Sooting Propensity Research Articles

Effects of fuel composition and octane sensitivity on polycyclic aromatic hydrocarbon and soot emissions of gasoline–ethanol blend surrogates

The sooting propensity of a fuel is closely coupled with the fuel composition and chemistry. A detailed understanding of their effects is, therefore, needed to develop next-generation fuels which can minimize particulate emissions. With this overarching goal, the present work numerically investigates the effects of fuel composition and octane sensitivity (S) on polycyclic aromatic hydrocarbons (PAH) and soot emissions, for four-component gasoline–ethanol blend surrogates comprising isooctane, n-heptane, toluene, and ethanol. A partially-premixed counterflow flame is chosen as the canonical configuration for this study and simulations are performed using CHEMKIN-Pro-employing a kinetic mechanism developed by Park et al. (2017). In addition, a, detailed soot model based on the sectional method is used to capture the spatial characteristics of soot emissions. The kinetic mechanism and soot model are validated using available experimental data for various targets. A total of 86 TPRF-E mixtures, spanning a wide range of concentration of each component, and a wide range of S are analyzed. The effect of each non-paraffinic fuel component on the resultant PAH and soot emissions is investigated. PAH and soot emissions are found to vary significantly depending upon the blend composition. Based on the parametric sweeps, a regression analysis is carried out to identify global parameters that govern the formation of PAHs and soot. The analysis shows that both toluene content and S have a prominent effect on the formation of PAHs and soot, with toluene content having a stronger impact. Moreover, larger PAHs have higher dependency on toluene content and S. Furthermore, a detailed analysis is carried out to understand the physical and chemical phenomena associated with the observed trends of PAH and soot emissions.

Effects of pressure on soot formation in laminar coflow methane/air diffusion flames doped with n-heptane and toluene between 2 and 8 atm

There is currently a lack of adequate understanding of soot formation in flames fueled with liquid hydrocarbons at elevated pressures. In this study, laminar coflow n-heptane and toluene doped CH4/air diffusion flames were numerically investigated under a constant carbon mass flow rate at pressures between 2 and 8 atm to understand how pressure affects the sooting propensity of these two main components of surrogate fuels mimicking gasoline. Numerical simulations were performed using a detailed reaction mechanism containing 175 species and 1175 reactions and a sectional soot model. Soot inception is modeled by collisions among pyrene, BAPYR and BGHIF. Soot surface growth and oxidation are modeled using the hydrogen abstraction acetylene addition (HACA) mechanism as well as PAH surface condensation. The predicted sensitivity of soot production to pressure is in better agreement with the measurements of Daca and Gülder (2017) when the soot aging effect is considered in HACA surface growth. The predicted soot concentrations are in overall good agreement with measurements. Propargyl recombination and propargyl reaction with propyne are important pathways for the formation of benzene. In methane+toluene flames, attack of toluene by H radical is an effective benzene formation pathway low in the flame, but the relative importance of benzene formation from toluene is reduced with increasing pressure. Although all the soot formation processes are enhanced with increasing pressure, PAH condensation is enhanced the most, followed by HACA and inception. At 6 and 8 atm, PAH condensation becomes comparable to HACA. The pressure dependence of the sooting propensity follows the order of methane+toluene < methane+n-heptane < methane, consistent with measurements. This result can be explained by the pressure dependence of benzene formation pathways, the kinetic effect of pressure, and the scrubbing effect of soot production on the gas-phase species involved in soot formation.

Effects of benzene, cyclo-hexane and n-hexane addition to methane on soot yields in high-pressure laminar diffusion flames

