Soot Structure Research Articles

Simulating the effect of post-flame agglomeration on the structure of soot

Simulating the effect of post-flame agglomeration on the structure of soot

Soot and Flame Structures in Turbulent Partially Premixed Jet Flames of Pre-Evaporated Diesel Surrogates with Admixture of OMEn

In this study, the soot formation and oxidation processes in different turbulent, pre-evaporated and partially premixed diesel surrogate flames are experimentally investigated. For this purpose, a piloted jet flame surrounded by an air co-flow is used. Starting from a defined diesel surrogate mixture, different fuel blends with increasing blending ratios of poly(oxymethylene) dimethyl ether (OME) are studied. The Reynolds number, equivalence ratio, and vaporization temperature are kept constant to ensure the comparability of the different fuel mixtures. The effects of OME addition on flame structures, soot precursors, and soot are investigated, showing soot reduction when OME is added to the diesel surrogate. Using chemiluminescence images of C2 radicals (line of sight) and subsequent Abel-inversion, flame lengths and global flame structure are analyzed. The flame structure is visualized by means of planar laser-induced fluorescence (PLIF) of hydroxyl radicals (OH). The spatial distribution of soot precursors, such as polycyclic aromatic hydrocarbons (PAHs), is simultaneously measured by PLIF using the same excitation wavelength. In particular, aromatic compounds with several benzene rings (e.g., naphthalene or pyrene), which are known to be actively involved in soot formation and growth, have been visualized. Spatially distributed soot particles are detected by using laser-induced incandescence (LII), which allows us to study the onset of soot clouds and its structures qualitatively. Evident soot formation is observed in the pure diesel surrogate flame, whereas a significant soot reduction with changing PAH and soot structures can be identified with increasing OME addition.

Soot evolution in ethylene combustion catalyzed by electric field: experimental and ReaxFF molecular dynamics studies

In this study, an electric field (E-field) was used to regulate the formation and oxidation of soot in combustion. In the procedure, an E-field is applied to the ethylene non-premixed flame, the flame is stretched horizontally and the flame height is reduced. By comparing the partially premixed flame to eliminate the effect of the E-field on the increase of combustion temperature caused by premixing, transmission electron microscope (TEM) detection revealed that the soot particles were more dispersed in the action of E-field, the “core-shell” structure of soot disappeared, and the markedly improved oxidation rate of soot in the flame. The evolution process of soot in ethylene combustion was studied by reactive molecular dynamics. The 9 ns simulation was performed at E-field strengths of 0, 0.001, and 0.1 V/Å by three successive modelings. The results showed that the E-field reduced the growth of the largest molecule in the system and increased the coagulation time of the initial soot particles. With increasing E-field strength, the “core-shell” structure of soot gradually broke. An increased hydrogen-carbon ratio and decreased number of six-membered rings are the internal factors for the more scattered structure of soot, which allows the soot to be more easily oxidized.

Internal Structure of Incipient Soot from Acetylene Pyrolysis Obtained via Molecular Dynamics Simulations.

A series of reactive molecular dynamics simulations is used to study the internal structure of incipient soot particles obtained from acetylene pyrolysis. The simulations were performed using the ReaxFF potential at four different temperatures. The resulting soot particles are cataloged and analyzed to obtain statistics of their mass, volume, density, C/H ratio, number of cyclic structures, and other features. A total of 3324 incipient soot particles were analyzed in this study. Based on their structural characteristics, the incipient soot particles are classified into two classes, termed type 1 and type 2 incipient soot particles in this work. The radial distribution of density, cyclic (5-, 6-, or 7-member rings) structures, and C/H ratio inside the particles revealed a clear difference in the internal structure between type 1 and type 2 particles. These classes were further found to be well represented by the size of the particles, with smaller particles in type 1 and larger particles in type 2. The radial distributions of ring structures, density, and the C/H ratio indicated the presence of a dense core region in type 2 particles. In contrast, no clear evidence of the presence of a core was found in type 1 particles. In type 2 incipient soot particles, the boundary between the core and shell was found to be around 50-60% of the particle's radius of gyration.

