POSTER SESSION 1
SPECTRAL RADIATION MEASUREMENTS OF A CONFINED TURBULENT NATURAL GAS DIFFUSION FLAME
B. GANZ**, P. SCHMITTEL*, R. KOCH**, B. LENZE*, S. WITTIG**
* Engler-Bunte-Institut, Bereich Feuerungstechnik
** Institut für Thermische Strömungsmaschinen
Universität Karlsruhe (T.H.)
Radiation heat transfer in flames depends strongly on the local quantities such as pressure,
temperature and concentration of participating species. In the present study, detailed measurements
of the concentration and temperature field of a model combustor are presented together with an
experimental analysis of the spectral radiative heat transfer. The combustor was defined by the flame
research group TECFLAM as a standardized flame for an extensive measurement campaign of the
reacting fluid flow. The geometry of the combustion chamber (D=0.5m), the flame configuration
(type-II swirling diffusion flame) and the highly turbulent flow conditions were selected to resemble
the characteristics of industrial combustors. The combustor is fired by natural gas. The thermal
power is about 150 kW at an air fuel ratio of 0.83 and a swirl number of 0.9. The walls of the
combustion chamber are cooled by water.
The concentration measurements of CO2 , H2O, CO, CH4 , NO, NOx , O2 and H2 were performed
isokinetically by means of a thermostatisized multihole suction probe and a gas analysis system. The
local mean temperatures and their fluctuations were measured by time-compensated thermocouples.
Temperatures and concentrations were recorded at 300 locations at 14 axial planes.
For the radiation measurements an IR-spectroscope has been employed. The radiation intensity
incident on the walls was measured spectrally and time resolved (sampling rate of 1000) at 11 axial
planes within the spectral range of 1.4 to 5.4 .
Induced by the swirl, a ring vortex flow structure with an inner and outer recirculation zone is
formed. The flame is located at the shear layer between the outer and the inner recirculation zone.
As typical examples, the results for two axial planes are presented. The plane x/D=0.5 represents the
beginning of the reaction zone. It is characterized by a highly complex flame structure, as it is
revealed by the temperature plot. The temperature records show a maximum at the edge of the inner
recirculation zone. This is the main reaction zone with near-stoichiometric conditions. Outside the
reaction zone, the temperature decreases as consequence of the injected cold combustion air, and
finally increases again in the outer recirculation zone. The plane x/D=5.0 represents the end of the
visible flame. Due to intensive mixing, the temperature profiles are almost even over the radius.
The radiation spectra and the corresponding fluctuations have also been recorded for both planes,
x/D=0.5 and x/D=5.0. Due to lean combustion conditions no soot radiation was detected and the
emission is restricted to the well known radiation bands of water vapour and carbon dioxide. In the
reaction zone, the 3.4 band of unburned methane is visible. Highest mean intensities and
fluctuations are found at x/D=5.0, at the end of the visible flame. This is due to the high temperatures
and concentration of emitting species at this location. Moreover, the hot gas zone is extended over
the whole cross-section of the combustor. Close to the nozzle at x/D=0.5, higher intensities as
expected from the flame geometry can be found. The reason is the recirculation of hot reaction
products within the outer recirculation zone.
AVAILABILITY OF BAND MODEL FOR THE CALCULATION OF SPECTRAL ABSORPTION COEFFICIENT OF HIGH TEMPERATURE COMBUSTION GAS
Shinya Kamiyama, Junko Konno and Masakazu Yamazaki
Thermal Energy and Combustion Engineering Department
National Institute for Resources and Environment
16-3 Onogawa, Tsukuba, 305 Japan
For radiative heat transfer in a furnace filled with high temperature combustion gas medium it has
been widely recognized that the spectral absorption/emission characteristics of CO2 and H2O should
be considered instead of the former gray body approximation. For the purpose of engineering
calculation of radiative heat transfer in a combustion furnace a number of band models have been
proposed1. In the present paper spectral absorption coefficients and total emissivities of CO2 and
H2O were numerically calculated by adopting the combination of Elsasser's narrow band model and
Edwards' wide band model for the range of temperature and partial pressure expected in a
combustion furnace. The availability of the relevant band models was discussed by comparing the
calculation results with existing experimental measurements.
