Soot sublimation studies in a premixed flat flame using laser-induced incandescence (LII) and elastic light scattering (ELS)

Laser-induced incandescence (LII) as a diagnostic technique is based on rapid heating of soot particles to temperatures of several thousand Kelvin and subsequent detection of the thermal radiation from the laser-heated particles. At such high temperatures, soot sublimation effects must be considered when estimating uncertainties in LII measurements. In this work we have investigated the use of various laser fluences in LII using a Nd:YAG laser at 1,064 nm. Using another Nd:YAG laser at 532 nm, the elastic light scattering (ELS) signal from soot particles heated by the 1,064-nm laser was monitored. This approach makes it possible to determine at which fluence of the LII laser soot sublimation starts to become visible as a decrease in the ELS signal. By performing the measurements at various heights in a premixed sooting flat ethylene/air flame, the fluence threshold above which the ELS signal decreased was found to be higher at the lower flame heights corresponding to younger, smaller and less aggregated particles. The results from this work indicate that the different fluence thresholds for sublimation may be explained by a lower absorption function E(m) for the younger soot particles.     

Two-color laser-induced incandescence and cavity ring-down spectroscopy for sensitive and quantitative imaging of soot and PAHs in flames

Laser-induced incandescence is a technique which enables the measurement of soot volume fractions. However, the laser-induced soot emission might be affected by a fluorescence background generally ascribed to the polycyclic aromatic hydrocarbon compounds (PAHs) present at the soot location. In this paper, spatially resolved distributions of PAH absorbance and soot are obtained in sooting diffusion flames. The original method developed here consists in comparing the emission distributions induced by two different laser wavelengths: (1) at 1064 nm emission signals are exempt from PAH fluorescence and (2) at 532 nm both soot incandescence and PAH emission contribute to the total signal. In addition, the absolute absorption coefficient of the PAH mixture is determined by comparing absorption measurements obtained by cavity ring-down spectroscopy (CRDS) at 1064 nm and 532 nm. The proposed method can provide highly sensitive 2D imaging of PAHs and soot using the fundamental and the second-harmonic frequencies of a single YAG laser. Finally, 2D distributions of PAH absorbance and soot volume fraction calibrated by CRDS are obtained in two diffusion flames, particularly in a very low-sooting flame exhibiting a maximum PAH absorbance of 6×10^-4 cm^-1 and a maximum soot volume fraction of 3 ppb only. The respective spatial distributions of PAHs and soot are shown to vary with the initial C/O ratio. PACS 33.20.Lg; 42.62.Fi; 44.40.+a     

2D aggregate sizing by combining laser-induced incandescence (LII) and elastic light scattering (ELS)

The combination of laser-induced incandescence and elastic light scattering has been further developed to allow for a quantitative two-dimensional determination of characteristic properties of soot aggregates, namely radius of gyration R _g and number N _p of primary particles per aggregate. In demonstrating the principle of the method, we have in a first approach approximated the particle ensemble as monodisperse and used a structure factor with an exponential cut-off function. Nonetheless, experiments performed on a laminar premixed ethene flame demonstrate basically good agreement with observations from literature and data from electron microscopy on thermophoretically obtained samples.     

Instantaneous in-flame measurement of soot volume fraction, primary particle diameter, and aggregate radius of gyration via auto-compensating laser-induced incandescence and two-angle elastic light scattering

A new combination of soot diagnostics employing two-angle elastic light scattering and laser-induced incandescence is described that is capable of producing non-intrusive, instantaneous, and simultaneous, in situ measurements of soot volume fraction, primary particle size, and aggregate radius of gyration within flames. Controlled tests of the new apparatus on a well-characterized laminar flame show good agreement with existing measurements in the literature. From a detailed and comprehensive Monte Carlo uncertainty analysis of the results, it was found that the uncertainty in all three measured parameters is dominated by knowledge of soot properties and aggregation behavior. The soot volume fraction uncertainty is dominated by uncertainty in the soot refractive index light absorption function; the primary particle diameter uncertainty is dominated by uncertainty in the fractal prefactor; while the uncertainty in the aggregate radius of gyration is dominated by the uncertainty in the width of the distribution of aggregate sizes.     

Optical and microscopy investigations of soot structure alterations by laser-induced incandescence

2 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 μs for fluences below the vaporization threshold. Received: 20 October 1997/Revised version: 16 February 1998     

Measurement of soot morphology by integrated LII and elastic light scattering

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.     