Formation mechanism of environmentally persistent free radicals during soot aging

Environmentally persistent free radicals (EPFRs) are a newly emerging class of pollutants that are known to be formed on metal oxides (MOs). Recent experimental investigations have suggested that EPFRs may also be formed on soot. However, little is known about the structural and compositional characteristics of soot surfaces conducive to EPFRs formation. Concurrently, the changes in the surface structure and composition of soot indicate its aging process, yet the detailed mechanism underlying EPFRs formation in this process remain obscure. Additionally, it is uncertain whether the formation mechanism of EPFRs on soot differs from that on MOs. In this study, the density functional theory (DFT) and ab initio molecular dynamics (AIMD) were used to investigate EPFRs formation on soot at different stages of aging. The results indicate that EPFRs could be formed on both the complete soot surface with hydroxyl groups and the defective surface. Correspondingly, during the aging process of soot, the formation trend of EPFRs increases in the early stage of the aging process and then decreases over time. Additionally, the formation mechanism on soot is different from that on CuO. On soot, the EPFRs formation is initiated by free hydroxyl radical formed through the desorption process, following the Eley-Rideal mechanism, while on CuO surfaces, its formation involves the surface-bound hydroxyl groups, and thus follows the Langmuir-Hinshelwood mechanism. Furthermore, EPFRs triggered by soot are less stable than those by CuO, due to the easier desorption process of phenoxyl radicals on soot. These findings provide valuable insights into the detailed formation mechanism of EPFRs on soot at different stages of aging, thereby contributing to control EPFRs emissions from a mechanism perspective.

Effect of co-combustion of coal with biomass on the morphology of soot

This study performed an experimental investigation to explore the effect of co-combustion of coal with biomass on the morphological parameters and nanostructure of soot. Three types of soot, namely Shenghua coal (SH) soot, Pinewood biomass (PW) soot, and Co-combustion of SH and PW (CC) soot, were collected from the combustion flames of Shenhua coal, Pinewood biomass, and a mixture of coal and biomass. Thereafter, the soot samples were observed and analyzed by means of transmission electron microscope (TEM) and scanning electron microscope (SEM) to obtain their morphological parameters and nanostructure characteristics. The TEM results showed that the size of nascent soot particles generated in co-combustion flame is the largest, rather than between the sizes of PW and SH soot. The nanostructure of the three types of soot primary particles is mainly composed of straight parallel crystal stripes. The separation distance of crystal stripes in SH soot was measured to be larger than that in PW soot, indicating that the crystal structure of soot generated in Pine biomass combustion flame is more compact. It was also observed that both coal-derived primary soot particles and biomass-derived primary soot particles were formed in the co-combustion flame. SEM results indicated that PW soot agglomerates have stronger aggregation performance and more compact structure then SH soot and CC soot. The primary particle size of SH and CC soot agglomerates was mainly concentrated between 50 and 70 nm, while that of PW soot agglomerates was relatively uniform. Furthermore, the number of primary particles in SH soot and CC soot was distributed over a wide range compared to PW soot.

Effects of polyoxymethylene dimethyl ethers (PODEn) on soot characteristics in isooctane inverse diffusion flames

