Cleaning wastewater is a massive industrial process. In large water treatment tanks, heavy dirt and solid particles (called sludge) naturally sink to the bottom. If this sludge just sits there, it creates toxic dead zones where the cleaning process fails entirely. To stop this, engineers install powerful submersible mixers to constantly stir the water. But how do we know if the mixer is strong enough before we build the tank? We use a Wastewater Sewage Mixer CFD simulation.

This report is a complete Wastewater Sewage Mixer fluent simulation tutorial. It is extremely important to state that this is an educational CFD analysis. While the geometry is inspired by real-world mixer designs, our goal here is to teach the physics of simulation. We use the ANSYS Fluent software to visualize how the spinning blades create powerful underwater wind to clean the tank. By mastering this CFD Analysis of Wastewater Sewage Mixer, engineers can design better tanks that never clog. For more lessons on how to simulate spinning machinery easily, please visit our MRF tutorials.

• Reference [1]: Tian, F., et al. “Mixing performance of sewage treatment mixer at different rotational speed.” ASME 2011 International Mechanical Engineering Congress and Exposition, IMECE 2011. 2011.
• Reference [2]: Błoński, D., et al. “Numerical simulation and experimental investigation of submersible sewage mixer performance.” Journal of Physics: Conference Series. Vol. 1741. No. 1. IOP Publishing, 2021.

Figure 1: Schematic of the mixer propeller zone inside the wastewater tank.

Simulation Process: The MRF Fluent Method

To build this Wastewater Sewage Mixer ANSYS Fluent project, we modeled a large water tank with a submersible propeller. The biggest challenge in simulating a Sewage Mixer is the spinning motion. If we force the computer to physically rotate the 3D grid (mesh) every single second, it will take weeks to calculate. Instead, we use the MRF fluent technique. MRF stands for Multiple Reference Frame. It is a highly clever math trick. We draw a small invisible cylinder around the propeller blades. We tell ANSYS Fluent to mathematically “spin” the fluid inside this cylinder at exactly 50 rpm (revolutions per minute), while the rest of the tank’s water stays in a normal, stationary frame. This allows the computer to calculate the complex mixing physics very quickly and accurately.

Post-processing: The Engine of Thrust and Mixing Vortex

To understand how a Wastewater Mixer works, we must first look at the Cause of the movement. The cause is the blade cutting through the water at 50 rpm. Look at the Pressure Contour (Figure 3) on the blades. When the angled blade smashes into the water, it creates a massive physical impact. The front of the blade (the leading edge) glows red. This is a high-pressure zone reaching +797 Pa. The blade is literally pushing the water. At the exact same time, the back of the blade leaves an empty space as it spins away. The water rushes in to fill this gap, creating a deep blue low-pressure suction zone dropping down to -1114 Pa. The giant mathematical difference between the +797 Pa push on the front and the -1114 Pa pull on the back is called Thrust. This pressure difference is the engine that powers the entire tank. However, this contour also shows a physical warning. The extreme dark blue tips (-1114 Pa) show where the pressure drops dangerously low. If pressure drops too low, the water will boil into tiny vapor bubbles that pop and destroy the metal blades—a dangerous effect called cavitation.

Now we look at the Effect of that thrust. Look at the Velocity Contour (Figure 2). The incredible pressure from the blades grabs the surrounding water and shoots it straight forward like a laser beam. The bright red color shows a perfectly focused, high-velocity jet of water traveling at 1.9 m/s. This 1.9 m/s axial jet is the ultimate weapon against sludge. As this fast jet shoots into the dark, still water of the tank, it creates friction. This friction acts like a vacuum, dragging the surrounding slow water along with it (a process called entrainment).

Because the tank has walls, this 1.9 m/s jet eventually hits the far side, turns around, and sweeps across the bottom floor before returning to the mixer. This creates a giant, invisible mixing wheel—a massive recirculation loop. This highly accurate CFD Analysis of Wastewater Sewage Mixer proves that you do not need to spin the whole tank; you only need to generate a 1.9 m/s jet to force the entire tank to stir itself, completely destroying all dead zones.

Figure 2: Velocity contour showing the powerful 1.9 m/s axial jet (red) generated by the propeller to drive tank circulation.

Figure 3: Pressure contour on the blades, proving how the +797 Pa high pressure (red) and -1114 Pa low pressure (blue) generate forward thrust.

Key Takeaways & FAQ

• Q: What is the MRF (Multiple Reference Frame) method?
  • A: It is a math tool in ANSYS Fluent that allows us to simulate the 50 rpm spinning of the mixer blades without physically moving the computer mesh, saving massive amounts of calculation time.
• Q: How does the mixer prevent sludge buildup?
  • A: The pressure of the blades generates a 1.9 m/s velocity jet. This fast jet pulls the surrounding water with it, creating a giant recirculation loop that continuously sweeps the tank floor.
• Q: What is cavitation and why is it bad?
  • A: As seen in our pressure contour, the back of the blade drops to -1114 Pa. If pressure drops too much, water forms vapor bubbles that violently collapse, eroding and destroying the metal propeller.