Abstract: <p>A compact experimental setup that integrates laser-induced incandescence (LII) and one-angle elastic light scattering (1A-ELS) to measure the size of polydisperse soot aggregates is described. A 532 nm laser and a detection angle of 35 degrees were employed, which provided sensitivity for aggregate radius of gyrations ( R g ) of R g ≤200 nm. Both lognormal and self-preserving distribution functions are compared with width parameters derived from both aggregation theory and transmission electron microscopy (TEM) measurements. Using these distributions, mean aggregate sizes derived from the scattering measurements are compared. The LII+1A-ELS technique is validated with a two-angle elastic light scattering (2A-ELS) approach with an additional detection angle at 145 deg. Unlike LII+1A-ELS, the 2A-ELS technique has the advantage of not requiring knowledge of soot optical properties. Good agreement is found between the two techniques for a given distribution. A fundamental discrepancy exists between distributions derived from TEM and those according to aggregation theory, limiting the accuracy of both 2A-ELS and LII+1A-ELS. The dependence of both techniques on laser fluence and hence soot temperature is examined and discussed.</p>
Abstract: <p>To investigate the transient change of soot optical properties resulting from pulsed laser heating of soot in a cooled exhaust plume we have simultaneously performed cw light extinction at 405 and 830 nm and elastic light scattering at 1064 nm. A reversible increase to the 830-nm light extinction of up to 7%, observed during the time period where the soot was hot, suggests a temperature-dependent light absorption refractive index function, E ( m λ ). At low fluence, small permanent increases of E ( m λ ) of <2% were also observed. 405-nm extinction measurements revealed that the soot likely contained material which continued to absorb 405-nm radiation when desorbed, thus complicating measurement interpretation. 1064-nm light scattering measurements showed a gradual decrease of scattering propensity with increasing laser fluence up to the point of material loss, which is consistent with the expected decrease of the structure factor of the soot aggregates as they expand. It is concluded that variations of the optical properties are occurring at the time of laser-induced incandescence (LII) emission, which should be accounted for in time-resolved LII measurement interpretation.</p>
Abstract: <p>Premixed laminar flat ethylene flames were investigated for nascent nanoparticles through photoionization mass spectrometry (PIMS). Using an atmospheric McKenna burner and ethylene air flames coupled to an atmospheric sampling system, within a relatively narrow C/O range two modes of these particles were found, which can be clearly distinguished with regard to their temperature dependence, their reactivity, and their ionization behaviour. Behind a diesel engine the same particles were observed. These results were corroborated using a low pressure ethylene–O 2 flame coupled to a high resolution mass spectrometer. In this case, due to a special inlet system, it was possible to operate the flame in a fairly wide C/O range without clogging of the inlet nozzles. This allowed pursuing the development of particle size distribution functions (PSDF) well into the regime of mature soot. In addition, on the low mass side of the particle spectra measurements with unity resolution were possible and this allowed gaining information concerning their growth mechanism and structure. Finally, in an attempt to mimic Laser Induced Incandescence (LII) experiments the soot-laden molecular beam was exposed to IR irradiation. This resulted in a near complete destruction of nascent particles under LII typical fluences. Small C clusters between 3 and 17 C atoms were found. In addition and with much higher intensities, clusters comprising several hundreds of C atoms were also detected, the latter even at very low fluences when small clusters were totally absent.</p>
Abstract: <p>The main objective of this work is to investigate the influence of high-pressure conditions on the determination of primary particle size distributions of laser-heated soot particles using pyrometrically determined temperature decays. The method is based on time-resolved laser-induced incandescence measurements carried out at two different wavelengths (two-colour TiRe-LII). The LII signals are transferred into a particle ensemble averaged (effective) temperature using Planck’s thermal radiation formula. Assuming that all particles within the size distribution possess a unique temperature at the end of the laser pulse, the size distribution can be determined by numerically simulating the measured temperature decay. From our investigations, for pressures up to a few bars it is obvious that this strategy can be successfully applied if standard laser pulses of nano-second duration are used as an LII-excitation source. At higher pressures the time scales of heat conduction are decreased to such an extent that a unique temperature for all particles within the ensemble cannot be assumed at the end of the nano-second laser pulse. However, further investigations show that the presented two-colour TiRe-LII technique can be successfully adopted under technical high-pressure conditions as well, if the pulse duration of the TiRe-LII-excitation source is reduced into the pico-second range.</p>
Abstract: <p>The evidence of the change of the complex refractive index function E ( m ) of carbon and iron nanoparticles as a function of their size was found from two-color time-resolved laser-induced incandescence (TiRe-LII) measurements. Growing carbon particles were observed from acetylene pyrolysis behind a shock wave and iron particles were synthesized by pulse Kr–F excimer laser photo-dissociation of Fe(CO) 5 . The magnitudes of refractive index function were found through the fitting of two independently measured values of particle heat up temperature, determined by two-color pyrometry and from the known energy of the laser pulse and the E ( m ) variation. Small carbon particles of about 1–14 nm in diameter had a low value of E ( m )∼0.05–0.07, which tends to increase up to a value of 0.2–0.25 during particle growth up to 20 nm. Similar behavior for iron particles resulted in E ( m ) rise from ∼0.1 for particles 1–3 nm in diameter up to ∼0.2 for particles >12 nm in diameter.</p>
Abstract: <p>This study presents the results of laser-induced incandescence (LII) measurements in an optically accessible gasoline direct injection engine. The focus was to evaluate LII as a particle measurement technique which is able to provide a deeper understanding of the underlying reaction and formation processes of soot in order to optimize the injection system to reduce exhaust gas emissions. A comparison of time-resolved LII, based on the model described by Michelsen, with an Engine Exhaust Particle Sizer (EEPS) was performed. In this context, the air–fuel ratio, the injection pressure and the injection timing have been varied while applying the measurement techniques in the exhaust system. In case of a variation of the air–fuel ratio, two-dimensional LII has been performed in the combustion chamber additionally. For each measurement, the Filter Smoke Number (FSN) was taken into account as well. Finally, a good agreement of the different techniques was achieved. Moreover, we found that by combining time-resolved LII and EEPS a differentiation of primary particles and agglomerates is possible. Consequently, a determination of the processes in the combustion chamber and agglomeration in the exhaust gas is feasible.</p>
Abstract: <p>Computational fluid dynamics (CFD) modelers require high-quality experimental data sets for validation of their numerical tools. Preferred features for numerical simulations of a sooting, turbulent test case flame are simplicity (no pilot flame), well-defined boundary conditions, and sufficient soot production. This paper proposes a non-premixed C 2 H 4 /air turbulent jet flame to fill this role and presents an extensive database for soot model validation. The sooting turbulent jet flame has a total visible flame length of approximately 400 mm and a fuel-jet Reynolds number of 10,000. The flame has a measured lift-off height of 26 mm which acts as a sensitive marker for CFD model validation, while this novel compiled experimental database of soot properties, temperature and velocity maps are useful for the validation of kinetic soot models and numerical flame simulations. Due to the relatively simple burner design which produces a flame with sufficient soot concentration while meeting modelers’ needs with respect to boundary conditions and flame specifications as well as the present lack of a sooting “standard flame”, this flame is suggested as a new reference turbulent sooting flame. The flame characterization presented here involved a variety of optical diagnostics including quantitative 2D laser-induced incandescence (2D-LII), shifted-vibrational coherent anti-Stokes Raman spectroscopy (SV-CARS), and particle image velocimetry (PIV). Producing an accurate and comprehensive characterization of a transient sooting flame was challenging and required optimization of these diagnostics. In this respect, we present the first simultaneous, instantaneous PIV, and LII measurements in a heavily sooting flame environment. Simultaneous soot and flow field measurements can provide new insights into the interaction between a turbulent vortex and flame chemistry, especially since soot structures in turbulent flames are known to be small and often treated in a statistical manner.</p>
Abstract: <p>Although the two-color laser-induced incandescence technique (2C-LII) has proved to be a significant tool for soot diagnostics, many efforts are still required to gain a whole understanding of the chemical and physical processes involved. Time-resolved two-color LII measurements are carried out in a rich ethylene/air premixed flame at different heights above the burner and by changing the laser fluence. The prompt LII at two wavelengths and the corresponding soot incandescence temperature are obtained at different stages of the soot growth and under different laser irradiations. The decay rate of the LII signals, as a method for soot sizing, is investigated at different laser fluence. The time-resolved LII curves, obtained in the low laser fluence regime, are analyzed by a numerical simulation, available on the web. By considering the gas/particle initial temperature obtained with thermocouple measurements and by knowing soot particle diameter with previous TEM and extinction/scattering measurements, information about soot parameters, such as absorption function and thermal accommodation coefficient are obtained. The presence of the so-called young or mature soot along the flame height is strictly related to different optical and heat-exchange properties necessary to fit all the experimental data available.</p>
Abstract: <p>We have measured time-resolved laser-induced incandescence of flame-generated soot under high-vacuum conditions (4.1×10 −6 mbar) at an excitation wavelength of 532 nm with laser fluences spanning 0.06–0.5 J/cm 2 . We generated soot in an ethylene/air diffusion flame, introduced it into the vacuum system with an aerodynamic lens, heated it using a pulsed laser with a spatially homogeneous and temporally smooth laser profile, and recorded LII temporal profiles at 685 nm. At low laser fluences LII signal decay rates are slow, and LII signals persist beyond the residence time of the soot particles in the detection region. At these fluences, the temporal maximum of the LII signal increases nearly linearly with increasing laser fluence until reaching a plateau at ∼0.18 J/cm 2 . At higher fluences, the LII signal maximum is independent of laser fluence within experimental uncertainty. At these fluences, the LII signal decays rapidly during the laser pulse. The fluence dependence of the vacuum LII signal is qualitatively similar to that observed under similar laser conditions in an atmospheric flame but requires higher fluences (by ∼0.03 J/cm 2 ) for initiation. These data demonstrate the feasibility of recording vacuum LII temporal profiles of flame-generated soot under well-characterized conditions for model validation.</p>
Abstract: <p>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.</p>
Abstract: <p>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.</p>
Abstract: <p>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.</p>
Abstract: <p>Laser induced incandescence experiments were carried out in a flame reactor during titania nanoparticle synthesis. The structure of the reactor employed allowed for a rather smooth particle growth along the flame axis, with limited mixing of different size particles. Particle incandescence was excited by the 4th harmonic of a Nd:YAG laser. The radiation emitted from the particles was recorded in time and checked by spectral analysis. Results were compared with measurements from transmission electron microscopy of samples taken at the same locations probed by incandescence. This was done covering a portion of the flame length within which a particle size growth of a factor of about four was detected . The incandescence decay time was found to increase monotonically with particle size. The attainment of a process control tool in nanoparticle flame synthesis appears to be realistic.</p>
Abstract: <p>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.</p>
Abstract: <p>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.</p>
Abstract: <p>This paper presents measurements of spectrally and temporally resolved laser-induced incandescence (LII) from soot. The second harmonic (532 nm) from a nanosecond Nd:YAG laser was used to heat the soot over a wide range of fluences. The emission was spectrally resolved using a spectrograph attached to an intensified CCD camera with a gate width of 1.5 ns. At fluences below 0.2 J/cm2, corresponding to the sublimation threshold, spectra demonstrate broadband featureless emission characteristic of laser-induced incandescence, whereas at higher fluences spectra show sharp features attributable to C2 Swan band emission, C3 Swings band emission, and other species. These features perturb the LII signal at wavelengths between 380 and 680 nm, suggesting that this detection region should be avoided for LII measurements made using a 532-nm laser beam at fluences of 0.2 J/cm2 and above. The detection wavelength regions to be avoided are much more extensive than previously believed.</p><!–StartFragment–><!–EndFragment–>
Abstract: <p>A numerical investigation was made of the generation and behaviour of the LII signal in optically dense combusting sprays at conditions similar to those in the combustion chamber of compression ignition engines and gas turbines. The influence of particle size, particle morphology and size distribution on the behaviour of the LII signal, and the scattering and absorption of light, and the consequences that different calibration procedures have on the accuracy of the results were studied. Results show that, as the particle size or aggregation increases, light extinction is not caused only by absorption but also by scattering, which contributes more than 10% to the total extinction of light. Particle shape effects are important, irrespective of particle size. The form, soot concentration gradients and optical thickness of the flame cause an uneven laser fluence across the measuring volume that affects the generation of the LII signal. In addition, the quotient between the transmitted and incoming laser pulses across the flame borders can be as small as a percentage of unity. The interpretation of the induced signal is further challenged by the loss of signal between the measuring volume and the detection arrangement, thus causing the detection of spectrally distorted and weaker signals with an erroneous profile of the local amount of carbonaceous particles. An appropriate calibration procedure must be followed to obtain results that are quantitatively representative. External calibration was found to be inappropriate for these systems since it can lead one to underestimate the local volume fraction for almost two orders of magnitude. Implementing an in situ calibration along a line can lead to underestimate or overestimate the local mean volume fraction by a factor of two. However, the use of an in situ calibration procedure using a laser sheet that propagates through the complete measuring volume can reduce the error in estimating the mean soot volume fraction to a 30%. The latter was found to be the most adequate among the studied calibration routines.</p>
