molecular transport effects of hydrocarbon addition on turbulent hydrogen flame propagation

SM(1)11-13 Sep 20072nd International Conference on Hydrogen Safety, San Sebastian, Spain

Molecular Transport Effects of HydrocarbonAddition on Turbulent Hydrogen Flame Propagation

Siva P R Muppala and Jennifer X WenFire and Explosion Research Group, Department of Mechanical Engineering

Kingston University, London, UK

Naresh K AluriGas Turbine Combustion group, ALSTOM (Baden), Switzerland

F DinkelackerInstitut Fluid- und Thermodynamik, Universität Siegen,

Paul-Bonatz-Str. 9-11, 57076 Siegen, Germany.

ThVSIEGENThVSIEGEN


Contents

1. Motivation2. Flames previously investigated3. Molecular transport effects in premixed turbulent

combustion4. Outwardly propagating spherical flames5. Various approaches to modelling of H2+HC flames 6. Algebraic flame surface wrinkling model 7. Results8. Conclusion


Study:Influence of molecular transport (preferential diffusion and Lewis number) effects in H2+HC mixtures

Quantitative evaluation of flame speed for mixed fuels

Motivation:

Safety considerations in hydrogen usage


Hydrocarbon flames previously investigated

Flame configurations

Effects investigated

Approach Experimental source

Bunsen Burner

High-Pressure, Fuel Type RANS, LES

Swirl Burner High-Pressure,

Fuel Type RANS

Dump Combustor High-Pressure RANS, LES

Swirl BurnerFlame Stability &

Dynamics RANS, LES

Bunsen Burner

High-pressure,H2-doping RANS, LES

Tohoku University

University of Orleans

Different configurations studied numerically using the Algebraic Flame Surface Wrinkling Premixed Turbulent model


Modelling Turbulent Premixed Combustion

lnE

k

ln k

lnE

k

ln k

/ Pressure:

Fine scale structures that can wrinkle the flame decreases

t xRe u l /

Fuel Variation (Lewis Number)

minor

Thermal DiffusivityLe

Mass Diffusivity D

0 75.k x tl / Re

Products

Reactants: Fuel + Air

Reaction zonePreheat zone

Le<1 Le=1 Le>1Dminor

Dminor

Dminor

Molecular Transport Effects

Aluri NK, Doctoral Dissertation, University of Siegen 2007


Flame Curvature, Mass Flow & Turbulent Flame Speed

Reaction Sheet

Burned

Unburned

A

A'

C

B'

B

Stream Lines

To Convex Flamelet toward the Burned Mixture

To Convex Flamelet toward the UnBurned Mixture

Unburned gas consumed by a

turbulent flamelet1. Premixed gas flows along

marked streamlines

2. Streamlines ┴ to flamefront

3. Ratio of mass flow flowing into the convex ‘BC unburned‘ / to convex ‘AC burned‘ ~ 3:1

4. The convex part of flamelet towards the unburned mix. affects the turbulent flame speed predominantly


:Stream L ine:H igher D iffusive R eactant; D:L ower D iffusive R eactant; D

R eactionSheet

B urned

U nburned

h

l

D R ichD P oor

hl

D P oorD R ich

hl

(F uel/O /N )2 2

I n the case of C H or H - mixtures,F uel is D and O is D .h

4

l

2

2

2I n the case of C H or C H - mixtures,F uel is D and O is D .h

6

l 2

83

Preferential Diffusion and Lewis number Effects


Outwardly Propagating Spherical flames – Kido database

Measured data:SL= Mean local burning velocity; = f(PD=Df/Do)

ST = Turbulent flame speed

H2/O2/N2; =1.2; Le=1.29 =0.8; Le=0.42SL0=25cm/s

Le decrease

u’/SL0=1.4

Spherical gaseous (H2) explosion

Mixture data:Hydrogen-methane & Hydrogen-propane lean mixturesEquivalence ratio: 0.8Turbulent velocity = 2 m/s (max)


DNS by Trouvé and Poinsot 1994 on lean H2/O2/N2 flames………………………………..

