Ethylene Counterflow Diffusion Flames Research Articles

Comprehensive kinetic study on ammonia/ethylene counter-flow diffusion flames: influences of diluents

This study aimed to investigate the effects of ammonia addition on ethylene counter-flow diffusion flames with different diluents on the fuel or oxidizer side, using kinetic analyses. A special emphasis was put on assessing the coupled chemical effects of NH3 and CO2 on C2H4 combustion chemistry. The chemical effects could be evaluated by comparing fictitious inert NH3 or CO2 with normal active NH3 or CO2. The results revealed that the addition of NH3 decreased the mole fractions and production rates of key soot precursors, such as acetylene, propynyl, and benzene. When CO2 was used as the dilution gas, the coupled chemical effects of NH3 and CO2 were affected by the chemical effects of CO2 to varying degrees. With the oxidizer-side CO2 addition, the coupled chemical effects of NH3 and CO2 reduced the mole fractions of H, O, OH radicals, acetylene, propynyl, and benzene, while the effects differed from the fuel-side CO2 addition. The coupled chemical effects of NH3 and CO2 also promoted the formation of aldehyde contaminants, such as acetaldehyde, to some extent, particularly with CO2 addition on the oxidizer side.

Effects of dimethyl ether and ethanol additions on soot transition in ethylene counterflow diffusion flames

Effects of dimethyl ether and ethanol additions on soot transition in ethylene counterflow diffusion flames

Effects of Ammonia Addition on the Sooting Characteristics of Ethylene Counterflow Diffusion Flames with Oscillating Strain Rates

Effects of Ammonia Addition on the Sooting Characteristics of Ethylene Counterflow Diffusion Flames with Oscillating Strain Rates

Soot nanoparticle sizing in counterflow flames using in-situ particle sampling and differential mobility analysis verified with two-colour time-resolved laser-induced incandescence

The emphasis of this work is on the development of an intrusive particle sampling system to track the evolution of soot nanoparticles in ethylene counterflow diffusion flames. The overarching objective is to determine the mobility size distributions in a spatially resolved manner using the developed probe system coupled with differential mobility analysis, i.e., a scanning mobility particle sizer (SMPS). The probe system involves a tailor-made quartz probe, gas supply, pressure control periphery, and a traverse system enabling a precise positioning along the flame axis. In preliminary experiments, the dilution ratio of the quartz probe as function of boundary conditions as well as particle losses during intrusive particle sampling are studied. To demonstrate the capability of the developed particle sampling system, results from ethylene counterflow diffusion flames with different fuel mass fractions and strain rates are presented and compared with results derived by non-intrusive laser-based diagnostics, i.e., two-colour time-resolved laser induced incandescence (2C-TiRe-LII). Results of these experiments indicate that the particle sampling system is capable of tracking the development of particle size distributions – independent of the distribution function, i.e., mono-, bi- or multimodal shape – in counterflow flames. Likewise, the agreement between soot volume fractions and particle size distributions measured via intrusive particle sampling coupled with differential mobility analysis and non-intrusive laser-based 2C-TiRe-LII is excellent at varying the fuel mass fractions and strain rates of the ethylene counterflow flames.

Synergistic effect on PAH and soot formation in ethylene counterflow diffusion flames by the addition of 1,3-dioxolane - a bio-hybrid fuel

