This paper presents the experimental and calculated results of the investigation of soot formation in a standard premixed ethylene/air flame with the addition of 15-100% dimethyl ether. The soot volume fraction was measured using the laser light extinction method at a wavelength of 520 nm. The flame temperature versus height above the burner was measured using standard Pt-Rh thermocouples. It was experimentally shown that the addition of 15% dimethyl ether to ethylene/air flame resulted in a 20% increase of soot volume fraction. The further replacement of ethylene by 60% dimethyl ether decreased the soot volume fraction at 5 fold relatively to a pure ethylene/air flame. Kinetic modeling of soot vol-ume fraction growth with height above the burner was carried out based on the kinetic mechanisms developed by CRECK group ( http://creckmodeling.chem.polimi.it/ ). The calculations were carried out us-ing open software package OpenSMOKE ++ , which includes the modeling of gas-dynamic processes in the premixed laminar flame reactor. Good agreement between the experimental and calculated data was obtained. The analysis of kinetic mechanism used in this study allowed determining the peculiarity of kinetic pathways of benzene formation, which is proposed as an initial substance for soot precursor. It was found, that increase of soot concentration at 15% dimethyl ether addition to the ethylene/air flame is caused by intensification of channel of benzene formation via C3H3 recombination, which additionally produced from C3H4-p (propyne). In turn, the increase of C3H4-p concentration occurred due to reactions of acetylene with CH3 radical formed during DME decomposition. These results obtained in a standard ethylene/air flame could be used for development and validation of various kinetic mechanisms of soot formation.(c) 2022 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

The growth of carbon particles was studied in heated flows of a burnt-gas flow reactor containing mixtures of N-2/C2H2, and N-2/C2H2 with addition of H-2 or CH4 surrounded by a rich C2H4/air flame. Soot particle sizes and volume fractions were measured by laser-induced incandescence (LII) between 50 and 130 mm above the nozzle exit. The measurements indicate a soot-inhibiting effect of adding H-2 to the C2H2/N-2 flow on both, particle sizes and soot volume fractions. The effect of CH4 addition to the C2H2/N-2 flows was ambivalent, depending on the methane-to-acetylene ratio. At gas mixtures with N-2:CH4:C2H2 = 0.42:0.35:0.23 and 0.39:0.32:0.29 by volume at fixed total flow rates, the measured soot volume fractions were substantially increased in presence of CH4, while the mean diameters of the particles were slightly decreased. Gas temperatures were measured by a generalized line-reversal method with Abel transformation. Temperatures of the surrounding C2H4/air flame were around 1600 K, and temperatures of the inner flows, where soot formation was measured, were between 1550 and 1630 K. Plug-flow reactor calculations provided a qualitative understanding of the influence of CH4 on the soot particle growth. (C) 2018 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

The formation of condensed particles in atmospheric ethylene-oxygen and acetylene-air premixed flames catalyzed by the vapours of Fe(CO)(5) and Mo(CO)(6) was studied in the wide range of C/O ratio. Laser scattering and extinction measurements were carried out in a wide frame of flame conditions. The structure and the morphology of formed particles were analyzed using electron microscope. Different formation of the particles takes place due to the fuels and also to the catalyst. In the C2H2/air flame catalyzed with Mo(CO)(6), particle formation starts at about threshold of soot formation (C/O similar to 0.6), while in the flames doped with Fe(CO)(5). particle formation starts already in the lean range of both premixed flames investigated here.

Formation of the waves of condensation of carbon nanoparticles at self-decomposition С3О2 and С2Н2, initiated by a shock wave, was experimentally observed. It was found that self-sustaining condensation wave in С2Н2 could be formed at much higher pressures than in С3О2 despite essential larger heat release of the process C2H2→H2+soot comparably to C3O2→2CO+carbon nanoparticles. The basic kinetic characteristics of process of pyrolysis and following growth of nanoparticles – induction time and rate constant of particle formation are determined for both mixtures. A connection between various stages of process and heat release of condensation is analyzed. It is shown that induction time in both mixtures is not accompanied by a noticeable heat release. For C2H2, duration of this stage is approximately two orders of magnitude larger than for С3О2. Against that, the following stage of carbon nanoparticle formation, accompanied by an essential heat release, proceeds in С3О2 and С2Н2 with comparable rates. It is demonstrated that distinction in formation of a self-sustained condensation wave in С3О2 and С2Н2 is caused by the long induction time in acetylene reflecting the growth of large polyhydrocarbon molecules. The increase in pressure leads to narrowing of this zone and a collapse of whole process to a detonation wave of condensation.

