1
Advanced Turbulence Models for
Emission Modeling in Gas Combustion
Ville Tossavainen, Satu Palonen & Antti Oksanen
Energy and Process Engineering
Tampere University of Technology
Funding: Tekes, Metso Power Oy, Andritz Oy, Vattenfall R&D AB (Sweden)
Network: ÅA University (Combustion and Materials Chemistry), HUT (Laboratory of
Applied Thermodynamics), Stanford University (Computational Energy Sciences)
Energy and Process Engineering
Ville Tossavainen & Satu Palonen, Advanced Turbulence Models for Emission Modelling in Gas Combustion
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Outline of Presentation
• Motivation
• Goals and contribution
• Measurements
• Computations
• Conclutions
• Plans for future research
Energy and Process Engineering
Ville Tossavainen & Satu Palonen, Advanced Turbulence Models for Emission Modelling in Gas Combustion
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Motivation for Research
• Computational Fluid Dynamics (CFD) already powerful
design tool in many engineering fields
& Provides compete information on flow conditions
& Modifications quite easily performed
' Accuracy limited by models and computer capacity
• Computational power increases exponentially according to
Moore’s law and single CPU load can be reduced using
computer clusters
• Comprehensive models can be applied to even more
complex and larger applications
Energy and Process Engineering
Ville Tossavainen & Satu Palonen, Advanced Turbulence Models for Emission Modelling in Gas Combustion
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Motivation for Research (…continues)
• Direct Numerical Simulation (DNS) remains in academic
realm for a long time
→ “Intermediate” approach so-called Large-Eddy
Simulation (LES) technique gotten more attention
• LES promising tool for combustion studies and academic
research work is very active
• Industry has great interest to lower emissions in combustion
processes (burners, internal combustion engines, turbines,
furnaces…)
Energy and Process Engineering
Ville Tossavainen & Satu Palonen, Advanced Turbulence Models for Emission Modelling in Gas Combustion
Why a Crossflow Reactor? What is
SNCR process?
• Jet-in-crossflow common phenomenon in combustion
applications
• Staged combustion and good mixing → lower emissions
• Selective non-catalytic reduction (SNCR) process a
secondary method to reduce NOx emissions
• Nitric oxide + Ammonia → Nitrogen + Water
• SNCR process sensitive to temperature:
• Too high temperature → more NO formed
• Too low temperature → NH3 passes unreacted
Energy and Process Engineering
Ville Tossavainen & Satu Palonen, Advanced Turbulence Models for Emission Modelling in Gas Combustion
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6
Goals and Contribution of Research
• Predict emission formation (NOx) and unburned fuel using
LES technique with appropriate turbulence-chemistry
interaction closure
• Contribution to study SNCR process with LES technique
• Detailed measurements with crossflow pilot reactor:
• 1) Cold flow + secondary air, 2) propane flame +
secondary air, 3) Case 2 + ammonia injection
• Different crossflow-to-jet velocity ratios studied
Energy and Process Engineering
Ville Tossavainen & Satu Palonen, Advanced Turbulence Models for Emission Modelling in Gas Combustion
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Measurements
Energy and Process Engineering
Ville Tossavainen & Satu Palonen, Advanced Turbulence Models for Emission Modelling in Gas Combustion
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Crossflow Pilot reactor
• OSBOURNE built in 2003-2004
• Burner:
• Oilon GP-6.20 H
• Power: 60-160 kW
• Liquefied Petroleum Gas, LPG
(98% propane, 2% butane)
• Dimensions:
• Inside width: 400 mm
• Outside width: 760 mm
• Total height: 2830 mm
Energy and Process Engineering
OSBOURNE pilot reactor
Ville Tossavainen & Satu Palonen, Advanced Turbulence Models for Emission Modelling in Gas Combustion
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Crossflow Pilot Reactor (continues...)
