1

EXPERIMENTAL INVESTIGATION ON SOOTY FLAMES

AT ELAVATED PRESSURES

School of Mechanical, Aerospace and Civil Engineering

The University of Manchester

A first year PhD progress reportpresented by:

Hamidreza Gohari Darabkhani

Supervisor :Dr. Yang Zhang

20.09.2007

2

Introduction

• Increasing efficiency and decreasing size in modern turbines and internal combustion engines requires higher operating pressures.

• At high pressure flames, the carbon (soot) particles are so dense that the flame is opaque to electromagnetic waves in the visible or near infrared.

• In the optical diagnostic methods, measuring the exact amount of combustion products is difficult but temperature and concentration of soot can be measured with high accuracy.

• The effect of pressure on thermo physical properties of laminar coflow diffusion flames (LCDF) was studied by optical diagnostic methods (Two-Colour Pyrometry) over the pressure range of 1 to 18 bar in ethylene-air and methane-air and 1 to 7 bar in propane–air LCDF.

3

Objectives of this Research 1. Investigation on physical sooty flame’s properties at elevated pressures (The

changes in the height, diameter and shape of the flames…).

2. Applying emission imaging techniques (two-colour pyrometry) by using just one

CCD digital camera and two narrow bands filters (CH and C2).

3. Applying the IR infratherm pyrometry in order to measure soot temperature

profiles in high pressure sooty flames for comparison with two-colour results.

4. Thermo acoustic or thermo diffusivity flame’s instability observation due to

elevated pressures.

5. Chemilominescent emission measurement by using optical fibre system.

6. Terahertz time domain spectroscopy (THz-TDS) to study the combustion

reactions in regimes inaccessible to optical diagnostics (if equipments will be

ready).

7. Cross-correlating the obtained data in order to gain unique physical insights into

the flame’s properties under high pressure and sooty conditions.

4

Burner TypesThree types of laminar diffusion flame

burners are commonly used by soot researchers:

1. Coflow : Axisymmetric 2-D flames with demonstrated stability at high pressures.

2. Counterflow (opposed jet); 1-D flames with instability problems at elevated pressures flames and critical location of the stagnation point.

3. Wolfhard–Parker; 1 and 2-D Flames with instability problems at elevated pressures

5

Measurement Method

Passive optical diagnostics (using light or laser sources)

Active optical diagnostics (uses natural flame

emission)

OPTICAL

Interferometry Holography Tomography Schelieren Shadowgraphy Rayleigh/Mie Scattering Laser Doppler Anemometry

(LDA) Particle Image Velocimetry

(PIV) Laser Induced Grating

Flame Photography High Speed

Photography Stereo Digital Imaging

SPECTROSCOPIC

Absorption Spectroscopy Laser Induced Fluorescence

(LIF) Coherent Anti Raman

Spectroscopy Raman Spectroscopy THz-time Domain

Spectroscopy

Flame Emission Spectroscopy

Narrow Band Photography

Summary of non-intrusive optical based combustion diagnostic techniques

6

Refs.Pressure

range [bar]

Fuel and fuel flow rate [ml/min]

Diagnostic Method

Pressure exponent n in [soot]∝Pn

Fraction of fuel's carbon converted to

soot

Path integrated maximum soot

Local maximum soot (location)

Macfarlane el al. (1964) 1-20

C5 and C6

hydrocarbons (premixed)

Glass fibre filter paper

2.53 - -

McArragher and Tanon (1972)

Elevated Pressures

Hydrocarbon Fuels

(Diffusion and Premixed)

Review paper 13 - -

Flower and Bowman (1983) 1-25 Ethylene LOSA 1.52 1.52 0.5-1.0

Flower and Bowman (1986)

1-10Ethylene,

LDF, 91,129.5, 211

Line of sight integrated

1.2±0.1 - -

Lee and Na (2000)

1-4 Ethylene, 163

Two-colour method and

thermocouple 1.26

2(20 mm above)

(the burner)-

McCrain and Roberts (2005)

1-16 Ethylene, 54LOSA & LII

1.2 1.7 -

1-25 Methane, 92 1 1.2 -

Thomson et al.(2005)

5-20Methane, 46

SSE and LOSA

1.3 2 1

20-40 0.9 1.2 0.1

Fengshan Liu et al. (2006)

5-40 Methane, 46SSE, LOSA

and numerical1.3 2 1

Bento et al.(2006)

