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1.
This study concerns the effect of soot-particle aggregation on the soot temperature derived from the signal ratio in two-color
laser-induced incandescence measurements. The emissivity of aggregated fractal soot particles was calculated using both the
commonly used Rayleigh–Debye–Gans fractal-aggregate theory and the generalized Mie-solution method in conjunction with numerically
generated fractal aggregates of specified fractal parameters typical of flame-generated soot. The effect of aggregation on
soot temperature was first evaluated for monodisperse aggregates of different sizes and for a lognormally distributed aggregate
ensemble at given signal ratios between the two wavelengths. Numerical calculations were also conducted to account for the
effect of aggregation on both laser heating and thermal emission at the two wavelengths for determining the effective soot
temperature of polydisperse soot aggregates. The results show that the effect of aggregation on laser energy absorption is
important at low fluences. The effect of aggregation on soot emissivity is relatively unimportant in LII applications to typical
laminar diffusion flames at atmospheric pressure, but it can become more important in flames at high pressures due to larger
primary particles and wider aggregate distributions associated with enhanced soot loading. 相似文献
2.
Fengshan Liu Kevin A. Thomson Gregory J. Smallwood 《Journal of Quantitative Spectroscopy & Radiative Transfer》2008,109(2):337-348
Absorption and scattering of laser-induced incandescence (LII) intensities by soot particles present between the measurement volume and the detector were numerically investigated at detection wavelengths of 400 and 780 nm in a laminar coflow ethylene/air flame. The radiative properties of aggregated soot particles were calculated using the Rayleigh-Debye-Gans polydisperse fractal aggregate theory. The radiative transfer equation in emitting, absorbing, and scattering media was solved using the discrete-ordinates method. The radiation intensity along an arbitrary direction was obtained using the infinitely small weight technique. The effects of absorption and scattering on LII intensities are found to be significant under the conditions of this study, especially at the shorter detection wavelength and when the soot volume fraction is higher. Such a wavelength-dependent signal-trapping effect leads to a lower soot particle temperature estimated from the ratio of uncorrected LII intensities at the two detection wavelengths. The corresponding soot volume fraction derived from the absolute LII intensity technique is overestimated. The Beer-Lambert relationship can be used to describe radiation attenuation in absorbing and scattering media with good accuracy provided the effective extinction coefficient is adequately. 相似文献
3.
Jun Hayashi Nozomu Hashimoto Noriaki Nakatsuka Hirofumi Tsuji Hiroaki Watanabe Hisao Makino Fumiteru Akamatsu 《Proceedings of the Combustion Institute》2013,34(2):2435-2443
Soot formation characteristics of a lab-scale pulverized coal flame were investigated by performing carefully controlled laser diagnostics. The spatial distributions of soot volume fraction and the pulverized coal particles were measured simultaneously by laser induced incandescence (LII) and Mie scattering imaging, respectively. In addition, the radial distributions of the soot volume fraction were compared with the OH radical fluorescence, gas temperature and oxygen concentration obtained in our previous studies [1], [2]. The results indicated that the laser pulse fluence used for LII measurement should be carefully controlled to measure the soot volume fraction in pulverized coal flames. To precisely measure the soot volume fraction in pulverized coal flames using LII, it is necessary to adjust the laser pulse fluence so that it is sufficiently high to heat up all the soot particles to the sublimation temperature but also sufficiently low to avoid including a too large of a change in the morphology of the soot particles and the superposition of the LII signal from the pulverized coal particles on that from the soot particles. It was also found that the radial position of the peak LII signal intensity was located between the positions of the peak Mie scattering signal intensity and peak OH radical signal intensity. The region, in which LII signal, OH radical fluorescence and Mie scattering coexisted, expanded with increasing height above the burner port. It was also found that the soot formation in pulverized coal flames was enhanced at locations where the conditions of high temperature, low oxygen concentration and the existence of pulverized coal particles were satisfied simultaneously. 相似文献
4.
