Flame Soot Research Articles

Generating an aerosol of homogeneous, non-spherical particles and measuring their bipolar charge distribution

The Aerodynamic Aerosol Classifier (AAC) and Differential Mobility Analyzer (DMA) are aerosol classifiers commonly used to generate a monodispersed aerosol by selecting particles within a narrow range of relaxation times or electrical mobilities, respectively. However, generating an aerosol of homogeneous particles, which has narrow ranges of particle mass, mobility and relaxation time simultaneously, with either of these methods is challenging. Particles classified by the DMA are often not homogeneous (or monodispersed) due to multiply-charged particles. While the AAC overcomes this challenge for spherical particles, homogeneity is not achieved with non-spherical particles due to their effective density varying with particle size. This study demonstrates using an AAC and DMA in tandem to generate an aerosol of homogeneous, non-spherical particles. This approach is validated using scanning electron microscope (SEM) images and electrical mobility measurements of the tandem-classified particles to highlight their homogeneity. To limit the effects of multiple charging during DMA classification, only a subset of DMA and AAC setpoints are permitted. While this subset is not representative of “average” non-spherical particles from the same aerosol source, this subset of low-density particles deviates the most from spherical morphology, and thus, provides insights into the upper bound of other particle properties, such as charging. Using this approach to select homogeneous particles, the bipolar charge distribution of low-density soot aggregates is then measured using another DMA. This AAC-DMA-DMA approach is demonstrated to measure up to 17 individual charge states (i.e. −8 to +8) after neutralization (with 85Kr) of size-resolved, soot aggregates with mobility diameters between 80 and 433 nm. The low-density soot aggregates obtain higher charges than predicted by theory, which overestimates the uncharged fraction (by 0.042–0.069) and, to a lesser extent, the single charge fractions (by up to 0.037) of the low-density soot aggregates, while underestimating their proportion of multiple charging (by up to 0.135 cumulatively at one particle size or up to 0.039 at one multiple charge state and size). These charging discrepancies represent an upper bound of the bipolar charging of average aggregates from the same source of flame soot.

Experimental analysis and modeling of Buoyancy-driven flame spread on cast poly(methyl methacrylate) in corner configuration

A new experimental setup was developed to study turbulent, buoyancy-driven flame spread on a corner wall. Two 146 × 50 × 0.58 cm panels of cast black poly(methyl methacrylate) (PMMA), whose pyrolysis properties were fully characterized in a separate study, were ignited using a triangular propane burner. The corner wall assembly was placed into a well-ventilated enclosure and heat release rate (HRR) and flame heat flux to the corner wall were measured in 7 repeated experiments. The HRR measurement was designed to achieve a fast response time (13 s) and was correlated with the flame heat flux, which was obtained using water-cooled heat flux gauges positioned in 28 locations distributed over the PMMA surface. Simultaneously, a monochromatic, 900 nm, video of the spreading flame was recorded and analyzed to determine the evolution of soot radiation intensity and flame geometry. Using collected data, an empirical model relating spatially resolved heat feedback from the flame to the solid fuel surface and HRR was developed and integrated with a detailed pyrolysis model. Simulations were conducted in an uncoupled and coupled mode. The uncoupled simulations, where the flame heat feedback was effectively prescribed to match the experimental measurements, revealed the importance of the knowledge of the split between the convective and radiative portions of the flame heat flux. The coupled simulations, where the flame heat feedback was computed from the simulated HRR, revealed that the coupling amplifies even relatively small uncertainties in the model parameters to produce large errors in the HRR predictions.

