Surrogate Mixtures Research Articles

A functional-group-based approach to modeling real-fuel combustion chemistry – I: Prediction of stoichiometric parameters for lumped pyrolysis reactions

Real fuels are complex mixtures of hundreds of molecules, which makes it challenging to unravel their combustion chemistry. Several approaches in the literature have helped to clarify fuel combustion, including multi-component surrogates, lumped fuel chemistry modeling, and functional-group based methods. This work presents an innovative advancement to the lumped fuel chemistry modeling approach, using functional groups for mechanism development (FGMech). Stoichiometric parameters of lumped fuel decomposition reactions dictate the population of the key pyrolysis products, previously obtained by fitting experimental data of real-fuel pyrolysis. In this work, a functional group-based approach is proposed, which can account for real-fuel variability and predict stoichiometric parameters without experimentation. A database of the stoichiometric parameters and/or yields of key pyrolysis products was first constructed for approximately 50 neat fuels, based on previous pyrolysis data and a lumped kinetic model we developed. The effects of functional groups on the stoichiometric parameters and/or yields of key pyrolysis products were then identified and quantified. A quantitative structure-stoichiometry relationship was developed by multiple linear regression (MLR) model, which was used to predict the stoichiometric parameters and/or yields of key pyrolysis products based on ten input features (eight functional groups, molecular weight, and branching index). Products from the pyrolysis of surrogate mixtures and real-fuels were predicted using the MLR model and validated against experimental data in the literature. Comparison with the stoichiometric parameters from the HyChem experiment-based approach (Xu et al., 2018) showed that the predicted values in this work were in reasonable agreement (generally within a factor of two). When the stoichiometric parameters in the jet fuel (POSF 10325) HyChem kinetic model were replaced with this functional-group based prediction, only minor discrepancies were observed in the predictions of key pyrolysis products and global combustion parameters (such as ignition delay times and laminar flame speeds). Sensitivity analysis on stoichiometric parameters revealed their different roles in predicting speciation and global parameters. The functional group approach for predicting stoichiometric parameters in this work was the first step towards developing FGMech for modeling real-fuel combustion chemistry. Further development of the FGMech model's thermodynamic, kinetic, and transport data will be presented in a following study.

A lumped kinetic model for high-temperature pyrolysis and combustion of 50 surrogate fuel components and their mixtures

Wide distillation fuels (WDF) and gasoline/diesel blends have been proposed as new fuel formulations for advanced combustion engines. Recent studies have shown that multi-component gasoline and diesel surrogate mixtures can accurately mimic the distillation curve, functional group or hydrocarbon class distribution, average molecular weight, and other combustion properties of real fuels. This work presents an updated decoupling methodology to construct a 50-component fuel model, which consists of a skeletal sub-mechanism for large hydrocarbon components present in gasoline, jet and diesel fuels, a reduced C5-Cn mechanism, and a detailed C0-C4 mechanism. The entire model contains 156 species and 1132 reactions. The chemical classes covered include n-alkanes, iso-alkanes, cyclo-alkanes and alkylbenzenes. Compared with the comprehensive detailed kinetic modeling approach, the present methodology largely reduces the model size due to the use of skeletal fuel mechanisms. Compared with the traditional decoupling methodology, our approach increases the model accuracy since it contains a detailed C0-C4 mechanism instead of a reduced C2-C3 mechanism. The present model was validated against experimental targets at high temperatures, including the pyrolysis and oxidation speciation data and global parameters such as ignition delay times and laminar flame speeds from 30 pure fuel components and 12 fuel mixtures. In general, the present model performs well against these experimental data, suggesting this methodology is a suitable approach for developing accurate kinetic models for multi-component fuels. Future work will extend the present framework to predict low-temperature combustion chemistry of multi-component fuels.