Energy and Process Engineering
Ville Tossavainen & Satu Palonen, Advanced Turbulence Models for Emission Modelling in Gas Combustion
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Crossflow Pilot Reactor (...continues)
• Air splitting
• Primary air : 70%
• Secondary air: 30%
• Secondary air system
• 8 nozzles on both sides
• 3 nozzle diameters: 8, 10, 12 mm
Modified Oilon LPG burner
Energy and Process Engineering
Secondary air pipe lines
Ville Tossavainen & Satu Palonen, Advanced Turbulence Models for Emission Modelling in Gas Combustion
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Feedings
• Seeding particles for LDA measurements:
• Ceriumoxide (CeO2), dp = 1 µm
• Secondary air: one inlet on both sides
• Compounds in measurement Case 3:
• NO in primary air
• NH3 in secondary air
Energy and Process Engineering
Particle flow to the
reactor
Ville Tossavainen & Satu Palonen, Advanced Turbulence Models for Emission Modelling in Gas Combustion
Pressurized air
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Measuring parameters
• Velocities with Laser-Doppler Anemometer (LDA)
• Two velocity components
• Concentrations with emission analyzers
• CO, CO2, O2, NO, NO2, HC
• Temperature measurements
• K-type thermocouples
• Flow rates
• Primary and secondary air
• Fuel
Optical measurement inlets
Energy and Process Engineering
Ville Tossavainen & Satu Palonen, Advanced Turbulence Models for Emission Modelling in Gas Combustion
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Combustion case - LDA measurements
MEASUREMENT POINTS:
• Secondary air:
Width of the reactor: 400 mm
• Nozzle diameter: 8 mm
x
X=0
• Temperature: 150°C
LINE 3
• Mean velocity: 30 m/s
260 mm
LINE 2
• Primary air:
• Mean velocity: ~1 m/s
130 mm
LINE 1
Secondary air level
• LDA:
• 11 measurement points
• Sample limit: 50,000 → measurement time 300 s in each point
• Measurements started when temperature reaches 1100°C in the
reaction zone
• Temperature of the reaction zone increases to the 1300°C during the
measurements
Energy and Process Engineering
Ville Tossavainen & Satu Palonen, Advanced Turbulence Models for Emission Modelling in Gas Combustion
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Velocity Vector Fields
• First LDA measurements were done at the
centerline of two opposing jets
(1.6 and 2.6 at the picture)
Jet 1.1
Jet 2.1
Jet 1.2
Jet 2.2
Jet 1.3
Jet 2.3
Jet 1.4
Jet 1.5
Jet 2.4
LDA measurement line 2
(Between)
Jet 1.6
Jet 1.7
Centerline
Jet 2.5
Jet 2.6
LDA measurement line 1
(Centerline)
Jet 1.8
Jet 2.7
Jet 2.8
Measurement window
• Measurements between two opposing jet
pairs were done to find out the behavior of
jets better
• The jets seems to be interlaced !
Between
Energy and Process Engineering
Ville Tossavainen & Satu Palonen, Advanced Turbulence Models for Emission Modelling in Gas Combustion
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Velocity Histograms on Jet Centerline
• Horizontal velocity histograms on line 1
Mean horizontal velocity
Histogram x = 50 mm
800
120,00 %
700
100,00 %
40
700
120,00 %
30
600
100,00 %
Frequency
20
60,00 %
300
40,00 %
200
20,00 %
100
0
0,00 %
10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44
0
-10
0
25
50
75
100 125 150 175 200 225 250 275 300 325 350 375 400
20,00 %
100
0,00 %
-30
Bin
Frequency
Distance [mm]
Cumulative %
Histogram x = 250 mm
120,00 %
50
100,00 %
40
80,00 %
30
60,00 %
20
40,00 %
10
20,00 %
0
0,00 %
120,00 %
90
80
100,00 %
70
Frequency
60
60
80,00 %
50
40
60,00 %
30
40,00 %
20
20,00 %
10
0
0,00 %
-2
-10
-18
-16
-14
-12
0
-8
-6
-4
-2
0
2
4
6
8
10
12
14
16
18
20
22
24
26
28
30
Frequency
40,00 %
200
-20
Histogram x = 150 mm
Bin
Bin
Energy and Process Engineering
60,00 %
300
0
Cumulative %
Frequency
80,00 %
400
-40
Bin
Frequency
10
500
-4 45
2,
5
-3 40
7,
5
-3 35
2,
5
-2 30
7,
5
-2 25
2,
5
-1 20
7,
5
-1 15
2,
5
-1
-7 0
,5
-2 5
,5
0
2,
5
5
7,
5
10
400
Velocity [m/s]
80,00 %
500
-2
-10
-18
-16
-14
-12
0
-8
-6
-4
-2
0
2
4
6
8
10
12
14
16
18
20
22
24
26
28
30
Frequency
600
Histogram x = 350 mm
Cumulative %
Frequency
Cumulative %