Bio-hybrid fuels combine the utilization of bio-based feedstocks and carbon dioxide with renewable electricity to achieve a carbon-neutral and low-emissions future for the transportation sector. A potential candidate is the heterocyclic acetal 1,3-dioxolane (C3H6O2). However, little is known about the mechanism behind the impact of 1,3-dioxolane on soot formation. In this study, we investigated ethylene counterflow diffusion flames with 1,3-dioxolane addition in different amounts. The equilibrium temperature, the stoichiometric mixture fraction, and the strain rate were kept nearly constant to highlight the chemical effect of 1,3-dioxolane addition. Measurements of soot volume fraction with laser-induced incandescence showed that the addition of 1,3-dioxolane up to 30 % leads to a synergistic effect on soot formation with a maximum at 10 %. To reveal the chemistry behind the synergistic effect and its attenuation, speciation measurements were performed on three representative flames (0 %, 10 %, and 30 % fuel mole fraction of 1,3-dioxolane) with gas chromatography-mass spectrometry. The experimental data were compared to kinetic model simulations for interpretation. It was found that the addition of 1,3-dioxolane enhances methyl radical formation, which promotes C3 species formation via C1+C2 pathways. As a consequence, C3 species increase monotonically, whereas some C4 species show a non-monotonic behavior. Pathway analyses revealed that benzene dominantly forms via the C3 route. The main soot precursor naphthalene primarily forms via C4+2C3 pathways through C7 species, and benzene-based pathways, i.e., C6+C4 and H-abstraction-C2H2-addition (HACA), play a minor role. The observed synergistic effect on naphthalene formation is promoted by C3 species for 1,3-dioxolane addition in small amounts. At higher fuel mole fractions of 1,3-dioxolane, C4 species become the bottleneck in naphthalene formation and the synergistic effect subsides. Naphthalene passes this behavior mainly via the HACA pathway to larger polycyclic aromatic hydrocarbons and subsequently to soot.

Detailed chemical effects of ammonia as fuel additive in ethylene counterflow diffusion flames

Ammonia is considered one of the most competitive fuels due to its carbon neutrality. The chemical effects of NH3 are distinguished by kinetic analysis via adding NH3 as reactive NH3 and fictitious inert NH3. The flame temperature and the mole fraction profiles affected by the chemical effects of NH3 addition for important species and soot are analyzed, with special emphasis on soot and its important precursor polycyclic aromatic hydrocarbons (PAHs). The results illustrate that NH3 addition inhibits the production of A1-A4. The chemical effects of ammonia decrease the hydrogen abstraction–C2H2–addition (HACA) surface growth rate and PAH condensation rate, which further reduces soot volume fraction and average particle diameter D63. The ammonia decomposition pathways interact with ethylene decomposition pathways via the four reactions: NH3 + C2H5 = C2H6 + NH2, HCN + C2H5 = C2H6 + CN, NH2 + C2H4 = C2H3 + NH3, and CH2CH2NH2 = C2H4 + NH2. The dilution and thermal effects of NH3 are dominant effects on soot reduction, while the chemical effects further inhibit soot formation.

Incipient sooting tendency of oxygenated fuels doped in ethylene counterflow diffusion flames

The incipient sooting tendencies of oxygenated fuels in counterflow diffusion flames were systematically assessed by doping selected oxygenate fuels into the baseline fuel of ethylene with various mixing ratios. Laser light scattering and planar laser-induced fluorescence (PLIF) techniques were adopted. The critical oxygen mole fractions at different mixing ratios were identified with the use of laser scattering. It is found that the critical oxygen mole fraction varies with oxygenates, suggesting that the effect of mixing ethylene with oxygenates is highly sensitive to the types of oxygenate. While the doping of acetone moderately promoted the incipient sooting tendency, the doping of methanol, acetic acid, formic acid, and diethyl carbonate suppressed the formation of incipient soot. On the other hand, the doping of ethanol, dimethyl carbonate, diethyl ether, and dimethoxymethane had weak influences on the incipient sooting tendency. The production of polycyclic aromatic hydrocarbons (PAHs) of different sizes were measured by PLIF. The results show there was a strong correlation between the incipient sooting tendency and the production of large PAHs. One important finding is that, at their sooting limits, the total oxygen-to-carbon ratio of a stoichiometric fuel/oxygen mixture is linearly correlated with the mixing ratio of the ethylene-oxygenate mixture. This slope of these two quantities can be defined as an incipient sooting index (ISI) of oxygenated fuels for ranking their tendency of incipient sooting formation. Having the slopes correlated well with the oxygenates’ hydrogen/oxygen ratio, a reasonably accurate prediction of critical oxygen mole fractions from the molecule information of oxygenated fuels can be achieved.