According to Elsasser's narrow band model2 the spectral absorption coefficient at a wave number
is given by
where S is the line intensity, d is the line spacing, is the line half-width and designates the
spectral location of each band. The parameters of S/d and in Eq.(1) are specified by
Edwards' wide band model2-3.
where is the integrated band intensity, is the exponential decay width, is the mean-line-width-
to-spacing parameter and Pe is the equivalent broadening pressure. Substituting Eqs.(2) and (3)
into Eq.(1) gives
For large the lines become broad compared to their spacing and the line structure is lost as
the lines strongly overlap and can be neglected. Under this condition Eq.(4) is
simplified and becomes
Adopting this simplified form of the model makes the numerical calculations very convenient4.
Using Eq.(4) or Eq.(5) the total emissivity of CO2 and H2O are calculated by the following
where T is the gas temperature, Ib is the spectral intensity of blackbody and is the optical path
CALCULATION RESULTS AND DISCUSSION
Fig.1 compares total emissivities calculated from Elsasser model ( Eq.(4)) and its simplified form (
Eq.(5)) with measured values by Hottel5 at several conditions of optical depth, in limit of zero
partial pressure in a mixture having a total pressure of 1 atm. In both cases of CO2 and H2O the
total emissivities calculated from Eq.(4) show good agreement with the experiments in the whole
temperature range. Comparison of calculated emissivities from Eq.(4) with those from Eq.(5)
shows discrepancy in relatively lower temperature range and at smaller optical depth conditions.
This feature is more significant for the emissivities of H2O than those of CO2.
Fig.1 Comparison of the total emissivities calculated from the band models with experimental measurements.
: Elsasser model
: simplified form of Elsasser model
: measured values by Hottel
Fig.2 shows the effect of the partial pressure on the discrepancy of calculated emissivities caused by the simplification included in Eq.(5). At the temperature conditions in Fig.2 almost no difference between two cases is shown for CO2, however, for H2O, calculated emissivity from Eq.(5) gives about the half of that from Eq.(4) at some extreme conditions.
Fig.2 Effect of the partial pressure on the discrepancy between calculated emissivityies from Eq.(4) and Eq.(5)
: Elsasser model
: simplified form of Elsasser model
The discrepancy of calculated emissivities shown in Figs.1 and 2 suggests that the values of the
parameter are not large enough and discrete line structure in each band can not be neglected
at low temperature and small partial pressure conditions.
For engineering calculation of radiative heat transfer in a combustion furnace spectral absorption
coefficients and total emissivities of CO2 and H2O calculated from the combination of Elsasser
model and Edwards model proved to be well available. The simplified form of Elsasser model is
very convenient for the numerical calculations, however its availability is limited and it should be
noted that the simplified calculation may lead to very significant errors especially at low
temperature and small partial pressure conditions for H2O.
- Modest, M.F., Radiative Heat Transfer, McGraw-Hill, 1993
- Irvine, T.F.Jr. and Hartnet, J.P., Advances in Heat Transfer, Volume 12, Academic Press, 1976
- Edwards, D.K. and Balakrishnan, A., Thermal Radiation by Combustion Gases, Int.J.Heat Mass Transfer, Vol.16, pp 25-40, 1973
- Kudo, K et.al., Improvement of Analytical Method on Radiative Heat Transfer in Nongray Media by Monte Carlo Method, Trans.JSME (B), Vol.59, No.560, pp 1265-1270, 1993
- Hottel, H.C. and Sarofim, A.F., Radiative Transfer, McGraw-Hill, 1967
U.V. AND VISIBLE EMISSION FROM DIESEL SPRAY IN IGNITION CONDITIONS
Cavaliere A.*, Ciajolo A. **, de Joannon M.*, Ragucci R.**
* Dip. Ingegneria Chimica, Università Federico II, Napoli Italia
** Istituto di Ricerche sulla Combustione, C.N.R., Napoli, Italia
The spectral characteristics of spontaneous emission of an igniting spray injected in high
pressure, high temperature ambience are of interest not only for identification of species present
during this part of combustion process, but also because they can help in the understanding of
electromagnetic energy transfer from emitting regions to adjacent region where fuel mixture is
not in the temperature range suitable for autoignition yet.
