X.S. Bai Turbulent premixed Flames
Lecture 8. Turbulent Premixed Flames
X.S. Bai Turbulent premixed Flames
Content
• 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 Turbulent premixed Flames
Experimental setup: Low swirl burner
Filtered Rayleigh Scattering setup
X.S. Bai Turbulent premixed Flames
Results - simultaneous PIV / PLIF
The combined PIV / OH-PLIF results showing the flame front structure and its position in the flow field.
X.S. Bai Turbulent premixed Flames
PLIF of fuel
X.S. Bai Turbulent premixed Flames
PLIF of fuel and OH
X.S. Bai Turbulent premixed Flames
Premixed jet flames
Douter
Dinner
methane/air(outer)
methane/air(inner)
Do=22mmDi=2.2mm
ceramicholder
X.S. Bai Turbulent premixed Flames
CH2O CHphoto
Vo=0.45 m/s, phi=1.17; Vin=11m/s, phi=1.1
Laminar flame
X.S. Bai Turbulent premixed FlamesVo=0.45 m/s, phi=1.17; Vin=120m/s, phi=1.0
CH2O CHphoto
Turbulent jet flame
X.S. Bai Turbulent premixed Flames
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 Turbulent premixed Flames
LeanH2/air mixture
door
Lean H2/air premixed flame in a room
X.S. Bai Turbulent premixed Flames
Lean H2/air premixed flame in a room
X.S. Bai Turbulent premixed Flames
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)
X.S. Bai Turbulent premixed Flames
Wrinking by turbulence eddies
X.S. Bai Turbulent premixed Flames
Flame – turbulence interaction andregimes turbulent premixed flames
X.S. Bai Turbulent premixed Flames
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 Turbulent premixed Flames
Physical interpretation of length, time and velocity scales
Chemical reaction and flame scales
600
800
1000
1200
1400
1600
1800
2000
tem
pera
ture
(K
)
−2 −1 0 1 2flame coordinate (mm)
0
0.05
0.1
0.15
0.2
mol
e fr
actio
n
T
O2
CO2
CO
C3H
8
inner layer
preheat zone oxidation layer
X.S. Bai Turbulent premixed Flames
Physical interpretation of length, time and velocity scales
Mean flow scales
Flow time >> molecularmixing time
alpha
SL = U sin(alpha)
U
SL
X.S. Bai Turbulent premixed Flames
Physical interpretation of length, time and velocity scales
Turbulence eddy scales
X.S. Bai Turbulent premixed Flames
Physical interpretation of length, time and velocity scales
Turbulence eddy scales
Eddy size – lEddy velocity – ulEddy turn over time – teddy=l/ul
Molecular mixing time of Material of size lk – tmixing=lk
2/D=lk/uk
ul
l
ηη
= =
= =
1/ 2Re
: ,
eddy kl
mixing l k
k k
t ult u lnote l u u
X.S. Bai Turbulent premixed Flames
Physical interpretation of length, time and velocity scales
ul
l
δL
heat
inlet, otherboundaries
Energy transfer at a‘constant’ rate ε
X.S. Bai Turbulent premixed FlamesVo=0.45 m/s, phi=1.17; Vin=120m/s, phi=1.0
CH2O CHphoto
Turbulent jet flame
X.S. Bai Turbulent premixed Flames
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.
