1,720,975 research outputs found
Growth of PAH in combustion systems
No full textA 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
Growth of PAH in combustion systems
No full textA 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
Numerical study on the temperature dependence of soot formation in acetylene pyrolysis blended with methane, formaldehyde, methanol, and dimethyl ether
No full textAbstract
This paper addresses the combined effects of varying C/H and C/O ratios as well as of the molecular structure of the fuels selected on the normalized soot volume fraction
f
V
. For the simulations, an already existing and validated reaction mechanism for the pyrolysis of C
2
H
2
in argon, Aghsaee et al. (
Combust. Flame
2014,
161
, 2263–2269), was used in the current work. It was extended with PAH reactions from coronene (C
24
H
12
) up to ovalene (C
32
H
14
), whereas general principles for the rapid build-up of
large
PAHs were presented. Soot formation was modeled according to Appel et al. (
Combust. Flame
2000,
121
, 122–136) by applying the method of moments. A validation of the extended reaction model was carried out for shock-wave-induced O
2
/C
2
H
2
mixtures from literature. In the following, the influence of blends of methane (CH
4
), formaldehyde (CH
2
O), methanol (CH
3
OH), and dimethyl ether (CH
3
)
2
O on soot formation during C
2
H
2
pyrolysis diluted in Ar was studied. Special emphasis was laid on the inception chemistry of soot formation. The role of intermediates, such as the propargyl radical (C
3
H
3
), leading towards benzene and polyaromatic hydrocarbon (PAH) formation and their interplay with hydrogen molecules (H
2
) to H atoms (H) ratio was examined. All blends increased the ratio of the concentrations of H
2
and H leading thus to reduced soot inception and soot formation. However, soot suppressing effects were overrun by supporting ones when the additives provided suitable molecular groups, such as methyl radicals (CH
3
), in sufficient high concentrations for early aromatic ring formation. Thus, a prominent synergistic effect on soot formation was found for the CH
4
/C
2
H
2
mixture only. Besides, species able to mirror characteristics of the soot formation process, such as the peak value of the normalized soot volume fraction, are presented. The findings of this work indicate the synergistic effect of H
2
/H and C/O ratios as well as of methyl radicals on the PAHs’ production of appropriate size able to initiate soot inception process in an aliphatic fuel
Experimental study of carbon particle charging at shock-wave pyrolysis of C3O2
No full textThe temporal variation in electron and ion concentrations have been measured in shock-heated mixtures of Ar + (0-2)% C3O2 in the 2000-3600 K temperature and 15-30 bar 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) x 10(-5) s and varied from 4 x 10(11) cm(-3) at T-5 = 2500 K to 5 x 10(12) cm(-3) at 3500 K. 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 40 mu s 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. (c) 2009 The Combustion Institute. Published by Elsevier Inc. All rights reserved.Gottingen Academy; RFB
Laser-induced incandescence for soot diagnostics at high pressures
No full textThe 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-resolve 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 fraction 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. (C) 2003 Optical Society of America
Laser-induced incandescence for soot-particle sizing at elevated pressure
No full textThis 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
Formation of nanoparticles by photolysis from metal and carbon bearing molecules
No full textThe 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
To the temperature dependence of carbon particle formation in shock wave pyrolysis processes
No full textIn this work the results of numerous experiments on carbon particle formation in combustion and pyrolysis of various carbon bearing molecules behind shock waves in the wide temperature range from 1200K to 3500K are analyzed. It is shown, that the discrepancy in the temperatures of the maximum particle yield could be attributed to the differences in the endothermicy of the pyrolysis of various molecules and the maximum optical density at 633 run in all mixtures can be related to the same temperture T = 1600 K. Based on this consideration, several statements were formulated. First particle growth in all mixtures can be described by the uniform dependence of optical density D (at 633 nm) on time D similar to atau (0.4) indicating, that particle formation proceeds via homogeneous condensation. The second - decrease of the optical density at 633 nm with the temperature rise is caused not by the decrease of particle yield, but the decrease of their size resulting in the fall of extinction at the given wavelength. Third - the reason of the fall of the final particle size with the temperature rise is the acceleration of the initial cluster formation process and a corresponding increase of the particle number density. And the last statement - the secondary particle growth, observed at T > 2200 K is completely determined by the primary clusters (nucleus) formed behind the incident wave and the coagulation of small carbon particles formed behind the reflected shock wave using these clusters
Formation of Condensed Particles in Premixed Flames Catalyzed by Metal Carbonyls
No full textAbstract
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 ~ 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.</jats:p
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