Soot Distribution Research Articles

Large eddy simulation of pollutant gas dispersion with buoyancy ejected from building into an urban street canyon

The dispersion of buoyancy driven smoke soot and carbon monoxide (CO) gas, which was ejected out from side building into an urban street canyon with aspect ratio of 1 was investigated by large eddy simulation (LES) under a perpendicular wind flow. Strong buoyancy effect, which has not been revealed before, on such pollution dispersion in the street canyon was studied. The buoyancy release rate was 5 MW. The wind speed concerned ranged from 1 to 7.5 m/s. The characteristics of flow pattern, distribution of smoke soot and temperature, CO concentration were revealed by the LES simulation. Dimensionless Froude number (Fr) was firstly introduced here to characterize the pollutant dispersion with buoyancy effect counteracting the wind. It was found that the flow pattern can be well categorized into three regimes. A regular characteristic large vortex was shown for the CO concentration contour when the wind velocity was higher than the critical re-entrainment value. A new formula was theoretically developed to show quantitatively that the critical re-entrainment wind velocities, u c, for buoyancy source at different floors, were proportional to −1/3 power of the characteristic height. LES simulation results agreed well with theoretical analysis. The critical Froude number was found to be constant of 0.7.

Multiphase and multiphysics particle in cell simulation of soot deposition inside a diesel particulate filter single channel

In the diesel particulate filters technology a key aspect is represented by the properties of the particulate matter that is collected inside their structure. The work presented is focused on the development of an innovative mathematical tool based on the particle-in-cell method (PIC) for the simulation of the soot distribution inside a single channel of a diesel particulate filter. The basic fluid dynamic equations are solved for the gas phase inside the channel using a novel technique based on the solution of the same set of equations everywhere in the system including the porous medium. This approach is presented as alternative to the more conventional methods of matching conditions across the boundary of the porous region where a Darcy-like flow is developed. The motion of the soot solid particles is instead described through a particle-by-particle approach based on Newton's equations of motion. The coupling between the dynamics of the gas and that of the soot particles, i.e. between these two sub-models, is performed through the implementation of the particle-in-cell technique. This model allows the detailed simulation of the deposition and compaction of the soot inside the filter channels and its characterization in terms of density, permeability and thickness. The model then represents a unique tool for the optimization of the design of diesel particulate filters. The details of the technique implementation and some paradigmatic examples will be shown.

Analysis of EGR Effects on the Soot Distribution in a Heavy Duty Diesel Engine using Time-Resolved Laser Induced Incandescence

Analysis of EGR Effects on the Soot Distribution in a Heavy Duty Diesel Engine using Time-Resolved Laser Induced Incandescence

Modeling nano-particle deposition in diesel engine filters

In this work, we develop a simple, fully analytical one-dimensional model based on lubrication theory to predict nano-particle deposition along the unit channel of a clean diesel particulate filter. Soot deposition along the channel depends on filter design parameters, which can be optimally selected to minimize the uneven distribution of soot. At the initial stage of the filtration process, the porous wall resistance is controlling; at later stages, the local resistance of the deposited soot is controlling. Typically, a hundredfold increase in filter resistance may be observed over time, and the maximum variability in the soot deposition is observed at the initial stages of the process. Therefore, in this work, we focus on the deposition of soot in a clean filter, focusing on the one dimensional variability along the main flow direction. In a clean filter, the porous wall resistance can be considered uniform along the filter length and the flow field and nano-particle transport equations can be solved analytically. Model predictions compare surprisingly well with those obtained from more complex three-dimensional simulations, thereby demonstrating that the essential physics is captured correctly. Based on the very simple framework identified in this paper, more complex models can be developed and used to predict the filter behavior over time (i.e. build-up of the soot cake and back-pressure rise up to the regeneration step) provided that (i) the variation in time of filter permeability due to local soot loading is implemented and (ii) equations are solved numerically.

