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Multidimensional simulations have been conducted to simulate atmospheric pressure, flat-flame/McKenna-burner-driven-flow tube experiments targeted to obtain NOx speciation data for predicting/analyzing syngas combustion emissions. In a prior work, we demonstrated the impacts of multidimensional transport on post flame region prediction departures from those assuming unidimensional flow/transport conditions. Here we develop and utilize a multidimensional laminar reacting flow solver to simulate the fully coupled flame and post flame regions to further elucidate the impacts of the earlier unidimensional modeling assumptions on interpreting post flame NOx experimental data. The model is used to simulate a lean, premixed syngas/air flame and its associated post flame regions within a cylindrical flow-tube-like arrangement. The combustion process takes place under atmospheric condition with trace amount of NOx seeding fed into the inlet gas stream. The spatial evolution of NOx species (NO and NO2) in the flame and in the post-combustion zone suggests two distinct regions: 1) a region encompassing the flame structure itself; and 2) a post flame region in which the temperature decays due to both axial and radial transport processes. The predictions show that for the conditions studied, a pulsatile flow field exists due to the formation of an expanding and contracting recirculation zone in the outer periphery of the flow tube. By resolving the nature of the flow, the resulting time-averaged temperature and species concentrations show improved agreement with existing experimental measurements. The flow-field interaction results in radial inhomogeneities in the NO2 profiles with the maximum concentration offset from the flow centerline. The location of the peak in NO2 is coupled with radial temperature gradients from wall cooling effects and their significant influence on NO/NO2 interconversion kinetics, producing notable NO2 accumulation in regions near the wall. Geometrical configurations capable of suppressing/minimizing the pulsatile nature are also investigated and the results are compared. Other experimental configurations could be considered in parametric simulations to determine the optimal configuration that would minimize non-idealities in the observations. The work shows the value in performing such computations in advance of settling on a particular design for flow tube/flow reactor experiments.

Operating combined cycle gas turbines and/or low speed diesel engines on light petroleum crudes rather than distillate or heavy fuels produced from them avoids the carbon emissions associated with refining while continuing to achieve high energy conversion efficiencies. However, crude assays generated for refinery feed stock characterization do not include an ignition propensity property indicator, and the wide distillation range of crudes disfavors the use of the Calculated Carbon Aromaticity Index or Cetane Index used to characterize heavy or distillate fuel ignition quality. Here, the derived cetane number (DCN) of a single (whole) light crude is reported and the liquid volume fractions and DCNs of light end distillation fractions of this and three other light crudes are compared. Using ASTM D86 distillation method, four crudes were distilled into four light fractions as typically reported in crude assays: 1) light naphtha; 2) heavy naphtha; 3) kerosene; 4) light gas oil. DCNs were obtained using an ignition quality tester (IQT) and ASTM D6890 procedures. The DCNs of the fractions 1) – 4) for all crudes increased from 23 to 35 for the lowest boiling point fraction to 50–60 for the highest tested, in the range of the DCN measured for one of the whole crudes (52.6). The large DCN variation with distillation temperature suggests that preferential vaporization is likely relevant to pre-vaporized/premixed gas turbine combustion behaviors near operational limits. 1H and 13C NMR spectra were also acquired for the distilled samples. A QSPR regression developed using a Scheffe simplex-polynomial descriptor indicates that n-paraffinic CH2 groups play the most significant role in determining crude global ignition propensity. Results suggest that the reactivity potential of crude oils can be estimated reasonably well by employing a well-trained QSPR prediction tool for DCN, based upon key chemical functional groups determined from 1H and 13C NMR spectral analyses.

