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In recent years, environmental problems such as global warming and air pollution have been recognized more seriously. Soot in the combustion products is one of the causes of air pollution. Hydrogen is now attracting attention as an alternative fuel to fossil fuels because it can be carbon-neutral fuel synthesized from renewable energy sources. If hydrogen is mixed with hydrocarbon fuels, it is possible to reduce CO2 emission. In this study, we investigated the effect of hydrogen addition on a coaxial jet diffusion flame. We used methane and propane as main fuels, and soot region was visualized using a laser induced incandescence (LII) method. In experiments, two-dimensional distribution of the soot concentration was obtained, and the maximum soot volume fraction and the total soot volume were evaluated from the LII signal. In order to clarify the effect of hydrogen addition, nitrogen was also added to the main fuel, and both results were compared.

The auto-compensating laser-induced incandescence (AC-LII) technique has been frequently used to measure soot volume fraction (f v) in flames. AC-LII relies on an assumption about the spectral variation of soot absorption function E (m , λ) at the two detection wavelengths for soot temperature determination. It has been a common practice to assume a constant E (m , λ) based on the believe that LII measures mature soot only. Since soot maturity in flames is a continuous phenomenon from young to mature, it is unclear how soot maturity affects the AC-LII measured f v. In this study, a correction to the AC-LII measured f v with the conventional hypothesis of mature soot is proposed based on the soot maturity coefficient β and absorption function E (m , λ) in a laminar coflow ethylene diffusion flame. The application of this correction is first demonstrated for synthetic time-resolved LII signals based on the spatial distribution of soot properties in the ethylene diffusion flame predicted by the CoFlame code. A significant decrease of the simulated incandescence signals S LII and soot temperature T eff is observed when the maturity coefficient is increased from 0 for fully mature soot to 1.2 for highly immature soot. Then the proposed correction is applied to AC-LII measured f v in the same flame with the help of spatial distributions of β and ratio of spectrally-resolved E (m , λ) by multiwavelength line-of-sight attenuation and emission (MW-LOSA/EMI) measurements. The uncorrected AC-LII measured f v is in general significantly lower than that measured by MW-LOSA/EMI, which inherently considers soot maturity. The corrected AC-LII f v results are in good agreement, within the experimental error, with those measured by MW-LOSA/EMI. Therefore, the proposed methodology can quantify the effect of soot maturity on AC-LII measured f v and largely eliminate the difference in the measurements of f v by AC-LII and MW-LOSA/EMI. [ABSTRACT FROM AUTHOR]

In this study, we experimentally investigated soot and NOx in a diffusion flame on a coaxial burner. By applying a dielectric-barrier-discharge (DBD) plasma, we tried to reduce these emissions. When the DBD plasma was exposed to the air flow, the height of the luminous flame slightly decreased. By considering the fact that the flame height without the DBD plasma is proportional to the fuel flow rate, the DBD plasma surely affects the soot concentration of the luminous flame. Based on the LII image, the soot region identified by the LII signal corresponds to the luminous flame zone. This soot distribution is similar even when the DBD plasma is activated, but the soot concentration of the luminous flame zone is reduced. As the air flow rate increases, the reduction of the integrated LII signal by the DBD plasma is smaller. This could be because the plasma is only active at the exit of the air flow, and the effect of the plasma is relatively weaker when the air flow rate increases. As for NOx emission, it is confirmed that the EINOx without plasma increases by increasing the air flow rate, showing that the thermal NOx could increase. Overall, when the plasma is activated, the EINOx always increases at any air flow rate. That is, the simultaneous reduction of soot and NOx cannot be achieved.

The control of PM emission is a rather complicated problem to be solved both for diesel engine and GDI engines. There are vast factors affecting PM emission, the fuel, A/F, combustion temperature and so on. But undoubtedly, most efforts are paid to exhaust aftertreatment, rather than the control of generation. This paper is to investigate the effect of dimethyl ether (DME, CH3OCH3) on soot generation in flame. Therefore, a laser induced incandescence (LII) measurement system for soot volume concentration measurements in flame is used.Two DME gaseous mixtures were prepared with equivalence ratios 1 and 2. To highlight its effect on soot formation, acetylene known as the precursor of soot, is added in different ratios. Similarly, CO2 is mixed to simulate exhaust gas recycling (EGR) effect in engines. The experimental results indicate that the fuel property and A/F ratio are the dominating factors for the generation of soot during combustion. The combustion of DME doesn't emit soot obviously even under rich mixture condition. When burning the mixture of DME and acetylene (C2H2) in different proportion, there is barely soot emission at equivalence ratio of 1. The soot emission increases as the proportion of C2H2 increases at equivalence ratio 2, and the DME addition reduces the soot emission of C2H2 flame. CO2 dilution doesn't lead to the increase of soot when burning DME at equivalence 1 and 2. Soot emission is lower when the mixture of DME and acetylene is diluted by CO2, though soot volume concentration increases slightly as the proportion of C2H2 increases.The kinetic analysis indicates that the combustion of DME does not produce the precursor of soot. Under fuel-rich combustion conditions, C2H2 reacts with H radical to form C2H3, which leads to soot. When DME is added, it competes for H radical, which reduces the formation of C2H3 and thus reduces soot formation.The research carried out in this paper indicates that DME fuel has obviously power to reduce soot formation during combustion, no matter whether it is diluted by CO2 or mixed with soot precursor substance C2H2.

