A calibration-independent laser-induced incandescence technique for soot measurement by detecting absolute light intensity

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.     

Laser-induced incandescence: detection issues

Theoretical predictions suggest that soot particle size and local gas temperature affect both the spectral intensity and the temporal evolution of laser-induced incandescence. A discussion of both the physical structure and the theoretical absorption models of soot aggregates is presented, suggesting that the soot particle size relevant to laser-induced incandescence (LII) is the primary particle size regardless of whether the primary particle exists individually or is assembled into an aggregate. Experimental results of LII measurements in a laminar gas-jet flame with different signal collection strategies for the LII are presented. These results suggest that (a) signal integration during the laser pulse is essential for minimizing particle size and local temperature bias in the LII signal, (b) signal integration times subsequent to the laser pulse produce a size and local gas-temperature-dependent bias in the LII signal with long integration times more sensitive to these effects, and (c) long wavelength detection produces less of a size and local gas-temperature-dependent bias than short wavelength detection.     

Laser-induced incandescence measurements of soot in turbulent pool fires

We present what we believe to be the first application of the laser-induced incandescence (LII) technique to large-scale fire testing. The construction of an LII instrument for fire measurements is presented in detail. Soot volume fraction imaging from 2?m diameter pool fires burning blended toluene/methanol liquid fuels is demonstrated along with a detailed report of measurement uncertainty in the challenging pool fire environment. Our LII instrument relies upon remotely located laser, optical, and detection systems and the insertion of water-cooled, fiber-bundle-coupled collection optics into the fire plume. Calibration of the instrument was performed using an ethylene/air laminar diffusion flame produced by a Santoro-type burner, which allowed for the extraction of absolute soot volume fractions from the LII images. Single-laser-shot two-dimensional images of the soot layer structure are presented with very high volumetric spatial resolution of the order of 10(-5)?cm3. Probability density functions of the soot volume fraction fluctuations are constructed from the large LII image ensembles. The results illustrate a highly intermittent soot fluctuation field with potentially large macroscale soot structures and clipped soot probability densities.     

Soot formation characteristics in a pulverized coal flame formed in a swirling flow

To understand the soot formation characteristics in a pulverized coal flame with a swirling flow, simultaneous imaging of Mie scattering of coal particles and laser induced incandescence (LII) of soot were performed in this study. The pulverized coal flame was stabilized by a hydrogen diffusion pilot flame. The characteristic structures of soot formation in the pulverized coal flame with a swirling flow were analyzed based on a comparison of the experimental results and two-dimensional numerical simulation in the mixing region of coal particles and the oxidizer. The interactions between coal particle clouds and soot formation are discussed in detail. The results clearly show that the averaged radial dispersions of scattering signals from coal particles and of the LII signals from soot are overlapped. The overlapping region appeared nearby the nozzle exit due to the turbulent mixing and the high temperature region formed by swirl-induced recirculation flow. This overlapping region radially expands with increasing the height from the burner. Additionally, the characteristic areas of soot formation were observed in the results of simultaneous imaging of Mie scattering and LII. These areas are 1) streaky soot formation areas around the particle clouds, 2) soot formation areas inside of the particle clouds and 3) soot formation areas around the large coal particles.     

Laser-induced incandescence: excitation intensity

Assumptions of theoretical laser-induced incandescence (LII) models along with possible effects of high-intensity laser light on soot aggregates and the constituent primary particles are discussed in relation to selection of excitation laser fluence. Ex situ visualization of laser-heated soot by use of transmission electron microscopy reveals significant morphological changes (graphitization) induced by pulsed laser heating. Pulsed laser transmission measurements within a premixed laminar sooting flame suggest that soot vaporization occurs for laser fluences greater than 0.5 J/cm(2) at 1064 nm. Radial LII intensity profiles at different axial heights in a laminar ethylene gas jet diffusion flame reveal a wide range of signal levels depending on the laser fluence that is varied over an eight fold range. Results of double-pulse excitation experiments in which a second laser pulse heats in situ the same soot that was heated by a prior laser pulse are detailed. These two-pulse measurements suggest varying degrees of soot structural change for fluences below and above a vaporization threshold of 0.5 J/cm(2) at 1064 nm. Normalization of the radial-resolved LII signals based on integrated intensities, however, yields self-similar profiles. The self-similarity suggests robustness of LII for accurate relative measurement of soot volume fraction despite the morphological changes induced in the soot, variations in soot aggregate and primary particle size, and local gas temperature. Comparison of LII intensity profiles with soot volume fractions (f(v)) derived by light extinction validates LII for quantitative determination of f(v) upon calibration for laser fluences ranging from 0.09 to 0.73 J/cm(2).     