…………………………………………………… and DNS of lean H2 flame by Bell et al. 2006 (not depicted here), confirm the Le influence on turbulent flame speed, especially in lean H2 mixtures

This substantial rise in flame speed may be due to sum of DL and PDT effects, or, can also be explained using Leading Point concept

Lewis Number Effects – DNS investigations


Modelling approaches to multi-fuel premixed turbulent flames

Weakly stretchedflamelet modelsbased on Ma

AnalyticalMethods(a submodelfor preferentialdiffusion effect)

Based onexp’taldata

Critically curvedflamelets imposedon flame-ballconcept byZel’dovich

exponential (Le-1) relation

Submodel for

time scale


Weakly stretched flamelets concepts

SL= SL0(1 – Ma ּ sּ(c)

SL= stretched laminar flame speed substituted for SL0

Ma = Markstein number = f(flame stretch, curvature)

s�ּc is commonly simplified to Ka.Ma or Ka.Le

s is stretch rate Chemical time scale ּc = laminar flame thickness/ (unstretched laminar flame speed)2

Measured Ma for CH4- and H2-air mixtures for =0.40, 0.43 and 0.50 are 0.7 and -0.3, respectively. Bechtold and Matalon Combust. Flame 1999


Analytical method – A submodel for preferential diffusion effect

0

0

0 0

(1 ) 1, if 1

(1 ) , if 11 (1 )

stlp lp

st

stlp lp

st st

C d dd C

C dC C d

SL(lp) = Mean local burning velocity at the leading point of the flamelet, substituted for SL0 in the turbulent flame speed model

lp is (1/ local equivalence ratio) at the leading point

stC mass stoichiometric coefficient

0 1/Initial equivalence ratio /f od D D = ratio of diffusivities of fuel and oxidant

Kuznetsov, V.R., and Sabel'nikov, V.A., Turbulence and Combustion, Hemisphere, 1990.

Assumption: DO,u = u


Critically curved flamelets: flame-ball concept by Zel'dovich

Asymptotically (activation temp ∞) exact solution of stationary 1D balance equations for the temperature and mass fraction of the deficient reactant, for single-step single-reactant chemistry.

For Lewis numbers < 1, the flame ball temperature is given by

r u b uT T T T Le

1.51

0

exp2

cr b b rcr c c

L r b r

R T T TLeT T T

0

exp( 1)cr

c

Le

The chemical time scale for the highest local burning rate is

0.25 0.2

0.8 0.20

0 0

0.461exp( 1)

tT L Lo

c

pS S u SLe p

Lipatnikov and Chomiak., Prog. in Energy, Comb Sci 2005Aluri, Muppala, Dinkelacker Comb Flame 2006

Muppala et al. Comb Flame 2005


Premixed Turbulent Combustion submodel

c u Lw s General reaction rate expression

Folding factor = Flame surface area / Volume

A

AT

Fuel+Air

T

T

cAA

A

A

Turbulent flame surface area

Averaged flame surface area

(Re , , ,...)tT f u p

AA

Muppala, Aluri, Dinkelacker -- Comb Flame 2005Aluri, Pantangi, Muppala,, Dinkelacker – Flow, Turb Comb 2005

Surface density functionc

~T T

L

A sA s

0.

1

3

00.2

0.25

AlgebraicFlameSurfaceWrinkling(AFSW)Model

0.46 '1 ee

RL tL

eT T

L

A s uA s s

because of Damköhlerhypothesis


{without the Lewis number effect}

{without Preferential Diffusion effect}

Model predictions

0.3

0.250

0

0.461 Reexp( 1)T L t

L

uS SLe s

0.30.251 0.46ReT L t

L

uS Ss

Algebraic Flame Surface Wrinkling model: two forms


0

0.5

1

1.5

2

2.5

0 0.5 1 1.5 2

Exp_00Model_00

S T, m

/s

u', m/s

Le = 0.38100%H2-0% CH4

0

0.5

1

1.5

2

2.5

0 0.5 1 1.5 2

Exp_05Model_05

S T, m

/s

u', m/s

Le = 0.4050%H2-50% CH4

0

0.5

1

1.5

2

0 0.5 1 1.5 2

Exp_10Model_10

S T, m

/s

u', m/s

Le = 0.890%H2-100% CH4

Turbulent flame speed vs. turbulence intensity for lean CH4–H2 flames. Model predictions based on SL0.