The results presented here evidence the presence of visible emission very early after
ignition time before the soot formation to which this visible emission was erroneously related in
A five-hole injector, in the class of light duty engines, injects 6 mg/stroke/hole of
tetradecane into a high pressure (3.6 MPa), high temperature (873K) nearly quiescent air
environment. The injection lasts 1.2 ms as it is shown in the "window" of Fig. l. Only one of
the five spray plumes, tilted 53o on the horizontal plane has been studied during experimental
The ignition delay, defined as the time from injection at which the first luminosity is
detected, is measured by means a UV-enhanced photodiod with a sensitivity spectral range from
200 to 1150 nm.
Fig. 1 Optical set-up.
The core of the diagnostic apparatus is a Jobin-Yvon spectrograph. The objective lenses
in front of the spectrograph collect the light from a linear objective field which is focused on the
entrance slit of the spectrograph. The holographic plate of the spectrograph separates along one
direction the different wavelengths components of the light, so that at the output section a 2-D
field is formed in which one direction represents a spatial linear image of the objective feld at
fixed wavelength whereas the perpendicular direction represents the spectral dispersion at a
fixed spatial position1. The photocathode entrance section of a micro channel plate is placed in
the image plane of the spectrograph outlet. This intensifies the incident light with adjustable gain
up to fixed intensity of electron excited phosphorescent light. A CCD-camera collects this light
from the phosphorescent screen generating a discretized analog image that is then converted to a
digital one using an A/D converter.
Stray light rejection, filtering of supernumerary wavelength contribution, spectral range
identification, intensity calibration have been obtained by means of light sealing, band pass
filter, filament and gas lamps.
RESULTS AND DISCUSSION
Some spatial-spectral patterns of light emitted from the whole spray, have been collected
at several times after ignition ( is the algebraic difference between the measuring time and
the ignition time). An example of the optical characterization is shown in Fig. 2 where two
single images (relative to two single injection events) of light emitted spontaneously from the
igniting spray taken at =0.17 ms and at =0.30 ms are reported For each image two profiles
of light intensity have been reported (in the right part) versus the collection wavelength () for
two spatial positions. Below the pictures, two profiles of light intensity versus the spatial
position in the spray at wavelengths of 550 and 350 nm have been reported. The spectral
profiles show as, just after the ignition has taken place, an ultraviolet contribution to the emitted
light can be detected along with a visible emission one. This latter increases with time becoming
the predominant one, but also an ultraviolet contribution can be detected over the whole
investigated time domain. The analysis of pattern like those reported in the figure, or of average
pattern, permits a thorough characterization of both spatial and spectral behaviour of light (due
to spontaneous or stimulated emission) coming from the burning spray. The emission could be
observed at all distances greater than 10 mm from the nozzle exit, but it seems to extend
downward at higher times due to the flame propagation. It must be underlined as the patterns
are not spatially uniform (for the single event images), testifying the intrinsic randomness of the
process. An extensive illustration of the emission characteristics with time is shown in the
results reported in the following part.
Fig.2 Examples of pattern analysis by means of spatial and spectral profiles determination.
Temporally averaged images have been reported in a false colour scale, on the left side of
Fig. 3 for from 0.04 to 0.36 ms. On the right side spatially averaged intensity profiles of
these patterns are reported.
