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Measurements and simulations of mixing and autoignition on an n-heptane plume in a turbulent flow of heated air. C.N. Markides, G. De Paola, E. Mastorakos( [email protected] ) http://www.eng.cam.ac.uk/~em257/. Introduction. Structure of presentation: Experimental Apparatus - PowerPoint PPT Presentation
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Engineering Department University of Cambridge1
Measurements and simulations of mixing and autoignition on an n-heptane plume in a turbulent
flow of heated air
C.N. Markides, G. De Paola,
E. Mastorakos ([email protected])
http://www.eng.cam.ac.uk/~em257/
Engineering Department University of Cambridge2
Introduction
Structure of presentation:
Experimental– Apparatus– Bulk observations
Simulations– The CFD– The CMC model
Results– Ignition lengths– Explanation of trends– Implications
Conclusions & suggestions for the future
Engineering Department University of Cambridge3
Why is autoignition important?
Fuel
Air
Air
0 1
Mixture
0 1
LPP gas turbines:
Premixing for low NOx, but danger of autoignition!
Diesel & HCCI engines:
Fast mixing for low emissions, but need to predict autoignition!
Engineering Department University of Cambridge4
Experiments
1. Apparatus
Air in, hot
grid
Fuel in, cold
Atmospheric pressureAir T up to 1100KBulk velocities up to 30m/s
Fuels: H2/N2, C2H2/N2
C7H16/N2
Techniques:
Hot wire for initial conditions
PLIF of acetone for
2D image of OH* with ICCD
Turbulence intensity boosted by grids.
“Diffusion from point source”.
(Markides & Mastorakos, 2005, Proc. Comb. Inst. 30)
Engineering Department University of Cambridge5
Experiments
2. Visualization
Ignition spot appears and then disappears.
Location of ignition spot is random.
Fuel
Hot air
OH chemiluminescence (0.2 ms exp.):Individual spots, not connected flame
Ignition spot development at 20kHz: nothing, spot, spherical flame, nothing (consistent with DNS!)
C2H2 ignition, natural light (1/125s exp.)
Engineering Department University of Cambridge6
Experiments
2. VisualizationQualitative regimes of operation (for all Ujet/Uair tested between 1 and 5):
T
U
RandomSpots
Flashback
NoIgnition
LiftedFlame
Individual short-lived autoignition kernels
Continuous flame sheet ? Stabilisation in mixture “almost ready to ignite”?This regime more likely at high Ujet/Uair. Similar to “Cabra” burner
Quick propagation back to nozzle
Autoignition not happeningdue to high strain?
Engineering Department University of Cambridge7
Experiments
2. Visualization
Localised autoignition
Statistically-steady
If ignition happens close, then it happens often
They always come in bursts
Engineering Department University of Cambridge8
Experiments
3. Mixing
Mean and variance of mixture fraction as expected
Two-component scalar dissipation measured at Kolmogorov resolution
<> satisfies global conservation (Bilger, 2004)
Data used for validating CFD & CMC model
(Markides & Mastorakos, to appear in Chem Eng Sci)
Engineering Department University of Cambridge9
Calculations
1. CFD – for mixing, neglecting reactions
STAR-CD
k- model
Very good resolution close to nozzle needed
Use experimental initial conditions
Use experimental Cd in model for <>
Engineering Department University of Cambridge10
Calculations
2. CMC model
Conditional Moment Closure equations:
||2
2
wQ
Nt
Q
jturb
jjj
turbj x
QDP
xPx
Q
x
Du
)(
~
)(~1~
~
~~
2
1