Effects of doping high pressure methane diffusion flames with benzene, cyclo-hexane and n-hexane were investigated to assess the sooting propensity of three hydrocarbons with six carbons at elevated pressures. Amount of liquid hydrocarbons added to methane constituted 7.5% of the total carbon content of the fuel stream. The pressure range investigated extended up to 10 bar and the experiments were carried out in a high pressure combustion chamber capable of establishing stable laminar diffusion flames with various fuels at elevated pressures and was used in similar experiments previously. Temperatures and soot volume fractions were measured using the spectral soot emission technique capturing spectrally-resolved line-of-sight intensities which were subsequently inverted using an Abel type algorithm to obtain radial distributions assuming that the flames are axisymmetric. The total mass carbon flow of the fuel stream was kept constant at 0.524 mg/s in neat methane, benzene-doped methane, cyclo-hexane-doped methane, and n-hexane-doped methane flames to have tractable measurements at all pressures. Measured maximum soot volume fractions and evaluated maximum soot yields showed that benzene-doped methane flame had the higher values than cyclo-hexane doped methane flames which in turn had higher values than n-hexane doped methane flames at all pressures. Sooting propensity dependence of the three hydrocarbons on pressure can be ranked as, in descending order, n-hexane, cyclo-hexane, and benzene; however, the difference between pressure dependencies of n-hexane and cyclo-hexane was within the measurement error margins. Ratio of soot yields of benzene to n-hexane doped flames changed from about 2 at 2 bar to 1.2 at 10 bar; the ratio of benzene to cyclo-hexane doped flames showed similar trends.

Biodiesel flames as a unique pyrolyzing carbon source for the synthesis of hydrophobic carbon films

We report the rapid single-step flame synthesis of hydrophobic carbon layers (C-layers) on the surface of stainless-steel (SS) substrates using vaporized biodiesel as the fuel. A co-flow canola methyl ester/air diffusion flame is used to generate a hydrophobic monolayer on the surface of the metal substrate upon its insertion into the reaction zone. Carbon deposition on the surface of the SS substrates varies by changing the SS disk’s position in the post-flame, and by varying its exposure time. The thickness and mass of the flame-formed monolayer varied depending on the substrate’s insertion point into the flame. However, the variation of mass did not significantly impact the C-layer’s uniformity or hydrophobicity. We hypothesize that a small “inner-cone” of the biodiesel flame along with a high soot propensity can result in an ideal medium to form uniform hydrophobic C-layers of unique hierarchical surface structure. This is supported by introducing SS substrates in methane/air flames formed using the same co-flow burner. The hydrophobic property of the carbon deposits was quantified by measuring the contact angle of water droplets placed on the film’s surface. A water droplet drop test was conducted on the flame-formed hydrophobic layers to study their wettability property.

Pressure dependence of sooting characteristics of m-xylene and n-octane doped laminar methane diffusion flames from 2 to 10 bar

The work reported here contributes to a wider effort that focuses on the issue of whether single ring aromatics and their same carbon number alkane counterparts have the same pressure dependence of their sooting characteristics. Influence of doping methane base fuel with either m-xylene or n-octane in laminar high-pressure co-flow diffusion flames was studied from 2 to 10 bar pressure in current work. Two liquid hydrocarbons added to the base fuel methane contributed 3% of the total carbon mass flow in the fuel stream, which was kept fixed at 0.524 mg/s at all pressures considered, to have tractable measurements. The methane flames doped with either m-xylene or n-octane were stabilized on a co-flow diffusion burner installed inside of a high pressure combustion chamber with optical access. Temperatures and soot volume fractions were inferred from spectrally-resolved measurements of soot radiation. Line of sight soot emission intensities were inverted at each measurement plane along the flame axis to obtain radially-resolved temperature and soot volume fraction. Soot yields, calculated from soot volume fractions, of m-xylene doped flames were found to be higher than those of n-octane doped flames in the pressure range of 2–10 bar; however, the ratio of soot yields of m-xylene to n-octane decreased with pressure, from 2.9 at 2 bar to 1.2 at 10 bar. This finding implies that m-xylene and n-octane, both with eight carbons in their molecular structure, do not have the same sooting propensity with increasing pressure; m-xylene’s pressure dependence is weaker than that of n-octane within the pressure range of 2–10 bar.

Burning of Algae-Derived Biofuel Droplets and Their Mixtures with Jet Fuel

This paper reports new results about the combustion dynamics of hydroprocessed renewable jet fuel droplets derived from heterotrophic algae (AHRJ) burning under conditions that promote one-dimensional transport dynamics and near spherically symmetric burning conditions. Results are compared between Jet-A and three AHRJ/Jet-A mixtures containing 25, 50, and 75% AHRJ by volume. The results show that AHRJ droplets burn slightly faster than Jet-A and have a significantly reduced sooting propensity. Detailed chemical analysis also shows that AHRJ contains virtually no aromatics while the aromatic content of Jet-A was 16% by volume. The influence of composition on the burning rate is explained using a simple scaling argument that illustrates the importance of liquid density and other properties.