Polyoxymethylene dimethyl ethers (PODEn, n = 1–3) such as dimethoxymethane (DMM) and PODE3 are considered highly promising alternative fuels due to their lack of C–C bonds and significant oxygen contents. Particularly, PODE3-blends with diesel have been proven to substantially reduce soot emissions and fuel consumption rates under optimized engine conditions, although the impact on soot nanostructure properties remains unclear. This study explored the effects of DMM and PODE3 on soot characteristics in isooctane (i-octane) inverse diffusion flames, focusing on soot morphology, soot nanostructure, soot nanocrystallite, and soot spectral features. To compare soot particles behavior, DMM and PODE3 blended as the isooctane substitute (by 20% and 40% volumetric fractions) were examined with various diagnostic techniques, including high-resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD), Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS).The results showed that DMM and PODE3 contributed to a reduction in sooting tendency from i-octane flames, whereas the average soot mass decreased significantly as the PODE3 ratio increased, consistent with a higher oxygen concentration. Therefore, 20% PODE3/i-octane exhibited growing disorganized arrangements of soot particles with shorter fringes, smaller crystallite sizes, and a relatively lower degree of graphitization than 20% and 40% DMM additions. However, XPS analysis indicated that soot produced from PODE3/i-octane mixtures had less oxygen atoms because PODE3 has multiple –CH2O- chain groups that can convert oxygen into carbon monoxide with little soot production. Moreover, HRTEM, XRD, and Raman spectra analyses of 40% PODE3-doped flames confirmed an exceptional correlation with turbostratic soot structure, as manifested by the shortest fringe length, longest fringe tortuosity, more amorphous soot with the smallest crystallite dimensions, and least degree of graphitization.

Assessment of magnetic effects on soot characteristics in inverse diffusion ethylene flames

The magnetic effect on soot properties in ethylene inverse diffusion flames (IDF) was investigated in this study. The investigation involved the use of high-resolution transmission electron microscopy analysis (HRTEM), Raman spectroscopy analysis, X-ray photoelectron spectroscopy (XPS) and X-ray diffraction spectroscopy (XRD) to examine the nanostructure and graphitization degree of soot collected from flame exhaust gas at various height above burner surface (HAB) under both magnetic and non-magnetic conditions. Increasing the magnetic flux density resulted in modifications to soot properties. When the sampling height was HAB = 4 cm, increasing magnetic flux density resulted in longer fringe lengths, lower fringe tortuosity, less amorphous carbon content and surface oxygen content for the collected soot from flame exhaust gas. These results indicated higher graphitization degrees, which may be attributed to longer residence time and oxidation of soot particle in flames. Moreover, when the sampling height increased from HAB = 4 cm to 6 cm under non-magnetic condition, the nanostructure of soot in the flame exhaust gas was more organized, possible due to the oxidation of soot at the region between HAB = 4 and 6 cm region. However, with the magnetic field on, the crystalline structure of soot in the flame exhaust gas became increasingly disordered as the sampling height increased. This suggested that the soot may also be affected by the magnetic effects in addition to oxidation at the region between HAB = 4 and 6 cm region. The magnetic effect could trigger the transformation of graphitic carbon to amorphous carbon in soot particles as they passed through the magnetic field. With more elaborated magnetic fields, the magnetic effect on soot could contribute to strategies controlling soot emissions and removal.

Enhanced Intracellular Delivery and Cell Harvest Using a Candle Soot-Based Photothermal Platform with Dual-Stimulus Responsiveness.

Intracellular delivery of bioactive macromolecules and functional materials plays a crucial role in fundamental biological research and clinical applications. Nondestructive and efficient harvesting of engineered cells is also required for some specific applications. In this work, we develop a multifunctional platform based on candle soot modified with copolymer brushes containing temperature-responsive poly(N-isopropylacrylamide) (PNIPAAm) and sugar-responsive phenylboronic acid (PBA) components. This platform possesses a high cell adhesion capacity due to the inherent hierarchical structure of candle soot and the formation of boronate ester bonds between the PBA groups and glycoproteins on the cell membrane. Under the irradiation of a near-infrared laser, the excellent light-to-heat conversion ability of candle soot enables the highly efficient delivery of macromolecules into diverse cells (including hard-to-transfect cells) attached to the surface via a photothermal-poration mechanism. Owing to the temperature-responsive properties of PNIPAAm and the sugar-responsive properties of PBA, the engineered cells could be harvested nondestructively from the platform by a mild treatment using a cold fructose solution. A proof-of-concept experiment demonstrates that fibroblasts attached to the surface could be transfected by a functional plasmid encoding basic fibroblast growth factor and then harvested efficiently and recultured with improved proliferation and migration ability. The whole delivery-harvesting process required less than 1 h, allowing the cells to be engineered without compromising their viability. This platform thus provides a widely applicable method for both the intracellular delivery of diverse macromolecules efficiently as well as harvesting engineered cells simply and safely, holding great potential for biomedical applications.