Results1 – Hydrocarbon + Hydrogen mixtures: CH4 + H2


0

0.5

1

1.5

2

2.5

0 0.5 1 1.5 2

Exp_00Model_00_SLModel_00_SL0_Le

S T, m

/s

u', m/s

Le = 0.38100%H2-0% CH4

0

0.5

1

1.5

2

2.5

0 0.5 1 1.5 2


S T, m

/s

u', m/s

Le = 0.4050%H2-50% CH4

0

0.5

1

1.5

2

0 0.5 1 1.5 2


S T, m

/s

u', m/s

Le = 0.890%H2-100% CH4

CH4–H2 mixtures. Model predictions are based on SL (mean local burning velocity with preferential diffusion) and Lewis number effect.

Results2 – Hydrocarbon + Hydrogen mixtures: CH4 + H2


0

0.5

1

1.5

2

2.5

0 0.5 1 1.5 2

Exp_00Model_00

S T, m

/s

u', m/s

100%H2-0% C3H8

0

0.5

1

1.5

2

0 0.5 1 1.5 2

Exp_05Model_05

S T, m

/s

u', m/s

50%H2-50% C3H8Le = 0.42 Le = 0.70

0

0.5

1

1.5

0 0.5 1 1.5 2

Exp_10Model_10

S T, m

/s

u', m/s

0%H2-100% C3H8Le = 1.57

Turbulent flame speed vs. turbulence intensity u’ for lean C3H8–H2 flames. Model predictions based on SL0.

Results1 – Hydrocarbon + Hydrogen mixtures: C3H8 + H2


0

0.5

1

1.5

2

2.5

0 0.5 1 1.5 2


S T, m

/s

u', m/s

Le = 0.42100%H2-0% C3H8

0

0.5

1

1.5

2

0 0.5 1 1.5 2


S T, m

/s

u', m/s

Le = 0.7050%H2-50% C3H8

0

0.5

1

1.5

0 0.5 1 1.5 2


S T, m

/s

u', m/s

Le = 1.570%H2-100% C3H8

C3H8–H2 mixtures. Model predictions are based on SL (mean local burning velocity with preferential diffusion) and Lewis number effect.

Results2 – Hydrocarbon + Hydrogen mixtures: C3H8 + H2


012345

6

0 1 2 3 4 5 6

ST/SL

- M

odel

ST/SL - Exp

H2 doped with CH4

01234567

0 1 2 3 4 5 6 7

H2 doped with C3H8ST/SL

- M

odel

ST/SL - Exp

Correlation plot for turbulent flame speed ST : Experimentally measured vs. model predicted, estimated based on SL for CH4–H2 and C3H8–H2 mixtures.

Correlation plots – turbulent flame speed: Exp vs. Model


An existing algebraic flame surface wrinkling reaction model was used to investigate the quantitative dependence of turbulent flame speed on molecular transport coefficients for two-component lean fuel (CH4–H2 and C3H8–H2) mixtures.

The model predictions were in good quantitative agreement with the corresponding experiments, if either mean local burning velocity SL or an exponential Lewis number term of the fuel mixture is used in the reaction model. The latter approach is a generalisation of earlier findings for single fuels and shows the applicability of the exponential Le term for dual-fuel mixtures.

The hydrocarbon substitutions to H2 mixtures are expected to suppress the leading flame edges, which are manifested by a decrease in mean local burning velocity, eventually preventing transition to detonation. Addition of hydrocarbons may also promote flame front stability of lean turbulent premixed H2 flames.

Summary


Thank you for your attention

molecular transport effects of hydrocarbon addition on turbulent hydrogen flame propagation

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