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Investigating the Links between Smoke Points, Sooting
Thresholds, and Particle Number and Size
This dissertation is submitted in accordance with the requirements for the
degree of Master of Philosophy.
Ayodeji Andrew AKERE
St Edmund’s College
August 2009
UNIVERSITY OF CAMBRIDGE
Department of Chemical Engineering
Preface
All of the work described in this dissertation is believed to be original except where explicit
reference is made to other authors. The work contained in this report was carried out at the
Department of Chemical Engineering, University of Cambridge between March 2009 and
August 2009.
No part of this dissertation has been, or is being submitted for any other degree other than the
degree of Master of Philosophy in the University of Cambridge.
This dissertation contains 9628 words and 40 pages.
Acknowledgment
The author will like to thank Drs Neal Morgan and Markus Kraft of the Department of
Chemical Engineering, University of Cambridge for their help and guidance through the
period of this research.
An award of the BG Chevening Cambridge Scholarship by the Cambridge
Commonwealth Trust for my MPhil study is gratefully acknowledged and appreciated.
2
Summary
In this thesis, the correlations between smoke points, sooting thresholds, and the number and
size distribution of particles given off by toluene/n-heptane/iso-octane blends and some
waxes are investigated. Blending rules are developed for liquid fuel blends, and correlations
between the TSIs, C/H and C/O ratios of the fuels are also investigated.
Concerns about the effects of combustion generated particles on health are leading to
changes in particulate emissions standards. Current standards regulate only the total mass of
Particulate Matter (PM) emitted per kilometre. This is effective for controlling only larger
sized particles, as the accompanying nanoparticles contribute very little to the total mass of
the emissions. Kittelson (1998) reported that particles with sizes <50 nm contribute only 0.1–
10% of the total particulate mass of emitted diesel particles, but make up more than 90% of
the total particle number. Rigorous emissions standards that will limit not just the mass of
particles emitted from a vehicle, but also the number of such particles are being introduced.
The European Commission has proposed a limit of 6 × 1011 particles/km (EC 2008) for light
duty vehicles; this will be incorporated into the EURO VI standards. It is expected that such
standards will be introduced in the aviation industry soon.
The current regulatory measure of particulate matter emissions from aircraft engines
is the ‘Smoke Point’. The smoke point is the maximum height of a diffusion flame that can be
achieved without smoking. Smoke points are a good indicator of the combustion qualities of
aviation fuels, but unfortunately, they can only distinguish between bad and acceptable fuels,
not between acceptable and extremely clean fuels. It is important to establish for a set of
standard fuel components whether or not the smoke point is sufficient for future emissions
legislation or whether new measurements should be made in conjunction with this metric.
Blending rules were developed for toluene/iso-octane/n-heptane blends. The sooting
tendencies of the fuels were found to increase with increases in the C/H ratio and C/O ratio.
The particles given off by these blends were analyzed using a fast particle spectrometer (the
DMS 500). For the fuels tested, no correlation was found between the smoke point, sooting
threshold and, the particle number and size. This implies that the smoke point may not be
sufficient for future emissions legislation and other metrics will have to be found.
3
Contents
Preface ....................................................................................................................................................2
Acknowledgment ....................................................................................................................................2
Summary .................................................................................................................................................3
1 Introduction..........................................................................................................................................5
1.1 Soot ...............................................................................................................................................5 1.2 Smoke Point ..................................................................................................................................6 1.3 Mixing Models for Multi-Components Fuels ...............................................................................7 1.4 Liquid Fuels ..................................................................................................................................8
1.4.1 Paraffins and Aromatics.........................................................................................................8 1.4.2 Oxygenates.............................................................................................................................8 1.4.3 Surrogate Fuels ......................................................................................................................9
1.5 Candles........................................................................................................................................10 1.6 Aims of this study .......................................................................................................................11 1.7 Structure of this Dissertation.......................................................................................................11
2 Methodology ......................................................................................................................................12
2.1 Experimental Setup and Procedure .............................................................................................12 2.1.1 Experimental Apparatus.......................................................................................................12 2.1.2 Preparation and Smoke Point Measurements of Liquid Fuels .............................................14 2.1.3 Preparation and Smoke Point Measurements of Waxes ......................................................16
2.2 DMS 500 Sampling.....................................................................................................................17 2.2.1 DMS Theory ........................................................................................................................19
2.3 Error Analysis .............................................................................................................................20
3. Results and Discussion .....................................................................................................................21
3.1 Validation of Rig Modifications .................................................................................................21 3.2 Liquid Fuels Results ...................................................................................................................22
3.2.1 Smoke points and TSIs ........................................................................................................22 3.2.2 The effect of oxygenates ......................................................................................................28 3.2.3 DMS results .........................................................................................................................29 3.2.4 Comparing DMS results with TSIs......................................................................................30
3.3 Condensed fuels results...............................................................................................................32 3.3.1 Smoke points and TSIs ........................................................................................................32 3.3.2 DMS Results ........................................................................................................................33
4. Conclusions.......................................................................................................................................35
5. Future Work ......................................................................................................................................36
References.........................................................................................................................................37
Nomenclature....................................................................................................................................40
4
1 Introduction
1.1 Soot
Soot is a black microscopic substance which is mostly carbon emitted as smoke from a flame.
It is formed when there is incomplete combustion of fuel.
Soot formation is a complex process. Excellent work has been done on the mechanism
of soot formation in combustion systems and estimates of fuel sooting tendency have been
made using various types of flames and chemical systems (Glassman, 1988). Calcote (1981)
defined three stages of soot formation, these are, nucleation, growth to spherical particles and
agglomeration. Nucleation is the transformation from a molecular system to a particulate
system; the particles then grow to spherical particles which are about 10 to 50 nm in
diameter. The final stage is the agglomeration (also known as accumulation) of the spherical
units to form chains. Fuel structure and composition has a significant effect on the sooting
tendency of diffusion flames (Glassman, 1988; Ladommatos et al, 1996; Yang et al, 2007).
Pure hydrocarbons with different molecular structures have different sooting performances
(Yang et al, 2007). A ranking of sooting tendencies determined by many researchers is
related to the fuel structure, from the highest to the lowest sooting tendency:
Polyaromatics > aromatics > alkynes > alkenes > alkanes > alcohols (Hamins, 1993).
The extent to which a given fuel system will produce soot also depends on the flame structure
and the temperature of the system (Glassman, 1988).
