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School of something FACULTY OF OTHER
Energy Technology and Innovation Initiative Faculty of
Engineering
Comparison of RANS and LES Turbulence Models for Predicting
Air-coal and Oxy-coal
Combustion Behaviours
2nd International Oxyfuel Combustion Conference 12th – 16th
September 2011, Yeppoon, Australia
Co-authors: D. Caridia, D. Coulingb, M. Gharebaghi, S.R. Gubba,
R.M.A. Ironsb,
L. Ma, M. Pourkashanian and A. Williams
a ANSYS UK b E.ON New Build and Technology, UK
Alessandro Pranzitelli
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Outline
Introduction
The E.ON Combustion Test Facility
Numerical modelling
Air-coal combustion
Oxy-coal combustion
Conclusions
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Introduction
Low
Low
High
Conc
ern
Ove
r av
aila
bilit
y of
Phy
sica
l Mod
el s
Radiation
HT
NOx-SOx
Soot Formation
Fouling Char
Oxidation
RANS Devolatilisation
TRI
Slagging TCI
LES
Volatile Combustion
Hg/Cl Metal release
LT Corrosion
Particle tracking
Ignition
Impact On Oxy-Fuel CFD Simulation Accuracy
High
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E.ON Combustion Test Facility (CTF)
Located in Ratcliffe-on-Soar, UK
1 MWth
Single, wall-fired, low NOx burner
Variety of fuels (coal, gas, biomass, … )
Time-temperature scaled to 500 MW facility
Particle separation by cyclones
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Test conditions
Air-coal (Thoresby)
Thoresby (UK) El Cerrejon (S. America) Cutacre (UK)
Ultimate analysis (%) DAF
C 80.6 78.9 78.5
H 6.2 5.5 4.9
O 9.6 13 7.5
N 1.9 1.7 1.7
S 1.7 0.9 7.4
Proximate analysis (%) AR
Ash 15.8 7.5 24.7
Moisture 2.8 3.4 6.1
VM 30 36.9 27.7
HCV(MJ/kg) 27.1 27.2 32.6
Oxy-coal (El Cerrejon + Cutacre)
Coal analysis
Flow rate (kg/h) Temperature (K)
Coal 131.5 ± 0.1 353
Primary 260 ± 4 353 ± 0.1
Secondary 1120 ± 12 563 ± 0.8
Flow rate (kg/h) Temperature (K)
Coal 146.3 ± 0.1 353
Primary 183 ± 4 353 ± 0.1
Secondary 1033 ± 12 509 ± 0.8
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CFD simulations: approach
commercial code: ANSYS® FLUENT V12.1
Preparation of the geometry
Mesh generation
Steady RANS simulation
LES
Hardware: • RANS: 1 quad-core intel® Q9550, 2.83GHz processor,
8GB RAM • LES: Leeds’ ARC1 HPC cluster, 50 intel® Nehalem X5560,
2.8GHz CPU cores, 2GB RAM/core
CPU time: • RANS: ≈25 days • LES: ≈15 days/second of
simulation
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Grid, the EON CTF
3M cells, RANS and LES
Unstructured, regular cell distribution Mostly hexahedral cells,
small number of polyhedral cells High quality, low cell aspect
ratio and squish, no tetrahedral cells
80% of Turbulent Kinetic Energy should be resolved
in the LES
Complex geometry for CFD simulations
Complex problem, several models required
High quality mesh required for the LES
ε
5.1
0
kL =
120Llcell =Cell sizing:
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Computational grid
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Sub-models
RANS: steady, standard k-ε, wall functions
LES: WALE sub-grid scale model
Radiation: discrete ordinate, WSGGM
Turbulence-chemistry interaction: eddy dissipation
Two-step global reaction for volatile combustion
Devolatilization: single rate
Char combustion: intrinsic model
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Air-coal combustion RANS
top view
side view
Predicted Temperature (K)
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Air-coal combustion RANS vs LES
RANS LES (instantaneous values)
Predicted Temperature distribution (K)
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Oxy-coal combustion RANS vs LES
RANS LES (instantaneous values)
Temperature distribution (K)
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Oxy-coal combustion flame
RANS LES (inst.) LES (mean)
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Oxy-coal combustion LES
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Gas temperature air- and oxy-coal combustion
vertical mid plane, distance from the burner centre line
Oxy-coal Air-coal
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Mean values at the exhaust
Exp. RANS LES
Air-fired
Temperature (k) 1180±50 1232 1282
O2 (%, dry) 3.1±0.5 3.7 2.2
CO (ppm) 11±5 25 50
NO (mg/MJ) 310±10 340 365
LOI (%) 3.9 3.0 2.0
Oxy-fired
Temperature (k) 1383±50 1533 1523
O2 (%, dry) 4.5±0.5 6.0 6.0
CO (ppm) 35±5 68 50
NO (mg/MJ) 151±10 406 380
LOI (%) NA 4.6 6.1
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Conclusions and way forward
Hybrid turbulence models Reduction of the number of cells /
reduction of the CPU time
Conclusions
Good results by RANS for air-coal combustion
Improvement of the results by LES for air-coal combustion
Temperature overestimation for oxy-coal combustion
Need of specific sub-models for oxy-coal combustion, i.e.
non-gray
gas radiation model
Way forward
Non-gray gas model (FSK) Improvement of the results
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Acknowledgements
Financial support from UK EPSRC (OXY-CAP UK/RCUK-China), Dorothy
Hodgkin Postgraduate Awards/EON is gratefully acknowledged.
Contact details: Dr Alessandro Pranzitelli
[email protected]
References: M. Gharebaghi, R.M.A. Irons, L. Ma, M.
Pourkashanian, A. Pranzitelli “Large eddy simulation of oxy-coal
combustion in an industrial combustion test facility”. Int. J.
Greenhouse Gas Control (2011), doi:10.1016/j.ijggc.2011.05.030
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