Turbulent jet flame
X.S. Bai Turbulent premixed Flames
Scale relationship
2/ 333
0 0 0 0
0 0
0 00
1/ 3
0 0 0 0 0
0 0
3/ 4 1/ 4 1/20 0 00 0 0
1 Re
Re ; Re ; Re ;
l
l l l
v vv l ll v
v v lv
vl l v l vv v l
l vv
η η
η
η
η
η
η
η η
τεη τ η η
ην η
ηη η ν
τη τ
⎛ ⎞⎟⎜ ⎟∝ ∝ ⇒ ∝ ∝ ⎜ ⎟⎜ ⎟⎜⎝ ⎠
∝ ⇒ ∝
⎛ ⎞⎟⎜ ⎟∝ ∝ ⎜ ⎟⎜ ⎟⎜⎝ ⎠
⇒ ∝ ∝ ∝
heat
inlet, otherboundaries
Energy transfer at a‘constant’ rate ε
X.S. Bai Turbulent premixed Flames
Non-dimensional numbersin turbulent premixed flames
0 00Relv lν
=
0 0
0
L
c L
l SDav
ττ δ
= =
2
c L L L L
L L L
v vDKaS S
η η
η
ητ δ δ δ δτ η δ ν ηη η
⎛ ⎞⎟⎜ ⎟= = = = ⎜ ⎟⎜ ⎟⎜⎝ ⎠Turbulent intensity
Karlovitz number
Damköhler number
Reynolds number
LSvTI 0=
X.S. Bai Turbulent premixed Flames
Non-dimensional numbers
( ) ( )
( ) ⎟⎟⎠
⎞⎜⎜⎝
⎛−=⎟⎟
⎠
⎞⎜⎜⎝
⎛⇒
∝∝=
∝∝∝⋅∝∴
=
Ll
L
LLl
LLLL
l
lSv
Slv
Dlvlv
DSrrDrrDS
lv
δ
δν
νδδν
00
000000
2/12/1
00
logReloglog
Re
,/,
Re
Reynolds number
Constant Rel lines in the Borghi-diagram
X.S. Bai Turbulent premixed Flames
Borghi-diagram
Constant Rel lines
( )0 0log log Re logl
L L
v lS δ⎛ ⎞ ⎛ ⎞⎟ ⎟⎜ ⎜⎟ ⎟= −⎜ ⎜⎟ ⎟⎜ ⎜⎟ ⎟⎜ ⎜⎝ ⎠ ⎝ ⎠
Rel=1
Rel=100
Rel=104
X.S. Bai Turbulent premixed Flames
Non-dimensional numbers
0 0
0
0 0log( ) log( ) log( )
L
c L
L L
l SDav
l vDaS
ττ δ
δ
= =
= −
Damköhler number
X.S. Bai Turbulent premixed Flames
Borghi-diagram
Constant Da lines
Rel=1
0 0log( ) log( ) log( )L L
l vDaSδ
= −
Da=1
Da=100
Da=0.01
X.S. Bai Turbulent premixed Flames
Non-dimensional numbers
0 0
0 0
1/2 1/2 3/2
0
0 0
0
Re ( ) ( )
1 2log( ) log( ) log( )3 3
c L L L
L L L
L Ll
L L
L L
v v vl u u lKaS S l u l S u
u ul S l S
u l KaS
η η η
η
τ δ δ δτ η η ηδ δ
δ
′ ′= = = =
′ ′
′ ′= =
′= +
Karlovitz number
X.S. Bai Turbulent premixed Flames
Borghi-diagram
Constant Ka lines
Rel=1
Ka=1
Ka=0.01
Ka=100
01 2log( ) log( ) log( )3 3L L
u l KaS δ′= +
X.S. Bai Turbulent premixed Flames
Borghi-diagram
dark region: GT enginessmall circle: GE LM6000squares: VR-1
I: laminar flamesII: wrinkled flameletIII: corrugated flameletIV: thin reaction zoneV: distributed reactions
Ka=100
X.S. Bai Turbulent premixed Flames
Flamelet regimes
• 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
2
, _
( ) ( )/
1
Kolmogrov L
reaction all layers L L
L
m u uDm D
η δ η ηδ δ ηδη<
∼ ∼
∼
Unburnedburned
2
1c L L L L
L L L
v vDKaS S
η η
η
ητ δ δ δ δτ η δ ν ηη η
⎛ ⎞⎟⎜ ⎟= = = = <⎜ ⎟⎜ ⎟⎜⎝ ⎠
X.S. Bai Turbulent premixed Flames
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.
