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• Novel radiation and soot model is proposed for buoyant diffusion flame of mixtures. • New relationship of radiation and soot interaction was found. • PDF model of soot volume fraction is valuable for numerical simulation validation. Simplified radiation and soot models were proposed to provide a simple yet effective method predicting flame radiation and soot characteristics for buoyant diffusion flames. To explore the quantitative effects of fuel mixing and validate the proposed models, LII measurement of soot distribution and multi-point measurement of radiation flux were conducted for different fuel mixtures and different mixing ratios. Characteristic length scales, including the height of soot inception, the height of maximum soot volume fraction and the height of soot oxidation in the soot distributions, were determined by the fuel type and mixing ratio, and resulted in different maximum soot volume fractions. The axial distribution of soot volume fraction, maximum soot volume fraction, soot volume and soot yield can be predicted by the simplified soot model based on the critical mixture fractions. The probability density function of axial soot volume fraction can be described by an exponential function related to the non-sooty probability and maximum soot volume fraction. In addition, flame radiation power presents a linear correlation with soot volume, and flame radiation fraction of buoyant diffusion flame of fuel mixtures presents a linear relationship with the maximum soot volume fraction, which is in agreement with the simplified radiation model proposed in this work. [ABSTRACT FROM AUTHOR]

Determining the soot volume fraction (f v) in combustion environments requires detailed knowledge of the optical properties of the soot particles, and in particular of their absorption function E(m). This study addresses a fundamental lack of information on the optical properties of 2–4 nm soot particles. Recent works based on the modeling of the photoelectron emission yields and UV-vis-NIR-absorption measurements found a sharp decrease of E(m) with the particle size in the vis-NIR spectral region, which is inconsistent with the in situ detection of 2–4 nm particles in the near-infrared region by laser-induced incandescence (LII) or sensitive absorption methods like cavity ring-down extinction (CRDE). The objective of this study is twofold: first, an original method for the determination of E(m) of soot particles, including 2–4 nm particles is proposed. Then, the dynamic of two widespread in situ diagnostics, LII and CRDE, are compared over three orders of magnitude of f v in atmospheric premixed ethylene/air flames with different flow rates and C/O. The determination of the absolute value of E (m) and of its variation in the flames is derived from an original analysis, which does not require complex LII modeling. This analysis is based on the comparison between the experimental and calculated LII/LII max signals in the low fluence regime, LII max being the plateau value of the fluence curve, which is reached at fluence larger than 1 J/cm2 for the smallest C/O. E(m) is found to vary between 0.15 at low C/O up to 0.36 for the richest flames. Concerning the comparison of the dynamics of LII and CRDE, an excellent agreement is found above a threshold (C/O) limit , while LII exhibits a stronger decrease with C/O below (C/O) limit. This discrepancy is attributed to the spectral dependence of E(m) which is negligible above (C/O) limit , but increases when C/O decreases below (C/O) limit. The particle size distribution function (PSD), measured by scanning mobility particle sizing, reveals monomodal or bimodal PSDs with soot having mobility diameter in the range 2.3–7.5 nm depending on the flame conditions. It is suggested that the particles contained in the first PSD mode, which is dominant in the low C/O range, could be affected by a significant spectral dependence of E(m) in comparison with the second PSD mode. • Absolute value of the soot absorption function E(m) by using an original way of fitting the experimental LII fluence curves. • Comparison of the dynamics of LII and CRDE over a range of three order magnitude of the soot volume fraction. • Quantitative determination of the soot volume fraction of 2–4 nm soot particles in flames. [ABSTRACT FROM AUTHOR]

To better understand the formation and evolution processes of soot, the two-color laser induced incandescence diagnostic method was applied on a single cylinder optical direct injection spark ignition engine. Soot volume fraction was measured, and soot distribution was imaged as cyclic fuel quantity changes. The results show that 45.5 mg/cycle generates the most soot at the same measure plane. Pool fire dominates the formation of soot in the tested engine and generates more soot on the top surface of the piston near the injector. In-cylinder soot increases until 42°CA ATDC and then reduces due to oxidation. Pool fire continues through the expansion stroke till 52°CA ATDC, and then soot cloud gathers near the 10 mm plane. After 82°CA ATDC, in-cylinder soot basically in equilibrium, and residual soot moves follow the in-cylinder flow randomly and evenly distributes within the whole combustion chamber. With increasing cyclic fuel quantity, particles number concentration gradually increases and their distribution present dual-peak shape. In detail, 45.5 mg/cycle emits the most accumulation mode particles while 52 mg/cycle emits the most nucleation mode particles. [ABSTRACT FROM AUTHOR]