In this work a study of the actual gas-phase temperatures during carbon particle formation from different carbon precursors behind shock waves has been carried out. A pyrolysis of C3O2, CCl4, C2Cl4 and C6H6 at initial (“frozen”) temperatures behind the reflected shock wave in the range 1500 - 3000 K have been studied. A significant difference between frozen and real temperatures behind shock waves was found, which was related to the heat consumption of the precursor decomposition and the heat release during particles formation. The obtained results have shown that the real temperature dependence of particle yield in all measured mixtures does not depend on the kind and initial concentration of precursors and close to that measured in premixed flames.

An experimental observation of a detonation wave driven by the energy of condensation of supersaturated carbon vapor is reported. The carbon vapor was formed by the thermal decay of unstable carbon suboxide C3O2 behind shock waves in mixtures containing 10-30% C3O2 in Ar. In the mixture 10% C3O2+Ar the insufficient heat release resulted in a regime of overdriven detonation. In the mixture 20% C3O2+Ar measured values of the pressure and wave velocity coincident with calculated Chapman-Jouguet parameters were attained. In the richest mixture 30% C3O2+Ar an excess heat release caused the slowing down of the condensation rate and the regime of underdriven detonation was observed.

This paper describes the applicability of laser-induced incandescence (LII) as a measurement technique for primary soot particle sizes at elevated pressure. A high-pressure burner was constructed that provides stable, laminar, sooting, premixed ethylene/air flames at 1–10 bar. An LII model was set up that includes different heat-conduction sub-models and used an accommodation coefficient of 0.25 for all pressures studied. Based on this model experimental time-resolved LII signals recorded at different positions in the flame were evaluated with respect to the mean particle diameter of a log-normal particle-size distribution. The resulting primary particle sizes were compared to results from TEM images of soot samples that were collected thermophoretically from the high-pressure flame. The LII results are in good agreement with the mean primary particle sizes of a log-normal particle-size distribution obtained from the TEM-data for all pressures, if the LII signals are evaluated with the heat-conduction model of Fuchs combined with an aggregate sub-model that describes the reduced heat conduction of aggregated primary soot particles. The model, called LIISim, is available online via a web interface.

The temporal variation in electron and ion concentrations have been measured in shock-heated mixtures of Ar + (0–2)% C3O2 in the 2000–3600K temperature and 15–30bar pressure range. Experiments in pure argon proved that the observed free electrons and ions originate from inherent impurities of sodium. The equilibrium concentrations of free charges in argon were established during (1–3)×10−5s and varied from 4×1011cm−3 at T5=2500K to 5×1012cm−3 at 3500K. In the reactive mixtures, containing C3O2, the time profiles of electron and ion concentrations showed a more complicate behavior—a fast rise to a maximum followed by a gradual decay. The maximum ion concentrations were much higher and electron concentrations were much lower than in similar conditions in argon. The extent of the subsequent decay of electron concentration increased proportionally to the square of the C3O2 concentration. In the mixture with 2% C3O2 the final electron concentration was about 100 times less than in pure argon. The characteristic decay time of free charges varied from 400 to 40μs and decreased proportionally to the square root of the charge concentration. The data analysis is based on the assumption that the observed redistribution of electron and ion concentrations is caused by charging of the carbon particles formed during pyrolysis of C3O2. The kinetics of particle charging and the final distribution of charges were evaluated by the analysis of electron and ion fluxes to the particles in accordance with the electric potentials of charged particles and corresponding sodium ionization. A predominance of negatively charged particles, caused by the high electron mobility, resulted in their much higher concentration than the concentration of free electrons.

The process of heat release during carbon particle formation and growth after pyrolysis of carbon suboxide C3O2 behind shock waves was investigated. For this goal, temperature and optical density of gas-particle mixtures initially consisting of 3% C3O2+5% CO2 in Ar were measured as a function of time. The temperature was determined by two-channel emission–absorption spectroscopy at λ=2.7±0.4μm, corresponding to the CO2 (1,0,1) vibrational band. In the range of initial temperatures behind the shock waves from 1600 up to 2200K a significant heating of the mixture during particle formation and growth was observed that increased towards higher temperatures. The analysis of the obtained data in combination with previous results about the temperature dependence of the particle size shows a decrease of the heat release of condensation from ∼200kJ/mol per atom for particles containing ∼1000 atoms to ∼50kJ/mol per atom for particle containing ∼106 atoms.

In this work the processes of nonequilibrium radiation and ionization in the weak shock waves (2 < M < 4) in argon or helium, containing a small admixture (0.1–2%) of Fe(CO)5 are experimentally studied. The spectra- and time-resolved measurements, performed using a ICCD camera (StreakStar II, LaVision GmbH) have shown, that the unresolved radiation spectra are situated in the range 400–700 nm. The maximum of the spectra lies approximately at 615 nm. The radiation appeared immediately at the propagating shock front and lasted about 8–12 μs. The following time-resolved measurements, performed using photomultipliers (at 615 ± 10 nm) and calibrated electric probes have shown intensive peaks of emission and electric current with duration of few μs, that correlates with the characteristic time of active growth of iron clusters. The analysis of the results obtained allowed to conclude that the real mechanism of generation of the observed peaks of radiation and ionization is the instant dissociation of Fe(CO)5 causing an active condensation process of a supersaturated vapor of iron atoms, which results in the formation of excited and ionized iron clusters.

