Research Papers: Multiphase Flows

Numerical Investigations of Unsteady Flows and Particle Behavior in a Cyclone Separator

[+] Author and Article Information
Osamu Akiyama

2-18-1 Toyogaoka, Tama-si,
Tokyo 206-0031, Japan
e-mail: acky036@gmail.com

Chisachi Kato

Institute of Industrial Science,
The University of Tokyo,
4-6-1 Komaba, Meguro-ku,
Tokyo 183-8505, Japan
e-mail: ckato@iis.u-tokyo.ac.jp

Contributed by the Fluids Engineering Division of ASME for publication in the JOURNAL OF FLUIDS ENGINEERING. Manuscript received June 15, 2016; final manuscript received March 12, 2017; published online June 28, 2017. Assoc. Editor: John Abraham.

J. Fluids Eng 139(9), 091302 (Jun 28, 2017) (11 pages) Paper No: FE-16-1368; doi: 10.1115/1.4036589 History: Received June 15, 2016; Revised March 12, 2017

Mechanism of particle separation in a cyclone separator is fully clarified by one-way coupled numerical simulations of large eddy simulation and particle tracking. The former resolves all the important vortical structures, while the latter inputs the computed flow fields and tracks trajectories of particles by considering Stokes drag as well as gravity. The computed axial and tangential velocities of the swirl flow in a cyclone compare well with the ones measured by particle image velocimetry (PIV). The precession frequency of the vortex rope computed for Stairmand cyclone also matches with the one measured by Darksen et al. The predicted collection efficiencies reasonably agree well with the measured equivalents for two cylindrical cyclones with different diameters and inflow conditions. Detailed investigations on the simulated vortical structures in the test cyclones and predicted trajectories of the particles have revealed that there are three major paths of trajectories for those particles that are not collected and exhausted from the cyclone. More than half of the exhausted particles are trapped by longitudinal vortices formed in the periphery of the vortex rope. Namely, the precession motion of the vortex rope generates a number of longitudinal vortices at its periphery, which trap particles and move them into the region of the upward swirl.

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Fig. 1

Schematic view of experimental system with stereo particle image velocimetry

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Fig. 2

Measured diameter distributions of test particles

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Fig. 3

Computational model together with computed typical streamlines and mesh shown at every five grids for CY70

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Fig. 4

Computed instantaneous distributions of tangential velocity uθ in z/D = −0.53, −2.43, and −3.57 planes (left) and y = 0 plane (right) for CY70

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Fig. 5

Comparisons of computed and measured time-averaged tangential velocity uθ (left) and its fluctuation uθ′ (right) in z/D = −2.43 (top) and −3.57 (bottom) planes for CY70

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Fig. 6

Comparison of computed and measured frequency spectra of velocity fluctuations in z/D = −2.0 plane for Stairmand cyclone

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Fig. 7

Collection efficiency curves for CY70 and CY60

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Fig. 8

Comparisons of overall collection efficiency for CY70 (top) and CY60 (bottom)

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Fig. 9

Comparisons of time-averaged tangential velocity

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Fig. 10

Balance of mass flow (top) and computed mass flow ratio of downward flow (bottom)

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Fig. 11

Computed instantaneous distributions of vortical structures (left—∇2CP = 1200) and skin-friction coefficient on inner surface of cyclone (right) for CY70

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Fig. 12

Computed instantaneous distribution of vortical structure (left—∇2CP = 1200) and skin-friction coefficient on inner surface of cyclone (right) for TAN70

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Fig. 13

Typical particle trajectories tracked in averaged flow in CY70. The shaded area of the middle graph indicates region with upward swirl flow.

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Fig. 14

Typical particle trajectories tracked in unsteady flow in CY70. The shaded area of the middle graph indicates region with upward swirl flow.

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Fig. 15

Instantaneous vortical structures visualized by isosurface of ∇2CP = 1200 together with particles with a diameter of 2.5 μm in CY70

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Fig. 16

Motion of particle (sphere) trapped by a longitudinal vortex (cross)

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Fig. 17

Ratio of number of exhausted to released particles categorized in terms of minimum z position reached by each particle

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Fig. 18

Comparison of collection efficiency curves between averaged and unsteady flows for CY70 and TAN70




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