Predicting the soot emission tendency of real fuels – A relative assessment based on an empirical formula

Significant progress is going on in the development of non-fossil based alternative fuels which are designed to be sustainable, have potential to substitute (fully or partially) the fossil-based fuels and importantly are targeted to reduce emissions. Though soot emission characteristics has not been focus of fuel certification process yet, due to growing environmental concerns it is important to consider this aspect in the fuel design and understand its emission patterns.It is known that soot formation in combustion is linked to the degree of branching, degree of unsaturation or the degree of cyclization of a fuel molecule. This information is used to extract soot emission properties of complex real fuel mixtures. To do so, we use the definition of hydrogen deficiency (HD) which gives information on the number of double bonds or cyclic structures present in a fuel. With the help of one measured emission property, the same property of different fuels is predicted using HD. This way, we obtain a relative assessment of fuels’ sooting tendency compared to other known fuels. In this work, soot emission pattern of six jet fuels (reference fuels and synthetic fuels) is predicted based on HD and compared them with the recent ground-test measurements of V2527-A5 engine of an Airbus A320. In addition, two more test cases are provided: aviation fuels and diesel fuels, to predict sooting tendency using HD to support our concept. The prediction of the sooting propensity based on HD was found in excellent agreement with the measurements.

Effects of the Bluff-Body Diameter on the Flow-Field Characteristics of Non-Premixed Turbulent Highly-Sooting Flames

ABSTRACT This paper presents a joint experimental and computational study of the effect of the bluff-body diameter on the flow field and residence time distribution (RTD) in a set of turbulent non-premixed ethylene/nitrogen flames with a high soot load. A novel optical design has been developed to undertake the PIV measurements successfully in highly-sooting turbulent flames, using polarizing filters. The mean velocity components and turbulent intensity are reported for three bluff-body burners with different bluff-body diameters (38, 50, and 64 mm), but which are otherwise identical in all other dimensions. The central jet diameter of 4.6 mm was supplied with a mixture of ethylene and nitrogen (4:1, by volume) to achieve a bulk Reynolds number of 15,000 for the reacting cases. Isothermal cases were also investigated to isolate the effect of heat release on the flow fiel. Pure nitrogen was utilized in the isothermal cases where the Reynolds number was kept the same as the reacting cases. The annular bulk velocity of the co-flowing air was kept constant at 20 m/s for all experiments. Computationally, a 2-D RANS model was developed, validated against the experimental data and was mainly used to investigate the effect of the bluff body diameter on the residence time in the recirculation zone. The flow structure for both isothermal and reacting cases was found to be consistent with the literature, exhibiting similar vortical structures of the recirculating zone and mixture fraction distribution, for similar momentum flux ratios. The flame length and volume were found to decrease by 20% and 9%, respectively, as the bluff diameter was increased from 38mm to 64mm. The length of the recirculation zone for the isothermal cases was found to be ~1.2DBB, while for the reacting cases it was ~1.5–1.75DBB. A stochastic tracking model was employed to estimate the pseudo-particles’ residence time distribution in the recirculation region. The model revealed that an increase in the bluff body diameter from 38mm to 64 mm leads to tripling of the mean residence time within the recirculation zone. Thermal radiation measurements from the recirculation zone show a 35% increase as the bluff body diameter is increased from 38 mm to 64 mm, whilst the total radiation from the whole flame drops by 15%, which is deduced to be due mostly to the decrease in flame volume. The effect of these differences on soot propensity and transport are described briefly and will be the subject of future investigations.

