Soot Primary Particle Diameter Research Articles

Effects of [formula omitted]-pentanol blending on soot formation in swirl-stabilized turbulent spray flames of Jet A-1 in a laboratory gas turbine combustor

Although the influence of alcohol supplement on soot in hydrocarbons fuel combustion in laminar flames and transportation engines is widely investigated, the information from well-controlled spray flames with clean boundary conditions are scarce, especially with kerosene type fuels doped with n-pentanol. Favorable physical properties of n-pentanol makes it an attractive oxygenated fuel extender for transportation fuels. We investigated the effects of adding 10% and 20% n-pentanol, by energy content, to a JetA-1 fuel on sooting characteristics of spray combustion in a swirl-stabilized model combustor. At an identical fuel feed rate, two different air flow rates were considered yielding global equivalence ratios of 0.64 and 0.69. Flow field generated by the combustion of swirling air and the injected fuel was measured using stereoscopic particle image velocimetry, and a diffraction based instrument was utilized to quantify the spray droplet size distribution. Laser-induced incandescence was used to measure the soot concentration distribution within the combustor and to estimate the primary soot particle sizes. Very small but measurable changes were observed in the spray droplet characteristics caused by the addition of n-pentanol to JetA-1. 10% replacement of JetA-1 by n-pentanol reduced the soot volume fractions and the total soot loading in the combustor significantly when the global equivalence ratio was 0.69, whereas with global equivalence ratio of 0.64, soot loading was almost zero. When the n-pentanol replacement increased to 20%, no soot was detected for both air flow rates, probably due to soot volume fractions well below the lower limits of laser-induced incandescence. The primary soot particle diameters covered a range from 25 to 50 nm mostly unaffected by the presence of n-pentanol in JetA-1.

Comparative study of soot properties and pressure sensitivity in n-dodecane and decalin laminar flames

The main objective of this study is to compare the soot properties and pressure sensitivity of the high-carbon-number n-alkane and cycloalkane in laminar diffusion flames at elevated pressures. As typical high-carbon-number components of aviation kerosene, n-dodecane and decalin have been used in surrogate fuel kinetic models for aviation kerosene such as Jet-A and Chinese RP-3. In this study, quantitative experiments on soot properties are conducted in nitrogen-diluted n-dodecane and decalin laminar diffusion flames at up to 5.0 atm, which has scarcely been reported in the literature. Soot morphology and detailed soot volume fraction distribution are studied by transmission electron microscopy analysis and laser extinction method. Based on the soot volume fraction results, the soot yield is evaluated to compare the pressure sensitivity of the soot propensity between the two fuels. The results show that from 1.0 to 5.0 atm, the primary soot particle diameter, maximum soot volume fraction and soot yield in decalin flames are higher than those in n-dodecane flames. However, with increasing pressure, the growth rates of these soot properties are higher for n-dodecane flames than for decalin flames. The ratio of maximum soot yield of decalin to n-dodecane decreases from 2.2 at 1.0 atm to approximately 1.2 at 5.0 atm. The quantitative results indicate that the pressure sensitivity of soot properties of n-dodecane is stronger than that of decalin. The quantitative experimental results of this work are helpful in understanding the relevant fuel kinetic mechanisms suitable for simulating soot formation at elevated pressures.

Experimental study on soot formation and primary particle size in oxy-combustion ethylene diffusion flames under CO2 substitution for N2

This paper reports an experimental study on the effects of O2 concentration and CO2 concentration on soot volume fraction and primary particle size of flame centre in ethylene laminar co-diffusion flame. The incandescent light was induced by a two-color band method and laser-excited by wavelength 1064 nm. Previous studies have shown that the oxygen concentration affects the soot volume fraction in the flame. However, the effects of O2 concentration on the primary particle size under CO2 atmosphere are unclear and need to be further discussed. Seven flames were optically detected, in which the oxygen concentration of the accompanying gas varied from 21% to 50% by volume. One primary flame was burned in co-flowing ethylene/air. Six ethylene flames were burned in an oxygen-rich O2/N2 or O2/CO2 atmosphere with an oxygen mole fraction from 30% to 50%. The C2H4 flow rate remained constant in all operating conditions. The results show that the SVF observed by the LII method and spectroscopy in the oxygen-enriched flame are in good agreement. Furthermore, the soot's primary particle diameter measured by the time-resolved LII method is in good agreement with the transmission electron microscope image analysis.With the increase of oxygen concentration, the flame height becomes shorter, the luminosity becomes brighter, and the soot yield increases. Compared with an N2 atmosphere with the same oxygen concentration, the soot yield in a CO2 atmosphere was significantly inhibited. In all the flames studied, the SVF along the flame axis showed a similar distribution: it first increased to the maximum value and then decreased rapidly until the flame tip. The maximum SVF on the axis appears between 0.64 and 0.78 based on the normalized height of the visible flame. In all ethylene/(O2/N2) flames, the maximum primary particle diameter and average primary particle diameter increase with oxygen content. For ethylene/(O2/CO2) flame, the maximum diameter of primary particles decreases with the increased oxygen content. The uncertainty of primary diameter is mainly caused by the thermal adaptation coefficient, the lower LII signal of smaller particles and the uncertainty of local flame temperature.

