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.     

The evolution of soot morphology in a laminar coflow diffusion flame of a surrogate for Jet A-1

An experimental study is performed to investigate the evolution of soot morphology in an atmospheric pressure laminar coflow diffusion flame of a three-component surrogate for Jet A-1. The laser extinction measurement method and the rapid thermocouple insertion technique are used to obtain soot volume fraction profiles and temperature profiles, respectively. Thermophoretic sampling followed by transmission electron microscopy and atomic force microscopy is used to study the morphology of soot particles at different locations inside the flame. Soot formation on the centerline appears to be different from conventional models. Liquid-like particles, which are transparent at the wavelength of 623 nm, are formed and grow up to a volume equivalent diameter of d_p = 60 nm at temperatures below T = 1500 K. When the temperature exceeds 1500 K, transition of the transparent particles to the mature agglomerated particles happens immediately, i.e. in less than 12 ms. The volume of the liquid-like particles just before the start of their transformation to solid is about five times larger than the volume of mature primary particles. This significant size difference suggests that a large liquid-like particle does not transform into a single primary particle. In addition, multiple dark nuclei are observed in the liquid-like particles prior to carbonization. The significant size discrepancy and the presence of multiple dark nuclei may indicate that primary particle formation and agglomeration on the centerline happen inside the liquid-like particles. In contrast to the centerline, on another streamline with a significantly different temperature history, soot particles form from relatively small liquid-like particles. These particles have the same size as mature primary particles. Carbonization happens early on the streamline. A single dark nucleus grows inside each liquid-like particle and primary particles agglomerate after carbonization is completed. Most of the currently used computational soot models consider a single evolution process for all of the streamlines inside the flame which may not be an accurate assumption. This study shows that soot evolution processes may be different across the flame and are a function of temperature and the concentration of specific species inside the flame.     

Hydrogen addition to acetylene-air laminar diffusion flames: Studies on soot formation under different flow arrangements

Axisymmetric co-flowing acetylene/air laminar diffusion flames have been experimentally investigated to study the effect of hydrogen addition on soot formation and soot morphology. An acetylene-hydrogen jet burning in co-flowing air at atmospheric pressure has been studied under different flow arrangements, i.e., premixed and with separate addition of acetylene and hydrogen. Thermophoretic sampling and analysis by transmission electron microscopy are employed for soot diagnostics. Soot microstructure, primary particle size, soot volume fraction, and fractal geometry results are reported. The effect of hydrogen addition on the temperature field is moderate (maximum increase ∼100 K), the effect being greater when hydrogen is premixed with acetylene. Soot volume fraction decreases with hydrogen addition. A shift was noted in the soot volume fraction peak with change in the Reynolds and Froude numbers at the burner exit. The primary soot particle diameter is in the range of 20-35 nm. Soot particles are larger in size close to the burner for the pure acetylene flame. A reverse trend is observed with hydrogen addition. The fractal dimension of the soot aggregates is about 1.7-1.8. It is unaffected by hydrogen addition and location in the flame. Soot aggregate size tends to decrease with hydrogen addition. The results of the present study on the effect of hydrogen addition on soot volume fraction and mean primary particle size are in good correlation with the results of other investigators for ethylene-, propane-, and butane-air flames, which have been described with regard to the HACA mechanism of soot nucleation and growth and enhanced soot oxidation in fuel-rich flames by increased OH radical concentration.     