Just few milliseconds after ignition an emission signal, ranging from ultraviolet to visible,
could be detected, as it is shown by intensity profile at = 0.04 ms. At this time luminous
signal extends in a limited part of the spray, as the blue zone in the visible range of
corresponding spectral pattern evidences.
Fig 3 Average emission patterns and intensity profiles at different time after ignition (), at T=873K, P=3.6 MPa.
At = 0.08 ms the signal intensity increases and its visible part expands along the
spray axis. The light intensity reaches a relative maximum at = 0.12 ms and slightly
decreases at = 0.16 ms. After this time it grows continuously in the whole investigated
In the full presentation the comparison of the temporal profile of the emission intensity
with the temporal profile of scattering intensity is also presented. This shows that visible
emission precedes the first formation of soot so that thermal emission from carbonaceous
particles can be neglected.
- Ragucci R., de Joannon M., Cavaliere A.: 26th Symp. (Int) on Combustion (in press) The Combustion Institute, Pittsburgh ( 1996).
PYROMETRIC MEASUREMENTS OF FUEL PARTICLE TEMPERATURE AND SIZE IN A PRESSURISED ENTRAINED FLOW REACTOR
Timo Joutsenoja, Jari Stenberg, Rolf Hernberg
Tampere University of Technology, Department of Physics, Plasma Technology Laboratory
P.O.Box 692, FIN-33100 Tampere, Finland
ABSTRACT. A two-colour pyrometric method for simultaneous in situ measurement of temperature
and size of individual fuel particles in a pressurised entrained flow reactor (PEFR) has been developed.
The method includes a novel technique for particle discrimination based on a two-focus optical
system. This technique can be used for the identification of measured particles through a single optical
port. The particle sizing is based on the proportionality of the measured radiative flux and the cross
sectional area of a particle at known temperature. Several series of measurements were made at a
PEFR at different process conditions and some typical results are presented.
The combustion of coal is a complex phenomenon involving the effects on heterogeneous and
homogenous reactions, heat and mass transfer and two-phase fluid flow. In recent years several
research groups have been studying the combustion of coal and char in order to get better fundamental
understanding of the combustion process and aiding in the development and evaluation of
comprehensive combustion models. The temperature of burning fuel particles affects both the
combustion rate and the characteristics of combustion products. The fuel particle temperature and size
also have a significant role, in particular, when the radiative heat transfer is studied. In consequence,
the accurate measurement of fuel particle temperature has an important contribution to this research.
Optical two-colour and three-colour pyrometry has been widely used for particle temperature
measurements in flow reactors. Simultaneous non-intrusive in situ particle size measurements have
been performed in combustion reactors using methods based on the coded aperture method and
particle imaging. A common feature of these sizing techniques, as well as techniques based on light
scattering, is that they require optical access from at least two non-collinear directions. This
requirement cannot always be met, especially in pressurised installations. Therefore, there was an
interest to develop a technique by which the particle size can be inferred from optical measurement
through a single port.
The industrial scale pulverised coal boilers typically burn particles around 100 mm. However, there is
scatter in particle size, and laboratory scale studies have included particles that are typically at the
range from 30 to 300 . The gas temperature in commercial pulverised coal boilers is typically 1500-
1700 K, but the laboratory studies often have lower temperatures down to 1000 K. The observed fuel
particle temperatures vary significantly due to strong dependence on the process conditions and
properties of fuel. Measured temperatures have varied from the gas temperature up to 2500 K.
The optical setup of the pyrometric measurements in a flow reactor is shown in Fig. l. Only one
optical port is necessary, in principle. Here the opposite port is forms the cold background for the
optical field of view. The absence of radiation from the plug is of some advantage for the accuracy,
particularly if particle temperatures are measured under pyrolysis conditions. In such a case, namely,
the fuel particles may be of the same temperature as or even colder than the reactor wall. Under
combustion conditions the particle temperatures are typically several hundred degrees above the wall
temperature, and the temperature measurement against a hot wall does not cause problems.
