Incipient Soot Particles Research Articles

The effect of sodium chloride on the charge state of soot particles in a laminar diffusion flame

This study investigates the effect of sodium chloride on the charge state of soot nanoparticles formed in a laminar diffusion flame by measuring the soot particle size distribution, average charge per particle, and charge fraction at various heights within the flame. The well-studied Santoro burner with methane as the fuel (at 0.35 L/min) and co-flow air (at 70 L/min) was used, which produced a stable laminar diffusion flame with flame height of 61 mm. Samples of soot nanoparticles were extracted via a 0.3 mm orifice in a 3-mm stainless steel tubular probe at various heights above the burner and were immediately diluted by a factor of a few thousand for aerosol measurement. The effect of sodium chloride on a methane flame was investigated by comparing the experimentally measured data for methane-only and methane-NaCl flames. The addition of NaCl particles to the laminar diffusion flame did not have a significant effect on the particles in the nucleation region of the flame. The majority of the incipient soot particles in both flames are uncharged, and their sizes are nearly the same, with diameters of approximately 5 nm or less. However, the size of soot particles differs by approximately 10 % to 25 % between methane-only and methane-NaCl flames in the coagulation-dominated region of the flame. The net charge on soot particles within the coagulation region of the methane-only flames is negative, while it is positive with NaCl addition. The fraction of charged particles and ion concentration decreases with NaCl addition within the coagulation region. The study indicates that the smaller particle size observed in methane-NaCl flames may be attributed to reduced coagulation via altered particle charge states. These factors could be the major contributor to the variations in soot formation between methane-only and methane-NaCl flames. Novelty and Significance StatementThis research addresses a notable knowledge gap concerning the influence of NaCl, a prevalent component of hydraulic fracturing fluids, on soot particle behaviour in flames. By explaining the impact of NaCl on soot formation and charge states, the findings contribute to a deeper understanding of combustion processes in environments influenced by hydraulic fracturing operations, thereby informing strategies for reducing emissions and improving environmental sustainability in energy production. Overall, this article represents a significant advancement in the field of combustion science, with implications for environmental stewardship in energy production.This study provides valuable insights into how the introduction of NaCl alters the electrostatic properties of soot particles. This investigation expands upon existing knowledge by shedding light on the intricate mechanisms underlying soot formation and evolution in the presence of NaCl, elucidating its potential implications for combustion processes involving fossil fuels and industrial flaring.

Physical, chemical and morphological evolution of incipient soot obtained from molecular dynamics simulation of acetylene pyrolysis

Incipient soot particles obtained from a series of reactive molecular dynamics simulations were studied to understand the evolution of physical, chemical, and morphological properties of incipient soot. Reactive molecular dynamics simulations of acetylene pyrolysis were performed using ReaxFF potential at 1350, 1500, 1650, and 1800 K. A total of 3324 incipient soot particles were extracted from the simulations at various stages of development. Features such as the number of carbon and hydrogen atoms, number of ring structures, mass, C/H ratio, radius of gyration, surface area, volume, atomic fractal dimension, and density were calculated for each particle. The calculated values of density and C/H ratio matched well with experimental values reported in the literature. Based on the calculated features, the particles were classified in two types: type 1 and type 2 particles. It was found that type 1 particles show significant morphological evolution while type 2 particles undergo chemical restructuring without any significant morphological change. The particle volume was found to be well-correlated with the number of carbon atoms in both type 1 and type 2 particles, whereas surface area was found to be correlated with the number of carbon atoms only for type 1 particles. A correlation matrix comparing the level of correlation between any two features for both type 1 and type 2 particles was created. Finally, based on the calculated statistics, a set of correlations among various physical and morphological parameters of incipient soot was proposed.

Internal Structure of Incipient Soot from Acetylene Pyrolysis Obtained via Molecular Dynamics Simulations.

A series of reactive molecular dynamics simulations is used to study the internal structure of incipient soot particles obtained from acetylene pyrolysis. The simulations were performed using the ReaxFF potential at four different temperatures. The resulting soot particles are cataloged and analyzed to obtain statistics of their mass, volume, density, C/H ratio, number of cyclic structures, and other features. A total of 3324 incipient soot particles were analyzed in this study. Based on their structural characteristics, the incipient soot particles are classified into two classes, termed type 1 and type 2 incipient soot particles in this work. The radial distribution of density, cyclic (5-, 6-, or 7-member rings) structures, and C/H ratio inside the particles revealed a clear difference in the internal structure between type 1 and type 2 particles. These classes were further found to be well represented by the size of the particles, with smaller particles in type 1 and larger particles in type 2. The radial distributions of ring structures, density, and the C/H ratio indicated the presence of a dense core region in type 2 particles. In contrast, no clear evidence of the presence of a core was found in type 1 particles. In type 2 incipient soot particles, the boundary between the core and shell was found to be around 50-60% of the particle's radius of gyration.

