Sooting Characteristics Research Articles

Sooting limits of nonpremixed n-heptane, n-butanol, and methyl butanoate flames: Experimental determination and mechanistic analysis

The sooting limits of nonpremixed n-heptane, n-butanol, and methyl butanoate flames were determined experimentally in a liquid pool stagnation-flow configuration. In addition, complementary simulations with detailed polycyclic aromatic hydrocarbon (PAH) chemistry and a detailed soot model, based on the Hybrid Method of Moments (HMOM), were performed and compared with the experimental critical strain rates for the sooting flames. Argon dilution was used to keep the thermal environment for the three fuel cases nearly the same to elucidate the chemical effects. Both experiment and simulation showed that n-heptane and n-butanol had similar sooting characteristics, while methyl butanoate had the least sooting propensity. Further sensitivity and reaction path analysis demonstrates that the three fuels share similar PAH chemical pathways, and C5 and C6 ring formation from the intermediate chain species is found to be the rate-limiting step. The differences in sooting propensity lie in the fuel breakdown processes. Specifically, the oxygen bounded in n-butanol does not reduce soot precursor concentrations but is primarily involved in intramolecular water elimination reactions. On the contrary, the fuel bound oxygen in methyl butanoate shortens the carbon chain of the soot precursors and promotes their oxidation, which reduces the total carbon available for soot formation.

Simulating the Sooting Propensity of JP-8 with Surrogate Fuels from Hydrocarbon Fluids

Surrogate fuel mixtures were formulated using hydrocarbon fluids to investigate the ability of different surrogate formulations to match the sooting characteristics of a specific jet fuel (JP-8 POSF 5699). Three different surrogates were studied: two binary mixtures and a ternary mixture. The first surrogate matched the threshold soot index and average molecular weight of the JP-8; the second matched the threshold soot index and hydrogen-to-carbon; and the third matched the threshold soot index, hydrogen-to-carbon, and derived cetane number. For the second and third surrogates, average molecular weight was within 10% of the estimated average molecular weight of the target JP-8. Soot distributions of JP-8 and its surrogates were compared in wick-fed coflow diffusion flames, and net soot production was compared in a model gas turbine combustor. Line-of-sight and local soot volume fraction measurements were obtained using laser extinction on the wick burner for flames at their smoke point. Line-of-sight soot volume fractions for the fuels were measured at different equivalence ratios in the model gas turbine combustor. All three surrogates were found to match the soot production of JP-8 within the experimental uncertainty of in the flame and model gas turbine combustor experiments.

Numerical and experimental study of soot formation in laminar diffusion flames burning simulated biogas fuels at elevated pressures

The effects of pressure and composition on the sooting characteristics and flame structure of laminar diffusion flames were investigated. Flames with pure methane and two different methane-based, biogas-like fuels were examined using both experimental and numerical techniques over pressures ranging from 1 to 20atm. The two simulated biogases were mixtures of methane and carbon dioxide with either 20% or 40% carbon dioxide by volume. In all cases, the methane flow rate was held constant at 0.55mg/s to enable a fair comparison of sooting characteristics. Measurements for the soot volume fraction and temperature within the flame envelope were obtained using the spectral soot emission technique. Computations were performed by solving the unmodified and fully-coupled equations governing reactive, compressible flows, which included complex chemistry, detailed radiation heat transfer and soot formation/oxidation. Overall, the numerical simulations correctly predicted many of the observed trends with pressure and fuel composition. For all of the fuels, increasing pressure caused the flames to narrow and soot concentrations to increase while flame height remained unaltered. All fuels exhibited a similar power-law dependence of the maximum carbon conversion on pressure that weakened as pressure was increased. Adding carbon dioxide to the methane fuel stream did not significantly effect the shape of the flame at any pressure; although, dilution decreased the diameter slightly at 1atm. Dilution suppressed soot formation at all pressures considered, and this suppression effect varied linearly with CO2 concentration. The suppression effect was also larger at lower pressures. This observed linear relationship between soot suppression and the amount of CO2 dilution was largely attributed to the effects of dilution on chemical reaction rates, since the predicted maximum magnitudes of soot production and oxidation also varied linearly with dilution.