Evolution of the soot particle size distribution along the centreline of an n-heptane/toluene co-flow diffusion flame

A newly developed experimental set-up for studying liquid hydrocarbon combustion in the well-established Yale burner was used to investigate the correlation between fuel composition and its sooting propensity. Soot particle size distributions (PSDs) and flame temperatures along the centreline of an n-heptane/toluene co-flow diffusion flame are reported. The results are compared to soot temperature and volume fraction profiles obtained using colour ratio pyrometry. The addition of toluene (0, 5, 10, and 15 mol%) to heptane moved soot inception to lower heights above the burner (HAB). The earlier inception extended the soot growth zone in the toluene-laden flames, leading to larger soot primary and agglomerate particles. Toluene addition had little influence on the maximum soot number density, indicating that the observed increase in soot volume fraction can mainly be attributed to the increase in particle size. The reported PSDs inside a vapour-fed diffusion flame are the first of their kind and provide a comprehensive dataset for future studies of combustion chemistry and soot particle models.

Radiant fraction from sooting jet fires

The objective of this article is to investigate numerically the radiative structure of methane, ethylene and acetylene lab-scale jet flames ranging from the transitional to the momentum-driven regimes. The numerical model involves a hybrid flamelet/transported PDF method coupled to an acetylene-benzene soot production model and a Wide-Band Correleted-K gas radiation model. Model predictions in terms of mean and rms soot volume fraction and temperature, integrated soot volume fraction and radiant fraction are in reasonable agreement with the available experimental data. In particular, the model reproduces quantitatively the decrease in radiant fraction observed as the flow becomes momentum driven. This behavior results mainly from two mechanisms: (i) an increase in flame self-absorption due to an enhancement in flame volume and (ii) for the ethylene and acetylene flames a reduction in the soot emission per unit flame volume owing to a strong decrease in soot production. In addition, for a given fuel, gas emission per unit flame volume remains approximatively constant as the exit strain rate increases whereas the soot emission per unit flame volume and the characteristic soot volume fraction scale with the Kolmogorov time scale. It was also found that competitive mechanisms govern the effects of Turbulence-Radiation Interaction (TRI) on radiant fraction. Enhancement mechanisms are due to gas emission TRI and temperature self-correlation effects on soot emission whereas inhibiting mechanisms results from the negative correlation between soot volume fraction and temperature. Enhancement mechanisms dominate in weakly sooting methane flames and taking TRI into account increases the radiant fraction. On the other hand, inhibiting mechanisms become significant in moderately and highly sooting fuels, with their importance increasing with both the fuel sooting propensity and the Reynolds number. For flames dominated by soot radiation, the inhibiting mechanisms prevail and taking TRI into account reduces the radiant fraction.

Experimental investigation on alternative fuel combustion performance using a gas turbine combustor

In order to ensure reliable combustion performance and low emissions profiles for prospective alternative fuels, this study was conducted in order to evaluate the impact of fuel properties and their composition on lean blowout performance and emission characteristics of various fuels. Significant contributions to knowledge have been achieved using visual images of the flames at the occurrence of lean blowout (LBO) that were captured by a high-speed camera and analysed to develop a new model for the evaluation of the LBO of current and future alternative fuels.The results show that high-density and high aromatic content fuel has the potential to produce higher soot formation. In addition, a low lean blowout equivalence ratio and low soot propensity is unlikely to be achieved at the same time by simply altering the value of fuel properties. It was discovered that the Derived Cetane number was observed to have a considerable effect on improving the LBO performance without inducing heavier soot emissions. An analysis of the impact of the fuel aromatic content (type and proportion) presents a hypothesis that not all aromatics species produce the same levels of particulate matter emissions.This work provides a comprehensive data foundation, analytical concepts and potential research techniques for the application of expeditious fuel screening tools to assess the combustion behaviour of current fuels, and also facilitates the potential for further operational fuel development. The visualisation method used in this work could contribute towards developing applications for flame detection in fuel performance assessment.