Temperature dependence of chemical effects of ethanol and dimethyl ether mixing on benzene and PAHs formation in ethylene counter-flow diffusion flames

The present work investigates the chemical, thermal, and dilution effects of ethanol and dimethyl ether mixing (10%/20%) on aromatics formation in ethylene counter-flow diffusion flames (CDFs) under different thermal conditions (Tmax≈1729/2000/2063 K). The fuel mixing effects were studied experimentally by gas chromatography measurements and computationally using two detailed chemical reaction mechanisms. The effects of ethanol and DME mixing on benzene and PAHs formation was found to be temperature-dependent in ethylene CDFs. Ethanol and dimethyl ether blending promoted benzene formation in high-temperature flames (2000 K and 2063 K) in comparison to low-temperature flames (1729 K). A slight reduction of benzene concentration was found after dimethyl ether addition under low-temperature conditions. The total effects of ethanol and dimethyl ether mixing on typical PAHs (C10H8, C14H10, C16H10) in low-temperature ethylene flames were also weaker than those in high-temperature flames. The temperature dependence of the total fuel mixing effects was dominated by chemical effects, while dilution and thermal effects were not significantly affected by the temperature. Pathway analysis showed that the temperature dependence of chemical mixing effects on benzene and PAHs formation originated from the C4+C2→C6→PAHs and C3+C3→C6→PAHs pathways.

Effects of diluent gases on sooting transition process in ethylene counterflow diffusion flames.

The impacts of adding diluent gases (nitrogen, carbon dioxide, and helium) either to the fuel side or the oxidizer side on the sooting transition process in ethylene counterflow diffusion flames are investigated experimentally and numerically. A series of ethylene flames ranging from non-sooting to heavy-sooting are studied by gradually increasing the oxygen concentration on the oxidizer side. The optical method is used to analyze flame images, determining the sooting transition process. It is found that whether CO2 is added to the fuel side or the oxidizer side, the sooting transition process is delayed significantly. This process is slightly delayed when He is added to the fuel side, however, it is promoted when He is introduced to the oxidizer side. The numerical results show that in CO2-diluted flames, the mole fraction of the main soot precursors C2H2, C3H3, and C6H6 are reduced, which leads to the delay of soot formation. In addition, the H radical decreases while the OH radical increases, both of them are important for soot formation. In He-diluted flames, the concentration of C2H2, C3H3, and C6H6 decreased, as well as H and OH radicals. Moreover, adding He obviously changes the distribution area of products.

Chemical speciation and soot measurements in laminar counterflow diffusion flames of ethylene and ammonia mixtures

Ammonia is considered as one of the most promising alternative fuels due to its carbon neutrality and the existing infrastructure for its mass production and delivery. However, burning neat ammonia has the issue of poor flame stability and high NOx emissions, making co-firing ammonia with conventional fuel a more feasible approach. The present work investigated the sooting characteristics of counterflow diffusion flames of ethylene/ammonia mixtures. Experimentally, soot volume fraction (SVF) and average soot particle diameter in the neat ethylene, ammonia- and nitrogen- doped flames were non-intrusively measured. Both SVF and average soot particles diameter were found to decrease with the addition of ammonia. Flame temperature were measured with tunable diode laser absorption spectroscopy and the results suggested that the inhibiting effect of ammonia on soot formation was chemical instead of thermal. For further kinetic insights, numerical simulation with newly-constructed reaction mechanisms were performed and the results were compared against chemical speciation data from gas chromatography (GC) measurements; the results showed that ammonia doping would lead to more significant reduction of benzene concentration than nitrogen doping. Kinetic pathways of the chemical suppressing effect of ammonia addition on soot and its precursor formation were then explained based on numerical results. The major contribution of the present work can be summarized in the following aspects: 1) New comprehensive experimental data on sooting characteristics, important intermediate species concentrations of diffusion counterflow flames of ethylene/ammonia mixtures were provided for model validation; 2) One coupled mechanism with detailed hydrocarbon-nitrogen interactions was established to predict PAHs formation and soot formation; 3) Detailed chemical kinetics insight of ammonia effect on soot formation was presented in the counterflow flame.