Isolated droplet burning behaviors of fuel mixtures that all share the fully pre-vaporized combustion behaviors of the same “global” Jet-A real fuel are investigated numerically to elucidate the effects of preferential vaporization. Predictions are generated using three such multi-component hydrocarbon mixtures (Mixture-1: n-decane/iso-octane/toluene 42.7/33.0/24.3, Mixture-2: n-dodecane/iso-octane/1,3,5 trimethyl benzene 49.0/21.0/30.0 and Mixture-3: n-hexadecane/iso-octane/1,3,5 trimethyl benzene 36.5/31.0/32.5 molar ratios), each having very different distillation properties. Simulations are performed using a transient one-dimensional sphero-symmetric model, involving numerically reduced detailed chemical kinetics, and multi-component gas-phase diffusive transport. The interactions among the different liquid-phase components are modeled using UNIFAC activity coefficient methodology. The predictions using Mixture-1 are found to be in good agreement with previously published drop-tower experiments for 550 µm droplet combustion in air. Stagnant and internally mixed liquid-phase behaviors are both considered. Near fully mixed internal conditions and including sooting effects (observed in the experiments) result in predictions that are in good agreement with experimental drop diameter and flame-standoff ratio histories. By comparing predictions for the three mixtures, preferential vaporization effects exhibit strong non-linear dependences on initial droplet diameter and ambient pressure. At low pressures and for droplet sizes typical of those found in multi-phase gas turbine combustors, comparable diffusion and vaporization characteristic times favor frozen limit vaporization. However, at pressures typical of those found in applications, batch distillation dominates and preferential vaporization chemical property differences are found to become as significant as physical property effects in influencing combustion behaviors.

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.

Transient, isolated n-alkane droplet combustion is simulated at elevated pressure for helium-diluent substituted-air mixtures. We report the presence of unique quasi-steady, three-stage burning behavior of large sphero-symmetric n-alkane droplets at these elevated pressures and helium substituted ambient fractions. Upon initiation of reaction, hot-flame diffusive burning of large droplets is initiated that radiatively extinguishes to establish cool flame burning conditions in nitrogen/oxygen “air” at atmospheric and elevated pressures. However, at elevated pressure and moderate helium substitution for nitrogen (XHe > 20%), the initiated cool flame burning proceeds through two distinct, quasi-steady-state, cool flame burning conditions. The classical “Hot flame” (∼1500 K) radiatively extinguishes into a “Warm flame” burning mode at a moderate maximum reaction zone temperature (∼ 970 K), followed by a transition to a lower temperature (∼765 K), quasi-steady “Cool flame” burning condition. The reaction zone (“flame”) temperatures are associated with distinctly different yields in intermediate reaction products within the reaction zones and surrounding near-field, and the flame-standoff ratios characterizing each burning mode progressively decrease. The presence of all three stages first appears with helium substitution near 20%, and the duration of each stage is observed to be strongly dependent on helium substitutions level between 20–60%. For helium substitution greater than 60%, the hot flame extinction is followed by only the lower temperature cool flame burning mode. In addition to the strong coupling between the diffusive loss of both energy and species and the slowly evolving degenerate branching in the low and negative temperature coefficient (NTC) kinetic regimes, the competition between the low-temperature chain branching and intermediate-temperature chain termination reactions control the “Warm” and “Cool” flame quasi-steady conditions and transitioning dynamics. Experiments onboard the International Space Station with n-dodecane droplets confirm the existence of these combustion characteristics and predictions agree favorably with these observations.

Derived Cetane Numbers (DCNs) of engine lubricating oil/multicomponent 95 Research Octane Number (RON) gasoline surrogate mixtures were measured in an Ignition Quality Tester (IQT). Measurements separately assess the effects of calcium- and magnesium-based detergent additive fraction, oil viscosity, oil degradation, and base oil classification on mixture ignition propensity at conditions with relevance to low speed pre-ignition (LSPI) in gasoline engines. Testing of 0–25% (by mass) oil blended into a six-component surrogate mixture representing an unleaded “average” European gasoline blend is used to determine sensitivity of DCN responses to variations in the properties. With one exception, mixture DCNs were found to increase with lubricating oil content. Despite variation in calcium and magnesium concentrations, DCN responses for all oil blends indicate no statistically significant effect of either calcium or magnesium. Similarly, neither aging of nor peroxide addition to the oil yields significant DCN changes compared to untreated oils. However, a distinct response is found for variations in the base lubricant chemical structural properties. At 25% oil blending with gasoline surrogate, the measured DCNs (RONs) of different group base oils range from 19.6 (95.7) to 42.1 (46.2). The DCN increases with increasing base oil API Group Number (I through IV); however, mixture DCN was found to decrease for a 25% blend of Group V-B with the gasoline surrogate. Using quantitative 1H NMR, the Group Number trend is interpreted to be a consequence of linear vs. branched character of the paraffinic base oil composition. Taken together, the present results indicate that at ASTM D6890 DCN test conditions, there is no significant ignition effect attributable to reasonable variations in the lubricant's calcium or magnesium content, viscosity, or degree of degradation. Instead, the isomeric character of the paraffinic base oil appears to be most significant in controlling lubricant autoignition properties relative to those of gasolines.

