Effects of n-Propylbenzene Addition on Soot Formation in an n-Dodecane Laminar Coflow Diffusion Flame
by
Liyun Zhao
A thesis submitted in conformity with the requirements for the degree of Masters of Applied Science
Graduate Department of Mechanical and Industrial Engineering University of Toronto
© Copyright by Liyun Zhao 2016
ii
Abstract
Effects of n-Propylbenzene Addition on Soot Formation in an n-
Dodecane Laminar Coflow Diffusion Flame
Liyun Zhao
Masters of Applied Science
Graduate Department of Mechanical and Industrial Engineering
University of Toronto
2016
The first part of this thesis addresses the validation of combined laser extinction and two-angle
elastic laser scattering diagnostics for soot characterization. The results from three measurement
heights (30, 40, 50 mm) of a non-smoking ethylene-air laminar coflow diffusion flame were found
to agree well with those from the literatures.
The second part of this thesis applies the optical diagnostics mentioned above to investigate the
effects of n-propylbenzene addition on soot formation in an n-dodecane laminar coflow diffusion
flame. All of the tested flames had similar temperature profiles. Soot volume fraction was found
to increase at all flame heights as the mole fraction of n-propylbenzene increases. Along the
centerline, the increase of the soot formation was mainly caused by the combined effect of higher
soot inception rate and surface growth rate, while along the wing, the higher soot formation was
mainly because of the higher surface growth rate.
iii
Acknowledgments
Firstly, I would like to express my sincere thanks to my supervisor Professor Murray J. Thomson
for his constant supports and motivation. His guidance and patience helped me in all the time of
my studies and research.
Besides my supervisor, I would like to thank my laboratory colleagues for the stimulating
discussions and the warm research environment. Special thanks to Tongfeng Zhang, who
collaborated with me to make experimental measurements, gave me insightful comments and
encouragement. Also, I would like to thank Jason Weingarten, Anton Sediako who helped me with
the temperature measurements.
Many thanks to the staff in the Machine shop and Purchasing Office of Mechanical & Industrial
Engineering at University of Toronto. Their help was important in the development of the research
facilities for my thesis.
I am also grateful to my great family for supporting me spiritually throughout my last two years.
The accomplishment could not be possible without the people mentioned above.
iv
Table of Contents
Abstract .......................................................................................................................................... ii
Acknowledgments ........................................................................................................................ iii
Table of Contents ......................................................................................................................... iv
List of Tables ............................................................................................................................... vii
List of Figures ............................................................................................................................. viii
Chapter 1 ....................................................................................................................................... 1
Introduction ................................................................................................................................... 1
1.1 Motivation ........................................................................................................................ 1
1.2 Objectives ......................................................................................................................... 4
Chapter 2 ....................................................................................................................................... 5
Literature Review ......................................................................................................................... 5
2.1 Soot Evolution Mechanism .............................................................................................. 5
2.1.1 Soot Formation .......................................................................................................... 5
2.1.1.1 Fuel Pyrolysis .................................................................................................... 5
2.1.1.2 Polycyclic Aromatic Hydrocarbon (PAH) Formation ....................................... 5
2.1.1.3 PAH Growth ...................................................................................................... 6
2.1.1.4 Particle Inception ............................................................................................... 7
2.1.2 Soot Growth .............................................................................................................. 7
2.1.3 Soot Coagulation and Agglomeration ....................................................................... 8
2.1.4 Soot Oxidation .......................................................................................................... 9
2.2 The Combustion of Aromatics ......................................................................................... 9
2.3 Jet Fuels and Surrogates ................................................................................................. 14
2.3.1 Jet Fuels .................................................................................................................. 14
2.3.2 Surrogates Formulation ........................................................................................... 16
2.3.3 Surrogates for Real Fuels ........................................................................................ 18
Chapter 3 ..................................................................................................................................... 20
Experimental Methodology ........................................................................................................ 20
3.1 Coflow Burner ................................................................................................................ 20
v
3.2 Fuel and Oxidizer Delivery System ............................................................................... 21
3.2.1 Oxidizer Delivery System ....................................................................................... 21
3.2.2 Gaseous Fuel Delivery System ............................................................................... 22
3.2.3 Liquid Fuel Delivery System .................................................................................. 22
3.3 Soot Optical Diagnostics ................................................................................................ 24
3.3.1 Laser Extinction Technique .................................................................................... 25
3.3.2 Elastic Laser Scattering Technique ......................................................................... 26
3.4 Optical Configuration ..................................................................................................... 29
3.5 Detection and Data Acquisition Equipment ................................................................... 30
3.6 Constants ........................................................................................................................ 31
3.6.1 Refractive Index ...................................................................................................... 32
3.6.2 Fractal Dimension and Fractal Prefactor ................................................................ 34
3.7 Temperature Measurements ........................................................................................... 37
3.8 Optics Alignment ........................................................................................................... 39
3.9 Scattering Calibration ..................................................................................................... 40
3.10 Test Conditions .............................................................................................................. 41
3.11 Uncertainty Analysis ...................................................................................................... 41
Chapter 4 ..................................................................................................................................... 44
Results and Discussion ................................................................................................................ 44
4.1 Validation of Experimental Apparatus ........................................................................... 44
4.1.1 Validation of Laser Extinction Apparatus .............................................................. 44
4.1.2 Validation of Two-angle Elastic Laser Scattering Apparatus ................................. 46
4.2 Investigation of the Effects of n-Propylbenzene Addition on Soot Formation in an n-
Dodecane Laminar Coflow Diffusion Flame ................................................................. 49
4.2.1 Flame Descriptions ................................................................................................. 50
4.2.2 Soot Volume Fraction Profiles ................................................................................ 50
4.2.3 Primary Particle Diameter and Number Density Profiles ....................................... 53
4.2.4 Temperature Profiles ............................................................................................... 56
4.2.4.1 Comparison Among Different Liquid Fuel Mixtures ...................................... 56
vi
4.2.4.2 Comparison Between Different Techniques .................................................... 58
Chapter 5 ..................................................................................................................................... 60
Conclusions and Recommendations .......................................................................................... 60
5.1 Conclusions .................................................................................................................... 60
5.2 Recommendations .......................................................................................................... 61
Attributions ................................................................................................................................. 62
Bibliography ................................................................................................................................ 63
Appendices ................................................................................................................................... 78
Appendix A MATLAB Code .................................................................................................... 78
A.1 MATLAB Code for Soot Volume Fraction .................................................................... 78
A.2 MATLAB Code for Temperature ................................................................................... 81
Appendix B Procedure of Calculating Soot Properties ............................................................. 85
Appendix C Optics Alignment .................................................................................................. 86
Appendix D Soot Volume Fraction Profiles ............................................................................. 89
D.1 Pure n-Dodecane ............................................................................................................. 89
D.2 Pure n-Dodecane Doped with 15 mol. % n-Propylbenzene ........................................... 90
D.3 Pure n-Dodecane Doped with 30 mol. % n-Propylbenzene ........................................... 91
D.4 Pure n-Dodecane Doped with 45 mol. % n-Propylbenzene ........................................... 92
Appendix E Temperature Profiles ............................................................................................. 93
E.1 Pure n-Dodecane ............................................................................................................. 93
E.2 Pure n-Dodecane Doped with 15 mol. % n-Propylbenzene ............................................ 94
E.3 Pure n-Dodecane Doped with 30 mol. % n-Propylbenzene ............................................ 95
E.4 Pure n-Dodecane Doped with 45 mol. % n-Propylbenzene ............................................ 96
vii
List of Tables
Table 2. 1: Properties of common jet fuels. .................................................................................. 15
Table 2. 2: Proposed components for jet fuels, formulas and molecular structures. .................... 18
Table 3. 1: Gas pressures used in the measurements. ................................................................... 24
Table 3. 2: Complex refractive index of soot used or determined by various researchers. .......... 34
Table 3. 3: Fractal dimension and fractal prefactor of soot used or determined by various
researchers. .................................................................................................................. 36
Table 3. 4: Experimental test conditions. ...................................................................................... 41
Table 3. 5: Values and errors of each component of soot volume fraction. ................................. 43
Table 3. 6: Sources and values of errors for temperature measurements. .................................... 43
Table 4. 1: Ratios of dp3 from HAB = 50 mm to HAB = 70 mm on the flame centerline. .......... 55
Table 4. 2: Ratios of dp3 from HAB = 30 mm to HAB = 50 mm on the flame wing. .................. 55
Table 4. 3: Ratios of Np from HAB = 50 mm to HAB = 70 mm on the flame centerline. ........... 55
Table 4. 4: Ratios of Np from HAB = 30 mm to HAB = 50 mm on the flame wing. ................... 55
viii
List of Figures
Figure 2. 1: HACA mechanisms of PAH growth. .......................................................................... 6
Figure 2. 2: Schematic picture displaying three possible consumption pathways of benzene in
flames. Pathways (1) and (2) can occur with or without oxygen and pathway (3) can
only occur with oxygen. BZ: benzene, C5H5: cyclopentadienyl radical, C6H5: phenyl
radical, C6H5O: phenoxy radical, C8H6: phenylacetylene, C10H8: naphthalene. Arrow
do not represent elementary reactions. ....................................................................... 10
Figure 2. 3: Oxidation pathways of n-propylbenzene in a JSR at 1 atm (1200 K). ...................... 14
Figure 2. 4: Compositions of a sample jet-A fuel. ........................................................................ 15
Figure 2. 5: C-C bond and C-H bond dissociation energies (kcal/mol) of the n-propylbenzene
side chain. The red bold italicized numbers represent the C-C bond energies. Plain
numbers denote C-H bond energies. .......................................................................... 19
Figure 3. 1: Air supply apparatus. ................................................................................................. 21
Figure 3. 2: Schematic of the vaporizer system and coflow diffusion burner used in the current
study. .......................................................................................................................... 24
Figure 3. 3: Layout of combined laser extinction and two-angle elastic laser scattering apparatus.
.................................................................................................................................... 30
Figure 3. 4: Experimental apparatus of the rapid thermocouple insertion method. ...................... 37
Figure 3. 5: Thermocouple readings at soot free regions and soot containing regions inside
flames. ........................................................................................................................ 38
Figure 4. 1: Transmittance profiles at different heights above burner (HAB) compared with those
from Santoro et al. ...................................................................................................... 45
Figure 4. 2: Soot volume fraction values of different heights above burner (HAB) at the
centerline of ethylene-air diffusion flame compared with those from Santoro et al. . 46
Figure 4. 3: Scattering cross section (30° and 150°) profiles at different heights above burner
(HAB) compared with those from Santoro et al. ....................................................... 47
Figure 4. 4: Values of primary particle diameter, primary particle number density, aggregate
number density, average number of primary particles per aggregate of different
heights above burner (HAB) at the centerline of ethylene-air diffusion flame
compared with those from literatures. ........................................................................ 49
Figure 4. 5: Visible flame images for the four levels of n-propylbenzene addition. .................... 50
ix
Figure 4. 6: Soot volume fraction profiles at different flame heights (HAB) of the four flames
studied. ....................................................................................................................... 51
Figure 4. 7: Soot volume fraction profiles along the centerline and the locations of peak soot
concentration of the four flames studied. ................................................................... 52
Figure 4. 8: Primary particle diameter and number density profiles along the centerline and the
locations of peak soot concentration of the four flames studied. ............................... 54
Figure 4. 9: Temperature profiles at different flame heights of the four flames studied. ............. 57
Figure 4. 10: Comparisons of temperature profiles of pure n-dodecane laminar coflow diffusion
flame at HAB = 50 mm, 60 mm obtained by rapid thermocouple insertion
technique and by soot spectral emission (SSE) technique. ..................................... 58
Figure 4. 11: Thermocouple readings at the centerline position and peak value position for HAB
= 50 mm of pure n-dodecane laminar coflow diffusion flame. ............................... 59
Figure B. 1: Procedure of calculating soot properties for combined laser extinction and two angle
elastic laser scattering experiments. ......................................................................... 85
Figure C. 1: Schematic of optics alignment for two-angle elastic laser scattering part. ............... 88
Figure D. 1: Soot volume fraction profiles with error bars for pure n-dodecane. ........................ 89
Figure D. 2: Soot volume fraction profiles with error bars for pure n-dodecane doped with 15
mol. % n-propylbenzene. ......................................................................................... 90
Figure D. 3: Soot volume fraction profiles with error bars for pure n-dodecane doped with 30
mol. % n-propylbenzene. ......................................................................................... 91
Figure D. 4: Soot volume fraction profiles with error bars for pure n-dodecane doped with 45
mol. % n-propylbenzene. ......................................................................................... 92
Figure E. 1: Temperature profiles with error bars for pure n-dodecane. ...................................... 93
Figure E. 2: Temperature profiles with error bars for pure n-dodecane doped with 15 mol. % n-
propylbenzene. ......................................................................................................... 94
Figure E. 3: Temperature profiles with error bars for pure n-dodecane doped with 30 mol. % n-
propylbenzene. ......................................................................................................... 95
Figure E. 4: Temperature profiles with error bars for pure n-dodecane doped with 45 mol. % n-
propylbenzene. ......................................................................................................... 96
1
Chapter 1
Introduction
1.1 Motivation
Soot refers to carbonaceous solid particulates which may contain varying quantities of oxygen and
hydrogen, and is formed by the combustion of hydrocarbon fuels under fuel rich conditions where
oxygen is not enough to completely convert the fuel into carbon dioxide (CO2) and water (H2O)
[1]. The presence of soot in flames can be observed by the characteristic yellow luminosity under
various operating conditions [2]. Soot primarily comes from furnaces, gas turbines, diesel engines,
and other combustion appliances that burn liquid fuels. The hydrocarbon fuels typically contain
alkanes, alkenes, cycloalkanes, and aromatics, whose carbon atoms vary from 5 to 20 [3].
It was found that the first stage of soot formation is characterized by the formation of particles
with diameters of 5-10 nm by the coagulation of polycyclic aromatic hydrocarbons (PAHs) [4].
Later stages include surface growth, coalescence, and coagulation resulting in the increase of
particle size. “Soot nuclei” whose structure has more condensed aromatic rings and more compact
shape is formed by simultaneously rearranging soot precursor particles [5]. Then surface reactions
and coagulation of these nuclei generate aggregates. From transmission electron microscopy (TEM)
observation, soot particles consist of approximately spherical, randomly arranged primary particles
with a certain degree of overlap [6], which depends on combustion conditions [7].
In practical combustion appliances, soot would reduce the device efficiency and influence the
maintenance of the device due to its deposition on the exhaust systems and the generation of dark
exhaust plumes [8], thus soot emission implies poor combustion conditions [9]. Besides, radiative
heat transfer from soot to engines walls contributes significantly to the total heat loss in diesel
engines and lower flame temperature influences NOx formation pathways [10]. In addition, soot
would increase the emission of CO because of the competition with CO for OH in flames [11].
It has been found that many PAHs are tumorigenic or mutagenic [12-17]. Soot emission often
associates with PAHs thus it is harmful to human health [9], and can cause lung cancer and
cardiopulmonary disease [18]. The death caused by the toxicity of fine particles per year can reach
up to 60,000 in the United States [19], which is much more than that caused by homicide or traffic
2
accidents (around 15,000 and 40,000 per year, respectively) [20]. The size and the number
concentration of particles affect health effects more than their mass concentration [21]. Fine
particles (PM2.5: diameter smaller than or equal to 2.5 μm) and ultra-fine particles (PM0.1:
diameter smaller than 0.1 μm) [22] are more toxic than larger particles and can penetrate more
deeply into human lungs, thus they have a greater risk to human health [23]. Many regulations
have been proposed to limit the number concentration of particle emission instead of mass
concentration of particle emission [21]. The United States Environmental Protection Agency has
set the upper limit of the concentration of fine particulates [24].