Soot plays a primary role in global climate change (Berry and Roberts, 2008); black
carbon heats up the atmosphere by directly absorbing solar radiation. When soot particles
settle on glaciers or ice, the ability of the ice to reflect sunlight is reduced, and as a result it
melts faster. Soot is also important in air pollution and heat transfer by radiation (Shahad and
Mohammed, 2000). It has been known for a long time that soot has adverse effects on human
health (Hamins, 1993; Donaldson et al, 1998; Ormstad, 2000). Significant health problems
that have been linked with particulate matter include premature death, respiratory related
hospital admissions, aggravated asthma, difficult or painful breathing, chronic bronchitis and
decreased lung function (USEPA, 1997). According to Allan (2007), environmental soot kills
more people than any other pollutant owing to its association with respiratory illness and
cancer. On the average, diesel particulates contribute about 84% of the total air toxic risk
(SCAQMD, 2008). Kennedy (2007) found that when ultrafine particles are inhaled by
humans, they could find their way into other organs like the heart and liver. These particles
generate reactive oxygen species which can cause inflammation and ultimately lead to
5
respiratory problems and/or worsening of pre-existing health conditions. Besides health
effects, soot enhances contrail formation, contributes to thermal radiation loads on combustor
liners and turbine blades and enhances the tactical visibility of military aircraft (McEnally,
1998).
1.2 Smoke Point
The smoke point has been used for years as a relative measurement of the tendency of fuels
to produce soot, and as an indication of the quality of liquid fuels (Olson et al, 1985). The
smoke point is the height of a flame at the point of incipient smoking; this is occurs when
there is too much fuel for the oxygen available. At the smoke point, the production of soot is
exactly offset by its oxidation (Berry and Roberts, 2006).
The smoke points of jet fuels are regulated and there is an American Society for
Testing and Materials (ASTM) method for finding the smoke points of liquid fuels using a
standard wick fed smoke point lamp (ASTM D-1322). Aviation jet fuels are required to have
a smoke point in the standard wick lamp (ASTM, 2002) of at least 25 mm (or at least 18 mm
when naphthalene content is 3% by volume or below) (ASTM, 2003). A higher smoke point
indicates a lower sooting tendency and better burning quality. For many fuels, as the wick
length is increased, ‘sooting wings’ appear on both sides of the flame. These grow until a thin
trail of smoke breaks out above the flame. Figure 1 shows how smoke points are determined.
Correct Smoke Point
Too High
Too Low
Figure 1: Flame Shapes for Smoke Point Determination.
According to Glassman (1988), smoke points occur when an increase in the fuel flow
rate gives a smaller increase in soot oxidation time compared to the increase in soot
formation time. Smoke points have been found to correlate with soot volume fractions (Kent,
1986).
6
Zhmykhova (1973) expressed the smoke point of kerosene distillates as a function of
density through linear and quadratic equations. The equations are:
h = 22 - 150 (ρ - 0.8100) (1)
h = 21.5 - 165 (ρ - 0.8100) - 1260 (ρ - 0.8100)2 (2)
where h is the smoke point in mm and ρ is the relative density of the fuel.
A threshold sooting index (TSI) was defined by Calcote and Manos (1983). The TSI
is an artificially defined number that represents the sooting tendency of a fuel and is
independent of the experimental apparatus used to obtain the data. The range of TSI values
was artificially assigned from 0 to 100, with ethane as the least sooting (TSI = 0) and
methylnaphthalene as the most sooting (TSI = 100). The TSI correlates linearly with the ratio
of fuel molecular weight and smoke point in a diffusion flame. The TSI for diffusion flames
is defined as,
(3)
bSP
MWaTSI
where a and b are constants which depend on the apparatus used for the smoke point
determination, MW is the molecular weight of the fuel and SP is the fuel’s smoke point.
The TSI is normally preferred to the smoke point because it follows a simple mixing
rule:
(4)
k
kkmix TSIXTSI
where TSImix is the TSI of the mixture, and Xk and TSIk are the mole fraction and the TSI of
the pure component k, respectively. This mixing rule was proposed by Gill and Olson (1984).
The TSI has been proposed by Yang et al (2007) as a new lumped parameter in soot
formation correlations; they found that the TSI model correlated excellently with
hydrocarbon compositions over a wide range of fuel samples.
1.3 Mixing Models for Multi-Components Fuels
Product formulation normally involves the mixing of two or three raw materials in specific
proportions to give a product that meets defined quality specifications. The smoke point of
fuels is a very important indicator of fuel quality; therefore, a means of predicting the smoke
points of fuel mixtures from the proportions and smoke points of the base components would
be very useful.
Mixture experiments are an aspect of Response Surface Methodology (RSM) which is
a collection of statistical and mathematical techniques useful for developing, improving, and
7
optimizing processes. RSM is important in the design and formulation of new products
(Myers and Montgomery, 2002). Mixture experiments are carried out to determine the
optimal proportions of the constituents (factors) of a mixture that maximize a response in the
product. The results of these experiments can be used to develop an empirical model (a first-
order or second-orde
(5) ε
r polynomial) of the form:
where y, x, β and ε are the response, factors, regressor coefficients and statistical error
respect
1.4 Liquid Fuels
Aromatics
saturated hydrocarbons with the general formula CnH2n+2.
position.
Aroma
1.4.2 Oxygenates
t methods of soot reduction is preventing the formation of soot in the first
xxβxxβxxβxβxβxβy 322331132112332211
ively. The empirical model approximates the relationship between the factors and the
response, this empirical model is called a response surface model.
1.4.1 Paraffins and
Paraffins are linear or branched
Paraffins are major components of petroleum. The simplest paraffin is methane (CH4).
Aromatics are unsaturated chemical compounds characterized by one or more planar rings of
atoms joined by covalent bonds. Benzene and toluene are the best known aromatics.
The combustion quality of a fuel is largely dependent on its hydrocarbon com
tics greatly increase the sooting tendencies of fuels; paraffins are known to have better
burning properties than aromatic compounds. (Gomez et al, 2000 and McEnally et al, 2006).
Calcote and Manos (1983) found that TSI of diffusion flames increased by 25-60 by changing
a saturated ring to an aromatic one.
