Threshold Sooting Index Research Articles

Development of two diesel surrogates focusing on the soot emission under engine-like conditions

ABSTRACT Threshold sooting index (TSI) is one of the critical properties related to the soot emission of diesel. Investigating the effects of TSI on the prediction of soot emission and the response of the soot model to TSI may improve our understanding of the soot formation process and model establishment during the combustion of diesel under engine-like conditions. To meet this need, two different d.iesel surrogates, named S1 and S2 were developed. S1 comprised n-dodecane, iso-octane, methylcyclohexane, and 1-methylnaphthalene, while S2 replaced 1-methylnaphthalene with toluene. Both were adjusted for similar cetane numbers, H/C ratios, and heating values, differing in TSI to analyze sooting tendencies to ensure the S1 and S2 had different soot tendencies but close ignition quality. Then, the skeletal mechanism containing the kinetics of the components in S1 and S2 was developed and validated against ignition delay times, speciation, laminar flame speeds and the 3-D spray combustion with results agreeing well with predictions. Finally, the soot performances of S1 and S2 in different soot models were compared in the constant volume combustion chamber (CVCC). The results showed that the comparison of S1 and S2 in different soot models could promote the understanding of soot models.

Formulation of a Jet Fuel Surrogate and Its Kinetic Chemical Mechanism by Emulating Physical and Chemical Properties of Real Jet Fuel

The application of jet fuel in gas turbines and diesel engines adheres to the Army’s single-fuel forward policy, streamlining supply chains. To ensure precise engine combustion numerical studies, surrogate fuels and mechanisms should faithfully replicate real fuel properties and combustion traits. In this work, a new four-component jet fuel surrogate containing 39.05% n-dodecane/21.79% isocetane/11.49% decalin/27.67% toluene by mole fraction is formulated based on a property optimizer. The new-formulated fuel surrogate can satisfactorily emulate the chemical and physical properties of real jet fuel, including cetane number (CN), threshold sooting index (TSI), molecular weight (MW), lower heating value (LHV), the ratio of hydrogen and carbon (H/C), liquid density, viscosity, and surface tension. Furthermore, a reduced and robust kinetic chemical mechanism (containing 124 species and 590 reactions) that could be directly employed in practical engine combustion simulations has also been developed for the proposed surrogate jet fuel. The mechanism is validated through comprehensive experimental data, including ignition delay time (IDT) determined in shock tubes and rapid compression machines (RCMs), species mole fractions measured in premixed flames and jet stirred reactors (JSRs), and laminar flame speeds. Generally, the property deviations of the jet fuel surrogate are less than 2% except for MW (10.73%), viscosity (5.88%), and surface tension (8.71%). The comparison results between the predictions and measurements are in good agreement, indicating that the current kinetic mechanism is capable of reflecting the oxidation process of real jet fuel. The current mechanism can accurately capture variations in the ignition delay time in the negative temperature coefficient (NTC) region as well. In the future, the proposed surrogate jet fuel could be applied in practical engine computational fluid dynamic (CFD) simulations.

A simple method for the quantitative assessment of soot production rate

Sooting tendency has been characterized with various empirical approaches such as smoke height and Threshold Sooting Index (TSI), that can be regarded as qualitative, as well as Yield Sooting Index (YSI), that is semiquantitative since it relies on the measurements of at least the peak soot volume fraction in the flame. All these techniques have the convenience of being easy to implement and of relying on inexpensive equipment. In the present work we present a comparatively easy but quantitative alternative to determine the soot production rate in counterflow flames. The approach is rooted in the use of: a) soot volume fraction measurements by pyrometry, b) the well established one-dimensional computational modeling of such flames for the determination of temperature and velocity profiles and c) the use of the soot governing equation. The technique is applied to several aliphatics, including methane, propane, ethylene, propene and acetylene. Soot production rate per unit flame area for the tested aliphatics ranges between 10−4 and 10−7 g/(cm2s) and, when normalized with respect to the carbon flux, between 10−5 and 10−2. On a logarithmic scale it correlates linealry with the peak temperature for all fuels. Soot yield scales as alkanes< alkenes < alkynes, with acetylene showing the highest sooting tendency even in flames at relatively low temperatures.

