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energies
Article
Investigation of Soot Formation in a Novel Diesel
Fuel Burner
Natascia Palazzo 1, 2, *, Matthias Kögl 1,2, Philipp Bauer 1, Manu Naduvil Mannazhi 3,
Lars Zigan 1,2, Franz Johann Thomas Huber 1,2 and Stefan Will 1,2
1Lehrstuhl für Technische Thermodynamik (LTT), Friedrich-Alexander-Universität
Erlangen-Nürnberg (FAU), 91058 Erlangen, Germany; matthias.koegl@fau.de (M.K.);
philipp.bauer@fau.de (P.B.); lars.zigan@fau.de (L.Z.); franz.huber@fau.de (F.J.T.H.); stefan.will@fau.de (S.W.)
2Erlangen Graduate School in Advanced Optical Technologies (SAOT), Friedrich-Alexander-Universität
Erlangen-Nürnberg (FAU), 91052 Erlangen, Germany
3Combustion Physics, Faculty of Engineering, Lund University, P.O. Box 118, SE-221 00 Lund, Sweden;
manu.mannazhi@forbrf.lth.se
*Correspondence: natascia.palazzo@fau.de; Tel.: +49-913-185-29-779
Received: 16 April 2019; Accepted: 15 May 2019; Published: 24 May 2019
Abstract:
In the present work, a novel burner capable of complete pre-vaporization and stationary
combustion of diesel fuel in a laminar diffusion flame has been developed to investigate the effect of
the chemical composition of diesel fuel on soot formation. For the characterization of soot formation
during diesel combustion we performed a comprehensive morphological characterization of the
soot and determined its concentration by coupling elastic light scattering (ELS) and laser-induced
incandescence (LII) measurements. With ELS, radii of gyration of aggregates were measured within a
point-wise measurement volume, LII was employed in an imaging approach for a 2D-analysis of the
soot volume fraction. We carried out LII and ELS measurements at different positions in the flame for
two different fuel types, revealing the effects of small modifications of the fuel composition on soot
emission during diesel combustion.
Keywords:
laser-induced incandescence; elastic light scattering; pre-vaporized diesel combustion;
fuel comparison
1. Introduction
The extensive use of fossil fuels as major energy carriers and the correlated emissions have
caused serious environmental concerns among fuel producers, engine manufacturers, and public
administration institutions. Emissions from engines include different chemical compounds like carbon
monoxide (CO), nitric oxides (NO
x
), volatile organic compounds (VOCs), and particulate matter (PM).
In particular, the majority of PM emitted by engines is composed of carbonaceous particles known
as soot. These particles, as many scientific studies have demonstrated, can provoke harmful effects
on human health and the environment [
1
]. In response to the forecasted emission regulations, much
effort has been put into developing new engine generations as well as optimizing existing fuels [
2
]. In
particular, improving fuel characteristics is a promising solution to reduce emissions. Two strategies
have been proposed to achieve that: (1) Complete replacement of the petroleum-derived fuels with
synthetic ones, and (2) inserting fuel additives capable of altering the fuel’s physical and chemical
properties [
3
]. To further develop and optimize such strategies a deep comprehension of the soot
formation mechanisms in the engine is necessary. There are many factors influencing soot emission
from engines. Besides the chemical composition of the fuel, which is in the focus of the present study,
soot formation in a diesel engine is influenced by physical processes. These include the atomization
Energies 2019,12, 1993; doi:10.3390/en12101993 www.mdpi.com/journal/energies
Energies 2019,12, 1993 2 of 17
and resulting shape of the spray, the method of air supply, the mixture proportion, turbulence level,
pressure, injection time, and ignition delay [
4
,
5
]. Consequently, a complete understanding of the
extremely complex nature of this process is only achievable by studying the single factors affecting
soot formation inside the engine separately. For example, identifying the chemical effects is decisive in
order to determine the impact of additive insertion. Although significant progress has been made in the
last years, a complete comprehension of the effect of the fuel chemical composition on soot generation
is still missing. Fundamental studies on the chemical effects of fuel composition on soot production
are possible by pre-vaporizing the diesel fuel before combustion, in order to avoid spray-related effects.
Moreover, the investigation of a laminar diffusion flame from completely pre-vaporized diesel offers
several advantages. Laminar diffusion flames provide simplified conditions that include the key steps
of soot formation, growth and oxidation. Additionally, laminar flames are easier to model and permit
easy access to apply diagnostic methods for the determination of species concentration, temperature,
and soot parameters [6–8].
The goal of the present work is the realization and characterization of a system that allows to
single-out the effect of the diesel fuel chemical composition on soot formation. The effect of minor
modifications of the fuel chemical composition on the soot produced is of particular interest in this work.
To that end, the strategy of complete vaporization of the fuel was adopted and a novel laminar burner
was developed and built. Mainly two optical diagnostic techniques, laser-induced incandescence
(LII) and elastic light scattering (ELS) were employed for the characterization of the soot produced.
While LII allows for the 2D evaluation of soot concentration, expressed as volume fraction f
v
[
9
],
ELS permits the measurement of crucial morphological characteristics (i.e., radii of gyration of the
aggregates) [
10
]. The set of fuels tested include two reference diesels with a modest variation in the
chemical composition (i.e., with a different content of aromatic species). The first fuel, CEC RF-06-03, is
a certified diesel used as a reference (it has a total content of aromatics species of 24.4 wt.%, 20.2 wt.%
mono aromatics) while the second fuel, CEC RF-79-07, is a certified reference fuel typically used as a
baseline for additive insertion (total content of aromatics: 21.6 wt.%, 16.9 wt.% mono aromatics). For
simplicity, CEC RF-06-03 will be referred to as “reference” fuel and CEC RF-79-07 as “baseline” fuel in
the following. The soot particulates generated at various heights above the burner were characterized
and compared for these fuels.
2. Background and Methods
2.1. Soot Characterization for Liquid Fuels
The sooting tendency of liquid fuels under various conditions has been the topic of a number of
studies. Fundamental investigations on fossil fuel surrogates as well as on biodiesel surrogates have
been performed both in shock tubes [
11
,
12
], premixed [
13
,
14
], and diffusion flames [
15
–
17
]. The studies
of soot formation in diesel flames are limited; recent investigations, including the work of Solero [18]
and Lemaire et al. [
2
,
19
], employed an atomizer to produce a fuel spray for direct vaporization of the
fuel in the flame itself. Du et al. [
20
] studied the effects of injection pressure, nozzle orifice diameter,
and gas density on soot formed in a turbulent diffusion diesel flame. The authors utilized a continuous
gas flow high-pressure/high-temperature spray chamber. Matti Maricq [
21
] used a system composed
of a syringe pump for fuel supply (diesel surrogate, biodiesel) and an injection nozzle to create a spray
flame. In 2009, Lemaire et al. [
22
] designed a McKenna-Hybrid burner where a high-speed spray of
micro-sized fuel droplets was formed at the nebulizer tip and directly injected in the burner, allowing
it to generate a turbulent diffusion flame. However, for the investigation of the chemical mechanisms
of soot formation it is especially important to reduce the effects of the fuel
´
s physical properties, such
as density, viscosity, and surface tension of droplets, that significantly influence atomization and
evaporation time in a spray flame. Only very few examples of non-spray flame burners, designed for
combustion of high-temperature boiling point fuels, are present in the literature. Gerstmann et al. [
23
]
described the concept of a vaporizing diesel burner for camp stoves and military field kitchens. In this
Energies 2019,12, 1993 3 of 17
set-up the fuel is vaporized in a vertical tube, directed through a super-heater tube, and then burned
on a cylindrical screen flame holder. The flame generated is not easily controllable/reproducible and;
therefore, not suitable for the quantification of the soot volume fraction or morphological investigations,
but is restricted to practical applications. In the work of Bryce et al. [
24
], a diesel flame (with ~13 mm
length) was generated in a burner system consisting of a vaporizer vessel, kept at 300
◦
C, joint to a
co-flow burner by a heated tube where the fuel is pulse injected. The height of the flame produced was
controlled by a needle valve. This set-up allowed for a stable flame; however, on the other hand, the
pulsed injection had to be continuously controlled in order to preserve steady flame conditions.