Experimental Study of Oxygen Depletion Effects on Soot Morphology and Nanostructure in Coflow Diffusion Aviation Fuel (RP-3) Flames

Oxygen concentration is a significant factor affecting soot formation and oxidation. However, there are few studies that have focused on the morphology and nanostructure characteristics of soot in aviation kerosene, oxygen-depleted combustion flames. In the present paper, five coflow flames under initial oxygen volume concentrations of 18.5%, 19%, 20%, 21%, and 23.5% were studied. The pneumatic probe sampling method and high-resolution transmission electron microscopy (HRTEM) analysis were conducted to quantify the morphology and nanostructure parameters, and laser extinction (LE) was applied to determine the soot volume fraction. Among the cases of different oxidizer oxygen concentrations (23.5% to 18.5%), the change in soot volume fraction was quantified, and the degree of graphitization of soot particles, i.e., the maturity, were compared. The results show that the peak value of soot volume fraction of the flames increased by 0.73 ppm as the oxygen concentration increased from 21% to 23.5%, and decreased by 1.25 ppm as the oxygen concentration decreased from 21% to 18.5%. When the oxygen concentration decreased from 23.5% to 18.5%, the soot primary particle diameter at the same dimensionless height decreased and then increased, which was attributed to the competition between the changes in the residence time and the growth rate of the soot particles. The quantitative analysis results of the soot nanostructure suggested that reduced oxygen concentration inhibited the graphitization process of carbon lattices and decreased the maturity and oxidation resistance of soot. When the oxygen concentration decreased from 23.5% to 18.5% at the same dimensionless height, the mean fringe length decreased by an average of 0.18 nm, and the mean value of fringe tortuosity and spacing increased by an average of 0.053 and 0.035 nm.

Effects of blending methane, propane, and propylene on soot evolution in ethylene diffusion flames based on optical diagnostics

Soot evolution of the ethylene-based laminar coflow diffusion flame doping with methane, propane, and propylene with the blending ratio ranging from 0 to 50% has been investigated experimentally. These fuels were selected for their different fuel structure. Spatially resolved distribution maps of the flame temperature, soot volume fraction, the fluorescence signals of polycyclic aromatic hydrocarbons (PAHs), and primary soot particle diameter and number density were determined utilizing laser-induced incandescence (LII), laser-induced fluorescence (LIF), and ratio pyrometry (RP). It was found that the flame height surged and the flame temperature dropped dramatically when the propylene blending ratio was 20% while they were almost constant with the increasing addition of methane or propane. The soot/PAHs loading decreased monotonically with the addition of methane and the opposite was true in the cases of blending propylene. The synergistic effect of mixing propane into ethylene was found on both the soot/PAHs loading. The same trends were observed in the percentage of carbon conversion to soot and the primary soot particle diameter. It can be seen from the soot evolution analysis that methane addition suppressed the soot nucleation and surface growth simultaneously. The enhanced soot surface growth for the flames doping with propane at a low addition level and for the whole flame sets doping with propylene were attributed to the earlier inception time and the longer mass growth time. The preferential diffusion and weakened oxidation of soot particles because of the decreased flame temperature perhaps accounted for the transition from a non-smoking flame to a smoking flame when the propylene blending ratio was greater than 40%.

