Formation Of Soot Precursors Research Articles

Numerical investigation of soot emission sources in a direct-injection spark-ignition engine based on comprehensive breakup model validation

Direct injection system is widely adopted in spark-ignition engines to achieve higher thermal efficiency, but it accompanies a penalty in particulate emission, especially when engine is not fully warmed-up. Split injection strategy is known to be an effective measure to reduce engine-out particulate emissions. To better understand the role of split injections, this study aims to analyze the effect of split injection strategy on the sources of soot formation using computational fluid dynamics simulation. To accurately predict changes in particulate mass and number associated with split injection strategy, it is vital that spray models be carefully validated against the experimental data since spray dynamics govern the formation of soot emission sources, such as local fuel-rich mixtures and wall-deposited fuel-films. To this end, a set of spray experiments for free sprays is performed to measure liquid penetration length and droplet size distribution, and hence a comprehensive validation is conducted for spray breakup models. Then, engine simulations are carried out to predict the change in soot sources according to split injection, and the trend of simulation results is compared against the measured engine-out particulate mass and number. Simulation results indicate that breakup model validation using both penetration length and droplet size data is critical for predicting fuel spray dynamics and formation of sources of soot emission. It is also revealed that the piston wetting decreases as the number of injections increases because less amount of fuel is injected when piston is closer to the injector. Lastly, the late evaporation of heavy gasoline components from fuel-film appears to be a significant contributor to soot precursors formation.

Effects of ammonia and hydrogen on the sooting characteristics of laminar coflow flames of ethylene and methane

Hydrogen and its derivatives, including ammonia, are gaining increasing attention as carbon-neutral fuel alternatives. An intermediate step in the transition to hydrogen and ammonia is the blending of these fuels with hydrocarbons, introducing the challenge of soot formation. The impact of ammonia on soot formation has recently been the focus of several studies, but a complete understanding of its chemical effects is lacking. Hydrogen, by comparison, has received significant attention from the soot community. However, controversy remains with regards to hydrogen’s chemical impact, and the dependence of this impact on fuel and flame configuration. This work investigates the effect of both hydrogen and ammonia on soot formation in laminar coflow flames of both ethylene and methane. Hydrogen or ammonia are introduced either by addition or substitution, with parallel studies of helium and argon, in order to isolate their chemical effects. Time- and spectrally-resolved laser-induced emissions from UV and IR excitation are used to quantify differences in soot and soot precursor formation. Additionally, chemical kinetics calculations and analyses are used to elucidate the effects of ammonia introduction to ethylene flames. Ammonia is found to chemically inhibit soot when mixed with either ethylene or methane, with increasing effects on larger precursors. Calculations suggest that this suppression is due to carbon consumption in the formation of HCN and CN. Hydrogen is found to chemically enhance soot formation in both ethylene and methane flames.

Doping rich ethylene premixed flames: Influence of C[formula omitted][sbnd]C[formula omitted] alcohols on the structure of the steady one-dimensional laminar flame

The addition of alcohols to fueled mixture has been extensively documented as a practical way to significantly mitigate soot emissions by internal combustion engines. While being crucial to the initiation of some soot precursors formation, some chemical pathways attributed to the presence of alcohol-derived species remain unclear. Consequently, atmospheric steady flat laminar rich (equivalence ratio ϕ=1.7) ethylene/air premixed flames were established over a McKenna burner. The unburnt stream was seeded by prevaporized alcohols ranging from C3 to C5. Every flame was probed and sampled gases were investigated using a gas chromatography (GC) device equipped with a flame ionization detector (FID). Thus, quantitative mole fraction profiles are reported as a function of the distance to the burner tip. In total, 17 chemical compounds, including isomeric species (butene and pentene) have been detected quantitatively. Significant discrepancies among the intermediate species pool were observed, especially the amount of propene, butene, and pentene isomers (produced mostly due to the addition of propanol, butanol, and pentanol, respectively). Several kinetic mechanisms support the experimental results, none of them do reproduce properly the alkene production measured. Further numerical investigations were conducted focusing on the pathways that lead to the production of the alkenes identified. To the authors’ knowledge, the results reported are the first that (i) provide experimental results for the addition of 2,2-dimethyl-propanol, (ii) document unambiguously the mole fraction profiles of the three butene and five pentene isomers, and (iii) unveil similar trends on alkene production due to the addition of C3 to C5 alcohols, i.e. a clear hierarchy among primary, secondary, and tertiary alcohols for the rich atmospheric premixed flames studied.