For the present study, a continuous fuel flow as well as a compact system, allowing for enhanced
homogenous conditions of the flame, are desirable. Furthermore, the complete vaporization of the
realistic multi-component diesel fuel considered in the present study requires temperatures of at least
370
◦
C. This is a challenge due to low self-ignition temperature (for commercial diesel at 210
◦
C), an
issue discussed in Section 3.1.
2.2. Laser-Induced Incandescence
Laser-induced incandescence (LII) is based on the heating of (usually carbonaceous) particles with
a short laser pulse up to the sublimation temperature and the analysis of the subsequent enhanced
thermal radiation signal [
25
]. This technique has received increasing attention as it allows for a
temporally- and spatially-resolved visualization of soot concentration [
26
]. Theoretical analysis has
shown that, for sufficient intensities of the laser pulse, the maximum LII signal is approximately
proportional to the local soot volume fraction f
V
[
25
]. To determine f
V
the system has to be calibrated,
usually this is done by laser extinction on a sooting “reference” lab flame [
27
] or on the flame under
investigation [
28
]. Generally, LII signals are highly nonlinear with laser fluence and the optimum
fluence must be determined first [
25
]. Thanks to the possibility of performing temporally- and
spatially-resolved soot measurements, pulsed LII has been widely applied for the evaluation of
soot concentration in a variety of flames, from laminar premixed [
29
] to laminar diffusion [
30
,
31
],
and jet-diffusion flames [
32
]. In particular, jet-diffusion flames have been used extensively for the
two-dimensional visualization of soot distribution and properties employing LII. Furthermore, LII has
been widely used for the analysis of influences of buoyancy and fuel type on the soot produced in
premixed and non-premixed laminar diffusion flames [
33
,
34
]. Finally, the use of LII has been extended
to particle characterization inside the cylinder of a diesel engine, at the exhaust and in a combusting
diesel jet [35–37].
2.3. Elastic Light Scattering
Although the aggregation of soot particles greatly influence the behavior of the LII signal [
10
],
aggregate parameters cannot be inferred by LII. For the evaluation of aggregate size, expressed
as radius of gyration R
g
and fractal dimension D
f
, another optical technique, namely elastic light
scattering (ELS), may be favorably employed [
10
]. Generally, two different types of experimental ELS
set-ups may be distinguished. The first approach uses a scanning goniometer [
38
], which allows for a
potentially high angular resolution, but comes with the drawback of a sequential, slow measurement
procedure, which restricts this method to stationary objects. The second experimental set-up uses
multiple detectors [
39
–
41
] for the simultaneous detection of scattering signals under several angles.
Unfortunately, the number of detectors and, consequently, the angular resolution is limited. More
recently, the approach of wide-angle light scattering (WALS) was introduced, which overcomes the
limitations previously listed as it combines the advantages of a fast measurement and a high angular
resolution [
6
]. In this case an ellipsoidal mirror is used to image the scattered light over a wide
range of angles and with high angular resolution onto a planar detector. For the determination of
aggregate parameters from WALS data, the theory of Rayleigh–Debye–Gans for fractal aggregates
(RDG-FA) is commonly employed [
10
], assuming that primary particles scatter independently. Under
these assumptions, the intensity of the scattered light I
sca
for monodisperse aggregates is directly
Energies 2019,12, 1993 4 of 17
proportional to the structure factor S(qR
g
), where qis the magnitude of the scattering vector. The
scattering vector
→
q
is defined as the difference between the incident and the scattered wave vector
with wavelength λand scattering angle θ, its magnitude qcan be calculated by
q=4πn
λsinθ
2. (1)
The functional behavior of the structure factor can be approximated for mainly three regimes [
10
]:
SqRg≈1, qRg1 Rayleigh regime
SqRg1−q2R2
g
3,qRg≤1 Guinier regime
SqRgCqRg−Df,qRg1 Power-law regime
(2)
From the Guinier regime it is possible to derive the radius of gyration of the aggregates R
g
. From
a Taylor-expansion one, it obtains
Isca(0)
Isca(q)=S(0)
SqRg1+1
3q2R2
g. (3)
The radius of gyration is then computed from the slope of the line after plotting the inverse
scattering intensity I
sca−1
(q) against q
2
. As real particle ensembles are polydisperse, the R
g
obtained
will be an effective value which depends on the sizes distribution, and it is strongly dominated by
large aggregates. A detailed description of the influence of polydispersity on the measured radius
of gyration is given by Sorensen et al. [
38
,
42
], while Huber et al. [
43
] investigated the possibility of
inferring size distribution parameters from WALS-measurements. This technique has been applied in
both laminar premixed and non-stationary flames [6,44].
3. Experimental Apparatus and Approach
3.1. Laminar Diesel Burner
Figure 1shows the novel burner designed. It was constituted of two main sections: The first
section was a vaporization zone, 100 mm in length, where the fuel was inserted and pumped upward
by a gear pump (HNPM mzr
®
-2921) through a 4 mm diameter tube. The tube was surrounded by
five heating elements (380
◦
C heating temperature) which allowed for the pre-vaporization of the
diesel fuel before entering into the intermediate zone. After passing through a metallic grid coupled
with a metallic porous set, the vaporized fuel, 0.175 L/min gas volume flow at 380 ◦C (corresponding
to 0.45 g/min), was inserted into the intermediate zone where it was mixed with N
2
. A constant
volume flow of 0.3 L/min of nitrogen (at 380
◦
C) was used as carrier gas instead of air in order to
avoid auto-ignition (as the auto-ignition temperature is in the same range as the boiling temperature)
inside the mixing zone of the burner. N
2
was pre-heated up to 380
◦
C and inserted from both sides
between the vaporization zone and the mixing section; in this way, a swirling flow was generated in
the mixing zone for better homogenization of the fuel–nitrogen blend. The fuel–nitrogen mixture then
entered the second section, which was a mixing chamber with 180 mm total length and 45 mm inner
diameter. A second ceramic porous set and a flow homogenizer, preceded by a metallic grid, were
arranged at the end of the section before the flame zone. A constant temperature between 370 and
380
◦
C was achieved by a heating jacket which surrounded the mixing section. This temperature range
was chosen for complete evaporation and, at the same time, to minimize pyrolysis (which mainly
occurs between 400 and 800
◦
C [
45
]). The inner flow temperature was constantly monitored by two
inner thermocouples positioned at the beginning of the modular stage and before the flame zone. The
45 mm diameter of the inlet tube, which allowed the generation of the desired homogeneous mixture
of the N
2
–fuel blend, was subsequently decreased by a conical nozzle to 2.5 mm to stabilize the flame.
Energies 2019,12, 1993 5 of 17
The Reynolds number at the burner outlet was approximately 100. Furthermore, a co-flow of air, 18
L/min volume flow at standard temperature, and pressure was inserted in order to efficiently shield
the flame. The air was pre-heated up to 380
◦
C and introduced from both sides around the nozzle. The
co-flow cylinder was filled with glass spheres for homogenization of the air co-flow. The laminar flame
obtained exhibited a height of 95 mm and a width of 8 mm.
