Department of Clean Air

 

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.

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.

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.

"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.

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. 

 

We have performed a comparison of ten models that predict the temporal behavior of laser-induced incandescence (LII) of soot.  In this paper we present a summary of the models and comparisons of calculated temperatures, diameters, signals, and energy-balance terms.  The models were run assuming laser heating at 532 nm at fluences of 0.05 and 0.70 J/cm² with a laser temporal profile provided.  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.  Preliminary calculations were performed with a fully constrained model.  The comparison of unconstrained models demonstrates a wide spread in calculated LII signals.  Many of the differences can be attributed to the values of a few important parameters, such as the refractive index function E(m) and thermal and mass accommodation coefficients.  Constraining these parameters brings most of the models into much better agreement with each other, particularly for the low-fluence case.  Agreement among models is not as good for the high-fluence case, even when selected parameters are constrained.  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.

 

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.  Laser-induced nanoparticle production was observed at fluences above 0.12 J/cm² at 532 nm and 0.22 J/cm² at 1064 nm.  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. 

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.

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.

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.

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.

An auto-compensating laser-induced incandescence (AC-LII) technique was applied for the first time to measure soot volume fraction (SVF) and effective primary particle diameter (dpeff) in a high pressure methane/air non-premixed flame. The measured dpeff profiles had annular structures and radial symmetry, and the particle size increased with increasing pressure. LII-determined SVFs were lower than those measured by a line of sight attenuation (LOSA) technique. The LOSA measured soot volume fractions were corrected for light scattering using the Rayleigh–Debye–Gans polydisperse fractal aggregate (RDG-PFA) theory, the dpeff data, and assumptions regarding the soot aggregate size distribution. The correction dramatically improved agreement between data obtained using these two measurement techniques. Qualitatively, soot volume distributions obtained using LII had more annular shapes than those obtained using LOSA. Nonetheless, it has been demonstrated that the AC-LII technique is very well suited for application in media where attenuation of the excitation laser pulse energy can exceed 45%. This paper also underlines the importance of correcting LOSA SVF measurements for light scattering in high pressure flames.

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 μs 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 μs, the large particles fragment into smaller entities. We have also modelled the incandescence behaviour using well established methods.

Editorial

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 ≤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.

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.

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.

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.

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 (C₃O₂) 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 C₃O₂ molecules on carbon particles.

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.

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 3300–3400 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.