Abstract: <p>The sensitivity and relative sensitivity of soot temperature and soot volume fraction inferred from the two-color laser-induced incandescence technique to different variables were systematically investigated to quantitatively understand how the detection wavelengths affect the behavior of the detection system. The effects of signal shot noises on the derived soot temperature and soot volume fraction were also analyzed. The detection wavelengths are in general between about 400 nm for the lower band and near infrared for the upper one. Numerical calculations were conducted for seven detection wavelength selections commonly used in two-color laser-induced incandescence experiments reported in the literature. To achieve a better accuracy for soot temperature and volume fraction measurements, it is desirable to use a shorter lower detection wavelength and a longer upper detection wavelength in the spectral range of about 400 nm to near infrared. The lower detection wavelength has a stronger impact on the detection system performance than the upper one. The sensitivity and shot noise analyses are valuable tools to assess the relative performance of different detection wavelengths and should be used in combination with other considerations to design an optimal detection system in a two-color laser-induced incandescence experiment.</p>
Abstract: <p>Laser-induced incandescence has been rapidly developed into a powerful diagnostic technique for measurements of soot in many applications. The incandescence intensity generated by laser-heated soot particles at the measurement location suffers the signal trapping effect caused by absorption and scattering by soot particles present between the measurement location and the detector. The signal trapping effect was numerically investigated in soot measurements using both a 2D LII setup and the corresponding point LII setup 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 contribution of scattering to signal trapping was found to be negligible in atmospheric laminar diffusion flames. When uncorrected LII intensities are used to determine soot particle temperature and the soot volume fraction, the errors are smaller in 2D LII setup where soot particles are excited by a laser sheet. The simple Beer–Lambert exponential attenuation relationship holds in LII applications to axisymmetric flames as long as the effective extinction coefficient is adequately defined.</p>
Abstract: <p> </p><p><!–StartFragment–></p><p>This paper presents a derivation of an expression to estimate the accommodation coefficient for gas collisions with a graphite surface, which is meant for use in models of laser-induced incandescence (LII) of soot. Energy transfer between gas molecules and solid surfaces has been studied extensively, and a considerable amount is known about the physical mechanisms important in thermal accommodation. Values of accommodation coefficients currently used in LII models are temperature independent and are based on a small subset of information available in the literature. The expression derived in this study is based on published data from state-to-state gas-surface scattering experiments. The present study compiles data on the temperature dependence of translational, rotational, and vibrational energy transfer for diatomic molecules (predominantly NO) colliding with graphite surfaces. The data were used to infer partial accommodation coefficients for translational, rotational, and vibrational degrees of freedom, which were consolidated to derive an overall accommodation coefficient that accounts for accommodation of all degrees of freedom of the scattered gas distributions. This accommodation coefficient can be used to calculate conductive cooling rates following laser heating of soot particles.</p><p><!–EndFragment–></p>
Abstract: <p>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/mm 2 ) 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.</p>
Abstract: <p>The accuracy of laser-induced incandescence (LII) measurements is significantly influenced by the calibration process and the laser profile degradation due to beam steering. Additionally, the wavelength used for extinction measurements, needed for LII calibration, is critical and should be kept as high as possible in order to avoid light absorption by molecular species in the flame. The influence of beam steering on the LII measurement was studied in turbulent sooting C 2 H 4 /air flames at different pressures. While inhomogeneities in the laser profile become smoothed out in time-averaged measurements, especially at higher pressure, the corresponding single-shot beam profiles reveal an increasing effect of beam steering. In the current configuration it was observed that the resulting local laser fluence remains within certain limits (30% to 200%) of the original value. A sufficiently high incident laser fluence can thus prevent the local fluence from dropping below the LII threshold value of approximately 0.3 J/cm 2 at the cost of increased soot surface vaporization. A spatial resolution in the dimension of the sheet thickness of below 1 mm cannot be guaranteed at increased pressure of 9 bars due to beam steering. A feasibility study in a combustor at technical conditions demonstrates the influence of both effects beam steering and choice of calibration wavelength and led to the conclusion that, however, a shot-to-shot calibration of LII with simultaneously measured extinction can be realized.</p>
Abstract: <p align="left"><font face="AdvSTP_PSTimBI" size="2"><font face="AdvSTP_PSTimBI" size="2">Particle size distribution functions (PSDF) and mean particle sizes have been determined </font></font><font face="AdvSTP_PSTimBI" size="2"><font face="AdvSTP_PSTimBI" size="2">in a laminar premixed ethylene/air flame with three different experimental approaches: </font></font><font face="AdvSTP_PSTimBI" size="2"><font face="AdvSTP_PSTimBI" size="2">photo-ionization mass spectrometry (PIMS), scanning mobility particle sizing (SMPS), </font></font><font face="AdvSTP_PSTimBI" size="2"><font face="AdvSTP_PSTimBI" size="2">and laser-induced incandescence (LII). The main goal of this investigation was the crossvalidation </font></font><font face="AdvSTP_PSTimBI" size="2"><font face="AdvSTP_PSTimBI" size="2">of these three methods used at our institute for the determination of particle sizes </font></font><font face="AdvSTP_PSTimBI" size="2"><font face="AdvSTP_PSTimBI" size="2">in a great variety of flames or exhaust gases. We found good agreement between the three </font></font><font face="AdvSTP_PSTimBI" size="2"><font face="AdvSTP_PSTimBI" size="2">methods in the ranges where they are comparable as well as a complementary behavior for </font></font><font face="AdvSTP_PSTimBI" size="2"><font face="AdvSTP_PSTimBI" size="2">the different size ranges. PIMS and SMPS are able to measure the particle size distribution </font></font><font face="AdvSTP_PSTimBI" size="2"><font face="AdvSTP_PSTimBI" size="2">functions with good resolution. PIMS is favorable in detecting the smallest particles </font></font><font face="AdvSTP_PSTimBI" size="2"><font face="AdvSTP_PSTimBI" size="2">(</font></font><font face="AdvSTP_PSTimB" size="2"><font face="AdvSTP_PSTimB" size="2"><</font></font><font face="AdvSTP_PSTimBI" size="2"><font face="AdvSTP_PSTimBI" size="2">6 nm) and thereby able to detect even bimodal distributions of the soot precursor </font></font><font face="AdvSTP_PSTimBI" size="2"><font face="AdvSTP_PSTimBI" size="2">particles. SMPS and LII are suitable in the mid- and upper range of the particle sizes </font></font><font face="AdvSTP_PSTimBI" size="2"><font face="AdvSTP_PSTimBI" size="2">(</font></font><font face="AdvSTP_PSTimB" size="2"><font face="AdvSTP_PSTimB" size="2">></font></font><font face="AdvSTP_PSTimBI" size="2"><font face="AdvSTP_PSTimBI" size="2">2 nm and </font></font><font face="AdvSTP_PSTimB" size="2"><font face="AdvSTP_PSTimB" size="2">></font></font><font face="AdvSTP_PSTimBI" size="2"><font face="AdvSTP_PSTimBI" size="2">3 nm, respectively). LII offers the particular advantage of being a non-intrusive </font></font><font face="AdvSTP_PSTimBI" size="2"><font face="AdvSTP_PSTimBI" size="2">method. This makes it applicable in extreme environments, such as high pressure flames, </font></font><font face="AdvSTP_PSTimBI" size="2"><font face="AdvSTP_PSTimBI" size="2">as well as in very sensitive flames because no probe is needed. </font></font></p>
Abstract: <p>The effect of sub-nanosecond fluence fluctuations and triggering on time-resolved laser-induced incandescence (LII) from soot has been studied using an injection-seeded pulsed Nd:YAG laser that produces a smooth laser temporal profile. Without injection seeding, this multi-mode laser generates pulses with large intensity fluctuations with sub-nanosecond rise times. The experimental results described here demonstrate that at fluences below 0.6 J/cm 2 LII signals are insensitive to fluence fluctuations on nanosecond time scales. At fluences above 0.6 J/cm 2 fluctuations in the laser profile cause the rising edge of the LII profile to move around in time relative to the center of the laser pulse causing a broader average profile that shifts to earlier times. Such fluctuations also lead to a decrease in the average LII temporal profile by up to 12% at a fluence of 3.5 J/cm 2 . A timing jitter on the trigger of the data acquisition, such as that produced by triggering on the laser Q-switch synchronization pulse, has a negligible effect on the shape and temporal maximum of the LII signal. Additional jitter, however, considerably reduces the peak of the LII temporal profiles at fluences as low as 0.15 J/cm 2 . Neither fast fluence fluctuations nor trigger jitter have a significant effect on gated LII signals, such as those used to infer soot volume fraction.</p>
Abstract: <p>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.</p>
Abstract: <p>This paper presents an analysis of several equations used to model laser-induced incandescence (LII) of soot. The analysis focuses on sub-models of the change in particle enthalpy during sublimation, conduction, and oxidation. Assuming that pressure is constant, expressing the conductive cooling rate in terms of enthalpy instead of energy, thereby accounting for expansion work, increases the signal decay rate and has an effect comparable to increasing the thermal accommodation coefficient from 0.30 to 0.38. Accounting for oxidative heating decreases the signal decay rate and has an effect comparable to decreasing the accommodation coefficient from 0.30 to 0.25. As an estimate of magnitude of these effects, primary particle sizes inferred from signal decay rates measured at low fluences may be over-predicted by as much as 17% if oxidation is neglected in the model at O2 partial pressures of 0.2 bar, under-predicted by 24% if expansion work is neglected, and under-predicted by only 9% if both are neglected. This paper also provides updated parameterizations for average enthalpies of formation, molecular weights, and total pressures of sublimed carbon clusters for use in LII models.</p><!–StartFragment–><!–EndFragment–>
Abstract: <p>"The laser-induced incandescence (LII) signal is proportional to soot volume fraction" is an often used statement in scientific papers, and it has – within experimental uncertainties – been validated in comparisons with other diagnostic techniques in several investigations. In 1984 it was shown theoretically in a paper by Melton that there is a deviation from this statement in that the presence of larger particles leads to some overestimation of soot volume fractions. In the present paper we present a detailed theoretical investigation of how the soot particle size influences the relationship between LII signal and soot volume fraction for different experimental conditions. Several parameters have been varied; detection wavelength, time and delay of detection gate, ambient gas temperature and pressure, laser fluence, level of aggregation and spatial profile. Based on these results we are able, firstly, to understand how experimental conditions should be chosen in order to minimize the errors introduced when assuming a linear dependence between the signal and volume fraction and secondly, to obtain knowledge on how to use this information to obtain more accurate soot volume fraction data if the particle size is known.</p>
Abstract: <p> </p><!–StartFragment–><p><span style="font-size:12.0pt;font-family:Times; mso-ansi-language:EN-US;mso-fareast-language:EN-US">We have performed a comparison of ten models that predict the temporal behavior of laser-induced incandescence (LII) of soot.<span style="mso-spacerun: yes"> </span>In this paper we present a summary of the models and comparisons of calculated temperatures, diameters, signals, and energy-balance terms.<span style="mso-spacerun: yes"> </span>The models were run assuming laser heating at 532 nm at fluences of 0.05 and 0.70 J/cm<sup>2</sup> with a laser temporal profile provided.<span style="mso-spacerun: yes"> </span>Calculations were performed for a single primary particle with a diameter of 30 nm at an ambient temperature of 1800 K and pressure of 1 bar.<span style="mso-spacerun: yes"> </span>Preliminary calculations were performed with a fully constrained model.<span style="mso-spacerun: yes"> </span>The comparison of unconstrained models demonstrates a wide spread in calculated LII signals.<span style="mso-spacerun: yes"> </span>Many of the differences can be attributed to the values of a few important parameters, such as the refractive index function <i>E</i></span><span style="font-size:12.0pt; font-family:Times;mso-ansi-language:EN-US;mso-fareast-language:EN-US">(<i>m</i></span><span style="font-size:12.0pt;font-family:Times;mso-ansi-language:EN-US;mso-fareast-language: EN-US">) and thermal and mass accommodation coefficients.<span style="mso-spacerun: yes"> </span>Constraining these parameters brings most of the models into much better agreement with each other, particularly for the low-fluence case.<span style="mso-spacerun: yes"> </span>Agreement among models is not as good for the high-fluence case, even when selected parameters are constrained.<span style="mso-spacerun: yes"> </span>The reason for greater variability in model results at high fluence appears to be related to solution approaches to mass and heat loss by sublimation.</span></p><!–EndFragment–>
Abstract: <p> </p><!–StartFragment–><p><span style="font-size:12.0pt;font-family:"Times New Roman"; mso-ansi-language:EN-US;mso-fareast-language:EN-US">We investigated the physical and chemical changes induced in soot aggregates exposed to laser radiation using a scanning mobility particle sizer, a transmission electron microscope, and a scanning transmission x-ray microscope to perform near edge x-ray absorption fine structure spectroscopy. <span style="mso-spacerun: yes"> </span>Laser-induced nanoparticle production was observed at fluences above 0.12 J/cm<sup>2</sup> at 532 nm and 0.22 J/cm<sup>2</sup> at 1064 nm.<span style="mso-spacerun: yes"> </span>Our results indicate that new particle formation proceeds via (1) vaporization of small carbon clusters by thermal or photolytic mechanisms, followed by homogeneous nucleation, (2) heterogeneous nucleation of vaporized carbon clusters onto material ablated from primary particles, or (3) both processes. </span></p><!–EndFragment–>
Abstract: <p>Temporal behavior of pulses from a Q-switched Nd:YAG laser with an unstable resonator can vary significantly with radial position in the beam. Our laser provides pulses with position-dependent durations spanning 8-11.5 ns at 1064 nm and 7-10 ns at 532 nm. Pulses emerge first and have the longest duration at the center of the beam; they are shorter (by up to 4 ns) and increasingly delayed (by up to 10 ns) with increasing radial distance from the center. This behavior can have a dramatic effect on time-sensitive experiments, such as laser-induced incandescence of soot, if not taken into account. </p><!–StartFragment–><!–EndFragment–>
Abstract: Time-resolved laser-induced incandescence (TR-LII) measurements have been performed inside the combustion chamber of a heavy-duty diesel engine running at low load and with regular diesel fuel. The LII traces were interpreted in terms of primary particle sizes, comparing two different assumed particle-size distributions: a mono-disperse and a log-normal distribution. The initial temperature of the particles (immediately after the laser pulse) is estimated by two-color pyrometry. We conclude that the initial temperature of the particles is not very critical for the outcome of the fitting procedure for the (mean) radius. Under the high-pressure conditions in the engine, the time dependence of the LII intensity contains sufficient structure to allow retrieval of details of the particle-size distribution.
