lecture 8. turbulent premixed flames - lth · x.s. bai turbulent premixed flames results -...

X.S. Bai Turbulent premixed Flames

Lecture 8. 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

Experimental setup: Low swirl burner

Filtered Rayleigh Scattering setup

Results - simultaneous PIV / PLIF

The combined PIV / OH-PLIF results showing the flame front structure and its position in the flow field.

PLIF of fuel

PLIF of fuel and OH

Premixed jet flames

Douter

Dinner

methane/air(outer)

methane/air(inner)

Do=22mmDi=2.2mm

ceramicholder

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

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

LeanH2/air mixture

Lean H2/air premixed flame in a room

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)

Wrinking by turbulence eddies

Flame – turbulence interaction andregimes 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

Physical interpretation of length, time and velocity scales

Chemical reaction and flame scales

−2 −1 0 1 2flame coordinate (mm)

inner layer

preheat zone oxidation layer

Mean flow scales

Flow time >> molecularmixing time

SL = U sin(alpha)

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

1/ 2Re

eddy kl

mixing l k

t ult u lnote l u u

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

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

Scale relationship

2/ 333

0 0 0 0

0 0 0 0 0

3/ 4 1/ 4 1/20 0 00 0 0

Re ; Re ; Re ;

v vv l ll v

v v lv

vl l v l vv v l

τεη τ η η

ην η

ηη η ν

τη τ

⎛ ⎞⎟⎜ ⎟∝ ∝ ⇒ ∝ ∝ ⎜ ⎟⎜ ⎟⎜⎝ ⎠

∝ ⇒ ∝

⎛ ⎞⎟⎜ ⎟∝ ∝ ⎜ ⎟⎜ ⎟⎜⎝ ⎠

⇒ ∝ ∝ ∝

inlet, otherboundaries

Energy transfer at a‘constant’ rate ε

Non-dimensional numbersin turbulent premixed flames

0 00Relv lν

l SDav

ττ δ

c L L L L

v vDKaS S

ητ δ δ δ δτ η δ ν ηη η

⎛ ⎞⎟⎜ ⎟= = = = ⎜ ⎟⎜ ⎟⎜⎝ ⎠Turbulent intensity

Karlovitz number

Damköhler number

Reynolds number

LSvTI 0=

Non-dimensional numbers

( ) ( )

( ) ⎟⎟⎠

⎞⎜⎜⎝

⎛−=⎟⎟

⎞⎜⎜⎝

⎛⇒

∝∝=

∝∝∝⋅∝∴

DSrrDrrDS

νδδν

000000

2/12/1

logReloglog

Reynolds number

Constant Rel lines in the Borghi-diagram

Borghi-diagram

Constant Rel lines

( )0 0log log Re logl

v lS δ⎛ ⎞ ⎛ ⎞⎟ ⎟⎜ ⎜⎟ ⎟= −⎜ ⎜⎟ ⎟⎜ ⎜⎟ ⎟⎜ ⎜⎝ ⎠ ⎝ ⎠

Rel=100

Rel=104

0 0log( ) log( ) log( )

l SDav

l vDaS

ττ δ

Damköhler number

Borghi-diagram

Constant Da lines

0 0log( ) log( ) log( )L L

l vDaSδ

Da=100

Da=0.01

1/2 1/2 3/2

Re ( ) ( )

1 2log( ) log( ) log( )3 3

c 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

τ δ δ δτ η η ηδ δ

′ ′= = = =

′ ′

′ ′= =

′= +

Karlovitz number

Borghi-diagram

Constant Ka lines

Ka=0.01

Ka=100

01 2log( ) log( ) log( )3 3L L

u l KaS δ′= +

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

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

( ) ( )/

Kolmogrov L

reaction all layers L L

m u uDm D

η δ η ηδ δ ηδη<

∼ ∼

Unburnedburned

1c L L L L

v vDKaS S

⎛ ⎞⎟⎜ ⎟= = = = <⎜ ⎟⎜ ⎟⎜⎝ ⎠

Turbulent jet flame

Thin reaction zone regime

( ) ( ) ( )/ /

Kolmogrov L

reaction all layers L L L

m u u uDm D

η η δ η ηδ δ δ ηδη

∼ ∼ ∼

( ) ( )/

0.1 0.1 1

kolmogrov L inn

reaction inner layer L in

m u S um D

η δ η ηδ δ ηδ δη η

∼ ∼

∼ ∼ ∼

Unburnedburned

,1 100c L L L L

v vDKa KaS S

⎛ ⎞⎟⎜ ⎟= = = = < <⎜ ⎟⎜ ⎟⎜⎝ ⎠

Reactant pocketsMay not pass theInner layer ofThe reaction zone

Turbulent jet flame

Distributed reaction zone regime

( ) ( )/

Kolmogrov L

reaction all layers L L

m u uDm D

η δ η ηδ δ ηδη>

∼ ∼

( ) ( )/

0.1 0.1 1

kolmogrov L inn

reaction inner layer L in

m u S um D

η δ η ηδ δ ηδ δη η

∼ ∼

∼ ∼ ∼

, 100c L L L L

v vDKa KaS S

⎛ ⎞⎟⎜ ⎟= = = = >⎜ ⎟⎜ ⎟⎜⎝ ⎠

Unburnedburned Reactant pockets

May pass theReaction zone Without fullconsumption

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

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)

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

Turbulent burning velocity: flamelet regime

Fall-offFlamelet theory

10 100−∼ / Lu S′

/T LS S

/ 1 /T L LS S u S′= +

Turbulent burning velocity: thin reaction zone regime

Fall-offFlamelet theory

10 100−∼ / Lu S′

1/ 20/ / ' / ReT L tS S D D u ν =∼ ∼

/T LS S

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

Propagation of turbulent premixed flames

TG v G S Gt

∂+ ⋅∇ = ∇

LG v G S Gt

∂+ ⋅∇ = ∇

Propagation of the instantaneous flame front

Propagation of the mean flame front

Mean planar flame in a tube

• Stable flame • Flashback• Blowoff• Quenching distance

USTUnburned x< 0 burned

x= 0 (at t=0) x

( )Tx U S t= −

Flame position

Mean conical flame

G(x,r,t)=0

2( ) T

U Sx R r

Rim stablizationLiftoffBlowoffflashback

Flame stabilization

Schematic of the conical burnerSchematic of the conical burner

TemperatureTemperature fieldfieldandand streamlinesstreamlines((whitewhite) ) fromfrom LESLES

Flame stabilization

AEV burner

ABB/Alstom EV burner

Between 1990 and 2005, all new gas turbines of ABB (later Alstom/Siemens) haveImplemented EV burners.

AEV burner: flame stabilization by swirling flow

Swirl combustor, dump combustor, bluff-body stabilization

lecture 8. turbulent premixed flames - lth · x.s. bai turbulent premixed flames results -...

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