Hydrogen enrichment enhances soot formation in swirl-stabilized non-premixed turbulent combustion of ethylene in a model gas turbine combustor

A switch from fossil fuels to hydrogen is currently not feasible mostly due to supply and infrastructure issues. One of the possible approaches, and this is now practiced to a limited extent in industrial gas turbines, is to blend relatively small amounts of hydrogen with fossil fuels curbing the carbon dioxide emissions. However, studies assessing the influence of modest amounts of hydrogen blending with hydrocarbon fuels on soot processes yielded contradictory results. Most of these experimental and numerical studies were performed on laminar diffusion flames and studies on turbulent flames are scarce. One of the confounding factors in assessing the influence of hydrogen is selection of a control experiment in which the fossil fuel is blended with the same amount of an inert diluent. Using helium in the control experiment is preferable because of its similar transport properties and heat capacity to those of hydrogen. Hence, we studied the soot processes in a model gas turbine combustor in which the flame is stabilized by an air swirl. Swirl-stabilized platform ensures that with and without hydrogen/helium dilution, the hydrodynamics of the combustor stays fixed. Base fuel ethylene is supplemented with hydrogen or helium by the same amount to separate the dilution affects and assess the direct chemical interaction of hydrogen related to soot formation. Soot volume fraction and primary soot particle diameters were measured by auto-compensating laser induced-incandescence for all cases. Flow field data obtained using stereoscopic particle image velocimetry is utilized to ascertain the hydrodynamic effects on soot distribution due to addition of lighter species. Soot formation was found to be enhanced by the addition of hydrogen when allowance was made for the dilution effect using the helium doped flame experiments. Possible causes of this observation including the molecular diffusivities of hydrogen and helium, and chemical interaction are discussed.

Investigating the oxidative reactivity and nanostructural characteristics of diffusion flame generated soot using methyl crotonate and methyl butyrate blended diesel fuels

Combustion-generated particulate matter has a consequential fallout on climate, environment, and human health. Thus, comprehension of soot formation and mitigation processes using biodiesel additives is a focal point of modern combustion research. This study presents an intricate investigation of enhanced oxidative reactivity and its dependence on the soot nanostructural properties induced by distinctive blending of a saturated and an unsaturated biodiesel surrogates namely, methyl butyrate (MB) and methyl crotonate (MC) respectively with pure diesel. 25% MB-75% diesel fuel resulted in the lowest sooting propensity and lowest activation energy (155.4 ± 2 kJ/mol) for soot oxidation in comparison to pure diesel fuel and diesel soot (175.1 ± 2 kJ/mol). The soot was collected using a smoke point apparatus involving a wick-fed laminar diffusion flame at atmospheric pressure after meticulous observation of the smoke points. The detailed nanostructural characterization of the flame generated soots were performed using high-resolution transmission electron microscopy, X-ray diffraction, elemental analysis and Raman spectroscopy. BET surface area analysis, and thermogravimetric analysis were carried out for surface area distinction and activation energy calculations respectively. It revealed that, addition of MB to diesel resulted in improved fuel combustion with reduction in primary soot particle diameter, greater inter-planar separation, increased fringe tortuosity and greater crystal structure disorder resulting in its enhanced reactivity with O2. The 25% MC-75% diesel blend resulted in relatively greater sooting propensity while its soot exhibited lower crystal structure disorder and greater activation energy (165.0 ± 2 kJ/mol) for O2 induced oxidation compared to 25% MB-75% diesel, possibly due to the resonance stabilized radical (RSRs) formation that altered the fuel combustion chemistry. Therefore, this study successfully depicts that the structural differences in the surrogate fuels do influence the soot formation and oxidation kinetics. These structural effects therefore need to be considered when formulating the global multiphase kinetic models for biodiesel-diesel fuel combustion.11Dedicated to Prof. Purnakala Samant on the occasion of her 50th birthday celebration.

On the effects of varying coflow oxygen on soot and precursor nanoparticles in ethylene laminar diffusion flames

