Conditional moment closure modelling of soot formation in turbulent, non-premixed methane and propane flames

Presented are results obtained from the incorporation of a semi-empirical soot model into a first-order conditional moment closure (CMC) approach to modelling turbulent, non-premixed methane-air and propane-air flames. Soot formation is determined via the solution of two transport equations for soot mass fraction and particle number density, with acetylene and benzene employed as the incipient species responsible for soot nucleation, and the concentrations of these calculated using a detailed gas-phase kinetic scheme involving 70 species. The study focuses on the influence of differential diffusion of soot particles on soot volume fraction predictions. The results of calculations are compared with experimental data for atmospheric and 3 atm methane flames, and propane flames with air preheated to 323 K and 773 K. Overall, the study demonstrates that the model, when used in conjunction with a representation of differential diffusion effects, is capable of accurately predicting soot formation in the turbulent non-premixed flames considered.     

Numerical simulation of soot filtration and combustion within diesel particulate filters

The numerous benefits offered by diesel engines, compared to gasoline ones, are balanced by a drawback of increasing concern, namely soot emissions. Nowadays, soot emissions can be reduced by physically trapping the particles within on-board diesel particulate filters (DPF). The filter gets progressively loaded by filtering the soot laden flue gases, thus causing an increasing pressure drop, until regeneration takes place. The aim of this work is to develop a fully predictive three-dimensional mathematical model able to accurately describe the soot deposition process into the filter, the consequent gradual modification of the properties of the filter itself (i.e. permeability and porosity), the formation of a soot filtration cake, and the final regeneration step. The commercial computational fluid dynamics (CFD) code Fluent 6.2.16, based on a finite-volume numerical scheme, is used to simulate the gas and particulate flow fields in the DPF, whereas particle filtration sub-models and regeneration kinetics are implemented through user-defined-subroutines (UDS).Model predictions highlight uneven soot deposition profiles in the first steps of the filtration process; however, the very high resistance to the gas flow of the readily formed cake layer determines the evolution into an almost constant layer of soot particles. The ignition of the loaded soot was simulated under different operating conditions, and two regeneration strategies were investigated: a “mild regeneration” at low temperature and oxygen concentration, that operated a spatially homogeneous ignition of the deposited soot, and a “fast regeneration”, with an uneven soot combustion along the axial coordinate of the filter, due to strong temperature gradients inside the filter itself. These findings are supported by comparison and validation with experimental data.     

Distribution characteristics of exhaust gases and soot particles in a wall-flow ceramics filter

An inhomogeneous soot distribution in a diesel particulate filter may deteriorate its behavior and result in higher pressure drops and fuel consumption. This will cause mechanical stresses on the filter due to temperature gradients resulting from the non-uniformly burning of soot during regeneration. The purpose of this paper is to investigate the flow distribution of the exhaust gas entering into a diesel particulate filter, the turbulent motion of diesel soot particles in the inlet header, and their deposition and distribution in the front surface of a diesel particulate filter. A Lagranian continuous random walk (CRW) model is developed to simulate soot particulate motion, which considers a succession of uncorrelated random forcing and drift corrections. The effects of particle inertia, turbulent fluctuation, and lift on the particle motion and trajectory are analyzed. Correlations of the uniformity index of the exhaust gas and soot particles with the flow rate, soot loading, and inlet expansion angle are evaluated. The results show that there is a two-peak phenomenon in the soot distribution at the front entrance of the filter, which is comprised of a peak in the central area due to inertia and a second peak in the periphery owing to diffusion and recirculation action. Exhaust flow rates and the inlet expansion angle have a major influence on the flow uniformity and soot uniformity, while soot loading has a slightly smaller effect on soot uniformity.     

Sheet-like carbon particles with graphene structures obtained from a Bunsen flame

Carbon particle structures containing only a few graphene layers have been collected from a Bunsen (propane) diffusion flame in the low particle concentration pale yellow luminous regions close to the soot inception. These particles were sampled directly on transmission electron microscopy grids for structural and elemental analysis. They were found to be several hundreds of nanometers in size. Such large structures are not easily explained from gas-phase kinetic models, yet the sheets occurred relatively frequent in the images. Some pictures also showed interesting polygonal few-layered graphitic structures perpendicular to the graphene planes.     