Quantum confinement and size resolved modeling of electronic and optical properties of small soot particles

Electronic and optical properties of flame-generated carbon nanoparticles were modeled based on quantum confinement and amorphous semiconductor theory of monodispersed particles. The ionization energies and optical band gaps of polydispersed particles 4–23 nm in volume median diameter (Liu et al., Proc. Natl. Acad. Sci. U.S.A. 116 (2019) 12692–12697) were re-analyzed by deconvolution to obtain particle-size specific properties. The complex refractive index was derived from optical absorption and the Kramers–Krönig relations over the wavelength range of 185–1400 nm. Dependence of the refractive index on the primary particle size is shown for the first time. The real component of the refractive index is weakly dependent on the particle size, but the imaginary component is found to be particularly sensitive to the particle size. The refractive indices of relatively large soot particles are found to be in close agreement with literature values in the visible spectrum; those of smaller particles (〈 ∼ 15 nm in diameter) show a significant difference from the literature values. The current result highlights the need for accounting for the size effect in the refractive index in laser diagnostics of soot in flames. It also suggests that some of the earlier extinction and scattering measurements of flame soot may have to be re-evaluated, especially for lightly sooting flames and during the early stage of soot growth.

On the use of PIV, LII, PAH-PLIF and OH-PLIF for the study of soot formation and flame structure in a swirl stratified premixed ethylene/air flame

Soot formation and oxidation are investigated in swirl stratified premixed ethylene/air flames at atmospheric pressure. The effects of both swirl and stratification are studied to understand the relationship between the flame structure, soot precursors and soot. The topology of the flame is obtained with particle image velocimetry (PIV) and planar laser-induced fluorescence (PLIF) on hydroxyl radical (OH). The production of polycyclic aromatic hydrocarbons (PAHs) is investigated using PLIF by mainly probing the aromatic compounds with two benzene rings (i.e. naphthalene) that are known to actively participate in soot nucleation and growth. Soot production is investigated using laser-induced incandescence (LII), giving quantitative data on the soot volume fraction. Extensive information on the flame structure and the mechanisms of formation/consumption of soot is gathered based on the coupling of these laser diagnostics. An image analysis of velocity, OH, PAHs and soot distributions enables us to propose a scenario that describes the link between the inception, growth, aggregation and oxidation processes. In particular, the data reveal the presence of distinct zones for these processes: a thermal decomposition region in which PAHs contributes to nascent soot formation, organised as filaments along the interface between the PAH region and the inner recirculation zone (IRZ); a mixing region controlled by large moving structures that favour the growth and aggregation of nascent soot into mature soot; and an oxidation region leading to the fast consumption of soot particles. These processes are impacted to varying extents by the intensities of swirl and stratification.

Effects of circulation and buoyancy on the transition from a fire whirl to a blue whirl

Author(s): Hariharan, SB; Hu, Y; Gollner, MJ; Oran, ES | Abstract: The relative influence of circulation and buoyancy on fire whirls (FWs), blue whirls (BWs), and the transition between these regimes of a whirling flame is investigated using a combination of experimental data and scaling analyses. FWs are whirling, turbulent, cylindrical yellow (sooting) flame structures that form naturally in fires and are here created in laboratory experiments. In contrast, a BW is a laminar, blue flame (nonsooting) with an inverted conical shape. Measurements of the circulation and heat-release rate are combined with measurements of the flame geometry, defined by the flame width and the height, to provide characteristic length scales for these whirling-flame regimes. Using these, a nondimensional circulation (Γf) and a heat-release rate (Qf) were defined and shown to correspond to azimuthal and axial (buoyancy driven) momenta, respectively. The ratio R=Γf/Qf, a quantity analogous to the swirl number used to characterize swirling jets, was evaluated for FWs and BWs. For FWs, Rl1, so that axial momentum is greater than azimuthal momentum and the flame is dominated by buoyant momentum. For BWs, Rg1, so that the flame is circulation dominated. This is argued to be consistent with vortex breakdown being an important part of the transition of FWs to BWs. This work presents a basis for predicting when a BW will form and remain a stable regime.