The chemical functional group approach is investigated to verify the fundamental applicability of low-dimensional descriptors in the prediction of global combustion behavior, as described by homogeneous reflected shock ignition delay times. Three key chemical functional groups, CH2, CH3 and benzyl-type, are used to represent n-alkyl, iso-alkyl, and aromatic functionalities, respectively. To examine whether such descriptors can appropriately reflect the influences of these functionalities on ignition delay, Quantitative Structure-Property Relationship (QSPR) regression analysis is performed with the formulation of analytical models based on a fundamental Arrhenius-type description. The models are trained using literature measurements of reflected shock ignition delay times for stoichiometric fuel/air mixtures at 20 atm. Sensitivity analyses applied to the QSPR regression models show that the CH2 functional group dominates chemical kinetic behaviors at low temperature, while the chemical kinetic impacts of CH2, CH3, and benzyl-type functional groups all diminish as temperature increases. Further analyses of constant-volume adiabatic ignition delay predictions using detailed chemical kinetic models demonstrate influences of n-alkyl, iso-alkyl, and aromatic functionalities at both low and high temperature, consistent with those found for the QSPR regression models. Finally, 1H and 13C Nuclear Magnetic Resonance (NMR) spectroscopy is used to directly quantify the chemical functional group compositions of both petroleum-derived and alternative jet fuels. Combining the QSPR model with NMR spectra interpretation, the applicability of current approach as an expeditious tool to accurately characterize the ignition propensity of real transportation fuels is demonstrated by comparison with experimental measurements.

Cool flame combustion of individual and isolated sub-millimeter sized n -heptane ( n -C 7 H 16 ) and n -decane ( n -C 10 H 22 ) droplets are computationally investigated for atmospheric and higher operating pressure (25 atm) conditions with varying levels of ozone (O 3 ) mole fractions in the surroundings. A sphero-symmetric, one-dimensional, transient, droplet combustion model is utilized, employing reduced versions of detailed chemical kinetic models for the fuel species and an appended ozone reaction subset. Comprehensive parametric computations show that the regime of the cool flame burning mode and the transition from cool to hot flames are sensitive to the changes of O 3 loading, pressure, diluent variation, the strength of initiation source, and the influence of fuel vapor pressure at the ambient condition. For both fuels and over a range of O 3 concentrations in the ambient, sustained cool flame burning can be directly produced, even for sub-millimeter sized droplets. Over some range of O 3 concentrations, operating pressure, and drop diameter, a self-sustaining, continuous cool flame burn can be produced without incurring a hot flame transition. For sufficiently high O 3 concentrations, combustion initiation is always followed by a hot flame transition. Fuel volatility is also shown to be important for initiation and transition to cool flame and hot flame initiation. For fuels having a flash point lower than the ambient temperature (e.g. n- heptane), atomic O radicals formed by O 3 decomposition react with the partially premixed, flammable gas phase near the droplet surface, leading to OH radicals, water production, and heat that auto-thermally accelerates the combustion initiation process. For fuels with flashpoints higher than the ambient temperature (e.g. n -decane), the reaction progress is limited by the local fuel vapor concentration and the necessity to heat the droplet surface to sufficiently high temperatures to produce locally flammable conditions. As a result, the initial transient for establishing either cool flame or hot flame transition is significantly longer for high flash point fuels. The transition of locally partially premixed reaction to diffusive burning conditions is more evident for high flash point conditions.

The global chemical character of complex chemical fuel mixtures is explicitly determined by evaluating the abundances of chemical functional groups present within them rather than by applying a traditional interpretation based upon molecular species composition. Statistical analyses of the relationships among each chemical functional group and specific combustion property targets (CPTs) of the fuel are rigorously developed. The results demonstrate that the four CPTs currently used in aviation kerosene surrogate formulation - H/C molar ratio, derived cetane number (DCN), average molecular weight (MW), and threshold sooting index (TSI) - effectively constrain the chemical functional group distribution of the fuel, and, hence, the global combustion behaviors of pre-vaporized fuel/air mixtures. Successful emulation of the CPTs for a target real fuel involves developing a surrogate mixture that defines an “equivalent” chemical functional group distribution to that of the target fuel. Among the CPTs used for real fuel surrogate development, DCN does not abide by a linear blending rule, which generally frustrates development of surrogates. However, a quantitative structure–property relation (QSPR) regression for DCN is demonstrated here using the chemical functional group approach. Results of the regression reveal that the (CH 2 ) n group plays the most significant role in determining the fuel autoignition propensity, followed by the influences of CH 3 and benzyl-type groups. The QSPR functional group approach extends to provide a powerful tool to address potential preferential vaporization effects dictated by fuel distillation characteristics. Further analysis of fuel chemical property variation (DCN and H/C ratio) over the distillation curve (and other physical properties) provides a foundation for understanding the complex combustion behaviors of multi-phase and multi-component fuels relevant to real gas turbine engine applications.

