Sooting Propensity Research Articles

A novel atom tracking algorithm for the analysis of complex chemical kinetic networks

A novel numerical algorithm is presented that enables the tracking of specific atoms during the simulation of simple combustion reactors as those atoms are transferred from initial reactants to final products. Tracking is performed by labeling individual atoms or groups of atoms of interest, and solving, in addition to the coupled ordinary differential equations describing the time evolution of species concentrations, appropriate transfer equations providing the concentration of tracked atoms at each possible location on all chemical species involved. The transfer equations for a given chemical kinetic mechanism are automatically generated using simple structural and energy-based arguments to characterize and quantify how, in each elementary reaction, atoms are re-organized as reactants are converted into products. The capabilities of the tracking algorithm are illustrated by analyzing soot precursors formation in a constant-volume homogeneous reactor, providing new, quantitative evidence of well-known behaviors, such as the link between the molecular structure of a fuel and its sooting propensity.

The effect of soot modeling on thermal radiation in buoyant turbulent diffusion flames

Radiative impact of buoyant turbulent diffusion flames is the driving force in fire development. Radiation emission and re-absorption is controlled by gaseous combustion products, mainly CO2 and H2O, and by soot. Relative contribution of gas and soot radiation depends on the fuel sooting propensity and on soot distribution in the flame. Soot modeling approaches incorporated in big commercial codes were developed and calibrated for momentum-dominated jet flames, and these approaches must be re-evaluated when applied to the buoyant flames occurring in fires. The purpose of this work is to evaluate the effect of the soot models available in ANSYS FLUENT on the predictions of the radiative fluxes produced by the buoyant turbulent diffusion flames with considerably different soot yields. By means of large eddy simulations, we assess capability of the Moss-Brooks soot formation model combined with two soot oxidation submodels to predict methane- and heptane-fuelled fires, for which radiative flux measurements are available in the literature. We demonstrate that the soot oxidation models could be equally important as soot formation ones to predict the soot yield in the overfire region. Contribution of soot in the radiation emission by the flame is also examined, and predicted radiative fluxes are compared to published experimental data.

Stochastic Eulerian field method for radiative heat transfer in a propane oxygen-enhanced turbulent diffusion flame

The stochastic Eulerian field (SEF) method is applied to solve the transport equation of the joint probability density function (PDF) of mixture fraction, enthalpy defect, scalar dissipation rate and soot quantities in order to predict radiative heat transfer from sooting turbulent jet diffusion flames. The flow field is computed by solving the Favre-averaged Navier–Stokes equation coupled with the standard k-ϵ model and a steady laminar flamelet (SLF) model. The modelling of soot production is carried out by using a flamelet-based semi-empirical acetylene/benzene soot model. Radiative heat transfer is modelled by using a wide-band correlated-k model and turbulent radiation interactions (TRIs) are accounted for by using the optically thin fluctuation approximation (OTFA). The model is applied to simulate an oxygen-enhanced jet turbulent diffusion flame fuelled by propane. Predicted soot volume fraction, radiant wall heat flux distributions and radiant fraction are in good agreement with the experimental data. A special emphasis was put on the evolution of the cross correlations between soot volume fraction and temperature and mixture fraction and enthalpy defect over the flame. Model results show that the former is strongly negative in regions where soot radiation is significant, which may affect turbulence–radiation interaction. The analysis of the latter showed that mixture fraction and enthalpy defect are strongly correlated over the flame. Propane flames with the same injection characteristics and different oxygen index were considered, showing that the conclusions drawn for these correlations can be generalised to flames with different sooting propensity.