Importance of resonance-stabilized radicals in soot formation mechanism of diphenyl ether pyrolysis

Soot formation during fuel pyrolysis can pose environmental and human health hazards. As a typical coal-related model compound, diphenyl ether (DPE) represents the specific structures in coal. In this study, soot formation mechanism of DPE pyrolysis was systematically investigated from theoretical principle. The molecular dynamics (MD) simulations show that the variety and number of DPE pyrolysis products increase with the raising temperature. In addition, the higher temperature also promotes the soot formation. Quantum mechanics (QM) calculations indicate that the energy barrier of CO removal from the phenoxy radical is lower than that of the phenyl isomerization reaction, leading to the easier depletion of the phenoxy radical. It is consistent with the result observed from the MD simulation that the phenoxy is consumed more rapidly than phenyl. The main pathways for initial ring formation are the isomerization of polyyne-like chains and the decomposition of DPE molecules which form the ring molecules of C6H5 and C5H5. C2H2 molecule and resonance-stabilized radical (RSR) species like C3H3, C6H5 and C5H5 are precursors for the growth of polycyclic aromatic hydrocarbons (PAHs), which have an important role in the growth process of soot. Subsequently, larger PAHs aggregate with each other to form incipient soot particles which will undergo surface growth and graphitization processes. We hope this work can provide worthy insights into the mechanism of soot formation in DPE and the suppression of carbon soot.

Study on the nano-morphology and chemical characteristics of soot produced by the combustion of kerosene mixed with ethanol

In this paper, we independently designed and built an experimental platform, including a combustion system and a soot particles collection system. Soot generated inside the flame and soot emitted outside the flame were collected during the combustion of kerosene and kerosene mixed with ethanol fuel. Transmission electron microscopy, X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, Raman spectroscopy, and thermogravimetry were used to characterize both soot samples. The results show that the addition of ethanol to kerosene, i.e., the formation of mixed fuel, causes the lag of soot emission. With the increase in ethanol addition, the incipient soot particles were generated inside the flame, and small, unoxidized, and heat-aging soot particles were emitted outside the flame. The oxidation activity of soot particles produced by adding ethanol decreased, which was related to their particle size.

Effects of Iron Addition on the Collision of Polycyclic Aromatic Hydrocarbon Clusters: A Molecular Dynamics Study

In this work, soot particle size distributions in iron-doped premixed ethylene flames are examined using scanning mobility particle sizer measurements. It is found that iron addition promotes the growth in soot particle size, and the enhanced particle coagulation is inferred to be an important reason. To support that, the influence of iron addition on the coagulation of polycyclic aromatic hydrocarbon (PAH) clusters, the analogue of incipient soot particles, is further investigated using molecular dynamics simulations. Based on the results of hundreds of binary head-on collision simulations, the collision between two coronene-Fe-coronene dimers is found to have a significantly higher coagulation efficiency than that between two coronene dimers. However, this enhancement effect weakens as the size of the PAH monomer increases. Although the coagulation efficiency can be increased by iron addition, the collision frequency is almost unaffected, as revealed from the binary off-central collision simulations. Moreover, the simulation results of coronene cluster growth via coagulation show that iron addition promotes coronene cluster growth, leading to larger cluster size, which may explain the larger soot particle size observed in iron-doped flames.

Morphology and electronic properties of incipient soot by scanning tunneling microscopy and spectroscopy

Soot nucleation is one of the most complex and debated steps of the soot formation process in combustion. In this work, we used scanning tunneling microscopy (STM) and spectroscopy (STS) to probe morphological and electronic properties of incipient soot particles formed right behind the flame front of a lightly sooting laminar premixed flame of ethylene and air. Particles were thermophoretically sampled on an atomically flat gold film on a mica substrate. High-resolution STM images of incipient soot particles were obtained for the first time showing the morphology of sub-5 nm incipient soot particles. High-resolution single-particle spectroscopic properties were measured confirming the semiconductor behavior of incipient soot particles with an electronic band gap ranging from 1.5 to 2 eV, consistent with earlier optical and spectroscopic observations.

