Size and charge of soot particles in rich premixed ethylene flames

A nano differential mobility analyzer (DMA) is used to measure both the size and electrical charge distributions of soot particles generated during rich premixed combustion. The size distributions are bimodal. One mode peaks at diameters below the 3 nm lower limit of the nano DMA and falls off nearly exponentially with increasing particle diameter. The intensity of this mode persists with increasing height above the burner suggesting that it represents the continued formation of new particles. The second mode is lognormal in shape. Its intensity decreases and the mean diameter increases with increasing height above the burner due to coagulation and surface growth as the particles rise in the flame. The DMA measurements show that a substantial fraction of the soot particles are electrically charged in the flame, predominantly with a single charge per particle and with essentially equal numbers of positive and negative particles. These charged particles belong solely to the upper mode, whereas the lower mode remains charge neutral, suggesting that ions do not act as soot nuclei. Following soot inception, the fraction of charged particles quickly increases with height above the burner and stabilizes at ∼30% of the upper mode for each polarity.     

Detection of electrically charged soot particles in laminar premixed flames

A particle mass spectrometer has been used to investigate the formation of electrically charged species and soot particles in laminar premixed flames. The mass range was from 600–6 × 10⁵ amu and extends from high molecular hydrocarbons to soot particles of 10 nm diameter. The flames were stabilized on cooled porous plate burners. Acetylene/oxygen flames were investigated at low pressure (30 mbar), and ethylene/air flames were investigated at atmospheric pressure. Soot particles could only be detected in flames showing yellow luminosity, i.e. above the critical C/O-ratio for soot formation. Both positively and negatively charged particles were found, the positive charge dominating in the low pressure acetylene/oxygen flames, the negative charge dominating in the atmospheric ethylene/air flames. With the assumption of spherical shape and constant density, the mass spectra were converted to size spectra. Usually, they show a multiple peak structure which is somewhat difficult to interpret. There are indications that particles may carry multiple (1–2) charges, and also that particles of different types may coexist beside polyaromatic and polyhedral species in the early stage of particle inception.     

Soot size distributions in rich premixed ethylene flames

Electrical mobility measurements of the soot particles generated in rich premixed ethylene/air flat flames reveal a characteristic lognormal size distribution that is distinct from the self preserving distribution expected from coagulation dominated aerosol dynamics. The distribution changes to a bimodal form as the equivalence ratio and the height above the burner increase. The soot particles are sampled using a three-stage ejector pump, with an overall dilution of the soot mole fraction by 3000, that quenches the flame chemistry and dilutes the sample for analysis by a nano-differential mobility analyzer. The measured soot volume fraction is in good agreement with optical extinction data, quantitatively reproducing the increases previously noted with respect to increasing equivalence ratio and height above the burner. The trends for mean particle diameter are also reproduced, but the mobility diameters are roughly threefold smaller than their light scattering counterparts. Particle number density is found to increase with height, whereas the optical data exhibit the opposite trend. Residence chamber experiments show that with sufficient time the quenched soot sample evolves from the lognormal shape found in the flame to the expected self preserving distribution.     

Modeling of soot particles growth in fuel-rich premixed flame

The soot formation process and the structure of a fuel-rich premixed flame stabilized in the downstream side of a porous medium have been investigated by experimental measurements and numerical analysis. In the numerical analysis, a modified Tesner's model for the reaction rate of soot formation has been introduced through a comparison between measured and calculated distributions of the temperature and the species mole fractions. Furthermore, a novel model for soot growth is developed, taking both surface and coalescence growths into account. On the basis of this model, the following results are obtained. Surface growth becomes dominant immediately after the beginning of soot formation. In the downstream side, the soot particle increases due to grow by the coalescence with smaller soot particles, and decrease as a result of collision with radicals and coalescence with larger soot particles.     

