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Measuring the size distribution and temperature of high-temperature dispersed particles, particularly in-flame soot, holds paramount importance across various industries. Laser-induced incandescence (LII) stands out as a potent non-contact diagnostic technology for in-flame soot, although its effectiveness is hindered by uncertainties associated with pre-determined thermal properties. To tackle this challenge, our study proposes a multi-parameter inversion strategy—simultaneous inversion of particle size distribution, thermal accommodation coefficient, and initial temperature of in-flame soot aggregates using time-resolved LII signals. Analyzing the responses of different heat transfer sub-models to temperature rise demonstrates the necessity of incorporating sublimation and thermionic emission for accurately reproducing LII signals of high-temperature dispersed particles. Consequently, we selected a particular LII model for the multi-parameter inversion strategy. Our research reveals that LII-based particle sizing is sensitive to biases in the initial temperature of particles (equivalent to the flame temperature), underscoring the need for the proposed multi-parameter inversion strategy. Numerical results obtained at two typical flame temperatures, 1100 K and 1700 K, illustrate that selecting an appropriate laser fluence enables the simultaneous inversion of particle size distribution, thermal accommodation coefficient, and initial particle temperatures of soot aggregates with high accuracy and confidence using the LII technique. [ABSTRACT FROM AUTHOR]

Snapshot multispectral imaging of chemical species in the flame is essential for improved understanding of the combustion process. In this article, we investigate the different configurations of a structured laser sheet-based multispectral imaging approach called the Frequency Recognition Algorithm for Multiple Exposures (FRAME). Using FRAME, a snapshot of Laser-Induced Fluorescence (LIF) of Polycyclic Aromatic Hydrocarbons (PAH) excited by 283.5 nm laser and Laser-Induced Incandescence (LII) of soot particles excited by 532 nm laser are acquired simultaneously on a single FRAME image. A laminar diffusion flame of acetylene produced by a Gülder burner is used for the experiments. The standard FRAME approach is based on creating two spatially modulated laser sheets and arranging them in a cross-patterned configuration (X). However, the effect of using different configurations (angles) of the two laser sheets on the multispectral planar imaging of the flame has not yet been studied. Therefore, we have compared the FRAME approach in four different configurations while keeping the same flame conditions. First, we have compared the relation between laser fluence and LII signals with and without spatial modulation of the 532 nm laser sheet and found that both detections follow the same curve. When comparing the maps of flame species reconstructed from the standard FRAME configuration and other configurations, there are some dissimilarities. These differences are attributed to minor changes in the imaging plane, optical alignment, laser path length, different modulation frequencies of the laser sheet, laser extinction, laser fluence, etc. [ABSTRACT FROM AUTHOR]

In this paper, the dependence of the sublimation temperature of soot particles synthesized during the combustion of various hydrocarbons, depending on their size and structure, is obtained. The experimental approach is based on the analysis of the thermal radiation of particles heated to the sublimation temperature by a nanosecond laser pulse. The sublimation temperature of soot particles was measured using the two-color pyrometry method. In this paper, it is proposed to use the average size of primary particles to compare data in different flames. It is established, that the sublimation temperature of soot particles depends mainly on the stage of their formation, which is characterized by an increase in average size. It is shown, that with an increase in the average particle size from 12 to 23 nm, their sublimation temperature increases significantly from 2700 to 4500 K. This reflects a significant difference in the thermodynamic and optical properties of the so-called "young" and "mature" soot particles, which must be taken into account when developing methods of soot diagnostics and in the thermo-physical analysis of combustion and pyrolysis processes with the formation of soot. [ABSTRACT FROM AUTHOR]

This paper investigates the soot sheet characteristics of buoyant turbulent fires using laser-induced incandescence technique. Experiments are carried out for four fires using propane and nitrogen hybrid gases as fuel with propane fractions of 1.0-0.4. An equivalent ellipse method in previous literature was employed to evaluate soot sheet characteristics. The results show that soot sheet mainly exists in two kinds of morphology: one is small piece and the other is narrow and long piece. Both the probability density functions of the length and width of the equivalent ellipse and soot volume fraction of soot sheet are strongly dependent on height. In addition, the effect of propane fraction on the probability density functions is always opposite in the lower and middle part of the fires, while no obvious effect can be found in the upper part of the fires. Soot sheet number is found to increase with height except when propane fraction is 0.4. The total soot sheet number increases linearly with height within the scope of this paper. [ABSTRACT FROM AUTHOR]

To comprehensively understand the effect of horizontal opening size on the soot evolution and transportation of flames in the confined compartment, 2D soot distribution and characteristic soot volume fraction of non-premixed propane flames were theoretically and experimentally investigated in the confined compartment (0.2 m × 0.2 m × 0.2 m) with different horizontal openings (0.03 m to 0.10 m) in this work. The oxygen concentration supplied by the horizontal opening was determined by the ratio of dimensionless opening size and dimensionless flame volume within a range of 14.5% to 20.7%. Results showed that the soot distribution of non-premixed propane flames in the confined compartment with a horizontal opening complied with the classical three-zone assumption. The maximum soot volume fraction, the characteristic length of the soot formation zone and the characteristic length of the soot oxidation zone were all positively related to the dimensionless flame volume and negatively correlated with the dimensionless opening size. There was a positive correlation between the ratio of two characteristic lengths and dimensionless flame volume, and the correlation was independent of the dimensionless opening size. Moreover, a linear correlation of characteristic soot volume fraction was proposed to predict the maximum soot volume fraction of non-premixed flames in the confined compartment with a horizontal opening. [ABSTRACT FROM AUTHOR]

Snapshot multispectral imaging of chemical species in the flame is essential for improved understanding of the combustion process. In this article, we investigate the different configurations of a structured laser sheet-based multispectral imaging approach called the Frequency Recognition Algorithm for Multiple Exposures (FRAME). Using FRAME, a snapshot of Laser-Induced Fluorescence (LIF) of Polycyclic Aromatic Hydrocarbons (PAH) excited by 283.5 nm laser and Laser-Induced Incandescence (LII) of soot particles excited by 532 nm laser are acquired simultaneously on a single FRAME image. A laminar diffusion flame of acetylene produced by a Gülder burner is used for the experiments. The standard FRAME approach is based on creating two spatially modulated laser sheets and arranging them in a cross-patterned configuration (X). However, the effect of using different configurations (angles) of the two laser sheets on the multispectral planar imaging of the flame has not yet been studied. Therefore, we have compared the FRAME approach in four different configurations while keeping the same flame conditions. First, we have compared the relation between laser fluence and LII signals with and without spatial modulation of the 532 nm laser sheet and found that both detections follow the same curve. When comparing the maps of flame species reconstructed from the standard FRAME configuration and other configurations, there are some dissimilarities. These differences are attributed to minor changes in the imaging plane, optical alignment, laser path length, different modulation frequencies of the laser sheet, laser extinction, laser fluence, etc.