This paper reports an experimental investigation of the effects of varying the oxygen concentration in the oxidizer stream on the formation and evolution of soot and precursor nanoparticles in an axisymmetric ethylene laminar coflow diffusion flame. The optical diagnostics employ temporally and spectrally resolved point-measurements of 266 nm excited scattering and laser-induced fluorescence, combined with 1064 nm excited laser-induced incandescence. Previous studies have indicated that oxygen enhancement changes soot volume fraction in sooting flames. However, the effects of coflow oxygen concentration on soot precursors are not well understood, and this aspect is addressed explicitly here. A set of eight flames is studied where the oxygen concentration in the coflow stream is varied from 19 to 40%, by volume, while the chemical composition of the jet stream is kept constant at 60% C2H4 / 40% N2. Oxygen enhancement in the coflow leads to a change in flame temperatures, luminosity, and flame length. It is found that the concentration of precursor nanoparticles increases with coflow oxygen. A combination of higher temperatures, red-shifting, and increased decay-times suggest structural growth in precursor nanoparticles with increasing coflow oxygen. In all flames, a similarity of particle evolution is noted with axial location, and this includes the presence of an intermediate stage during the transformation of precursors to soot. Moreover, the structural transformation from aliphatic to aromatic features is shifted towards the beginning of the precursor region when coflow oxygen is increased. It is found that structural growth starts at a similar temperature, regardless of coflow oxygen concentration. Soot volume fraction and primary soot particle diameter increase with coflow oxygen enrichment up to a threshold oxygen volume fraction, after which both the soot volume fraction and diameter decrease with further oxygen enrichment.

Soot formation in turbulent swirl-stabilized spray flames in a model combustor fueled with n-butanol/Jet A-1 blends

Information on sooting characteristics of n-butanol/middle distillate fuels from well-described spray flames is lacking but desirable as a benchmark which could help in better interpretation of the reciprocating engine data and can be used in numerical simulation validation efforts. The effects of n-butanol addition to turbulent swirl-stabilized JetA-1 spray flames were studied experimentally in a model gas turbine combustor with 94 mm × 94 mm cross-section and 188 mm length. n-Butanol was added in concentrations of 10% and 20% by energy content such that the total thermal power output was maintained at 10 kW in the globally fuel lean flames. Spray droplet size distributions and velocity fields were obtained using a Fraunhofer diffraction based droplet sizer and stereoscopic particle image velocimetry, respectively. Spatially resolved soot volume fraction and mean primary particle sizes were measured with auto-compensating laser-induced incandescence. Measurable differences in droplet size distributions were observed with n-butanol addition. Flow fields were similar in all flames. Time-averaged soot volume fraction peaks were observed in all flames at radial positions of approximately 0 mm and 27 mm and axial heights of approximately 20 mm and 40 mm, respectively. Soot production decreased monotonically throughout the flames with n-butanol addition. Primary soot particle diameters were generally within the range of 25 to 50 nm. Potential mechanisms explaining the changes observed with n-butanol substitution in jet fuel are discussed.

Soot aggregate morphology deduced from thermophoretic sampling in coflow laminar methane diffusion flames at pressures up to 30 bar

A high pressure combustion chamber, housing a co-flow burner and a novel thermophoretic soot sampling system, was used to collect soot samples from methane-air diffusion flames at pressures from 5 bar to 30 bar. To have tractable diffusion flames with the same residence times, constant methane flow rate of 0.55 mg/s was maintained throughout the experiments. Soot aggregates were collected on 3 mm diameter transmission electron microscope (TEM) grids and the images of the soot aggregates were captured by a TEM device. Sampling was done at three heights above the burner rim at 3, 6, and 8 mm. An automated image processing method was used to analyze the TEM images of soot aggregates to infer the primary soot particle diameters and the aggregate morphology. The mean primary soot particle diameters increased from 5 to 15 bar at all three sampling locations and seemed to reach a peak around 15–20 bar at 6 and 8 mm height and start decreasing with further increase in pressure. On the other hand, mean primary soot diameter at 3 mm height increased slightly beyond 15 bar. In view of the measured soot concentrations and temperatures at the sampling locations at 3, 6, and 8 mm above the burner, the soot number density is argued to be increasing with increasing pressure. Throughout the all experimental conditions, fractal dimension of the aggregates in these methane diffusion flames did not exceed 1.4 which is much smaller than the accepted universal value of about 1.85. Morphological characteristics inferred from TEM images indicate that the aggregation is less prominent in methane flames in comparison to ethylene diffusion flames under similar conditions.

The effect of elevated reactant temperatures on soot nanostructures in a coflow diffusion ethylene flame

The relationship between soot surface growth, soot nanostructure and reactant temperature (Tr) in a coflow diffusion ethylene flame was investigated with multiple experimental techniques. The Tr was raised by heating the coflow air. Three cases, with 300K, 473K, and 673K Tr, respectively, were studied. Laser-induced Incandescence revealed that increasing Tr promotes soot formation. Although soot primary particle diameter (dp) also increases with Tr, the increase in dp slows down after 473K Tr, suggesting that there is a deceleration in soot surface growth. Transmission Electron Microscopy (TEM) imaging showed that increased Tr promotes soot aggregation and yields larger and more mature primary particles. The assessment of the Selected Area Electron Diffraction (SAED) patterns indicated that, at 673K Tr, there is a growth of lattice planes. Raman spectroscopy revealed further structural details. By assessing the band intensity ratios, soot for the Tr of 673K has more curved nanostructures. The deceleration of soot surface growth may be explained by surface aging, which is characterized by an increase in curved nanostructures.

