Model Assumptions in Light of CFD and Experiment

In order to obtain anal dical models for cyclone performance, investigators have made some rather sweeping assumptions about the flow pattern that actually exists within cyclone separators. We can get some indication of whether these assumptions are reasonable from measmements and numerical simulations. 

One important assumption in models predicting cyclone flow pattern and separation performance is that the surface CS defines the boundary between axial upflow and downflow. We can test this both experimentally and by CFD. We shall discuss CFD (computational fluid dynamics) for cyclone modeling in Chap. 7. 

The qualitative picture is the same in the two figures, in fact the measurements and the simulations agree excellently for the larger vortex finder Dx is a little larger than 1/2 D, the surface appears to be cylindrical in agreement with the model assumption. For the smaller vortex finder, however, the inner part of the vortex widens under the vortex finder wall, and, in the main part of the body, it becomes practically the same diameter as the larger vortex finder. 

Although the general picture therefore is in agreement with the model assumptions, the results in Fig. 4.4.1 indicate that the diameter of the boundary between up- and downward axial flow is determined by D rather than Dx, except right under the vortex finder wall. 

In much of the cyclone literature the radial position of the boundary between up- and downward flow is held to determine the cut size of the cyclone or swirl tube. However, for a particle caught in the upward flow to be lost, it 

---