• The spatial distributions of coal particles and soot were measured simultaneously. • Imaging of Mie scattering and LII shows three types of soot formation areas. • The soot formation areas trapped inside coal particle clouds was newly observed. To understand the soot formation characteristics in a pulverized coal flame with a swirling flow, simultaneous imaging of Mie scattering of coal particles and laser induced incandescence (LII) of soot were performed in this study. The pulverized coal flame was stabilized by a hydrogen diffusion pilot flame. The characteristic structures of soot formation in the pulverized coal flame with a swirling flow were analyzed based on a comparison of the experimental results and two-dimensional numerical simulation in the mixing region of coal particles and the oxidizer. The interactions between coal particle clouds and soot formation are discussed in detail. The results clearly show that the averaged radial dispersions of scattering signals from coal particles and of the LII signals from soot are overlapped. The overlapping region appeared nearby the nozzle exit due to the turbulent mixing and the high temperature region formed by swirl-induced recirculation flow. This overlapping region radially expands with increasing the height from the burner. Additionally, the characteristic areas of soot formation were observed in the results of simultaneous imaging of Mie scattering and LII. These areas are 1) streaky soot formation areas around the particle clouds, 2) soot formation areas inside of the particle clouds and 3) soot formation areas around the large coal particles. [ABSTRACT FROM AUTHOR]

Abstract The aim of this study is to investigate the detailed soot formation characteristics of crude Jatropha oil (CJO) and heavy fuel oil (HFO) blends (lower heating value (LHV) basis: 50%/50%) in a 550 kW-class combustion test furnace equipped with an industry-scale steam atomizing burner using optical measurement systems. In the present study, time-resolved flame images through the high-speed photography, flame radiation intensity through Hottel & Broughton (H&B) two-color method, instantaneous soot volume fraction and the velocity field around the fuel nozzle through laser induced incandescence (LII) and particle image velocimetry (PIV), and the soot absorbance through the dust analyzer were obtained. The results of this study show that soot volume fraction around the fuel nozzle became lower with blending CJO in HFO, thereby the flame radiation intensity decreasing. This seems to be caused by fewer polycyclic aromatic hydrocarbons (PAHs) in the CJO. By the exhaust gas recirculation (EGR), a larger soot volume fraction around the fuel nozzle and a lower soot volume fraction in the downstream region were obtained, which are probably due to a decrease in oxygen concentration slowing the soot oxidation rate around the fuel nozzle and the increase in oxygen concentration behind overfire air holes, respectively. Graphical abstract Image 1 Highlights • Soot formation characteristics of a pure HFO and a CJO/HFO blend are compared. • Soot volume fraction and the velocity are measured by LII/PIV measurement. • Soot volume fraction and flame radiation intensity become lower with blending CJO. [ABSTRACT FROM AUTHOR]

Different non-intrusive optical and intrusive non optical diagnostic methods are used to measure flame and soot properties in a laminar coflow diffusion flame in order to compare and analyze the sensitivity of each technique to soot particles with different age and morphology. Flame temperature is measured using rapid thermocouple insertion (RTI) method and also by measuring soot spectral emissions (SSE). Soot volume fraction ( f v ) is measured quantitatively with laser extinction (LE), time resolved laser induced incandescent (TiRe-LII) and SSE methods and qualitatively from the transmission electron microscope (TEM) images of the thermophoretically sampled soot particles. Particle internal/aggregate nanostructure, and primary particle diameter are also analyzed based on TEM images from the sampled particles and TiRe-LII. It is shown that the optical methods are only sensitive to mature soot particles with solid appearance and cannot detect either temperature or f v in regions where liquid like nascent soot particles are dominant. f v measured by LE and TiRe-LII agree well while the values measured by SSE are lower. This discrepancy is attributed to the high sensitivity of f v measured by SSE to the measured temperature values. Temperature profiles measured by SSE are considerably higher than the values measured by RTI. It is shown that not considering the change of the surface emissivity of the thermocouple junction due to particle deposition for estimating radiation loss in regions where nascent or mature soot particles are dominant contributes to this discrepancy. Primary particle sizes measured based on TEM images and TiRe-LII agree reasonably well. Soot aggregate fractal dimension is shown to decrease as the soot particles age and become more mature. [ABSTRACT FROM AUTHOR]