Predicting the physical and chemical properties of sustainable aviation fuels using elastic-net-regularized linear models based on extended-wavelength FTIR spectra

A Fourier transform infrared (FTIR) spectra-based prescreening approach was developed for estimating nine important physical and chemical properties of sustainable aviation fuels (SAFs): molecular weight (MW), hydrogen-to-carbon (H/C) ratio, density, net heat of combustion (NHC), derived cetane number (DCN), threshold sooting index (TSI), initial boiling point (IBP), flash point, and kinematic viscosity (KV). A training dataset containing the vapor-phase FTIR absorption spectra of pure hydrocarbons spanning four molecular classes, blends of neat hydrocarbons, conventional jet fuels and SAFs, across the 2–15.38 μm wavelength range, was compiled. The physical and chemical properties of the fuels in the training dataset were sourced either from experimental data reported in the literature or calculated using relevant property blending correlations. Elastic-net-regularized linear models were trained for each property by optimizing the model parameters using a cross-validated grid search. The results from these models were compared with the results from previous models (Lasso-regularized linear models developed previously by Wang et al. at Stanford), which were trained on FTIR absorption spectra across the limited wavelength range of 3.3–3.55 μm. Use of the extended wavelength range and the new model-parameter optimization strategy results in significant improvement in predictive performance of the current models compared to the previous models for all nine properties. The model performance is also evaluated on three candidate SAFs. The current models are found to achieve higher property prediction accuracy than the previous models for these test fuels. Overall, this work demonstrates the utility of predictive models based on extended-wavelength FTIR spectra and their potential use as a low-volume prescreening method for characterizing properties of new sustainable aviation fuel candidates.

Formulation of Surrogates of Hydrocarbon Fuels Using Selected Physico-Chemical Properties Related to Atomization, Heating, Evaporation and Combustion Behaviours

ABSTRACT The aim of this work is to describe an approach to the formulation of surrogates of hydrocarbon fuels, taking into account the properties related to atomization, heating, evaporation and combustion behaviors. These properties include the density, viscosity, lower calorific value, distillation curve, saturated vapor pressure, heat capacity, thermal conductivity, surface tension, molar ratio of hydrogen to carbon, molar mass, derived cetane number, and threshold soot index. Three diesel surrogates were developed using the proposed approach. Methods for calculating the physicochemical properties of both individual hydrocarbons and their mixtures are presented. The discrepancy between calculated results and experimental data for all properties is shown not to exceed 3%. Experimental studies were carried out in a lab-scaled combustion chamber, in which the properties of one of the three developed surrogates, DSSU3, were compared with those of diesel fuel. Also, the simulation results for DSSU3 were compared with published experimental data. It is shown that the properties of DSSU3 are a good match for those of diesel fuel. In addition, a kerosene surrogate was formed using the proposed approach. This allowed us to conclude that this approach to determining the properties of multi-component fuels is suitable for the formulation of both kerosene and diesel fuel surrogates.

Study of Sooting Propensity and Related Indices Based on Tests With Isolated Droplets

Abstract This work compares two approaches to characterize the sooting propensity of liquid fuels, namely, the Threshold Soot Index (TSI) obtained from smoke point measurements and the Isolated Droplet Soot Yield (IDSY), an original metric expressing the mass of soot collected from single droplets vaporizing under a hot atmosphere representative of conventional flames. The TSI of different fuels was experimentally obtained by means of a computer-vision method, whereas their IDSY was extracted using a free-falling droplet combustion facility. The results clearly pointed to a linear dependency of the IDSY with mass fraction, allowing to propose a mixture rule for this index for a significantly broader measuring range than that allowed by the smoke point lamp. A nonlinear correlation was found between IDSY and TSI, with relevant deviations for some compounds. These deviations could advise to consider which of the available indices (TSI, IDSY, others) would be more adequate to emulate the sooting behavior.