Soot Reduction Under DC Electric Fields in Counterflow Non-Premixed Laminar Ethylene Flames

The effects of DC electric fields on non-premixed ethylene flames in a counterflow burner were studied experimentally with a focus on the reduction of soot particles. The experiment was conducted by connecting a high voltage terminal and a ground terminal to a lower (fuel) and upper (oxidizer) nozzle, respectively. We applied direct current (DC) potentials in a range of -5 kV < Vdc < 5 kV. Uniform electric fields were then generated in the gap between the two nozzles. The experimental conditions were selected to cover both soot formation (SF) and soot formation oxidation (SFO) flames. The flames subjected to the negative electric fields moved toward the fuel nozzle because of an ionic wind due to the Lorentz force acting on the positive ions in the flames. In addition, the yellow luminosity significantly decreased, indicating changes in the sooting characteristics. To analyze the sooting characteristics under the electric fields, planar laser induced incandescence (PLII) and fluorescence (PLIF) techniques were used to visualize the soot, polycyclic aromatic hydrocarbons (PAHs), and OH radicals. The sooting limits in terms of the fuel and oxygen mole fractions were measured. No substantial soot formation due to the effects of the DC electric fields for the tested range of voltages and reactant mole fractions could be identified. The detailed flame behaviors and sooting characteristics under the DC electric fields are discussed.

Sooting tendency of paraffin components of diesel and gasoline in diffusion flames

The influence of the chemical structure on the sooting characteristics of some paraffin class hydrocarbons which are found in gasoline and diesel fuel is studied experimentally. The experiment involves the combustion of the paraffin in a wick-fed burner. Differential mobility spectrometry is used to measure the particle size distribution (PSD) at different flame heights. The wick-fed laminar diffusion flame is sampled at the tip; the flame height is modified systematically from small heights to large heights. Normal, iso and cyclo paraffins PSDs evolve in a similar way with flame height. At very low flame heights the PSD is unimodal, but rapidly evolves into a multi-modal one. The total number of particles peaks at small heights, and then decreases as flame height increase until it approaches constant values for all considered fuels. The mean soot particle diameter increases with flame height until a height where a maximum is achieved and sustained. Among each type of fuel, a systematic decrease in the maximum mean soot particle diameter was observed as the number of carbon atoms in the molecule increased. At all flame heights, comparing fuels with the same carbon number, cyclic paraffins presented the largest mean soot particles sizes, followed by iso-paraffins and the smallest particles for normal paraffins.

Mixture condition, combustion and sooting characteristics of ethanol–diesel blends in diffusion flames under various injection and ambient conditions

Blend of oxygenated alcohol fuels to diesel has drawn great attention as a way to promote the use of bio-derived alternative fuels and for reduction of soot emissions in direct-injection diesel engines. This study compares the mixture condition, combustion and sooting characteristics of ethanol–diesel blends (E30 and E50: 30% and 50% ethanol blend to diesel in volume respectively) to those of a diesel in diffusion flames under various injection and ambient conditions. The comparisons were made in terms of liquid-phase penetration, flame structure, ignition delay, flame lift-off length, mixture condition at flame lift-off location, and soot concentration (KL factor) that obtained using Mie-scattering, natural flame luminosity imaging and two-color method.The liquid-phase penetration of the ethanol–diesel blends was almost equivalent to that of diesel under diesel-like injection and ambient conditions. The ethanol–diesel blend (E30) showed longer ignition delay than diesel, but this difference became insignificant at high ambient temperatures over 1000K. The flame lift-off length of the E30 was longer than that of diesel that formed less fuel-rich mixture of E30 at the flame lift-off location. The difference in mixture condition between E30 and diesel became larger with increase in injection pressure, but smaller with increase in ambient temperature. The E30 suppressed the soot formation near the flame lift-off location, and also promoted the soot oxidation process. This soot reduction potential of E30 became larger with increase in injection pressure, but insignificant at high ambient temperatures. Increasing ethanol blend ratio decreased the flame temperature slightly, but further enhanced the soot reduction potential.

Effect of electron injection for soot suppression in hydrocarbon flames

Numerical simulation of one-dimensional, laminar, premixed flat flame of acetylene was conducted to elucidate influence of ionic reactions on sooting characteristics. For reaction kinetics of the sooting flame, we adopted the mechanism developed by Frenklach et al. and the mechanism developed by Brown et al. The former consisted of recent developed in gas-phase reactions, and aromatic reactions. The latter included chemiionization, proton transfer, ion–molecule reactions, and charge recombination reactions. Numerical predictions were compared with previous experimental and numerical results. In addition, using the developed reaction mechanism, we simulated the plasma by corona discharge in flame. To compare the normal prediction with discharged prediction, we investigated the effect of plasma by corona discharge. Effect of electron injection in a hydrocarbon flame on soot precursor reduction was confirmed. In addition, the reaction “C6H5+ + e− = c-C6H4 + H” was intensified and more dominant through the electron injection.

