Laser-Induced Incandescence of Soot for High …...Abstract Laser-Induced Incandescence of Soot for High Pressure Combustion Diagnostics Daniel Dennis Emile Cormier Master of Applied

Laser-Induced Incandescence of Sootfor High Pressure Combustion Diagnostics

by

Daniel Dennis Emile Cormier

A thesis submitted in conformity with the requirementsfor the degree of Master of Applied Science

Graduate Department of Aerospace Science and EngineeringUniversity of Toronto

Copyright c© 2011 by Daniel Dennis Emile Cormier

Abstract

Laser-Induced Incandescence of Soot

for High Pressure Combustion Diagnostics

Daniel Dennis Emile Cormier

Master of Applied Science

Graduate Department of Aerospace Science and Engineering

University of Toronto

2011

Accurate determination of soot emissions from combustion is of interest in both fun-

damental research and industries that rely on combustion. Laser-induced incandescence

of soot particles is a young technique that allows unobtrusive measurements of both soot

volume fraction and particulate size. An apparatus utilizing this technique has been

brought to function for both atmospheric and high pressure measurements. Proof of

concept measurements of an atmospheric ethylene-air laminar diffusion flame at 35, 42,

and 47 mm above the burner exit correlate well with literature findings. Profile trends

of a methane-air diffusion flame at 10, 20, and 40 atm at 6 mm above the burner are

similar to reports in literature and are compared to trends from spectral soot emission

measurements. Particle size is found to be roughly proportional to pressure. Discussion

on the errors of laser-induced incandescence as well as recommendations for improving

the apparatus are herein.

ii

Acknowledgements

Studying under Professor Omer L. Gulder has been a pleasure; he maintained high

standards with amiable character, providing a rewarding learning environment and mem-

orable experience. Thank you, Professor.

To my family, who have always provided enduring, loving support, I am forever

indebted.

iii

Contents

1 Motivation 1

2 Introduction 4

2.1 Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

2.1.1 Alternative Methods . . . . . . . . . . . . . . . . . . . . . . . . . 4

3 Theory 6

3.1 Soot Precursors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

3.2 Soot Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

3.2.1 Thermal Accommodation Coefficient . . . . . . . . . . . . . . . . 7

3.2.2 Soot Density . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

3.2.3 Soot Absorption Function . . . . . . . . . . . . . . . . . . . . . . 7

3.2.4 Soot Morphology . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

3.3 Thermal Models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

3.4 Calibration Factor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

3.5 Temperature Decay . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

3.6 Soot Volume Fraction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

3.7 Particle Size . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

4 Apparatus 17

4.1 Layout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

iv

4.2 Support and Mounting . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

4.2.1 Table Frames . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

4.2.2 Optical Breadboards . . . . . . . . . . . . . . . . . . . . . . . . . 20

4.3 Excitation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

4.3.1 Laser . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

4.3.2 Laser Beam Attenuation . . . . . . . . . . . . . . . . . . . . . . . 22

4.3.3 Imaging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

4.4 Detection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

4.4.1 Imaging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

4.4.2 Image Filtering . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

4.4.3 Detectors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

4.4.4 Shielding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

4.5 Diagnostics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

4.5.1 Beam Profiler . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

4.5.2 Power Meter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

4.6 Calibration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

4.6.1 Integrating Sphere . . . . . . . . . . . . . . . . . . . . . . . . . . 30

4.6.2 Power Supply . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

4.6.3 Spectrometer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

4.7 Burners . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

4.7.1 Atmospheric . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

4.7.2 High Pressure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

5 Experimental Procedure 32

5.1 Atmospheric . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

5.2 High Pressure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

v

6 Results: Atmospheric 34

6.1 Signal Quality . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34


6.3 Primary Soot Particle Size . . . . . . . . . . . . . . . . . . . . . . . . . . 35

7 Results: High Pressure 39

7.1 Signal Quality . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39


7.3 Primary Soot Particle Size . . . . . . . . . . . . . . . . . . . . . . . . . . 39

8 Discussion 43

8.1 Proof of Concept: Atmospheric Measurements . . . . . . . . . . . . . . . 43

8.2 Proof of Concept: High Pressure Measurements . . . . . . . . . . . . . . 44

8.3 Addressing Complications . . . . . . . . . . . . . . . . . . . . . . . . . . 46

8.3.1 Noise . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48

8.3.2 Laser Profiling . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50

8.3.3 Laser Stability and Imaging . . . . . . . . . . . . . . . . . . . . . 51

8.3.4 Flame Stability . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51

8.4 Acknowledging Recommendations . . . . . . . . . . . . . . . . . . . . . . 52

8.4.1 Detection Volume . . . . . . . . . . . . . . . . . . . . . . . . . . . 52

8.4.2 Electromagnetic Shielding . . . . . . . . . . . . . . . . . . . . . . 52

8.4.3 Apparatus Layout . . . . . . . . . . . . . . . . . . . . . . . . . . . 52

8.4.4 Avoiding DC Saturation . . . . . . . . . . . . . . . . . . . . . . . 53

8.4.5 Supplemental Techniques . . . . . . . . . . . . . . . . . . . . . . . 53

8.5 Error . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54

8.5.1 Physical Properties . . . . . . . . . . . . . . . . . . . . . . . . . . 54

8.5.2 Experimental Error . . . . . . . . . . . . . . . . . . . . . . . . . . 56

8.6 Recommendations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60

vi

9 Conclusion 61

Appendices 62

A LII Operations Manual 62

A.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62

A.2 Setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62

A.3 Basic Operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65

A.3.1 The Laser . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65

A.3.2 PMT Power Supply . . . . . . . . . . . . . . . . . . . . . . . . . . 66

A.4 Alignment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66

A.4.1 Independent Excitation and Detection Alignment . . . . . . . . . 66

A.4.2 Flame and Laser Alignment . . . . . . . . . . . . . . . . . . . . . 69

A.4.3 Flame and Detector Alignment . . . . . . . . . . . . . . . . . . . 71

A.5 Calibration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72

A.5.1 Components and Setup . . . . . . . . . . . . . . . . . . . . . . . . 72

A.5.2 Experimental Procedure . . . . . . . . . . . . . . . . . . . . . . . 74

A.5.3 Using the Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76

A.6 Measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77

A.6.1 Measurement Position . . . . . . . . . . . . . . . . . . . . . . . . 77

A.6.2 Taking Measurements . . . . . . . . . . . . . . . . . . . . . . . . 77

A.6.3 Saving Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79

A.7 Data Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80

A.8 Maintenance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81

Bibliography 81

vii

List of Tables

5.1 Parameters used for atmospheric ethylyene/air and high pressure methane/air

LII analysis. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

A.1 Power supply channel configuration for each of the PMTs’ input wires. . 66

A.2 Spectrometer parameters used during calibration. . . . . . . . . . . . . . 75

A.3 Oscilloscope channel settings used for LII measurments. . . . . . . . . . . 78

A.4 Oscilloscope trigger settings used for LII measurements. . . . . . . . . . . 79

A.5 Oscilloscope file settings used for LII measurements. . . . . . . . . . . . . 80

viii

List of Figures

3.1 Soot particles heated by 0.15 Jcm2 fluence laser as imaged by a transmission

electron microscope. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

4.1 Schematic illustrating the optical layout of the two-colour LII apparatus

setup for atmospheric measurements. . . . . . . . . . . . . . . . . . . . . 18

4.2 Schematic illustrating the optical layout of the two-colour LII apparatus

setup for high pressure measurements. . . . . . . . . . . . . . . . . . . . 19

4.3 A photograph showing the optical components of the LII apparatus set up

for high pressure measurements. . . . . . . . . . . . . . . . . . . . . . . . 20

4.4 Rear-illuminated photograph of melted alumina ceramic excitation slit. . 24

6.1 A normalized atmospheric LII signal with low signal-to-noise ratio. . . . 35

6.2 A normalized atmospheric LII signal with high signal-to-noise ratio. . . . 36

6.3 Atmospheric ethylene-air soot volume fraction at 35 and 47 mm above the

burner exit on the flame centreline. . . . . . . . . . . . . . . . . . . . . . 37

6.4 Atmospheric ethylene-air primary particle size at 35 and 47 mm above the

burner exit on the flame centreline. . . . . . . . . . . . . . . . . . . . . . 38

7.1 A normalized high pressure LII signal with high signal-to-noise ratio. . . 40

7.2 Methane-air soot volume fraction at 6 mm above the burner exit on the

flame centreline at pressures of 10, 20, and 40 atm. . . . . . . . . . . . . 41

ix

7.3 Methane-air primary particle size at 6 mm above the burner exit on the

flame centreline at pressures of 10, 20, and 40 atm. . . . . . . . . . . . . 42

8.1 Comparison of present results with those of Thomson et al. and Joo. . . 47

A.1 Schematic illustrating the optical layout of the available two-colour LII

apparatus setup for atmospheric measurements. . . . . . . . . . . . . . . 63

A.2 Schematic illustrating the optical layout of the available two-colour LII

apparatus setup for high pressure measurements. . . . . . . . . . . . . . . 64

x

Chapter 1

Motivation

Laser-induced incandescence (LII) is an established and widely-used technique for

measuring soot particle size and concentration [1]. The technique involves using a high

power pulse laser to heat soot particles to near-sublimation temperature, causing the

heated particles to emit broadband radiation that can be detected as a raw LII signal

[2]. The magnitude of the signal depends on soot concentration whereas the decay rate

of the signal is related to the size of the cooling particles [3, 4].

Atmospheric soot particles have been shown to be detrimental to respiratory health

and contributors to climate change [5–11]. While ground-level emissions of soot particu-

late provoke pulmonary diseases [7, 12], emissions released high in the atmosphere, from

jet turbines for example, may affect climate change [5, 8]. Meanwhile, thermal absorption

by soot accumulated on the interior of internal combustion engines can reduce operation

efficiency; conversely, the formation of soot particles can be important for efficient furnace

operation and desired in the formation of carbon black [1, 13, 14].

The detection of soot and the understanding of soot formation in combustion is there-

fore a relevant pursuit and LII is an apt diagnostic tool for this purpose [1]. Widely-

utilized commercial methods for measuring soot concentrations in flames often rely on

mechanical means and therefore are limited in accuracy. The Society of Automotive

1

Chapter 1. Motivation 2

Engineers (SAE) Smoke Number test and Transmission Electron Microscopy (TEM) are

common examples. It is impossible to implement these tests in a real-time and non-

intrusive manner and in situ sampling requires expensive modifications to the appara-

tus [13]. LII is uniquely capable of yielding high temporal and spatial resolution data

describing both soot volume fraction and primary particle size. The technique is also

non-intrusive in that measurements do not noticeably alter the behaviour or properties

of the flame in a way that would invalidate the accuracy of subsequent measurements.

The LII technique is not alone in the field of optical combustion techniques. Its

advantages over other techniques are numerous including sensitivity, accuracy, and ver-

satility. Instead of just time-averaged results, instantaneous readings of both particulate

concentration and diameter are possible. The sampling rate of the detection equipment

determines temporal resolution; several billion signals per second are obtained, result-

ing in temporal resolution on the order of picoseconds. The high temporal resolution of

LII makes the technique particularly well suited to in situ measurements and turbulent

targets in general. LII is not limited to flame diagnostics and can probe any volume

containing light-absorbing particles.

LII has a large dynamic range of detection and has been used to detect particu-

late concentrations ranging from 0.01 ppt to 10 ppm in a single experiment [15]. The

spatial resolution with LII is diffraction-limited and can be changed within or between

experiments by simply changing apertures.

Objective

The objective of this thesis is to ready an LII apparatus for the investigation of flames

within a high pressure combustion chamber. The extent of this preparation should be

such that measurements and data analysis are possible with little knowledge of LII theory.

This thesis should serve as preparation for using the apparatus to its developed potential.

This thesis builds upon the groundwork of Trevor Kempthorne who completed exten-

Chapter 1. Motivation 3

sive literature survey and assembled a non-functioning LII apparatus [16]. This work is to

address the problems preventing the successful implementation of the apparatus and ac-

knowledge Kempthorne’s observations and recommendations. Further recommendations

for future improvements on this project are to be given herein.

Chapter 2

Introduction

2.1 Background

LII is not the lone method of determining either particle size or concentration; other

methods exist and can be more reliable. However, LII is at an advantage due to its

robustness and almost non-intrusive nature.

2.1.1 Alternative Methods

Filtered Rayleigh scattering, laser attenuation, and transmission electron microscopy

(TEM) are techniques frequently employed in place or in complement of LII. Rayleigh

scattering works by measuring the fraction of incident light that was scattered from

molecules within a flame. A detector develops a signal from the scattered light that

can be used, upon calibration of the system, to provide particle temperatures [13, pp.

155-165]. For particulate-heavy flames, however, Rayleigh scattering no longer provides

a viable model. Instead, a mechanism known as Mie scattering takes over and the weak

Rayleigh signal is drowned out. It becomes difficult to distinguish between there being

many small particles and fewer large particles.

Laser attenuation measurement is a straightforward diagnostic technique used to de-

4

Chapter 2. Introduction 5

termine soot volume fraction. The technique relies on determining the ratio of absorbed

to transmitted incident light [17]. When particulate concentration becomes very high or

when turbulent conditions with high-frequency pressure and density fluctuations exist,

however, the accuracy of the attenuation measurement decreases [13, 18].