On the importance of species selection for the formulation of fuel surrogates

Fuel surrogates are mixtures of simple compounds that emulate the combustion characteristics of more complex fuels, with the primary objective to enable detailed combustion modeling of very complex real fuels. Current efforts in surrogate development aim at optimizing the compositions of pure hydrocarbons to emulate multiple combustion related properties. In doing so, weights are assigned when defining optimization problem to reflect the importance of each property. In this study, we report on the relative importance of species selection and their weights on the overall performance of the optimized surrogate. Using experimental data of a reference jet fuel as target, we designed a study using a surrogate optimizer that imposes orthogonal perturbations on the surrogate components and weights and analyzed their impact on the optimized surrogate mixtures. Results from 3600 cases show that perturbations of surrogate components, rather than weights, induce far greater variability in the optimized composition and target property agreement. While the Derived Cetane Number (DCN) agreement shows a greater variability from the weight perturbation, the main reason for such high sensitivity is due to the wide range of values for pure component DCN of the individual components, which is also a result of the surrogate component selection. Further, the results show that the selection of surrogate components nearly predefines the overall shape of the distillation curves regardless of the weight values. The current study quantitatively supports the idea that appropriate selection of surrogate components that capture the physical and chemical characteristics of actual constituents of target fuel will increase the possibility of successful surrogate formulation and will mitigate the impact from arbitrary weight assignment.

Counterflow ignition and extinction of FACE gasoline fuels

The demand for petroleum-derived gasoline in the transportation sector is on the rise. For better knowledge of gasoline combustion in practical combustion systems, this study presents experimental measurements and numerical prediction of autoignition temperatures and extinction limits of six FACE (fuels for advanced combustion engines) gasoline fuels in counterflow flames. Extinction limits were measured at atmospheric pressures while the experiments for autoignition temperatures were carried out at atmospheric and high pressures. For atmospheric pressure experiment, the fuel stream consists of the pre-vaporized fuel diluted with nitrogen, while a condensed fuel configuration is used for ignition experiment at higher chamber pressures. The oxidizer stream is pure air. Autoignition temperatures of the tested fuels are nearly the same at atmospheric pressure, while a huge difference is observed as the pressure is increased. Unlike the ignition temperatures at atmospheric pressures, minor difference exists in the extinction limits of the tested fuels. Simulations were carried out using a recently developed gasoline surrogate model. Both multi-component and n-heptane/isooctane mixtures were used as surrogates for the simulations. Overall, the n-heptane/isooctane surrogate mixtures are consistently more reactive as compared the multi-component surrogate mixtures. Transport weighted enthalpy and radical index analysis was used to explain the differences in extinction strain rates for the various fuels.

A method for determining hydrogen–methane–nitrogen mixtures for laboratory tests of syngas-fuelled internal combustion engines

An original method for formulating surrogate fuels from actual syngas mixtures is presented and formalised. The method is the first example in the scientific literature of a rather complete tool for planning and setting up a laboratory syngas-fuelled engine test when some components of the syngas mixture are not available. Basically, the method allows a map to be built that provides the composition for a surrogate fuel once the composition of a syngas mixture is assigned, the components of a surrogate fuel are selected and the equivalence parameters are defined. The laminar flame speed, the energy density of the fuel–air mixture and the methane number are identified as equivalence parameters in the study. In particular, the proper laminar flame speed and energy density ensure that an engine fuelled by the surrogate mixture produces the same indicated power as it would when fuelled by the original syngas. Instead, the methane number allows for checking the fact that the tendency of the engine to knock is the same or greater than the knock tendency during syngas operation. In this article, the method is used to determine the hydrogen–methane–nitrogen mixtures corresponding to six five-component syngas mixtures, resulting from actual gasification processes. The laminar flame speed and methane number of each syngas mixture are estimated by means of simple original models aimed at either improving the predicting capabilities of existing models or allowing for a prompt application of the procedure. The results show that four of the six surrogate fuels are equally or more knock-prone than the original syngas mixtures, whereas only one of the two remaining surrogate fuels likely imposes a retardation of the spark advance in the final setup of the engine for actual syngas operation.

