Laminar Diffusion Flames Research Articles

Double blue zones in inverse and normal laminar jet diffusion flames

Past experimental and computational works indicate that double blue zones are possible in hydrocarbon diffusion flames. This study examines these zones in inverse (IDF) and normal (NDF) laminar gas jet diffusion flames. Over 100 diffusion flames were observed with various fuels, diluents, dilution levels, and flow rates. Upon close examination, the IDFs and NDFs are seen to have double blue zones, separated at the flame tip by up to 1.6 and 0.9 mm, respectively, with a relatively dark region in between. By partially premixing the fuel and oxidizer streams, it was observed that, for both IDFs and NDFs, the zone toward the fuel side is rich, while that toward the oxidizer side is stoichiometric. Chemiluminescence was investigated using cameras and bandpass filters emphasizing emissions from OH* (310 nm), CH* (430 nm), CO2* (455 nm), and C2* (515 nm). The images were deconvolved using onion peeling. Broadband CO2* emissions were subtracted from the images of OH*, CH*, and C2* emissions to find profiles of spectral emissive power for these species. The thin, blue-green zone on the fuel side coincides with the peaks in CH* and C2* emissions in the fuel-consumption zone, while the broader, blue stoichiometric zone coincides with the peaks in OH* and CO2* emissions in the oxygen-consumption zone. The spectral emissive power peaks for C2*, CO2*, CH*, and OH* are typically 3 times as high in the IDF as in the NDF, which is attributed to the higher scalar dissipation rates.

Effects of Detection Wavelengths on Soot Volume Fraction Measurements Using the Auto-Compensating LII Technique

ABSTRACT Soot particles in the detection volume in general have different temperatures due to non-uniform laser fluence and particle size distribution. The thermal radiation intensity displays different temperature dependence at different wavelengths. The effective soot temperature inferred from the ratio of laser-induced incandescence (LII) signals in the auto-compensating LII (AC-LII) technique is dependent on the detection wavelengths and affects the measured soot volume fraction. This paper numerically investigates the effects of detection wavelengths on the inferred soot effective temperature and volume fraction under conditions relevant to laminar diffusion flames at atmospheric pressure and for three representative laser fluence distributions. Numerical calculations were conducted using an LII model for a laser pulse of 5.8 ns FWHM and 1064 nm and assuming a lognormal primary particle size distribution and neglecting the aggregation effect. LII signals were modeled at four wavelength bands centered at 420, 560, 680, and 790 nm and the effective soot temperature was derived over three pairs of LII signals, namely [420, 560 nm], [560, 680 nm], and [680, 790 nm]. The two shortest and two longest detection wavelengths respectively result in the highest and lowest effective soot temperature when the laser fluence is non-uniform. The inferred soot volume fraction displays the opposite trend as the effective soot temperature. The effective soot temperature is biased toward the highest values and AC-LII always underestimates the soot volume fraction. The detection wavelengths should be carefully selected to minimize the impact of non-uniform laser fluence and at the same time to maximize the accuracy of effective soot temperature.

The effect of oxygen concentration in the co-flow of laminar ethylene diffusion flames

This paper reports an experimental study of the effects of oxygen concentration (OC) in the oxidant stream on the characteristics of laminar co-flow ethylene diffusion sooting flames. Oxygen concentration was varied from 16.8% to 36.8% (by volume) by either diluting the air co-flow with nitrogen or by adding oxygen. Planar optical measurements were performed, with high spatial resolution, of flame luminosity at 700 nm, soot volume fraction, flame temperature, primary soot particle diameters and OH* chemiluminescence. All measured parameters are found to vary significantly with OC over the investigated range. The soot volume fraction measurements quantify the extent to which soot formation and oxidation are enhanced by increasing OC, revealing that the former effect dominates in the upstream region, while the latter effect dominates the downstream region, close to the flame tip. The changes to flame luminosity, soot volume fraction and flame temperature are strongly correlated with each other, while the primary soot particle diameter correlates well with the soot residence time. In addition, the rate of soot evolution, including its formation and oxidation, correlates with flame temperature along the axis of flames and a second reaction zone emerges in the tip of flames with high OC, as revealed by OH* chemiluminescence. The experimental data are well suited to the development and validation of soot models because of the well-defined boundary conditions, high spatial resolution, systematic nature and combination of the measured scalars.

