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QNET 2002Application Challenge – Cyclone Separator
MDS 16/05/01
QNET - CFD
Contract No.: GTC1 - CT99 - 10030 24th May 2002
Application Challenge
Cyclonic SeparatorPresented
by
Dr. Michael Slack
FLUENT EUROPE LTD Sheffield Airport Business Park
Europa Link . Sheffield . S9 1XU . UK
QNET 2002Application Challenge – Cyclone Separator
MDS 16/05/01
Contents• Introduction
• how it works• Typical applications
• Relevance to industrial sector• Complexities of the flow and modelling approach which may be
taken.• The Application Challenge• Experimental Test Case• CFD Modelling Solution Strategies
• Development of best practice• Boundary conditions • Validation of turbulence models• Numerical accuracy
• Comparison with experimental results• Conclusions and sensitivity discussion (LES findings)
QNET 2002Application Challenge – Cyclone Separator
MDS 16/05/01
The Cyclone• The cyclone is a separation device that induces swirl rotation in a liquid and
therefore imposes an enhanced radial acceleration on a particulate or liquid suspension for the purpose of separation or classification. When applied to liquids they are termed hydrocyclones.
• Uses• Particle Classifier• Phase Separation • Thickener
• Example applications• Mineral Processing (Sorting course material from fine in grinding circuits)• Oil Industry (oil water, sand and gas separation)• Cement Industry (reactor, classifier and dust extraction)
QNET 2002Application Challenge – Cyclone Separator
MDS 16/05/01
Slurry In
Coarse
Fine
Underflow
Inlet
Overflow
Inlet
Vortex Finder
Dust in
QNET 2002Application Challenge – Cyclone Separator
MDS 16/05/01
Advantages•Easy of use
•High volume throughput
•Simple and Compact mechanical design
•Reliable
QNET 2002Application Challenge – Cyclone Separator
MDS 16/05/01
cl
cl
Locus ofzero velocity
+ ve
Tangential velocity
cl
cl
Axial velocity
cl
cl
Inlet
Apex
Axisymmetric Flow patterns
Vortex finder
Showing the tangential and axial velocity distributions in a cyclone basedon published experimental observations
Illustration of how the tangential velocity profile consists of both freeand forced vortex swirl
Tangential velocity profile
0
Wall
Radius
Tang
entia
l vel
ocity
forcedvortex
vr
cons t= tan
freevortex
v r cons t⋅ = tanTransistion
QNET 2002Application Challenge – Cyclone Separator
MDS 16/05/01
Cyclone design and operation•Analytical modelling
•Requires simplifying assumptions to linearise the fundamental equations, (Batchelor).
•Physical modelling
•Expensive and slow
•Numerical modelling
•Recent advances in computers and numerical techniques has led to the application of numerical techniques for cyclone design, (Boysan).
QNET 2002Application Challenge – Cyclone Separator
MDS 16/05/01
Modelling complexities•Physical
•Large number of influencing parameters
•The flow field is essentially 3-dimensional and can only be assumedaxisymmetric in the simplified case.
•High stream line curvature
•Anisotropic turbulence (high shear associated with free vortex motion in main flow)
•High volume concentrations of dispersed phase.
•Numerical
•The elliptic partial differential equations governing fluid flow are non- linear and when swirl is present strongly coupled.
QNET 2002Application Challenge – Cyclone Separator
MDS 16/05/01
Modelling Approaches.• Analytical Modelling
• Improves awareness of the physical mechanics involved but can only be applied to simple geometry's• Bloor, M.I.G. and Ingham, D.B., 1973, 1986 Leeds University.• Batchelor, G. K., 1967, An introduction to fluid dynamics, Cambridge University Press, pp. 543-547.
• Numerical modelling• Allows a wider scope of boundary conditions to be applied and is much more flexible.• Workers in this field: F. Boysan 1984 Pericleous, K. A. and Rhodes, N., 1984, 1986. Davidson, M. R., 1988. Hargreaves, J. H. and Silvester, R. S., 1990. Rajamani, R. K. et al., 1990, 1992. Dyakowski, T. and Williams, R.A., 1993. Fernando Concha A., 1997. Slack, M. D. and Wraith A. E., 1997. Slack, M. D. and F. Boysan. 1998.
QNET 2002Application Challenge – Cyclone Separator
MDS 16/05/01
Turbulence Modelling
Engineers are not interested in the details of the turbulent motions but on the mean properties of the flow.
There is then no need to solve equations for instantaneous variables if averaged values are all that is required.
The numerical treatment of the statistical averaged Navier-Stokes equations are required to provide a value for the Reynolds stress at each point in the flow.
QNET 2002Application Challenge – Cyclone Separator
MDS 16/05/01
Types of Turbulence model• The choice of turbulence model should be the one best suited to the particular flow problem.
• Which one ?• Prandtl Mixing length model
• Treat the N-S Equations as lamina but solve them for a modified turbulent viscosity derived from aprandtl mixing length model
• Eddy Viscosity Models• 2-equation k-epsilon model.• Renormalisation group [RNG] k-epsilon model.
• Second Moment Closure Models• Uses differential transport equations to solve the stresses at each point in the system, eg. The Reynolds
Stress Model [RSM].• Large Eddy LES !
• Current state of the art Research level turbulence model. Transient calculation on a fine mesh to solve the large eddies directly with a sub-grid scale model to represent those eddies smaller than the mesh and faster than the time step.
QNET 2002Application Challenge – Cyclone Separator
MDS 16/05/01
Tangential velocity profiles at 0.41 m below the vortex finder predicted usingSKE, RNG and RSM compared with experimental data.