One of the simples
place by modifying the fuel. Oxygenates are hydrocarbons containing one or more oxygen
atoms. They are used as fuel additives, this is because they increase the octane rating of
gasoline and reduce emissions. Oxygenates are normally alcohols or ethers, examples
including ethanol, Isopropyl alcohol, methanol, methyl tertiary butyl ether (MTBE), ethyl
tertiary butyl ether (ETBE) and tertiary amyl methyl ether (TAME). Table 1 gives typical
properties of some oxygenates.
8
able 1: Typical properties of some oxygenates.
ETBE TAME
T
Ethanol MTBE
Chemical formula H (CH3)3 2OC(CH3)3 CH2OCH3 CH3CH2O CH3OC CH3CH (CH)3C
Oxygen content, percent by weight
Octane rating
115 110 111 105
ur
34.73 18.15 15.66 15.66
Blending vapopressure, RVP
18.0 8.0 4.0 1.5
Source: National Petr(Washing
oleum il, U.S. Petroleum Refining: Meeting Requirements for Cleaner Fuels and Refineries ton, DC, August 1993), Appendix L.
found to reduce the sooting tendencies of fuels.
1.4.3 Surrogate Fuels
ed as mixtures of few hydrocarbon compounds designed to
al fuels
ity
been extensively studied in the
de up of blends of n-decane and
alkyl-su
Counc
Oxygenated molecules have been
Hong et al (2009) investigated the effect of oxygenates on soot formation in rich heptane
mixtures and found that heptane fuel mixtures with oxygenate additives had significantly
lower levels of soot yield. Pepiot-Desjardins et al (2008) obtained similar results with
n-heptane/toluene/oxygenate mixtures. This trend was not followed by ethylene non
premixed flames; McEnally and Pfefferle (2007) reported that ethanol and dimethyl ether
actually enhanced soot concentrations in ethylene non premixed flames. This was attributed
to the decomposition of the oxygenates to methyl radicals which indirectly enhanced the
formation of benzene.
Surrogate fuels are defin
reasonably describe the important combustion characteristics of commercial fuels. Re
are complex mixtures of a large number of hydrocarbon compounds. This multi-component
nature introduces complexity to experimental studies and modelling. The use of surrogate
fuels made up of only a small number of components can significantly reduce this complex
and enhance the development of accurate numerical designs.
Mixtures of toluene, iso-octane and n-heptane have
past, and are traditionally used as automotive surrogates (Andrae et al, 2007; Pitz et al, 2007;
Sakai et al, 2009). In this study, different volume fractions of toluene, iso-octane and n-
heptane are used as components of ternary surrogate fuels.
Aksit and Moss (2005) worked on model fuels ma
bstituted aromatics, selected to reproduce the sooting behaviour of aviation kerosene.
Their results suggest that the sooting behaviour of kerosene can be satisfactorily reproduced
by such blends. Honnet et al (2009) found that a mixture of 1, 2, 4-trimethylbenzene 20% and
9
n-decane 80% by weight also accurately reproduced the soot volume fraction and critical
auto-ignition conditions of kerosene.
1.5 Candles
ry useful in illustrating the complicated chemical and physical processes that
at high
temper
ig 2: Candle Flame Reaction Zones, Emissions, and Temperature. lt Lake City, UT.
indoor
Candles are ve
occur during combustion. “There is not a law under which any part of this universe is
governed which does not come into play, and is touched upon in these phenomena. There is
no better, there is no more open door by which you can enter the study of natural philosophy,
than by considering the physical phenomena of a candle.” – Michael Faraday (1861).
Oxygen and the fuel (wax vapour) mix at the surface of the flame to burn
atures; oxygen diffuses into the flame from the surrounding air. Heat from the flame
melts the wax below and liquid wax moves up the wick through capillary action. Soot
particles formed in the region between the wick and the flame are carried upward by
convection currents, they give the flame its yellow colouration. Figure 2 shows the regions of
a typical candle flame.
F Source: “The Science of Flames” poster, National Energy Foundation, Sa
The physical and chemical properties of particle emissions from burning candles in
air have been investigated by Pagels et al (2009). They reported that distinctly
different particle types are emitted during the different modes of candle burning. The soot
emitted during burning of wax candles can contribute significantly to the number of ultrafine
particles in indoor air (Afshari et al, 2005; Wright et al, 2007).
10
Allan (2007) developed a method for measuring the smoke point of candles, measured
the smoke points of diverse waxes, considered the effects of wick diameter and length on
smoke points, and identified wick diameter and length requirements for smoke-free
conditions. It was found that candle smoke points increased with wick diameter and also that
there were no smoke points for wick diameters less than 1.8 mm or wick lengths less than
6 mm. Allan’s method was used in this work and is described extensively in the
methodology.
1.6 Aims of this study
The aims of this study are to:
1. Develop blending models/rules for the smoke point and threshold sooting index of
surrogate fuels which are used in industry/academia (toluene/iso-octane/n-heptane).
2. Measure additional metrics along with the smoke point. Particle size distribution
measurements will be determined using a Cambustion DMS 500 fast particle
spectrometer.
3. Determine if smoke point measurements are sufficient for future emissions legislation.
1.7 Structure of this Dissertation
The experimental setup and procedure, fuel blends and waxes used for this study are
described in section 2.1; section 2.2 briefly discusses the theory underlying differential
mobility spectrometry and its use in this work. The results obtained are presented and
discussed in section 3.
Sections 4 and 5 present the conclusions drawn from this research and directions for
future work respectively.
11
2 Methodology
2.1 Experimental Setup and Procedure
2.1.1 Experimental Apparatus
Two main pieces of equipment were used for this study: the modified smoke point lamp and a
DMS 500. The DMS 500 is a fast particle spectrometer produced by Cambustion Ltd,
Cambridge; it was used for particle size and number determination. The smoke points of the
fuel blends were determined by the ASTM standard smoke point method (ASTM, 2002). The
smoke point lamp used for this research was modified in order to accommodate a sampling
probe for the particles emitted by the flames. The modified smoke point rig is shown in
figure 3.
The chimney and scale of the standard smoke point lamp were removed and a boxlike
rig (F) with dimensions 11 cm *13 cm * 50 cm was made for the wick candle (D) and socket
(B). The rig helped to keep out draft so that the flame burned smoothly without flicker. The
top of the rig had a mesh grille which allowed air needed for burning to diffuse into the rig.
The back of the rig was white, which allowed for easy identification of soot emission from
the flames, while the front was transparent to allow for visual examination of the flames. To
eliminate parallax error, two vertically placed rulers (A) were used to read off the flame
lengths, one in front of the flame and the other behind. The wick lamp was mounted on a
movable platform (E) which enabled flame sampling at different heights. Holes were made at
the sides of the rig for the sampling probe and water jacket assembly (C).