Ville Tossavainen & Satu Palonen, Advanced Turbulence Models for Emission Modelling in Gas Combustion
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Concentration measurement points
• 9 below the secondary air jets
• 18 above the secondary air jets
side
front
top
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Ville Tossavainen & Satu Palonen, Advanced Turbulence Models for Emission Modelling in Gas Combustion
3D
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Concentration Profiles (8 mm Nozzle)
• Concentration profile 380 millimeters below secondary air feeding
CO concentration, 380 mm below
CO2 concentration, 380 mm below
7
14
6
12
5
10
4
8
CO [%]
CO2 [%]
3
6
2
4
300
1
200
0
100
200
mm
100
300
2
mm
200
0
100
THC concentration, 380 mm below
300
200
mm
100
mm
NO concentration, 380 mm below
300
50
120
40
100
80
30
THC [ppm]
NO [ppm]
60
20
40
• O2 ~ 0
10
300
200
0
100
200
mm
Energy and Process Engineering
100
mm
20
300
200
0
100
200
300
mm
Ville Tossavainen & Satu Palonen, Advanced Turbulence Models for Emission Modelling in Gas Combustion
100
300
mm
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Concentration Profiles (8 mm Nozzle)
• Concentration profile 68 millimeters above secondary air feeding
CO concentration, 68 mm above
CO2 concentration, 68 mm above
30000
14
25000
12
20000
10
8
CO [ppm] 15000
CO2 [%]
6
10000
0
30
100
200
mm
300
4
300
266
200
mm
133
100
5000
0
O2 concentration, 68 mm above
370
300
266
200
mm
133
100
NO concentration, 68 mm above
2
30
100
mm
200
300
370
120
4
3,5
100
3
80
2,5
O2 [%]
2
NO [ppm]
1,5
• THC ~ 0
40
1
300
266
200
mm
133
100
0,5
0
30
100
mm
Energy and Process Engineering
60
200
300
370
300
266
200
mm
133
100
20
0
30
100
200
mm
Ville Tossavainen & Satu Palonen, Advanced Turbulence Models for Emission Modelling in Gas Combustion
300
370
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Concentration Profiles (8 mm Nozzle)
• Concentration profile 188 millimeters above secondary air feeding
CO concentration, 188 mm above
CO2 concentration, 188 mm above
3000
14
2500
12
10
2000
8
CO2 [%]
CO [ppm] 1500
6
1000
4
500
266
200
0
132
200
mm
266
2
mm
200
0
132
133
200
O2 concentration, 188 mm above
268
mm
mm
133
268
NO concentration, 188 mm above
4,0
120
3,5
100
3,0
2,5
80
O2 [%] 2,0
NO [ppm]
1,5
• THC ~ 0
40
1,0
266
0,5
200
0,0
132
200
mm
Energy and Process Engineering
60
133
268
20
266
mm
200
0
132
200
mm
Ville Tossavainen & Satu Palonen, Advanced Turbulence Models for Emission Modelling in Gas Combustion
133
268
mm
20
Concentration Profiles (8 mm Nozzle)
• Concentration profile 230 millimeters above secondary air feeding
CO concentration, 230 mm above
CO2 concentration, 320 mm above
14
3000
12
2500
10
2000
8
CO2 [%]
CO [ppm] 1500
6
1000
4
500
300
200
0
100
200
mm
300
2
mm
200
0
100
100
O2 concentration, 320 mm above
300
200
mm
mm
100
300
4
NO concentration, 320 mm above
120
3,5
100
3
80
2,5
O2 [%]
2
NO [ppm]
60
1,5
• THC ~ 0
40
1
300
0,5
200
0
100
200
mm
Energy and Process Engineering
100
300
mm
20
300
200
0
100
200
mm
Ville Tossavainen & Satu Palonen, Advanced Turbulence Models for Emission Modelling in Gas Combustion
100
300
mm
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Concentration Profiles (8 mm Nozzle)
• Concentration profile 308 millimeters above secondary air feeding
CO concentration, 308 mm above
CO2 concentration, 308 mm above
3000
14
2500
12
2000
10
8
CO [ppm] 1500
CO2 [%]
6
1000
0
30
100
200
mm
300
4
300
266
200
mm
133
100
500
0
O2 concentration, 308 mm above
370
300
266
200
mm
133
100
NO concentration, 308 mm above
2
30
100
mm
200
300
370
120
4
3,5
100
3
80
2,5
O2 [%]
2
NO [ppm]
1,5
• THC ~ 0
40
1
300
266
200
mm
133
100
0,5
0
30
100
mm
Energy and Process Engineering
60
200
300
370
300
266
200
mm
133
100
20
0
30
100
200
mm
Ville Tossavainen & Satu Palonen, Advanced Turbulence Models for Emission Modelling in Gas Combustion
300
370
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Concentration Profiles (8 mm Nozzle)
• Concentration profile 720 millimeters above secondary air feeding
O2 concentration, 720 mm above
CO2 concentration, 720 mm above
14
4
12
3,5
NO concentration, 720 mm above
120
100
3
