stabilized three-stage oxidation of gaseous n-heptane/air ....../25 stabilized three-stage oxidation...

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Stabilized three-stage oxidation of gaseous

n-heptane/air mixture in a micro flow reactor

with a controlled temperature profile

August 5, 2010

The 33rd International Symposium on Combustion

Akira Yamamoto, Hiroshi Oshibe, Hisashi Nakamura,

Takuya Tezuka, Susumu Hasegawa and Kaoru Maruta

Institute of Fluid Science, Tohoku University

(4C01)

/25Institute of Fluid Science, Tohoku Univ. 2

It is important to know the ignition characteristics of various fuels.

Typical ignition process of practical hydrocarbon fuels

Time

Cool flame

Hot flame

Two-stage ignition at

different temperature range

BackgroundH

eat

rele

ase

rate

Cool flame

• Low temperature oxidation (LTO)

• Small heat release

Hot flame

• High temperature oxidation (HTO)

• Main heat release

Two-stage ignition phenomena strongly affect the combustion control.

⇒ Improvement of internal combustion engines.• Spark ignition engines

• Compression ignition (Diesel, HCCI) engines

• Gas turbines etc.


Major approach: e.g., Rapid Compression Machines (RCMs)

Approaches to investigate the ignition characteristics

• Heat loss to chamber wall

• Roll-up vortices by piston motion

• Ignition in local spot → Propagation

Disparities with

the modeling that assumes

homogeneous combustion

Difficulties in RCM experiments…

Auto-ignition of methane/air mixture, Strozzi et al., CST 180 (2008)

Fuel/air

mixture

Rapid compression

Multi-zone/dimensional model should be required to reproduce the

experiments more accurately. But it needs higher computational cost.


• Imposed wall-temperature profile in the flow direction

• Inner diameter of the tube < Ordinary quenching diameter

• Laminar flow (Re ≈ 1 ~ 100)

• Constant pressure

※Maruta et al., PCI 30, 32

Fuel/Air d < Quenching

diameter

x

Tw(x)

Test section0

Flame

External heat

Wall temperature profile

Quartz tube

Micro flow reactor with controlled temperature profile


At extremely low velocity Gas-phase temperature ≒ wall temperature

※Maruta et al., PCI 30, 32

Fuel/Air d < Quenching

diameter

x

Tw(x)

Test section0

Flame

External heat

Wall temperature profile

Quartz tube

Micro flow reactor with controlled temperature profile

CH4/air, 1atm, U = 0.2 cm/s

Tsuboi et al., PCI 32

Stable weak flames exist at certain positions in temperature profile.


Objectives

n-Heptane (n-C7H16)

– A basic liquid hydrocarbon fuel that shows typical multi-stage ignition

– One of the primary reference fuel of automotive gasoline

Investigate ignition and combustion characteristics of

gaseous n-heptane/air mixture using a micro flow reactor

with a controlled temperature profile.


n-C7H16/Air

Flat flame

for heating

H2/Air

d = 1-2 mm

x

Tw

300 K

1300 K

Test section0

Quartz tube

Flame

Experimental setup

• Stoichiometric gaseous n-heptane/air mixture, Pressure = 1-4 atm

• Flame images were taken by CH-filtered digital still camera

• Gas sampling analysis by GC

Pressure

Regulator


Flame code PREMIX-based 1-D steady code

Reaction scheme n-Heptane, Reduced mechanism from LLNL

(159 species, 1540 steps) ※Seiser et al., PCI 28 (2000)

Conditions • Stoichiometric gaseous n-heptane/air mixture

• Experimentally measured wall-temperature profile

as a boundary condition

Gas-phase energy equation

21 1

1 4( ) 0

K K

k k pk k k k w g

k kp p p p

dT d dT A dT A A NuM A Y V c h W T T

dx c dx dx c dx c c d

Heat transfer with the wall

Flame position Peaks in heat-release-rate (HRR) [W/cm3] profile

Computational method


Combustion characteristics at atmospheric pressure

Results and Discussion

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400

600

800

1000

1200

1400

1

10

100

-14 -12 -10 -8 -6 -4 -2 0 2 4 6

Wal

l te

mp

erat

ure

[K

]

Mea

n f

low

vel

oci

ty [

cm/s

]

x [mm]

Normal flame FREI_ignition FREI_extinction1st weak flame 3rd weak flame Wall temp.

