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Modeling with CFD

Modeling with CFD

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    CFD software provides insight to what is actually happening inside a structure, evident in this model of the San Diego Miramar water treatment plant's flow splitter box that shows velocity contours. Photos: COM

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    Shown here is construction of the San Diego Miramar water treatment plant, which provides 140 mgd to nearly 500,000 residents.

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    Above: CFD results, such as this graphic of velocity magnitude contours in the chlorine contactor at the Los Angeles West Basin Municipal Water District water recycling plant, illustrate fluid flow within a structure. Left: A CFD analysis at the West Basin Municipal Water District plant revealed that the residence time of the existing chlorine contactor at the increased flow rate was sufficient, and adding another contactor was unnecessary.

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    These two graphics illustrate the modeling of the velocity contours of the Scott Candler water filter plant's channel with (top) and without (bottom) a taper. Results showed that a rectangular channel was more effective than the tapered design, saving more than $75,000 in additional costs.

Utility managers will continue to face questions about servicing the needs of their communities, but as CFD's applications become even more increasingly far reaching, these questions will now have answers.

Knatz is an environmental engineer and CFD modeler with CDM, Carlsbad, Calif.

A revealing analysis

More than 600,000 Georgia citizens rely on DeKalb County for potable water treatment and distribution. The service area's dense population and southern climate create an obstacle to daily water delivery. As such, the county had to impose outdoor water use restrictions for two summers in a row, because the Scott Candler water filter plant—the facility serving DeKalb County—was at capacity. The county's decision to expand treatment capacity at the plant from the current maximum of 128 mgd to 150 mgd was an imperative strategy.

The proposed facility included an intermediate channel between the flocculation and sedimentation basins. The channel configuration was designed with a taper intended to improve the flow distribution into the sedimentation basin. CFD analysis was used because of the high cost of creating the taper and reinforcing the channel bottom.

The channel was modeled with and without the taper; the CFD results illustrated that the taper actually provided poorer flow distribution than the non-tapered design. At first, these results came as a surprise in that tapers traditionally facilitate flow. However, further consideration of the design suggested that the direction of inlet flow determines whether a tapered design facilitates flow distribution. If the inlet flow had been parallel to the channel, the momentum of the fluid would want to carry the majority of the flow to the far end of the channel.

In this instance, providing a taper would force more flow to the beginning of the channel. For the Scott Candler plant, the design provided inlet flow perpendicular to the channel, not parallel. Therefore, the momentum of the fluid was already directing the flow to leave the channel at the beginning.

This is a perfect example of how CFD is helping engineers accurately predict the behavior of fluid flow. The CFD analysis ended up saving the county more than $75,000 in concrete, reinforcing of the channel taper, and labor. In addition, the final design produced a better distribution of water into the sedimentation basin.