Lateral Flame Spread over PMMA Under Forced Air Flow

In wildland and other flame spread scenarios a spreading fire front often forms an elliptical shape, incorporating both forward and lateral spread. While lateral flame spread is much slower than forward rates of spread, it still contributes to the growth of the overall fire front. In this work, a small-scale experiment is performed to investigate the mechanisms causing this lateral spread in a simple, small-scale configuration. PMMA strips with thicknesses ranging from 1 mm to 3.1 mm and widths of 5 cm and 10 cm were ignited under forced flow in a laminar wind tunnel. Unlike traditional concurrent or opposed flame spread experiments, flames were allowed to progress from one side of the sample to the other, perpendicular to the wind direction. An infrared camera was used to track the progression of the pyrolysis front by estimating the surface temperature of the PMMA. The flame spread rate, depth of the burning region, thermal diffusion length, and radiant heat flux were determined and analyzed. Based on a theory of heat and mass transfer for a laminar diffusion flame, a thermal heat transfer model was developed for the preheating region to predict the lateral flame spread rate. Results show that the thermal diffusion length decreases with wind velocity, ranging from 4.5 mm to 3 mm. Convection dominates the flame-spread rate, accounting for more than 80% of the total heat flux. The theoretical flame spread rate agrees well with experimental data from all but the thinnest samples tested, overpredicting the lateral flame spread rate for 1 mm thick samples. The resulting model for lateral flame spread under concurrent flow works for forced-flow dominated flame spread over thermally-thin fuels and helps provide physical insight into the problem, aiding in future development of two-dimensional, elliptical fire spread models.

Experimental and numerical study of soot formation in counterflow diffusion flames of gasoline surrogate components

Soot formation is experimentally and numerically investigated in laminar counterflow diffusion flames burning ethylene and three typical gasoline surrogate components; n-heptane, iso-octane, and toluene. Laser-induced incandescence and a light extinction technique are employed to determine the soot volume fraction within the well-controlled region of the burner. The experiments are performed across a wide range of strain rates and stoichiometric mixture fractions. From the experimental data, sensitivities of soot formation on strain rate and stoichiometric mixture fraction are derived for each fuel. The fuels show significantly different sensitivities. For iso-octane and n-heptane, a higher sensitivity of soot production on the strain rate is observed as compared to ethylene and toluene. Moreover, the sensitivities of soot formation on the strain rate increase with increasing stoichiometric mixture fraction. One-dimensional simulations of the flames investigated experimentally were performed using two different detailed chemical kinetic mechanisms, detailed chemical soot models, and the hybrid method of moments as well as a discrete sectional method to describe soot dynamics. The models are capable of predicting the soot volume fraction of the ethylene flames with remarkable accuracy, whereas for the gasoline surrogate components, the overall soot volume fractions are overpredicted for all tested models. In iso-octane flames, soot nucleation and PAH condensation rates are particularly enhanced. A reaction pathway analysis shows that in ethylene flames, the formation of benzene mostly originates from acetylene, while for iso-octane, large amounts of iso-butenyl form propyne, propargyl, and then benzene.

Influence of water-vapor in oxidizer stream on the sooting behavior for laminar coflow ethylene diffusion flames

The effects of adding water-vapor to the oxidizer stream on soot production in laminar coflow diffusion flames under different oxygen indices (OI) were investigated both experimentally and numerically. A modified coflow Gülder-type burner was employed to produce the laminar ethylene flames for fifteen different conditions of the oxidizer stream from oxygen-deficient (OI 17%) to oxygen-enriched (OI 25%) conditions, and also without and with adding water-vapor into the oxidizer stream up to 10% on mole basis. The measured soot volume fractions were compared with numerical predictions obtained using the CoFlame code and a chemical kinetic mechanism that consists of reaction pathways up to 5-ring PAHs. The sectional soot model used to simulate the soot particles dynamics considers soot nucleation, surface growth, PAH condensation, oxidation, particle coagulation and fragmentation. Fairly good agreement between experimental and numerical results was found, as close as 1% and 5% of difference in the peak soot volume fraction for OI21% with 0% and 10% of water-vapor addition, respectively. Clear trends of increasing the soot volume fraction (peak and also overall) was observed with increasing OI, while a significant reduction was obtained with the addition of the water-vapor. For the cases analyzed, a reduction of the total soot content was up to 60%. The chemical effects were numerically isolated using non-reacting water-vapor and analyzed, contributing especially to the soot oxidation rates. Finally, a study of the main reaction pathways was performed to better understand the chemical effects of water vapor. The results show that water vapor addition affects the concentrations of H and OH radicals and alters the formation and oxidation of soot precursors.