Energies 2019, 12, x FOR PEER REVIEW 5 of 17
The co-flow cylinder was filled with glass spheres for homogenization of the air co-flow. The laminar
flame obtained exhibited a height of 95 mm and a width of 8 mm.
Figure 1. Schematic design of the laminar burner for diesel vaporization and combustion.
3.2. Laser-Induced Incandescence Set-Up
The optical LII set-up is shown in Figure 2. A Q-switched pulsed Nd:YAG laser (Quantel, Q-
smart 850), with a repetition rate of 10 Hz and a pulse width of 5 ns was employed as the light source.
The fundamental wavelength, 1064 nm, was chosen to avoid interferences with laser-induced
fluorescence (LIF) from polycyclic aromatic hydrocarbons. The laser energy is regulated by a
combination of a half-wave plate and a thin film polarizer. The beam, 6 mm in diameter, is then
shaped into a laser sheet by a series of three cylindrical lenses. The first two lenses, f
1
= −50 mm and
f
2
= 200 mm, expand the beam vertically while the last lens, f = 1000 mm, compresses the beam and
focuses it into the flame. Finally, a 30 mm aperture is positioned before the burner, which cuts the
edges to ensure that the final beam profile was approximately top hat. This setup generates a laser
sheet of 30 mm height and 0.6 mm thickness for 2D-LII measurements. The homogeneity of the laser
sheet generated was checked by using a beam profile camera (SP620U Spiricon, OPHIR Photonics).
The fluence used is varied between 0.05 and 0.3 J/cm
2
. The detection system consists of an intensified
CCD camera (Andor USB iStar model DH334T-18F-E3, 16 bit) with a resolution of 354 × 301 pixels
(maximum resolution 1024 × 1024 pixels). The gate width used is 20 ns while the repetition rate is 10
Hz. A band-pass filter at 425 nm with 50 nm FWHM is employed. For the quantification of the soot
volume fraction by LII, the absolute incandescence intensity was calibrated by extinction
measurements using a CW laser wavelength (532 nm) on a flat flame from a McKenna burner and,
subsequently, compared with the incandescence intensity measured in the diesel burner. From the
ratio between the beam intensity before (I
0
) and after (I) the flame, and knowing the path length of
the beam in the flame d, the extinction coefficient K
λ
is obtained by
𝐾=ln
𝐼
𝐼1
𝑑. (4)
and subsequently the soot volume fraction f
v
is determined through
Figure 1. Schematic design of the laminar burner for diesel vaporization and combustion.
3.2. Laser-Induced Incandescence Set-Up
The optical LII set-up is shown in Figure 2. A Q-switched pulsed Nd:YAG laser (Quantel, Q-smart
850), with a repetition rate of 10 Hz and a pulse width of 5 ns was employed as the light source. The
fundamental wavelength, 1064 nm, was chosen to avoid interferences with laser-induced fluorescence
(LIF) from polycyclic aromatic hydrocarbons. The laser energy is regulated by a combination of a
half-wave plate and a thin film polarizer. The beam, 6 mm in diameter, is then shaped into a laser
sheet by a series of three cylindrical lenses. The first two lenses, f
1
=
−
50 mm and f
2
=200 mm, expand
the beam vertically while the last lens, f =1000 mm, compresses the beam and focuses it into the
flame. Finally, a 30 mm aperture is positioned before the burner, which cuts the edges to ensure that
the final beam profile was approximately top hat. This setup generates a laser sheet of 30 mm height
and 0.6 mm thickness for 2D-LII measurements. The homogeneity of the laser sheet generated was
checked by using a beam profile camera (SP620U Spiricon, OPHIR Photonics). The fluence used is
varied between 0.05 and 0.3 J/cm
2
. The detection system consists of an intensified CCD camera (Andor
USB iStar model DH334T-18F-E3, 16 bit) with a resolution of 354
×
301 pixels (maximum resolution
1024
×
1024 pixels). The gate width used is 20 ns while the repetition rate is 10 Hz. A band-pass filter
at 425 nm with 50 nm FWHM is employed. For the quantification of the soot volume fraction by LII,
the absolute incandescence intensity was calibrated by extinction measurements using a CW laser
wavelength (532 nm) on a flat flame from a McKenna burner and, subsequently, compared with the
incandescence intensity measured in the diesel burner. From the ratio between the beam intensity
Energies 2019,12, 1993 6 of 17
before (I
0
) and after (I) the flame, and knowing the path length of the beam in the flame d, the extinction
coefficient Kλis obtained by
Kλ=lnI0
I1
d. (4)
and subsequently the soot volume fraction fvis determined through
fv=Kλλ
6πE(m). (5)
Energies 2019, 12, x FOR PEER REVIEW 6 of 17
𝑓
=𝐾𝜆
6𝜋𝐸𝑚. (5)
Uncertainties in the determination of soot volume fraction are mainly introduced by two factors,
namely the value of the absorption function E(m), where m is the complex index of refraction, and
the influence of scattering. For E(m), we chose a common value of 0.3 that is generally used for 1064
nm wavelength [46-48]. It should be noted that absolute values of f
v
derived for the diesel burner
might be influenced by possible differences in the composition of the soot particles, thus the value of
E(m) between the burners used for calibration and measurement. Nonetheless when comparing
relative differences in the diesel burner the influence of chemical composition should be small as
formation conditions are similar, although an influence can—as a matter of principle—not be ruled
out completely [25]. When a collimated beam passes through a volume, its intensity will decrease
due to absorption and scattering. For aggregates in a size range of 150–200 nm, the scattering process
usually carries between 20% and 30% weight on the overall extinction process [10]. Consequently,
after the estimation of the effective aggregates size by WALS measurements, the extinction coefficient
was corrected for the scattering, which in this case resulted to be about 22% of the total beam
attenuation.
Figure 2. Schematic top-view of the optical set-up for 2D-LII measurements.
3.3. Wide Angle Light Scattering Set-Up
The optical set-up used to adapt the WALS technique to the burner configuration is shown in
Figure 3. The ellipsoidal mirror required for WALS measurements can be described as a slice cut out
of a spheroid with 32 mm height. The body of the mirror is made out of aluminum, while the reflective
surface has an amorphous nickel plating. The distance between the two focal points of the mirror is
∆f = 600 mm and the diameter of the respective plane is 250 mm. Two slits of 10 mm width are placed
oppositely to each other in order to allow the beam to pass through. The mirror is positioned above
the burner, while the flame generated from the burner is located in the first focal point of the
ellipsoidal mirror. The scattered light from the first focal point of the mirror is imaged onto the CCD-
camera (AV PIKE F-100B, 1000 × 1000 pixels, 16 bit) by the mirror and a camera lens. An aperture in
the second focal point of the mirror defines the size of the measurement volume in the first focal
point, which is about 0.3 mm under 90° scattering angle. The exposure time of the camera for the
Figure 2. Schematic top-view of the optical set-up for 2D-LII measurements.
Uncertainties in the determination of soot volume fraction are mainly introduced by two factors,
namely the value of the absorption function E(m), where m is the complex index of refraction, and
the influence of scattering. For E(m), we chose a common value of 0.3 that is generally used for
1064 nm wavelength [
46
–
48
]. It should be noted that absolute values of f
v
derived for the diesel burner
might be influenced by possible differences in the composition of the soot particles, thus the value
of E(m) between the burners used for calibration and measurement. Nonetheless when comparing
relative differences in the diesel burner the influence of chemical composition should be small as
formation conditions are similar, although an influence can—as a matter of principle—not be ruled
out completely [
25
]. When a collimated beam passes through a volume, its intensity will decrease
due to absorption and scattering. For aggregates in a size range of 150–200 nm, the scattering process
usually carries between 20% and 30% weight on the overall extinction process [
10
]. Consequently, after
the estimation of the effective aggregates size by WALS measurements, the extinction coefficient was
corrected for the scattering, which in this case resulted to be about 22% of the total beam attenuation.