1-2Propane, 15

SSE and LOSA

3.4(requires further experiments for confirmation)

-3.3(requires

further experiments for confirmation)

2-7.3 1.4 1.8 1.1

High Pressure Soot Diagnostics (pressure dependence of soot)

7

Schematic diagram of apparatus used for Line of Sight measuring

the temperature of soot by Flower (1989)

8

Optical Layout of Spectral Soot Emission (SSE) Diagnostic

Optical Layout of the Line-of-Sight Attenuation (LOSA) diagnostic

9

Two-Colour Soot Temperature Measurement Theory

1exp. 25

1

T

C

CI b

1exp. 25

1

T

C

CII b

)....36exp(1 LfF

v

22222 .42

.

knkn

knF

bII 111 bII 222

2

2

1

1

21

2

252

11

1

251 1exp

1

1exp

1

FF

IC

T

C

IC

T

C

2

2

11

2

25

1

1

1exp1

1ln)(..36

.

F

v IC

T

C

FLf

1

2

5

1

221

122 )(

I

ILn

CT

(refractive index ) m=n-ik n,k from Lee and

Tien(1981) report

10

Test Setup Schematic

CCD Camera

Narrow Band Filter

Infratherm Pyrometer

Sooty Flame

Optical Windows

High Pressure Chamber

11

Experiment Arrangement

Olympus E-100RS Visible Windows Infrathem Pyrometer

Filters (C2 or CH)

High Pressure Chamber

12

CH filter (430 ± 5 nm) C2 filter (516 ± 2.5 nm)

Two-colour IR Pyrometer

Narrow band filters and IR Pyrometer

13

Camera calibration setup with tungsten ribbon lamp

Tungsten Lamp

Digital Camera

Digital Voltmeter

Rheostat

12V Battery

CH and C2 Filters

14

Test Results

Ethylene (100 ml/min)-Air (20 l/min) coflow diffusion flame

Ethylene(100 ml/min)-Air(20 l/min) Diffusion Flame(Flame Heights at Different Pressures)

0

5

10

15

20

25

0 2 4 6 8 10 12 14 16 18 20

Chamber Pressure (bar)

Fla

me

Hei

gh

t (m

m)

1 bar 2 bar 4bar 6 bar 8 bar 10 bar 14 bar 16 bar 18 bar

15

Flame Heights in Diffrent Pressures for Methan(120 ml/min)-Air(20 l/min) DF

0

5

10

15

20

25

30

0 2 4 6 8 10 12 14 16 18 20

Pressures (Bar)

Fla

mes

Hei

gh

t (m

m)

Propane (60 ml/min)-Air (20 l/min) DF

0

5

10

15

20

25

30

35

40

0 1 2 3 4 5 6 7 8

Pressures (bar)

Fla

me

, blu

e p

art

an

d s

oo

t lin

e

he

igh

ts in

fla

me

(m

m)

Blue Flame

Flame Heights

Soot Line

Flame Heights in Methane and Propane Flames

16

The cross-sectional area of the Propane flame

Propane (60 ml/min)-Air (20 l/min)

0

5

10

15

20

25

30

35

0 1 2 3 4 5 6 7 8

Chamber Pressures(bar)

Cro

ss

-Se

cti

on

al A

rea

of

Fla

me

(m

m^

2)

H=1 mm

H=5 mm

H=10 mm

H=15 mm

H=20 mm

H=25 mm

As the pressure was increased, axial flame diameters decreased, giving an overall stretched appearance to the flame.

In This Study: The cross-sectional area of the flame (Acs) inverse dependence on pressure to the power of 0.6±0.1

Glassman (1998): Acs inverse dependence on pressure to the power of 0.5

Thomson et al. (2006), Bento et al.(2006) and McCrain and Roberts(2005): Acs inverse dependence on pressure(1/P).

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Flame Visualisation:

1. The height of the flames increases gradually as pressure increases and then

decrease with further increases in pressure.

2. The shape of the flame changes dramatically with increasing pressure. At

atmospheric pressure, the flame has a bulbous appearance and is wider than

the exit diameter of the burner nozzle. By increasing the pressure the flame

changes in shape from wide and convex to slender and concave.

3. As the pressure was increased, axial flame diameters decreased, giving an

overall stretched appearance to the flame. The cross-sectional area of the

propane flame was observed to decrease with pressure as AcsP-n , where ,

n=0.6±0.1

4. The flame can be considered as a laminar axisymmetrical coflow diffusion

flame.