M. Leschowski K. A. Thomson D. R. Snelling C. Schulz G. J. Smallwood 《Applied physics. B, Lasers and optics》2015,119(4):685-696
The measurement of soot and soot precursors is important for understanding the formation of soot particles in flames. In this paper, we use the difference between laser-induced incandescence (LII) and two-dimensional extinction measurements to assess the contribution of soot precursors to the extinction measurement. LII measurements are performed with a high spatial resolution of 100 µm to determine the soot volume fraction (f V) in a laminar ethylene/air non-premixed flame at the standard Gülder conditions. While LII is specific to mature soot only, the extinction data represent attenuation due to mature and young soot (absorption and elastic scattering) and also absorption by soot precursors. The difference between the two measurements indicates the contribution of soot precursors and allows a determination of the maturity of soot. This is important knowledge for those using extinction techniques to measure soot concentration, as the contribution from soot precursors may lead to an overestimation of the mature soot concentration. Further, regions with high soot-precursor concentrations, which lead to soot formation, can be identified. 相似文献
5.
Q. N. Chan P. R. Medwell Z. T. Alwahabi B. B. Dally G. J. Nathan 《Applied physics. B, Lasers and optics》2011,104(1):189-198
Nonlinear excitation regime two-line atomic fluorescence (NTLAF) is a laser-based thermometry technique that has application
in turbulent flames with soot. However, no assessment of the various interferences from soot or its precursors in flames with
high soot loadings on the technique is available. To examine these issues, both on- and off-wavelength NTLAF measurements
are presented and compared for laminar nonpremixed ethylene-air flames. Laser-induced incandescence (LII) measurements were
used to determine the corresponding soot concentration and location in the investigated flames. The measurements indicate
that interferences, such as spurious scattering and laser-induced incandescence from soot, are not significant for the present
set of flame conditions. However, interferences from soot precursors, predominantly condensed species (CS) and perhaps polycyclic
aromatic hydrocarbons (PAH), can be significant. Potential detection schemes to correct or circumvent these interference issues
are also presented. 相似文献
6.
M. Hofmann B.F. Kock T. Dreier H. Jander C. Schulz 《Applied physics. B, Lasers and optics》2008,90(3-4):629-639
This paper describes the applicability of laser-induced incandescence (LII) as a measurement technique for primary soot particle
sizes at elevated pressure. A high-pressure burner was constructed that provides stable, laminar, sooting, premixed ethylene/air
flames at 1–10 bar. An LII model was set up that includes different heat-conduction sub-models and used an accommodation coefficient
of 0.25 for all pressures studied. Based on this model experimental time-resolved LII signals recorded at different positions
in the flame were evaluated with respect to the mean particle diameter of a log-normal particle-size distribution. The resulting
primary particle sizes were compared to results from TEM images of soot samples that were collected thermophoretically from
the high-pressure flame. The LII results are in good agreement with the mean primary particle sizes of a log-normal particle-size
distribution obtained from the TEM-data for all pressures, if the LII signals are evaluated with the heat-conduction model
of Fuchs combined with an aggregate sub-model that describes the reduced heat conduction of aggregated primary soot particles.
The model, called LIISim, is available online via a web interface.
PACS 65.80.+n; 78.20.Nv; 42.62.-b; 47.70.Pq 相似文献
7.
Billy Kaldvee Christian Brackmann Marcus Aldén Joakim Bood 《Applied physics. B, Lasers and optics》2014,115(1):111-121
A novel concept for remote in situ detection of soot emissions by a combination of laser-induced incandescence (LII) and light detection and ranging (lidar) is presented. A lidar setup based on a picosecond Nd:YAG laser and time-resolved signal detection in the backward direction was used for LII measurements in sooty premixed ethylene–air flames. Measurements of LII–lidar signal versus laser fluence and flame equivalence ratio showed good qualitative agreement with data reported in literature. The LII–lidar signal showed a decay consisting of two components, with lifetimes of typically 20 and 70 ns, attributed to soot sublimation and conductive cooling, respectively. Theoretical considerations and analysis of the LII–lidar signal showed that the derivative was proportional to the maximum value, which is an established measure of soot volume fraction. Utilizing this, differentiation of LII–lidar data gave profiles representing soot volume fraction with a range resolution of ~16 cm along the laser beam propagation axis. The accuracy of the evaluated LII-profiles was confirmed by comparison with LII-data measured simultaneously employing conventional right-angle detection. Thus, LII–lidar provides range-resolved single-ended detection, resourceful when optical access is restricted, extending the LII technique and opening up new possibilities for laser-based diagnostics of soot and other carbonaceous particles. 相似文献
8.