Measuring the size distribution and temperature of high-temperature dispersed particles, particularly in-flame soot, holds paramount importance across various industries. Laser-induced incandescence (LII) stands out as a potent non-contact diagnostic technology for in-flame soot, although its effectiveness is hindered by uncertainties associated with pre-determined thermal properties. To tackle this challenge, our study proposes a multi-parameter inversion strategy—simultaneous inversion of particle size distribution, thermal accommodation coefficient, and initial temperature of in-flame soot aggregates using time-resolved LII signals. Analyzing the responses of different heat transfer sub-models to temperature rise demonstrates the necessity of incorporating sublimation and thermionic emission for accurately reproducing LII signals of high-temperature dispersed particles. Consequently, we selected a particular LII model for the multi-parameter inversion strategy. Our research reveals that LII-based particle sizing is sensitive to biases in the initial temperature of particles (equivalent to the flame temperature), underscoring the need for the proposed multi-parameter inversion strategy. Numerical results obtained at two typical flame temperatures, 1100 K and 1700 K, illustrate that selecting an appropriate laser fluence enables the simultaneous inversion of particle size distribution, thermal accommodation coefficient, and initial particle temperatures of soot aggregates with high accuracy and confidence using the LII technique.

This review is conducted to assess the effectiveness of the laser extinction method (LEM) calibrated laser-induced incandescence (LII) to quantitatively determine the soot volume fraction in flames. This article mainly focuses on the discussion and analysis of the existing experimental results in typical co-flow laminar diffusion flames. Initially, a brief introduction of the background and application of sooting tendency in co-flow laminar diffusion flames by using the combination of LII and LEM is presented. Then, the general theoretical backgrounds of LII and LEM techniques used for soot diagnostics is introduced. This is followed by a detailed summary and comparison of the maximum soot volume fraction in laminar diffusion flames obtained by a combination of LII and LEM techniques as well as the various optical methods. The maximum soot volume fraction in flames measured by the combined technique of LII and LEM exhibits high consistency with those of other optical methods, but the uncertainty derived from this approach is relatively large in soot concentration measurement. Minimizing the measurement uncertainty of soot volume fraction obtained by the combination of LII and LEM is still an important and non-negligible issue. The determination of the soot refractive index function E(m) for mature soot is one of the major sources of measurement uncertainty when using the combined technique of LII and LEM. Whereas it is of great challenge to confirm an accurate E(m) for mature soot of flames in the visible and near-infrared spectrum, and it still needs a large number of experimental investigations to reveal the accurate value of E(m). Finally, the conclusion and perspective for investigating sooting tendency in flames by the LEM-calibrated LII technique are presented.

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

McKenna burner providing premixed, laminar, steady flat flame is widely used in the combustion community. The ethylene/air flame with equivalent ratio of 2.34 is considered as one of the target flames for the development and calibration of optical diagnostics of soot particles. In this work, the accurate soot particle sizing in dependence on the height above a burner was performed using the methods of transmission electron microscopy and laser-induced incandescence. It was found, that shielding co-flow gas type and rate together with small variation of the height of stabilization plate in flat premixed flame burner is not influenced on the center flame temperature and on the soot particle size measurement results. Also, the small variation of the velocity of the cold gas in the 5% range at the same equivalence ratio is not influenced by the results of flame temperature and soot particle size measurements. From the other side, the variation of the burner stabilization plate configuration and material of the sintered plate results in the essential differences of the measured values. The accomplished data analysis resulted in a reliable dependence of soot size in the target flame on the height above a burner, which can be applied for validation of soot formation models. [ABSTRACT FROM AUTHOR]

As a carbon-neutral fuel, ethanol is commonly used as a partial replacement for gasoline, but its impact mechanism on the soot formation of gasoline remains unclear, especially under elevated-pressure conditions. In this work, the soot formation of ethanol/n-heptane was experimentally and numerically investigated in laminar diffusion flames in the pressure range of 1.2–5 bar. For all experimental conditions, the total carbon flow rate in the fuel was maintained at 0.353 mg/s with 0%, 10%, 20%, and 30% ethanol contributions. The soot volume fraction and flame temperature were obtained by laser-induced incandescence coupled with light extinction and color-ratio soot pyrometry technique. In the considered pressure range, the synergistic effect on the soot formation was observed with 10% ethanol addition and became more pronounced at higher pressures. The soot formation was suppressed when the amount of ethanol was increased to 20% or 30%. Numerical simulations are performed using a reduced n-heptane/ethanol chemical mechanism and a soot model based on the bivariate Hybrid Method of Moments. The synergy effect was successfully predicted by the model. The simulation results indicate that the reason for the synergistic effect is the increase in acetylene and methyl caused by ethanol pyrolysis, which increases the mass fraction of soot precursor species and consequently promotes PAH condensation. [ABSTRACT FROM AUTHOR]