A reliable technique for determining particulate size and distribution is to physically

look at the soot with a microscope. To collect samples, a microscopic grating must be

mechanically inserted into the flame of interest for microsecond durations. Using TEM,

the particle size and distribution can be manually determined. The problem with this

technique, aside from it being labourious and time-consuming, is that it is an intrusive

method; physically inserting the grating into the flame noticeably alters the surrounding

flame properties. The technique is often used as a benchmark for more robust techniques,

however, such as in ensuring accurate measurements with non-intrusive methods such as

LII [19, 20].

These techniques, although simple and capable of high accuracy measurements, are

often difficult to apply to a practical or diverse problem. LII is robust and can be used

from very high to very low pressures and concentrations of particulate to give accurate

results with high dynamic range [4, 17, 21–23]. The other techniques are often utilized

for calibrating or verifying the accuracy of LII setups.

Chapter 3

Theory

3.1 Soot Precursors

The mechanisms involved in soot formation are not well understood; fortunately,

LII does not rely on understanding these mechanisms. Large soot-precursor molecules

are of interest, however, as they may skew calculations if they contribute too much to

signals. Such precursors, polycyclic aromatic hydrocarbons (PAHs) are formed following

the breakdown of fuel molecules [24, 25, pp. 343-346]. If these PAHs remain unoxidized,

other hydrocarbons in the mixture can adhere to them to eventually form large primary

soot particles ranging between 5 and 50 nm in atmospheric diffusion flames [1].

Like soot particles, PAHs are susceptible to heating by ultraviolet and visible light

which could cause them to incandesce and interfere with the signal from the soot particles

of interest [26]. Because of this, 1064 nm lasers have grown in popularity for LII; using

near-infrared light defeats the significant interference observed from PAHs and other

hydrocarbons in the detection volume [1, 13].

6

Chapter 3. Theory 7

3.2 Soot Properties

Soot density, thermal accommodation coefficient, and the soot absorption function

must each be known to fully process an LII signal.

3.2.1 Thermal Accommodation Coefficient

The thermal accommodation coefficient, α, is the least understood property of soot

essential for LII analysis. The calculated primary soot particle size is proportional to this

constant. Rather than using a physically-meaningful value, many researchers use the co-

efficient as a fitting parameter to match results for particle size to physically-measured

particles in identical flame conditions [27]. To simplify analysis, this property is often

assumed to be temperature-independent despite expectations of it being dependent on

gas and soot temperatures [27]. Recent studies have proposed temperature-dependent

values, although gas temperature-dependence seems to dominate over soot particle tem-

perature [27, 28]. Common values used for α range from 0.28 [29] to 0.9 [1].

3.2.2 Soot Density

Soot density, ρ ( kgm3 ), is used to calculate particle size; values in the literature com-

monly range from 1850 [30] to 2260 kgm3 [31] where soot particles are roughly approximated

as graphite sheets in density.

3.2.3 Soot Absorption Function

The soot absorption function, E(m), is a function of the complex index of refraction of

soot, m, and incident wavelength. The index of refraction is suggested to be temperature-

dependent [1]. Values of E(m) commonly used by researchers range between 0.24 [1] and

0.4 [32, 33].

Chapter 3. Theory 8

3.2.4 Soot Morphology

The adopted model makes the common assumption [33–37] that aggregates of primary

soot particles have a surface area to volume ratio similar to an equal mass of individ-

ual spherical primary particles. This assumption simplifies the aggregates as groups of

spheres touching at single points, which keeps aggregate surface area to volume ratio

constant.

50 nm

Figure 3.1: Soot particles heated by 0.15 Jcm2 fluence laser as imaged by a transmission

electron microscope by Vander Wal et al. [38].

Chapter 3. Theory 9

3.3 Thermal Models

Understanding the heating and cooling processes of soot particles is required to prop-

erly analyze LII signals. At atmospheric pressure, the magnitude of the LII signal from

a given detection volume has been shown to be proportional to primary soot particle

size, incident laser fluence, and the number density of soot particles [39–41]. This result,

the McCoy and Cha model, uses the assumption that the velocity distribution of the gas

molecules is independent of soot particle presence – that the Knudsen number is much

greater than one for the free molecular regime [42, 43]. Laser fluence plays an important

role in heating. The resulting LII signal is roughly linear with respect to fluence until

the point when the soot particles begin to evaporate [34, 44]. This occurs at roughly

4000 K, which corresponds to approximately 0.3 to 0.4 Jcm2 at 1064 nm [1, 45]. It is

generally accepted that measurements should be taken at whatever fluence corresponds

to the highest LII signal, which is usually the level at which the soot is just starting to

evaporate [4, 21, 29, 34, 45–47].

Although radiation forms the measured LII signal, conduction is the primary mecha-

nism responsible for soot particle cooling [1]. Conductive cooling rate is proportional to

the surface area of the particles and so large particles, with a low surface area to volume

ratio, will cool slower than smaller particles. Because of this, large soot particles’ overall

contribution to the LII signal will be greater than that of the smaller particles and will

increase as the signal decays, when only the large particles are still emitting detectable

radiation [42]. Most models make the assumption that aggregates of primary soot par-

ticles have a surface area to volume ratio similar to an equal mass of individual primary

particles. This assumption simplifies the aggregates as groups of spheres touching at

single points, which keeps aggregate surface area to volume ratio constant [33–37].

Another model, Fuchs’ method, has been recommended for transition Knudsen num-

bers on the order of unity. This model considers particles as having two concentric layers.

The inner layer is modeled as being in the free molecular regime while in the outer layer

Chapter 3. Theory 10

the heat conduction is modeled as existing in the continuum regime where collisions are

frequent. This method is unnecessary at atmospheric pressure, however, and the McCoy

and Cha model results in just 5 % error of particle temperature at 80 atm [42].

3.4 Calibration Factor

To calibrate the photomultiplier tubes (PMTs) that detect the radiant cooling of

soot particles in the flame, a light source of known intensity must be observed. Without

this step, the PMT signals yield only relative intensity that can not be correlated to

a meaningful spectral radiance. It is this radiance that will eventually be compared to

blackbody radiation curves to determine the approximate temperature of the cooling soot

particles. This calibration of the PMTs is therefore independent of flame type or signal

intensity.

The calibration technique was developed by Snelling et al. [48]. For a given PMT

gain voltage and incident light intensity, a unique voltage signal is produced by the

PMT. Knowing all three of these values during calibration therefore yields a calibration

factor η for each detection wavelength given by

η =VCAL

RSGCAL

(3.1)

where RS is the spectral radiance of the light source and VCAL and GCAL are the associated

PMT signal and gain voltages, respectively. This calibration factor can then be used in

evaluating the temperature decay and soot volume fraction.


3.5 Temperature Decay

Finding the temperature of the cooling particles relies on two-colour optical pyrome-

try [48]. Using the calibration factor, the ratio of two signals of different wavelength can

be related to the ratio of power emission by the particles at each wavelength as

Pp(λ1)

Pp(λ2)=λ6

2E(mλ1)

λ61E(mλ2)

exp

[− hc

kTs

(1

λ1

− 1

λ2

)]=VEXP1

VEXP2

η2

η1

GEXP2

GEXP1

(3.2)

where λ1 and λ2 are the two detection wavelengths which correspond to unique power

emissions Pp, index of refraction functions E(m), signal voltages VEXP, PMT gains GEXP,

and calibration factors η. The speed of light, Planck constant, and Boltzmann constant

are represented by c, h, and k respectively, while Ts is the particle temperature. The

index of refraction function is approximated as being constant between the two wave-

lengths. From Equation (3.2) the particle temperature can easily be found as an explicit

function of the PMT signal ratio and thus a function of time:

Ts = −hck

[1

λ1

− 1

λ2

] [ln

(VEXP1

VEXP2

η2

η1

λ61

λ62

)]−1

. (3.3)

With this information, the soot volume fraction can also be found.

3.6 Soot Volume Fraction

Assuming uniform heating – which is achieved with sufficient laser fluence – the vol-

ume fraction of soot in the probed volume is proportional to the magnitude of the signal


produced by the PMTs at any given wavelength. A large signal corresponds to a large

amount of radiating soot. Using the particle temperature and calibration factor from

above, the soot volume fraction fv is calculated by

fv =VEXP

ηGEXP

λ6C

12πhc2

exp(

hckλCTp

)− 1

E(mλC)wb

(3.4)

where λC is the centre wavelength of detection and wb is the width of the laser sheet [48].

Similarly, the primary particle diameter can be found using the temperature decay profile.

3.7 Particle Size

According to the McCoy and Cha transition regime heat conduction model [43], the

ratio of transition regime heat flux Φ to continuum regime heat flux ΦC for soot particles

can be expressed as

Φ

ΦC

=1

1 +GKn(3.5)

where Kn is Knudsen number and G is the heat transfer factor given by

G =8f

α(γ + 1)(3.6)

wherein α is the thermal accommodation coefficient, γ is the adiabatic constant at local

gas temperature, and the Euchen factor f is given by

f =9γ − 5

4(3.7)


For heat conduction between two concentric spheres with radii R1 < R2 where the

region between the spheres is filled with an arbitrary gas, the continuum value of heat

flux at the surface of the inner sphere is given by

ΦC =κa∆T

R1 (1−R1/R2)(3.8)

where κa is the thermal conductivity coefficient of the surrounding gas and ∆T is the

difference in surface temperature between the concentric spheres. Because the boundary

distance R2 is much greater than the particle radii, R2 → ∞ and R1 is taken to be the

average radius of primary soot particulate, rp. Then the continuum flux becomes

ΦC =κa∆T

rp

. (3.9)

With the Knudsen number defined in terms of mean free path and particle radius,

Kn =λMFP

2rp

, (3.10)

and substituting for the continuum flux, Equation (3.5) can be rewritten as

Φ =2κa∆T

dp +GλMFP

(3.11)

where

dp = 2rp (3.12)

is the diameter of primary soot particulate. Integrating the heat flux over the entire

particle surface S,

∮S

~Φ · ~dS =∂Qin

∂t− ∂Qout

∂t, (3.13)


yields the time rate of change of particulate heat energy Q in and out of the system. Since

the total flux is being considered as the outward flux normal to the particles’ surface,

this becomes

∮S

ΦdS = −∂Q∂t. (3.14)

Integrating the RHS of Equation (3.11):

−∂Q∂t

=

∮S

2κa∆T

dp +GλMFP

dS (3.15)

=

∮S

2κa∆T

dp +GλMFP

r2p sin θdθdφ

=2κa∆T

dp +GλMFP

4πr2p

Rearranging the above relation yields

− 2κa

dp +GλMFP

4πr2p =

∂Q

∂t

1

∆T(3.16)

where, via the chain rule,

∂Q

∂t

1

∆T=∂Q

∂Ts

∂Ts

∂t

1

∆T(3.17)

Soot particle heat capacity at constant volume Cs is defined as

Cs =

(∂Q

∂Ts

)V

(3.18)

and

τT ≡∂Ts

∂t

1

∆T(3.19)

can be defined as the time, t, decay rate of temperature divided by ∆T , given by

∆T = Ts − Tg (3.20)


where Ts and Tg are the temperatures of soot particles and local gases respectively.

Equation (3.16) can then be written in terms of the specific heat capacity of soot particles

at constant volume, Cs:

− 2κa

dp +GλMFP

4πr2p = τTCsρsVs (3.21)

where ρs is the particulate mass density and

Vs =4

3πr3

p (3.22)

is the assumed mean particle volume. Finally, rearranging the above equation gives an

implicit solution for the particle diameter:

−τ−1T

2κa

(dp +GλMFP)Csρs

=(4/3)πr3

p

4πr2p

(3.23)

=1

6dp

or

dp = −τ−1T

12κa

(dp +GλMFP)Csρs

. (3.24)

To obtain the mean free path, a general expression for the transition regime total

flux [43] is used:

Φ =ΦC(

1 + ΦC

ΦK

) (3.25)

where the Knudsen flux [43], ΦK, is given by

ΦK =αPCV(γ + 1)∆T

2√

2πmkBT(3.26)


where kB is the Boltzmann constant and P , CV, and m are the pressure, heat capacity at

constant volume, and molecular mass of the combustion gas, respectively. Substituting

this and Equation (3.9) into Equation (3.25) yields

Φ =2κa∆T

dp

(1 +

4κa

√2πmkBT

dpαPCV(γ + 1)

)−1

. (3.27)

Equating the above to Equation (3.11) and rearranging gives

GλMFP =4κa

√2πmkBT

αPCV(γ + 1). (3.28)

Finally, substituting for G and rearranging yields

λMFP =κa

PCVf

√πmkBT

2. (3.29)

Using molar values for heat capacity and molecular mass, and noting temperature

dependencies, this becomes

λMFP =κa(Tg)

PCV(Tg)f(Tg)

√πRuTg

2W(3.30)

where Ru is the universal gas constant and W is the molar mass of the combustion gas.