Study on the application of laser diagnosis technology in the rapid real time measurement of soot

Based on previous soot studies, this paper proposes a new method to quantify the evolution of the soot generation mechanism in flames. The soot distribution in inverse diffusion flame at different fuel gas velocities was quantitatively measured by laser-induced incandescent (LII) technique, and the soot structure evolution was analyzed by Raman spectroscopy and field emission transmission electron microscopy (FETEM). Laser-induced breakdown spectroscopy (LIBS) and numerical simulation were used to investigate the association between the intensity ratio of hydrocarbon atoms of at different positions of the flame and soot formation. Meanwhile, a detailed chemical kinetic simulation of the formation process of soot was carried out, and the flame structure was divided in more detail. The flame state in different regions and the key reactions of soot formation were analyzed. The results presented that the volume fraction of soot measured by LII is consistent with the height-dependent graphitization of the flame and is identical to the yellow definition of the flame. The variation of the C/H atomic intensity ratio reflected the soot formation in flame. The CH intensity ratio in the flame axial direction showed a bimodal distribution, corresponding to the blue phase of soot precursor formation and the yellow phase of soot formation, respectively. In addition, the CH atomic intensity ratio and the CH atomic molar ratio showed a parabolic transition from incipient to mature soot in the soot production region. The CH intensity ratios at different heights are linear in the radial direction, and the main core region of soot production can be obtained when the CH intensity ratio is 0.10 ∼ –0.13. With radial distance increases, it is clear results that the most sensitive reaction of (precursor) A1 changes from C6H5OH + H = A1 + OH to C2H3 + HCO=C3H3 + OH. It reveals that with the increase of yellow light, the soot structure along the flame center line is gradually graphitized, and the graphitization degree is the highest at the axial height of 18.0De ∼ 22.4De. This is consistent with the distribution of CH atomic intensity ratio. The present results demonstrate that LIBS can quickly achieve the initial measurement of soot and provide a new method for the study of flame soot during its evolution.

Conductive particulate films fabricated by electrospray deposition of candle soot suspensions with acid treatment

The electrical properties of carbon-based materials have attracted significant interest due to their wide range of potential applications. This study investigated the effect of nitric acid treatment on the electrical conductivity of candle soot particle films. Electrospray deposition was used to deposit the candle soot dispersion on a solid substrate. The electrical conductivity of the films increased as the concentration of nitric acid increased, with the highest conductivity of 27.65 S cm−1 observed in the film treated with 5 M nitric acid. The increase in conductivity was attributed to the enhancement of interparticle bonding between soot particles after acid treatment. Functional groups such as carboxylic or nitro groups were observed in the films by Fourier transform infrared (FTIR) spectroscopy. Raman spectroscopy showed a broadening of the D bands, suggesting a defect in the crystalline structure due to the formation of functional groups on the soot structure. These results suggest that nitric acid treatment can be used to improve the electrical conductivity of candle soot particle films.

Characterization of atmospheric aerosols by SEM-EDX in a rural-continental environment-a seasonal approach

The vital information on the morphology and composition of the atmospheric aerosols over any region is critical to understand their climatic and health effects. This work aims at the physicochemical characteristics of atmospheric aerosols at a rural continental site in northeastern India. We collected the aerosol samples from January to December 2018 and applied a seasonal approach to investigate the physical and elemental characteristics of aerosol particles. A Scanning Electron Microscope (SEM) has been used to examine the morphology of particles that, includes shape, size, and structure information. Energy-dispersive X-ray analysis (EDX) is used to investigate the elemental composition information. By using the SEM-EDX, we have noticed a variation in the morphology of particles and elemental composition from season to season. The different particle shapes vary from regular to irregular, including spherical, nearly spherical, triangular, capsule-like, and platy shapes have been observed. We have seen a variety of particle clusters, including chain-like structures, porous textures, soot structures, and irregular structures. A size range of particles with a diameter ranging from 100 nm to 30 µm has been observed. The presence of elements C, Si, O, B, K, Ca, P, S, Fe, Na, N, Cl, Mg, and Al has been examined in different seasons. We have noticed that road dust, crustal origin particles, and carbonaceous particles dominate the study region. We have observed a clear seasonal variation in the percentage of different elements. A small contribution by sea salt origin elements has also been noticed during the pre, monsoon, and post-monsoon seasons when winds flow from the marine region, and air mass back trajectory results support this.

