Soot Particle Surface Growth Research Articles

Near-threshold soot formation in premixed flames at elevated pressure

Soot formation at lean-threshold conditions referred to as “near-threshold sooting conditions” (i.e., with stoichiometry, φ, around 1.90 for ethene as a fuel) are studied in laminar premixed ethylene/air flames at pressure from 1 to 10 bar. Laser extinction is used to measure the soot volume fraction. Time-resolved laser-induced incandescence (TiRe-LII) is used to determine particle diameters from the LII signal temporal decay after pulsed laser heating. Thermophoretic sampling is applied to extract particle samples from the flame and ex situ transmission electron microscopy (TEM) is used to measure particle sizes and morphology. The soot volume fraction scales with pressure in a power-law function with the parameter n as 1.4 to 1.9 for flames at the equivalence ratio (φ = 2.1) even at the onset of soot formation. The elevated dependence of soot volume fraction on height above burner is detected with increasing pressure in the near-threshold sooting conditions. The measured soot diameter increases with pressure and equivalence ratio and its sensitivity to the equivalence ratio increases with increasing pressure. The TiRe-LII signal decay varies only little with height above burner and laser fluence in the near-threshold sooting flame (φ = 1.90–1.95), which indicates that the soot particle surface growth and oxidation are balanced. For a slightly sooting flame, TEM measurements from thermophoretically-sampled soot agree well with the LIIsim-evaluated particle size, indicating the reliability of TiRe-LII particle diameter determination under near-threshold conditions.

Effects of Acetylene Addition to the Fuel Stream on Soot Formation and Flame Properties in an Axisymmetric Laminar Coflow Ethylene/Air Diffusion Flame.

The effects of adding acetylene to the fuel stream on soot formation and flame properties were investigated numerically in a laminar axisymmetric coflow ethylene/air diffusion flame using the open-source flame code Co-Flame in conjunction with an elementary gas-phase chemistry scheme and detailed transport and thermodynamic database. Radiation heat transfer of the radiating gases (H2O, C2H2, CO, and CO2) and soot was calculated using a statistical narrow-band correlated-k-based wide band model coupled with the discrete-ordinates method. The soot formation was described by the consecutive steps of soot nucleation, surface growth of soot particles via polycyclic aromatic hydrocarbons (PAHs)-soot condensation or the hydrogen abstraction acetylene addition (HACA) mechanism, and soot oxidation. The added acetylene affected the flame structure and soot concentration through not only chemical reactions among different species but also radiation effects. The chemical effect due to the added acetylene had a significant impact on soot formation. Specifically, it was confirmed that the addition of 10% acetylene caused an increase in the peak soot volumetric fraction (SVF) by 14.9% and the peak particle number density by about 21.1% (z = 1.5 cm). Furthermore, increasing acetylene concentration led to higher concentrations of propargyl, benzene, and PAHs and consequently directly enhanced soot nucleation rates. In addition, the increased H mole fractions also accentuated the soot surface growth. In contrast, the radiation effect of the addition of 10% acetylene was much weaker, resulting in slightly lower flame temperature and SVF, which in turn reduced the radiant heat loss.

Impact of the competition between aggregation and surface growth on the morphology of soot particles formed in an ethylene laminar premixed flame

Soot particle surface growth may have a significant impact on aggregation kinetics, particle morphology, and both aggregate and monomer size distributions. Particularly, the morphological effects of surface growth are currently poorly understood. This is complex since surface growth also affects the kinetics of aggregation resulting in a time-dependent competition between both phenomena, producing very different morphological markers. The present study aims at improving our understanding of the morphological impact of surface growth by implementing this effect in a Monte Carlo discrete element code for the simulation of soot formation in an ethylene premixed flame. An asymptotic average primary particle overlapping ∼30% is observed for different flame conditions. The primary particle coordination number is highly sensitive to surface growth rate and particle volume fraction, and maximum values are on average within the 4 - 8 range. The particle local compacity, as quantified by the packing factor, is also considerably increased when aggregates experience surface growth, contrarily to the fractal structure of individual aggregates that seems more dependent on the aggregation regime. Finally, surface growth narrows both the primary particle and aggregate size distributions.

Enhancement of radiation characteristics and reduction of NOx emission in natural gas flame through silver-water nanofluid injection

The aim of the present work is to investigate the effect of low-flow-rate injection of silver-water nanofluid on the enhancement of thermal and radiative characteristics and reduction of NOx emission in natural gas flame. In our method, 0.009 Lit/min of 100-ppm silver-water nanofluid is directly injected into the natural gas flame at the equivalence ratio of 0.65. An SBG01 water cooled heat flux sensor is used to measure the total radiation of flame and IR photography method and luminosity measurement are used to determine the spectral emissivity of flame in infrared and visible wavelengths. Furthermore, a KIGAS 310 gas analyzer measures the concentrations of pollutant emissions. The results indicate that nanofluid injection enhances the IR radiation of natural gas flame and creates a high-value and uniform radiative heat flux distribution for flame. Solid nanoparticles, as foreign surfaces, enhance the nucleation rate and surface growth of soot particles, which are the best radiative species in flame. The other parameters which affect the flame radiation are increase in H2O concentration, as another radiative species, and increased number of tiny high temperature dark nanoparticles, as highly emissive bodies. Furthermore, although the nanofluid injection does not significantly change the concentration of carbonaceous pollutant emission, it decreases the NOx emission as much as 22.6%. The reduction of NOx emission is caused by decrease in the maximum flame temperature, due to absorption of heat by nanofluid droplets, and increase in the concentration of OH radicals, due to dissociation of H2O at the high temperatures of flame reaction zone.