An experimental study on the laminar burning velocities of RP-3 kerosene and its surrogate fuel at elevated pressures and temperatures

This work develops a new surrogate fuel for RP-3 kerosene. Based on five optimizing targets (molecular weight, hydrogen/carbon ratio, cetane number, lower heating value and threshold sooting index) to emulate the chemical and physical properties of RP-3 kerosene, the surrogate fuel was determined to contain 22% n-dodecane, 15% iso-cetane, 48% decalin and 15% 1,2,4-trimethylbenzene. The laminar burning velocities of RP-3 kerosene and its surrogate fuel were investigated at the unburnt temperature of 413, 443, 473, 503 K, pressure of 1, 2, 5 bar and equivalence ratio of 0.6 ∼ 1.6 in a constant volume combustion bomb. The results show that the laminar burning velocities of the surrogate fuel were in good agreement with that of RP-3 kerosene. Meanwhile, a kinetic model was adopted, which well predicted the laminar burning velocities of RP-3 kerosene and the surrogate fuel. Finally, the flame propagation process and flame instability of RP-3 kerosene under elevated temperatures and pressures conditions were analyzed. It was found that the surrogate fuel was slightly different from RP-3 kerosene in terms of the combustion instability under fuel-rich conditions.

Fundamental insights into the effect of blending hydrogen flames with sooting biofuels

A major challenge to using hydrogen as a carbon-free energy carrier to replace fossil fuels in high-temperature practical processes is the reduced thermal radiative transfer of hydrogen flames due to the absence of soot. To address the potential issue of low radiative heat transfer from hydrogen flames, the effect of blending highly sooting biofuels to turbulent nonpremixed hydrogen-based flames on flame luminosity and radiant heat flux enhancement is investigated in this paper. The sooting propensities of aromatic bio-oil surrogates (toluene, anisole and guaiacol) and monoterpene essential oil surrogates (eucalyptol and D-limonene) are evaluated. The Threshold Sooting Index (TSI) and Oxygen Extended Sooting Index (OESI) are calculated and compared with the data sets in the literature. The sooting propensities of the reported fuels are found to follow a decreasing order: aromatics > monoterpenes > alkenes > alkanes > aldehydes. The effect of chemical structure and functional group on sooting propensity tends to be more significant than the oxygen content and effective C/H ratio. Turbulent nonpremixed hydrogen–nitrogen (9:1 mol) jet flames are blended with 0.2 and 1 mol% (based on the molar concentration of hydrogen) vapourised biofuels. The results show that adding 0.2 and 1 mol% vapourised biofuels to the hydrogen-based flame non-linearly improves the flame luminosity and radiant fraction by 61%–253% and 2%–19%, respectively. Toluene is the most effective additive in luminosity and radiant fraction enhancement among all tested biofuels. Adding biofuels with an aromatic structure increases the radiant fraction by 10%–19%, which is more effective than cyclic monoterpenes (2%–9%). Adding non-oxygenated biofuels generally have larger increases in radiant fraction (9%–19%) than oxygenated biofuels (2%–10%).

Fuel oxygenation as a novel method to reduce sooting propensity of fuels: An investigation with gasoline surrogate fuels

Blending transportation fuels like gasoline with biofuels reduce our reliance on fossil fuels and pollutant emission due to the presence of fuel-bound oxygen. However, biofuel availability is limited to regions with an abundance of biomass and land for energy crops. To gain the advantage of fuel-bound oxygen on emission reduction, this work explores the effect of replacing a fraction of soot-producing aromatics present in gasoline with oxygenated aromatics to check if a fuel formulation strategy, where a fraction of aromatics-rich stream in a refinery is catalytically oxygenated to form oxygenated aromatics before blending with other hydrocarbon streams to produce gasoline, could be advantageous in producing clean fuels. A gasoline surrogate containing 17% n-heptane, 47% isooctane, and 36% toluene, which closely predicted gasoline properties such as density, molecular weight, threshold sooting index, and octane number, is used for the experiments. To mimic the above fuel formulation strategy, a fraction of toluene in gasoline surrogate is replaced with oxygenated aromatics, benzyl alcohol and anisole that are structurally similar to toluene. The oxygenated blends showcased lower sooting tendency than the surrogate fuel. Soot particles, collected from the three fuel blends, are analyzed using TGA, XRD, HRTEM, and elemental analyzer, where soots from oxygenated blends are found to have higher reactivity with lower particle diameter and crystallite size. The results indicate that such fuel formulation can not only reduce soot emission, but also improve soot physicochemical properties to reduce its lifetime in the environment.

