Particle-size measurements in flames using light scattering and dissymmetry ratios

Particle-size measurements have been performed above a flat-flame burner using C₂H₆/O₂ and C₂H₄/O₂ mixtures and the assumption that a self-preserving distribution (s.p.d.) obtains for spherical particles. The absence of significant deviations from sphericity for the particles has been verified. Dissymmetry ratios were measured for parallel polarized light from an argon-ion laser at 4880 Å. The measured results are compared with previously obtained data for CH₄/O₂ mixtures and show differences in the rates and amounts of particulate growths, with the growth curves for all three gases leveling off at about 40–50 mm above the burner rim. The rates of particulate growths downstream of the flame are ordered as follows in the gas mixtures: C₂H₄/O₂>C₂H₆/O₂>CH₄/O₂.     

Particle-size measurements in flames using light scattering; comparison with diffusion-broadening spectroscopy

Particle-size measurements have been performed above a flat-flame burner using CH₄/O₂ mixtures. Size estimates were made on the assumption that particle sizes are defined by the self-preserving distribution (s.p.d.). Desymmetry ratios were measured for parallel and perpendicularly polarized argonion-laser radiation at 4880 A. These ratios for the assumed s.p.d. yield mean radii when the Mie theory is applied to spherical particles for a known value of the complex index of refraction (m = 1.57-0.44 i).The measured results are found to be in good agreement with our earlier studies using diffusion-broadening spectroscopy.     

Numerical investigation of the effect of signal trapping on soot measurements using LII in laminar coflow diffusion flames

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.     

Effects of soot absorption and scattering on LII intensities in laminar coflow diffusion flames

Absorption and scattering of laser-induced incandescence (LII) intensities by soot particles present between the measurement volume and the detector were numerically investigated at detection wavelengths of 400 and 780 nm in a laminar coflow ethylene/air flame. The radiative properties of aggregated soot particles were calculated using the Rayleigh-Debye-Gans polydisperse fractal aggregate theory. The radiative transfer equation in emitting, absorbing, and scattering media was solved using the discrete-ordinates method. The radiation intensity along an arbitrary direction was obtained using the infinitely small weight technique. The effects of absorption and scattering on LII intensities are found to be significant under the conditions of this study, especially at the shorter detection wavelength and when the soot volume fraction is higher. Such a wavelength-dependent signal-trapping effect leads to a lower soot particle temperature estimated from the ratio of uncorrected LII intensities at the two detection wavelengths. The corresponding soot volume fraction derived from the absolute LII intensity technique is overestimated. The Beer-Lambert relationship can be used to describe radiation attenuation in absorbing and scattering media with good accuracy provided the effective extinction coefficient is adequately.     

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.     

Combined soot optical characterization using 2-D multi-angle light scattering and spectrally resolved line-of-sight attenuation and its implication on soot color-ratio pyrometry

Soot characterization using multiple techniques has been performed in a series of nitrogen-diluted ethylene coflow laminar diffusion flames. Soot aggregate sizes have been measured in two dimensions, as opposed to traditional point measurements, by a newly developed two-dimensional multi-angle light scattering technique where image processing was applied to align images for Guinier analysis. Extinction measurements have also been performed using spectrally resolved line-of-sight attenuation with an imaging spectrometer. Spectrally and spatially resolved extinction measurements have been obtained as well. Combined with previously obtained time-resolved laser-induced incandescence measurements of primary particle diameters, the scattering and absorption components of extinction can be estimated. The so-called dispersion exponent that describes the wavelength dependence of spectral emissivity was determined in two dimensions and found to improve the accuracy of soot color-ratio pyrometry measurements.     

Fundamental aspects of light scattering and optical Kerr effect spectroscopy

We describe the principles of two spectroscopic methods of non-resonance light scattering (LS) and optical-Kerr-effect (OKE) spectroscopies in detail, and review basic but truly important phenomena observed by these two methods. Particularly, we focus on the following three experiments: 1) Response functions determined by frequency-domain LS and time-domain OKE spectroscopies, almost completely agree with each other, indicating that the quantum-mechanical fluctuation-dissipation theorem (QM-FDT) holds well in this system. 2) Femtosecond time responses of liquids clearly show an initial rise process even though they show an exponential response at a later time. This indicates that the Debye relaxation model does not hold in such an early time region. 3) From the measurement of the Stokes and anti-Stokes LS intensity ratios, it is found that the LS spectra of relaxation modes in liquids and solids are symmetric with respect to the spectral origin of the scattered light and hence the ratio does not satisfy the Boltzmann distribution rule expected from QM-FDT. These experimental results which contain apparently contradictory data are closely related with the nature of relaxation modes examined, which more or less assume a macroscopic character, and also with the physical basis of relaxation, which is inevitably connected to the observation problem of quantum mechanics. We discuss these points in relation to the physical reality of macroscopic quantity.     

