Determination of Soot Volume Fraction and Primary Particle Size in ...

Determination of Soot Volume Fraction and Primary Particle Size in Laminar
Non-Premixed Hydrocarbon Flames under Microgravity Using Laser-Induced
Incandescence
Monika Wendler ∗ , Roland Sommer, and Alfred Leipertz
Lehrstuhl für Technische Thermodynamik
Universität Erlangen-Nürnberg
Erlangen, Germany
Abstract
Time-resolved laser-induced incandescence (TIRE-LII) has been applied to measure the volume concentration and
the primary particle sizes of soot under different gravity levels during parabolic flights. This has been carried out
for laminar non-premixed ethene and methane flames for different fuel flow and air co-flow conditions. It could be
shown that there is a strong influence of the amount of air co-flow on soot volume concentration but only a minor
influence on primary particle size under microgravity. This is in contrast to normal gravity and 1.8 g, in which no
influence of the amount of co-flow could be found, neither on soot volume concentration nor on primary paricle
size. Under microgravity, in ethene flames the primary particle sizes are about double the size of those at normal
gravity. Methane flames show all-over smaller primary particle sizes than ethene flames.
Introduction
Soot formation is one of the most important
processes in combustion systems, but it is still not
fully understood [1, 2]. Under normal gravity these
processes are mainly dominated by the natural convective
flows and other processes. Diffusion effects, however,
have nearly no influence. This is different under
microgravity. Microgravity generally offers unique
prospects for the understanding and improvement of
combustion processes, as the absence of buoyancy allows
measurements with low-speed flows resulting in
an enlargement of spatial and temporal scales. The
possibilities of a full control of residence times and of
an observation of basic combustion phenomena without
buoyancy-induced flow superimposed, make it also
possible to establish and improve models for soot formation.
The mathematical treatment of combustion
processes is simplified [3], and therefore they are studied
in different microgravity facilities like drop towers,
parabolic flights with aircrafts or in space [4].
Laminar non-premixed flames have been of special
interest in microgravity combustion research as they
provide a good model system for numerical studies.
For that reason, they have also been chosen for the experiments
presented in this paper. For an assessment
of particle growth, which is essential for a detailed
comprehension and modelling of soot formation, not
only soot concentration profiles but also primary particle
sizes have to be determined. A measurement technique
which is able to measure both quantities simultaneously
is time-resolved laser-induced incandescence
(TIRE-LII) [5, 6, 7].
∗ Corresponding author: mwe@ltt.uni-erlangen.de
Associated Web site: http://www.ltt.uni-erlangen.de
Proceedings of the European Combustion Meeting
The basic principle of laser-induced incandescence
(LII) is to heat up particles with a short laser pulse of
high energy up to their vaporization temperature, followed
by the detection of the enhanced thermal radiation
[8]. The heated particles release the absorbed energy
via three paths: vaporisation, heat conduction to
the surrounding medium and thermal radiation. For the
whole process, the energy balance is set up which leads
to a differential equation which can be solved numerically
for obtaining the temporal behaviour of the LII
signal [6, 9].
It has been shown that the intensity in the moment
of the laser pulse is nearly proportional to soot volume
concentration which has also been proven in several
experiments, see, e.g., [10, 11]. Moreover, the evaluation
of the temporal signal decay provides primary
particle sizes [5, 7, 12], because later than 100 ns after
the laser pulse conductive heat loss to the surrounding
gas is the dominant cooling process. This is mainly
governed by the particles’ specific surface area, i.e.,
smaller particles cool down faster corresponding to a
faster decreasing signal. Primary particle size is determined
by comparison of experimental decay rates with
calculated model signal decay rates. For the determination
of a two-dimensional distribution of primary particle
size the signal is measured at two different moments
after the laser pulse. From the ratio of these two signals
the primary particle size can be calculated. Renouncing
spatial resolution the whole signal decay can be evaluated
which results in improved statistics.
Some studies concerning soot volume concentrations
in flames under microgravity using LII have al-