10
80
2,5
8
CO2 [%]
O2 [%]
6
NO [ppm]
2
1,5
4
60
40
1
300
2
200
0
100
200
mm
100
300
mm
300
0,5
200
0
100
200
mm
100
300
mm
20
300
200
0
100
200
mm
• THC ~ 0
• CO ~ 0
Energy and Process Engineering
Ville Tossavainen & Satu Palonen, Advanced Turbulence Models for Emission Modelling in Gas Combustion
100
300
mm
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Computations
Energy and Process Engineering
Ville Tossavainen & Satu Palonen, Advanced Turbulence Models for Emission Modelling in Gas Combustion
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What Is LES?
• In LES technique, Large (energy-containing) turbulent scales
solved while the smallest scales modeled
• Large scales geometry-dependent while smallest scales
universal → good approximation
log(E(k))
solved in LES
modeled in LES
modeled in RANS
solved in DNS
solved in LES
modeled in LES
ine
rtia
l
energy
containing
integral scales
large scales
sub
-ra
nge
,s
lop
e
filter length
~ grid size
~5/ 3
viscous sub-range
dx
log(k)
Turbulence energy in wave number space
Energy and Process Engineering
Schematic of scale separation in LES
Ville Tossavainen & Satu Palonen, Advanced Turbulence Models for Emission Modelling in Gas Combustion
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Why to Use LES in Reactive Flows?
• Accurate turbulence modeling needed describing interaction
between turbulence and chemical reactions
• Large scale mixing fully solved → LES able to outperform
RANS turbulence models that can employ more
sophisticated chemistry models
• Chemical reactions occur mostly in the smallest scales
• LES scale separation approximation acceptable
• Thermal NO formation slow but highly temperature sensitive
process and hence strongly influenced by flow field
Energy and Process Engineering
Ville Tossavainen & Satu Palonen, Advanced Turbulence Models for Emission Modelling in Gas Combustion
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Turbulent Non-Premixed Models
• Real propane-air flame consists of several hundred species
and thousands of reactions — How to model it properly?
• Reaction rate approach comprehensive but unpractical in
LES → approach limited to one- or two-step reactions
• Conserved scalar (mixture fraction) based methods usually
chosen in LES computations
• Decouples chemistry (mixture fraction) and mixing (scalar
dissipation)
• Solutions can be based on several methods: flameletbased presumed PDF, conditional moment closure,
transport equation PDF
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Ville Tossavainen & Satu Palonen, Advanced Turbulence Models for Emission Modelling in Gas Combustion
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Non-Premixed Flamelet-Based
PDF Method
• Turbulent flame composed of sheet of laminar flames called
“flamelets” where solutions pre-computed from 2D flames
• Model applicable if turbulent scales larger than flame
thickness → satisfied in most industrial applications
Instantaneous stoichiometric
turbulent flame front
Fuel
Oxidizer
Instantaneous 2D laminar stagnation point flame
flamelet
Fuel
temperature
PDF
look-up
table
Energy and Process Engineering
Laminar flame
solutions
sca
lar d
issip
atio
n
on
racti
ure f
t
x
i
m
Ville Tossavainen & Satu Palonen, Advanced Turbulence Models for Emission Modelling in Gas Combustion
Oxidizer
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LES Flamelet PDF Solver
LES solver
• Pressure
• Velocities
• Mixture fraction
• Density
• Viscosity
• (Temperature)
• (Species mass fractions)
•…
p(z)
p(χ)
z
Presumed PDF for mixture fraction
from flamelet solutions
Model for scalar dissipation rate
Energy and Process Engineering
Presumed PDF of stoichiometric
scalar dissipation from flamelet
solutions
z
Model for stoichiometric scalar dissipation
rate
Ville Tossavainen & Satu Palonen, Advanced Turbulence Models for Emission Modelling in Gas Combustion
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Computational Tools
• Experimental cases simulated using CFD using LES
technique and steady-RANS
• LES simulations performed with structured solver by
