an experimental study of autoignition in turbulent co-flows of heated air c.n. markides & e....
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An Experimental Study of Autoignitionin Turbulent Co-Flows of Heated Air
C.N. Markides & E. Mastorakos
Hopkinson Laboratory, Department of Engineering,University of Cambridge, U.K.
INTRODUCTION• Theory:
Motivated by the DNS work of Mastorakos et al, 1997(and similar)– Re-examination of laminar, inhomogeneous Linan, Linan/Crespo, mid-70’s– Maximizing local reaction rate through ξMR (most reactive mixture fraction)
– AND –– Minimizing local heat losses through χ (effect of scalar dissipation rate)– “Turbulence” may accelerate autoignition– Autoignition was always observed at a finite τIGN (ignition delay time)
• Experiment:Turbulent, inhomogeneous counterflows of Law et al, from late-90’s(and similar)– Turbulent, hot air opposite cold fuel, including hydrogen (elliptic problem)– Enhanced turbulence and increased strain rate increase “autoignition
temperature” necessary for autoignition – and even more interestingly –– Higher strain rates completely preclude autoignition
OBJECTIVES• Aforementioned results are not entirely consistent and there is
an inability to properly explain why
• This is a reflection of a more general situation:– Insufficient current knowledge concerning turbulent,
inhomogeneous autoignition– Limited number of relevant, well characterized experiments for
validation– THUS –
• In order to understand the fundamental underlying physics of the coupling between turbulent mixing and the chemistry of autoignition, we experimentally:– Observe autoignition in a turbulent, co-flow configuration
(parabolic problem, easier to model)– Investigate the temporal and topological features of the
phenomenon– Results directly available for modelling
APPARATUS• Air continuously through
Perforated Grid (3mm, 44%) & Insulated Quartz Tube (24.9mm):– Velocity: up to 40m/s– Temperature: up to 1200K– Turbulence Intensity: 12–14%– Integral Length-scale: 3–4mm– Returb: 80 - 220
• Atmospheric Pressure• Fuel continuously through
S/Steel Injector (2.24/1.185mm):– Velocity(*): 20–120m/s– Temperature(*): 650–1000K– Limited control of temperature
• Bluff bodies (10.0 & 14.0mm):– Used with 24.8 & 34.0 mm tubes
to give a single blockage ratio 0.17
INDEPENDENT VARIABLES:EXPERIMENTAL ACCURACY
• Set all rates to get a steady and repeatable flow– AIR- and FUEL-MFC (excellent, <0.6%)– N2 Flow Meter (average, <5%)
• Measure all flow rates accurately– AIR- and FUEL-MFC (excellent, ~0.9% and ~1.9%)– N2 Flow Meter (average, <6%)
• Set heaters to get steady temperature conditions– Active Heater Controllers (excellent, <1K)
• Measure Tair and Tfuel accurately– Air stream (excellent, <4K(random)+6K(systematic), or <1%)– N2-diluted fuel stream (good, <14K+2K, or <2-3%)– N2-diluted fuel stream & small injector (average, <14K+12K, or <3-
4%)• Measure geometry accurately
– Quartz tubes (excellent, <0.03mm or <0.1%)– Normal injectors (good, <0.03mm or <1%)– Small injectors (excellent, <0.005mm, or <0.4%)
• Measure the ambient pressure• Use accurate 2nd Order Virial Equation of State (error<1%) for
densities
INDEPENDENT VARIABLES:CHARACTERIZATION
• PITOT TUBE and HOT WIRE– Profiles at various axial locations for different Returb
– Mean velocity field uniformity– Magnitude of turbulence intensity– Integral lengthscale from Taylor hypothesis– Turbulence spectra estimation– Kolmogorov scales (dissipation) from variance of the velocity spatial
gradients
• THERMOCOUPLE– Profiles at various axial locations– Heat losses– Extent of thermal boundary layer (profile uniformity)– Estimate temperature fluctuations
• HIGH TEMPERATURE HOT WIRE– Attempt to get simultaneous fluctuations of temperature and
temperature/velocity fluctuation cross-correlations
BULK BEHAVIOUR• CTHC: Four regimes of operation identified for
given Yfuel:
1. ‘No Ignition’2. ‘RANDOM SPOTS’3. ‘Flashback’4. ‘Lifted Flame’
• CTHAJ: Similar, with exception of ‘SPOT-WAKE INTERACTIONS’
T
U
RandomSpots
Flashback
NoIgnition
LiftedFlame
• Looking at effects of:– Fluid mechanics
• Uair and Ufuel
– Chemistry• Tair and Tfuel(*)• Fuel dilution with N2 (Yfuel)
Flow Direction
Injector
Quartz Tube
UNSTEADY BEHAVIOUR
0 10 20 30 40 50 600
50
100
150
Time (s)
Min
imum
Aut
oign
itio
n L
engt
h (m
m) Spot-Wake Interaction
Random Spots
• CTHAJ:‘Spot-Wake Interactions’
• Velocity/Mixing PDFs crucial
• CTHC:‘Unsteady Regime’?