The ‘’Wang-type’’ sampling probe (Zhao et al, 2005), originally used to sample soot
and the ‘’Fennell-type’’ probe (Fennell et al, 2007), used for low concentration nanoparticles
of metal oxides, were considered for this experiment. The Wang-type sampling probe was
preferred due to rapid expansion in the Fennell-type probe which caused a large amount of
particle deposition, thus reducing the accuracy of the results. The sampling probe used for
this work is essentially a straight stainless steel tube with a small orifice for sample intake
(figure 4). An orifice diameter of 0.3 mm was used for this work. The sampling probe was
connected to the DMS with a conductive rubber tube about 1.5 m long.
12
(a) The rig used for this work. 11 cm F
50cm
(b) Schematic diagram of the smoke point: A) rulers for flame length determination,
B) candle socket C) water jacket and sampling probe assembly, D) candle, E)
movable platform, F) rig.
Figure 3: The modified smoke point lamp.
13
500 mm
N2 from the DMS
N2 + sample to the DMS 0.75 mm
= 9.5 mmφ = 0.3 mmφ
P = atmospheric
Fig 4: Schematic of the sampling probe used for this work. Flame Gases
Figure 5 shows a schematic of the sampling probe and water jacket. The sampling
orifice is at A. Filtered N2 from the DMS enters the probe at D, mixes with the sample at A
and then leaves the probe at E for the DMS. During sampling, water is pumped into the water
jackets at B and out through C. Sampling Probe
19 cm
4 cm
Water Jackets
Figure 5: Schematic of the sampling probe and water jacket assembly – A) sampling orifice,
B) water inlet, C) water outlet, D) inlet for filtered N2, E) outlet for N2 and sample
to DMS.
2.1.2 Preparation and Smoke Point Measurements of Liquid Fuels
Blends of n-heptane, toluene and iso-octane, shown in table 2, were prepared according to an
augmented simplex experimental design. To ensure safety, the blends were mixed in a fume
cupboard. 30 ml of each blend was prepared. Glass pipettes were used to transfer the required
volume of the pure components into glass bottles.
About 15 ml of the sample to be tested was pipetted into the wick candle which is a
metallic cylinder with an air vent. It was ensured that the air vent was free from fuel. A wick-
trimmer assembly was used to place a wick in the wick tube. The wick-trimmer holder was
14
Table 2: Fuel blends used for this work.
e Fraction Fuel Volum
Blend No. Toluen ptane e Iso-octane n-he
1 1.000 0.000 0.000
2 0.667 0.167 0.167
3 0.500 0.500 0.000
4 0.500 0.000 0.500
5 0.333 0.333 0.333
6 0.167 0.667 0.167
7 0.167 0.167 0.667
8 0.000 1.000 0.000
9 0.000 0.500 0.500
10 0.000 0.000 1.000
serted over the top of the wick tube and the long-nosed triceps were inserted through the
fuel being tested. The
candle
in
tube and holder. The wick was grasped and carefully pulled through the tube without
twisting. A clean knife was used to cut the wick at the face of the holder; clippers were used
to remove any frayed ends. The holder was then removed and the wick tube was screwed into
the candle. The use of the wick-trimmer assembly ensured that 6 mm of the wick was left
above the holder and that the wick was free of twists and frayed ends.
The burning end of the wick was soaked with some of the
was put into the candle socket, placed in the rig and left for about 30 seconds to allow
the wick become soaked with fuel. The candle was lit with rig open to reduce the danger of
building up explosive mixtures. The rig was closed after about 30 seconds and the candle was
allowed to burn till a quasi-steady flame was observed. The wick was raised until a smoky
trail appeared, and was then gradually reduced until the flame just stopped sooting and a
slightly blunted flame tip was observed. This was taken as the smoke point and the length of
the flame was read off using the two rulers and a photograph of the flame was taken. This
cycle was repeated three times for each of the fuels and the results were averaged. The wick
height reductions were done gradually and time was allowed for the flame to adjust to these
reductions. A new wick was used for each fuel and the wicks were stored in bottles to be
reused for only the corresponding fuel blend.
15
2.1.3 Preparation and Smoke Point Measurements of Waxes
3 with some of their known
Table 3: Summary of waxes tested.
Fami Formula Molecular
ol
Melting
)
Boiling
C)
Fourteen waxes were tested in this work; they are shown in table
properties. These are the same waxes used in Allan (2007) and were obtained in granular
form from Sigma-Aldrich.
ly Wax
Weight, g/m Point (oC Pointa (o
Commercial Beeswax 63
Candelilla 40
50-430
ormal Alkane e C24H50 39 52
ane
rimary
1-octadecanol C18H38O 270 58 21015
arboxylic
Myristic acid C14H28O2 228 54 250100
0
68 2
Carnauba 83
Paraffin 66 3
N n-tetracosan 3 49 - 391
n-octacosane C28H58 395 61.1 431.6
n-hexatriacont C36H74 507 75 265
P
Alcohol
1-hexadecanol C16H34O 242 49.2 312
1-docosanol C22H46O 327 70 1800.22
C
Acid
Lauric acid C12H24O2 200 43.8 299.2
Palmitic acid C16H32O2 256 62 271.510
Stearic acid C18H36O2 284 69.3 370
a boiling points g except wh t pressure is shown in supers he pressure
ot plate in 50 ml beakers, but not allowed to boil. A
stirrer w
are at 760 mmH ere a differen cript. T s shown in superscripts have units of mmHg.
The waxes were melted on a h
as used. The beakers containing the molten waxes were cooled in an ice bath which
ensured rapid cooling and also made removal of the candles easier. The candles were
removed from the beakers by gently hitting them against a flat surface. Holes were drilled at
the centre of the candles using a hand drill with a 5 mm diameter drill bit. The wicks were
stiffened with the wax corresponding to the candles they were to be used in. A 5 mm drill bit
was chosen because 4.5 mm wicks (ASTM standard smoke point wicks) were to be used, and
16
the holes had to be slightly larger than the wicks. The extra space accommodated the wax
used to stiffen the wicks and made wick adjustment easier.
