The Influences of Preheat Temperature and Mixture Compositions ...

Lean Combustion Conference II
April 25-29, 2004, Tomar, Portugal
The Influences of Mixture Composition
and Preheat Temperature on
Combustion Regime
ChulJu AHN*, Fumiteru AKAMATSU*,
Masashi KATSUKI *, Akio KITAJIMA **
*Department of Mechanical Engineering, Osaka University, Japan
** Combustion Reaction Control Research Group, Institute for Energy
Utilization, National Institute of Advanced Industrial and Science ce Technology,
Japan

Background(1)
Energy Efficiency V.S. NOx Emission
Combustion air preheating by regenerater
Emerging new combustion technology
⇒ called mild combustion, flameless oxidation
or highly preheated air combustion
Advanced furnace technology resolving the dilemma
of thermal efficiency and nitric oxide emission
has been developed

Highly preheated air combustion
air
fuel
spark plug
open
open
pilot burner A pilot burner B
on
flow and combustion
off
close
close
air
fuel
spark plug
heat transfer
fuel
open
slab
close
fuel
burner A
burner B
4-way switching valve
insulation
ceramic honeycomb
air
open
G
gate valve
open
exhaust gas

Variation of NOx Emissions Level with Air Temperature
200
NOx [ppm] at 0 % O2
100
Conventional
Air Staging Burner
Excess Air : 20 %
HiPAC Burner
Excess Air : 30 %
0
0 200 400 600 800 1000 1200
Air Temperature [℃]

Flammable domain of mixture
♦ Flammable domain as a function of T mix , Y O2 , and φ
Present work
T mix = 1173K
T mix
Low O 2 O 2 21%
φ φ st
300K
Rich Lean
YO 2
Conventional region
of study on premixed
turbulent combustion

Background(2)
Principal feature of mild combustion
Mixing of fuel and air or air/fuel mixture with large
amounts of exhaust gas in the furnace by high speed
ejection of combustion air (Internal EGR)
Formation of lean diluted mixture with
burned product before combustion reaction
Decrease of thermal NOx formation

Far-lifted jet flame
Oxygen concentration in the co-flow : 21%
Fuel : CH4, Nozzle diameter :0.4mm
V jet,fuel : 80m/s
V jet,fuel :160m/s
1235K 1194K 1165K
1337 K 1297 K

Direct photograph of flames
1400
Air temperature K
1300
1200
1100
900
200 ~100
90 ~40
Vf
Va1
Va0
Preheated Air
Fuel
Nozzle
Fuel
600
21 17 15 11 8 5
O2 2mole fraction %

Autoignition and blow-off temperatures
of propane in air and air diluted with nitrogen
1800
Air temperature K
1600
1400
1200
1000
800
600
400
Pa = 0.12
Re: air = 1.2 × 10
Re: fuel = 4.2 × 10
×
×
×
○
■
×
Flammable region
×
No flame region
×
Autoignition Boundary
Stable Boundary
Auto-ignition point
Non Auto-ignition point
Unstable Point (CO>100 ppm)
× × × ×
φ=1
× × 0
200
20
15 10
5
O 2
concentration %

Objective(1)
To observe the change of combustion regime of
turbulent premixed flames with various mixture
composition and preheat temperature

Experimental apparatus
Air +N 2
H 2
O 2
fuel
T1
Q T =Q a +Q N2
Nozzle mixing
Duct
T2

Flammability limit
φ equivalence ratio
0.2
0.3
0.4
0.5
0.6
0.7
0.8
20 18 16 14 12 10 8 6
O 2 co ncentratio n %
C H 4 5 7 3 K
C H 4 7 7 3 K
C H 4 9 7 3 K
C H 4 1 0 7 3 K
C H 4 1 1 7 3 K

Flame temperature at extinction
Y CH4
%
5.0
4.5
4.0
3.5
3.0
2.5
Tad
2.0
500 600 700 800 900 100011001200
Mixture temperature K
2000
1800
1600
1400
1200
1000
800
600
400
Adiabatic flame temperature K