Effects of ammonia addition on soot formation in ethylene laminar premixed flames

This paper presents particle size distributions and chemical compositions of soot measured using a Scanning Mobility Particle Sizer (SMPS) and a Fourier-Transform Ion Cyclotron Resonance Mass Spectrometer (FT-ICR-MS), respectively, in flames of ethylene-oxygen and ethylene-oxygen doped with ammonia. The inhibitory effect of ammonia addition on soot formation in ethylene flames, as well as the influence on soot particle size are discussed. The results of particle size distributions showed that doping with ammonia decreases soot particle size and the volume fraction of soot particles. In this work, the flames were carefully studied under similar temperature conditions, so all observed changes could be attributed to chemical effects. The results of FTICR-MS analysis showed that polycyclic aromatic hydrocarbons (PAHs) were the main compositions in both pure ethylene and ammonia doped ethylene flames. Nitrogen-containing PAHs (N-PAHs) did not contribute significantly to soot nucleation or mass growth processes. Chemical kinetic analysis revealed that CN species generated in ethylene-ammonia flames removed carbon from participating in soot precursor formation, thereby reducing soot formation.

Ultrafine Particulate Matter in Methane-Air Premixed Flames With Oxygen Enrichment

A complementary computational and experimental study is carried out on the formation of ultrafine particulate matter in premixed laminar methane air flames. Specifically, soot formation is examined in premixed stretch-stabilized flames to observe soot inception and growth at relatively high flame temperatures common to oxygen enriched applications. Particle size distribution functions (PSDF) measured by mobility sizing show clear trends as the equivalence ratio increases from Φ = 2.2 to Φ = 2.4. For a given equivalence ratio, the measured distribution decreases in median mobility particle size as the maximum flame temperature increases from approximately 1,950–2,050 K. The median mobility particle size is 20 nm or less for all flame conditions studied. The volume fraction decreases with increasing flame temperature for all equivalence ratio conditions. The Φ = 2.2 condition is close to the soot inception limit and both number density and volume fraction decrease monotonically with increasing flame temperature. The higher equivalence ratio conditions show a peak in number density at 2,000 K which may indicate competing soot inception processes are optimized at this temperature. Flame structure computations are carried out using detailed gas-phase combustion chemistry of the Appel, Bockhorn, Frenklach (ABF) model to examine the connection of the observed PSDF to soot precursor chemistry. Agreement between measured and computed flame standoff distances indicates that the ABF model could provide a reasonable prediction of the flame temperature and soot precursor formation for the flames currently studied. To the first order, the trends observed in the measured PSDF could be understood in terms of computed trends for the formation of benzene, naphthalene and other soot precursors. Results of the current study inform particulate matter behavior for methane and natural gas combustion applications at elevated temperature and oxygen enriched conditions.

Detailed examination of the combustion of diesel and glycerol emulsions in a compression ignition engine

This study examines glycerol as an additive to diesel fuel to demonstrate it has the potential to suppress the formation of soot/PM. The investigation of a diesel/glycerol emulsion included an engine trial, high-speed imaging in an optical combustion chamber and a fundamental chemical kinetic study examining soot precursor formation. The emulsion had a longer ignition delay but higher AHRR with increasing load. There was no impact on the brake thermal efficiency. CO and THC were higher with the emulsion at the lower engine loads. The emulsion emitted a smaller number of particles with diameters greater than 25 nm, with a significant drop in the number of particles at 60 nm. The number of particles with diameters greater than 25 nm is reduced by 61% at 20 Nm, by 56% at 80 Nm, and by 11% at 140 Nm. A large peak of sub 10 nm particles, 2 orders of magnitude greater than with diesel alone, was observed, hypothesised to be semi-volatile organic compounds that have started to condense. A thermogravimetric analysis supported a larger semi-volatile content. Ignition delay time, determined from the OH* flame emission, was always longer for the emulsion at all conditions. In-flame soot was always lower with the emulsion at all conditions. Flame lift-off length decreased with increasing temperature and pressure of the ambient gas whilst soot increased. The concentration of known soot precursors, C2H2 and C2H4 was reduced but the concentrations of C3H6 and PC3H4 were not significantly affected.