Effects of Magnetic Fields on Morphology and Nanostructure Evolution of Incipient Soot Particles from n-heptane/2,5-dimethylfuran Inverse Diffusion Flames

Differences of the morphology and nanostructure evolution of incipient soot particles generated in n-heptane/2,5-dimethylfuran (DMF) inverse diffusion flames (IDFs) with/without magnetic fields were investigated. Utilizing a high resolution transmission electron spectroscopy, the morphology and nanostructures of soot sampled from spatial locations at different heights in IDFs were analyzed. The graphitization and the oxidation reactivity of soot were tested by an X-ray diffraction and a thermogravimetric analyzer, respectively. Results demonstrated that the magnetic force on paramagnetic species, such as oxygen molecules, can modify the soot formation and oxidation. More incipient soot particles with larger diameters appeared in chains or branches or tufted forms on the flame wing region and the higher position than that on the flame centerline region and the lower position. With magnetic fields, greater amounts of clustered soot particles displayed more crowded distribution and larger diameters. Soot particles with typical structures of the core-shell were promoted to own more orderly bordered lamellae with longer fringe length and smaller fringe tortuosity by the magnetic force acting on oxygen at the same sample position. These modifications resulted in relatively larger diffraction angle of the peak, higher graphitization degree and slightly lower oxidation reactivity of soot.

Soot formation and oxidation mechanisms in a diesel engine separated swirl combustion system

The separated swirl combustion system (SSCS) is a new combustion system developed to improve fuel efficiency and reduce soot emissions in diesel engines. While soot emissions from the SSCS can be tested in a real-world single-cylinder engine, soot formation and oxidation cannot be; therefore, the mechanisms behind those processes have not yet been researched. To understand the mechanisms of soot formation and oxidation in the SSCS, soot evolution must be investigated using a simulation model. To bridge this gap, this study analyzed the combustion and emission performance of the SSCS under different speeds in a real-world single-cylinder engine. Then, a new phenomenological model was developed and used to simulate soot formation and oxidation and reveal the mechanisms behind those processes. The SSCS with the new model was validated against a double swirl combustion system (DSCS). The experiment results show that at 2100 r/min, the SSCS significantly reduced fuel consumption (about 6.54 g/(kW h)) and soot emission (0.17 FSN) by 2.89% and 6.31%, respectively. The simulation results show that the DSCS generates more incipient soot particles and soot mass than the SSCS, and surface soot oxidation is faster in the SSCS. The mechanisms analysis shows that the equivalence ratio is smaller in the SSCS than in the DSCS, so fewer incipient soot particles are generated, and the temperature is higher, which speeds up soot oxidation. Furthermore, the combustion duration is much shorter in the SSCS than in the DSCS, which means that the time between the end of combustion and the exhaust valve opening is longer, granting more time for soot oxidation. Due to the decreased soot formation and faster soot oxidation, the soot mass is also lower in the SSCS than in the DSCS.

Evaluation of the effect of nickel clusters on the formation of incipient soot particles from polycyclic aromatic hydrocarbons via ReaxFF molecular dynamics simulations.

In the present study, the ReaxFF reactive molecular dynamics simulation method was applied to investigate the effect of a small nickel cluster (Ni13) on the formation of nascent soot from polycyclic aromatic hydrocarbon (PAH) precursors. A series of NVT simulations was performed for systems of a Ni13 cluster and various PAH monomers, namely, naphthalene, anthracene, pyrene, coronene, ovalene, and circumcoronene, at temperatures from 400 to 2500 K. At low temperatures, the PAHs form soot particles via binding and stacking around nickel clusters. Larger soot particles are formed due to the early initiation of clustering provided by nickel compared to those observed in homogenous PAH systems. At 1200-1600 K, the PAH monomers show a chemisorption tendency onto the nickel surface, which results in incipient soot particles. Chemical nucleation was observed at 2000 K where nickel-assisted dehydrogenation and chemisorption of PAH led to the growth of stable soot particles, which did not occur in the absence of Ni-clusters. At a high temperature (2500 K), nickel significantly accelerates the ring-opening and graphitization of PAH molecules and increases the size of the fullerene-type soot as compared to that of homogenous systems.