eady been done [13, 14]. In the present work, not only
soot volume concentrations but also primary particle
sizes have been measured spatially resolved in laminar
non-premixed ethene and methane flames. A special
interest was set on the influence of the variation of air
co-flow on soot parameters under different gravity conditions.
The emphasis of this paper is the description
of the phenomena discovered. Only some of the results
obtained could have been explained so far and will be
the subject of future experimental an theoretical work.
Experimental Setup
The experiments have been carried out during an
ESA parabolic flight campaign aboard an Airbus A300
aircraft. Each parabola was initiated and closed by a
time period of increased gravity (1.8 g), embedding an
about 20 second period of microgravity. Thus, including
the time between two parabolas, the flames could be
observed under three different gravity conditions. Due
to safety reasons aboard the aircraft, the setup had to
be completely integrated in a rack, as shown in Fig. 1.
The burner was installed inside a chamber connected
to the exhaust line of the overboard ventilation system.
The burner chamber was equipped with four optical
windows for the access to the measurement location.
Magnetic and flame-non-return valves were installed to
ensure a safe operation of the gas supply, and all relevant
parameters were controlled automatically, especially
the amount of fuel and air co-flow, which was varied
systematically to determine the influence of these
parameters on soot formation under microgravity. The
burner itself consisted of two concentric tubes, the inner
one (2.2 mm in diameter) for fuel (ethene or methane)
the outer one (30 mm in diameter) for air co-flow (synthetic
air 20.5 % O2, 79.5 % N2). Besides, a second air
co-flow was integrated to prevent the dimming of the
optical windows by condensed vapour.
Laser power supply
Laser head
Monitor #1 Monitor #2
Electronics Camera controller #1 Camera controller #2
ICCD camera #2
burner
chamber
Figure 1: Schematical drawing of the experimental setup
The setup was chosen as a result of the experience
of an earlier campaign with a different burner
configuration, where the flame structures had been disturbed
by short periods of negative gravity forces during
the parabolas, resulting in a complete collapse of the
ICCD camera #1
2
flames. The actual layout was constructed to be able to
increase the gas flows, providing more stable flames.
Nevertheless, also in this new experimental approach
the flame shape was affected by g-jitter which
will be described in detail later. The soot particles were
heated up to their vaporisation temperature by using
a frequency-doubled Nd:YAG laser with a pulse frequency
of 10 Hz and a pulse duration of 6 ns. The beam
was formed to a light sheet with a width of 3 cm and
a thickness of 0.5 cm which leads to an irradiance of
1·10 8 W/cm 2 for a typical pulse energy of 75 mJ. Modifications
of the laser were done by the manufacturer
to ensure functionality under microgravity conditions.
Thus, a variation of laser output was not expected and
there was no continuous control of pulse energy during
the flight installed. It was only measured prior and after
every flight indicating no differences. In the course of
the data evaluation, however, indications for laser pulse
energy fluctuations appeared. This poses an uncertainty
especially on the results of the primary particle size determination.
The reason for this unreliable behaviour
of the laser under microgravity has not been found yet.
The LII signal was detected by using two ICCD cameras,
placed opposite each other, perpendicular to the
laser beam, as it can be seen in Fig. 1. It turned out
that the camera sensitivity varied up to 25 % between
two following images. In addition, the extent of this effect
was dependent on the chosen intensifier gain value.
Thus, the results for ethene flames and methane flames
cannot be compared quantitatively as different gain values
had to be used due to lower soot concentrations in
the methane flame. Nevertheless, a lot of valuable information
could be acquired, which will be presented
hereafter.
The maximum signal was captured by one of the
cameras and compared later to a single line-of-sight extinction
measurement for calibration in order to get information
on the absolute soot volume concentration.
For the determination of the primary particle sizes the
LII signal decay was evaluated by detecting at two different
times, 100 ns and 400 ns after the laser pulse.
From the signal ratio the signal decay time constant
can be calculated. Taking into consideration the ambient
temperature this ratio is unambiguously related
to the primary particle size. Unfortunately, due to experimental
problems, we have no information about the
temperature distribution in the examined flames. Therefore,
temperature information from measurements performed
at the ZARM drop tower in Bremen, Germany,
by project partners using the same burner configuration,
fuel and air co-flow parameters were used for data evaluation.
Results and Discussion
Flame behaviour During most of the parabolas the
influence of reduced gravity and the typical behaviour

under microgravity could be observed [15, 16]. All investigated
flames were broadened under microgravity
and longer as their counterparts under normal gravity.
However, the change of flame shape under µg was dependent
on the chosen gas and air flows. Some ethene
flames show no open tip under microgravity (see Fig. 3
and Fig. 4).
Figure 2: Sequence of LII-Signals during a collapse of a
flame due to g-jitter
Nevertheless, in some cases, short periods of negative
g-forces induced a fast breakdown of the flame.
Fig. 2 shows the LII-signal of an ethene flame in an exemplary
case. Unfortunately, even without flame breakdown,
the exact evaluation of the measurements still reveals
a strong influence of fluctuations in the quality
of microgravity on the flame shape. The behaviour of
the flame during the parabolas is not only determined
by the absolute value of remaining gravity, but depends
also on the course of the g-level during the beginning of
the parabola. Thus, a steep gradient with a slight overturn
into the region of negative accelerations causes a
different flame shape than a smooth beginning of the
µg-phase with some time of negative g. The latter behaviour
results more often in a collapsing flame. As
the effect of g-jtter is stronger in flames with low flow
rates, in future campaigns a compromise must be found
in chosing flow rates both leading to stable flames and
showing the effects of microgravity.
3
Soot volume concentration Figures 3 and 4 show
the soot volume concentration for ethene flames under
variation of air co-flow rate and ethene flow rate respectively.
Figure 3: Soot volume concentration for different ethene
flow rates (in ppmv): 75 scm3 /min (a and c) and
100 scm3 /min (b and d); air co-flow rate: 10 sl/min
Figure 4: Soot volume concentration for different air co-flow
rates (in ppmv): 15 sl/min (a and d), 10 sl/min (b and e),
and 8 sl/min (c and f); ethene flow rate: 75 scm3 /min
For the variation of air co-flow in ethene flames a
rather strong influence of the amount of air co-flow on
soot volume fraction and its distribution in the flame
can be observed under microgravity. As it can be seen
in Fig. 4, soot volume concentration increases with
rising ethene flow rate in outer flame regions and decreases
with falling air co-flow in the inner flame regions
resulting in an increased soot formation in a narrow
band at the outer regions. This is in contrast to
observations under normal gravity. A variation of air
co-flow has no significant influence on the amount of
soot produced in the examined flames under buoyant
conditions.
The influence of the variation of ethene flow rate is
shown in Fig. 3. Under normal gravity, only the amount
of soot produced rises with an increase of ethene flow
rate, the distribution of soot in the flames remains extensively
constant. In contrast to that, there is a decrease
of soot volume concentration in inner flame regions and
an increase in outer flame regions. The results obtained
during the 1.8 g phase are not shown, as the flames behave
similar to the 1 g flames. An explanation for this
difference in soot distribution between 1 g and µg can
be found in the temperature distribution in the flames
which is several hundred degrees lower in the micro-

