Effects of Aromatic Fuel Blending on theExtinction Limits and Soot Formation of n-

Decane Diffusion Flames

Sang Hee Won, Wenting Sun, Xiaolong Gou and Yiguang Ju

Mechanical and Aerospace EngineeringPrinceton University

Introduction: Surrogate Fuel Model Development

• Jet Fuel includes various hydrocarbon molecules: – normal, branched, and cyclo alkanes, and aromatics

• MURI team selected surrogate jet fuel components– n-decane, methyl-cyclohexane, n-propyl- and trimethyl-

benzenes, toluene…By matching H/C ratio, sooting index, burning rates, ignition time…

• Example: Gasoline Surrogate model (PRF+1)– Blending of toluene to n-heptane and iso-octane to meet both

burning and soot formation targets

• Challenge 1: How does the kinetic coupling between aromatics and normal alkanes affect burning rate, extinction limit, and soot formation?

• Challenge 2: How to generate an efficient kinetic reduction method for direct numerical simulation?

Objectives

• Investigate the kinetic and transport coupling effects of toluene and trimethyl benzene blending in n-decane on extinction limits and heat release rateusing counterflow diffusion flames

• Measure the effects of toluene blending in n-decane on soot formation, flame temperature, and OH concentration by using laser induced incandescence (LII), Rayleigh scattering, and LIF

• Develop a dynamic multi-scale model for kinetic mechanism reduction

1. Experimental investigation of aromatics and n-decane kinetic coupling on diffusion flame extinction limits

2. Numerical investigation of n-decane and toluene kinetic coupling on extinction limits

3. Measurements of soot formation of toluene blended n-decane diffusion flames

Research Projects in 2008

4. A dynamic multi-scale model and P-MARS

1. Experimental Studies of Diffusion FlameExtinction Limits

(Fuel Vaporization and Counterflow Burner)• Integration of liquid fuel evaporation system with

counterflow flame burner– Using FTIR measurements to check thermal decomposition and

concentration variation: Concentration fluctuation < 1%

Heater & insulationHeater

Nitrogen

FuelTo burner

T1 T2

Schematics of evaporation system

Extinction Strain Rates of Toluene- and n-Decane-AirDiffusion Flames: Validation

0

100

200

300

0.15 0.2 0.25 0.3 0.35

Present expDryer's mechanismseshadri's expseshadri's num

Extin

ctio

n st

rain

rate

[1/

s]

Fuel mass fraction YF

n-decane

toluene

Extinction Strain Rates of Toluene Blended n-Decane–Air Mixtures

96

128

160

192

224

0.15 0.2 0.25 0.3 0.35

n-decane80% n-decane & 20% toluene60% n-decane & 40% toluene40% n-decane & 60% toluenetolueneJet-A, Ref [5]JP-8, Ref [5]

Extin

ctio

n st

rain

rate

[1/

s]

Fuel mass fraction YF

n-decane

toluene

Extinction Strain Rates of 1,2,4- and 1,3,5-Trimethylbenzene-air Diffusion Flames

100

120

140

160

180

200

0.25 0.3 0.35 0.4 0.45

Toluene1,2,4 trimethylbenzene1.3.5 trimethylbenzene

Extin

ctio

n st

rain

rate

[1/

s]

Fuel mass fraction YF

What controls theFuel oxidationrate?

2. Numerical Simulation of n-Decane/Toluene Extinction Limit (Understanding of kinetic coupling between aromatic/alkane)

How does H/C ratio affect radical pool concentration?

1600

1650

1700

1750

1800

0

0.0006

0.0012

0.0018

0.0024

0.003

0 0.2 0.4 0.6 0.8 1

Tmax [K]

[H]max/HC ratio

[O]max/HC ratio

[OH]max/HC ratio

Max

imum

tem

pera

ture

[K

] Mole fraction / H

C ratio

Blending ratio of n-decane Xb

[OH]max

/ HC ratio

[O]max

/ HC ratio

[H]max

/ HC ratioXf = 0.1, a = 60 [1/s]

40 80 120 1601500

1550

1600

1650

1700

1750

1800

0.00.010.030.08

0.9N2+(0.1-x) n-decane+x*toluene

E

xtin

ctio

n fla

me

tem

pera

ture

, K

Extinction stretch rate, 1/s

x=0.1

Flame Temperature and Extinction Limit vs. Stretch Rate

What is the roleof OH concentration?

n-Decane oxidationis kinetically affectedby toluene addition.

0.00 0.02 0.04 0.06 0.08 0.10

50

100

150

Toluene mole fraction

Ext

inct

ion

stra

tch

rate

, 1/s 0.9N2+(0.1-x) n-decane+x*toluene

1560

1580

1600

1620

1640

1660 M

aximum

flame tem

preature, K

Extinction Strain rate vs. Toluene Blending Mole Fraction

0.00 0.02 0.04 0.06 0.08 0.101.0x10-5

2.0x10-5

3.0x10-5

4.0x10-5

5.0x10-5

6.0x10-5

7.0x10-5

8.0x10-5

OH

Toluene mole fraction

Max

imum

H m

ole

fract

ion

0.9N2+(0.1-x) n-decane+x*toluene

H

2.0x10-4

4.0x10-4

6.0x10-4

8.0x10-4

1.0x10-3

1.2x10-3

1.4x10-3 Maxim

um O

H concentration

Maximum OH and H Mole Fractions on Toluene Blending

N2O2H2

H2OH

OHO

COCO2CH3CH4

C2H4C2H6C2H2

CH2COpC3H4aC3H4C4H81

A1C6H5OH

C3H8C3H6

C10H22C5H10-1

C6H5CH3

-0.1 0.0 0.1 0.2Diffusion sensitivity

0.9N2+0.09n-decane+0.01toluene

∫=+−

δ

ωρ0

00 dx

dxdYD i

Fi

Jump condition of diffusion flames

Thermal effect via diffusion: n-decane, toluene, O2, CO2, CO

Kinetic effect via diffusion: H, O, OH…

Sensitivity of thermal conductivity

3. Experimental Measurements of Soot Formation & TemperatureUsing Laser Induced Incandescence & Rayleigh Scattering

• Nd:YAG Laser : Quanta-Ray (532 nm), Cobra-stretch dye laser• ICCD Camera : PIMAX-Gen II (Princeton Instrument)• LII : narrow band pass filter (420 nm)