Insights into incipient soot formation by atomic force microscopy

Combustion-generated soot particles can have significant impact on climate, environment and human health. Thus, understanding the processes governing the formation of soot particles in combustion is a topic of ongoing research. In this study, high-resolution atomic force microscopy (AFM) was used for direct imaging of the building blocks forming the particles in the early stages of soot formation. Incipient soot particles were collected right after the particle nucleation zone of a slightly sooting ethylene/air laminar premixed flame at atmospheric pressure and analyzed by AFM after a rapid sublimation procedure. Our data shed light on one of the most complex and still debated aspect on soot formation, i.e., the nucleation process. The molecular constituents of the initial particles have been individually analyzed in detail in their chemical/structural characteristics. Our data demonstrate the large complexity/variety of the aromatic compounds which are the building blocks of the initial soot particles. Nevertheless, some fundamental and specific characteristics have been clearly ascertained. These include a significant presence of penta-rings as opposed to the purely benzenoid aromatic compounds and the noticeable presence of aliphatic side-chains. In addition, there were indications for the presence of persistent π radicals. Incipient soot was also investigated by Raman spectroscopy, the results of which agreed in terms of chemical and structural composition of the particles with those obtained by AFM.

Morphological Change and Number-Size Distributions of Particulate Matter (PM) from a Diesel Generator Operated with Wood Pyrolysis Oil-Butanol Blended Fuel

This report details our experimental study investigating particulate matter (PM) emissions from a diesel generator fueled with wood pyrolysis oil (WPO)–butanol blended fuel for electricity generation. Particle number-size distributions and PM mass concentrations from diesel, n-butanol, and WPO-butanol blended fuels were investigated via aerosol measurements using a fast mobility particle sizer and an aerosol monitor with three generator outputs (0, 3.3, and 6.6 kWe). For the n-butanol and WPO-blended fuels, the total number concentrations of exhaust particles were higher than that of conventional diesel combustion; however, the PM mass was observed to be nearly zero for all the engine operating conditions due to the higher number concentration in the nuclei mode. The morphology of the exhaust particles was investigated by analyzing transmission electron microscopy (TEM) micrographs. The morphology of the particles was drastically changed according to the test fuels and engine loads. Two types of particles were observed, including soot and coke shaped particles. These results were directly related to the immaturity of incipient soot particles due to the different physical properties and chemical compositions of the fuels.

Formation of incipient soot particles from polycyclic aromatic hydrocarbons: A ReaxFF molecular dynamics study

In this study, we present the results from a series of ReaxFF molecular dynamics (MD) simulations to uncover the underlying mechanisms behind the nucleation and growth of incipient soot particles from polycyclic aromatic hydrocarbons (PAHs). PAHs, namely, naphthalene, anthracene, pyrene, coronene, ovalene and circumcoronene, are selected for ReaxFF MD simulations over a range of temperatures from 400 to 2500 K. Distinctive mechanisms of incipient soot formation are identified with respect to PAH mass and temperature. At low temperatures (e.g., 400 K), all types of the above PAHs can nucleate into incipient soot particles in stacked structures due to physical interactions. With the increase of temperature, the possibility of physical nucleation decreases for each PAH. At moderate temperatures (e.g., 1600 K), it becomes difficult for these PAH monomers, except circumcoronene grows into incipient soot particles. When the temperature increases to 2500 K, all the PAHs become chemically active, which not only leads to the formation of incipient soot particles but also takes the graphitization with the increase of the carbon-to-hydrogen (C/H) ratios in the particles. In addition to the formation of fullerene-like soot particles, stacked particles connected by ‘carbon bridges’ are also observed for large PAHs like coronene, ovalene and circumcoronene.

Investigation of the size of the incandescent incipient soot particles in premixed sooting and nucleation flames of n-butane using LII, HIM, and 1 nm-SMPS

ABSTRACTLaser-induced incandescence (LII) measurements were conducted to explore the ability of LII to detect small soot particles of less than 10 nm in two sooting flat premixed flames of n-butane: a so-called nucleation flame obtained at a threshold equivalence ratio Φ = 1.75, in which the incipient soot particles undergo only minor soot surface growth along the flame, and a more sooting flame at Φ = 1.95. Size measurements were obtained by modeling the time-resolved LII signals detected using 1064 nm laser excitation. Spectrally-resolved LII signals collected in the nucleation flame display a similar blackbody-like behavior as mature soot. Soot particle temperature was determined from spectrally-resolved detection. LII modeling was conducted using parameters either relevant to those of mature soot or derived from fitting the modeled results to the experimental LII data. Particle size measurements were also carried out using (1) ex situ analysis by helium-ion microscopy (HIM) of particles sampled thermo...