2D and 3D TEM Characterisation of Benzene and Ethylene Soot

Flame-generated soot particles from two different fuels, benzene (B) and ethylene (E), at different ageing conditions, were analysed to assess their morphological and structural features. Samples were collected at 6, 10 and 14 mm from the nozzle location. Traditional 2D transmission electron microscopy (TEM) and a novel 3D TEM were used to investigate morphology variations. High-resolution TEM (HRTEM) was used to capture structural characteristics. Samples were then placed on lacey carbon microgrids. A field emission gun TEM was used to capture images of the agglomerates. Tilt series of ±60 degrees were acquired at 1-degree steps to perform the 3D reconstruction. IMOD software and backwards projection were used to reconstruct the tomogram from the tilt series. The 2D analysis revealed that soot particles’ agglomerations for both fuels change from a bundle to a chain-like structure as they “age”, i.e., extracted at a higher “flame height”. In B, the primary particle diameter increases as they get “older,” whereas in E, the opposite happens, and overall, E particles are bigger than B ones. The nanostructure presents ordered regions with parallel-stacked layers of carbon lamellae. This is more evident in the aged soot samples, with the difference that in E a thick amorphous layer is present at the edge of the particles, which is not observable for B. A nanostructure analysis suggests a trend of increasing fringes length as the soot ages, going from 1.04 nm of B6 to 1.22 nm of B10 to 1.05 nm of B14 and from 1.139 nm of E6 to 1.20 nm of E14 (±0.02 nm). The tortuosity does not vary greatly across all the samples, ranging between 1.132 and 1.149 (±0.004). Separation is also quite similar everywhere, with 0.404 nm of B6, 0.392 nm of B10, 0.399 nm of B14, 0.397 nm of E6 and 0.396 nm of E14 (±0.002 nm). Ring structures and particle overlaps, two examples of geometrical characteristics concealed in 2D, can be seen in the 3D reconstructions. Furthermore, the comparison between 3D and 2D volume and surface area raises questions about the reliability of those parameters as derived from 2D measurements. This study advances knowledge of how soot structure can be affected by the fuel type and emphasises the significance of how soot is investigated.

Soot structure and flow characteristics in turbulent non-premixed methane flames stabilised on a bluff-body

The soot properties of methane in turbulent regimes are not well characterised but are highly desirable. Methane is the main constituent of natural gas that is broadly used in many industrial combustors. Investigation of turbulent methane flames under well-defined boundary conditions is therefore useful for interpreting soot formation in practical burners and can be used for further model development. This study presents a joint experimental and numerical study of a series of turbulent non-premixed bluff-body flames fuelled with pure methane for three values of the momentum flux ratio of fuel jet to co-flowing air. Soot volume fraction (SVF) and flowfield are measured simultaneously using planar laser-induced incandescence (P-LII) and 2D-polarised particle image velocimetry (P-PIV). Additionally, time-averaged temperature, mixture fraction, OH and C2H2 concentrations are estimated numerically using RANS models. The global flame structure for all three flames features a recirculation zone with a double-vortex structure, a jet-propagating zone, and a neck zone connecting the two regions. The soot distribution within the recirculation zone shows clear distinct features, which is attributed to the mean mixture fraction distribution in this zone. Increasing the momentum flux ratio shifts the location of the mean stoichiometric mixture fraction to the rich inner vortex core, leading to a distinct peak of the total integrated soot in the inner vortex of the recirculation zone that is not observed in other cases. Also, it is deduced that the soot inception starts earlier in the recirculation zone for the flame with the highest momentum flux ratio and in the jet zone for the other two flames. Much higher soot concentration and lower intermittency are found with ethylene-based flames stabilised on the same burner and with the same operating conditions. In addition, the study has generated a database of soot and flowfield results, which can be helpful for future model validations.