Modelling gas-phase synthesis of single-walled carbon nanotubes on iron catalyst particles

A simple model for the gas-phase synthesis of carbon nanotubes on iron catalyst particles has been developed. It includes a growth model for the catalyst particles and describes nanotube growth processes through carbon monoxide disproportionation and hydrogenation. Models for particle-particle interactions and sintering are also included. When carbon arrives at a catalyst particle it can either dissolve in the particle until a saturation limit is reached, or form a graphene layer on the particle, or go on to form a nanotube. Two models for incipient nanotube growth are considered. The first allows nanotubes to form once a catalyst particle reaches the saturation condition. The second only allows nanotubes to form on the collision of two saturated particles. The particle system is solved using a multivariate stochastic solver coupled to the gas-phase iron chemistry using an operator splitting algorithm. Comparison with experimental data gives a good prediction of the nanotube length, and reasonable values of catalyst particle diameter and nanotube diameter. A parametric study is presented in which the carbon monoxide reaction rate constants are varied, as is the fraction of carbon allowed to form nanotubes relative to surface layers. The assumptions of the coagulation and sintering models are also discussed.     

A phase field model of pressure-assisted sintering

The incorporation of an efficient contact mechanics algorithm into a phase field sintering model is presented. Contact stresses on the surface of arbitrarily shaped interacting bodies are evaluated and built into the model as an elastic strain energy field. Energy relaxation through deformation is achieved by diffusive fluxes along stress gradients and rigid body motion of the deforming particles maintain contact between the particles. The proposed model is suitable for diffusion deformation mechanisms occurring at stresses below the yield strength of a defect-free material; this includes Nabarro-Herring creep, Coble creep and pressure-solution. The effect of applied pressure on the high pressure-high temperature (HPHT) liquid phase sintering of diamond particles was investigated. Changes in neck size, particle coordination and contact flattening were observed. Densification rates due to the externally applied loads were found to be in good agreement with a new theory which implicitly incorporates the effect of applied external pressure.     

The compaction of soot particles generated by spark discharge in the propene ozonolysis system

The compaction of soot aggregates generated by spark discharge is reported in the propene ozonolysis experiment. Since propene is not an organic aerosol precursor, the compaction cannot be explained by an earlier hypothesis attributing the compaction to the capillary forces of condensed organic matter in the small angle cavities of the aggregates [Saathoff, Naumann et al. (2003). Coating of soot and (NH₄)₂ SO₄ particles by ozonolysis products of alpha-pinene. Journal of Aerosol Science, 34, 1297–1321]. Several species among the gas-phase products from propene ozonolysis are proposed to contribute to the structural change of soot particles. As the compaction would directly change the morphology and the size distribution of soot particles, it is expected to induce important changes to the soot particles’ physical and chemical properties, which in turn would have significant atmospheric implications.     

Intra-layer temperature gradients during regeneration of diesel particulate filters

Wall-flow filters are worldwide recognized as the most efficient devices for the abatement of particulate emissions from automotive diesel engines. Mathematical models simulating the particulate thermal oxidation process in the filters are already applied for system optimization. This paper deals with the appropriateness of a specific assumption inherently used in all relevant published models, namely the temperature uniformity in the soot and wall layer. A new mathematical model is developed to predict the temperature gradients under various operating conditions. Based on the model results, it is shown that significant temperature gradients inside the soot layer may exist under some practical operating conditions. These conditions are associated with high flow rates and high soot loadings. In these cases, the uniform temperature assumption may lead to erroneous results for the prediction of the overall regeneration process. The error of this assumption is assessed as function of the soot porosity.     

Grain Growth Control and Dopant Distribution in ZnO-Doped BaTiO3

ZnO additions to BaTiO₃ have been studied in order to determine the role of this dopant on sintering and microstructure development. As a consequence of a better initial dopant distribution, samples doped with 0.1 wt% zinc stearate show homogeneous fine-grained microstructure, while a doping level of 0.5 wt% solid ZnO is necessary to reach the same effect. When solid ZnO is used as the dopant precursor, ZnO is redistributed among the BaTiO₃ particles during heating. Since no liquid formation has been detected for temperatures below 1400°C in the system BaTiO₃-ZnO, it is proposed that dopant redistribution takes place by vapor-phase transport and grain boundary diffusion. Shrinkage and porosimetry measurements have shown that grain growth is inhibited during the first step of sintering for the doped samples. STEM-EDX analysis revealed that solid solubility of ZnO into the BaTiO₃ lattice is very low, being strongly segregated at the grain boundaries. Grain growth control is attributed to a decrease in grain boundary mobility due to solute drag. Because of its effectiveness in controlling grain growth, ZnO appears to be an attractive additive for BaTiO₃ dielectrics.     

Calculations of Internal Stresses during Sintering in Two Dimensions

The internal stresses which may arise during sintering are modeled by calculating the behavior of coupled strings of circular particles. Strings consisting of large and small particles are connected in parallel and are allowed to sinter by surface and grain boundary diffusion. The strings consisting of the smaller particles experience tensile stresses, since their shrinkage is inhibited by the slower sintering of the larger particles. These internal stresses are followed as a function of time. The calculations provide a first-order estimate of the internal stresses which may arise during the sintering of irregular arrays of particles.     

