Lecture 8. Turbulent Premixed Flames

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Lecture 8. Turbulent Premixed Flames
X.S. Bai
Turbulent premixed Flames
Content
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•
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•
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Feastures of turbulent premixed flames
Mechanisms of flame wrinkling
Regimes of turbulent premixed flames
Turbulent burning velocity
Propagation of turbulent flames in flow field
Turbulent premixed flame stabilization
X.S. Bai
Experimental setup: Low swirl burner
Filtered Rayleigh Scattering setup
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Results - simultaneous PIV / PLIF
The combined PIV / OH-PLIF results
showing the flame front structure and
its position in the flow field.
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PLIF of fuel
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PLIF of fuel and OH
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Premixed jet flames
D outer
ceramic
holder
Do=22mm
Di=2.2mm
D inner
methane/air
(outer)
methane/air
(inner)
X.S. Bai
Laminar flame
photo
CH2O
CH
Vo=0.45 m/s, phi=1.17; Vin=11m/s, phi=1.1
X.S. Bai
Turbulent jet flame
photo
CH2O
CH
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The shape of a turbulent premixed flame: a closer look
•
Planar single pulse OH radical
concentration, 50mm above the
burner. Field size: 150x110 mm
•
natural gas/air premixed flame
measured by Buschmann et al (26th
symp. Comb., pp.437, 1996)
•
OH peak denotes the flame zone. Why
flame zone is wrinkled? See next slide
X.S. Bai
Lean H2/air premixed flame in a room
door
Lean
H2/air mixture
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Lean H2/air premixed flame in a room
X.S. Bai
Basic features of TPF
• TPF can be divided to three zones
– Preheat zone
– Reaction zone
– Postflame zone
• The reaction zone in typical TPF is thin
– CH layer;
– fuel consumption layer
• The reaction zone is highly wrinkled
– Due to turbulence eddies
– Due to self-instability
• Hydrodynamic instability (Landau-Darrieus)
• Diffusion-thermal instability
• Bouyancy effect (Rayleigh-Taylor)
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Wrinking by turbulence eddies
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Flame – turbulence interaction and
regimes turbulent premixed flames
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Different scales in turbulent premixed flames
• Flow scales
– Mean flow scales
• Length (L), velocity (U), time (t=L/U)
– integral scales
• length (l0), velocity (v0=u(l0)), time (τ0= l0 /v0),
– Kolmogrov scales
• length (η), velocity (vη=u(η)), time (τη= η /vη),
• Flame scales
– flame speed (SL)
– flame thickness (δL)
– time scale (tc)
• flame thickness/flame speed
• chemical reaction time
– the flame structure may not be laminar
X.S. Bai
Physical interpretation of length, time and velocity scales
Chemical reaction and flame scales
oxidation layer
mole fraction
0.2
2000
1800
T
1600
inner layer
0.15
1400
0.1
O2
0.05
CO2
1200
1000
800
C3H8
CO
600
0
−2
temperature (K)
preheat zone
−1
0
1
2
flame coordinate (mm)
X.S. Bai
Mean flow scales
U
alpha
SL = U sin(alpha)
SL
Flow time >> molecularmixing time
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Turbulence eddy scales
X.S. Bai
Physical interpretation of length, time and velocity
scales
Turbulence eddy scales
Eddy size –
Eddy velocity –
Eddy turn over time –
l
ul
teddy=l/ul
Molecular mixing time of
Material of size lk –
tmixing=lk2/D=lk/uk
teddy
ul
l
tmixing
l uk
=
= Rel 1 / 2
ul l k
note : lk = η, uk = u η
X.S. Bai
Physical interpretation of length, time and velocity
scales
inlet, other
boundaries
Energy transfer at a
‘constant’ rate ε
heat
δL
ul
l
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Turbulent jet flame
photo
CH2O
CH
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Turbulent jet flame
PLIF images of
formaldehyde (green)
and CH (red) in
laminar (a) and
turbulent (b-f) flames
with different gas
supply speeds in the
inner tube.