Dual/scram-mode combustion limits of ethylene and surrogate endothermically-cracked hydrocarbon fuels at Mach 8 equivalent high-enthalpy conditions

This paper examines the scram/dual-mode combustion limits of hydrocabon fuels within a Mach 8, scramjet combustor. Flight-equivalent flows were delivered to the axisymmetric, cavity combustor via a reflected shock tunnel. Two scramjet fuels were examined: ethylene and a surrogate mixture representing endothermically cracked n-dodecane. Combustion modes were examined via static pressure sensors and through both chemiluminescence imaging, and planar laser induced fluorescence (PLIF) of the OH combustion radical in the combustor exhaust plume. Ethylene-fuelled experiments developed scram-mode combustion under reduced fuelling conditions, experiencing shock wave dominated flowfields. OH PLIF diagnostics indicated such combustion modes developed a ring-like structure of combustion products, primarily axisymmetrically adjacent to the combustor wall. Increased fuelling anchored combustion downstream of the fuel injector, while further increases instigated dual-mode combustion. In this mode, subsonic combustion regions combine with the supersonic coreflow to permit the transfer of information upstream with substantially increased pressure encountered. Optical diagnostics indicate broadly asymmetric, unsteady combustion features. The surrogate mixture representing endothermically cracked n-dodecane experienced rapid onset from no-combustion (optically confirmed) to fully developed dual-mode combustion at critical fuelling rates. OH PLIF signals and chemiluminescence of this fuel were weaker than comparable ethylene cases, indicating potential differences in combustion pathways.

Variation of gas phase combustion properties of complex fuels during vaporization: Comparison for distillation and droplet scenarios

Surrogate mixtures for modeling real fuels are formulated based on gaseous combustion property targets and liquid physical properties. A batch distillation model was developed to evaluate the vaporization characteristics of some existing surrogates for Jet-A. Chinese aviation fuel RP-3 was then used as a target to experimentally obtain distillation curves and variation of chemical functional groups. A 24-component surrogate was formulated mainly to capture the distillation behavior of RP-3. This surrogate was then used in two droplet vaporization models (Finite Thermal Conductivity/Finite Diffusivity (FTC/FD), Infinite Thermal Conductivity/Infinite Diffusivity (ITC/ID)) to investigate the effects of preferential vaporization on gaseous combustion properties of a complex real fuel. The results obtained from FTC/FD and ITC/ID provide regional bounds for droplets in a vaporizing spray. Four combustion property targets (Molecular Weight (MW), Hydrogen to Carbon ratio (H/C), Derived Cetane Number (DCN) and Threshold Sooting Index (TSI)) were employed as indicators of gas combustion properties. It was found that due to a wide distribution of compounds’ volatility, gaseous combustion properties vary significantly during droplet vaporization. The results suggest development of vaporization models that well capture preferential vaporization of a target real fuel for further spray modeling.

Evaluation of a New Chemical Mechanism for 2-Amino-2-methyl-1-propanol in a Reactive Environment from CSIRO Smog Chamber Experiments.

Amines are considered as an emerging class of atmospheric pollutants that are of great importance to atmospheric chemistry and new particle formation. As a typical amine, 2-amino-2-methyl-1-propanol (AMP) is one of the proposed solvents for capturing CO2 from flue gas streams in amine-based post-combustion CO2 capture plants, and it is expected to result in AMP emission and secondary product formation in the atmosphere. However, the current knowledge of its atmospheric chemistry and kinetics is poorly understood, particularly in a reactive environment. In this work, we used the CSIRO smog chamber to study the photo-oxidation of AMP in the presence of volatile organic compound (VOC)-NOx surrogate mixtures over a range of initial amine concentrations. O3 formation was significantly inhibited when AMP was added to the surrogate VOC-NOx mixtures, implying that AMP could alter known atmospheric chemical reaction pathways and the prevailing reactivity. Simultaneously, a large amount of AMP-derived secondary aerosol was formed, with a considerably high aerosol mass yield (i.e., ratio of aerosol formed to reacted AMP) of 1.06 ± 0.20. Based on updated knowledge of its kinetics, oxidation pathways, and product yields, we have developed a new mechanism (designated as CSIAMP-19), integrated it into the Carbon Bond 6 (CB6) chemical mechanism, and evaluated it against available smog chamber data. Compared with the existing AMP mechanism (designated as CarterAMP-08), the modified CB6 with CSIAMP-19 mechanism improves prediction against AMP-VOC-NOx experiments across a range of initial AMP concentrations, within ±10% model error for gross ozone production. Our results contribute to scientific understanding of AMP photochemistry and to the development of the chemical mechanism of other amines. Once some potential limitations are considered, the updated AMP reaction scheme can be further embedded into the chemical transport model for regional modeling scenarios where AMP-related emissions are of concern.

