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1. Detailed Study of the Formation of Soot Precursors and Soot in Highly Controlled Ethylene(/Toluene) Counterflow Diffusion Flames

   https://doi.org/10.1021/acs.jpca.2c06538  

   Gleason, Kevin; Gomez, Alessandro (January 2023, The Journal of Physical Chemistry A)

   We perform spatially resolved measurements of temperature, gaseous species up to three-ring Polycyclic Aromatic Hydrocarbons (PAHs), and soot in atmospheric pressure counterflow diffusion flames. First, we characterize fully a baseline ethylene flame and then a toluene-seeded flame in which an aliquot of ethylene in the feed stream is replaced with 3500 ppm of prevaporized toluene. The goal is twofold: to investigate the impact of a common reference fuel component of surrogates of transportation fuels and bypass the main bottleneck to soot formation from aliphatic fuels, that is, the formation of the first aromatic ring. The composition of the fuel and oxidizer streams are adjusted to maintain a constant stoichiometric mixture fraction and global strain rate, thereby ensuring invariance of the temperature–time history in the comparison between the two flames and decoupling the chemical effects of the fuel substitution from other factors. Major combustion products and critical radicals are fixed by the baseline flame, and profiles of critical C2–C5 species precursors to aromatic formation are invariant in both flames. On the other hand, doping with toluene boosts the aromatic content and soot volume fraction, increasing the mole fraction of benzenoid structures and soot volume fraction by a factor of 2 or 3, relative to the baseline ethylene flame. This finding is consistent with the expectation that the formation of the first aromatic ring is no longer a bottleneck to soot formation in the doped flame. In addition, toluene bypasses completely benzene formation, opening a radical recombination pathway to soot precursors through the production of C14H14 (via dimerization of benzyl radical) and pyrene (through dimerization of indenyl radical). 

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2. PAHs controlling soot nucleation in 0.101—0.811MPa ethylene counterflow diffusion flames

   https://doi.org/10.1016/j.combustflame.2021.01.015  

   Gleason, Kevin; Carbone, Francesco; Gomez, Alessandro (May 2021, Combustion and Flame)

   On the heels of a recent study in an atmospheric pressure ethylene diffusion flame in which the transition from parent fuel molecule to Polycyclic Aromatic Hydrocarbons (PAHs) and, eventually, soot was studied by spatially resolved measurements of PAH concentrations and soot quantities, we extended the focus to diffusion flames with self- similar structure in the 0.101–0.811 MPa pressure range. To that end, we complemented pyrometry based measurements of soot volume fraction with light scattering measurements that, once corrected for beam steering, yielded soot particle size and number concentration profiles. A chemistry model, that had been validated for all species up to 3 ring aromatics in one of the flames investigated at each pressure and up to 4-rings for the flame at atmospheric pressure, was used to compare profiles of chemical species to those of soot quantities. Further analysis yielded the assessment of number nucleation rates of soot and their comparison to the dimerization rates of PAHs. Soot nucleation rate is consistent only with the dimerization of one- and two-ring PAHs, an observation that confirms findings in the atmospheric pressure flame. Changes in pressure and temperature have a progressively larger impact on the concentration of aromatics of increasingly larger molecular weight and, even more so, on soot volume fraction and nucleation rate. A four-fold increase in pressure from 0.101 MPa to 0.405 MPa increases the soot nucleation rate and PAH dimerization rate in flames with constant peak temperature, which is primarily a concentration effect on bimolecular collision rates; a similar but less pronounced effect is observed in the higher (0.405–0.811 MPa) pressure range. Changes in pressure and temperature tend to be progressively more consequential on aromatics of increasing molecular weight and soot. 

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3. Effect of equivalence ratio and temperature on soot formation in partially premixed counterflow flames

   https://doi.org/10.1016/j.combustflame.2022.112088  

   Gleason, Kevin; Carbone, Francesco; Gomez, Alessandro (August 2022, Combustion and Flame)