Measurement of the dimensionless extinction coefficient of soot within laminar diffusion flames

The dimensionless extinction coefficient (K_e) of soot must be known to quantify laser extinction measurements of soot concentration and to predict optical attenuation through smoke clouds. Previous investigations have measured K_e for post-flame soot emitted from laminar and turbulent diffusion flames and smoking laminar premixed flames. This paper presents the first measurements of soot K_e from within laminar diffusion flames, using a small extractive probe to withdraw the soot from the flame. To measure K_e, two laser sources (635 nm and 1310 nm) were coupled to a transmission cell, followed by gravimetric sampling. Coannular diffusion flames of methane, ethylene and nitrogen-diluted kerosene burning in air were studied, together with slot flames of methane and ethylene. K_e was measured at the radial location of maximum soot volume fraction at several heights for each flame. Results for K_e at both 635 nm and 1310 nm for ethylene and kerosene coannular flames were in the range of 9–10, consistent with the results from previous studies of post-flame soot. The ethylene slot flame and the methane flames have lower K_e values, in some cases as low as 2.0. These lower values of K_e are found to result from the contributions of (a) the condensation of PAH species during the sampling of soot, (b) the wavelength-dependent absorptivity of soot precursor particles, and, in the case of methane, (c) the negligible contribution of soot scattering to the extinction coefficient. RDG calculations of soot scattering, in combination with the measured K_e values, imply that the soot refractive index is in the vicinity of 1.75–1.03i at 635 nm.     

Burnout of soot particles in a two-stage burner with a JP-8 surrogate fuel

This work focuses on the understanding of the oxidation of soot particles which were the result of using a JP-8 surrogate fuel in a two-stage burner. The two-stage system consists of an initial premixed burner where soot was generated with an air/fuel mixture, specifically a JP-8 surrogate (m-xylene and n-dodecane), under a variety of conditions. Downstream, the soot-laden combustion gases were passed through a second, flat-flame burner where soot was burned out under fuel-lean or slightly fuel-rich conditions. Soot oxidation in the secondary burner was determined by investigating particle size distribution (PSD), flame temperature, gas-phase composition, soot surface area, and soot morphology and nanostructure as a function of the height above the second burner (HAB). Measurements of soot size distribution and number concentration as a function of the HAB under fuel lean (Φoverall = 0.8) and slightly rich (Φoverall = 1.14) conditions showed a decrease in particle mean diameter and a significant increase in number concentration in the region where O₂ concentration decreased. In this region, the effectiveness factor for O₂ was found to be 1, indicating the potential for internal oxygen diffusion and burning. This caused both the breakup of the bridges cementing primary particles and the rupture of the primary particles. Higher in the burner, where modeling suggested the presence of OH^*, soot oxidation was attributed to OH^* mechanisms which are faster as compared to O₂ oxidation.     

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.     

Studies of soot oxidative reactivity using a diffusion flame burner

Oxygen addition to a laminar diffusion flame burner was carried out by intake oxygen enrichment and by fuel oxygenation in order to study the relation between soot oxidative reactivity and the combustion process. For this work, oxidative reactivity of n-heptane-derived soot was analyzed with respect to adiabatic flame temperature at various oxygen concentrations of the oxidizer stream, and compared to those of ethylene-derived soot and soot derived from an oxygenated fuel (a mixture of 70 vol.% n-heptane and 30 vol.% monoglyme (C₄H₁₀O₂), monoglyme mixture). There is a clear inverse relation observed between adiabatic flame temperature and soot oxidative reactivity. However, detailed experiments suggest that the primary factors impacting soot oxidative reactivity are the soot inception limit, which is closely related to soot formation rate, and the soot oxidation process: the earlier soot is formed, the less reactive it becomes. Consequently, ethylene-derived soot is shown to be less reactive than n-heptane-derived soot under the same carbon flow rate and the same adiabatic flame temperature, because soot is formed at an earlier stage with ethylene than with n-heptane. In addition, increasing oxygen concentration in the oxidizer stream made soot less reactive, because not only is the soot inception limit increased by the increase in the flame temperature, but also the soot oxidation process is enhanced by increased abundance of oxidizing gases during the combustion process. From these results, it is concluded that the reason why soot derived from the monoglyme mixture is more reactive than soot from n-heptane is related to the longer time for soot to be incepted, due to the reduced concentration of soot precursors with the oxygenated fuel.     