Performance and emissions of a DISI engine fueled with gasoline/ethanol and gasoline/C-4 oxygenate blends – Development of a PM index correlation for particulate matter emission assessment

The increasing attention devoted to biomass-derived oxygenated molecules is among others related to their potential to reduce pollutant emissions at the exhaust of spark-ignition engines particularly when combined with the latest direct injection technologies. The impact of adding such compounds to gasoline on engine performances, however, strongly depends on the composition, the chemical structure and the physical properties of the considered additives. The formation of particulate matter (PM), whose emissions are subject to increasingly stringent standards, is especially influenced by such factors thus prompting the need for the formulation of predictive multi-parameter PM indexes helping to support the development of advanced and cleaner engine fuels. Within this context, the present work proposes to analyze the performance and emissions of a direct injection spark-ignition engine fueled with mixtures made of gasoline with five different oxygenated fuels under homogeneous injection condition. Ethanol and four C-4 oxygenates (1-butanol, butanone, butanal and methyl propionate) have been selected to be blended with gasoline in proportions of 10, 20 and 40 vol%. Engine performances and exhaust emissions have been characterized in terms of specific fuel consumption, flame development angle, fully developed combustion duration and concentrations of CO, HC, NOx and PM. While the addition of oxygenated fuels increases the specific fuel consumption proportionally to the mixture lower heating value, it tends to decrease the flame development angle while not significantly influencing the fully developed combustion phase. Besides, relatively unchanged CO and HC emissions have been observed contrary to NOx and PM releases which have been found to decline with increasing oxygenate contents. Measured particulate concentrations have been compared with the predictions of four PM and sooting indexes from the literature including the Particulate Matter Index (PMI), the Threshold Sooting Index (TSI), the Oxygen Extended Sooting Index (OESI) and the Fuel Equivalent Sooting Index (FESI). While the FESI has been found to be the most efficient to reproduce experimentally monitored trends, it has still been coupled with five different fuel properties and volatility metrics to propose, through a multi-parameter optimization procedure, a generic PM index correlation allowing reproducing well measured PM emissions. By this way, it has been shown that the heat of vaporization and the fuel energy content are key factors to be combined with a sooting propensity indicator so as to capture engine exhaust PM releases.

Downstream evolution of n-heptane/toluene flames in hot and vitiated coflows

The scenario of fuel injected into hot surrounds is found in a range of practical combustion applications. These flame conditions have been emulated using a jet-in-hot-coflow-burner using prevaporised n-heptane and mixtures of n-heptane and toluene, relevant to gasoline and diesel fuel surrogates. This paper reports measurements of six lifted, turbulent flames, with a constant jet flow of a prevaporised fuel/N2 mixture at 380 K into various hot and vitiated coflow conditions. Five of these flames issued into coflows generated by the combustion of different mixtures of ethylene/air and one had a coflow from a natural gas/air flame. Two n-heptane/toluene fuel blends were also measured to study the effect of soot propensity. Gas sampling, non-linear excitation regime two-line atomic fluorescence (NTLAF) and laser-induced incandescence (LII) were used to characterise the flames, investigate the mixing between the hot coflow and the surrounding air, and measure the flame temperature for the different coflow configurations. A comparison of results of the flames issuing into hot coflows is presented, indicating that the hottest flame is not associated with the coflow containing the highest concentration of O2, but with the minimum soot loading and, consequently, the minimum radiative heat loss. Subsequent numerical simulations of canonical opposed-flow flames demonstrate that the soot loading in the downstream region of the flames is strongly dependent on PAH formation in the hot coflow region and further analyses reveal the chemical pathways which are most impacted by small variations in hot coflow composition.

The blending effect on the sooting tendencies of alternative/conventional jet fuel blends in non-premixed flames