• Absorption non-uniformities within aggregates increase with increasing aggregate sizes, and may contribute to excessive absorption and anomalous cooling effects observed in TiRe-LII experiments on metal nanoparticles. • For the aggregates considered, primary particles within the aggregate may be shielded from the E-field at certain orientations leading to a larger degree of absorption and emission non-uniformity, however, orientation has a less than 10 % impact on the random orientation value. • Sintering-induced overlap can enhance the absorption properties of the aggregate due to increased electrical conductivity and morphological changes towards an elongated structure that promotes the antenna effect. Analyzing time-resolved laser-induced incandescence (TiRe-LII) data from metal nanoparticle aerosols requires a detailed understanding of their absorption and emission characteristics. This work investigates how non-uniform absorption within metal nanoparticle aggregates, aggregate morphology and orientation, and sintering of primary particles may affect TiRe-LII signals from metal nanoparticle aerosols. The multi-sphere T-matrix method is used to compute the absorption properties of aggregates with point contact between primary particles, while the discrete dipole approximation method is used when primary particles overlap. It was found that absorption non-uniformities within aggregates increase with increasing aggregate sizes, and may contribute to excessive absorption and anomalous cooling effects. For the anisotropic aggregates considered, the total absorption cross-section depends weakly on orientation. It was also found that the sintering of primary particles can enhance the absorption cross-section of metal aggregates. [ABSTRACT FROM AUTHOR]

• Inferring primary particle size distribution and TAC is validated for in-flame soot. • LII model with sublimation and thermionic emission is optimal for trivariate inversion. • Increased laser fluence enhances trivariate inversion accuracy and reliability. • Spatial inhomogeneity of flame temperature is solved by the proper laser fluence. Laser-induced incandescence (LII) technique has proven to be a powerful tool for non-invasive, in-situ particle sizing of soot aggregates in flames, and the key lies in interpreting LII signals with a heat transfer model. However, the reliability of model-derived primary particle size distribution (PPSD) is sensitive to the uncertainty of thermal accommodation coefficient (TAC), which is typically regarded as a pre-determined parameter in the LII heat transfer model. To overcome this problem, we have proposed a trivariate inversion strategy, i.e., inferring two PPSD parameters and TAC simultaneously from LII signals, which has been theoretically validated at room temperature, but its feasibility in the flame case remains to be investigated. In this study, the trivariate inversion strategy is extensively evaluated over a wide distribution of both ambient temperature and laser fluence. For different cases ranging from room temperature to flame temperature, numerical tests show that the inversion results of all three variables exhibit significantly high accuracy and low uncertainty with proper selection of laser fluence. Furthermore, our study shows that the trivariate inversion strategy can maintain robust performance in ambient temperature fluctuation caused by spatial inhomogeneity of the flame. All these results indicate that this trivariate inversion strategy has a promising application in combustion scenarios. [ABSTRACT FROM AUTHOR]

The impact on the use of carbon dioxide (CO2) as a diluent in the fuel and oxidizer streams on the formation and evolution of precursor nanoparticles and soot in axisymmetric ethylene laminar coflow diffusion flames is experimentally explored using optical diagnostics in this paper. The optical diagnostics employed are based on temporally and spectrally resolved point measurements of laser-induced fluorescence, elastic scattering, and laser-induced incandescence. The measurements provide insight to indicative structure and concentration of precursor nanoparticles, and enable the determination of volume fraction and mean diameter of soot. Previous studies have indicated that the use of CO2 as a diluent compared to nitrogen can reduce soot formation through both chemical and thermal effects. However, the effect of CO2 addition on precursor nanoparticles' concentrations and structure is not well understood, hence exploring the impact of CO2 on precursor nanoparticles is one of the primary aims of this study. Flames with CO2 as a diluent in the jet only, coflow only, and both streams are identified and investigated. For the flames studied with respect to the replacement of N2 with CO2: i), the soot volume fraction and mean soot diameter is reduced, and ii) precursor nanoparticles concentration is reduced, and the structural transformation is delayed. [ABSTRACT FROM AUTHOR]

The objective of this study is to derive morphological and nanostructural properties of soot as well as the reactivity against low-temperature oxidation by O 2 from easily measurable optical properties. First, ex-situ experiments utilizing thermogravimetric analysis (TGA) and high-resolution transmission electron microscopy (HRTEM) serve to evaluate the kinetics of soot oxidation with O 2 and relate reactivity to particle morphology and nanostructure. Second, ultraviolet–visible (UV-VIS) absorption spectra provide wavelength-dependent absorption cross sections and refractive-index functions E (m ~ , λ). From these, optical band gap energies, E OG , and coefficients ξ∗ for single parameter functions describing the wavelength-dependency of E (m ~ , λ) are obtained. Third, from time-resolved laser-induced incandescence (TR-LII) ratios of the refractive-index functions E (m ~ , λ i) / E (m ~ , λ j) at three excitation wavelengths and primary particle size distributions are acquired. The ex-situ experiments show that the size of the graphene layers predominantly determines soot reactivity against oxidation. Graphene layer size and, therefore, soot reactivity are reflected in the UV-VIS absorption spectra and E (m ~ , λ) , E OG , and ξ∗ , respectively. Similarly, scattering-corrected ratios E (m ~ , λ i) / E (m ~ , λ j) from TR-LII also reflect graphene layer size and, hence, soot reactivity. The established strong correlations between the optical properties, nanostructural characteristics and reactivity against oxidation make UV-VIS spectroscopy as well as TR-LII useful fast in-situ diagnostic methods for soot reactivity. [Display omitted] [ABSTRACT FROM AUTHOR]

Laser-induced incandescence (LII) was used to quantify the soot volume fraction in five piloted turbulent non-premixed C 2 H 4 -N 2 jet flames at elevated pressures. In one series of flames, the bulk jet velocity was held constant as the pressure was increased from 1 bar (Re = 10,000) to 3 bar (Re = 30,000), and then 5 bar (Re = 50,000). In the other series, a Reynolds number of 10,000 was held constant at 1 bar, 3 bar, and 5 bar. LII measurements were calibrated with laminar diffusion flames at each pressure using 2D diffuse line-of-sight attenuation. Mean and RMS soot volume fractions along the flame centerline exhibit Gaussian profiles for all of the flames. The magnitude of soot volume fraction and the spatial soot distribution are enhanced by increases in pressure. In both series of flames, the peak mean soot volume fraction scales with the pressure as p 2.2. In the constant velocity series, the scaling drops to p 1.5 if the volume-integrated mean soot volume fraction on a per fuel mass basis is considered instead. At 3 bar, a threefold increase in Re leads to a decrease in the mean soot volume fraction despite the decrease in soot intermittency. At 5 bar, a fivefold increase in Re results in soot intermittency near zero at the flame's center and a slight increase in the mean soot volume fraction. [ABSTRACT FROM AUTHOR]