Returning to Equation (3.24) and explicitly solving for dp produces

dp = −1

2GλMFP +

√(GλMFP

2

)2

− τ−1T

12κa

Csρs

. (3.31)

In cases where dp GλMFP, which occurs at the low pressures generally considered,

an inexact explicit solution for dp can be found by ignoring the diameter term in the

RHS of Equation (3.24) such that the approximate particle size does not depend on the

thermal conductivity or adiabatic constant of the surrounding gas:

dp = −τ−1T

3αP

ρs

[2CP(Tg)−Ru]

Cs(Ts)

√W

2πRuTg

(3.32)

where CP is the heat capacity of the local combustion gas at constant pressure.

Chapter 4

Apparatus

4.1 Layout

The apparatus consists of a very simple and compact arrangement of optical com-

ponents; the atmospheric and high pressure LII optical configurations can be seen in

Figures 4.1 and 4.2 respectively. These layouts do not show electrical or support com-

ponents including power supplies, oscilloscope, cables, and computers. A photograph of

the apparatus (Figure 4.3) better illustrates scale.

4.2 Support and Mounting

The apparatus’s optical components are mounted on several optical breadboards

which are secured to mobile aluminum tables.

4.2.1 Table Frames

The table frames that support the apparatus’s optical breadboards consist of 60 mm

cross-section aluminum framing manufactured by Bosch Rexroth. The tables are built

to support several hundred kilograms of equipment including heavy optical breadboards,

17

Chapter 4. Apparatus 18

Nd:YAG Laser (1064 nm)

PMT

PMT

250 mm

Relay

lens pairs

Half wave plate Thin-film polarizer

3 mm x 50 µm Vertical slits

Burner

692 nm

Band-pass

filter

440 nm Band-

pass filter Dichroic filter

150 mm

Collimating lens

150 mm

Achromats

Electromagnetic-

shielding box

Figure 4.1: Schematic illustrating the optical layout of the two-colour LII apparatus setup

for atmospheric measurements. Oscilloscope, power supplies, cables, and gas delivery

system are not shown.

support electronics, and shielding.

The tables ride on rubber caster wheels to provide mobility between experiments,

and are set stationary by manually raising several metal feet. The feet are independently

height-adjustable to allow the optical breadboards to be level during experiments.

For atmospheric measurements, the apparatus sits on a single table fitted with two

optical breadboards. This table is used for the detection optics in the high pressure

configuration and a second table with a single breadboard is used for the excitation

optics.


Nd:YAG

Laser

(1064 nm)

PMT

PMT

400 mm

Relay

lens pairs

3 mm x 100 µm Vertical slit

High pressure

combustion

chamber

692 nm

Band-pass

filter

440 nm Band-


150 mm

Collimating lens

150 mm

Achromats

Electromagnetic-shielding box

Half wave plate

Thin-film polarizer

100 µm

Aperture

Burner

Quartz

viewports

Figure 4.2: Schematic illustrating the optical layout of the two-colour LII apparatus setup

for high pressure measurements. Oscilloscope, power supplies, cables, and gas delivery

system are not shown.


Figure 4.3: A photograph showing the optical components of the LII apparatus set up for

high pressure measurements. Detection optics are on the foreground table and excitation

optics including partial laser head are visible on the background table. Oscilloscope,

power supplies, and fuel and gas delivery system are not in full view. In the background,

the high pressure chamber is partially visible between the excitation and detection optical

tables. The electromagnetic shielding box that is normally present is omitted so that

optical components are visible. Metal shielding boxes built around the detectors are,

however, present.

4.2.2 Optical Breadboards

Optical components are mounted on several Thorlabs PerformancePlusTM series op-

tical breadboards. In the atmospheric setup, a small (300 mm × 900 mm) and a large

(600 mm × 1200 mm) breadboard lie side by side to create a 300 mm × 300 mm space

for a burner to reside. A 600 mm x 1200 mm breadboard on its own frame is used in


the high pressure setup for the excitation optics while the detection optics remain on the

other two breadboards.

4.3 Excitation

The excitation component of the apparatus includes the laser and beam-manipulation

optics including power-attenuation, beam shaping, and relay optics.

4.3.1 Laser

Because PAHs absorb strongly in the visible and ultraviolet, which could cause over-

estimation of soot volume fraction, an Nd:YAG laser operating at 1064 nm is used in

the apparatus. Several groups have used 532 nm lasers for LII; however, light at this

frequency may induce fluorescence in addition to incandescence [1, 26]. The laser used

is a multimode Continuum Surelite II-10 modified to use a graded reflectivity mirror

(GRM). The GRM’s non-uniformity allow for the superpositioning of different modes

resulting in a stable super-Gaussian spatial profile that has a flatter centre peak than a

normal Gaussian distribution.

The laser has a measured divergence of 5 × 10−4 radians and a 3.6 mm initial 1/e2

spot size. With a 0.18 m cavity length and 1 cm−1 laser linewidth, the Rayleigh range1

of the laser beam is 3.83 m.

A major weakness of using near-infrared light is the difficulty inherent to aligning and

focusing an invisible beam.

1The propagation distance from a beam’s waist at which the cross sectional area of the beam hasdoubled.


4.3.2 Laser Beam Attenuation

The default laser output, over 2 Jcm2 , is more powerful than is needed; the maximum

fluence required is roughly 0.4 Jcm2 and therefore the beam power must be attenuated.

Although the laser itself can be configured to alter its power output by changing the

Q-switch delay or pump voltage, this would also alter the spatial and temporal profiles

of the laser beam. Because the LII phenomenon will be independent of polarization state

of incident light, a half wave plate (HWP) and thin-film polarizer are used to reduce the

laser output without affecting the beam profile. This also provides the ability to readily

change the resulting beam power.

Half Wave Plate

The Thorlabs half wave plate can be rotated to alter the portion of p-polarized light

that is rotated to s-polarized light. Anywhere between all of and none of the p-polarized

light will be rotated depending on the HWP rotation angle.

Thin Film Polarizer

The Thorlabs thin film polarizer, when rotated to Brewster’s angle2 from the beam’s

propagation axis, will reflect all s-polarized light. The remaining light will be transmitted

without an alteration in beam profile. By changing the portion of s-polarized light

transmitted through the half wave plate, the portion of light transmitted through the

thin film polarizer is changed, thereby attenuating the laser beam.

2The angle of incidence for a transparent dielectric surface at which light will be either perfectlytransmitted or reflected depending on if the light has a particular polarization or not, respectively.Roughly 56 for glass in air.


4.3.3 Imaging

The detection volume is partially defined by the excitation beam shape. A small slit

is used to shape the beam; this slit of light is then imaged into the flame. Imaging the

resulting image with the detection optics will define the detection volume.

Aperture

For atmospheric measurements, a highly light-tolerant alumina ceramic 3 mm ×

50 µm (± 10 %), 125 µm thick slit manufactured by Lenox Laser was used to define

the shape of the laser beam to be imaged onto the detection volume within the flame.

Diffraction caused by the thin slit is detectable at the burner location using a beam

profiler; however, the intensities of non-primary maxima are small (< 5 % total fluence).

This component reflects or absorbs most of the incident beam while allowing the

small slit portion to be imaged into the flame. Because it is being bombarded with

high intensity light, prolonged use will result in damage as can be seen in Figure 4.4.

Likewise, the damage threshold of the aperture somewhat limits the fluence used during

measurements, though this limit accommodates that required.

For high pressure measurements the excitation slit is 100 µm wide rather than 50 µm

but is otherwise identical to the atmospheric slit.

Relay Lenses

On both the excitation and detection sides of the burner, relay lenses focus on the

intended measurement point within the flame and simultaneously onto the aperture on

that lens pair’s side of the apparatus. The intersecting shape of the two apertures’ images

composes the detection volume.

In the atmospheric setup, two f/10 250 mm focal length achromatic lenses are used

on either side of the burner to produce unity magnification of the detection volume.

These are swapped with f/16 400 mm lenses for high pressure measurements. The larger


1 mm

Figure 4.4: Rear-illuminated photograph of the alumina ceramic excitation slit, 3 mm ×

50 µm. Prolonged exposure to high-fluence light has visibly ablated the 125 µm thick

ceramic material near the slit. White light from behind reveals holes in the disc, bored

over time by laser light.

focal length accommodates the size of the high pressure chamber, although the larger

f-number means that less light is detected.

Normally relay imaging is susceptible to creating a slightly softer (or more out-of-

focus) detection volume since both the excitation and detection relay lenses must be

manually focused to a single vertical axis in space. A larger f-number increases the lenses’

depth of field which eases alignment and may result in greater measurement accuracy

since the detection volume will be in sharper focus for all measurement positions. This

comes at the compromise of reducing measurement sensitivity; the minimum soot volume

fraction detectable with the high pressure setup is therefore higher than in atmospheric

conditions. Likewise, for any measurement, the signal-to-noise ratio will be inherently

lower with the high pressure setup. On the other hand, this may come as a boon for very

large signals; because less light is detected, detector saturation will occur at higher soot


concentrations (which are common at high pressure).

The lenses are all manufactured by Thorlabs; excitation-side lenses have an anti-

reflection coating for infrared wavelengths. The calculated depth of field of the 250 mm

and 400 mm lens pairs is roughly 0.25 mm and 0.63 mm, respectively.

4.4 Detection

The laser-induced incandescent light is detected at two distinct colors. This requires

two detectors, each measuring a color-filtered portion of the total emitted light spectrum.

This light is represented by voltages produced by the detectors, and measured by an

oscilloscope to produce the LII signals.

4.4.1 Imaging

The imaging of the detection volume occurs in the same way that the laser light is

relayed to the flame. By focusing the image of an aperture onto the laser-heated particles

within the flame, the detection volume is defined. The shape of the detection volume

will be dictated by the shape of both the excitation and detection apertures.

Relay Lenses

The detection-side relay lenses are identical to the excitation-side with the exception

of having visible-wavelength anti-reflective coating rather than infrared.

Aperture

A Lenox stainless steel aperture is used to image the detection volume with respect to

the detection optics. For atmospheric measurements, a vertical 3 mm × 50 µm (± 5 %)

slit was used; the intersection of the images of the excitation and detection apertures

define a 50 µm × 50 µm × 3 mm vertical line as a 7.5 × 10−12 m3 detection volume.


For high pressure measurements, a Lenox 100 µm (± 3 %) diameter circular stainless

steel aperture is used. The detection volume in this case is a 100 µm × 100 µm right

cylinder with axis parallel to the detection optics, approximately 7.85 × 10−13 m3. Proof

of concept measurements utilized a 50 µm × 1 mm vertical slit.

The smaller detection volume at high pressures means higher precision and smaller

signals. Because the high pressure flames are smaller and sootier than atmospheric flames,

this is an apt compromise.

4.4.2 Image Filtering

To obtain two distinct signals, the emitted LII light must be split into two beams,

and each beam must be filtered for a single color.

Dichroic Filter

An achromatic lens is used to collimate the broadband LII signal imaged from the

detection-side aperture. The collimated light then passes through a Thorlabs short-pass

dichroic mirror that transmits light below 488 nm and reflects the remaining incident

beam.

Band Pass Filters

The split signal then passes through two Thorlabs 40 nm full width at half maxi-

mum (FWHM) band-pass filters centred at 440 and 692 nm, respectively, before each is

focused to a photomultiplier tube (PMT) by another achromatic lens resulting in unity

magnification. This choice of detection wavelengths avoids interference of C2 Swan band

emissions at 473 nm, 516 nm, 563 nm, and 618 nm [1, 24, 49]. The large separation

between detection wavelengths also allows for more precise discrimination between emis-

sion intensities at these wavelengths. The FWHM value is higher than the typical 20

nm found in the literature, and was chosen to detect a larger signal given that the small


detection volume being utilized provides little light.

4.4.3 Detectors

Because measurements are to be time-resolved, fast photo-detectors are required to

resolve the short lifetime of the LII signal. Charge-coupled devices (CCDs) lack the

required temporal resolution, and so photomultiplier tubes are used.

Photomultiplier Tubes

The Hamamatsu PMTs have peak detection efficiency between 400 and 800 nm which

fits the 440 nm and 692 nm detection colours. The PMTs have a rise time of 1.4 ns which

accommodates the Nyquist-Shannon theorem which implies that sampling should occur

at a frequency of half the electronic rise time.

Circuit Boards

The PMTs are mounted on circuit boards manufactured by Artium, a company spe-

cializing in the production of commercial emissions diagnostics LII apparatuses. These

boards feature noise filters and op-amps that help to reduce signal noise from the PMTs.

Power Supply

Each PMT requires a mainline ± 15 V supply in addition to a variable gain between

0 and 5 V. The gain voltage is manually set before taking measurements, and can be the

same for each PMT. A single GW Instek GPS-4303 quad-output power supply is used

for both PMTs. This power supply is best kept within the same shielding box as the

detection optics to shield exposed wire leads.


Oscilloscope

The LeCroy Wavesurfer 64Xs oscilloscope used has 600 MHz bandwidth, sampling

rate of 2.5 × 109 samples per second, and 11 bits of vertical resolution. The oscillo-

scope monitors both PMT outputs in addition to their gain voltage. Shielded bayonet

Neill-Concelman connector coaxial cables are used with the oscilloscope and have a 0.75

velocity factor, yielding approximately 4 ns of temporal offset between signals per metre

of path difference. This offset is insignificant because cable lengths are equal to within

less than a centimetre.