An Experimental Comparison of the Sooting Behavior of Synthetic Jet Fuels

An experimental study is conducted for the laminar, atmospheric pressure, sooting, coflow diffusion flames of prevaporized Jet A-1 and four synthetic jet fuels to compare their sooting characteristics and flame structures. Soot volume fraction, species concentration, and temperature profiles are measured, using the laser extinction measurement method, gas chromatography, and fine wire thermocouples, respectively. To evaluate the sooting tendency of the fuels, their smoke point heights are measured using the ASTM D1322 method and compared. The synthetic jet fuels under investigation are (1) Fully Synthetic Jet Fuel (FSJF), which is a coal-to-liquids (CtL) kerosene plus some coal tar derived material from Sasol, (2) Fischer–Tropsch Synthetic Paraffinic Kerosene (FT-SPK), which is a gas-to-liquids (GtL) product from Shell, (3) SPK plus naphthenic cut (50% by volume), and (4) SPK plus hexanol (20% by volume). The threshold sooting index (TSI) of the proposed surrogate mixtures for the fuels are calculated and...

A numerical study on the effects of pressure and gravity in laminar ethylene diffusion flames

The effects of pressure and gravity on sooting characteristics and flame structure were studied numerically in coflow ethylene–air laminar diffusion flames between 0.5 and 5 atm. Computations were performed by solving the unmodified and fully-coupled equations governing reactive, compressible, gaseous mixtures which include complex chemistry, detailed radiation heat transfer, and soot formation/oxidation. Soot formation/oxidation was modeled using an acetylene-based, semi-empirical model which has been verified with previously published experimental data to correctly capture many of the observed trends at normal-gravity. Calculations for each pressure considered were performed for both normal- and zero-gravity conditions to help separate the effects of pressure and buoyancy on soot formation. Based on the numerical predictions, pressure and gravity were observed to significantly influence the flames through their effects on buoyancy and reaction rates. The zero-gravity flames have higher soot concentrations, lower temperatures and broader soot-containing zones than normal-gravity flames at the same pressure. The zero-gravity flames were also found to be longer and wider. Differences were observed between the two levels of gravity when pressure was increased. The zero-gravity flames displayed a stronger dependence of the maximum soot yield on pressure from 0.5 to 2 atm and a weaker dependence from 2 to 5 atm as compared to the normal-gravity flames. In addition, flame diameter decreased with increasing pressure under normal-gravity while it increased with pressure in the zero-gravity cases. Changing the prescribed wall boundary condition from fixed-temperature to adiabatic significantly altered the numerical predictions at 5 atm. When the walls were assumed to be adiabatic, peak soot volume fractions and temperatures increased in both the zero- and normal-gravity flames, emphasizing the importance of heat conduction to the burner rim on flame structure.

Effects of gravity and pressure on laminar coflow methane–air diffusion flames at pressures from 1 to 60 atmospheres

The effects of pressure and gravity on the sooting characteristics and flame structure of coflow methane–air laminar diffusion flames between 1 and 60 atm were studied numerically. Computations were performed by solving the unmodified and fully-coupled equations governing reactive, compressible flows which include complex chemistry, detailed radiation heat transfer and soot formation/oxidation. Soot formation/oxidation was modeled using an acetylene-based, semi-empirical model which was verified with previously published experimental data to correctly capture many of the observed trends at normal-gravity. Calculations for each pressure considered were performed under both normal- and zero-gravity conditions to help separate and identify the effects of pressure and buoyancy on soot formation. Based on the numerical predictions, pressure and gravity were observed to significantly influence the sooting behavior and structure of the flames through their effects on buoyancy and temperature. Zero-gravity flames generally have lower temperatures, broader soot-containing zones, and higher soot volume fractions than normal-gravity flames at the same pressure. Buoyancy forces caused the normal-gravity flames to narrow with increasing pressure while the increased soot concentrations and radiation at high pressures caused the zero-gravity flames to lengthen. Low-pressure flames at both gravity levels exhibited a similar power-law dependence of the maximum carbon conversion on pressure that weakened as pressure was increased. In the zero-gravity flames, increasing pressure beyond 20 atm caused the maximum carbon conversion factor to decrease.