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Ethylene-Air DF soot temperatures along centre line at different pressures

700

800

900

1000

1100

1200

1300

1400

1500

1600

1700

0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32

Flames Heights (mm)

So

ot T

em

pe

ratu

re (

ºC)

1 bar

2 bar

4 bar

6 bar

8 bar

10 bar

12 bar

16 bar

Soot temperatures along the centre line at different pressures

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Soot Temperature Measurement

1. In atmospheric pressure most centre parts of flames was blue and the

presence of soot is limited to the region near the tip of the flame.

2. By increasing the chamber pressure the overall soot temperature was

decreased.

3. Temperature increment by increasing the flame height from nozzle tip and

temperature drop after a certain height was observed for all flames.

4. The temperature trend line plots show steep radial temperature gradients

across the soot annulus and a general axial increase in temperature.

5. It is found that at lower pressures the temperature of soot annuals are more

than centreline soot temperature and after a critical pressure this trend will

be changed.

6. It is shown that rate of temperature dropping are more in lower pressures

in compare with higher pressures.

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Two-Colour Soot Temperature Results (Ethylene-Air)

Ethylene (100 ml/min)-Air (20 l/min)Flame Centre Line

P=1 bar

1300

1350

1400

1450

1500

1550

1600

1650

4 5 6 7 8 9 10 11 12 13 14 15 16

Flame Heights (mm)

So

ot

Tem

per

atu

re (

ºC)

Pyrometery

Two-Colour Method

Ethylene (100 ml/min)-Air (20 l/min)Flame Centre Line

P=2 bar

1000

1100

1200

1300

1400

1500

1600

1700

2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

Flame Heights (mm)

So

ot

Tem

per

atu

re (

ºC)

Pyrometry

Two-Colour Method

Ethylene (100 ml/min)-Air (20 l/min)Flame Centre Line

P=4 bar

1000

1100

1200

1300

1400

1500

1600

2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

Flame Heights (mm)

So

ot

Tem

per

atu

re (

ºC)

Pyrometry

Two-Colour Method

Ethylene (100 ml/min)-Air (20 l/min)Flame Centre Line

P=6 bar

1260

1280

1300

1320

1340

1360

1380

1400

1420

1440

0 1 2 3 4 5 6 7 8 9 10 11 12 13

Flame Heights (mm)

So

ot

Tem

per

atu

re (

ºC)

Pyrometry

Two-Colour Method

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Propane (60 ml/min)-Air (20 l/min)P=2 bar, H=15 mm

1200

1250

1300

1350

1400

1450

1500

1550

1600

1650

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2

Radial Distance (mm)

So

ot

Tem

per

atu

re (

ºC)

Two-Colour Method

Pyrometry

Trend Line

Propane (60 ml/min)-Air (20 l/min) P=4 bar , H= 15 mm

1000

1100

1200

1300

1400

1500

1600

1700

1800

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8

Radial Distance (mm)

So

ot

Te

mp

era

ture

(ºC

)

Two-Colour MethodPyrom etryTrend Line

Propane (60 ml/min)-Air (20 l/min) P=7 bar , H=15 mm

1315.0

1320.0

1325.0

1330.0

1335.0

1340.0

1345.0

1350.0

1355.0

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

Radial Distance (mm)

So

ot

Te

mp

era

ture

(ºC

)

Two-Colour Method

Pyrom etry

Two-Colour Soot Temperature Results (propane-Air)

22

Soot Concentration Results by Applying Two-Colour Method(Ethylene-Air)

Ethylene (100 ml/min)-Air (20 l/min)Soot Concentration (fv .L ) in flame centre line

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

0.18

0.2

0 2 4 6 8 10 12 14 16 18

Flame Heights (mm)

fv.L

P=1 bar

P= 2bar

p=4 bar

P=6 bar

p=16 bar

Ethylene (100 ml/min)-Air (20 l/min)Soot volume fraction profile

0

20

40

60

80

100

120

140

160

180

200

220

240

0 2 4 6 8 10 12 14 16 18

Flame Heights (mm)

fv-S

oo

t V

olu

me

Fra

cti

on

(p

pm

) P=1 bar

P=2bar

P=4 bar

P=6 bar

p=16 bar

23

Soot volume fraction Results by Applying Two-Colour Method(Propane-Air)