Previous studies have demonstrated that methyl ester-based biodiesels produce significantly less soot compared to petroleum diesel in standard diffusion flames. However, the roles of the oxygen content and the degrees of unsaturation (DoU) of the biodiesels on their sooting propensity have not been independently analysed. In the present study, the relationship between soot yield of biodiesels and the DoU of the fuel is systematically analysed in laminar pool flames and pre-vaporised diffusion flames. The spatial distribution of soot volume fraction of the flames fuelled with four biodiesel fuels, two methyl esters and their blends with diesel are quantitatively measured using laser induced incandescence. All six biofuels have different DoU but similar oxygen mass fractions, so that the effect of the DoU on soot formation could be separately considered. The soot yield using biodiesels was measured to be lower than that of diesel fuel by a factor up to 7.5 in pool flames and by a factor of 4.0 in vapour flames. In both flame setups, the soot reduction effect of biodiesels ranks in order of DoU. The more saturated the fuel molecule is, the lower the soot yield. The soot reduction effect of the blending ratio with biodiesel was found to be non-linear, and different in pool and vapour flames. The morphology, size, and number density of soot particles were analysed using scanning electronic microscopy. The results suggest that the shape of primary soot particles is close to spherical and the structure of the point-contact aggregates is similar in all cases. Biofuels not only produce smaller particle sizes, but also fewer particles compared to diesel. Chemical kinetic simulations suggest that blending of biodiesels reduces the soot yield primarily by slowing the soot surface growth process, and secondarily by prohibiting the soot nucleation and inception. Both the nucleation (C 3 H 3 and benzene) and growth species (C 2 H 2) for soot formation take place more quickly and intensely for the combustion of more unsaturated fuels, which results in the correspondingly larger soot yield. • Blending of biodiesels suppresses soot formation in both pool and vapour flames. • In both flame setups, the soot reduction effect of biodiesels ranks in order of DoU. • The soot reduction effect of all biofuels is not linear in pool and vapour flames. • Biofuels with lower DoU produce smaller soot particles. • Nucleation and growth species for soot appear quicker in more unsaturated fuels. [ABSTRACT FROM AUTHOR]

In the present paper, three different soot-measuring techniques, namely laser extinction method (LEM), 2-color pyrometry (2C) and laser-induced incandescence (LII) have been simultaneously employed to characterize soot distribution inside a diesel flame. Two single-component fuels ( n -Decane and n -Hexadecane) and two derived blends (50%Dec/50%Hex and 30%Dec/70%Hex) have been used. Tests have been performed at an optical diesel engine, under different in-cylinder conditions. The study has been complemented with the measurement of ignition delay and Lift-off length. The present work pursues a twofold objective. On the one hand, the effect of fuel properties on soot formation has been analysed, under different engine operating conditions. On the other hand, sensitivity and performance of the three optical techniques has been evaluated, identifying their main advantages and drawbacks in the framework of the current study. LEM has been considered as the reference technique, as the measurement principle can be implemented without important limitations associated to the other two. Results highlight that larger molecules produce more soot than the smaller ones, with both reactivity and soot formation changing with the proportion of the heavier fraction. Despite describing similar trends, LEM and 2C do not provide the same KL values, with the pyrometry reaching some sort of saturation when increasing flame soot. A detailed analysis confirms that 2-Color measurements are strongly biased by soot and temperature distribution inside the flame. Nevertheless, it could still be a good option for low sooting conditions. On the other hand, an attempt to calibrate LII signal by means of LEM measurements has been reported. This approach should make it possible to obtain additional information on the soot spatial distribution. However, inconsistencies have been identified which stem from the inherent limitations of LII technique in highly sooting conditions. [ABSTRACT FROM AUTHOR]

