Turbulent Nonpremixed Combustion
Transcript of Turbulent Nonpremixed Combustion
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School of Aerospace Engineering
Copyright © 2004-2005, 2020-2021 by Jerry M. Seitzman.
All rights reserved.
AE/ME 6766 Combustion
Introduction to Turbulent Combustion:
Turbulent Nonpremixed Flames
Jerry Seitzman
Methane Flame
0
0.05
0.1
0.15
0.2
0 0.1 0.2 0.3
Distance (cm)
Mo
le F
racti
on
0
500
1000
1500
2000
2500
Te
mp
era
ture
(K
)
CH4
H2O
HCO x 1000
Temperature
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School of Aerospace Engineering
Copyright © 2004-2005, 2020-2021 by Jerry M. Seitzman.
All rights reserved.
AE/ME 6766 Combustion
Overview
• The specific goals of this section:
1. Describe turbulent non-premixed jet flames based on experimental results
– flame length
– flame structure
– mixture fraction relations and finite-rate chemistry effects, including local extinction
2. Introduce non-premixed flamelet modeling
3. Present flame index parameter for differentiating non-premixed and premixed combustion in non-premixed combustors
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School of Aerospace Engineering
Copyright © 2004-2005, 2020-2021 by Jerry M. Seitzman.
All rights reserved.
AE/ME 6766 Combustion
Nonpremixed Combustion
• Most common form employed in high power and industrial (especially liquid fueled) combustors
– reactants entering combustor initially nonpremixed
– simpler (no requirement to prevaporize/mix reactants)
– easier to control flame stability
Process Heating (flare) generon.com
Yuan et al., Intl J Spray & Comb Dynamics 2018
(diesel)
energy.gov
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School of Aerospace Engineering
Copyright © 2004-2005, 2020-2021 by Jerry M. Seitzman.
All rights reserved.
AE/ME 6766 Combustion
Nonpremixed Combustion
• Engineering issues
– flame shape, size, location (combustor length,…)
– flame stability (liftoff, blowout, …)
– heat transfer (radiation-soot, distance to walls,…)
– pollutant emissions (NOx, UCH/CO,…)
Process Heating
Yuan et al., Intl J Spray & Comb Dynamics 2018
(flare) generon.com
(diesel)
energy.gov
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School of Aerospace Engineering
Copyright © 2004-2005, 2020-2021 by Jerry M. Seitzman.
All rights reserved.
AE/ME 6766 Combustion
Nonpremixed Combustion• Combustion science issues
– local mixing (fuel, oxidizer, hot products,…)
• scalar dissipation, turbulent transport, partial premixing, differential diffusion
– highly nonlinear dependence of reaction rates on T, Yi,…
– density/buoyancy, flow instabilities
Process Heating
Yuan et al., Intl J Spray & Comb Dynamics 2018
(flare) generon.com
(diesel)
energy.gov
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School of Aerospace Engineering
Copyright © 2004-2005, 2020-2021 by Jerry M. Seitzman.
All rights reserved.
AE/ME 6766 Combustion
Common (Jet) Burner Configurations
• Typical to use fuel jets flowing into
– quiescent oxidizer (air)
“jet flame”
– coaxial oxidizer jet
– swirled coaxial oxidizer
• Many other practical configurations exist
– piloted jets, impinging jets, jets-in-crossflow,…
fuel
fuelair air
fuelair air
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School of Aerospace Engineering
Copyright © 2004-2005, 2020-2021 by Jerry M. Seitzman.
All rights reserved.
AE/ME 6766 Combustion
Turbulent Nonpremixed Jet Flames
from Hottel and Hawthorne, 1949
• Based on visual and photographic observations
– transition
from
laminar
to turb.
– change
in Lf
growth
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School of Aerospace Engineering
Copyright © 2004-2005, 2020-2021 by Jerry M. Seitzman.
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AE/ME 6766 Combustion
• Measurements of maximum (average) height of
flame for different tube diameters and exit
velocities
Turbulent Jet Flame Length
Lf
Qe=ueRe2
Lf
de/2=Re Re=2
Re=1
(in.)
(cm3/s)
de (in.)
ue
Lf, ded1
~2d1
~3d1
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School of Aerospace Engineering
Copyright © 2004-2005, 2020-2021 by Jerry M. Seitzman.
All rights reserved.
AE/ME 6766 Combustion
Jet Flame Length Relations
• Empirical relations available for flame lengths of
turbulent jet flames (Kalghati)
– flame Froude number
21
*
e
stoich
ef
f
dLL
stoich
eef
fD
duL
2
flameefn ,
523
507.01
5.13512
52
*
f
f
f
f
Fr
FrFr
Fr
L
2141
23
TTTgd
fuFr
fee
stoichef
momentum dominated
buoyancy effected
compare to laminar case(X.13)
(X.13a)
(X.13b)
no longer dependent on convection vs. diffusion velocities(ue vs. D/de )
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School of Aerospace Engineering
Copyright © 2004-2005, 2020-2021 by Jerry M. Seitzman.
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AE/ME 6766 Combustion
Nonpremixed Jet Flame Structure• Instantaneous flame structure
- laser sheet imaging
• Reaction zone relatively thin and highly wrinkled
• Large vortical structures increase interfacialarea between fuel and oxidizer
– bring reactantscloser together faster
diffusion
– structures grow (larger downstream)
from Ö. Gülder
FuelAir
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Copyright © 2004-2005, 2020-2021 by Jerry M. Seitzman.
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AE/ME 6766 Combustion
Mixture Fraction Relations• Recall for laminar nonpremixed jet flames we could
relate local scalar field (e.g., T, Yi) to local f
• H2-airexampleassuming fast chem.and Lei=1
– can alsoget resultsif no reactions
Mixing Lines
(no reaction)
T
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School of Aerospace Engineering
Copyright © 2004-2005, 2020-2021 by Jerry M. Seitzman.