THz imaging techniques for nondestructive inspections

Abstract We have suggested a wide range of real-life applications using novel terahertz imaging techniques. A high-resolution terahertz tomography has been demonstrated by ultra short terahertz pulses using optical fiber and a nonlinear organic crystal. We also describe a nondestructive inspection system that can monitor the soot distribution in a ceramic filter using millimeter-to-terahertz wave computed tomography. Further, we report on the thickness measurement of very thin films using high-sensitivity metal mesh filter. These techniques are directly applicable to the nondestructive testing in industries.

Non-destructive Characterization of Soot in Exhaust Filters Using Millimeter-wave Imaging

It is necessary to monitor the distribution of the soot that accumulates in the soot removal filter of an internal combustion engine to design the highly efficient filtering structure and optimize the timing of the filter renewal. However, there is no non-destructive inspection technique at present. In this study, we describe a non-destructive inspection system that can monitor the distribution of soot that accumulates in the filter using combination of millimeter-to-terahertz wave imaging and computed tomography. The transmittance of the filter shows a strong dependence on the angle of incidence because of its two-dimensional periodical shape. We identified a frequency at which photonic band gaps did not occur, and measured the three-dimensional tomographic image of the filter in a non-destructive manner.

Effect of a uniform electric field on soot in laminar premixed ethylene/air flames

The effect of a nominally uniform electric field on the initially uniform distribution of soot has been assessed for laminar premixed ethylene/air flames from a McKenna burner. An electrophoretic influence on charged soot particles was measured through changes to the deposition rate of soot on the McKenna plug, using laser extinction (LE). Soot volume fraction was measured in situ using laser-induced incandescence (LII). Particle size and morphologies were assessed through ex situ transmission electron microscopy (TEM) using thermophoretic sampling particle diagnostics (TSPD). The results show that the majority of these soot particles are positively charged. The presence of a negatively charged plug was found to decrease the particle residence times in the flame and to influence the formation and oxidation progress. A positively charged plug has the opposite effect. The effect on soot volume fraction, particles size and morphology with electric field strength is also reported. Flame stability was also found to be affected by the presence of the electric field, with the balance of the electrophoretic force and drag force controlling the transition to unstable flame flicker. The presence of charged species generated by the flame was found to reduce the dielectric field strength to one seventh that of air.

Radiation characteristics and turbulence–radiation interactions in sooting turbulent jet flames

A comprehensive modeling strategy including detailed chemistry, soot and radiation models coupled with state-of-the-art closures for turbulence–chemistry interactions and turbulence–radiation interactions is applied to various luminous turbulent jet flames. Six turbulent jet flames are simulated with Reynolds numbers varying from 6700 to 15,000, two fuel types (pure ethylene, 90% methane–10% ethylene blend) and different oxygen concentrations in the oxidizer stream (from 21% O2 to 55% O2). All simulations are carried out with a single set of physical and numerical parameters (model constants). A Lagrangian particle Monte Carlo method is used to solve a modeled joint probability density function (PDF) transport equation, which allows accurate closure for turbulence–chemistry interactions including nonlinear soot subprocesses. Radiation is calculated using a particle-based photon Monte Carlo method that is coupled with the PDF method to accurately account for both emission and absorption turbulence–radiation interactions (TRI). Line-by-line databases are used for accurate spectral radiative properties of CO2 and H2O; soot radiative properties also are modeled as nongray. For the flames that have been investigated, soot emission can be almost 45% of the total emission, even when the peak soot volume fraction is of the order of a few parts-per-million (ppm) and up to 99% of soot emission can escape the domain without re-absorption. Turbulence–radiation interactions have a strong effect on the net radiative heat loss from these sooting flames. For a given temperature, species and soot distribution, TRI increases emission from the flames by 30–60%, and the net heat loss from the flame increases by 45–90% when accounting for TRI. This is higher than the corresponding increase in radiative heat loss due to TRI in nonsooting flames. Absorption TRI was found to be negligible in these laboratory-scale sooting flames with soot levels on the order of a few ppm, but may be important in larger industrial-scale flames and in higher-pressure systems.