K.A. Thomson D.R. Snelling G.J. Smallwood F. Liu 《Applied physics. B, Lasers and optics》2006,83(3):469-475
An auto-compensating laser-induced incandescence (AC-LII) technique was applied for the first time to measure soot volume fraction (SVF) and effective primary particle diameter (dpeff) in a high pressure methane/air non-premixed flame. The measured dpeff profiles had annular structures and radial symmetry, and the particle size increased with increasing pressure. LII-determined SVFs were lower than those measured by a line of sight attenuation (LOSA) technique. The LOSA measured soot volume fractions were corrected for light scattering using the Rayleigh–Debye–Gans polydisperse fractal aggregate (RDG-PFA) theory, the dpeff data, and assumptions regarding the soot aggregate size distribution. The correction dramatically improved agreement between data obtained using these two measurement techniques. Qualitatively, soot volume distributions obtained using LII had more annular shapes than those obtained using LOSA. Nonetheless, it has been demonstrated that the AC-LII technique is very well suited for application in media where attenuation of the excitation laser pulse energy can exceed 45%. This paper also underlines the importance of correcting LOSA SVF measurements for light scattering in high pressure flames. PACS 07-60.-j; 47.70.Pq; 65.80.+n; 78.67.-n 相似文献
9.
F. Migliorini S. De Iuliis S. Maffi F. Cignoli G. Zizak 《Applied physics. B, Lasers and optics》2009,96(4):637-643
Theoretical papers predict that prompt LII signals are weakly dependent on the soot size due to the fact that larger particles
reach higher temperatures during the heating process by nanosecond laser pulses. This question is of crucial importance for
establishing LII as a practical technique for soot volume fraction measurements. In this work two-color prompt LII measurements
have been performed in several locations of diffusion and rich premixed ethylene-air flames. The experimental apparatus was
carefully designed with a probe volume of uniform light distribution and sharp edges, a 4 ns integration time around the signal
pulse peak and narrow spectral bandwidth. Measurements did not confirm the theoretical predictions concerning an increase
of temperature for larger particles. On the contrary, larger particles in richer premixed flames exhibit a lower 400/700 signal
ratio. This can probably be attributed to small differences in the refractive index of soot. 相似文献
10.
A novel technique for two-dimensional measurements of soot volume fraction and particle size has been developed. It is based
on a combined measurement of extinction and laser-induced incandescence using Nd:YAG laser wavelengths of 532 nm and 1064 nm.
A low-energy laser pulse at 532 nm was used for extinction measurements and was followed by a more intense pulse at 1064 nm,
delayed by 15 ns, for LII measurements. The 532-nm beam was split into a signal beam passing the flame and a reference beam,
both of which were directed to a dye cell. The resulting fluorescence signals, from which the extinction was deduced, together
with the LII signal, were registered on a single CCD detector. Thus the two-dimensional LII image could be converted to a
soot volume fraction map through a calibration procedure during the same laser shot. The soot particle sizes were evaluated
from the ratio of the temporal LII signals at two gate time positions. The uncertainty in the particle sizing arose mainly
from the low signal for small particles at long gate times and the uncertainty in the flame temperature. The technique was
applied to a well-characterized premixed flat flame, the soot properties of which had been previously thoroughly investigated.
Received: 21 June 2000 / Revised version: 11 September 2000 / Published online: 7 February 2001 相似文献
11.
