Soot Particle Size Distribution Research Articles

Kinetic modeling of particle size distribution of soot in a premixed burner-stabilized stagnation ethylene flame

A detailed model of soot formation is proposed, which consists of a gas-phase kinetic model for the pyrolysis and oxidation of selected hydrocarbon fuels and a kinetic mechanism of soot nucleation and mass/size growth through coagulation and surface reactions. The gas-phase model (Ranzi et al., 2012) was expanded to include the chemistry of Polycyclic Aromatic Hydrocarbons (PAHs) up to four-to-five ring PAHs, with a modular and hierarchical approach. The discrete sectional method was employed to solve the size evolution of the particle size distribution function (PSDF). Analogy and similarity rules were employed to describe heterogeneous reaction kinetics of soot surface reactions. A variable collision efficiency was assumed for the coalescence of small soot particles. Larger particles were assumed to undergo aggregation. The predicted PSDFs are found to be in reasonably good agreement with the experimental data for nascent soot measured in an atmospheric-pressure premixed ethylene–oxygen–argon flame in the burner-stabilized stagnation flame configuration. Sensitivity analyses of the PSDF, number density, and volume fraction were carried out with respect to the rate parameters of addition reactions of acetylene, PAHs, resonantly stabilized radical reactions, and coalescence and aggregation. The results show that the reaction of PAHs and acetylene with soot surfaces and the kinetics of coalescence and aggregation exhibit dominant effects on the detailed and global soot properties for the flame studied, in agreement with conclusions of a large range of previous modeling studies.

Simulation of Soot Size Distribution in a Counterflow Flame

Abstract Soot formed during the rich combustion of fossil fuels is an undesirable pollutant and health hazard. A newly developed Monte Carlo method is used to simulate the soot formation in a counterflow diffusion flame of ethylene. The simulation uses a new reaction mechanism available in literature, which focuses on modeling the formation of large polycyclic aromatic hydrocarbons (PAHs) up to coronene (C24H12). Nascent soot particles are assumed to form from the collision of eight different PAH molecules. Soot surface growth includes the hydrogen-abstraction-C2H2-addition mechanism and the condensation of the PAHs. Soot coagulation is in the free-molecular regime because particles are small (not more than a hundred nanometer). The coupling between vapor consumption and soot formation is handled by an interpolative moment method. Soot particle diffusion is found negligible throughout the counterflow flame, except for a very narrow region right around the stagnation plane. The soot particle size distribution (PSD) generally exhibits a bimodal shape. The first peak corresponds to a large number of nascent particles, while the second peak results from the competition between nucleation and coagulation. Surface growth affects the PSD quantitatively, but does not change the modality. A comparison with experimental data is also provided.

Application of a semi-detailed soot modeling approach for conventional and low temperature diesel combustion – Part I: Model performance

In the present study a previously developed soot model has been tested extensively for conventional and low temperature diesel combustion engine simulations. The soot model framework, which was implemented in the Computational Fluid Dynamics (CFD) code KIVA-CHEMKIN, is based on four fundamental steps, viz., soot inception through a four ring polycyclic aromatic hydrocarbon species, soot surface growth through acetylene and benzene, soot coagulation, and oxygen and OH-induced soot oxidation. The proposed model can be used not only for soot mass predictions, but also for soot number density and soot particle diameter predictions. Diesel combustion was simulated using reduced n-heptane/primary reference fuel (PRF) chemistry mechanisms and a reduced polycyclic aromatic hydrocarbon (PAH) chemistry mechanism, while the n-heptane chemistry mechanism was used for modeling constant volume n-heptane combustion. Soot model performance was evaluated by comparing the model predictions with available constant volume combustion chamber optical diagnostic experiments, optical engine in-cylinder soot data, and a light-duty single cylinder metal engine-out smoke data. In particular, a variety of combustion regimes were tested, including conventional diesel, premixed charge compression ignition (PCCI), and high exhaust gas recirculation (EGR) low temperature combustion (LTC). In general, the model was able to predict well the trends in soot mass over the range of operating conditions. With increasing degree of premixing, the model was able to predict relatively lower soot concentrations compared to diffusion combustion. In terms of soot particle diameter, the model computed particle size was seen to increase with increasing simulated EGR when the ambient oxygen concentration was varied from 21% to 8% by volume and with increasing in-cylinder density (14.8kg/m3 vs. 30kg/m3 at the same in-cylinder O2 level) under steady-state constant volume combustion conditions. It is also observed from the optical and metal engine studies that the soot particle size is larger for high temperature combustion conditions that are marked with high soot formation rates during the combustion process and may or may not lead to smaller particles at exhaust valve opening (EVO) depending on the oxidation rates. The soot size for LTC is generally lower through-out the combustion process including EVO conditions. All but one data-point show uni-modal type soot particle size distribution with respect to both soot mass and particle number.