+ k-epsilon, � RNG, –— RSM, ∆ experiment [LDA]
QNET 2002Application Challenge – Cyclone Separator
MDS 16/05/01
+ k-epsilon, � RNG, –— RSM, ∆ experiment [LDA]
Axial velocity profiles at 0.41 m below the vortex finder predicted usingSKE, RNG and RSM compared with experimental data.
QNET 2002Application Challenge – Cyclone Separator
MDS 16/05/01
0.205m
0.103m
0.103m
0.308m
0.82m
0.154m
0.074m
0.1m
0.04m 0.32m0.35m0.38m0.41m
0.59m0.62m0.66m
0.77m0.80m
0 mRadial Profile measurement locations Tangential Inlet
Boundary (velocity)
Underflow or spigot (wall boundary)
Overflow/vortex finder (Outflow boundary)
Stairmand Cyclone geometry used for application challenge
QNET 2002Application Challenge – Cyclone Separator
MDS 16/05/01
Model Inputs
• Air inflow rate 0.08 m3/s• Release Density 1.225 kg/m3
• Viscosity 1.7894e-5 kg m-1
• Inlet Reynolds Number 10e+4 – 10e+5
• Average residence time in cyclone 0.25 seconds
QNET 2002Application Challenge – Cyclone Separator
MDS 16/05/01
• LDA tangential and axial Velocity measurements recorded on radial lines at vertical distances 0.32, 0.38, 0.41, 0.59, 0.62,0.66, 0.77, 0.8 m from top of cyclone.
• Closest approach to walls 0.00924 m• Error on peak velocities are estimated to be ± 0.25 m/s• Experimental results were presented by Ayers et al. 1983,
Theoretical modelling of cyclone performance, Filtech Conference
•
Experimental Strategy
Measurements taken along this line
Inflow
QNET 2002Application Challenge – Cyclone Separator
MDS 16/05/01
• Highly swirling flow in cyclone separator • Course 40,000 cell hexahedral mesh• High-order upwind scheme was used.• Computed with Reynolds Stress anisotropic turbulence model
with the standard wall functions. • Solved in a Cartesian coordinate system as apposed to a
cylindrical coordinate system therefore requiring higher order discretisation to avoid numerical diffusion.
• PRESTO (Pressure staggered Option) Pressure interpolation scheme. Essential for high swirl.
• QUICK discretisation scheme for • Momentum• Turbulent Kinetic energy• Turbulent disipation rate• Reynolds Stress
•
CFD Modelling strategy
QNET 2002Application Challenge – Cyclone Separator
MDS 16/05/01
Pressure contours Path Lines depicting typical airflow patterns expected
Grid
QNET 2002Application Challenge – Cyclone Separator
MDS 16/05/01
Tangential Axial
35 m/s
0
14 m/s
0
0 cm
32353841
596266
7780
Tangential and axial profiles predicted at all the experimental axial locations.
QNET 2002Application Challenge – Cyclone Separator
MDS 16/05/01
Position 32. Axial & tangential velocity experimental v Predicted .
Position 35. Axial & tangential velocity experimental v Predicted .
–— CFD, ∆ experiment [LDA]
QNET 2002Application Challenge – Cyclone Separator
MDS 16/05/01
Position 38. Axial & tangential velocity experimental v Predicted .
Position 41. Axial & tangential velocity experimental v Predicted .
–— CFD, ∆ experiment [LDA]
QNET 2002Application Challenge – Cyclone Separator
MDS 16/05/01
Position 59. Axial & tangential velocity experimental v Predicted .
Position 66. Axial & tangential velocity experimental v Predicted .
–— CFD, ∆ experiment [LDA]
QNET 2002Application Challenge – Cyclone Separator
MDS 16/05/01
Position 77. Axial & tangential velocity experimental v Predicted .
Position 81. Axial & tangential velocity experimental v Predicted .
–— CFD, ∆ experiment [LDA]
QNET 2002Application Challenge – Cyclone Separator
MDS 16/05/01
Conclusions DiscussionA best practice approach for cyclonic flow modelling has produced.
This documented has been compiled based on modelling experience in this area.
The RSM turbulence combined with high order discretisation schemes provides an engineering tool suitable for this type of flow problem. And can give satisfactory results on relatively course meshes.
But what about sensitivity to grid density and more advanced turbulence models ? (LES) ?
QNET 2002Application Challenge – Cyclone Separator
MDS 16/05/01
Large Eddy Simulation (LES)• The contribution of large, energy containing eddies to
momentum and energy transfer is exactly computed while the effect of small (subgrid) scales is modeled
• LES involves transient, three-dimensional simulations• Computational requirements are intermediate between those
required for RANS and DNS• Very useful in simulating many transient physical processes• Advantage of a Cyclone for LES is the short residence time
of the fluid. Sub grid scales are therefore not as important.
QNET 2002Application Challenge – Cyclone Separator
MDS 16/05/01
Problem Description• Hexahedral mesh of 700,000
cells• QUICK discretization scheme • Second-order time marching
scheme• LES-RNG subgrid model• Time step = 100 µs• 4 days of CPU time on an 8-
Processor HP machine
QNET 2002Application Challenge – Cyclone Separator
MDS 16/05/01
Contours of Velocity Magnitude from LES
Instantaneous Instantaneous Averaged over T=0.25 s
QNET 2002Application Challenge – Cyclone Separator
MDS 16/05/01
LES Cyclone Animations
Velocity magnitude Axial vorticity Velocity vectors
QNET 2002Application Challenge – Cyclone Separator
MDS 16/05/01
Comparison between the tangential velocity at various stations as predicted by the RSM model, the time averaged LES results, and the experimental data (∆ Experimental data; RSM model; - - - Time averaged LES results).
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