Observations of “ Cool Flame ” burning for large diameter isolated droplets on board the International Space Station have stimulated interest in combustion initiation/generation of non-premixed combustion modes. For a number of n -alkane fuels at large initial droplet diameters, the initiation process was observed to first establish a hot flame condition that radiatively extinguished, followed by a quasi-steady, “ Cool Flame ” droplet burning mode. However, recent large diameter n -decane experiments show that depending on the ignition energy supplied, the first stage hot flame condition was absent, with an apparent, direct establishment of a “ Cool Flame ” burning mode that continued to diffusive extinction. Here we report these experimental observations and elucidate the underlying parameters resulting in dual and single stage “ Cool Flame ” burning. Detailed, transient sphero-symmetric droplet combustion modeling is applied to interpret the experiments. The simulations indicate that the balance and duration of the ignition energy applied, the energy release associated with reaction of partially premixed fuel vapor surrounding the droplet, heat flux to the drop surface, and far field diffusive heat loss all play key roles as to whether a dual stage, radiatively extinguished hot-flame-to- Cool - Flame -transition for only large droplets or direct establishment of “ Cool Flame ” burning for all droplet sizes occurs. The rate at which the reactive partially premixed vapor layer surrounding the droplet is formed, its volume, and its subsequent reaction significantly influence the observed transition to “ Cool Flame ” burning. The initial droplet temperature relative to saturation and flash point temperatures of the fuel and the liquid phase heat capacity contribute to the thermal transport requirement at the droplet surface for establishing the partially premixed reactive layer surrounding the droplet, which through its reaction history defines whether a transition to “ Cool Flame ” burning can be initiated without a requirement for radiative extinction of a hot flame burning mode.

We report experiments and detailed numerical modeling of mixtures of primary reference fuel (PRF) droplets consisting of n -heptane and iso -octane with initial droplet diameters of 0.5 and 3.51 mm. The results show a quasi-steady, low temperature (or “Cool Flame” ( CF )) droplet burning mode that stems from a varying two-stage chemical kinetic behavior of the combustion chemistry. The simulations further illustrate, that the CF droplet burning mode in 1 atm air is dependent upon the iso -octane fraction and droplet size. CF droplet burning is predicted to be absent for large diameter droplets containing more than 50% (by volume) iso -octane (>PRF50), and for all droplet diameters that exhibit hot flame burning without radiative extinction. The model predictions are in agreement with new, large diameter PRF50 experiments reported here, as well as previous ground-based PRF50 sub-millimeter diameter experiments. The effects of PRF mixture fraction are further analyzed numerically. Additional simulations show that replacing small amounts of inert (nitrogen) with ozone can sufficiently modify the low temperature kinetic activity of PRF50 droplets to promote CF droplet burning, even for sub-millimeter droplet diameters (with no hot flame transition). The implications are that with proper experimental configurations, CF droplet burning might be studied in ground-based, sub-millimeter diameter, isolated droplet burning experiments.

This study reports results of experiments on the isolated droplet burning characteristics of butanol isomers ( n -, iso -, sec -, and tert -) under standard atmosphere conditions in an environment that promotes spherical combustion. The data are compared with predictions from a detailed numerical model (DNM) that incorporates complex combustion chemistry, radiative heat transfer, temperature dependent variable fluid properties, and unsteady gas and liquid transport. Computational predictions are generated using the high temperature kinetic models of Sarathy et al. (2012) and Merchant et al. (2013). The experiments were performed in a free-fall facility to reduce the effects of buoyancy and produce spherical droplet flames. Motion of single droplets with diameters ranged from 0.52 mm to 0.56 mm was eliminated by tethering them to two small-diameter SiC filaments (∼14 µm diameter). In all the experiments, minimal sooting was observed, offering the opportunity for direct comparison of the experimental measurements with DNM predictions that neglect soot kinetics. The experimental data showed that the burning rates of iso - and sec -butanol are very close to that of n -butanol, differing only in flame structure. The flame stand-off ratios (FSR) for n -butanol flames are smaller than those for the isomers, while tert -butanol flames exhibited the largest FSR. DNM predictions based upon the kinetic model of Sarathy et al. over-predict the droplet burning rates and FSRs of all the isomers except n -butanol. Predictions using a kinetic model based upon the work of Merchant et al. agree much better with the experimental data, though relatively higher discrepancies are evident for tert -butanol simulation results. Further analyses of the predictions using the two kinetic models and their differences are discussed. It is found that the disparity in transport coefficients for isomer specific species for Sarathy et al. model fosters deviation in computational predictions against these newly acquired droplet combustion data presented in this study.