An 8 cm * 8.5 cm * 7 cm holder (figure 6) with a 4 cm diameter hole at the top was
made for the candles; the candles were secured in the holder by 4 nuts. Each candle was
placed in the holder and the holder was mounted on the stand in the modified smoke point rig
(figure 3) used to determine the smoke points of the liquid fuels. The candle was lit and
allowed to burn till sufficient wax melted. The wick was then gradually adjusted from
underneath until the smoke point was reached. The smoke point was read off the rulers and a
photograph of the flame was taken. Three readings were taken for each candle and the results
were averaged.
Figure 6: Candle holder.
2.2 DMS 500 Sampling
The flames were sampled ‘below’, ‘at’ and ‘above’ their smoke points using the DMS 500
with the flame tip just touching the probe. Nitrogen was used as the dilution gas. Dilution is
essential because it inhibits agglomeration and condensation, and it increases the instrument’s
cleaning interval. The flame gases were sampled continuously through the orifice in the
sampling probe, and were immediately diluted by N2 which had been filtered and metered by
the DMS. Water was continuously circulated through the water jackets with a pump
delivering 35 litres per min during the duration of sampling. Standard non-conductive
sampling hose was used to transport the sampled gas, after cooling and dilution in the
sampling probe, to the DMS 500.
17
The DMS was allowed to warm up for 30 minutes before sampling began and was
autozeroed before each sampling. High gain range was used and the dilution factor was set to
obtain a flow rate of 8 litres per minute into the DMS. To increase the dilution factor further,
a ‘‘leaky HEPA’’ filter was used to reduce the concentration of the sample going into the
DMS. The use of the leaky HEPA filter was suggested by Chris Nickolaus of Cambustion Ltd
due to the fact that some of the flames tested had extremely high concentration of soot.
Figure 7 shows a schematic diagram of the experimental set up.
Figure 7: Schematic diagram of the experimental set-up.
The output from the DMS was accessed through a computer with the data processing
software from Cambustion which saves the results as data files. The output to the screen was
in form of a particle size distribution function plot of dN/dlogDp against Dp. The plot showed
the nucleation and accumulation modes, and the total of both modes as a continuous
spectrum. The data was logged into files for about 30 seconds for each of the samples tested.
The flames were not sampled for too long because the flame lengths were observed to
increase during prolonged sampling. The nucleation mode diameter, accumulation mode
diameter and the total number of particles per cm3 were obtained from the data files.
The HEPA filter was calibrated by recording files of aerosol sample concentration on
the low gain range with the HEPA filter fitted and then without the filter. The same flame and
flame height were used in both cases. Sampling without the filter gave the full concentration
of the particles, and was done for only 10 seconds to prevent saturating the machine with
18
particles. The concentrations from both files were compared to obtain the dilution ratio of the
filter. The figure obtained was used to multiply all other concentrations returned by the DMS.
The DMS was operated at all times within the ‘’green band’’ on the dynamic range
indicator on the software interface screen indicating that the machine was not being saturated
with particles and so producing erroneous results. The cyclone, HT rod, classifier and space
charge guard were cleaned after a couple of samplings. Regular cleaning removes build-up of
deposits in the column which could lead to spurious signals. The sampling orifice and the
leaky HEPA were also cleaned after each sampling to dislodge particle deposits and to ensure
a clean passage of the particles into the machine.
2.2.1 DMS Theory
Differential mobility spectrometry is used for separation and characterization of gas-phase
ions based on the difference in their electrical mobilities (charge/drag ratio) under electric
fields. Electrical mobility analysis is the most efficient technique for measuring aerosol
particle size distributions in the submicron size range (Biskos et al, 2005). Instruments that
measure particle size spectra based on electrical mobility methods usually consist of three
main parts, the aerosol charger, the classification column, and the detection system.
The DMS 500 (from Cambustion) gives a number and size distribution of particles
between 5 nm and 1000 nm. The sample is drawn into the instrument through a conductive
rubber tube, and enters a cyclone which removes particles with diameters >1000 nm. The
sample then passes through a corona discharge charger which places a positive charge on the
particles; the charge on each particle is proportional to its surface area. The charged sample
then moves through a strong radial electric field in a classifier column (Figure 8).
Figure 8: Schematic of the DMS500 classifier column and charger. Source: DMS500 User Manual, Version 2.7.
19
The electric field separates the charged particles onto grounded electrometers based
on their aerodynamic drag/charge ratio. The charged particles lose their charges to the
electrometers and the resulting currents are translated into particle size/number spectra by the
computer connected to the DMS (DMS 500 User Manual, Version 2.7). The DMS 500
operates at sub-atmospheric pressure (250 mbar); this increases the possible size range to
5 nm–1000 nm, improves the time response and reduces particle agglomeration (Symonds et
al, 2007).
2.3 Error Analysis
The possible sources of uncertainty in the results reported here are, the error of the DMS,
error in reading the smoke points with the rulers, loss of particles in the sampling probe and
coagulation of particles before classification. For large smoke point flames which flicker a
lot, error could arise due to uncertainty in determining when sooting begins. The uncertainty
in the smoke points reported in this study is + 0.5 mm. All three smoke point values
determined for each fuel did not vary over a range greater than 1.0 mm.
20
3 Results and Discussion
3.1 Validation of Rig Modifications
Smoke points of the reference fuel blends specified in the ASTM D 1322-97 were determined
using the modified smoke point rig. This was done to investigate the effect of the
modification of the rig on the results. Table 4 and figure 9 show the results obtained.
Excellent agreement was found between our results and values given in the ASTM standard;
all readings were within 1 mm of the corresponding ASTM standard values. This shows that
the modified apparatus had little or no effect on the smoke points of fuels.
Table 4: Smoke points of toluene/Iso-octane blends.
Smoke Point (mm) Toluene %(v/v)
Iso octane %(v/v) Modified Rig
Results ASTM standard
Difference
40 60 14.8 14.7 0.1
25 75 19.2 20.2 -1.0
20 80 21.8 22.7 -0.9
15 85 25.3 25.8 -0.5
10 90 31.0 30.2 0.8
5 95 35.3 35.4 -0.1
0 100 43.0 42.8 0.2
10
15
20
25
30
35
40
45
10 15 20 25 30 35 40 45
ASTM standard /mm
Mo
dif
ied
rig
res
ult
s /m
m
Slope of line = 1
Figure 9: ASTM standard values plotted against modified rig results.