Abstract: In this study experimental single-pulse, time-resolved laser-induced incandescence (TIRE-LII) signal intensity profiles acquired during transient Diesel combustion events at high pressure were processed. Experiments were performed between 0.6 and 7 MPa using a high-temperature high-pressure constant volume cell and a heavy-duty Diesel engine, respectively. Three currently available LII sub-model functions were investigated in their performance for extracting ensemble mean soot particle diameters using a least-squares fitting routine, and a â\texteuroœquick-fitâ\texteuro? interpolation approach, respectively. In the calculations a particle size distribution as well as the temporal and spatial intensity profile of the heating laser was taken into account. For the poorly characterized sample environments of this work, some deficiencies in these state-of-the-art data evaluation procedures were revealed. Depending on the implemented model function, significant differences in the extracted particle size parameters are apparent. We also observe that the obtained â\texteuroœbest-fitâ\texteuro? size parameters in the fitting procedure are biased by the choice of their respective â\texteuroœfirst-guessâ\texteuro? initial values. This behavior may be caused by the smooth temporal profile of the LII cooling curve, giving rise to shallow local minima on the multi-parameter least squares residuals, surface sampled during the regression analysis procedure. Knowledge of the gas phase temperature of the probed medium is considered important for obtaining unbiased size parameter information from TIRE-LII measurements.
Abstract: This paper provides an overview of a workshop focused on fundamental experimental and theoretical aspects of soot measurements by laser-induced incandescence (LII). This workshop was held in Duisburg, Germany in September 2005. The goal of the workshop was to review the current understanding of the technique and identify gaps in this understanding associated with experimental implementation, model descriptions, and signal interpretation. The results of this workshop suggest that uncertainties in the understanding of this technique are sufficient to lead to large variability among model predictions from different LII models, among measurements using different experimental approaches, and between modeled and measured signals, even under well-defined conditions. This article summarizes the content and conclusions of the workshop, discusses controversial topics and areas of disagreement identified during the workshop, and highlights recent important references related to these topics. It clearly demonstrates that despite the widespread application of LII for soot-concentration and particle-size measurements there is still a significant lack in fundamental understanding for many of the underlying physical processes.
Abstract: 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.
Abstract: Laser-induced incandescence (LII) is a relatively new experimental method for studying the soot formation process in flames. LII is based on the quasi-instantaneous heating of soot particles, by means of a high-energy pulsed laser beam, to almost their vaporization temperature, resulting in a strong but transient increase in their incandescence. After the laser pulse the particles cool down, at a rate which is dependent on their surface-to-volume ratio. The decay rate of the laser-induced incandescence intensity thus contains information on the particle size distribution within the irradiated volume. In this communication we report on the characterization of soot by time resolved LII (Tire-LII) measurements in a heavy-duty diesel engine, with peak pressures up to 6 MPa, paying particular attention to the correction required for the finite time resolution of the hardware, and to the role of the initial particle temperature.
Abstract: The paper by De Iuliis et al. [Appl. Opt. 44, 7414 (2005)] contains a mistake in the printing of the parentheses in Eq. (9). The correction of the equation is given here.
Abstract: Time-resolved laser-induced incandescence (TR-LII) was applied for the determination of particle sizes during carbon-particle formation from supersaturated atomic carbon vapor that was generated by laser photolysis of carbon suboxide (C<sub>3</sub>O<sub>2</sub>) at room temperature. Thus, the solid carbon particles were formed under hydrogen-free conditions. The TR-LII technique was used for in situ size measurement of growing carbon particles and samples of final particles were analyzed by transmission electron microscopy (TEM). It was found that the particles grow to a final size of 4–12 nm within 0.02–1 ms. The properties of the obtained particles depend on the initial conditions in the reaction volume, i.e. concentration of carbon suboxide, pressure and type of gas diluter, photolysis wavelength, and laser pulse energy. The comparison of TR-LII and TEM particle sizing results yields information about the effective thermal energy accommodation coefficients for He, Ar, CO, and C<sub>3</sub>O<sub>2</sub> molecules on carbon particles.
Abstract: An original approach of laser-induced incandescence consisting in the simultaneous recording of the two-color-time-resolved and 2D LII signal is described in this paper. The application of this approach in an atmospheric pressure diffusion flame fueled with isooctane as well as inside the combustion chamber of a diesel engine is presented. Soot volume fraction and primary particle diameters are calculated, and the results are discussed. The mean diameter estimated by fitting the LII modeled curve on the experimental one is compared with the results obtained through soot sampling and microscope analyzing. The influence of the thermal accommodation coefficient and soot refractive index function is also discussed.
Abstract: This paper presents measurements of time-resolved laser-induced incandescence (LII) from soot recorded on a picosecond time scale. The 532-nm output from a picosecond Nd:YAG laser was used to heat the soot, and a streak camera was used to record the LII signal. The results are compared with data collected on a nanosecond time scale and with a time-dependent model that solves the energy- and mass-balance rate equations. Relative to the laser timing, the picosecond and nanosecond results are very similar. Signals increase during the laser pulse as soot temperatures increase and decrease after the laser pulse. The signal decay rates increase significantly with increasing laser fluence. The LII model gives good agreement with the nanosecond data at fluences â\textperthousand€0.2 J/cm2 and underpredicts the signal decay rates at higher fluences. The picosecond temporal profiles increase significantly faster and earlier in the laser pulse than predicted by the model. This disagreement between the model and picosecond LII data may be attributable to perturbations to the signal by laser-induced fluorescence from polycyclic aromatic hydrocarbons or other large organic species. The excited state or states responsible for this fluorescence appear to be accessed via a two-photon transition and have an effective lifetime of 55 ps.
Abstract: In-situ measurements of soot volume fraction in the exhausts of jet engines can be carried out using the laser-induced incandescence (LII) technique in backward configuration, in which the signal is detected in the opposite direction of the laser beam propagation. In order to improve backward LII for quantitative measurements, we have in this work made a detailed experimental and theoretical investigation in which backward LII has been compared with the more commonly used right-angle LII technique. Both configurations were used in simultaneous visualization experiments at various pulse energies and gate timings in a stabilized methane diffusion flame. The spatial near-Gaussian laser energy distribution was monitored on-line as well as the time-resolved LII signal. A heat and mass transfer model for soot particles exposed to laser radiation was used to theoretically predict both the temporal and spatial LII signals. Comparison between experimental and theoretical LII signals indicates similar general behaviour, for example the broadening of the spatial LII distribution and the hole-burning effect at centre of the beam due to sublimation for increasing laser pulse energies. However, our comparison also indicates that the current heat and mass transfer model overpredicts signal intensities at higher fluence, and possible reasons for this behaviour are discussed.
Abstract: Temperature histories of nanosecond pulsed laser heated soot particles of different primary particle size distributions were calculated using a single primary particle based heat and mass transfer model under conditions of a typical atmospheric laminar diffusion flame. The critical peak soot particle temperatures beyond which soot particle sublimation cannot be neglected were identified to be about 3300â\texteuro\textquotedblleft3400 K. Knowledge of this critical soot particle temperature is required to conduct low-fluence laser-induced incandescence experiments in which soot sublimation is avoided. After the laser pulse, the temperature of smaller primary soot particles decreases faster than that of larger ones as a result of larger surface area-to-volume ratio. Unlike the common belief that the peak soot particle temperature is independent of the primary particle diameter, the numerical results indicate that this assumption is valid only when soot sublimation is negligible and for primary soot particle diameters greater than about 20 nm. The effective temperature of a soot particle ensemble having different primary particle diameters in the laser probe volume was calculated based on the ratio of the total thermal radiation intensities of soot particles at 400 and 780 nm to simulate the experimentally measured soot particle temperature using two-color optical pyrometry. In the non-sublimation regime, the initial effective temperature decay rate after the peak soot temperature is related to the Sauter mean diameter of the primary soot particle diameter distribution. At longer times, the effective temperatures of soot particle ensembles start to display different decay rates for different soot primary particle diameter distributions. A simple approach was proposed in this study to infer the two parameters of lognormal distributed primary soot particle diameter. Application of this approach was demonstrated in an atmospheric laminar ethylene diffusion flame with the inferred primary soot particle diameter distribution compared with independent ex situ measurement.
Abstract: An improved aggregate-based low-fluence laser-induced incandescence (LII) model has been developed. The shielding effect in heat conduction between aggregated soot particles and the surrounding gas was modeled using the concept of the equivalent heat transfer sphere. The diameter of such an equivalent sphere was determined from direct simulation Monte Carlo calculations in the free molecular regime as functions of the aggregate size and the thermal accommodation coefficient of soot. Both the primary soot particle diameter and the aggregate size distributions are assumed to be lognormal. The effective temperature of a soot particle ensemble containing different primary particle diameters and aggregate sizes in the laser probe volume was calculated based on the ratio of the total thermal radiation intensities of soot particles at 400 and 780 nm to simulate the experimentally measured soot particle temperature using two-color optical pyrometry. The effect of primary particle diameter polydispersity is in general important and should be considered. The effect of aggregate size polydispersity is relatively unimportant when the heat conduction between the primary particles and the surrounding gas takes place in the free-molecular regime; however, it starts to become important when the heat conduction process occurs in the near transition regime. The model developed in this study was also applied to the re-determination of the thermal accommodation coefficient of soot in an atmospheric pressure laminar ethylene diffusion flame.
Abstract: Laser-induced incandescence (LII) of nano-second pulsed laser heated nano-particles has been developed into a popular technique for characterizing concentration and size of particles suspended in a gas and continues to draw increased research attention. Heat conduction is in general the dominant particle cooling mechanism after the laser pulse. Accurate calculation of the particle cooling rate is essential for accurate analysis of LII experimental data. Modelling of particle conduction heat loss has often been flawed. This paper attempts to provide a comprehensive review of the heat conduction modelling practice in the LII literature and an overview of the physics of heat conduction loss from a single spherical particle in the entire range of Knudsen number with emphasis on the transition regime. Various transition regime models developed in the literature are discussed with their accuracy evaluated against direct simulation Monte Carlo results under different particle-to-gas temperature ratios. The importance of accounting for the variation of the thermal properties of the surrounding gas between the gas temperature and the particle temperature is demonstrated. Effects of using these heat conduction models on the inferred particle diameter or the thermal accommodation coefficient are also evaluated. The popular McCoy and Cha model is extensively discussed and evaluated. Based on its superior accuracy in the entire transition regime and even under large particle-to-gas temperature ratios, the Fuchs boundary-sphere model is recommended for modeling particle heat conduction cooling in LII applications.
Abstract: Time-resolved LII (TIRE-LII) measurements are performed simultaneously at two different wavelengths in a sooting, premixed, flat acetylene flame under atmospheric pressure conditions. The influence of temporal response of the detection system on the measured evolution of the LII signal is discussed. The effect of the temporal response on the determination of particle size distributions is quantified for data evaluation starting some nanoseconds after the maximum particle ensemble temperature. Furthermore, it is investigated how the temporal response of a slow detection system affects the determination of accommodation parameters, e.g. thermal accommodation coefficients, and evaporation coefficients, if TIRE-LII signals are modelled including particle heating as well as particle cooling, and if deconvolution techniques are not applied to the measured LII signal.
Abstract: This paper reports on a study of laser-induced incandescence of carbon particles in free space within a high vacuum (<10-3 mbar) excited by an Nd:YAG laser pulse. We have conducted an experimental study using samples of carbon black placed within an evacuated, sealed glass vessel which is slowly tumbled to cause a cascade of carbon black particles in free space. Our experiments show that under a high vacuum two important phenomena are observed. Due to the absence of gaseous conduction, in comparison to particles in ambient air, incandescence lifetime in a vacuum is dramatically extended to more than 50 ÃŽ\textonequarters with a corresponding increase of a factor of over 104 in the integrated or total number of photons emitted by each soot primary particle. For large aggregates and/or agglomerates in a vacuum after a delay of the order of 2 to 10 ÃŽ\textonequarters, the large particles fragment into smaller entities. We have also modelled the incandescence behaviour using well established methods.
Abstract: 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 Rayleighâ\texteuro\textquotedblleftDebyeâ\texteuro\textquotedblleftGans 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.
Abstract: We report on spatially and temporally resolved optical diagnostic measurements of propagation and combustion of diesel sprays introduced through a single-hole fuel injector into a constant volume, high-temperature, high-pressure cell. From shadowgraphy images in non-reacting environments of pure nitrogen, penetration lengths and dispersion angles were determined for non-vaporizing and vaporizing conditions, and found to be in reasonable agreement with standard models for liquid jet propagation and break-up. Quasi-simultaneous two-dimensional images were obtained of laser elastic light scattering, shadowgraphs and spectrally integrated flame emission in a reacting environment (cell temperature 850 K). In addition laser-induced incandescence was employed for the identification of soot-loaded regions. The simultaneously recorded spray images exhibit remarkable structural similarity and provide complementary information about the spray propagation and combustion process. The measurements also reveal the fuel vapor cloud extending well beyond the liquid core and close to the nozzle tip. Ignition takes place close to the tip of the spray within the mixing layer of fuel vapor and surrounding air. Soot is formed in the vapor core region at the tip of the liquid fuel jet. Our results support recently developed phenomenological model on diesel spray combustion.
Abstract: A two-color version of the laser-induced incandescence (2C-LII) technique was implemented for measuring absolute soot volume fraction in flames. By using a calibrated tungsten ribbon lamp, soot peak temperatures were measured as a function of fluence at several locations in an ethylene diffusion flame by using a steeply edged laser beam profile. Above a certain fluence threshold, peak temperatures were tightly distributed just above 4000 K independent of the particle size and number density. Radial profiles of soot volume fraction were obtained and compared (not calibrated) with results from the laser extinction technique. Good agreement showed the validity of the 2C-LII technique at a controlled fluence.