Stability of lifted flames in centerbody burner

The centerbody burner was designed with the objective of understanding the coupled processes of formation, growth, and burn-off of soot through decoupling them using recirculation zones (RZs). Experimentally it was found that the sooting characteristics of the centerbody burner could be altered dramatically via simple changes in the operating conditions. One of the interesting operating regimes in which a flame lifts off and forms a column of soot was identified when oxygen in the annulus air jet was reduced sufficiently. This paper describes the numerical studies performed to aid the understanding of lifted flames in the centerbody burner. A time-dependent, axisymmetric, detailed-chemistry CFD model is used. Combustion and PAH formation are modeled using the Wang–Frenklach (99 species and 1066 reactions) mechanism, and soot is simulated using a two-equation model of Lindstedt. Calculations have predicted the structure of the lifted flame very well. Two RZs [outer (ORZ) and inner (IRZ)] are formed between the fuel and air jets. A diffusion flame that is lifted-off the centerbody plate anchors steadily to the outer periphery of the ORZ. A near-perfect match between the computed and measured flame lift-off heights is achieved. RZs transport soot that is formed in the flame toward the face of the centerbody and create the soot column. Ethylene and its lighter fuel fragments that are formed in the RZs diffuse toward the annulus air jet and establish a mixing layer with the inwardly diffusing oxygen. Heat diffusing away from the RZs initiates autoignition reactions in the mixing layer. A flame with a triple-flame-base structure becomes established at a location where the ignition-delay time matches the residence time. Soot that is transported into the RZs is found to have a significant effect on the flame lift-off height. Numerical experiments are performed to aid the understanding of the relationship between soot and flame lift-off. Radiation from the soot decreases the temperature, slows the autoignition process, and increases the lift-off height. Soot oxidation consumes O and OH radicals, slows the autoignition reactions, and increases the lift-off height.

Experimental and computational study on partially premixed flames in a centerbody burner

The centerbody burner was designed with the objective of understanding the coupled processes of soot formation, growth, and burnout. Fuel that issues from the center of the burner establishes two flame zones – one associated with the recirculation zone (RZ) and the other, with the trailing jet. The sooting characteristics in these two flame zones can be quite different because of variations in residence time and transport of reactants and products. Calculations performed for this burner operating under a partially premixed fuel jet suggested that soot in the RZ decreases and that soot in the trailing jet flame increases with the amount of premixing. An experimental and numerical study is performed to aid the understanding of these differences. A time-dependent, axisymmetric, detailed-chemistry computational-fluid-dynamics (CFD) model known as Unsteady Ignition and Combustion using ReactioNs (UNICORN) is used for simulating flames under different equivalence-ratio conditions. Combustion and PAH formation are modeled using the Wang–Frenklach (99 species and 1066 reactions) mechanism, and soot is simulated using a two-equation model of Lindstedt. A Lagrangian-based particle-tracking model is used for understanding the evolution of soot-like particles. Flame and recirculation-zone structures and soot in the experiments are identified using direct photographs taken with and without Mie scattering from soot particles as well as laser-induced-incandescence (LII) measurements. Calculations predict the structures of the partially premixed centerbody flames for various equivalence ratios reasonably well. Experiments confirm the predicted soot suppression in the RZs and enhancement of soot in the trailing jet flame when air is added to the fuel jet. It is found that flame movement in the RZ increases soot-particle burnout and, thereby, reduces the amount of soot within the RZ. As the flame moves closer to the fuel jet, more soot becomes entrained into the inner vortex. Motion of soot-like particles explained the spiral rings observed in the experiment. Increased particle burnout with partial premixing leads to shrinkage of soot spirals.

Synthesis, droplet combustion, and sooting characteristics of biodiesel produced from waste vegetable oils

In light of the potential of fatty acid methyl ester (FAME, i.e. biodiesel) as a renewable energy source, an innovative acid catalyzed process was developed for the synthesis of biodiesel from waste vegetable oils. The synthesized biodiesels were analytically characterized for their major components, molar fraction and molecular weight of each component, the average molecular weight, and the heat of combustion. Their droplet combustion characteristics in terms of the burning rate, flame size, and sooting tendency were subsequently determined in a high-temperature, freely-falling droplet apparatus. Results show that the biodiesel droplet has higher burning rate, and that biodiesel in general has a lower propensity to soot because its molecular oxygen content promotes the oxidation of the soot precursors.

