Aggregation Of Soot Particles Research Articles

Influence of Global Operating Parameters on the Reactivity of Soot Particles from Direct Injection Gasoline Engines

The aim of this study is to investigate the impact of global operating parameters, e.g., engine speed, brake mean effective pressure, and air–fuel ratio, of a turbocharged 4-cylinder GDI engine on the reactivity of soot particles against oxidation. The knowledge of soot reactivity is crucial for optimizing gasoline particulate filter regeneration strategies and is, consequently, a key parameter for reducing fuel consumption and CO2 emissions. In this work, time-resolved in-cylinder soot concentrations and exhaust particle size distributions are measured by using two-color pyrometry, engine exhaust particle sizer and smoke meter, respectively. Reactivity against oxidation by molecular oxygen is determined by temperature programmed oxidation analysis. To derive a physicochemical explanation for varying soot reactivity, the morphological and nanostructural primary particle structure of collected samples is analyzed using high-resolution electron microscopy and image analysis algorithms. The results reveal that engine operating parameters affect soot reactivity differently. While engine speed has only a slight effect, a reduction of air/fuel ratio (λ < 1.0) or an increase of BMEP > 10 bar significantly reduces the soot oxidation reactivity. These findings give evidence, that the quality of the fuel/air mixture is a significant parameter influencing soot reactivity. Measured soot concentrations substantiate the hypothesis that low-sooty homogeneous premixed combustion of a homogeneous fuel/air mixture favors formation of high-reactive soot particle fractions. Reactive soot particle aggregates are composed of multiple soot fractions of different reactivity. Reactive primary particles are composed of short graphene-like layers and vice versa, providing a physicochemical explanation for varying soot reactivity depending on engine operating conditions.

Impact of the maturation process on soot particle aggregation kinetics and morphology

Mature soot aggregates exhibit a morphology which is currently mimicked by diffusion-limited aggregation (DLCA) codes, i.e. with a sticking probability = 1 when two aggregates collide. Nevertheless, nascent soot particles may grow in the reaction-limited aggregation regime (RLCA), i.e. with a sticking probability ≪ 1. Yet, it remains to be seen how fast the transition from RLCA to DLCA occurs for soot particles and what is the impact on aggregation kinetics, particle size distribution and morphology. This work intends to fill this gap by exploring the aggregation of soot particles formed in a laminar premixed flame through numerical simulations. Results show that the transition from RLCA to DLCA is very fast and produces a moderate impact on soot formation dynamics and morphology. However, soot particles mass bulk density is found to play an important role and should be considered in future simulations of soot formation in flames.

(UV, VIS) Laboratory evaluation of the lidar depolarization ratio of freshly emitted soot aggregates from pool fire in ambient air at exact backscattering angle

In this paper, a controlled-laboratory experiment is carried out to evaluate the lidar depolarization ratio of freshly emitted soot aggregates in the exact backward scattering direction at 180.0°. The experiment is performed at two wavelengths simultaneously, namely 355 and 532 nm, often used in polarimetric lidar remote sensing. The soot aggregates are generated from a kerosene JET A-1 pool fire in laboratory ambient air and microscopic images confirm the fractal morphology of generated soot aggregates. Then, the Superposition T-Matrix (STM) method is applied to numerically simulate the soot aggregates backscattering properties for different soot particles refractive indices, monomer radii and monomer numbers. The range of these parameters which ensures the lowest discrepancy between the laboratory measurement and the STM-computations is discussed within experimental and numerical error bars. We find that the polydisperse monomers model gives an overall better evaluation of the ratio F22(π)/F11(π). In the polydisperse case, our numerical and laboratory experimental findings agree at both wavelengths for a refractive index m = 2.65 + i1.32 and monomer number Nm > 40 at a mean monomer radius of rp = 30 nm (Nm > 160 at rp = 27.5 nm). We believe this work may be useful for the light scattering and remote sensing communities and may also help future studies aimed at better understanding the impact of soot particle aggregates on the Earth's climate, which still needs to be precisely quantified.