Investigating the Effects of C3 and C4 Alcohol Blending on Ignition Quality of Gasoline Fuels

The study of the ignition quality of alcohol blends with petroleum fuels is a subject of practical interest. It is well known that the ignition delay time (IDT), as well as the octane number (ON), increases when gasoline fuels are blended with ethanol. This study focuses on the impact on inverse ignition delay time (IDT–1) when alcohols, such as n-propanol and n-butanol, are blended with gasoline fuels. A nonlinear decrease in the IDT–1 of the blends was observed. Predicting the extent of nonlinearity in blends is complicated because it involves unknown intermolecular interactions between base fuel components and the blended components. The purpose of this study is to establish the dependence of base fuel composition (in terms of functional groups) on observed nonlinearity. Gasoline fuel contains hundreds of compounds (predominantly hydrocarbons), making it a challenge to understand observed nonlinearity when they blend with other components. In this study, the IDTs of primary reference fuels (PRFs, a binary mixture of iso-octane and n-heptane) and FACE (fuel for advanced combustion engines) gasolines blended with two alcohols (n-propanol and n-butanol) were obtained with an ignition quality tester (IQT) following ASTM D6890 standards. A mole-based Gaussian fit was used to model the blending effects of alcohol with gasoline. The synergistic effect of the different mixtures tested in this study was investigated by analyzing the Gaussian parameters. A multiple linear regression model was formulated to provide information about the impact of the structural composition (functional group) on the synergistic blending effects of gasoline–alcohol mixtures. Constant-volume homogeneous batch reactor simulations were also conducted, using Chemkin-Pro for alcohols blended with a FACE J surrogate mixture to provide kinetic information about the blending effects observed in the IQT measurements.

Study of PAH formation in laminar premixed toluene and C8H10 aromatics flames

Toluene and C8H10 aromatic fuels are widely proposed as components of surrogate mixtures and usually employed as the prototypical fuels to investigate the combustion properties of aromatic fuels. In this study, the PAH formation of pure C8H10 aromatics (xylenes and ethylbenzene) and toluene was studied in a laminar premixed flame at atmospheric pressure using laser-induced fluorescence (LIF) diagnostic. The experimental results indicated the aromatic structure has a significant influence on the PAH tendency, and the observed LIF intensities were sequenced by toluene < xylenes < ethylbenzene. A detailed kinetic model was employed to validate against the experimental results based on the ROP analysis. The predicted mole fraction profiles of PAHs were consistent with the measured PAH tendency for toluene, o-xylene, and ethylbenzene. The chemical kinetic analysis indicated that benzyl is the most dominant intermediate product in toluene decomposition and precursor in PAH formation, corresponding to methylbenzyl in o-xylene flame, styrene and benzyl in ethylbenzene flame. In the comparison of o-xylene and toluene, the increased methylated groups on the benzenoid ring make the yield of benzylic radicals (mostly methylbenzyl) enhanced, increasing the probability of radical–radical recombination on PAH formation. In the ethylbenzene flame, two secondary CH bonds in the ethyl site are much easier to be abstracted than primary CH bonds in methyl sites of o-xylene. The different bond dissociation energies (BDEs) depended on the aromatic structure can change the distribution of the intermediate species in fuel decomposition, which can result in either an increase or decrease of PAH formation. Additionally, the weak benzylic CC bond makes the initiation of ethylbenzene decomposition easier, which could also explain the fated PAH formation in contrast with toluene and o-xylene. The experimental and kinetic analysis for C8H10 aromatics and toluene conducted in the present study contributed a deep insight into the PAH formation of aromatics fuels.

An experimental and computational study on multicomponent evaporation of diesel fuel droplets

In this study, the preferential evaporation behavior of diesel fuel is investigated using a discrete continuous multicomponent (DCMC) evaporation model. By analyzing real diesel fuel composition data from a 2D gas chromatography (GCxGC) measurement, 180 individual species from 11 different hydrocarbon families could be identified. To utilize these detailed composition data in the DCMC model, four continuous distribution functions for normal paraffins, mono-naphthenics, mono-aromatics, and naphthenic-mono-aromatics have been derived, which cover 80% of the total diesel composition. A specific property database for these hydrocarbon groups was established and suitable correlations were derived. The DCMC model was validated with experimental results of evaporating acoustically levitated single diesel droplets. Gas temperatures up to 200 °C were investigated, where the diesel fuel did not evaporate completely due to the presence of very long-chain compounds with low volatilities. The DCMC model using the experimentally-based diesel composition successfully captures this trend. To prove the importance of using the DCMC model with experimentally based fuel composition data at diesel engine conditions, a single droplet was exposed to a gas environment of 700 °C. Here, the diesel droplet showed the longest lifetime, while dodecane, which is commonly used in diesel surrogate mixtures, evaporated significantly faster.