Besides the influence on practical combustion appliances and human health, soot also affects
atmospheric visibility as a major contributor to anthropogenic aerosols [25]. In addition, since
soot can absorb light, it increases the melting of polar ice by depositing on it. It has been studied
that soot may result in as much as 94% of Arctic warming [26] and the warming of atmosphere
caused by carbon black is 0.5-0.8 W/m2 [27, 28], while that caused by the most important
greenhouse gas (carbon dioxide) is 1.46 W/m2, and that caused by the second most important
greenhouse gas (CH4) is 0.48 W/m2 [ 29 ]. Furthermore, soot is one of the causes of the
photochemical smog formation due to its dispersion into atmosphere [30] and soot particles in the
upper atmosphere deplete the ozone layer significantly [31].
Several soot formation processes are still not well understood such as the detailed process about
the formation and growth of aromatic species, particle inception, surface growth, coagulation and
oxidation. Quantitative and qualitative understanding about these processes are important to design
the operating conditions and modify technology to reduce aerosols emission [21]. Besides, better
understanding of soot formation can assist in increasing the performance of combustion devices
[2] and reducing the air pollution. To accurately determine the properties of soot, reliable values
of soot refractive index, soot structure and soot dimensions are required [32, 33], which can be
obtained using laser diagnostics. With the application of elastic laser scattering technique, more
properties of soot can be investigated such as primary particle diameter and primary particle
number density.
Aromatics are toxic hydrocarbons that contain benzenoid ring structures [3]. Aromatics contribute
20-40% (30-35% average) in the commercial US diesel fuels [34]. Aromatics are mainly 1-ring
structures, for example, alkyl-benzenes (15%), with 5% substituted 2-ring structures. The
3
concentrations of 3-ring cyclo-paraffins and naphtha-aromatics are relatively small and are
probable to decrease in the future [35].
The formation of small aromatic hydrocarbon is one of the essential steps towards soot generation
[3]. If the fuel is non-aromatic, the aromatic ring will be produced by the precursors cyclization
[36]. The reaction of the first ring formation from small aliphatics is a rate-controlling step [37]
and is much more sensitive to the molecular structure of fuel than growth process [3]. For aromatic
fuel, it is the addition of additional benzenoid rings to the initial structures that forms larger
aromatics, not the decomposition reaction of the initial structures to non-aromatic structures and
the generation of new rings [3]. Small aromatics (≤3 benzenoid ring) are produced by adding the
first new benzenoid ring to the single-ring, and two-ring hydrocarbons which make up the bulk of
fuels, while large aromatics (>3 benzenoid ring) are produced by subsequent soot growth process
[3].
Although the formation of small aromatics only accounts for a small part in the whole soot
formation process, it strongly affects soot concentration in flames [3]. The molecular structures of
hydrocarbons significantly affect soot formation in flames [38]. Particularly, aromatic components
in hydrocarbon fuels greatly influence soot formation process in flames [38]. Variations in the rate
of benzene formation lead to the corresponding difference in the rate of soot formation [3]. Soot
generated in flames of aromatic hydrocarbon fuels is much more than that generated in flames of
non-aromatic hydrocarbon fuels [3]. Aromatic fuels soot heavily because the relatively slow step
of creating the initial ring is evited [3]. Fuel pyrolysis and the formation of one-ring to two-ring
aromatic structures are essential steps in soot formation. Each aromatic structure has several
generation pathways [3]. The detailed fuel pyrolysis and formation pathways of aromatics are not
understood completely [3].
Since turbulent fluctuations in turbulent flames make the study of aromatic formation much more
complicated, most of the research on aromatics formation is conducted in laminar flames [3].
Besides, the advantage of using coflow flames to study fuel decomposition and aromatic formation
is that these processes occur throughout the fuel-rich core of the flame, whose dimensions are
comparable to the flame height and tube-diameter or slot-width [3]. The spatial resolution
accessible with probe samples is always smaller than these dimensions, thus the spatial behaviour
of hydrocarbon fuels can be easily detected [3].
4
1.2 Objectives
The first objective of this work is to validate the experimental apparatus for combined laser
extinction (LE) and two-angle elastic laser scattering (ELS) diagnostics to study soot formation in
laminar coflow diffusion flames. The validation was performed by comparing the results of the
current apparatus with those of the literatures for a non-smoking ethylene-air laminar coflow
diffusion flame. The results include experimental raw data from laser extinction and laser
scattering measurements, as well as the information about soot properties, such as soot volume
fractions, primary particle diameters and primary particle number densities.
The second objective of this work is to understand the underlying mechanisms of the effects of n-
propylbenzene addition on soot formation in an n-dodecane laminar coflow diffusion flame. To
achieve this, methane was used as carrier gas, n-dodecane was selected to establish the base flame
environment and n-propylbenzene was mixed into n-dodecane with mole fractions from 0 mole. %
to 45 mol. %. The total inlet carbon flow rate was held constant for all of the flames. The combined
laser extinction and two-angle elastic laser scattering diagnostics was applied to obtain information
about soot volume fraction, primary particle diameter, and primary particle number density. Rapid
thermocouple insertion method was used to obtain the temperature profiles.
5
Chapter 2
Literature Review
2.1 Soot Evolution Mechanism
2.1.1 Soot Formation
2.1.1.1 Fuel Pyrolysis
Soot precursors are formed by fuel pyrolysis, which is controlled by a gas-phase chemical kinetic
mechanism. In this process, large hydrocarbon structures decompose into smaller components,
such as acetylene, polyacetylenes, unsaturated hydrocarbons and polycyclic aromatic
hydrocarbons [39] at high enough temperature. Temperature as well as molecule concentration
strongly affects the decomposition rate [39].
Fuel pyrolysis process generates initial precursors to form soot. Important precursors (gaseous)
may involve polyacetylenes [40], ionic species [41] (a carbon vapor formed from dehydrogenation
of initial hydrocarbon molecules), or polycyclic aromatic hydrocarbons (PAHs) [9]. Currently, the
majority of opinions support the conclusion that carbon nuclei is formed from PAHs [37].
2.1.1.2 Polycyclic Aromatic Hydrocarbon (PAH) Formation
PAH formation was debated among combustion researchers until Bittner and Howard [42] found
that unsaturated aliphatic species are produced from the thermal decomposition of the fuel and
then PAH is formed via reactions between acetylenic species and aromatics using a molecular
beam mass spectrometer system.
Soot formation and the chemistry of primary combustion zone are connected by the formation and
growth of aromatic species [37]. The formation of the initial aromatic ring is limited by rate [37]
and is slower than the growth process to larger aromatic ring structures [43]. Thus the rate of soot
formation is controlled by the formation of initial aromatic ring. Miller and Melius [44] proposed
that it is the reactions of two propargyl radicals that form the first ring.
C3H3 + C3H3 → benzene or phenyl + H (2. 1)
6
The assumption that aromatics are formed via propargyl has long been adopted [45, 46]. The
reaction of CH3 with C5H5 [47] and the recombination and rearrangement of two C5H5 radicals
[48] are other pathways suggested to form benzene:
C5H5 + CH3 → benzene + H + H (2. 2)
C5H5 + C5H5 → naphthalene + H + H (2. 3)
2.1.1.3 PAH Growth
For the fuels that already contain aromatics, active reactants to form aromatics can be formed in
relatively large concentrations during the fuel pyrolysis process. However, for aliphatic
hydrocarbon fuels, the first aromatic ring is produced by a series of reactions of products from the
fuel decomposition process and the active reactants to form ring structures are in relatively small
concentrations [18]. Benzyl-type radical is generated by the abstraction of a hydrogen atom from
the reacting aromatics. Cyclization is continued by adding acetylene. The increase of the
replacement rate and the number of reactive sites results in a catalytic process [ 49 ]. This
mechanism is called “H-abstraction-C2H2-addition” (HACA) mechanism, which is shown in
Figure 2. 1. This implies the addition of a gaseous acetylene molecule to the radical site follows
the abstraction of a hydrogen atom [37]. This is a two-step process. The first step involves
converting a molecule to a radical to activate the molecule for further growth. The primary feature
of this step is its reversibility. The reverse reaction can be the reverse direction of the H abstraction
itself, combination with a gaseous H or other reactions. The degree of the reversibility of acetylene
addition determines whether this step will lead to molecular growth [37].
Figure 2. 1: HACA mechanisms of PAH growth [50].
7
Kinetic models for the multi-ring molecules formation have been studied and the mechanisms of
three-dimensional networks needs more attention [49]. The local composition, burning conditions
and the temperature of the combustion system determine whether a group of soot precursors
become soot [49].
2.1.1.4 Particle Inception
Currently the particle inception process (Nucleation) is the most poorly understood part in the
whole soot formation process [37]. Some researchers [51, 52] proposed that the appearance of
nascent soot particles results from the continued growth of PAH groups involving physical and
chemical coalescence. PAH species at some size start to stick to each other via collisions producing
PAH dimers. Then these dimers continue colliding with PAH molecules generating PAH trimers
or with other PAH dimers generating PAH tetramers and so on. During the collision of PAH
species, the sizes of individual PAH species keep increasing through reactions of molecular
chemical growth. In this mechanism, clusters of PAH evolve into solid particles.
2.1.2 Soot Growth
Although soot particle inception is an important part in soot formation process, it only contributes
to 10% of the soot mass produced. The other 90% is from the surface growth process [49]. While
the number of nascent particles and the evolution of the particle number density are determined by
the nucleation kinetics and coagulation process, the carbon mass accumulated on soot is mainly
controlled by surface reaction, growth and oxidation [53].
The surface growth process occurs under conditions with high amount of acetylene [37]. In the
soot surface growth process, H reacts with soot surface and then the hydrogen atom is abstracted
from the carbon-hydrogen bond, leading to the formation of an active site on soot surface. If
acetylene exists, the active site will react with acetylene, and thus the carbon amount of particles
are increased. This process is similar to the HACA mechanism [37], as is discussed in section 2.1.
1.3. In this process, surface migration of H atom is found to be rapid enough to dominate the
formation of final product from initial adduct [54, 55]. The collision and condensation of PAH
species on the soot surface is another path of soot growth, which is called PAH-soot surface
condensation [55, 56].
8
Surface growth appears to occur both on the individual particle and on the aggregates. In the soot
growth process, the mass of the soot particle increases while the number of soot particles remain
constant [39]. The majority of soot mass is accumulated in the soot growth process. Since smaller
particles have more reactive radical sites, the rate of soot growth for smaller particles is higher
than that for larger particles [57].
2.1.3 Soot Coagulation and Agglomeration
A great number of particles collide due to the Brownian motion and then generate larger spherical
particles. This process is called coagulation, which produces fractal-like primary particles [58]. It
is different from the soot surface growth process, because the kinetics for the coagulation is
physical in nature while surface growth is driven by chemical mechanisms [59]. Coagulation
decreases soot number density, and changes size distribution and soot morphology while leaving
the total soot mass unchanged [58].
The fast restructuring of small and young particles is called coalescence [60]. In this process, the
two small particles coalesce into each other completely resulting a larger particle. As for the larger
particles, the restructuring is relatively slow and the colliding particles just merge into each other
partially resulting a transition region connecting them together. In this case, soot particles become
larger aggregates. The collision and sticking of two aggregates which consist of several primary
particles will lead to the formation of a larger aggregate. This process is called agglomeration or
aggregation [59]. The mechanisms of restructuring depend on particle size, particle material
property and local temperature. An important issue regarding coagulation is its efficiency.
Traditionally, it was assumed that every collision results in a successful coagulation. However,
some studies showed that the coagulation cannot always be successful. Under some flame
conditions, the collision of particles cannot result in sticking particles, which is called thermal
rebound effect [31, 61]. No matter what kinds of conditions are, the ultimate number density of
agglomerates is similar around 1016 /m3 [62]. It has been found that soot coagulation process takes
place almost immediately after the soot formation process, or when particles are relatively young
or small [6]. However, soot agglomeration usually occurs relatively later when there is no
coagulation [63].
9
2.1.4 Soot Oxidation
Soot oxidation and soot formation process may occur simultaneously, as in well-mixed combustors
or premixed flames, soot oxidation may occur after soot formation process as in staged combustors
or diffusion flames [18]. In the soot oxidation process, soot particles react with oxidizer back into
gaseous states, and the carbon accumulated on the soot particles is depleted [6]. The oxidation of
soot or soot precursors always competes against the production of soot or soot precursors [49].
The net amount of soot from soot formation and oxidation process determines the final particulate
emission from combustion devices [59].
Both mass transport and chemical mechanisms with heat transfer are involved in the oxidation
process. Surface intermediates are formed by the absorption of gaseous oxidizer on the surface.
Then the intermediates rearrange and desorb into gaseous products [8]. Soot can be oxidized either
by O2 or OH. The efficiency of the collision between OH and soot is relatively high. The reaction
usually occurs in the fuel-rich region. For O2, the efficiency is much lower. And O2 is a major
contributor in fuel-lean region [64]. Under some conditions, species such as O, CO2, H2O may act
as important oxidizers in soot oxidation process [65].
2.2 The Combustion of Aromatics
Aromatic hydrocarbons have a high tendency to generate soot in both diffusion flames [66] and
premixed flames [67]. Aromatics normally constitute 5 to 60% of the hydrocarbons in unleaded
gasolines, jet fuels and diesel fuels [68, 69] and generate more PAH and soot compared with non-
aromatic fuels [70], presumably because the aromatic ring can stay intact and avoid the slow
process of the first ring formation [71]. This behaviour shifts the rate-limiting step to the formation
of the second ring (naphthalene) and arouses interests in which pathways are more important for
the naphthalene formation from fuels containing monoaromatic hydrocarbons [72].
The oxidative pyrolysis mechanism of aromatic hydrocarbons is very different from pure pyrolysis
mechanism [68, 73]: pyrolysis of aromatic hydrocarbons leads to the destruction of six-membered
ring through ring rupture reactions and the formation of C4 and C2 hydrocarbons, as is shown in
pathway (1) of Figure 2. 2; phenoxy radical (C6H5O) is produced by oxidation process and then
goes through the ring contraction to generate cyclopentadienyl radical (C5H5) and CO, as is shown
in pathway (3) of Figure 2. 2.
10
Figure 2. 2: Schematic picture displaying three possible consumption pathways of benzene in
flames. Pathways (1) and (2) can occur with or without oxygen and pathway (3) can only occur
with oxygen. BZ: benzene, C5H5: cyclopentadienyl radical, C6H5: phenyl radical, C6H5O:
phenoxy radical, C8H6: phenylacetylene, C10H8: naphthalene. Arrow do not represent elementary
reactions [74].
The formation of the first aromatic ring and two-ring species are important steps in soot formation
process [72, 75]. Frenklach et al. [76] concluded that naphthalene is mainly generated from
phenylacetylene (C8H6) by the HACA mechanism. The step that generates naphthalene from one-
ring aromatic hydrocarbons is also a rate-limiting step and will lead to the production of
carcinogenic PAH and soot [36]. There are mainly three formation pathways of naphthalene. The
first one is HACA mechanism, which includes the abstraction of a ring H from phenylacetylene,
the addition of an acetylene to the resulting sites, the cyclization of two side chains leading to the
formation of naphthyl radical, and the addition of H to the naphthyl radical forming naphthalene
[76, 77]. This process can be summarized as
C8H6 + C2H2 → C10H8
(2. 4)
11
The second pathway is the reaction between propargyl radical and benzyl radical, which is
proposed by Colket and Seery [78]. The reaction can be represented as
C7H7 + C3H3 → C10H8 + 2H
(2. 5)
Propargyl and benzyl can coexist in relatively large concentrations due to their resonant
stabilization. Thus reaction (2. 5) have a high overall reaction rate. Alkyl side chains exist in many
aromatic hydrocarbons in real diesel and jet fuels [69], thus aromatics can readily decompose into
benzyl [68].
The third pathway is the reaction between two cyclo-pentadienyl radicals [79], which can be
expressed as
2C5H5 → C10H8 + 2H
(2. 6)
Anderson et al. [72] studied the formation pathways of the second ring in the combustion of
monoalkylbenzenes by separately doping a non-premixed nitrogen-diluted methane flame with
500 ppm of ethylbenzene, toluene, and the structural isomers of butylbenzene and propylbenzene.