A comparison of soot size and charge distributions from ethane, ethylene, acetylene, and benzene/ethylene premixed flames

The size and electrical charge distributions of soot particles generated in rich premixed flames are examined using a nano-differential mobility analyzer (nDMA). The aim is to investigate how these distributions vary with the choice of fuel, diluent, and flame gas velocity. The measured soot size distributions are typically bimodal. The dynamics of the upper size mode is qualitatively independent of the fuel. At increasing heights above the burner this mode increases in diameter and volume fraction, but decreases in number concentration, as expected from surface growth and coagulation. At about 6 mm above the burner a small fraction of the particles, <10%, acquire a bipolar charge. The charge is associated with the upper mode, where the fractions of positive and negative particles evolve to a Boltzmann distribution, whereas the lower mode remains electrically neutral. The existence of the lower mode depends sensitively on the choice of fuel, the flame gas velocity, and the diluent. Comparison to model calculations of flame structure reveals that as each of these are varied, the lower mode exhibits an inverse correlation with temperature, hydrogen atom, and pyrene concentrations.     

The influence of ethanol addition on a rich premixed benzene flame at low pressure

The aim of the study is to analyze the effect of ethanol in rich benzene flame, to observe the influence of this oxygenated species and to understand the kinetics of ethanol in the benzene combustion. Two premixed rich benzene/oxygen/argon (11.5% C₆H₆, 43.2% O₂, 45.3% Ar) and benzene/ethanol/oxygen/argon (10.7% C₆H₆, 2.1% C₂H₅OH, 43.2% O₂, 44.0% Ar) flat flames are stabilized at low pressure (45 mbar) on a burner with the same equivalence ratio of 2.0. Identification and monitoring of signal intensity profiles of species within the flames are carried out using molecular beam mass spectrometry (M.B.M.S.). The substitution of some C₆H₆ by C₂H₅OH is responsible for a reduction of the maximum concentrations of main intermediate species such as C₂H₂, C₄H₂, C₄H₄ and C₅H₆. The UCL mechanism is extended to heavier hydrocarbons, tested against these flames to check its validity and used to underline the effect of ethanol on soot precursors formation. It contains 1028 elementary reactions and 184 chemical species.     

Numerical investigation into the decoupling effects of hydrogen blending on flame structure and soot formation in a laminar ethylene diffusion flame

Numerical calculations were conducted to explore the various effects of hydrogen blending on flame properties and soot behaviors in an ethylene coflow diffusion flame, based on a fully step-by-step decoupling method, by introducing several virtual species into the gas-phase mechanism. Results show that the concentration of OH increases under the chemical effect of hydrogen due to an enhanced rate of H₂ + O ↔ OH + H. The soot yield, primary number density, and average primary number per aggregate decrease under dilution effect while these increase under chemical effect. The enhancements of hydrogen-abstraction-carbon-addition (HACA) rates and polycyclic aromatic hydrocarbon (PAH) condensation rates are responsible for soot mass addition under chemical effect. Both the oxidation rates by O₂ and OH are delayed under the chemical effect because of lower concentrations of O₂ and OH in the sooting zone. The overall effect of higher surface growth rates and delayed oxidation rates results in an increased soot volume fraction (SVF).     

Reduction of PAH and soot in premixed ethylene-air flames by addition of ethanol

Results are presented from a combined experimental and modeling study undertaken to understand the pathways by which the addition of ethanol to fuel-rich ethylene flames causes reductions in PAH and soot. The experimental work was conducted in a flat-flame burner at equivalence ratios of 2.34 and 2.64. Ethanol was added to the ethylene at two levels corresponding to 5 and 10% oxygen by weight in the fuel. Soot was measured by laser-induced incandescence calibrated with light extinction, and aromatic species were measured using laser-induced fluorescence. Modeling was based on a 1-D premixed flame model and kinetic mechanisms available in the literature. The modeling work captures the trends in aromatic species with changes in equivalence ratio and oxygen concentration in the fuel. However, the soot predictions do not match the increases observed at the higher equivalence ratio. Analysis of the modeling results for the lower equivalence ratio shows that the addition of ethanol to the ethylene reduces the aromatic species mainly by reducing the amount of carbon that is available to form precursor species.     