21
3.2 Liquid Fuels Results
3.2.1 Smoke points and TSIs
Figure 10 shows an iso-octane flame 5 mm below its smoke point, at its smoke point
(43.0 mm) and 5 mm above its smoke point. Tiny sooting wings can be seen in figure 10a,
the wings grow bigger and become very prominent at the smoke point (figure 10b). Above
the smoke point, the flame loses its slightly rounded tip and becomes elongated as a smoky
Figure 10: ) at its smoke point (43.0 mm),
(c) 5 mm above its smoke point.
slightly improved this situation and enabled a reading to be taken.
trail breaks out.
(a) (b) (c)
(a) iso-octane 5 mm below its smoke point, (b
During the test, it was noted that the n-heptane flame flickered a lot. This could be
attributed to its high smoke point which means it needs more air to burn than the other tested
fuels. The air in the rig was quickly used up by the flame and more air could not flow in fast
enough through the mesh grille at the top. Additional holes made on the walls of the rig
22
Table 5 gives the C/H ratios, smoke points and TSI values of the ten toluene/iso-
octane/n-heptane blends used in this work. Since the values of ‘a’ and ‘b’ in the TSI formula
(equation 3) are apparatus specific, new values had to be calculated for the modified rig.
Based on various studies, Olson et al (1985) recommended TSI values for n-heptane (2.6),
iso-octane (6.4) and toluene (44). The square error between our values and these
recommended values were found and minimized by altering the values of a and b to obtain
new parameters a = 3.42 and b = -2.37 for the modified rig. These were then used to
determine the TSIs given in table 5 for the remaining blends.
Table 5: Smoke points and TSI values of n-heptane/Toluene/iso-octane blends.
Blend No. Molar Mass (g/mol) C/H Ratio
Smoke Point (mm) TSI
1 92.134 0.875 6.8 44.0
2 95.860 0.693 9.5 32.1
3 100.756 0.621 10.3 31.1
4 95.525 0.616 10.8 27.9
5 100.585 0.552 13.7 22.7
6 106.696 0.496 21.5 14.6
7 100.400 0.492 25.8 10.9
8 114.224 0.444 43.0 6.7
9 106.773 0.441 62.8 3.4
10 100.198 0.438 73.2 2.3
The smoke points and TSIs in table 5 are shown on contour plots (figures 11 and 12).
These plots can be used to predict the smoke point or TSI of any toluene/iso-octane/n-heptane
blend. Properly chosen design points increase the accuracy of the predictions. The figures
show that toluene has a huge effect on the sooting tendency of the blends. All blends with
toluene fractions greater than 0.5 have smoke points less than 12 mm. This effect is due to
toluene’s aromaticity. A plot of the TSIs against the carbon/hydrogen ratio is shown in figure
13. Correlations between C/H ratio and sooting tendency have been investigated in the past
(Calcote and Manos, 1983; Schug et al, 1980). The figure shows that for the blends tested, the
sooting tendency increases with the C/H ratio as expected. However, this trend is not a
general one for fuels (Calcote and Manos, 1983).
23
Figure 11: Contour plot of the predicted smoke points (in mm) of toluene/iso-
octane/n-heptane blends. The solid points are the design points.
Figure 12: Contour plot of the predicted TSI values of toluene/iso-octane/n-heptane
blends. The solid points are the design points.
0
0
0
0.1
0.1
0.1
0.2
0.2
0.2
0.3
0.3
0.3
0.4
0.4
0.4
0.5
0.5
0.5
0.6
0.6
0.6
0.7
0.7
0.7
0.8
0.8
0.8
0.9
0.9
0.9
1
1
1
5.7755.775
9.25
9.25
12.725
12.725
16.2
16.2
19.675
19.675
23.15
23.15
26.625
30.1
33.575
37.05
iso-Octane
n-H
epta
ne Toluene
5
10
15
20
25
30
35
40
45
0
0
0.1
0.1
0.1
0.2
0.2
0.2
0.3
0.3
0.3
0.4
0.4
0.4
0.5
0.5
0.5
0.6
0.6
0.6
0.7
0.7
0.7
0.8
0.8
0.8
0.9
0.9
1
1
0
0.9
1
12.3333
17.866717.8667
23.4
23.428.9333
28.933334.4667
34.4667
40
40
40
45.533345.533351.0667 51.066756.6
56.662.133367.6667
iso-Octane
n-H
epta
ne Toluene
10
20
30
40
50
60
70
80
24
SI of the liquid fuel blends.
(6)
TSI = -182.06x2 + 329.12x - 105.29
R2 = 0.9832
0
5
10
15
20
25
30
35
40
45
0.40 0.45 0.50 0.55 0.60 0.65 0.70 0.75 0.80 0.85 0.90
Carbon-Hydrogen Ratio, x
TS
I
Figure 13: Effect of C/H ratio on the T
The special cubic model (Myers and Montgomery, 2002) was fitted with the smoke
point results. The fitted model obtained is:
32xx1323121321 73320101116554270641110 x.xx.xx.xx.x.x.x.SP
where SP is the smoke point in mm and x1, x2 and x3 are the volume fractions of
toluene, iso-octane and n-heptane in the mixture respectively.
From the model, because β3 > β2 > β1, we can conclude that n-heptane (component 3)
has the greatest effect on the smoke point of the mixture. β12, β13, β23 and β123 are all
negative, this means that blending any two or all three of these fuels together will give a
lower smoke point than would be expected from averaging the smoke points of the pure
blends. This is an antagonistic blending effect (Myers and Montgomery, 2002).
Table 6 shows the actual smoke point values, the corresponding fitted values and the
residuals from this model. Some of the residuals obtained are large compared to the actual
smoke points (blends 2 and 7). The special cubic model used does not accurately describe the
smoke points of the blends. Figure 14 shows a plot of the actual smoke points against those
predicted by the model.
25
Table 6: The actual and predicted smoke point values for a special cubic model.
Blend No. Actual Smoke Predicted ) Residual
1 6.8
Point (mm) Value (mm
10.1 -3.3
2 9.5 0.5 9.0
3 10.3
12.2 -1.9
4 10.8 11.1 -0.3
5 13.7 10.5 3.2
6 21.5 26.8 -5.3
7 25.8 35.9 -10.1
8 43.0 41.6 1.4
9 62.8 58.4 4.5
10 73.2 70.2 3.0
Figure 14: The actual and predicted smoke points.