Liquid-phase penetration, flame lift-off location and soot volume fraction distribution of gasoline–diesel blended fuel jets (gasoline volume fraction 0%, 20%, 40% and 60%) were measured in a constant volume vessel to investigate the combustion and soot formation processes of the wide-distillation fuel. The test was conducted under a constant ambient condition ( T a = 830 K, P a = 4 MPa) with fixed injection parameters ( d = 168 μm, P inj = 80 MPa). Mie-scatter imaging, OH chemiluminescence imaging along with coupled Laser Induced Incandescence (LII) and Laser Extinction Method (LEM) were used to investigate the liquid spray length, lift-off length and quantitative soot concentration, respectively. It was found that the increase of gasoline proportion in gasoline–diesel blends results in the following effects: the liquid spray length decreases with a nearly linear tendency while the flame lift-off length increases non-linearly; the general soot concentration decreases significantly and the initial soot-formation location moves downstream; the peak soot volume fraction decreases and the soot inception time increases; the initial soot point and the peak concentration region of soot shift from the periphery to the center of the jet. It was also found that the lift-off length, first-soot distance, peak soot concentration and soot inception time vary more significantly at higher gasoline proportions. [ABSTRACT FROM AUTHOR]

The Laser Induced Incandescence technique (LII) is an emerging optical method for the reliable spatially and temporally resolved measurement of soot concentration and potentially for monitoring primary soot particle size. Due to its origin, this method appears to be suitable for the measurement of fast transient soot emissions from Diesel engines, which form the main fraction of total emissions during standardised test cycles. Current existing commercial LII devices require modifications in the exhaust gas flow, dilution, and measuring with a partial stream in a preconditioned cell. The results from the development of a single window access in-situ LII setup for rapid measurement of soot emission during the combustion process from a Diesel production engine, suitable for direct full stream, measurements in the tail-pipe without the need for dilution or a sampling cell are presented here. Furthermore, the issue of the optical window cleaning, background incandescence due to the laser – tailpipe interaction and possible suppression of disrupted emission by means of time delay constant is addressed. The obtained and corrected in-situ LII results are further compared to commercially available devices for the measurement of soot emissions. Static and dynamic emission tests have been performed in order to demonstrate the viability and applicability of proposed single access optical probe for full stream fast transient soot emission measurement in real-time. [ABSTRACT FROM AUTHOR]

Highlights: [•] An LPG flame is investigated with TC and LII for soot volume fraction measurement. [•] The goal is to apply TC and LII to an LPG flame by an innovative imaging approach. [•] TC flame temperature is compared with a thermocouple measurements on the same flame. [•] LII reduction is evaluated by extinction measurements and laser power loss in flame. [•] Good agreement is found between soot volume fraction calculated through TC and LII. [ABSTRACT FROM AUTHOR]

Highlights: [•] We present measurements of OH, PAH and Soot in flames of vaporized liquid fuels. [•] Flames of n-heptane and n-decane showed similar combustion characteristics. [•] For both fuels the threshold for soot formation was observed around ϕ ∼3.3. [•] The maximum PAH LIF signal is a good predictor of the soot volume fraction from LII. [•] Influence of particle size for the delayed gate time appeared not significant. [Copyright &y& Elsevier]

Abstract: The effects of fuel droplet size on soot formation in spray flames formed in a laminar counterflow are investigated experimentally and numerically. Sauter mean diameter (SMD) of quasi-monodispersed fuel spray (n-decane) is carefully controlled independently from the other spray characteristics using a frequency-tunable vibratory orifice atomizer, and the two-dimensional spatial distributions of soot volume fraction and soot particle size are measured by laser induced incandescence (LII) and time resolved LII (TIRE-LII), respectively. In addition, the soot formation processes are examined in detail by a two-dimensional direct numerical simulation (DNS) employing a kinetically based soot model with flamelet model. The results show that the soot formation area and location are strongly affected by the SMD of the fuel spray. As the SMD of the fuel spray increases, the average soot formation area expands, whereas local suppression of soot formation is instantaneously observed in the spray flames because of the appearance of groups of unburned droplets. The size of soot particles tend to be larger in the outer part of the soot formation area compared to soot in the inner part. This is because the surface growth of soot particles markedly proceeds compared to the coagulation and oxidation. [Copyright &y& Elsevier]