Experimental condition (Tu=1173K)
Q ox (L/min) Q CH4 (L/min) O 2 (%) T ad (K) S L (m/s)
F3Hox 65 3.15 19.4 2112 3.934
F3Mox 65 3.15 13.0 2118 3.220
F2Hox 65 2.40 19.4 1923 3.003
F2Mox 65 2.40 13.0 1927 2.625
F2Lox 65 2.40 7.4 1917 1.555
F1Hox 65 1.68 19.4 1719 1.952
F1Mox 65 1.68 13.0 1721 1.806
F1Lox 65 1.68 7.4 1724 1.322
Since Q ox was fixed as 65, turbulence level is
almost same at each condition

Approach
To investigate flame structure of turbulent
premixed flames with high preheat temperature
(1173K)
Flame structure
→ Flame visualization by OH-LIPF
→ Temperature measurement by Thermocouple
Mixing time (turbulent time) estimation
→ 1-D LDV :Estimation by calculating auto-correlation coefficient using
slot-method
Chemical time estimation
→Laminar flame calculation of S L , δ th by PREMIX with GRI-Mech3.0

Methane flames
Mixture temperature : 1173K
O2
7.4%
φ 1.0 φ 0.9 φ 0.8 φ 0.7
O2
13.0%
φ 0.7 φ 0.6 φ 0.5 φ 0.4
O2
19.4%
φ 0.6 φ 0.5 φ 0.4 φ 0.3

Instantaneous OH-LIPF images(1)
F3Hox
F3Mox
Image
region
-18-16-14-12-10 -8 -6 -4 -2 0 2 4 6 -18-16-14-12-10 -8 -6 -4 -2 0 2 4 6

Instantaneous OH-LIPF images(2)
F2Hox F2Mox F2Lox
-18-16-14-12-10 -8 -6 -4 -2 0 2 4 6 -18-16-14-12-10 -8 -6 -4 -2 0 2 4 6 -18-16-14-12-10 -8 -6 -4 -2 0 2 4 6

Instantaneous OH-LIPF images(3)
F1Hox F1Mox F1Lox
-18-16-14-12-10 -8 -6 -4 -2 0 2 4 6 -18-16-14-12-10 -8 -6 -4 -2 0 2 4 6 -18-16-14-12-10 -8 -6 -4 -2 0 2 4 6

OH-LIPF images and PDF of temperature
Ka=0.44
Ka=0.65
Ka= (δ F /η) 2
(a) F3Hox
(b) F3Mox
-11 -9 -7 -5 -3 -1 1 3 5 7 9 11
125
100
75
50
25
a.u.
T(F3Hox)
T(F3Mox)
2000
1600
1200
800
PDF %
2000
1600
1200
800
PDF %
0 2 4 6 8 10
Time ms
R=3.5mm
6
4
2
0
800 1000 1200 1400 1600 1800 2000
Temperature K
8
6
4
2
0
0 2 4 6 8 10
Time ms
R=4mm
800 1000 1200 1400 1600 1800 2000
Temperature K

OH-LIPF images and PDF of temperature
Ka=0.74
Ka= (δ F /η) 2
Ka=0.97
Ka=2.76
(a) F2Hox
(a) F2Mox
(a) F2Lox
-4 -2 0 2 4 6 8 10 12 14 16 18
R mm
60
50
40
30
20
10
a.u.
18
16
14
12
10
8
6
4
2
Z mm
T(F2Hox)
T(F2Mox)
T(F2Lox)
2000
1600
1200
800
8
PDF %
6
4
2
0
2000
1600
1200
800
4
2
0
PDF %6
0 2 4 6 8 10
Time ms
R=6mm
800 1000 1200 1400 1600 1800 2000
Temperature K
0 2 4 6 8 10
Time ms
R=6mm
800 1000 1200 1400 1600 1800 2000
Temperature K
2000
1600
1200
800
0 2 4 6 8 10
8
6 R=7.5mm Time ms
4
2
0
8800 1000 1200 1400 1600 1800 2000
PDF %
Temperature K

Calculated chemical properties in experimental
conditions (T u =1173K)
F3Hox F3Mox F2Hox F2Mox F2Lox F1Hox F1Mox F1Lox
T AD K( ± 5K ) 2120 1920 1720
T i K( ± 5K ) 1550 1520 1420
T OH K( ± 5K ) 1780 1600 1540
Density ratio γ 1.81 1.64 1.47
Density ratio γ OH
1.52 1.36 1.31
O 2 %
Equivalence ratio
S L m/s
S OH m/s
δ th µm
δ OH µm
τ F =δ th /S L µs
τ OH =δ OH /S OH µs
19.4 13.0 19.4 13.0 7.4 19.4 13.0 7.4
0.5 0.75 0.38 0.57 1.0 0.27 0.4 0.7
3.934 3.220 3.003 2.625 1.555 1.952 1.806 1.322
6.1 5.0 4.31 3.78 2.25 2.56 2.38 1.81
363 423 430 478 687 583 614 748
222 236 254 264 324 316 322 355
92 131 143 182 442 299 340 566
36 47 59 67 144 123 135 196