0
10
20
30
40
50
60
70
0 10 20 30 40 50 60 70
Predicted Smoke Point /mm
Act
ual
Sm
oke
Po
int
/mm
Slope of line = 1
26
A linear model (Myers and Montgomery, 2002) was fitted with the TSI values. The
fitted model obtained is:
(7 ) 321 1348247 x.x.x.TSI
where TSI is the threshold sooting index and x1, x2 and x3 are the volume fractions of
toluene, iso-octane and n-heptane in the mixture respectively.
The model shows that component 1 (toluene) contributes most to the sooting tendency
of the blend. The TSI values, the corresponding fitted values and the residuals from this
model are given in table 7. The linear model approximates the TSIs better than the special
cubic approximated the smoke points. This linear dependence of the smoke point of diffusion
flames on the composition of the fuel blend has previously been reported by Gill and Olson
(1984).
Table 7: The actual and predicted TSI values for a linear model.
Blend No. Actual TSI Predicted TSI Residual
1 44.0 47.2 -3.3
2 32.1 33.4 -1.3
3 31.1 27.8 3.3
4 27.9 25.2 2.7
5 22.7 19.6 3.2
6 14.6 14.0 0.6
7 10.9 11.3 -0.4
8 6.7 8.4 -1.6
9 3.4 5.7 -2.3
10 2.3 3.1 -0.8
A plot of the actual TSIs against those predicted by the linear model are shown in
figure 15, the figure further shows the good agreement between these two sets of values.
27
0
5
10
15
20
25
30
35
40
45
50
0 5 10 15 20 25 30 35 40 45 50
Predicted TSI
Act
ual
TS
I
Slope of line = 1
Figure 15: The actual and predicted TSI values.
3.2.2 The effect of oxygenates
The effect of ethanol (an oxygenate) on the smoke points of toluene and iso-octane was also
investigated. Hong et al(2009), Wang et al(2009) and Pepiot-Desjardins et al(2008) have
investigated the effects of oxygenates on fuels, they all agree that oxygenates reduce the
sooting tendencies of fuels. The results (figures 16a and b) obtained in this work show that
ethanol does indeed increase the smoke points of toluene and iso-octane in an approximately
linear manner, confirming the findings of the above papers.
6
8
10
12
14
16
0 10 20 30 40
Percenta
50
ge Ethanol
Sm
oke
Po
int
/mm
Figure 16a: The effect of ethanol on the smoke point of toluene.
28
40
45
50
55
60
65
0 5 10 15 20 25
Percentage Ethanol
Sm
oke
Po
int
/mm
Figure 16b: The effect of ethanol on the smoke point of Iso octane.
3.2.3 DMS results
Figure 17 presents the total number of particles given off per cm3 by the fuel blends at
different flame heights. Generally, it is expected that the number of particles emitted should
increase as the flame height increases, but as these results show, this is not always the case.
This could be due to lots of deposition in the sampling probe. The results do not follow any
recognizable trend. Kumar et al (2008), while studying the loss of ultrafine aerosol particles
0.0E+00
5.0E+08
1.0E+09
1.5E+09
2.0E+09
2.5E+09
1 2 3 4 5 6 7 8 9 10
Blend
Par
ticl
e n
um
ber
den
sity
, #/c
m3
Flame 5mm below SP
Flame at SP
Flame 5mm above SP
Figure 17: Particle number density of the blends.
29
in long sampling tubes, found that losses of particles smaller that 20 nm were important and
needed to be taken into account when using sampling tubes greater that 1 m in length
The n-heptane flame could not be sampled above its smoke point because the flame
was highly unstable at this point. Sampling at different heights along the flame was also not
possible because the orifice sucked in the flames as they were raised. This is may be due to
the suction from the DMS being too high.
3.2.4 Comparing DMS results with TSIs
Figures 18, 19 and 20 show plots of the total number of particles per cm3, the nucleation and
the accumulation mode diameters against the TSIs of the fuels respectively.
In figure 18, the particle density seems to increase as the TSI increases, peaks at about
a TSI of 15 and then decreases again. This trend is weak and it is unlikely that it has any
predictive value. The particle number densities for the blends range from 2.35 x 108 to 2.12 x
109 particles/cm3. The nucleation (figure 19) and accumulation (figure 20) diameters do not
show any trend or correlation with the TSI values either.
0.0E+00
5.0E+08
1.0E+09
1.5E+09
2.0E+09
2.5E+09
0 10 20 30 40 5
TSI
Par
ticl
e n
um
ber
den
sity
, #/c
m3
0
Flame 5mm above SP
Flame at SP
Flame 5mm below SP
Figure 18: Particle number density plotted against TSI.
30
0.0E+00
2.0E+01
4.0E+01
6.0E+01
8.0E+01
1.0E+02
1.2E+02
0 10 20 30 40 5
TSI
Nu
clea
tio
n m
od
e d
iam
ete
r /n
m
1
0
Flame 5mm above SP
Flame at SP
Flame 5mm below SP
Figure 19: Nucleation diameter plotted against TSI
0.0E+00
2.0E+02
4.0E+02
6.0E+02
8.0E+02
1.0E+03
0 10 20 30 40
TSI
Acc
um
ula
tio
n m
od
e d
iam
ete
r /n
m
i
50
Flame 5mm above SP
Flame at SP
Flame 5mm below SP
Figure 20: Accumulation diameter plotted against TSI.
31
The lack of correlation between the DMS results and the TSIs could be due to the fact
that, since a wick fed lamp was used for this work, the different fuels flow up the wick at
different rates and therefore the air fuel ratio (AFR) is different in each flame. The smoke
point and the air fuel ratio are dictated by the flow dynamics, as such viscosity, volatility and
other variables play a role and these cannot be deconvoluted easily in the analysis.
The nucleation diameter should always be less than the accumulation diameter
because at accumulation the particles are agglomerating to form chains of soot. Some of the
nucleation diameters obtained here were larger than the accumulation diameters. This is
probably an error of the software and how it calculates the diameter of particles. The results
shown in figure 20 have two outlier values (485 nm and 923 nm). These two diameters are
not of the order expected based on the other results. They could be flaked particles from the
deposition on the tubing or wall of the sampling probe.
3.3 Condensed fuels results
3.3.1 Smoke points and TSIs
The smoke point and TSI values obtained for the condensed fuels tested are given in table 8.
The table also shows Allan’s results (Allan, 2007) for these waxes. As with the n-heptane
flame, most of the condensed fuels’ flames flickered a lot during tests, so readings had to be
taken quickly.