4.4.4 Shielding

Shielding detection equipment from electromagnetic interference (EMI) is crucial;

visible light leaks will artificially inflate the LII signal and decrease the signal-to-noise

ratio of measurements. In addition to visible light, invisible EMI is of greatest concern;

interference can be caused by any electronic equipment in the room including lights,

motors, power supplies, and the LII laser.

The detection equipment shown in Figures 4.1 and 4.2 is covered with a black felt-

board box mounted to the breadboards in attempt to reduce noise introduced by ambient

light; furthermore, the box is lined with 0.5 mm thick copper mesh to absorb and reflect

low frequency electromagnetic waves. Additionally, the PMTs and circuit boards are

encased in 6.4 mm thick conductive metal boxes to further prevent noise caused by EMI.

4.5 Diagnostics

Having a properly-shaped laser beam is desired for LII analysis, and so the beam’s

profile is measured using a beam profiler. Furthermore, the total fluence of the beam can

be found using a power meter.

For crude diagnostics, burn paper can be used to determine both beam shape and


power. The burn paper will ablate upon being exposed to high intensity light with

wavelengths between 100 and 1500 nm. The visible depth of ablation gives insight into

the intensity of incident light, allowing for crude analysis of spatial power distribution.

Shooting the burn paper is also an effective way of verifying the location of the laser

beam, because the beam is invisible.

4.5.1 Beam Profiler

A beam profiler will reveal the spatial distribution of laser power in the laser beam.

Likewise, it reveals the overall shape of the laser beam. This is important since LII

analysis requires a measurement of the width of the excitation laser sheet. The Newport

LBP-2 USB laser beam profile used has a very low damage threshold of 1 µJ/cm2, there-

fore the laser beam must be attenuated before reaching the profiler. A glass Thorlabs 3

wedge is used to reflect approximately 3 % of the laser beam toward the profiler. In front

of the profiler, three neutral-density filters, with 1.653, 3.42, and 15.1 % transmittances,

yield a total transmittance of 8.54 × 10−3 %. Together, these attenuators allow roughly

2.56 × 10−4 % of the laser beam’s original intensity to reach the profiler’s sensor.

4.5.2 Power Meter

A Gentec Electro-Optics SOLO-PE laser power and energy meter is used together

with a QEAX-25 attenuator. The attenuated is placed in front of the power meter and

allows 24.56 % of incident light to be transmitted. This is required because the laser’s

fluence is greater than the damage threshold of the meter.

Although the exact power or fluence of the laser beam is not required for signal

analysis, it is important to have a rough idea of the fluence to avoid under or over

heating the soot particles.


4.6 Calibration

The calibration equipment is identical between atmospheric and high pressure setups.

In addition to the LII detection equipment, the calibration apparatus consists of very few

components: an integrating sphere, a power supply, and a spectrometer with software.

4.6.1 Integrating Sphere

The calibration method [48] requires a reliable source of light. For this, an integrating

sphere is ideal. The integrating sphere used is a halogen lamp-illuminated SphereOptics

SPH-6-2 (serial number 3925). It has a 15.2 cm inner diameter spherical OptowhiteTM

Lambertian surface3 with two ports. In addition to a 1 mm diameter fibre optic port

used for spectrometric measurements, it has a 3.81 cm diameter knife-edge port which is

used to illuminate the LII detectors during calibration.

4.6.2 Power Supply

An Agilent E3634A 200 W 0-25 V, 7 A / 0-50 V, 4 A DC power supply was used to

supply the integrating sphere lamp with a steady current of 4.166 A.

4.6.3 Spectrometer

A SphereOptics SMS-500 Spectral Measurement System was used to determine an

accurate spectral radiant output of the integrating sphere. It interfaces via USB with

the integrating sphere via a 1 mm fibre optic cable. The spectrometer interfaces with

proprietary SMS-500 software loaded on a computer to measure and output the spectral

radiance of the integrating sphere as a function of emission wavelength.

3A surface that adheres to Lambert’s cosine law; its observed scattered light intensity is independentof the angle of observation.


Spectrometer Software

The SMS-500 software is loaded onto the oscilloscope connected to the PMTs. The

software produces a text file containing a table of precise time-averaged spectral radiance

values that is used to determine the calibration factor for LII analysis.

4.7 Burners

Although all measurements were of non-premixed laminar flames, atmospheric and

high pressure measurements relied on different burner types and fuels.

4.7.1 Atmospheric

Atmospheric proof of concept measurements were taken from a Gulder burner using

an ethylene-air non-premixed flame. This provides a stable, highly-sooting flame ideal for

testing the apparatus. There are several published soot volume fraction and particulate

size measurement results using this burner and fuel with standardized flow rates and

measurement location [1, 17, 48, 50–52].

The burner, burner stage, and gas delivery apparatus are the same used and described

by Bohan [53, pp. 25-29].

4.7.2 High Pressure

The gaseous fuel burner that was used for testing is the same burner that was used by

Joo [54]. Measurements by Joo were taken via SSE and include full flame measurements

of methane, which are used in part as a metric for proof of concept LII measurements

taken under the same experimental conditions. The burner is based on the burner used

by Miller and Maahs [55] for its high flame stability.

The burner stage and gas delivery apparatuses are as previously described by Joo for

methane-air flames [54, pp. 19-29].

Chapter 5

Experimental Procedure

Detailed procedures followed for experimental work are presented in Appendix A.

5.1 Atmospheric

Flow conditions were standard for the burner type (Gulder burner): 194 standard

mL/min ethylene and 284 standard L/min of (coflow) air.

Measurements were taken at a rate of 10 Hz in 50 µm intervals; at each location, 400

shots were taken and averaged. Measurements share an axis with the detection optics

to show signal attenuation effects. Parameters used in analysis of the signals are as in

Table 5.1.

5.2 High Pressure

The methane and air laminar co-annular non-premixed flame used fuel and air flow

rates of 0.55 mg/s and 0.4 g/s respectively.

Measurements were taken at a rate of 10 Hz in 50 µm and 100 µm intervals; at each

location, 400 shots were taken and averaged. Measurements were taken along the axis of

the detection optics. Parameters used in analysis of the signals are as in Table 5.1.

32

Chapter 5. Experimental Procedure 33

Table 5.1: Parameters used for atmospheric ethylyene/air and high pressure methane/air

LII analysis.

Property Atmospheric Value High Pressure Value

E(m) 0.25 0.25

α 0.3 0.3

W 0.02896 kgmol

a 0.02896 kgmol

a

Tg 2369 K [56] Pressure-dependent [54]

ρs 2030 kgm3 [57]b 2030 kg

m3 [57]b

κa Not usedc Temperature-dependent [58]a

Cs Not usedc Temperature-dependent [59]

Cp Temperature-dependent [60]a Temperature-dependent [60]a

a Value for atmospheric air

b Value for graphite

c See Equation (3.32)

Chapter 6

Results: Atmospheric

6.1 Signal Quality

Initial LII signals from the apparatus appeared stretched over time. Figure 6.1 shows

a characteristic sinusoidal disturbance predominantly in the 692 nm channel that was

detected whenever the laser was fired.

A signal with much less disturbance was acquired; this quality was repeatable and

ultimately representative of signals used for final measurements. Figure 6.2 shows those

smoother signals that are also sharp in time.


Soot volume fraction at any given location was found to be constant in time, with

maximum deviations being less than 5 %. The resulting soot volume fraction spatial

profile was as in Figure 6.3. A flat plateau in soot volume fraction was found at a height

35 mm above the burner (HAB) approximately 3.4 ppm in volume. At 47 mm HAB,

a flame tip-shaped soot volume fraction profile was observed, peaking at approximately

3.2 ppm in volume. Measurements at 42 mm HAB resulted in peaks at 4.7 and 4.8 ppm

with a non-flat centre profile ranging between 3.7 and 4.1 ppm in soot volume fraction.

34

Chapter 6. Results: Atmospheric 35

0 1 0 0 2 0 0 3 0 0 4 0 0 5 0 0 6 0 0 7 0 0 8 0 0

0 . 0

0 . 2

0 . 4

0 . 6

0 . 8

1 . 0

No

rmaliz

ed LI

I sign

al

T i m e ( n s )

4 4 0 n m s i g n a l 6 9 2 n m s i g n a l

Figure 6.1: A normalized atmospheric LII signal with low signal-to-noise ratio. Its large

time width is indicative of under heating. Significant electromagnetic disturbance is

evident in the 692 nm channel, caused by interference from the laser.

6.3 Primary Soot Particle Size

Particulate size as a function of time at each measurement position appeared to

increase nearly linearly by approximately 10 % over 300 ns after the peak signal. The

particulate size profile of the flames were as as in Figure 6.4.


0 1 0 0 2 0 0 3 0 0 4 0 0 5 0 0 6 0 0 7 0 0 8 0 0

0 . 0

0 . 2

0 . 4

0 . 6

0 . 8

1 . 0

Norm

alized

LII s

ignal

T i m e ( n s )


Figure 6.2: A normalized atmospheric LII signal with high signal-to-noise ratio. Elec-

tromagnetic interference is minimal and the sharp peak is indicative of heating to near-

sublimation temperature.


- 3 - 2 - 1 0 1 2 30

1

2

3

4

5

6

Soot

volum

e frac

tion (

ppm)

R a d i a l p o s i t i o n ( m m )

3 5 m m H A B 4 7 m m H A B

Figure 6.3: Atmospheric ethylene-air soot volume fraction at 35 and 47 mm above the

burner exit on the flame centreline. Laser beam propagation is into the page with detec-

tors on the left; signal attenuation is apparent on the right-side 35 mm HAB peak.


- 3 - 2 - 1 0 1 2 305

1 01 52 02 53 03 54 04 55 05 5

Prima

ry pa

rticle

size (

nm)


3 5 m m H A B 4 7 m m H A B

Figure 6.4: Atmospheric ethylene-air primary particle size at 35 and 47 mm above the

burner exit on the flame centreline. Laser beam propagation is into the page with detec-

tors on the left.

Chapter 7

Results: High Pressure

7.1 Signal Quality

LII signals from both PMTs were smooth and sharp, characterized by the normalized

signals from each PMT as seen in Figure 7.1.


As with atmospheric measurements, soot volume fractions were found to be constant

in time at any given location within the flame. Although generally smoother than the

atmospheric results, deviations were roughly equal at 5 %. The soot volume fractions as

a function of radial position were as in Figure 7.2 for 10, 20, and 40 atm flames measured

at 6 mm HAB. At each radial position, soot volume fraction roughly doubled with each

step in pressure.

7.3 Primary Soot Particle Size

For each location at any given location within the flame, particulate size appeared to

increase linearly as a function of time within any given measurement. Increases ranged

39

Chapter 7. Results: High Pressure 40

- 2 0 0 0 2 0 0 4 0 0 6 0 0 8 0 0 1 0 0 0

0 . 0

0 . 2

0 . 4

0 . 6

0 . 8

1 . 0

Norm

alized

LII s

ignal

T i m e ( n s )


Figure 7.1: A normalized high pressure LII signal with high signal-to-noise ratio. Noise

is insignificant and the peaks are sharp.

from 10 % at 10 atm to 30 % at 30 atm over 300 ns. Particulate sizes as a function of radial

position were as in Figure 7.3 for 10, 20, and 40 atm flames measured at 6 mm HAB.

Particle size greatly increased at each radial position for each step in pressure.

Particulate sizes at 20 atm were an average of 2.14 times larger than at 10 atm.

Diameters at 40 atm were 4.50 times larger than at 10 atm and 2.12 times larger than

at 20 atm, on average.


- 1 . 0 - 0 . 8 - 0 . 6 - 0 . 4 - 0 . 2 0 . 0 0 . 2 0 . 4 0 . 6 0 . 8 1 . 00

5

1 0

1 5

2 0

2 5

3 0

3 5

4 0

Soot

volum

e frac

tion (

ppm)


1 0 a t m 2 0 a t m 4 0 a t m

Figure 7.2: Methane-air soot volume fraction at 6 mm above the burner exit on the flame

centreline at pressures of 10, 20, and 40 atm. Laser beam propagation is into the page

with detectors on the right.


- 1 . 0 - 0 . 8 - 0 . 6 - 0 . 4 - 0 . 2 0 . 0 0 . 2 0 . 4 0 . 6 0 . 8 1 . 00

5 0

1 0 0

1 5 0

2 0 0

2 5 0

3 0 0

3 5 0

4 0 0

Prima

ry pa

rticle

size (

nm)


1 0 a t m 2 0 a t m 4 0 a t m

Figure 7.3: Methane-air primary particle size at 6 mm above the burner exit on the flame

centreline at pressures of 10, 20, and 40 atm. Laser beam propagation is into the page

with detectors on the right.

Chapter 8

Discussion

8.1 Proof of Concept: Atmospheric Measurements

Standard measurements were taken at atmospheric pressure to compare the results

with similar measurements that have been published. For the burner and flame used, the

results agreed well with similar literature values [1]. Of particular interest was matching

results to those of Snelling et al. [48] on which work this apparatus is largely based.