The effective density and mixing state of particles emitted from a natural gas–diesel dual fuel engine are investigated. Measurements were conducted at three different fuel compositions including 100% diesel fuel (0% NG), 75% diesel–25% natural gas (25% NG) and 50% diesel–50% NG (50% NG). The particle effective density was measured using a differential mobility analyzer in series with a centrifugal particle mass analyzer. A catalytic stripper at 350 °C was employed upstream of the centrifugal particle mass analyzer in order to remove the semi-volatile material from the solid particles to measure the effective density of non-volatile particles as well as the particle mixing state. A scanning mobility particle sizer was used to measure the particle size distribution. The particle mass concentration was also measured using several techniques including cavity-attenuated phase-shift particulate matter single-scattering albedo, laser-induced incandescence, thermal-optical analysis, photoacoustic spectroscopy, and integrated particle size distribution. The semi-volatile number and mass fractions are found to be lower than 15%. The effective density functions of particles at 0% and 25% NG are within 6% of each other; however, the effective density values of particles at 50% NG are lower than those of the 0% NG by up to 35%. The mass-mobility exponent varies in the range of 2.42–2.51 and 2.38–2.54 for undenuded and denuded particles, respectively. For the mass concentration measurements, photoacoustic spectroscopy agrees with thermal-optical analysis within 5%, while all the other techniques measure up to 50% deviations relative to thermal-optical analysis. [ABSTRACT FROM AUTHOR]

Dimethyl ether (DME) has received considerable attention as a fuel additive to reduce the emission of particulate matter (PM) due to its low-temperature chemistry, molecularly bound oxygen atom and the absence of C C bonds. However, the effect of DME addition on the evolution of soot and particularly soot precursors is not entirely understood. This study aims to shed light on this issue by blending different proportions of DME with diffusion, E60, and partially premixed, PP12, base cases of laminar ethylene flames using the Yale benchmark burner. Laser-induced fluorescence (LIF) intensity and decay time are used to characterize the structure and evolution of soot precursors, while laser-induced incandescence (LII) is utilized to determine the soot volume fraction (SVF) and the effective primary particle diameter (D p). For the diffusion flames, the addition of 10% DME increases the concentrations of both soot and soot precursors. With the further addition of DME to 20%, the SVF decreases to levels similar to those of E60 and then decreases further with 30% DME addition. All diffusion flames with DME addition exhibit higher concentrations of soot precursors than those of the reference E60 case. For PP12, the addition of 10% DME shows similar concentrations of soot precursors and a slight reduction in the SVF which continues to decrease with further increases in DME additions to the PP12 flame. The addition of DME seems to have little effect on the soot particle diameters for all the studied flames. Overall, the PP flames result in smaller mean particle diameters than the diffusion flame counterparts. [ABSTRACT FROM AUTHOR]

Two-dimensional laser-induced incandescence (LII) measurements were used for quantitative soot volume fraction (f v) measurements in methane-air diffusion flames at pressures ranging from 0.1 to 0.5 MPa. Additionally, laser-induced fluorescence (LIF) was used for visualization of polycyclic aromatic hydrocarbons (PAHs) considered as important soot precursors. A heat and mass transfer-based LII model was used for the analysis of experimental LII fluence curves to evaluate soot absorption functions, E (m) , at various spatial positions in the flames. Based on previous work, variations in the evaluated E (m) was assumed to be related to soot maturity. Generally, the most mature soot was found at the spatial position of maximum f v along the central axis and at the flame edges. Also, the soot maturity at the position of maximum f v at each pressure was found to increase from the flame at 0.1 MPa to 0.5 MPa. A critical examination of the assumptions and uncertainties in the E (m) analysis was made. The present study emphasizes the limitation of assigning soot a constant E (m) in the overall flame for temperature and soot concentration evaluation using the optical methods spectral soot emission (SSE) and line-of-sight attenuation (LOSA), which are commonly used non-intrusive optical diagnostic techniques in sooting high-pressure flames. The study also demonstrates the critical choice of fluence in quantitative imaging LII measurements of f v when the E (m) of soot spans over large range of values. [ABSTRACT FROM AUTHOR]

Laser-induced incandescence (LII) is a widely used technique for measuring soot concentrations. For flame applications LII is frequently deployed as a planar diagnostic to measure the two-dimensional soot field. However, when the laser sheet is focused, as is typical to reach the requisite laser fluence level and achieve good spatial resolution, the complex laser power dependence of the LII signal generation process can introduce a large variation in LII signal sensitivity across an LII image. In this work, this effect is quantified for the first time as a function of laser pulse fluence, using a typical planar LII excitation scheme with a clipped Gaussian YAG laser beam focused with a 1 m focal length lens. Furthermore, the cross-sectional energy distribution in the laser sheet was measured across the image plane, to relate the details of the laser sheet focal properties with the resultant LII behavior. The results show that a unique laser fluence level (referenced to the focal plane) exists whereby there is essentially no dependence of LII signal on position relative to the focal plane. However, at lower or higher fluences, the radial signals either decrease (low fluence) or increase (high fluence) rapidly with increasing distance away from the focal point. For measurements using an LII 'plateau' laser fluence level, as is usual in environments with significant optical depth (i.e. sufficiently strong soot levels), the LII signals are found to be 2.5X larger 40 mm away from the focal point. An analysis conducted by combining a previously measured LII fluence dependence for a top-hat laser profile with the laser sheet cross-sections measured in this work shows general agreement with the measured results for LII signal variation. Further, the sensitivity of LII signals at high fluences to the laser beam spatial profile, particularly away from the sheet focus, is highlighted. [ABSTRACT FROM AUTHOR]