Laser-induced incandescence for soot particle size measurements in premixed flat flames

Measurements of soot properties by means of laser-induced incandescence (LII) and combined scattering-extinction were performed in well-characterized premixed ethylene-air flames. In particular, the possibility of using LII as a tool for quantitative particle sizing was investigated. Particle sizes were evaluated from the temporal decay of the LII signal combined with heat balance modeling of laser-heated particles, and these sizes were compared with the particle sizes deduced from scattering-extinction measurements based on isotropic sphere theory. The correspondence was good early in the soot-formation process but less good at later stages, possibly because aggregation to clusters began to occur. A critical analysis has been made of how uncertainties in different parameters, both experimental and in the model, affect the evaluated particle sizes for LII. A sensitivity analysis of the LII model identified the ambient-flame temperature as a major source of uncertainty in the evaluated particle size, a conclusion that was supported by an analysis based on temporal LII profiles.     

Assessment of soot particle vaporization effects during laser-induced incandescence with time-resolved light scattering

Although laser-induced incandescence (LII) has been successfully used for soot volume fraction and particle size measurements, uncertainties remain regarding issues of soot vaporization leading to mass loss and morphological changes occurring in soot due to intense heating. Prompt LII detection schemes are often based on the assumption that the associated time scale is shorter than the time scale of soot vaporization or sublimation. The validity of such assumptions is the focus of the current study. Time-resolved light-scattering measurements were made in combination with LII measurements to quantify soot particle vaporization effects resulting from the LII laser pulse. The light-scattering measurements revealed a sharp decrease in total soot particle mass during the time course of the 25 ns full-width LII laser pulse for fluences in the range of 0.5 J/cm2. Light-scattering theory was used to invert the scattering data, revealing approximately 80%-90% reductions in the soot particle volume for LII fluences of 0.47 and 0.61 J/cm2. In addition, the time-resolved scattering measurements show that the time scale of soot vaporization is completely confined to the LII laser pulse itself. Light scattering revealed no soot vaporization only for fluences of approximately 0.1 J/cm2, which is consistent with recent work on low-fluence LII. Possible mechanisms for soot vaporization are discussed, notably for near-threshold fluences.     

Optical diagnostics on sooting laminar diffusion flames in microgravity

Laser-induced incandescence was successfully applied to the investigation of soot formation in both buoyant and non-buoyant laminar jet diffusion flames. Microgravity experiments were conducted in the Drop Tower Bremen, Germany. By the use of imaging laser-induced incandescence (LII) it was possible for the first time to obtain simultaneously two-dimensional information on soot concentration and primary particle size under microgravity. Additionally, temperature fields were measured by 2-color emission pyrometry. Results for the fuels propane and ethene show that soot formation and oxidation is drastically altered under microgravity. Maximum temperatures are reduced by roughly 220 K and 120 K, respectively, which in the case of ethene results in a termination of oxidation processes and the emission of soot. The distribution of soot within the non-buoyant flames is always concentrated in relatively small bands. For all non-buoyant flames investigated the maximum primary particle size roughly doubles compared to the buoyant ones.     

Two-color laser-induced incandescence (2C-LII) technique for absolute soot volume fraction measurements in flames

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.     