The actual work deals with the present state of research on flame ions, polyaromatic hydrocarbons, nanotubes, fullerenes, and soot particles in premixed flames. Experimental arrangements for detection and quantitative investigation of flame ions are presented. In addition, the influence of ions on flame chemistry and on formation of carbon particles under non-sooting and sooting conditions is discussed. The study also focuses on the formation pathway from flat polyatomic hydrocarbons to fullerenes, nanotubes, and soot particles. In this connection, the features of arched “aromers,” which are high reactive metastable species and leading candidates for soot precursors and fullerene formation, are reported. These aromers seem to be a kind of “switch” capable of producing either fullerenes or soot particles, depending on the reaction conditions. At lower flame temperatures and a high number density of small unsaturated hydrocarbons, bimolecular reactions are favored, and the formation of soot particles exceeds that of fullerenes. It is also shown how the further growth of soot particles can be described, namely, by soot mass growth and by coagulation processes in strong sooting flames. Typical values for soot volume fractions and particle diameters under various reaction conditions are given.

A modelling study of the formation routes of polycyclic aromatic hydrocarbons (PAHs) is performed for three combustion systems: a benzene pyrolysis in a shock tube, a premixed propene/oxygen/argon flames and a strained methane/air counter-flow diffusion flame. The differences and common features concerning the favoured reaction pathways of PAH growth in these very different combustion systems are examined based on reaction path analysis. It is found that the reactivity of the PAHs is very much dependent on their molecular structure, in particular to the availability of 4-carbon bays. That is, H-rich PAHs featuring these structures play a decisive role in the growth of large PAHs. The maximum concentrations of PAHs versus their C atom number calculated by the model for PAH growth show a similar behaviour as found previously in the literature.

The results of various experiments on carbon nanoparticle formation during gas phase pyrolysis behind shock waves in the temperature range 1200 K ⩽ Tf ⩽ 3500 K are analysed. In the wavelength range 0.22 μm ⩽ λ ⩽ 1.31 μm, the optical properties of particles and their current sizes, measured by laser-induced incandescence, are compared. For a correct knowledge of the actual temperature during particle formation, spectral emission–absorption measurements in the IR range were performed. It was found that the light extinction due to particle growth in different mixtures and at different temperatures can be described by a global rate law for optical density. The observed decrease in optical density at 633 nm with increasing temperature is not a result of the decrease in particle yield, as was assumed earlier, but is caused by the size dependent refractive index. Furthermore, the final particle size decrease with the temperature rise is most likely the deceleration of the coagulation rate of the primary clusters and correspondingly, the increase of the particle number density.

Various carbon particles formed by the pyrolysis of C 3 O 2 and C 2 H 2 behind shock waves in the temperature range 1200–3800 K are studied. The formation of the condensed carbon particles is observed directly by the multichannel detection of the time profiles of the extinction of the medium in the UV, visible, and near-IR spectral regions. The samples of carbon material deposited on the walls of a shock tube after an experiment are analyzed using transmission electron microscopy with different resolutions and electron microdiffraction. Particles formed from C 3 O 2 and C 2 H 2 at 1500–2000 K are 10–30 nm in size and look like usual soot. The absence of molecular hydrogen in C 3 O 2 only results in faster formation and graphitization. At 2100–2600 K, the formation of particles is retarded, and the yield of the carbon particles decreases for both substances. After experiments on pyrolysis of C 3 O 2 at these temperatures, giant spherical particles up to 700 nm in size are found on the walls of the shock tube. Carbon particles formed at the highest temperatures (2700–3200 K) in C 3 O 2 pyrolysis have the high degree of crystallinity of particles.

The influence of pressure on laser-induced incandescence (LII) is investigated systematically in premixed, laminar sooting ethylene/air flames at 1-15 bar with wavelength-, laser fluence-, and time-resolved detection. In the investigated pressure range the LII signal decay rate is proportional to pressure. This observation is consistent with the prediction of heat-transfer models in the free-molecular regime. Pressure does not systematically affect the relationship between LII signal and laser fluence. With appropriate detection timing the pressure influence on LII signal's proportionality to soot volume faction obtained by extinction measurements is only minor compared with the variation observed in different flames at fixed pressures. The implications for particle sizing and soot volume fraction measurements using LII techniques at elevated pressures are discussed.