Theoretical analysis and numerical calculations were conducted to investigate the relationship between soot volume fraction and laser-induced incandescence (LII) signal within the context of the auto-compensating LII technique. The emphasis of this study lies in the effect of primary soot particle diameter polydispersity. The LII model was solved for a wide range of primary soot particle diameters from 2 to 80 nm. For a log-normally distributed soot particle ensemble encountered in a typical laminar diffusion flame at atmospheric pressure, the LII signals at 400 and 780 nm were calculated. To quantify the effects of sublimation and differential conduction cooling on the determined soot volume fraction in auto-compensating LII, two new quantities were introduced and demonstrated to be useful in LII study: an emission intensity distribution function and a scaled soot volume fraction. When the laser fluence is sufficiently low to avoid soot mass loss due to sublimation, accurate soot volume fraction can be obtained as long as the LII signals are detected within the first 200 ns after the onset of the laser pulse. When the laser fluence is in the high fluence regime to induce significant sublimation, however, the LII signals should be detected as early as possible even before the laser pulse reaches its peak when the laser fluence is sufficiently high. The analysis method is shown to be useful to provide guidance for soot volume fraction measurements using the auto-compensating LII technique. 相似文献
12.
H. Bladh J. Johnsson J. Rissler H. Abdulhamid N.-E. Olofsson M. Sanati J. Pagels P.-E. Bengtsson 《Applied physics. B, Lasers and optics》2011,104(2):331-341
Laser-induced incandescence (LII) is a versatile technique for quantitative soot measurements in flames and exhausts. When
used for particle sizing, the time-resolved signals are analysed as these will show a decay rate dependent on the soot particle
size. Such an analysis has traditionally been based on the assumption of isolated primary particles. However, soot particles
in flames and exhausts are usually aggregated, which implies loss of surface area, less heat conduction and hence errors in
estimated particle sizes. In this work we present an experimental investigation aiming to quantify this effect. A soot generator,
based on a propane diffusion flame, was used to produce a stable soot stream and the soot was characterised by transmission
electron microscopy (TEM), a scanning mobility particle sizer (SMPS) and an aerosol particle mass analyzer coupled in series
after a differential mobility analyzer (DMA-APM). Despite nearly identical primary particle size distributions for three selected
operating conditions, LII measurements resulted in signal decays with significant differences in decay rate. However, the
three cases were found to have quite different levels of aggregation as shown both in TEM images and mobility size distributions,
and the results agree qualitatively with the expected effect of diminished heat conduction from aggregated particles resulting
in longer LII signal decays. In an attempt to explain the differences quantitatively, the LII signal dependence on aggregation
was modelled using a heat and mass transfer model for LII given the primary particle and aggregate size distribution data
as input. Quantitative agreement was not reached and reasons for this discrepancy are discussed. 相似文献
13.
Francesca Migliorini Silvana De Iuliis Silvia Maffi Giorgio Zizak 《Applied physics. B, Lasers and optics》2013,112(3):433-440
A portable instrument based on two-color laser-induced incandescence (LII) technique has been designed and developed for the detection of carbonaceous particles for environmental applications. The instrument has been calibrated by performing LII measurements at the exhaust of a home-made soot generator. The incandescence signal from particles sampled into the instrument has been compared with in situ, calibrated, LII measurements to correlate the incandescence signal by the instrument with particles concentration. Measurements of particulate with the LII instrument were then conducted in different environmental conditions, covering a wide range of concentration (from ambient air to cars’ exhaust). The detection limit of the LII instrument has been estimated to be in the range of 200 ng/m3. These measurements have been also compared with results obtained with a commercial aethalometer. The results show a linear relationship between the two sets of measurements, also in the case where significant variation of the carbon particles concentration has been observed over time. These observations allow us to infer that the two instruments are responding in the same way to different carbon particles load, size and nature. 相似文献
14.
Nils-Erik Olofsson Jonathan Johnsson Henrik Bladh Per-Erik Bengtsson 《Applied physics. B, Lasers and optics》2013,112(3):333-342
Laser-induced incandescence (LII) as a diagnostic technique is based on rapid heating of soot particles to temperatures of several thousand Kelvin and subsequent detection of the thermal radiation from the laser-heated particles. At such high temperatures, soot sublimation effects must be considered when estimating uncertainties in LII measurements. In this work we have investigated the use of various laser fluences in LII using a Nd:YAG laser at 1,064 nm. Using another Nd:YAG laser at 532 nm, the elastic light scattering (ELS) signal from soot particles heated by the 1,064-nm laser was monitored. This approach makes it possible to determine at which fluence of the LII laser soot sublimation starts to become visible as a decrease in the ELS signal. By performing the measurements at various heights in a premixed sooting flat ethylene/air flame, the fluence threshold above which the ELS signal decreased was found to be higher at the lower flame heights corresponding to younger, smaller and less aggregated particles. The results from this work indicate that the different fluence thresholds for sublimation may be explained by a lower absorption function E(m) for the younger soot particles. 相似文献
15.