Experimental investigation and detailed modeling of soot aggregate formation and size distribution in laminar coflow diffusion flames of Jet A-1, a synthetic kerosene,and n-decane

A fully-coupled soot formation model is developed to predict the concentration, size, and aggregate structure of soot particles in the atmospheric pressure laminar coflow diffusion flames of a three-component surrogate for Jet A-1, a three-component surrogate for a Fischer–Tropsch Synthetic Paraffinic Kerosene (SPK), and n-decane. To model the chemical structure of the flames and soot precursor formation, a detailed chemical kinetic mechanism for fuel oxidation, with 2185 species and 8217 reactions, is reduced and combined with a Polycyclic Aromatic Hydrocarbon (PAH) formation and growth scheme. The mechanism is coupled to a highly detailed sectional particle dynamics model that predicts the volume fraction, structure, and size of soot particles by considering PAH-based nucleation, surface growth, PAH surface condensation, aggregation, surface oxidation, fragmentation, thermophoresis, and radiation. The simulation results are validated by comparing against experimental data measured for the flames of pre-vaporized fuels. The objectives of the present effort are to more accurately simulate the physical soot formation processes and to improve the predictions of our previously published jet fuel soot formation models, particularly for the size and aggregate structure of soot particles. To this end, the following improvements are considered: (1) addition of particle coalescence submodels to account for the loss of surface area, reduction of the number of primary particles, and increase of primary particle diameters upon collision, (2) consideration of a larger PAH molecule (benzopyrene instead of pyrene) for nucleation and surface growth to enhance the agreement between the soot model and the measured chemical composition of soot particles, and (3) implementation of a dimerization efficiency in the soot inception submodel to account for the collisions between PAH molecules that do not lead to dimerization. The results of two different particle coalescence submodels show that this process is too slow to account for the growth of primary particles, mainly because of the limited rate of particle collisions. Soot volume fraction predictions on the wings and at lower flame heights are considerably improved by using benzopyrene, due to the different distribution of the soot forming PAH molecule in the flame. The computed number of primary particles per aggregate and the diameters of primary particles agree very well with the experimentally measured values after implementing the dimerization efficiency for PAH collisions, because of the reduced rate of soot inception compared to growth by PAH condensation. Concentrations of major gaseous species and flame temperatures are also well predicted by the model. The underprediction of soot concentration on the flame centerline, observed in previous studies, still exists despite minor improvements.     

Evidence of soot superaggregates in a turbulent pool fire

We report experimental observations of extremely large, 10–100 μm, soot aggregates in a blended methanol/toluene fueled turbulent pool fire, which are believed to be the first observation of “superaggregates” in a turbulent flame. Laser-induced incandescence images of soot volume concentration, at the center of the fire plume and at a height within the active flaming region, reveal the appearance of large-scale particle-like features across a broad range of apparent volume fraction, which emit at an intensity that is comparable with that of the laser-heated soot particles. We argue that the features in the incandescence images result from very large soot aggregates. This observation is supported by scanning electron microscope imaging of extracted soot that reveals large soot structures composed of much smaller chains of individual primary particles. Analysis of the soot aggregate structure from the electron-microscope images reveals a 1.8 fractal dimension at micron scales, comparable with commonly reported soot aggregate sizes from hydrocarbon flames. At larger scales of 10s of microns, comparable with the total aggregate size, a larger volume-filling fractal dimension of 2.5–2.6 is observed. This type of fractal structure is consistent with reported, but apparently rare, observations of soot superaggregates in heavily sooting laboratory laminar diffusion flames, but is encountered in the much larger meter-scale pool fire at much lower soot volume concentrations.     