In order to study the blending effect on sooting tendencies for practical alternative/conventional jet fuel blends, the present study has investigated soot formation in non-premixed flames of Jet-A blended with three alternative jet fuels (AJFs), i.e., Hydroprocessed Esters and Fatty Acids from camelina (HEFA-Camelina), Fischer-Tropsch Synthetic Paraffinic Kerosene (FT-SPK), and Alcohol-to-Jet (ATJ). The smoke points of the AJF/Jet-A blends were measured in a wick lamp per ASTM D1322 standard, and their Threshold Sooting Indices were derived. Soot volume fraction profiles of the AJF/Jet-A fuel blends were also measured using the laser-induced incandescence diagnostics in the counterflow non-premixed flames. The effects of AJF blending ratio, strain rate, and reactant concentration on the soot formation were examined. The experimental results show that the Threshold Sooting Indices of the AJF/Jet-A blends exhibit a linear relationship with AJF mole fraction. While the maximum soot volume fractions of fuel blends show a fairly linear relationship with AJF blending ratio, the nonlinearity becomes prominent at high strain rates and low reactant concentrations. Comparing the three AJF/Jet-A fuel blends, the sooting tendencies are ranked as ATJ/Jet-A > FT-SPK/Jet-A > HEFA-Camelina/Jet-A. Correlations of the maximum soot volume fraction with aromatics content, hydrogen content, smoke point, and Threshold Sooting Index were all also examined. It is found that neither aromatics content nor hydrogen content can correlate with the maximum soot volume fraction for all the fuel blends at a given condition; conversely, smoke point shows a nonlinear correlation and Threshold Sooting Index shows an approximately linear correlation with the maximum soot volume fraction for all the fuel blends. It is suggested that the different sooting tendencies of AJF/Jet-A blends are mainly caused by the different sooting propensities of individual AJFs, which are strongly dependent on their distinct paraffinic compositions.

Soot formation in diluted laminar ethene, propene and 1-butene diffusion flames at elevated pressures

Soot formation characteristics of ethene, propene, and 1-butene, the most abundant unsaturated intermediates in thermal decomposition of paraffinic hydrocarbons, were investigated in laminar diffusion flames stabilized on a co-flow burner installed in a high-pressure combustion chamber with optical access. All three olefins were diluted with nitrogen to produce sooting but non-smoking diffusion flames at desired pressures. Pressure range was 1–2.5 bar with 1-butene, and 1–8 bar with propene and ethene. Upper pressure limits of 1-butene and propene were established by their respective vapour pressure characteristics. The spectral soot emission technique, in which radiation emitted by the soot within the flame was collected as line-of-sight intensity and spectrally resolved over the range 690–945 nm, was used to measure radially-resolved temperature and soot volume fraction. The carbon mass flow rates of the three fuels were kept constant at 0.505 mg/s to facilitate direct comparison among the fuels at elevated pressures. With the same dilution level, the sooting propensity increased from ethene to 1-butene as expected; however, the pressure sensitivity of propene and 1-butene differed significantly from that of ethene. Soot yields in both propene and 1-butene flames showed a much weaker dependence on pressure than the soot in ethene flames. In the decomposition of propene and 1-butene, allyl radical and 1,3-butadiene are known to form in critical quantities leading to formation of higher molecular growth species specifically six-membered ring aromatics, and presence of these simple aromatics is argued to play a role in lowering the pressure sensitivity of the soot in C3 and C4 olefin flames.

The distillation curve and sooting propensity of a typical jet fuel

Real jet fuels are complex mixtures of many organic components, some of which are aromatic compounds. Towards the high-temperature end of the distillation curve, some of the fuel components are multi-ring compounds. A small amount of these high molecular weight species in the fuel could impact soot nucleation in practical engines especially when the fuel is injected as a spray. This work aims to highlight the variation of the sooting propensity of jet fuels as a function of distillate fractions and to examine the validity of a surrogate fuel in emulating soot production from real fuels. Particle size distribution functions and soot volume fractions are studied in a series of laminar premixed stretch-stabilized ethylene flames doped with Jet A, its various distillate fractions, and the 2nd generation MURI surrogate. Soot formation as a result of doping real jet fuel and its distillate fractions is also investigated in counterflow and coflow diffusion flames. The results show that the higher-boiling distillates mostly influence soot nucleation and produce substantially more soot in nucleation controlled flames than the light molecular fraction and jet fuel as received, while such an effect is seen to be small in flames where soot production is controlled by surface growth. The potential impact of distillate fractions on soot nucleation propensities is discussed.