Propane (60 ml/min)-Air (20 l/min)P=2 bar, H=15 mm

0

50

100

150

200

250

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2

Radial Distance (mm)

So

ot

Vo

lum

e F

rac

tio

n, f

v (

pp

m)

Propane (60 ml/min)-Air (20 l/min) P=4 bar , H=15 mm

0

50

100

150

200

250

300

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8

Radial Distance (mm)

So

ot

Vo

lum

e F

rac

tio

n, f

v (

pp

m)

Propane (60 ml/min)-Air (20 l/min) P=7 bar , H=15 mm

0

50

100

150

200

250

300

350

400

450

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

Radial Distance (mm)

So

ot

Vo

lum

e F

ract

ion

, fv

(mm

)

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Soot Formation

1. It is found that the methane-air diffusion flame is less sooty in compare

with ethylene and propane diffusion flames.

2. It was observed that by increasing the pressure the soot concentration and

proportionally soot volume fraction dramatically increased.

3. Soot formation at lower pressures was occurred mainly at the tip of the

flame and in an annular band near the burner rim, as the pressure was

increased, the luminous carbon zone moved downward, filling an

increasingly large portion of the flame.

4. It is found that the more sooty flame the less soot temperature. In our

measurements for example in 4 bar the maximum temperature which was

recorded for methane-air diffusion flame is 1552 ºC, however for same

pressure in ethylene-air diffusion flame the maximum recorded soot

temperature is 1444 ºC and for propane-air diffusion flame 1400 ºC.

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Two-Colour Pyrometry

1. The values of soot volume fraction measured using two-colour method are

coupled to the measured soot temperatures, any errors in measured

temperatures will lead to errors in soot volume fractions.

2. The main problem in applying two-colour method in our experiments was

spectrum region of two selected narrow band filters.

3. Also in separate pictures of flame by CH and C2 filters it is inherently difficult to

find exactly the same points in two pictures of flame, for intensity evaluation.

4. The sensitivity of the main equation was tested on all the parameters and was

found that F1 and F2 have the less effect; however the I1 and I2 show

maximum change.

5. calibration of two colour optical setup (camera with filters) was performed on a

certified tungsten ribbon lamp.

6. The maximum average error that is recorded in our temperature measurement in

two-colour calculation was about 10%.

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Instability Observations

1. Pressure is very influence on stability behaviour of different gaseous flame.

2. The ethylene flames with fuel flow rates of 100ml/min and 115ml/min exhibited good, long term stability at all pressures up to 16 bars.

3. In Methane flame from 8 bar flame dramatically changed to an unstable flame.

4. It was observed that propane presented more stable flame in comparison with ethylene and methane.

5. Fuel and air flow rates play an important role in instability behaviour of gaseous flames.

Methane (120 ml/min)-Air (20 l/min) at 18 bar Ethylene(300 ml/min)–Air(20 l/min) at P=16 bar)

27

Future Works (1)

Applying Modified and Specified Two-Colour Pryometry

Filters (Two-colours and natural density)

Tungsten Ribbon Lamp

CCD Camera

Anti-Heat Filter

Infratherm Pyrometer

Optical Windows

High Pressure Chamber

Roof PrismQuartz Plate

28

Future Works (2)

Chemilominescent Emission Measurement (evaluation of flame dynamic)

Collection Lens

Fibre Optic Cable

3D-Traverse gear

Digital CCD Camera Infrathem Pyrometer

Maas Flow Meters

29

Terahertz Time-Domain Spectroscopy of Sooty Flames at High-Pressure Future Works (3)

•Removing the effects of ambient water vapour• Improving dynamic range and measurable bandwidth •Reducing thermal lensing effects to increase THz transmission

30

Thermo-Acoustic Instability Simulation at Elevated Pressures

• Combustion instability is the main problem in developing new low

emission combustors and burners.

• In our experiments instability was observed for ethylene flame in

higher flow rates and in methane flame after 8 bar flame started

flicking and in higher pressures it became totally unstable.

• Feasibility study of thermo aquatic simulation in this high pressure

burner by using Gambit and Fluent software.

• It means in addition with Chemilominescent emission measurement,

we can predict and discussed the effect of pressure on flame

instabilities, flame buoyancy and Reynolds number

Future Works (4)

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Thank you for your attention