Photo for Rayleigh scattering and LII

Measured Soot Volume Fractiontoluene diffusion flame

0

0.6

1.2

1.8

2.4

3

0.3 0.32 0.34 0.36 0.38 0.4

Max

imum

LII

inte

nsity

I max

[a.

u.]

Fuel mass fraction Yf

Toluene, a = 154 [1/s]

0

0.6

1.2

1.8

2.4

3

4.5 5 5.5 6 6.5

Max

imum

LII

inte

nsity

I max

[a.

u.]

TSItoluene

Xf

Toluene, a = 154 [1/s]TSI

toluene = 40

Measured Soot Formation vs. Predicted TSItoluene diffusion flame

Flame Temperature Measurement using Rayleigh Scattering

Cold flow

Reacting flow

Temperature

4. A Dynamic Multi-Scale (DMS) Kinetic Reduction Model

Time scales in reactive flow

1 s

10-2s

10-4s

10-6s

10-8s

10-10s

Flow time

Transport timeMolecularTurbulent

NO formation

Product formation H2O

Radical formation H

Radical formation H2O2quasi-steady state?

Radiation transferquasi-steady state

Chemical time Physical time

DNS Time-step

Physical interest

?

0 1 2 3 4

x 10-4

0

0.01

0.02

0.03

t(s)

YH

2

H2

standarddynamic

0 1 2 3 4

x 10-4

0

0.1

0.2

0.3

0.4

t(s)

YO

2

O2

standarddynamic

0 1 2 3 4

x 10-4

0

0.05

0.1

0.15

0.2

t(s)

YH

2O

H2O

standarddynamic

0 1 2 3 4

x 10-4

0

2

4

6x 10-3

t(s)

YH

H

standarddynamic

0 1 2 3 4

x 10-4

0

0.005

0.01

0.015

0.02

t(s)

YO

O

standarddynamic

0 1 2 3 4

x 10-4

0

0.01

0.02

0.03

t(s)

YO

H

OH

standarddynamic

0 1 2 3 4

x 10-4

0

2

4

6

8x 10-5

t(s)

YH

O2

HO2

standarddynamic

0 1 2 3 4

x 10-4

0

0.5

1

1.5x 10-5

t(s)

YH

2O2

H2O2

standarddynamic

0 1 2 3 4

x 10-4

1000

1500

2000

2500

3000

t(s)

Tem

p/K

Temp

standarddynamic

Validation of DMS Method (Hydrogen)

0 1 2 3 4 5

x 10-3

1000

1500

2000

2500

3000

t(s)

Tem

p/K

Temp

standarddmts

0 1 2 3 4 5

x 10-3

0

0.1

0.2

0.3

0.4

t(s)

YO

2

O2

standarddmts

0 1 2 3 4 5

x 10-3

0

0.02

0.04

0.06

t(s)

YC

H4

CH4

standarddmts

0 1 2 3 4 5

x 10-3

0

0.05

0.1

t(s)

YC

O2

CO2

standarddmts

0 1 2 3 4 5

x 10-3

0

0.05

0.1

0.15

0.2

t(s)

YH

2O

H2O

standarddmts

0 1 2 3 4 5

x 10-3

0

0.005

0.01

t(s)

YO

O

standarddmts

0 1 2 3 4 5

x 10-3

0

0.5

1x 10

-3

t(s)

YH

H

standarddmts

0 1 2 3 4 5

x 10-3

0

0.005

0.01

0.015

t(s)

YO

H

OH

standarddmts

0 1 2 3 4 5

x 10-3

0

1

2

3x 10

-3

t(s)

YC

H3

CH3

standarddmts

Validation of DMS Method (Methane)

Further Kinetic Mechanism Reduction: A Path Flux Analysis Method

),max( AXXABAr P

AB →→→

≡

A

B

consumption production

Reaction path coefficients

A

B

G

C D

Case B),max( AXXAABrC

AB →→→

≡

Reaction path

Case A

DRG: Lu & Law, 2005DRG-EP: Pitsch, 2008

MPA: Bendtsen 2001

0.60 0.66 0.72 0.78 0.841E-5

1E-4

1E-3

0.01P=1 atm

τ ig (s

ec)

1000/T (1/K)

detail (121) PFA (48) DRG (50)

φ=2

Results and Comparisons on n-Decane Ignition

Conclusions•The extinction limits of 1,3,5 and 1,2,4 trimethy-benzenes are much narrower than toluene and n-decane. Blending of aromatics in alkanes can effectively control the extinction limit of surrogate fuel components.

•The extinction limit of toluene blended n-decane is found linearly dependent on the maximum OH concentration. The OH concentration is controlled by the kinetic coupling between aromatics and alkanes.

•Transport affects extinction limit in both thermal and kinetic ways via major species and active radicals.

•The soot volume fraction increases with the concentration of toluene. The measured soot volume fraction by LII agrees well with the TSI prediction. These linear dependences on aromatics blending level provide a convenient way to construct surrogate models.

•A dynamic multi-scale model and a P-MARS mechanism reduction model are developed.