School of Aerospace Engineering
Flame Structure and
Flame Speed Measurements
Jerry Seitzman
2500
2000
0.15
1500
CH4
H2O
HCO x 1000
Temperature
0.1
1000
0.05
500
Temperature (K)
Mole Fraction
0.2
Methane Flame
0
0
0
0.1
0.2
0.3
Distance (cm)
AE/ME 6766 Combustion
FlameSpeedMeasurementsLimits -1
Copyright © 2004-2005 by Jerry M. Seitzman. All rights reserved.
School of Aerospace Engineering
1D Laminar Flame Structure
• Recall assumed flame structure
unburned burned
T
Ti q&′′′
T2
T1
preheat zone reaction
zone
x
Discussion point:
SL for Le≠
≠1
FlameSpeedMeasurementsLimits -2
Copyright © 2004-2005 by Jerry M. Seitzman. All rights reserved.
AE/ME 6766 Combustion
1
School of Aerospace Engineering
CH4 Flame Structure
• Calculations
using Premix
(Chemkin)
– stoich.
CH4/air
– 298K, 1 atm
– “full”
mechanism
ref: Glassman
AE/ME 6766 Combustion
FlameSpeedMeasurementsLimits -3
Copyright © 2004-2005 by Jerry M. Seitzman. All rights reserved.
School of Aerospace Engineering
CH4 Flame Structure
• Calculations
using Premix
(Chemkin)
– stoich.
CH4/air
– 298K, 1 atm
– “full”
mechanism
ref: Glassman
FlameSpeedMeasurementsLimits -4
Copyright © 2004-2005 by Jerry M. Seitzman. All rights reserved.
AE/ME 6766 Combustion
2
School of Aerospace Engineering
H2 Flame Structure
• Calculations
using Premix
(Chemkin)
– stoich.
H2/air
– 298K, 1 atm
– “full”
mechanism
ref: Glassman
AE/ME 6766 Combustion
FlameSpeedMeasurementsLimits -5
Copyright © 2004-2005 by Jerry M. Seitzman. All rights reserved.
School of Aerospace Engineering
H2 Flame Structure
• Calculations
using Premix
(Chemkin)
– stoich.
H2/air
– 298K, 1 atm
– “full”
mechanism
ref: Glassman
FlameSpeedMeasurementsLimits -6
Copyright © 2004-2005 by Jerry M. Seitzman. All rights reserved.
AE/ME 6766 Combustion
3
School of Aerospace Engineering
Flame Structure
• Most hydrogen/hydrocarbon flames have at least
three discernible zones
1. a preheat/diffusion dominated zone
2. an initial reaction zone
• radical production begins, intense heat release
3. a final oxidation (burnout) zone
• thicker than initial reaction zone
• slow approach to final equilibrium
• final heat release
AE/ME 6766 Combustion
FlameSpeedMeasurementsLimits -7
Copyright © 2004-2005 by Jerry M. Seitzman. All rights reserved.
School of Aerospace Engineering
Laminar Flame Speed
• Flame speed
m& ′′
∝ α RR
Sb ρu Tb
=
~
m& ′′
S L ρb Tu
Sb =
SL =
unburned burned
ρu
ρb
• Assumptions
T
q&′′′
Sb
SL
u=0
Tu=T1
SL=u1
T2=Tb
u2=Sb
preheat zone reaction
zone
x
– one-dimensional flame structure
• no cross-stream diffusion
• only changes along one direction
– adiabatic
• all energy “losses” from reaction zone go into oncoming
reactant gases
FlameSpeedMeasurementsLimits -8
Copyright © 2004-2005 by Jerry M. Seitzman. All rights reserved.
AE/ME 6766 Combustion
4
School of Aerospace Engineering
Flame Speed Measurements
• Measurements of “SL” are difficult
– hard to achieve adabatic and 1-d conditions
– usually compromise and try to correct
• Various methods
–
–
–
–
–
–
Bunsen-type burners (“simplest”)
traveling tube
spherical “bombs”
soap bubbles
flat flame burners
stagnation flames
AE/ME 6766 Combustion
FlameSpeedMeasurementsLimits -9
Copyright © 2004-2005 by Jerry M. Seitzman. All rights reserved.