ベンゼンの層流拡散火炎の内外におけるPM構成成分の分析(熱工学,内燃機関,動力など)

Nano size particulate matter (PM) has harmful effects on human health. Investigations of fundamental characteristics of PM and its compositions inside and outside of flame were required to develop reduction technologies for nano-PM emission. In this paper, relationship between PM characteristic and its compositions such as soot and soluble organic fraction (SOF) were investigated using laser diagnostic and direct sampling methods. A laser induced incandescence (LII) method was used to obtain spatial distribution of soot in a benzene flame. PM at the LII region in flame and emission smoke from flame tip were sampled by a direct sampling system. Further, soot and SOF in the sampled PM were measured using a combustion type PM analyzer (MEXA-1370 PM). It was confirmed that main composition of upstream-PM was nuclei mode particles, but downstream-PM and PM emitted from the flame tip were aggregation mode particles. Volatility of SOF in PM was investigated by a mass reduction ratio of SOF in an image furnace. Main composition of SOF in PM sampled from upstream part (upstream-PM) was lighter molecules, though those of PM at downstream part (downstream-PM) was heavier molecules.

Concentration distributions of intermediate hydrocarbons and soot in propane–air triple flames

AbstractAn experimental investigation has been conducted to study the distribution of intermediate hydrocarbons (CH4, C2H2, C2H4 and C2H6) and soot in propane/air triple flames at different inner and outer equivalence ratios. The flames are established on an axisymmetric, coflowing burner. A rich mixture of propane and air flows through the inner port of the burner, whereas a lean mixture flows through the annular port surrounding the central jet. The concentrations of hydrocarbons at different locations in the flames are measured with gas chromatography and the soot volume fractions are measured using a thermocouple particle densitometry (TPD) method. The flame structures have been described using the visual images of the flames as well as with the distributions of fuel (C3H8) and CO2 concentrations and temperature across the flames. The peak concentrations of the intermediate hydrocarbons are observed at the inner premixed flame location, which decay down rapidly also at the inner flame. The intermediat...

Validation of a Grid Independent Spray Model and Fuel Chemistry Mechanism for Low Temperature Diesel Combustion

Spray and combustion submodels used in a Computational Fluid Dynamics (CFD) code, KIVACHEMKIN, were validated for Low Temperature Combustion (LTC) in a diesel engine by comparing measured and model predicted fuel spray penetrations, and in-cylinder distributions of OH and soot. The conditions considered were long ignition delay, early and late fuel injection cases. It was found that use of a grid independent spray model, called the GASJET model, with an improved n-heptane chemistry mechanism can well predict the heat release rate, not only of the main combustion stage, but also of the cool flame stage. Additionally, the GASJET model appropriately predicts the distributions of OH and soot in the cylinder even when the resolution of the computational mesh is decreased by half, which significantly reduces the required computational time.

Evolution of Soot Particle Size Distribution Function in Burner-Stabilized Stagnation n-Dodecane−Oxygen−Argon Flames

We investigate the evolution of particle size distribution of incipient soot formed in laminar premixed n-dodecane−oxygen−argon flames. The flames are established on a porous flat flame burner with equivalence ratio equal to 2 and a maximum flame temperature of 1800 and 1870 K. Detailed size distributions are obtained by the burner-stabilized stagnation (BSS) flame sampling approach using a scanning mobility particle sizer. The flame temperature profiles are determined for each separation distance between the burner surface and stagnation surface/probe orifice. It is shown that the flames can be modeled closely using an opposed jet flame code without having to estimate the effect of probe perturbation. The measured and simulated temperature profiles show good agreement. The evolution of the soot size distributions for n-dodecane flames are similar to those observed in ethylene flames. The size distributions are characteristically bimodal, indicating strong, persistent nucleation over a large range of resi...