Numerical study of particle dynamics in laminar diffusion flames of gasoline blended with different alcohols

Alcohols have been considered as promising clean alternative fuels for IC engines. At present, the difference and mechanism of particle dynamics for different alcohols is unclear. In this work, numerical simulation of laminar diffusion flame of gasoline blended with different alcohols was carried out by coupling a gas-phase chemical kinetic model and fixed sectional particle model, then the micro-processes of particle nucleation, growth and oxidation were analyzed. The results show that the dilution effect of different alcohol blending on the reduction of soot is quantitatively similar under the same blending ratio, while the chemical effect increases with the increase of alcohol carbon chain length. The molecular structure of alcohols increases the chemical action during the formation of soot particles. At the same blending ratio, the difference of temperature field caused by adding different alcohols has little effect on the growth of particles. The proportion of PAHs condensation in surface growth increases with the increase of carbon chain length of alcohols. PAHs condensation rate together with HACA rate determines the inhibiting ability of different alcohols on soot particle growth, but PAHs condensation plays a dominant role. With the addition of alcohols, the oxidation rate of O2 decreases, but the oxidation rate of OH increases. However, as the particles formed in laminar diffusion flames of alcohol/gasoline is mainly oxidized by O2, the increase of OH oxidation rate has little effect on particle reduction. From the oxidation region, the particles are oxidized first by OH, then by the co-oxidation of O2 and OH.

The measurement of SVF of ethanol gasoline based on two-dimensional light extinction method

ABSTRACT In this paper, a two-dimensional(2D) parallel optical extinction experiment system was adopted. The ethanol gasoline was selected as the fuel, which was blended with 20% ethanol. The laminar diffusion combustion was selected as the combustion mode to test the soot volume fraction (SVF). The results showed that the SVF was 0 in the range of 0 ~ 7 mm flame height. It rose up firstly and then decreased from 7 to 32 mm flame height, reaching the maximum of 14.6 × 10−6 at 16.6 mm. At the same time, the structure of the laminar diffusion flame was redefined in combination with the flame image. In the range of 0 ~ 1.5 cm flame radial, the SVF was 0.7 × 10−6, which belonged to the soot condensation forming zone. In the range of 1.5 ~ 4 mm, the SVF rose and then dropped sharply until it was completely burned, which belonged to the transition zone of soot formation oxidation.

Sooting tendency of n-heptane/anisole blends in laminar counter-flow flames with oxygen enrichment

Anisole has been recognized as a potential alternative fuel to lower soot emission in internal combustion engines. Thus, in the present study, soot formation by n-heptane/anisole blends in a counter-flow laminar diffusion flame have been measured by using the Line of Sight Attenuation (LOSA). Several main factors related to the sooting tendency were analyzed in detail, including the anisole proportion, oxygen content, and fuel and oxidant flow velocities. Increasing the anisole proportion (0–30% in mole fraction) and oxygen content (30–40% in volume fraction) in oxidant promoted soot formation in the flame, and the maximum soot volume fraction could be fitted as an exponential function of the anisole proportion. Higher fuel and oxidant flow velocities resulted in greater strain rates and a shorter residence time for the reaction mixture, which ultimately inhibited soot formation.