School of Aerospace Engineering
Bunsen (Tube) Burner Method
• Premixed fuel/air in
cylindrical tube
– laminar
– various velocity
profiles
– stationary flame
when normal
component of local
approach velocity=SL
α
SL
un
u
FlameSpeedMeasurementsLimits -10
Copyright © 2004-2005 by Jerry M. Seitzman. All rights reserved.
color schlieren
inner
cone
outer flame
(nonpremixed)
ref: Sébastien Ducruix
CH4/air
φ=1.05
AE/ME 6766 Combustion
5
School of Aerospace Engineering
Bunsen (Tube) Burner Method
α
• Approaches
SL
particle
tracks
– measure α, u (or un)
un u
• u not necessarily same as uexit
• u, α may not be constant
(depends on exit profile)
– measure flame area
m& = ρuA ⇒ S L = m& exit ρ u A flame
• how to identify flame surface
– schlieren, shadow,
luminosity,…
luminousity
φ<1
ref: Echekki and Mungal, Phys. Fluids A 2, 1523 ( 1990)
AE/ME 6766 Combustion
FlameSpeedMeasurementsLimits -11
Copyright © 2004-2005 by Jerry M. Seitzman. All rights reserved.
School of Aerospace Engineering
Bunsen (Tube) Burner Method
Visible
• Flame surface
measurement
Schlieren
– luminous,
schlieren,
shadow
can give
similar results
– shadow closest to
unburned region
Shadowgraph
• Other non-1d issues
– curvature, crossstream diffusion
⇒Su≠SL
ref: Ibarreta and Sung, 3rd Joint US Section Meeting Comb. Instit.
FlameSpeedMeasurementsLimits -12
Copyright © 2004-2005 by Jerry M. Seitzman. All rights reserved.
AE/ME 6766 Combustion
6
School of Aerospace Engineering
Propagating Methods
uf
• Transparent Tube
ur
– fill tube with premix gases, ignite, watch flame
propagate (make sure it is sustained propagation)
– gas moves ahead of flame front (compressed)
• uf > SL, and gases may be slightly preheated
• non-1d (boundary layer), buoyancy
uf =
• Spherical Bomb
dR f
dt
– fill closed spherical chamber,
Rf
watch flame propagate
– pressure, unburned velocity continuously increasing
– not 1-d (planar), affected by curvature and strain
FlameSpeedMeasurementsLimits -13
Copyright © 2004-2005 by Jerry M. Seitzman. All rights reserved.
AE/ME 6766 Combustion
School of Aerospace Engineering
Flat Flame Burner
• Construct uniform velocity profile using porous
plate, sintered metals, or small flow passages
• Burner is either water cooled or naturally cooled
• Flame can be essentially
SL
flat except at edges of burner
• Measurement of flame
Q
velocity easy (uexit)
• Nonadiabatic
– measure cooling, extrapolate to Q=0
– or T profile and compare with
computation
FlameSpeedMeasurementsLimits -14
Copyright © 2004-2005 by Jerry M. Seitzman. All rights reserved.
ref: Glassman
AE/ME 6766 Combustion
7
School of Aerospace Engineering
Stagnation Flames
Nozzle
Stagnation plane
Flame
Nozzle
• Nearly flat flames (no curvature),
but with aerodynamic “strain” due
to deceleration caused by
stagnation plane
2
1.6
Axial velocity (m/s)
• Opposed
jets or jet and
stagnation
plate
1.2
0.8
Plug
K=-du/dx
0.4
– can try to extrapolate to zerostrain to attain SL
FlameSpeedMeasurementsLimits -15
X
L
D
U
Nozzle
0
1
ref: J. Natarajan PhD thesis
Copyright © 2004-2005 by Jerry M. Seitzman. All rights reserved.
Flame
Su
3
5
7
9
Distance from stagnation plane X (mm)
11
AE/ME 6766 Combustion
8