   Soot formation is quantified in detail (volume fraction, particle size, number concentration, and light emissivity dispersion exponent) in a series of partially premixed counterflow flames of ethylene at equivalence ratios equal to 6.5, 5.0, and 4.0, and with maximum temperature spanning approximately 200 K. The focus is to investigate the effect of peak temperature and equivalence ratio on soot formation while maintaining constant global strain and stoichiometric mixture fraction. Oxygen is progressively displaced from the oxidizer to the fuel stream of a diffusion flame to stabilize partially premixed flames of decreasing, showing a double-flame structure consisting of a rich premixed flame component stabilized on the fuel side of the stagnation plane and a diffusion flame component stabilized on the oxidizer side. Soot is detected in the region sandwiched between the two flame components, is formed in both of them, and is convected away radially at the Particle Stagnation Plane (PSP). At fixed , raising the peak temperature invariably raises the soot volume fraction throughout the probed region. Vice versa, at fixed peak temperature, lowering the equivalence ratio causes the premixed flame component to shift away from the diffusion flame component, with the consequent broadening of the soot forming region and an increase in both soot volume fraction as well as soot particle sizes through an enhancement of surface growth. Detailed probing of the region in the vicinity of the PSP offers evidence of soot oxidation from molecular oxygen. Furthermore, when the maximum temperature is sufficiently low, the net soot production rate turns negative because surface oxidation overwhelms surface growth. Comparing the soot number production rate inferred from experiments to the dimerization rate of benzene, naphthalene, and pyrene reveals that only the smallest aromatics are present in flames at sufficiently large concentrations to account for soot nucleation. This observation applies to both the diffusion flame and the premixed flame components and confirms previous findings in strictly diffusion flames. 

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4. Spatially resolved measurements of soot and gaseous precursors in ethylene counterflow diffusion flames up to 32 atm

   https://doi.org/10.1016/j.proci.2020.06.284  

   Gleason, Kevin; Carbone, Francesco; Gomez, Alessandro (January 2021, Proceedings of the Combustion Institute)

   We investigate the effect of pressure on both flame structure and soot formation in nitrogen diluted counterflow diffusion flames of ethylene in the 8–32atm pressure range. Capillary-probe gas sampling is performed to resolve spatially the profiles of gaseous species up to three-ring aromatics by GC/MS analysis and multi-color pyrometry is used to quantify the soot volume fraction and dispersion exponent. Self-similarity of flames is preserved by keeping constant mixture fraction and strain rate, so that profiles of concentrations and temperature, normalized with respect to their peak values, are unaffected by changes in pressure, once the axial coordinate is nondimensionalized with respect to the pressure-dependent diffusion length scale. When conditions are chosen so that the overall soot loading is approximately constant and compatible with the diagnostics, it is found that both the soot volume fraction and the profiles of key aromatics in the high-temperature nucleation region are virtually invariant. For it to happen, a twofold increase in pressure must be compensated by a ∼100 K decrease in peak flame temperature and, therefore, in the temperature across the soot forming region. The implication is that from the perspective of the chemical kinetics of soot formation these two actions counterbalance each other. As pressure increases (and temperature decreases) the peak production rate of the high-temperature soot mechanism decreases and, further downstream, towards the particle stagnation plane, a low-temperature soot mechanism sets in, yielding an increase in soot H/C content. This mechanism is enhanced as the pressure is raised, causing a higher overall soot volume production rate in the 16atm flame and, especially, in the 32atm one. The role of C4/C2 species in the formation of C6H6 increases with increasing pressure and dominates over the recombination of propargyl radical at sufficiently high pressures. A comprehensive database is established for soot models at high pressures of relevance to applications. 

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5. Small aromatic hydrocarbons control the onset of soot nucleation

   https://doi.org/10.1016/j.combustflame.2020.08.029  

   Gleason, Kevin; Carbone, Francesco; Sumner, Andrew J.; Drollette, Brian D.; Plata, Desiree L.; Gomez, Alessandro (January 2021, Combustion and Flame)

   The gas-to-particle transition is a critical and hitherto poorly understood aspect in carbonaceous soot particle formation. Polycyclic Aromatic Hydrocarbons (PAHs) are key precursors of the solid phase, but their role has not been assessed quantitatively probably because, even if analytical techniques to quantify them are well developed, the challenge to adapt them to flame environments are longstanding. Here, we present simultaneous measurements of forty-eight gaseous species through gas capillary-sampling followed by chemical analysis and of particle properties by optical techniques. Taken together, they enabled us to follow quantitatively the transition from parent fuel molecule to PAHs and, eventually, soot. Importantly, the approach resolved spatially the structure of flames even in the presence of steep gradients and, in turn, allowed us to follow the molecular growth process in unprecedented detail. Noteworthy is the adaptation to a flame environment of a novel technique based on trapping semi-volatile compounds in a filter, followed by off-line extraction and preconcentration for quantitative chemical analyses of species at mole fractions as low as parts per billion. The technique allowed for the quantitation of PAHs containing up to 6 aromatic rings. The principal finding is that only one- and two-ring aromatic compounds can account for soot nucleation, and thus provide the rate-limiting step in the reactions leading to soot. This finding impacts the fundamental understanding of soot formation and eases the modeling of soot nucleation by narrowing the precursors that must be predicted accurately. 

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