Nascent soot particle size distributions down to 1 nm from a laminar premixed burner-stabilized stagnation ethylene flame

In this study, spatially resolved measurement of soot particle size distribution functions (PSDFs) down to ∼1nm from a laminar premixed burner-stabilized stagnation ethylene flame was made by paralleling a commercial 3936 Scanning Mobility Particle Spectrometer (3936 SMPS) and a Diethylene Glycol (DEG) SMPS. While the 3936 SMPS may detect particles with a mobility diameter of 3–150nm, DEG SMPS can be used to measure incipient soot particles of 1–10nm. We found that the minimum diameter of the incipient soot particles appeared at ∼1.5nm (though with some uncertainty caused by the classification device). A complete bimodality of the PSDFs was observed quantitatively when the burner-to-stagnation surface separation distance (Hp) was greater than 0.6cm. Characterized by a lognormal distribution, the first peak appears to be relatively stable at different Hp, with the geometric standard deviation varying from 1.1 to 1.3 and the peak diameter ranging from 1.9 to 2.9nm. The absolute number density of particles no bigger than the first peak diameter was found to be positively related to the first peak diameter and the geometric mean diameter of these particles.

Radical–radical reactions, pyrene nucleation, and incipient soot formation in combustion

We present a combined experimental and probabilistic simulation study of soot-precursor. The experiments were conducted using aerosol mass spectrometry coupled with tunable vacuum ultraviolet radiation from the Advanced Light Source at Lawrence Berkeley National Laboratory. Mass spectra and photoionization efficiency (PIE) curves of soot precursor species were measured at different heights in a premixed flat flame and in a counter-flow diffusion flame fueled by ethylene and oxygen. The PIE curves at the pyrene mass from these flames were compared with reference PIE scans recorded for pyrene. The results demonstrate that other C16H10 isomers than pyrene are major components among species condensed onto incipient soot in this study, which is in agreement with the simulations. Species with mass 202u only have a high prevalence in incipient soot particles drawn from the premixed flame, but hydrocarbon species with sizes in the range 200–400u are important to incipient-soot formation in both flames. The simulations predict that some species form through combination reactions involving relatively large radicals and bypass traditional molecular-growth pathways through addition of small hydrocarbon species. The experimental results support this prediction; they demonstrate that these species have higher relative abundances in particles formed close to the fuel outlet than smaller, lighter molecular species and indicate that these species are important to early formation of incipient-soot precursors. The results also imply that a leading role in incipient-soot precursor formation is played by species with lower thermal stability than the even-carbon numbered, unsubstituted polycyclic aromatic hydrocarbons known as “stabilomers”.

Effects of n-propylbenzene addition to n-dodecane on soot formation and aggregate structure in a laminar coflow diffusion flame

The effects of n-propylbenzene addition to n-dodecane on soot formation and aggregate structure in a laminar coflow diffusion flame is investigated. The goal is to gain insight on the influence of n-propylbenzene addition to n-dodecane on the underlying mechanisms of soot formation processes. The methane laminar coflow diffusion flame doped with pure n-dodecane establishes the base environment in this study. n-Propylbenzene is mixed into n-dodecane at three levels: 15%, 30% and 45% in mole fraction. The inlet total carbon flow rate is held constant for all of the flames. The combined laser extinction and two-angle elastic light scattering method is used to measure soot volume fraction, primary particle diameter and number density. As expected, soot volume fraction is shown to increase at all heights in the flames with increasing mole fraction of n-propylbenzene in the liquid fuel mixture. The differences in profiles of soot volume fraction suggest a non-linear relationship between the mole fraction of n-propylbenzene in the liquid fuel mixture and the increase in soot production. The relative importance of soot inception and surface growth affected by n-propylbenzene addition is shown to be different along the flame wing and centerline. Along the wing, the aromatic fuel molecular structure primarily influences the initial soot inception process at earlier times by providing a larger population of incipient soot particles for the subsequent surface growth. Along the centerline, the aromatic fuel chemistry effect is stronger, as it accelerates both the soot inception and the subsequent surface growth processes. The experimental data presented in this study is also useful for the validation of soot models of the liquid fuel surrogate components.

Effect of Drive Cycle and Gasoline Particulate Filter on the Size and Morphology of Soot Particles Emitted from a Gasoline-Direct-Injection Vehicle.