Abstract: The emission regulations for diesel particulate matter (PM) are becoming increasingly strict. The focus of regulations is turning to reducing the number of nanosized particles as well as the total mass. A more precise measurement technique for particle numbers and mass must be developed to meet these new regulations. In this study, a new method for estimating the mass weighted size distribution of diesel PM was investigated by measuring the size of primary particles and the number concentration distribution of particle aggregates. Time-resolved laser-induced incandescence was used for primary particle size measurement and a scanning-mobility particle sizer was used to quantify the number concentration of aggregates. The results from these two conventional measurement techniques were combined using fractal analysis formulas to relate the electrical mobility diameter, the number of primary particles per aggregate, primary particle size, and fractal dimension. This method, applied to single-cylinder diesel engine exhaust with various engine loads and injection pressures, successfully estimated the mass weighted size distribution of particle aggregates. The procedure is very simple and the estimations are comparable with those based on effective density, making this method a useful and reliable tool for estimating mass weighted size distribution of fractal particles such as diesel PM. [Copyright &y& Elsevier]

Abstract: Particle size is a crucial parameter in nanopowder use and production. A method to obtain information about this during the particle synthesis would be very valuable in order to optimize the process. In the field of nanoparticles flame synthesis, an extension of the techniques used in soot diagnostics sounds rather obvious, but it is far from being easily accomplished. In this paper investigations on the application of the laser induced incandescence to TiO2 nanoparticles in a flame reactor are reported. The work basically concerns the effects of laser fluence on the spectral structure of the laser-excited radiation emitted from the particles. It will be shown that in many cases such a radiation cannot be plainly assumed as an incandescence signal. Measurements of the signal time decay are reported and interpreted in the light of the spectral indications. Finally, the dependence of such a time decay on the particle size is experimentally demonstrated. [Copyright &y& Elsevier]

Abstract: Temperature histories of nanosecond-pulsed laser-heated soot particles of different primary particle diameters and different aggregate sizes were calculated using an aggregate-based heat transfer model. Relatively low laser fluences were considered to ensure maximum particle temperatures were below about 3800K to avoid soot particle sublimation. After the laser pulse, the temperature of soot particles in larger aggregates decreases more slowly than that of particles in smaller aggregates due to the increased shielding effect. For a given aggregate size, the temperature of particles of smaller diameter decays faster as a result of a larger surface area-to-volume ratio. The effective temperature of soot particles in the laser probe volume was calculated based on the ratio of thermal radiation intensities of soot particles at 400 and 780nm to simulate the experimentally measured soot particle temperature using two-color optical pyrometry. The effect of aggregate size distribution of soot particles on the effective particle temperature was investigated under different initial temperatures. [Copyright &y& Elsevier]

Abstract: Soot volume fraction () is measured quantitatively in a laminar diffusion flame, with either methane or ethylene as fuel, at elevated pressures up to 2.5 MPa in order to gain a better understanding of the effects of pressure on the soot formation process. Soot continues to be of interest because it is a sensitive indicator of the interactions between combustion chemistry and fluid mechanics and known to be detrimental to human health. To examine the effects of increased pressure on soot production, laser-induced incandescence (LII) is used to obtain the desired spatially resolved measurements of as the pressure is incrementally increased to 2.5 MPa. The effects of pressure on the physical characteristics of the flame are also observed. Using a laser light extinction technique, the path-integrated soot volume fraction scales with pressure as p1.0 and p1.2 for the methane–air and ethylene–air flames, respectively, at 65% of the flame height. From the LII images, it is observed that the soot layer radius decreases with increasing pressure, scaling as approximately p-0.5 at 65% of the flame height, for both methane and ethylene flames. The local peak is found to scale with pressure as p1.2 for methane and p1.7 for ethylene flames, which is different than the path-integrated soot pressure dependence. The location of peak soot is observed to move from the edges toward the tip of the flame as the pressure is increased for both fuels. [Copyright &y& Elsevier]