X.S. Bai
Scale relationship
Energy transfer at a
‘constant’ rate ε
inlet, other
boundaries
heat
⎛ l 0 ⎞⎟
v η3
v0
l0 vη
τ0
ε∝
∝
⇒
∝
∝ ⎜⎜ ⎟⎟
⎜⎝ η ⎠⎟
η
τη
l0
v0 η
vη η
vl
∝ 1 ⇒ 0 0 ∝ Rel 0
ν
vη η
2/3
3
l0
l0 v0 vη
l 0v 0 ⎛⎜ η ⎞⎟
∝
∝
⎜⎜ ⎟⎟
η
η vη v0
ν ⎝ l 0 ⎠⎟
1/ 3
⇒
τ
l0
v
∝ Rel 0 3 / 4 ; 0 ∝ Rel 01/ 4 ; 0 ∝ Rel 01/ 2 ;
η
τη
vη
X.S. Bai
Non-dimensional numbers
in turbulent premixed flames
Reynolds number
Damköhler number
Karlovitz number
v 0l 0
Rel 0 =
ν
τ0
l0 S L
Da =
=
τc
v 0 δL
τc
δL v η
δL δL D v η η ⎛⎜ δL ⎞⎟
Ka =
=
=
= ⎜ ⎟⎟
⎜⎝ η ⎠⎟
SL η
S L δL ν ηη
τη
Turbulent intensity
v0
TI =
SL
2
X.S. Bai
Reynolds number
Re l =
v0 l0
ν
∴ S L ∝ (D ⋅ rr ) , δ L ∝ (D / rr ) , S Lδ L ∝ D ∝ ν
1/ 2
1/ 2
v0 l0
v0 l0
v0 l0
∝
∝
Re l =
D
S Lδ L
ν
⎛ v0 ⎞
⎛ l0 ⎞
⇒ log ⎜⎜ ⎟⎟ = log (Re l ) − log ⎜⎜ ⎟⎟
⎝ SL ⎠
⎝δL ⎠
X.S. Bai
Constant Rel lines in the
Borghi-diagram
Borghi-diagram
Rel=104
Constant Rel lines
Rel=100
⎛ v0 ⎞⎟
⎛ l0 ⎞⎟
⎜
log ⎜ ⎟⎟ = log (Rel ) − log ⎜⎜ ⎟⎟
⎜⎝S ⎠⎟
⎝⎜ δ ⎠⎟
L
Rel=1
X.S. Bai
L
Damköhler number
τ0
l0 S L
Da =
=
τc
v 0 δL
l0
v0
log(Da ) = log( ) − log( )
δL
SL
X.S. Bai
Da=0.01
Borghi-diagram
Constant Da lines
Da=1
Da=100
l
v
log(Da ) = log( 0 ) − log( 0 )
δL
SL
Rel=1
X.S. Bai
Karlovitz number
τc
δL v η
δL l 0 v η u ′
δL u ′ l 0 v η
=
=
=
Ka =
SL η
S L l0 η u ′
l0 S L η u ′
τη
δL u ′
δL 1/ 2 u ′ 3 / 2
1/ 2
=
Rel 0 = ( ) ( )
l0 S L
l0
SL
1
2
u′
l0
log( ) = log( ) + log(Ka )
3
3
SL
δL
X.S. Bai
Borghi-diagram
Ka=100
Constant Ka lines
Ka=1
u′
1
l
2
log( ) = log( 0 ) + log(Ka )
SL
3
δL
3
Rel=1
X.S. Bai
Ka=0.01
Borghi-diagram
Ka=100
dark region: GT engines
small circle: GE LM6000
squares: VR-1
I: laminar flames
II: wrinkled flamelet
III: corrugated flamelet
IV: thin reaction zone
V: distributed reactions
X.S. Bai
Flamelet regimes
τc
δL v η
δL δL D v η η ⎛⎜ δL ⎞⎟
Ka =
=
=
= ⎜ ⎟⎟ < 1
⎜⎝ η ⎠⎟
SL η
S L δL ν ηη
τη
2
• The thickness of reaction zone + preheat zone is thinner than the Kolmogrov
scale, i.e. δL<η
– Tranport of mass and heat between the reaction zone and preheat zone is
by molecular mixing
– As a good approximation the local flame propagates at laminar flame
speed and the thickness of the flame is laminar
burned
Unburned
X.S. Bai
mKolmogrov
mreaction ,all _ layers
∼
u(η) D
δ u(η)η
/ 2 ∼ L
δL
δL
η D
∼
δL
<1
η
Turbulent jet flame
PLIF images of
and CH (red) in
laminar (a) and
with different gas
inner tube.
