By Elise VeCasey and Kevin Linfield, Ph.D., P.Eng., P.E.
Water treatment tanks are used to incorporate sanitizing chemicals into water for both residential and industrial applications. These tanks often use a series of internal structures or “baffles” to redirect the flow for residence time. The baffling factor is a way of characterizing the effectiveness of water flow through a given tank by quantifying the mixing of the chemicals. By finding the time for the tracer concentration to reach 10% of the total flow at the tank exit and comparing it to the theoretical time for the entire fluid volume to fill the tank, the baffling factor can be calculated as a simple ratio. This baffling factor can range from 0 to 1.0, with higher values corresponding to better mixing.
By adjusting the design of the baffles within a water tank, flow patterns can be curated to yield an ideal baffling factor for the specific application. For example, a tank used to incorporate chlorine into water may need a baffling factor of 0.7 to be considered sufficient and to help ensure adequate contact time.
One way to find the baffling factor is by putting dye in the inlet and observing the concentration throughout the tank over time to understand the flow patterns. To predict these, Computational Fluid Dynamics (CFD) tools can be used to simulate the complex behavior. CFD simulations can predict the water velocity, flow direction, and dye concentration at any location within the tank during a set duration of time.
The first step to determine the baffling factor for a water tank is to construct a 3-D CAD model of the tank, including inlet and outlet pipes and the interior baffles. The water tank analyzed for this example consists of a cylindrical tank 33 feet in diameter with an 8 foot water depth. The interior consists of 3 baffles, each 24 feet long and spaced 7 feet apart. Water enters the domain through the inlet pipe, flows through the tank past the baffles, and exits through the outlet pipe. The second step is to prepare a grid or mesh throughout the domain. This model was subsequently divided into a mesh consisting of many millions of computational cells. Areas of interest, such as inlet and outlet pipes, were refined to visualize key areas of the flow and increase the accuracy of desired calculations.
Using AzoreCFD®, the fluid movement inside the tank was modeled to determine the time taken for dye to travel from the inlet to the outlet. The simulation process began with a steady-state run to establish the basic flow profile, with boundary conditions specifying a constant mass flow of water through the inlet. Results of the steady-state simulation were followed by a transient (time dependent) model, as shown in Figure 1 and Figure 2. This simulates dye being added to the inlet, showing its concentration changes and movement over time, as seen in Figure 3.
From the results of the transient simulation, the numerator of the baffling factor formula can be found by monitoring the mass flow fraction of dye at the outlet and extracting the time at which dye makes up 10% of the total flow at the outlet. The denominator is determined by dividing the volume of the tank by the design flow, giving the ideal tank fill time. The resulting ratio of these two times yielded a baffling factor of 0.6 for this water tank. As above, the baffling factor goal for this tank is 0.7, demonstrating that the current design would not satisfy the industry standards for its intended application. Additional tank designs may require adjusting the number, size, and/or spacing of the baffles inside the tank to encourage more mixing between fluids, and consequently increasing the baffling factor.
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Airflow Sciences Corporation (ASC) is a fluid dynamics solutions company, specializing in the design and optimization of equipment and processes involving flow, heat transfer, combustion, and mass transfer. Now celebrating 50 years as an industry leader, ASC has focused on testing and simulation of air, gas, liquid, and particulate flows since 1975.
ASC also manufactures standard and custom test equipment, including probes and wind tunnels, enabling customers to collect data accurately and efficiently. ASC's primary CFD software, AzoreCFD®, is also available for customers with in-house CFD personnel. ASC offers comprehensive flow solutions and optimization and serves a wide range of industries including HVAC, power, auto, rail, and food processing.