Isobutene is an important intermediate in the pyrolysis and oxidation of higher-order branched alkanes, and it is also a component of commercial gasolines. To better understand its combustion characteristics, a series of ignition delay time (IDT) and laminar flame speed (LFS) measurements have been performed. In addition, flow reactor speciation data recorded for the pyrolysis and oxidation of isobutene is also reported. Predictions of an updated kinetic model described herein are compared with each of these data sets, as well as with existing jet-stirred reactor (JSR) species measurements.IDTs of isobutene oxidation were measured in four different shock tubes and in two rapid compression machines (RCMs) under conditions of relevance to practical combustors. The combination of shock tube and RCM data greatly expands the range of available validation data for isobutene oxidation models to pressures of 50 atm and temperatures in the range 666-1715 K. Isobutene flame speeds were measured experimentally at 1 atm and at unburned gas temperatures of 298-398 K over a wide range of equivalence ratios. For the flame speed results, there was good agreement between different facilities and the current model in the fuel-rich region. Ab initio chemical kinetics calculations were carried out to calculate rate constants for important reactions such as H-atom abstraction by hydroxyl and hydroperoxyl radicals and the decomposition of 2-methylallyl radicals.A comprehensive chemical kinetic mechanism has been developed to describe the combustion of isobutene and is validated by comparison to the presently considered experimental measurements. Important reactions, highlighted via flux and sensitivity analyses, include: (a) hydrogen atom abstraction from isobutene by hydroxyl and hydroperoxyl radicals, and molecular oxygen; (b) radical-radical recombination reactions, including 2-methylallyl radical self-recombination, the recombination of 2-methylallyl radicals with hydroperoxyl radicals; and the recombination of 2-methylallyl radicals with methyl radicals; (c) addition reactions, including hydrogen atom and hydroxyl radical addition to isobutene; and (d) 2-methylallyl radical decomposition reactions. The current mechanism accurately predicts the IDT and LFS measurements presented in this study, as well as the JSR and flow reactor speciation data already available in the literature.The differences in low-temperature chemistry between alkanes and alkenes are also highlighted. in this work. In normal alkanes, the fuel radical (R) over dot adds to molecular oxygen forming alkylperoxyl (R(O) over dot(2)) radicals followed by isomerization and chain branching reactions which promote low-temperature fuel reactivity. However, in alkenes, because of the relatively shallow well (similar to 20 kcal mol(-1)) for R(O) over dot(2) formation compared to similar to 35 kcal mol(-1) in alkanes, the (R) over dot+O-2 (sic) R(O) over dot(2) equilibrium lies more to the left favoring (R) over dot+O-2 rather than R(O) over dot(2) radical stabilization. Based on this work, and related studies of allylic systems, it is apparent that reactivity for alkene components at very low temperatures (1300 K), the reactivity is mainly governed by the competition between hydrogen abstractions by molecular oxygen and OH radicals. (C) 2016 The Combustion Institute. Published by Elsevier Inc. All rights reserved. The work at NUI Galway was supported by Saudi Aramco under the FUELCOM program. Collaboration between NUI Galway and LRGP enters in the frame the COST Action CM1404. peer-reviewed 2017-03-17