Abstract: <p>An experimental investigation into soot formation in laminar premixed flames at atmospheric and elevated pressures (1–5 bar) has been conducted. The flames were produced in a dual-flame burner enclosed in a pressure housing. Quantitative soot volume fraction measurements were obtained using laser-induced incandescence coupled with a quasi-simultaneous absorption measurement for calibration; the data were corrected for signal trapping using an ‘‘onion peeling’’ algorithm. Temperature measurements were obtained using shifted vibrational coherent anti-Stokes Raman scattering, which yields well-resolved, accurate temperature measurements in sooting and nonsooting environments. Results are presented for stable homogeneous flames using air as oxidizer and ethylene, propylene, and toluene as fuels. The variation of soot volume fraction and temperature with height above burner and as a function of fuel,equivalence ratio, and pressure are presented and discussed. The present soot data are well represented by a first-order growth rate law. The data identify trends and features useful for the validation of numerical models of soot formation.</p>
Abstract: Laminar nonpremixed methaneâ\texteuro\textquotedblleftair flames were studied over the pressure range of 0.5 to 4 MPa using a new high-pressure combustion chamber. Flame characterization showed very good flame stability over the range of pressures, with a flame tip rms flicker of less than 1% in flame height. At all pressures, soot was completely oxidized within the visible flame. Spectral soot emission (SSE) and line-of-sight attenuation (LOSA) measurements provided radially resolved measurements of soot volume fraction and soot temperature at pressures from 0.5 to 4.0 MPa. Such measurements provide an improved understanding of the influence of pressure on soot formation and have not been reported previously in laminar nonpremixed flames for pressures above 0.4 MPa. SSE and LOSA soot concentration values typically agree to within 30% and both methods exhibit similar trends in the spatial distribution of soot concentration. Maximum soot concentration depended on pressure according to a power law, where the exponent on pressure is about 2 for the range of pressures between 0.5 and 2.0 MPa, and about 1.2 for 2.0 to 4.0 MPa. Peak carbon conversion to soot also followed a power-law dependence on pressure, where the pressure exponent is unity for pressures between 0.5 and 2.0 MPa and 0.1 for 2.0 to 4.0 MPa. The pressure dependence of sooting propensity diminished at pressures above 2.0 MPa. Soot concentrations measured in this work, when transformed to line-integrated values, are consistent with the measurements of Flower and Bowman for pressures up to 1.0 MPa [Proc. Combust Inst. 21 (1986) 1115â\texteuro\textquotedblleft1124] and Lee and Na for pressures up to 0.4 MPa [JSME Int. J. Ser. B 43 (2000) 550â\texteuro\textquotedblleft555]. Soot temperature measurements indicate that the overall temperatures decrease with increasing pressure; however, the differences diminish with increasing height in the flame. Low down in the flame, temperatures are about 150 K lower at pressures of 4.0 MPa than those at 0.5 MPa. In the upper half of the flame the differences reduce to 50 K.
Abstract: Quasi-simultaneous measurements of temperature and soot volume fraction in pressurized and atmospheric flames are presented. A dual-flame burner concept yielded stable laminar flames for a variety of equivalence ratios, pressures, and fuels, and permitted the investigation of flames without the influence of soot oxidation. A CARS-based technique (shifted vibrational CARS) for temperature measurements, which offers high accuracy over the entire relevant temperature and soot concentration range, is described. Comparison of temperature measurements in the nonsooting part of a laminar diffusion flame at atmospheric pressure by SV-CARS and conventional N2 Q-branch CARS yielded excellent agreement. This new technique was applied to quasi-1D laminar flames with soot concentrations up to 10 ppm and pressures up to 5 bar. The temperature profiles measured in these flames were combined with soot concentration measurements using LII; calibration and correction for signal trapping yielded quantitative soot volume fraction data. The temperature and soot concentration data were combined to generate a comprehensive dataset for the validation of an improved kinetic soot model for the prediction of soot formation in premixed combustion at elevated pressure.
Abstract: Laser-induced incandescence (LII) has proved to be a useful diagnostic tool for spatially and temporally resolved measurement of particulate (soot) volume fraction and primary particle size in a wide range of applications, such as steady flames, flickering flames, and Diesel engine exhausts. We present a novel LII technique for the determination of soot volume fraction by measuring the absolute incandescence intensity, avoiding the need for ex situ calibration that typically uses a source of particles with known soot volume fraction. The technique developed in this study further extends the capabilities of existing LII for making practical quantitative measurements of soot. The spectral sensitivity of the detection system is determined by calibrating with an extended source of known radiance, and this sensitivity is then used to interpret the measured LII signals. Although it requires knowledge of the soot temperature, either from a numerical model of soot particle heating or experimentally determined by detecting LII signals at two different wavelengths, this technique offers a calibration-independent procedure for measuring soot volume fraction. Application of this technique to soot concentration measurements is demonstrated in a laminar diffusion flame.
Abstract: Temperature histories of nanosecond-pulsed laser-heated soot particles of different primary particle diameters and different aggregate sizes were calculated using an aggregate-based heat transfer model. Relatively low laser fluences were considered to ensure maximum particle temperatures were below about 3800 K to avoid soot particle sublimation. After the laser pulse, the temperature of soot particles in larger aggregates decreases more slowly than that of particles in smaller aggregates due to the increased shielding effect. For a given aggregate size, the temperature of particles of smaller diameter decays faster as a result of a larger surface area-to-volume ratio. The effective temperature of soot particles in the laser probe volume was calculated based on the ratio of thermal radiation intensities of soot particles at 400 and 780 nm to simulate the experimentally measured soot particle temperature using two-color optical pyrometry. The effect of aggregate size distribution of soot particles on the effective particle temperature was investigated under different initial temperatures.
Abstract: Double-pulse laser-induced desorption with elastic laser scattering (LIDELS) is a diagnostic technique capable of making time-resolved, in situ measurements of the volatile fraction of diesel particulate matter (PM). The technique uses two laser pulses of comparable energy, separated in time by an interval sufficiently short to freeze the flow field, to measure the change in PM volume caused by laser-induced desorption of the volatile fraction. The first laser pulse of a pulse-pair produces elastic laser scattering (ELS) that gives the total PM volume, and also deposits the energy to desorb the volatiles. ELS from the second pulse gives the volume of the remaining solid portion of the PM, and the ratio of these two measurements is the quantitative solid volume fraction. In an earlier study, we used a single laser to make real-time LIDELS measurements during steady-state operation of a diesel engine. In this paper, we discuss the advantages and disadvantages of the two LIDELS techniques and present measurements made in real diesel exhaust and simulated diesel exhaust created by coating diffusion-flame soot with single-component hydrocarbons. Comparison with analysis of PM collected on quartz filters reveals that LIDELS considerably underpredicts the volatile fraction. We discuss reasons for this discrepancy and recommend future directions for LIDELS research.
Abstract: The two-color version of time-resolved laser-induced incandescence (TR-LII) as well as rapid particle probing and transmission electron microscopy (TEM) were applied to the size measurement of chain-like iron particles, synthesized by thermal decomposition of ironpentacarbonyl (Fe(CO)5, IPC) in a hot-wall flow reactor. Both argon and nitrogen were used as carrier gases in different experiments. TR-LII theory considers particle heat transfer and particle evaporation for the interpretation of the measured signals in terms of particle size. The heat transfer from the particle to the surrounding was assumed to proceed under free molecular conditions, which requires the knowledge of the translational energy accommodation coefficient aT during particle cooling. By fitting calculated TR-LII cooling curves to the measured signals, it was possible to determine both aT and the mean primary particle diameter of an assumed lognormal size distribution. Close to the TR-LII measurement section, particles were rapidly sampled and analyzed by TEM. For the obtained chain-like agglomerated particle structures, the TR-LII measured size is in excellent agreement with the TEM determined primary particle size. The method was further validated by variation of the heat-up laser energy density in a wide range of conditions, and the resulting TR-LII diameter was found to be independent of it.
Abstract: The method and apparatus of laser-induced incandescence (LII) to analyze characteristics of submicron-sized particles are described. LII is recognized as a good tool for determining the characteristics of small particles in a gas, e.g., volume fraction, particle size, and specific surface area. It uses the fact that the incandescence signal is proportional to the volume of the particles. It also uses the fact that transient cooling is dependent on the specific surface area of the particle, which is related to diameter of the particle. In LII, particles are heated by a pulsed laser light beam to a temperature where incandescence from the particles can be distinguished from ambient light. The temperature of particles and their volume fraction governs the incandescence. The temperature decay rate is proportional to the primary particle size. The invention uses an optical arrangement that ensures a near-uniform laser energy distribution spatial profile. The invention also uses a low fluence laser beam pulse to avoid evaporation of particles. Without significant evaporation and with a uniform energy profile, accurate and precise measurements can be conducted more easily and reliably.
Abstract: Effective temperatures of pulsed-laser-heated soot particles were derived from their thermal emission intensities using optical pyrometry in a laminar ethylene coflow diffusion flame. The present study concerns conditions of relatively low laser fluences under which soot particles are heated to temperatures below 3500 K to avoid complications of soot particle vaporization in both the experiment and the numerical calculations. The current nanoscale heat transfer model for laser-induced incandescence (LII) of soot was improved to account for the effect of the fractal structure of soot aggregates on the rate of heat loss to the surrounding gas. Mean primary soot particle diameter and mean aggregate size at the location of measurement were determined using the technique of thermophoretic sampling/transmission electron microscopy analysis. Numerical calculations based on the improved LII model were conducted to predict the soot particle temperature with known gas temperature, the heat conduction coefficient, the primary particle diameter, and the mean aggregate size, as well as values of assumed soot absorption function E(m) and the thermal accommodation coefficient of soot. The experimentally observed soot temperature history, characterized by the peak value and the temporal decay rate, cannot be reproduced numerically using the values of E(m) and a found in the literature. By utilizing the experimental peak temperature and temporal decay rate new values of E(m) at 1064 nm and the thermal accommodation coefficient were determined. Uncertainties in the derived values of E(m) and the thermal accommodation coefficient caused by the uncertainty in the primary soot particle diameter and the mean aggregate size were analyzed. A novel method to determine the values of the soot absorption function E(m) and the thermal accommodation coefficient was developed in the present study.
Abstract: Experimental data on the probability distribution of N, from which Ng and Ã\textflorin2g are derived, for soot aggregates sampled within a laminar diffusion flame environment have not been published. The objective of the present investigation is to report such experimental data and to gain a better understanding of the distribution of N of soot aggregates thermophoretically sampled from a laminar ethylene/air diffusion flame by analyzing thousands of aggregates in TEM (transmission electron microscopy) images.
Abstract: This paper describes a model for analyzing and predicting the temporal behavior of laser-induced incandescence (LII) from combustion-generated soot, carbon black, and other carbonaceous particles on a nanosecond time scale. The model accounts for particle heating by absorption of light from a pulsed laser and cooling by sublimation, conduction, and radiation. The model also includes mechanisms for oxidation, melting, and annealing of the particles and nonthermal photodesorption of carbon clusters from the particle surface. At fluences above 0.1 J/cm2, particle temperatures during the laser pulse are determined by the balance between absorption and sublimation, whereas at lower fluences particle temperatures do not reach the sublimation temperature, and temperatures are predominantly controlled by absorption and conduction. After the laser pulse, temperatures are predominantly controlled by conductive cooling rates. Oxidative heating may compete with conductive cooling on these time scales. Annealing of the particles to a more ordered phase of carbon is predicted to occur at fluences as low as 0.02 J/cm2. Annealing may strongly influence sublimation rates, and changes in emissivity during annealing are predicted to increase signal decay rates. Supersonic expansion of the carbon clusters sublimed from the surface is calculated to occur at fluences above 0.12 J/cm2. When compared with LII measurements recorded in a flame at atmospheric pressure, the model reproduces the shapes and relative magnitudes of LII temporal profiles over a wide range of laser fluences. Comparisons between model predictions and experimental observations suggest that the particles do not melt at laser fluences that lead to melting of bulk graphite. These comparisons also indicate that the energy released during particle annealing is much smaller than that released during annealing of neutron- or electron-irradiated graphite. Despite good agreement between model and experimental results, large uncertainties exist for input parameters used to calculate annealing rates and rates of oxidation, conduction, absorption, emission, and photolytic desorption of carbon clusters for both the initial and annealed particles.
Abstract: The UV-VIS-NIR spectral optical properties of soot and soot containing aerosols were investigated in detail during the AIDA Soot Aerosol Campaign 1999. One aim of the campaign was a comprehensive comparison of the microphysical properties of Diesel and spark generator soot. The mass specific extinction cross section at Î\guillemotright=450 nm of Diesel soot is 10.6Â\textpm0.5 m2 gâ\textasciicircum\textquoteright1 which is almost a factor of two larger than the corresponding value of 5.7Â\textpm0.3 m2 gâ\textasciicircum\textquoteright1 measured for spark generator soot. Coagulation-induced particle growth does not affect the soot extinction cross section and has a weak influence on the scattering properties of the soot aggregates. Atmospheric processing of freshly emitted soot was simulated in mixing experiments. The formation of mixed Diesel soot and dry ammonium sulfate particles by coagulation has only a minor effect on the soot absorption cross section. The coating of spark generated soot with organic material results in a strong increase of the single scattering albedo. A significant increase of the absorption coefficient at Î\guillemotright=473 nm during the coating process can be attributed to an enhancement of the specific soot absorption cross section by more than 30%.
Abstract: We present a data set for testing models of time-resolved laser-induced incandescence of soot. Measurements were made in a laminar ethene diffusion flame over a wide range of laser fluences at 532 nm. The laser was seeded to provide a smooth temporal profile, and the beam was spatially filtered and imaged into the flame to provide a homogeneous spatial profile. The particle incandescence was imaged onto a fast photodiode. The measurements are compared with the standard Melton model and with a new model that incorporates physical mechanisms not included in the Melton model.