The role of DME addition on the evolution of soot and soot precursors in laminar ethylene jet flames

Dimethyl ether (DME) has received considerable attention as a fuel additive to reduce the emission of particulate matter (PM) due to its low-temperature chemistry, molecularly bound oxygen atom and the absence of CC bonds. However, the effect of DME addition on the evolution of soot and particularly soot precursors is not entirely understood. This study aims to shed light on this issue by blending different proportions of DME with diffusion, E60, and partially premixed, PP12, base cases of laminar ethylene flames using the Yale benchmark burner. Laser-induced fluorescence (LIF) intensity and decay time are used to characterize the structure and evolution of soot precursors, while laser-induced incandescence (LII) is utilized to determine the soot volume fraction (SVF) and the effective primary particle diameter (D_p). For the diffusion flames, the addition of 10% DME increases the concentrations of both soot and soot precursors. With the further addition of DME to 20%, the SVF decreases to levels similar to those of E60 and then decreases further with 30% DME addition. All diffusion flames with DME addition exhibit higher concentrations of soot precursors than those of the reference E60 case. For PP12, the addition of 10% DME shows similar concentrations of soot precursors and a slight reduction in the SVF which continues to decrease with further increases in DME additions to the PP12 flame. The addition of DME seems to have little effect on the soot particle diameters for all the studied flames. Overall, the PP flames result in smaller mean particle diameters than the diffusion flame counterparts.     

The effect of the optical anisotropy of scattering media on the polarization state of scattered light

The effect of the optical anisotropy of scattering media on the polarization state of scattered light is studied. The study is performed using a simple polarization method based on the comparison of the spectral composition of the co-and cross-polarized components of transmitted light measured for samples differently oriented with respect to the plane of polarization of probe linearly polarized light. The experimental results obtained are interpreted theoretically in terms of ordinary methods used in optics of birefringent media. Using rat skin as an example, it is shown that surface tissues can be characterized by a high degree of orientational order of the local optical axis of a medium within large areas (with a size of 5 mm or more), which manifests itself in macroscopic optical measurements. In such measurements in the spectral range 550–700 nm, whole rat skin behaves as a partially depolarized phase plate with a difference between the principal refractive indices Δn ≈ 0.00023.     

Combination of LII and extinction measurements for determination of soot volume fraction and estimation of soot maturity in non-premixed laminar flames

The measurement of soot and soot precursors is important for understanding the formation of soot particles in flames. In this paper, we use the difference between laser-induced incandescence (LII) and two-dimensional extinction measurements to assess the contribution of soot precursors to the extinction measurement. LII measurements are performed with a high spatial resolution of 100 µm to determine the soot volume fraction (f _V) in a laminar ethylene/air non-premixed flame at the standard Gülder conditions. While LII is specific to mature soot only, the extinction data represent attenuation due to mature and young soot (absorption and elastic scattering) and also absorption by soot precursors. The difference between the two measurements indicates the contribution of soot precursors and allows a determination of the maturity of soot. This is important knowledge for those using extinction techniques to measure soot concentration, as the contribution from soot precursors may lead to an overestimation of the mature soot concentration. Further, regions with high soot-precursor concentrations, which lead to soot formation, can be identified.     

Calculating the soot particle size distribution function in turbulent diffusion flames using a sectional method

Soot formation in a turbulent jet diffusion flame is modeled using an unsteady flamelet approach in post-process. In the present work, we apply a detailed kinetic soot model with a sectional method, and study the evolution of the particle size distribution. Detailed information on the evolution of the soot particle size distribution function is acquired. It is found that the particle size distribution function is bimodal throughout the flame. The transition from the small to large particle size distributions is strongly influenced by surface growth and oxidation reactions. We find that large particles are most likely to be emitted from the flame.     

Influence of the optical axis distribution in the anisotropic layer surrounding a spherical particle on the scattering of light

The problem of light scattering from an anisotropic layer with a spherically symmetric distribution of the optical axis is solved exactly. The dependence of the scattering efficiency on the particle size, the anisotropy parameter, and the layer thickness is studied numerically for various anisotropy types. It is shown that the scattering cross section is strongly affected by the type of anisotropy and that the presence of disclinations increases the scattering efficiency. As an additional effect specific to anisotropic scatterers, it is found that, in the case of configurations with broken central symmetry, the scattering amplitudes contain a phase shift that scales logarithmically with the thickness of the anisotropic layer.     