Statistical relationship between soot volume fraction, temperature, primary particle diameter and OH radicals along transects normal to the local reaction zone in a turbulent flame

We present a new method to extract experimental data from simultaneous planar images in turbulent sooting flames, to derive transects of key species based on local coordinates that are aligned normal to the instantaneous, fluctuating reaction zone. Such statistics represent a milestone in the assessment of turbulent sooting flames, because they are one-dimensional and based on local coordinates conditioned on the distribution of soot sheets, in contrast to previous statistics, which are typically Eulerian and single point in nature. The local coordinate system was determined from instantaneous images of soot volume fraction (fv) based on a varying threshold and the local reaction zone was identified by further referring OH images. Quantitative experimental data of fv, temperature (T) and primary particle diameter (Dp), together with qualitative OH intensities, were extracted along the direction normal to the local reaction zone from simultaneous images of the four parameters. These results were measured in a pre-vaporized toluene diffusion flame (Reynolds number = 10,000) using advanced laser-based techniques. Statistical data were combined together and used to generate the profiles for these four parameters with high accuracy and level of resolution that is not possible using previous statistics methods based on Eulerian coordinates. These new kind results showed that the spatial relationship between fv, T and OH remains broadly similar anywhere in the flame, while the profiles of Dp significantly shift away from those of the other three parameters with the increase in the height of the flame. The mean values of fv, Dp and T also show a clear relationship as a function of the height, even though the trends of mean fv and Dp along the height are different from that of T, also suggesting a strong relationship exists between soot and temperature in the local reaction zone.

On the relative contributions of soot to radiative heat transfer at different oxygen indices in ethylene – O2/CO2 laminar diffusion flames

Soot concentration measurements in ethylene-O2/CO2 diffusion flames (fuel Re 120) encompassing oxidizer oxygen mole fractions (oxygen index, OI) of 35% to 90% were employed to validate a methodology for modeling soot production. A skeletal 10-step reaction mechanism and modeling the soot nucleation parameter (Cα) in the Moss-Brookes soot model as a function of OI provided adequate agreement with measurements. Cross-validation of the OI based Cα model was ascertained through ethylene-O2/CO2 diffusion flame simulations in a different experimental configuration (fuel Re 43, 35%-50% O2) by ensuring that peak soot concentrations were in good agreement with previously published results. A mesh resolution of 0.0625 mm within the flame was deemed necessary to obtain grid independent results. Despite increases in peak soot volume fractions and flame temperatures with OI, the radiant fraction (χR) (in-flame) decreases due to decreasing flame lengths. Although in-flame radiation is dominated by gases, the spectrally-continuous soot radiation plays an important role on χR (when it is assessed across the entire domain) due to the attenuation of flame radiation by the cold, radiatively-participating, co-flow. Consequently, in-flame radiation show a strong sensitivity to gas-radiative property models whereas χR when assessed across the entire domain is impacted by the choice of soot radiative property model.

Nanostructural Disorder and Reactivity Comparison of Flame Soot and Engine Soot Using Diesel and Jatropha Biodiesel/Diesel Blend as Fuels

Soot is a major anthropogenic air pollutant that affects human health and contributes to global warming. To understand its formation pathways and reduce emission, several flame and engine studies exist in the literature, though the fundamental differences in the characteristics of engine and flame soots are not well understood. This study presents a detailed comparative investigation of soot nanostructural properties and their relationship with the oxidative reactivity of soots from an engine and a diffusion flame using diesel and 20% Jatropha biodiesel/diesel blend fuels. X-ray diffraction, Raman spectroscopy, high-resolution transmission electron microscopy, electron energy loss spectroscopy, and thermogravimetric analyses confirm that engine soot has greater primary particle diameter, higher concentration of loosely held aliphatics, greater degree of graphitized nanocrystallites with lower interplanar separation, longer fringe lengths, lower tortuosity, and greater resistance for oxidation than the flame soot, though the differences in several properties were minor. The effects of biodiesel addition to diesel on soot properties and sooting tendency were predicted very well with both flame and engine setup. Moreover, the enhanced soot oxidation in the combustor catalyzed by fuel-bound oxygen in biodiesel further reduces the nanostructural and reactivity differences between engine and flame soots. Though engine soot properties have more relevance to anthropogenic particulate matter, flame setups appear to be suitable for screening and studying the effect of fuel additives on the sooting propensity and physicochemical properties of soot prior to their testing and utilization in engines.