Abstract: <p>The influence of pressure on laser-induced incandescence (LII) is investigated systematically in premixed, laminar, sooting ethylene/air flames at 1-15 bar with wavelength-, laser fluence-, and time-resolved detection. In the investigated pressure range the LII signal decay rate is proportional to pressure. This observation is consistent with the prediction of heat-transfer models in the free-mol. regime. Pressure does not systematically affect the relation between LII signal and laser fluence. With appropriate detection timing the pressure influence on LII signal&$#$39;s proportionality to soot vol. fraction obtained by extinction measurements is only minor compared with the variation obsd. in different flames at fixed pressures. The implications for particle sizing and soot vol. fraction measurements using LII techniques at elevated pressures are discussed.</p>
Abstract: The influence of pressure on laser-induced incandescence (LII) is investigated systematically in premixed, laminar, sooting ethylene /air flames at 1-15 bar with wavelength-, laser fluence-, and time-resolved detection. In the investigated pressure range the LII signal decay rate is proportional to pressure. This observation is consistent with the prediction of heat-transfer models in the free-molecular regime. Pressure does not systematically affect the relationship between LII signal and laser fluence. With appropriate detection timing the pressure influence on LII signal "s proportionality to soot volume fraction obtained by extinction measurements is only minor compared with the variation observed in different flames at fixed pressures. The implications for particle sizing and soot volume fraction measurements using LII techniques at elevated pressures are discussed., url = http://ao.osa.org/abstract.cfm?id=71880, author = Hofmann, Max and Bessler, Wolfgang G. and Schulz, Christof and Jander, Helga
Abstract: A three-dimensional (3-D) imaging system for studies of reactive and non-reactive flows is described. It can be used to reveal the topology of turbulent structures and to extract 3-D quantities, such as concentration gradients. Measurements are performed using a high repetition rate laser and detector system in combination with a scanning mirror. In this study, the system is used for laser-induced incandescence measurements to obtain quantitative 3-D soot volume fraction distributions in both laminar and turbulent non-premixed flames. From the acquired data, iso-concentration surfaces are visualised and concentration gradients calculated.
Abstract: CONTEXT: Associations have been found between day-to-day particulate air pollution and increased risk of various adverse health outcomes, including cardiopulmonary mortality. However, studies of health effects of long-term particulate air pollution have been less conclusive. OBJECTIVE: To assess the relationship between long-term exposure to fine particulate air pollution and all-cause, lung cancer, and cardiopulmonary mortality. DESIGN, SETTING, AND PARTICIPANTS: Vital status and cause of death data were collected by the American Cancer Society as part of the Cancer Prevention II study, an ongoing prospective mortality study, which enrolled approximately 1.2 million adults in 1982. Participants completed a questionnaire detailing individual risk factor data (age, sex, race, weight, height, smoking history, education, marital status, diet, alcohol consumption, and occupational exposures). The risk factor data for approximately 500 000 adults were linked with air pollution data for metropolitan areas throughout the United States and combined with vital status and cause of death data through December 31, 1998. MAIN OUTCOME MEASURE: All-cause, lung cancer, and cardiopulmonary mortality. RESULTS: Fine particulate and sulfur oxide–related pollution were associated with all-cause, lung cancer, and cardiopulmonary mortality. Each 10-microg/m(3) elevation in fine particulate air pollution was associated with approximately a 4%, 6%, and 8% increased risk of all-cause, cardiopulmonary, and lung cancer mortality, respectively. Measures of coarse particle fraction and total suspended particles were not consistently associated with mortality. CONCLUSION: Long-term exposure to combustion-related fine particulate air pollution is an important environmental risk factor for cardiopulmonary and lung cancer mortality.
Abstract: A new diagnostic technique is described that has the capability of making real-time, in situ measurements of the volatile fraction of diesel particulate matter (PM). LIDELS uses two laser pulses of comparable energy, separated in time by an interval sufficiently short to freeze the flow field, to measure the change in PM volume caused by laser-induced desorption of the volatile fraction. The first laser pulse produces elastic light scattering (ELS) that gives the volume of the total PM, and also deposits the energy to desorb the volatiles. ELS from the second pulse gives the volume of the remaining solid portion of the PM, and the ratio of these two measurements is the quantitative solid volume fraction. Calibration is required for the individual total PM and solid fraction to be quantitative. Applicability of the technique is demonstrated for load and EGR sweeps for a turbocharged, direct-injection diesel engine.
Abstract: A multiwavelength flame emission technique is developed for high spatial resolution determination of soot temperature and soot volume fraction in axisymmetric laminar diffusion flames. Horizontal scans of line-integrated spectra are collected over a spectral range of 500-945 nm. Inversion of these data through one-dimensional tomography using a three-point Abel inversion yields radial distributions of the soot radiation from which temperature profiles are extracted. From an absolute calibration of the flame emission and by use of these temperature data, absorption coefficients are calculated, which are directly proportional to the soot volume fractions. The important optical parameters are discussed. It is shown that a uniform sampling cross section through the flame must be maintained and that variations in sampling area produce inconsistencies between measurements and theory, which cannot be interpreted as spatial averaging of the property field. The variations in cross-sectional sampling area have the largest influence on the measurements at the edges of the flame, where the highest resolution is required. Emission attenuation by soot has been shown to have minor influence on the soot temperature and soot volume fraction for the soot loading of the axisymmetric flame tested. An emission correction scheme is outlined, which could be used for more heavily sooting flames. For a refractive index absorption function E(m) = Im[(m2 - 1)(m2 + 2)] that is independent of wavelength, the soot temperatures and soot volume fractions measured with this technique are in excellent agreement with data obtained by coherent anti-Stokes Raman scattering nitrogen thermometry and two-dimensional soot extinction in the same ethylene coflow diffusion flame. The agreement of the results suggests a limit of the slope of the spectral response of E(m) to be between 0 and 20% over the spectral range examined.
Abstract: A comparison of scanning mobility particle sizer (SMPS) and laser-induced incandescence (LII) measurements of diesel particulate matter (PM) was performed. The results reveal the significance of the aggregate nature of diesel PM on interpretation of size and volume fraction measurements obtained with an SMPS, and the accuracy of primary particle size measurements by LII. Volume fraction calculations based on the mobility diameter measured by the SMPS substantially over-predict the space-filling volume fraction of the PM. Correction algorithms for the SMPS measurements, to account for the fractal nature of the aggregate morphology, result in a substantial reduction in the reported volume. The behavior of the particulate volume fraction, mean and standard deviation of the mobility diameter, and primary particle size are studied as a function of the EGR for a range of steady-state engine speeds and loads for a turbocharged direct-injection diesel engine. Both the SMPS and LII techniques demonstrate good repeatability and consistency with each other. Increasing the EGR results in a sharp rise in the volume fraction of particulates for all engine speeds and loads. At all speed and load conditions the primary particle size decreases with increasing EGR.
Abstract: A computational study of soot formation in an undilute axisymmetric laminar ethylene-air coflow jet diffusion flame at atmospheric pressure was conducted using a detailed gas-phase reaction mechanism and complex thermal and transport properties. A simple two-equation soot model was employed to predict soot formation, growth, and oxidation with interactions between the soot chemistry and the gas-phase chemistry taken into account. Both the optically thin model and the discrete-ordinates method coupled with a statistical narrow-band correlated-K based wide band model for radiative properties of CO, CO2, H2O, and soot were employed in the calculation of radiation heat transfer to evaluate the adequacy of using the optically thin model. Several calculations were performed with and without radiative transfer of radiating gases and/or soot to investigate their respective effects on the computed soot field and flame structure. Radiative heat transfer by both radiating gases and soot were found to be important in this relatively heavily sooting flame studied. Results of the optically thin radiation model are in good agreement with those obtained using the wide band model except for the flame temperature near the flame tip.
Abstract: The present invention relates to a method and apparatus for applying laser induced incandescence (LII) to determine a primary particle size of submicron sized particles. The present invention has found that in addition to volume fraction information, particle size can be determined using LII due to the fact that transient cooling is dependent on the diameter of the particle. The ratio of a prompt and a second time integrated measurement from the same laser pulse has been found to be a function of the particle size. A modeling process involves a solution of the differential equations describing the heat/energy transfer of the particle and surrounding gas, including parameters to describe vaporization, heat transfer to the medium, particle heating etc. The solution gives temperature and diameter values for the particles over time. These values are then converted to radiation values using Planck\textquoterights equation. Thus the technique in accordance with the invention is able to provide a more accurate particle measurement than previous LII techniques, particularly where time averaging is not possible and size measurements must be obtained from a single laser pulse. Simultaneously a particle volume fraction can be obtained in accordance with the invention. Calibration is needed to obtain a quantified volume fraction measurement. In a further embodiment of the present invention, a technique for providing absolute intensity calibration is included in the method.
Abstract: Laser-induced incandescence (LII) and laser elastic-scattering measurements have been obtained with subnanosecond time resolution from a propane diffusion flame. Results show that the peak and time integrated values of the LII signal increase with increasing laser fluence to maxima at the time of the onset of significant vaporization, beyond which they both decrease rapidly with further increases in fluence. This latter behavior for the time-integrated value is known to be characteristic for a laser beam with a rectangular spatial profile and is attributed to soot mass loss from vaporization. However, there is no apparent explanation for the corresponding large decrease in the peak value. Analysis shows that the peak value occurs at the time in the laser pulse when the time-integrated fluence reaches approximately 0.2 J/cm2 and that the magnitude of the peak value is strongly dependent on the rate of energy deposition. One possible explanation for this behavior is that, at high laser fluences, a cascade ionization phenomenon leads to the formation of an absorptive plasma that strongly perturbs the LII process.
Abstract: ABSTRACT Diesel and gasoline engines face tightening particulate matter emissions regulations due to the environmental and health effects attributed to these emissions. There is increasing demand for measuring not only the concentration, but also the size distribution of the particulates. Laser-induced incandescence has emerged as a promising technique for measuring spatially and temporally resolved soot volume fraction and size. Laser-induced incandescence has orders of magnitude more sensitivity than the gravimetric technique, and thus offers the promise of real-time measurements and adds information on the increasingly desirable size and morphology information. Quantitative LII is shown to provide a sensitive, precise, and repeatable measure of the soot concentration over a wide measurement range. The current research determined the tailpipe particulate emissions characteristics from a DISI (direct injection spark ignition) vehicle, including identifying the relative contributions of various engine modes to the total particulate emissions. The volume concentration measurements were obtained in the undilute exhaust with laser-induced incandescence (LII). Particulate measurements were also performed with ELPI instrumentation, sampling from a mini-diluter. Gravimetric filter sampling was performed to measure mass emission rate, organic/elemental carbon, and sulphates/nitrates/trace elements. The LII technique was demonstrated to be capable of real-time particulate matter measurements over all vehicle transient conditions. The wide measurement range and lower detection limit of LII make it a potentially preferred standard instrument for soot measurements.
Abstract: Aerosols affect the Earth\textquoterights temperature and climate by altering the radiative properties of the atmosphere. A large positive component of this radiative forcing from aerosols is due to black carbonâ\texteuro\textquotedblrightsootâ\texteuro\textquotedblrightthat is released from the burning of fossil fuel and biomass, and, to a lesser extent, natural fires, but the exact forcing is affected by how black carbon is mixed with other aerosol constituents. From studies of aerosol radiative forcing, it is known that black carbon can exist in one of several possible mixing states; distinct from other aerosol particles (externally mixed1, 2, 3, 4, 5, 6, 7) or incorporated within them (internally mixed1, 3, 7), or a black-carbon core could be surrounded by a well mixed shell7. But so far it has been assumed that aerosols exist predominantly as an external mixture. Here I simulate the evolution of the chemical composition of aerosols, finding that the mixing state and direct forcing of the black-carbon component approach those of an internal mixture, largely due to coagulation and growth of aerosol particles. This finding implies a higher positive forcing from black carbon than previously thought, suggesting that the warming effect from black carbon may nearly balance the net cooling effect of other anthropogenic aerosol constituents. The magnitude of the direct radiative forcing from black carbon itself exceeds that due to CH4, suggesting that black carbon may be the second most important component of global warming after CO2 in terms of direct forcing.
Abstract: 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 extinctionwas 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.
Abstract: The ambiguity and incorrect treatment of the evaporation term among some LII models in the literature are discussed. This study does not suggest that the correct formulation presented for the evaporation model is adequate, or that it reflects the soot evaporation process under intense evaporation. The emphasis is that the current evaporation model must be used correctly in the evaluation of the LII model against experimental data. Numerical results are presented to demonstrate the significance of the molecular weight associated with the heat of evaporation and the thermal velocity of carbon vapor on the results obtained with the evaporation model. Other errors frequently repeated in the literature are also identified.
Abstract: Extinction and scattering properties at wavelengths of 250â\texteuro\textquotedblleft5200 nm were studied for soot emitted from buoyant turbulent diffusion flames in the long residence time regime where soot properties are independent of position in the overfire region and characteristic flame residence times. Flames burning in still air and fueled with gas (acetylene, ethylene, propane, and propylene) and liquid (benzene, toluene, cyclohexane, and n-heptane) hydrocarbon fuels were considered. Measured scattering patterns and ratios of total scattering/absorption cross sections were in good agreement with predictions based on the Rayleigh-Debye-Gans (RDG) scattering approximation in the visible. Measured depolarization ratios were roughly correlated by primary particle size parameter, suggesting potential for completing RDG methodology needed to make soot scattering predictions as well as providing a nonintrusive way to measure primary soot particle diameters. Measurements of dimensionless extinction coefficients were in good agreement with earlier measurements for similar soot populations and were independent of fuel type and wavelength except for reduced values as the near ultraviolet was approached. The ratios of the scattering/absorption refractive index functions were independent of fuel type within experimental uncertainties and were in good agreement with earlier measurements. The refractive index function for absorption was similarly independent of fuel type but was larger than earlier reflectometry measurements in the infrared. Ratios of total scattering/absorption cross sections were relatively large in the visible and near infrared, with maximum values as large as 0.9 and with values as large as 0.2 at 2000 nm, suggesting greater potential for scattering from soot particles to affect flame radiation properties than previously thought.
Abstract: Nonintrusive measurements of the optical properties of soot at visible wavelengths (351.2-800.0 nm) were completed for soot in the overfire region of large (2-7 kW) buoyant turbulent diffusion flames burning in still air at standard temperature and pressure, where soot properties are independent of position and characteristic flame residence time for a particular fuel. Soot from flames fueled with gaseous (acetylene, ethylene, propylene, and butadiene) and liquid (benzene, cyclohexane, toluene, and n-heptane) hydrocarbon fuels were studied. Scattering and extinction measurements were interpreted to find soot optical properties using the Rayleigh-Debye-Gans/polydisperse-fractal-aggregate theory after establishing that this theory provided good predictions of scattering patterns over the present test range. Effects of fuel type on soot optical properties were comparable to experimental uncertainties. Dimensionless extinction coefficients were relatively independent of wavelength for wavelengths of 400-800 nm and yielded a mean value of 8.4 in good agreement with earlier measurements. Present measurements of the refractive index function for absorption, E(m), were in good agreement with earlier independent measurements of Dalzell and Sarofim and Stagg and Charalampopoulos present values of the refractive index function for scattering, F(m), however, only agreed with these earlier measurements for wavelengths of 400-500 nm but otherwise increased with increasing wavelength more rapidly than the rest. The comparison between present and earlier measurements of the real and imaginary parts of the complex refractive index was similar to E(m) and F(m).