They found that a great number of the added aromatic rings kept intact and thus promoted the
formation of the second ring. Primarily, the additives break down in two routes: when the
secondary carbon attached to the aromatic ring, the alkylbenzene would quickly break down into
benzyl radical; when the tertiary or quartary carbon attached to the aromatic ring, the pyrolysis or
decomposition through H abstraction would lead to the formation of styrene or methylstyrene,
which further broke down into phenylacetylene. The second ring was generated through the HACA
pathway. They concluded that which pathways are more important depends on the main
decomposition products of the additives and the second ring formation is also an important rate-
limiting step in combustion of fuels with alkylbenzene hydrocarbons.
12
McEnally and Lisa [75] investigated the relative importance of different formation pathways of
naphthalene in non-premixed flames with fuels separately doped with 1700 ppm of carbon-13-
labeled styrene, toluene and benzene. They observed that styrene was converted to phenylacetylene
by the side chain dehydrogenation and phenylacetylene was converted to naphthalene by the
HACA mechanism. They summarized that the HACA pathway and propargyl addition to benzyl
are feasible routes to form naphthalene in flames, because carbon attached to an ethynyl side chain
of benzene and carbon attached to a methyl side chain of benzene can be directly converted into
naphthalene in real flames.
Brezinsky [68] studied the oxidation mechanisms of aromatic hydrocarbons at 875-1500 K, 1 atm
in Princeton flow reactor. It was found that the oxidation of phenyl radical and benzene follows
phenoxy radial (C6) - cyclopentadienyl radical (C5) - butadienyl radical (C4) sequence. For the
oxidation of alkylated aromatics, such as propylbenzene and ethylbenzene, the alkylated aromatics
are initially attacked by styrene, benzyl radical or benzene. Then the styrene reacts further leading
to the formation of a benzene radical or benzene.
Tregrossi et al. [80] described the structures of two premixed benzene-air flames with different
C/O ratios (0.72 and 0.77) in fuel rich condition at atmospheric pressure using the concentration
profiles of reactants and combustion products which were measured along the axes of the two
flames. They found that different C/O ratios mainly affect flame temperature and increase pyrolytic
products such as acetylene, PAHs and soot. However, the relative distributions of PAHs and the
light hydrocarbons are not influenced. The main light hydrocarbons generated in the studied flames
are acetylene and methane, which have larger concentrations later on compared with unsaturated
hydrocarbons (C3-C4). PAHs form in large amounts at the end of the main oxidation zone, while
in the burned gas region, PAHs significantly decrease.
Laurent et al. [81] investigated laminar premixed methane/air flames, and methane mixed with
benzene (1.5%)/air flames at low pressure (5.33 kPa). They concluded that benzene is mainly
consumed by hydrogen abstraction with OH and H as reactants, and its oxidation by O significantly
contributes to the formation of phenoxy. It was identified that the reaction between phenyl and O2
is a major contribution to the consumption of phenyl and the formation of phenoxy. The
chemistries of phenoxy and phenol are strongly coupled. The dominant consumption pathway of
phenoxy is a unimolecular decomposition generating cyclopentadienyl radicals (C5H5) and carbon
13
monoxide. They predicted that phenanthrene (C14H10) and naphthalene (C10H8) are generated in
the reaction zone.
Defoeux et al. [82] experimentally determined the structure of a one-dimensional premixed
benzene-oxygen-argon flame with a fuel equivalence ratio of 2.0 at a pressure of 50 mbar. They
compared their results with those from an ethylene flame with a fuel equivalence ratio of 2.5 [83]
and showed that benzene as the initial fuel strongly increases the formation of cyclopentadiene and
heavier hydrocarbons, for example, the maximum concentration of naphthalene is more than 100
times larger in flames whose initial fuels contain benzene structure. And the quantities of light
species (<C3) are similar in all studied flames.
Gudiyella and Brezinsky [84] investigated kinetics of n-propylbenzene under high pressure and
temperature. In their experiments, the pressures, temperatures and equivalence ratios varied from
25 atm to 50 atm, 838 K to 1669 K, 0.5 to 1.9, respectively. They found that the concentration of
the oxidizer would influence the formation of the intermediates and the fuel decay appears to be
insensitive to the pressure changes. They also concluded that at high temperatures, the majority of
the fuel is mainly decayed by the homolysis pathway, while at low temperatures, the majority of
the fuel is consumed by hydrogen abstraction reactions on the n-propyl side chain.
Dagaut et al. [85] performed experiments in a jet-stirred reactor (JSR) at atmospheric pressure to
study the oxidation of n-propylbenzene over high temperature range (900-1250 K), for different
equivalence ratios from 0.5 to 1.5. They presented 23 species concentration profiles by probe
sampling and GC analyses. Figure 2. 3 depicts the oxidation pathways of n-propylbenzene in a
JSR at 1 atm. Under stoichiometric condition and at lower fuel conversion (950 K), the depletion
of n-propylbenzene occurs through thermal decomposition and its reaction with H, O, OH, and
CH3, while at higher fuel conversion (1200 K), it occurs via thermal decomposition and its reaction
with H.
14
Figure 2. 3: Oxidation pathways of n-propylbenzene in a JSR at 1 atm (1200 K) [85].
2.3 Jet Fuels and Surrogates
2.3.1 Jet Fuels
Jet fuel or aviation turbine fuel (ATF) is a generic name for aviation fuels and is different from
traditional fuels in both physical and chemical properties due to different operating conditions and
requirements [86]. Jet fuels are used in both the civilian and military aircrafts. Jet fuels are usually
Kerosene fuels. The boiling points of jet fuels typically vary from 160 to 260 [87]. Jet fuels
are manufactured to satisfy certain American Society for Testing & Materials (ASTM)
requirements such as smoke point, flash point, density, etc [88]. Jet fuels are usually complex
mixtures of alkyl aromatics, n-paraffins, weakly branched (iso-) paraffins, or cyclo-paraffins [89].
Table 2. 1 summarizes common jet fuels and their properties [90].
15
Table 2. 1: Properties of common jet fuels [90].
Name Description Specification Freeze
point, C
Flash
point, C
Jet A U.S domestic jet fuel ASTM D1655 <-40 >38
Jet A-1 Standard commercial jet
Fuel
ASTM D1655, UK
DefStan 91-91 <-47 >38
JP-8 U.S. military jet fuel (Jet
A-1 + 3 additives) MIL-DTL-83133 <-47 >38
JP-5 U.S. Navy high flash jet
Fuel MIL-DTL-5624 <-46 >60
TS-1 Russian jet fuel GOST 10227-86 <-50* >28
Jet A and Jet A-1 fuels are the most widely used fuels in industry. Jet A is a kerosene fuel
designated by ASTM [91]. Jet A provides baseline specifications for other commercial jet fuels
[87] and is considered as the standard fuel for jet fuels in US, while Jet A1 is the standard fuel
adopted by the rest of the world [91]. The difference between Jet A and Jet A1 fuel is that Jet A
fuel has higher freezing point and Jet A-1 has mandatory anti-static additives [92].The difference
between JP-8 and Jet A-1 is an additive to meet military requirements [90]. Since jet fuels are not
specified, compositional variations exist between jet fuels.
Figure 2. 4 shows molecular compositions of a sample Jet-A fuel [93]. Jet-A fuels usually consist
of up to 75% paraffins and up to 26% aromatics [93, 94]. Aromatics are usually the main sources
of soot formation in combustion engines [88].
Figure 2. 4: Compositions of a sample jet-A fuel [93].
Since the amount of fossil fuels is decreasing, combining with environmental issues, it is necessary
to develop clean and sustainable fuels [95-97]. The synthetically produced jet fuels, as well as the
hydro-processed bio-derived oils are the only alternative fuels that can be used in the current engine
16
design. Synthetic jet fuels can be produced from coal, biomass or natural gas by the Fischer-
Tropsch (FT) process [98].
2.3.2 Surrogates Formulation
Since real fuels are usually mixed with hundreds of components in detail and the components of
different fuels are significantly different, thus enormous computational resources are required to
model them. It is out of reach to numerically simulate and describe the combustion of all the
components. In addition, data is limited on the chemical reaction pathways, thermodynamic
parameters and kinetic rate constants of a number of components [93]. Using a surrogate fuel
mixture which can emulate both the physical and chemical properties of the real target fuel is a
prevalent method adopted by combustion research [93].
The compositions of real fuel are usually complex and variable [93]. Most aviation fuels are mixed
with a great number of hydrocarbons, usually from four hydrocarbon classes-nomal paraffins, iso-
paraffins, cyclo-paraffins, and aromatics [90, 99, 100]. Surrogate formulation should be flexible
enough to study a wide range of real fuels. A surrogate fuel usually contains one to ten pure
hydrocarbon components from these representative hydrocarbon classes found in real fuels. These
components are chosen to replicate the same combustion properties in real conditions, sometimes
physical properties as well [90, 101-105]. Surrogate compositions have large variations due to the
wide variety of jet fuel applications and the composition sensitivity to these applications [90]. One
component would be enough for estimating simple properties such as combustion efficiency.
However, for applications which depend on chemistry such as radiation loading, soot formation
and emission, more complex surrogates are required [90]. Physical properties of fuels such as
distillation characteristics can also be simulated if suitable number of components are selected [90].
The combustion community has been working on searching the surrogate fuels which can replicate
the performance and emissions of real jet fuels for decades [90]. Before formulating surrogates for
real target fuels, detailed chemical kinetic mechanism of each component must first be studied.
Many researchers tested these components in flow reactors, shock tubes, and rapid compression
machines to develop kinetics models of these components. Then comparisons can be made
between surrogates and real fuels in the mentioned devices [88]. Combustion kinetics are
principally driven by the ability of fuel components to generate important radical species which
17
influence both chain branching reactions of radicals and primary heat production and release in the
combustion process [93]. The formation properties and various chemical properties of individual
radical species make up the phenomena of combustion kinetics [93]. Thus the main target of
formulating surrogate is to reproduce those radicals. To formulate surrogates for real fuel, several
combustion property targets are considered to specify the identity and fraction of each component
[106, 107]. These combustion property targets include average fuel molecular weight (MW),
hydrogen/carbon molar ratio (H/C), threshold sooting index (TSI), derived cetane number (DCN).
MW strongly influences the diffusive properties of gas phase [108]. Therefore, similar average
molecular weight is required to emulate the diffusive properties of real fuels in gas phase [93]. For
real jet fuels, the average carbon number is approximately 12 [89].
H/C molar ratio determines the ratio of CO2 to H2O formed in combustion process and influences
reaction enthalpy. Besides, the ratio of hydrogen/carbon also describes the diversity of molecule
structure which determines the air fuel stoichiometry. In addition, hydrogen/carbon ratio also
strongly affects total radical population [93].
TSI is proposed by Calcote and Manos [109] for describing sooting tendency which considers
molecular weight. It is defined as:
𝑇𝑆𝐼 = 𝑎 (𝑀𝑜𝑙𝑒𝑐𝑢𝑙𝑎𝑟 𝑤𝑒𝑖𝑔ℎ𝑡
𝑆𝑚𝑜𝑘𝑒 𝑝𝑜𝑖𝑛𝑡) + 𝑏 (2. 7)
Where smoke point is the maximum diffusion flame height (mm) when there is no soot breaking
through the flame [110], molecular weight is in g/mol, a is in mol mm/g and b is a dimensionless
experimental constant. The constant a and constant b were determined so that different studies can
fall on a single scale [88]. It was found that TSI strongly depends on aromatic component fractions
[38]. A reference database has been developed for the TSI values of common surrogate
components [111].
DCN was selected to replicate the auto-ignition property of real fuels [88].
The hydrocarbons shown in Table 2. 2 are usually used as surrogate components of jet fuels [106,
112].
18
Table 2. 2: Proposed components for jet fuels, formulas and molecular structures [112].
Formula Component Formula Component
n-Decane
C10H22 n-Dodecane
C12H26
Iso-octane
C8H18
Iso-cetane
C16H34
Methyl cyclohexane
C7H14
Propyl cyclohexane
C10H18
Toluene
C7H8
n-Propylbenzene
C9H12
1,3,5-Trimethylbenzene
C9H12
1-Methyl naphthalene
C11H10
2.3.3 Surrogates for Real Fuels
Dodecane and decane are usually primary surrogate components describing alkanes for jet fuels
[58]. Dagaut et al. [101] built a detailed kinetic reaction mechanism for the oxidation of n-decane
to study kerosene and reproduced experimental results. However, decane can only be used as a
simple fuel surrogate, not in the study of aromatics combustion [113]. Alkanes with higher carbon
number such as n-dodecane are chosen to match the ratio of hydrogen to carbon and the
autoignition properties of jet fuels [58]. Adding aromatic components to n-dodecane can emulate
the entire fuel properties for different real gas turbine fuels [107].
1,3,5-Trimethylbenzene, n-propylbenzene and toluene have been selected as aromatic surrogate
components which are complex enough to describe the monocyclic alkylaromatic content class in
the real fuel [93, 105, 111, 114]. Soot formation is greatly affected by the structure and amount of
the aromatic component [88]. Figure 2. 5 shows the C-C bond and C-H bond dissociation energies
of the n-propyl side chain of n-propylbenzene [84]. It can be inferred that the hydrogen abstraction
routes mainly generate styrene and methyl radical, while the amount of benzyl radical and ethane
is smaller.
19
Figure 2. 5: C-C bond and C-H bond dissociation energies (kcal/mol) of the n-propylbenzene
side chain. The red bold italicized numbers represent the C-C bond energies [115]. Plain
numbers denote C-H bond energies [85].
Several researchers have shown that typically jet fuels average around 20 vol. % n-paraffins and
the palette contains n-dodecane, n-tetradecane and n-decane [116]. 40.4% n-Dodecane, 22.8% n-
propylbenzene, 29.5% iso-octane and 7.3% 1, 3, 5-trimethylbenzene are four components of
MURI Jet A1 surrogate proposed by Princeton University [93]. Dodecane was selected in MURI
surrogate to match the molecular weight with real fuel [93]. The mixture of n-dodecane/n-
propylbenzene/iso-octane/1, 3, 5-trimethylbenzene is called 2nd generation (POSF 4658)
surrogate, which can exhibit combustion behaviour nearly consistent to that of the target real fuel
[93]. In this mixture, n-dodecane with a higher carbon number can allow sufficient addition of
aromatic components to elucidate various fuel properties which are already known for real fuels
[106]. It was measured that DCN, TSI, H/C and average MW of this mixture are 47.1, 21.4, 1.96
and 138.7 g/mol, respectively [93].
20
Chapter 3
Experimental Methodology
3.1 Coflow Burner
The laminar coflow diffusion flame in this work was generated by a coflow diffusion burner, which
was designed for a stable and axisymmetric diffusion flame. Fuels passed through the inner
stainless steel fuel tube with an inner diameter of 10.90 mm. Oxidizer flowed through the
concentric annulus with an inner diameter of 90 mm. The fuel tube is long enough to ensure a fully
developed velocity profile of the fuel stream when it reaches the exit. The lower part of the annulus
is filled with 5 mm spherical glass beads enclosed by a porous metal disk. The beads and the porous
disk can unify laminar flow velocity profiles and stabilize flames. In the combustion research lab
at University of Toronto, a ceramic honeycomb with the size of 150×150×100 mm was used to
straighten the flow from the exit of the fuel tube and prevent air circulation down the side walls,
thus it can help obtain a stable flame. The flame was shielded from the outside lab air currents by
an optically clear acrylic tube with a 304.8 mm length, 3.175 mm wall thickness, and 152.4 mm
outside diameter. Different ports were machined on the tube for the laser extinction and scattering
experiments. Because the signal of scattering is relatively low and the influence of the tube
reflections on scattering signal is relatively large, we covered black aluminum foil tape both inside
and outside of the tube. This tape is flame-retardant, non-reflective and can be exposed to a 20 W
laser beam for 10 seconds [117].