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AE/ME 6766 Combustion
• Let’s examine what happens if
we take many measurements at a
single point in the flame
• Each time local flame conditions
change
• Next examples based on
– laser Raman scattering for f
– laser Rayleigh scattering for T
Scalar Measurements in Jet Flame
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Copyright © 2004-2005, 2020-2021 by Jerry M. Seitzman.
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AE/ME 6766 Combustion
• Nonpremixed hydrogen jet flames
– look at two ue cases low, high urms
– turb.large f fluctuations (near flame)
H2-Air Jet Flame Measurements
Magre and Dibble, Comb.
and Flame 73, 195 (1988)
high Da
(ue=U)
lower Da
(ue=3U)
results near equil.
relationship
finite rate (slow)
chemistry effects
each point represents instantaneous measurement at same point in the flowfield
chemoDa
rmseoud
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School of Aerospace Engineering
Copyright © 2004-2005, 2020-2021 by Jerry M. Seitzman.
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AE/ME 6766 Combustion
CH4-Air Jet Flame Measurements
Masri et al., Combustion and Flame 73, 261 (1988)
x/D=10
• Compare to equil. & stag. flame calc. (depends on strain)
usually nearfast chem.
low straincalcs.
slow chemistry, often get
incomplete combustion,
even localextinction
x/D=20
change ofx scale
a=1 s-1
320 s-1
ue=48m/s, D=7.2 mm
CH4
chemistry
slower
than H2
same conditions, new location
mixing line
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School of Aerospace Engineering
Copyright © 2004-2005, 2020-2021 by Jerry M. Seitzman.
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AE/ME 6766 Combustion
Local Flame Extinction
• Can observe localized breaks in flame front
– localized extinction
– can grow: larger region of extinction
– can shrink: ignition of unburned (mixed) reactants and propagation to close “holes”
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School of Aerospace Engineering
Copyright © 2004-2005, 2020-2021 by Jerry M. Seitzman.
All rights reserved.
AE/ME 6766 Combustion
Nonpremixed Flamelet Approach• Repeating general
turbulent combustion regime diagram
• For Ka ~< 1, all turbulent scales slower than chemistry
– region near reaction zone retains laminar-like structure
Da
oRe
0.1
1
10
102
103
102 103 104101 105
Ka=1
Ka=100
Ka=0.01
flamelets
broken flamelets
distributed reaction zones
21Reo
DaKa 1 kDaKa
chemoDa
• Can characterize turbulent flame as ensemble of laminar flamelets
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School of Aerospace Engineering
Copyright © 2004-2005, 2020-2021 by Jerry M. Seitzman.
All rights reserved.
AE/ME 6766 Combustion
Transport Equation for f• Can write conservation equation for mixture fraction
• Common engineering approach to solve for time-averaged conditions
0~~
fDfufu
t
dttxft
f0
t,
1lim average time
ff average Favre~
density weighted
ffff ~
n fluctuatio Favre
(X.15)
0
fDfuf
t
(X.14)
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School of Aerospace Engineering
Copyright © 2004-2005, 2020-2021 by Jerry M. Seitzman.
All rights reserved.
AE/ME 6766 Combustion
Transport Equation for f
• Need models for last two correlated terms (“closure”)
– for turbulent transport, one simple approach isgradient transport model
– often assume molecular diffusion term negligible compared to turbulent transport
fufu
~~
ffuT
~
T = eddy diffusivitye.g., from k- model
0~~
fDfufu
turb. transport diffusion
desired solutions
from mass/mom.(coupled)
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School of Aerospace Engineering
Copyright © 2004-2005, 2020-2021 by Jerry M. Seitzman.
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AE/ME 6766 Combustion
Nonpremixed Flamelet Modeling
• Need approach to relate f to T, Yi
– generally assume no external heat losses
– could use chemical equil., but chem. often not that fast
• Flamelet models
– use local scalar dissipation rate, , e.g.
– rely on strained non-premixed laminar flameletcalculations, = (a); a = strain rate
2
2~ fD
fYY F
ii
)(
ff
YY
F
i
i
iiii
mYDYuYt
i
F
i
F
i mf
YfDfDfuf
tf
Y
2
22
0
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School of Aerospace Engineering
Copyright © 2004-2005, 2020-2021 by Jerry M. Seitzman.
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AE/ME 6766 Combustion
Nonpremixed Flamelet Modeling
• Flamelet equation
– to get average chemical species formation/destruction
need average flamelet equation
– one approach: probability distributions (PDFs) of f
i
F
i mf
YfD
2
22
dfdfPf
Ym
F
i
i ,
~
2
12
21
0 0
from laminar experiments or
calculations(on-line or library)
Favre-avg. Joint PDF
shape=?
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School of Aerospace Engineering
Copyright © 2004-2005, 2020-2021 by Jerry M. Seitzman.
All rights reserved.
AE/ME 6766 Combustion
Non-Premixed Combustors
• Can get premixed and non-premixed combustion in same system/combustor
• Lifted (gaseous) jet flames– leads to partial premixing
• helps stabilize flame
• Reignition of local extinction regions• Liquid-fueled combustors
– fast vaporization can produce regions of premixed combustion if vaporized fuel mixes with oxidizer before reacting
• How to differentiate?– alignment of fuel and
oxidizer gradients atflame
airair fuel
f=fstoich
premixed
non-premixed
FO F
O
premixed non-premixedmax,max,
max,max,
OF
OF
YY
YY
Takeno Flame Index
= 1 = 1