3.3. Wide Angle Light Scattering Set-Up
The optical set-up used to adapt the WALS technique to the burner configuration is shown in
Figure 3. The ellipsoidal mirror required for WALS measurements can be described as a slice cut out of
a spheroid with 32 mm height. The body of the mirror is made out of aluminum, while the reflective
surface has an amorphous nickel plating. The distance between the two focal points of the mirror is
∆
f
=600 mm and the diameter of the respective plane is 250 mm. Two slits of 10 mm width are placed
Energies 2019,12, 1993 7 of 17
oppositely to each other in order to allow the beam to pass through. The mirror is positioned above the
burner, while the flame generated from the burner is located in the first focal point of the ellipsoidal
mirror. The scattered light from the first focal point of the mirror is imaged onto the CCD-camera (AV
PIKE F-100B, 1000
×
1000 pixels, 16 bit) by the mirror and a camera lens. An aperture in the second
focal point of the mirror defines the size of the measurement volume in the first focal point, which is
about 0.3 mm under 90
◦
scattering angle. The exposure time of the camera for the measurements was
set to 43
µ
s. A bandpass filter (535 nm wavelength to compensate the blueshift of the tilted illumination
under 12.5
◦
) placed before the lens blocked the ambient light and light from soot luminescence. The
camera-mirror system is installed in a rotatable construction and tilted by an angle of 15
◦
, with respect
to the vertical axis of the burner, as it can be observed in Figure 4, in order to avoid the signal being
completely blocked by the burner. In this manner, the scattered light is collected by one half of the
mirror, which contains all the information, while the redundant signal from the other half is discarded.
A cw-laser (Qioptiq Nano-250) at 532 nm wavelength, 200 mW, (35 mW used for measurements), is
used as the light source. The beam is focused into the measurement volume by a f =300 mm lens to
obtain a small measurement volume with constant flame conditions. The beam polarization is cleaned
by a Glan–Taylor prism and adjusted to vertical relative to the mirror plane. The optical set-up had to
be calibrated to account for the 1/sin(
θ
) size dependence of the measurement volume as well as for
aberrations caused by the mirror’s surface. For this purpose, calibration measurements on nitrogen
molecules, scattering in the Rayleigh regime, were carried out. The irradiation was performed using
vertically polarized light; in this manner the scattering of the gas molecules is expected to be isotropic
in the scattering plane. For calibration with nitrogen, the laser-power was increased to 200 mW, while
the exposure time was increased to 3 s. These settings guarantee sufficient scattering signal as well as
constant conditions during a single camera exposure to avoid signal blurring.
Energies 2019, 12, x FOR PEER REVIEW 7 of 17
measurements was set to 43 µs. A bandpass filter (535 nm wavelength to compensate the blueshift of
the tilted illumination under 12.5°) placed before the lens blocked the ambient light and light from
soot luminescence. The camera-mirror system is installed in a rotatable construction and tilted by an
angle of 15°, with respect to the vertical axis of the burner, as it can be observed in Figure 4, in order
to avoid the signal being completely blocked by the burner. In this manner, the scattered light is
collected by one half of the mirror, which contains all the information, while the redundant signal
from the other half is discarded. A cw-laser (Qioptiq Nano-250) at 532 nm wavelength, 200 mW, (35
mW used for measurements), is used as the light source. The beam is focused into the measurement
volume by a f = 300 mm lens to obtain a small measurement volume with constant flame conditions.
The beam polarization is cleaned by a Glan–Taylor prism and adjusted to vertical relative to the
mirror plane. The optical set-up had to be calibrated to account for the 1/sin(θ) size dependence of
the measurement volume as well as for aberrations caused by the mirror’s surface. For this purpose,
calibration measurements on nitrogen molecules, scattering in the Rayleigh regime, were carried out.
The irradiation was performed using vertically polarized light; in this manner the scattering of the
gas molecules is expected to be isotropic in the scattering plane. For calibration with nitrogen, the
laser-power was increased to 200 mW, while the exposure time was increased to 3 s. These settings
guarantee sufficient scattering signal as well as constant conditions during a single camera exposure
to avoid signal blurring.
Figure 3. Schematic side-view of the optical set-up for WALS measurements.
Figure 4. Rotatable camera-mirror system. Camera, mirror, and optical set-up are tilted by 15° with
respect to the vertical axis of the burner. In this way the signal collected by half of the mirror is imaged
into the camera without being blocked by the burner itself.
4. Results and Discussion
Figure 3. Schematic side-view of the optical set-up for WALS measurements.
Energies 2019, 12, x FOR PEER REVIEW 7 of 17
measurements was set to 43 µs. A bandpass filter (535 nm wavelength to compensate the blueshift of
the tilted illumination under 12.5°) placed before the lens blocked the ambient light and light from
soot luminescence. The camera-mirror system is installed in a rotatable construction and tilted by an
angle of 15°, with respect to the vertical axis of the burner, as it can be observed in Figure 4, in order
to avoid the signal being completely blocked by the burner. In this manner, the scattered light is
collected by one half of the mirror, which contains all the information, while the redundant signal
from the other half is discarded. A cw-laser (Qioptiq Nano-250) at 532 nm wavelength, 200 mW, (35
mW used for measurements), is used as the light source. The beam is focused into the measurement
volume by a f = 300 mm lens to obtain a small measurement volume with constant flame conditions.
The beam polarization is cleaned by a Glan–Taylor prism and adjusted to vertical relative to the
mirror plane. The optical set-up had to be calibrated to account for the 1/sin(θ) size dependence of
the measurement volume as well as for aberrations caused by the mirror’s surface. For this purpose,
calibration measurements on nitrogen molecules, scattering in the Rayleigh regime, were carried out.
The irradiation was performed using vertically polarized light; in this manner the scattering of the
gas molecules is expected to be isotropic in the scattering plane. For calibration with nitrogen, the
laser-power was increased to 200 mW, while the exposure time was increased to 3 s. These settings
guarantee sufficient scattering signal as well as constant conditions during a single camera exposure
to avoid signal blurring.
Figure 3. Schematic side-view of the optical set-up for WALS measurements.
Figure 4. Rotatable camera-mirror system. Camera, mirror, and optical set-up are tilted by 15° with
respect to the vertical axis of the burner. In this way the signal collected by half of the mirror is imaged
into the camera without being blocked by the burner itself.
4. Results and Discussion
Figure 4.
Rotatable camera-mirror system. Camera, mirror, and optical set-up are tilted by 15
◦
with
respect to the vertical axis of the burner. In this way the signal collected by half of the mirror is imaged
into the camera without being blocked by the burner itself.
Energies 2019,12, 1993 8 of 17
4. Results and Discussion
4.1. Determination of Soot Concentration
The resulting diesel flame was laminar and sufficiently stable for soot characterization studies.
Nevertheless, the flame had minor fluctuations of
±
0.7 mm from the central position in the radial
direction. For LII measurements, 5000 images were acquired and, after a selection based on symmetry
criterion, around 1000 LII images at every height were considered and evaluated to quantify the soot
distribution. A background image of the flame with the laser turned offwas recorded, accounting
for flame luminosity, ambient light, and thermal noise of the camera. LII single-shot images of the
reference and baseline diesel flame between 30 and 90 mm height above the burner surface (HAB) are
shown in Figure 5, while the averaged images of 1000 LII-single-shots at the same position are shown
in Figure 6. The red dots in Figure 6indicate the location of the WALS measurements.