Radiation from a particle is collected via a lens system and focused to the ends of two optical fibres.
One of these fibres, called the primary fibre, is used for pyrometry. The other one, called the reference
fibre, is used for particle discrimination. The radiation entering the primary fibre is measured in a
radiometric unit on two wavelength bands. The lens system has been optimised so that the effective
focal length is the same at centre wavelengths of both bands. The used wavelengths were chosen so
that emission bands of combustion gases and absorption of media gas do not affect the temperature
When a fuel particle passes through the probe's field of view a pulse is detected in the pyrometric
signals. The particle temperature is solved using the ratio of the pulse heights measured at the two
wavelength bands. The pyrometric response signal is proportional to the particle cross sectional area in
the FOV when the particle temperature is fixed. This proportionality can be used for particle sizing.
The evaluation must be performed at a moment when all of the particle is visible to the detecting
device. The crucial part of the method is to be able to determine when full visibility is at hand. This is
done using the particle discrimination procedure, which is based on dual-focus optics.
Figure 1 shows the end-faces of the primary optical fibre, which is used for the transport of the
pyrometric signals, and the thinner reference flbre, which is used for particle discrimination. The
reference fibre is located next to the primary fibre in the flow direction. The discrimination procedure
is based on the ratio of intensities measured on the same wavelength band from reference and primary
fibres. The optical probe can be optimised so that the measuring volume and detection limit are the
most practical for the fuel particle number density, size distribution and expected temperatures of the
experiment. This is made by selecting the f-number of the optics, the magnification and the diameters
of the fibres.
The uncertainty of the particle temperature measurement is ± 30 K, which include both the accuracy of
calibration and uncertainty due to electronic noise. The uncertainty of particle size measurement is at
most ± 10 % for temperatures above 1500 K. The sources of uncertainty in sizing are the uncertainty in
temperature determination, emissivity of particle, solid angle and noise. At lower temperatures the
uncertainty of particle sizing grows with decreasing temperature. The method has been described in
greater detail in Ref. [ 1 ].
Figure 1. The optical setup of pyrometric measurements at a pressurised entrained flow reactor.
Figure 2. Typical results from the fuel particle temperature and size measurements at a pressurised entrained flow combustion reactor. Fuel particle temperature (a) and size (b) distribution, individual fuel particles in temperature-size plane (c). (Gardanne lignite, Tg =1153 K, c[O2] =15 vol%)
Several series of measurements have been made at the PEFR of VTT Energy in Jyväskylä, Finland,
with various fuels and process conditions. Figure 1 shows the optical setup and instrumentation. The
reactor consists of a heated ceramic tube of 60 mm diameter and about 2 m length. A typical reactor
temperature is 1000-1300 K. The reactor is contained inside a pressure vessel designed for operation
up to 2 MPa pressure. Observation ports with 20 mm diameter are located at five levels. The
background of the measurement was either cold or at the reactor wall temperature, depending on
whether the measurement was made through one of two paired collinear observation ports or through
an unpaired port. Figure 1 shows the case of a paired port.
Figure 2 shows typical distributions of fuel particle temperature (a) and size (b). Dp and Tp of
individual particles of the same experiment are plotted in Fig. 2c. The fuel in the experiment was
Gardanne lignite that was sieved to a nominal particle size fraction of 140-180 mm. The location of
the measurement downstream of the beginning of the reactor tube corresponds to a particle residence
time in the reactor of 130 ms. The temperature distribution is relatively narrow (standard deviation 95
K) due to the relatively homogenous fuel sample, the uniform environment and dilute suspension. The
correlation between temperature and size was usually weak. Detailed report of the experiments at VTT
Energy is given in Ref. .
- Joutsenoja, T., Stenberg, J., Hernberg, R. and Aho, M., Pyrometric measurement of the temperature and size of individual combusting fuel particles, Applied Optics, Vol. 36, pp.1525-1535,1997.