A Modeling Study on the Nucleation, Growth and Coagulation of Soot Particles in Combustion Flame

Soot, a crucial component of PM2.5 in the ambient air, is a prime product of incomplete combustion of fule. The present paper uses a series of models, including nucleation of soot precursors, surface growth of soot particles as well as particles coagulation, to do numerical simulation of soot particles formation mechanism over time in combustion flame, with the aid of MATLAB platform by C language. Simulation results indicate that the concentration distribution of soot precursors like polycyclic aromatic hydrocarbons (PAHs) have direct effect on the concentration distribution of soot nuclei. Mass addition rate on soot particles surface rises over time since the enlargement of the surface area. In coagulation process, the number of small diameter particles decreases, while that of large diameter particles increases.

Effects of fuel droplet size on soot formation in spray flames formed in a laminar counterflow

The effects of fuel droplet size on soot formation in spray flames formed in a laminar counterflow are investigated experimentally and numerically. Sauter mean diameter (SMD) of quasi-monodispersed fuel spray ( n-decane) is carefully controlled independently from the other spray characteristics using a frequency-tunable vibratory orifice atomizer, and the two-dimensional spatial distributions of soot volume fraction and soot particle size are measured by laser induced incandescence (LII) and time resolved LII (TIRE-LII), respectively. In addition, the soot formation processes are examined in detail by a two-dimensional direct numerical simulation (DNS) employing a kinetically based soot model with flamelet model. The results show that the soot formation area and location are strongly affected by the SMD of the fuel spray. As the SMD of the fuel spray increases, the average soot formation area expands, whereas local suppression of soot formation is instantaneously observed in the spray flames because of the appearance of groups of unburned droplets. The size of soot particles tend to be larger in the outer part of the soot formation area compared to soot in the inner part. This is because the surface growth of soot particles markedly proceeds compared to the coagulation and oxidation.

Experimental study of ethylene counterflow diffusion flames perturbed by trace amounts of jet fuel and jet fuel surrogates under incipiently sooting conditions

The structure of an ethylene counterflow diffusion flame doped with 2000 ppm on a molar basis of either jet fuel or two jet fuel surrogates is studied under incipient sooting conditions. The doped flames have identical stoichiometric mixture fractions ( z f = 0.18) and strain rates ( a = 92 s −1), resulting in a well-defined and fixed temperature/time history for all of the flames. Gas samples are extracted from the flame with quartz microprobes for subsequent GC/MS analysis. Profiles of critical fuel decomposition products and soot precursors, such as benzene and toluene, are compared. The data for C7–C12 alkanes are consistent with typical decomposition of large alkanes with both surrogates showing good qualitative agreement with jet fuel in their pyrolysis trends. Olefins are formed as the fuel alkanes decompose, with agreement between the surrogates and jet fuel that improves for small alkenes, probably because of an increase in kinetic pathways which makes the specifics of the alkane structure less important. Good agreement between jet fuel and the surrogates is found with respect to critical soot precursors such as benzene and toluene. Although the six-component Utah/Yale surrogate performs better than the Aachen surrogate, the latter performs adequately and retains the advantage of simplicity, since it consists of only two components. The acetylene profiles present a unique multimodal behavior that can be attributed to acetylene’s participation in early stages of formation of soot precursors, such as benzene and other large pyrolysis products, as well as in the surface growth of soot particles.

Study on Soot Formation in Spray Flames (Effect of Droplet Size Distribution)

Effect of the droplet size distribution of fuel spray on the soot formation process in spray flames is investigated. The vibratory orifice atomizer is used to control the droplet size distribution of fuel spray. Two-dimensional soot volume fraction and primary soot particle size are measured by the Time-Resolved Laser Induced Incandescence (Time-Resolved LII) method and direct numerical simulation. The relationship between spatial distribution of fuel droplets and soot formation area are observed by the LII method and Interferometric Laser Imaging Droplet Sizing (ILIDS) method. The results show that flame structures of spray flame and soot formation process are affected critically by the mean droplet size of fuel spray. The mean soot formation area is increased by increase of the mean droplet size of fuel spray. The primary soot particles become large at around the edge of soot formation area. This trend mainly occurs from surface growth of soot particles. The group of some droplets and its vapor forms the soot formation area, and the large droplet of liquid fuel deforms shape of the soot formation area.