Chemical species associated with the early stage of soot growth in a laminar premixed ethylene-oxygen-argon flame

The roles of aliphatic and aromatic chemical species in soot mass growth were studied in a burner-stabilized premixed ethylene-oxygen-argon flame at equivalence ratio ?=2.5. Temperature, soot size distribution, and volume fraction were measured as a function of distance from the burner surface. The chemical composition of the soot was determined using a novel aerosol mass spectrometric technique, photoionization aerosol mass spectrometry (PIAMS), spatially resolved as a function of height above the burner surface (HAB). At lower HABs, the soot chemical composition was dominated by polycyclic aromatic hydrocarbons (PAHs) containing 16 to 30 carbon atoms. These measurements confirm that during particle inception and initial growth, the increase in particle mass results predominantly from an increase in the amount of PAH mass. Somewhat unexpected, ions corresponding to saturated and/or unsaturated hydrocarbons in the soot increased substantially as HAB increased. At the late stage of soot mass growth, the masses of aliphatic and aromatic components are similar. These observations indicate that for the flame tested, aliphatic compounds make a notable contribution to soot mass growth. The large aliphatic contribution coincides with a liquidlike particle morphology observed by TEM.     

Effect of a uniform electric field on soot in laminar premixed ethylene/air flames

The effect of a nominally uniform electric field on the initially uniform distribution of soot has been assessed for laminar premixed ethylene/air flames from a McKenna burner. An electrophoretic influence on charged soot particles was measured through changes to the deposition rate of soot on the McKenna plug, using laser extinction (LE). Soot volume fraction was measured in situ using laser-induced incandescence (LII). Particle size and morphologies were assessed through ex situ transmission electron microscopy (TEM) using thermophoretic sampling particle diagnostics (TSPD). The results show that the majority of these soot particles are positively charged. The presence of a negatively charged plug was found to decrease the particle residence times in the flame and to influence the formation and oxidation progress. A positively charged plug has the opposite effect. The effect on soot volume fraction, particles size and morphology with electric field strength is also reported. Flame stability was also found to be affected by the presence of the electric field, with the balance of the electrophoretic force and drag force controlling the transition to unstable flame flicker. The presence of charged species generated by the flame was found to reduce the dielectric field strength to one seventh that of air.     

Quantitative measurement of soot particle size distribution in premixed flames - The burner-stabilized stagnation flame approach

A burner-stabilized, stagnation flame technique is introduced. In this technique, a previously developed sampling probe is combined with a water-cooled circular plate such that the combination simultaneously acts as a flow stagnation surface and soot sample probe for mobility particle sizing. The technique allows for a rigorous definition of the boundary conditions of the flame with probe intrusion and enables less ambiguous comparison between experiment and model. Tests on a 16.3% ethylene-23.7% oxygen-argon flame at atmospheric pressure show that with the boundary temperatures of the burner and stagnation surfaces accurately determined, the entire temperature field may be reproduced by pseudo one-dimensional stagnation reacting flow simulation using these temperature values as the input boundary conditions. Soot particle size distribution functions were determined for the burner-stabilized, stagnation flame at several burner-to-stagnation surface separations. It was found that the tubular probe developed earlier perturbs the flow and flame temperature in a way which is better described by a one-dimensional stagnation reacting flow than by a burner-stabilized flame free of probe intrusion.     

Field measurements of soot volume fractions in laminar partially premixed coflow ethylene/air flames

The conditions under which soot is formed vary widely and depend upon several factors, including pressure, temperature, fuel type, combustor geometry, and extent of premixing. Although it is known that partially premixed flames (PPFs) can become either more or less sooting than their nonpremixed or premixed counterparts, the impact of partial premixing on soot formation across a large equivalence ratio and flow range is still inadequately understood. Comprehensive experimental data are relatively sparse for this important configuration. Herein, we report on soot formation in various ethylene/air PPFs utilizing full-field light extinction. The dimensionless extinction coefficient Kext is an important calibrated constant for the determination of the soot volume fraction for this measurement technique. We find that a value of Kext=7.1 provides results that are in good agreement with benchmark literature data for a nonpremixed flame. We examined the soot microstructures for two flames established at ?=∞ (i.e., nonpremixed) and 5. In both cases, the primary particles were found to be nearly spherical. In case of the nonpremixed flame the average primary soot particle diameter was ∼35 nm, but for the ?=5 flame it was ∼20 nm. However, the parameter responsible for the value of Kext is the average aggregate size and not that of the primary particles. The aggregate sizes are similar for the two flames. We consider this as verification of a constant Kext value over the entire equivalence ratio range. The addition of air to the fuel stream produces an initial increase in the flame height. Further air addition gradually decreases the flame height, which is followed by a more rapid decrease with larger premixing. Likewise, the peak soot concentration first increases with small amounts of air addition (or partial premixing of the fuel stream) and reaches a maximum value at ?∼24. With further air addition, as ? decreases below a value of 20, the soot volume fraction considerably decreases.     