Energies 2019, 12, x FOR PEER REVIEW 8 of 17
4.1. Determination of Soot Concentration
The resulting diesel flame was laminar and sufficiently stable for soot characterization studies.
Nevertheless, the flame had minor fluctuations of ±0.7 mm from the central position in the radial
direction. For LII measurements, 5000 images were acquired and, after a selection based on symmetry
criterion, around 1000 LII images at every height were considered and evaluated to quantify the soot
distribution. A background image of the flame with the laser turned off was recorded, accounting for
flame luminosity, ambient light, and thermal noise of the camera. LII single-shot images of the
reference and baseline diesel flame between 30 and 90 mm height above the burner surface (HAB)
are shown in Figure 5, while the averaged images of 1000 LII-single-shots at the same position are
shown in Figure 6. The red dots in Figure 6 indicate the location of the WALS measurements.
Figure 5. Single-shot raw LII images of the laminar non-premixed flame obtained from the
combustion of the reference and the baseline diesel.
Figure 6. Mean LII images, average of 1000 single-shots.
Under flame conditions for pulsed LII, the peak-LII signal typically increased by increasing the
fluence until it reached a maximum. In fact, the peak-LII signal started to saturate when the
sublimation temperature was reached [49]. For quantitative LII measurements, the prompt LII signal
Figure 5.
Single-shot raw LII images of the laminar non-premixed flame obtained from the combustion
of the reference and the baseline diesel.
Energies 2019, 12, x FOR PEER REVIEW 8 of 17
4.1. Determination of Soot Concentration
The resulting diesel flame was laminar and sufficiently stable for soot characterization studies.
Nevertheless, the flame had minor fluctuations of ±0.7 mm from the central position in the radial
direction. For LII measurements, 5000 images were acquired and, after a selection based on symmetry
criterion, around 1000 LII images at every height were considered and evaluated to quantify the soot
distribution. A background image of the flame with the laser turned off was recorded, accounting for
flame luminosity, ambient light, and thermal noise of the camera. LII single-shot images of the
reference and baseline diesel flame between 30 and 90 mm height above the burner surface (HAB)
are shown in Figure 5, while the averaged images of 1000 LII-single-shots at the same position are
shown in Figure 6. The red dots in Figure 6 indicate the location of the WALS measurements.
Figure 5. Single-shot raw LII images of the laminar non-premixed flame obtained from the
combustion of the reference and the baseline diesel.
Figure 6. Mean LII images, average of 1000 single-shots.
Under flame conditions for pulsed LII, the peak-LII signal typically increased by increasing the
fluence until it reached a maximum. In fact, the peak-LII signal started to saturate when the
sublimation temperature was reached [49]. For quantitative LII measurements, the prompt LII signal
Figure 6. Mean LII images, average of 1000 single-shots.
Under flame conditions for pulsed LII, the peak-LII signal typically increased by increasing the
fluence until it reached a maximum. In fact, the peak-LII signal started to saturate when the sublimation
temperature was reached [
49
]. For quantitative LII measurements, the prompt LII signal intensity
was evaluated. To minimize effects of fluence fluctuations from the laser and laser extinction, first
the fluence for a maximum LII signal had to be adjusted. The LII peak behavior as a function of the
Energies 2019,12, 1993 9 of 17
fluence was; therefore, determined for the diesel flame under study; Figure 7shows the resulting LII
fluence curve. The LII signal was measured at 50 mm above the burner and resulted from the average
of 200 single-shot measurements for every fluence. A fluence of 0.15 J/cm
2
, at the beginning of the
plateau region of the curve, was chosen for the subsequent quantitative measurements. At this fluence,
the LII peak was barely dependent on the fluence itself, allowing for a proper evaluation of the soot
volume fraction.
Energies 2019, 12, x FOR PEER REVIEW 9 of 17
intensity was evaluated. To minimize effects of fluence fluctuations from the laser and laser
extinction, first the fluence for a maximum LII signal had to be adjusted. The LII peak behavior as a
function of the fluence was; therefore, determined for the diesel flame under study; Figure 7 shows
the resulting LII fluence curve. The LII signal was measured at 50 mm above the burner and resulted
from the average of 200 single-shot measurements for every fluence. A fluence of 0.15 J/cm2, at the
beginning of the plateau region of the curve, was chosen for the subsequent quantitative
measurements. At this fluence, the LII peak was barely dependent on the fluence itself, allowing for
a proper evaluation of the soot volume fraction.
Figure 7. LII peak signal versus fluence curve. The vertical error bars are obtained from the standard
deviation of the 1000 LII single-shot images evaluated, while the horizontal error bars are the standard
deviation of the laser fluence.
Figures 8 and 9 show the radial soot volume fraction distribution estimated from the averaged
LII signal intensity of 1000 single-shot images after calibration by extinction. The measurements were
carried out from 30 to 90 mm HAB, in intervals of 10 mm. The overall height of the flame did not
change when the fuel was changed; therefore, it was possible to compare the measurements carried
out at different heights in the flame between the two fuels. In the chart only 30, 50, 70, and 90 mm
were reported. For both fuels, the soot volume fraction reached its maximum in the annular region
of the flame, while the central section of the flame exhibited a lower concentration or absence of soot.
As expected for diffusion flames, soot was formed in the fuel-rich regions (i.e., below the
stoichiometric surface in which fuel and air is perfectly mixed). Accordingly, the LII signal exhibited
two peaks in the annular areas, from 30 to 80 mm HAB. The two typical peaks of diffusion flames,
which were visible until 80 mm HAB, joined into one peak higher in the flame (i.e., at 90 mm),
reflecting the typical “bell” trend of soot in diffusion flames.
Figure 7.
LII peak signal versus fluence curve. The vertical error bars are obtained from the standard
deviation of the 1000 LII single-shot images evaluated, while the horizontal error bars are the standard
deviation of the laser fluence.
Figures 8and 9show the radial soot volume fraction distribution estimated from the averaged LII
signal intensity of 1000 single-shot images after calibration by extinction. The measurements were
carried out from 30 to 90 mm HAB, in intervals of 10 mm. The overall height of the flame did not
change when the fuel was changed; therefore, it was possible to compare the measurements carried
out at different heights in the flame between the two fuels. In the chart only 30, 50, 70, and 90 mm were
reported. For both fuels, the soot volume fraction reached its maximum in the annular region of the
flame, while the central section of the flame exhibited a lower concentration or absence of soot. As
expected for diffusion flames, soot was formed in the fuel-rich regions (i.e., below the stoichiometric
surface in which fuel and air is perfectly mixed). Accordingly, the LII signal exhibited two peaks in the
annular areas, from 30 to 80 mm HAB. The two typical peaks of diffusion flames, which were visible
until 80 mm HAB, joined into one peak higher in the flame (i.e., at 90 mm), reflecting the typical “bell”
trend of soot in diffusion flames.