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

We investigate the effect of pressure on both flame structure and soot formation in nitrogen diluted counterflow diffusion flames of ethylene in the 8–32atm pressure range. Capillary-probe gas sampling is performed to resolve spatially the profiles of gaseous species up to three-ring aromatics by GC/MS analysis and multi-color pyrometry is used to quantify the soot volume fraction and dispersion exponent. Self-similarity of flames is preserved by keeping constant mixture fraction and strain rate, so that profiles of concentrations and temperature, normalized with respect to their peak values, are unaffected by changes in pressure, once the axial coordinate is nondimensionalized with respect to the pressure-dependent diffusion length scale. When conditions are chosen so that the overall soot loading is approximately constant and compatible with the diagnostics, it is found that both the soot volume fraction and the profiles of key aromatics in the high-temperature nucleation region are virtually invariant. For it to happen, a twofold increase in pressure must be compensated by a ∼100 K decrease in peak flame temperature and, therefore, in the temperature across the soot forming region. The implication is that from the perspective of the chemical kinetics of soot formation these two actions counterbalance each other. As pressure increases (and temperature decreases) the peak production rate of the high-temperature soot mechanism decreases and, further downstream, towards the particle stagnation plane, a low-temperature soot mechanism sets in, yielding an increase in soot H/C content. This mechanism is enhanced as the pressure is raised, causing a higher overall soot volume production rate in the 16atm flame and, especially, in the 32atm one. The role of C4/C2 species in the formation of C6H6 increases with increasing pressure and dominates over the recombination of propargyl radical at sufficiently high pressures. A comprehensive database is established for soot models at high pressures of relevance to applications.

Flame structure of supercritical ethanol/water combustion in a co-flow air stream characterized by Raman chemical analysis

The thermochemical properties of the ethanol hydrothermal flame (30%-v ethanol-water solution), established in air co-flow under supercritical water conditions, are measured by a quantitative Raman diagnostic technique. The unique spectroscopic features of the chemical compounds are revealed to aid understanding of the fuel decomposition and oxidation process in supercritical phase. A point-wise fiber-optic probe is designed to resolve the needle-like thin flame and identify the spatial profiles – across the flame thickness and over the height of the visible flame – of the combustion species at different fuel/air ratios and airflow rates. An attempt is made to analyze the steep thermal gradients in the primary reaction zone near the fuel nozzle by estimating thermodynamic temperatures from the measured fluid density. The test cell was held at conditions above the critical point of water, at a nominal pressure of 25 MPa and a nominal temperature of 723 K (450 °C). The flames were over ventilated with an excess air:fuel ratio between two and five times stoichiometric. Depending upon the air:fuel ratio non-sooting blue flames or yellow sooting flames are observed. The results of the Raman diagnostic provide temperature and species profiles that will prove useful for validating numerical models using an idealized laboratory hydrothermal flame. The Raman diagnostic results are augmented with imaging from two orthogonal camera views: a backlit shadow-graphic image of the co-flow jet and a color image of the flame.

Population balance modelling and laser diagnostic validation of soot particle evolution in laminar ethylene diffusion flames

Laminar diffusion flames present an elementary configuration for investigating soot formation and validating kinetic models before these are transferred to turbulent combustors. In the present article, we present a joint experimental and modelling investigation of soot formation in a laminar co-flow burner. The diffusion flames are analysed with the aid of laser diagnostic techniques, including elastic light scattering (ELS), planar laser-induced fluorescence of OH (OH-PLIF) and line-of-sight attenuation (LOSA), to measure the spatial distribution of soot, gas phase species and the line-of-sight integrated soot volume fraction (ISVF), respectively. The experimental dataset is supplemented by location-specific TEM images of thermophoretically sampled soot particles. The simulation of the sooting flames is carried out with a recently developed discretisation method for the population balance equation (Liu and Rigopoulos, 2019, Combust. Flame 205, 506-521) that accomplishes an accurate prediction of the particle size distribution, coupled with an in-house CFD code. By minimising numerical errors, we ensure that the discrepancies on the modelling side are mainly due to kinetics and are able to carry out an investigation of alternative models. We include a complete set of soot kinetics for PAH-based nucleation and condensation, HACA-based surface growth and oxidation as well as size-dependent aggregation, and consider three different gas phase reaction mechanisms (ABF, BBP and KM2). Based on predictions of the gas phase composition and particle size distribution of soot, modelled counterparts of the laser diagnostic signals are computed and compared with the experimental measurements. The approach of directly predicting signals circumvents the difficulties of explicitly representing the OH concentration in terms of the measured OH-PLIF data and avoids using ‘hybrid’ modelled and measured values to approximate the OH concentration. Moreover, the LOSA signal is directly converted to the line-of-sight ISVF instead of a measure of local soot volume fraction to avoid tomographic inversion errors. Lastly, the predicted ELS signal is computed in terms of the particle size distribution resolved by the population balance model, thus circumventing the approximation of an integral soot property using a presumed size distribution. While we cannot obtain quantitative agreement between experiments and simulations, the accuracy of the numerical approach and the direct prediction of experimental signals allow us to conduct sensitivity analyses of key empirical parameters and investigate the importance of the PAH chemistry and its influence on the competition between nucleation, condensation and surface growth.