An Investigation of Soot Volume Fraction and Temperature for Natural Gas Laminar Diffusion Flame Established From a Honeycomb Gaseous Burner

The present work is an experimental investigation that aims at studying the effects of different fuel additives on the soot volume fraction and temperature in a well-defined vertical laminar diffusion flame configuration, and these additives include a diluent (argon) that suppresses the formation of soot and a soot promoter (acetylene) that accelerates and intensifies the soot formation. Three different measuring techniques are employed throughout the whole experimental program, namely, a high-resolution digital camera (up to 3.7 fps) for flame visualization, a bare wire Pt/Pt-13% rhodium fine thermocouple of 15 µm wire diameter for measuring the mean gas temperature inside the flame region and a laser system for measuring the in-flame soot volume fraction. The results indicated that the soot inception zone (deep dark parabolic shape) occurs at the immediate vicinity of the burner. The soot oxidation zone is characterized by high luminosity, and it begins after the fuel is largely consumed. The increased percentages of acetylene in the fuel mixture would lead to extending the length of this zone to ultimately occupy the whole visible flame length, where the luminosity becomes independent of the amount of soot. The temperature within the soot surface growth zone (orange color) continues increasing but at a lower rate that reflects the domination of diffusion combustion mode. Limited partial oxidation may be anticipated within this zone due to the relatively high temperature, which is not high enough to cause luminosity of the soot particles.

Online in situ prediction of 3-D flame evolution from its history 2-D projections via deep learning

Online in situ prediction of 3-D flame evolution has been long desired and is considered to be the Holy Grail for the combustion community. Recent advances in computational power have facilitated the development of computational fluid dynamics (CFD), which can be used to predict flame behaviours. However, the most advanced CFD techniques are still incapable of realizing online in situ prediction of practical flames due to the enormous computational costs involved. In this work, we aim to combine the state-of-the-art experimental technique (that is, time-resolved volumetric tomography) with deep learning algorithms for rapid prediction of 3-D flame evolution. Proof-of-concept experiments conducted suggest that the evolution of both a laminar diffusion flame and a typical non-premixed turbulent swirl-stabilized flame can be predicted faithfully in a time scale on the order of milliseconds, which can be further reduced by simply using a few more GPUs. We believe this is the first time that online in situ prediction of 3-D flame evolution has become feasible, and we expect this method to be extremely useful, as for most application scenarios the online in situ prediction of even the large-scale flame features are already useful for an effective flame control.

Influence of the n-dodecane chemical mechanism on the CFD modelling of the diesel-like ECN Spray A flame structure at different ambient conditions

Encouraged by the diversity of n-dodecane chemical mechanisms currently available, this investigation focuses on analysing the impact of using different fuel oxidation schemes on the diesel-like Engine Combustion Network (ECN) Spray A flame structure, simulated by means of an Unsteady Flamelet Progress Variable (UFPV) combustion model. The present research discusses systematically the characteristics of four n-dodecane chemical mechanisms in perfectly stirred reactors and counterflow laminar diffusion flames (flamelets) before the final evaluation in turbulent reacting sprays in order to describe the effects of adding different physical levels of complexity to the ignition of the mixtures. In addition, this analysis is complemented with the description of the effect of the boundary conditions on the flame structure.Results evidence the extreme importance of the low temperature chemistry including the period for which the cool flame extends. The different prediction of this stage between mechanisms leads to noticeable different laminar flame structures which in turn produce substantially distinct turbulent flames, especially in the vicinity of the lift-off length (LOL) in terms of reactivity and positioning in the Z-T map. Finally, simulations confirm the strong effect of the boundary conditions, especially for the ambient temperature, on the ignitable mixtures which directly impacts on the soot precursors formation.

A post processing technique to predict primary particle size of sooting flames based on a chemical discrete sectional model: Application to diluted coflow flames

A numerical post-processing strategy to reconstruct the size of soot primary particles is presented in this work in the context of the chemical sectional models. The proposed technique is based on solving the transport equation of the primary particle number density for each considered aggregate size. The chemical source terms are derived from the corresponding chemical reactions.The validity of the proposed approach is tested on target flames of the International Sooting Flame (ISF) Workshop. In particular, first, a laminar premixed ethylene flame is considered. Profiles of soot volume fraction and mean primary particle size are compared between simulation and measurements and a satisfactory agreement is observed, validating the proposed post-processing strategy. Second, a laminar coflow ethylene flame is put under the scope. Numerical results are compared to experimental data once again in terms of soot volume fraction and primary particle size. The sensitivity to model parameters, such as accounting for surface rounding and the choice of the smallest aggregating particle size, is explored.Once validated, the effect of dilution on the mean primary particle diameter in laminar diffusion flames is examined. The general trends observed experimentally are recovered. The correlation between temperature, precursor concentrations, soot volume fraction and primary particle diameter is explored. Finally, formation rates and residence time along the particle trajectories are investigated to explain the effect of dilution on the spatial localization of the biggest particles along the flame. The relation between the soot volume fraction and the mean primary particle diameter is examined.