Modeling of soot deposition in wavy-fin exhaust gas recirculator coolers

This work presents one of the first CFD studies carried out to understand the fouling of exhaust gas recirculator (EGR) cooler surfaces. The deposition of soot particles in wavy-fin EGR coolers is studied by way of simulations carried out in a periodic framework. In the presence of very high temperature gradients, usually prevalent in EGR flows, the particle deposition process is dominated by the thermophoretic force. Calculations are performed for 10 and 100 nm particles at various Reynolds numbers and wall temperature gradients ranging from 1.0 to 9.45 × 10⁶ K/m. It is seen that for the sub-micron particle sizes considered, the deposition process is independent of the particle size. Simulations in the wavy-fin geometry indicate the presence of preferential deposition patterns, corresponding to the regions of higher heat transfer. At lower Reynolds numbers, the amount of deposition increases considerably due to the higher particle residence times. Also, the amount of deposition exhibits a linear relationship with the applied wall temperature gradient, thus confirming the importance of thermophoresis in the soot deposition process.     

A stochastic approach to calculate the particle size distribution function of soot particles in laminar premixed flames

We introduce an efficient stochastic approach to solve the population balance equation that describes the formation and oxidation of soot particles in a laminar premixed flame. The approach is based on a stochastic particle system representing the ensemble of soot particles. The processes contributing to the formation and oxidation of soot particles are treated in a probabilistic manner. The stochastic algorithm, which makes use of an efficient majorant kernel and the method of fictitious jumps, resolves the entire soot particle distribution (PSDF) without introducing additional closure assumptions. A fuel-rich laminar premixed acetylene flame is computed using a detailed kinetic soot model. Solutions are obtained for both, the stochastic approach and the method of moments combined with a modified version of the Premix, CHEMKIN code. In this manner, the accuracy of the method of moments in a laminar premixed flame simulation is investigated. It is found that the accuracy for the first moment is excellent (5% error), and mean error for rest of the moments is within 25%. Also the effect of the oxidation of the smallest particles (burnout) has been quantified but was found not to be important in the flame investigated. The time evolution of computed size distributions and their integral properties are compared to experimental measurements and the agreement was found to be satisfactory. Finally, the efficiency of the stochastic method is studied.     

Soot formation and temperature structure in small methane–oxygen diffusion flames at subcritical and supercritical pressures

An experimental study was conducted to examine the characteristics of laminar methane–oxygen diffusion flames up to 100 atmospheres. The influence of pressure on soot formation and on the structure of the temperature field was investigated over the pressure range of 10–90 atmospheres in a high-pressure combustion chamber using a non-intrusive, line-of-sight spectral soot emission diagnostic technique. Two distinct zones characterized the appearance of a methane and pure oxygen diffusion flame: an inner luminous zone similar to the methane–air diffusion flames, and an outer diffusion flame zone which is mostly blue. The flame height, marked by the visible soot radiation emission, was reduced by over 50% over the pressure range of 10–100 atmospheres. Between 10 and 40 atmospheres, the soot levels increased with increasing pressure; however, above 40 atmospheres the soot concentrations decreased with increasing pressure.     

Experimental investigation of soot morphology and primary particle size along axial and radial direction of an ethylene diffusion flame via electron microscopy

The effect of oxygen concentrations on the formation and evolution of soot particles was investigated by analyzing soot morphology using SiC fiber deposition technique and thermophoretic sampling method in a co-flow diffusion ethylene flame. Soot particles were examined via transmission electron microscopy at different heights along the flame centerline. Results show that the flame temperature exhibits a bimodal distribution. As the flame height increases, the flame distribution gradually changes from bimodal to unimodal, and the increase in oxygen concentration not only causes the flame height to decrease, but also causes the bimodal distribution of the flame more pronounced. The morphological evolution of soot deposits are strongly dependent on the oxygen concentration, radial and axial position, and flame temperature. Soot deposits along the flame centerline begin to be oxidized into dense flocculent and fibrous mesh structures with the flame temperature increases. In front of the flame, the oxidation is enhanced with temperature rise at the same height, resulting in more dense morphology of the soot deposits and the decrease in primary particle size. The results of thermophoretic sampling show that soot growth undergoes various stages of nucleation, growth, coagulation, agglomeration and oxidation, and the average particle size distributions of soot increase first and then decrease. The increase in oxygen concentration leads to advances in all stages of soot formation, including surface growth, agglomeration and oxidation. Additionally, the flame temperatures increase sharply as the increase of flame heights, leading to the soot aggregates to be oxidized to loose chain-like agglomerates.     