Flame classification by Borghi
Re L = u’L/ν
u’/S L
Stirred
reactor
DRZ
u'/
S L
Ka= (δ F /η) 2
Da
τ
f
= =
τ
c
LE/ u'
δ / S
F
L
L
δ
Da
λ
=
λ / u'
δ /
S L

Flame classification diagram proposed by Peters
Hatched region is the area observed as thin reaction zone by others
at room temperature condition.
u’/ S L
100
10
1
Broken reaction zone
Thin reaction zone
Ka=100
Wrinkled laminar flame
Ka=10
Ka=3
Ka=1
Corrugated flamelet
0.1
0.1 1 10 100 1000
F3Hox
F3Mox
F2Hox
F2Mox
F2Lox
F1Hox
Mansour (1998)
φ=1.0, fuel(CH4)
T u = 300K, O 2 21%
Ka = 17
Thin reaction zone
F2 7.4% Flame
φ=1.0, fuel(CH4)
T u = 1173, O 2 7.4%
Ka = 2.76
Broken reaction zone
L E / δ F

⎛
Ka C L
F
=
m⎜
⎝ δ
C
τ t
τ
m
th
⎞
⎟
⎠
05
= ( S δ / 15ν
) . ≅1
m L th u
= L / u'
f
= δ / S
F th L
L m
Dimensionless number for flame classification by
≡ ε
12 / 32 /
τ F
Da
F
Chen(1)
τ L u
t f
= = ⎛ ⎝ ⎜ / ' ⎞
⎟
τ δ / S ⎠
F
th
L
(Turbulent)
Preheat zone
−1
δ th
Re
= Lu' f
ν u
Burnt gas
Mixing length scale:
maximum distance that
preheated fluid can be
transported ahead of the flame
L m
Preheated material
Reaction zone

Regime Diagram by Chen et al. (1)
100
10
u' / S L
1
0.1
Re=100
5
well stirred
flame front
Re=1
laminar flow
distributed flame front
4
2
3
Da F
=1
turbulent
flame front
Ka F
=1 or L m
=δ th
complex strain flame front
η u
=δ th
wrinkled laminar flamelets
1
Chen
Mansour
Dinkelacker
Buschmann
Present data
0.01
0.1 1 10 100
L f
/ δ th

What is difference
Ka F
Da F
L m
L m,OH
L m /δ th
mm
mm
Present flames(1173K)
L m,OH /δ OH
F3Hox F3Mox F2Hox F2Mox F2Lox F1Hox F1Mox F1Lox
0.74 1.06 1.15 1.47 3.56 2.41 2.74 4.56
5.41 3.80 3.49 2.74 1.13 1.67 1.47 0.88
0.35 0.6 0.68 0.97 3.68 2.04 2.48 5.33
0.09 0.13 0.18 0.23 0.68 0.54 0.62 1.09
0.97 1.41 1.58 2.04 5.35 3.51 4.04 7.13
0.39 0.54 0.71 0.88 2.11 1.72 1.93 3.06
Flames of Chen et al.(300K)
MM1 MM2 ML3 MM3 MH3 MM4
Ka F
Da F
L m
L m,OH
L m /δ th
L m,OH /δ OH
mm
mm
0.86 1.26 1.25 2.58 3.76 3.83
7.5 5.1 4.0 2.5 2.0 1.7
0.43 0.76 1.05 2.22 3.23 3.85
0.0102 0.0194 0.0265 0.0558 0.0812 0.0897
0.98 1.51 1.65 3.46 5.04 5.20
0.054 0.091 0.107 0.226 0.329 0.346