Soot formation characteristics of diffusion flames of methane doped with toluene and n-heptane at elevated pressures

Laminar co-flow diffusion flames of methane doped with n-heptane and toluene were studied experimentally to assess the sooting characteristics of two liquid fuels with increasing pressure. Experiments were conducted in the high-pressure combustion chamber that had been used previously for high-pressure soot formation studies in laminar diffusion flames. Either toluene or n-heptane was added to the methane such that 7.5% of the total carbon would be from the liquid fuel so that the results could be used to infer the pressure dependence of sooting propensities of the two liquid fuels. Pressure range was from atmospheric to 8 atm for methane and methane+n-heptane flames, whereas for methane+toluene mixture it was from atmospheric to 6 atm. A constant carbon mass flow rate of 0.41 mg/s for the three fuels was maintained at all pressures to have tractable measurements. Visible flame heights, as marked by the luminous soot radiation, were constant at all pressures except for methane at 1 atm. Variation of the maximum soot volume fractions, maximum soot yields, and the line-of-sight averaged soot temperatures of the three flames, pure methane, toluene-doped methane, and n-heptane-doped methane, with pressure were evaluated from soot spectral emission measurements which were collected line-of-sight but converted to radially-resolved values by using an Abel type inversion algorithm assuming axisymmetry of the laminar diffusion flames. Maximum soot volume fractions and maximum soot yields in n-heptane- and toluene-doped flames showed the higher sooting propensity of toluene in comparison to n-heptane at elevated pressures. Sooting propensity, in terms of both maximum soot yield and maximum soot volume fraction, of the methane+toulene flame displayed a relatively weaker dependence on pressure as compared to those of methane and methane+n-heptane mixture.

Effects of methyl group on aromatic hydrocarbons on the nanostructures and oxidative reactivity of combustion-generated soot

The substituted and unsubstituted aromatic hydrocarbons, present in transportation fuels such as gasoline and diesel, are thought to be responsible for most of the soot particles produced during their combustion. However, the effects of the substituted alkyl groups on the aromatic hydrocarbons on their sooting tendencies, and on the physical and chemical properties of soot produced from them are not well understood. In this work, the effect of the presence of methyl groups on aromatic hydrocarbons on their sooting propensity, and on the oxidative reactivity, morphology, and chemical composition of soot generated from them in diffusion flames is studied using benzene, toluene, and m-xylene as fuels. Several experimental techniques including high resolution transmission electron microscopy and X-ray diffraction are used to identify the morphological changes in soot, whereas the elemental and thermo-gravimetric analyses, electron energy loss spectroscopy, and Fourier transform infrared spectroscopy are used to study the changes in its chemical properties and reactivity. The activation energies for soot oxidation are calculated at different conversion levels, and a trend in the reactivity of soots from benzene, toluene and m-xylene is reported. It is observed that the sizes of primary particles and graphene-like sheets, and the concentrations of aliphatics and oxygenated groups in soot particles decreased with the addition of methyl group(s) on the aromatic ring. The physicochemical changes in soot are found to support the oxidative reactivity trends.

Transported scalar PDF modeling of oxygen-enriched turbulent jet diffusion flames: Soot production and radiative heat transfer

Turbulent jet flames fueled either by propane or a methane/ethylene blend and burning under different oxygen indexes (OI) in the range from 21% (air) to 100% (pure oxygen), studied experimentally by Wang et al. (2002), were simulated by using an hybrid flamelet/Stochastic Eulerian Field (SEF) method, an acetylene/benzene-based two-equation soot model, and a wide-band correlated-k (WBCK) method to model the spectral dependence of the absorption coefficient of the gaseous radiatively participating species and soot. Emission Turbulent Radiation Interactions (TRIs) are taken into account by means of the PDF method, whereas absorption TRIs are modeled using the optically-thin fluctuation approximation (OTFA). Model predictions in terms of soot volume fraction, radiant heat flux distribution along the wall of the combustion chamber, and radiant fraction are compared with the experimental data of Wang et al. (2002), showing a reasonable agreement. The non-monotonic evolution of the soot production with the OI, increasing sharply when the OI is enhanced from 21% to about 40% and then being significantly reduced as the OI is further increased, and the effects of the fuel sooting propensity, with the propane flames producing much more soot than the methane/ethylene blend flames, are well reproduced by the numerical model. The non-monotonic influence of the OI on soot production can be explained by several competing mechanisms. The soot production tends to be enhanced owing to, on the one hand, the increase in temperature and, on the other hand, a positive feedback between surface growth process and soot surface area observed for moderate level of oxygen enrichment. Oppositely, the soot production tends to be reduced due to a decrease in residence time and an enhancement in the oxidation of both soot precursors and soot particles.