X.S. Bai
Thin reaction zone regime
τ
δ v
δ δ D v η η ⎛⎜ δL ⎞⎟
= ⎜ ⎟⎟ ,1 < Ka < 100
Ka = c = L η = L L
⎜⎝ η ⎠⎟
τη
SL η
S L δL ν ηη
2
burned
Reactant pockets
May not pass the
Inner layer of
The reaction zone
Unburned
mKolmogrov
mreaction ,all −layers
δ u(η)η
u(η)
u(η) D
/Ω ∼
/ 2 ∼ L
∼
δL
δL
δL
η D
∼
X.S. Bai
δL
>1
η
mkolmogrov
mreaction ,inner _ layer
∼
u(η) S L
δ u(η)η
/
∼ inn
δL
δin
η D
∼
δinn
δ
∼ 0.1 L ∼ 0.1Ka 1 / 2 < 1
η
η
Turbulent jet flame
PLIF images of
and CH (red) in
laminar (a) and
with different gas
inner tube.
X.S. Bai
Distributed reaction zone regime
τ
δ v
δ δ D v η η ⎛⎜ δL ⎞⎟
Ka = c = L η = L L
= ⎜ ⎟⎟ , Ka > 100
⎜⎝ η ⎠⎟
τη
SL η
S L δL ν ηη
2
Reactant pockets
May pass the
Reaction zone
Without full
consumption
burned
Unburned
mKolmogrov
mreaction ,all _ layers
X.S. Bai
∼
u(η) D
δ u(η)η
/ 2 ∼ L
δL
δL
η D
∼
δL
>1
η
mkolmogrov
mreaction ,inner _ layer
∼
u(η) S L
δ u(η)η
/
∼ inn
δL
δin
η D
∼
δinn
δ
∼ 0.1 L ∼ 0.1Ka 1 / 2 > 1
η
η
How does TPF propagate
• TPF propagates due to
– Heat transfer to preheat zone to heat up the fuel/air to above
’cross-over’ temperature
– Fuel/air mass transfer to the reaction zone to provide fuel/air
for combustion and releasing heat
• Transport of mass and heat between the reaction zone and the
preheat zone
– can be different from laminar premixed flames
– Depending on the thickness of the zones
X.S. Bai
X.S. Bai
• Also called turbulent flame speed
• Defined as the propagation speed of the mean flame front, relative
to the unburned mixture
• Is equal to the consumption rate of the unburned mixture (volume
flow per unit area of the mean flame front)
X.S. Bai
Turbulent burning velocity: flamelet regime
• Also called turbulent flame speed
• Defined as the propagation speed of the mean flame front, relative
to the unburned mixture
• Is equal to the consumption rate of the unburned mixture (volume
flow per unit area of the mean flame front)
AL
sT
unburned
side
AM
burned
side
sL
η
X.S. Bai
low-intensity, large
scale
Turbulent burning velocity: flamelet regime
ST / S L = 1 + u ′ / S L
ST / S L
Flamelet
theory
Fall-off
1
∼ 10 − 100
X.S. Bai
u′ / S L
Turbulent burning velocity: thin reaction zone regime
ST / S L ∼ Dt / D ∼ u '
0 / ν = Re
ST / S L
Flamelet
theory
Fall-off
1
∼ 10 − 100
X.S. Bai
u′ / S L
1/ 2
0
Burning velocities
• Laminar burning velocity
– Depending on Molecular diffusion
– Depending on Chemical reactions
– Independent of flow
• Turbulent burning velocity
– Depending on Molecular diffusion
– Depending on Chemical reactions
– Depending on flow
X.S. Bai
Propagation of turbulent premixed flames
Propagation of the instantaneous flame front
∂G
+ v ⋅ ∇G = S L ∇ G
∂t
Propagation of the mean flame front
ST
SL
X.S. Bai
∂G
+ v ⋅ ∇G = ST ∇G
∂t
Mean planar flame in a tube
Unburned
x< 0
ST
U
x= 0 (at t=0)
•
•
•
•
Stable flame
Flashback
Blowoff
Quenching distance
X.S. Bai
burned
x> 0
x
Flame position
x = (U − ST )t
Mean conical flame
G(x,r,t)=0
U
x
R
r
ST
U 2 − ST 2
x = (R − r)
ST 2
U
X.S. Bai
Rim stablization
Liftoff
Blowoff
flashback
Flame stabilization
Zst
x
Ssgs
v
Temperature field
and streamlines
(white) from LES
Schematic of the conical burner
X.S. Bai
Flame stabilization
X.S. Bai
AEV burner
Between 1990 and 2005, all new gas turbines of ABB (later Alstom/Siemens) have
Implemented EV burners.
ABB/Alstom EV burner
X.S. Bai
AEV burner: flame stabilization by swirling flow
X.S. Bai
Swirl combustor, dump combustor, bluff-body stabilization
X.S. Bai

Lecture 8. Turbulent Premixed Flames

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