A machine learning model for predicting threshold sooting index (TSI) of fuels containing alcohols and ethers

In this work, a machine learning based model using artificial neural networks (ANN) was developed for the prediction of threshold sooting index (TSI) of fuels containing oxygenated chemical classes like alcohols and ethers, along with hydrocarbon classes such as paraffins, olefins, naphthenes, aromatics, and their mixtures. Experimental TSI data of 342 fuels including 124 pure compounds, 212 fuel surrogate mixtures and 6 gasolines was used as a dataset for developing the model. Ten features (eight functional groups, molecular weight (MW) and branching index (BI)) have been used as inputs in this model. The eight functional groups and the two structural parameters (MW and BI) represent the composition and structure of the fuel. The ANN model was trained, validated, and finally tested on randomly split sets of 70%, 15%, and 15% of the data, respectively. The observed regression coefficient (R2) between the real and predicted TSI values was 0.97 as obtained for the test set. The absolute error of prediction obtained was 2.46, which is promising as this number is closed to the uncertainty observed in experimental measurements. The results indicate that a fuel’s TSI is dependent on the fuel functional groups, and thus can be used as modeling criteria. The model can be applied towards the prediction of TSI of pure compounds, fuel surrogate mixtures and petroleum fuels containing alcohols and ethers.

Effects of the addition of a high energy density fuel, adamantane to diesel on its cetane number, sooting propensity, and soot nanostructural properties

Soot particles, released during diesel combustion, adversely affect human health and the environment. Some studies suggest blending diesel with oxygenated fuels to reduce soot emissions, though it also reduces fuel economy and may produce new pollutants such as aldehydes. Another method to reduce soot could be to increase its reactivity by altering its physicochemical properties (e.g., introducing curvatures in its nanostructures) so that it oxidizes inside the engine or flame. In this direction, a cyclic high-density hydrocarbon fuel, adamantane is studied as an additive for diesel to determine its effect on cetane number, threshold sooting index, soot reactivity, and soot nanostructural properties. The addition of up to 6.75 wt% of adamantane in diesel significantly increased the smoke point of diesel from 18 to 29 mm (and threshold sooting index decreased by 44%), indicating a drastic drop in fuel sooting tendency. Adamantane addition in a small amount to diesel minimally affected the cetane number. To understand the impact of adamantane addition to diesel on the characteristics of soot particles, different techniques including TGA, HRTEM, XRD, EDX, and EELS were used. It is observed that adamantane enhances curvatures in soot nanostructures and reduces the size of fringes in soot and its aromatic content, and all of these property variations improve soot reactivity to improve its oxidation in flames or engines and reduce soot emission.

Threshold Sooting Index of Sustainable Aviation Fuel Candidates from Composition Input Alone: Progress toward Uncertainty Quantification

A quantitative structure–property relationship model has been developed to predict the threshold sooting index (TSI) of arbitrary mixtures of aliphatic and aromatic hydrocarbons of known composition. The model employs contributions from eight molecular fragments plus a global shift and a penalty factor for naphthenic compounds. For each coefficient, the contributions were determined by a constrained regression to data from five different experimental campaigns, which were stitched together by setting the TSI of methylcyclohexane to 5 and the TSI of 1-methylnaphthalene to 100. Unique to this study; the TSI of 1,3,5-trimethylbenzene was restricted to the range of 54.7 and 63.1, which significantly constrains uncertainty. Within the composite training dataset, which contained 65 molecules and 124 data points, including simple mixtures, the model was found to match 95% of the data within 8.9 TSI. Validation of the model against n-butylcyclohexane, dimethylcyclooctane, and a six-component surrogate jet fuel shows prediction to be well within the 95-percentile confidence band of the experiment. This model is the first to integrate the linear blending rule for the TSI with a linear quantitative structure–property relationship model for the TSI, and the first time that referee controls have been applied to ensure that all datasets, experimental and modeled, are normalized to the same scale.