2nd reaction zone

Institute of Fluid Science, Tohoku Univ. 10

(a) Normal flame

(b) FREI

(c) Weak flames

Experimental flame positions against various flow velocities

Three different flame responses were observed by changing inlet flow velocities.

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400

600

800

1000

1200

1400

1

10

100

-14 -12 -10 -8 -6 -4 -2 0 2 4 6

Wa

ll tem

pera

ture [K

]

Mea

n f

low

velo

cit

y [cm

/s]

x [mm]


2nd reaction zone


(a) Normal flame

(b) FREI

(c) Weak flames

Normal flame in high velocity condition

Flow

Normal flame Local mixture flow velocity = Flame propagating velocity

30

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400

600

800

1000

1200

1400

1

10

100

-14 -12 -10 -8 -6 -4 -2 0 2 4 6

Wa

ll tem

pera

ture [K

]

Mea

n f

low

velo

cit

y [cm

/s]

x [mm]


2nd reaction zone

(c) Weak flames


(a) Normal flame

(b) FREI

Flow

t [msec] 0

1

2

3

4

6

10

23

IgnitionExtinction

FREI Unstable Flames with Repetitive Extinction and Ignition

Unstable FREI in middle velocity condition

30

4

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600

800

1000

1200

1400

1

10

100

-14 -12 -10 -8 -6 -4 -2 0 2 4 6

Wa

ll tem

pera

ture [K

]

Mea

n f

low

velo

cit

y [cm

/s]

x [mm]


2nd reaction zone


(a) Normal flame

(b) FREI

(c) Weak flames

Weak flames in low velocity condition

Flow

2nd reaction zone

Three reaction zones in the flow directionStable weak flames

4


Three HRR peaks in the flow direction

Observed three weak flames were reproduced.

U = 2.0 cm/s

Twall

HRR

Computational profiles of Tg and HRR of weak flames

Tgas

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300

400

500

600

700

800

900

1000

1100

1200

1300

0

5

10

15

20

25

3.5 4 4.5 5 5.5 6

Wa

ll t

em

pera

ture

[K

]

Mo

le f

ra

cti

on

[%

]

x [cm]

O2

CO2

CH2O×10 CO

CH4×20

n-C7H16×10

H2O2×10

Tw


Form CH2O, H2O2, CO, CH4 etc.

U = 2.0 cm/s

Oxidation of fuel (n-C7H16)

Computational species profiles -1st reaction-

Low temperature oxidation

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400

500

600

700

800

900

1000

1100

1200

1300

0

5

10

15

20

25

3.5 4 4.5 5 5.5 6

Wa

ll t

em

pera

ture

[K

]

Mo

le f

ra

cti

on

[%

]

x [cm]

O2

CO2

CH2O×10 CO

CH4×20

n-C7H16×10

H2O2×10

Tw


U = 2.0 cm/s

Computational species profiles -2nd reaction-

CH2O + OH ⇒ HCO + H2OH2O2 (+M) ⇒ 2OH (+M)

H2O2 decomposition

HCO + O2 ⇒ CO + HO2

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500

600

700

800

900

1000

1100

1200

1300

0

5

10

15

20

25

3.5 4 4.5 5 5.5 6

Wa

ll t

em

pera

ture

[K

]

Mo

le f

ra

cti

on

[%

]

x [cm]

O2

CO2

CH2O×10 CO

CH4×20

n-C7H16×10

H2O2×10

Tw


U = 2.0 cm/s

Computational species profiles -3rd reaction-

Oxidation of CH4 & CO to CO2: CO + OH ⇒ CO2 + H

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0

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

0.09

0.1

0

5

10

15

20

25

300 500 700 900 1100 1300

Ma

ss c

on

cen

tra

tio

n o

f C

H2O

[%

]

Vo

lum

etr

ic c

on

cen

tra

tio

n [

%]

Wall temperature [K]

O2

CO2

CH2O

CO

CH4×20 300

400

500

600

700

800

900

1000

1100

1200

1300

0

5

10

15

20

25

3.5 4 4.5 5 5.5 6

Wa

ll t

em

pera

ture

[K

]

Mo

le f

ra

cti

on

[%

]

x [cm]

O2

CO2

CH2O×10 CO

CH4×20

n-C7H16×10

H2O2×10

Tw


Measurement with GC (U = 2.0 cm/s)

Computation (U = 2.0 cm/s)

Comparison of experimental & computational species profiles

Three-stage oxidation process was experimentally confirmed

by gas sampling analysis with GC.