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

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.

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.

The role of CO2 dilution on soot formation and combustion characteristics in counter-flow diffusion flames of ethylene

The effect on soot formation of carbon dioxide addition as diluent in ethylene/O2/Ar laminar counter flow diffusion flames has been examined both experimentally and with the help of a multi-sectional kinetic model. Different concentrations of CO2 have been used in the oxidizer stream to replace argon, whereas the amount of oxygen has been kept constant. Optical techniques have been adopted to measure particulate matter: laser-induced fluorescence (LIF) to detect carbon nanoparticles and laser-induced incandescence (LII) to detect soot. Significant reductions of both soot and carbon nanoparticles have been observed with the addition of CO2: by increasing the amount of CO2, the reduction of particulate matter increases. In the case of 100% CO2 as diluent, soot particles are completely depleted, whereas carbon nanoparticles are still formed but at a lower rate. Therefore, the net result of CO2 addition is the reduced emission of total particulate matter enriched in carbon nanoparticles. On the basis of the results of the kinetic model, particle reduction has been mostly attributed to a thermal effect due to the reduced adiabatic flame temperatures at increasing amounts of CO2 added and only partially to the chemical effect of CO2 in enhancing OH radical formation.

Chemical effects of hydrogen addition on soot formation in counterflow diffusion flames: Dependence on fuel type and oxidizer composition

We experimentally studied and kinetically modeled the effects of hydrogen addition on soot formation in methane and ethylene counterflow diffusion flames (CDFs). To isolate the chemical effects of hydrogen in such flames, we also ran a set of experiments on flames of the same base fuels but with the addition of helium. Specifically, we measured the soot volume fractions of the flames using the planar laser-induced incandescence technique. We simulated detailed sooting structures by coupling the gas-phase chemistry with the polycyclic aromatic hydrocarbon (PAH)-based soot model, using a sectional method to resolve the soot particle dynamics. Our experimental and numerical results show that hydrogen chemically inhibits soot formation in ethylene CDFs. While in methane flames, it is interesting to observe that the difference in soot production between the hydrogen- and helium-doped cases became much smaller when the oxygen concentration in the oxidizer stream (XO) was reduced. This suggests that for methane CDFs the chemical soot-inhibiting effects of hydrogen was highly dependent on oxidizer composition (i.e., XO). Explanations are provided through detailed kinetic analysis concerning the effects of hydrogen addition on the growth of PAH, soot inception, and soot surface growth processes. Our results suggest that hydrogen's chemical role in soot formation not only depends on fuel type (ethylene or methane), it also may be sensitive to oxidizer composition.

Coupled Effects of Carbon Dioxide and Water Vapor Addition on Soot Formation in Ethylene Diffusion Flames

An experimental and numerical investigation on the effects of the addition of CO₂ and H₂O on soot formation in ethylene counterflow diffusion flames was performed. Experimentally, axial profiles of soot volume fractions were measured using a light extinction technique. A numerical model, coupling a detailed gas-phase reaction mechanism and a polycyclic aromatic hydrocarbon (PAH)-based sectional soot model, was used to investigate the underlying mechanisms of the observed experimental trend. It was observed that addition of either CO₂ or H₂O led to a monotonic reduction of soot volume fraction. The results also showed that when added in the fuel stream, H₂O was more effective than CO₂ in inhibiting soot formation. However, CO₂ became a more effective soot inhibitor than H₂O when added in the oxidizer side of the flame. Simultaneous addition of H₂O and CO₂, in either the fuel or oxidizer side, retained the soot suppression benefits from H₂O/CO₂ individual addition, suggesting negligible chemical interactions between the dopants in terms of their effects on soot formation. The chemical suppressing effects of CO₂ and H₂O addition on soot formation were identified and attributed to reduced rates of PAH growth and soot inception. However, numerical analysis showed that the specific kinetic pathways responsible for soot inhibition were different for CO₂ and H₂O addition.