Global combustion characteristics of iso-dodecane (2,2,4,6,6-pentamethylheptane, iC12) are measured and compared to those of iso-octane (2,2,4-trimethylpentane, iC8), iso-cetane (2,2,4,4,6,8,8-heptamethylnonane, iC16) and a 50/50 molar blend of iC8/iC16. The fuels are experimentally characterized by measuring combustion property targets (derived cetane number and smoke point/threshold sooting index), reflected shock ignition delay times at 20 and 40 atm, and extinction limits of strained laminar diffusion flames at 1 atm. The derived cetane number (DCN) and threshold sooting index (TSI) of iC12 are measured to be 16.8 (±1) and 15.4 (±0.5), respectively. In addition to average molecular weight (MW) and overall hydrogen-to-carbon ratio (H/C ratio), the combustion property targets for iC12 are very nearly molar averages of those for iC8 and iC16. Values agree very well with the measured combustion property indicators for the 50/50 blend of iC8/iC16. Further analysis of fuel chemical functional group distributions also finds that the abundances of methylene and methyl groups in iC12 and the 50/50 blend of iC8/iC16 are identical, further supporting that the global combustion characteristics of iC12 can be emulated by the molar averaged mixture of iC8 and iC16. Measurements of reflected shock ignition delays show that the ignition delay times of iC12 are in close agreement with those of the 50/50 molar mixture of iC8/iC16 over a broad range of temperature conditions. It is also found that the ignition delays of the three neat iso-alkanes exhibit quantitatively identical behaviors for both high and low temperature regimes, an observation that can guide the construction of combustion kinetic models for iC12. Measurements of strained diffusion flame extinction and their interpretation by the transport-weighted enthalpy (TWE) metric also support that the high temperature chemical kinetic potentials of the three iso-alkanes are essentially identical. Moreover, it is shown that molecular diffusivity (inherent in molecular weight) is the major parameter that differentiates flame extinction behaviors amongst the three iso-alkanes. In summary, this experimental study further supports the utility and characteristics of combustion property target formulation concepts for producing mixtures that emulate the pre-vaporized global combustion properties of a specific target fuel. The work also points to a strong correlation of the combustion property target data for these iso-alkanes with methylene and methyl functional group distributions for each fuel.

Pre-vaporized global combustion behaviors of a petroleum-derived jet fuel (JP-8), five alternative jet fuels (Shell SPK, Sasol IPK, HRJ Camelina, HRJ Tallow, and Gevo ATJ), and five 50/50 (liquid volume) blends of JP-8/alternative fuels are experimentally examined and compared. Three experiments are performed to investigate the gas-phase combustion behaviors of the tested fuel samples: (1) global oxidative species profiles in a variable pressure flow reactor, (2) diffusion flame extinction in a counterflow burner, and (3) premixed flame initiation in a heated spherical combustion chamber. Multivariate linear regression methods have been applied to investigate the sensitivities of pre-vaporized global combustion behaviors to individual combustion property targets of the fuels, including Derived Cetane Number (DCN), H/C ratio, mean molecular weight, and smoke point. As a proof of concept for fuel screening tool based on the standardized fuel property measurements, a “combustion property target (CPT) index” based upon this regression analysis is found to show promise as a rapid means to evaluate the global pre-vaporized combustion behaviors of the tested fuel samples against each other as well as the spectrum of JP-8 fuels found in use. The present work suggests the applicability of such a methodology not only as an expeditious fuel screening tool for assessing the fully pre-vaporized, kinetically coupled behaviors of emerging alternative jet fuel candidates, but further supports the use of combustion property targets in developing kinetic models that are specific to each real fuel.

Internal combustion engines running on liquid fuels will remain the dominant prime movers for road and air transportation for decades, probably for most of this century. The world’s appetite for liquid transportation fuels derived from petroleum and other fossil resources is already immense, will grow, will at some future time become economically unsustainable, and will become infeasible only in the very long term. The ongoing process of augmenting and eventually replacing petroleum-derived fuels with liquid alternative fuels must necessarily involve approaches that result in comparatively much lower net carbon cycle emissions from the transportation sector, most likely through a combination of carbon sequestration and renewable fuel production. The successful growth and establishment of a sustainable, profitable alternative fuels industry will be best facilitated by approaches that integrate alternative products into petroleum-derived fuel streams (i.e., gasolines, diesel, and jet fuels) and consider synergistic evolution of and integration with prevailing refining and liquid fuel distribution infrastructures. The emergence of low temperature combustion strategies, particularly those implementing dual fuel methods to achieve Reaction Controlled Compression Ignition (RCCI), offers the potential to significantly improve operating efficiency and reduce emissions with minimal aftertreatment. For all advanced combustion engine technologies, but especially for RCCI, a clear understanding of fuel property influences on combustion behaviors will be important to achieving projected engine performance and emissions. To achieve the benefits projected by emerging engine technologies, the properties of petroleum-derived fuels themselves must be modified over time, but the effects of blending candidate alternative fuels with these conventional fuels must also be understood. Predicting the coupled physical and chemical property effects of real fuels on energy conversion system performance and emissions is a daunting problem, even for petroleum-derived real fuels, since each is composed of several hundred to thousands of individual chemical species typically belonging to one of a few organic classes (e.g., n-paraffins, iso-paraffins, cyclo-paraffins, olefins, aromatics). For specific combustion applications, it is often the global combustion response to variations in the composition of fuel mixtures – inclusive of those occurring by blending petroleum-derived fuel with alternative fuel candidates – that is of interest for fuel property optimization. This paper presents an overview of tools used for evaluating and emulating combustion-relevant properties of real fuels and alternative fuel candidates. New analytical and statistical methods can provide important insights as to how the ensembles of distinct molecular structures found in a given fuel mixture contribute to the physical and chemical kinetic properties that govern its combustion in energy conversion processes. Such tools can in turn assist in screening candidate alternative fuels for more detailed study.