Comparative study of soot formation on the centerline of axisymmetric laminar diffusion flames: Fuel and temperature effects

The appearance of soot on the centerline of axisymmetric laminar diffusion flames has been studied by monitoring (i) the gas temperature by thermocouples; (ii) the soot particle field by laser scattering/extinction; (iii) the presence of polycyclic aromatic hydrocarbons (PCAH) by laser induced fluorescence. Four fuels were used: butene, acetylene, butadiene, and benzene. All but one flame were at the smoke height condition and were characterized by different levels of N₂ dilution aimed at controlling the temperature field.It was observed that (i) soot nucleation occurs at the centerline; (ii) the soot onset on the centerline occurs when a characteristic temperature of 1350K is measured, regardless of fuel type or level of dilution; (iii) butene and benzene have similar fluorescence patterns, in contrast with premixed flame environments. These last two observations are consistent with the proposal that, though the extent of conversion of fuel into soot may significantly change from fuel to fuel, there is a common mechanism of soot formation for all fuels.The centerline fluorescence measurements indicate that the fluorescing species may contribute to the nucleation phase or, to a minimal extent, to surface growth in these diffusion flames. Time resolved measurements indicate that the fluorescence decay time is ≤ 10 ns, consistent with a possible identification of the fluorescing species with polycyclics such as acenaphthalene and pyrene.     

Numerical investigation of soot formation mechanisms in partially-premixed ethylene–air co-flow flames

Recently, an improved chemical mechanism of PAH growth was developed and tested in soot computations for a laminar co-flow non-premixed ethylene–air diffusion flame [Dworkin et al., Combust. Flame 158(9) (2011) 1682–1695]. With the intention of testing the robustness of the solution methodology on partially-premixed systems, this work used the same algorithm as that in the study of Dworkin et al. for computations of two sets of sooting partially-premixed co-flow laminar ethylene–air flames. The results show very good qualitative and good quantitative agreement with the experimental results for soot volume fractions and soot precursors, without any changes to the parameters of the model. The soot yield was found to initially increase with decreasing primary equivalence ratios, and then to decrease for Φ < 24, reaching levels lower than the non-premixed case for Φ < 10. On the flame centerline, both PAH and acetylene-related processes were found to be important for soot growth. The initial increase in the soot yield was linked to higher inception rates. On the wings of the flame the dominant soot growth process was found to be HACA growth. The initial increase in the soot yield was mostly due to higher acetylene yield leading to faster surface growth. The primary air was also found to influence the soot oxidation process by increasing OH radicals in both the centerline and the wings region.     

Experimental and numerical study on the occurrence of off-stagnation peak in heat flux for laminar methane/air flame impinging on a flat surface

A combined experimental and numerical study has been conducted to investigate the occurrence of off-stagnation peak for laminar methane/air flame impinging on a flat surface. Experiments were conducted for three tube burners of internal diameter 8 mm, 9.7 mm and 12 mm. Radial heat flux distributions were compared (experimentally) for different burner diameters under identical operating conditions (with firing rates of 0.25 kW, 0.40 kW and 0.50 kW, ? = 1 and H = 40 mm). An off-stagnation point peak in heat flux was observed for some of the configurations in the present study which is in accordance with the previous findings. This off-stagnation point peak is a function of stand-off distance between the exit plane of the burner and the plate and also the distance between flame-tip and the plate. A satisfactory explanation is presented to explain the existence of this off-stagnation peak with the help of results of numerical simulation carried out with commercial CFD code FLUENT. It is concluded that this off-stagnation peak in heat flux is primarily due to the peak in the axial velocity profile close to the impingement surface.     