Simulations of sooting turbulent jet flames using a hybrid flamelet/stochastic Eulerian field method

The stochastic Eulerian field method is applied to simulate 12 turbulent C1−C3 hydrocarbon jet diffusion flames covering a wide range of Reynolds numbers and fuel sooting propensities. The joint scalar probability density function (PDF) is a function of the mixture fraction, enthalpy defect, scalar dissipation rate and representative soot properties. Soot production is modelled by a semi-empirical acetylene/benzene-based soot model. Spectral gas and soot radiation is modelled using a wide-band correlated-k model. Emission turbulent radiation interactions (TRIs) are taken into account by means of the PDF method, whereas absorption TRIs are modelled using the optically thin fluctuation approximation. Model predictions are found to be in reasonable agreement with experimental data in terms of flame structure, soot quantities and radiative loss. Mean soot volume fractions are predicted within a factor of two of the experiments whereas radiant fractions and peaks of wall radiative fluxes are within 20%. The study also aims to assess approximate radiative models, namely the optically thin approximation (OTA) and grey medium approximation. These approximations affect significantly the radiative loss and should be avoided if accurate predictions of the radiative flux are desired. At atmospheric pressure, the relative errors that they produced on the peaks of temperature and soot volume fraction are within both experimental and model uncertainties. However, these discrepancies are found to increase with pressure, suggesting that spectral models describing properly the self-absorption should be considered at over-atmospheric pressure.

Soot formation in non-premixed counterflow flames of butane and butanol isomers

Soot formation processes for butane and butanol isomers have been experimentally investigated and compared in a counterflow non-premixed flame configuration at atmospheric pressure. Non-intrusive laser-based diagnostic techniques, including Laser Induced Incandescence and Light Extinction, have been adopted for soot volume fraction measurements. Experimental results of these C4 fuels have been compared to illustrate the effects of hydroxyl (–OH) group and the isomeric structures on soot formation process. Under the investigated conditions, butane isomers were observed to form more soot than butanol isomers, thereby showing the effect of the hydroxyl group. The effects of isomeric structural differences on sooting propensity were also observed within the butane and butanol isomers. In addition, while soot volume fraction was seen to increase with increasing fuel mole fraction, the ranking of sooting propensity for these C4 fuels remained unchanged. Effects of varying strain rate and oxygen mole fraction on soot formation were studied herein as well. For each isomeric class, two chemical kinetic models available in the literature were used to simulate and compare the spatially-resolved profiles of the soot precursors. The amount of soot precursors predicted by the models was different and the initial fuel breaking pathways were also found to differ significantly. Furthermore, no direct correlation between the computed mole fractions of soot precursors and the measured amount of soot formed in the present experimental conditions was observed.

Influence of diesel fuel blended with biodiesel produced from waste cooking oil on diesel engine performance

