Application Area 3: Chemical & Process, Thermal Hydraulics & Nuclear Safety
Application Challenge AC3-03
Cyclone chambers (cyclones) are used in many industrial processes. Typical applications for cyclones include two phase separation, liquid clarification, slurry thickening, solids washing, degassing of liquids, solids classification/sorting and for the separation of immiscible liquids.
This wide range of applications has resulted in the development of many alternative designs of cyclone with the design optimised in respect of the specific application.
A cyclone design type commonly used in industry is cono-cylindrical. A design of this type is shown in Figure 1. The key geometrical features of this type of cyclone are identified in the figure.
The type of cyclone shown in Figure 1 uses either a single or a multiple set of tangential inlets. The inflowing fluid rotates within the main body of the chamber and is constrained to follow a spiral flow path. Any particles suspended within the fluid are subjected to an enhanced radial acceleration. Cyclones are commonly used when the density of the inflowing fluid (the carrier phase) is less than the particle phase. In cyclones of this type the larger particles migrate outwards to the cone wall where they travel in a downward spiral to the base of the chamber and exit at the underflow. The smaller particles migrate more slowly and therefore their distribution across the flow changes little. Those in the centre are captured in the upward flow and spiral upward and out through the vortex finder, see Figure 1. The remainder are discharged with the coarse fraction at the underflow or spigot.
The application challenge (AC) described in this document focused on a cyclone of the type shown in Figure 1. CFD simulations were carried out and the predicted axial and tangential velocity components were compared with published experimental data. The domain was represented using a relatively coarse unstructured hexahedral mesh. The Reynolds stress turbulence model was used.
Fluent CFD software version 5.0 was used throughout the study.
The absence of moving parts and simple compact construction combined with a high volume of material throughput make the cyclone a convenient and practical tool for industrial applications, many of which are still being explored.
Rietema carried out some of the first experimental optimisation studies that built upon the work of Kelsall. The results of such studies have been used to develop empirical based design criteria. Numerical and analytical studies of the flow in cyclones have also been carried out, the most notable by Bloor and Ingham. As stated by Slack and Wraith, this approach requires simplifying assumptions about the Navier-Stokes equations aimed at establishing a solvable analytical model. Clearly, therefore, CFD can be used to further develop the design of cyclones and for the design of non-standard cyclones where empirical or simplified models may be inadequate. The development of an accurate CFD cyclone modelling technique would also address the need for a reliable computer design model for cyclones called for by Knowlton.
The assessment parameters used for the purpose of this AC were the mean axial and tangential velocity profiles on a number of radial lines through a cyclone chamber with the location of each radial profile identified in Figure 2.
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