The effect of temperature on the condensed phases formed in fuel-rich premixed benzene flames

The sooting structure of premixed fuel-rich atmospheric pressure benzene flames burning at the same C/O molar ratio = 0.8 was studied in different temperature conditions (T_max = 1720 K and 1810 K) by changing the cold gas velocity. Compositional profiles of gaseous and condensed phases, measured by probe sampling and chemical analysis, indicated that pyrolytic routes leading to higher soot formation are more favoured in the lower temperature conditions.The structural analysis of condensed phases, including condensed species and soot, has been carried out by using FT-IR and UV–Visible spectroscopy sensitive to the hydrogen bonding and carbon network, respectively.The very low hydrogenated character, as evaluated by FT-IR and elemental analysis, and the high aromatic/graphitic nature of the benzene soot, as shown by a detailed examination of UV–Visible spectral parameters, confirmed the effect of benzene fuel on the internal structure of soot particles already in the early stages of particle inception.     

Measurement and analysis of soot inception limits of oxygen-enriched coflow flames

This study explores the criteria for soot inception in oxygen-enriched laminar coflow flames. In these experiments we select an axial height in the coflow flame at which to identify the sooting limit. The sooting limit is obtained by varying the amount of inert until luminous soot first appears at this predefined height. The sooting limit flame temperature is found to increase linearly with stoichiometric mixture fraction, regardless of fuel type. To understand these results, the relationships between flame structure, temperature, and local C/O ratio is explored through the use of conserved scalar relationships. Comparison of these relationships with the experimental data indicates that the local C/O ratio is a controlling parameter for soot inception in diffusion flames (analogous to the global C/O ratio in premixed flames). Analysis of experimental results suggests that soot inception occurs when the local C/O ratio is above a critical value. The values for critical C/O ratios obtained from the analysis of experiments using several fuels are similar in magnitude to the corresponding C/O ratios for premixed flames. In addition, temperatures and PAH fluorescence were measured to identify regions in these flames most conducive to particle inception. Results indicate that the peak PAH concentration lies along a critical iso-C/O contour, which supports a theory that soot particles first appear along this critical contour, given sufficient temperature.     

The effect of CO2/H2O on the formation of soot particles in the homogeneous environment of a rapid compression facility

Previous investigations show that soot particle volume fraction and number density were significantly reduced by exhaust gas recirculation (EGR) diluents CO₂ and H₂O. However, these investigations were often convoluted by their experimental flame configurations and primarily focused on soot volume fraction rather than soot inception. To isolate the effects on soot inception and the corresponding chemistry, the current study measured the reactivity of CO₂ (up to 9.5% volume fraction) for both C₂H₂ (1.00% volume fraction) and CH₄ (1.85% volume fraction) fuels in homogeneous mixtures. Computed effect of H₂O on these and other fuels are also presented. Experiments were performed at high temperature (1640 K and 1770 K) and high equivalence ratios (Φ = 55 and 75) to understand the effect of CO₂ on polycyclic aromatic hydrocarbons (PAH) and formation of nascent soot particles with negligible oxygen influence. Experimental results show that CO₂ enhanced the soot inception rate when added to C₂H₂ but had an undetectable affect on CH₄. Gas chromatography confirmed that CO₂ increases CO mole fraction and reduces C₂H₂ fuel concentration. Chemical kinetic simulations showed that the C₂H₂ was being converted to soot precursors. CO₂ enhanced the soot inception rate for C₂H₂ by producing OH radicals. Images of nascent soot particles produced in the presence of CO₂ were used to determine the size of PAH molecules in the particles and particle morphology. Both attributes were similar to particles formed without CO₂. CO₂ had little impact on the long reaction pathway from CH₄ to PAH molecules because H and CH₃ radicals propagated these reactions more readily than OH radicals.