Deposit morphology on SiC fibers in methane-acetylene/air laminar diffusion flames

The morphologies of soot deposit on 15 μm diameter silicon carbide (SiC) fibers have been investigated with a scanning electron microscope (SEM) in methane-acetylene/air laminar diffusion flames with co-flowing air. The morphologies are shown to be strongly dependent on the fuels ratio. Two kinds of processes by which mature soot particles are produced were proved to exist in a sooting flame: one is the transition from the condensed-phase deposits; the other is the aggregation of the smaller soot particles (or chains of them) carried along the particle path line. Different transition processes are compared between the present work and previous work done by other researchers that used propane/air laminar diffusion flames. It seems the presence of C=C in methane-acetylene laminar diffusion flames is the key factor that causes the difference of transition processes in those two kinds of flames.     

Influence of Temperature Gradients on Sintering: Experimental Tests of a Theory

The H₂O-catalyzed sintering of MgO in temperature gradients between 1303 and 1233 K is compared to sintering of isothermal samples. Both the changes in sample dimensions in planes normal to the temperature gradients and the increases in density for a fixed sintering time are greater than predicted on the assumption that densification in a temperature gradient is a function only of each local temperature in the gradient. Under the conditions used, neither vapor transport nor transport through a liquid phase is important. The results, therefore, support a recent prediction that temperature gradients supplement surface energy changes in driving sintering and related processes not only by vapor transport, as expected from earlier studies, but also by a surface, grainboundary, or bulk diffusion path.     

Morphological Changes in Process-Related Large Pores of Granular Compacted and Sintered Alumina

Large pore defects clearly develop in Al₂O₃ ceramics during sintering. These large pores originate from voids caused by the incomplete deformation and adhesion of powder particles in collapsed dimples at the centers and boundaries of granules in the green compacts. The coalescence of pores, with limited shrinkage, during densification and grain growth in the late intermediate to final stages of sintering, is considered responsible for the development of the large pores. The mechanism of pore coalescence is explained by thermodynamic arguments, which demonstrate that the largest pores result in a stable system.     

Measuring aerosol active surface area by direct ultraviolet photoionization and charge capture in continuous flow

Abstract

Direct ultraviolet photoionization electrically charges particles using a mechanism distinct from diffusion charging. The purpose of this study is to evaluate aerosol photoemission theory as a function of aerosol particle size, concentration, material, and morphology. Particles are classified using an aerodynamic aerosol classifier (AAC) and subsequently measured with a scanning mobility particle sizer (SMPS) and photoionization measurement system in parallel. This configuration allows direct comparison of photo-emission from high concentrations of initially neutral, monodisperse aerosols with different morphologies or materials. Under all examined conditions, the overall photoelectric yields of particles of self-similar material (silver and unconditioned soot) and morphology (sintered spheres and agglomerates) are each linearly proportional to the second moment of the mobility-equivalent diameter distribution, even in the transition regime (mobility diameter 30–200?nm), with agglomerate silver particles resulting in 5× higher photoelectric yield than unconditioned soot from a propane flame. It is shown for the first time that the photoelectric yield is significantly higher (2.6×) for fractal-like agglomerate silver particles than sintered, close-packed spherical particles of the same material and mobility-equivalent diameter, which is inferred to be due to the larger material surface area exposed externally to the particle surroundings. It is demonstrated that photoelectric measurements of aerosols reflect the photoelectrically active surface area which depends on the particle morphology and therefore the state of sintering.

Copyright © 2019 American Association for Aerosol Research     

Gas-phase reactions during CVD synthesis of carbon nanotubes: Insights via numerical experiments

A series of computational calculations to understand the gas-phase reactions of xylene, a typical feedstock for carbon nanotubes, were conducted. A xylene reaction model was combined with a soot formation model, allowing us to calculate the detailed xylene pyrolysis reactions as well as the growth of polycyclic aromatic hydrocarbons (PAHs) and soot. The model was validated against soot formation experiments that were conducted by other researchers under the conditions similar to CVD nanotube synthesis; their experimental results confirmed the computed soot yield and soot surface area to be reasonable. Our calculations showed that xylene and toluene were the major gas-phase species at temperatures lower than 973 K, implying that nanotubes were formed through the interaction between catalyst and xylene and/or toluene at these temperatures. At higher temperatures, however, a considerable amount of acetylene was found which likely enhances the growth of PAHs and soot as evidenced at 1373 K.     