To make experimental measurements at different positions inside flames, it is more convenient to
move the position of the burner instead of the LE and ELS system. Two linear translation stages
(Newport Model No. M-436) with low-profile crossed-roller bearing and a lab jack (Newport
Model No. M-EL120) were used to make the burner move horizontally and vertically. The travel
range of the two stages is 50.8 mm and the load capacity is 556 N. The lab jack has a travel length
of 120 mm and a load capacity of 500 N. Different from the stages, the lab jack cannot provide
direct position readings. To determine different flame heights, an absolute digimatic scale unit
(Mitutoyo Model No. 572-571) with a range of 152.4 mm and an accuracy of 0.0254 mm was
mounted to the lab jack.
21
3.2 Fuel and Oxidizer Delivery System
3.2.1 Oxidizer Delivery System
Several devices were used in the oxidizer gas line to obtain repeatable, accurate stream pressures
and flow rates.
Figure 3. 1: Air supply apparatus.
Air came from a common compressor in the building. After the compressor, various devices were
used to control the air pressures, flow rates and remove contaminants. The first device after the
building air supply was a filter coalescer, which removed aerosols and oil vapours produced by
the common compressor. Then a regulator was used to regulate air pressure. Downstream of this
regulator, air was supplied to different experimental setups in the combustion research lab at
University of Toronto. For laser extinction and scattering experimental system, a second regulator
was used to further reduce air pressure. The line pressure was reduced step by step. We used an
air thermal mass flow controller (Brooks Model No. SLA 5851) to control the flow rate of air. The
flow range of this unit is from 20 lpm to 100 lpm (N2 eq.) and the rated accuracy is 0.7% of rate
and 0.2% of full scale (FS). A digital controller (Brooks Model no. 0254) was used to control the
mass flow meter. The inlet pressure of the thermal mass flow meter was around 50 psig in our
22
system. There were two filters in the air gas line. The first one was placed downstream of the
coalescer and could remove larger contaminants down to 5 μm. Very fine particles with size of 1-
2 μm cannot be captured by this step. As a result, another micron filter (Swagelok Model No. SS-
SCF3-VR4-P-225) was placed upstream of a digital mass flow controller. The particle removal
rating of this filter is greater than 99.9999999% at 0.003 μm at maximum flow rate. Besides the
filters, a chemical resistant air dryer was used to remove moisture, oil and oil vapor. The
arrangement of the devices is shown in Figure 3. 1.
3.2.2 Gaseous Fuel Delivery System
For the validation of experimental apparatus, the ethylene-air diffusion flame data from Santoro et
al. [118] was used as a benchmark to evaluate our laser extinction and scattering system.
Ethylene used in this study was supplied from a compressed cylinder with a purity of 99.5%. A
regulator was used to reduce the high pressure of the cylinder to the operating pressure of around
40 psig. A high-accuracy rotameter (Matheson Gas Model No. FM1050) was used to measure the
fuel flow rates. A needle valve was attached to the rotameter to set the desired flow rates. A small
glass ball in the glass tube displayed the fuel flow rate. When the ball suspended in the glass tube,
the drag force from the flow was equal to the weight of the ball. The scale on the rotameter is linear.
The accuracy of the rotameter does not fluctuate a lot with day-by-day use under the experimental
conditions. As the rotameter itself cannot indicate the accurate fuel flow rate, another flow
measurement device was used to calibrate it. The rotameter was calibrated by a primary gas flow
calibrator-BIOS piston prover (Mesalabs Model No. Definer 220). The BIOS calibrator can
measure flow rates from 0.005 liters to 30 liters and can work as a primary flow standard with an
accuracy of ±1% of readings standardized for temperature and pressure, and 0.75 ± 0.75% of
volumetric flow.
3.2.3 Liquid Fuel Delivery System
Most research on soot formation use gas fuels, such as methane, ethylene, and acetylene, instead
of liquid fuels. From an experimental point of view, studying liquid fuels requires a vaporizer
system to vaporize the fuel and liquid fuels generate heavy soot, which makes experiments much
more complicated. However, these gas fuels lack most of the molecular structures that characterize
liquid fuels: allylic bonds, alkyl rings, alkyl carbon-carbon bonds, and benzenoid rings. Thus
23
studying flames of liquid fuels can provide a more complete picture of fuel decomposition and
aromatics formation chemistry [3].
In the investigation of n-propylbenzene addition on soot formation in an n-dodecane laminar
coflow diffusion flame, liquid fuel was vaporized by a Bronkhorst® Controlled-Evaporator-Mixer
(CEM) unit, including a gas mass flow controller (EL-Flow Model No. F-201-CV-5K0-AAD-
22V), a liquid mass flow controller (LIQUI-FLOW Model No. L13-AAD-22K-10S), and a 3-way
mixing evaporator (CEM Model No. W-102A-222-K). Methane with a purity of 99.99% was
selected as carrier gas. The quantities of soot produced by methane itself is very small. The peak
soot volume fraction value of pure methane flame was about 0.33 ppm with a flow rate of 0.33
L/min. This peak value was less than 25% of that of pure n-dodecane flame. The adiabatic flame
temperature of pure methane under stoichiometric condition was 2321 K, which was 150 K higher
than that of 5 mol.% n-dodecane in nitrogen. Pure methane which produces very small amount of
soot made the flame temperatures more representative of that in practical conditions compared to
nitrogen used as carrier gas. Nitrogen with a purity of 99.998% was used to flush and cool the
vaporizer system and the burner after each measurement was finished. The temperature of the
vaporizer and mass flow rates were controlled by a digital readout (Bronkhorst Model No. E-7120).
Liquid fuel was supplied using a dispensing pressure vessel (Millipore Model No. XX6700P01),
which was pressurized by helium with a purity of 99.999%. It was assumed that the solubility of
helium in liquid fuel can be neglected at the temperatures and pressures used in this study.
A heated tube from Unique Heated Products INC was used to deliver the vaporized gas mixtures
to the coflow diffusion burner. The heated tube was wrapped with heating tapes (Omegalux
Catalog No. SWH171 - 020) at the outlet of the vaporizer and inlet of the burner. The fuel tube of
the burner was also heated using coil heaters (O.E.M. Heaters Model No. K460182). All of these
heaters were used to prevent fuel condensation. The temperatures of vaporizer, heated tube, and
coil heaters were set to around 460 K, 550 K, and 590 K, respectively. A schematic of the vaporizer
and burner apparatus is illustrated in Figure 3. 2.
24
Figure 3. 2: Schematic of the vaporizer system and coflow diffusion burner used in the current
study.
The pressures used for gases are shown in Table 3. 1.
Table 3. 1: Gas pressures used in the measurements.
Gas Pressure (psig)
Air 50
Helium 42
Methane 40
Ethylene 40
Nitrogen 40
3.3 Soot Optical Diagnostics
Appropriate sampling, such as TEM technique, can characterize particle morphology
comprehensively. In these methods, a sampling probe is inserted directly into a flame. The
temperature of the tube is much lower than that of the flame. Thus it will influence the reactions
inside the flame [119]. Besides, these methods need time-consuming evaluation work and cannot
achieve on-line measurement. Researchers have to infer parameters of three-dimensional clusters
such as radius of gyration of soot aggregates and fractal dimension from two-dimensional images.
Optical methods do not disturb flames, can achieve real-time measurements with appropriate
25
sensitivity, and can remotely sense even in hostile environments [119]. Therefore, optical methods
are developed to improve the field of combustion research.
Optical techniques used for combustion research include laser extinction (LE) technique, soot
spectral emission (SSE) technique, laser-induced Incandescence (LII) technique, and elastic laser
scattering (ELS) technique. Optical methods are conducted in an in situ, non-intrusive way. The
energy they add to the flame field is considered as negligible [120]. Thus they can provide
information without disturbing the flame. Optical methods are highly sensitive and have high
spatial resolution. LE method can be used to obtain information of soot volume fraction. SSE
method can be used to study soot temperature and soot volume fraction in flames. LII method can
be used to determine soot volume fraction and reduced primary particle diameter. ELS method can
provide information about primary particle diameter and primary particle number density. LE and
ELS methods are used in this work.
3.3.1 Laser Extinction Technique
LE technique measures the extinction of a collimated laser beam after it passes through a field
containing particles by optical detectors. The loss of light intensity from the light source to the
detector is caused by both the absorption and scattering by the particles along the light path.
Extinction = Absorption + Scattering (3. 1)
The extinction of laser beam is related to the length of laser path and the extinction coefficient of
particles, which can be described by Beer-Lambert law:
𝐼𝜆 = 𝐼𝜆0𝑒𝑥𝑝 (− ∫ 𝐾𝑒,𝜆
𝐿
0
𝑑𝑥) (3. 2)
Where λ is laser wavelength, Ke,λ is local extinction coefficient, Iλ,0 is the laser beam intensity
before passing through the flame chord, Iλ is the laser beam intensity after passing through the
flame chord, L is the length of the flame chord that laser passes through.
Soot volume fraction can be calculated as formula (3. 3) with the assumption that soot particles
are almost spherical [121]:
𝑓𝑣 =𝜆
𝐾𝑒𝐾𝑒,𝜆 (3. 3)
26
Where Ke is dimensionless optical extinction coefficient, which can be presented as:
𝐾𝑒 = 6𝜋(1 + 𝜌𝑠,𝑎)𝐸() (3. 4)
Where ps,a is the ratio of scattering to absorption. E is a function of the soot refractive index ,
which can be calculated as:
𝐸(𝑚) = −𝐼𝑚 [𝑚2 − 1
𝑚2 + 2] (3. 5)
According to equation (3. 3) and (3. 4), soot volume fraction can be expressed as:
𝑓𝑣 =𝐾𝑒,𝜆𝜆
6𝜋(1 + 𝜌𝑠,𝑎)𝐸() (3. 6)
The transmittance Iλ/Iλ0 is equal to the integrated value of local extinction coefficient along the
chord of flame that laser beam passes through. Thus the local extinction coefficient can be
calculated by inverting the measured transmittance, which is called tomographic reconstruction
technique. In this method, the flame was assumed symmetric. Saffaripour et al. [122] has used the
three-point Abel inversion method to obtain local extinction coefficient. LE method is flexible and
simple to conduct. However, it cannot be applied to asymmetric and non-steady flame due to
tomographic reconstruction technique and line-of-sight approach.
3.3.2 Elastic Laser Scattering Technique
Previous study from Dobbins [123] showed that the ratio of scattering to absorption can reach to
40% under a population of polydisperse aggregates. Zhu et al. [124] measured the ratios of
scattering to extinction cross-section at different light wavelength (543.6 nm, 632.8 nm, and 856
nm). The average values are 24.5%, 19.5%, and 19.5% for ethane and 31.1%, 22.8%, and 23.7%
for acetylene, respectively. It has been found that soot usually contains aggregates which consists
of a number of monomers or spherules [123]. In this case, light scattering should be considered in
the total extinction [123].
It has been observed that the light intensity absorbed by a soot particle is proportional to its volume,
and the light intensity scattered by a soot particle is proportional to the square of its diameter. Thus
the particle diameter can be obtained by calculating the ratio of absorption to scattering. This
theory has been the foundation of many research efforts in the recent past [123, 125-132]. More
detailed work is discussed below.
27
Most of previous studies use Rayleigh theory to study soot scattering. This method is suitable when
the particle size is smaller than the wavelength of incident laser beam. And Mie theory is suitable
when the size of particles is comparable to the wavelength of the light source. However, primary
soot particles usually stick together to form aggregates instead of existing separately. The size of
aggregates may be larger than the wavelength of laser light, because the number of primary
particles in an aggregate varies from 10 to 104 [133]. Thus it is inaccurate to use Rayleigh theory
and Mie theory to study soot in flames. The classical Rayleigh-Debye-Gans scattering theory has
been generalized for fractal aggregates by using fractal ideas along with certain assumptions about
multiple scattering and primary particle properties in aggregates. There are analytical expressions
in the formulation which directly relate optical cross sections to aggregate size, particle size and
morphology [134]. For elastic light scattering, Rayleigh theory and Mie theory has substantial
disadvantages in accurately determining the aggregates scattering cross sections [135]. On the
contrary, Rayleigh-Debye-Gans Fractal Aggregate (RDG/FA) theory [136] is more reliable to
analyze soot aggregate properties because it considers the optical cross sections of particulate
aggregates and aggregate polydispersity. In the current work, we used an improved data analysis
[137] approach to relate the various measured optical cross sections to soot aggregate properties
based on the RDG-FA theory. In this approach, we assumed that the soot primary particles are
spherical scatterers that only make point contact with each other. The full calculation procedure is
shown in appendix B.
Puri et al. [128] analyzed a coannular ethane diffusion flame using laser extinction technique and
laser scattering technique at multiple angles (45°, 90°, 135°) combined with additional information
from TEM measurements. It was shown in this study that the data reduction is quite different
between those based on aggregate cross sections and those using Rayleigh or Mie theory. Based
on Mie and Rayleigh theory, the volume mean diameter increases much more modestly in the
growth region and decreases quite moderately in the oxidation region. It was found that the particle
number concentration shows a slight increase in the growth region in the Rayleigh sphere data
reduction and is constant along most streamline in Mie data reduction using scattering/extinction
cross sections, which indicates cluster-cluster aggregation (CCA) is absent or CCA is offset by the
increase of aggregate population through inception. Besides, Mie theory with dissymmetry
measurements yields a much lower number concentration, which is unreasonable due to the
generation of more than twice theoretical aggregation rate. For peak soot volume fraction, the
28
result obtained from Rayleigh theory is around 30% higher and that from Mie theory is 15% lower
at the intermediate flame heights. Mie theory overestimates the contribution of scattering to
extinction, while Rayleigh theory ignores the contribution of scattering compared to RDG/FA
theory.
Link et al. [135] studied the ability of multi-angle scattering measurements to determine soot
aggregate properties including the fractal dimension and the size distribution. The range of angles
spans from 10° to 160°. They found that it is difficult to unambiguously determine the size
distribution parameters using multi-angle scatter intensities and the measured part of the overall
structure factor is significantly limited by the corresponding range of scattering wave vector. It
was shown that relative multi-angle (10° to 160°) scattering experiments with laser wavelength in
the visible region can only obtain possible combinations of the size distribution of soot aggregates
and fractal dimension instead of determining their values simultaneously.
Iuliis et al. [138] compared the results of three angle laser scattering and laser extinction technique
with those from TEM analysis. The primary particle diameter and radius of gyration with the
presence of mature soot agree well with each other. However, in the relatively lower region of the
flame, difference was found due to different structures from mature soot. There are substantial
limitations to both optical and TEM methods at these regions due to different optical properties
and the splashing of the liquid-like structure respectively. For young soot, more structural and
optical information are required.
Teng et al. [139] suggested two optimum scattering angles (30° and 150°) by considering both the
sensitivity analysis and the spatial resolution. From sensitivity analysis aspect, the further the two
scattering angles are separate, the more accurate the aggregate size inversion is; from spatial
resolution aspect, since the scattering volume detected by the detector increases with 1/sin θ (θ:
scattering angle), too small or too large scattering angles will result in a poor spatial resolution.
Besides, the scattering length and volume detected by the detector are determined by the pinhole
in front of the photomultiplier. On one hand, to ensure enough scattered light is collected, the size
of pinhole should be large enough; on the other hand, to ensure the solid angle is not too large, the
size of pinhole should be small enough. A scattering angle of more than 150° or less than 30°
would possibly make the radial distance that the signal averages exceeds the desirable limit of
spatial resolution. Thus 30° and 150° were selected for the optical arrangement of scattering
29
experiments. They evaluated the measurements using exact scattering computations on fractal-like
aggregates with convenient form of structure factor. The aggregate gyration radius inferred from
computations of dissymmetry ratios of 30° and 150° agrees well with the initially prescribed values
and the spherule diameters also agree well with the simulation values, except for relatively small
aggregates, since the dissymmetry ratio of them is close to unity. Their computational results
showed that inverse analysis of laser scattering at only two angles can provide information about
spherule and aggregate sizes with minor errors. In the current work, two scattering angles (30° and
150°) were selected to study soot formation in laminar coflow diffusion flames. The two scattering
angles combined with the improved data analysis approach [137] allow us to remove the
assumption made about the regime in which the scattering measurements are made.
