In the design of water and wastewater treatment plants, proper flow and solids distribution can be as critical as process design considerations. Insufficient treatment and even plant failures can result from unequal and unmanageable flow and solids distribution. Computational fluid dynamics (CFD) modeling is a valuable tool in the evaluation of flow distribution to multiple units within a treatment process. This article reviews the benefits achieved by performing a CFD analysis of an Infilco high-rate dissolved air flotation (DAF) influent channel prior to finalizing the design of the plant. The CFD model was used to optimize the DAF influent channel configuration with respect to flow distribution to 10 identical process units that were inserted into an existing facility footprint. For the initial configurations modeled, the largest deviation of flow rate to an individual DAF unit was over 60%. Using CFD, design engineers developed a DAF influent channel configuration predicted to achieve less than 10% deviation. The upgraded facility is constructed and in service and the results of the CFD model were confirmed using actual turbidity data, which indicate that the solids are evenly distributed to the DAF process trains.
Experience has shown the predictive reliability of two commonly used equations for the prediction of headloss (known as the Bernoulli principle and the Kirschmer method) may be questionable under certain conditions, especially at higher flow velocities through fractional openings. To quantify this observed disparity, a series of tests were executed to compare predicted headloss-employing the results of both these equations-to actual results under controlled conditions. Test results indicate that the headloss predicted by the Bernoulli principle consistently overstates actual headloss through screens by a significant amount. The Kirschmer method correlated much closer to actual headloss, however it proved to be less than satisfactory for some openings or bar shapes. Study of the results of these tests, along with careful examination of the two equations, indicate that better correlations for headloss prediction versus actual measured values could be obtained by modifying elements of both the Bernoulli and Kirschmer equations, or by applying Computational Fluid Dynamics (CFD).
A capacity increase at the West Basin water recycling plant in California was necessary in order to handle an increasing population and ensuing demand. The disinfection process included a chlorine contactor, operating at 5 million gallons per day. It was believed that an additional train would be necessary to accommodate the increased load of 6.9 million gallons per day. Goals and ObjectivesRather than initially performing a costly physical tracer test at the higher flow volume, a computational fluid dynamics (CFD) model of the existing contactor was created followed by a virtual tracer study to determine the residence time, the objective being to verify the need for an additional train. Inadequate residence time results would indicate the need to move forward in contactor design and construction, whereas a result showing sufficient residence time would be followed by a physical tracer test confirmation. MethodologyAfter creating CAD geometry to represent the contactor and meshing the volume with the GAMBIT [1] preprocessing tool, FLUENT version 6.1 [2] was used to calculate steady state flow through the contactor. Uniform flow was assumed at the inlet, and a standard k-epsilon turbulence model was used to account for the effect of unsteady turbulent fluctuations on the average flow field. Figure 1 shows flow velocity contours on a horizontal slice through the contactor volume. A residence time calculation was performed using the Discrete Phase Model (DPM), in which massless particles were released at the contactor inlet and tracked throughout the domain. The time at which each particle left the contactor volume was recorded. A standard method for evaluating residence time in a contactor is the t 10 /T calculation, where t 10 is the time at which ten percent of a tracer passes through the tank, and T is the theoretical time it takes for all of the tracer to pass through the tank assuming uniform, or "plug" flow. Thus, for the virtual tracer study, t 10 is the time at which ten percent of the particles have exited. Generally, a value of 0.6 or greater for t 10 /T indicates an effective contactor design. Findings and SignificanceThe model results suggested that there was sufficient residence time to handle the increased flow rate in the existing contactor. West Basin then performed a physical tracer study, which confirmed the CFD results and reaffirmed that the residence time met the Department of Health Services' requirements. Through saving the utility an estimated $3 7874
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