An optimized evaluation strategy for a comprehensive morphological soot nanoparticle aggregate characterization by electron microscopy

For a comprehensive understanding of nanoparticle formation in gas phase processes, such as soot formation, morphological parameters of fractal-like particle aggregates, like the radius of gyration, the fractal dimension and the primary particle size, have to be determined. Often transmission electron microscopy (TEM) is employed for the investigation of particle characteristics as it not only allows to investigate ensemble averages but single particle aggregates and thus also to determine statistical properties, such as the size distribution within a sample. Many different evaluation methods can be found in the literature. We investigated different approaches for the determination of morphological parameters from TEM images of soot and compared the results to derive an optimized evaluation strategy for TEM nanoparticle characterization. We compared four methods for the determination of the radius of gyration – three length- and one pixel-based methods – showing good agreement within 9% deviation for the median of the recovered lognormal size distribution. Furthermore, the fractal dimension was determined via a sample-based and various box counting methods with different limiting box sizes. Here, we could show that the upper and lower bounds (aggregate size in terms of radius of gyration and primary particle size in terms of its radius, respectively) of self-similarity of fractal-like aggregates should be accounted for by choosing corresponding upper and lower box sizes. Using box counting, we could show that for small aggregates the fractal dimension as well as its span are increased, yet with increasing aggregate size the fractal dimension converges towards 1.6. Furthermore, we could show the potential of semi-automatic aggregate detection through Trainable Weka Segmentation. However, image noise resulting in erroneous aggregate splitting often leads to smaller aggregate sizes by automatic detection compared to manual segmentation. Generalized Hough Transformation for the semi-automatic determination of primary particle sizes performs well for large soot particle aggregates as those often show spherical primary particles. For leaner combustion conditions, the primary particles of the formed clumpy soot aggregates cannot be detected well via semi-automatic detection. TEM images were taken on soot samples from premixed laminar flat flames (burner type: McKenna) under various conditions to provide comprehensive reference data.

Fractal Diffusion Limited Aggregation of Soot Particles Based on Fuzzy Membership Functions

In this paper, the membership function in fuzzy systems is used in the Diffusion Limited Aggregation (DLA) model to investigate the fractal diffusion of soot particles from diesel engine emissions. The transformation of the morphology of soot particle aggregates and the control of fractal diffusion of soot particles are investigated by analyzing the nonlinear relationship between the motion steps and angles of diffusing particles. The simulation results demonstrate that the morphology of the aggregates varies from loose to compact by changing the particles’ motion steps and angles in membership functions. Meanwhile, the Ballistic Aggregation (BA)-like aggregates are obtained. Furthermore, the control of the morphology of soot particle aggregates is realized, which makes the settlement of the aggregates become easier. This will provide a reference for further understanding the growth mechanism of soot particle diffusion and enhancing the purification technology of the soot particles.

ESPResSo 4.0 – an extensible software package for simulating soft matter systems

ESPResSo 4.0 is an extensible simulation package for research on soft matter. This versatile molecular dynamics program was originally developed for coarse-grained simulations of charged systems Limbach et al., Comput. Phys. Commun. 174, 704 (2006). The scope of the software has since broadened considerably: ESPResSo can now be used to simulate systems with length scales spanning from the molecular to the colloidal. Examples include, self-propelled particles in active matter, membranes in biological systems, and the aggregation of soot particles in process engineering. ESPResSo also includes solvers for hydrodynamic and electrokinetic problems, both on the continuum and on the explicit particle level. Since our last description of version 3.1 Arnold et al., Meshfree Methods for Partial Differential Equations VI, Lect. Notes Comput. Sci. Eng. 89, 1 (2013), the software has undergone considerable restructuring. The biggest change is the replacement of the Tcl scripting interface with a much more powerful Python interface. In addition, many new simulation methods have been implemented. In this article, we highlight the changes and improvements made to the interface and code, as well as the new simulation techniques that enable a user of ESPResSo 4.0 to simulate physics that is at the forefront of soft matter research.

Controlling Fractal Diffusion of Differently-Sized Soot Particles

Fractal diffusion of soot particles is widespread in nature. We propose a mathematical model of interference control for diffusion systems of differently-sized soot particles that can be used to predict and control fractal diffusion. The effect of the source of the interference on diffusion is investigated by analyzing the numerical relationship between the growth probability and the different interferences, a nonlinear logarithm interference and an internal source. Simulation demonstrates the effective control of fractal diffusion for differently-sized soot particles. Meanwhile, analyzing the multifractal spectra of soot particle aggregates under different controlling parameters lends to a quantitative description of the impact on the morphology and distribution of aggregated soot particles due to environmental interferences. This is useful not only for selecting controlling parameters and regions, but also for further developing studies into the kinetics of soot particle diffusion and the improvement of aggregation mechanisms.