- Joutsenoja, T., Stenberg, J., Hernberg, R., Aho, M., Richard, J.R., Mallet, C. and Bonn, B., Pyrometric fuel particle measurements in pressurised reactors, JOULE II Extension programme, Novel Approaches in Advanced Combustion Technologies, Contract JOU2-CT93-0331, Final Report,1996
A NUMERICAL AND EXPERIMENTAL STUDY OF RADIATION FROM A TURBULENT PROPANE-AIR FLAME
D. Eklund, A. Haghighi, T. Badinand, T. Fransson
Department of Energy Technology,
The Royal Institute of Technology
S-100 44 Stockholm, Sweden
A long term effort has been started to investigate the role that radiation plays in the overall heat
transfer to wall liners and in the rate of emissions of NOx and other pollutants within gas turbine
combustion chambers. In the present paper radiation from combustion gases with cylindrical
geometries is studied. The finite volume technique is used to solve the axisymmetric Radiative
Transfer Equation (RTE). The first case considered is an absorbing, emitting but nonscattering gas
at a constant temperature within a black-walled cylindrical enclosure at zero temperature. Two
different discretization schemes are used to relate the intensities along a control volume face to the
cell center values: the step scheme and the higher order scheme of Raithby and Chui (1990). The
accuracy (relative to the exact solution) and efficiency (in terms of cpu time) of the two
discretization schemes are compared. The finite volume technique for the RTE is then coupled to a
three dimensional CFD code to examine the radiative heat transfer from three axisymmetric
chemically reacting flow fields. The first two cases are a turbulent flame stabilized behind a
cylindrical body and a furnace. These cases were presented by a 1994 ERCOFTAC Workshop on
turbulent non premixed flames. The third case is an experiment performed at KTH in which a
propane jet is injected into a concentric coflow of air within a cylindrical enclosure. The fuel and
air are originally separated by a bluff body that is used to stabilize the flame. Measurements and
calculations of the intensity are compared.
MODELLING OF RADIATION FLUCTUATIONS: ONE-DIMENSIONAL STUDIES
W M G Malalasekera and J C Jones
Department of Mechanical Engineering
Loughborough University, Loughborough
Leicestershire LE11 3TU, United Kingdom
In the area of combustion modelling, there are a number of issues which need further
research and improvements. Among them, the accurate description of turbulence/radiation
interaction and the effect of radiation fluctuations (up to 25-30% in certain practical
instances of turbulent combustion) on combustion have received very little attention. Since
radiation is not a local effect, evaluation of radiation on the basis of local mean properties in
a highly turbulent situation can give rise to significant errors on mean radiation intensities.
Moreover, the radiative fluctuations are dependent on the spectral radiative properties of the
combustion gases (CO2, H2O, CxHy), which are affected by temperature. The presence of
spectral dependence greatly increases the complexity of the problem. In addition, since all the
different geometrical locations surrounding a point contribute to the fluctuations of the
radiative intensity, local fluctuations are the result of integration over all possible radiation
The present study attempts to provide quantitative evidence of the effects of
fluctuations and develop a modelling approach that will account for turbulence/radiation
interactions in combustion. The work seeks to develop a modelling approach for
turbulence/radiation interaction effects requiring modest computing resources based on the
discrete transfer method of radiative transfer. The discrete transfer method is based on a ray
tracing technique and offers a way of incorporating local variations of temperature and
species concentration as well as fluctuating effects along the path contributing to the local
intensity. For a control volume of concern, the information regarding the radiative
contributions from the surrounding points is available as part of the calculation procedure.
This can be used as a basis to develop a technique to account for the effect of radiative
fluctuations due to fluctuations of temperature and species concentrations.
