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• Novel chemistry model to predict morphology and nanostructure of CNPs. • Carbonization process describes transition from liquid-like to primary particles. • Predicted morphological features reproduce flame reactor experiments. • Findings motivate further research towards a priori determination of CNP properties. Carbon nanoparticle (CNP) formation from hydrocarbons combustion is of high interest not only for the study of pollutant (soot) emissions, but, above all, in the area of advanced materials. CNP optical and electronical properties, relevant for practical applications, significantly change with their size, morphology, and nanostructure. This work extends a detailed soot kinetic model, based on the discrete sectional approach, to explicitly incorporate the description of CNP polydispersity, maintaining the CHEMKIN-like format. The model considers various nanosized primary particles, generated from liquid-like counterparts through the carbonization process, which successively grow or aggregate forming fractal structures. The model is validated against experimental measurements from the literature including CNP volume fraction, several morphological characteristics, number density and particle H/C ratio. Data are taken from 19 laminar flames, in different configurations (counterflow diffusion flames, premixed flat flames established on the McKenna-type burner and burner-stabilized stagnation flames) and over a wide range of operating conditions (P=1–10 atm, T max =1556-2264 K). The model captures the measured trends of all the analyzed CNP properties as a function of equivalence ratio, residence time and fuel type in premixed flames, and pressure and strain rate in counterflow flames. Model deviations from the experiments are discussed, also in comparison with other state-of-the-art soot models based on different approaches. Sensitivity analyses are performed on carbonization, coalescence, and aggregation rates, which have the largest impact on CNP morphology and are characterized by larger uncertainty compared to elementary chemical pathways. [Display omitted] [ABSTRACT FROM AUTHOR]

Spherically-symmetric, isolated droplets are ideal systems to investigate the physics and the chemistry of combustion of liquid fuels. Despite their simplicity, most phenomena involved in spray combustion are still accounted for: evaporation and diffusion-induced transport, complex liquid thermodynamics, radiation, aerosol chemistry. In this work we analyzed the formation and evolution of soot particles and aggregates from the combustion of n-heptane isolated droplets. A 1D mathematical model including detailed description of thermodynamics and transport properties of liquid and gaseous phases, radiative heat transfer, and detailed homogeneous chemistry is adopted. Soot formation and evolution is described via a discrete sectional method, accounting for nucleation, surface growth, coagulation, aggregation, and oxidation. The mathematical model was utilized in this study to investigate how the initial diameter of droplets affects their tendency to form soot. The simulations were specifically designed to examine the experimental findings of Choi et al. (2001), which indicated that droplets with intermediate diameters (∼ 1.9 mm) result in the highest soot volume fraction. The numerical results reveal that this maximum is due to a competition between two phenomena: (i) increasing droplet diameter leads to a longer lifetime, thereby promoting soot formation, but (ii) larger diameters cause more radiative losses, lower flame temperatures, and subsequently, a decrease in soot formation. • Soot formation around isolated fuel droplets was simulated via a detailed kinetic mechanism. • The link between soot formation tendency and initial diameter was numerically explained. • Radiative heat transfer was recognized as a key-element for explaining soot formation tendency. • Kinetic analysis highlighted the different evolution spherical particles and aggregates. • Very large soot aggregates were observed and associated to the large residence times. [ABSTRACT FROM AUTHOR]

Soot formation of n -heptane is experimentally and numerically studied in low-strain rate (K = 50 s−1) counterflow soot formation (SF) flames. Flame temperatures and soot volume fractions at different oxygen mole fractions on the oxidizer stream (xO 2 =0.3–0.45) are measured by using thermocouple-calibrated OH two colors laser-induced fluorescence (2C-PLIF) and laser-induced incandescence (LII) calibrated by light extinction methods, respectively. Mono (MAHs) and polycyclic aromatic hydrocarbons (PAHs) are also qualitatively measured by using the PAHs-LIF technique. Good agreement between measured and predicted maximum flame temperature (T max) and peak soot volume fraction (f v ,peak) is obtained. However, discrepancies in the shape of the entire soot volume fraction profiles along the axial centerline are observed between 1D and 2D simulations. This result is found to be primarily related to the thermophoretic and radial effects in the low strain rate flames investigated, which hamper neglecting the underlying 2D effects. MAH/PAH peak mole fractions and f v ,peak increase from xO 2 =0.3 to xO 2 =0.45 due to the related increase of flame temperature, which fosters n -heptane pyrolysis near the fuel nozzle leading to higher concentrations of C 1 C 4 hydrocarbons. The latter are found to grow to MAHs/PAHs and finally to soot particles through analogous pathways independently of xO 2. However, the higher flame temperature in the sooting region at xO 2 =0.45 leads to soot particles and aggregates characterized by larger sizes and more dehydrogenated than those produced at xO 2 =0.3. [ABSTRACT FROM AUTHOR]