3.4 Optical Configuration
The optical apparatus for LE and ELS measurement is shown in Figure 3. 3. It consists of a
Coherent CW high-power optically pumped semiconductor laser with a wavelength of 639 nm.
The incident laser power was set to 150 mW for LE measurements and 1000 mW for ELS
measurements. Laser beam was modulated by a mechanical chopper at 1015 Hz. The modulated
beam was then enlarged using a plano-concave lens with a focal length of 25 mm and a plano-
convex lens with a focal length of 150 mm. Then the enlarged beam passed through a polarizer
with an extinction ratio of 100,000:1 to ensure the light was polarized in the vertical direction. A
beam splitter with a split ratio of 90:10 was used to partially transmit the polarized beam to a
photodiode to monitor the power fluctuations of laser. This measurement was used to correct the
detected signals in laser power. The direction of the other portion of the laser beam passing through
beam splitter was changed by a mirror and then focused to the center of the coflow burner by a
750 mm focal length lens. After the beam passed through the flame, a 100 mm plano-convex lens
was used to collimate the beam. Then the collimated beam was focused into an integrating sphere
by a plano-convex lens with the same focal length. This integrating sphere was used to uniformly
distribute the incoming light by multiple reflections inside the sphere. After that, the uniform
distributed laser beam was collected by a photodiode. For the scattering part, the scattered light
was first collimated using a 200 mm focal plano-convex lens. Then the collimated light passed
through a Glan-Laser Calcite polarizer to make sure that only vertically polarized light was
transmitted. Then a plano-convex lens with the same focal length focused the beam into a 500 μm
pinhole located in front of the photomultiplier tubes (PMTs). The pinhole was used to define the
30
entrance aperture and spatial resolution of the measurements. Bandpass filters were used in front
of each photodiode and photomultiplier (PMT) to prevent the influence of flame luminosity and
room light. Neutral density filter was used in front of the first lens in the scattering part when
necessary.
Figure 3. 3: Layout of combined laser extinction and two-angle elastic laser scattering apparatus.
3.5 Detection and Data Acquisition Equipment
There were two sets of detection equipment for laser extinction and scattering measurements. The
first set consists of two Si-detectors with high speed used for laser extinction measurement. One
of the detectors was mounted to detect reference signals which could monitor the fluctuations of
laser power. The other Si-detector collected laser signals after the laser passed through the laminar
coflow diffusion flame. When operating them, it is necessary to center the incident light on the
active area of the detectors because the edges of the active area are not homogeneous, thus they
will generate unexpected capacitance and resistance. To ensure the intensity of the collected laser
was in the region where the detectors behave linearly, neutral density filters were mounted before
each detector when necessary.
The second set of detection equipment includes two PMTs with high gain, wide dynamic range
and high-speed response to detect several orders of magnitude lower scattered light signal at 30°
31
and 150° to the incident beam. In the scattering part, the first element was an aperture with a
diameter range from 0.8 mm to 12 mm, which was used to define the solid angle for collecting
scattered light. A plano-convex lens with a focal length of 200 mm was mounted at a distance of
200 mm from the center of the fuel tube to collimate the scattered light. The aperture, lens,
polarizer, pinhole and band pass filter were all mounted inside a tube to prevent unwanted light
from entering the PMT.
There were four data channels in total in the current LE and two-angle ELS setup. Two of them
were for LE measurements from two Si-detectors, the other two were for ELS measurements from
two PMTs. BNC terminators were used to convert the current signals to voltage signals. Then the
signals were transmitted to the lock-in amplifiers (Stanford Research Systems, Model No. SR-830).
The measured signals had both the alternating current (AC) component and the direct current (DC)
component. The AC signal was from the scattered light or the transmitted light and the DC signal
was from extraneous light, such as flame radiation. The incident light was modulated at a specified
frequency (1015 Hz) by a chopper. Lock-in amplifiers were capable to select these AC signals and
recover the signals that were mixed with other signals. The time constant used in the lock-in
amplifier was 1s.
In the current study, data acquisition system includes the LabVIEW software installed in a
ThinkPad computer and National instruments (NI) data acquisition (DAQ) system. All of the above
mentioned data channels-two for scattering measurements from PMTs and two for extinction
measurements from Si-detectors were transmitted into a National Instruments (NI) model. This
module has 4 analog input channels (Al) with a sampling rate of from 1 S/s up to 102.4 kS/s and
1 analog output channel (A0) with an update rate of 96 kS/s. All of the Al channels were sampled
simultaneously [140]. All the data was saved to an Excel file by the LabVIEW code.
3.6 Constants
In the current study, soot aggregate properties were determined using the measured volumetric
extinction coefficients and scattering coefficients through the Rayleigh-Debye-Gans Fractal
Aggregate (RDG/FA) theory. This calculation requires the values of soot refractive index (m),
fractal dimension (Df) and fractal prefactor (kf). Many researchers have attempted to determine the
values of complex refractive index, fractal dimension and fractal prefactor. Some of these studies
are discussed below.
32
3.6.1 Refractive Index
Erickson et al. [141] measured the concentrations and sizes of soot particles in a premixed laminar
benzene-air flame using laser scattering technique and compared the experimental results with
theoretical calculated parameters. They found that the best fit between experimental value and
theoretically calculated value for monodisperse spheres occurs when refractive index m = 1.40-
1.00i at x = 1.05 (x = particle perimeter/wavelength, πd/λ) was selected to calculate the properties
of soot particles. However, this only demonstrated that it is possible to match the results of
experiments and theoretical calculations for monodisperse spheres, but it did not mean the values
for m and x are correct.
Lee and Tien [142] developed a dispersion model to analyze the optical constants of soot based on
a more rigorous consideration of the dispersion constants and the electronic band structures. They
concluded that soot optical properties are not sensitive to increasing temperatures and are relatively
independent of the fuel hydrogen/carbon ratio.
Bockhorn et al. [143] investigated soot concentrations and particle sizes in a propane-oxygen flame
with additives of hydrogen and ammonia at atmospheric pressure by laser scattering and probe
measurements. They evaluated the measurement results of light scattering by nonlinear regression
analysis. It was found that the regression can be improved if the real part and imaginary part of the
refractive index are not assumed constant. They obtained refractive index m = 1.1-0.37i for
propane-oxygen flame, m = 1.3-0.74i for propane-oxygen flame with additive of hydrogen, m =
1.3-0.94i for propane-oxygen flame with additive of ammonia at the height of 25 mm above the
burner.
Charalampopoulos and Felske [144] studied a fuel-rich premixed methane-oxygen flame and
inferred complex refractive index of Rayleigh size soot particles from extinction and classical
scattering data. The particle size distribution was separately determined from the refractive index
and particle concentration. For monodisperse, the real parts of the refractive index they obtained
are between 1.37 and 1.79 and the imaginary parts of the refractive index are between 0.41 and
0.74; for polydisperse, the real parts of the refractive index they obtained are between 1.38 and
1.86, the imaginary parts of the refractive index are between 0.42 and 0.78.
33
Mullins and Williams [145] measured refractive index of soot in flames of four fuels: n-heptane,
propane, methane and toluene (methyl benzene) at two laser wavelengths, 450 nm and 633 nm,
using two techniques, light attenuation and light reflectance. They found that the contribution of
light scattering to light attenuation is approximately 5%. By comparing their experimental results
with the theoretical values calculated by Lee and Tien [142], they concluded that Mie scattering
theory, which is usually applied to describe light attenuation by small spherical particles, can
successfully determine the complex refractive index of soot when the density and mean particle
size are provided. They also found that soot collected from four different flames through means of
impingement on a plate has similar optical properties at 300 K by using light attenuation technique,
which are consistent with the theoretical values of the complex index at 300 K. However, the light
reflectance technique resulted in higher values of the imaginary part of the complex refractive
index compared to the light attenuation technique. They proposed that this difference occurs
because light reflectance technique is sensitive to the surface roughness degree. In their results,
the real part of refractive index varies between 1.88 and 1.93 in both methods, the imaginary part
of refractive index varies between -0.78 and -0.51 in the light reflectance method, and between -
0.46 and -0.39 in light attenuation method.
Vaglieco et al. [146] evaluated the optical property dispersion of absorbing submicronic aerosols
in premixed flames at atmospheric pressure by simultaneously measuring scattering and extinction
coefficients in the near UV and visible spectrum. They used non-aromatic fuels such as CH4, C2H2,
and C2H4 at different flow rates and C/O ratios. They assumed the particles are not agglomerated
and not considered as Rayleigh scatterers, and the contribution of molecules is negligible when
determining the spectral properties of the real and imaginary parts of the complex refractive index
of soot. Their method required that the optical properties at a reference wavelength, the average
size and number density of the soot particles are known independently, laser scattering and
extinction are primarily caused by solid submicronic particles, and fluorescence and absorption
from aromatic gaseous compounds are negligible. They found that the real part of the complex
refractive index displays a strong dispersion in the visible region and decreases from the visible
region to the UV region while the imaginary part decreases when wavelengths are shorter than 300
nm and remains constant in the visible region.
Smyth and Shaddix [147] indicated that refractive index m = 1.57-0.56i is still by far the most
often cited value in the visible wavelengths by combustion community. They found that the value
34
of m = 1.57-0.56i may underestimate the soot volume fraction inferred from extinction results.
Dalzell and Sarofim [148] found that refractive index of soot is essentially constant in the
wavelength range between 435.8 nm and 806.5 nm with mean values of 1.57-0.50i and 1.56-0.46i
for soot in propane and acetylene diffusion flames, respectively. Dalzell et al. [119] obtained m =
1.60-0.60i in propane flames at 435.8 nm later.
Table 3. 2 summarizes the values of complex refractive index of soot used or determined by various
researchers. Williams et al. [149] found that the soot refractive index is close to 1.75-1.03i at 635
nm by comparing soot scattering calculated by Rayleigh-Debye-Gans theory and the measured
dimensionless extinction coefficient using several refractive indexes. Since the wavelength used
in the current work is close to 635 nm, a value of 1.75-1.03i was selected as the refractive index.
Table 3. 2: Complex refractive index of soot used or determined by various researchers.
Researchers Wavelength
(nm) Real Part Imaginary Part
Danzell and Sarofim (1969) [148] 650 1.57 -0.56
Chippet and Gray (1978) [150] visible 1.9 to 2.0 -0.5 to -0.35
Roessler and Faxvog (1980) [151] 515 1.75 -0.5
Bockhorn et al. (1981) [143] 633 1.1 -0.37
Lee and Tien (1981) [142] visible 1.8 to 2.0 -0.65 to -0.45
Charalampopulus and Felske (1987) [144] 488 1.4 to 1.9 0.4 to 0.8
Stagg and Charalampopoulos (1993) [152] 633 1.53 0.38
Koylu and Faeth (1996) [153] 514 1.51 0.48
Mulholland and Choi (1998) [33] -- 1.55 0.8
Snelling et al. (1999) [154] 577 1.59 0.566
Williams et al. (2007) [149] 635 1.75 -1.03
3.6.2 Fractal Dimension and Fractal Prefactor
Mountain and Mulholland [155] studied how light scattering experiments can be used to deduce
the size, the radius of gyration (Rg), the concentration, and the fractal dimension (df) of the
agglomerates. They used Langevin dynamics, a computer simulation technique, to construct
agglomerates, each of which contains primary particles with number varying from 10 to 1000 and
then the scattered light intensity was calculated by Rayleigh-Debye approximation. This method
can relate fractal dimension with integrated intensity and the angular dependence of the scattered
35
light. They found that the fractal dimension lies in the interval between 1.7 and 1.9. They showed
that the radius of gyration (Rg) is a function of the number (N) of primary particles as N =
5.8(Rg/σ)1.9 when selecting fractal dimension as 1.9 and fractal prefactor as 5.8. The value 5.8 for
fractal prefactor was empirically determined.
Meakin [156] simulated diffusion-limited aggregation (DLA) process and indicated that the fractal
dimensions may vary from 1.35 to 1.85. Jullien [157] simulated cluster-cluster aggregation. In
their model, all clusters stuck at their first contact resulting in diffusing and growth. This model
can be used to describe the aggregation of aerosols or colloids. The experimentally estimated value
of fractal dimension (D = 1.49 ± 0.05) agrees well with their calculation value. They also found
that the value of fractal dimension is between 1.44 and 1.78.
Dobbins and Megaridis [158] investigated the absorption, scattering and differential scattering
cross sections for polydisperse aggregates with prescribed fractal dimension. The value they used
for fractal dimension varies from 1.7 to 1.9 for aggregated materials generated by cluster-cluster
aggregation and the value used for fractal prefactor is constant to be 5.8. Dobbins et al. [123] also
proposed that the value of fractal dimension is in the range from 1.7 to 1.9 when aggregates grow
through the processes of cluster-cluster collision.
Puri [128] determined the fractal dimension of aggregates as 1.74 in a coannular ethane diffusion
flame using laser extinction and laser scattering measurements at multiple angles and the value 9.0
of fractal prefactor was obtained approximately by a regression analysis. Koylu [132] inferred
fractal dimension in the fuel-rich region of laminar ethylene flame and the post-flame region of
turbulent ethylene flame as 1.73 and 1.83 respectively based on RDG/PFA scattering theory with
prefactor as 8.5 and refractive index as 1.54 + 0.48i.
Koylu and Faeth [129] investigated soot structure in the overfire (fuel-lean) region of buoyant
turbulent diffusion flames using transmission electron technique. They concluded that fractal
dimensions of aggregates are less dependent on fuel type and vary between 1.70 and 1.79 with the
assumed value of prefactor as 5.8.
It can be found from the above literatures that generally the value of fractal prefactor was assumed
by researchers while the value of fractal dimension can be deduced by combining experimental
data and computer simulations. Table 3. 3 summarizes the values of fractal dimension and fractal
36
prefactor used or determined by different researchers. In this work, a value of 1.75 was used as
fractal dimension based on the review of Sorensen about light scattering by fractal aggregates
[136]. A value of 2.2 was selected as fractal prefactor based on the mean values used by recent
publications [135, 139, 163, 166].
Table 3. 3: Fractal dimension and fractal prefactor of soot used or determined by various
researchers.
Researchers Method Fuel Flame Type
Fractal
Dimension
(Df)
Fractal
Prefactor
(kf)
Gangopadhyay
et al. (1990) [159]
In Situ Light
Scattering
CH4/O2
Premixed Flame 1.6 to 1.8 --
Charalampopoulos
and Chang (1991)
[160]
In Situ Light
Scattering
C3H8/O2
Premixed Flame 1.7 ± 0.08 --
Sorensen et al.
(1992) [161]
In Situ Light
Scattering
CH4/O2
Premixed Flame 1.70 to 1.75 --
Puri et al. (1993)
[128]
In Situ Light
Scattering and
TEM
C2H4
Diffusion Flame 1.74 ± 0.1 --
Koylu et al.
(1995) [162]
In Situ Light
Scattering and
TEM
Diffusion Flame 1.7 ± 0.15 2.4 ± 0.4
Sorensen (2001)
[136]
In Situ Light
Scattering -- 1.75 --
Yang and Koylu
(2005) [163]
In Situ Light
Scattering
C2H4
Diffusion Flame 1.8 2.2
Teng and Koylu
(2006) [139]
In Situ Light
Scattering Laboratory Flame 1.7 ± 0.1 2 ± 0.2
Iyer et al. (2007)
[164]
In Situ Light
Scattering
C2H4
Diffusion Flame 1.74 5.8
Iuliis et sl. (2011)
[165]
In Situ Light
Scattering and
TEM
C2H4
Premixed Flame 1.67 6.34
Snelling et al.