Formation of nanoparticles in flames; measurement by particle mass spectrometry and numerical simulation

The size distributions of nanoparticles in flames are measured using a novel particle mass spectrometer (PMS), which is developed for the size range between 0.3 and 50?nm and for number concentrations between 10(9) and 10(13). Using this instrument the particles are sampled without prior dilution from the flame into a molecular beam. The charged nanoparticles are then deflected by an electric field, to determine the mass according to the time-of-flight principle. The PMS is installed in a low pressure combustion chamber operated at 30?mbar. Measurements are made on primary soot particles and iron oxide particles in a laminar, premixed acetylene/oxygen flame. The soot particles increase in size as a function of the height above the burner and the C/O ratio from 2 up to 10?nm. Iron oxide particles of 3-5?nm are detected as a function of burner height. The soot particles form more rapidly than the iron oxide particles. A model calculation for the formation of silica and iron oxide in hydrogen/oxygen flames is developed, based on previously published reaction mechanisms. On adding a mono-disperse particle coagulation scheme, the time history of the particle number concentration and the particle size is calculated. In agreement with experimental data, the calculations show that iron oxide particles are formed more slowly than silica particles.     

Laser-induced incandescence applied to droplet combustion

Laser-induced incandescence (LII) is ideally suited for obtaining high temporally and spatially resolved measurements of soot volume fraction in transient combustion phenomena. We demonstrate qualitative two-dimensional nonintrusive optical measurements of the soot evolution versus time from single fiber-supported burning fuel droplets of heptane and decane. Quantitative measurement of the soot volume fraction is also demonstrated through calibration of the LII signal against a small coflow ethylene diffusion flame.     

Laser-induced incandescence for soot diagnostics at high pressures

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 faction 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.     

Conductivity for soot sensing: possibilities and limitations

In this study we summarize the possibilities and limitations of a conductometric measurement principle for soot sensing. The electrical conductivity of different carbon blacks (FW 200, lamp black 101, Printex 30, Printex U, Printex XE2, special black 4, and special black 6), spark discharge soot (GfG), and graphite powder was measured by a van der Pauw arrangement. Additionally the influence of inorganic admixtures on the conductivity of carbonaceous materials was proven to follow the percolation theory. Structural and oxidation characteristics obtained with Raman microspectroscopy and temperature programmed oxidation, respectively, were correlated with the electrical conductivity data. Moreover, a thermophoretic precipitator has been applied to deposit soot particles from the exhaust stream between interdigital electrodes. This combines a controlled and size independent particle collection method with the conductivity measurement principle. A test vehicle was equipped with the AVL Micro Soot Sensor (photoacoustic soot sensor) to prove the conductometric sensor principle with an independent and reliable technique. Our results demonstrate promising potential of the conductometric sensor for on-board particle diagnostic. Furthermore this sensor can be applied as a simple, rapid, and cheap analytical tool for characterization of soot structure.     

Cavity ringdown and laser-induced incandescence measurements of soot

Currently laser-induced incandescence (LII) is widely used for the measurement of soot volume fraction. A particularly important aspect of the technique that has received less attention, however, is calibration. The applicability of cavity ringdown (CRD) for measurement of soot volume fraction f(v) is assessed, and the calibration of LII by means of CRD is demonstrated. The accuracy of CRD for f(v) determination is validated by comparison with traditional light extinction and path-integrated LII. By use of CRD, the quantification of LII for parts in 10(9) (ppb) f(v) levels is demonstrated. Results are presented that demonstrate the accuracy of CRD for a single laser pulse to be better than ?5% for measurement of ppb soot volume-fraction levels over a 1-cm path length. By use of CRD, spatially resolved LII signals from soot within methane-air diffusion flames are calibrated for ppb f(v) levels, thereby avoiding the extrapolation required of less sensitive methods in current use.     

Light scattering and absorption by soot in the presence of sulfate aerosols

The radiative properties of aerosol-soot mixtures, both internal and external, are determined in the visible and near-infrared bands by use of exact indirect mode-matching solutions to electromagnetic-wave scattering from a sphere with an eccentric spherical inclusion and from a cluster of spheres. Spherical sulfate droplets are assumed to represent aerosol particles. Soot particles are represented by volume-equivalent carbon spheres, the size distribution of which is obtained from the number distribution of the primary carbon particles that aggregate into soot grains. The mean gram-specific absorption cross section and the mean albedo of aerosol-soot mixtures are obtained by integration of the corresponding characteristics of composite sulfate-carbon particles over the size range of carbon spheres. Enhanced absorption of light by soot in aerosol-soot mixtures, a result of lensing by sulfate droplets, is highlighted by maps of the electromagnetic field in a sulfate-carbon particle.     

