Effect of fuel/air ratio and aromaticity on the molecular weight distribution of soot in premixed n-heptane flames (original) (raw)
Related papers
2004
In this work, the molecular growth process leading to high molecular weight aromatic species and soot has been experimentally studied for sooting premixed hexane and benzene flames. This description was obtained by measuring the axial concentration profiles of polycyclic aromatic hydrocarbons (PAH) up to 300 u, and total particulate (dry soot and soot extract). A detailed kinetic model combining the different mechanisms of benzene and PAH formation has been tested on the experimental data in order to give information on the possible routes through which soot inception occurs. Moreover, a mechanism for the formation of particulate including high molecular weight (HMW) species and soot has been added to the model and compared to the experimental data relative to the particulate concentration and molecular weight (MW) distribution of soot extract and dry soot measured by Size Exclusion Chromatography (SEC).
Modeling of soot particle inception in aromatic and aliphatic premixed flames
Combustion and Flame, 2004
The growth of hydrocarbon molecules up to sizes of incipient soot is computed in premixed laminar flames using kinetic Monte Carlo and molecular dynamic methodologies (AMPI code). This approach is designed to preserve atomistic scale structure (bonds, bond angles, dihedral angles) as soot precursors evolve into three-dimensional structures. Application of this code to aliphatic (acetylene) and aromatic (benzene) flame environments is able to explain results in the literature on the differences in properties of soot precursors from these two classes of flames, particularly relating to H/C ratio, particle sphericity, and depolarization ratio.
Experimental and Modeling Study on Soot Inception in Rich Premixed Aliphatic and Aromatic Flames
Particle formation in fuel-rich combustion has been the subject of many studies in the last years in order to clarify the controlling pathways leading to their nucleation. Studies on premixed laminar flat flames have allowed to focus the attention on the physical-chemical processes occurring, almost neglecting the fluid-dynamic effects due to different and more complicated geometries. In rich premixed flames a bimodal particle size distribution function has been generally observed. The modal peak at around 20nm is universally accepted as soot mode, whereas the origin and evolution of the smaller particle mode, which has typical size of 2-3nm, is still controversial. The smaller particles can be formed through fast radical reactions occurring just downstream of the flame front and can survive in flame as consequence of their low coagulation rate. Moreover a competition between a persistent nucleation and a coagulation mechanism might be responsible for the evolution of the small particles along the flame. Particles formation process can also vary with flame temperature and fuel composition. The object of the present work is to perform a comparative study on the effect of fuel chemical structure on particle inception in rich premixed flames. Both experimental and modeling approach are used. In situ optical techniques based on the interaction of UV-light with particles have been adopted. Modeling of flame structure and particle formation has been performed by using a complete detailed scheme that provides both for gas phase and for particles reactions, the latter adopting a sectional method. This approach allows to model simultaneously major and minor species in gas phase and to predict the size distribution functions for particles. Benzene and n-hexane have been studied. Particle inception in these flames has showed a sensitivity to the chemical nature of the fuels. Results obtained from measurements have been confirmed by the modeling.