Formation of soot particles in methane and ethylene combustion: A reactive molecular dynamics study

A numerical analysis of the complete soot formation process was conducted in the combustion condition with methane and ethylene as initial fuel and oxygen as oxidant by applying the reactive molecular dynamics. The relative time of different periods during soot formation was quantitatively analyzed by a normalized time (t/t_g), and the effects of fuel type, temperature, and equivalent ratio on soot formation were further studied. The simulation results showed that there is little difference of the normalized time (t/t_g) of the different periods during soot formation between CH₄ and C₂H₄ combustion. The normalized time of the periods for fuel pyrolysis and initial PAHs formation, soot nucleation, and soot surface growth and particle coalescence are 27.7%, 12.3%, and 60%, respectively. The carbon atom number and C/H ratio of the final soot particles in ethylene combustion were approximately twice that of methane combustion, which was due to the high acetylene concentration during ethylene combustion promoting the formation of initial polycyclic aromatic hydrocarbons (PAHs). Increasing temperature significantly accelerated soot formation mainly by promoting soot coalescence process, while excessive temperature inhibited soot formation by accelerating soot fragmentation. Decreasing equivalence ratio mainly inhibited soot formation by reducing the concentrations of acetylene and PAHs during fuel pyrolysis and initial PAHs formation period. Assuming CH₃ and C₂H₃ free radical as the initial fuel could speed up soot formation process, while the hydroxyl (OH) had no obvious effect on soot formation process.     

Effects of turbulent combustion modeling errors on soot evolution in a turbulent nonpremixed jet flame

Due to the long time scales associated with soot evolution and its sensitivity to the background thermochemical state, even small errors in a turbulent combustion model have the potential to lead to large errors in soot evolution. For example, in turbulent jet flames, small upstream errors in the temperature and species concentrations could lead to large errors in soot volume fraction downstream. In this work, an algorithm is developed for propagating upstream errors in the thermochemical state, specifically, the temperature, into soot predictions downstream. The algorithm is based on a stochastic collocation approach that perturbs the reaction progress variable in the flamelet model at an upstream location and lets this error passively propagate downstream in the soot and combustion models (i.e., the hydrodynamic field is unaffected). The approach is applied to the simulation of Delft Flame III, a natural gas turbulent nonpremixed piloted jet flame for which both upstream temperature measurements and downstream soot volume fraction measurements are available. The results indicate that upstream errors in temperature, which are within the experimental uncertainty, can lead to errors in the soot volume fraction downstream up to 30%; the downstream error in the temperature is comparable in magnitude to the upstream perturbation. Further analysis reveals that the primary source of the downstream error in soot volume fraction is the accumulation of errors in the soot precursor mass fraction with downstream distance.     

On the effect of carbon monoxide addition on soot formation in a laminar ethylene/air coflow diffusion flame

The effect of carbon monoxide addition on soot formation in an ethylene/air diffusion flame is investigated by experiment and detailed numerical simulation. The paper focuses on the chemical effect of carbon monoxide addition by comparing the results of carbon monoxide and nitrogen diluted flames. Both experiment and simulation show that although overall the addition of carbon monoxide monotonically reduces the formation of soot, the chemical effect promotes the formation of soot in an ethylene/air diffusion flame. The further analysis of the details of the numerical result suggests that the chemical effect of carbon monoxide addition may be caused by the modifications to the flame temperature, soot surface growth and oxidation reactions. Flame temperature increases relative to a nitrogen diluted flame, which results in a higher surface growth rate, when carbon monoxide is added. Furthermore, the addition of carbon monoxide increases the concentration of H radical owing to the intensified forward rate of the reaction CO + OH = CO₂ + H and therefore increases the surface growth reaction rates. The addition of carbon monoxide also slows the oxidation rate of soot because the same reaction CO + OH = CO₂ + H results in a lower concentration of OH.     