In this paper, recent developments of the devolatilization model and soot-formation model for the numerical simulations of pulverized-coal combustion fields, and the technology used to measure soot particles in pulverized-coal combustion fields are reviewed. For the development of new models, the validation of the developed models using measurement is necessary to check the accuracy of the models because new models without validation have a possibility to make large errors in simulations. We have developed the tabulated devolatilization process model (TDP model) that can take into account the effect of particle heating rate on the volatile matter amount and the devolatilization-rate parameters. The accuracy of the developed TDP model was validated by using the laser Doppler velocimetry data for the bench-scale coal combustion test furnace. The soot-formation model combined with TDP model for the large eddy simulation (LES) has been also developed. The spatial distributions of both the soot-volume fraction and the polycyclic aromatic hydrocarbons were measured by virtue of laser-induced incandescence (LII) and laser-induced chemiluminescence (PAHs-LIF). The accuracy of the developed soot-formation model was validated by using the measured data.

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]

Soot formation process during coal combustion is one of a key phenomenon to achieve a high-performance furnace. This is because the thermal radiation of soot influences the temperature distribution and the heat transfer process in the furnaces. In this study, polycyclic aromatic hydrocarbons (PAHs) and soot were visualized by a 10 kHz planar laser induced fluorescence (PLIF), laser induced incandescence (LII) and laser induced scattering (LIS) to investigate the soot formation processes in the volatile flame of single pulverized coal particle combustion. In addition, the magnified high-speed imaging was conducted to capture the combustion behavior of the single coal particle. Pulverized coal particles entered high temperature region formed by hydrogen/air diffusion flame in the counterflow. Results showed that five typical combustion behaviors were observed by magnified high-speed imaging. This paper succeeded in visualizing the process from the release of volatile matter to soot formation of single pulverized coal particles with high-speed PAHs-PLIF, LII and LIS measurements. The results showed that the size of PAHs and soot areas was similar, but the duration of PAHs was longer than soot. Also, the PAHs signal was locally high, suggesting that PAHs were directly released from pulverized coal.

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]

In this paper, recent developments of the devolatilization model and soot-formation model for the numerical simulations of pulverized-coal combustion fields, and the technology used to measure soot particles in pulverized-coal combustion fields are reviewed. For the development of new models, the validation of the developed models using measurement is necessary to check the accuracy of the models because new models without validation have a possibility to make large errors in simulations. We have developed the tabulated devolatilization process model (TDP model) that can take into account the effect of particle heating rate on the volatile matter amount and the devolatilization-rate parameters. The accuracy of the developed TDP model was validated by using the laser Doppler velocimetry data for the bench-scale coal combustion test furnace. The soot-formation model combined with TDP model for the large eddy simulation (LES) has been also developed. The spatial distributions of both the soot-volume fraction and the polycyclic aromatic hydrocarbons were measured by virtue of laser-induced incandescence (LII) and laser-induced chemiluminescence (PAHs-LIF). The accuracy of the developed soot-formation model was validated by using the measured data. [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]

Biodiesel and blends with petroleum diesel are promising renewable alternative fuels for engines. In the present study, the soot concentration generated from four biodiesels, two pure methyl esters, and their blends with petroleum diesel are measured in a series of fully pre-vapourised co-flow diffusion flames. The experimental measurements are conducted using planar laser induced-incandescence (LII) and laser extinction optical methods. The results show that the maximum local soot volume fractions of neat biodiesels are 24.4% - 41.2% of pure diesel, whereas the mean soot volume fraction of neat biodiesel cases was measured as 11.3% - 21.3% of pure diesel. The addition of biodiesel to diesel not only reduces the number of inception particles, but also inhibits their surface growth. The discretised population balance modelling of a complete set of soot processes is employed to compute the 2D soot volume fraction and size distribution across the tested flames. The results show that the model also demonstrates a reduction of both soot volume fraction and primary particle size by adding biodiesel fuels. However, it is not possible to clearly determine which factors are responsible for the reduction from the comparison alone. Moreover, analysis of the discrepancies between numerical and experimental results for diesel and low-blending cases offers an insight for the refinement of soot formation modelling of combustion with large-molecule fuels. [ABSTRACT FROM AUTHOR]