In the work presented here simple one-dimensional parametric studies have been
considered as an initial attempt to identify the most important parameters in radiation
fluctuating situations. One-dimensional problems involving fluctuating temperatures and
absorption coefficients have been solved to calculate resulting wall flux distributions and
source distributions. A number of test cases have been considered with sinusoidal
fluctuations at a local layer and the Discrete Transfer method has been used to solve for wall
flux distributions and source term distributions. The results obtained using the discrete
transfer method have been validated using the Monte Carlo method and steady state (non-
fluctuating) solutions of both methods have been compared with analytical solutions.
Further, one-dimensional simulations have been repeated using random fluctuations of
temperature and absorption coefficients and resulting mean flux distributions have been
calculated. These results will be used to identify the effects of fluctuations and their
significance on mean flux and intensity distributions. One-dimensional results will also be
used to identify suitable assumptions for averaging the transfer equations and authors intend
to develop a technique of incorporating radiation fluctuations in multi-dimensional systems
using the discrete transfer method.
THE SPECTROSCOPIC CHARACTERISATION OF DIFFERENT KINDS OF CARBONACEOUS AEROSOLS PRODUCED IN PREMIXED LAMINAR ETHYLENE/AIR FLAMES
S. Carlucci*, A. D'Alessio *, G. Gambi* , P. Minutolo**
* Dip. di Ingegneria Chimica, Università degli Studi di Napoli Federico II, Napoli, Italy
** Istituto di Ricerche sulla Combustione, C.N.R., Napoli, Italy
In the last few years it has been found that the formation of soot particles in flames, i.e. highly
absorbing particles with a typical size around 20 nm, is accompanied and sometimes preceded by the
formation of particles with a much smaller size around 2-3 nm, absorbing in the ultraviolet region
but transparent to the visible light (1). Both classes of particles contribute to the presence of
carbonaceous aerosols in the atmosphere and it is therefore of great interest to characterise them
The object of this communication is the spectroscopic characterisation, in the UV-visible region, of
ethylene/air premixed flames with C/O ratios ranging from the stoichiometric to the rich sooting
conditions by employing absorption spectroscopy, laser induced fluorescence and elastic light-
scattering. With regard to the presence of different aerosols different regimes are identified.
EXPERIMENTAL AND METHODS
Flat premixed laminar ethylene/air flames, with C/O ratio ranging from the 0.33 to 0.77, and the
material sampled in the flame were examined by employing a Nd:YAG laser (o=266 and 532 nm)
and a high pressure Xenon lamp as light sources.
The condensable material produced in flame, sampled by employing a stainless steel water cooled
probe, was stopped in a cold trap containing distilled water placed on the sample line. Soot particles
were sampled on a quartz plate inserted into the fully-sooting zone of the flame.
In analogy to the aromatic molecules, in carbonaceous structures the energy gap between the highest
occupied electronic level in the valence band and the lowest unoccupied level in the conduction
band, namely the band gap, decreases with the increase of the degree of aromatization of the species.
The band gap has been evaluated from the measured absorption spectra following the procedure
described in (2).
The absorption spectra of flames with C/O ratios ranging from the stoichiometric (C/0=0.33) up to
0.5 show a high signal at =200 nm and yet they fall off near 230 nm. No ultraviolet laser induced
fluorescence signal is detectable.
Flames with C/O ratios in the interval between 0.5 and 0.7 show an absorption spectrum extended
from 200 nm up to 250 nm and a broad band UV fluorescence spectrum with a maximum around
320 nm. The scattering coefficient measured in these flames is appreciably higher than that due to
gaseous combustion products. The strong dependence of the light scattering coefficient on the mean
size of the scatterers suggests that high molecular mass structures are present in these flames. The
high value of the band gap found in these flames indicates that the transparent particles contain
islands with only few condensed aromatic rings.
Finally, flames with a C/0 ratio higher than 0.7 have an absorption spectrum which extends from the
near UV down to the visible and a visible fluorescence signal in addition to the ultraviolet feature.