It was also observable that for both fuels soot volume fraction increased with increasing HAB, until
80 mm HAB, because of ongoing soot formation. At 90 mm, HAB soot oxidation became dominant;
therefore, a reduction of volume fraction occurred. The behavior observed is consistent with data
in the literature [
50
]. The averaged soot volume fraction measured along the width of the flame
(~10 mm, from
−
5 mm to +5 mm) and the maximum f
v
for the HAB considered, expressed as ppm,
are reported in Table S1 in the Supplementary Material. In the diesel flame studied, the maximum
f
v
found was between 1.5 ppm for the baseline diesel and 1.8 ppm for the reference diesel. A more
detailed comparison for the two diesel fuels is shown in Figure 10. The combustion of the reference
diesel CEC RF-06-03 exhibited between 17% and 40% (depending on the HAB) more soot production
with respect to the baseline diesel CEC RF-79-07, especially in the soot formation zone. The error bars,
calculated from the standard deviation of the sample distribution, were always below 9%. The two
fuels tested had a similar cetane number (52.4 for the reference, 53.3 for the baseline), viscosity, and
density (about 2.8 mm
2
/s at 40
◦
C for the viscosity and 834 kg/m
2
at 15
◦
C for the density for both
Energies 2019,12, 1993 10 of 17
fuels). The difference found between the two fuels can be explained by a higher content of aromatic
species (mainly mono-aromatics) by about 3% in the CEC RF-06-03 reference diesel. In fact, the
phenomenological models of soot formation [
51
–
53
] postulate that the propensity of a fuel to generate
soot is determined mainly by the rate of incipient soot nucleation. The nucleation can involve several
paths: Cyclization of chain molecules into rings or dehydrogenation (at low temperatures) of aromatic
rings and subsequent formation of polycyclics, as well as breakup and subsequent recyclization of
rings at higher temperatures. Fuels with a higher content of aromatic species are; thus, expected to
show a higher potential for particle nucleation. As a consequence, also a little difference in aromatics
content, like in our case, can cause a significant increase of soot emission, which arises from a growth
in the number of soot nucleation paths. Overall flame temperature has a significant impact on the
soot formation process. Though it may be assumed that temperatures in the flame were very similar
for both fuels, and thus effects resulting from temperature were minor, we also intend to perform
temperature measurements in future work.
Energies 2019, 12, x FOR PEER REVIEW 10 of 17
Figure 8. Comparison of soot volume fraction (fv) radial profiles at 30, 50, 70, and 90 mm for the
reference diesel.
Figure 9. Comparison of soot volume fraction (fv) radial profiles at 30, 50, 70, and 90 mm for the
baseline diesel.
It was also observable that for both fuels soot volume fraction increased with increasing HAB,
until 80 mm HAB, because of ongoing soot formation. At 90 mm, HAB soot oxidation became
dominant; therefore, a reduction of volume fraction occurred. The behavior observed is consistent
with data in the literature [50]. The averaged soot volume fraction measured along the width of the
flame (~10 mm, from −5 mm to +5 mm) and the maximum fv for the HAB considered, expressed as
ppm, are reported in Table S1 in the Supplementary Material. In the diesel flame studied, the
maximum fv found was between 1.5 ppm for the baseline diesel and 1.8 ppm for the reference diesel.
A more detailed comparison for the two diesel fuels is shown in Figure 10. The combustion of the
reference diesel CEC RF-06-03 exhibited between 17% and 40% (depending on the HAB) more soot
production with respect to the baseline diesel CEC RF-79-07, especially in the soot formation zone.
The error bars, calculated from the standard deviation of the sample distribution, were always below
9%. The two fuels tested had a similar cetane number (52.4 for the reference, 53.3 for the baseline),
viscosity, and density (about 2.8 mm2/s at 40 °C for the viscosity and 834 kg/m2 at 15 °C for the density
for both fuels). The difference found between the two fuels can be explained by a higher content of
Figure 8.
Comparison of soot volume fraction (f
v
) radial profiles at 30, 50, 70, and 90 mm for the
reference diesel.
Energies 2019, 12, x FOR PEER REVIEW 10 of 17
Figure 8. Comparison of soot volume fraction (fv) radial profiles at 30, 50, 70, and 90 mm for the
reference diesel.
Figure 9. Comparison of soot volume fraction (fv) radial profiles at 30, 50, 70, and 90 mm for the
baseline diesel.
It was also observable that for both fuels soot volume fraction increased with increasing HAB,
until 80 mm HAB, because of ongoing soot formation. At 90 mm, HAB soot oxidation became
dominant; therefore, a reduction of volume fraction occurred. The behavior observed is consistent
with data in the literature [50]. The averaged soot volume fraction measured along the width of the
flame (~10 mm, from −5 mm to +5 mm) and the maximum fv for the HAB considered, expressed as
ppm, are reported in Table S1 in the Supplementary Material. In the diesel flame studied, the
maximum fv found was between 1.5 ppm for the baseline diesel and 1.8 ppm for the reference diesel.
A more detailed comparison for the two diesel fuels is shown in Figure 10. The combustion of the
reference diesel CEC RF-06-03 exhibited between 17% and 40% (depending on the HAB) more soot
production with respect to the baseline diesel CEC RF-79-07, especially in the soot formation zone.
The error bars, calculated from the standard deviation of the sample distribution, were always below
9%. The two fuels tested had a similar cetane number (52.4 for the reference, 53.3 for the baseline),
viscosity, and density (about 2.8 mm2/s at 40 °C for the viscosity and 834 kg/m2 at 15 °C for the density
for both fuels). The difference found between the two fuels can be explained by a higher content of
Figure 9.
Comparison of soot volume fraction (f
v
) radial profiles at 30, 50, 70, and 90 mm for the
baseline diesel.
Energies 2019,12, 1993 11 of 17
Energies 2019, 12, x FOR PEER REVIEW 11 of 17
aromatic species (mainly mono-aromatics) by about 3% in the CEC RF-06-03 reference diesel. In fact,
the phenomenological models of soot formation [51-53] postulate that the propensity of a fuel to
generate soot is determined mainly by the rate of incipient soot nucleation. The nucleation can
involve several paths: Cyclization of chain molecules into rings or dehydrogenation (at low
temperatures) of aromatic rings and subsequent formation of polycyclics, as well as breakup and
subsequent recyclization of rings at higher temperatures. Fuels with a higher content of aromatic
species are; thus, expected to show a higher potential for particle nucleation. As a consequence, also
a little difference in aromatics content, like in our case, can cause a significant increase of soot
emission, which arises from a growth in the number of soot nucleation paths. Overall flame
temperature has a significant impact on the soot formation process. Though it may be assumed that
temperatures in the flame were very similar for both fuels, and thus effects resulting from
temperature were minor, we also intend to perform temperature measurements in future work.
Figure 10. Comparison of the maximum f
v
measured for the reference diesel and baseline diesel
between 30 and 90 mm above the burner (HAB), with corresponding standard deviations.
4.2. Characterization of Morphological Soot Properties
The WALS-measurement technique requires homogeneous flame-properties within the
observed measurement volume, a condition not always fulfilled when the flame is flickering. From
the ring-shaped scattering data from the CCD-image, scattering data with an angular resolution of 1°
is generated. A total of 5000 images of the ring-shaped scattering data were collected from the point-
wise measurement volume in the annular region of the flame at the same heights as LII (i.e., from 30
to 70 mm above the burner, every 10 mm; at 80 and 90 mm HAB it was not possible to collect enough
data for the evaluation due to the tip fluctuation). As calibrated scattering data from fractal
aggregates should monotonously decay with increasing scattering angle, data with positive slope
could be discarded. A filter prevented the further evaluation of scattering data that exhibited an
increase in intensity between 10° and 90°. For the backward scattering, a less rigorous criterion can
be employed as predominantly forward scattering contains the data for the R
g
-evaluation.
Consequently, between 90° and 170° a larger number of non-decaying data-points (i.e., five) were
allowed. Data that passed the filter were evaluated in the Guinier-regime according to Equation (2).
Within the 5000 single-images acquired, about 200 images were selected and evaluated for every
measurement point. In Figures 11 and 12, one of the single images is shown (Figure 11) in comparison
with the corresponding normalized intensity-angle chart. It can be observed that forward scattering
is predominant over backward scattering. For the determination of effective aggregate sizes, the
measured scattering data I(q) is inverted according to
Figure 10.
Comparison of the maximum f
v
measured for the reference diesel and baseline diesel
between 30 and 90 mm above the burner (HAB), with corresponding standard deviations.