(2011) [166]
In Situ Light
Scattering and
laser-induced
incandescence
C2H4
Diffusion Flame 1.704 1.904
Link et al. (2011)
[135]
In Situ Light
Scattering
Diffusion and
Premixed Flame 1.78 1.94, 2.4
37
3.7 Temperature Measurements
Temperature profiles at different radial and axial positions inside flames were measured using a
rapid thermocouple insertion method. The experimental apparatus shown in Figure 3. 4 consists
of an OMEGA® fine gage R-type uncoated thermocouple with a diameter of 0.075 mm and a
junction diameter of 0.18 mm, two high-temperature ceramic thermocouple insulators with a
diameter of 1 mm. The errors caused by thermal conduction along the wires and catalytic effects
of the thermocouple junction were expected to be insignificant [167, 168]. The thermocouple was
uncoated because of the negligible catalytic effects, and the reason we used a 0.075 mm
thermocouple is that beyond this critical wire diameter, conduction effects will be important [168].
Figure 3. 4: Experimental apparatus of the rapid thermocouple insertion method [58].
Figure 3. 5 shows two possible temperature readings at different positions inside flames. Both of
the curves display the initial response time of the thermocouple to reach the measured temperature.
Curve (a) represents the readings of soot free regions inside flames. The temperature first increases
to the maximum value during the thermocouple response time and then keeps constant. However,
in the soot containing region shown in curve (b), the temperature would decrease after it reaches
to the maximum value because soot deposition on the thermocouple increases heat radiation from
the junction. The temperature decrease includes two stages. First, due to soot deposition, both the
emissivity and the diameter of the junction increase. Second, once the junction is completely
38
covered by soot particles. Further soot deposition only increases the junction diameter while its
emissivity stays constant.
Figure 3. 5: Thermocouple readings at soot free regions and soot containing regions inside
flames [58].
Since soot deposition on the thermocouple would increase the emissivity and the effective diameter
of the thermocouple junction [168], the thermocouple was rapidly inserted into the desired position
inside the flame, held for over two seconds, and then brought to flame front where the temperature
was high to burn off the remaining soot on the thermocouple. Besides, the influence of soot
deposition was corrected by calculating the radiation losses from the surface of the thermocouple
suggested by Shaddix [169]. Radiation losses from the thermocouple junction are considered equal
to the heat transferred from the gas to the thermocouple in steady state condition. Equation (3. 7)
calculates the heat transfer balance of a thermocouple.
𝑇𝑔 = 𝑇𝑚 +휀𝜎(𝑇𝑚
4 − 𝑇𝑊4)𝑑
𝑘𝑁𝑢 (3. 7)
Where Tm is the measured temperature or the thermocouple junction temperature in this case, Tg is
the gas temperature, TW is the ambient temperature (300 K) which heat is radiated to, σ is the
Stefan-Boltzman constant (5.67 ×10−8 W/m2K2), ε is the emissivity of the thermocouple junction,
d is the diameter of the thermocouple junction, k is the thermal conductivity of the gas and Nu is
the Nusselt number.
39
The diameter of the thermocouple junction, the emissivity of the thermocouple junction and the
Nusselt number are three main factors that influence radiation losses. The effect of junction
diameter was minimized by using a thin 0.075 mm thermocouple. The emissivity value of uncoated
thermocouple at different temperatures was obtained from Bradley and Entwistle [170]. The
estimated value 2 was used for the Nusselt number based on the correlation suggested by Shaddix
[169].
3.8 Optics Alignment
Proper optics alignment is one of the main factors that affect the accuracy of data sets. Since it is
difficult for the researcher to distinguish if the deviations of profiles from the expected trends are
caused by improper optics alignment or some other experimental errors, a careful effort should be
made to make sure all of the optics and the coflow diffusion burner are appropriately positioned.
Two factors should be considered when aligning optics: efficiency and position. Efficiency means
that we should consider if the transmitting efficiency meets expectation after laser passes through
each element in an optical system. Position means that we should consider if the laser passes
through the center of a symmetrical system and if the position of focal point satisfies the design
requirement.
Before performing optics alignment, the adjusters of each mount should be checked if they are in
the middle position, which is the balance position. This step is to leave enough allowance to align
the optics. The basic principles for optics alignment are described as following:
1. Sequence principle: Usually, an optical system consists of fixing element and adjustable
element. Fixed elements should be handled first, then determine the alignment sequence of
adjustable elements. For our experimental system, we fixed the position of semiconductor
laser. Our optical system consists of laser extinction part and laser scattering part. We first
aligned the laser extinction part and determined the center position of flames, and then
aligned the laser scattering part.
2. Middle principle: When performing optics alignment, enough space and allowance should
be left. The deviation of the laser beam from the center of all the optics should be as small
as possible.
40
3. Safety principle: To ensure both personal safety and instrumental safety, when aligning
optics, the output power of laser should be set as low as possible to prevent harm to eyes,
as well as the damage to system elements due to improper behaviours.
The detailed optics alignment procedure is described in Appendix C.
3.9 Scattering Calibration
The intensity of scattered light which is measured by PMT is related to the number density of
scatterers (np), the incident laser power (I0), and the size and the shape of the scatterers which are
soot particles in the probe volume for the current study. The output of PMT is a function of the
solid angle (∆Ω), the size of the probe volume (∆V), the quantum efficiency of the PMT (ηpmt), the
efficiency of the optics (ηopt), and the above mentioned parameters.
𝑉𝑠 = 𝐼0𝜂𝑝𝑚𝑡𝜂𝑜𝑝𝑡𝑛𝑝𝐶𝑣𝑣(𝜃)∆𝑉∆Ω (3. 8)
Here, Vs is the measured signal, Cvv(θ) is the differential cross section of a single scatterer in the
direction 𝜃. In equation (3. 8), I0, ηpmt, ηopt, ∆V, and ∆Ω are unknown, these constants are combined
into a single constant C. Through the scattering calibration procedure, C can be determined by
using a scatter whose cross section is already known, such as ethylene and propane. Thus, the
scattering cross section can be determined without obtaining the value of each constant separately.
Equation (3. 8) can be written as:
𝑉𝑠 = 𝐶𝑛𝑝𝐶𝑣𝑣(𝜃) (3. 9)
In this study, we used ethylene and propane as calibration gases, the number density of each gas
can be calculated by the ideal gas equation. Scattering calibration was conducted at room
temperature and pressure. We flowed the ethylene and propane in the fuel tube of the burner. To
ensure that we only detected the light scattered by the calibration gas and the light scattered by the
lip of the burner did not enter the solid angle, the calibration was carried out at the height of 7 mm
above the centerline of the burner. Then we compared the ratio of the signal obtained from propane
to that from ethylene with theoretical value. If the error was within 5%, then the scattering
alignment was considered to be accurate enough [133].
41
3.10 Test Conditions
For the ethylene-air diffusion flame, the air flow rate was set to 42.7 L/min, while the fuel flow
rate was set to 0.231 L/min at 293.15 K, 101.3 kPa.
For the flame of n-dodecane doped with n-propylbenzene, the tested experimental conditions are
shown in Table 3. 4. The flow rates of methane, air and total carbon at inlet were held constant for
all of the four flames. The mole fraction of n-dodecane in fuel stream at inlet was 3% for the base
flame. The mole fractions of added n-propylbenzene in the liquid fuel mixtures varied from 0% to
45%. The main purpose was to distinguish the effects of n-propylbenzene addition on soot
formation pathways under nearly unchanged thermodynamic and chemical environments. The
variations of soot refractive index with soot maturity in flames and the influences of PAH
absorption on extinction measurements were not considered in the current study. These
experimental uncertainties should be similar for the four flames.
Table 3. 4: Experimental test conditions.
Liquid fuel Molecular fraction CH4 flow
rate (L/min)a
Air flow
rate (L/min)a
Liquid fuel
flow rate (g/h) n-C9H12 n-C12H26
Pure n-dodecane 0% 100% 0.32 60 4
15 mol. % n-propylbenzene 15% 85% 0.32 60 3.97
30 mol. % n-propylbenzene 30% 70% 0.32 60 3.94
45 mol. % n-propylbenzene 45% 55% 0.32 60 3.91 a 293.15 K, 101.3 kPa.
3.11 Uncertainty Analysis
The uncertainty in this work was calculated based on the Root-Sum-Squares (RSS) method,
combining with the error propagation equation [171]. In the RSS method, the uncertainty of a
result is affected by the interaction of each individual error with the other errors. This method
assumes that the square of an uncertainty is a measure of the variance assigned to an error. And
these variances propagate to produce a probable estimate of the final result uncertainty. The final
uncertainty is reported to be at 95% probability level. If the measured variable Y is in a relationship
with several dependent variables (X1, X2, …, Xn), represented in equation (3. 10), and each
individual error is stated as 𝛿n, where n = 1, 2, …, n, then the total uncertainty of Y can be
represented in equation (3. 11).
Y = f (X1, X2, …, Xn) (3. 10)
42
𝛿𝑌 = √𝛿12 + 𝛿2
2+. . . +𝛿𝑛2 (3. 11)
In the error propagation method, if a result Y is determined by one variable X, then the uncertainty
of Y is related to the uncertainty of X by
𝛿𝑌 = (𝑑𝑌
𝑑𝑋)
𝑋 =
𝛿𝑋 (3. 12)
Extend this idea to a result which is effected by several variables. The uncertainty propagation
from the variables to the final result can be represented by
𝛿𝑌 = ∑ (𝜕𝑌
𝜕𝑋𝑖𝛿𝑋𝑖
)2𝑛
𝑖 = 1
1/2
(3. 13)
As a result, the estimate of the true value Y would be as:
Y = 𝑌 ± 𝛿𝑌 (3. 14)
where is the value determined using the measured values (𝑋1 , 𝑋2 , …, 𝑋𝑛 ) according to equation
(3. 10). In this study, we used the RSS method for the uncertainty analysis.
Take error calculation of soot volume fraction for example, soot volume fraction was calculated
using the following equation:
𝑓𝑣 =𝐾𝑒,𝜆𝜆
6𝜋(1 + 𝜌𝑠,𝑎)𝐸() (3. 15)
Where Ke,λ is the local extinction coefficient, ρs,a is the ratio of scattering to absorption, λ is the
laser wavelength, and E() is a function of the soot complex refractive index . The uncertainty
of soot volume fraction was analyzed based on the uncertainty of the local extinction coefficient,
the ratio of scattering to absorption, the laser wavelength and the function of soot complex
refractive index. Thus, the uncertainty for soot volume fraction can be calculated by
𝛿𝑓𝑣= (
𝜕𝑓𝑣
𝜕𝐾𝑒𝑥𝑡𝛿𝐾𝑒𝑥𝑡
)2
+ (𝜕𝑓𝑣
𝜕𝜆𝛿𝜆)
2
+ (𝜕𝑓𝑣
𝜕𝜌𝑠𝑎𝛿𝜌𝑠𝑎
)2
+ (𝜕𝑓𝑣
𝜕𝐸(𝑚)𝛿𝐸(𝑚))
2
1/2
(3. 16)
43
= (𝜆
6𝜋(1 + 𝜌𝑠𝑎)𝐸(𝑚)𝛿𝐾𝑒𝑥𝑡
)2
+ (𝐾𝑒𝑥𝑡
6𝜋(1 + 𝜌𝑠𝑎)𝐸(𝑚)𝛿𝜆)
2
+ (−6𝜋𝐾𝑒𝑥𝑡𝜆𝐸(𝑚)
(6𝜋(1 + 𝜌𝑠𝑎)𝐸(𝑚))2𝛿𝜌𝑠𝑎
)2
+ (−𝐾𝑒𝑥𝑡𝜆
6𝜋(1 + 𝜌𝑠𝑎)𝐸(𝑚)2𝛿𝐸(𝑚))
2
1/2
The uncertainty for the laser wavelength was ±3 nm. The average ratio of scattering to extinction
was 10% calculated based on the Rayleigh-Gans-Debye scattering theory with an error of 20%.
16% uncertainty was considered for extinction coefficient. The variation of refractive index, which
comes from different soot properties in the soot formation process, was considered in the current
error calculation as it affects the accuracy of experimental measurements [172]. An error of 15%
was chosen for the function of the refractive index. Table 3. 5 shows the values and errors for each
component.
Table 3. 5: Values and errors of each component of soot volume fraction.
Component λ Ke,λ ρs,a E(m)
Value 639 nm - 10% 0.373
Error 3 nm 16% 20% 15%
The errors for temperature, primary particle diameter and primary particle number density are also
estimated based on the RSS method. For temperature measurements, the sources of errors and their
corresponding values are listed in Table 3. 6. An error of ±50 K is added for the impact of the
radiative heat transfer between the thermocouple junction and the luminous flame zone [173]. The
coating on the thermocouple junction increases the diameter, emissivity, and the response time of
the junction [174, 175], and catalytic effects are found to be minimal when making temperature
measurements, however, ±30 K is considered for the error caused by the catalytic effects in the
current study based on [167]. The effect of soot deposition on the junction is evaluated as ±10 K
[176]. By comparing the results of a 0.075 mm thermocouple and a more thinner but fragile 0.050
mm thermocouple, ±10 K is obtained for the conductive heat transfer effect along the 0.075 mm
thermocouple [168].
Table 3. 6: Sources and values of errors for temperature measurements.
Component Radiation
Correction
Catalytic
Effects
Soot
Deposition
Conductive
Heat Transfer Precision
Error ±50 K ±30 K ±10 K ±10 K ±5%
44
Chapter 4
Results and Discussion
4.1 Validation of Experimental Apparatus
We used a non-smoking ethylene-air laminar coflow diffusion flame to validate the experimental
apparatus. The LE and two-angle ELS measurements were conducted at three different heights
above burner (HAB): HAB = 30, 40, and 50 mm. The flow rates of air and ethylene were set to the
same values used by Santoro et al. [118]. The radial profiles for both laser extinction and scattering
tests were obtained from R = -4 mm to R = +4 mm. Results were compared with the profiles from
Santoro et al. [118]. This reference was selected because they presented complete and reliable
results, including raw data, for both laser extinction and scattering measurements. The flame was
found to be steady and repeatable between multiple tests on different days.
4.1.1 Validation of Laser Extinction Apparatus
Since the measured transmittance (Iλ,/Iλ,0) is not influenced by the assumed parameters in the post
processing code, such as soot refractive index, transmittance profiles were used to validate the
experimental apparatus of laser extinction technique. Figure 4. 1 shows the measured transmittance
profiles as a function of radius for ethylene-air diffusion flame compared with those of Santoro et
al. [118]. The radial position refers to the distance from the centerline of the flame to the chord
that laser beam passes through. Measured transmittance is the integrated value of the local
extinction coefficient along the optical path through the flame. These integrated values were then
inverted to the local values to calculate the extinction coefficient by three-point Abel inversion
technique [177]. It is promising that both data sets display a similar trend. For both trends,
transmittance is much higher at HAB = 30 mm compared to HAB = 40 mm, 50 mm. Our results
appear to be larger than those of Santoro et al. This is probably due to systematic errors, for
example, the laser, the burner and the optical system we used are different from those used by
Santoro et al. [118]. The higher laser wavelength (639 nm) we used compared with the one (514.5
nm) used by Santoro et al. [118] is the main factor causing the differences. According to equation
(3. 6), if the laser wavelength is higher, the local extinction coefficient will be lower when the
same flame region is studied. According to equation (3. 2), a lower local extinction coefficient will
be accompanied by a higher transmittance for the same flame length that laser passes through.
45
Figure 4. 1: Transmittance profiles at different heights above burner (HAB) compared with those
from Santoro et al. [118].
Figure 4. 2 shows the soot volume fraction values of different heights above burner (HAB) at the
centerline of ethylene-air diffusion flame compared with those of Santoro et al. [178]. It seems that
46
our results are higher than those of Santoro et al., but the differences are within error bars.
Figure 4. 2: Soot volume fraction values of different heights above burner (HAB) at the
centerline of ethylene-air diffusion flame compared with those from Santoro et al. [178].