Visualization of soot formation from evaporating fuel films by laser-induced fluorescence and incandescence

Late-evaporating liquid fuel wall-films are considered a major source of soot in spark-ignition direct-injection (SIDI) engines. In this study, a direct-injection model experiment was developed to visualize soot formation in the vicinity of evaporating fuel films. Isooctane is injected by a multi-hole injector into the optically accessible part of a constant-flow facility at atmospheric pressure. Some of the liquid fuel impinges on the quartz-glass windows and forms fuel films. After spark ignition, a turbulent flame front propagates through the chamber, and subsequently sooting combustion is detected near the fuel films. Overlapping laser light sheets at 532 and 1064 nm excite laser-induced fluorescence (LIF) of polycyclic aromatic hydrocarbons (PAH) -potential soot precursors- and laser-induced incandescence (LII) of soot, respectively. The 532 nm light sheet has low fluence to avoid the excitation of LII. The LII and LIF signals are detected simultaneously and spectrally separated on two cameras. In complementary line-of-sight imaging, the fuel spray, chemiluminescence, and soot incandescence are captured with a high-speed color camera. In separate experiments, toluene is added to the isooctane as a fluorescent tracer and excited by pulsed 266 nm flood illumination. From images of the LIF signal, the fuel-films’ thickness and mass evolutions are evaluated. The films survive the entire combustion event. PAH LIF is found in close vicinity of the evaporating fuel films. Soot is found spatially separated from, but adjacent to the PAH, both with high spatial intermittency. Average images additionally indicate that soot is formed with a much higher spatial intermittency than PAH. Images from the color camera show soot incandescence earlier and in a similar region compared to soot LII. Chemiluminescence downstream of the soot-forming region is thought to indicate the subsequent oxidation of fuel, soot, and PAH.     

Simultaneous Rayleigh scattering and laser-induced CH fluorescence for reaction zone imaging in high-speed premixed hydrocarbon flames

4 /air flames where CH concentration is on the order of 1 ppm based on flamelet calculations. The present experimental conditions are also examined and shown to be suitable for quantitative measurements of CH radical based on the two-level model analysis. A linear relationship can be found between the measured CH signal intensity and the calculated CH concentration within a maximum 30% uncertainty range. The FWHM thickness of the CH profile in a stoichiometric laminar methane flame was shown to be less than 0.3 mm, which is the smallest ever achieved. Simultaneous image pairs of flame temperature and concentration of CH radicals from a premixed turbulent Bunsen flame at an exit velocity of 65 m/sec are obtained to demonstrate the system superiority of application on high-speed reacting flows. Received: 29 January 1996/Revised Version: 3 May 1996     

Application of a pulsed laser for soot measurements in premixed flames

A pulsed Nd: YAG laser has been used to perform scattering/extinction measurements in flat, premixed methane/oxygen flames to determine soot particle sizes, number concentrations and soot volume fractions. A discussion is given on accuracies in these determinations with respect to experimental and theoretical parameters. Absorptions were measured through a referencing method which yielded a single-pulse relative standard deviation in the normalized signal of 0.7%. The incident laser fluence was kept below 0.1 J/cm² in the focusing point in the flame to avoid soot vaporization effects. Interference in the measurements from polyaromatic hydrocarbons (PAH) is also discussed.     

Numerical simulations of soot aggregation in premixed laminar flames

In this paper we make use of a detailed particle model and stochastic numerical methods to simulate the particle size distributions of soot particles formed in laminar premixed flames. The model is able to capture the evolution of mass and surface area along with the full structural detail of the particles. The model is validated against previous models for consistency and then used to simulate flames with bimodal and unimodal soot particle distributions. The change in morphology between the particles from these two types of flames provides further evidence of the interplay among nucleation, coagulation, and surface rates. The results confirm the previously proposed role of the strength of the particle nucleation source in defining the instant of transition from coalescent to fractal growth of soot particles.     

Derivation of a temperature-dependent accommodation coefficient for use in modeling laser-induced incandescence of soot

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.     

Modeling laser-induced incandescence of soot: a summary and comparison of LII models

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 a 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. PACS 65.80.+n; 78.20.Nv; 42.62.-b; 44.05.+e     

Optimization of measurement angles for soot aggregate sizing by elastic light scattering,through design-of-experiment theory

In multiangle elastic light scattering (MAELS) experiments, the morphology of aerosolized particles is inferred by shining collimated radiation through the aerosol and then measuring the scattered light intensity over a set of angles. In the case of soot-laden-aerosols MAELS can, in principle, be used to recover the size distribution of soot aggregates, although this involves solving an ill-posed inverse problem. This paper presents a design-of-experiment methodology for identifying the set of angles that maximizes the information content of the angular scattering measurements, thereby minimizing the ill-posedness of the underlying inverse problem. While the optimized angles highlight the physical significance of the scattering regimes, they do not improve the accuracy of size distributions reconstructed from simulated experimental data.