The effect of the contact between platinum and soot particles on the catalytic oxidation of soot deposits on a diesel particle filter

Experiments were performed with two model soot aerosols brought into different forms of contact with Pt aerosol particles, to investigate the effectiveness of this contact in lowering the catalytic soot oxidation temperature. The contact was either generated between individual particles in the aerosol state (Pt-doped soot to simulate a fuel borne catalyst), or by sequential or simultaneous deposition of separately generated soot and Pt aerosols onto a sintered metal filter. (Formation of a soot cake on previously deposited Pt aerosol would simulate a catalyst coated diesel particle filter.) The catalytic activity was determined in all cases from temperature ramped oxidation in air of the filtered particles, and defined as the 50% conversion temperature.

It was found that Pt-doped soot and simultaneously filtered aerosols were both equally effective in reducing the oxidation temperature by up to 140–250 °C for the spark discharge soot (with 3–47 wt% Pt concentration in the soot cake), and by up to 140 °C for the pyrolysis soot (3 wt% Pt). Conversely, the deposition of a thin soot layer of 5–10 μm thickness onto Pt, or vice versa, produced only a slight temperature reduction on the order of about 13–42 °C. These results suggest that the distance between soot and Pt particles plays a key role in promoting an effective oxidation on the filter, which is consistent with the role of Pt particles as local generators of activated oxygen.     

Morphologies and properties of NiO particles prepared from NiCl<Subscript>2</Subscript>·6H<Subscript>2</Subscript>O by spray pyrolysis

Nickel oxide particles were prepared by spray pyrolysis of aqueous solution of NiCl₂·6H₂O. In the reactor the salt droplets were first converted to hollow particles by drying and then they were collapsed by oxidation to reduce their size. Each oxide particle was composed of many small nuclei with voids among them due to extremely low rate of sintering. The particle size decreased with the temperature as the sintering and crystallization proceeded. The size as well as the crystallinity of the particles increased with the initial salt concentration. When the salt droplets were preliminarily dried in diffusion dryer before entering the reactor, the collapse of the particles was considerably reduced, resulting in lower hollowness and higher sphericity. Numerical simulation on the drying of the droplets provided insight on the initial stage of spray pyrolysis.     

Initiation mechanism for flame flashover within premises with a seat of combustion

Conditions are analyzed for the occurrence of flame flashover during the development of a fire within premises and secondary ignition for gas-phase combustion of PMMA in air with additions of tetrafluorodibromoethane, which intensifies the release of soot. It is shown that the phenomena in question arise with a high concentration of soot particles which in both cases play an important role in initiating flame propagation.All-Union Research Institute of Fire Protection (VNIIPO), Balashikha-6, 143900. Translated from Fizika Goreniya i Vzryva, Vol. 30, No. 6, pp. 37–39, November–December, 1994.     

Soot formation in inverse diffusion flames of diluted ethene

Experimental measurements of flame structure and soot characteristics were performed for ethene inverse diffusion flames (IDFs). IDFs are considered as an ideal flow field for studying incipient soot because the soot particles do not experience the oxidation process. In this study, an laser-induced fluorescence image clarified the reaction zone of IDFs with OH signal and polycyclic aromatic hydrocarbon distribution. A laser scattering technique also identified the soot particles. To address the degree of soot maturing, the carbon to hydrogen ratio and morphology of the soot sample were investigated. From the measurements, the effect of flow residence time and temperature on soot inception could be estimated, and more details on soot characteristics in the IDFs were determined according to the fuel dilution ratio and air/fuel ratio. Fuel dilution results in a decrease of temperature and an enhancement of residence time, but the critical dilution mole fraction is found for which temperature does not effect soot growth. Soot inception is also found to be weakly dependent on temperature as influenced by fuel dilution.     

Particle-vapor interaction in deposition systems: influence on deposit morphology

We have here presented methods to study interactions of vapors and particles in systems involving simultaneous deposition of vapors and particles. Besides estimating vapor and particle concentration profiles in the boundary layer adjacent to the deposition surface, their deposition rates are also calculated. In particular, we consider formation of porous preforms by deposition of silica particles and germania particles/vapors during the manufacturing of optical fibers. The process conditions not only dictate the relative rates of germania particle and vapor deposition on the deposition surface, but also controls the fraction of germania crystallinity in the resulting deposit. Moreover, the loss and migration behavior of the deposited germania during the sintering of the porous preform is extremely sensitive to the germania crystalline fraction. Our methods predict the germania weight percent deposited during the deposition process as a function of the deposition conditions, along with the fraction of germania that is crystalline/amorphous. The germania loss and migration behavior during the sintering step is also estimated. In predicting the germania loss and migration behavior, we have developed methods to systematically take into account the simultaneous heating of the preform, sintering of the porous preform, diffusion of gas species through the pores and the gas-solid reaction. Based on the methods developed here for deposition and sintering, processes have been developed which have resulted in sintered glasses with very high germania content (50 weight percent), and, without any glass quality issues of glass seeds, blank splitting or glass crizzling (devitrification).