Effect of Mesh Structure in the KIVA-4 Code with a Less Mesh Dependent Spray Model for DI Diesel Engine Simulations

Two different types of mesh used for diesel combustion with the KIVA-4 code are compared. One is a well established conventional KIVA-3 type polar mesh. The other is a non-polar mesh with uniform size throughout the piston bowl so as to reduce the number of cells and to improve the quality of the cell shapes around the cylinder axis which can contain many fuel droplets that affect prediction accuracy and the computational time. This mesh is specialized for the KIVA-4 code which employs an unstructured mesh. To prevent dramatic changes in spray penetration caused by the difference in cell size between the two types of mesh, a recently developed spray model which reduces mesh dependency of the droplet behavior has been implemented. For the ignition and combustion models, the Shell model and characteristic time combustion (CTC) model are employed. The calculated spatial distribution of droplets, fuel vapor and soot are compared against high-speed in-cylinder imaging obtained from an optical access diesel engine. Heat release rate and computational time are also compared between the two types of mesh. The results show that the uniform-sized mesh reduces computational times significantly while maintaining almost the same prediction accuracy as the KIVA-3 type polar mesh.

Modeling of multi-component fuel vaporization and combustion for gasoline and diesel spray

The present study applied a continuous thermodynamics approach to consider the multi-component nature of petroleum fuels during the vaporization process. A gamma distribution was used to describe the molecular weight of the fuel. The model was first used to study the vaporization of single diesel and gasoline drops. Results showed that the mean molecular weight of the fuel drop kept increasing, indicating that the lighter components vaporized earlier in the process. The present vaporization model was also integrated with an engine simulation code for diesel spray combustion study. Results of diesel spray modeling showed that heavy fuel components survived during the early vaporization process such that the drops in the outer regions of the spray were mostly composed of heavier components. In this study, detailed chemistry was used for diesel combustion modeling. Results showed that good levels of agreement between experiments and predictions were obtained in flame structures and soot distributions. Effects of ambient temperature in the sooting tendency of diesel spray were also predicted by the present model. Under the conditions studied ( ρ = 14.8 kg / m 3 ) , soot emissions were not seen for ambient temperature less than 850 K, which is consistent with the concept of low-temperature engine combustion for low emissions.

Soot volume fraction in a piloted turbulent jet non-premixed flame of natural gas

Planar laser-induced incandescence (LII) has been used to measure soot volume fraction in a well-characterised, piloted, turbulent non-premixed flame known as the “Delft Flame III”. Simulated Dutch natural gas was used as the fuel to produce a flame closely matching those in which a wide range of previous investigations, both experimental and modelling, have been performed. The LII method was calibrated using a Santoro-style burner with ethylene as the fuel. Instantaneous and time-averaged data of the axial and radial soot volume fraction distributions of the flame are presented here along with the Probability Density Functions (PDFs) and intermittency. The PDFs were found to be well-characterised by a single exponential distribution function. The distribution of soot was found to be highly intermittent, with intermittency typically exceeding 97%, which increases measurement uncertainty. The instantaneous values of volume fraction are everywhere less than the values in strained laminar flames. This is consistent with the soot being found locally in strained flame sheets that are convected and distorted by the flow.

Modelling of diesel particulate filtration in wall-flow traps

The present work addresses the modelling aspects of soot deposition inside diesel particulate filters (DPFs). These wall-flow monoliths are commonly employed in the automotive sector to reduce particulate emissions of Diesel engines. Simulations were carried out using computational fluid dynamics under different operating conditions by varying the exhaust-gas inlet velocity, the filter permeability and the soot particle size. The numerical results show that the flow field inside the filter is highly responsible for the soot distribution along the axial coordinate: the velocity of the soot laden gas through the porous wall of the bare filter is not constant along the channel, which in turn promotes an uneven deposition of particles inside the filter itself. Moreover, it has been shown that the fraction of the particles that impact the wall surface and are trapped by the porous media is sensitive to the particle size: the collection efficiency was estimated, and the most penetrating particles were found to range from 200 nm to 500 nm. The accumulation of soot particles in the porous media (depth filtration) and the subsequent formation of a soot particles layer on the top of the filter wall (cake filtration) generates an additional pressure drop. Hence, the gas through-wall velocity evolves towards an “iso-permeability” profile characterized by an almost constant soot layer thickness, since its permeability is two orders of magnitude higher than that of the bare filter. Preliminary experimental results have confirmed this behaviour. In addition, it has been shown that the velocity profile at the inlet of the filter is highly responsible for different soot loadings of the inlet filter channels.