Investigation of natural gas/hydrogen mixtures for exergy storage in a piston engine

The conversion of mechanical to chemical energy offers an option for long-term and versatile energy storage. It was already proven that piston engines can be used as flexible reactors for energy conversion. Here, a novel method for energy conversion in piston engines is investigated, the pyrolysis of natural gas/hydrogen mixtures for energy storage. The supplied energy is stored by chemical conversion into hydrogen and higher energy hydrocarbons. The storage efficiency and the product composition are addressed here. To reach sufficiently high temperatures after compression, a dilution with 85–99% argon is used. The main products are hydrogen, acetylene, ethylene and benzene but also soot precursors are formed. The piston engine is simulated as a time-dependent four-stroke single-zone model with detailed chemical kinetics. The intake pressure is kept constant at 2 bar, while intake temperature, intake argon mole fraction and the hydrogen/natural gas ratio is varied. The hydrogen addition allows a reduction of the intake temperature and argon dilution but also reduces the storage power and efficiency. Yields of acetylene or ethylene are increased and the formation of soot precursors is suppressed. A storage power of 1.59 kW is reached with an efficiency of 52%.

Technical application of a ternary alternative jet fuel blend – Chemical characterization and impact on jet engine particle emission

The use of alternative fuels is an essential element in future aviation. There are currently different approaches with regard to fuel production processes and feedstock materials. This requires flexible handling of available fuel quantities. The production of fuel mixtures (blends) is inevitable for an effective usage of commercially available fuel components. Within the framework of the project DEMO-SPK approx. 600 t of a ternary mixture (multiblend) of alternative jet fuels was produced and used in a real airport infrastructure at the airport Leipzig/Halle. Production and application were accompanied by extensive R&D activities to show that on-spec, semi-synthetic multiblends can be produced from several different synthetic fuels. One important aspect is the emission of soot particles from aircraft engines. The Multiblend Jet A-1 has been analyzed in a flow reactor setup regarding the formation of soot precursor compounds in comparison to a reference Jet A-1. The experiment revealed a lower formation of relevant soot precursors which also corresponds to the higher hydrogen content of the Multiblend Jet A-1 compared to the reference fuel. The lower soot formation in a real aircraft engine has been proven during ground runs of an A300-600 aircraft with PW4158 engines. The Multiblend Jet A-1 showed lower particle number and particle mass emission than the reference fuel. The difference to the reference fuel decreased with increasing power settings. In summary, the emission of particle mass is reduced by ~ 29 % and the number of emitted particles is reduced by ~ 37 % if the ICAO landing-and-take-off cycle is used for evaluation.

Experimental study of the influence of hydrogen as a fuel additive on the formation of soot precursors and particles in atmospheric laminar premixed flames of methane

We report here the experimental investigation of lightly sooting methane premixed flames with and without hydrogen. Two different approaches were considered to introduce hydrogen in the methane flame, either by keeping the total gas flow rate constant or not. Speciation data were obtained using a set of analytical tools including Gas Chromatography, Fourier-Transform Infrared Spectroscopy, Jet-Cooled Laser-Induced Fluorescence, Laser-induced Incandescence coupled with Cavity Ring-Down Spectroscopy.The results include mole fraction profiles of gaseous species (C0-C16) and soot volume fraction (fv) measured in all studied flames. These results demonstrate that the introduction of hydrogen to the flame insignificantly impacts the maximum mole fractions of small species (<C6) while it strongly influences the concentration of aromatics and fv. Interestingly, this influence is different, depending on how hydrogen was added to the reference methane flame. An excellent linear relationship between pyrene squared concentration [C16H10]2 and fv is demonstrated in all investigated flames. Moreover, hydrogen addition changes the slope of these data indicating a non-negligible impact of hydrogen on the rate of the soot nucleation process. These data and the observed linearity between fv and [C16H10]2 supports therefore the idea of a soot nucleation step initiated by a dimerization process implicating moderate-size polyaromatic hydrocarbons (PAHs) as pyrene. Finally, we note that PAH mole fractions at the beginning of soot formation highlight similar values regardless the flame conditions, which suggests that the inception of the soot formation might be dependent of a minimum concentration threshold of aromatic precursors required to initiate the soot nucleation process.