This paper examines the evolution of particle size and morphology of plasma-synthesized carbonaceous nanoparticles for various reactants, reactant concentrations, reactor temperatures, and positions within the reactor. Aerosol particles were characterized at various positions downstream from the plasma zone by spatially-resolved thermophoretic sampling and transmission electron microscopy, Raman spectroscopy, and by in situ time-resolved laser-induced incandescence. Depending on the carbon-bearing reactant (ethanol or toluene) and its concentration, either pure few-layer graphene (FLG) or soot-like particles, or a mixture of both, were generated. The initial carbon nucleation has been found to commence less than 12.4 cm downstream from the plasma nozzle. In the case of FLG formation, particles show an increasing level of crumpling with increasing distance downstream from the plasma zone. The process was numerically simulated with a 1D plug-flow reactor model employing a joint population balance model for graphitic and amorphous-like particles (FLG and soot-like, respectively). The simulation includes modified versions of inception, hydrogen-abstraction-carbon-addition (HACA), and polycyclic-aromatic hydrocarbon (PAH) adsorption models adapted from a pre-existing soot model. The simulations showed that the ratio of HACA/PAH adsorption determined by post-plasma C 2 H 2 is the main factor that affects the soot-to-FLG ratio. This is also consistent with experimental observations as reducing the rate at which carbon precursor is supplied to the reactor leads to less C 2 H 2 in the post-plasma zone, which results in less PAH adsorption, and consequently suppressed soot formation in favor of FLG synthesis. [ABSTRACT FROM AUTHOR]

Flame structure of laminar inverse diffusion flames (IDFs) was studied to gain insight into soot formation and growth in underventilated combustion. Both ethylene-air and methane-air IDFs were examined, fuel flow rates were kept constant for all flames of each fuel type, and air flow rates were varied to observe the effect on flame structure and soot formation. Planar laser-induced fluorescence of hydroxyl radicals (OH PLIF) and polycyclic aromatic hydrocarbons (PAH PLIF), planar laser-induced incandescence of soot (soot PLII), and thermocouple-determined gas temperatures were used to draw conclusions about flame structure and soot formation. Flickering, caused by buoyancy-induced vortices, was evident above and outside the flames. The distances between the OH, PAH and soot zones were similar in IDFs and normal diffusion flames (NDFs), but the locations of those zones were inverted in IDFs relative to NDFs. Peak OH PLIF coincided with peak temperature and marked the flame front. Soot appeared outside the flame front, corresponding to temperatures around the minimum soot formation temperature of 1300 K. PAH appeared outside the soot layer, with characteristic temperature depending on the wavelength detection band. PAH and soot began to appear at a constant axial position for each fuel, independent of the rate of air flow. PAH formation either preceded or coincided with soot formation, indicating that PAH are important components in soot formation. Soot growth continued for some time downstream of the flame, at temperatures below the inception temperature, probably through reaction with PAH.

The structure of laminar inverse diffusion flames (IDFs) of methane and ethylene was studied using a cylindrical co-flowing burner. Several flames of the same fuel flow-rate yet various air flow-rates were examined. Heights of visible flames were obtained using measurements of hydroxyl (OH) laser-induced fluorescence (LIF) and visible images. Polycyclic aromatic hydrocarbon (PAH) LIF and soot laser-induced incandescence (LII) were also measured. In visible images, radiating soot masks the blue region typically associated with the flame height in normal diffusion flames (NDFs). Increased air flow-rates resulted in longer flames. PAH LIF and soot LII indicated that PAH and soot are present on the fuel side of the flame and that soot is located closer to the reaction zone than PAH. Ethylene flames produced significantly higher PAH LIF and soot LII signals than methane flames, which is consistent with the sooting propensity of

Soot formation and oxidation in a turbulent n -heptane jet spray flame are investigated through simultaneous imaging of quantitative soot volume fraction (f v), flame-front (OH), and soot-precursor marked by smaller (2–4 ring) polycyclic aromatic hydrocarbons (PAH). Laser-induced incandescence (LII) and laser-induced fluorescence (LIF) techniques are used. The spray flame is composed of a dual branch axisymmetric structure. The inner composite branch (B1) consists of fuel-lean and non-premixed reaction zones, whereas combustion in the outer branch (B2) occurs in a non-premixed mode. Frequent local flame extinctions occur along B1, while no extinctions are observed in B2. Consequently, the soot inception occurs near B2. In the onset of soot detection (f v ≥ 0.03 ppm) region, soot layers are thinner than those from downstream regions where soot occupies nearly the same area as of PAH. Soot generally oxidizes across the flame-front as evidenced from the absence of soot-OH overlap. The soot structures are intermittent with peak soot probability of 60%. The most probable peak f v measures 1 ppm, while the conditional mean is 0.42 ppm which is 2.5 times that of time-averaged f v. Conditional mean f v shows monotonically increasing trend with height. The flame lift-off height and local extinctions do not instantaneously influence the soot concentration, possibly due to larger convective and soot formation timescales. Soot-PAH correlations are explored. However, no strong instantaneous correlations are noted, most likely due to different chemical timescales of soot and the imaged smaller-PAH. Nevertheless, a few distinct trends are noted in soot-PAH correlations. f v weakly increases with the PAH-LIF intensity. The PAH intensity from soot onset to peak region decreases, while both soot and PAH are nearly consumed at a local flame-tip (oxidation) region. The soot-PAH overlap decreases consistently from soot onset to oxidation region. The reported database and deduced insights can aid in the development and validation of soot models in two-phase reacting flows. [ABSTRACT FROM AUTHOR]

This paper focuses on the soot inception region in laminar coflow diffusion flames of methane and ethylene stabilised on the Yale diffusion burner. Earlier studies of these flames have focused on the downstream regions where soot has already developed. Laser-induced fluorescence (LIF) and elastic scattering measurements from 266 nm excitation are combined with laser-induced incandescence (LII) excited at 1064 nm. The structure and evolution of the soot precursor particles are characterised using the LIF intensity, decay time and the relative spectral emission in the ultraviolet and visible. The LIF decay times indicate that the majority of 266 nm excited LIF originates from nanostructures rather than gas phase polycyclic aromatic hydrocarbons (PAH). A similarity in the particle evolution for the low and high sooting flames in the upstream regions is found, indicating a general transition towards larger structures with more aromatic features as the nanostructures advect downstream in the fuel rich pyrolytic conditions. Higher nanostructure concentrations are found to precede the higher soot volume fractions (SVF) found in fuel rich sootier flames, although not proportionally, suggesting that surface growth strongly contributes to SVF in the high sooting flames. In the heavier sooting flames the majority of particle formation shifts from the centreline to the wings at the outer edges of the flame closer to stoichiometry. Particle formation in the wings of the flames occurs in the presence of oxygen and higher temperatures, resulting in particles with spectroscopic properties resembling those formed toward the oxidiser side of counter-flow diffusion flames. In the heavier sooting flames, particles produced in the wings appear to mix with particles formed along the centreline of the flame at the flame tip, resulting in a broad range of nanostructures and soot occurring in this region. [ABSTRACT FROM AUTHOR]