This paper investigates the autoignition of C3/C4 hydrocarbon mixtures in a CFR octane rating engine. The four species examined – propane, propylene (propene), n-butane and iso-butane – are the primary constituents of Liquefied Petroleum Gas (LPG), and are also important intermediates in the oxidation of larger hydrocarbons. In-cylinder pressure data was acquired for both autoigniting and non-autoigniting engine operation at the same test conditions. The latter was used to calibrate a two-zone model of the CFR engine in a prior work, thus enabling the inclusion of the unburned charge chemical kinetics for further examination in this paper. The in-cylinder heat transfer and residual gas composition are both shown to affect autoignition significantly. In particular, physically reasonable concentrations of nitric oxide (NO) are found to be a strong promoter of autoignition in almost all cases, in keeping with several, more fundamental studies. The inclusion of NO in the residual gas is also required to obtain good agreement between the measured and modelled autoignition timing. This in turn suggests that kinetic interaction between hydrocarbon fuels and NO plays a vital role in octane rating, and its inclusion is important when modelling the autoignition of hydrocarbons in spark-ignition engines more generally.

Ignition characteristics in the form of derived cetane numbers (DCN) of (hydro) furanic species are investigated experimentally in an Ignition Quality Tester. Further, quantum chemistry calculations at CBS-QB3 level of theory are applied to determine bond dissociation energies (BDEs) and thereby suggest the initial reactions of the ignition process for all of these fuels. Using the calculated BDEs, it is found that the ignition characteristics are similar among furans and among tetrahydrofurans, but strongly differ between these molecular classes. It is shown that the ignition behavior of aromatic furans is determined by the ring structure, which correlates with a negligible side chain influence. Hence, furan fuel structures can be chosen with respect to feasibility of the production pathways and engine compatibility. On the contrary, the side chain length for tetrahydrofurans defines the potential application paradigm. Tetrahydrofurans with short side chains are candidates for SI application, whereas 2-butyltetrahydrofuran may be a candidate for diesel application. The influence of the number and location of double bonds in the ring is illustrated with the additional study of dihydrofurans, and the influence of other functional groups is evaluated for (tetrahydro) furfuryl alcohols. To investigate possible fuel application scenarios, a second part of this study investigates DCNs of furanic fuel blends in n-heptane and diesel fuel. A DCN mixing rule is found to be approximately linear and for blends with up to 20 mol% furans, the side chain structure (alkyl, alcohol) has no distinct influence on the blend DCN.

Recent interest in alternative and bio-derived fuels has emphasized butanol over ethanol as a result of its higher energy density, lower vapor pressure and more favorable gasoline blending properties. Numerous efforts have examined the combustion of butanol from the perspective of low dimensional gas-phase transport configurations that facilitate modeling and validation of combustion kinetics. However, fewer studies have focused on multiphase butanol combustion, and none have appeared on isolated droplet combustion that couples experiments with robust modeling of the droplet burning process. This paper presents such an experimental/numerical modeling study of isolated droplet burning characteristics of n -butanol. The experiments are conducted in an environment that simplifies the transport process to one that is nearly one-dimensional as promoted by burning in a reduced gravity environment. Measurements of the evolution of droplet diameter ( D o = 0.56–0.57 mm), flame standoff ratio ( FSR ≡ D f / D ) and burning rate ( K ) are made in the standard atmosphere under reduced gravity and the data are compared against numerical simulation. The detailed model is based on a comprehensive time-dependent, sphero-symmetric droplet combustion simulation that includes spectrally resolved radiative heat transfer, multi-component diffusive transport, full thermal property variations and detailed chemical kinetic. The simulations are carried out using both a large order kinetic mechanism (284 species, 1892 reactions) and a reduced order mechanism (44 species, 177 reactions). The results show that the predicted burning history and flame standoff ratios are in good agreement with the measurements for both the large and reduced order mechanisms. Additional simulations are conducted for varying oxygen concentration to determine the limiting oxygen index and to elucidate the kinetic processes that dictate the extinction of the flame at these low oxygen concentrations.