• We studied soot concentration distributions of ethanol blended fuels using 2C-LII. • E20 shows the highest soot concentration on the tested plane. • Ethanol blending can significantly reduce the total particle number concentration. • E0, E40, E60 show similar behavior between the results of exhaust particle emissions and LII. Soot volume fraction distribution of ethanol blended fuels (ethanol volume fraction 0%, 20%, 40% and 60%, named as E0, E20, E40 and E60) were measured in an optical direct injection spark ignition (DISI) engine by combined laser induced incandescence (LII) and two-color (2C) method. Exhaust particulate matter (PM) emissions were measured by a Combustion DMS500 Fast Particulate Analyzer. For DISI engines, pool fire and the resulting diffusion flame are the main sources of soot in the cylinder. Choosing a suitable measurement plane and detection time, laser induced incandescent can be used to quantitatively measure the soot concentration produced by the pool fire. This research explores the effect of ethanol blending on engine particulate matter by comprehensively analyzing the soot process in the cylinder and exhaust particulate emissions. According to the two-dimensional quantitative soot volume fraction results, the maximum average soot volume fraction of the same fuel on the 10 mm plane is greater than the 5 mm plane; on the same plane, E20 shows the highest soot concentration compared to the other fuel blends which could be the physical properties of the ethanol become apparent. For exhaust particle emissions, ethanol can significantly reduce the total particle number and accumulation mode particle number. E0, E40 and E60 show fairly similar behavior between the exhaust particle emissions and the soot-LII imaging measurements. For E20, however, this is in poor agreement when comparing the soot-LII imaging results to the results of exhaust particle emission measurements. [ABSTRACT FROM AUTHOR]

We investigate the sooting propensity of an Alcohol-to-Jet-Synthetic Kerosene with Aromatics (ATJ-SKA) biojet fuel. The soot volume fraction and primary particle size in the pre-vaporised diffusion flames using ATJ-SKA biojet and blends with Jet A-1 at atmospheric conditions were measured experimentally and compared to numerical simulations. The measurements were conducted using extinction calibrated laser induced incandescence (LII). The soot volume fractions measured using the ATJ-SKA fuel do not show significant differences relative to measurements with Jet A-1. A comparison of the chemical composition of the fuels suggests that the Degree of Unsaturation (DoU) may not determine the sooting propensity of biojet fuels. The SEM analysis shows that diffusion flames using neat Jet A-1 produce finer soot particles and larger number density compared to the biojet and biojet surrogate. The soot model employs a semi-detailed chemical kinetic mechanism and a physical model which integrates the population balance equation governing the soot particle size distribution with an in-house reactive flow solver for multicomponent ideal gases. The model predicts the maximum soot volume fraction (SVF m) in the neat biojet case and the blended cases with Jet A-1 fuels within an error margin of 13% of the measured values. However, the predicted soot volume fraction distribution patterns differ from the measured one and the possible causes are discussed. [ABSTRACT FROM AUTHOR]

• Effects of coflow oxygen concentration on the evolution of soot precursors are studied. • Optical diagnostics use temporally and spectrally resolved point-measurements of scattering, fluorescence and incandescence. • Growth of precursor nanoparticles begins at a similar temperature range regardless of coflow oxygen level. • An intermediate stage is observed while transforming from precursor to soot, regardless of coflow oxygen level. • Aromatization shifts towards lower flames regions with higher oxygen levels This paper reports an experimental investigation of the effects of varying the oxygen concentration in the oxidizer stream on the formation and evolution of soot and precursor nanoparticles in an axisymmetric ethylene laminar coflow diffusion flame. The optical diagnostics employ temporally and spectrally resolved point-measurements of 266 nm excited scattering and laser-induced fluorescence, combined with 1064 nm excited laser-induced incandescence. Previous studies have indicated that oxygen enhancement changes soot volume fraction in sooting flames. However, the effects of coflow oxygen concentration on soot precursors are not well understood, and this aspect is addressed explicitly here. A set of eight flames is studied where the oxygen concentration in the coflow stream is varied from 19 to 40%, by volume, while the chemical composition of the jet stream is kept constant at 60% C 2 H 4 / 40% N 2. Oxygen enhancement in the coflow leads to a change in flame temperatures, luminosity, and flame length. It is found that the concentration of precursor nanoparticles increases with coflow oxygen. A combination of higher temperatures, red-shifting, and increased decay-times suggest structural growth in precursor nanoparticles with increasing coflow oxygen. In all flames, a similarity of particle evolution is noted with axial location, and this includes the presence of an intermediate stage during the transformation of precursors to soot. Moreover, the structural transformation from aliphatic to aromatic features is shifted towards the beginning of the precursor region when coflow oxygen is increased. It is found that structural growth starts at a similar temperature, regardless of coflow oxygen concentration. Soot volume fraction and primary soot particle diameter increase with coflow oxygen enrichment up to a threshold oxygen volume fraction, after which both the soot volume fraction and diameter decrease with further oxygen enrichment. [ABSTRACT FROM AUTHOR]