Light scattering and absorption by fractal-like carbonaceous chain aggregates: comparison of theories and experiment

This study compares the optical coefficients of size-selected soot particles measured at a wavelength of 870 nm with those predicted by three theories, namely, Rayleigh-Debye-Gans (RDG) approximation, volume-equivalent Mie theory, and integral equation formulation for scattering (IEFS). Soot particles, produced by a premixed ethene flame, were size-selected using two differential mobility analyzers in series, and their scattering and absorption coefficients were measured with nephelometry and photoacoustic spectroscopy. Scanning electron microscopy and image processing techniques were used for the parameterization of the structural properties of the fractal-like soot aggregates. The aggregate structural parameters were used to evaluate the predictions of the optical coefficients based on the three light-scattering and absorption theories. Our results show that the RDG approximation agrees within 10% with the experimental results and the exact electromagnetic calculations of the IEFS theory. Volume-equivalent Mie theory overpredicts the experimental scattering coefficient by a factor of approximately 3.2. The optical coefficients predicted by the RDG approximation showed pronounced sensitivity to changes in monomer mean diameter, the count median diameter of the aggregates, and the geometric standard deviation of the aggregate number size distribution.     

Kinetics of growth of γ′- precipitates in Ni–6·75AI alloy

Abstract

The size distributions of γ precipitates in a Ni-based alloy containing 6·75 wt–% aluminium have been measured, using dark-field electron microscopy, at five temperatures ranging from 600 to 825°C and the corresponding volume fractions have been assessed. From these distributions, the average particle sizes were obtained and plotted against time according to the kinetics laws proposed by theory for diffusion–controlled growth, interface-controlled growth, and growth in the transition region where neither process dominates. The particle-size distributions were reconstructed in terms of the ratio of the actual particle size/average size, and the resulting distributions were compared with those predicted by theory for the three controlling conditions. Although the values of activation energy obtained from the plots of particle size against annealing time are equally consistent for the three processes, the agreement between the predicted distributions and the experimental distribution is distinctly better for growth in the transition region than for the two alternative processes.

MST/167     

Observation of sooting behavior in single droplets combustion in direct current electric fields under microgravity

Because of ions and electrons in flame, flow is induced around flame in electric filed. It affects combustion phenomena, and the effect has seemed to be significant for droplet combustion in previous study. This study is focused on movement of soot particle in single droplets combustion in direct current electric fields under microgravity in order to obtain some information about flow field around droplet. The large soot particles were observed by high-speed CCD camera, and the soot velocities are measured by PTV method. The single droplets were burned in the center of electrodes, which are two parallel rectangular wire nettings. Distance between electrodes is 50mm and applied voltages between electrodes are from 0kV to 6kV. Fuels tested are n-octane and toluene and the initial droplet diameters are around 0.8mm. All experiments were performed in microgravity in order to eliminate the natural convection. The results shows the velocities of soot particle are mainly in the direction of electric field, and most soot particles move to the cathode but some move to the anode. This indicates some soot particles have some negative charges. The velocities of soot particle increase with increasing in the distance from the droplet in both directions, and the velocity to cathode is larger than that to anode. From these results, the change of electric field and flow field around droplet are discussed.     

Application of the Hough transform for the automatic determination of soot aggregate morphology

We report a new method for automated identification and measurement of primary particles within soot aggregates as well as the sizes of the aggregates and discuss its application to high-resolution transmission electron microscope (TEM) images of the aggregates. The image processing algorithm is based on an optimized Hough transform, applied to the external border of the aggregate. This achieves a significant data reduction by decomposing the particle border into fragments, which are assumed to be spheres in the present application, consistent with the known morphology of soot aggregates. Unlike traditional techniques, which are ultimately reliant on manual (human) measurement of a small sample of primary particles from a subset of aggregates, this method gives a direct measurement of the sizes of the aggregates and the size distributions of the primary particles of which they are composed. The current version of the algorithm allows processing of high-resolution TEM images by a conventional laptop computer at a rate of 1-2 ms per aggregate. The results were validated by comparison with manual image processing, and excellent agreement was found.