The soot particle formation process inside the piston bowl of a small-bore diesel engine

The present study unveils the soot formation processes occurring inside the piston-bowl of a small-bore diesel engine by conducting the thermophoresis-based soot sampling experiments at various locations along the flame development path. Based on planar laser-induced incandescenece (PLII) and planar laser-induced florescence of hydroxyl (OH-PLIF) imaging performed in the same optical engine previously, it was understood that the sooting flame impinges on and then flows along the bowl wall, suggesting a soot growth and persistence near the fuel-rich wall region. In the present study, a soot sampling probe is placed in five different locations including the flame–wall impingement point and four further downstream regions: two 60° and two 120° from the jet axis with two different distances from the bowl wall in each angle. Methyl decanoate is selected as a surrogate fuel due to its low-sooting propensity and thus reduced laser attenuation in the reference PLII images; however, the fuel produces high enough number of soot particles for the in-flame sampling and their statistical analysis. The transmission electron microscope (TEM) images of the sampled soot particle aggregates and their statistical analysis of sizes and fractal dimensions as well as nanoscale internal pattern of the soot primary particles show that precursor-like, small soot particles with amorphous internal carbon layer structures form in the flame–wall impingement region, which grow in size and become large soot aggregates as travelling along the bowl wall. The detailed analysis clearly indicates that the soot precursors underwent the surface growth, aggregation and coagulation to produce large, long-stretched soot aggregates during which the amorphous soot carbon layers transformed into a typical core–shell structure. At further downstream locations, the continued surface growth increases the size of soot primary particles in the core region of the soot aggregates while the oxidation of the soot primary particles located in the outer region tends to reduce the aggregate size, resulting in more compact structures. In the outer region of the flame, the intensive soot oxidation induced by the hydroxyl attack further reduces the size of large soot aggregates and at the same time, eliminates the small soot aggregates. Throughout these soot formation/oxidation processes, the soot carbon layer gaps continue to decrease, indicating more mature soot primary particles.

Physico-chemical properties and biological effects of diesel and biomass particles

Diesel combustion and solid biomass burning are the major sources of ultrafine particles (UFP) in urbanized areas. Cardiovascular and pulmonary diseases, including lung cancer, are possible outcomes of combustion particles exposure, but differences in particles properties seem to influence their biological effects.Here the physico-chemical properties and biological effects of diesel and biomass particles, produced under controlled laboratory conditions, have been characterized. Diesel UFP were sampled from a Euro 4 light duty vehicle without DPF fuelled by commercial diesel and run over a chassis dyno. Biomass UFP were collected from a modern automatic 25 kW boiler propelled by prime quality spruce pellet. Transmission electron microscopy (TEM) and scanning electron microscopy (SEM) images of both diesel and biomass samples showed aggregates of soot particles, but in biomass samples ash particles were also present. Chemical characterization showed that metals and PAHs total content was higher in diesel samples compared to biomass ones.Human bronchial epithelial (HBEC3) cells were exposed to particles for up to 2 weeks. Changes in the expression of genes involved in xenobiotic metabolism were observed after exposure to both UFP already after 24 h. However, only diesel particles modulated the expression of genes involved in inflammation, oxidative stress and epithelial-to-mesenchymal transition (EMT), increased the release of inflammatory mediators and caused phenotypical alterations, mostly after two weeks of exposure.These results show that diesel UFP affected cellular processes involved in lung and cardiovascular diseases and cancer. Biomass particles exerted low biological activity compared to diesel UFP. This evidence emphasizes that the study of different emission sources contribution to ambient PM toxicity may have a fundamental role in the development of more effective strategies for air quality improvement.

Characterization of a multiculture in-vitro cell exposure chamber for assessing the biological impact of diesel engine exhaust

In order to study the various health influencing parameters related to particulate as well as to gas-phase pollutants emitted by Diesel engine exhaust, there is an urgent need for appropriate sampling devices and methods for cell exposure studies and associated biological and toxicological tests. In a previous paper [1], a specific concept for a cell culture exposure chamber was introduced to allow the uniform exposure of cell cultures to diesel aerosols. In the present work, this cell culture exposure chamber is evaluated and characterized with state-of-the-art nanoparticles measurement instrumentation to assess the local deposition of soot aggregates on the cell cultures and any losses due to particle deposition on the cell culture exposure chamber walls, and in addition an upgraded Multiculture Exposure Chamber (MEC) for in vitro continuous flow cell exposure tests is introduced with improved, compared to the previous version, features. Analysis and design of the MEC employs CFD and true to geometry representations of soot particle aggregates.