Development of High Fidelity Soot Aerosol Dynamics Models using Method of Moments with Interpolative Closure

The method of moments with interpolative closure (MOMIC) for soot formation and growth provides a detailed modeling framework maintaining a good balance in generality, accuracy, robustness, and computational efficiency. This study presents several computational issues in the development and implementation of the MOMIC-based soot modeling for direct numerical simulations (DNS). The issues of concern include a wide dynamic range of numbers, choice of normalization, high effective Schmidt number of soot particles, and realizability of the soot particle size distribution function (PSDF). These problems are not unique to DNS, but they are often exacerbated by the high-order numerical schemes used in DNS. Four specific issues are discussed in this article: the treatment of soot diffusion, choice of interpolation scheme for MOMIC, an approach to deal with strongly oxidizing environments, and realizability of the PSDF. General, robust, and stable approaches are sought to address these issues, minimizing the use of ad hoc treatments such as clipping. The solutions proposed and demonstrated here are being applied to generate new physical insight into complex turbulence-chemistry-soot-radiation interactions in turbulent reacting flows using DNS. Copyright 2014 American Association for Aerosol Research

Sooting behaviors of n-butanol and n-dodecane blends

This work focuses on understanding the formation and oxidation of soot when adding n-butanol, an oxygenated fuel, to n-dodecane. A two-stage burner was used to characterize the oxidation of soot from different n-butanol blends, 10%, 30%, and 60mol% in n-dodecane. The two-stage burner isolates the soot oxidation process from the formation process. Soot is formed in a first-stage premixed burner under fuel-rich conditions, while in a second stage, the soot is oxidized under slightly fuel-rich conditions. A scanning mobility particle sizer (SMPS) was used to measure the soot particle size distributions in the flame at different heights during oxidation. Results showed a decrease in particle mass concentration (g/cm3) as the fraction of n-butanol increased, which indicates the capability of n-butanol to reduce soot particle number and mass. On the other hand, the results demonstrated that the increasing n-butanol reduces the difference between initial mass of soot particles entering and the final mass of soot particles leaving the second burner. This result implies that increasing the n-butanol concentration decreases the rate of soot oxidation. Two different fuel quality indicators are used to quantify our observations. The first one, “sooting tendency”, is calculated to show how the amount of soot formed in the flame is affected by using different n-butanol percentages. The second one, “sooting stability”, is defined for quantifying the stability of soot particles against oxidation. The results demonstrated that by increasing the n-butanol percentage, soot formation was suppressed. However, sooting stability increased with higher concentrations of n-butanol. The soot nanostructure was quantified by high-resolution electron microscopy and digital image processing. Image analysis revealed layer arrangement is in correlation with sooting stability. The results of interlayer spacing showed a decrease by increasing n-butanol at the same sampling height.

Size Distribution and Structure of Wall-Deposited Soot Particles in an Automotive-Size Diesel Engine

Size Distribution and Structure of Wall-Deposited Soot Particles in an Automotive-Size Diesel Engine

Non-catalytic after-treatment for diesel particulates using carbon-fiber filter and experimental validation

Recently, the stricter diesel emission standards have been setting and an after-treatment of exhaust gas is needed. In this study, as a potential non-catalytic system, the after-treatment using a carbon-fiber filter was investigated, where soot particulates are burned by an electric heater. In the vehicle tests, the experimental validation for our proposed system was conducted, where the soot volume fraction and particle size distribution in diesel exhaust gas were measured. Since the filter was attached to the exhaust pipe, the filter backpressure increased with higher soot emission. Thus, the performance of the filter was evaluated under the same pressure condition. It is confirmed that, the soot emission is greatly reduced by trapping the diesel soot. In addition, the particle number concentration is smaller, so that most of soot particulates are trapped and burned inside the filter. However, in the after-treatment process, particulates less than 30nm are newly formed. In experiments, the information on the soot oxidation process was limited, and the numerical simulation was performed to investigate temperature field and soot oxidation rate in the filter. It is found that the continuous regeneration is largely promoted by increasing the filter wall temperature. Hence, it is necessary to pay more attention on the effect of filter wall temperature on the particle size for more reduction of smaller particulates.