Expanding for the scalar such as OH, CH(2)
Ka
ψ
= ⎛ ⎝ ⎜ ⎜
L m , ψ
δ ψ
⎞
⎟
⎠
Da
ψ
τ
L
/ u'
t f
= = ⎛ τψ ⎝ ⎜ δψ / Sψ
⎞
⎟
⎠
−1
Re
= Lu' f
ν u
τ t
τ
= L / u'
f
= δ / S
ψ ψ ψ
/ /
L m, ψ
≡ ε
12 32
τψ
(Turbulent)
Preheat zone
δ ψ
Burnt gas
Mixing length scale for scalar:
maximum distance that
scalar can be transported
ahead of the reaction zone
L m,ψ
Reaction zone
L
1/ 2 3/
2

Definition of flame thickness
Temperature K
Temperature
CH
OH
δ th
Y ψ
δ
th
=
Tad
− Tu
( ∂T
/ ∂x)| max
Temperature K
T OH
x coordinate mm
Temperature
OH
δ OH
Y ΟΗ
T
S
δ
OH
OH
OH
YOH
= T
⎛ ⎝ ⎜ ∂ ⎞
⎟
∂x
⎠
max
TOH
= ⎛ S
⎝ ⎜ ⎞
⎟ =γ
T ⎠
=
u
( YOH
|
max
)
( ∂Y
/ ∂x)|
OH
S
L OH L
max
x coordinate mm

Modified Diagram by Ka ψ , Da ψ (300K)
Ka ψ =1, Da ψ =1 for methane (300K, φ=1)
1000
u' / S L
100
10
1
0.1
Re=100
Re=10
Re=1
Quenching
Da F
=1
0.01
0.1 1 10 100
L f
/ δ th
4
3
Da CH
=1
Da OH
=1
Ka CH
=1 or L m,CH
=δ CH
Ka F
=1 or L m
=δ th
Ka OH
=1 or L m,OH
=δ OH
η u
=δ th
Distributed flame front
(with broken reaction zones)
Turbulent flame front
(with thin reaction zone)
Chen
Mansour
Dinkelacker
Buschmann

Changes of critical dimensionless number with
increasing T u (φ=1.0, CH4, 1atm)
Ka F,OH_crit
30
25
20
15
10
5
Ka
=
C
F, ψ_
crit m
32 /
ψ
0.5
0.4
0.3
0.2
0.1
0
0.0
400 600 800 1000
Mixture Temperature T u
K
1
γ
⎛ δ
ψ
Da F , ψ _ crit
= ⎜
γ
ψ ⎝ δth
⎛ δ
⎜
⎝ δ
⎞
⎟
⎠
th
ψ
⎞
⎟
⎠
Da F,OH_crit
T AD
, T OH
K
2800
2600
2400
2200
2000
1800
1600
1400
1200
1000
density ratio γ, γ OH
8
7
6
5
4
3
2
T AD
T OH
S OH
S L
400 600 800 1000
Mixture Temperature
u
TK
δ
δ
γ ΟΗ
th
OH
γ
δ th / δ OH
TAD
− Tu
=
( ∂T
/ ∂x)| max
YOH
|
max
=
( ∂Y
/ ∂x)|
OH
max
400 600 800 1000
Mixture Temperature
u
TK
20
18
16
14
12
10
8
6
4
2
0
8
7
6
5
4
3
2
1
0
S L
, S OH
m/s
scale ratio δ th
/δ OH

Modified Diagram by Ka ψ , Da ψ (1173K)
Ka ψ =1, Da ψ =1 for for present F3Hox
1000
u' / S L
100
10
1
Re=100
Re=10
Re=1
Quenching
Da F
=1
4
3
Da CH
=1
Da OH
=1
Ka CH
=1 or L m,CH
=δ CH
Ka F
=1 or L m
=δ th
Ka OH
=1 or L m,OH
=δ OH
Distributed flame front
(with broken reaction zones)
Turbulent flame front
(with thin reaction zone)
0.1
η u
=δ th
0.01
0.1 1 10 100
Present data
L f
/ δ th

Summary
• Extinction limit is mainly affected by mole fraction of
fuel (thus, flame temperature) and is less affected by O2
concentration in premixed turbulent combustion
• The criterion between flamelet regime and broken
reaction zones changes depending on initial condition of
mixture such as mole fractions of fuel, oxygen and initial
temperature.
• Mild combustion in premixed type is observed in high
temperature and low oxygen concentration with the
characters of small temperature fluctuations, low sound
emission, relatively distributed reaction zone.

Thank you.