Charles Pierce (Stanford University, USA)
• Restrictions for grid size and time step apply → only
center section of pilot reactor simulated
• Steady-RANS simulations performed with commercial
ANSYS FLUENT 6.2/6.3 solver
• Laminar flamelets pre-computed with FlameMaster (by
Heinz Pitsch) using elemental reaction mechanism
files
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Ville Tossavainen & Satu Palonen, Advanced Turbulence Models for Emission Modelling in Gas Combustion
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LES and RANS Computation Geometries
Side view of pilot reactor
Top view of pilot reactor
RANS simulations
LES simulations
Earlier LES cold flow
investigation by Kari
Koskinen (2004)
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Ville Tossavainen & Satu Palonen, Advanced Turbulence Models for Emission Modelling in Gas Combustion
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Steady-RANS Simulations
• Block-structured mesh with several Chimera (nonconformal) grid interfaces
→ Best compromise between grid size (~3M) and
accuracy
• Computations being performed with commercial
FLUENT 6.2/6.3 software
• Non-premixed Flamelet-based PDF
• Realizable k-e turbulence model
Non-conformal grid interfaces
Closer look on secondary air
grid interface
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Ville Tossavainen & Satu Palonen, Advanced Turbulence Models for Emission Modelling in Gas Combustion
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Progress on LES Computations
Özcan & Larsen (2001)
– single jet-in-crossflow
Propane-air flame test case
Pilot reactor
Energy and Process Engineering
Ville Tossavainen & Satu Palonen, Advanced Turbulence Models for Emission Modelling in Gas Combustion
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PDF Chemistry Table Tabulation Process
Table optimization
PDF
look-up
table
Second mixture fraction tabulation
Chemistry look-up table
LES computations
Flamelet Library
Interface to FLUENT
RANS simulations
temperature
sca
lar d
issip
atio
n
ion
fract
e
r
u
mixt
Chemical mechanisms:
• C3H8 + NO
• Ammonia + NO
Energy and Process Engineering
Radiation
Lewis number effect:
• Le=1, Le≠1, Le unequal
Ville Tossavainen & Satu Palonen, Advanced Turbulence Models for Emission Modelling in Gas Combustion
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Stoichiometric temperature [K]
Flamelet Computation Results
1 / Stoichiometric Scalar Dissipation Rate [s]
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Ville Tossavainen & Satu Palonen, Advanced Turbulence Models for Emission Modelling in Gas Combustion
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Pilot Reactor Run
• Dimensionalized by primary air flow conditions:
• Re=36700, Lref=0.10 m, Uref=5.45 m/s,
t=0.001T, djet=8 mm
Axial velocity on centerline (z=0)
Energy and Process Engineering
Ville Tossavainen & Satu Palonen, Advanced Turbulence Models for Emission Modelling in Gas Combustion
36
Conclusions
• Measurements with 8 mm secondary air jet performed and
preliminary LDA and concentration results are satisfactory
• Flow field complex due to instabilities (jets interlaced)
• CO and TCH ~0 in the highest concentration
measurement points
• NOx level raise 20ppm → 80ppm near the secondary air
feeding and remain unchanged
• With 12 mm secondary air nozzle, burner power reduced
160 kW → 90 kW due to too high temperatures (>1300 K)
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Ville Tossavainen & Satu Palonen, Advanced Turbulence Models for Emission Modelling in Gas Combustion
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Conclusions (continues…)
• Only small differences found between propane elementary
reaction mechanisms…
• Differences between different chemical species need to be studied
• Pilot reactor computations started in November and will be
performed till end of March 2007
• Results will be reported on project home page:
• ”http://webhotel.tut.fi/~tossavav/”
Energy and Process Engineering
Ville Tossavainen & Satu Palonen, Advanced Turbulence Models for Emission Modelling in Gas Combustion