A numerical study of soot aggregate formation in a laminar coflow diffusion flame

Soot aggregate formation in a two-dimensional laminar coflow ethylene/air diffusion flame is studied with a pyrene-based soot model, a detailed sectional aerosol dynamics model, and a detailed radiation model. The chemical kinetic mechanism describes polycyclic aromatic hydrocarbon formation up to pyrene, the dimerization of which is assumed to lead to soot nucleation. The growth and oxidation of soot particles are characterized by the HACA surface mechanism and pyrene-soot surface condensation. The mass range of the solid soot phase is divided into thirty-five discrete sections and two equations are solved in each section to model the formation of the fractal-like soot aggregates. The coagulation model is improved by implementing the aggregate coagulation efficiency. Several physical processes that may cause sub-unitary aggregate coagulation efficiency are discussed. Their effects on aggregate structure are numerically investigated. The average number of primary soot particles per soot aggregate np is found to be a strong function of the aggregate coagulation efficiency. Compared to the available experimental data, np is well reproduced with a constant 20% aggregate coagulation efficiency. The predicted axial velocity, OH mole fraction, and C₂H₂ mole fraction are validated against experimental data in the literature. Reasonable agreements are obtained. Finally, a sensitivity study of the effects of particle coalescence on soot volume fraction and soot aggregate nanostructure is conducted using a coalescence cutoff diameter method.     

Study of carbonaceous nanoparticles in premixed C2H4-air flames and behind a spark ignition engine

Nanoparticle size distributions and their concentrations were studied in atmospheric premixed ethylene/air flames using photo ionization mass spectrometry (PIMS) and total organic carbon (TOC) calibration supplemented by differential mobility analysis (DMA). Focus of this study is the evolution of nanoparticles as a function of height above burner (HAB) and of the C/O ratio of the unburned gases. It was found that especially particles of the cluster type exhibit a sharp concentration drop by more than two orders of magnitude within a narrow C/O window which is close to the sooting threshold. Using DMA a decline by two orders of magnitude was found. These results suggest that at best only small concentrations of nanoparticles should be formed significantly below the sooting threshold. As these conditions prevail in a homogeneously charged IC engine no or only very small nanoparticle emissions are expected in the exhaust gas. This was indeed found for a small Otto engine driving a power generator unit. Using flame nanoparticle profiles as standard, absolute concentrations for their emissions could be deduced. These data were supported by additional DMA measurements. The calibration using TOC did not completely match the one based on the condensation particle counter of the DMA apparatus.     

Experimental studies on dynamics of methane–air premixed flame in meso-scale diverging channels

The propagation characteristics of a laminar premixed flame front in meso-scale straight and diverging channels of 5°, 10° and 15° with inlet dimension of 25 mm × 2 mm are reported in this paper. The downstream part of the channels was heated with an external heat source, to maintain a positive wall temperature gradient along the direction of fluid flow. These investigations show that planar flames are observed near flash back limits. Negatively stretched flames were observed for moderate flow rates and rich mixtures and for high flow rates, flames were positively stretched. These flames were either symmetric or asymmetric in nature. Partially stable flames were observed at high velocities for rich mixtures, whereas for lean mixture partially stable flames were observed for all flow rates. All the divergent channels showed an improvement in high velocity limits compared with the straight channel for the same mixture. Planar flames observed in the experiments helped in determining the laminar burning velocities for these mixtures at different preheat temperatures. A co-relation of laminar burning velocity with mixture preheat temperature is also obtained for a stoichiometric methane–air mixture. This co-relation S_u/S_u,o = (T_u/T_u,o)^1.558 is in good agreement with the earlier co-relations.