Combustion and Flame, 2007
An investigation was conducted on the evolution of products of incomplete combustion (PIC) emitted from onedimensional, laminar, atmospheric-pressure ethylbenzene flames in the vicinity of the soot onset threshold. The objective of this study was to identify the role of the fuel-to-air equivalence ratio in the evolution of polycyclic aromatic hydrocarbons (PAH) and other PIC as soot precursors, just prior to and subsequent to soot onset in premixed flames. Liquid ethylbenzene was prevaporized in nitrogen and blended with an oxygen-nitrogen mixture. Upon ignition, premixed flat flames were stabilized over a burner at atmospheric pressure. Temperature measurements and product sampling were conducted at various heights above the burner. Collected samples were analyzed for soot, PAH, oxygenated species, fixed gases, and light hydrocarbons. Three flames were investigated in the vicinity of the observed soot onset threshold, at equivalence ratios of φ 1 = 1.68, φ 2 = 1.74, and φ 3 = 1.83. By adjusting the amounts of oxygen, nitrogen, and fuel, both the maximum measured flame temperature and the spatial profile of the temperature were kept nearly constant as the equivalence ratio was varied. The cold gas velocity through the burner was also kept nearly constant. Changes in species concentration profiles prior to, at and beyond the sooting limit were evaluated. The results indicated that the soot onset limit is not a function of flame temperature alone; i.e., while the maximum measured flame temperatures was kept fairly constant, the flame could be either sooting, at the sooting limit or nonsooting depending on the equivalence ratio. A detailed chemical kinetic model, previously tested against sooting premixed benzene and ethylbenzene flames, was used to gain insight in chemical processes involved in soot formation. A reaction flux analysis was conducted to determine the pathways for ethylbenzene consumption, as well as for benzene and naphthalene formation. Examination of experimental measurements of species along the axis of the flame, in view of the theoretical predictions, showed a rather direct correlation of acetylene to soot formation. Moreover, a correlation between the consumption of ethylbenzene pyrolyzates, such as styrene, and soot formation at the soot onset was also apparent. Whereas the model's results were very encouraging, additional development is deemed necessary to improve its predictive capability in the challenging regime of soot inception. * .
The Characteristics of Soot Formed in Premixed Flames by Different Fuels
The internal structure of soot sampled from fuel-rich methane and benzene premixed flames burning in different temperature conditions was investigated. UV-visible spectroscopy was used for its sensitivity to the carbon network in terms of sp(2) and sp(3) sites and of extension of sp(2) aromatic moieties. The UV-visible absorption maximum and the optical band gap have shown to be affected by the fuel characteristics rather than by flame temperature. The important role of fuel aromaticity on soot internal structure from particle inception throughout the soot formation region was demonstrated.
Combustion and Flame, 2018
Aromatic hydrocarbons in liquid transportation fuels help in suppressing auto-ignition and knocking tendency in engines, but are toxic and can generate polycyclic aromatic hydrocarbons (PAHs) and soot during combustion that are harmful for human health and the environment. Benzene, toluene, and xylenes are present in noticeable quantities in liquid fuels. This paper reports the sooting propensity of benzene and m-xylene, and the role of substituted aliphatic chains on soot emissions at different fuel flow rates along with their effects on soot characteristics. Soot particles collected from the aromatic diffusion flames at different fuel flow rates are characterized using high resolution transmission electron microscopy and X-ray diffraction to examine their nanostructural changes, and using, electron energy loss spectroscopy, Raman spectroscopy, and elemental analysis to determine their chemical properties. With increasing fuel flow rates in benzene and m-xylene flames, the size of the primary particles in soot increased, but that of PAHs reduced. The chemical changes introduced by increasing fuel flow rates in both the diffusion flames were the reduction in the concentrations of oxygen and hydrogen content. Though aliphatics are less sooting than aromatics, the presence of methyl groups on the aromatic ring in m-xylene not only increased the soot production rate, but also led to soot particles with less hydrogen content than those from benzene flames. Soot modeling studies are conducted to study the sooting behaviors of benzene an m-xylene fuels. The results from benzene and m-xylene flames suggest that increasing the rate of fuel consumption (which happens while increasing engine load) can significantly affect the physicochemical properties of soot from the same fuel.