Moreover, the scattering signal is much higher than that due to gaseous compounds thus indicating
the presence of soot particles. These structures are characterised by a low value of the band gap that
indicates a strong growth of the aromatic islands inside the particles. From the absorption spectra it
appears that the class of structures with a higher band gap is still present.
The "in situ" absorption and fluorescence measurements have been compared with those obtained on
particles sampled in flame and suspended in water. A very good agreement between the spectra was
found in flame regions where only transparent nanoparticles are produced.
When soot particles are also present the comparison of the absorption spectra measured in flame
with those of the soot particles collected on the quartz plate allows to separate the contribution of
soot from that due to nanoparticles.
- A.D'Alessio, A.D'Anna, A.D'Orsi, P.Minutolo, R.Barbella, A.Ciajolo, "Precursors formation and soot inception in premixed ethylene flames". Proc. Twenty-Fourth Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, PA, pp. 973-980, 1992.
- P. Minutolo, G. Gambi and A. D'alessio, "The Optical Band Gap Model In The Interpretation Of The U.V.-Visible Spectra Of Rich Premixed Flames". Proc. Twenty-Sixth Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, PA, pp.951-957, 1996.
SOME PROBLEMS IN APPLICATION OF MONTE-CARLO METBODS TO THE CALCULATION OF RADIATION TRANSFER NEAR REFLECTING SURFACES
Rumynsky A.N., Katasonov A.A., Voronin V.V., Itina T.E.
Russia, Kaliningrad, Moscow Region, TsMIIMASh
It is well known that in exact mathematical formulation the problem of
statistical simulation of radiation transfer in scattering substance can be reduced to
the calculation of the following value 
where x is the phase vector (for example photon position r and direction of its
movement v), collisions phase density is a decision of integral equation
k(x',x) may be regarded as a probability distribution of photon transition x'--> x,
f(x) is phase density of initial collisions. The appearance of k(x',x), f(x) and
function h(x) (for calculation of various physical values) is well studied.
If scattering medium contains reflecting surface S the appearance of k(x',x)
has to be modified. We write it in the following form:
where among obvious and common indications R is a point where ray r = r'+vs
crosses the surface S, ls(r',v) is a distance along this ray to the surface S, s(r) is
surface -function, F(r',r) =1 if one point is seen from another one and F(r',r)=0
The most attractive feature of Monte-Carlo method is the possibility of
calculation of radiation intensity, which is proportional to . The method of
"double local estimator"  is the most universal one for this purpose. According to
this method one constructs the function h(x) only for photons arriving to the selected
ray through intermediate collision (calculations for other ones are trivial). The
presence of reflecting surface results in necessity of calculation for three various
functions, which for protuberant correspond to the following photons transitions
The calculation of the second and third values is connected with "local
estimators" for points on the surface S.
- from medium to the selected ray;
- from reflecting surface to the selected ray;
- from medium to the point where selected ray crosses reflecting surface;
If one does not take some special precessions the dispersion of all three values
is infinite [l, 2] this leading to inadequate accuracy of the calculations.
The infinity of the dispersion for the first value may be eliminated by
re-selection procedure  for free point of integration along the selected ray.
The most convenient method for calculation of "local estimators" type for
points in medium is proposed in the article . It is constructed in such a way that
combination of contributions from consecutive point of Markov's chain leads to final
value of estimator dispersion. The application of this method for points on a surface is
not obvious because "next" point of Markov's chain may be not seen from the point
under consideration. We have shown that corresponding estimator has final value of
dispersion also for points on the surface.
- Ermakov S.M., Mikhailov G.A. Statistical simulation. Moscow, "Science" 1986,
- Calos M., Steinberg H. Bounded estimators for a llux at a point in Monte-Carlo. Nuclear Science and Engineering 1971, 44, 406-412.
- Voronin A.F., Khisamutdinov A.I. Estimatíons with additional casual sample for calculation of particles flux "in a point". Journal of Calculation Mathematics and Mathematical Physics,1985, 25, 8,1155-1163.