Gas and soot formed in the dimethoxymethane pyrolysis. Soot characterization

The many simultaneous processes occurring within in a diesel engine make difficult a thorough understanding of the mechanisms responsible for reduction of soot and/or NOX when an oxygenated compound is added to diesel fuel. Thus, in order to explore the use of oxygenated compounds as biofuels/additives, it is interesting to study their conversion under well-controlled laboratory conditions, together with kinetic studies that help to interpret and understand the reaction schemes that occur during such processes. The aim of this work has been to contribute to the knowledge of the dimethoxymethane (DMM) pyrolysis, one of the oxygenated compounds proposed in literature as alternative fuel. In this way, the influence of pyrolysis temperature (1075–1475 K) and inlet fuel concentration (33,333 and 50,000 ppm DMM) on the sooting propensity of DMM, soot reactivity and its properties is analyzed. Therefore, this work includes pyrolysis experiments under different experimental conditions, focusing on the gas-phase analysis and the soot formation, together with a gas-phase model. Additionally, the interaction of soot with O2 and with NO has been studied, and since soot properties are important on the oxidation rate, selected soot samples have been characterized by different instrumental techniques (elemental analysis, physical adsorption with N2, Transmission Electron Microscopy (TEM), X-Ray Diffraction (XRD), and Raman spectroscopy).

Measurements of sooting limits in laminar premixed burner-stabilized stagnation ethylene, propane, and ethylene/toluene flames

The systematic investigation of the effect of two major experimental parameters (equivalence ratio and temperature) as well as fuel structure on the sooting limits of ethylene, propane, and ethylene/toluene flames was carried out in premixed burner-stabilized stagnation flames through qualitatively visual observations of flame yellow luminosity and quantitatively direct measurements of incipient soot particle size distributions by a scan mobility particle sizer. A strong temperature dependence of incipient soot formation was observed in the three flames. Detailed chemical kinetic modeling of the flame structure and the soot precursor chemistry was performed to explain the experimental observations. The results show that the incipient soot formation is prohibited in either low or high temperature flames. While the former is due to slow reactions induced by both low temperatures and small concentrations of soot precursors, the latter is attributed to the thermodynamic reversibility of polycyclic aromatic hydrocarbons (PAHs) at high temperatures. Sooting limits have been found to be not a function of concentrations of PAHs alone at a fixed flame temperature; sooting propensity generally increases with increasing fuel carbon/hydrogen atom ratio, presenting the following order: propane < ethylene < ethylene/toluene.

Ignition delay time and sooting propensity of a kerosene aviation jet fuel and its derivative blended with a bio-jet fuel

Ignition delay time and sooting index of kerosene blended with a bio-jet fuel is measured for a comparative study with general aviation fuels. The new blended fuel is similar to a kerosene jet fuel (Jet A-1 or Korean domestic jet fuel) in terms of properties, H/C ratio, density, and heat of combustion. But, its ignition characteristics and sooting propensity are different from those of Jet A-1. Ignition delay time is measured by a shock tube and it is found that the blended fuel of kerosene and a bio-jet fuel has NTC (negative temperature coefficient) behavior. Ignition delay times of the blended fuel are compared with those of jet fuels (Jet A-1 and Jet A) over a wide range of temperature from 700 K to 1200 K at 20 atm. The blended fuel has shorter ignition delay time at low temperature below 900 K. It can be explained by short ignition delay of components in a bio-jet fuel at low temperature. In terms of sooting propensity, blending with a bio-jet fuel reduces the propensity remarkably to a half.

Analysis of Soot Propensity in Combustion Processes Using Optical Sensors and Video Magnification.

Industrial combustion processes are an important source of particulate matter, causing significant pollution problems that affect human health, and are a major contributor to global warming. The most common method for analyzing the soot emission propensity in flames is the Smoke Point Height (SPH) analysis, which relates the fuel flow rate to a critical flame height at which soot particles begin to leave the reactive zone through the tip of the flame. The SPH and is marked by morphological changes on the flame tip. SPH analysis is normally done through flame observations with the naked eye, leading to high bias. Other techniques are more accurate, but are not practical to implement in industrial settings, such as the Line Of Sight Attenuation (LOSA), which obtains soot volume fractions within the flame from the attenuation of a laser beam. We propose the use of Video Magnification techniques to detect the flame morphological changes and thus determine the SPH minimizing observation bias. We have applied for the first time Eulerian Video Magnification (EVM) and Phase-based Video Magnification (PVM) on an ethylene laminar diffusion flame. The results were compared with LOSA measurements, and indicate that EVM is the most accurate method for SPH determination.