Turbulent jet flame
X.S. Bai Turbulent premixed Flames
Thin reaction zone regime
2
,
( ) ( ) ( )/ /
1
Kolmogrov L
reaction all layers L L L
L
m u u uDm D
η η δ η ηδ δ δ ηδη
−
Ω
>
∼ ∼ ∼
∼
, _
1/ 2
( ) ( )/
0.1 0.1 1
kolmogrov L inn
reaction inner layer L in
inn L
m u S um D
Ka
η δ η ηδ δ ηδ δη η
<
∼ ∼
∼ ∼ ∼
Unburnedburned
2
,1 100c L L L L
L L L
v vDKa KaS S
η η
η
ητ δ δ δ δτ η δ ν ηη η
⎛ ⎞⎟⎜ ⎟= = = = < <⎜ ⎟⎜ ⎟⎜⎝ ⎠
Reactant pocketsMay not pass theInner layer ofThe reaction zone
X.S. Bai Turbulent premixed Flames
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.
Turbulent jet flame
X.S. Bai Turbulent premixed Flames
Distributed reaction zone regime
2
, _
( ) ( )/
1
Kolmogrov L
reaction all layers L L
L
m u uDm D
η δ η ηδ δ ηδη>
∼ ∼
∼
, _
1/ 2
( ) ( )/
0.1 0.1 1
kolmogrov L inn
reaction inner layer L in
inn L
m u S um D
Ka
η δ η ηδ δ ηδ δη η
>
∼ ∼
∼ ∼ ∼
2
, 100c L L L L
L L L
v vDKa KaS S
η η
η
ητ δ δ δ δτ η δ ν ηη η
⎛ ⎞⎟⎜ ⎟= = = = >⎜ ⎟⎜ ⎟⎜⎝ ⎠
Unburnedburned Reactant pockets
May pass theReaction zone Without fullconsumption
X.S. Bai Turbulent premixed Flames
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 Turbulent premixed Flames
Turbulent burning velocity
X.S. Bai Turbulent premixed Flames
Turbulent burning velocity
• 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 premixed Flames
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)
low-intensity, large scale
burned side
unburned side
AL
AMsT
η
sL
X.S. Bai Turbulent premixed Flames
Turbulent burning velocity: flamelet regime
Fall-offFlamelet theory
1
10 100−∼ / Lu S′
/T LS S
/ 1 /T L LS S u S′= +
X.S. Bai Turbulent premixed Flames
Turbulent burning velocity: thin reaction zone regime
Fall-offFlamelet theory
1
10 100−∼ / Lu S′
0
1/ 20/ / ' / ReT L tS S D D u ν =∼ ∼
/T LS S
X.S. Bai Turbulent premixed Flames
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 Turbulent premixed Flames
Propagation of turbulent premixed flames
TG v G S Gt
∂+ ⋅∇ = ∇
∂
ST
SL
LG v G S Gt
∂+ ⋅∇ = ∇
∂
Propagation of the instantaneous flame front
Propagation of the mean flame front
X.S. Bai Turbulent premixed Flames
Mean planar flame in a tube
• Stable flame • Flashback• Blowoff• Quenching distance
USTUnburned x< 0 burned
x> 0
x= 0 (at t=0) x
( )Tx U S t= −
Flame position
X.S. Bai Turbulent premixed Flames
Mean conical flame
R r
U
G(x,r,t)=0
x
2 2
2( ) T
T
U Sx R r
S−
= −
U
ST
Rim stablizationLiftoffBlowoffflashback
X.S. Bai Turbulent premixed Flames
Flame stabilization
Ssgsv
x
Zst
Schematic of the conical burnerSchematic of the conical burner
TemperatureTemperature fieldfieldandand streamlinesstreamlines((whitewhite) ) fromfrom LESLES
X.S. Bai Turbulent premixed Flames
Flame stabilization
X.S. Bai Turbulent premixed Flames
AEV burner
ABB/Alstom EV burner
Between 1990 and 2005, all new gas turbines of ABB (later Alstom/Siemens) haveImplemented EV burners.
X.S. Bai Turbulent premixed Flames
AEV burner: flame stabilization by swirling flow
X.S. Bai Turbulent premixed Flames
Swirl combustor, dump combustor, bluff-body stabilization