The size and morphology of particulate matter emitted from a light-duty gasoline-direct-injection (GDI) vehicle, over the FTP-75 and US06 transient drive cycles, have been characterized by transmission-electron-microscope (TEM) image analysis. To investigate the impact of gasoline particulate filters on particulate-matter emission, the results for the stock-GDI vehicle, that is, the vehicle in its original configuration, have been compared to the results for the same vehicle equipped with a catalyzed gasoline particulate filter (GPF). The stock-GDI vehicle emits graphitized fractal-like aggregates over all driving conditions. The mean projected area-equivalent diameter of these aggregates is in the 78.4-88.4 nm range and the mean diameter of primary particles varies between 24.6 and 26.6 nm. Post-GPF particles emitted over the US06 cycle appear to have an amorphous structure, and a large number of nucleation-mode particles, depicted as low-contrast ultrafine droplets, are observed in TEM images. This indicates the emission of a substantial amount of semivolatile material during the US06 cycle, most likely generated by the incomplete combustion of accumulated soot in the GPF during regeneration. The size of primary particles and soot aggregates does not vary significantly by implementing the GPF over the FTP-75 cycle; however, particles emitted by the GPF-equipped vehicle over the US06 cycle are about 20% larger than those emitted by the stock-GDI vehicle. This may be attributed to condensation of large amounts of organic material on soot aggregates. High-contrast spots, most likely solid nonvolatile cores, are observed within many of the nucleation-mode particles emitted over the US06 cycle by the GPF-equipped vehicle. These cores are either generated inside the engine or depict incipient soot particles which are partially carbonized in the exhaust line. The effect of drive cycle and the GPF on the fractal parameters of particles, such as fractal dimension and fractal prefactor, is insignificant.

The impact of combustion characteristics and flame structure on soot formation in oxy-enhanced and oxy-fuel diffusion flames

Based on a detailed chemical mechanism, impacts of combustion characteristics and flame structure on soot formation in opposed-flow diffusion ethylene flames was studied with different stoichiometric mixture fractions in O2/N2 and O2/CO2 atmospheres. The results showed the followings. 1) In both atmospheres, with the increase of stoichiometric mixture fraction, the flame structure changed significantly. The stagnation plane shifted toward the oxidizer side. Furthermore, there were less C2H2 but more O and OH to occur in the soot inception zone, therefore the amount of soot in the flame decreased. 2) Compared with N2, CO2 had a suppression effect on soot formation, which was mainly due to thermal and direct chemical interaction effects of CO2. This is because the specific heat capacity of CO2 is higher than that of N2, which will cause the flame temperature to drop, and mole fractions of C2H2, H, O, OH and main PAHs to decrease. Soot oxidation played a dominant role, while soot surface growth was attributed to the secondary position. Meanwhile, when CO2 abounded in the flame, OH concentration was increased through the backward reaction of CO+OH=CO2+H, and this would be conducive to the oxidation of soot precursor and incipient soot particles. In addition, the results of maximum particle density indicated the thermal effect of CO2 on soot formation is more important than the direct chemical interaction effect.

The effect of oxygen enrichment on incipient soot particles in inverse diffusion flames

To investigate the characteristics of incipient soot particles with respect to oxygen mole fractions in oxidizer streams, experimental measurements of soot zone structure and degree of carbonization were performed for ethene in inverse diffusion flames (IDFs). IDFs are alternative flames used for research with incipient soot particles because the soot in IDFs does not experience the oxidation process significantly. In the present study, polycyclic aromatic hydrocarbons (PAHs)-planar laser induced fluorescence (PLIF) signals and soot scattering images were used to clarify the soot zone structure of IDFs. The flame height was measured by OH-PLIF with respect to the oxygen mole fraction in the oxidizer stream. The flame height decreased slightly as the oxygen mole fraction in the oxidizer increased. The two-color pyrometry showed that soot temperature along the soot growth path below the flame height was between 2000K and 2200K regardless of the oxygen enrichment. Beyond the flame height, soot formation in IDFs remained effective until the temperature maintained a sufficiently high level. The carbon to hydrogen (C/H) ratio, which was obtained through the element analysis of soot sampling, was measured to assess the degree of carbonization. The C/H ratio increased with respect to residence time at which the soot mostly resided in the region below the flame height. In addition, the C/H ratio increased with regard to the oxygen enrichment in the oxidizer. However, regardless of the oxygen enrichment, the C/H ratio increased linearly according to the modified residence time, which considered inception region and high temperature region beyond the flame height. The morphology of the soot sample was investigated by transmission electron microscopy (TEM) micrographs. A micrograph typical of the young soot particles was detected when the soot particles had a C/H ratio of about 2.