Control of the structure and sooting characteristics of a coflow laminar methane/air diffusion flame using a central air jet: An experimental and numerical study

Multi-port co-annular burners are widely used in practice to achieve low NOx and soot emissions from combustion devices. However, there is lack of fundamental studies on the structure and flame regime under different flow conditions. A conventional laminar axisymmetric coflow diffusion flame burner was modified by introducing a central air jet inside the fuel tube to investigate how the central air jet velocity affects the structure and sooting characteristics of a coflow methane/air diffusion flame. The modified burner produces a double flame structure: an inner inverse diffusion flame or an inner partially premixed flame and an outer normal diffusion flame. Experiments were conducted to observe the effect of the central air jet velocity on the appearance of the flame. At a given and relatively low central air jet flow rate the inner flame can be either a partially premixed one or an inverse diffusion one, depending on how the central air jet flow rate is adjusted. The overall flame structure and sooting characteristics can be controlled effectively by varying the flow rate of the central air jet. Detailed numerical calculations were conducted using GRI-Mech 3.0 without the NOx chemistry and a simplified soot model. Numerical results reproduce the experimental observations and provide detailed information on the flow field, temperature, and species concentration distributions. The central air jet is an effective aerodynamic means to control the flame size, structure, and sooting characteristics.

Sooting characteristics of surrogates for jet fuels

Currently, modeling the combustion of aviation fuels, such as JP-8 and JetA, is not feasible due to the complexity and compositional variation of these practical fuels. Surrogate fuel mixtures, composed of a few pure hydrocarbon compounds, are a key step toward modeling the combustion of practical aviation fuels. For the surrogate to simulate the practical fuel, the composition must be designed to reproduce certain pre-designated chemical parameters such as sooting tendency, H/C ratio, autoignition, as well as physical parameters such as boiling range and density. In this study, we focused only on the sooting characteristics based on the Threshold Soot Index (TSI). New measurements of TSI values derived from the smoke point along with other sooting tendency data from the literature have been combined to develop a set of recommended TSI values for pure compounds used to make surrogate mixtures. When formulating the surrogate fuel mixtures, the TSI values of the components are used to predict the TSI of the mixture. To verify the empirical mixture rule for TSI, the TSI values of several binary mixtures of candidate surrogate components were measured. Binary mixtures were also used to derive a TSI for iso-cetane, which had not previously been measured, and to verify the TSI for 1-methylnaphthalene, which had a low smoke point and large relative uncertainty as a pure compound. Lastly, surrogate mixtures containing three components were tested to see how well the measured TSI values matched the predicted values, and to demonstrate that a target value for TSI can be maintained using various components, while also holding the H/C ratio constant.

Development and validation of a new soot formation model for gas turbine combustor simulations

In this paper a new soot formation model for gas turbine combustor simulations is presented. A sectional approach for the description of Polycyclic Aromatic Hydrocarbons (PAHs) and a two-equation model for soot particle dynamics are introduced. By including the PAH chemistry the formulation becomes more general in that the soot formation is neither directly linked to the fuel nor to C 2-like species, as it is the case in simpler soot models currently available for CFD applications. At the same time, the sectional approach for the PAHs keeps the required computational resources low if compared to models based on a detailed description of the PAH kinetics. These features of the new model allow an accurate yet affordable calculation of soot in complex gas turbine combustion chambers. A careful model validation will be presented for diffusion and partially premixed flames. Fuels ranging from methane to kerosene are investigated. Thus, flames with different sooting characteristics are covered. An excellent agreement with experimental data is achieved for all configurations investigated. A fundamental feature of the new model is that with a single set of constants it is able to accurately describe the soot dynamics of different fuels at different operating conditions.