Notes: Univ Michigan, Dept Aerosp Engn, Ann Arbor, MI 48109 USA
Abstract: A common view is that the current global warming rate will continue or accelerate. But we argue that rapid warming in recent decades has been driven mainly by non-CO2 greenhouse gases (GHGs), such as chlorofluorocarbons, CH4, and N2O, not by the products of fossil fuel burning, CO2 and aerosols, the positive and negative climate forcings of which are partially offsetting. The growth rate of non-CO2 GHGs has declined in the past decade. If sources of CH4 and O3 precursors were reduced in the future, the change in climate forcing by non-CO2 GHGs in the next 50 years could be near zero. Combined with a reduction of black carbon emissions and plausible success in slowing CO2 emissions, this reduction of non-CO2 GHGs could lead to a decline in the rate of global warming, reducing the danger of dramatic climate change. Such a focus on air pollution has practical benefits that unite the interests of developed and developing countries. However, assessment of ongoing and future climate change requires composition-specific long-term global monitoring of aerosol properties.
Abstract: Diesel engines face tightening particulate matter emissions regulations due to the environmental and health effects attributed to these emissions. There is increasing demand for measuring not only the concentration, but also the size distribution of the particulates. Laser-induced incandescence has emerged as a promising technique for measuring spatially and temporally resolved particulate volume fraction and size. Laser-induced incandescence has orders of magnitude more sensitivity than the gravimetric technique, and thus offers the promise of real-time measurements and adds the increasingly desirable size and morphology information. The usefulness of LII as a diagnostic instrument for the precise measurement of particulate concentration and primary particle size has been demonstrated. Measurements have been performed in the exhaust of a single cylinder DI research diesel engine. Simultaneous gravimetric filter measurements were made for direct comparison with the LII technique. Quantitative LII is shown to provide a sensitive, precise, and repeatable measure of the particulate concentration over a wide dynamic range. LII and gravimetric measurements are shown to correlate well over a wide range of operating conditions. A novel method for determining the primary particle size is shown to be precise enough to distinguish particle sizes for different engine operating conditions, and subsequently the number density of primary particles was determined. LII has also been shown to be sensitive in differentiating the PM performance between four different fuels. The LII technique is capable of real-time particulate matter measurements over any engine transient operation. The wide dynamic range and lower detection limit of LII make it a potentially preferred standard instrument for particulate matter measurements.
Abstract: The invention relates to a method and an apparatus for the determination of particle volume fractions with laser induced incandescence (LII) using absolute light intensity measurements. This requires a knowledge of the particle temperature either from a numerical model of particulate heating or experimental observation of the particulate temperature. Further, by using a known particle temperature a particle volume fraction is calculated. This avoids the need for a calibration in a source of particulates with a known particle volume fraction or particle concentration. The sensitivity of the detection system is determined by calibrating an extended source of known radiance and then this sensitivity is used to interpret measured LII signals. This results in a calibration independent method and apparatus for measuring particle volume fraction or particle concentrations. A modeling process involves a solution of the differential equations describing the heat/energy transfer of the particle and surrounding gas, including parameters to describe vaporization, heat transfer to the medium, particle heating etc. The solution gives temperature and diameter values for the particles over time. These values are then converted to radiation values using Planck\textquoterights equation.
Abstract: A number of modern technological applications require a detailed calculation of the physical properties of aggregated aerosol particles. For example, in probing soot aerosols by the method called laser-induced incandescence (LII), the soot clusters are suddenly heated by a short, powerful laser pulse and then cool down to the temperature of the carrier gas. LII sizing is based on rigorous calculation of the soot aggregate heat-up and cooling and involves prediction of laser light absorption and energy and mass transfer between aggregated particles and the ambient gas. This paper describes results of numerical simulations of the mass or energy transfer between the gas and fractal-like aggregates of N spherical particles in either the free-molecular or continuum regime, as well as the light scattering properties of random fractal-like aggregates, based on Rayleigh-Debye-Gans (RDG) theory. The aggregate geometries are generated numerically using specially developed algorithms allowing "tuning" of the fractal dimension and prefactor values. Our results are presented in the form of easily applicable scaling laws, with special attention paid to relations between the aggregate gyration radius and the effective radius describing various transport processes between the aggregates and the carrier gas. (C) 2000 Academic Press.
Abstract: In the present paper a simple and straightforward recipe for characterizing the structural and fractal properties of aggregates from their projected images is presented. Starting from geometrical properties that are directly measured from the projected image-such as primary particle mean diameter, maximum projected length, projected area, and overlap coefficient-important three-dimensional properties including number of primary particles in an aggregate, radius of gyration, aggregate surface, or fractal dimensions, D-f and k(g), can be inferred. Expressions proposed in the recipe to relate three dimensional with projected properties were obtained from an extensive investigation of the structure of numerically simulated cluster-cluster fractal-like aggregates. This involved the simulation of statistically significant populations of aggregates having appropriate fractal properties and prescribed numbers of primary particles per aggregate in order to characterize three-dimensional morphological properties of aggregates. Specific ranges of aggregate properties considered were as follows: number of primary particles per aggregate up to 512, fractal dimension, D-f approximate to 1.78, overlap coefficient in the range 0-0.33 and fractal pre factor between 1.5 and 3.1. (C) 1999 Elsevier Science Ltd. All rights reserved.
Abstract: Laser-induced incandescence has emerged as a promising technique for measuring spatially and temporally resolved particulate volume fraction and size. Laser-induced incandescence has orders of magnitude more sensitivity than the gravimetric technique, and thus offers the promise of real-time measurements and adds the increasingly desirable size and morphology information. Particulate matter emissions have been measured by laser-induced incandescence and the standard gravimetric procedure in a mini dilution tunnel connected to the exhaust of a single-cylinder diesel engine. The engine used in this study incorporates features of contemporary medium- to heavy-duty diesel engines and is tuned to meet the U.S. EPA 1994 emission standards. The engine experiments have been run using the AVL 8-mode steady-state simulation of the U.S. EPA heavy-duty transient test procedure. Results of the measurements using the two methods are compared and the suitability of the laser-induced incandescence for particulate mass measurements in diesel exhaust is demonstrated.
Abstract: This paper reviews the optical techniques for in-cylinder combustion temperature measurement, particularly soot measurements in diesel engines. The review starts with the two-colour method for in-cylinder soot and combustion temperature measurement. The principle and implementation of the two-colour technique are described in detail. Both signal point and full-field temperature and soot measurements by the two-colour method are considered. In the second part, the soot diagnostics based on light scattering, especially the light extinction method for in-cylinder soot concentration measurements, are discussed. In the third part, optical techniques for spatially resolved two-dimensional measurements of soot particles in diesel engines are introduced. Since laser induced incandescence (LII) is a relatively new technique and is particularly suitable for the two-dimensional imaging of soot distribution, the operating principle and implementation of LII are discussed in detail. At the end of each part, examples are given to illustrate the understanding gained about diesel combustion as a result of the application of these optical techniques. This paper provides a comprehensive review for those who are interested in using optical diagnostics for in-cylinder soot and combustion temperature measurement in diesel engines. (C) 1998 Elsevier Science Ltd. All rights reserved.
Abstract: Understanding the physical process of LII is central to practical implementation and accurate theoretical modelling of LII. The LII dependence upon laser fluence is shown to depend upon detection conditions thereby not providing direct information about the soot temperature or structural changes. Transmission electron microscopy, used to investigate the morphological changes induced in the soot at different laser fluences, shows increasing graphitization of the soot with increasing laser fluence. For laser fluences above 0.45-0.05 J/cm2 at 1064 nm, vaporization/fragmentation of soot primary particles and aggregates occurs. Optical measurements are performed using a second laser pulse to probe the effects of these changes upon the LII signal. With the exception of very low fluences, the structural changes induced in the soot lead to a decreased LII intensity produced by the second laser pulse. These two-pulse experiments also show that these changes do not alter the LII signal on timescales less than 1 7s for fluences below the vaporization threshold.
Abstract: Most of the particle number emitted by engines is in the nanoparticle range, Dp<50 nm, while most of the mass is in the accumulation mode, 50 nm<Dp<1000 nm, range. Nanoparticles are typically hydrocarbons or sulfate and form by nucleation during dilution and cooling of the exhaust, while accumulation mode particles are mainly carbonaceous soot agglomerates formed directly by combustion. Emission standards on diesel engines have led to dramatic reductions in particle mass emitted. However, a new HEI study shows that some low-emission diesel engines emit much higher concentrations of nanoparticles than older designs and other low-emission designs. Many recent studies suggest that at similar mass concentrations; nanometer size particles are more dangerous than micron size particles. This has raised questions about whether nanoparticle (number based) emission standards should be imposed. Unlike mass, number is not conserved. It may change dramatically by nucleation and coagulation during dilution and sampling, making it very difficult to design a standard. Furthermore, if nanoparticles are a problem, spark ignition engines may also have to be controlled.
Abstract: Laser-induced incandescence from soot was analyzed with a time-dependent, numerical model of particle heating and cooling processes that includes spatial and temporal intensity profiles associated with laser sheet illumination. For volume fraction measurements, substantial errors result primarily from changes in gas temperature and primary soot particle size. The errors can be reduced with the proper choice of detection wavelength, prompt gating, and high laser intensities. Two techniques for primary particle size measurements, based on ratios of laser-induced incandescence signals from a single laser pulse, were also examined. Compared with the ratio of two integration times, the newly proposed ratio of two detection wavelengths is better suited for simultaneous volume fraction and size measurements, because it is less temperature sensitive and produces stronger signals with, however, a lower sensitivity to size changes.
Abstract: CARS measurements of gas temperature profiles performed at low pressure (about 1 Torr) in a PECVD RF reactor and in a CVD reactor reveal the thermal accommodation phenomenon between the gas and the surfaces. A one-dimensional thermal model has been developed to calculate the temperature profiles in the PECVD and CVD reactors and the results are compared with the experimental measurements. In addition to the thermal conduction and accommodation, the model takes into account the radiative exchange between the surfaces. The influence of the pressure on the temperature jump between the gas and the surfaces was investigated in the CVD reactor. Thermal accommodation probabilities for five gas/surface couples have been determined: 0.07â\texteuro\textquotedblleft0.13 for H2/stainless steel, 0.05+/-0.01 for H2/Si, 0.17+/-0.02 for H2/graphite, 0.38+/-0.03 for N2/stainless steel and 0.26+/-0.02 for N2/graphite. In the PECVD reactor, the influence of the electrical power deposited in the plasma on the temperature profile between the electrodes was studied.
Abstract: An in situ particulate diagnostic/analysis technique is outlined based on the Rayleigh-Debye-Gans polydisperse fractal aggregate (RDG/PFA) scattering interpretation of absolute angular light scattering and extinction measurements. Using proper particle refractive index, the proposed data analysis method can quantitatively yield all aggregate parameters (particle volume fraction, fv, fractal dimension, Df, primary particle diameter, dp, particle number density, np, and aggregate size distribution, pdf(N)) without any prior knowledge about the particle-laden environment. The present optical diagnostic/interpretation technique was applied to two different soot-containing laminar and turbulent ethylene/air nonpremixed flames in order to assess its reliability. The aggregate interpretation of optical measurements yielded Df, dp, and pdf(N) that are in excellent agreement with ex situ thermophoretic sampling/transmission electron microscope (TS/TEM) observations within experimental uncertainties. However, volume-equivalent single particle models (Rayleigh/Mie) overestimated dp by about a factor of 3, causing an order of magnitude underestimation in np. Consequently, soot surface areas and growth rates were in error by a factor of 3, emphasizing that aggregation effects need to be taken into account when using optical diagnostics for a reliable understanding of soot formation/evolution mechanism in flames. The results also indicated that total soot emissivities were generally underestimated using Rayleigh analysis (up to 50%), mainly due to the uncertainties in soot refractive indices at infrared wavelengths. This suggests that aggregate considerations may not be essential for reasonable radiation heat transfer predictions from luminous flames because of fortuitous error cancellation, resulting in typically a 10 to 30% net effect.
Abstract: Quantitative experimental measurements of soot concentrations and soot scattering are presented for a series of steady and flickering coflowing methane, propane, and ethylene flames burning at atmospheric pressure. Flickering diffusion flames exhibit a wide range of time-dependent, vortex-flame sheet interactions, and thus they serve as an important testing ground for assessing the applicability of chemical models derived from steady flames. Acoustic forcing of the fuel flow rate is used to phase lock the periodic flame flicker close to the natural flame flicker frequency caused by buoyancy-induced instabilities. For conditions in which flame clip-off occurs, the peak soot concentrations in the methane flickering flames are 5.5 to 6 times larger than measured in a steady flame burning with the same mean fuel flow rate, whereas the enhancement for the flickering propane and ethylene flames is only 35 to 60%, independent of the flicker intensity. Soot concentration profiles and full Mie analysis of the soot volume fraction/scattering results reveal significant differences in the structure of the soot fields and in the roles of soot inception, growth, and oxidation for the different hydrocarbon fuels. The soot concentrations have been measured using laser-induced incandescence (LII). Since this is the only technique currently available for making time- and spatially-resolved soot concentration measurements in time-varying flow fields, considerable effort has been devoted to developing LII for quantitative applications. Important considerations include (1) proper calibration measurements, (2) signal detection which minimizes interferences from C2 Swan-band emission and broadband molecular fluorescence, (3) correction for the laser beam focus/spatial averaging effect in line image measurements, and (4) correction for LII signal extinction within the flame.