Effect of hydrogen enrichment of laminar ethylene diffusion flames on thermal structure and soot yields at pressures up to 10 bar

A series of high-pressure experiments were conducted to assess the influence of hydrogen enrichment of laminar diffusion flames of nitrogen-diluted ethylene on the thermal flame structure and soot yields at pressures above atmospheric. In parallel experiments, added hydrogen is replaced by helium, either in equal mole fractions or in mass fractions, to evaluate the thermal, dilution, and direct chemical interaction effects of hydrogen in soot formation. Experiments covered pressures from atmospheric to 10 bar. In the first set of experiments, conducted at 3, 6, and 10 bar pressure, base fuel was an ethylene-nitrogen mixture with 33.3% ethylene and 66.7% nitrogen (by mole as well as by mass). This base fuel was doped with either hydrogen or helium such that hydrogen and helium mass fractions and mole fractions in the fuel stream are matched in two cases. In the second set of experiments, which were conducted at 1.2 bar pressure with ethylene as the base fuel, hydrogen or helium is added such that additive mole fraction in the fuel stream was 44%. Temperature measurements in the first set of experiments indicate that, when hydrogen is added to nitrogen-diluted ethylene, the changes in the temperature field of the co-flow diffusion flames are negligible, except at lower in the flame where hydrogen added flames display slightly higher temperatures. When helium is added instead of hydrogen, however, the temperatures were measurably lower than those of the base fuel. Results show that, once the dilution effects are accounted for, the hydrogen addition to ethylene does not suppress soot formation by direct chemical interaction at elevated pressures. These findings, which are not in agreement with the previous experimental results obtained at atmospheric pressure, are discussed in terms of the higher molecular diffusivity of hydrogen and shorter residence times of high-pressure flames.

Experimental assessment of the sudden-reversal of the oxygen dilution effect on soot production in coflow ethylene flames II: soot radiation and flame transition analysis

The current study focuses on the radiative characteristics of heavy smoking flames that are formatted by oxygen being added to the fuel stream of a steady laminar ethylene diffusion flame. In such a scenario, based on the experimental local soot temperature and volume fraction results, the soot local radiative fields are obtained by solving the Radiative Transfer Equation (RTE). Along with the diluted range (oxygen volume fraction XO2ax from 0 to 32%), the flame undergoes two significant transitions, i.e., smoking and non-smoking; however, only one remarkable radiative intensity transition between 30% and 32% is identified. The radiative loss due to the oxygen chemical effect is further assessed by comparing with that of N2 diluted flames. It is found that the chemical effect on radiative loss promotion is undermined by approximately 25%, compared with that on soot formation promotion. In contrast, the oxygen dilution effect results in a similar reduction level on soot formation propensity and soot radiative loss. Furthermore, the correlations between relative soot consumption rate and cumulated radiative loss below the flame tip region are explored, and the results indicate that the initial relative oxidation rate at the position of zFv,max (the peak location of flame cross-section mean soot volume fraction) and its residence time are two important factors that may affect the final flame smoking/non-smoking transition. Eventually, these local soot radiative intensity fields, together with the local soot temperature and volume fraction fields, provide a more comprehensive benchmark to refine the soot oxidation model as the ISF workshop required and understand the flame transition mechanism in the sooting flames.