Influence of soot particle aggregation on time-resolved laser-induced incandescence signals

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.

The effect of particle aggregation on the absorption and emission properties of mono- and polydisperse soot aggregates

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.

In situ study of aggregation of soot particles in an acetylene flame by small-angle x-ray scattering

The nucleation and growth of soot particles in an acetylene flame were investigated by synchrotron small-angle x-ray scattering. The scattered intensity spanning over a wide scattering vector (q) range, 10−3nm−1⩽q⩽0.5nm−1, was analyzed using the unified scattering function involving up to three structural levels corresponding to primary particles, aggregates, and agglomerates. The primary particles have a compact morphology with a terminal radius of gyration (Rg) of about 27nm and their growth dynamics is consistent with the nucleation and growth process. The evolution of aggregate Rg reveals a diffusion limited growth mechanism resulting in a fractal dimension (df)≈2 and Rg∼250nm. Although the production of soot dramatically increases with the flow rate, the aggregation dynamics, terminal Rg, and df remain nearly the same. The flow rate used in this study extended to the regime of aerogelation of soot but the aggregation dynamics is found to be similar to the nongelling case.

Comment on “Sorption Nonlinearity for Organic Contaminants with Diesel Soot: Method Development and Isotherm Interpretation”

An experimentally practical and precise flocculation-based method was developed, tested, and applied to determine phenanthrene and 1,2,4-trichlorobenzene sorption with NIST SRM 2975 diesel particulate matter. Following an initial equilibration period, polyaluminum chloride (PACI) solution was added to the sorption tubes in order to facilitate the formation of flocculated aggregates of soot particles. After separation of the solids through centrifugation, supernatant concentrations were determined as with conventional batch methods. The flocculation-based method was tested on three kinds of soot and then used to evaluate sorption kinetics and equilibrium with SRM 2975. Kinetic results showed that wetting of the soot required more than 20 days, but that 60 days was sufficient to achieve equilibration with both water and phenanthrene. Sixty-day isotherms for both phenanthrene and 1,2,4-trichlorobenzene were strongly nonlinear. At approximate 10(-3) of solubility, carbon-normalized distribution coefficients (Koc) were 10-20 times higher than those for absorption to sediment organic matter. Measurements at closer to solubility indicated much lower Koc, suggesting a total sorption capacity at aqueous solubility that is of similar magnitude to that in sediment organic matter. Independent analysis of extractable hydrocarbons suggests that absorption into a native hydrocarbon phase was not a major component of sorption.

Kinetic modeling of soot formation with detailed chemistry and physics: laminar premixed flames of C 2 hydrocarbons

The present study reports an updated detailed chemical kinetic model for soot formation. The model combines recent developments in gas-phase reactions, aromatic chemistry, soot particle coagulation, soot particle aggregation, and develops a new submodel for soot surface growth. The model was tested against experimental profiles of major and minor chemical species, aromatics, soot volume fractions, and soot particle diameters reported in the literature for nine laminar premixed flames of ethane, ethylene, and acetylene. The numerical agreement between the model predictions and experimental data is generally within a factor of 3. This level of agreement is very encouraging, considering the current uncertainties in the thermodynamics and kinetics of aromatics and soot chemistry. The principal accomplishment of the present study is that the demonstrated level of agreement all around—main flame environment, aromatics, and soot—can be attained with a single reaction model.

Experimental study of light scattering and extinction on the aggregates of soot particles

Experimental study of light scattering and extinction on the aggregates of soot particles

The size and optical properties of soot particles

The emissivity of soot particles makes an important contribution to heat transfer by radiation from industrial flames. This investigation has involved the measurement of the size distribution of aggregates of soot particles from both a soot generator and a flame. The techniques involved sampling by electrostatic precipitation, filtration and impingement. Subsequent measurements of particle size were carried out with the aid of an electron microscope and an image analysing microscope (Quantimet). Spectral transmission measurements in the visible range were also made on both the flame and the generator. Calculations, based on the Mie theory, were made for the measured volume-equivalent particle size distributions to predict the variation of the spectral transmission with wavelength and the ratio of polarised components of scattered light with angle. It was concluded that spectral transmission measurements are more sensitive to particle size distribution than measurements of angular scattering. The measured particle sizes were found to follow a log-normal distribution and a complex refractive index for soot of 1.9-0.35 i gave the closest agreement between experimental and calculated results.