Time-lapse visualization of shrinking soot in diesel particulate filter during active-regeneration using field emission scanning electron microscopy.

Regeneration of diesel particulate filter (DPF) is a complicated process due to its high operating temperature and associated oxidation of soot on the filter substrate. Several oxidation mechanisms of diesel soot have been proposed based on reaction kinetics, but more information about the oxidation phenomena is needed in a practical system, that is, soot oxidation on a DPF substrate. In this work, the DPF regeneration process was, for the first time, visualized at the particle scale ex-situ, in time-lapse series, using field emission scanning electron microscopy (FE-SEM). Time-lapse transformation of soot cake layer on a real DPF was followed from initiation through the complete regeneration process. In parallel, transformation of the soot primary particle diameter was directly measured using high-resolution transmission electron microscopy. FE-SEM visualization clearly showed shrinkage of the soot cake layer on the DPF wall as oxidation progressed. Furthermore, diameter distribution analysis revealed a trend of shrinkage of the nanoscale soot primary particles during oxidation, supporting the FE-SEM observations of the micron-scale shrinkage of the agglomerated soot cake layer. Fragmentation of the shrunken soot cake layer was also observed and is suspected to be a result of locally higher gas flow that depends on surface pore morphology of the filter substrate. LAY DESCRIPTION: In this work, the DPF regeneration process was, for the first time, visualized at the particle scale ex-situ, in time-lapse series, using field emission scanning electron microscopy (FE-SEM). Time-lapse transformation of soot cake layer on a real DPF was followed from initiation through the complete regeneration process. In parallel, transformation of the soot primary particle diameter was directly measured using high resolution transmission electron microscopy. FE-SEM visualization clearly showed shrinkage of the soot cake layer on the DPF wall as oxidation progressed. Furthermore, diameter distribution analysis revealed a trend of shrinkage of the nano-scaled soot primary particles during oxidation, supporting the FE-SEM observations of the shrinkage of the micron-scale agglomerated soot cake layer. Fragmentation of the shrunken soot cake layer was also observed and is suspected to be a result of locally higher gas flow that depends on surface pore morphology of the filter substrate. Further detail mechanism about transformations of the soot cake layer was additionally discussed based on electrostatic attraction.

The effect of oxygen concentration in the co-flow of laminar ethylene diffusion flames

This paper reports an experimental study of the effects of oxygen concentration (OC) in the oxidant stream on the characteristics of laminar co-flow ethylene diffusion sooting flames. Oxygen concentration was varied from 16.8% to 36.8% (by volume) by either diluting the air co-flow with nitrogen or by adding oxygen. Planar optical measurements were performed, with high spatial resolution, of flame luminosity at 700 nm, soot volume fraction, flame temperature, primary soot particle diameters and OH* chemiluminescence. All measured parameters are found to vary significantly with OC over the investigated range. The soot volume fraction measurements quantify the extent to which soot formation and oxidation are enhanced by increasing OC, revealing that the former effect dominates in the upstream region, while the latter effect dominates the downstream region, close to the flame tip. The changes to flame luminosity, soot volume fraction and flame temperature are strongly correlated with each other, while the primary soot particle diameter correlates well with the soot residence time. In addition, the rate of soot evolution, including its formation and oxidation, correlates with flame temperature along the axis of flames and a second reaction zone emerges in the tip of flames with high OC, as revealed by OH* chemiluminescence. The experimental data are well suited to the development and validation of soot models because of the well-defined boundary conditions, high spatial resolution, systematic nature and combination of the measured scalars.