Effects of gas and soot radiation on soot formation in counterflow ethylene diffusion flames

Numerical study of soot formation in counterflow ethylene diffusion flames at atmospheric pressure was conducted using detailed chemistry and complex thermal and transport properties. Soot kinetics was modelled using a semi-empirical two-equation model. Radiation heat transfer was calculated using the discrete-ordinates method coupled with an accurate band model. The calculated soot volume fractions are in reasonably good agreement with the experimental results in the literature. The individual effects of gas and soot radiation on soot formation were also investigated.     

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.     

分形结构对随机取向烟尘团簇粒子光散射特性的影响

利用蒙特卡罗方法对不同分形维数和分形前向因子的随机取向烟尘团簇粒子的分形结构进行了仿真,采用离散偶极子近似（DDA）方法对随机取向烟尘团簇粒子的缪勒矩阵元进行了数值计算,并与球形粒子模型进行了比较,深入探讨了烟尘团簇粒子的分形维数和分形前因子对其散射特性的影响。研究表明,等效球形粒子的光散射特性与随机取向烟尘团簇粒子的光散射特性存在很大差别,并且此差别随着团簇粒子的分形维数以及分形前向因子的增大而减小;分形维数对表征团簇粒子散射特性的缪勒矩阵元的影响在一定散射角范围内均比较明显,分形前向因子对团簇粒子的缪勒矩阵元角分布的影响与分形维数的影响类似,不过其影响相对分形维数较弱。     

The nanoparticle shape's effect on the light scattering cross-section

In the framework of kinetic approach we develop a theory for light scattering by ellipsoidal metallic nanoparticles whose dimensions are less than those of a free electron path. In this case, the surface of the particle starts to play a dominant role in electron scattering. When the size of the particle decreases below the free electron path at least in one direction, the optical conductivity becomes a tensor quantity, and the diagonal components of this tensor define the half-widths of the plasmon resonances peaks. Thus, the effect of the particles' shape both on the frequencies of plasmon resonances and on their half-widths is considered. Additionally, the expression for a cross-section of the light scattering by a collection of chaotically oriented spheroidal nanoparticles is obtained and averaged over different directions of particles in the collection. Our results highlight the plasmonic properties of metallic nanospheroids, notably, the spectrum of the light scattering has two peaks at the frequencies of plasmon resonances even if there is no preferential direction in the collection of particles.     

Characterization of the light scattering by ensembles of randomly distributed soot aggregates

Soot particles formed in combustion processes commonly exist in the form of ensembles of randomly distributed aggregates of small, nearly spherical monomers. In this paper, these randomly distributed aggregates are numerically generated using a combination of the cluster–cluster aggregation algorithm with the Monte Carlo method. Moreover, an efficient and accurate numerical method is presented for characterizing the light scattering by these complex soot particles illuminated by plane wave and Gaussian beam. This method exploits the unique features of the hybrid finite element-boundary integral method and, more importantly, the unique features of soot aggregates. It is designed in such a manner that it first decomposes the original problem into many sub-regions, where each primary particle is regarded as a sub-region, and then it employs the edge-based finite element method to deal with each sub-region. The sub-regions communicate through the near-field Green’s function. To reduce computational burdens, an iterative domain decomposition method in combination with parallel conjugate gradient method is adopted to solve the coupling system of equations. As an illustration, we present some of our preliminary numerical results. The results are expected to provide useful insights into the optical properties of soot particles formed in combustion processes.     

Pulse compression effect based on stimulated Brillouin scattering light storage in an optical fiber

The pulse compression effect in SBS light storage is numerically investigated. We demonstrate theoretically that the compressed width of retrieved data-pulses is not only related with temporal profile of data-pulse, but also spectrum and temple profile of control-pulse. The data-pulses with steep rising edge can be compressed after they are retrieved. The optimum compression effect takes place when a 2-ns-long exponential data-pulse interacts with a 1.5-ns-long chirped Gaussian control-pulse, where the data-pulse width is compressed to 1.73-ns-long and the readout efficiency is the biggest. A 2-ns-long rectangular data-pulse is also compressed to 1.73-ns-long when it encounters to a 1.5-ns-long rectangular control-pulse, but the readout efficiency isn’t maximum value. The results are significant for increasing storage capacity and variable bits all-optical buffering.