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CFD Fixes Faulty Mixer
Posted Fri January 11, 2002 @02:05PM
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Application Philadelphia Mixers used MixSim, a dedicated mixing analysis software package, to solve a problem with a pH adjustment system in a baffled mixing tank. The complete analysis, which required several simulations, took only one day, considerably less than the week or more that would have been required if general-purpose computational fluid dynamics (CFD) software and grid generation tools had been used. Most of this time would have been required to build the model because of the complex geometry of the system and the fact that it contained both stationary and moving parts. The mixing-specific software used set up the model automatically in response to parameters entered by the user. "After I entered a few specifications, the software created the geometry and the analysis mesh in less than 5 minutes," says Wojciech Wyczalkowski, director of technology at Philadelphia Mixers. After the first analysis showed the reason for the fluctuating pH, Wyczalkowski ran seven more analyses to improve the design. By the end of one day, he had an optimized solution that the customer has since implemented successfully.

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Based in Palmyra, Pennsylvania, Philadelphia Mixers is one of the world's most experienced manufacturers of mixers and aerators. With a full range of globally proven designs, the company supplies mixers for chemical processing, water and wastewater treatment, mining, food, pharmaceuticals and others. The company is more than a supplier of products, however, offering expertise in all phases of process technology. Its services include consulting and mixing design, scale up and testing, on-site start up and employee training, inspection and repair.

Fixing a faulty design

In the project mentioned above, Philadelphia Mixers was contacted by a steel mill operator, who was having a problem with a mixer that had been purchased from another supplier. The mixer was used in a wastewater pretreatment process in which the fluctuating pH of the incoming stream was adjusted to a constant level. From there, the wastewater went on to further treatment before being discharged. The system was not delivering a constant pH, however, so the mill's management asked Philadelphia Mixers to find out if the problem was in the design of the mixer itself or in some aspect of its operation.

The mixing system consisted of a vessel and two impellers, an axial impeller and below it, a radial impeller. The influent port was located near the radial impeller. "The first thing we wanted to do was determine the mixing efficiency of this design," says Wyczalkowski. One option for doing that was to build and test a scale model of the system. That would have been costly and time-consuming. Another problem with a scale model is that, due to the nature of fluid behavior, results of testing models like flow, dissipation and surface effects are not directly scalable.

Another option was to simulate the mixing in the existing system with general-purpose CFD software. CFD involves the solution of the governing equations for fluid flow, heat transfer and chemistry in tens or hundreds of thousands of computational cells in the defined flow domain. The use of CFD enables engineers to obtain solutions for problems with complex geometry and boundary conditions. A CFD analysis yields values for species concentration, fluid velocity, and other variables throughout the solution domain. A key advantage of CFD is that it provides the flexibility to readily change design parameters and evaluate new configurations quickly.

A major challenge in using CFD to model this type of problem is the continual motion of the impellers in the baffled vessel. The challenge has been met in recent years by the use of the multiple reference frames (MRF) method in which the spinning agitator is simulated in a rotating frame while the remainder of the tank (including the baffles and inflow port) is simulated in a stationary frame. The solution proceeds with a steady transfer of information across a pre-defined interface between the two frames. As with any CFD simulation, this approach requires creating the geometry of the impellers and vessel and then creating an analysis mesh. It also requires the user to specify other information about the problem such as fluid properties and performance parameters like shaft speed. Setting up a complicated mixing problem in this manner can take up to a week.

Philadelphia Mixers overcame this limitation by using dedicated mixing software to determine the mixing efficiency of the malfunctioning pH system. The software they chose, MixSim, is available from Fluent Incorporated, Lebanon, New Hampshire, supplier of the popular CFD program, FLUENT. The software's user interface is designed to simplify the creation of a mixing analysis model. "One of the main advantages of MixSim for us is its ability to create the geometry and mesh automatically according to specifications entered by the user," Wyczalkowski says. Seamlessly integrated with the FLUENT solver, MixSim specifically addresses mixing and related flow phenomena for complete hydrodynamic simulation of agitated mixing vessels. It predicts the complete blending and motion in batch, semi-batch, and continuous stirred tanks, and incorporates additional transport phenomena important in mixing applications. Users can also visualize mass, momentum, and/or heat transfer in single or multiple phase mixtures in the vessel and obtain integral quantities, such as blending time and minimum suspension speed.

Simple Setup

Wyczalkowski decided to start with a 2D analysis of the existing pH system. Because he had laser Doppler velocimetry data of the system, he decided to use the velocity data method initially. This simplified method substitutes time-averaged velocity components and turbulence quantities for the actual impellers in the fluid cells of the CFD model. It is a steady-state formulation that allows for the modeling of other time-dependent processes such as blending and free surface prediction. "The laser data showed how fast the fluid was moving when it left the impellers," Wyczalkowski explains. "With that input, it wasn't necessary to model the impellers exactly." He simply entered the laser data into MixSim, specified the liquid properties, and ran the analysis. The solution time took only 15 minutes. A particle flow plot clearly showed why the steel mill was having problems with the mixer. A design flaw caused a short circuit in which the pH adjustment fluid went out of the vessel before mixing with the entire volume. "CFD is an excellent visualization tool that helps you understand a problem," says Wyczalkowski. "Once you understand it, you can apply physics to it to solve it."

Wyczalkowski's next step was to change the design to see if the mixing could be improved. Using the velocity data method once again, he made several design changes and was able to evaluate them quickly. Keeping the vessel configuration fixed, he added a third impeller and changed the direction of the pumping action of the original axial impeller. These changes were implemented easily through the MixSim interface by entering parameters such as impeller location, size, and pumping direction. For each design change, the software built the geometry and created the analysis mesh automatically. "The entire process of setting up each model took five minutes," Wyczalkowski says. He went through five iterations to try out different designs, still working in 2D. "My goal was to configure the system so that the pH adjustment stream was pushed into an impeller instead of the wall," he explains. When he achieved one that thoroughly mixed the pH adjustment material with the entire contents of the vessel, he repeated the analysis in 3D to capture any 3D effects and make it easier for the client to visualize. The solution time for the 3D analysis was three hours.

The customer was convinced by the analysis results that Philadelphia Mixers' new impeller's configuration would work. They replaced the existing impeller configuration with Wyczalkowski's new design. Since then, the pH adjustment system has achieved the constant values it was designed to produce. Using dedicated mixing analysis software on this problem allowed Philadelphia Mixers to find this solution much sooner than if they had used general-purpose CFD analysis because it simplified the creation of the analysis model. "Mixing-specific software makes CFD practical in a situation such as this," says Wyczalkowski. "It is so fast that it you can use the computer to evaluate many different designs."

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