Highly radiating hydrogen flames: Effect of toluene concentration and phase

Adapting hydrogen as a carbon-free fuel for industrial applications requires new, innovative approaches, especially when radiant heat transfer is required. One possible option is to dope hydrogen with bio-oils, containing aromatics that help produce highly sooting flames. This study investigates the potential doping effects of toluene on a hydrogen-nitrogen (1:1 vol) flames. Flames with 1–5% toluene, based on the mole concentration of hydrogen, are measured using a combination of techniques including: still photographs and laser-based techniques. Toluene was mixed with hydrogen-nitrogen fuel mixture as either a vapour carried by nitrogen, or as a dilute spray. Spray flames are found to produce substantially more polycylic aromatic hydrocarbons, with significantly more soot near the nozzle exit plane, than the prevaporised flames. Increasing the dopant concentration from 1 to 3% of the hydrogen has a marked effect on soot loading in the flame, although the further increasing the dopant concentration to 5% has a far smaller effect on the soot produced in the flame. Simulations of laminar flames using detailed chemical kinetics support the above findings and reveal details of the competition between soot precursor formation and hydrocarbon oxidation. Correlations of formation rates are non-linear with toluene concentration in cases where toluene represents less than 10% of the fuel, although expected linear relationships are noted beyond this regime up to 1:1 toluene/hydrogen blends. The study provides insight and explanation into effects of toluene as a dopant, comparison between flame doping in gaseous or liquid phases and suggests that flame doping and blending should be treated as different regimes for their global effect on flame sooting characteristics.

Effects of ethanol and 2, 5-dimethylfuran addition on the morphology and nanostructure evolution of soot in gasoline primary reference fuel-air coflow diffusion flames

The sediment morphology and oxidation activity of soot at different heights above the burner are studied for three fuels. The three fuels are n-heptane/iso-octane mixtures (0% 2, 5-dimethylfuran and 0% ethanol, DE0), n-heptane/isooctane mixture with 15% ethanol (E15) and n-heptane/iso-octane mixture with 15% 2, 5-dimethylfuran (DMF15). Oxygenated fuel increases flame height and decreases flame brightness. DMF15 has the highest flame and has a brighter waist edge. From the field emission scanning electron microscopy (FESEM) images, ethanol and 2–5, dimethylfuran (DMF) make the deposits particle size of soot smaller and uniform. The soot deposits at the height of the flame became more orderly, E15 soot with the smallest fibrous deposits and pore structure. “Donut” and “Steamed bun” structures are found in soot sediments of DE0 and DMF15, which had not been previously reported. The results of thermogravimetric analysis (TGA) show that the oxidation activity of soot in different flame positions is very different. Generally, the activation energy of soot increases with the increase of sampling position, but the activation energy of DMF15 soot decreases first and then increases. Finally, the soot activation energy of E15 and DMF15 is much less than that of DE0, and E15 has the highest oxidation activity. It is inferred that the structure of the special soot deposits is caused by the inhomogeneous oxidation activity on the surface of the deposits, therefore, a free radical erosion sediments model is proposed.

On the Effects of Carbon Dioxide as a Diluent on Precursor Nanoparticles and Soot in Axi-symmetric Laminar Coflow Diffusion Flames

ABSTRACT 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.

Candle flame soot sizing by planar time-resolved laser-induced incandescence

Soot emissions from flaming combustion are relevant as a significant source of atmospheric pollution and as a source of nanomaterials. Candles are interesting targets for soot characterization studies since they burn complex fuels with a large number of carbon atoms, and yield stable and repeatable flames. We characterized the soot particle size distributions in a candle flame using the planar two-color time-resolved laser induced incandescence (2D-2C TiRe-LII) technique, which has been successfully applied to different combustion applications, but never before on a candle flame. Soot particles are heated with a planar laser sheet to temperatures above the normal flame temperatures. The incandescent soot particles emit thermal radiation, which decays over time when the particles cool down to the flame temperature. By analyzing the temporal decay of the incandescence signal, soot particle size distributions within the flame are obtained. Our results are consistent with previous works, and show that the outer edges of the flame are characterized by larger particles (approx 60,hbox {nm}), whereas smaller particles (approx 25,hbox {nm}) are found in the central regions. We also show that our effective temperature estimates have a maximum error of 100 K at early times, which decreases as the particles cool.