Soot emission reduction in oxygenated co-flow jet flames

A computational study was performed for ethylene/air non-premixed laminar co-flow jet flames using an axisymmetric CFD code to explore the effect of oxygenation on PAH and soot emissions. Oxygenated flames were established using N2 diluted fuel stream along with O2 enriched air stream such that the stoichiometric mixture fraction (Ζst) is varied but the adiabatic flame temperature is not materially changed. Simulations were carried out using a spatially and temporally accurate algorithm with detailed chemistry and transport. A detailed kinetic model involving 111 species and 784 reactions and a fairly detailed soot model were incorporated into the code. Two different approaches, one with constant flame height and other with constant inlet velocity are comprehensively examined to bring out the effects of changes in flame structure and residence time on soot emissions with respect to Zst. With increase in Ζst, a drastic reduction in the formation of soot precursors (acetylene and benzene) and thus in soot emissions are observed. In the present study, oxygenated flames with Ζst ≥ 0.424 are considered as blue flames or completely soot free. For various oxygenated flames a C/O ratio between 0.45 and 0.6 is found to be most favorable for soot formation.

Investigation of structural effects of aromatic compounds on sooting tendency with mechanistic insight into ethylphenol isomers

Small aromatic molecules with oxygen-containing functional groups are a promising class of fuel additives, as they can be readily sourced from depolymerized lignin. These oxygenated aromatic compounds (OACs) show a lower sooting tendency than aromatic hydrocarbons, but OACs having alkyl groups such as ethylphenol show a higher sooting tendency than other OACs such as phenol and anisole, despite the oxygen moiety. In this study, we investigate the relationship between chemical structure and soot precursor formation to explain observed differences in the sooting tendency of OACs and to gain insight into how alkyl or oxygenated substituents on the aromatic ring affect soot precursor formation. The weakest bond for 15 aromatic compounds was identified and cleavage of these bonds was shown to generate either benzyl or phenoxy radicals. A linear relationship between standard enthalpy of formation (ΔHfo) of these radicals and the yield sooting index (YSI) was found, and thus ΔHf° can be applied as a metric to estimate YSIs of various aromatic compounds; higher ΔHfo of a radical indicates an increase in the radical reactivity and leads to more soot precursor formation. Flow reactor experiments were performed for 2-ethylphenol and 3-ethylphenol to elucidate how ortho and meta substitution effects the sooting tendency. Soot precursors were identified from the experiment and their formation pathways were investigated computationally. 2-ethylphenol produces more oxygenated products than 3-ethylphenol since the ortho position has increased resonance stabilization of radical intermediates, which leads to lower YSI. These results further inform the selection of potential biomass-derived fuel blendstocks that have favorable sooting tendencies.

Effects of 2, 5–dimethylfuran/ethanol addition on soot formation in n-heptane/iso-octane/air coflow diffusion flames

The effects of 2, 5-dimethylfuran (DMF)/ethanol addition on the atomized combustion soot formation characteristics of n-heptane/iso-octane coflow diffusion flames are systemically studied. The changes of soot particles and deposition morphology at different heights in the flame of reference fuel with 15% ethanol (E15) and 15% DMF (DMF15) are analyzed. Results show that the addition of oxygenated fuels can increase the flame heights and decrease the brightness. With the addition of 15% ethanol and DMF, the flame heights are increased by 11.29% and 19.35% respectively, and the brightness of the edge region of the middle section of DMF15 flame is enhanced. At the nozzle, the average temperature of E15 flame increases by 80 K. The high temperature region of E15 flame enlarged and the average temperature difference of different heights is higher than 50 K. As for DMF15, the high temperature region of flame is constant, and the average temperature difference between various heights is smaller than 35 K. For three fuels, the size of the sediment particles formed at the low height above burner (HAB = 20 mm) is large, and the obvious pore structure can be seen at HAB = 60 mm. The addition of oxygenated fuels makes the pore structure distribution regular and the porosity decreases, and DMF15 (porosity = 0.2670) has larger pore structure than E15 (porosity = 0.1557). The oxygenated fuel can inhibit the formation of soot precursors and the agglomeration of soot particles, but the increase of particle size in the middle region of flame is promoted because of the increase of flame height and residence time of soot particles. As a result, the soot particle size of oxygenated fuel is decreased greatly in the oxidation stage, and the soot particle size of DMF15 (40.4%) is decreased more than that of E15 (37.1%). The effect of ethanol and DMF addition on the activation energy of low position soot is not obvious, which is about 130 kJ/mol. In the middle position of the flame, the activation energy of E15 is the highest (159.32 kJ/mol), and the activation energy of DMF15 is the smallest (148 kJ/mol). At the high position, E15 (212.07 kJ/mol) and DMF15 (226.73 kJ/mol) are less than that of DE0 (235.03 kJ/mol) respectively, indicating that the activation energy of the mature soot reduces, which is beneficial to the oxidative removal. Ethanol can dilute the fuel and increase the oxygen concentration in the mixture, while DMF can delay the ignition and prolong the residence time of soot and increase the content of disordered carbon in mature soot particles.