On the opposing effects of methanol and ethanol addition on PAH and soot formation in ethylene counterflow diffusion flames

An experimental and numerical analysis of the effects of methanol and ethanol addition on polycyclic aromatic hydrocarbon (PAH) and soot formation in non-premixed ethylene flames is reported here. Laser-induced incandescence (LII) and laser-induced fluorescence (LIF) techniques were used to measure soot volume fractions and relative PAH concentrations in counterflow diffusion flames, respectively. A comprehensive chemical kinetic analysis was performed by modeling soot with detailed gas-phase chemistry and a sectional method. The results showed that although both methanol and ethanol are typically regarded as clean fuels, their presence in ethylene diffusion flames had the opposite effects on PAH and soot formation. The LIF and LII signals decreased significantly as methanol fraction increased, suggesting a soot-inhibiting role for methanol. Apart from the fact that methanol addition reduced the carbon supply for soot thus having a fuel-dilution effect (methanol converted primarily to CO), the increased H2 concentration from methanol decomposition was seen to chemically suppress incipient benzene ring formation and subsequent PAH and soot growth processes. In contrast, a small amount of ethanol addition enhanced soot formation, which was well captured by the numerical model. Reaction pathway analysis showed that ethanol decomposition produced a relatively large amount of methyl radicals, enhancing the chemical interaction between CH3 and C2 species and, thereby promoting the formation of propargyl and C4 species. As a result, benzene formation was promoted through reactions between C2H2 and C4 species and via C3H3 recombination reaction, leading sequentially to the enhancement of PAH growth and soot formation.

An experimental study on the spectral dependence of light extinction in sooting ethylene counterflow diffusion flames

A light extinction technique is widely-adopted for quantitative measurement of soot volume fractions. The measurement accuracy is dependent on the optical properties of soot, which are expected to vary with the wavelength of incident light and the physicochemical environments in which soot is formed. In the present study, a diode laser–based light extinction setup, capable of providing light with variable wavelengths ranging from 405 to 1064 nm, was utilized to investigate the in-situ spectral dependence of light absorption for soot formed in counterflow diffusion flames. Soot volume fractions (FV) were inferred from the extinction level of these laser beams for a series of flames parameterized by oxygen/fuel mole fractions, nozzle exit velocities, and fuel types. Special attention was given to distinguish between the soot formation (SF) and soot formation/oxidation (SFO) flames, considering their notable differences in soot evolutions. It was found that the inferred FV as measured with visible light (405–670 nm) was always significantly higher than those measured with near-infrared light (>780 nm). In addition, the quantitative decrease of FV with the increase in light wavelength (λ) was found to be different for soot particles formed at different flame locations and/or flame conditions, even in the spectral range above 780 nm for which polycyclic aromatic hydrocarbon (PAH) interferences are expected to be minimal. This confirms the wavelength dependence of the soot optical property E(m). In particular, the value of E(m) tends to decrease with increasing wavelength and the rate of decrease is lower for more mature soot particles. Furthermore, by fitting the extinction coefficient with wavelength in the near-infrared range, the quantitative relation of E(m) with λ was derived and compared among various flame conditions. The present study demonstrates that soot formed at different conditions have different optical properties. The results are also expected to provide essential information for uncertainty evaluation in literature FV data as measured in counterflow diffusion flames with light extinction, especially for those performed with visible light sources where PAH interference may not be negligible.

Effect of dimethyl ether (DME) addition on sooting limits in counterflow diffusion flames of ethylene at elevated pressures