Abstract: The spectral extinction coefficients of soot aggregates were studied in the fuel-lean (overfire) region of buoyant turbulent diffusion flames. Extinction measurements were carried out in the wavelength region of 0.2-5.2 mu m for flames fueled with acetylene, propylene, ethylene, and propane, burning in air. The present measurements were combined with earlier measurements of soot morphology and light scattering at 0.514 mu m in order to evaluate the spectral soot refractive indices reported by Dalzell and Sarofim (1969), Lee and Tien (1981), and Chang and Charalampopoulos (1990). The specific extinction coefficients and emissivities were predicted based on Rayleigh-Debye-Gans theory for polydisperse fractal aggregates, which has been recently found to be the best approximation to treat optical cross sections of soot aggregates. The results indicated that available refractive indices of soot do not predict the spectral trends of present measurements in the ultraviolet and infrared regions. Soot complex refractive index was inferred to be m = 1.54 + 0.48i at 0.514 mu m, which is surprisingly in best agreement with the values reported by Dalzell and Sarofim (1969). Additionally, specific extinction coefficients of soot aggregates varied with wavelength as lambda(-0.83) from the visible to the infrared. Finally soot refractive indices were found to be relatively independent of fuel type for the visible and infrared spectral regions over the H/C ratio range of 0.08-0.22.
Notes: Univ michigan, dept aerosp engn, ann arbor, mi 48109, usa
Abstract: <p>Simultaneous measurements of laser-induced incandescence (LII) and elastic scattering from soot particles in diesel engine exhaust have been made. The LII signal scaled linearly with the mass concentration of the non-volatile particulate mass fraction over the entire range of engine operating loads. Over this range of conditions, the volume mean diameter of the soot particles varied from 0.07 to 0.11 mu m, but the size change did not appear to affect the signal response. The scattering response did not scale linearly with the mass concentration of soot. Mass concentrations of 0.2 mg/m3 were easily detectable, with even lower values possible. Additional techniques for determining the volatile fraction of particulate mass are described. (Author abstract). EiPLUS (c) 1996 Engineering Information Inc.</p>
Notes: Caterpillar, Inc, Pontiac, IL, USA Experimental
Abstract: <p>A recently developed laser-induced incandescence technique is used to make novel planar measurements of soot volume fraction within turbulent diffusion flames and droplet flames. The two-dimensional imaging technique is developed and assessed by systematic experiments in a coannular laminar diffusion flame, in which the soot characteristics have been well established. With a single point calibration procedure, agreement to within 10% was found between the values of soot volume fraction measured by this technique and those determined by conventional laser scattering-extinction methods in the flame. As a demonstration of the wide range of applicability of the technique, soot volume fraction images are also obtained from both turbulent ethene diffusion flames and from a freely falling droplet flame that burns the mixture of 75% benzene and 25% methanol. For the turbulent diffusion flames, approximately an 80% reduction in soot volume fraction was found when the Reynolds number of the fuel jet increased from 4000 to 8000. In the droplet flame case, the distribution of soot field was found to be similar to that observed in coannular laminar diffusion flames.</p>
Abstract: The structure of soot aggregates was investigated, emphasizing the fractal properties as well as the relationships between the properties of actual and projected soot images. This information was developed by considering numerically simulated soot aggregates based on cluster-cluster aggregation as well as measured soot aggregates based on thermophoretic sampling and analysis by transmission electron microscopy (TEM) of soot for a variety of fuels (acetylene, propylene, ethylene, and propane) and both laminar and turbulent diffusion flame conditions. It was found that soot aggregate fractal properties are relatively independent of fuel type and flame condition, yielding a fractal dimension of 1.82 and a fractal prefactor of 8.5, with experimental uncertainties (95% confidence) of 0.08 and 0.5, respectively. Relationships between the actual and projected structure properties of soot, e.g., between the number of primary particles and the projected area and between the radius of gyration of an aggregate and its projected image, also are relatively independent of fuel type and flame condition.
Abstract: <p>Laser-induced incandescence is used to obtain spatially resolved measurements of soot volume fraction in a laminar diffusion flame, excellent agreement. In addition, the laser-induced incandescence signal is observed to involve a rapid rise in intensity followed by a relatively long (ca. 600 ns) decay period subsequent to the laser pulse, while the effect of laser fluence is manifest in nonlinear and near-saturated response of the laser-induced incandescence signal. Laser-induced incandescence can be used as an instantaneous, spatially resolved diagnostic of soot volume fraction without the need for the conventional line-of-sight laser extinction method, while potential applications in two-dimensional imaging and simultaneous measurements of laser-induced incandescence and light-scattering to generate a complete soot property characterization are significant. (Edited author abstract).</p>
Notes: Pennsylvania State Univ, University Park, PA, USA Article Jun . p 384-392. 14 Refs DRS First Santoro LII paper. Shows that LII and extinction give very similar (within 10%) soot profiles in conventional LDF from 10 to 70 mm height. Decay time constant 600 ns in heavily sooting region. Claim this indicates approximately 100 nm particle size quoting Melton. They also measure soot size from 90 degree scattering. Find d=60-110 nm as r=0-2.5 at 40 mm height.
Abstract: Optical cross-sections of carbonaceous aggregates (smoke) formed by combustion sources have been computed based on fractal concepts. Specific extinction depends upon the primary particle size, the structure of the aggregates as represented by the fractal dimension, the fractal prefactor, and the real and imaginary components of the refractive index of the particle material. While the fractal dimension and primary particle diam. are narrowly defined, the refractive index, to which the results are highly sensitive, are disputed. Specific extinction was measured at .lambda. = 450, 630, and 1000 nm in a smoke-filled chamber with an optical path length of 1.0 m that was equipped to continuously monitor both particle mass and no. concn. as the smoke aged during a 90-120 min interval. The smoke was generated by the burning of crude oil in a pool fire. Specific extinction at all three values of .lambda. was const. even though the aggregate no. concn. decreases by a factor of 24 owing to cluster-cluster aggregation. The refractive indexes at several wavelengths that are required to give agreement with the measured specific extinction are compared with literature values. The inadequacy of Mie theory for spheres in predicting the optical properties of soot aggregates is reiterated.
Abstract: Laser-Induced Incandescence (LII) occurs when a high-energy pulsed laser is used to heat soot to incandescent temperatures. Theoretical calculations predict and experimental tests demonstrate the resulting incandescence to be a measure of soot-volume fraction. Practical implementation of the technique is detailed by examining the spectral character, temporal behavior, and excitation-intensity dependence of the resulting thermal emission from the laser-heated soot in both premixed and diffusion flames. Spatial and temporal capabilities of LII are demonstrated by obtaining one- and two-dimensional images of soot-volume fraction via laser-induced incandescence in both types of flames.
Abstract: Raman spectra of annealed carbon soot reveal strong structural changes. Downshifts of the graphite-like phonon bands to lower energies after annealing are suggested to be related to strained or curved graphitic planes. The effect of curvature on the energy of the in-plane optical phonon mode is quantitatively estimated by applying the semi-empirical interatomic Tersoff potential. A weighted average curvature corresponding to a bond bending of 2.1[o] is deduced for spherical shells with 20.6 \r A radius. These findings are consistent with high-resolution electron microscopy images which reveal closed-shell carbon particles in the same size range
Abstract: <p>A technique was developed that allows the determination of the temperature dependence of the refractive indices of carbonaceous materials from ellipsometric intensity measurements on bulk samples. The refractive indices of the carbonaceous samples pyrolytic graphite, amorphous carbon and flame soot were determined over the temperature range 25-600 degree C and the spectral region 400-700 nm. For all three samples it was found that the inferred refractive index shows insignificant variation with temperature for this range of temperature and wavelength. These results differ by 30 percent or more from the predictions of the Drude Lorentz dispersion model which has been used extensively to predict the variation of the optical properties of carbonaceous particulates. A new set of dispersion constants is presented that accurately predict the indices in the temperature range 25-600 degree C and in the wavelength range 400-700 nm. (Author abstract).</p>
Notes: Louisiana State Univ, Baton Rouge, LA, USA Article Sep . 37 Refs
Abstract: The authors discuss the development and application of 2D imaging methods for the study of fuel-air mixing and the in-situ measurement of soot in flames. Fuel imaging is based on planar laser-induced fluorescence (PLIF) with pulsed UV lasers. Fuel concns. may be estimated from the fluorescence of common fuel components or through the addition of a fluorescent species or seed. PLIF of fuel is shown to be applicable to both premixed and non-premixed combustion. The choice of fluorescent seed and the role of interfering species, such as combustion derived polyarom. hydrocarbons (PAH) and laser-induced incandescence from soot, are discussed. The application of the method is illustrated with an example of an isothermal mixing study in a gas turbine combustor sector rig and measurements in a simple flame. The results demonstrate that PLIF can achieve high spatial resolution, 0.4 mm, in a combustor with dimensions of order of 400 mm. The dynamic range of the measured intensities exceeds 1500 with typical signal-to-noise ratios of better than 100:1. A major source of interference in PLIF studies of non-premixed flames is laser-induced incandescence (LII) from soot particles. The authors discuss the role of LII both as an interference in PLIF imaging and as an imaging method for soot volume fraction. A detailed anal. of the physics of LII is presented on a theor. model developed. Results from the model and supporting exptl. data are presented. LII images from a highly turbulent sooting flame indicate that soot structures, probably in the form of thin sheets as small as 100 mm across, are formed through vortex mixing.
Abstract: It is shown that the Abel inversion, onion-peeling, and filtered backprojection methods can be intercompared without assumptions about the object being deconvolved. If the projection data are taken at equally spaced radial positions, the deconvolved field is given by weighted sums of the projections divided by the data spacing. The weighting factors are independent of the data spacing. All the methods are remarkably similar and have Abelian behavior: the field at a radial location is primarily determined by the weighted differences of a few projections around the radial position. Onion-peeling and an Abel inversion using two-point interpolation are similar. When the Shepp-Logan filtered backprojection method is reduced to one dimension, it is essentially identical to an Abel inversion using three-point interpolation. The weighting factors directly determine the relative noise performance: the three-point Abel inversion is the best, while onion peeling is the worst with approximately twice the noise. Based on ease of calculation, robustness, and noise, the three-point Abel inversion is recommended.
Abstract: <p>The mass density normalized absorption and total scattering coefficients have been measured using in situ techniques at selected wavelengths from the visible to 1 cm for soot generated by the open combustion of diesel fuel. Particle morphologies are complex although similar to those of soots of other hydrocarbons and methods of generation. An ellipsoidal model has been applied as an approximation to the often multiconnected, chainlike aerosol and then compared with the measured results. The experimental results show an approximate (lambda)-1 dependence over more than five decades of wavelength data. There is only general agreement with the simplified calculations in this feature as well as in the magnitude.</p>
Notes: White Sands Missile Range, U.S. Army Atmos. Sci. Lab., Las Cruces, Article
Abstract: <p>This article proposes a new method for the evaluation of the dispersion of the optical properties of absorbing submicronic aerosols, starting from the simultaneous measurements of the scattering and extinction coefficients in the near UV and visible. The experiments were on premixed flat flames at atmospheric pressure with such nonaromatic fuels as CH4, C2H4, and C2H2 and different C-O ratios and flow rates. A quantitative determination of the special behavior of the real and imaginary parts of the complex refractive index of soot was obtained in conditions where the molecular contribution could be neglected and where the particles were not agglomerated and behaved as Rayleigh scatterers. (Edited author abstract).</p>
Notes: Univ di Napoli P.le Techio, Naples, Italy Article Mar p 259-271. 31 Refs
Abstract: Through numerical calculations we have investigated the possibility of developing soot diagnostics based on laser heating of the soot particles. Two strategies, one using the laser-modulated incandescence of the particles, and the other using direct detection of the evaporated C2 molecules, were examined. Both strategies can yield size distribution and volume fraction information provided the laser wavelength is near the graphite absorption band at 260 nm; otherwise, only volume fractions can be obtained.
Abstract: Laser-modulated particulate (soot) incandescence has been studied by measuring the laser-driven particle surface temperature in a propane diffusion flame as a function of laser focal flux. The experimental results display fair agreement with an analytical model of the process. Quite importantly, the absolute incandescence level displays a saturation behavior with increasing laser pulse energy. For laser Raman scattering diagnostics, this behavior means that the S/N ratio will increase with increasing laser flux level. Thus, for highest S/N ratio, it is preferable to operate at the highest possible flux short of gas breakdown and/or optical-component damage levels.
Abstract: For rarefied gases the collision frequency of the Krook kinetic equation is expressed as the sum of wall and gas collision frequencies. Effective transport coefficients arise from the Chapmanâ\texteuro\textquotedblrightEnskog theory, and are similar to those proposed by other methods. The transport of either energy, momentum, or mass is analysed for several geometries. Profiles of temperature, velocity, and concentration, which involve jump and slip at the wall, are derived, and accommodation effects are included. The expressions have the proper limits for the continuum and free-molecule regimes, and compare well with other theories and experimental data for transition regime. Heat conduction formulas apply to polyatomic gases. Plane, concentric cylinder, and spherical geometries are considered.