Highly radiating hydrogen flames: Effect of toluene concentration and phase

Adapting hydrogen as a carbon-free fuel for industrial applications requires new, innovative approaches, especially when radiant heat transfer is required. One possible option is to dope hydrogen with bio-oils, containing aromatics that help produce highly sooting flames. This study investigates the potential doping effects of toluene on a hydrogen-nitrogen (1:1 vol) flames. Flames with 1–5% toluene, based on the mole concentration of hydrogen, are measured using a combination of techniques including: still photographs and laser-based techniques. Toluene was mixed with hydrogen-nitrogen fuel mixture as either a vapour carried by nitrogen, or as a dilute spray. Spray flames are found to produce substantially more polycylic aromatic hydrocarbons, with significantly more soot near the nozzle exit plane, than the prevaporised flames. Increasing the dopant concentration from 1 to 3% of the hydrogen has a marked effect on soot loading in the flame, although the further increasing the dopant concentration to 5% has a far smaller effect on the soot produced in the flame. Simulations of laminar flames using detailed chemical kinetics support the above findings and reveal details of the competition between soot precursor formation and hydrocarbon oxidation. Correlations of formation rates are non-linear with toluene concentration in cases where toluene represents less than 10% of the fuel, although expected linear relationships are noted beyond this regime up to 1:1 toluene/hydrogen blends. The study provides insight and explanation into effects of toluene as a dopant, comparison between flame doping in gaseous or liquid phases and suggests that flame doping and blending should be treated as different regimes for their global effect on flame sooting characteristics.

Nanoscale inspection on carbon particles from commercial RP-3 kerosene combustion with different dilutions

RP-3 kerosene fuel is the dominant hydrocarbon fuel used in civilian airplanes in China. Lately, many administration and universal organizations imposed a stringent standard for civilian airplanes pollutants emission. The amount of pollutants emission can be affected by the introduction of a variety of additives into flames, such as inert diluents, gaseous additives. This paper reports the effects of different inert gases (nitrogen and helium) and carbon dioxide dilutions in the oxidizer side on soot characteristics generated from RP-3 kerosene inverse diffusion flames. Flame diluted by carbon dioxide produced less amount of soot compared to flames diluted by nitrogen and helium, possibly because of changes in flame shape and properties such as flame diameter and availability of oxygen atoms. Carbon dioxide suppressed soot precursors formation in inception zone and released particles with smaller diameter due to lack of hydrogen radicals, an increase of hydroxyl radicals pool concentration, and decrease in flame temperature. Flame diluted by helium generated much soot aggregates with bigger primary particles diameter compared to base flame (flame diluted by nitrogen) as a result of high temperature. Soot particles produced in carbon dioxide diluted flame had more amorphous nanostructure, longer interlayer spacing and nanocrystallite height, shorter nanocrystallite width, lesser number of layers, and higher oxygen content compared to flame diluted by nitrogen and helium, which reflected a better oxidation reactivity among other diluted flames.