Monte‐Carlo Simulation of Soot Particle Size Distributions in Turbulent Natural Gas/Air Flames

Rieselbettreaktoren unterliegen einerseits inharenten Limitierungen bei Gasloslichkeit und Gasphasenstoffubertragung an den Katalysator sowie andererseits verminderter Katalysatorausnutzung bis zur Ausbildung von Hotspots infolge inhomogener Phasenverteilungen und Benetzung. Zur Prozessintensivierung wurde u. a. die periodische Betriebsweise vorgeschlagen, wodurch der Benetzungsgrad periodisch variiert werden kann und der Zugang der Gasphase an den Katalysator verbessert wird. In Reaktoren im industriellen Masstab sind jedoch nur geringe Effekte zu erwarten, da die aufgepragte Periodizitat mit der Reaktorlange abnimmt. Beim neuen Reaktorkonzept (Abb.) fuhrt die Neigung zu einer Phasensegregation und damit jeweils zu einem direkten Zugang beider fluider Phasen an den Katalysator. Die zusatzlich aufgepragte Rotation bewirkt eine periodische Beund Entnetzung des Katalysators, wobei neben der Periodizitat auch die Verweilzeit eingestellt werden kann. Zur Bewertung des neuen Reaktorkonzepts wurden hydrodynamische Untersuchungen bei unterschiedlichen Neigungswinkeln und Reaktordrehzahlen sowie bei verschiedenen Durchsatzen durchgefuhrt. Mittels kompakter Gamma-Tomographie wurde die Zweiphasenstromung hinsichtlich der Stromungsform charakterisiert. Neben dem neuen Reaktorkonzept werden die eingesetzte bildgebende Messtechnik vorgestellt und die Ergebnisse zusammengefasst.

Modeling of soot particle size distributions in premixed stagnation flow flames

Soot formation in five laminar premixed ethylene flames was studied computationally in order to investigate the ability of a sectional model to reproduce measured soot particle size distributions (PSDs). The flames were modelled using a burner-stabilised stagnation flow configuration due to the influence of the sampling probe. In the vicinity of the probe, the residence time is relatively long and the temperature reduced. It was found that under such conditions the coagulation efficiency for small (<10nm) particles becomes important due to the combination of a relatively low nucleation rate with a significant collision frequency. In previous studies, the impact of the probe was only accounted for by shifting the position of the measured PSDs. A model for the collision efficiency was introduced with a particle size dependent interpolation between a lower limit, based on the Lennard–Jones potential, and an upper limit, based on collision stabilisation caused by the surrounding gas. The developed model is consistent with a low collision efficiency for PAH coagulation and explains the measured high number densities of small particles. The size of particles affected by a reduced collision efficiency were found to decrease with increasing flame temperature. The effect is consistent with temperature dependent internal carbonisation processes changing the polarisability of particles leading to a strengthening of attractive forces and enhanced collisional stabilisation. The calculated PSDs were found to be in reasonable agreement with measurements.

Performance of Particle Oxidation Catalyst and Particle Formation Studies with Sulphur Containing Fuels

The aim of this paper is to analyse the quantitative impact of fuel sulphur content on particulate oxidation catalyst (POC) functionality, focusing on soot emission reduction and the ability to regenerate. Studies were conducted on fuels containing three different levels of sulphur, covering the range of 6 to 340 parts per million, for a light-duty application. The data presented in this paper provide further insights into the specific issues associated with usage of a POC with fuels of higher sulphur content. A 48-hour loading phase was performed for each fuel, during which filter smoke number, temperature and back-pressure were all observed to vary depending on the fuel sulphur level. The Fuel Sulphur Content (FSC) affected also soot particle size distributions (particle number and size) so that with FSC 6 ppm the soot particle concentration was lower than with FSC 65 and 340, both upstream and downstream of the POC. Conversely, FSC did not have major effects on the soot particle number reduction efficiency of the POC. Soot and other exhaust compounds accumulated within the POC during this phase, gradually built a pressure drop across the POC. The final mass of collected matter in the POC differed significantly according to the sulphur content. The efficiency of removal of gaseous pollutants by the POC was found to be markedly worse for the fuels with higher sulphur content, although this deterioration was observed to be non-linear. Following the accumulation phase, a duty cycle was applied that caused the POC to commence passive regeneration. The time taken for the POC to cleanse itself of accumulated matter and thereby eliminate the pressure drop was observed to increase with increasing fuel sulphur content. The proportion of NOx leaving the POC in the form of NO2 was also found to vary as a strong function of fuel sulphur content.