Biodiesel from edible waste cooking oil (waste cooking oil methyl ester – WCOME) has been produced via trans-esterification process. The effect of various fuel blends containing the conventional diesel fuel and the produced WCOME on diesel engine performance has been evaluated experimentally. The study investigated the effect of WCOME blending ratio on viscosity and sooting propensity of the biodiesel–diesel fuel mixture. The engine performance has been expressed in terms of the in-cylinder pressure data as well as the engine mechanical and environmental aspects measured at engine rated speed (1500rpm) and different engine loads. The main results of the current work showed that, the location and value of the in-cylinder peak pressure depends mainly on the engine load and the biodiesel blending ratio. The best value of Brake Specific Energy Consumption (BSEC) is attained at blended fuel containing 20% WCOME (B20) where the maximum brake thermal efficiency is also observed. While there was a range of blending ratio from B20 to B50 throughout the best engine environmental behavior is attained. Results indicated that, the use of neat biodiesel fuel (B100) at different engine loads leads to an increase of BSEC by about 8%, an increase of engine smoke opacity by about 15%, a decrease of NOx emissions by about 10% with slight decrease of CO emissions and a decrease of the unburned hydrocarbons by about 15%. The most recommended WCOME biodiesel blending ratios vary from 30% to 50% for better engine performance and emission characteristics. In this recommended blending ratios, the engine performance provides the following results in comparison with the corresponding values for neat fuel around 10% higher BSFC, insignificant change in ηbth, around 3% higher BSEC, and 2% lower TExh, while the corresponding engine emissions include 25% lower CO, 20% lower UHC, 6% lower NOx, and 20% higher smoke opacity.

Experimental and numerical study of soot formation in laminar coflow diffusion flames of gasoline/ethanol blends

This paper reports the experimental and numerical results of soot formation in laminar coflow diffusion flames of vaporized gasoline/ethanol blends at atmospheric pressure to gain improved understanding of ethanol addition to gasoline on soot formation. Four gasoline/ethanol blends of different ethanol mole fractions in the fuel stream ranging from 0% up to 85%, E0, E20, E50, and E85, were investigated to quantify how soot loading varies with the amount of ethanol blending in the fuel. The laminar coflow diffusion flames were generated using a burner system designed for vaporized liquid fuels. The fuel stream was heavily diluted with nitrogen to lower the boiling points and to prevent the flames from smoking. The soot volume fraction distributions were measured using a 2D line-of-sight attenuation technique. These flames were also modeled numerically using the extensively validated CoFlame code and a recently developed reaction mechanism for gasoline surrogates including PAH formation. The results of experiment and numerical modeling agree quite well in terms of the levels of soot volume fraction and they both show a decrease in the soot loading as more ethanol is added in the fuel stream. The measured peak soot volume fraction occurs in the flame centerline region. The soot model used in this study is unable to capture this experimental feature and predicts the peak soot volume fraction in the co-annular region. Ethanol addition reduces the soot loading primarily by lowering soot inception and PAH condensation rates through decreasing the concentrations of aromatics. Additional measurements in gasoline surrogate/ethanol blend flames indicate that the gasoline surrogate model emulates well the sooting propensity of the real gasoline fuel.

Effects of 2,5-dimethylfuran addition to diesel on soot nanostructures and reactivity

With the improvements in technologies to produce 2,5-dimethyfuran (DMF) from biomass such as cellulose, the potential of this biofuel to completely or partially replace fossil fuels in order to reduce our dependence on them and to curb pollutant emissions has been explored in some recent studies. In this work, the effect of the addition of DMF (up to 15% by volume) to diesel on the sooting propensity and on the oxidative reactivity and nanostructures of soot particles is studied using a diffusion flame. With increasing amount of DMF in the DMF/diesel blends, soot emission was found to reduce, while the oxidation reactivity of soot particles increased. The activation energies of soot oxidation from blended fuels were significantly lower than that of diesel soot. The increase in soot reactivity is linked with the changes in physical and chemical properties of soot nanostructures. Several experimental techniques such as high resolution transmission electron microscopy, electron energy loss spectroscopy, thermo-gravimetric analysis, elemental analysis, Fourier transform infrared spectroscopy, and X-ray diffraction have been used to identify the changes in structures, functional groups such as oxygenates and aliphatics, π and σ bonding, O/C and H/C ratios, and crystallite parameters of soot particles. With increasing DMF concentration in diesel, the sizes of primary particles and the polycyclic aromatic hydrocarbons (PAHs) comprising soot decreased, while the aliphatic character (sp3 hybridization) and the amount of oxygenated functional groups in it increased. The observations from soot characterization are used to explain the high susceptibility of soot particles from blended fuels toward oxidation.