Demonstration of a cost-effective single-pixel UV camera for flame chemiluminescence imaging.

Combustion is the dominant form of energy conversion for a span of power systems such as engines and power plant boilers. It is an extremely complicated process which produces a huge number of intermediate products that usually feature non-uniform spatial distributions. Among those intermediate products, free radicals emitting in the UV band are of special interest because they contain abundant useful information of the target flame. Imaging methods such as planar laser-induced fluorescence or chemiluminescence imaging with UV cameras are of paramount importance to resolve the non-uniformities for a better understanding of combustion. However, a major limitation of UV cameras is that they are usually expensive, especially when multiple cameras are needed, such as in a tomographic system. In this work, we report the attempt of flame imaging with a cost-effective single-pixel UV camera which enables 2D spatial resolution of a single-pixel detector through light modulation to overcome this limitation. Meanwhile, numerical studies were conducted to systematically assess the performance of a few representative reconstruction algorithms. The impact of important factors such as sampling ratio and measurement matrix were also investigated. A validation as well as a demonstrative experiment was also conducted to reconstruct a laminar diffusion flame. The high similarity between the reconstruction and the image taken by a CMOS camera proves the feasibility of flame imaging with a single-pixel camera.

Numerical study of a laminar hydrogen diffusion flame based on the non-premixed finite-rate chemistry model; thermal NOx assessment

The present study examines thermal NOx formation on a laminar hydrogen diffusion flame based on a patented multi-flame diffusion burner installed in a domestic boiler. The analysis is performed through CFD modeling by means of ANSYS-FLUENT 19.0. Detailed chemical, thermal, and transport parameters are implemented, in order to obtain accurate results using finite-rate chemistry together with laminar and low-Reynolds turbulence models. The Damköhler number is calculated using a Matlab code developed in-house. The chemical kinetics of the thermal NOx route are analyzed for several inlet flame power levels, so as to demonstrate the effectiveness of the flame-splitting method at reducing the formation of thermal NOx. The numerical results are contrasted through measurements from literature for validation purposes. In the present work, the flame power is varied from 0.05 kW up to 0.8 kW. In contrast to turbulent non-premixed hydrogen flames, the numerical results provide a quasi-constant trend in thermal NOx formation at higher power levels (0.4–0.8 kW). The heat exchange rate between the flame and the combustion chamber and its influence on thermal NOx formation are all carefully analyzed.

Detector calibration and measurement issues in multi-color time-resolved laser-induced incandescence

Time-resolved laser-induced incandescence is used to infer the size distribution of gas-borne nanoparticles from time-resolved pyrometric measurements of the particle temperature after pulsed laser heating. The method is highly sensitive to aspects of the measurement strategy that are often not considered by practitioners, which often lead to discrepancies between measurements carried out under nominally identical conditions. This paper therefore presents a well-documented calibration procedure for LII systems and quantifies the uncertainty in pyrometric temperatures introduced by this procedure. Calibration steps include corrections for: (1) signal baseline, (2) variable transmission through optical components, and (3) detector characteristics (i.e., gain and spectral sensitivity). Candidate light sources are assessed for their suitability as a calibration reference and the uncertainty in calculated calibration factors is determined. The error analysis is demonstrated using LII measurements made on a sooting laminar diffusion flame. We present results for temperature traces of laser-heated particles determined using two- and multi-color detection techniques and discuss the temperature differences for various combinations of spectral detection channels. We also summarize measurement artifacts that could bias the LII signal processing and present strategies for error identification and prevention.

