of baffle used are not enough for the cases of h/b= 2~5 to converge.
• The formulation of the K_baffle and Pi_cell are valid and consistent. They captures some of the characteristics of flocculator performance.
• Optimal values of h/b=4 is observed in terms of Pi_cell/(h/b) value.
• Relationships between (theta*epsilon^(1/3))/(h/b) and h/b is unexpectedly simple and needs further investigation.

Future Work

• Simulations with more baffles and investigate the sensitivity to the number of baffles used, especially for h/b=4.
• More detailed comparison between symmetry boundary conditions and the wall boundary condition at the water-air interface.
• Simulations with different Reynolds numbers.
• Non-dimensionalize the results.
• Characterize the maximum energy dissipation rate to characterize floc break-up

2D Performance Parameters Analysis

Goals and Hypothesis

Based on our understanding of flocculation model, we use Pi-cell, G-theta and epsilon-theta as important design parameters that characterize the performance of hydraulic flocculation tanks. By comparing the performance parameters that derived from or assumed according physical understanding or intuition with the values that are calculated from the discrete numerical solutions from CFD simulation, we want to verify the validity of the flocculation model and draw more insights of how to design hydraulic flocculators.

Methods and Procedures

preliminary simulation experiments

• To investigate the appropriate boundary condition settings and the convergence level

Simulations with various h over b ratios
According to the results of the preliminary simulation experiments, the simulation are design in following ways:

• Mesh generation
  • The mesh files for various h over b ratios are generated using the journal file
• Define the problem in FLUENT
  • Click here for a report summary of all parameters of FLUENT model
• Formulations of the performance parameters

  • Wiki Markup
    Click !here^CFD tasks.ppt|width=32,height=32! for a summary of the derivation and formulation of the performance parameters

Results and Discussion

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