Radiative emissions measurements from a buoyant, turbulent line flame under oxidizer-dilution quenching conditions

This work features the suppression of buoyant, turbulent, methane- and propane-fueled diffusion flames. Flames are stabilized above a 5×50cm2 slot burner surrounded by a co-flowing oxidizer. Nitrogen gas is added to the oxidizer to achieve suppression. Mean flame height, measured using a digital camera, increases with reducing oxidizer oxygen mole fraction (XO2), in agreement with scaling predictions. Visible emissions, measured using a photodiode, are found to decrease by six orders of magnitude with reducing XO2. This decrease is attributed to diminishing soot radiation, where sharp curves in the trends for both fuels coincide with changes in flame color from yellow to blue. Methane, but not propane, flames are found to experience a period of soot-free (blue) combustion prior to extinction. Infrared emissions are measured using a heat flux transducer and are interpreted using an infrared camera and multipoint radiation source model. Radiative loss fraction is found to decrease linearly with reducing XO2, where the slope of decline is affected by fuel sooting propensity. Flame extinction occurs as liftoff at XO2=0.151 for methane and XO2=0.138 for propane. An oxygen anchor, explored to resist liftoff, extended the flammable domain to XO2=0.130 for both fuels.

Analysis of the sooting propensity of C-4 and C-5 oxygenates: Comparison of sooting indexes issued from laser-based experiments and group additivity approaches

Oxygenated biofuels have received a particular attention during the last few years due to their ability to reduce soot emissions. Numerous experimental and numerical studies focusing on the sooting propensity of such alternative fuels especially revealed that the ability of oxygenates to limit the soot production was strongly related to their molecular structure in addition to their oxygen content. The objective of the present work is thus to analyze and quantify the potential of soot reduction of several C-4 and C-5 oxygenated molecules used as additives in a Diesel surrogate (the IDEA fuel composed of 70vol% n-decane and 30vol% α-methylnaphthalene). To do so, soot concentration measurements have been carried out by Laser-Induced Incandescence (LII) in 33 turbulent spray flames burning mixtures of IDEA with 12.5, 25 and 50vol% of various oxygenated additives including alcohols, aldehydes, ketones and esters. The different oxygenates have been selected to assess the impact of the position of the oxygenated functional group, the number of oxygen atoms and the type of CO link (single or double) on the production of soot. Based on the peak soot volume fractions measured in standardized jet flames, a Fuel Equivalent Sooting Index (FESI) has been inferred for each considered oxygenated additive and compared with corresponding Yield Sooting Index (YSI), Threshold Sooting Index (TSI) and Oxygen Extended Sooting Index (OESI) values measured and/or estimated based on structural group contribution methods. Obtained results show that all the tested oxygenates reduce the sooting propensity of the base fuel in which they are added. For a given number of carbon atom in the molecule, the sooting tendency of oxygenated compounds increases in the following order: esters<aldehydes<ketones<alcohols. Reasons explaining such trends have been discussed considering the different effects that may impact the soot formation (i.e. the chemical effect induced by the presence of oxygenated functional groups in the fuel mixtures and the dilution of the base fuel by the additives which remains the most important factor for all the considered molecules). Finally, FESI values derived from a previous database obtained by analyzing flames burning gasoline-surrogate/ethanol blends [1] have been confronted with recently published OESI values proposed in [2] for gasoline/ethanol blends.