4.2. Characterization of Morphological Soot Properties
The WALS-measurement technique requires homogeneous flame-properties within the observed
measurement volume, a condition not always fulfilled when the flame is flickering. From the
ring-shaped scattering data from the CCD-image, scattering data with an angular resolution of 1
◦
is
generated. A total of 5000 images of the ring-shaped scattering data were collected from the point-wise
measurement volume in the annular region of the flame at the same heights as LII (i.e., from 30 to 70 mm
above the burner, every 10 mm; at 80 and 90 mm HAB it was not possible to collect enough data for the
evaluation due to the tip fluctuation). As calibrated scattering data from fractal aggregates should
monotonously decay with increasing scattering angle, data with positive slope could be discarded. A
filter prevented the further evaluation of scattering data that exhibited an increase in intensity between
10
◦
and 90
◦
. For the backward scattering, a less rigorous criterion can be employed as predominantly
forward scattering contains the data for the R
g
-evaluation. Consequently, between 90
◦
and 170
◦
a larger
number of non-decaying data-points (i.e., five) were allowed. Data that passed the filter were evaluated
in the Guinier-regime according to Equation (2). Within the 5000 single-images acquired, about 200
images were selected and evaluated for every measurement point. In Figures 11 and 12, one of the
single images is shown (Figure 11) in comparison with the corresponding normalized intensity-angle
chart. It can be observed that forward scattering is predominant over backward scattering. For the
determination of effective aggregate sizes, the measured scattering data I(q) is inverted according to
1
I(q)=1
I(0)+q2R2
g,eff
3I(0). (6)
By fitting 1/I(q) against q
2
within the limit of the Guinier regime (I(q)/I(0)
≤
2), the values I(0) and
the slope mare obtained and processed to calculate the value of the effective radius of gyration R
g,eff
.
Figure 12 displays the single-shot scattering data as a function of qR
g,eff
at different heights in the flame
for the reference diesel RF-06-03.
The results for the effective radii of gyration distribution from 30 to 70 mm above the burner for
reference and baseline diesel are depicted in Figures 13 and 14, respectively. The statistical distribution
of the radii of gyration shows a lognormal shape, which was also observed in slightly turbulent
flames [
44
]. In general, a slight increase of the median
µg,eff
with increasing HAB can be observed
for both fuels, which can be explained by an increasing aggregation time of the particles. As for the
reference diesel CECRF-06-03, the median values
µg,eff
obtained ranged from 178 nm at 30 mm HAB to
211 nm at 70 mm HAB. In the case of the baseline diesel CECRF-79-07, the variation of µg,effbetween
the lowest height at 30 mm HAB and the upper one at 70 mm HAB is approximately 40 nm.
Energies 2019,12, 1993 12 of 17
Energies 2019, 12, x FOR PEER REVIEW 12 of 17
1
𝐼𝑞=1
𝐼0+𝑞
𝑅
,
3𝐼0. (6)
By fitting 1/I(q) against q
2
within the limit of the Guinier regime (I(q)/I(0) ≤ 2), the values I(0) and
the slope m are obtained and processed to calculate the value of the effective radius of gyration R
g,eff
.
Figure 12 displays the single-shot scattering data as a function of qR
g,eff
at different heights in the
flame for the reference diesel RF-06-03.
Figure 11. Single-shot calibrated image of half (0–180°) of the ring-shaped WALS scattering data (left)
and the corresponding normalized scattering intensity (right).
Figure 12. Comparison of single-shot scattering data versus qR
g,eff
from 30 to 70 mm above the burner.
The scattering data can be evaluated in the Guinier regime.
The results for the effective radii of gyration distribution from 30 to 70 mm above the burner for
reference and baseline diesel are depicted in Figures 13 and 14, respectively. The statistical
distribution of the radii of gyration shows a lognormal shape, which was also observed in slightly
turbulent flames [44]. In general, a slight increase of the median µ
g,eff
with increasing HAB can be
observed for both fuels, which can be explained by an increasing aggregation time of the particles.
As for the reference diesel CECRF-06-03, the median values µ
g,eff
obtained ranged from 178 nm at 30
mm HAB to 211 nm at 70 mm HAB. In the case of the baseline diesel CECRF-79-07, the variation of
µ
g,eff
between the lowest height at 30 mm HAB and the upper one at 70 mm HAB is approximately 40
nm.
Figure 11.
Single-shot calibrated image of half (0–180
◦
) of the ring-shaped WALS scattering data (
left
)
and the corresponding normalized scattering intensity (right).
Energies 2019, 12, x FOR PEER REVIEW 12 of 17
1
𝐼𝑞=1
𝐼0+𝑞
𝑅
,
3𝐼0. (6)
By fitting 1/I(q) against q
2
within the limit of the Guinier regime (I(q)/I(0) ≤ 2), the values I(0) and
the slope m are obtained and processed to calculate the value of the effective radius of gyration R
g,eff
.
Figure 12 displays the single-shot scattering data as a function of qR
g,eff
at different heights in the
flame for the reference diesel RF-06-03.
Figure 11. Single-shot calibrated image of half (0–180°) of the ring-shaped WALS scattering data (left)
and the corresponding normalized scattering intensity (right).
Figure 12. Comparison of single-shot scattering data versus qR
g,eff
from 30 to 70 mm above the burner.
The scattering data can be evaluated in the Guinier regime.
The results for the effective radii of gyration distribution from 30 to 70 mm above the burner for
reference and baseline diesel are depicted in Figures 13 and 14, respectively. The statistical
distribution of the radii of gyration shows a lognormal shape, which was also observed in slightly
turbulent flames [44]. In general, a slight increase of the median µ
g,eff
with increasing HAB can be
observed for both fuels, which can be explained by an increasing aggregation time of the particles.
As for the reference diesel CECRF-06-03, the median values µ
g,eff
obtained ranged from 178 nm at 30
mm HAB to 211 nm at 70 mm HAB. In the case of the baseline diesel CECRF-79-07, the variation of
µ
g,eff
between the lowest height at 30 mm HAB and the upper one at 70 mm HAB is approximately 40
nm.
Figure 12.
Comparison of single-shot scattering data versus qR
g,eff
from 30 to 70 mm above the burner.
The scattering data can be evaluated in the Guinier regime.
Energies 2019, 12, x FOR PEER REVIEW 13 of 17
Figure 13. Distribution of the effective R
g,eff
at 30, 40, 50, 60, and 70 mm HAB for the reference diesel
CEC RF-06-03.
Figure 14. Distribution of the effective R
g,eff
at 30, 40, 50, 60, and 70 mm HAB for the baseline diesel
CEC RF-79-07.
In Figure 15 the comparison of the median values µ
g,eff
obtained at various HAB for the two fuels
under study is reported. To further compare the results, arbitrary subsamples of about 200 are drawn
from the full sample at each HAB, yielding a standard deviation of the µ
g,eff
, which on average is 7
nm. Although µ
g
seems to be slightly higher for the baseline diesel between 40 and 60 mm above the
burner, within the statistical uncertainty of the results a difference in aggregate size between the two
fuels cannot be stated clearly, the aggregation process itself does not seem to alter for the different
fuels.
Figure 13.
Distribution of the effective R
g,eff
at 30, 40, 50, 60, and 70 mm HAB for the reference diesel
CEC RF-06-03.
Energies 2019,12, 1993 13 of 17
Energies 2019, 12, x FOR PEER REVIEW 13 of 17
Figure 13. Distribution of the effective R
g,eff
at 30, 40, 50, 60, and 70 mm HAB for the reference diesel
CEC RF-06-03.
Figure 14. Distribution of the effective R
g,eff
at 30, 40, 50, 60, and 70 mm HAB for the baseline diesel
CEC RF-79-07.
