Project overview
A cyclone separator has no moving parts. It spins the incoming mixture so hard that the heavier phase is thrown to the wall and drops out of the bottom, while the lighter phase leaves through the top. Whether it works well depends on a column of air that forms down the middle of the spinning flow: the air core.
The air core's size and stability control how the flow splits between the overflow at the top and the underflow at the bottom, and that split sets the separation efficiency. It cannot be seen from outside a running unit, and a hand calculation will not give it. In this project we ran a multiphase computational fluid dynamics (CFD) analysis in Ansys Fluent to capture the air core directly and see how the separator behaves inside.
Objectives
How we built the multiphase model
A cyclone is a demanding CFD problem: the flow swirls strongly and two phases have to be tracked at once.
Volume of Fluid model
The VOF approach solves the air volume fraction, tracking the boundary between air and liquid instead of blurring them, so the air core comes out of the solution rather than being assumed.
Turbulence for strong swirl
A turbulence model suited to highly swirling flow, since simpler two-equation models struggle with the strong rotation and tend to wash the core out.
Mesh
Refined along the central axis, where the core forms, and through the cone and near the walls, where the swirl and its velocity gradients are strongest.
Solution until the core settles
The solution ran until the air core had formed and settled into a steady shape, because a core that is still growing gives misleading numbers for the flow split.
| Boundary | Setting | Purpose |
|---|---|---|
| Inlet | Tangential feed | At the operating flow rate; it generates the swirl |
| Overflow | Vortex finder (top) | Open to the surrounding pressure |
| Underflow | Bottom outlet | Open to the surrounding pressure |
Leaving both outlets open to the surrounding pressure lets air be drawn in and the flow split naturally, as it does in a real unit.
Results: a stable, full-length air core
The air volume fraction result at the top of this page is the clearest picture of what happens inside:
What it means for the design
The separator generates a stable, full-length air core at its operating condition, so the geometry is fundamentally sound. The model is now a cheap way to test changes and see straight away how the core diameter and flow split respond:
That is the practical route to better separation efficiency, without building and testing one unit after another.
Why multiphase CFD matters for separators
Cyclones, hydrocyclones and separators are used everywhere from mineral processing and oil and gas to water treatment and food production, and they all rely on the same physics: swirl, an air core and a flow split. Because none of it is visible from outside, multiphase CFD is the practical way to size a new separator or find out why an existing one is underperforming. We used the same VOF method in our water tank filling and siphon emptying CFD.
Frequently asked questions
What is the air core in a cyclone separator?
A column of air that forms along the axis of the spinning flow. Its size and stability control how the flow splits between the overflow and the underflow, which sets the separation efficiency of the whole device.
How does CFD capture the air core?
With a multiphase Volume of Fluid (VOF) model, which solves the air volume fraction and tracks the air-liquid interface, together with a turbulence model suited to strong swirl and a mesh refined along the axis and through the cone.
Why isn't the simulated air core perfectly straight?
Because a real air core precesses around the axis rather than standing still, so slight wandering and waviness are physically real. What matters is that it stays continuous and does not collapse or pinch off, which it did not.
What can the model be used for next?
Testing design changes cheaply, such as the vortex finder depth, the cone angle or the feed rate, and seeing how the core diameter and flow split respond, to improve separation efficiency before building.