On the relevance of surface growth in soot formation in premixed flames
Proceedings of the Combustion Institute, 2000
The role of surface growth mechanisms in particle mass accumulation was investigated in rich, premixed, ethylene/air flames from non-sooting to moderately sooting conditions using in situ optical diagnostics and predictions from a detailed chemical kinetic model. Particles formed just after the flame front, which are transparent to the visible light but absorb in the UV range, have been detected in large amounts in nonsooting flames and earlier in the flame than soot particles in sooting-flames, using UV-visible light optical diagnostics. For C/O Ͻ 0.8, the amount of visible-transparent particles accounts for the total mass of soot detected later in the flame, indicating that surface growth processes are negligible and that soot formation is a rearrangement of the carbonaceous material already present in the form of smaller particles. Furthermore, predictions from the kinetic model, which does not include surface growth reactions, agree well with experiments for C/O Ͻ 0.8. The model is able to predict the total carbon contained in particles observed in the non-sooting flames and in slightly sooting flames, as well as that observed up to the onset of soot formation in richer flames. For C/O Ͼ 0.8, additional soot growth mechanisms need to be included in the mechanism to account for the amount of soot observed in the later part of the flames. Interestingly enough, this late growth mechanism occurs almost simultaneously with a strong increase in the coagulation rate of the particles, thus indicating that both effects are related and are probably due to a major change in the chemical nature of the particle surface.
Precursor formation and soot inception in premixed ethylene flames
Symposium (International) on Combustion, 1992
Absorption, fluorescence and scattering measurements were applied both in the ultraviolet and visible to rich premixed C2H4/O2 flames in the C/O range from 0.5 to 0.8. The concentration of soot and condensed hydrocarbon species were also measured by direct sampling along a lightly-sooting (C/O = 0.65) and sooting (C/O = 0.75) flame. The early formation in the flames of "particles" with typical average size around 2 nm (2500 a.m.u.), which do not absorb in the visible and fluoresce in the ultraviolet, was followed. These "transparent" particles were considered soot precursors on the basis of their decreasing concentration profile in correspondence of soot inception, as evaluated by both optical and ~chemical measurements. The soot inception process is characterized by the progressive aromatization of the "transparent" particles as shown by the progressive shift toward the visible of absorption and fluorescence. The internal rearrangement of a partially aromatic polymeric structure, more than a progressive formation of very large PAH structures, is proposed as the mechanism of soot inception.
Combustion and Flame, 2009
This work investigates the similarities/differences of one-dimensional, laminar, atmospheric pressure premixed ethylbenzene flames at their soot onset threshold (φ critical ). The goals of this investigation are to contrast the entire temperature profiles of the flames at various critical equivalence ratios, φ critical , and to report on hydrocarbon specie profiles. Liquid ethylbenzene was pre-vaporized in nitrogen, blended with an oxygen-nitrogen mixture and, upon ignition, premixed flat flames were stabilized over a burner at atmospheric pressure. The C/O ratio was regulated and simultaneously the temperature profile was adjusted by varying the heat lost from the flame to the burner, to obtain visually similar flames, all at their apparent soot onset limit. Temperature profiles of five such flames were determined with thermocouples. The results indicate that as the axial temperature profiles in the flames rose, φ critical ((C/O) critical ) also increased. Sampling was performed at various heights along the axes of three of these flames (φ c1 = 1.68, φ c2 = 1.74 and φ c3 = 1.83) to monitor changes in chemical speciation. The mole fractions of CO increased in the order of increasing equivalence ratio and hence increasing flame temperature. The CO 2 mole fractions of the three flames were similar. However, the mole fractions of light hydrocarbons and polycyclic aromatic hydrocarbons (PAH), which are suspected soot precursors, decreased in the order of increasing equivalence ratio and hence increasing flame temperature. Whereas direct measurements of particulate matter in the flame were not made in this work, these observations on soot precursors along with theoretical flame emissivity considerations suggest that particulate loadings at the sooting limit also decreased in the order of increasing equivalence ratio and hence increasing flame temperature. (Y.A. Levendis). mospheric pressure ethylbenzene flames. The results indicate that the soot onset limit depends on both T and φ, both having independent effects on the flame chemistry. Within the investigated parameter range: (1.68 φ 1.83) [1] and (1826 K T max 1921 K) [2], temperature was found to be more influential in affecting the chemical compositions of species.