Study of soot functional groups and morphological characteristics in laminar coflow methane and ethylene diffusion flames with hydrogen addition

The soot in laminar coflow methane and ethylene diffusion flames with different hydrogen addition ratios is sampled by a capillary soot sampling system and analyzed by Fourier transform infrared spectroscopy (FTIR) and field emission scanning electron microscopy (FESEM). Soot functional groups and morphological characteristics are studied. The results show that with hydrogen addition, the relative contents of aromatic and aliphatic functional groups in the soot of methane flames increase, while the relative contents of the same functional groups in the soot of ethylene flames mostly show different trends, indicating that H2 has a chemical promotion and inhibition on soot in methane and ethylene flames, respectively. The relative content of oxygen-containing functional groups in the soot of both methane and ethylene flames with hydrogen addition increases. By adding H2, the coagulation and condensation of soot low in the methane flames are greatly enhanced, indicating that H2 promotes the formation and growth of PAH molecules. This is an important reason for the chemical promotion effect of H2 in methane flames. For the soot of C2H4 hydrogen-added flames, oxidation is relatively thorough such that a large number of soot branches are oxidized. This is an important reason for the chemical inhibition of H2.

Benchmark evaluation of tomographic algorithms for simultaneous reconstruction of temperature and volume fraction fields of soot and metal-oxide nanoparticles in non-uniform flames

With the growing applications of nanofluid flame, the monitoring and controlling of its combustion process is of paramount importance. Thus, it is necessary to develop diagnosing methods which can simultaneously image important parameters such as temperature and volume fractions of soot, metal-oxide nanoparticles. Tomographic emission spectroscopy is an effective method which has been proposed for this purpose. However, the inversion process was only reported with least-squares QR decomposition (LSQR) so far and there are numerous well-established reconstruction algorithms which have not been utilized yet. Thus, this work aims to perform systematic comparative studies on several representative algorithms for the inversion process. In the simulative studies, algorithms including Tikhonov regularization, algebraic reconstruction technique (ART), LSQR, Landweber algorithm, maximum likelihood expectation maximization (MLEM), and ordered subset expectation maximization (OSEM) were discussed. The effects of the number of iterations, the signal-to-noise ratio, and the number of projections and the calibration error in projection angles on the performance of the algorithms were investigated. Advice on selecting the suitable algorithms under different application conditions is then provided according to the extensive numerical studies.

On the effect of pressure on soot nanostructure: A Raman spectroscopy investigation

Although the majority of existing combustion devices operate at high pressure conditions, most of our understanding of the soot formation process and soot physicochemical properties rely on studies performed at atmospheric pressure. Pressure is known to have nonlinear effects on combustion processes and a significant influence on soot formation; soot loading increases with increasing combustion pressure. Soot characteristics directly affect soot oxidation and optical/radiative properties, and it is desirable to have a better insight into them under high-pressure conditions. Due to scarcity of information on soot primary particles, aggregate morphology, and soot nanostructure relevant to high-pressure combustion, there are challenges in predicting soot oxidation and radiation, particularly at engine-relevant conditions.In this study, we perform Raman spectroscopy measurements on soot sampled from a set of laminar diffusion flames of ethylene at various pressures, ranging from atmospheric pressure to 12 bar. Our results show an increase in soot maturity as the pressure increases within the range of investigated pressures. In the examined co-flow flames, pressure seems to have an indirect influence on soot nanostructure through an earlier inception of soot, resulting in longer residence times of the carbon soot particles in the hot and reactive flame environment. It is found that soot maturity, tracked through the size of graphitic domains, La, increases linearly with residence times. The longer residence time of soot in high-pressure flames could be the main cause of the higher degree of graphitization observed, which suggests a greater resistance to oxidation with increasing pressure.