A predictive group-contribution model for the viscosity of aqueous organic aerosol

Abstract. The viscosity of primary and secondary organic aerosol (SOA) has important implications for the processing of aqueous organic aerosol phases in the atmosphere, their involvement in climate forcing, and transboundary pollution. Here we introduce a new thermodynamics-based group-contribution model, which is capable of accurately predicting the dynamic viscosity of a mixture over several orders of magnitude (∼10-3 to &gt;1012 Pa s) as a function of temperature and mixture composition, accounting for the effect of relative humidity on aerosol water content. The mixture viscosity modelling framework builds on the thermodynamic activity coefficient model AIOMFAC (Aerosol Inorganic–Organic Mixtures Functional groups Activity Coefficients) for predictions of liquid mixture non-ideality, including liquid–liquid phase separation, and the calorimetric glass transition temperature model by DeRieux et al. (2018) for pure-component viscosity values of organic components. Comparing this new model with simplified modelling approaches reveals that the group-contribution method is the most accurate in predicting mixture viscosity, although accurate pure-component viscosity predictions (and associated experimental data) are key and one of the main sources of uncertainties in current models, including the model presented here. Nonetheless, we find excellent agreement between the viscosity predictions and measurements for systems in which mixture constituents have a molar mass below 350 g mol−1. As such, we demonstrate the validity of the model in quantifying mixture viscosity for aqueous binary mixtures (glycerol, citric acid, sucrose, and trehalose), aqueous multicomponent mixtures (citric acid plus sucrose and a mixture of nine dicarboxylic acids), and aqueous SOA surrogate mixtures derived from the oxidation of α-pinene, toluene, or isoprene. We also use the model to assess the expected change in SOA particle viscosity during idealized adiabatic air parcel transport from the surface to higher altitudes within the troposphere. This work demonstrates the capability and flexibility of our model in predicting the viscosity for organic mixtures of varying degrees of complexity and its applicability for modelling SOA viscosity over a wide range of temperatures and relative humidities.

Determining the Thermal Properties of Military Jet Fuel JP-5 and Surrogate Mixtures Using Differential Scanning Calorimetry/Thermogravimetric Analysis and Differential Scanning Calorimetry Methods

Heat capacity and heat of vaporization (HoV) are thermal properties that affect the combustion of fuels in engines. This study highlights new calorimetric methods to measure the thermal properties ...

Spray combustion simulation study of waste cooking oil biodiesel and diesel under direct injection diesel engine conditions

Spray combustion characteristics of waste cooking oil biodiesel (WCO) and conventional diesel fuels were simulated using a RANS (Reynolds Averaged Navier Stokes) based model. Surrogates were used to represent WCO and diesel fuels in simulations. N-tetradecane (C14H30) and n-heptane (C7H16) were used as surrogates for diesel. Furthermore for WCO, surrogate mixtures of methyl decanoate, methyl-9-decenoate and n-heptane were used. Thermochemical and reaction kinetic data (115 species and 460 reactions) were implemented in the CFD code to simulate the spray and combustion processes of the two fuels. Validation of the spray liquid length, ignition delay, flame lift-off length and soot formation data were performed against previous published experimental results. The modeled data agreed with the trends obtained in the experimental data at all injection pressures. Further investigations, which were not achieved in previous experiments, showed that prior to main ignition, a first stage ignition (cool flame) characterized by the formation formaldehyde (CH2O) species at low temperature heat release occurred. The main ignition process occurred at high temperature with the formation of OH radicals. Furthermore, it was observed that the cool flame played a greater role in stabilizing the downstream lifted flame of both fuels. Increase in injection pressure led to the cool flame location to be pushed further downstream. This led to flame stabilization further away from the injector nozzle. WCO had shorter lift-off length compared to diesel as a result of its cool flame which being closer to the injector. Soot formation followed similar trends obtained in the experiments.