The TSIs of the commercial waxes could not be calculated due to their unknown
molecular weights. For some of the waxes, large differences are observed between our results
and Allan’s results. These apparent discrepancies may be due to the highly unstable flames
caused by air flow problems in the modified rig and/or the methods used to determine the
length of the flames. In this work, rulers were used to read the flame lengths while Allan
(2007) obtained the flame lengths from images of the flames using a pixel conversion factor.
The effect of the C/H ratio on the sooting tendency of the condensed fuels is also
investigated. In figure 21, the TSIs of the condensed fuels are plotted against their C/H ratios.
The commercial waxes are not shown in the figure because of their unknown molecular
formulae. The figure shows that for alkanes and alcohols, the sooting tendency generally
increases with increasing carbon-hydrogen ratio. The four carboxylic acids used in this work
all have a C/H ratio of 0.5 and do not follow this trend because of their increased oxygen
content. Allan et al (2009) also reached the same conclusions. The increased oxygen content
leads to a reduction in the sooting tendency of the acids. This is analogous to the effect
oxygenates have on liquid fuels.
32
Table 8: Smoke points and TSIs of the condensed fuels.
Waxes Molecular
weight (g/mol)
C/H
ratio
C/O
ratio
Smoke Point
(mm)
Allan’s
Results (mm) TSI
beeswax 53.7 53
candelilla 34.3 40
carnauba 46.5 48
paraffin 50.3 53
n-tetracosane 338.640 0.480 46.3 63 22.6
n-octacosane 394.744 0.483 57.3 57 21.2
n-hexatriacontane 506.952 0.486 37.7 57 43.7
1-hexadecanol 242.432 0.471 16 56.0 65 12.4
1-octadecanol 270.484 0.474 18 55.3 63 14.4
1-docosanol 326.588 0.478 22 51.0 65 19.5
lauric acid 200.312 0.500 6 58.7 75 9.3
myristic acid 228.364 0.500 7 64.3 77 9.8
palmitic acid 256.416 0.500 8 58.0 70 12.8
stearic acid 284.468 0.500 9 59.7 70 13.9
Figure 22 shows a plot of the TSIs of the condensed fuels against their C/O ratios.
The commercial waxes and alkanes are not shown in the figure because of unknown
molecular formulae and absence of oxygen respectively. The general trend of the figure is
that sooting tendency increases with C/O ratio as expected. This means that, as more oxygen
becomes available in a fuel, its sooting tendency reduces.
3.3.2 DMS Results
Due to the highly unstable nature of some of the flames, accurate sampling of the
condensed fuels with the DMS was not possible.
33
0
10
20
30
40
0.465 0.470 0.475 0.480 0.485 0.490 0.495 0.500 0.505Carbon-Hydrogen Ratio
TS
I
Alkanes
Alcohols
Acids
Figure 21: The effect of C/H ratio on the TSI of the condensed fuels.
5.0
7.5
10.0
12.5
15.0
17.5
20.0
5 10 15 20 2
Carbon-Oxygen Ratio
TS
I
5
Alcohols
Acids
Figure 22: The effect of C/O ratio on the TSI of the condensed fuels.
34
4 Conclusions
In this thesis, blending rules have been developed for the smoke points and TSIs of
toluene/iso-octane/n-heptane blends. The correlations between smoke points, sooting
tendencies and the size distribution of particles emitted by some standard liquid fuel
components and waxes have also been investigated.
The modification of the smoke point apparatus to accommodate the sampling probe
did not significantly affect the results obtained; comparisons were made between the rig’s
results and standard ASTM results. Flames with smoke points greater than 65 mm were
found to flicker a lot in the rig.
Blending any two or all of toluene, iso-octane and n-heptane produces an antagonistic
blending effect. Toluene, due to its aromaticity, was found to have a significant effect on the
smoke points and TSIs of the liquid fuel blends. All blends with toluene fractions greater than
0.5 had smoke points less than 12 mm and TSIs greater than 31. A special cubic model did
not provide an adequate approximation of smoke points, but the linear model was an
adequate fit for the TSIs. Ethanol was found to increase the smoke points of toluene and iso-
octane flames in a near linear manner.
For the liquid fuels, alkanes and alcohols used in this work, sooting tendency
generally increased with increasing C/H ratio. The carboxylic acids tested had the same
carbon to hydrogen ratio. They did not follow this trend because of their increased oxygen
content. Threshold sooting indices reduce with increasing oxygen content. The sooting
tendencies of the alcohols and acids were also found to increase with increasing C/O ratios.
The total number of particles given off by the flames of the liquid fuels blends did not
increase as the flames got longer for some of the fuels tested. No correlations were found
between the sooting tendency, nucleation mode diameter, accumulation mode diameter and
the number of particles emitted by the flames. This implies that the smoke point
measurements may not be sufficient for future emissions legislation and new metrics will
have to be found.
35
5 Future Work
The work presented in this thesis represents important steps in understanding the links
between sooting thresholds and the particle size distribution of fuels, but more work still
needs to be done.
Fuels and Environment
This study presents results of toluene, iso-octane and n-heptane flames, it would be
beneficial to test other fuel mixtures in order to generalize these findings.
This research was carried out at atmospheric pressure, it might be of interest to extend
it to lower and higher pressures.
Additional metrics like Transmission Electron Microscope (TEM) images of the
particles emitted can also be investigated for correlations related to particle
morphology.
The Rig
The modified smoke point rig could be further modified in the following ways to get
more accurate results:
The air flow into the rig should be improved to eliminate flickering in long smoke
point flames.
The apparatus should be designed such that the wick length can be adjusted without
opening the rig. This will greatly reduce sampling time and increase the accuracy of
results.
The DMS
The DMS results can be improved in the following ways:
The orifice diameter could be increased, to ensure that all emitted particles are sucked
in.
Using a shorter conductive tubing will reduce loss of particles in the tubing.
Better dilution can be achieved by using a secondary diluter, obtainable from
Cambustion Ltd. This diluter provides dilution ratios from 20:1 to 500:1. Its use will
increase the dynamic range of the instrument, reduce the need for cleaning and make
the use of a ‘’leaky’’ HEPA filter unnecessary.
36
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Nomenclature
β Regressor coefficient
ε Statistical Error
ρ Relative density
AFR Air Fuel Ratio
MW Molecular Weight (g/mol)
SP Smoke Point (mm)
TSI Threshold Sooting Index
x Volume Fraction
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