Measurements were made at 42 mm that correspond well with the results of Snelling

et al.. Soot volume fractions reported by Snelling et al. peaked at roughly 5.0 ppm while

measurements in this study peaked at 4.8 ppm. The centre profile of the flame reported

by Snelling et al. ranged from approximately 3.8 to 4.0 ppm compared to a 3.7 and

4.1 ppm range reported in this study. The profiles of the flames share a similar shape.

Measurements in this study, however, display a lower dynamic range. While peak soot

volume fractions are similar, the minimum detected soot volume fraction was 0.3 ppm

while Snelling et al. reports values near 0 and 0.1 ppm.

The discrepancy in dynamic range could be attributed to several factors. In addition

to possessing larger f-number lenses, the apparatus used by Snelling et al. featured a

detection volume several times larger than in this study. These two factors result in

43

Chapter 8. Discussion 44

much more light being detected, hence the disagreement in minimum detected signal.

Discrepancies in soot volume fraction can also be partially attributed to a difference

in detection volume dimensions, as slightly different parts of the flame are being detected

between studies.

8.2 Proof of Concept: High Pressure Measurements

Upon successful atmospheric measurements, high pressure measurements were made,

again, in attempt to match results reported in the literature. As with atmospheric

pressure, of great interest was comparing results with Thomson et al. [17] on which

work this apparatus is largely based. Along with similar LII theory, the chamber and

burner used by Thomson et al. is similar to that used in this study. The same chamber

and burner as used in this study was used for extensive SSE-based soot volume fraction

measurements by Joo [54].

Comparisons of results between the current study, Thomson et al. [17], and Joo [54]

are presented in Figure 8.1. Soot volume fraction and particle size trends are similar to

those of Thomson et al. [17]. Ratios of peak to centreline values are similar to those pre-

sented by Thomson et al. despite differences in magnitude. Soot volume fractions were

consistently lower than those presented by Thomson et al. while particulate sizes were

consistently larger. Differences in aperture dimension are likely a major contributor to

this inconsistency. The greatest contributor may be that values for the thermal accom-

modation coefficient and index of refraction function were kept constant – 0.3 and 0.25,

respectively – for both atmospheric and high pressure analysis. It is typical for studies to

adjust these to scale both soot volume fraction and particulate size to acceptable values

while maintaining a constant trend [1]. The values were kept the same across flames

and pressures to be consistent. While this is almost certainly in error, there is currently

no consistency for these values reported in literature as functions of either pressure or


temperature.

Attenuation effects appear low in Figure 7.2. This may indicate that the soot volume

fraction is not significantly more than an order of magnitude higher than was seen in

Figure 6.3, which displays significant attenuation. The high pressure profile’s width is

approximately a third of the atmospheric profile’s width, so high pressure attenuation

should be smaller for similar soot volume fractions.

The particulate size profile at 40 atm was less symmetric than other profiles. Thomson

et al. also note asymmetry at this pressure and measurement height. Poor flame stability

at heightened pressure may be suspect.

Joo [54] reports soot volume fraction profiles much smoother and larger in magnitude

with SSE than either LII analysis. Additionally, measurements presented by Joo appear

to have much higher dynamic range than either LII results. The ratio of peak to centreline

signal is much smaller than what is yielded by LII. This results in different profile shapes

between the two techniques. While difference in spatial resolution may again be to blame

for inconsistencies between values, the differences in profiles requires further explanation.

The comparison of these results clearly illustrates the advantages of each technique.

SSE results are not characterized by the asymmetry inherent to LII due to attenuation

losses which results in each peak having a different magnitude. Instead, SSE data may

be averaged to present a single, averaged peak, resulting in smoother profiles.

Minimum soot volume fraction measurements with SSE appear to go to zero, giving

SSE a much higher apparent dynamic range than LII. This is due to the ability to record

any ambient light as a signal, including a dark current as zero. LII lacks this ability;

the smallest LII signals will be lost to ambient light and attenuation. The lack of a

detectable LII signal is merely a line-of-sight SSE measurement and therefore does not

necessarily correspond to zero soot volume fraction. While this ambient light produces

an SSE signal, it is subtracted from the LII signal.

The cause to SSE’s advantage in apparent dynamic range also leads to one of the


technique’s weaknesses that results in profiles that do not match LII measurements. Be-

cause SSE is a line-of-sight measurement, light is collected at every measurement position

within the flame regardless of local emission intensity. Measuring along the centreline

of the flame results in light being collected from both the near and far annular peaks

surrounding the flame’s centre. Additionally, the technique assumes perfect axial sym-

metry, which is not observed in practice. Although proper SSE analysis algorithmically

accounts for this, it is apparent that there is inherent error in this technique. An at-

tempt to quantify this error finds it to be on the order of ± 12 % [61]. However, the error

may show systematic bias: while low-sooting locations may be over-represented, peaks

could be under-represented. If this were to occur, the result would be that flame profiles

measured by SSE would not appear as hollow as those measured by LII, which measures

soot locally within the flame; the ratio of peak to centreline signal would be consistently

smaller from SSE than LII. This is observed to occur for most comparisons in Figure 8.1.

Likewise, SSE values are consistently significantly higher than those measured by LII.

Both of these effects may be explained by the line-of-sight nature of SSE.

8.3 Addressing Complications

Kempthorne [16] observed lower than desired values of heated soot particle temper-

ature, ranging between 2800 and 3000 K, when attempting to obtain near-sublimation

temperatures. Possible explanations include inaccurate fluence measurement, a phase

shift between the two detection channels, or an improper value of the absorption func-

tion. However, the two signals also showed different levels of EMI, which could make

comparison of the magnitudes of the channel outputs problematic. At 300 mJ/cm2 laser

fluence, the apparatus now heats particles to near-sublimation temperatures, as calcu-

lated by subsequent data analysis.

Soot volume fractions for the atmospheric ethylene flame studied initially ranged


L I I [ T h o m s o n e t a l . ] L O S A ( c o r r e c t e d ) [ T . e t a l . ] L O S A ( u n c o r r e c t e d ) [ T . e t a l . ] L I I S S E [ J o o ]

- 1 . 0 - 0 . 8 - 0 . 6 - 0 . 4 - 0 . 2 0 . 0 0 . 2 0 . 4 0 . 6 0 . 8 1 . 002468

1 01 21 41 61 82 0

So

ot vo

lume f

ractio

n (pp

m)


(a) Soot volume fraction at 10 atm.

L I I [ T h o m s o n e t a l . ] L O S A ( c o r r e c t e d ) [ T . e t a l . ] L O S A ( u n c o r r e c t e d ) [ T . e t a l . ] L I I S S E [ J o o ]

- 1 . 0 - 0 . 8 - 0 . 6 - 0 . 4 - 0 . 2 0 . 0 0 . 2 0 . 4 0 . 6 0 . 8 1 . 00

1 0

2 0

3 0

4 0

5 0

6 0

Soot

volum

e frac

tion (

ppm)


(b) Soot volume fraction at 20 atm.

L I I [ T h o m s o n e t a l . ] L O S A ( c o r r e c t e d ) [ T h o m s o n e t a l . ] L O S A ( u n c o r r e c t e d ) [ T h o m s o n e t a l . ] L I I S S E ( J o o )

- 1 . 0 - 0 . 8 - 0 . 6 - 0 . 4 - 0 . 2 0 . 0 0 . 2 0 . 4 0 . 6 0 . 8 1 . 00

2 0

4 0

6 0

8 0

1 0 0

1 2 0

1 4 0

1 6 0

Soot

volum

e frac

tion (

ppm)


(c) Soot volume fraction at 40 atm.

1 0 a t m [ T h o m s o n e t a l . ] 2 0 a t m [ T h o m s o n e t a l . ] 4 0 a t m [ T h o m s o n e t a l . ] 1 0 a t m 2 0 a t m 4 0 a t m

- 1 . 0 - 0 . 8 - 0 . 6 - 0 . 4 - 0 . 2 0 . 0 0 . 2 0 . 4 0 . 6 0 . 8 1 . 0

5 0

1 0 0

1 5 0

2 0 0

2 5 0

3 0 0

3 5 0

4 0 0

Prima

ry pa

rticle

size (

nm)


(d) Particle diameter at 10, 20, and 40 atm.

Figure 8.1: Comparison of present results (black markers) with those of Thomson et al.

[17] (white markers) and Joo [54] (black and white markers) for a methane/air co-flow

diffusion flame at 6 mm HAB.

between 100 and 4000 ppm [16], where values near 4 ppm are typical [1]. Also observed

was the steady 4-fold gain in soot concentration at any given location in the flame over the

course of a measurement, whereas the volume fraction should remain constant. Similar

phenomena have been reported in the literature [48], but on a much smaller scale with


10 % increases. The pattern of soot volume fraction within the flame was also suspect

and followed no physically-meaningful or smooth trend within the flame as is seen in the

literature. These issues have been remedied as discussed in Section 8.1.

Previous particle size measurements partially agreed with literature; the majority of

values matched ranges the literature, within experimental error; however, the spatial dis-

tribution of particle size was again not meaningful and did not match the literature. Using

shorter decay timescales resulted in calculating larger particles [16]. Shorter timescales

should result in smaller particulate size calculation, as small particles will cool faster,

early within the LII signal; as time passes, only large particles contribute to the signal.

Particle size should therefore appear to increase as a function of time at any given posi-

tion. These problems have been remedied, resulting in meaningful trends, as discussed

in Section 8.1.

8.3.1 Noise

During lasing, there is often a large sinusoidal disturbance to the PMTs’ DC gain

voltage which is reflected in the PMT signals. During the disturbance, the voltage

alternates positively and negatively by up to 100 % of its DC value. This was attributed

to Q-switch noise produced at the laser head. The characteristic shape of this disturbance

is evident in an LII signal presented in Figure 6.1. There was much random noise as seen

by the PMTs regardless of whether measurements were being taken; however, this was

not attributed to anything particular.

The oscilloscope, PMT power supply, and laser power supply were initially all within

close proximity to each other. The PMT power supply was sitting on top of the laser

computer while the laser flash lamp was firing at 10 Hz; at that time, a small spike in

the PMT gain voltage was observed at exactly 10 Hz. This did not occur when the PMT

power supply was in proximity to the laser head. It was concluded that most noise during

measurements was originating from the laser computer and not the laser head itself.


Moving the laser computer far from the apparatus reduces noise during measurements.

Furthermore, it was noted that keeping the laser’s single-shot cable, which usually resides

next to the oscilloscope, near the PMT power supply or the oscilloscope increased the

noise. Moving all cables leading to the laser power supply away from the oscilloscope

and PMT power supply reduces the noise drastically; however, the relative positioning of

the oscilloscope and PMT power supply has a large effect on the magnitude of this noise

and these two items should be kept as far away as possible from each other and from all

other electronic equipment and cords.

The two control units that operate the atmospheric burner’s movement stage motors

were each creating significant high-frequency electronic noise that appeared random while

both units were plugged in. Unplugging the control units eliminates the noise. The high

pressure chamber motors do not seem to create similar noise.

Although the noise during measurements has not been completely eliminated, it has

been reduced drastically. To further reduce or eliminate this noise, the PMT power

supply should be properly shielded in a metal box to isolate the detection and excitation

equipments.

The box surrounding the detection optics and PMTs poorly blocks ambient light and

contributes to random noise seen by the PMTs; furthermore, its thin shielding poorly

blocks the laser Q-switch noise. This box should be constructed of mostly iron to shield

the PMT power supply and wires which are not currently being shielded by the metal

PMT circuit board housings.

Much of the detection equipment was rewired with shielded coaxial cables which fur-

ther reduced noise. This meant re-soldering connections to the PMT circuit boards,

changing oscilloscope connections to have proper electromagnetic shielding, and chang-

ing the wiring at the PMT power supply to be less exposed and more organized. These

changes resulted in decreasing random noise in the measurements by almost a full or-

der of magnitude and to a typically negligible amount. Bare wires at PMT and PMT


power supply terminals are still suspect to propagating random noise and non-random

disturbances to the data.

Systematic disturbance persisted even with the above rewiring. The source was the

laser Q-switch. From firing the laser, a significant sinusoidal disturbance to the PMT

signals can be easily seen. This was almost entirely resolved (Figures 6.1 and 6.2) by

careful positioning of the PMT power supply to ensure it was shielded, and by moving

the laser power supply far from the apparatus. The problem persists to a minor degree

and is sensitive to the spatial positioning of the apparatus.

In contrast to Figure 6.2, the large FWHM of the signal curves in Figure 6.1 is

indicative of under-heated soot particles. Because the particles’ peak temperature is well

below what is required for sublimation, the cooling profile is what is observed at the

tail of a proper curve (Figure 6.2) where temperatures are low and cooling has slowed.

In Figure 6.2, particles are peaking near 4000 K, as desired, and cool to 3000 K within

300 ns.

Analysis Software

Despite a high quality signal, the existing analysis code was still unable to produce

meaningful results such as the temperatures mentioned above. Thus, a new analysis

model was developed which is based on Snelling et al. [48] and Thomson et al. [17]. The

methodology being employed is unique to this project and therefore new analysis software

has been written and described in Section A.7.