Blending of hydrocarbon fuels with hydrogen is a practically feasible way to rapidly integrate a carbon neutral energy vector in the transport sector and to effectively reduce harmful particulate matter emissions. This work investigates the effects of hydrogen (H 2) addition on soot formation in n -heptane in low strain rate counterflow diffusion flames with different oxygen mole fractions (X O2 =0.3 ∼ 0.45). Flame temperature, soot volume fraction (f v), monocyclic (MAHs), and polycyclic aromatic hydrocarbons (PAHs) are measured by using OH-2C-PLIF/thermocouple, LII/LE, and PAH-LIF methods, respectively. Measurements of n -heptane flames with addition of helium (He), as an inert gas with a specific heat capacity similar to that of H 2, are also carried out and compared. Numerical simulations are then performed using a soot discrete sectional model coupled with detailed gas-phase kinetics to interpret the data obtained. Temperature and soot volume fraction profiles are better predicted by 2D simulations, which account for the non-negligible impact of radial diffusion and buoyancy under low strain rate conditions. Then, it is shown that both the f v and PAH yields almost linearly decrease by increasing H 2 or helium (He) addition. The reduction of PAHs and soot is larger in n -heptane/H 2 flames due to H 2 chemical effects, absent in the n -heptane/He flames, hampering the formation of peri‑condensed structures. The effect of O 2 concentration in the oxidizer stream is also studied. Experimental and simulation results show that PAHs and soot increase with X O2 for all flames. Finally, the effects of H 2 /He addition varying X O2 on soot formation are investigated. With the increase of X O2 , the difference in the peak flame temperatures of n -heptane/H 2 and n -heptane/He flames decreases. Consequently, compared to the neat n -heptane flame, the reduction of peak f v becomes more and more pronounced in the case of hydrogen addition rather than the helium addition since the H 2 soot inhibiting chemical effect prevails over the H 2 temperature effect which instead promotes soot formation. [ABSTRACT FROM AUTHOR]

Based on recent theoretical and experimental evidences, this work proposes a detailed discrete sectional soot model considering particles and aggregates behaving as stable radicals, paving the way for the implementation of a game-changing assumption in soot kinetic models. The concentration of large closed-shell and open-shell polycyclic aromatic hydrocarbons (PAHs) levels out with the increase of their aromatic structure and, contrary to conventional assumptions in existing soot models, their reactivity approaches that of persistent radical species. These findings were implemented in the model here proposed to describe soot formation (Part I) and oxidation (Part II). While the number of lumped pseudo-species involved is significantly reduced compared to previous discrete sectional soot models since no distinction between the kinetics of closed-shell and open-shell particles is taken into account, the physical meaning and consistency with recent evidences improves together with model performances. Rate parameters of gas-phase reactions involving resonantly stabilized radical PAHs were used as a reference for soot particle kinetics. The model was validated against 65 target ethylene laminar premixed flames and counterflow diffusion flames, with a general good agreement between experimental data and numerical simulations at different pressures (P = 1–8 atm), maximum flame temperatures (T max ∼1500–2400 K) and soot yields (f v ∼0.01 to ∼170 ppm). Parity diagrams allow to identify systematic deviations of the present model from the experiments, while the comparison between all the flames included in the database highlights the relative contribution to soot formation and growth of the main reaction classes constituting the soot model, spotting analogies and peculiarities of the two flame configurations considered. Similar kinetic patterns characterize most of the flames within the database, independently of flame configuration and sooting tendency of the specific case. Large PAH condensation dominates the formation of the first soot nuclei, while the condensation of smaller PAHs up to four aromatic rings is primarily responsible for the growth of larger soot particles and aggregates. Finally, the contribution of the different fluid dynamics in premixed and counterflow flames to particle evolution, resulting in a higher hydrogenation level of soot particles in the latter, is discussed. [ABSTRACT FROM AUTHOR]