Particulate emissions and soot characterisation of diesel engine exhaust for steady-state operating condition using dioctyl phthalate blends with diesel

The study investigates the particulate emission and soot characterisation of particles originating from a six-cylinder common-rail direct injection (CRDI), turbocharged diesel engine. The experiments were conducted using steady-state operating conditions, a constant rpm of 1500 and four different engine loads (25, 50, 75, and 100%). Three different fuels were tested: neat diesel and two dioctyl phthalate (DOP) blends with diesel (DOP10 and DOP20). Particulate emissions were measured using DMS500 and collected on transmission electron microscope (TEM) grids for further analysis of the morphology and nanostructure of the soot structure. The particle number and mass concentrations for neat diesel were found to be lower than DOP fuel blends for all four loads. The primary particle diameter increases with increase in the engine load and decreases with an increase in DOP fuel content in the blend. A compact structure of soot aggregate was observed for DOP fuel blends compared to that of diesel and it increased with an increase in the engine load. Short and curved fringes were observed for DOP fuel blends. Also, an increase in fringe length and a reduction in fringe tortuosity was observed with an increase in engine load. The fringe separation distance results show an opposite trend with respect to fringe length and fringe tortuosity for DOP fuel blends i.e. less inter-planar spacing was observed for DOP fuel blends compared to that of diesel. This would result in an overall decrease in the oxidation reactivity of soot particles.

Decomposition of fullerite during the high-temperature heating

X-ray diffraction, scanning electron microscopy, Mossbauer and Raman spectroscopy have been used for studying structural changes arising due to the thermal decomposition of fullerite C60/70 at heating to 1600 °C in CO medium. The scheme of the structural changes of fullerite at the initial graphitization temperatures is given. It is shown that the thermal disordering of fullerite with the formation of a fullerite-like amorphous phase is observed at heating above 900 °C. The subsequent ordering of carbon takes place at the temperature of 1600 °C with the formation of a graphite-like amorphous phase. The graphitization process proceeds with the formation of turbostratic carbon and crystalline graphite with a low degree of ordering. The nature of the formation of carbon substructures is considered. Carbon structural changes are accompanied by morphological changes. Comparative studies of changes in the structure of soot, graphite and glass-like carbon during heating have been presented. The catalytic effect of Fe on the processes of the disordering of initial fullerite is considered. It is shown that in the Fe presence the temperature of the phase transition ‘fullerite-graphite’ decreases from 1600 to 760 °C. Graphite, formed after the Fe3C decomposition, is characterized by a higher degree of ordering.

Detonation Synthesis Nanodiamond Soot as a Promising Filler for Polymer Composites

In this work, the results of a complex investigation of structure and properties of nanodiamond soot (NDS) of detonation synthesis are presented. Size distribution of NDS particles, dispersed in different liquid media, was investigated using dynamic light scattering and laser diffraction analysis methods. The results of the investigation, as well as the results of zeta-potential measurements, allowed us to characterize the agglomeration process of the NDS particles as independent of the medium, making NDS a good model filler for research of composite-modified nanosized particles. Additional data obtained using scanning electron microscopy, scanning tunneling microscopy, atomic force microscopy, X-ray diffraction, and Raman spectroscopy, demonstrated that in NDS the spherical nanodiamond (ND) particles with diameter ~5 nm are densely packed into strong-coupled aggregates with diameter ~300 nm, surrounded by graphite nanoribbons. X-ray diffraction analysis estimated the volume fraction of NDs in NDS as ~45 vol.%, simultaneously showing that the graphite is not defective, which was confirmed with the electron diffraction method. It was demonstrated that this structure of NDS allows to efficiently use NDS as a filler for polymer composites to increase polymer characteristics such as electrical conductivity or tribological characteristics, similarly to conventionally applied fillers such as carbon black.

