The gas‐phase flow field in a cyclone separator with a cylindrical body was measured using Laser Doppler Velocimetry (LDV) and characteristics of the natural vortex length were analyzed. The results show that the swirling flow is obviously attenuated along the axial direction downward in the cyclone, and both the tangential velocity and the axial velocity gradually decrease. Meanwhile, the radial position of the maximum tangential velocity in a cross‐section moves towards the wall of the cyclone, so the quasi‐free vortex zone reduces while the forced vortex zone extends. Finally, the outer quasi‐free vortex disappears in a certain axial section. Accordingly, a new prediction model of the natural vortex length is proposed. The natural vortex length is defined as the distance between the bottom end of the vortex finder and the section at which the free vortex zone disappears. The natural vortex length is further explained based on the attenuation characteristics of the swirling flow and energy transfer between the two types of vortexes.
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