Abstract: Optical Constants of Soot and Their Application to Heat Flux Calculations Data on the roorn temperature optical constants of soot are present.ed for the wavelength regions 0.4-0.8 micro m and 2. 7 -10.0 micro m Dispersion formlas are developed for interpolating the data between 0.8 and 2.5 micro m. The results are used to calculate the spectral absorption coefficient and the locall emissivties of soot suspensions. It is shown that the correct values of the optical constants are neeeded in thc use of light-scatterirrg techniques for the measurement of the soot concentration but that uncertainties introduced in flux calculations by use of approximate values of the optical constants are not greater than those inlroduced hy the present uncertainties in the valucs of the soot concentration
Abstract: <ol><li><h6><a href="http://sunsite.informatik.rwth-aachen.de/Publications/CEUR-WS//Vol-211/proceedings_2006.pdf">Complete Proceedings</a></h6></li></ol><h6>Papers</h6><ol><li><a href="http://sunsite.informatik.rwth-aachen.de/Publications/CEUR-WS//Vol-211/Modeling_Liu.pdf">Theoretical considerations in modeling LII at low pressures</a><br /><em>F. Liu, K. J. Daun, G. J. Smallwood</em> </li><li><a href="http://sunsite.informatik.rwth-aachen.de/Publications/CEUR-WS//Vol-211/particle_size.pdf">Inverse analysis of time-resolved LII data</a><br /><em>K. J. Daun, F. Liu, G. J. Smallwood, B. J. Stagg, and D. R. Snelling</em> </li><li><a href="http://sunsite.informatik.rwth-aachen.de/Publications/CEUR-WS//Vol-211/conduction.pdf">Heat conduction from spherical nano-particles</a><br /><em>F. Liu, K. J. Daun, G. J. Smallwood, and D. R. Snelling</em> </li><li><a href="http://sunsite.informatik.rwth-aachen.de/Publications/CEUR-WS//Vol-211/Zizak.pdf">Absorption correction of two-color LII-signals</a><br /><em>F. Migliorini, S. De Iuliis, F. Cignoli, G. Zizak</em> </li><li><a href="http://sunsite.informatik.rwth-aachen.de/Publications/CEUR-WS//Vol-211/Black.pdf">Longer laser pulses for practical LII</a><br /><em>J. D. Black</em> </li><li><a href="http://sunsite.informatik.rwth-aachen.de/Publications/CEUR-WS//Vol-211/Tribalet_final.pdf">TR-LII and PMS particle sizing applied to soot particles synthesized in a low-pressure flame reactor</a><br /><em>B. Tribalet, B.F. Kock, P. Ifeacho, P. Roth, C. Schulz</em> </li><li><a href="http://sunsite.informatik.rwth-aachen.de/Publications/CEUR-WS//Vol-211/Stirn.pdf">Time-resolved LII in comparison with mass spectrometry measurements in a premixed ethylene/air flame</a><br /><em>R. Stirn, K.P. Geigle, W. Meier, T. Gonzalez-Baquet, H.H. Grotheer, M. Aigner</em> </li><li><a href="http://sunsite.informatik.rwth-aachen.de/Publications/CEUR-WS//Vol-211/wendler.pdf">Temperature measurements for LII evaluation in non-premixed flames – comparison between emission spectroscopy and CARS</a><br /><em>M. Wendler, G. Guevara, M.C. Weikl, R. Sommer, F. Beyrau, T. Seeger and A. Leipertz</em> </li><li><a href="http://sunsite.informatik.rwth-aachen.de/Publications/CEUR-WS//Vol-211/Boiarciuc.pdf">Pressure effects on LII-signals</a><br /><em>A. Boiarciuc, F. Foucher, C. Mounaïm-Rousselle</em> </li><li><a href="http://sunsite.informatik.rwth-aachen.de/Publications/CEUR-WS//Vol-211/Stagg.pdf">Effect of pressure on thermal accommodation coefficient</a><br /><em>B.J. Stagg</em> </li><li><a href="http://sunsite.informatik.rwth-aachen.de/Publications/CEUR-WS//Vol-211/Delhay.pdf">Laser-induced incandescence for measuring soot particle emission from aero-gas turbines</a><br /><em>J. Delhay, P. Desgroux, E. Therssen, John Black</em> </li><li><a href="http://sunsite.informatik.rwth-aachen.de/Publications/CEUR-WS//Vol-211/Doerr.pdf">Two-color time-resolved LII study of iron oxide nanoparticle formation in a premixed flat low pressure flame</a><br /><em>H. Dörr, H. Bockhorn, and R. Suntz</em></li></ol><h6>Posters</h6><ol><li><a href="http://sunsite.informatik.rwth-aachen.de/Publications/CEUR-WS//Vol-211/Bougie.pdf">In-cylinder particulate sizing with combined TR-LII/2C pyrometry</a><br /><em>B. Bougie, L.C. Ganippa, A.P. van Vliet, W.L. Meerts, N.J. Dam, J.J. ter Meulen</em> </li><li><a href="http://sunsite.informatik.rwth-aachen.de/Publications/CEUR-WS//Vol-211/accommodationcoefficient.pdf">Measuring accommodation coefficients using laser-induced incandescence</a><br /><em>K. J. Daun, G. J. Smallwood, F. Liu, and D. R. Snelling</em> </li><li><a href="http://sunsite.informatik.rwth-aachen.de/Publications/CEUR-WS//Vol-211/Erenim.pdf">Comparison of TR-LII sizing for pure carbon and hydrogen-containing carbon particles</a><br /><em>A. Eremin, M. Falchenko, E. Gurentsov, B. Kock, R. Starke, C. Schulz</em> </li><li><a href="http://sunsite.informatik.rwth-aachen.de/Publications/CEUR-WS//Vol-211/Greenhalgh.pdf">LII in a high vacuum and up date and LII in carbon black from a particle generator</a><br /><em>V. Beyer and D. A. Greenhalgh, D. Clavel, K. Daun, F. Liu, B. Sawchuck, G. Smallwood, D. Snelling and K. Thomson</em> </li><li><a href="http://sunsite.informatik.rwth-aachen.de/Publications/CEUR-WS//Vol-211/Hadef.pdf">Particle size measurements with two color TIRE-LII</a><br /><em>R. Hadef, R. Stirn, KP. Geigle, M. Aigner</em> </li><li><a href="http://sunsite.informatik.rwth-aachen.de/Publications/CEUR-WS//Vol-211/Hofmann.pdf">A web-based interface for modeling laser-induced incandescence (LIISim)</a><br /><em>M. Hofmann, B.F. Kock, C. Schulz</em> </li><li><a href="http://sunsite.informatik.rwth-aachen.de/Publications/CEUR-WS//Vol-211/Extinction_Liu.pdf">Effects of soot absorption and scattering on LII intensities in a laminar coflow ethylene/air diffusion flame</a><br /><em>F. Liu, K. A. Thomson, G. J. Smallwood</em> </li><li><a href="http://sunsite.informatik.rwth-aachen.de/Publications/CEUR-WS//Vol-211/Dreier.pdf">Time-resolved laser-induced incandescence (TIRE-LII) coupled with spectral emission measurements for particle sizing in high-pressure diesel combustion environments</a><br /><em>R. Ryser, T. Gerber, T. Dreier</em> </li><li><a href="http://sunsite.informatik.rwth-aachen.de/Publications/CEUR-WS//Vol-211/Thomson.pdf">Absolute intensity calibration of LII detectors</a><br /><em>K. A. Thomson, D. R. Snelling, G. J. Smallwood</em></li></ol>
Abstract: <h3>Papers </h3><ol><li><a href="http://sunsite.informatik.rwth-aachen.de/Publications/CEUR-WS//Vol-195/kuhlmann.pdf">Heat conduction issues in laser-induced incandescence</a><br /><em>S.-A. Kuhlmann, J. Reimann, S. Will</em> </li><li><a href="http://sunsite.informatik.rwth-aachen.de/Publications/CEUR-WS//Vol-195/bladh.pdf">A detailed experimental and theoretical comparison of spatially-resolved laser-induced incandescence signals</a><br /><em>H. Bladh, J. Delhay, Y. Bouvier, E. Therssen, P-E. Bengtsson, P. Desgroux</em> </li><li><a href="http://sunsite.informatik.rwth-aachen.de/Publications/CEUR-WS//Vol-195/michelsen.pdf">Investigations of the mechanisms involved in LII particle detection</a><br /><em>H. A. Michelsen, M. Y. Gershenzon, P.O. Witze</em> </li><li><a href="http://sunsite.informatik.rwth-aachen.de/Publications/CEUR-WS//Vol-195/liu.pdf">Influence of polydisperse distributions of both primary particle and aggregate sizes on soot temperature in low-fluence laser-induced incandescence</a><br /><em>F. Liu, M. Yang, F. A. Hill, G. J. Smallwood, D. R. Snelling</em> </li><li><a href="http://sunsite.informatik.rwth-aachen.de/Publications/CEUR-WS//Vol-195/stagg.pdf">2-Color LII measurements of carbon black: Interpretation for quantitative measurement of fineness</a><br /><em>B.J. Stagg</em> </li><li><a href="http://sunsite.informatik.rwth-aachen.de/Publications/CEUR-WS//Vol-195/bouvier.pdf">Wavelength-dependence of refractive index function of soot particle by two-color laser induced incandescence</a><br /><em>Y. Bouvier, E. Therssen, P. Desgroux</em> </li><li><a href="http://sunsite.informatik.rwth-aachen.de/Publications/CEUR-WS//Vol-195/snelling.pdf">An LII technique independent of ex-situ calibration by detecting absolute light intensity</a><br /><em>D. R. Snelling, G. J. Smallwood, F. Liu, Ö. L. Gülder, W. D. Bachalo</em> </li><li><a href="http://sunsite.informatik.rwth-aachen.de/Publications/CEUR-WS//Vol-195/black.pdf">Laser-induced processes in carbon generated in an argon arc</a><br /><em>J.D. Black, M.P. Johnson</em> </li><li><a href="http://sunsite.informatik.rwth-aachen.de/Publications/CEUR-WS//Vol-195/beyer.pdf">An investigation of soot nanoparticulate in a vacuum</a><br /><em>V. Beyer, D.A. Greenhalgh</em> </li><li><a href="http://sunsite.informatik.rwth-aachen.de/Publications/CEUR-WS//Vol-195/thomson.pdf">Laser-induced incandescence measurements in a laminar co-annular non-premixed methane/air flame at pressures of 0.5 to 4.0 MPa</a><br /><em>K. A. Thomson, D. R. Snelling, G. J. Smallwood, F. Liu</em> </li><li><a href="http://sunsite.informatik.rwth-aachen.de/Publications/CEUR-WS//Vol-195/geigle.pdf">Laser-induced incandescence and shifted vibrational CARS in laminar premixed flames at atmospheric and elevated pressures</a><br /><em>K.P. Geigle, M.S. Tsurikov, W. Meier, V. Krüger, R. Hadef</em> </li><li><a href="http://sunsite.informatik.rwth-aachen.de/Publications/CEUR-WS//Vol-195/hofmann.pdf">Laser-induced incandescence and multi-line NO thermometry for soot diagnostics at high pressures</a><br /><em>M. Hofmann, H. Kronemayer, B. F. Kock, C. Schulz</em> </li><li><a href="http://sunsite.informatik.rwth-aachen.de/Publications/CEUR-WS//Vol-195/bougie.pdf">Soot particulate size measurements in a heavy duty Diesel engine</a><br /><em>B. Bougie, L.C. Ganippa, A.P. van Vliet, N.J. Dam, W.L. Meerts, J.J. ter Meulen</em> </li><li><a href="http://sunsite.informatik.rwth-aachen.de/Publications/CEUR-WS//Vol-195/dreier.pdf">Modeling of time-resolved laser-induced incandescence (TIRE-LII) transients for particle sizing in high-pressure spray combustion environments</a><br /><em>T. Dreier, B. Bougie, L. Ganippa, N. Dam, T. Gerber, J.J. ter Meulen</em> </li><li><a href="http://sunsite.informatik.rwth-aachen.de/Publications/CEUR-WS//Vol-195/eremin.pdf">Application of TR-LII for the study of carbon vapor condensation at room temperature</a><br /><em>A. Eremin, E. Gurentsov, M. Hofmann, C. Schulz</em> </li><li><a href="http://sunsite.informatik.rwth-aachen.de/Publications/CEUR-WS//Vol-195/lucas.pdf">Planar laser-induced incandescence of iron particles in welding fumes</a><br /><em>O. Lucas, Z. Alwahabi, V. Linton</em> </li><li><a href="http://sunsite.informatik.rwth-aachen.de/Publications/CEUR-WS//Vol-195/kock.pdf">Time-resolved laser-induced-incandescence (TR-LII) for iron-particle sizing</a><br /><em>B. Kock, J. Knipping, H.R. Orthner, C. Kayan, C. Schulz, P. Roth</em> </li><li><a href="http://sunsite.informatik.rwth-aachen.de/Publications/CEUR-WS//Vol-195/schittkowski.pdf">Laser-induced incandescence of free and surface-adsorbed particles</a><br /><em>T. Schittkowski, D. Böker, D. Brüggemann</em> </li><li><a href="http://sunsite.informatik.rwth-aachen.de/Publications/CEUR-WS//Vol-195/charwath.pdf">In-situ determination of gas-to-particle reaction generated nanoscaled particles</a><br /><em>M. Charwath, T. Lehre, R. Suntz, H. Bockhorn</em> </li><li><a href="http://sunsite.informatik.rwth-aachen.de/Publications/CEUR-WS//Vol-195/qamar.pdf">Two-dimensional imaging of soot volume fraction and OH in turbulent jet diffusion flames spanning low to high mixing rates</a><br /><em>N. H. Qamar, Z.T. Alwahabi, G. J. Nathan, K. D. King</em> </li></ol><h3>Posters</h3><ol><li><a href="http://sunsite.informatik.rwth-aachen.de/Publications/CEUR-WS//Vol-195/deiuliis.pdf">Peak soot temperature in laser-induced incandescence measurements</a><br /><em>S. De Iuliis, F. Cignoli, G. Zizak</em> </li><li><a href="http://sunsite.informatik.rwth-aachen.de/Publications/CEUR-WS//Vol-195/boiarciuc.pdf">Soot volume fractions and primary particle size estimations by means of simultaneous time-resolved and 2D laser-induced incandescence</a><br /><em>A. Boiarciuc, F. Foucher, C. Mounaïm-Rousselle</em> </li><li><a href="http://sunsite.informatik.rwth-aachen.de/Publications/CEUR-WS//Vol-195/kockp.pdf">Time-resolved laser-induced incandescence applied to in-cylinder Diesel particle sizing</a><br /><em>B. F. Kock, C. Schulz, P. Roth</em> </li><li><a href="http://sunsite.informatik.rwth-aachen.de/Publications/CEUR-WS//Vol-195/kronemayer.pdf">Gas-phase temperature imaging in sooting flames by multi-line NO-LIF thermometry</a><br /><em>H. Kronemayer, M. Hofmann, K. Omerbegovic, C. Schulz</em> </li><li><a href="http://sunsite.informatik.rwth-aachen.de/Publications/CEUR-WS//Vol-195/liup.pdf">A critical evaluation of the thermal accommodation coefficient of soot determined by the laser-induced incandescence technique</a><br /><em>F. Liu, D. R. Snelling, G. J. Smallwood</em> </li></ol>