Assessment of semi-empirical soot production models in C1–C3 axisymmetric laminar diffusion flames

The objective of this work is to assess the accuracy and limitations of three different semi-empirical soot models: an acetylene/benzene flamelet based two-equation soot model developed by Lindstedt (1994) [4] (in: H. Bockhorn, pp. 417–441) and two laminar smoke point (LSP) based soot models developed by Lautenberger et al. (2005) [15] (Fire Saf. J., 40, pp. 141–176) and by Yao et al. (2011) [14] (Fire Saf. J., 46, pp. 371–387). This task is performed by simulating twenty-seven C1–C3 hydrocarbon-air flames, consisting of normal (NDF) and inverse (IDF) diffusion flames and covering a wide range of residence times and fuel sooting propensities. The acetylene/benzene based soot model predicts the maximum soot volume fractions and the peaks of the integrated soot volume fraction within a factor of 2 of measurements. However, two pre-exponential factors for the surface growth process, one for the C1–C2 hydrocarbons and the other for the C3 hydrocarbons, were necessary in order to obtain this agreement. LSP soot model results are also within a factor of 2 of the measurements for most of the propane, ethylene, propylene and acetylene NDFs. For weakly sooting fuels, such as methane and ethane, the LSP concept overestimates soot production rates. In addition, the model of Lautenberger et al. was found to provide a better description of the soot production in the ethylene IDFs than that of Yao et al. Eventually, computed radiant fractions were compared with experimental data for propane, ethylene and propylene NDFs, showing that predictions using the models of Lindstedt and Lautenberger are on the whole within 10% of the experimental data.

Sooting propensities of some gasoline surrogate fuels: Combined effects of fuel blending and air vitiation

The sooting propensities of binary mixtures of n-heptane and toluene on one hand, and isooctane and toluene on the other hand are evaluated in atmospheric laminar diffusion flames. These hydrocarbons are indeed major components of any gasoline surrogate fuel. The sooting propensities are here measured in terms of Yield Sooting Indices (YSIs). To this end, two-dimensional maps of soot volume fraction are extracted using Light Extinction Method (LEM) applied to methane diffusion flames doped with the vapors of the aforementioned mixtures. Burning in a coflowing oxidizer stream, these flames are established over the Santoro’s axis-symmetric burner. The experimental setup allows the air stream to be diluted by carbon dioxide at a content normally encountered in internal combustion engines that use exhaust gas recirculation (EGR). The evolution of the sooting propensity as a function of both toluene mole fraction in the binary mixtures and CO2 content of the air stream can then be revealed. The sooting tendencies of both kinds of blends decrease in a very linear way with increasing CO2 content of the air stream. On the opposite, the sooting tendencies of isooctane/toluene blends follow a strongly non-linear relationship with the toluene mole fraction due to synergistic effects while YSIs of n-heptane/toluene blends show very linear trends. Interestingly, these trends match those observed on other configurations of diffusion flames, further strengthening the consistency of the YSI methodology. The combined effects of fuel blending and CO2 dilution are also exhibited. All these trends can be of great importance for further investigations as toluene and isooctane mole fractions found in commercially available gasoline and gasoline surrogate fuels are prone to affect soot formation through a strong synergistic effect as identified in current study.

Dependence of sooting characteristics and temperature field of co-flow laminar pure and nitrogen-diluted ethylene–air diffusion flames on pressure