Profile of each species qualitatively agreed well.

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500

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700

800

900

1000

1100

1200

1300

0

5

10

15

20

25

3.5 4 4.5 5 5.5 6

Wa

ll t

em

pera

ture

[K

]

Mo

le f

ra

cti

on

[%

]

x [cm]

O2

CO2

CH2O×10 CO

CH4×20

n-C7H16×10

H2O2×10

Tw


Present three-stage oxidation

Interpretation of the three-stage oxidation

Typical two-stage oxidation: Cool flame + Hot flame

Present three-stage oxidation: Cool flame + Separated hot flames

Time

Con

centr

atio

n

Cool flame Hot flame

CO

H2O2

CH4CH2O

Typical two-stage oxidation

From the species profiles…

(Blue flame & Hot flame)

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500

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800

900

1000

1100

1200

1300

0

5

10

15

20

25

3.5 4 4.5 5 5.5 6

Wa

ll t

em

pera

ture

[K

]

Mo

le f

ra

cti

on

[%

]

x [cm]

O2

CO2

CH2O×10 CO

CH4×20

n-C7H16×10

H2O2×10

Tw


Present three-stage oxidation

Interpretation of the three-stage oxidation

Time

Con

centr

atio

n

Cool flame Hot flame

CO

H2O2

CH4CH2O

Typical two-stage oxidation

Ordinary approach

Strongly transient ignition

Micro flow reactor

Stabilized spatial profile

Micro flow reactor can contribute to chemical kinetic studies.


Pressure dependence of three-stage oxidation

Results and Discussion

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800

900

1000

1100

1200

1300

0

20

40

60

80

100

4 4.5 5 5.5 6

Wa

ll t

em

pera

ture

[K

]

Hea

t re

lea

se r

ate

[W

/cm

3]

x [cm]

HRR (6 atm) Tw

HRR×6 (1 atm)


1 atm: 3rd HRR peak is the strongest.

6 atm: 1st and 2nd HRR peaks are stronger than 3rd peak.

U = 1.0 cm/s

Pressure dependence of HRR profile (Computation)

Main heat release shift to the lower temperature side at high pressure.

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1300

0

1

2

3

4

5

6

7

4 4.2 4.4 4.6 4.8 5 5.2 5.4 5.6

Wa

ll tem

pera

ture [K

]

Press

ure [a

tm]

x [cm]

1st HRR peak 2nd HRR peak 3rd HRR peak

Tw


U = 1.0 cm/s

Increase of pressure • 1st & 2nd peak shift to lower temperature side.

• 3rd peak shifts to higher temperature side at 2-5 atm.

Pressure dependence of HRR peak positions (Computation)


Pressure

U = 3.0 cm/s, d = 1 mm

Low and middle temperature reactions affect the whole ignition process

more significantly at higher pressure conditon.

Flow

• 1st and 2nd flames become strong, 3rd flame becomes weakened.

• 2nd and 3rd flames are clearly separated.

Weak flame images at high pressures

As increasing pressure from 1 to 4 atm…

1 atm

2 atm

3 atm

4 atm


Ignition and combustion characteristics of a gaseous n-heptane/air

mixture were examined using a micro flow reactor with a

controlled temperature profile.

• Three different flame responses were observed by changing an inlet mean

flow velocity. Especially at the low velocity condition, three reaction

zones were observed in the flow direction.

• Three peaks of heat release rate in the flow direction were obtained in the

computation. Computational species concentration profiles agreed well

with the results of gas sampling analysis.

Conclusions

• Under the high pressure condition, 1st and 2nd flames become stronger,

and 3rd flame becomes weakened. This indicates that low and middle

temperature reactions affect the whole ignition process more significantly

at higher pressure condition.


Thank you for your kind attention!

stabilized three-stage oxidation of gaseous n-heptane/air ....../25 stabilized three-stage oxidation...

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