Effects of CO and H2 addition on OH* chemiluminescence characteristics in laminar rich inverse diffusion flames

Optical diagnostic technique based on chemiluminescence emissions is a promising alternative flame monitoring method to indicate the combustion regime and related characteristic parameters. In this work, laminar rich inverse diffusion flames were numerically studied to elucidate the effects of CO and H2 addition on OH* chemiluminescence characteristics. A jet flame simulation based on CFD was carried out to determine the effects of CO and H2 addition on OH* chemiluminescence distribution, and a 1-D counterflow diffusion flame calculation by CHEMKIN-PRO was performed to analyze the effects of CO and H2 addition on the formation characteristic of OH* chemiluminescence. The results show that OH* chemiluminescence is mainly concentrated at the central axis of the downstream region in inverse diffusion flames. With the increase of oxy-fuel equivalence ratio, the intensity of OH* chemiluminescence gradually decreases, and location of peak OH* intensity gradually moves from the downstream flame to the upstream. When the oxy-fuel equivalence ratio is fixed, the increase of the CO content in the fuel will reduce the H concentration in the flame, thereby reducing the concentration of OH* radicals. But the increase of the H2 addition would only accelerate the burning velocity of the flame without having a significant impact on the concentration of OH* radicals. When the flow rates of fuel and oxygen are fixed, the distribution and formation characteristics of OH* chemiluminescence are affected by the synergetic impact of fuel component and oxy-fuel equivalence ratio. The effect of H2 addition plays the most important role in the upstream of flame, and CO addition mainly affects the OH* radical production in the downstream region.

Effects of fuel compositions on the heat generation and emission of syngas/producer gas laminar diffusion flame

Demand for the clean and sustainable energy encourages the research and development in the efficient production and utilisation of syngas for low-carbon power and heating/cooling applications. However, diversity in the chemical composition of syngas, resulting due to its flexible production process and feedstock, often poses a significant challenge for the design and operation of an effective combustion system. To address this, the research presented in this paper is particularly focused on an in-depth understanding of the heat generation and emission formation of syngas/producer gas flames with an effect of the fuel compositions. The heat generated by flame not only depends on the flame temperature but also on the chemistry heat release of fuel and flame dimension. The study reports that the syngas/producer gas with a low H2:CO maximises the heat generation, nevertheless the higher emission rate of CO2 is inevitable. The generated heat flux at H2:CO = 3:1, 1:1, and 1:3 is found to be 222, 432 and 538 W m-2 respectively. At the same amount of heat generated, H2 concentration in fuel dominates the emission of NOx. The addition of CH4 into the syngas/producer gas with H2:CO = 1:1 also increases the heat generation significantly (e.g. 614 W m-2 at 20%) while decreases the emission formation. In contrast, adding 20% CO2 and N2 to the syngas/producer gas composition reduces the heat generation from 432 W m-2 to 364 and 290 W m-2, respectively. The role of CO2 on this aspect, which is weaker than N2, thus suggests CO2 is preferable than N2. Along with the study, the significant role of CO2 on the radiation of heat and the reduction of emission are examined.

Combustion characterization of the mixtures biogas-syngas, strain rate and ambient pressure effects

The use of biofuels in applications of interest requires knowledge of behavior of their flames under various working conditions. This paper aims at demonstrating the effect of strain rate and ambient pressure on laminar diffusion flames structure and extinction temperature limits of biogas-syngas mixtures. Opposed-flow configuration is adopted over a wide interval of strain rate (nearly from ignition to extinction) and ambient pressure (from 1 to 10 atm). Chemical kinetics is described by GRI-Mech 3.0 mechanism and Chemkin program used to solve the problem. Results revealed that increase of hydrogen volume in syngas highly enlarges strain rate extinction limits and keeps extinction temperature nearly constant. On the other hand, methane volume augmentation in biogas preserves approximately extinction strain rate and improves extinction temperature. Besides, it has been observed that increasing strain rate enhances non-equilibrium effects and radical production; whereas, it reduces temperature and final combustion products. It has been established that ambient pressure has opposite role of strain rate except for flame thickness. Results showed that in this kind of flames, for the cases of low strain rate and pressure or high strain rate and pressure, the NO species production is governed by the NO2 route. For the other cases, the prompt route is preferred and the thermal path is the least favored and disabled at elevated pressures. Reaction zone thickness is almost constant for all compositions; however, it is reduced by pressure and strain rate augmentations.

Investigation of Soot Formation in a Novel Diesel Fuel Burner

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