An experiment on the effect of oxygen content and air velocity on soot formation in acetylene laminar diffusion flame produced in a burner with a parallel annular coaxial oxidizer flow

The effect of oxygen content and of the combustion air velocity on soot formation was studied in acetylene diffusion flames. These flames were produced in a burner with a parallel annular coaxial flow of oxidizer. The effect on the flame axial temperature profile was also evaluated. The soot volume fraction was calculated by the laser light extinction methodology. The oxygen content in the combustion air was smaller than 30%, which does not require significant retrofit of existent equipment when the combustion conditions are varied. The results suggest that the parallel manipulation of the oxygen content and of the oxidizer velocity can provide means for managing soot formation and distribution. The formation of soot in industrial combustion systems is of interest in engineering, because the presence of soot in the flame enhances the heat transfer from the combustion gases by thermal radiation, increases the need for burner maintenance, and constitutes an environmental problem when emitted in the atmosphere.

Simulating low temperature diesel combustion with improved spray models

Current spray models based on the Lagrangian-droplet and Eulerian-fluid (LDEF) method in the KIVA-3V code are strongly mesh dependent due to errors in predicting the droplet–gas relative velocity and errors in describing droplet–droplet collision and coalescence processes. To reduce the mesh dependence, gas-jet theory is introduced to predict the droplet–gas relative velocity, and a radius of influence (ROI) of collision methodology is established for each gas phase cell to estimate the collision probability for each parcel in the cell. Spray and combustion processes in a low temperature combustion diesel engine under early and late injection strategies with a fine mesh were predicted using the conventional LDEF model and compared with the measurements of soot, OH, fuel liquid and vapor distributions obtained by laser based diagnostics including, PLIF, LII, and Mie scattering. Then, the KIVA-3V code implemented with the improved spray model based on the gas-jet model and modifications of the spray models was utilized to simulate the processes on a relatively coarse numerical mesh. Comparison of the simulations between the fine and coarse meshes shows that the improved spray model can greatly reduce the mesh dependence for low temperature combustion diesel engine CFD simulations.

Transient Development of Flame and Soot Distribution in Laminar Diffusion Flame With Preheated Air

A numerical investigation of the transient development of flame and soot distributions in a laminar axisymmetric coflowing diffusion flame of methane in air has been carried out considering the air preheating effect. The gas phase conservation equations of mass, momentum, energy, and species concentrations along with the conservation equations of soot mass concentration and number density are solved simultaneously, with appropriate boundary conditions, by an explicit finite difference method. Average soot diameters are then calculated from these results. It is observed that the soot is formed in the flame when the temperature exceeds 1300 K. The contribution of surface growth toward soot formation is more significant compared with that of nucleation. Once the soot particles reach the high temperature oxygen-enriched zone beyond the flame, the soot oxidation becomes important. During the initial period, when soot oxidation is not contributing significantly, some of the soot particles escape into the atmosphere. However, under steady condition the exhaust product gas is nonsooty. Preheating of air increases the soot volume fraction significantly. This is both due to more number of soot particles and the increase in the average diameter. However, preheating of air does not cause a qualitative difference in the development of the soot-laden zone during the flame transient period.

The reconstructing of soot agglomerates in propene ozonolysis and photooxidation systems

The soot in the atmosphere exhibits irregular fractal agglomerate structure. In this study, the reconstruction of agglomerate structure is observed for soot generated by spark discharge in both propene ozonolysis and photooxidation systems. This process makes the agglomerate soot more compact, and is possibly caused by the peroxy radicals and hydroxyl radicals generated from propene oxidation. The result furthers our understanding of the atmospheric aging process of soot agglomerate. Since the reconstruction directly changes the size distribution of soot, it may induce important changes to the soot's physical and chemical properties in the atmosphere.