대용 디젤연료의 대향류 비예혼합 화염에서 Methyl Decanoate 첨가가 방향족 생성에 미치는 영향

Numerical simulations have been conducted to investigate the effect of biodiesel addition on polycyclic aromatic hydrocarbon (PAH) formation in diesel surrogate fuel with the detailed chemical reaction mechanism which was developed for n-dodecane and m-xylene as a diesel surrogate and methyl decanoate as a biodiesel surrogate. Flame temperatures or species concentrations calculated from n-dodecane or methyl decanoate counterflow diffusion flames were relatively well predicted compared to the results of other researchers. The addition of methyl decanoate to the diesel surrogate mixture resulted in a reduction of the maximum flame temperature because of the low calorific value of methyl decanoate. Because methyl decanoate was mainly decomposed according to β-scission, its bond energy between atoms played an important role. Increasing the amount of methyl decanoate in the diesel surrogate mixture reduced the production of PAH while decreasing the maximum concentration of C₂H₂ and C₃H₃ which were important precursors to benzene or phenyl. As with hydrocarbon fuels, the increase of the strain rate in the methyl decanoate-added counterflow diffusion flame reduced the maximum concentration of PAH.

Thermodynamic model of fast pyrolysis bio-oil advanced distillation curves

Fast pyrolysis bio-oil (FPBO) is a complex organic liquid with over 1000 organic compounds. Therefore, little research exists on the prediction and modelling of liquid physicochemical properties and vapour-liquid phase behaviour. A thermodynamic model of the FPBO vapour-liquid equilibrium (VLE) in distillation will improve the understanding of bulk and FPBO fraction properties, and therefore, aid in the enhancement of FPBO for desired applications, such as condensation of pyrolysis vapours, fractionation, and evaporation during combustion. FPBO produced at 450 °C from softwood sawmill residues at a feed rate of 4 kg/h (~20 s residence time) in a pilot-scale auger reactor was distilled in an advanced distillation curve (ADC) apparatus at atmospheric pressure and at a vacuum of 5 kPa (abs). The ADC equipment allows for the data collection of true thermodynamic state points, including vapour phase composition data. UNIQUAC was the selected equation of states (EoS) to model FPBO distillation VLEs in a 20–flash series in VMGSim™. A surrogate mixture of water, 3,4,4′-biphenoltriol (pyrolytic lignin), inert solids, and multiple organic components was used to model and predict the VLE and consequently bulk physicochemical properties. It was found that GC-FID composition data (in area%), water content (Karl Fisher titration), distillation residue, and solid content were sufficient to inform the model (surrogate composition). This work improved the understanding of FPBO phase behaviour, composition, and bulk properties. The method allows for the identification of target components and fractions to enhance FPBO quality for desired applications.

Intermediate and high ethanol blends reduce secondary organic aerosol formation from gasoline direct injection vehicles

We evaluated the secondary aerosol formation from two current technology flex fuel vehicles (FFVs) equipped with gasoline direct injection engines when operated on different fuel formulation on a chassis dynamometer. This study explored the influence of ethanol fueling and aromatic concentration on secondary organic aerosol (SOA) production by utilizing two E10 fuels with different aromatic levels and two higher ethanol blends (i.e., E30 and E78). The diluted emissions from both FFVs were photooxidized in a 30 m3 mobile atmospheric chamber with two distinct initial conditions, while the vehicles operated over cold-start and hot-start LA92 test cycles. Testing revealed that one vehicle did not produce secondary aerosol when emissions were photooxidized with a single precursor oxidant. However significant SOA formation occurred in the presence of an anthropogenic volatile organic carbon surrogate and NOx mixture in the chamber, despite the fact that both vehicles had similar primary particulate emission levels. The secondary aerosol formed in an urban airshed was mainly composed of ammonium nitrate and significantly exceeded primary tailpipe PM emissions. Cold-start operation from both vehicles led to greater secondary aerosol production compared to hot-start LA92 operation. SOA production showed a strong relationship with the tailpipe non-methane hydrocarbon (NMHC) emissions and overall reduced with higher ethanol blending in the fuel. The impact of aromatics was clear, with the higher aromatic E10 fuel showing higher SOA formation than the lower aromatic E10 fuel.