• Velocity/Mixing PDFs crucial
0 10 20 30 40 50 600
20
40
60
80
100
120
Time (s)
Min
imum
Aut
oign
itio
n L
engt
h - L
MIN
(mm
)
Unsteady Regime: Uair
=Ufuel
=24.8m/s,Tair
=877K,YC2H2
=0.66U
air=U
fuel=25.4m/s,T
air=902K,Y
C2H2=0.55
Uair
=Ufuel
=24.8m/s,Tair
=874K,YC2H2
=0.53
Confined Turbulent Flows of Hot AirConfined Turbulent Flows of Hot Air
Quartz Tube:24.9mm
Insulation: Blanket,‘Jacketed’ Tube,Heat Exchanger
Injectors: 2.24&1.185mm
Fuels: H2,C2H2, C2H4,
n-C7H16
Mixing w/ Acetone PLIF and Link w/ LIGN
MEASURE:LIGN, τIGN and fIGN
Quartz Tubes:24.9&34.0mm
Insulation:Blanket,
‘Jacketed’ Tube
Injector &Bluff-bodies:
2.24&10.0/14.0mm
Fuels:
C2H4
only
MEASURE:LIGN ONLY and fIGN
Confined Turbulent Hot Co-FlowsConfined Turbulent Hot Annular Jets
REVIEW
OPTICAL MEASUREMENTS – I SPECTROSCOPY
• CTHC and CTHAJ similar
1. Nothing-to-Spots Transition: C2H4
2. Random Spots: H2
3. Comparison: C2H2 and H2
1
2 3
OPTICAL MEASUREMENTS – II IMAGING
Injector
2.5 mm
~ 4 mm ø
Flow Direction
Flow Direction
Flow Direction
OPTICAL MEASUREMENTS – IIIPMT
• Fast imaging and PMT with all fuels including H2
• Reveal characteristic autoignition event profiles:explosion, propagation and quench
• Obtained fIGN from PMT timeseries; strong correlations with LIGN
OPTICAL MEASUREMENTSOVERVIEW
• Post-ignition flamelet propagation images consistent with DNS
– Spherical shell shape– Propagation velocities ~ 15–20m/s for C2H2 (not considered
in depth)
• Life-span of spots ~ 0.1–0.2s for C2H2 but can vary across fuels
• Autoignition kernel propagation velocities ~ Uair
• Exposure times important because they determine theautoignition information that can be retrieved from the raw images
Flow directionEarliest
Mean
IMAGING DATA ANALYSIS
Earliest
Mean
• Lower U (~ 20 m/s)• And/or Higher T (~ 1010
K)
• Higher U (~ 26 m/s)• And/or Lower T (~ 1000
K)
PDFs from“OH Snapshots”
• From PDF image get lengths:– Mean L⟨ IGN. and ⟩ Standard Deviation LRMS
– Earliest LMIN
• Attempt to define corresponding times
LMIN
⟨LIGN.⟩
Flow Direction
PRELUDE TO RESULTS• In-homogenous autoignition of fuels
in a turbulent co-flow of hot airwith/without an additional bluff-body
• Various regimes possible, depending on conditions– We concentrate on the ‘Random Spots’
• Three types of experiments (mixing):– Equal velocities in CTHC– Jet in Co-Flow in CTHC– Jet in CTHAJ
• (Mostly) optical OH chemiluminescence measurements (images)– To get PDF of autoignition– Define suitable “autoignition lengths”– And calculate corresponding “residence times until autoignition” or
“autoignition delay times”
CTHC RESULTS – I (H2)• Lengths:
– Equal Velocity Case (Uair = Ufuel)
– Increased Tair shifts autoignition UPSTREAM
– Increased U shifts autoignition DOWNSTREAM• LMIN ~ 60–70% of L⟨ IGN⟩
• Times:– Define τMIN “minimum autoignition time” simply as: LMIN/U (~ 1 ms)
– Increased Tair → EARLIER autoignition
– Increased U → DELAYED autoignition
• Similarly for Jet in Co-Flow:– Not easy to define an unambiguous “autoignition time”– Consider the centreline velocity decay in the jet and integrate
IncreasingTair
IncreasingU
U
T
Increasing U
T
U
840 850 860 870 880 890 900 910 9200
20
40
60
80
100
120
Air Temperature (K)
LM
IN,
LM
IN
and L
IGN
(m
m)
U=17.5,Y=0.60 U=20.7,Y=0.62U=29.8,Y=0.62
10 12 14 16 18 200
1
2
3
4
5
6
7
8
9
10
Uair
(m/s)
MIN
(m
s)
T=1026,Y=0.73T=1043,Y=0.72
CTHC RESULTS – II (HnCm)
0.4 0.5 0.6 0.7 0.8 0.9 10
20
40
60
80
100
120
140
Yfuel
(-)
LM
IN a
nd
LM
IN
(mm
)
T=821,U=11.2T=876,U=24.8T=902,U=25.5
T=1026,U=14.8T=1042,U=18.3
1. Effect of fuel dilution (C2H2&C2H4 ):– LIGN decreases as Yfuel increases
2. Effect of Uair (C2H4):– τIGN increases as Uair increases
3. Effect of Tair and small injector (C2H2):– LIGN decreases as Tair increases
– Sensitivity of Tair lost for small injector
• On the effect of Uair:– Autoignition delayed by increase in Uair (and hence) u’,
(because u’ increases with Uair so that u’/U ~ const. behind the
grid)
– BUT –
– Direct comparison with DNS pre-mature until ξ and χ measurements are considered
– In other words:u’ increases, but does χ ~ u’/Lturbξ’’2 also locally increase?