Pressure dependence of sooting characteristics and the flame temperature field of pure ethylene and ethylene diluted with nitrogen in co-flow laminar diffusion flames was investigated experimentally. The pressure range for ethylene was from atmospheric to 7atm and to 20atm for nitrogen-diluted ethylene flames. Spectrally-resolved line-of-sight soot radiation emission measurements were used to obtain radially resolved temperatures and soot volume fractions by using an Abel type inversion algorithm. A constant mass flow rate of ethylene was maintained at 0.48mg/s at all pressures to match the carbon flow rates of gaseous alkane fuels experiments reported previously. Visible flame heights, as marked by the luminous soot radiation, initially increased with pressure, but changed little above 5atm. Maximum local soot volume fraction of ethylene flames seems to scale with pressure raised to the third power (about 2.8). This is argued to be a relatively stronger pressure dependence of maximum soot volume fraction as compared to other gaseous fuels. A similarly higher pressure dependence was observed when the maximum soot yields of ethylene and other gaseous fuels were compared. It was shown that the soot yield dependence of ethylene flames does not conform to the unified dependence on pressure which was demonstrated for gaseous alkane fuels recently. The sooting propensity of nitrogen-diluted ethylene flames was shown to be less than that of n-heptane flames diluted with similar amount of nitrogen. Flame temperature profiles and averaged temperatures of ethylene flames showed similar characteristics as the other gaseous fuels, however radial temperature gradients in ethylene flames were much higher than those in gaseous alkane fuel flames.

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.

Challenges and artifacts of probing high-pressure counterflow laminar diffusion flames

An experimental study was conducted on two C2H4/O2/N2 laminar diffusion flames with identical compositions and strain rate, but operated at 0.29MPa and 2.5MPa, respectively, to assess pressure effects on the onset of soot formation. The low-pressure flame was permanently blue, whereas the high-pressure one was under conditions of incipient sooting. Both flames had nominally identical temperature–time history. Using gas sampling through quartz microprobes followed by GC/MS analysis and addressing potential artifacts associated with the diagnostic intrusiveness and probe-induced chemistry, we demonstrated that the flame structure was resolved even at the highest pressures, as shown by the good agreement with respect to major species between the experimental results and a one-dimensional computational model of the flame with detailed chemistry and transport. The increase in soot propensity correlates with an increase in the concentration of soot precursors like the aromatics and acetylene. Comparison with model predictions shows that the concentration of aromatics is systematically overpredicted. An ancillary computational study to estimate the sensitivity to changes in the velocity boundary conditions showed that minor species like the aromatics are sensitive to these details under certain conditions, because of changes in the temperature–time history that affects predominantly the slow chemistry.

Sooting tendencies of primary reference fuels in atmospheric laminar diffusion flames burning into vitiated air

In this study, the sooting tendencies of primary reference fuels (PRFs) are measured in term of yield sooting indices (YSIs) in methane diffusion flames doped with the vapors of PRFs. The present paper represents an incremental advance complementing the original methodology prescribed by McEnally and Pfefferle. The influence of both PRF formulation and CO2 dilution of the coflowing air on the YSIs is also assessed. The diffusion flames burning in a coflowing oxidizer stream are established over the Santoro’s burner and vapor of the liquid fuel to be investigated is injected into the fuel stream. Laser extinction measurements are performed to map the two-dimensional field of soot volume fraction in the flame. For the pure liquid hydrocarbons investigated, i.e., n-hexane, n-heptane, isooctane, and benzene, the YSI reported in the original paper by McEnally and Pfefferle quantitatively predict the sooting propensities, measured here at much higher dopant concentrations. The present study therefore extends the consistency of the YSI methodology on the Santoro’s burner. For blends of n-heptane and isooctane, the sooting tendency of doped flames exhibits regular and monotonic trends and decreases with increasing n-heptane mole fraction or CO2 dilution. Interestingly, the evolution of YSI with the isooctane mole fraction exhibits a strong similarity for varying CO2 mole fraction. A quadratic least-squares fit is then derived, providing a phenomenological model of YSI as a function of both isooctane mole fraction in the fuel stream and CO2 mole fraction in the oxidizer. A non-negligible cross effect of PRF formulation and CO2 dilution on YSI is revealed. The method elaborated within the framework of the present paper could be extended to surrogate fuels. This would help develop a comprehensive database and empirical correlations that could predict the sooting propensities of different surrogate fuels, therefore their potentially mitigationed soot production through control of fuel composition and/or exhaust gas recirculation. This database would also be useful for the validation of CFD simulations incorporating sophisticated model of soot production.