Recent experimental and theoretical studies have shed light on the persistent radical behavior of large polycyclic aromatic hydrocarbons (PAHs) and soot particles. The first detailed soot model developed based on these findings is here proposed. While its good performances in predicting soot formation and growth when soot oxidation is negligible are presented in the companion paper of this work (Part I), the model is here validated against 45 cases of particle size distribution functions (PSDF) and soot volume fraction under oxidizing conditions. Notably, we show that by considering stable radical particles and aggregates it is possible to reproduce soot oxidation by O 2 in plug flow reactors under very diluted conditions, whereas existing models could not predict particle consumption in the absence of particle surface activating species. Therefore, this work paves the way for a different approach to soot modeling that could help to better interpret soot reactivity. Oxidation of soot produced from different fuels (ethylene, n-heptane, and toluene, i.e. leading to different particle graphitization) at different temperatures (950–1073 K) and O 2 concentrations (1000–10000 ppm) is successfully modeled adopting reference oxidation rates of gas-phase resonantly stabilized PAH radicals, featuring analogous reactivity compared to soot species, and by also accounting for different particle hydrogenation levels. Good model performances are also obtained in both rich and lean premixed ethylene flames (T max ∼1480–1510 K) and in two sets of counterflow ethylene soot formation/oxidation flames (SFO, T max ∼2560–2825 K), where OH radical becomes the dominant oxidizer. Finally, model limitations emerging in some of the cases considered are discussed through a sensitivity analysis and with references to other oxidation mechanisms proposed in the literature in order to highlight possible future improvement of the model predictive capability [ABSTRACT FROM AUTHOR]

The mechanism of evolution of polycyclic aromatic hydrocarbons (PAHs) into carbonaceous particles in combustion, atmosphere, and interstellar space has been the subject of intense debate. Recently, there has been emerging evidence supporting resonantly-stabilized radicals as key players in PAH growth. In this work, we build on this hypothesis and propose that, beyond a critical size, PAH reactivity can be assimilated to that of radicals. We found that odd-C-numbered PAHs embedding 5-membered rings rapidly lose a hydrogen atom to form resonantly-stabilized radicals in combustion conditions, while even-C-numbered PAHs react as open-shell rather than closed-shell molecules independently of temperature, as usually assumed. Acenes were used as molecular models of large even-C-numbered PAHs. The construction of a kinetic model including these findings allows to interpret experimental soot oxidation data otherwise irreconcilable with existing chemical kinetic mechanisms. [ABSTRACT FROM AUTHOR]

• New measurements of Benzene flame speed in air. • Prediction of UFL and LFL at different pressures, temperatures and dilutions. • Quantification of the relevance of soot for flammability limits. • First comprehensive interpretation of all existing benzene flammability data. • Approach applicable to other high-sooting fuels. This research introduces an innovative approach to predict benzene Lower and Upper Flammability Limits (LFL and UFL). The focus of this study is on predicting the flammable range of benzene/air/steam mixtures utilizing a freely-propagating flame method, incorporating an optically-thin approximation to model soot radiation. The investigation delves into the consequences of dilution by inert gases (N 2 and steam), along with the impacts of pressure and initial temperature. Soot is recognized as essential not only for its role in flame chemistry under rich conditions but also for its influence on radiation, thereby affecting the flammable region of hydrocarbons especially at higher temperatures and pressures. To address the significant formation of soot during benzene combustion near the UFL, the study integrates the kinetic model for benzene combustion with a recently developed soot mechanism based on the discrete sectional method, which has been validated extensively against a large database of sooting flames, encompassing various hydrocarbons, including benzene. To limit the computational effort associated with predicting flammability limits, a skeletal version (with 136 species and 4788 reactions) of the overall kinetic model covering benzene combustion and soot formation is developed and validated in this work. The kinetic model was first validated against new and existing benzene flame speed data at different pressures and initial temperatures. Then it was used to investigate the flammable range. The model predictions align remarkably well with the available experimental data in the literature for the LFL, for the effect of dilution with inert gases, and with some experimental measurements for the UFL. An extensive review of these experimental data revealed significant uncertainty in characterizing benzene's UFL experimentally, both in terms of absolute value and effect of initial temperature. The comprehensive model predictions provide valuable insights, enabling differentiation among various UFL datasets for benzene. [ABSTRACT FROM AUTHOR]