Influence of ethanol blending ratios on in-flame soot particle structures in an optical spark-ignition direct-injection engine

This study shows how ethanol addition impacts in-flame and exhaust soot in spark-ignition direct injection (SIDI) engines through direct and simultaneous sampling of particles from the flames and exhaust stream. The thermophoresis-based soot sampling and transmission electron microscope (TEM) imaging was performed for various ethanol blending ratios ranging from 0 to 60%. A quantitative analysis is performed in the images obtained from a standard TEM by yielding the in-flame and exhaust soot number counts, projection area, as well as morphological parameters such as soot primary particle diameter, aggregate radius of gyration, and fractal dimension. In addition, the internal structures and reactivity status of in-flame and exhaust soot particles are unveiled using a high-resolution TEM with carbon fringe length, tortuosity and fringe-to-fringe separation extracted from processed carbon layer fringe images. The results show that both the number counts and projection areas of exhaust soot are much lower than those of the in-flame soot at any fixed ethanol blending ratios. The exhaust soot is smaller in size for both the primary particles and aggregates with more compact aggregate structures, longer and straighter fringe, and lower fringe separation, all indicating significant soot oxidation occurred inside the cylinder before the particles exit through the exhaust. With increasing ethanol blending ratio, almost linear reduction in number counts and projection area is found for both the in-flame and exhaust soot particles, while both soot aggregates and primary particles become smaller with more stretched aggregate structures and shorter, more curved, and narrower gap fringe structures, indicating soot particles with higher reactivity. The significantly lower soot emissions achieved with higher ethanol blending ratio is found to be due primarily to the suppression of soot formation within the flame as evidenced by higher reduction rate of number counts, projection area, and primary particle and aggregate sizes for the in-flame soot than that of the exhaust soot.

Experimental investigation of soot evolution in a turbulent non-premixed prevaporized toluene flame

The formation, growth, and oxidation of soot in turbulent prevaporized toluene diffusion flames stabilized on a jet-in-hot-coflow (JHC) burner are investigated in this study. Flame structure, local gas temperature as well as local soot volume fraction and primary soot particle diameter, are simultaneously detected by means of OH planar laser-induced fluorescence (PLIF), non-linear excitation regime two-line atomic fluorescence (nTLAF) of indium, and time-resolved (TiRe) laser-induced incandescence (LII), respectively. The collected data sets were used to generate joint statistics of soot properties and flame characteristics and provided new insights into the interaction of the OH layer and soot in turbulent flames. The interaction of OH and soot as a driving mechanism for soot oxidation is of particular interest as it has been proven to be challenging to model. Statistics of soot volume fraction and primary particle size in the OH layer are employed to gain deeper insights into the soot oxidation process. Mean soot volume fraction and primary soot particle size conditioned on temperature and OH signal intensity indicate that, due to differential diffusion of soot with respect to the chemical species, high soot volume fraction and primary soot particle diameter of up to 50 nm are present at low temperatures and low OH concentration. In the soot oxidation region, statistical analysis of the soot parameters disclose that clusters of high soot volume fraction mostly consist of large primary particles. Observations from instantaneous images and the presence of large primary particles inside the OH layer suggest that the oxidation is not sufficiently fast to burn the soot completely.

Soot aggregate morphology in coflow laminar ethylene diffusion flames at elevated pressures

The effect of combustion pressure on soot aggregate morphology and on primary soot particle diameter was investigated by thermophoretic sampling in nitrogen-diluted ethylene flames. Soot aggregate samples were collected using a high-pressure thermophoretic sampling system installed inside the high-pressure combustion chamber. Collected samples were imaged by transmission electron microscopy followed by an automated image analysis process to yield aggregate morphology information. The experiments covered pressures from 3 to 6 bar, and samples were collected at heights of 2 and 5 mm above the burner exit in nitrogen-diluted ethylene (ethylene to nitrogen mole ratio 2:1) flames with nominal visible flame heights of about 20 mm. It was observed that average primary soot particle diameter increased with increasing pressure within the pressure range considered, at both measurement locations within the flame. Fractal parameters of the soot aggregates were inferred directly from the experimental data (as opposed to the usual practice of assuming a priori values for fractal dimension and pre-factor). Near to the soot nucleation and growth regions of the co-flow laminar diffusion flames, at 2 mm above the burner exit, the fractal dimension of the aggregates was not as high as the accepted universal value of about 1.9, in spite of the fact that number of primary particles per aggregate is quite high and pressure is well above atmospheric. At 5 mm above the burner exit, however, the fractal dimension of the soot aggregates approached the accepted universal value of 1.9, at high pressures; at relatively lower pressures, the fractal dimension was about 1.6 at the same location within the flame. In view of the inferred fractal results, caution should be exercised when applying fractal analysis to low heights in flames at lower pressures where soot inception and growth are relatively dominant.