Abstract Soot formation is a major challenge in the development of clean and efficient combustion systems based on hydrocarbon fuels. Fundamental understanding of the reaction mechanism leading to soot formation can be obtained by investigating the role of key reactive species such as atomic hydrogen taking part in soot formation pathways. In this study, two-dimensional laser induced incandescence (LII) measurements using λ = 1064 nm laser have been used to measure soot volume fraction (f V) in a series of rich ethylene (C 2 H 4)/air flames, stabilized over a McKenna burner fitted with a flame stabilizing metal disc. Moreover, a comparison of UV (λ = 283 nm), visible (λ = 532 nm) and IR (λ = 1064 nm) laser excited LII measurements of soot is discussed. Recently developed, femtosecond two-photon laser-induced fluorescence (fs-TPLIF) technique has been applied for obtaining spatially resolved H-atom concentration ([ H ]) profiles under the same flame conditions. The structure of the flames has also been determined using hydroxyl radical (OH) planar laser induced fluorescence (PLIF) imaging. The results indicate an inverse dependence of f V on [H] for a range of C 2 H 4 /air rich flames up to an equivalence ratio, Φ = 3.0. Although an absolute relationship between [H] and f V cannot be easily derived owing to the multiple steps involving H and other intermediate species in soot formation pathways, the present study demonstrates the feasibility to couple [H] and f V obtained using advanced optical techniques for soot formation studies. [ABSTRACT FROM AUTHOR]

Abstract Measurements of soot volume fraction (SVF) using planar Laser-Induced Incandescence (LII) were performed in a set of attached turbulent non-premixed jet flames, burning the same mixture of C 2 H 4 /H 2 /N 2. A novel post processing technique was used to identify and characterize the average width (W s) and volume fraction of the soot sheets and correlate it with key flame features and parameters. The axial distributions of both W s and SVF, within those sheets, in all flames are shown to grow with flame length up to the region where soot oxidation becomes significant, with only a secondary dependence on the inflow conditions. Probability density functions (pdfs) of W s are seen to exhibit far less variation than those of SVF , also indicating the soot structure morphology insensitivity to inflow conditions. The effects of fuel exit strain (U / D) and bulk exit Reynolds number (Re D) on soot sheets are also assessed for three flames, of which two share the same U / D and two share the same Re D. The variation in U / D at constant Re D is seen to only affect SVF within the sheets, while a variation in Re D , at constant U / D is shown to have negligible effect on both W s and SVF , in agreement with previous findings by the authors. Finally, a strong power-law correlation is deduced from joint statistics for W s and SVF through all flame locations and for all the flames investigated. [ABSTRACT FROM AUTHOR]

Abstract The growth of carbon particles was studied in heated flows of a burnt-gas flow reactor containing mixtures of N 2 /C 2 H 2 , and N 2 /C 2 H 2 with addition of H 2 or CH 4 surrounded by a rich C 2 H 4 /air flame. Soot particle sizes and volume fractions were measured by laser-induced incandescence (LII) between 50 and 130 mm above the nozzle exit. The measurements indicate a soot-inhibiting effect of adding H 2 to the C 2 H 2 /N 2 flow on both, particle sizes and soot volume fractions. The effect of CH 4 addition to the C 2 H 2 /N 2 flows was ambivalent, depending on the methane-to-acetylene ratio. At gas mixtures with N 2 :CH 4 :C 2 H 2 = 0.42:0.35:0.23 and 0.39:0.32:0.29 by volume at fixed total flow rates, the measured soot volume fractions were substantially increased in presence of CH 4 , while the mean diameters of the particles were slightly decreased. Gas temperatures were measured by a generalized line-reversal method with Abel transformation. Temperatures of the surrounding C 2 H 4 /air flame were around 1600 K, and temperatures of the inner flows, where soot formation was measured, were between 1550 and 1630 K. Plug-flow reactor calculations provided a qualitative understanding of the influence of CH 4 on the soot particle growth. [ABSTRACT FROM AUTHOR]

Abstract In-situ soot characterization has been successfully performed in a 100 kW th down-fired oxy-fuel test furnace using laser-induced incandescence (LII) and extinction measurements. Primarily non-premixed propane flames were investigated in oxy-fuel mode with various concentrations of oxygen in the oxidant. The turbulent flame character was manifested through two-dimensional single-shot LII signals from soot showing strong spatial variations as well as local temporal variations. The LII signals were calibrated to soot volume fractions, f v , using in-situ extinction in the same spatial regions of the furnace. The results show increased f v for increasing oxygen concentration in the oxidant, which is related to increased temperatures as well as decreased mixing inside the furnace due to lowered total flow. For some measurement cases, the influence of additives was studied for flames in oxy-fuel and air environments. The results showed increased f v for additions of SO 2 and NO for oxy-fuel conditions, while a decrease of f v was found for air-fed flames. Also, a large decrease in f v was found for water injection in the air-fed flames, and a slightly larger decrease for addition of KCl dissolved in water with the same amount of injected solution. Uncertainties in performing soot volume fraction measurements using LII and extinction in this large-scale furnace are discussed, and mainly considered to be uncertainties in E (m) for soot, the spatial variation of the laser fluence in the large imaged area, and the estimation of the absorption length during extinction calibration. [ABSTRACT FROM AUTHOR]