S. De Iuliis F. Migliorini F. Cignoli G. Zizak 《Applied physics. B, Lasers and optics》2006,83(3):397-402
In order to understand the processes involved in the laser-induced incandescence (LII) technique, the value of soot temperature at the peak of the incandescence signal has been studied. To this purpose, an absolute two-color LII technique has been applied on ethylene and methane diffusion flames, based on the comparison with a calibrated tungsten ribbon lamp. The dependence of peak temperature on the fluence has been investigated by using a sharply edged probe beam. Above a certain fluence threshold a value close to 4000 K was obtained for both flames at all locations, that means in largely different soot conditions. At a suitably selected laser fluence, radial and axial profiles of peak soot temperature and volume fraction were performed. Soot volume fraction data have been validated with results from laser extinction technique measurements. The quite low values observed for methane prove the sensitivity of the LII technique. Moreover, a discussion about soot refractive index is presented. In the visible region a test of its influence on both soot volume fraction and soot peak temperature was carried out, while in the infrared the heating process was analyzed. PACS 42.62.b; 42.87-d; 44.40+a 相似文献
16.
Time-resolved laser-induced incandescence (LII) has been developed rapidly during the last decade as a useful non-intrusive
technique for particle size determination. Still several parameters should be investigated in order to improve the accuracy
of LII for particle sizing and the spatial distribution of the laser energy is one of these. Generally a top-hat profile is
recommended, as this ensures a uniform heating of all particles in the measurement volume. As it is generally not straightforward
to create a uniform beam profile, it is of interest to establish the influence of various profiles on the evaluated particle
sizes. In this work we present both an experimental and a theoretical investigation of the influence of the spatial profile
on evaluated sizes. All experiments were carried out using a newly developed setup for two-colour LII (2C-LII) which provides
online monitoring of both the spatial and temporal profile as well as the laser pulse energy. The LII measurements were performed
in a one-dimensional premixed sooting ethylene/air flame, and evaluated particle sizes from LII were compared with thermophoretically
sampled soot particles analysed using transmission electron microscopy (TEM). The results show that although there is some
influence of the spatial laser energy distribution on the evaluated particle sizes both in modelling and experiments, this
effect is substantially smaller than the influence of the uncertainties in gas temperature and the thermal accommodation coefficient. 相似文献
17.
R.?L.?Vander?Wal 《Applied physics. B, Lasers and optics》2009,96(4):601-611
Successful implementation of laser-induced incandescence (LII) relies upon judicious choice of excitation and detection conditions.
Excitation conditions encompass choice of excitation wavelength and laser fluence. Detection conditions include choice of
detection wavelength, spectral band pass about the central wavelength, detection delay and duration relative to the excitation
laser pulse usually corresponding to the peak of the signal intensity. Examples of applying these parameters to LII are illustrated
by way of examples: soot/polycyclic aromatic hydrocarbon and metal aerosol systems. Tradeoffs must be recognized. Laser-induced
chemical and structural changes of the aerosol must be considered, particularly in light of heterogeneous aerosols. Diagnostics
of such changes are outlined as they will affect interpretation of the LII signal. Finally, calibration (for LII) must be
chosen to be appropriate for aerosols from practical sources as they may be mixed organic and inorganic composition.
A research paper in response to the call for papers from the Third International Discussion Meeting and Workshop on Laser-Induced
Incandescence July 30th–August 1st, Ottawa, Canada (2008). 相似文献
18.