Informing TiRe-LII assumptions for soot nanostructure and optical properties for estimation of soot primary particle diameter

Time-resolved laser-induced incandescence for primary particle size determination is tested using three model carbon blacks. Optical properties change as does the nanostructure upon laser annealing, whereas aggregate morphology and primary particle size remain equivalent to the original material, as shown by transmission electron microscopy (TEM). Primary particle diameters found from fitting experimentally measured time-resolved laser-induced incandescence (LII) signals with existing models do not match the particle diameters as directly visualized by TEM. The accommodation coefficient is shown to be a crucial parameter which can result in substantial variations in simulated conductive cooling profiles for particle sizing. Aggregate structure in the form of intra-aggregate connectivity and shielding is an additional underlying cause for erroneous particle sizing, not presently captured by LII models.

Soot primary particle size dependence on combustion pressure in laminar ethylene diffusion flames

A multi-probe thermophoretic soot sampling system, installed inside a high pressure combustion chamber, was used to collect soot samples at elevated pressures from ethylene–air laminar diffusion flames. Ethylene was diluted with nitrogen at a ratio of 1/3 by mass, and a constant mass flow rate of ethylene-nitrogen mixture was maintained at pressures of 5, 10, 15, and 20 bar. Selected mass flow rate of ethylene, 0.72 mg/s, provided stable and non-smoking laminar flames with a height of about 16 mm at all pressures considered. Soot samples were collected on transmission electron microscope grids at three heights above the burner rim, 3, 8, and 12 mm. The images of the soot particles were captured by transmission electron microscope and the primary soot particle diameters were determined using an automated edge detection method. The mean primary soot particle diameter increased from the sampling height of 3 mm to 8 mm, which corresponds to the mid-height of the flames where the peak soot volume fractions are observed. The soot diameters decreased from the mid-height of the flame to the sampling location of 12 mm, near the tip of the flame. The mean diameter of the primary soot particles increased with increasing pressure up to 15 bar; at 20 bar, the mean soot diameter seemed to reach a plateau or start decreasing. Measured soot volume fractions at sampling locations of 3 and 8 mm above the burner rim indicate that, in view of the measured mean soot diameters at the same locations, soot number density should be increasing intensely with pressure.

Soot concentration and primary particle size in swirl-stabilized non-premixed turbulent flames of ethylene and air

Turbulent non-premixed swirl-stabilized flames were investigated experimentally in a gas turbine model combustor with optical access. Velocity and soot concentration fields were measured for three levels of air flow for a fixed flow rate of ethylene as the fuel. Stereoscopic particle image velocimetry was used to get the three-dimensional velocity field data within the combustor. The time-averaged soot volume fractions and primary soot particles sizes were obtained using the laser induced incandescence technique. These two measurements were conducted separately, but under the identical experimental flow conditions. The velocity measurements showed a region of high velocity flow, sandwiched between inner and outer recirculation zones. The boundaries of the recirculation zones in all three cases displayed very high turbulence intensities. Most of the soot was found to be within the inner recirculation zone, and the regions having maximum time-averaged soot concentrations grew radially outward with axial height. The soot concentrations showed a strong dependence on air flow rate; a small increase in air flow rate caused a significant reduction in soot concentrations. The primary soot particle diameters inferred from laser induced incandescence measurements covered the range from 30 to 50 nm.

DQMOM approach for poly-dispersed soot formation processes in a turbulent non-premixed ethylene/air flame

To realistically treat the poly-dispersed soot formation system, the Direct Quadrature Method of Moment (DQMOM) in conjunction with the transient flamelet approach was adopted to simulate a turbulent non-premixed C2H4/air flame. The population balance equation of the soot particle distribution was approximated using the multi-environment probability density function which consists of multiple weights and abscissas in the physical space. The transient flamelet model was employed to treat the turbulence-chemistry interactions and the two-equation soot model was used to account for the soot/gas chemistry coupling allowing mass and energy exchange. In the framework of the transient flamelet model, the physical soot model terms such as nucleation, surface growth, and oxidation for the population balance equation of soot particle distribution were closed. In terms of the mean temperature, soot volume fraction, primary soot particle number density, primary soot aggregate number density, mean number of primary particle per aggregate, mean radius of soot aggregate, and mean primary soot particle diameter, the predicted profiles agreed reasonably well with the experimental data.