External gear pumps are simple but powerful machines used to move fluids. You can find them in car engines, hydraulic lifts, and chemical factories. They work by trapping liquid between two spinning gears. One gear is connected to a motor, and it turns the other gear. As they spin, they push the fluid from the inlet to the outlet. To understand how to design these pumps better, engineers use an External Gear Pump CFD Simulation.

In this tutorial, we simulate a 2D gear pump using ANSYS Fluent. This is a special simulation because the parts are moving. We cannot use a static (still) mesh. Instead, we use the Dynamic Mesh module. This allows the grid to change shape as the gears rotate. We also use a custom code called a User Defined Function (UDF) to tell the software exactly how fast the gears should spin in opposite directions. This training is essential for learning how to handle moving parts in CFD. For more simulations involving rotating machines, please check our Turbomachinery CFD tutorials.

• Reference [1]: Martínez, Javier. “Mesh handling for the CFD simulation of external gear pumps.” Positive displacement machines. Academic Press, 2019. 345-368.

Figure 1- Fluid transfer mechanism through a gear pump, showing how fluid is trapped between the gear teeth. [1]

Simulation Process: Gear Pump CFD Setup with Dynamic Mesh

For this External Gear Pump CFD Simulation, we started by drawing the geometry in SpaceClaim. Designing gears requires care. We must ensure the teeth touch perfectly but do not crash into each other, which would stop the simulation. A schematic of the model is shown in Figure 2. Next, we created the mesh using ANSYS Meshing. Because the gears move, the gap between the teeth changes constantly. Therefore, we used a “Proximity” size function to put very small cells in the tiny gaps. We generated a grid with 172,324 triangular elements. We used triangles because they are flexible and easy to “remesh” (fix) as the gears move.

The most critical setting in ANSYS Fluent is the Dynamic Mesh. We turned on “Smoothing” and “Remeshing.” : As the gear rotates, the cells get stretched and ugly. Fluent automatically deletes the bad cells and draws new, good ones. This keeps the simulation accurate. Although the flow is mostly smooth (laminar), we used the k-omega SST turbulence model. This helps the calculation stay stable and avoids errors. Finally, we loaded a UDF (C-code) to define the rotation speed and axis of gravity for the rigid gears.

Figure 2- Schematic of the designed computational domain showing the inlet, outlet, and the two meshing gears.

Post-processing: Pressure and Velocity Analysis of the Pump

Now we look at the results to understand how the pump performs. The External Gear Pump CFD Simulation reveals the invisible flow patterns inside the casing. First, let’s look at the Pressure Distribution in Figure 3b. You can see a clear difference between the left and right sides. The Suction Side (Blue): On the left, the pressure drops to -15 kPa. This low pressure acts like a vacuum cleaner. It sucks the fluid into the pump. However, this is a dangerous area. If the pressure drops too low, bubbles can form. This is called “Cavitation,” and it can damage the pump. The Discharge Side (Red): On the right, the pressure rises to +25 kPa. As the gears mesh together, they squeeze the fluid. This compression forces the liquid out of the pump with high energy. The simulation proves that the pump is working correctly as a “Positive Displacement” machine.

Figure 3- Distribution of (a) Velocity (b) Pressure and  around the gears. The contours show low pressure at the inlet and high velocity in the tooth gaps.

Next, we analyze the Velocity Profile in Figure 3a. The fluid speed changes drastically. In the small gaps between the gear teeth and the wall, the fluid moves very fast, reaching speeds of 0.2 m/s. This happens because the spinning gear carries the fluid along the wall. The streamline analysis shows that the fluid accelerates quickly as it enters the meshing zone. The results confirm that the Dynamic Mesh method is successful. It captured the complex motion without crashing. Even though the flow is simple, the pressure changes are violent. This simulation helps engineers choose the right speed for the gears to avoid cavitation while keeping the flow rate high.

Key Takeaways & FAQ

• Q: What is Dynamic Mesh in ANSYS Fluent?
  • A: It is a feature that allows the mesh (grid) to move and change shape during the simulation. It is essential for simulating things like pumps, fans, and valves.
• Q: Why do we use a UDF for this simulation?
  • A: Fluent does not know how gears spin by itself. We use a User Defined Function (UDF) to tell the software the exact speed and direction of rotation for each gear.
• Q: What does the negative pressure (-15 kPa) mean?
  • A: It means suction. This negative pressure pulls the fluid into the pump. However, if it is too negative, it can cause damage called cavitation.