Laser‐induced incandescence (LII) is introduced as a valuable tool for the characterization of nanoparticles. This optical measurement technique is based on the heating of the particles by a short laser pulse and the subsequent detection of the thermal radiation. It has been applied successfully for the investigation of soot in different fields of application, which is described here in the form of an overview with a focus on work done at the LTT‐Erlangen during the last 10 years. In laboratory flames the soot primary particle size, volume concentration, and relative aggregate size have been determined in combination with the number density of primary particles. Furthermore, the primary particle sizes of carbon blacks have been measured in situ and online under laboratory conditions and also in production reactors. Measurements with different types of commercially available carbon black powders, which were dispersed in a measurement chamber yielded a good correlation between LII results and the specified product properties. Particle diameters determined by LII in a furnace black reactor correlate very well with the CTAB‐absorption number, which is a measure for the specific surface area. It turned out that the LII method is not affected by variations of the aggregate structure of the investigated carbon blacks. The LII signal also contains information on the primary particle size distribution, which can be reconstructed by the evaluation of the signal decay time at, at least, two different time intervals. Additionally, soot mass concentrations have been determined inside diesel engines and online measurements were performed in the exhaust gas of such engines for various engine conditions simultaneously providing information about primary particle size, soot volume, and number concentration. The LII results exhibit good correlation with traditional measurement techniques, e.g., filter smoke number measurements. In addition to the soot measurements, primarily tests with other nanoparticles like TiO2 or metal particles are encouraging regarding the applicability of the technique for the characterization of such different types of nanoparticles. 相似文献
19.
S. De Iuliis F. Cignoli S. Maffi G. Zizak 《Applied physics. B, Lasers and optics》2011,104(2):321-330
The effect of multiple laser pulses reaching soot particles before an actual laser-induced incandescence (LII) measurement
is investigated in order to gain some insights on soot morphological and fine structure changes due to rapid laser heating.
Soot, extracted from a premixed and a quenched diffusion flames, is flowing through a tubular cell and undergoes a variable
number of pulses at different fluence. The response of soot is studied by the two-color LII technique. Transmission electron
microscopy (TEM) analysis of laser-modified soot aggregates from the diffusion flame is also presented. The results indicate
that even at low laser fluences a permanent soot transformation is induced causing an increase in the absorption function
E(m). This is interpreted as an induced graphitization of soot particles by the laser pulse heating. At high fluences the vaporization
process and a profound restructuring of soot particles affect the morphology of the aggregates. Soot from diffusion and premixed
flames behaves in a similar way although this similarity occurs at different fluence levels indicating a different initial
fine structure of soot particles. 相似文献
20.
D. R. Snelling K. A. Thomson F. Liu G. J. Smallwood 《Applied physics. B, Lasers and optics》2009,96(4):657-669
Laser-induced incandescence (LII) was used to derive temperatures of pulsed laser heated soot particles from their thermal
emission intensities detected at two wavelengths in a laminar ethylene/air co-annular diffusion flame. The results are compared
to those of a numerical nanoscale heat and mass transfer model. Both aggregate and primary particle soot size distributions
were measured using transmission electron microscopy (TEM). The model predictions were numerically averaged over these experimentally
derived size distributions. The excitation laser wavelength was 532 nm, and the LII signal was detected at 445 nm and 780 nm.
A wide range of laser fluence from very low to moderate (0.13 to 1.56 mJ/mm2) was used in the experiments. A large part of the temporal decay curve, beginning 12–15 nsec after the peak of the laser
excitation pulse, is successfully described by the model, resulting in the determination of accommodation coefficients, which
varies somewhat with soot temperature and is in the range of 0.36 to 0.46. However, in the soot evaporative regime, the model
greatly overpredicts the cooling rate shortly after the laser pulse. At lower fluences, where evaporation is negligible, the
initial experimental cooling rates, immediately following the laser pulse, are anomalously high. Potential physical processes
that could account for these effects are discussed. From the present data the soot absorption function, E(m), of 0.4 at 532 nm is obtained. A procedure for correcting the measured signals for the flame radiation is presented. It
is further shown that accounting for the local gas temperature increase due to heat transfer from soot particles to the gas
significantly improves the agreement in the temperature dependence of soot cooling rates between model and experiments over
a large range of laser fluences. 相似文献