8.3.2 Laser Profiling

The laser profile was measured using the apparatus described in Section 4.5.1. The

profile of the laser sheet appeared uniform with measured deviations from the maximum

fluence being less than 10 %; diffraction effects were negligible. Study on the influence

of highly heterogeneous spatial profiles reveals that minor deviations are not of signifi-


cance [62].

The excitation aperture has been replaced with a 100 µm wide slit of the same height

and material as shown in Figure 4.4. Effective deviations in slit width are well within

specification of 5 %, as measured with the beam profiler, and are generally on the order

of 1 to 2 %.

Operating the apparatus with fluences near or below that required for soot sublima-

tion has no noticeable wear on the alumina ceramic aperture.

8.3.3 Laser Stability and Imaging

The laser head was re-calibrated by the manufacturer and is operated without mod-

ification to the profile. This results in a profile within specifications. The laser travels

much less than 1 m before encountering the excitation aperture, where the measured

beam profile remains nearly uniform and stable between shots.

8.3.4 Flame Stability

Flame stability with the atmospheric Gulder burner was not a problem during ex-

periments; measured profiles seen in Figure 6.3 do not appear erratic. High pressure

flames often flickered and waved during pressure changes but appeared to remain stable

at constant pressure. Because the high pressure flame is much smaller than the atmo-

spheric flame, profile measurements are much more sensitive to small instabilities using

the high pressure burner. Both atmospheric and high pressure flow control apparatuses

were thoroughly checked for leaks before measurements. Flow controllers were calibrated

before each experiment and flow rates were monitored to ensure constant, unperturbed

flow conditions during measurements. Flame height was measured using a high resolution

CCD camera to ensure consistent flame size between experiments.


8.4 Acknowledging Recommendations

Kempthorne [16] made recommendations that should be acknowledged, based on

complications with the apparatus, with the intent to improve results.

8.4.1 Detection Volume

A larger aperture has been recommended to avoid diffraction effects and to improve

signal-to-noise (S/N) ratio.

The initial aperture slits were each 3 mm tall and 50 µm wide. Because neither

diffraction or S/N ratio were found to be an issue, smaller apertures were acquired to

improve spatial resolution. Because the high pressure flames are approximately 9 mm in

height, a 3 mm tall detection volume is too large. A 100 µm to 1 mm height is sufficiently

small, depending on signal strength, without compromising S/N ratio.

8.4.2 Electromagnetic Shielding

To suppress electromagnetic interference, a Faraday cage is recommended for the

PMT power supply and oscilloscope leads.

Although the oscilloscope cables and leads have been replaced to possess better shield-

ing, the leads are not fully protected. Grounding is currently done via the PMT power

supply ground, which is not yet adequately shielded as recommended. A full iron Faraday

cage would significantly improve the S/N ratio.

8.4.3 Apparatus Layout

It is recommended that the apparatus components, namely the laser from the detec-

tion equipment, be separated as much as possible to avoid electromagnetic interference.

The components are all situated to be as isolated as possible. With proper shield-

ing, the apparatus should be condensed and compartmentalized. The detection optics’


shielding box already contains the PMT power supply unit, which has helped to reduce

external interference.

8.4.4 Avoiding DC Saturation

A resistor in series with the PMT output is recommended to improve S/N ratio and

avoid DC saturation of the PMTs.

Because adding anything in series between the PMT and oscilloscope would introduce

a weakness in electromagnetic shielding, this may adversely affect S/N ratios. Having a

resistor here would compress signals on the oscilloscope, effectively reducing the dynamic

range of the apparatus. Additionally, DC saturation is best solved by increasing the

spatial resolution of the apparatus; a smaller detection volume will result in exponentially-

smaller signals.

Typically only one of the PMTs produces a signal near saturation, as is desired to

optimize dynamic range. To obtain near-saturation on both PMTs, the ideal solution is

to operate each PMT at a different gain voltage.

8.4.5 Supplemental Techniques

It has been recommended that a secondary diagnostic technique be used in parallel

with the LII to compare results.

The high pressure chamber has until now been operated with SSE as the sole diag-

nostic method; therefore, there exists a wealth of data to compare to as has been done

in this study. In addition to the benefits of comparing results between techniques, this

is indeed advantageous; because LII requires knowledge of local gas temperature in the

flame, a secondary diagnostic technique that can measure this is ideal.


8.5 Error

Sources of error have been implicitly mentioned throughout this text. LII suffers from

numerous random and systematic error sources. Most systematic sources are masked

through calibration and scaling optical properties to match results to accepted values.

Many random sources are negated through averaging.

8.5.1 Physical Properties

The assumptions made in Chapter 3 all result in error that is difficult to quantify.

Soot particles are not truly spherical or blackbodies, soot aggregates can not be ignored,

and the thermal and optical properties of soot are not well understood. In reality, all of

the considered intrinsic properties of soot most likely depend on temperature, pressure,

and wavelength despite being treated as constants or functions of only either temperature

or wavelength.

The McCoy and Cha model itself errs at high pressures, resulting in up to 5 % error

in particle temperature at 80 atm [42].

Soot Morphology

The assumptions of particle size and emission spectrum are largely accommodated by

averaging; although particles are not spherical, treating them as such on a large statistical

scale will result in predictable error. The same is true for the spectral emissions of the

soot.

Particles are assumed to aggregate as perfect spheres touching at single points and

any shielding effects caused by aggregation is ignored. This is clearly in error as large

aggregates will not heat evenly and will not emit radiation representative of their mass

due to shielding effects. An attempt to quantify this error finds it to be significant [63].


Optical and Thermal Properties of Soot

Attempts to produce accurate optical properties of soot, namely the refractive index

function, either as a constant or function of wavelength or temperature, have proven

highly inconsistent [34, 46, 64–69]. This is despite evidence that the function is at least

wavelength dependent [46, 65–67, 69–71]. The parameter is typically used for fitting soot

volume fractions to physically-measured data, often measured via physical sampling and

electron microscopy [1, 38]. This is convenient because soot volume fraction is inversely

proportional to this function, as seen in Equation (3.4). If it is used as a fitting parameter,

then this will conceal much of any systematic error in the calculations.

Like with the refractive index function, attempts to determine an accurate thermal

accommodation coefficient have proven inconsistent [27, 28, 46, 72–74]. While generally

represented as constant, it is thought to be a function of at least temperature [27, 75, 76].

Although not immediately obvious, particle size is approximately proportional to the

thermal accommodation coefficient, as seen in Equation (3.32), which may well lend the

variable to the scaling of results.

Using Equation (3.32) in lieu of Equation (3.31) results in less than 1 % difference in

calculated values at 40 atm.

Local Gas Properties

At each measurement position, the local gas temperature is required to calculate the

soot volume fraction and particle size. Additionally, the particle size calculation depends

on the heat capacity and thermal conductivity of the local gas surrounding the soot

particles.

Uncertainties in local gas temperature is one of the greatest errors and difficult to

quantify. Although the adiabatic flame temperature can be used for all measurement

positions in a flame, the temperature differs throughout the flame. Having a diagnostic

method work parallel to the LII that is able to measure temperature is advantageous


for this reason. Assuming constant temperature throughout the flame results in up to

± 35 % error [54].

Also dependent on local flame temperature is the thermal conductivity and heat

capacity of the surrounding gas. Errors in these values will be increased by errors in

the local temperature. With Equation 3.32, only the heat capacity contributes to the

calculation.

8.5.2 Experimental Error

Most of the errors introduced during experiments are eventually masked by either

averaging or PMT calibration. Averaging reduces the impact of random error while

calibration negates systematic error by producing a scaling constant – the calibration

constant, given by Equation (3.1) – that represents the optical state of the detection

apparatus.

Noise

Random noise is caused primarily by ambient light, the greatest of which originates

from the target flame’s luminosity due to the flame’s characteristic incandescence. Ambi-

ent electromagnetic noise is subtracted from measurements in post-processing, however,

and does not impose significant noise to measurements.

Random noise that expresses bias at any point within an LII measurement is not

typically subtracted and therefore contributes error. This type of noise is rarely seen,

small in magnitude, and nearly eliminated by averaging several hundred signals during

the measurement. Therefore, random noise is not considered a significant contributor to

error.

Systematic electromagnetic disturbances, generally referred to as noise, is the great-

est systematic experimental error. The primary cause of this interference is previously

referred to as Q-switch noise. This disturbance is caused by high electric current peaks


during lasing from both the laser’s head and power supply unit. Its characteristic im-

pact on signals is presented in Figure 6.1. Although generally not as prevalent as in the

aforementioned figure, it still poses a large error to small signals when the amplitude of

the interference is comparable to the magnitude of the LII signal. This contributes to a

flame’s profile having greatest relative error at its lowest sooting points.

Flame Stability

Instabilities in a flame result in the wrong part of the flame being measured, which

can result an infinite level of error. This is the greatest random experimental error.

Measurements consist of an average of several hundred LII signals taken in sequence at

10 Hz. This is in attempt of overcoming small fluctuations in the flame. Even with a large

number of signals, this averaging may not account for instabilities because fluctuations

are likely to be far from symmetric.

A flame that appears stable to the eye is often not stable to the LII detection appa-

ratus, which has a spatial resolution of 50 µm. Near the peak sooting region of a flame,

soot volume fraction can change by several factors in this small distance. Shot-to-shot

error fluctuates as does the flame, ranging from insignificant to infinite. Averaging does,

however, allow for repeatability of results with only a few percent error. Outlier data

points are most commonly attributed to uncommonly large flame fluctuations and gener-

ally occur near the peak sooting region of the flame, where soot volume fraction gradients

are highest.

Optics

The apparatus requires careful optical setup to avoid error; however, if setup within

certain thresholds, systematic flaws in optical components of the detection apparatus are

accounted for by calibrating the system.

Detection volume, defined by aperture dimensions, poses significant error when com-


paring results with measurements taken with a different volume. This is because enlarg-

ing the detection volume will result in a larger portion of the flame being sampled and

averaged.

There is error in finding the correct vertical measurement position (HAB) in the

flame. Following the methodology described in Section A.4.2, this will be the error in

the measuring device. Typically ± 0.5 mm. The effect will be slightly altered trends

and values, dependent on the location within the flame. The effect is larger on smaller

flames.

Laser profile uniformity is desired, though as discussed in Section 8.3.2, minor devi-

ations are observed and not of significance.

The 400 mm relay lenses on either side of the flame have a 0.63 mm calculated depth

of field. Because the laser sheet is 100 µm wide, focusing errors are unlikely; additionally,

imperfect focus will be systematic and corrected by calibration.

Although the PMTs respond to light differently, calibration will account for the differ-

ences if systematic. If either of the PMTs produces inconsistent signal responses, which

may occur at certain gain voltages, calibration will not correct this. It may be difficult

to detect if this subtly occurs.

Aside from gross alignment errors or random electrical errors within the PMTs, cali-

bration will account for small errors in the physical apparatus. This includes wavelength-

dependent differences in the transmission properties of optical components.

Burner and Flow Control

Errors in gas flow result in inconsistent flame size between experiments. Flow regu-

lators are calibrated to within roughly ± 1 % before each experiment. Fluctuations in

flow are observed to be approximately ± 1 % during measurements. This contributes to

flame instability.

Joo [54] observed burner stage drift on the order of 5 µm per minute during ex-


periments at high pressures. This was not observed during present measurements at

40 atm. The translational stage has a resolution of ± 2.5 µm, contributing 5 % error in

measurement position.

Pressure is digitally monitored with precision of ± 0.5 psi. Pressure drift during

experiments was observed to be no more than 2.0 ± 0.5 psi at 40 atm.

Attenuation

LII emissions are subject to attenuation effects. When measuring a location on the

side of the flame furthest from the detection optics, the signal must pass through the

entire flame before reaching the detectors. Some of the light from the signal will be lost

due to absorption and scattering from the soot in the flame. A high-sooting flame will

therefore display higher attenuation effects than a low-sooting flame. Figure 6.3 clearly

demonstrates the effect of attenuation; the detectors are on the left side of the profile.

The left side of the flame profile peaks at 5.61 ppm while the right peaks at 5.06 ppm.

Attenuation has a negligible effect on particulate size measurements because this

calculation depends on the relative magnitudes of the two signals and not the absolute

intensity of the emissions.

Calibration

Although calibration masks many of the potential experimental errors, it does contain

error itself. Linear interpolation of calibration curves coincide with R2 values typically

above 0.96.

Time Window

Analysis of data requires the choice of a time window within the signal, which is

typically chosen shortly after the peak to avoid contributions from quick-cooling PAHs.

Changing the time window’s duration, start time, or end time can drastically change


both the trend and scale of profiles. Individual data points can unpredictably shift up to

± 4 % with a 10 ns shift in window time, with an average shift of ± 2 %.

8.6 Recommendations

With a limited conscience of practicality, the following changes to the apparatus would

yield the greatest improvement to signal quality and reduction of error:

A more compact and less powerful laser would be better suited for the apparatus.

The current laser’s default power is too high and must be reduced with additional optics.

The greatest source of signal noise is currently the laser. A smaller laser with better

electromagnetic shielding and lower power would have a smaller impact upon the signal.

An electrically-grounded solid ferrous shielding box for the detection equipment would

significantly reduce random noise and laser Q-switch noise. A 1 mm thick solid steel box

would theoretically block over 99.9 % of electromagnetic interference from the laser. This

box should shroud the detection optics, PMT power supply, and oscilloscope leads.