Laser-induced incandescence (LII) of a surface layer of GaSb/In x Ga 1−x Sb/GaSb (x = 0.1) sandwich heterostructure is investigated in the visible spectral region under irradiation by the third harmonic of radiation of a Q-switched YAG:Nd 3+ laser. Within the laser-induced hot spot on the irradiated surface, strong granularity of LII is observed. The experiments revealed that the intensity of surface-integrated LII in the In x Ga 1−x Sb region is twofold larger than in the GaSb regions of the investigated structure; this fact is interpreted as consequence of difference in thermal conductivities of GaSb and In x Ga 1−x Sb. Computer simulations were performed for LII of laser-melted surface layers. The results of simulations are in agreement with the experimental data. [ABSTRACT FROM AUTHOR]

Emissions from diesel engines can be limited and potentially decreased by modifying the fuel chemical composition through additive insertion. One class of additives that have shown to be particularly efficient in the reduction of the particulates from the combustion of diesel fuels are oxygenated compounds. In the present study we investigate the effect of tripropylene glycol methyl ether (TPGME) and two polyoxymethylene dimethyl ethers (POMDME or OMEs) on soot formation in a laminar diesel diffusion flame. From the evaluation of soot volume fraction by laser-induced incandescence (LII) measurements we could observe that OME additives have a substantial capability (higher compared to TPGME) to decrease the particle concentration, which drops by up to 36% with respect to the pure diesel fuel. We also note a reduction in particle aggregate size, determined by wide-angle light scattering (WALS) measurements, which is more pronounced in the case of OME–diesel blends. The effects we observe can be correlated to the higher amount of oxygen content in the OME molecules. Moreover, both additives investigated seem to have almost no impact on the local soot temperature which could in turn play a key role in the production of soot particles.

In this article, the application of the FRAME (Frequency Recognition Algorithm for Multiple Exposures) technique is presented for multi-species measurements in symmetric and asymmetric ethylene/air diffusion flames. Laminar Bunsen-type and swirled diffusion flames are investigated to gain a better understanding of sooting combustion. For this purpose, simultaneous imaging is conducted in terms of Laser-Induced Fluorescence (LIF) of Polycyclic Aromatic Hydrocarbons (PAH) and Laser-Induced Incandescence (LII) of soot particles. Subsequently, the approach is utilized for simultaneous imaging of hydroxyl (OH)-LIF and soot-LII. Here, the modulated LIF- and LII-signals are acquired together as a single sub-image—with a single exposure utilizing the full sensor size of a single camera. By employing the frequency-recognition algorithm on the single image, the LIF- and LII-signals are spectrally isolated—generating two individual LIF- and LII-images. The flame luminosity and out-of-focus light such as reflected surrounding laser light are detected as non-modulated signals in the unprocessed image. These unwanted signals are suppressed using the image post-processing, and, therefore, the image contrast of the two resulting images is improved. It is found that PAHs mainly exist in the inner region near the burner and are surrounded by soot. The majority of the OH is distributed on the outer edges of the flame—representing the reaction zone and soot-oxidation region of the flame.

Laser-induced incandescence (LII) is a powerful diagnostic technique allowing quantifying soot emissions in flames and at the exhaust of combustion systems. It can be advantageously coupled with modeling approaches to infer information on the physical properties of combustion-generated particles (including their size), which implies formulating and solving balance equations accounting for laser-excited soot heating and cooling processes. Properly estimating soot diameter by time-resolved LII (TiRe-LII), nevertheless, requires correctly evaluating the thermal accommodation coefficient α T driving the energy transferred by heat conduction between soot aggregates and their surroundings. To analyze such an aspect, an extensive set of LII signals has been acquired in a Diesel spray flame before being simulated using a refined model built upon expressions accounting for soot heating by absorption, annealing, and oxidation as well as cooling by radiation, sublimation, conduction, and thermionic emission. Within this framework, different conduction sub-models have been tested while a corrective factor allowing the particle aggregate properties to be taken into account has also been considered to simulate the so-called shielding effect. Using a fitting procedure coupling design of experiments and a genetic algorithm-based solver, the implemented model has been parameterized so as to obtain simulated data merging on a single curve with experimentally monitored ones. Eventually, values of the thermal accommodation coefficient have been estimated with each tested conduction sub-model while the influence of the aggregate size on the so-inferred α T has been analyzed.

Quantitative soot volume fraction measurements were conducted in a counterflow non-premixed flame configuration using ethylene/nitrogen as the fuel stream, oxygen/nitrogen as the oxidizer stream, and a pressure range of 1–8 atm. The laser-induced incandescence technique, calibrated using the light extinction method, was used to measure the soot volume fraction distributions. The variations of soot formation along the centerline of the counterflow flame with pressure were compared by keeping the density-weighted strain rate constant. Maintaining a constant density-weighted strain rate allows the overall flame thickness, as well as the reactant mass fluxes entering the flame, to remain unchanged for all pressures. As such, the effect of pressure on soot chemistry can be isolated from the effect of convective-diffusive transport. Based on the measured soot volume profiles, the soot layer thickness variation with pressure was determined. It was found that when keeping the density-weighted strain rate constant, the soot layer thickness remains similar over the pressure range investigated. However, the soot layer thickness was seen to decrease with increasing pressure when holding the strain rate fixed. In addition, the effects of fuel mole fraction and oxygen mole fraction on soot formation were investigated. Furthermore, the pressure scaling factors of soot formation under varying mixture conditions were deduced from experimental measurements. A literature gas-phase reaction mechanism including polycyclic aromatic hydrocarbon (PAH) chemistry up to pyrene was also used to simulate the experimental counterflow flames. The pressure effect on PAH formation was presented and discussed. [ABSTRACT FROM AUTHOR]

Multi-pass cavity line-of-sight extinction (MPC-LOSE) and laser-induced incandescence (LII) techniques are deployed to measure the soot volume fraction in a series of nitrogen-diluted flames, which produce only ppm volume mass fractions of soot. The separate suppression effects on soot formation of direct fuel dilution and indirect effects of temperature and residence time are interpreted by using a numerically calculated flow velocity and temperature field using a one-step fast chemistry model. The experimentally determined rate of soot formation is shown to obey approximately the same function of the local temperature for all dilution cases. The results show that a simple one-step reaction model using previously measured activation energies can account for the dilution effect with good accuracy. The results show that the direct effect of dilution on concentration is comparable to the effects of changing the temperature estimated local temperature and residence time. [ABSTRACT FROM AUTHOR]