Variation in sooting characteristics and cetane number of diesel with the addition of a monoterpene biofuel, α-pinene

Diesel fuel combustion generates soot particles, which are harmful for human health and the environment. To reduce soot emission, various solutions are proposed in the literature such as the use of metal-additives in fuels, fuel blending with biofuels, and the use of diesel particulate filters. This study analyses the effect of the addition of a bicyclic monoterpene hydrocarbon biofuel, α-pinene to diesel in different proportions on the fuel cetane number, sooting propensity, and the physicochemical properties of soot nanoparticles. The addition of 10% α-pinene to diesel exhibited a synergistic effect on sooting tendency and reduced the threshold sooting index of diesel by 21%, even though α-pinene is an unsaturated hydrocarbon, and had a minimal impact on cetane number, which reduced from 63.5 for diesel to 61.5 for the blended fuel. The influence of α-pinene addition to diesel on soot nanostructural characteristics and reactivity is determined through different characterization approaches including HRTEM, TGA, XRD, SEM-EDX, and EELS. The results indicate that α-pinene addition to diesel imposes curvatures in soot nanostructure, creates relatively smaller fringes (PAHs) in soot, and reduces soot aromatic content to improve soot oxidation rate.

Identification and Quantification of Hydrocarbon Functional Groups in Gasoline Using 1H-NMR Spectroscopy for Property Prediction.

Gasoline is one of the most important distillate fuels obtained from crude refining; it is mainly used as an automotive fuel to propel spark-ignited (SI) engines. It is a complex hydrocarbon fuel that is known to possess several hundred individual molecules of varying sizes and chemical classes. These large numbers of individual molecules can be assembled into a finite set of molecular moieties or functional groups that can independently represent the chemical composition. Identification and quantification of groups enables the prediction of many fuel properties that otherwise may be difficult and expensive to measure experimentally. In the present work, high resolution 1H nuclear magnetic resonance (NMR) spectroscopy, an advanced structure elucidation technique, was employed for the molecular characterization of a gasoline sample in order to analyze the functional groups. The chemical composition of the gasoline sample was then expressed using six hydrocarbon functional groups, as follows: paraffinic groups (CH, CH2 and CH3), naphthenic CH-CH2 groups and aromatic C-CH groups. The obtained functional groups were then used to predict a number of fuel properties, including research octane number (RON), motor octane number (MON), derived cetane number (DCN), threshold sooting index (TSI) and yield sooting index (YSI).

Ignition Delay Time and Oxidation of a Kerosene Aviation Fuel and a Blended Jet50-Bio50 Fuel.

Ignition delay and oxidation of two jet aviation fuels, Jet A-1 and its blended fuel with a bio-jet fuel in half, are investigated by experiments and numerical simulations. From their major combustion properties, derived cetane number and molecular weight of the blended fuel, Jet50-Bio50, are higher than those of Jet A-1, and its H/C ratio and threshold sooting index are lower because more n-alkanes are contained in a bio-jet fuel and aromatic compounds are not. The surrogate fuels of the two jet fuels are constructed for numerical simulations of their ignition and oxidation. Early ignition of the blended fuel measured in a shock tube experiment is investigated by comparing the speciation profiles of several products from the two fuels, and their global reactivity is measured in a laminar flow reactor. Oxidation of the blended fuel is initiated at a lower temperature than Jet A-1, and reaction pathways of the two fuels are analyzed at two temperatures of 600 and 1100 K, respectively. At a low temperature of 600 K, reaction pathways of the major surrogate components for the two fuels are significantly different, while they are almost the same at high temperatures. The active radical of OH is produced more by the oxidation of Jet50-Bio50, and its oxidation is initiated at a lower temperature than Jet A-1, leading to earlier ignition. At low temperatures, the difference between initiation times of oxidation of the two fuels is much larger than at high temperatures. At both temperatures, production rates of the major reaction steps, where OH is produced, are higher in Jet50-Bio50 than in Jet A-1.