Replacing relay lenses with lenses of smaller f-number would increase the amount of

light detected, thus increasing dynamic range and S/N ratio. Although the focal lengths

are restricted by the high pressure chamber, the lenses’ diameters could be increased.

A new PMT power supply with two channels for gain voltage would allow near-

saturation voltages for both PMTs simultaneously. This would improve the S/N ratio

for the lower-response channel by up to over 1 000 %.

Chapter 9

Conclusion

An LII apparatus has been fully developed for high pressure combustion diagnostics.

Proof of concept measurements at pressures from 1 atm up to 40 atm were taken that

correlate well with expected results.

Soot volume fraction and particle size trends and values at atmospheric pressure

agree well with literature findings. With the same parameters as used for atmospheric

analysis, trends at high pressure remain in agreement with what is found in literature

despite differences in magnitudes. This difference may be attributed at least partially

to not scaling results. As predicted by Equation (3.32), particulate size was found to

be roughly proportional to pressure; particles increased in diameter by 2.14 times from

10 atm to 20 atm and by 2.12 times from 20 atm to 40 atm.

Appendix A provides detailed documentation for operating the apparatus, and soft-

ware has been written with user-interface and documentation for straightforward data

analysis. Prior complications and acknowledgements have been addressed, inherent errors

were discussed, and new recommendations have been made for improving the apparatus.

61

Appendix A

LII Operations Manual

A.1 Overview

Working with the laser-induced incandescence (LII) apparatus and resolving its data

can be divided into seven primary categories: Setup, Basic Operation, Alignment, Cali-

bration, Measurement, Data Analysis, and Maintenance.

A.2 Setup

The LII apparatus can be setup for use with either atmospheric or high pressure

burners. Both setups use much of the same components with very few changes.

The general optical layout of the apparatus setup for atmospheric measurements

(Figure A.1) differs little from the high pressure setup (Figure A.2). The few changes

required between the two setups are due to the large size of the high pressure chamber

and the expected increase in soot volume fraction at high pressures.

The atmospheric setup is positioned on a single optical breadboard table frame for

compactness of the apparatus. Because of this, a mirror is utilized between the laser and

the burner to reflect the exciting laser beam ninety degrees. In the high pressure setup,

this mirror is not present because compactness is no longer possible with a single table

62

Appendix A. LII Operations Manual 63

Nd:YAG Laser (1064 nm)

PMT

PMT

250 mm

Relay

lens pairs

Half wave plate Thin-film polarizer

3 mm x 50 µm Vertical slits

Burner

692 nm

Band-pass

filter

440 nm Band-


150 mm

Collimating lens

150 mm

Achromats

Electromagnetic-

shielding box

Figure A.1: Schematic illustrating the optical layout of the available two-colour LII

apparatus setup for atmospheric measurements. Oscilloscope, power supplies, cables,

and gas delivery system are not shown.

frame; instead, the apparatus resides on two separate breadboard table frames that flank

the high pressure chamber at a ninety degree angle.

Another consequence of the high pressure chamber’s size is that the 250 mm relay

lenses used for compact atmospheric measurements must be changed to a higher focal

length. Because the chamber and its surrounding movement stage create, approximately,

a 35 cm footprint, relay lenses with 400 mm focal lengths are used. These function in

the same manner as the 250 mm lenses but have longer reach, enabling them to focus on

the same point inside a flame within the chamber.


Nd:YAG

Laser

(1064 nm)

PMT

PMT

400 mm

Relay

lens pairs

3 mm x 100 µm Vertical slit

High pressure

combustion

chamber

692 nm

Band-pass

filter

440 nm Band-


150 mm

Collimating lens

150 mm

Achromats

Electromagnetic-shielding box

Half wave plate

Thin-film polarizer

100 µm

Aperture

Burner

Quartz

viewports

Figure A.2: Schematic illustrating the optical layout of the available two-colour LII

apparatus setup for high pressure measurements. Oscilloscope, power supplies, cables,

and gas delivery system are not shown.

The bulk of measurements at high pressures are expected to reveal high soot volume

fractions relative to most measurements taken at atmospheric pressure. Because of this,


the half wave plate and thin film polarizer will need to transmit more light than in the

atmospheric case. These two components function together to enable the reduction of

the laser beam’s fluence as a function of the half wave plate’s angle of rotation:

Φ

Φ=

1 + sin 4ψ

2(A.1)

where Φ/Φ is the fraction of the maximum beam fluence Φ that is transmitted as a

function of the half wave plate rotation angle ψ in radians. Note that the surface of the

thin film polarizer should make a 56 angle, roughly Brewster’s angle for glass in air,

from the incident beam.

As long as the excited soot particles are not high enough temperature to sublimate,

measurements are independent of the laser beam’s fluence. Under its default configura-

tion, the current laser’s maximum fluence is roughly 550 mJ/cm2 at 1064 nm wavelength

and sublimation begins to occur at roughly 300 mJ/cm2 at this wavelength.

A.3 Basic Operation

This section will outline the basic operation of each of the minor apparatuses asso-

ciated with the LII apparatus. This includes the laser and the photomultiplier tubes’

(PMT) power supply.

A.3.1 The Laser

The laser should at all times be connected to a 240 V, 15 A power source. As such,

the lasers’ main breaker switch should be kept closed so that the system has continuous

power.

Instructions on operating the laser can be found in the manufacturer’s operation and

maintenance manual [77, p. 25].


A.3.2 PMT Power Supply

In addition to ± 15 V supply, the PMTs require a gain voltage between 0 and 5 V.

The cables leading from the PMT circuit boards to the power supply contain several

wires with coloured insulation. The wires connect to the power supply via colour-coded

banana plugs. The wires’ roles and placement on the power supply are as in Table A.1.

Table A.1: Power supply channel configuration for each of the PMTs’ input wires.

Insulation Colour Plug Colour Role Power Supply Channel

White White +15 V Channels 1 Pos and 4 Pos

Red Red -15 V Channel 1 Neg and 4 Neg

Black Black Gain Both to Channel 2 Pos

Black Brown Ground Both to Ground

Bare Black Shielding Both to Ground

Furthermore, a cable should run from the oscilloscope’s ground to the power supply’s

ground.

A.4 Alignment

Alignment can be achieved by any of several methods. By even the simplest method,

proper and complete alignment of the apparatus can sometimes take an entire day or

more to accomplish.

A.4.1 Independent Excitation and Detection Alignment

Independent of setup type, the first stage of the alignment is to ensure that each

component on each optical breadboard is aligned with every other component on the

table.


Before adjusting any of the optics, ensure that the breadboards are completely level.

This is important since even a slight angle will most likely cause issues later when trying

to align the optics with a flame at some remote burner.

Excitation Optics

The excitation optics should be aligned first. The first thing to ensure is that the

laser beam is parallel with respect to the optical breadboard. It is not important at this

stage whether the breadboard (and therefore the laser) is level; this will be done once

the tables are in their final positions for taking measurements. Checking if the laser is

parallel can be done by straightforward visualization. If the laser beam is just a few

degrees away from being parallel with the table, further adjustment is unnecessary as a

small angle alone will not impact the accuracy of measurements.

Once the laser head and table are parallel, the remainder of the detection optics must

be placed in the path of the laser beam. The procedure for this is largely trial and error;

because the laser beam is invisible and powerful enough to damage components, the

process is slow and great care must be taken. The position of the laser beam at a given

distance from the laser head can be determined by using optical ablation paper designed

for this purpose. High energy photons will visibly ablate the surface of the paper’s black

coating, creating a beige coloured pattern indicative of the spatial profile of the beam.

When performing this test, single laser shots should always be used.

When testing the location of the laser beam with ablation paper, the paper should

be kept in a sealed, transparent bag. This is to prevent any hot ablation material ejected

from the paper’s surface from contaminating any nearby optics. It should be noted that

ablation causes a distinct odour; this is normal.

Because the laser beam is capable of damaging equipment and people, the beam’s

approximate location should always be known before firing the laser. This means that

ablation testing should begin near the laser head to gain initial knowledge of the beam’s


location. Then the paper can be gradually moved away from the beam’s last known

location every shot.

Once the location of the laser beam is roughly at the same location as an optical

component, greater care should be taken to ensure greater accuracy in the measurement

of the beam’s location. For example, if the beam is known to be just in front of the

half wave plate, make sure with several successive trials that the beam will completely

pass through the crystal and not hit the crystal’s frame. This can be a slow process for

some components, notably the half wave plate; if it is very near the laser head, then it

is difficult to determine if the beam is truly aligned with the crystal. Once sure that the

beam will pass safely beyond the component, place the ablation paper just on the other

side of the optic and fire the laser. It is easy to see if the beam hit the component since

the surface will glow white and become ablated; this should be avoided.

While aligning optical components in the path of the laser beam, ensure that reflective

materials are angled a few degrees from perpendicular to the laser beam. This is to avoid

back-reflection of the beam into the laser cavity.

When positioning components, bear in mind the focal length of the lenses being used.

For perfect focus, the relay lenses must be positioned such that the distance between

the aperture and lens as well as between the lens and the probing position within the

flame are each equal to the focal length of the lenses. For high pressure that means the

aperture will be 40.0 cm away from the first relay lens. Also, the second relay lens must

be 40.0 cm away from the probing location. To ensure there is enough room on the table,

it is easiest to position (but not align) elements closest to the flame first, working toward

the laser head.

After components are positioned on the table, those closest to the laser should be

aligned first. For example the half wave plate and thin film polarizer, if present, should

be set first. Care must be taken with components that split the beam. The thin film

polarizer reflects s-polarized light and transmits p-polarized light. There are now two


beams to track the location of simultaneously; the reflected beam should be directed to

a beam dump so that it is no longer an issue.

Detection Optics

While aligning the detection optics, there is no guiding laser beam as there is for

the detection optics. For a high intensity light source, an integrating sphere is used.

Conveniently, calibration of the detection optics can be done with the same integrating

sphere immediately following optical alignment. This should be taken advantage of since

whenever the layout of the detection optics changes, calibration for that new configuration

must be performed.

Because the integrating sphere continuously emits high intensity visible light, it is

simple to determine the path of incident light. Similar to tracing the laser beam on

the excitation side of the apparatus with ablation paper, white paper can be used to

continuously trace the location of incident light on the detection side. Again, components

nearest the flame should be positioned first. The inclusion of three more lenses means

that distances between many components is fixed. In Figures A.1 and A.2, triangular

beam traces represent lens focusing and therefore these are fixed distances given by the

corresponding focal length. Rectangular beams in these figures represent collimated light

and therefore those distances are not fixed.

A.4.2 Flame and Laser Alignment

The procedure for positioning the image of the laser beam in the flame changes

between atmospheric and high pressure setups. With the atmospheric setup, the burner

is freely accessible and in fact alignment can be done without a flame present. With the

high pressure setup, however, the burner is surrounded by the high pressure chamber and

therefore completely inaccessible. Although the alignment can be done without a flame,

the procedure is complicated and timely. Alignment can instead be achieved relatively


quickly with a flame present.

Atmospheric Case

Aligning the laser with the flame can be done with or without a flame. While it is

ideal to have no flame present for vertically positioning the image of the laser, it is not

as advantageous for horizontal positioning to not have a flame.

To find the vertical position of the laser image, burn paper is used. A strip of paper

can be put in place of the flame and shot with the laser to reveal the image’s position and

focus. The optical components and the burner may have to be adjusted several times

to achieve proper beam focus at the flame’s position. Focus should be achieved along

the vertical axis in the centre of the flame. Once focused, the image’s vertical position

with respect to the burner exit should be evident. The burner can then be moved to the

height above burner (HAB) desired for measurements.

High Pressure Case

Positioning the image of the laser beam in the high pressure flame is not straight-

forward because the high pressure chamber prevents regular access to the burner exit.

Despite this, the same technique used for the atmospheric case can be used. The image’s

position on the burn paper can be determined using a camera that is roughly parallel

to the incident beam. This technique relies on opening and closing the chamber several

times to obtain proper alignment.

A quicker but less accurate method relies on a sooting flame present and roughly

aligned detection optics. The top of a flame can be determined by finding the highest

detectable LII signal and therefore the vertical position can be roughly known for a flame

of known height. The height of the flame can be determined using a high resolution

camera with the width of the burner exit known. This technique is not recommended

for measuring a specific height in the flame, but is adequate if the entire flame is being


profiled. Perfect focus is not ensured by this technique, although the larger depth of field

granted by the high-focal length lenses mitigates this concern.

A.4.3 Flame and Detector Alignment

The detection optics have to be aligned to both the flame and the laser beam’s image

within the flame. Although the former can be done without the laser, the latter obviously

relies on the flame already being aligned with the laser. The procedure for this is the

same for atmospheric and high pressure setups.

Alignment with the flame is straightforward for a highly-sooting flame. The flame

can be used as a guiding light source similar to how an integrating sphere is used to align

the optics. The flame’s image should be visible on the aperture, making crude alignment

simple. Focus should be ensured by having the detection optics’ table connected to the

excitation optics’ table so that the distance between lenses is finely controlled. Focus

can be judged by looking at the image hitting the PMTs using white paper. Fine tuning

can be done by moving the burner stage through the axis normal to the imaging plane

of the detection optics. This focus is much more crucial to measurement accuracy than

is the laser beam’s image’s focus. The position of the detection image along the axis of

the laser beam is irrelevant as long as it is within the flame.