There is a lack of published soot measurement data in buoyant turbulent flames under reduced oxygen conditions. In this work, the laser induced incandescence (LII) technique is used to investigate the soot volume fraction (f v) and soot sheet thickness in buoyant turbulent diffusion flames in atmospheres with different oxygen concentrations (OC, oxygen molar fraction in oxidizer). The experiments establish an extensive database of quantitative f v spatial distribution in ethylene flames under three OCs (20.9%, 16.8%, and 15.2%) and two heat release rates (HRR = 10 kW and 15 kW). Mean, fluctuation, and probability density function of f v , as well as local soot intermittency are studied and compared across different conditions. The results show that the maximum instantaneous f v in normal air can be ~8 ppm; but the corresponding mean f v only reaches up to 0.58 ppm due to turbulence and flame intermittency. As the oxygen concentration is reduced from 20.9% to 15.2%, the mean f v decreases, and this effect is more significant at lower heights above the burner (e.g., ~75% reduction in the mean f v at a height of half a burner outer diameter). Soot is found to be distributed unevenly in the flame, appearing in sheet-like regions with irregular and complicated shapes. The soot sheet thickness is quantified to better understand soot-turbulence interactions. This soot dataset, along with other accompanying radiation and flow field measurements, provides a comprehensive understanding of the dynamics of turbulent buoyant flame dynamics under normal and vitiated conditions, and can be used to develop and validate models for soot formation and radiative heat transfer in such flames. [ABSTRACT FROM AUTHOR]

• Spray combustion of diesel/PODE blends with PODE fraction of 0–100% was studied. • Ignition delay decreases more in higher ambient temperature and higher EGR condition. • Flame lift-off length shows increasing tendency when more PODE is added. • When the PODE fraction is more than 50%, nearly no soot was observed. • Soot reduction caused by PODE addition weakens in high temperature and EGR cases. Polyoxymethylene dimethyl ethers (PODE) have been reported as a promising alternative fuel for the potential to reduce soot, CO and HC emissions of the diesel engine and improve the engine performance of gasoline compression ignition and dual-fuel engines. However, the fundamental data of spray and combustion characteristics of fuels with PODE are still deficient. In this study, the ignition delay, flame lift-off length (LOL) and soot distribution of diesel/PODE blends with PODE volume fraction of 0–100% were studied using high-speed OH chemiluminescence and planar laser-induced incandescence under different ambient temperature and oxygen concentration conditions. Results show that the ignition delay decreases and the flame LOL increases with the increase in PODE content. The increase in LOL is more distinct in lower ambient temperature conditions, whereas the decrease in ignition delay is relatively larger in higher temperature and lower oxygen concentration conditions. The total soot amount in the tested region decreases more than 50% and 95% when PODE volume fraction reaches 20% and 50%, respectively. When the PODE fraction is larger than 50%, nearly no soot can be observed in all the tested cases. The influence of PODE on soot reduction is more significant in higher ambient oxygen concentration and lower ambient temperature conditions. The soot distribution profile of the soot shows that the addition of PODE can not only reduce the soot formation but also accelerate the soot oxidation. [ABSTRACT FROM AUTHOR]

Soot primary particle size distribution along the centerline of a laminar coflow methane/air diffusion flame doped with vaporized n-heptane at atmospheric pressure was studied using the planar two-color time-resolved laser-induced incandescence (TiRe-LII) technique and analysis of transmission electron microscope images. An improved thermophoretic probe sampling procedure was used to collect samples of soot particles. The LII signals captured at two wavelength bands in the visible are used to determine the soot effective temperature by two-color pyrometry. The methodology was first validated against the literature data obtained in a laminar coflow ethylene/air diffusion flame. The same methodology is then applied to the n-heptane doped methane flame along the flame centerline. The Sauter and geometric mean diameters of soot primary particles were obtained. Good agreement is found between the soot primary particle size distributions obtained by the two techniques. [ABSTRACT FROM AUTHOR]