In Figure 15 the comparison of the median values µ
g,eff
obtained at various HAB for the two fuels
under study is reported. To further compare the results, arbitrary subsamples of about 200 are drawn
from the full sample at each HAB, yielding a standard deviation of the µ
g,eff
, which on average is 7
nm. Although µ
g
seems to be slightly higher for the baseline diesel between 40 and 60 mm above the
burner, within the statistical uncertainty of the results a difference in aggregate size between the two
fuels cannot be stated clearly, the aggregation process itself does not seem to alter for the different
fuels.
Figure 14.
Distribution of the effective R
g,eff
at 30, 40, 50, 60, and 70 mm HAB for the baseline diesel
CEC RF-79-07.
In Figure 15 the comparison of the median values
µg,eff
obtained at various HAB for the two fuels
under study is reported. To further compare the results, arbitrary subsamples of about 200 are drawn
from the full sample at each HAB, yielding a standard deviation of the
µg,eff
, which on average is 7 nm.
Although
µg
seems to be slightly higher for the baseline diesel between 40 and 60 mm above the burner,
within the statistical uncertainty of the results a difference in aggregate size between the two fuels
cannot be stated clearly, the aggregation process itself does not seem to alter for the different fuels.
Energies 2019, 12, x FOR PEER REVIEW 14 of 17
Figure 15. Comparison of the median Rg (µg,eff) found at the different HABs investigated for the
reference diesel and the baseline diesel.
5. Conclusions
A novel prototype of a high-boiling-temperature liquid fuel burner at atmospheric pressure was
designed, manufactured, and tested with realistic diesel fuels. The burner was specifically designed
to investigate soot formation and, in particular, the influence of the fuel chemical composition on soot
formation. For this purpose, the fuel was completely pre-vaporized in order to avoid physical effects
on soot formation caused by, for example, spray formation. Soot emission from two diesel fuels, CEC
RF-06-03 (reference diesel) and CEC RF-79-07 (baseline diesel), slightly different in chemical
composition, were characterized by LII and WALS measurements in the laminar diffusion flame.
The results of the soot quantification at different heights of the flame indicated that with
increasing height above the burner, from 30 to 80 mm, soot concentration increased for both fuels;
the maximum volume fraction found at 80 mm HAB was 1.76 ppm for the reference and 1.47 ppm
for the baseline diesel. After 80 mm HAB, soot oxidizes; therefore, a reduction of soot concentration
was observed. As for aggregates size, the radius of gyration (Rg) increased slightly with increasing
height above the burner for both reference and baseline diesel. The maximum median of Rg was
observed at 70 mm HAB with µg = 211 nm for the reference and µg = 2 07 nm for the baseline; the
minimum median was measured at 30 mm HAB, with µg = 178 nm for the reference and µg = 169 nm
for the baseline. The comparison of the soot measured in the two diesels showed that the medium
radius of gyration did not vary significantly; however, the soot concentration decreased in the
baseline diesel flame. This baseline diesel contained a slightly reduced amount of aromatic species
leading to the reduction of soot emission, yet the aggregation process was not influenced.
It may be concluded that the novel burner system is capable of producing a sooting non-
premixed diesel flame, which forms a suitable basis for the analysis of soot formation for different
realistic diesel and other liquid fuels with a high vaporization temperature. Furthermore, the
combination of LII and WALS turned out as a suitable tool for the investigation of soot formation in
such a burner. Since soot formation in a vapor fuel flame is mainly influenced by the fuel chemical
composition, we are now able to single-out the effect of minor changes of fuel chemical composition
on soot formation, for example, in the case of diesel additives, which are a promising way to reduce
soot emission from engines. Investigations on the influence of diesel additives (which are typically in
the range of 1–3 wt.% or even lower) on particle emissions, as well as the analysis of biofuels are
planned for future experiments.
Figure 15.
Comparison of the median R
g
(
µg,eff
) found at the different HABs investigated for the
reference diesel and the baseline diesel.
Energies 2019,12, 1993 14 of 17
5. Conclusions
A novel prototype of a high-boiling-temperature liquid fuel burner at atmospheric pressure was
designed, manufactured, and tested with realistic diesel fuels. The burner was specifically designed
to investigate soot formation and, in particular, the influence of the fuel chemical composition on
soot formation. For this purpose, the fuel was completely pre-vaporized in order to avoid physical
effects on soot formation caused by, for example, spray formation. Soot emission from two diesel
fuels, CEC RF-06-03 (reference diesel) and CEC RF-79-07 (baseline diesel), slightly different in chemical
composition, were characterized by LII and WALS measurements in the laminar diffusion flame.
The results of the soot quantification at different heights of the flame indicated that with increasing
height above the burner, from 30 to 80 mm, soot concentration increased for both fuels; the maximum
volume fraction found at 80 mm HAB was 1.76 ppm for the reference and 1.47 ppm for the baseline
diesel. After 80 mm HAB, soot oxidizes; therefore, a reduction of soot concentration was observed.
As for aggregates size, the radius of gyration (R
g
) increased slightly with increasing height above the
burner for both reference and baseline diesel. The maximum median of R
g
was observed at 70 mm
HAB with
µg
=211 nm for the reference and
µg
=2 07 nm for the baseline; the minimum median was
measured at 30 mm HAB, with
µg
=178 nm for the reference and
µg
=169 nm for the baseline. The
comparison of the soot measured in the two diesels showed that the medium radius of gyration did
not vary significantly; however, the soot concentration decreased in the baseline diesel flame. This
baseline diesel contained a slightly reduced amount of aromatic species leading to the reduction of
soot emission, yet the aggregation process was not influenced.
It may be concluded that the novel burner system is capable of producing a sooting non-premixed
diesel flame, which forms a suitable basis for the analysis of soot formation for different realistic diesel
and other liquid fuels with a high vaporization temperature. Furthermore, the combination of LII and
WALS turned out as a suitable tool for the investigation of soot formation in such a burner. Since soot
formation in a vapor fuel flame is mainly influenced by the fuel chemical composition, we are now able
to single-out the effect of minor changes of fuel chemical composition on soot formation, for example,
in the case of diesel additives, which are a promising way to reduce soot emission from engines.
Investigations on the influence of diesel additives (which are typically in the range of 1–3 wt.% or even
lower) on particle emissions, as well as the analysis of biofuels are planned for future experiments.
Supplementary Materials:
The following are available online at http://www.mdpi.com/1996-1073/12/10/1993/s1,
Table S1: Maximum and mean fvwith the corresponding standard deviations for reference and baseline diesel.
Author Contributions:
M.K., N.P. and L.Z. designed the burner; N.P., P.B., M.N.M. performed the experiments;
N.P. and F.J.T.H. performed the data evaluation; N.P., P.B., M.N.M., M.K., L.Z., F.J.T.H. and S.W. wrote the paper.
Funding:
The financial support from the European Union’s Horizon 2020 research and innovation program
under the Marie Skłodowska-Curie grant agreement No. 675528 (Project: IPPAD) is gratefully acknowledged.
We acknowledge support by Deutsche Forschungsgemeinschaft and Friedrich-Alexander-Universität
Erlangen-Nürnberg (FAU) within the funding program Open Access Publishing.
Acknowledgments:
The authors thank Leo Bahr and Simon Aßmann for supporting the measurements and
post-processing. Furthermore, the authors thank Afton Chemical Corporation for the supply of the baseline diesel.
The authors acknowledge support by Deutsche Forschungsgemeinschaft and Friedrich-Alexander-Universität
Erlangen-Nürnberg (FAU) within the funding program Open Access Publishing.
Conflicts of Interest: The authors declare no conflicts of interest.
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