4.1.2 Validation of Two-angle Elastic Laser Scattering Apparatus
Since scattering cross section (Qvv) is not influenced by the assumed parameters in the post
processing code, such as fractal dimension, scattering cross section profiles were used to validate
the experimental apparatus of two-angle elastic laser scattering technique. Figure 4. 3 shows
scattering cross section (Qvv) profiles at 30° and 150° as a function of radius compared with those
of Santoro et al. at 90°. All of the data sets display a much lower scattering cross section in the
central region compared with the annular region, which is close to the edge of the yellow luminous
zone of the flame. The differences of Qvv between the central region and the annular region
decrease as the height above burner increases from 30 mm to 50 mm. The differences between the
current results and those of Santoro et al. [118] become smaller as height above burner increases.
This is probably because the influence of the different heat capacities of fuel tubes becomes less
as the regions of flame go further from the exit of the fuel tube. The material of fuel tube we used
is stainless steel, which is different from the brass fuel tube Santoro et al. used. The heat capacity
of stainless steel is 510 J/kgK, while that of brass is 370 J/kgK at 298 K [179]. Different heat
capacities of fuel tube would cause differences in flame temperatures, thus influence soot
formation process.
47
Figure 4. 3: Scattering cross section (30° and 150°) profiles at different heights above burner
(HAB) compared with those from Santoro et al. [118].
Figure 4. 4 shows the values of primary particle diameter, primary particle number density,
aggregate number density, average number of primary particles per aggregate of different heights
above burner (HAB) at the centerline of ethylene-air diffusion flame obtained from current study
48
compared with those from literatures. All of the differences between our results and those from
literatures are within error bars.
Continued on next page
49
Continued from previous page
Figure 4. 4: Values of primary particle diameter, primary particle number density, aggregate
number density, average number of primary particles per aggregate of different heights above
burner (HAB) at the centerline of ethylene-air diffusion flame compared with those from
literatures [2, 128 178, 180, 181].
These comparison results confirm that the current experimental apparatus for both LE and two-
angle ELS measurements is able to characterize soot generated in the laminar coflow diffusion
flames.
4.2 Investigation of the Effects of n-Propylbenzene Addition on
Soot Formation in an n-Dodecane Laminar Coflow Diffusion
Flame
This section discusses the influence of n-propylbenzene on soot formation in an n-dodecane
laminar coflow diffusion flame at atmospheric-pressure. n-Dodecane laminar coflow diffusion
flame was selected to provide the base flame environment. As was mentioned in chapter 1 and 2,
n-dodecane is usually considered as a main surrogate component which can describe alkanes for
jet fuels. Adding aromatic component to n-dodecane can emulate entire fuel properties for different
real fuels [107], and thus makes the current study more relevant to practical combustion conditions.
50
4.2.1 Flame Descriptions
Figure 4. 5 shows each of the four flames studied to investigate the effects of n-propylbenzene
addition. Each of the flames was approximately 90-95 mm high, with pure n-dodecane being the
shortest. Besides, with the increasing mole fraction of added n-propylbenzene, the visible flame
became longer, and the luminous area of each flame slightly increased and moved closer to the
fuel tube exit of the burner. This indicates that soot inception happens earlier and the time required
to oxidize soot is longer as the mole fraction of n-propylbenzene increases.
Figure 4. 5: Visible flame images for the four levels of n-propylbenzene addition.
4.2.2 Soot Volume Fraction Profiles
The radial profiles of soot volume fraction for all of the four diffusion flames at different heights
above burner (HAB) are compared in Figure 4. 6. The results of pure n-dodecane at HAB = 80
mm are not presented here because its soot volume level falls below the limit of the detector in the
LE measurements. The separate profiles of each flame with error bars are presented in Appendix
D. The peak soot volume fractions at each height above burner initially occur in the annular region
and then move to the central region as the heights above burner increase. Soot generated in the
four flames all begins to oxidize after it reaches its maximum value at HAB = 60 mm. The datasets
of all of the four flames display similar shape and trends, which suggests that the addition of n-
propylbenzene does not influence the overall structure of the base n-dodecane flame. As can be
seen from Figure 4. 6, soot volume fraction increases as n-propylbenzene mole fraction in the
51
liquid fuel mixture increases at all heights, which meets our expectation. As discussed in chapter
1 and chapter 2, aromatic ring can stay intact and avoid the slow process of first ring formation in
the combustion of aromatics [71]. Soot generated in flames of aromatic hydrocarbon fuels is much
more than that generated in flames of non-aromatic hydrocarbon fuels [3]. The discrepancy of soot
volume fraction becomes larger as the mole fraction of n-propylbenzene increases, which indicates
a non-linear relationship between the production of soot and the mole fraction of n-propylbenzene
in the n-dodecane base flame. This may be due to the nonlinear influence of concentrations of soot
precursors on soot formation rate. A non-linear relationship was found between soot increase and
the quantity of carbon added from m-xylene in a previous study [182].
Figure 4. 6: Soot volume fraction profiles at different flame heights (HAB) of the four flames
studied.
52
Figure 4. 7 shows soot volume fraction along the centerline and the locations of peak soot
concentration on the wing of the four studied flames, respectively. Error bars are only presented
for the results of n-dodecane doped with 45 mol. % n-propylbenzene to make the figures more
readable. Results along the locations of peak soot concentration on the wing are shown until 50
mm, because the peak radial soot volume fraction moves to the central region from this height.
The laser extinction diagnostics with a laser wavelength of 639 nm used in the current study can
only measure the volume fraction of mature soot [183]. Therefore, results along the centerline are
shown from 50 mm, since most of the soot particles are probably to be fully carbonized after this
height. Again the results of pure n-dodecane at HAB = 80 mm are not shown because the soot
volume falls below the detector limit. As is shown in Figure 4. 7, soot volume fraction increases
along both of the flame wing and the centerline as the mole fraction of n-propylbenzene increases.
Figure 4. 7: Soot volume fraction profiles along the centerline and the locations of peak soot
concentration of the four flames studied.
53
4.2.3 Primary Particle Diameter and Number Density Profiles
Figure 4. 8 displays the comparison profiles of primary particle diameter and primary particle
number density along the centerline and the locations of peak soot concentration on the wing of
the four studied flames, respectively. The primary particle diameter here refers to an equivalent
optical diameter, which considers a primary particle as an equivalent sphere having the same
optical properties. Due to the same reason mentioned in the previous section, error bars are only
included for the results of n-dodecane doped with 45 mol. % n-propylbenzene, results along the
wing of flames are shown until 50 mm and results along the centerline of flames are shown from
50 mm.
As is shown in Figure 4. 8 (a1) and (a2), as the n-propylbenzene mole fraction increases, the
primary particle diameter increases while the primary particle number density does not display any
significant change, implying that it is primarily the higher surface growth rate that results in the
higher soot formation along the wing. From Figure 4. 8 (b1) and (b2), along the centerline, both
of the primary particle diameter and number density display obvious increase as the mole fraction
of n-propylbenzene increases, which indicates that the increase in soot formation along the
centerline was caused by the combined effect of increase in soot inception and surface growth rate.
Besides, the central region of flames is typically a low temperature fuel rich region [184] and along
the centerline, with small addition of aromatics, the concentration of naphthalene increases while
the concentration of acetylene almost keeps unaltered [185], thus it can be inferred that the
structure of n-propylbenzene can be maintained instead of decomposing into smaller hydrocarbons
along the centerline of the studied flames. These maintained aromatic structure will increase the
amount of soot precursors.
54
Figure 4. 8: Primary particle diameter and number density profiles along the centerline and the
locations of peak soot concentration of the four flames studied.
Table 4. 1 and Table 4. 2 shows the change in ratios of the cube of primary particle diameter (dp3)
for different liquid fuel mixtures from HAB = 50 mm to HAB = 70 mm on the flame centerline
and from HAB = 30 mm to HAB = 50 mm on the flame wing, respectively. Table 4. 3 and Table
4. 4 shows the change in ratios of primary particle number density (Np) for different liquid fuel
mixtures from HAB = 50 mm to HAB = 70 mm on the flame centerline and from HAB = 30 mm
to HAB = 50 mm on the flame wing, respectively. Table 4. 1 and Table 4. 2 further show that the
primary particle diameter increases with increasing n-propylbenzene mole fraction along both the
centerline and the wing of flames. Table 4. 3 and Table 4. 4 further display that the differences in
primary particle number density along the wing of the flames are insignificant, while along the
centerline of the flames, the differences in primary particle number density are more obvious with
different n-propylbenzene addition. As can be seen in Table 4. 1 and Table 4. 3, from HAB = 50
mm to HAB = 55 mm along the centerline, the ratios of dp3 increase while the ratios of Np decrease
for all three levels of n-propylbenzene addition, which indicates that the soot coalescence rate is
higher than soot inception rate in the flame regions from HAB = 50 mm to HAB = 55 mm. High
55
level of coalescence increases the primary particle diameter and decreases the primary particle
number density.
Table 4. 1: Ratios of dp3 from HAB = 50 mm to HAB = 70 mm on the flame centerline.
HAB (mm) 50 55 60 65 70
15 % n-propylbenzene / Pure n-dodecane 1.0582 1.2697 1.1894 1.1166 1.2694
30 % n-propylbenzene / Pure n-dodecane 1.2059 1.6460 1.5047 1.3993 1.8423
45 % n-propylbenzene / Pure n-dodecane 1.4417 1.7896 1.8815 1.8843 2.2590
Table 4. 2: Ratios of dp3 from HAB = 30 mm to HAB = 50 mm on the flame wing.
HAB (mm) 30 35 40 45 50
15 % n-propylbenzene / Pure n-dodecane 1.0498 1.1643 1.1750 1.3469 1.3450
30 % n-propylbenzene / Pure n-dodecane 1.3235 1.4368 1.4076 1.6386 1.8050
45 % n-propylbenzene / Pure n-dodecane 1.5050 1.8087 1.8594 2.0247 2.2105
Table 4. 3: Ratios of Np from HAB = 50 mm to HAB = 70 mm on the flame centerline.
HAB (mm) 50 55 60 65 70
15 % n-propylbenzene / Pure n-dodecane 1.4305 1.0951 1.1013 1.0748 1.1195
30 % n-propylbenzene / Pure n-dodecane 1.7284 1.3316 1.1895 1.2400 1.2182
45 % n-propylbenzene / Pure n-dodecane 1.9577 1.4869 1.6012 1.6993 1.5614
Table 4. 4: Ratios of Np from HAB = 30 mm to HAB = 50 mm on the flame wing.
HAB (mm) 30 35 40 45 50
15 % n-propylbenzene / Pure n-dodecane 0.9686 0.9760 1.0097 1.0096 1.0651
30 % n-propylbenzene / Pure n-dodecane 1.0350 1.0078 1.0115 1.0560 1.0895
45 % n-propylbenzene / Pure n-dodecane 1.0709 0.9710 1.0742 1.0920 1.1985
56
4.2.4 Temperature Profiles
4.2.4.1 Comparison Among Different Liquid Fuel Mixtures
Temperature has a strong influence on soot formation process. To determine temperature effects
on sooting tendency of the four different fuels studied, temperature measurements were performed
at different radial and axial positions inside flames.
Figure 4. 9 shows comparison of temperature profiles of the four studied diffusion flames at
different heights above burner (HAB). The separate profiles of each flame with errors are shown
in Appendix E. As can be seen, there is no significant difference among the four temperature
profiles at each height above burner (HAB). The peak temperatures of all the four profiles move
from the annular regions of flames to the central regions of flames as the flame heights increase
and first increase to the maximum value with increasing flame heights until approximately HAB
= 40 - 50 mm and then decrease as flame heights further increase. The temperature profiles of pure
n-dodecane flame are the highest, which can be explained by the radiation effect of soot and the
relatively less soot in the pure n-dodecane flame compared with the other three flames with n-
propylbenzene addition. The differences in the radial positions of peak temperatures among the
four profiles can be attributed to the positioning errors (±0.4 mm) [58]. The temperature
uncertainties can reach to ±130 K in the current study by considering radiation effects, catalytic
effects, soot deposition, conductive heat transfer effects and the experimental precision. Generally,
all of the temperature profiles display similar trends, which indicates that temperature effects on
soot formation in different flames can be neglected and the differences in soot volume fraction
profiles, soot particle diameter profiles and number density profiles are caused by different fuel
compositions.
58
4.2.4.2 Comparison Between Different Techniques
The temperature profiles of pure n-dodecane at HAB = 50 mm, 60 mm measured using rapid
thermocouple insertion technique were compared with those obtained by soot spectral emission
(SSE) technique (Courtesy of Carson Chu), as is shown in Figure 4. 10. Since soot spectral
emission technique cannot provide accurate temperature results when soot volume is low, the
radial distances from the centerline position are only presented until 4 mm for HAB = 50 mm and
until 3 mm for HAB = 60 mm.
Figure 4. 10: Comparisons of temperature profiles of pure n-dodecane laminar coflow diffusion
flame at HAB = 50 mm, 60 mm obtained by rapid thermocouple insertion technique and by soot
spectral emission (SSE) technique.
The differences between rapid thermocouple insertion technique and soot spectral emission
technique decrease as the flame region moves from the centerline to the wing at HAB = 50 mm,
60 mm. Thermocouple readings of pure n-dodecane at HAB = 50 mm are presented at the
centerline position and the position of peak temperature on the wing in Figure 4. 11. As has been
discussed in Chapter 3, curve (a) of Figure 4. 11 represents the reading of soot-containing region
inside flames, which is the centerline position for HAB = 50 mm of pure n-dodecane laminar
coflow diffusion flame, and curve (b) of Figure 4. 11 represents the reading of soot-free region
inside flames, which belongs to radial position = 3.2 mm, HAB = 50 mm of pure n-dodecane
laminar diffusion flame. The larger differences at the centerline position between the two
techniques are attributed to soot deposition on the thermocouple, which increases the heat radiation
59
errors of rapid thermocouple insertion technique. The gas temperatures measured by rapid
thermocouple insertion technique have found to be much lower along the centerline of soot-
containing regions of flames by a previous study [186]. Boedeker and Dobbs [186] measured gas
temperatures of ethylene coflow flames using CARS and compared their results with Kent and
Wagner’s rapid thermocouple insertion data [187]. They found that maximum temperatures agree
well over the entire length of the flame, but along the centerline, the results from CARS can be
200 K higher than that obtained by rapid insertion technique, and they attributed this to the soot
deposition on the thermocouple in the measurements.
Figure 4. 11: Thermocouple readings at the centerline position and peak value position for HAB
= 50 mm of pure n-dodecane laminar coflow diffusion flame.
60
Chapter 5
Conclusions and Recommendations
5.1 Conclusions
The current study validated the experimental apparatus for combined LE and two-angle ELS
diagnostics to study soot formation in laminar coflow diffusion flames. Multiple radial
measurements were made at three heights above burner, HAB = 30, 40, and 50 mm of a non-
smoking ethylene air laminar coflow diffusion flame. Both the results of LE measurements and
the results of two-angle ELS display similar trends and shape with those from the literatures and
the differences are within error bars. The comparison profiles prove that the current experimental
apparatus is capable to characterize soot formation in laminar coflow diffusion flames.
The effects of n-propylbenzene addition in an n-dodecane laminar diffusion flame were
investigated by measuring soot volume fractions, primary particle diameters, primary particle
number densities using the validated experimental apparatus, and measuring temperatures using
the rapid thermocouple insertion method. To distinguish the effects of n-propylbenzene, n-
dodecane laminar coflow diffusion flame established the base flame environment with different
addition of n-propylbenzene from 0 mol.% to 45 mol.% in the liquid fuel mixtures. The total inlet
carbon flow rate was kept constant.
Generally, the soot volume fraction profiles, primary particle diameter profiles, primary particle
number density profiles, and flame temperature profiles are all very similar for all of the four
flames, which confirms that the overall flame structure is not affected by the different additions of
n-propylbenzene. Because of the similar flame temperature profiles, we can consider the
differences in soot volume fraction profiles, primary particle diameter profiles, primary particle
number density profiles are caused by different fuel compositions instead of different flame
temperatures.