Predicting the propensity to soot of hydrocarbons and oxygenated molecules by means of structural group contribution factors derived from the processing of unified sooting indexes

To limit the emission of particulate pollutants at the exhaust of combustion processes, we must improve our understanding of soot formation mechanisms while identifying promising low-sooting additives to be blended with conventional fuels. In order to characterize the sooting behavior of hydrocarbons, different sooting indexes (SI) have been developed including the threshold soot index (TSI), the oxygen extended sooting index (OESI), the yield sooting index (YSI) and the fuel equivalent sooting index (FESI). By processing data derived from the use of such indicators, one can assign sooting contribution factors to each structural group composing a given molecule based on regressions against measured SI. Results that have been reported using such a methodology, however, remain difficult to compare due to the presence of wide varied index scales and uncertainties. Moreover, many studies consider relatively limited panels of molecules, while the statistical relevance of group increment models strongly depends on the number and variety of species considered. To overcome this issue, SI values drawn from 15 studies representing more than 700 experimental points were unified on a same numeric scale. This statistical sample integrates new OESI and FESI measurements conducted with hydrocarbons and oxygenated molecules, including furanic species. Developing the most comprehensive group increment model ever built allowed characterizing the sooting tendency of 93 structural groups entering into the composition of the main classes of compounds encountered in practical fuels. While satisfactorily reproducing the SI values from the database, the model also captures well the impacts of the number of C atoms, the unsaturation degree, the chain branching or the presence of oxygenated groups in soot formation. The sooting propensity of non-oxygenated compounds was found to increase in the following order: alkanes < alkenes < alkynes < aromatics against esters < aldehydes < ketones < 1-alcohols < 2-alcohols for oxygenated molecules and tetrahydrofuran < furan ~ 2,3-dihydrofuran ~ 2,5-dihydrofuran < 2-methyltetrahydrofuran < 2-methylfuran < 2,5-dimethylfuran for furanic species. The proposed model was finally shown capable of properly simulating the sooting propensity of a gasoline surrogate, as well as its behavior towards the addition of oxygenated additives during both lab-scale and engine tests.

Measurement of combustion properties and ignition delay time of high performance alternative aviation fuels

High-energy density aviation fuels are tested to see feasibility as alternative fuels to conventional jet fuels. They are derivatives of the existing high-energy density hydrocarbon fuels, exo-tetrahydrodicyclopentadiene and exo-tetrahydrotricyclopentadiene. Combustion properties and ignition delay times of these alternative fuels, called high performance fuels (HPF-1 and 2) are measured. Densities of HPF-1 and HPF-2 are similar to density of exo-tetrahydrodicyclopentadiene, but their H/C ratio, molecular weight, heat of combustion, derived cetane number, and threshold sooting index are quite higher than those of exo-tetrahydrodicyclopentadiene. It is found that HPF-1 and 2 have higher heat of combustion and lower sooting propensity than conventional jet aviation fuels when they are compared at the same fuel density. On the other hand, ignition delay time is measured in a temperature range from 950 K to 1,300 K at 12 atm by using a shock tube. Results show that ignition delay time of the two fuels is shorter than exo-tetrahydrodicyclopentadiene. HPF-2 containing methylcyclohexane has much shorter ignition delay time than exo-tetrahydrodicyclopentadiene at high temperature over 1,050 K.

Development and validation of a detailed kinetic model for RP-3 aviation fuel based on a surrogate formulated by emulating macroscopic properties and microscopic structure

