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Gas and liquid cyclones, the latter often being referred to as hydrocyclones, are simple devices widely used in many industries for phase separation.

Usually, the goal is to separate a solid phase from a carrier gas or liquid flow. The tangential inlet of flow into the main body of the cyclone generates a flow pattern—and consequently a centrifugal force profile—which is responsible for the phase separation based on the density difference between the phases: generally, the higher the density difference, the higher the separation efficiency.

However, there are many additional factors affecting the separation efficiency, including the cyclone dimensions, particle size distribution, particle loading, feed flow rate, and split ratio. For some applications of liquid cyclones, where the cyclone is open to the atmosphere, air is sucked through the overflow/underflow ducts to the center of the cyclone, forming an air core whose shape and stability can also affect the separation performance.

To achieve the required centrifugal force profile in the cyclone, it is important to optimize the velocity field, which is very complex despite the fairly simple geometry of cyclones. Computational Fluid Dynamics (CFD) modeling of multiphase flows in cyclones, including the highly anisotropic turbulence modeling for swirling flow, requires special considerations. Upon validation of a CFD framework for such flows, further simulations can be employed to optimize operational conditions for a given cyclone design. Thus, the geometry of the cyclone can be improved to enhance separation performance, or performance changes can be assessed when scaling of the cyclone is considered.

The animation displays the evolution of an air core in a 75mm hydrocyclone operating at the overflow-to-underflow mass flow split ratio of 30. Note how the air core extends over the entire length of the hydrocyclone. The Reynolds Stress Model, with quadratic pressure strain, was used to determine the turbulence stresses and obtain an accurate CFD prediction for the air core.