0
),,(~
),,(),(~ dtxPtxQtxY
Conditional convection Conditional turbulent flux
Diffusion in -space & chemistry, closed at 1st order
Engineering Department University of Cambridge11
Calculations
2. Formulation of the CMC model for plume
Averaged across plume:
model AMC from |
),(~
2
),(~
|2|
|||
0
0*
2
2**
N
drrPr
drrPNrN
wQ
Nz
Qu
R
R
Engineering Department University of Cambridge12
Calculations
3. Code, chemistry, validation
31-scalar reduced heptane chemistry
(Bikas, PhD Thesis, Aachen)
Ignition times of homogeneous mixtures OK
Ignition times of spray with CMC OK
(Wright, De Paola, Boulouchos, Mastorakos, Comb. Flame, to appear)
Engineering Department University of Cambridge13
Results
1. Mixing: Good agreement
0 10 20 30 40 50 60 70 80 900
0.1
0.4
0.6
0.8
1
Axial Distance [mm]
Mix
ture
Fra
ctio
n [
-]
CFD
Experiment
0 10 20 30 40 50 60 70 80 900
0.02
0.04
0.06
0.08
0.1
Axial Distance [mm]
Mix
ture
Fra
ctio
n V
ari
an
ce [
-]
CFD
Experiment
0 10 20 30 40 50 60 70 80 900
5
10
15
20
25
Axial Distance [mm]
Scal
ar D
issi
pati
on R
ate
[s-1
]
CFD
Experiment
0 0.2 0.4 0.6 0.8 10
2
4
6
8
10
12
14
16
Mixture Fraction [-]
Con
diti
onal
Sca
lar
Dis
sipa
tion
Rat
e [s
-1]
<>
<>
variance
<>
Engineering Department University of Cambridge14
Results
2. Autoignition lengths: reasonably good agreement
1090 1100 1110 1120 1130 1140 11500
50
100
150
200
250
Aut
oig
nit
ion
Len
gth
(m
m)
Tair
(K)
OH LMODE
: U=13.8,=1.05
OH LMIN
: U=13.8,=1.05
LCMC
: U=13.8,=1.05
OH LMODE
: U=17.6,=1.20
OH LMIN
: U=17.6,=1.05
LCMC
: U=17.6,=1.20
Physics: • As U increases, ignition length L increases, but also L/U increases. Hence, not
simply chemistry-controlled!• Trend captured by model
0.875 0.88 0.885 0.89 0.895 0.9 0.905 0.91 0.9150.5
1
1.5
2
2.5
3
3.5
4
ln(A
uto
ign
itio
n T
ime)
(m
s)
1000/Tair
(1/K)
OH MODE
: U=13.8,=1.05
OH MIN
: U=13.8,=1.05
CMC
: U=13.8,=1.05
OH MODE
: U=17.6,=1.05
OH MIN
: U=17.6,=1.05
CMC
: U=17.6,=1.05
Engineering Department University of Cambridge15
Results
2. Conditional statistics
Ignition at the most-reactive mixture fraction, not at stoichiometry.As L increases, P()(-well-mixed).
Ignition time becomes long as P(MR) 0.
0 0.2 0.4 0.6 0.8 11000
1100
1200
1300
1400
1500
1600
1700
1800
1900
Mixture Fraction [-]
Tem
pera
ture
[K
]
0 0.2 0.4 0.6 0.8 11000
1100
1200
1300
1400
1500
1600
1700
1800
1900
Mixture Fraction [-]
Tem
per
atu
re [
K]
Low T: long L High T: short L
MR
Engineering Department University of Cambridge16
Results
3. Discussion
0 10 20 30 40 50 60 70 800
5
10
15
20
25
30
35
40
45
50
Max Conditional Scalar Dissipation Rate <N|> [s -1]
Ign
itio
n D
ela
y T
ime
[ms]
0 20 40 60 80 1000
10
20
30
40
50
60
70
Axial Distance [mm]
Co
nd
itio
na
l S
cala
r D
issi
pa
tio
n R
ate
at
M
R
17.64 m/s
13.79 m/s
Autoignition limit <N|=MR
>
Flamelet or CMC: ignition time increases
as N increases
In our flow: N increases with U
Hence: Ignition time in our flow increases as U increases
Also: N<Ncritical hence 2nd-order CMC not needed
Engineering Department University of Cambridge17
Conclusions
A novel autoignition rig is operational and has produced results for various fuels
Intense turbulence can delay autoignition due to increasing scalar dissipation rate
CMC model can capture all experimental trends
Crucial aspect: modelling of scalar dissipation
Future: Transport equation for <>
2D-CMC to capture spatial diffusion / flashback conditions
LES, PDF calculations