The sooting tendency increases at all measured flame heights as mole fraction of n-propylbenzene
in the liquid fuel mixtures increases. The differences of sooting tendency become larger with
increasing mole fraction of n-propylbenzene, indicating a non-linear relationship between the
increase of soot formation and the increase of mole fractions of n-propylbenzene in the fuel mixture.
61
By comparing the profiles of primary particle diameters and primary particle number densities, we
found out that along the wing of flames, the increase of sooting tendency is primarily caused by a
higher surface growth rate, while along the centerline of flames, it is the combined effect of the
higher soot inception and soot surface growth rates that results in the higher sooting tendency.
5.2 Recommendations
The current section proposes several recommendations for the future experiments.
1. The current laser with a wavelength of 639 nm can only be used to measure the soot volume
fraction of mature soot particles, which limits the ability to detect soot at low flame heights
and leads to the lower soot volume fraction results compared to the practical conditions. A
UV laser which can detect the transparent soot particles as well as PAHs can be combined
with the current laser to improve the accuracy of our results.
2. For some optical mounts used in the current study such as mirror mounts, the adjusters
cannot be locked once the optics are aligned. Thus the desired position can be changed.
Since the laser scattering signal is several orders of magnitude lower than the laser
extinction signal, even a slight move of the adjusters would result in a big difference in the
scattering signal. In the future, it is recommended by the author to replace the current
mounts to the ones on which the adjusters can be locked.
3. A shorter heated tube to transfer the vaporized fuel would improve the flame stability due
to a lower chance of flame flicker.
4. In the current work, the Three-Point Abel Inversion method was used to calculate the local
extinction coefficient from the measured signals. The results obtained by this method
display large errors in the central region of flames. Using the Tikhonov regularization
method to process the data can provide more accurate results [188].
62
Attributions
The experimental apparatus of combined laser extinction and two-angle elastic laser scattering
diagnostics was validated by the author and Tongfeng Zhang together. The laser extinction
measurements, two-angle elastic laser scattering measurements, and temperature measurements
were conducted by the author and Tongfeng Zhang together. The author designed the fuel and
oxidizer delivery system and upgraded the setup of rapid thermocouple insertion technique.
Tongfeng Zhang designed the optical system and developed the improved data analysis to calculate
soot aggregate properties based on Rayleigh-Debye-Gans Fractal Aggregate (RDG/FA) theory.
Jason Weingarten developed the LabVIEW code to collect data.
63
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78
Appendices
Appendix A MATLAB Code
A.1 MATLAB Code for Soot Volume Fraction
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
% Project: Laser Extinction Diagnostics Post-Processing
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
close all;
clc;
clear;
V1=csvread('main.csv');
V2=csvread('reference.csv');
I = Vcalc(V1);
I0 = Vcalc(V2);
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
I0corrected = I(1).*I0(1:end)./I0(1);
startpoint = 2;
I = I(startpoint:end);
I0corrected = I0corrected(startpoint:end);
ratio = I./I0corrected;
for i = 1:length(ratio)
if ratio(i) > 1
ratio(i) = 1;
end
end
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
[F,D,I,X,Fv]=main(ratio',0.2);
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
x = 0:0.2:(size(Fv,1)-1)*0.2;
figure;
hold all;
for j = 1:length(Fv);
if Fv(j) < 0;
Fv(j) = 0;
end
end
79
plot(x,Fv*10^6,'','LineWidth',4)
Fv = smooth(Fv,9,'loess');
axis([0 5 0 5]);
xlabel('Radial Position (mm)','FontSize',14);
ylabel('Soot Volumn Fraction (ppm)','FontSize',14);
set(gca,'FontSize',14);
Fraction = Fv*10^6;
xlswrite('Surrotate_40_1_Jul9.xls',Fraction);
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
function [I] = Vcalc(V)
for i=1:length(V)
%looks for when ambient light is read (2x drop in voltage)
if(((max(V)-V(i))/V(i))>1.5)
drop=i;
break;
end
end
V1=V(1:drop-1);
Lum=V(length(V1)+1:end);
for i=1:size(V1,1)
if (mod(i,120)==0)
I(i/120)=sum(V1(i-119:i))/120;
end
end
end
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
function [F,D,I,X,Fv]=main(II0,l)
i_i0=-log(II0);
x=0:l:l*(max(size(i_i0))-1);
[F,D,Fv]=Inversion(i_i0,x,l);
I=i_i0;
X=x;
End
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
80
function [F,D,Fv]=Inversion(I_I0,x,l)
i=0:max(size(x));
j=0:max(size(x));
I=i+1;
J=j+1;
for i=0:max(size(x))-1
for j=0:max(size(x))-1
if (j<(i-1))
D(i+1,j+1)=0;
else if ((i-1)==j)
D(i+1,j+1)=I0(i,j+1)-I1(i,j+1);
else if (i==j)
D(i+1,j+1)=I0(i,j+1)-I1(i,j+1)+2*I1(i,j);
else if ((i+1)<=j)
D(i+1,j+1)=I0(i,j+1)-I1(i,j+1)+2*I1(i,j)-
I0(i,j-1)-I1(i,j-1);
if (i==0 & j==1)
D(i+1,j+1)=I0(i,j+1)-
I1(i,j+1)+2*I1(i,j)-2*I1(i,j-1);
end
end
end
end
end
end
end
F=(1/(l*.001))*(D*I_I0);
m=1.75-1.03i;%1.57-.56i;
Fv=632.8e-9*F/(6*pi*(-imag((m^2-1)/(m^2+2))));
end
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
function Izero=I0(i,j)
if ((i==0 & j==0) | (j<i))
Izero=0;
end
if ((i==j) & (j~=0))
Izero=(1/(2*pi))*log((sqrt((2*j+1)^2-4*i^2)+2*j+1)/(2*j));
end
if (i<j)
Izero=(1/(2*pi))*log((sqrt((2*j+1)^2-
4*i^2)+2*j+1)/(sqrt((2*j-1)^2-4*i^2)+2*j-1));
end
end
81
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
function Ione=I1(i,j)
if (j<i)
Ione=0;
else if (i==j)
Ione=(1/(2*pi))*sqrt((2*j+1)^2-(2*i)^2)-2*j*I0(i,j);
else
Ione=(1/(2*pi))*(sqrt((2*j+1)^2-(2*i)^2)-sqrt((2*j-1)^2-
(2*i)^2))-2*j*I0(i,j);
end
end
end
A.2 MATLAB Code for Temperature
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
% Project: Thermocouple Temperature Post-Processing
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%Start from scratch
clear all
close all;
clc;
%original junction diameter
d_wire = 0.000075;
d_junction_orig = 0.000185;
v_junction_orig = (4/3)*pi*(d_junction_orig/2)^3;
%Variable alpha approximation
alpha_point1 = 0.000009;
alpha_t1 = 298;
alpha_point2 = 0.000009917;
alpha_t2 = 698;
%find the slope of alpha
alphaslope = (alpha_point2-alpha_point1)/(alpha_t2-alpha_t1);
%Ask user for file
file = uigetfile('.csv');
%Import file data
T = csvread(file);
%DAQ Settings- Ensure these match the collections settings
SampleRate = 100; %Hz
SampleTime = 2; %seconds
SamplesPerInsert = SampleRate * SampleTime;
82
j=1; %Counter
i=0; %Sample Counter
n=1; %Counter in array k
x=2; %Position in flame, starting position from centerline
NumAve = 26; %Number of points used in the average calculation
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
Devfwd = 20;
Devbk = 5;
extra = 0;
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%Creates an array that stores all the T array positions for the
%maximum temp values
MaxPosition = zeros((size(T,1))/200);
Tj = zeros((size(T,1))/200);
Tmax = zeros((size(T,1))/200);
displacement = 0.2; % the dist the stage was programmed to move
each time
CtoK = 273.15; %Convert degC to degK
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
Te=[900
1000
1100
1200
1300
1400
1450
];
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
emis=[0.1723
0.1837
0.1937
0.2032
0.2122
0.2206
0.2243
];
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
83
Tk=[800
900
1000
];
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
k=[57.25
62.54
67.68
];
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
E=polyfit(Te,emis,1);% fits a linear curve to emis data
K=polyfit(Tk,k,1);% fits a linear curve to k data
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
% Calculates the size of the whole data file and loops through it
for i=1:SamplesPerInsert:size(T,1)
Tmax(j)=max(T(i:i+SamplesPerInsert-1))+CtoK;
for m = i:(i + SamplesPerInsert)
if floor(T(m,1))== floor((Tmax(j)-CtoK))
MaxPosition(n) = m;
break
end
end
if i < SamplesPerInsert
if Devbk > MaxPosition(n)
extra = Devbk - MaxPosition(n) + 1;
end
Tj(j) = ((sum(T(((MaxPosition(n))-
Devbk+extra):((MaxPosition(n))+Devfwd+extra)))) / NumAve) + CtoK;
else
if Devbk > (MaxPosition(n) - i)
extra = Devfwd - MaxPosition(n) - i + 1;
end
Tj(j) = ((sum(T(((MaxPosition(n))-
Devbk+extra):((MaxPosition(n))+Devfwd+extra)))) / NumAve) + CtoK;
end
84
%Correction
alpha(j) = alpha_point1 + alphaslope*(Tj(j)-
alpha_t1); %Find local alpha
deltaV(j) = 3*alpha(j)*v_junction_orig*(Tj(j)-
298); %Find delta volume
V_2 (j) =
v_junction_orig+deltaV(j); %Find new volume
d_junction(j) =
2*(((3/4)*V_2(j)/pi)^(1/3)); %Find new bead diameter
%now run correction with updated diameter
Tg1(j)=Tj(j)+polyval(E,Tj(j))*0.0000000567*d_junction(j)*(Tj(j)^4
-296^4)/(.001*2*polyval(K,Tj(j)));
Tg(j)=Tj(j)+polyval(E,Tj(j))*0.0000000567*d_junction(j)*(Tj(j)^4-
296^4)/(.001*2*polyval(K,Tg1(j)));
%Increments n to store next max temp in MaxPosition
n = n + 1;
j = j + 1;
end
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
x=0 : displacement : displacement*(size(Tg,2)-1);
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
hold all
plot(x,Tg,'LineWidth',2)
axis([0 10 0 3000])
xlabel('R (mm)')
ylabel('T (K)')
Data_to_file = transpose(Tg);
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
xlsFileName = input('Enter the name for the xls file: ', 's');
xlswrite(xlsFileName,Data_to_file);
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
85
Appendix B Procedure of Calculating Soot Properties
Figure B. 1: Procedure of calculating soot properties for combined laser extinction (LE) and two
angle elastic laser scattering (ELS) experiments [137].
86
Appendix C Optics Alignment
The optics used in our setup are listed below (in sequence starting from the first optics after the
semiconductor laser):
1. Periscope;
2. Mirror-1;
3. Lens-1;
4. Lens-2;
5. Polarizer;
6. Beamsplitter;
7. Aperture-1;
8. Mirror-2;
9. Lens-3;
10. Aperture-2;
11. Lens-4;
12. Aperture-3;
13. Lens-5;
14. Integrating sphere;
15. Photodetector;
Optics alignment in our setup started from periscope. The steps for optics alignment of laser
extinction part are as following:
1. Place the periscope at the right position according to the exit position of laser to make sure
laser arrived at the center of the lower mirror. At the same time, rotate the upper and lower
housing to make sure the direction of laser light was changed as desired.
2. Position lens-1and lens-2 according to the requirement of focal length. According to the
design requirement, the laser comes out of lens-2 should be a collimated beam with larger
beam diameter compared to the laser beam that comes into lens-1. Lens-1 is concave lens
and lens-2 is convex lens. These two lenses constitute laser beam expanders. The lens
mounts were adjusted so that laser passed through the center of the lenses.
87
3. Align the polarizer until laser passed through its center. The plate on the polarizer was
rotated roughly to allow the passing of vertically polarized laser. More accurate
polarization would be conducted later by slightly adjusting the adjusters on the polarizer
mount while observing the signal change in LabVIEW program.
4. Determine the right mounting position of the beam splitter, then adjust the position of the
beamsplitter until laser pointed the center of the input face of beamsplitter, and came out
of the beamsplitter from the center of both output faces.
5. Position aperture-1, mirror-2, lens-3, aperture-2, and lens tubes to make laser beam pass
through the center of these elements.
6. Adjust the distance between lens-3 and the center of the burner, the distance between lens-
4 and the center of the burner until the design requirement was satisfied;
7. Adjust the distance between lens-4 and the integrating sphere so that the focal point of lens-
4 located inside the integrating sphere, and then make laser beam pass through the center
of these elements.
8. Turn on detectors, use LabVIEW program to check the signal of each detector, and use the
adjusters on the mount to make sure the desired signal was obtained.
During alignment, we used alignment plates-LMR1AP from Thorlabs to observe the change of
laser beam to determine the proper position of the optics. When aligning the optics, pay attention
not to get the optics dirty. The dust on the optics will damage the surface of optics and can scatter
light. And do not touch the coatings of optics, only handle the optics by edges with gloves. If the
optics get dirty, clean the optics immediately. Because if the dust is collected on the optics, the
surface of optics will be scratched during cleaning.
For the alignment of the scattering part, since the scattering signal was collected at 30° and 150°
to the incident beam, firstly the two scattering part should be placed at 30° and 150° to the incident
beam. In this step, the relationship between the side and angle in right triangle was used. Take the
30° scattering part for example, the center of the fuel tube of the burner was placed at point A. We
measured the length of the scattering part, which was the length of side-AB in the Figure C. 1.
According to the value (30°) of ∠BAC and the length of side AB, we calculated the length of side
AC by function ‘cos’ and the length of BC by function ‘sin’. Then we drew line-AC on our optical
table and marked point C. According to the value (90°) of ∠BCA and point C, we drew line-BC
88
on our optical table and marked point B. Then we placed the midpoint of the end of the 30°
scattering part at point B. After that the 30° scattering part was fixed on the optical table.
Figure C. 1: Schematic of optics alignment for the 30° elastic laser scattering part.
In the next step, we placed a pin at the exit of the fuel tube of the burner and demounted PMT,
then a beam of light from a laser pen whose output power is less than 50 mW passed through the
centerline of the 30° scattering part from the pinhole. We adjusted the position of the 30° scattering
part slightly using adjusters until the light shot on the pin. After that, we mounted the PMT back
and connected the output of PMT to the lock-in-amplifier. Then we turned on the laser with output
power of around 10 mW and detected laser light reflected by the pin at the exit of the fuel tube.
According to signal fluctuations from LabVIEW software, we used the adjusters on the mount to
align the scattering part in a more accurate way. Then we locked all the adjusters on the 30°
scattering part. The alignment of the 150° scattering part was conducted using the same method.
89
Appendix D Soot Volume Fraction Profiles
D.1 Pure n-Dodecane
Figure D. 1: Soot volume fraction profiles with error bars for pure n-dodecane.
90
D.2 Pure n-Dodecane Doped with 15 mol. % n-Propylbenzene
Figure D. 2: Soot volume fraction profiles with error bars for pure n-dodecane doped with 15
mol. % n-propylbenzene.
91
D.3 Pure n-Dodecane Doped with 30 mol. % n-Propylbenzene
Figure D. 3: Soot volume fraction profiles with error bars for pure n-dodecane doped with 30
mol. % n-propylbenzene.
92
D.4 Pure n-Dodecane Doped with 45 mol. % n-Propylbenzene
Figure D. 4: Soot volume fraction profiles with error bars for pure n-dodecane doped with 45
mol. % n-propylbenzene.
93
Appendix E Temperature Profiles
E.1 Pure n-Dodecane
Figure E. 1: Temperature profiles with error bars for pure n-dodecane.
94
E.2 Pure n-Dodecane Doped with 15 mol. % n-Propylbenzene
Figure E. 2: Temperature profiles with error bars for pure n-dodecane doped with 15 mol. % n-
propylbenzene.
95
E.3 Pure n-Dodecane Doped with 30 mol. % n-Propylbenzene
Figure E. 3: Temperature profiles with error bars for pure n-dodecane doped with 30 mol. % n-
propylbenzene.