Experimental study of hydrogen production and soot particulate matter emissions from methane rich-combustion in inert porous media

Abstract A detailed experimental study of stationary Thermal Partial Oxidation (TPOX) within inert porous media has been conducted. The reaction zone of the tested TPOX reformer is designed so as to enable stationary conversion of fuel/air mixtures for a wide range of operational conditions. Operating characteristics of the process have been examined for two different porous matrices, with different thermal and transport properties, namely SiSiC open foam structure and a packed bed of pure Al2O3 packing material in the form of cylindrical rings. The influence of reactants preheating was also examined since the reformer is meant for integration within high temperature fuel cell systems. The operating regime was scanned for reactants' inlet temperature of 400 °C and 550 °C, varying the thermal load in a range from 350 kW/m2 up to 2600 kW/m2 and the equivalence ratio from 1.9 up to 2.9. Temperature profiles within the reaction region of the reformer were recorded for all tested conditions while gas samples were on-line analyzed for the major species H2, CO, CO2, and minor species CH4, C2H2. At reactants' inlet temperatures of 400 °C and 600 °C, for a fixed thermal load of 1540 kW/m2 and for selected equivalence ratios around the sooting limit of the process (φ = 2.2–2.6), soot particle size distributions were measured in the exhaust gas with a Scanning Mobility Particle Sizer (SMPS). The results show that the better thermal properties and the higher porosity in the case of the SiSiC matrix enables longer residence times for slow reforming reactions to evolve towards equilibrium and yields syngas with significantly less soot in terms of particle numbers and mass concentration.

Monte Carlo simulation for soot dynamics

A new Monte Carlo method termed Comb-like frame Monte Carlo is developed to simulate the soot dynamics. Detailed stochastic error analysis is provided. Comb-like frame Monte Carlo is coupled with the gas phase solver Chemkin II to simulate soot formation in a 1-D premixed burner stabilized flame. The simulated soot number density, volume fraction, and particle size distribution all agree well with the measurement available in literature. The origin of the bimodal distribution of particle size distribution is revealed with quantitative proof.

Soot particle size and pollutant emissions characterization from a LD diesel engine equipped with high pressure and low pressure EGR system and operated with conventional and PCCI combustion

The paper analyses the soot Particle Size Distribution Function (PSDF) measurements carried out at the exhaust of a modern automotive diesel engine equipped with a low-pressure Exhaust Gas Recirculation (EGR) system and operating with conventional and Premixed Charge Compression Ignition (PCCI). The results have indicated significant benefits in terms of exhaust raw particle number emission with the adoption of low-pressure EGR system and PCCI calibration and, thanks to the peculiarity of the operating mode of Low Pressure EGR (LPEGR) system, have highlighted the complex link between in-cylinder charge composition and the PSDF characteristics.

Prediction of soot particle size distribution for turbulent reacting flow in a diesel engine

There is increasing interest in obtaining the size distribution as well as the number density and mass fraction of soot particles created by diesel engines. This study addresses this problem and particle size distributions are computed and validated in typical operating conditions of a diesel engine. The program KIVA is modified and integrated with CHEMKIN in order to be able to consider detailed gas phase kinetics and soot precursors using the conditional moment closure (CMC) model for turbulence/chemistry coupling. The soot simulator, SWEEP is then used to calculate the size distribution as a post-processing step at any given mixture fraction. The conditional results obtained at a mixture fraction of 0.12 are generally consistent with measurements over a range of engine operating conditions. However, the effect of differential diffusion in the probability density function weighted quantities affects the quality of the results obtained as a function of engine speed.

Constraints of two-colour TiRe-LII at elevated pressures

The main objective of this work is to investigate the influence of high-pressure conditions on the determination of primary particle size distributions of laser-heated soot particles using pyrometrically determined temperature decays. The method is based on time-resolved laser-induced incandescence measurements carried out at two different wavelengths (two-colour TiRe-LII). The LII signals are transferred into a particle ensemble averaged (effective) temperature using Planck’s thermal radiation formula. Assuming that all particles within the size distribution possess a unique temperature at the end of the laser pulse, the size distribution can be determined by numerically simulating the measured temperature decay. From our investigations, for pressures up to a few bars it is obvious that this strategy can be successfully applied if standard laser pulses of nano-second duration are used as an LII-excitation source. At higher pressures the time scales of heat conduction are decreased to such an extent that a unique temperature for all particles within the ensemble cannot be assumed at the end of the nano-second laser pulse. However, further investigations show that the presented two-colour TiRe-LII technique can be successfully adopted under technical high-pressure conditions as well, if the pulse duration of the TiRe-LII-excitation source is reduced into the pico-second range.