Computational and experimental investigation of the interaction of soot and NO in coflow diffusion flames

A combined computational and experimental investigation that examines the relationship of soot formation and NO in coflow ethylene air diffusion flames is presented. While both NO and soot formation are often studied independently, there is a need to understand their coupled relationship as a function of system parameters such as fuel type, temperature and pressure. The temperature decrease due to radiative losses in systems in which significant soot is produced can affect flame length and other temperature-dependent processes such as the formation of NO. The results of a computational model that includes a sectional representation for soot formation with a radiation model are compared against laser-induced fluorescence measurements of NO. The sooting characteristics of these flames have been studied previously. Experimentally, a laser near 225.8nm is used to excite the γ(0,0) band in NO. Spectrally resolved fluorescence emission is imaged radially, for the (0,0), (0,1), (0,2), (0,3), and (0,4) vibrational bands, at varying axial heights to create a two-dimensional image of NO fluorescence. A reverse quenching correction is applied to the computational results to determine an expected fluorescence signal for comparison with experimental results. Modeling results confirm that Fenimore NO is the dominant mechanism for NO production and suggest that for lightly sooting flames (peak soot volume fraction<0.5ppm), soot reduces only the Zeldovich NO formation (by a factor of two). For flames with increased soot levels (peak soot volume fraction∼4ppm), the model indicates not only that Zeldovich NO decreases by a factor of 2.5 through radiation loss, but that non-Zeldovich NO is reduced in the top center of the flame by about 30% through the oxidation of soot.

Soot formation and oxidation in normal and inverse diffusion flames

A numerical study has been carried out to address the sooting characteristics of normal and inverse diffusion flames. The simulation was performed on the basis of single-step kinetics with a laminar flow assumption and non-unity Lewis number. Five different coaxial normal and inverse flame combinations with different momentum ratios have been investigated and compared. The results were experimentally validated using an identical burner with concentric cylindrical ports, where flame temperature measurements were employed for flame length comparison while a paper filter was utilized for soot concentration measurement. It was revealed that the momentum diffusion between the parallel streams governs the flame length and size. The results also showed that the single-step kinetics is able to predict the maximum soot formation in the normal flame annular region, provided that the Lewis number is less than unity. In inverse diffusion flames, the region of the soot surface growth is too small and the flow residence time is too short to allow significant soot production. For all combined inverse/normal diffusion flames, the soot emitted by the inner inverse flame had almost no influence on the total soot production. The thermal radiation by soot increased with the increase in both inner and outer flame lengths. The inner flame contributed to a 13 per cent increase in the total thermal radiation with keeping the same exhaust soot concentration level.

Sooting characteristics of isolated droplet burning in heated ambients under microgravity

This paper presents an investigation into the sooting characteristics of isolated droplets (for fuel n-decane) burning in heated ambients in microgravity. A backlit video view of the droplet was taken to determine the soot shell size and to judge the transient soot generation according to qualitative amount of soot. The independent experiment variables were the ambient temperature and initial droplet diameter. Soot generation was higher for initially larger droplets when compared at the same burning time normalized with the initial droplet diameter squared (called normalized burning time). At the same absolute burning time there existed an obvious initial transient period after ignition in which the stated relationship was not satisfied. This transient time increased with increasing the ambient temperature. There was a peak in the soot generation at about 1000 K throughout the lifetime of the droplet. The soot shell size was generally larger for an initially bigger droplet at the same instantaneous droplet diameter or normalized burning time. At the same absolute burning time, however, an initially smaller droplet exhibited larger relative soot shell sizes (the soot shell size normalized with the initial droplet diameter). The soot shell size increased monotonically with increasing ambient temperature. This is due to the increase in the Stefan flow drag with the larger burning rate at the higher temperature. The consequent result is that the soot shell sizes are much larger for droplets burning in heated ambients than for droplets burning in room-temperature ambients.

에틸렌/프로판 대향류 확산화염에서 PAH 와 매연의 생성특성

Sooting characteristics of counterflow ethylene/propane mixture flames have been experimentally studied to investigate the fuel structure effect on PAH and soot formation. Laser-induced incandescene and laser induced fluorescene techniques were employed to measure soot volume fraction and polycyclic aromatic hydrocarbon (PAH) concentration, respectively. Importance of C₃-species on PAH growth as well as the Habstraction-C₂H₂ addition (HACA) mechanism has been emphasized, considering that PAH growth rate is greater for with mixed fuel than for pure fuel flames. It was also con finned that HACA pathways are the dominant soot growth mechanism. A new PAH growth model including both C₂-and C₃-growth mechanisms is proposed based on the experimental results.<br/>