This work proposed a kinetic model for RP-3, the most widely used military-civilian aviation fuel in China. Four hydrocarbons within the typical size of major components in RP-3, i.e., n-dodecane, iso-dodecane (2,2,4,6,6-pentamethylheptane), decalin and n-butylbenzene, were included in the component palette to improve the ability of the surrogate to mimic the properties related to the molecular weight. Seven properties, cetane number, molecular weight, H/C ratio, threshold sooting index, lower heating value, the proportion of -CH3 and -CH2 in the total carbons were selected as targets aiming at the comprehensive emulation of the chemical and physical propensities of RP-3. By sequential use of the genetic algorithm and a local search method, a surrogate containing 27.44% n-dodecane, 28.81% iso-dodecane, 26.12% decalin and 17.63% n-butylbenzene by mole was formulated. The autoignition delay times of the surrogate were measured using a heated rapid compression machine at pressures of 10, 15, 20 bar and equivalence ratios of 0.5, 1.0 and 2.0 over low-to-intermediate temperature range. Results show that the surrogate can not only emulate the target properties but also the key non-targeted properties. A kinetic model of 3065 species and 11,898 reactions was then developed based on the proposed surrogate to describe the chemical process during the combustion of RP-3. Simulations show that the model can predict the fundamental combustion datasets in the present work and literature satisfactorily, suggesting the rationality and applicability of the model. Rate of production and evolution histories of OH and HO2 were then conducted using the kinetic model to provide insight into the combustion of RP-3. Analysis suggests that negative temperature coefficient behavior also exists in the first stage ignition.

Effect of Lagrangian-phase Modelling on Charge Stratification and Spatial Distribution of Threshold Soot Index for Toluene Reference Fuel Surrogates

Nowadays, soot emissions are one of the major concerns in Direct Injection Spark Ignition engines. Soot prediction models can be computationally expensive, especially when particle mass, number, and size distribution are to be forecast. While soot formation heavily depends on the chemical and physical characteristics of the fuel, the simulation of the exact composition of a real gasoline is computationally unfeasible. Thus, it is essential to find simplified yet representative pathways to reduce the computational cost of the simulations. On the one hand, the a-priori investigation of the factors influencing particulate onset can be a simplified approach to compare different solutions and strategies with much cheaper costs than the modelling of soot formation and oxidation mechanisms. On the other hand, the use of surrogate fuels is a practical approach to cope with the fuel chemical nature. Although they poorly mimic the evaporation properties of a real gasoline, Toluene Reference Fuels are broadly adopted to match combustion relevant properties of the real fuels. In this study, the spatial distribution of the Threshold Soot Index in the fluid domain is investigated for three surrogates characterized by an increasing content of toluene (0 mol%, 30 mol%, 60 mol%). The correlation between the sooting tendency and the fuel distribution in the combustion chamber before spark ignition time can provide useful preliminary indications, without spending the computational effort of the whole soot model multicycle resolution. In particular, two approaches for the lagrangian description of the injected fuel are investigated: a multicomponent approach and a single component one, this last driven by a high-fidelity lumped modelling of the surrogate properties for both liquid and vapor phase.

Sooting characteristics of hydrocarbon compounds and their blends relevant to aviation fuel applications

The sooting characteristics of neat hydrocarbon compounds from different molecular classes and their blends that are relevant to aviation fuels were systematically investigated in the present work. Smoke points and maximum soot volume fractions of 10 neat hydrocarbon compounds and 33 binary fuel blends were measured in a standard wick-fed flame burner. The threshold sooting indices of test fuels were derived based on the fuel uptake flow rate with threshold imaging method. At the smoke point conditions, flames of all test fuels were noted to have relatively close maximum soot volume fractions, thereby suggesting that the maximum soot concentration contained by a diffusion flame without escaping smoke is relatively insensitive to the effects of fuel molecular structure. At the constant fuel carbon flow rate conditions, it was found that the soot formation generally follows: naphthalenes > monocyclic aromatics > cycloalkanes >iso-alkanes >n-alkanes. The relationships between soot formation and various sooting tendency indices were further examined. For given constituents, their binary blend result at varying blending ratio showed that the maximum soot volume fraction can be approximately linearly correlated with aromatics content, hydrogen content, hydrogen deficiency, and threshold sooting index; the slope varies with different blends. At a given constant fuel carbon flow rate, the results of all test fuels demonstrated that the reciprocal of smoke point is able to correlate linearly with the maximum soot volume fraction through a single curve. Therefore, the present findings indicated that smoke point is more effective even than threshold sooting index in predicting soot formation in diffusion flames.