Effect of Engine Load on Size and Number Distribution of Particulate Matter Emitted from a Direct Injection Compression Ignition Engine

Particle size and number distribution from an engine tailpipe has a direct bearing on the residence time of the particles in the atmosphere and their toxicity. This study presents the number concentration and size distributions of nano-particles emitted from naturally aspirated, water cooled, single cylinder, diesel fuelled direct injection compression ignition (DICI) engine. The engine exhaust particle sizer (EEPS) was used for measurement of number, surface area and mass distributions of soot particles. It measures particle sizes ranging from 5.6 to 560 nm. Reading the size distribution 10 times per second allows for the measurement of transient emissions of soot particles. The experiments were conducted at a constant engine speed (1500 rpm) with varying engine load. It was found that (a) number and size distribution, (b) surface area and size distribution, and (c) mass and size distribution of soot particles varies significantly with the engine load. The width of the emitted particle size distribution increases with increasing engine load.

Exploring the Role of PAHs in the Formation of Soot: Pyrene Dimerization

A critical step in currently accepted models for soot formation in combustion is the dimerization of polycyclic aromatic hydrocarbons as small as pyrene, which is necessary within these models to reproduce correctly the soot particle size distribution. We present experimental measurements on the kinetics of pyrene dimerization performed in low-temperature supersonic flows with photoionization mass spectrometric detection, coupled with theoretical results based on careful consideration of the intermolecular interaction energies, binding energy, equilibrium constant, and intermolecular dynamics. These results demonstrate that the equilibrium of the reaction strongly favors the dissociation of the pyrene dimer at high temperature and that physical dimerization (involving van der Waals forces) of pyrene cannot be a key step in carbon particle formation in hot environments such as flames and circumstellar shells.

Detailed Investigation on Soot Particle Size Distribution during DPF Regeneration, using Standard and Bio-Diesel Fuels

The particle number and size distribution are important aspects to qualify diesel engine emissions, considering that new limits, in term of particle number, are expected for Euro 6 regulations. In this scenario it is important to study particulate matter (PM) emissions, not only during engine normal operating mode, but also during diesel particulate filter (DPF) regeneration processes. The aim of this work is therefore to analyze PM emissions throughout the whole exhaust system of a small displacement Euro 5 common rail automotive diesel engine, during both normal operating conditions and DPF regeneration mode. Because the test engine was equipped with a close-coupled after-treatment system, featuring a Diesel Oxidation Catalyst (DOC) and a DPF integrated in a single canning, the exhaust gas was sampled at the engine outlet, at the DOC outlet, and at the DPF outlet to fully characterize the PM emissions throughout the exhaust line. After a two-stage dilution, the sampled gas was analyzed by means of a TSI 3080 SMPS, in the range 6 to 225 nm range. The particle number and size distribution were evaluated at part load, both under normal operating conditions and at DPF regeneration mode, to highlight the impact of the different combustion processes on the PM characteristics. Finally, the particle number and size distribution at the engine outlet were also evaluated while fueling the engine with neat fatty acid methyl ester (FAME) to evaluate the impact of alternative fuels on PM characteristics during normal operating conditions. The results have shown that, under normal operating conditions, the engine and DOC outlet particle number and mass size distributions appear to be very similar, while the DPF exhibits high values of filtration efficiency on a particle number basis, even in the nanoparticle range. Regeneration mode caused a particle number increase of 1 order of magnitude, with a substantial shift of the number distribution peaks toward larger diameters. The particle number across the DOC showed a remarkable reduction for particles larger than 40 nm, with a reduction of 1 order of magnitude in their concentrations. Finally, fueling the engine with FAME leads to remarkable reductions in terms of particle number and mass (up to 80% and 90%, respectively) under normal operating mode conditions.

On the influence of bimodal size distributions in particle sizing using laser-induced incandescence

Time-resolved laser-induced incandescence (LII) is a technique for in-situ soot particle size distribution measurements with the limitation that the distribution function has to be assumed (often monodisperse or lognormal) in the signal analysis. Since it is established that size distributions sometimes are bimodal, it is of interest to understand the influence from such distributions on the LII signal, and consequently what information can be extracted from experimental LII signals. The influence of the parameters of the bimodal lognormal distribution on the evaluation using a monodisperse or lognormal distribution was investigated. A range of bimodal distributions were tested and it was found that a unimodal evaluation of a signal from a bimodal distribution is only slightly affected by its small-size mode, meaning that LII can be used to determine the approximate parameters of the large-size mode.

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...