Once the flame is aligned and focused with respect to the detectors, the detection and

excitation images must be superimposed within the flame; this is the ultimate alignment

procedure. Once both the excitation and detection images are focused to the center of

the flame, alignment in the horizontal planes is complete. All that remains is the vertical

alignment of the images.

If the laser and detectors reside on the same table, vertical offset will be minimal

and can be adjusted by moving the apertures up or down. If this is not the case, then

the vertical offset can be crudely adjusted by adjusting the height of the tables. When

the offset is on the order of a few millimeters, the apertures can be moved up or down


to finely tune vertical alignment. Measuring the maximum magnitude of an LII signal

with a stable sooting flame while adjusting the apertures’ height can help to determine

alignment. It is impossible to ensure that the apertures are perfectly vertically aligned;

however, this is not a crucial issue since analysis does not require knowledge of the

detection volume height. This is also the time to ensure that the tables and burner stage

are all completely level and resting on solid feet (and not wheels).

A.5 Calibration

The detectors are the only part of the apparatus that require calibration. Calibration

will yield a constant parameter associated with each detector that will be used to calculate

soot volume fraction and particle temperature (and by extension, particle size). Although

the procedure for calibrating the detectors is identical for both atmospheric and high

pressure setups, the calibration parameter will change between configurations. In fact,

calibration must be performed any time any of the detection-side optical components are

altered or moved in the slightest.

One of the advantages to this calibration technique [48] is that it accounts for many

defects that could otherwise be hindering the detection optics. For example, it does not

matter if one of the detectors is more sensitive than the other or if the image of the flame

is not in perfect focus. As long as a defect is consistent across all measurements and does

not show bias to a particular portion of the detection volume, the calibration will often

negate the problem.

A.5.1 Components and Setup

In addition to the base apparatus, calibration requires two primary components: an

incandescent light source of uniform and high intensity, and a spectrometer to quantify

the light source’s emissions. A halogen lamp within an integrating sphere is used with a


power supply for use as a reliable light source.

Integrating Sphere

The integrating sphere will output a predictable and steady spectrum of light. The

spectrum is not truly blackbody but is meant to simulate an LII signal. The light is sup-

posed to simulate the voltage responses seen by the PMTs at two particular wavelengths.

Ideally, the integrating sphere’s output port should be in the same position as the

flame to be measured. Actual placement of the integrating sphere depends on the burner

being used.

In atmospheric setup, the sphere can be placed on top of the burner stage and posi-

tioned such that the port is in the same plane as LII measurements.

The integrating sphere is housed in a large, unwieldy cube. This makes placing

it inside the high pressure combustion chamber impossible. There are alternatives to

placing the sphere in the same position as the burner exit, though not all methods are

practical.

The simplest method is to simply calibrate before positioning the detection optics

for LII measurements. The sphere can then be placed in front of the detection optics

at a distance equal to that during measurements. Although this simulates the proper

distance, the chamber is absent. Most importantly, the quartz viewport is missing. A

spare viewport needs to be substituted and placed in front of the detection optics in the

line of sight of the integrating sphere. Preferably, the spare window will be placed in the

same relative position as would be the case during LII measurements.

Other alternatives to placing the sphere inside the chamber require relay optics much

like those used in the base LII apparatus. This method is more complicated and in-

troduces additional optical components not typically present; this adds to error. The

benefit is that calibration can be done when the detection optics are in position for LII

measurements. The first, simpler method should usually be chosen.


Spectrometer

The spectrometer collects and measures light directly from the integrating sphere dur-

ing calibration. Measurements are sent to a computer, averaged over time, and plotted.

A fibre optic cable should be connected to both the integrating sphere and spectrom-

eter to collect light, and the spectrometer should be plugged into a computer with USB

port and proprietary SphereOptics SMS-500 spectrometer software installed.

Power Supply

The power supply provides a constant 25 V 4.166 A supply to the integrating sphere

lamp. It should be properly grounded and connected to the integrating sphere via the

sphere’s two gator clips.

A.5.2 Experimental Procedure

The calibration procedure is straightforward and requires a small set of measurements.

Two distinct data sets are acquired: the integrating sphere output spectrum, and the

PMT voltage response curve.

Warmup and Cooldown

Quickly switching the integrating sphere lamp on at full power will stress the filament.

Instead, the power supply should be slowly dialed up from zero to the required amperage

(roughly 4.166 A) until the potential sits at 25 V. This should be done over a period

of approximately one or two minutes. This process should be repeated in reverse when

powering down the lamp after calibration.

Obtaining Spectrum

Before the integrating sphere spectrum is obtained, the spectrometer must be cal-

ibrated. This is a simple procedure done by the spectrometer’s software. To do this,


close the spectrometer input cap so that it sees no light. With the software, select

Tools → System Zero to calibrate dark current. This takes several minutes. Once fin-

ished, the fibre optic cable leading to the integrating sphere can be reconnected to the

spectrometer.

To configure the number of spectrum samples, select Setup → Setup Spectral

Parameters. The settings found in Table A.2 are sufficient.

Table A.2: Spectrometer parameters used during calibration.

Setting Value

Integration Time 2 ms

Samples 5 - 15

Boxcar Smoothing 2

Auto Range On

Wavelength 335 - 1100 nm

To preview a spectrum scan, select Start Auto Scan, wait a few moments, and then

select Stop Auto Scan. A spectrum should be shown. To obtain the full spectrum

measurement, select Acquire Data. To save the data set to a text file (in the software’s

installation folder), select Save New Data.

Collecting PMT Data

Collecting PMT voltage responses can be done in parallel with obtaining the spectrum

using the spectrometer.

As with the spectrometer, the PMT dark current should be measured. This must

be done individually for each data point. Dark current can be obtained by completely

blocking the entrance to the detection optics’ shielding box.

In addition to the spectral data obtained from the spectrometer, each data point must

include the gain voltage for each PMT, the dark current voltage response for each PMT


at the pertinent gain voltage, and the voltage response for each PMT.

The gain voltage should initially be set low, near 2.3 V as measured by the oscilloscope

and increased by roughly 0.1 V for each new data point. At each gain voltage, the cor-

responding voltage response and dark current voltage for each PMT should be recorded.

Collecting points up to between 2.7 and 3.0 V gain is sufficient, yielding between 5 and

10 data points.

A.5.3 Using the Data

The calibration factor at any given gain voltage is given by

η =VCAL

RSGCAL

(A.2)

where RS is the spectral radiance of the light source and VCAL and GCAL are the associated

PMT signal and gain voltages, respectively.

The PMT response voltage VCAL is calculated by subtracting the dark current voltage

at GCAL by the measured response voltage.

The spectral radiance is listed in the table output by the spectrometer software. There

is a unique calibration factor for each PMT; each corresponds to a single colour. When

reading the spectral radiance from the output file, the correct value is not given by a

single number but an average across the range of wavelengths. This is because the filters

let through light as a function of wavelength across a narrow spectrum. For example, the

440 nm filters used in the apparatus have a true centre wavelength of 443 nm based on the

transmission spectrum supplied by the manufacturer and transmit significant amounts of

light between 415 and 464 nm. The net transmission spectrum should be multiplied by

the corresponding integrating sphere spectral radiances; the resulting spectral radiance

should be averaged over the sum of wavelength transmittances to obtain a single value


for that colour.

The averaged response voltages can then be plotted against the corresponding gain

voltages. The slope of this plot should be linear and can be used to interpolate the

response voltage and appropriate calibration factor for any given gain voltage. This plot-

ting and interpolation is done automatically by the LII analysis software which requires

only a list of the gain voltages, response voltages, and spectral radiances.

A.6 Measurement

Measuring an LII signal consists of shooting the desired target with the laser while

looking for a resulting peak in light detection from the oscilloscope. Correctly setup, the

oscilloscope will identify the LII signal automatically.

A.6.1 Measurement Position

Measurements are typically along the flame axis or perpendicular to it to obtain

centreline or profile measurements, respectively. In the former case, the initial position

is typically at the base of the flame. For profile measurements, the initial position can

be on either edge of the flame at the desired HAB.

Profile measurements should be in the same plane as the excitation laser sheet. This

means moving the burner stage in the axis of the laser beam between measurement

positions. This is to sacrifice laser attenuation for a lower average signal attenuation.

Step sizes for measurement positions are typically equal to the height or width of the

detection optics’ aperture. Step size can be smaller than the aperture if the aperture is

large, yielding a rolling average.

A.6.2 Taking Measurements

Before detecting a signal, the oscilloscope must be correctly setup.


Channel Vertical Adjust

The Channel Vertical Adjust window contains the setting options for signal inter-

pretation. Appropriate settings for measurements are as in Table A.3. Averaging should

typically be set to 1 unless taking final measurements.

Table A.3: Oscilloscope channel settings used for LII measurments.

Setting Value

Volts/div Adjust to fit signal

Variable gain No

Offset Adjust to fit signal

Bandwidth Full

Invert No

Coupling DC50Ω

Deskew 0

Probe Attenuation ÷1

Averaging 100 to 400 sweeps

Interpolation Linear

Noise Filter None

Timebase

Suitable time resolution (Time/Division) for LII signals is 100 ns with a delay of

approximately -350 ns to fit the whole usable signal while leaving lead time for baseline

averaging that is done by the analysis software for each measurement.


Trigger

Detecting the LII signal is done automatically by the oscilloscope if the trigger is setup

correctly. Although the laser can be connected to the oscilloscope to trigger measurements

upon firing, this leads to unreliable signal lag.

Suitable trigger settings are as in Table A.4. Source should be the larger of the two

channels, which is typically C1 (440 nm). These settings will trigger measurement at the

onset of the initial LII signal peak, although correct setup of the trigger often involves

fine tuning between laser shots.

Table A.4: Oscilloscope trigger settings used for LII measurements.

Setting Value

Type Edge

Source C1

Level Half of peak signal

Slope Positive

Coupling DC

With the preceding setup and triggering set to normal, the oscilloscope will detect

any present LII signal upon a single laser shot. If a signal is found, the laser should be set

to automatically fire at 10 Hz until the oscilloscope collects enough sweeps for averaging.

A.6.3 Saving Data

After the oscilloscope has averaged a suitable number of sweeps, the signals may be

saved (File → Save Waveform...) with the settings found in Table A.5 by selecting

Save Now. Two files must be saved manually for each measurement: one for Source set

to C1 and another for C2. The file, position, and directory formats are dictated by the

analysis software.


Table A.5: Oscilloscope file settings used for LII measurements.

Setting Value or Format

Save To File

Source C1 or C2

Trace Title PosNa

Format Excel

SubFormat Time & Amplitude

Auto Save Off

Save files in directory . . . /MonDDYYYY/Raw

a Pos1, Pos2, . . . , PosN for N measurement posi-

tions

A.7 Data Analysis

Data analysis is performed automatically by the analysis software. The software will

output time-resolved temperature, soot volume fraction, and particulate size spreadsheets

for each measurement position as well as position-resolved time-averaged spreadsheets for

plotting.

The software, implemented with Python, features a user-friendly interface and is

independently documented with instructions on use. The software takes a single comma-

separated value (CSV) file for each channel at each measurement position as input.

Experiment-specific parameters are requested upon the first attempt to analyze a data

set. These parameters are saved in a CSV file in the data set’s folder for future calcula-

tions. Physical constants that do not change from experiment to experiment are saved in

the program script itself, and may be easily edited. Calculations for temperature, soot

volume fraction, and particle size are done automatically in series. Temperature, soot

volume fraction, and particle size are calculated as functions of time for each location and


output as separate CSV files for each location. A single number for each soot volume

fraction and particle size is then calculated for each measurement position; these are

collated and saved in a soot volume fraction CSV file and particle size CSV file. More

detailed information about each of the numerous functions in the software can be found

in the software’s inline documentation.

A.8 Maintenance

Maintenance requirements do not vary between atmospheric and high pressure setups.

The apparatus in either case is largely maintenance free; the primary exception is the laser

itself. In addition to laser maintenance, the optical components must be inspected and

cleaned before each experiment. Maintenance of burners and the high pressure chamber

are beyond the scope of this manual.

The laser maintenance is straightforward and covered in detail within its user manual.

The most frequent maintenance is ensuring that the laser’s breaker switch is always closed

so that there is power to the system. Other regular maintenance includes checking the

water level every few months and replacing the flash lamp as required. Refer to the

laser’s manual for detailed procedures and maintenance schedules [77, p. 63].

Before each experiment, each optical component should be inspected and cleaned if

any dust is present. This is especially important for components coming in contact with

the laser beam because damage can occur if dust particles are heated on the surface of a

piece of glass by the laser. Cleaning is done by wetting a new lens cleaning tissue with

isopropyl alcohol and gently dragging the tissue along the surface of the glass. To avoid

scratching the surface, do not re-use the tissue and do not apply pressure while cleaning.

To avoid leaving residue or damaging optical coatings, only isopropyl alcohol should be

used.

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