1839 対向流噴霧火炎中の燃料噴霧特性とすす生成に関する研究(J15-2 噴霧による混合気形成および燃焼(2),J15 噴霧による混合気形成および燃焼)

Effect of the droplet size distribution of fuel spray on the soot formation process in spray flames is investigated. The vibratory orifice atomizer is used to control the droplet size distribution of fuel spray. Two-dimensional soot volume fraction, primary soot particle size and relationship between spatial distribution of fuel droplets and soot formation area are observed by optical measurements. The results show that flame structures of spray flame and soot formation process are affected critically by the mean droplet size of fuel spray. The mean soot formation area is increased by increase of the mean droplet size of fuel spray. The primary soot particles become large at around the edge of soot formation area. This trend occurs from coagulation and surface growth of soot particles. The group of some droplets and its vapor forms the soot formation area, and the large droplet of liquid fuel deforms shape of the soot formation area.

Hydrogen migration in polyaromatic growth

New reaction pathways for polyaromatic growth in combustion environments are explored theoretically. The analysis is based on semiempirical quantum calculations of potential energy barriers and vibrational frequencies, followed by standard transition state theory evaluation of reaction rates. The reaction systems considered include formation of five-and six-member aromatic rings, their interconversion, and migration of cyclopenta rings along zigzag edges of aromatic surfaces. All reactions have a distinctive mechanistic feature: the reaction pathways are induced or assisted by hydrogen atom migration. The present calculations show that such migration is rapid and leads to a faster surface growth than reaction pathways previously considered. The results extend the mechanism for the involvement of five-member aromatic rings in surface growth of soot particles. The migrating cyclopenta rings propagate the growth as a continuous replicating front. This is achieved by extending a six-member-ring step upon encounter or converting to a six-member ring at an open end, thus providing a nucleus for the next aromatic layer. Collisions of the propagating fronts may be responsible for creation of sites that cannot be filled by cyclization and thus cannot support further growth. Formation of such surface defects may be responsible for the loss of reactivity of soot particle surfaces to growth, consistent with a prior suggestion.

On surface growth mechanism of soot particles

A sterically resolved kinetic Monte Carlo technique was applied to model surface growth of soot particles under conditions typical of flame environments. The employed elementary surface reaction mechanism was based on analogous reactions of gaseous aromatic species. The simulations indicated that the growth rate of soot particle surface increases rather than decreases with temperature, even when the reaction reversibility is fully accounted for. This implies that the decreasing thermal stability of surface intermediates is unlikely to be responsible for the decline in surface reactivity of soot particles. In contrast, the growth of gaseous aromatics by the same reaction mechanism is impeded by the reversibility of cyclization reactions and was shown to result inzigzag edges, inconsistent with the armchair substrate assumed for the particle surface growth. The inconsistency was resolved by invoking the formation of five-member rings that serve as surface nuclei for the subsequent growth. The computational results also showed that the surface growth rate is proportional to the C 2 H 2 concentration and independent of H at high H concentrations but declines with the decrease in the abundance of hydrogen atoms. This concurs with the previous analysis identifying the decreasing H concentration as one of the primary causes for the decay in the surface reactivity of soot particles.

Surface Growth of Soot Particles in Premixed Ethylene/Air Flames

Abstract The surface growth rate of soot increases steeply with increasing equivalence ratio. We have found that the increased growth rate is accounted for primarily by the increased surface area available for growth rather than by increased concentrations of surface growth species. We have shown that the surface growth species are not depleted by the process of surface growth. Our data suggest that the surface growth seen in our flames can be accounted for primarily by reaction of acetylene with the soot particles.

Optical studies of soot particle growth in hydrocarbon oxygen flames

Number densities, particle sizes and flame temperatures were measured in flat, premixed, laminar hydrocarbon oxygen flames at low pressure. Methods used were light scattering, extinction and sodium-D-line reversal. Soot particles in sooting hydrocarbon-oxygen flames are comparable to particles of a coagulating aerosol. This is indicated by testing the long time particle dynamics for particle number densities in the measured flames and by solving numerically the particle balance using a model based on free molecular coagulation which comprises surface growth and particle inception. The computations yield particle size distributions tending to a constant relative standard deviation after short reaction times though particle inception and surface growth should counteract the broadening of the size distributions. Changes of soot loading appear as changes of the soot mass increase by surface reactions because about 90% of the soot is built by surface reactions. Surface growth of soot particles does not follow simple pyrolytic mechanism involving thecomponents of a hydrocarbon pool. This can be concluded from the concentration profiles of those components in the soot forming region of the flames and the surface growth rates. Particle number densities and particle sizes are governed by particle inception over a small interval at incipient soot formation meaning that once solid particles are formed the intermediates of the hydrocarbon pool preferably react with the surfaces of the soot particles than with themselves to form larger intermediates or new particles.