gravity flames than at normal gravity, resulting in overall,
but also local larger soot production.
The results of the experiments with methane reveal
a different soot distribution under microgravity than
for the ethene flames (see Fig. 5). The soot distribution
is more concentrated in the inner flame regions
and nearly independent of the magnitude of the air coflow.
The soot concentration is overall lower than in
ethene flames. Regarding the influence of the variation
of the air co-flow no difference between the 1 g and
1.8 g phase could be observed.
Figure 5: Soot volume concentration (in a. u.) for methane
flames under microgravity (left) and normal gravity
(right)
Primary particle sizes Furthermore, the primary
particle size distribution has been determined inside the
flame for different fuels and air co-flow rates. As mentioned
above, there were indications for unstable laser
operation during the µg phase resulting in a clearly
lower pulse energies. The presented results are based
on the assumption of an irradiance of 5 MW/cm 2 . A temperature
distribution with a temperature gradient from
1700 K to 1400 K in microgravity was chosen in agreement
with drop-tower measurements, and an uniform
temperature of 1800 K for 1 g (obtained in previous
ground-based measurements with the same setup).
The reduced flame temperature under microgravity
can be expected due to the larger residence time of particles
in the flame and the increased radiation losses.
Figure 6: Primary particle sizes (in nm) for different air coflow
rates: 8 sl/min (a, d), 7 sl/min (b, e), 5 sl/min (c, f);
fuel flow rate: 75 scm3 /min under microgravity (a-c)
and 1 g (d-e)
It can be seen in Fig. 6 that there is only a minor
influence of air co-flow at 1 g as well as at 0 g. Under
4
microgravity, the largest particles can be found in the
same narrow band at outer flame regions where large
soot concentrations have been measured (see Fig. 3 and
Fig. 4). Compared to microgravity, under normal gravity
the maximum primary particle sizes are about half
the values which have been found under microgravity.
This may be due to the extended residence time of particles
in the flame under microgravity and the lower surrounding
gas temperatures.
For the methane flame, the results under microgravity
are displayed in Fig. 7. The particles are smaller
than in the ethene flames and more equally distributed
over the flame body which has also been found for the
soot volume concentration (Fig. 5). Again, the influence
of air co-flow on primary particle sizes is very
low. Under normal gravity, the soot volume concentration
was too low to measure particle sizes.
Figure 7: Primary particle sizes (in nm) in methane flames
under microgravity for different air co-flow rates:
a) 15 sl/min, b) 10 sl/min; methane flow rate: 110 scm3 /min
Conclusions
Experiments on ethene and methane flames in different
gravity environments have been performed. In
ethene flames, under microgravity soot volume fraction
increases with rising fuel flow rates and decreases with
falling air co-flow in inner flame regions resulting in an
increased soot formation in a narrow band in the outer
regions. The influence of a variation of air co-flow on
primary particle sizes is rather low, compared to 1 g,
primary particle sizes are roughly doubled under µg.
Neither under normal gravity nor under increased gravity
any influence of air co-flow on the sooting behaviour
could be found, the flames are in general very similar
under 1 g and 1.8 g. In methane flames, the soot volume
concentration showed a different behaviour under
microgravity than in ethene flames, being more concentrated
in the inner regions of the flame and nearly
independent on the air co-flow. The absolute values
were overall lower than in ethene flames, and significantly
smaller particle sizes have been found. TIRE-LII
has been proven to be a suitable non-invasive technique
for measuring the sooting characteristics of flames under
microgravity, providing spatially resolved informa-

tion on soot volume concentration and primary particle
sizes.
Acknowledgements
The authors gratefully acknowledge financial support
for parts of the work by the European Space
Agency (ESA) and by the German National Science
Foundation (DFG).
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