The effects of dimethyl ether (DME) addition to ethylene fuel on sooting tendencies with varying pressure were investigated in counterflow diffusion flames by using a laser scattering technique. Sooting limit maps were determined in the fuel (XF) and oxygen (XO) mole fraction plane, separating sooting and non-sooting regions. The results showed that when DME is mixed to ethylene, the sooting region was appreciably shrank, especially in the cases of soot formation/oxidation (SFO) flames as compared with the cases of soot formation (SF) flames. This indicated an inhibiting role of DME on sooting. An interesting observation was that the critical XO required for sooting initially decreased and then increased with the DME mixing ratio to ethylene β for the cases of SF flames, exhibiting a non-monotonic behavior. This implied a promoting role of DME on sooting when small amount of DME is mixed to ethylene. As the pressure increased, the sooting region generally expanded. Specifically, the range of β in promoting soot formation extended with pressure. This implies that a strategy in reducing soot by adding DME to ethylene at high pressures required a large amount of DME addition. To interpret the observed phenomena, kinetic simulations including reaction pathway and sensitivity analyses were conducted with the opposed-flow flames model using the KAUST-Aramco PAH Mech. The results showed that the thermal effect of DME addition on sooting tendency monotonically decreases with β. The chemical effect was found to be the main contributor to the DME addition effect on sooting tendency, resulting in the non-monotonic sooting limt behavior. The pathway analysis showed the role of methyl radicals generated from DME promoted incipient benzene ring formtion when small amount of DME was added, which can be attributed to the soot promoting role of DME addition for small β.

Experimental and soot modeling studies of ethylene counterflow diffusion flames: Non-monotonic influence of the oxidizer composition on soot formation

Previous soot studies in counterflow diffusion flames revealed that the sooting limit curve in regions with large oxygen mole fractions (XO) exhibited marked bending behaviors that indicated a non-monotonic variation of sooting tendency with oxygen concentration. The underlying mechanisms of this bending behavior remained unclear. In this regard, the present study systematically investigated the effect of oxygen mole fraction in the oxidizer stream on the sooting characteristics of ethylene counterflow diffusion flames. We used the near-infrared light extinction technique to measure soot volume fractions of two types of flames that significantly differed in sooting structures: soot formation oxidation (SFO) flames with a fixed fuel mole fraction (XF) of 0.28 and varying XO between 0.5 and 1.0 and soot formation (SF) flames with pure ethylene (XF = 1.0) in the fuel stream and varied XO from 0.25 to 0.3. We also conducted detailed soot modeling studies by combining the gas-phase chemistry with a sectional soot model that accounted for soot inception, surface growth, particle coagulation as well as soot oxidation. Our experimental and modeling results demonstrated the non-monotonic relationship between soot volume fractions and XO in SFO flames. A detailed analysis of the evolutionary process of soot formation revealed that the suppression of soot inception and the enhancement of the soot oxidation process with increasing XO led to a reduction of soot volume fraction. On the contrary, the surface growth rates increased with XO, resulting in an increase in soot mass concentration. These competing effects led to the non-monotonic variation of soot volume fractions with XO in SFO flames. On the other hand, in SF flames both the inception and surface growth of soot increased as XO increased, resulting in the observed monotonic relationship between soot volume fraction and XO. We also analyzed the soot zone structures and made comparisons between SFO and SF flames.

Effect of temperature on soot inception in highly controlled counterflow ethylene diffusion flames

Soot volume fraction and dispersion index measurements were performed by multi-color pyrometry in a series of highly controlled ethylene counterflow diffusion flames in which the peak temperature was varied in a 300 K range, while maintaining constant stoichiometric mixture fraction and global strain rate. In so doing, the temperature-convective time history remained nearly constant, when the temperature was normalized with respect to its peak value. Therefore, the peak temperature was isolated as the key controlling parameter. We observed two distinct trends in the soot measurements: a high-temperature, high activation energy process near the flame front, for temperatures larger than ∼1600 K, that can be rationalized with conventional nucleation and surface growth models; and a low temperature zero-activation process that is hypothesized as dimerization (homogenous nucleation) of aromatics. The first process is accompanied initially by large values of the dispersion index, denoting small C/H ratios, followed by a rapid decrease towards a plateau, as soot carbonization occurs. The second process is accompanied by a sharp increase in the dispersion index, consistent with a new inception process that is well distinct from that in the high temperature region. In one of the flames studied the soot measurements supplement an extensive set of measurements of gaseous species from a previous study, thereby offering a comprehensive database for modelers.