Investigation on the dual-fuel active-thermal atmosphere combustion strategy based on optical diagnostics and numerical simulations

The dual-fuel active-thermal atmosphere combustion strategy has the potential to make the homogeneous combustion compression ignition more controllable, achieving low fuel consumption and engine emissions. However, the detailed combustion process and chemical interaction between the high- and low- reactivity fuels have not been fully understood. This work investigated the premixed iso-octane combustion assisted by the two-stage reaction of n-heptane on an optical engine. High-speed imaging technique was applied to visualize the natural flame luminosity and a data-processing method based on Cantera was proposed to evaluate the detailed chemical kinetics process. Results indicate that the high-temperature heat release stage changes from partially premixed combustion with distinct blue flames into the mixing-controlled combustion, showing evident signs of soot radiation when the two-stage ignition transits into three-stage ignition. Heat release from the premixed n-heptane is initially dominated by reaction HCO + O2 = CO + HO2 and turns into hydrogen-oxygen reactions with increasing temperature. In the primary reacting regions for the cases with latter injection timings, reactions C3H5 + H(+M) = C3H6(+M) and C3H3 + H(+M) = C3H4(+M) play the major role, which promotes the formation of soot precursors. Formaldehyde can represent the mixture reactivity. Reaction pathway of premixed n-heptane generated reactive species that promote the consumption of iso-octane. Reactions of iso-octane also enhance the formation of reactive species that further accelerate the combustion process. Retardation of the iso-octane injection timing reduces the local mixture reactivity and weakens the chemical interaction between iso-octane and n-heptane. This lowers the heat release rate, and, objectively, results in the transition of two-stage ignition to three-stage ignition.

Synergistic effects on soot formation in counterflow diffusion flames of acetylene-based binary mixture fuels

This study reports experimental data regarding the effects of the addition of propene and propane in the fuel stream on soot formation in acetylene counterflow diffusion flames. Compared to the baseline acetylene flame, a continuous reduction of soot volume fraction was observed as more propene was mixed in the fuel. However, the addition of 1% propane resulted in an enhancement of soot formation, exhibiting an interesting fuel synergistic effect. The propane-doped flames were seen to have a higher sooting tendency compared to the propene-doped one when the molar doping ratio was less than 5%. These experimental results were rather unexpected considering that propene has a much higher sooting tendency than propane. Through a kinetic simulation accounting for the formation of molecular soot precursors, it is shown that the previous CH3-dominated reaction routes in forming propargyl radicals, which were typically used to rationalize the synergistic effect of ethylene-based binary fuel mixtures, were insufficient to explain the present observation of propane-doped acetylene flame having high sooting tendency than the propene-doped one at low doping ratios. The role of H radicals in the formation of C4 species (via the paths C2H2 + H → C2H3 and C2H3 + C2H2 → n-C4H5) was found to be critical in understanding the sooting behavior of C2H2 flames.

Characterizing the diversity of aromatics in a coflow diffusion Jet A-1 surrogate flame

Understanding the formation of soot precursors requires properly taking into account their diverse structure and chemistry. To study the extent of this variety in a realistic combustion system, we simulated the growth of polycyclic aromatic compounds (PACs) in a diffusion coflow flame using a three-component Jet A-1 surrogate (n-decane/ propylbenzene/ propylcyclohexane) as fuel. A kinetic Monte Carlo software, which generates samples of the atomistic evolution of gas-phase molecules’ growth, was used to characterize the PACs’ growth in this flame. We leveraged our simulations to identify a number of aromatic structures. In addition to aromatic hydrocarbons, we were able to reproduce the diverse array of oxygenated PACs observed experimentally between 150u and 450u, specifically a number of PACs with five-membered carbon rings and furan, hydroxl, and ketone groups. The amount of PACs with masses between 450u and 600u was slightly over-predicted, indicating that in this mass range there are possibly other mechanisms (e.g., radical-radical interactions) that reduce the concentration of these PACs. As our growth mechanism is focused mostly on aromatic growth, we did not observe the more aliphatic molecules seen in experiment. Overall, our results suggest that there is a wide range of structures with different degrees of oxygenation and aromaticity that must be accounted for when modeling PACs’ growth in combustion environments.