PRELIMINARY DISCUSSION
TURBULENT MIXING:ΒACKGROUND
• Acetone PLIF for mixture fraction
• 266nm straight form Nd:Yag, 110mJ/pulse– Sheet thickness <0.1mm (Kolmogorov Length scales are >
0.15mm)
• Optimal linear de-noising (Wiener) of all images in the Wavelet domain before taking gradients for χ2D
• Consider justification for extending to χ3D
• We have <ξ>, <ξ‘2>, <χ>, <χ‘2> and (not shown) pdf(ξ), pdf(χ)
– Also conditional <χ|ξ>, <χ|ξ‘2>, pdf(χ|ξ)
TURBULENT MIXING: <ξ>
-2 0 20
2
4
6
8
10
r/d (-)
z/d
(-)
-2 0 20
2
4
6
8
10
r/d (-)
z/d
(-)
-2 0 20
2
4
6
8
10
r/d (-)
z/d
(-)
-2 0 20
2
4
6
8
10
r/d (-)
z/d
(-)
0.8
-2 0 20
2
4
6
8
10
r/d (-)
z/d
(-)
0.8
0.2
0.4
0.6
0.8
-2 0 20
2
4
6
8
10
r/d (-)
z/d
(-)
-2 0 20
5
10
15
20
r/d (-)
z/d
(-)
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
-2 0 20
5
10
15
20
r/d (-)
z/d
(-)
• BELOW:– All are equal velocity cases (Uair =
Ufuel) with varying Returb
• RIGHT:– Jet case (Ufuel = 3 and 4 Uair)
0 5 10 15 20 250
0.5
1
z/d (-)
(-
)
10-2
100
10210
-4
10-2
100
z/d (-)
(-
) -2 z/d=5
TURBULENT MIXING (Uair=Ufuel):<ξ>
-2 -1 0 1 20
0.5
1
r/d (-)
(-
)
z/d=1
z/d=4 z/d=5
z/d=6
0 0.05 0.1 0.1510
-1
100
(r/d)2/(z/d)2 (-)
/ (
r=0)
(-
)
z/d=1
z/d=4, 5 and 6
TURBULENT MIXING (Uair=Ufuel):<ξ'2>
-2 0 20
2
4
6
8
10
r/d (-)
z/d
(-)
-2 0 20
2
4
6
8
10
r/d (-)
z/d
(-)
0.02
0.06
-2 0 20
2
4
6
8
10
r/d (-)
z/d
(-)
0.02
-2 0 20
2
4
6
8
10
r/d (-)
z/d
(-)
0 5 10 150
0.05
0.1
0.15
z/d (-)
'
2 (
-)
Re=404, =0.93Re=532, =0.94Re=561, =0.97Re=561, =1.85Re=746, =1.89
TURBULENT MIXING (Uair=Ufuel):<χ>
-1 0 10
1
2
3
4
5
r/d (-)
z/d
(-)
10
20
30
40
50
60
70
-1 0 10
1
2
3
4
5
r/d (-)
z/d
(-) 0 5 10 15 20
0
50
100
z/d (-)
2D
(
1/s)
Re=404, =0.93Re=532, =0.94Re=561, =0.97Re=561, =1.85Re=746, =1.89
-1 0 10
50
100
150
200
r/d (-)
2D
(
1/s)
0 10 20 3002
5
10
15
z/d (-)
3D
/
'2 .
turb
TURBULENT MIXING (Uair=Ufuel):MODELLING – Isotropy and CD
10-3
10-2
10-1
100
101
10210
-3
10-2
10-1
100
101
102
axial
(1/s)
ra
dia
l (
1/s
)
• LEFT:– Isotropy (Radial and Axial Components of χ2D)
• RIGHT:– Timescale ratio model for <χ2D> only valid away from the
injector
CONCLUSIONS
• Length (both LMIN and L⟨ IGN.⟩):– Increase non-linearly with lower Tair and/or higher Uair
– Increase with Ufuel
• Residence Time until Autoignition:– Increases with lower Tair and/or higher Uair
• Enhanced turbulent mixing through u’ and through <χ>:DELAY AUTOIGNITION
An Experimental Study of Hydrogen Autoignition
in a Turbulent Co-Flow of Heated Air
C.N. Markides & E. Mastorakos
Hopkinson Laboratory, Department of Engineering,University of Cambridge, U.K.