An experimental investigation has been performed where the influence of metal salts on soot formation has been studied. By combining two-dimensional laser-induced incandescence (LII) and elastic light scattering (ELS), two-dimensional information could be obtained on soot properties in the flames. For these studies, seven metal salts (NaCl, MgCl 2 , AlCl 3 , KCl, CaCl 2 , FeCl 3 and ZnCl 2 ) were dissolved in water and aspirated into a premixed ethylene/air flame. At lower flame heights, in the soot inception region, the LII signal (representing soot volume fraction) was marginally affected by all additives, whereas the ELS signal strongly decreased with increasing additive concentration for the alkali salts. At higher heights, in the soot growth region, the soot volume fractions were lowered for the addition of potassium, calcium and sodium chloride, in order of significance. Some of the salts (MgCl 2 , AlCl 3 and FeCl 3 ) resulted in negligible influence on LII signals and slightly higher ELS signals throughout the flames, and we relate the increased ELS signals to salt particles propagating through the flame. Main focus in our study was on the addition of potassium chloride for which several parameters were investigated. For example, soot primary particle sizes were evaluated using combined LII and ELS, showing decreasing particle sizes for increasing concentrations of potassium, in reasonable agreement with particle sizes evaluated using transmission electron microscopy. Also, CARS thermometry showed slightly higher flame temperature, ∼30 K, for the potassium-seeded flame compared to the reference flame. [ABSTRACT FROM AUTHOR]

With increasing concerns over fuel cost, environmental pollutions, and energy security, the development of alternative fuels from renewable sources has gained considerable attention in the past decade. To evaluate the sooting propensities of representative conventional and alternative jet fuels, the present study has investigated soot formation in non-premixed combustion for three conventional reference jet fuels, i.e., Jet-A, JP-8, and JP-5, and three selected alternative jet fuels, i.e., Fischer-Tropsch Synthetic Paraffinic Kerosene (FT-SPK), Hydroprocessed Esters and Fatty Acids from camelina (HEFA-Camelina), and Alcohol-to-Jet (ATJ). The soot volume fraction profiles of these jet fuels in the atmospheric-pressure counterflow non-premixed flame configuration were measured using Laser-Induced Incandescence technique. In addition, the effects of global strain rate and reactant concentration on soot formation were investigated. At the same stoichiometric mixture fraction, the experimental results show that the sooting level increases with increasing fuel and O 2 concentrations, while it decreases with increasing global strain rate, as expected. It is also worth noting that the three alternative jet fuels tested demonstrated greater sensitivity to variations in global strain rate and reactant concentration, as compared to the three conventional jet fuels. Based on the measured soot volume fractions, the sooting propensity of the six jet fuels are ranked as follows: JP-5 > Jet-A > JP-8 > ATJ > FT-SPK > HEFA-Camelina. The observed ranking is consistent with the aromatics content in each jet fuel except when comparing FT-SPK and ATJ. While ATJ has a lower aromatics content than FT-SPK, it contains much heavier hydrocarbon components, thereby leading to higher sooting propensity. Furthermore, the correlations of soot volume fraction with aromatics content and hydrogen content are examined for all six jet fuels. Although such correlations work well for the three conventional jet fuels, the present results suggest that they cannot be directly transferable to the alternative jet fuels, as these alternative fuels are composed of negligible amounts of aromatics, as well as composed of distinct chemical compositions. [ABSTRACT FROM AUTHOR]

In previous work, we introduced the dual-pulse two-color time-resolved laser-induced incandescence (DP-2C-TiRe-LII), which is possibly suitable for on-line monitoring of the nanostructural topology of inhomogeneous soot particle ensembles in transient flows. The method relies on the quantitative relationship between the size of the basic structural units (BSUs) embedded within primary particles and the ratio of the refractive-index function for absorption at two wavelengths E (m , λ U V) / E (m , λ N I R). As the size of BSUs increases, the band gap energy for an electronic π → π * transition becomes narrower increasing the wavelength for absorption and, hence, forcing E (m , λ U V) / E (m , λ N I R) to decrease. Once quantitative correlations between the carbon nanostructure, i.e., size of the BSUs, and a structure-associated particle property are available, its on-line monitoring becomes accessible through monitoring of E (m , λ U V) / E (m , λ N I R). The objective of this paper is to provide the proof-of-concept for on-line monitoring of an exemplary structure-associated particle property, viz. soot reactivity against oxidation, in an application from practice. For this, DP-2C-TiRe-LII is employed downstream of a gasoline direct injection (GDI) engine running under steady-state and transient operating conditions. Intrusive particle sampling in combination with an analytical toolbox comprising high-resolution transmission electron microscopy (HRTEM), pattern recognition methods, elemental analysis (EA), and thermogravimetric analysis (TGA) was applied to validate the results from DP-2C-TiRe-LII. Excellent agreement is found between particle properties derived via ex situ and in situ diagnostics. Moreover, the transformation of the time series of E (m , λ U V) / E (m , λ N I R) into probability density distributions seems to represent the BSU size distributions of the investigated soot particle ensembles. Likewise, soot reactivity is reflected in the distributions of E (m , λ U V) / E (m , λ N I R). [ABSTRACT FROM AUTHOR]

In the present work, a novel burner capable of complete pre-vaporization and stationary combustion of diesel fuel in a laminar diffusion flame has been developed to investigate the effect of the chemical composition of diesel fuel on soot formation. For the characterization of soot formation during diesel combustion we performed a comprehensive morphological characterization of the soot and determined its concentration by coupling elastic light scattering (ELS) and laser-induced incandescence (LII) measurements. With ELS, radii of gyration of aggregates were measured within a point-wise measurement volume, LII was employed in an imaging approach for a 2D-analysis of the soot volume fraction. We carried out LII and ELS measurements at different positions in the flame for two different fuel types, revealing the effects of small modifications of the fuel composition on soot emission during diesel combustion.