Temperature dependence of the fuel mixing effect on soot precursor formation in ethylene-based diffusion flames

Soot formation is a complex process and can be affected by a variety of factors including base fuel type, fuel mixing, temperature and pressure. Under real-world conditions, these factors can interact with each other to influence soot formation jointly. The present work aims to provide an understanding on the temperature dependence of the fuel mixing effect on the formation of molecular soot precursors. In particular, the effects of propane and methane addition on aromatic formation in ethylene counterflow diffusion flames were studied experimentally by gas chromatography measurements and computationally utilizing a detailed chemical reaction mechanism. Two series of flames under high (with peak flame temperature ~2020 K) and low temperature conditions (~1660 K) were covered to investigate the fuel mixing effects and its interaction with flame temperature. The experimental results clearly demonstrated the temperature-dependence of propane’s mixing effects on benzene formation: Compared to the ethylene baseline flame a notable enhancement of benzene formation was observed with propane addition in the high temperature flames; while a reduction of benzene concentration was seen under low temperature conditions. For the methane mixing cases, the addition of methane decreased benzene formation in both the high and low temperature flames, with the effect being more pronounced under low temperature conditions. Numerical simulations of the tested flames were also conducted with a previously developed reaction mechanism (Wang et al, Combust. Flame, 160 (2013) 1667–1676). Through a detailed reaction pathway analysis based on the numerical data, it was found the addition of C3H8 and CH4 would inhibit the C4 + C2 but enhance the C3 + C3 benzene formation pathway. However, the relative importance of the two pathways to total benzene production rate was strongly dependent on flame temperature. This fact was identified to be the primary mechanism leading to the temperature dependence of the C3H8/CH4 mixing effect. The qualitatively different effect of C3H8 and CH4 addition on benzene formation in the high temperature flame was believed to originate from their different influences on CH3 and C3H3 radical concentrations. The present work highlighted the fact that the effects of fuel mixing on benzene formation can be highly dependent on flame thermal conditions. As a result, great care is needed when generalizing literature data of fuel doping effect on aromatic and sooting tendencies.

Effects of CO2 on soot formation in ethylene pyrolysis

Effects of CO2 on soot formation during ethylene pyrolysis were investigated in a laminar flow reactor with the addition of various amounts of CO2 (0–99.5% in mole fraction). Based on a quantitative dilution sampling technique and a scanning mobility particle sizer, soot particle size distributions and the associated global properties, including soot volume fraction, number density, and soot induction delay time, were examined. Results show that while addition of a small amount of the CO2 (0 - 10%) tends to promote soot formation as the total number and volume of soot particles increase and the soot induction delay time decreases, its further increase, from 10% to 99.5%, leads to an obvious reduction of the soot nucleation and mass growth rates. Subsequent kinetic modeling of the gas-phase chemistry showed that with increasing CO2 concentration, the corresponding concentrations of the soot precursors, namely benzene and pyrene, first increase and then decrease, which is consistent with the observed trend in soot formation. Further sensitivity and reaction path analyses of benzene formation indicate that CO2 addition produces more hydroxyl radicals, such that while the presence of a small amount of hydroxyl radicals increases the propargyl concentration and thereby promotes the formation of soot precursors, excessive hydroxyl radicals lead to more oxidation and hence inhibit soot formation.

Influence of water-vapor in oxidizer stream on the sooting behavior for laminar coflow ethylene diffusion flames

The effects of adding water-vapor to the oxidizer stream on soot production in laminar coflow diffusion flames under different oxygen indices (OI) were investigated both experimentally and numerically. A modified coflow Gülder-type burner was employed to produce the laminar ethylene flames for fifteen different conditions of the oxidizer stream from oxygen-deficient (OI 17%) to oxygen-enriched (OI 25%) conditions, and also without and with adding water-vapor into the oxidizer stream up to 10% on mole basis. The measured soot volume fractions were compared with numerical predictions obtained using the CoFlame code and a chemical kinetic mechanism that consists of reaction pathways up to 5-ring PAHs. The sectional soot model used to simulate the soot particles dynamics considers soot nucleation, surface growth, PAH condensation, oxidation, particle coagulation and fragmentation. Fairly good agreement between experimental and numerical results was found, as close as 1% and 5% of difference in the peak soot volume fraction for OI21% with 0% and 10% of water-vapor addition, respectively. Clear trends of increasing the soot volume fraction (peak and also overall) was observed with increasing OI, while a significant reduction was obtained with the addition of the water-vapor. For the cases analyzed, a reduction of the total soot content was up to 60%. The chemical effects were numerically isolated using non-reacting water-vapor and analyzed, contributing especially to the soot oxidation rates. Finally, a study of the main reaction pathways was performed to better understand the chemical effects of water vapor. The results show that water vapor addition affects the concentrations of H and OH radicals and alters the formation and oxidation of soot precursors.