Laminar burning velocities of surrogate components blended with ethanol

To provide a consistent database for all important classes of hydrocarbons considered as components of surrogate mixtures, laminar burning velocities of n-decane, p-xylene and methylcyclohexane blended with ethanol were determined using the heat flux method at atmospheric pressure and initial temperatures of 318–400 K. The results obtained at Samara University and Lund University are within overlapping experimental uncertainties. Moreover, data consistency for MCH + ethanol blends was assessed with the help of analysis of the temperature dependence of the laminar burning velocity. Present experiments were modelled using the PoliMi detailed kinetic mechanism. Its predictions were found in a good agreement with the measurements performed with n-decane, while burning velocities of methylcyclohexane and p-xylene were over- and underpredicted, respectively. In all cases, the burning velocity of ethanol is higher than that of the hydrocarbons studied, while the burning velocity of the blends is in-between the values for the neat components.

Realistic aviation fuel chemistry in computational fluid dynamics

The prediction of flame extinction, soot formation and heat transfer for kerosene (pool) fires is a key aspect for the characterisation of aviation fuels. In particular, one of the main numerical challenges is the description of complex interactions of chemical and physical phenomena. This work is devoted to the individuation of an optimised surrogate mixture for the development of a detailed chemical model describing liquid fuel pyrolysis, homogeneous combustion and the formation of soot precursors, as well as the implementation of the obtained kinetic mechanism in open source computational fluid dynamics (CFD). To this aim, the primary combustion products and corresponding rate determining step into the production paths are identified by means of sensitivity and reaction paths analyses under several initial conditions representative for the investigated scenarios. The results show that a binary mixture composed of 92%v n-decane and 8%v toluene reproduces the aviation fuel properties. Based on the numerical results obtained in this work for aviation fuel, it holds that the mass burning rate m''kgm-2s-1 dependence with pool diameter (Dp) expressed in meters is: m''=0.47∙1-exp-3.61∙Dp A reduced (detailed) kinetic mechanism consisting of 14 reactions is obtained and implemented in a CFD model. The numerical results are in accordance with experimental data retrieved from the literature. The observed data consistency validates the adopted procedure and allows for further considerations on soot precursor formation in case of diffusive flames of aviation fuel.

Preferential vaporization impacts on lean blow-out of liquid fueled combustors

Recent experimental works have shown that the global equivalence ratio defining lean blow-out (LBO) in model gas turbine combustors correlates with the derived cetane number (DCN) of the tested fuel, which represents the chemical reactivity potential of the fuel, but additional physical and kinetic parameters of the fuel also have influence. The current work explores the significance of preferential vaporization impacts on LBO behaviors; i.e., rather than parameterizing the fuel by overall averaged fuel properties, it looks at DCN correlations based upon distillation properties prior to full vaporization. Preferential vaporization potentials of six fuels are evaluated by measuring the DCN values of five distillation cuts (each of 20% liquid distillation volume recovered). In spite of relatively large disparities in total fuel DCN values (∼9.1), two petroleum-derived jet fuels are found to have nearly the same LBO equivalence ratios, which is attributed to the relatively indiscernible difference of DCN values (∼2) for the initial 20% distillation cut of each fuel. Trade-off impacts between fuel chemical and physical properties are demonstrated by comparing n-dodecane and Gevo-ATJ, which do not have preferential vaporization potential. LBO results suggest that fuel physical properties (particularly fuel boiling characteristics) predominantly control LBO behaviors at low air inlet temperature conditions, whereas fuel chemical properties appear to gain significance with increasing air inlet temperature. Further evidence of preferential vaporization effects on LBO is discussed with two surrogate mixtures formulated to emulate the fully pre-vaporized combustion behaviors of Jet-A, but having drastically different preferential vaporization potentials. Finally, the relationship between DCNs and LBO equivalence ratios is re-examined using the DCN values of initial 20% distillation cuts of all six fuels. The results display a significantly improved correlation, suggesting that the relevance of preferential vaporization on LBO can be significant for fuels that exhibit significant departure of the DCN for high volatile fractions (i.e., the initially vaporized constituents) in comparison to the overall fuel DCN.