The formation of particles following the photolysis of C3O2, Fe(CO)(5) and Mo(CO)6(,) diluted with Ar or He was registered at room temperature. Particle growth was followed by taking light extinction profiles at 633 nm and at 220 nm for various mixture compositions and pressures. The particles obtained at different conditions were analyzed using transition electron microscope (TEM) technique. It was found that in the pure undiluted gases at the partial pressures shown in the pictures no light absorption and no particle formation could be observed. Light absorption started for partial pressures of the diluent gas > 10 mbar. A comparison of particle size measured here at room temperature with data obtained at elevated temperature shows that the data obtained here fit well to the elevated temperature data.

The diversity of carbon particles, forming during pyrolysis Of C3O2 and C2H2 behind shock waves in the wide temperature range 1200-3800 K, was investigated. The process of condensed carbon particle formation was observed in situ by the multichannel registration of the time profiles of optical properties of media in the UV, visible, and near-IR ranges. Besides that, the probes of postshock material, deposited on the walls of the shock tube, were analyzed by low- and bigh-resolution transition electron microscopy (TEM) and by electron microdiffraction (MDF) measurements. The comparison of extinction properties of young, growing particles with the electron microscopic analysis of solidified substance gave a notion about the peculiarities of carbon particle formation process from the different carbon-bearing gases at various temperatures. Particles, forming from both substances at 1500-2000 K, look similar to usual soot, and the absence of hydrogen in C3O2 leads to faster formation and graphitization of particles. At the temperatures 2100-2600 K, the decrease of the particle formation rate and the fall of final particle yield in all mixtures is observed. After C3O2 pyrolysis experiments, gigantic film-like spheres with the size up to 700 run were observed on the walls. The peculiarity of the high-temperature (2700-3200 K) process of carbon particle formation in C3O2 pyrolysis is the high degree of crystallization of the final particles.

Laser-induced emissions obtained after excitation with a tunable KrF excimer laser at 248 nm were measured in well-defined sooting laminar high-pressure flames fueled with methane/air and ethylene/air up to 15 bar. A spectral analysis shows that Mie scattering, Raman scattering and laser-induced fluorescence (LIF) signals can be used for detailed flame studies under sooting high-pressure conditions. Mie scattering is correlated with soot, Raman signals can be used to measure spatially-resolved major species concentrations as well as temperatures. A LIF-scheme to measure NO was found to be applicable even under these conditions. The broadband fluorescence in the range from 270 to 290 nm, usually discarded as background, correlates well with the total concentration of polycyclic aromatic hydrocarbons (PAH) as measured via GC-MS methods.

In premixed C2H4-air-flames, optical measurements of the growth of soot particles were performed in the pressure region of 30 to 70 bar. From the laser-scattering and molar absorptivity measurements, particle number densities, N, particle diameter, d, and soot volume fraction, f(V), were obtained at different heights above the burner for several mixture compositions and unburnt gas velocities corresponding to different flame temperatures. In addition soot samples were taken for electron microscopic measurements. The particle diameters and their size-distributions were determined and the soot particle structures were studied. The electron microscopic measurements showed, that for the pressures applied, the primary particle diameters can be described by a log-normal size distribution with a mean standard deviation of sigma(g)approximate to 0.35 nm. In the high resolution electron micrographs, crystallite layers could be observed. These crystallite layers become more pronounced for long reaction times, high flame temperatures and high pressures. In the pressure region of 30 to 70 bar the particle number density towards the end of the growth process, N-infinity, increases with increasing C/O ratio and with pressure. At 70 bar N-infinity can reach 10(13) cm(-3) in strongly sooting flames. The final particle diameter, d(infinity), for otherwise fixed conditions decreases with pressure, so that at 70 bar small diameters of d(infinity)less than or equal to 50 nm result. Therefore the mean final soot surface, A(infinity), becomes very large, with values up to 250 cm(-1) observed (C/O greater than or equal to 0.70, T similar to 1850 K).

The contributions of different reaction pathways to the formation of polycyclic aromatic hydrocarbons (PAHs), the actual soot precursors, were investigated for the shock tube pyrolysis of acetylene, benzene and an acetylene-benzene (1:1) mixture. This study provides information on the principal reaction steps leading to the fast formation of high molecular weight PAHs before soot inception. It is found that reaction pathways to high molecular weight PAHs and soot precursors including only successive growth steps of aliphatic hydrocarbons are in disagreement with the measured induction time of soot formation taken from the literature. The limiting, most time-consuming step in this reaction sequence is shown to be the formation of "outer rings'', that is, PAHs consisting of four-carbon bay structures. In the model developed in the present study, these difficulties are overcome by introducing combinative reaction steps of aryls.