Abstract In this study, transient soot formation processes in a small-scale jet burner (CRIEPI burner) were investigated by simultaneous measurements of coal particles, polycyclic aromatic hydrocarbons (PAHs) and soot. Pairs of simultaneous measurements of "Mie scattering measurement for coal particles with laser induced fluorescence (LIF) for PAHs" and "LIF for PAHs with laser induced incandescence (LII) for soot" were performed to understand the transitive formation processes of soot formation in pulverized coal flame, whose signals were successfully separated. Findings in the present study are as follows. Coal particles, PAHs and soot were distributed in this order in radial direction from the central axis. Existing regions of coal particles, PAHs and soot were overlapped from the time averaged viewpoint while there were few overlapping areas of coal particles, PAHs and soot from the instantaneous viewpoint. This result indicates that a long time is required for the formation of soot from 2 to 3 rings PAHs through larger PAHs. [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]

Abstract A detailed understanding of the mechanisms underlying soot production is needed to control particle emissions from practical combustion devices. This requires fundamental investigations of these processes in turbulent combustion systems. This study reports experiments on soot production in a perfectly premixed turbulent flame operating under rich conditions. A swirled ethylene/air flame is stabilized in a confined combustor at atmospheric pressure with full optical access. Quantitative measurements of the soot volume fraction (f v) are carried out with the Laser Induced Incandescence (LII) technique, allowing the characterization of the soot spatial distribution and temporal intermittency. While f v is detected by LII mainly in the near-wall region of the burner, light scattering measurements bring complementary information on the presence of particles in the whole chamber. The investigation of a rich perfectly premixed turbulent flame represents an unexplored configuration that provides new insights on soot production in a situation where this process is not dominated by mixing between fuel and air. The data gathered in this situation may be of interest in future developments of modeling methods and their validation for soot prediction. [ABSTRACT FROM AUTHOR]

Abstract Biodiesel and blends with petroleum diesel have shown their potential as renewable alternative fuels for engines, with additional benefits of low particulate matter and low sulfate emissions. In this paper we measure the soot volume fraction produced by three different methyl esters processed biodiesels (extracted from palm (PME), soy (SME) and coconut (CME)), and their blends with petroleum diesel, in a series of co-flow stabilized laminar pool flames, using laser induced-incandescence (LII) and laser extinction optical methods. The soot volume fraction measurement results show that all neat biodiesels produce only up to 33% of the total soot volume compared to pure diesel, and that the total soot volume correlates directly with the degree of unsaturation of the biodiesels. Blending leads to approximately linear behaviour of total soot volume, with a shift in slope with smaller sensitivity towards neat diesel. [ABSTRACT FROM AUTHOR]

Abstract In view of the potential of bio-ethanol as an alternative fuel and the particulate matter (PM) issues during gasoline combustion, the soot distribution characteristics of ethanol-gasoline blends in laminar diffusion flames were studied on a Gülder liquid burner using the two-color laser induced incandescence (TC-LII) technique. During the experiments, the ethanol ratio in the blends was varied from 20% to 80% by volume in order to investigate quantitatively the soot reduction potential of ethanol. In order to study the effect of reduction in carbon content due to ethanol addition on soot formation, the experiments were performed under a fixed fuel mass flow rate and a fixed carbon mass flow rate. It was found that both peak and average soot volume fraction in the flame reduced significantly with increasing ethanol content under both fuel supplying modes, however, this effect was progressively less pronounced as ethanol content increased. By comparing the two fueling modes, it was found that the reduction in carbon content due to ethanol addition has little impact on soot reduction. For a given ethanol blending ratio, the soot reduction under the same carbon mass flow rate was only slightly smaller than that under the same fuel mass flow rate. In terms of flame characteristics, the initial height of soot formation increases with increasing ethanol content under both fuel supply modes mainly due to the increased fuel outlet velocity. Radially, the peak soot location moves from the outside towards the center gradually as height increases. However, along the center line of the flame, the initial height of soot formation decreases with increasing ethanol content under the same fuel flow rate, whereas the trend remained similar to that in the whole flame under the same carbon flow rate. Highlights • Soot distribution of ethanol-gasoline in laminar diffusion flame was studied by TC-LII. • The effect of ethanol on soot reduction is weakened with the ethanol ratio increasing. • Carbon reducing due to ethanol addition has little contribution to soot reduction. • The initial height of soot formation in flames increases with increasing ethanol ratio. • Radial peak soot moves from the flame periphery towards the center with increasing height. [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]