In-flame soot structure of a jet fuel with 24% aromatics in a small-bore optical compression-ignition engine

This study applies a multi-location soot sampling method in a small-bore optical compression ignition engine to show in-flame particle evolution of a surrogate jet fuel with 24% aromatics (AR24) and a conventional diesel fuel. Four soot sampling probes are installed on the piston-bowl wall with 60° spacing angles for simultaneous sampling from the same firing cycle. These sampling locations represent a jet-wall impingement point (JW), an up-swirl point (US), a down-swirl point 1 (DS1) and a down-swirl point 2 (DS2) along the sooting flame path identified by high-speed soot luminosity images and planar laser induced incandescence (PLII) images. The sampled soot particles are examined with transmission electron microscope (TEM) and the images are post processed for the analysis of morphology. The results showed both fuels produce a large amount of small soot aggregates at JW point due to fuel-rich mixtures while AR24 soot aggregate number count is much less than diesel, indicating less soot formation for AR24. They grow as the flame travels along the bowl wall through aggregation, which is more significant at DS1 point than that of US point. Simultaneously, some small soot aggregates disappear due to oxidation, which is more significant at US point. As the sooting flame reaches DS2 point, only large aggregates with compact structures survived intense oxidation. The soot sampling performed for high resolution TEM and internal structure analysis of AR24 showed that the core-shell particle structure becomes clearer and the carbon-layer fringe-to-fringe gap decreases during the flame penetration, indicating the soot oxidation continued. From the results, the in-flame soot evolution is characterised by high formation of small aggregates at JW point, simultaneous aggregation and oxidation at US and DS1 point with the latter more prone to aggregation, and significant oxidation at DS2 point.

Understanding the reaction kinetics of diesel exhaust soot during oxidation process

The purpose of the present study is to better understand the reaction kinetics of diesel exhaust soot during oxidation process. A thermogravimetric analyzer was used to oxidize real diesel exhaust soot generated from a Euro VI diesel engine under non-isothermal conditions. The Friedman-Reich-Levi method and the Sestak-Berggren model were used to determine the oxidation kinetics. Raman spectroscopy and high-resolution transmission electron microscopy were employed to follow the changes of the soot structure during oxidation. The activation energy gradually increased with increasing conversion level during soot oxidation. The oxidation process of diesel exhaust soot could be described as three-step kinetics, and the calculated conversions fitted the experimental results very well. The kinetic predictions of diesel soot oxidation that were obtained using the proposed kinetic models were more accurate and precise than those with the common first-order model. The structural order increased as oxidation progressed, which was responsible for the increased activation energy. The structural ordering was principally caused by the preferential oxidation of the disordered fraction in the diesel soot, especially for the amorphous carbon, which was oxidized in the initial stage of the oxidation reaction.

Time-resolved 2D angular scattering of soot particles in atmospheric turbulent flames

A better understanding of the formation of soot particles is essential to improve combustion-related processes. For this purpose, this work reports for the first time the measurement of the size of soot particles under formation in a turbulent atmospheric flame in a planar configuration. This is ensured by the detection of the elastic scattering of a laser sheet thanks to two cameras positioned at different scattering angles (45° and 135°). The size information is then used to interpret the scattering signal collected by an additional camera positioned at 90° in order to evaluate the soot number concentration. This last step relies on a calibration ensured by soot volume fraction and primary particle size previously determined by autocompensating laser-induced incandescence in the same flame. Moreover, the use of a 1 kHz repetition rate nanosecond laser and three high speed CMOS cameras enables the access to these physical parameters with a high temporal resolution. The 2D and time-resolved soot characterization is of interest because it enables the observation of the temporal and spatial modifications of soot structures when propagating in such turbulent flames. In particular, in this flame, the transport and mixing of soot pockets are clearly observed and the impact of these processes on the size and number concentration is quantified.