To develop accurate models for the numerical simulation of coal combustion field, detailed experimental data using laser techniques, which can figure out the basic phenomena in a coal flame, are necessary. In particular, soot is one of the important intermediate substances in a coal flame. This paper is the first paper in the world reporting soot particle size distributions in a coal flame. The spatial distribution of the primary soot particle diameter were measured by the combination of the time-resolved laser induced incandescence (TiRe-LII) method and the thermophoretic sampling (TS) method. The primary soot particle diameter distribution was expressed by the log normal function based on the particle diameter measurement using SEM images obtained from the TS samples. The relative function between the signal decay ratio obtained by TiRe-LII and the primary soot particle diameter was defined based on the log normal function. Using the relative function, the spatial distributions of the primary soot particle diameter with the soot volume fraction were obtained. The results show that the small isolated soot regions instantaneously exist in the entire combustion field. This characteristics is different from spray combustion field. From the ensemble-averaged TiRe-LII images, it was found that the soot volume fraction and the primary soot particle diameter increases with increasing the height above the burner in any radial distance. It was also found that the volumetric ratio of small particles decreases with increasing radial distance at the region close to the burner exit. However, the variation of the soot particle diameter distribution along the radial direction becomes small in the downstream region. This tendency is caused by the turbulent mixing effect. It is expected that the accurate soot formation model will be developed in the near future by using the data reported in this paper.

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]

A method to produce spatially resolved images of the distribution of absorbing particles in the exhaust plume of a modified helicopter gas turbine engine is presented. Over a small region of the plume, in situ sensing of soot particles by laser-induced incandescence (LII) is demonstrated using fiber lasers with higher power ( $\sim 10$ W), longer pulse duration (>100 ns), and higher pulse repetition rates (>10 kHz) than the conventional LII. The sensitivity of the method is illustrated by the detection of ambient absorbing particles in background conditions with engine at rest. With a running engine, single-beam images are obtained in 0.01 s. The feasibility of using long-pulsed fiber lasers for soot particle concentration measurement is investigated using a representative laboratory system. The time-resolved LII behavior and the measurement linearity are investigated, demonstrating the suitability of using fiber lasers for soot particle measurement for aero-engine emissions. Results for normalized soot concentration are compared with extractive measurements illustrating good correlation across a range of engine speeds. This paper is the first step toward the development of a non-intrusive system for the measurement of 2-D soot concentration in the cross section of an aero-engine exhaust plume. [ABSTRACT FROM PUBLISHER]

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]

Soot formation and oxidation in high-pressure combustion is of high practical relevance but still sparsely investigated because of its experimental complexity. In this work we present a high-pressure burner for studying sooting premixed flames at pressures up to 30 bar. An optically accessible vessel houses a burner that stabilizes a rich premixed ethylene/air flame on a porous sintered stainless-steel plate. The flame is surrounded by a non-sooting rich methane/air flame and an air coflow for reducing temperature gradients, buoyancy-induced instabilities, and heat loss of the innermost flame. Spectrally-resolved soot pyrometry was used for determining gas temperatures. These were introduced into model functions to fit the temporal signal decay curves obtained from two-color time-resolved laser-induced incandescence (TiRe-LII) measurements for extracting soot volume fractions and mean particle size as a function of height above burner and gas pressure. The derived mean particle sizes and soot concentrations were compared against thermophoretically sampled soot analyzed via transmission electron microscopy (TEM) and laser extinction measurements at 785 nm, respectively. Soot volume fractions derived from LII peak signal intensities need to be corrected for signal attenuation at the high soot concentrations present in the investigated flame. From the various heat conduction models employed in deriving mean soot particle diameters from TiRe-LII, the Fuchs model gave remarkably good agreement with TEM on sampled soot at various heights above the burner. [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: Soot formation characteristics of a lab-scale pulverized coal flame were investigated by performing carefully controlled laser diagnostics. The spatial distributions of soot volume fraction and the pulverized coal particles were measured simultaneously by laser induced incandescence (LII) and Mie scattering imaging, respectively. In addition, the radial distributions of the soot volume fraction were compared with the OH radical fluorescence, gas temperature and oxygen concentration obtained in our previous studies [1,2]. The results indicated that the laser pulse fluence used for LII measurement should be carefully controlled to measure the soot volume fraction in pulverized coal flames. To precisely measure the soot volume fraction in pulverized coal flames using LII, it is necessary to adjust the laser pulse fluence so that it is sufficiently high to heat up all the soot particles to the sublimation temperature but also sufficiently low to avoid including a too large of a change in the morphology of the soot particles and the superposition of the LII signal from the pulverized coal particles on that from the soot particles. It was also found that the radial position of the peak LII signal intensity was located between the positions of the peak Mie scattering signal intensity and peak OH radical signal intensity. The region, in which LII signal, OH radical fluorescence and Mie scattering coexisted, expanded with increasing height above the burner port. It was also found that the soot formation in pulverized coal flames was enhanced at locations where the conditions of high temperature, low oxygen concentration and the existence of pulverized coal particles were satisfied simultaneously. [Copyright &y& Elsevier]

Abstract: The effects of fuel droplet size distribution of the fuel spray on the soot formation of spray flames, which is stabilized in a laminar counterflow field, were investigated numerically and experimentally. To investigate the effects of droplet size distribution of fuel spray on soot formation, fuel sprays with quasi-mono-dispersed droplet size distribution (QM-DSD) spray and poly-dispersed droplet size distribution (P-DSD) spray were generated using a frequency-tunable vibratory orifice atomizer. The differences in the spray flame structures and soot formation characteristics between QM-DSD and P-DSD were examined in detail using an experiment with two-dimensional laser induced incandescence of soot and using three-dimensional direct numerical simulations (3D-DNS) employing a CIP method. Results showed that the soot formation area in the cases of P-DSD decreased compared to the cases of the QM-DSD under the same Sauter mean diameter (SMD) condition. This tendency was caused by differences of the flame structures. The results of the fuel and oxygen concentration and temperature obtained from the 3D-DNS indicated that the portion of the premixed-like flame structure increased in the case of P-DSD. As a result, the combustion reaction region of the spray flames is enlarged in axis direction in the case of P-DSD in the counterflow field. [Copyright &y& Elsevier]

The model of laser induced incandescence(LII) technique is established in this research. It plays an essential role in inferring the primary soot particle size. The variations of the particle size and temperature during heating process were calculated. The effects of factors, such as laser wavelength, laser fluence and initial diameter of the particles, on the heating process were then investigated. It was found that, the laser wavelength of 532 nm was to be a better choice, the laser fluence had a significant effect on the process and the particle temperature decay rate could be used to infer the primary particle size. The presented model and simulation results would be very useful for applications of LII to deduce soot particle size. [ABSTRACT FROM AUTHOR]

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]

With environmental concerns over emitted particulate matters(PM) from combustion systems, various researches have been conducted on reduction and measurement techniques of particulate matters. The LII analysis results from numerical models and experiments in an ethylene/air laminar diffusion flame at elevated pressure up to 2.0 MPa with laser fluence 0.07 J/cm2, detection wavelength 400 nm were compared to validate a modified LII numerical model. The lifetime of the LII signal decreased as the elevated pressure increased, so that LII decay time also decreased in both results. In the aspect of heat transfer mechanism, it becomes earlier that dominant conduction starts. This shows that the results matched well under the pressure conditions. It is concluded that the LII numerical model could be applied to decide particle size in TIRE-LII at the high-pressure condition. [ABSTRACT FROM AUTHOR]

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]

The characteristics of combustion and radiation heat transfer of an oxygen-enhanced diffusion flame was experimentally analyzed. An infrared radiation heat flux gauge was used to measure the thermal radiation of various types of flames with fuel, air and pure oxygen. And the Laser Induced Incandescence (LII) technique was applied to characterize the soot concentrations which mainly contribute to the continuum radiation from flame. The results show that an oxygen-enhanced inverse diffusion flame is very effective in increasing the thermal radiation compared to normal oxygen diffusion flame. This seems to be caused by overlapped heat release rate of double flame sheets formed in inverse flame and generation of higher intermediate soot in fuel rich zone of oxygen-fuel interface, which is desirable to increase continuum radiation. And the oxygen/methane reaction at slight fuel rich condition (φ=2) in oxygen-enhanced inverse flame was found to be more effective to generate the soot with moderate oxygen availability. [ABSTRACT FROM AUTHOR]