Recently, large diameter, isolated n-heptane droplet experiments under microgravity conditions (aboard the International Space Station) exhibited “Cool Flame” burning behavior, resulting from a heat loss mechanism that extinguishes hot combustion and a transition into a sustained, low temperature second stage combustion. In atmospheric pressure air, a single combustion mode transition to “Cool Flame” burning is followed by diffusive extinction. But with increasing pressure, multiple cycles of hot initiation followed by transition to “Cool Flame” burning are observed. This paper reports experimental observations that characterize the transition time histories of this multi-cycle, multi-stage behavior. Transient sphero-symmetric droplet combustion modeling that considers multi-stage detailed kinetics, multi-component diffusion, and spectral radiation is applied to analyze the experimental observations. The simulations indicate that as parameters change the chemical time scales dictating low temperature degenerate chain branching, multiple hot/cool flame burning transitions are induced by increasing the cool flame burning heat generation rate compared to the diffusive loss rate. The balance of these terms in the negative temperature coefficient kinetic regime defines whether reactions accelerate into re-ignition of a hot flame event, burn quasi-steadily in the cool flame mode, or diffusively extinguish. The rate of reactions controlling ketohydroperoxide formation and destruction are shown to be key re-ignition of hot combustion from the cool flame mode. Predictions are found to be in good agreement with the experimental measurements. Modeling is further applied to determine how these observations are dependent on initial experimental conditions, including pressure, and diluent species.

Few studies of formaldehyde flames are available, especially at pressures greater than 55 torr, due to the difficulties and hazards associated with producing formaldehyde vapor. This work experimentally and numerically investigates the flame properties of formaldehyde (CH 2 O) and formyl radical (HCO) at high O 2 loadings and both atmospheric and reduced pressure by measuring and modeling the laminar burning rates of 1,3,5-trioxane/O 2 /N 2 mixtures. Trioxane is shown to decompose nearly exclusively into high concentrations of formaldehyde early in the flame structure before subsequent flame chemistry reactions occur. Kinetic model predictions show that the flame properties are controlled by CH 2 O and HCO kinetics. Laminar burning rate predictions of several combustion kinetic models vary significantly in comparison to experimental data and each other; however, all simulations show that the present observations are particularly sensitive to the competition between reactions HCO + M = H + CO + M (R3) and HCO + O 2 = HO 2 + CO (R4). Monte Carlo optimization of these rate coefficients allows interpretation of the measured flame speeds as indirect rate coefficient measurements at flame relevant temperatures. Although results from simple A-factor optimization agree well with the present measurements, three-parameter optimization is shown to be necessary in order to accurately model kinetics across a wide temperature range, including high temperature flames and low temperature direct rate measurements. A-factor and three-parameter optimization both show that a reduced k 3 /k 4 branching ratio over the temperature range from 1100 to 1700 K improves model predictions compared to present measurements.

The present discourse is directed toward the community that wishes to generate or use flow reactor data from complex chemical reactions as kinetic model development and validation targets. Various methods for comparing experimental data and computational predictions are in evidence in the literature, along with limited insights into uncertainties associated with each approach. Plug flow is most often assumed in such works as a simple, chemically insightful physical reactor model; however, only brief qualitative justifications for such an interpretation are typically offered. Modern tools permit the researcher to quantitatively confirm the validity of this assumption. In a single complex reaction system, chemical time scales can change dramatically with extent of reaction of the original reactants and with transitions across boundaries separating low temperature, intermediate temperature, and chain branched (high temperature) kinetic regimes. Such transitions can violate the underlying assumptions for plug flow interpretation. Further, uncertainties in reaction initialization may confound interpretation of experiments for which the plug flow assumption may be appropriate. Finally, various methods of acquiring experimental data can also significantly influence experimental interpretations. The following discussions provide important background for those interested in critically approaching the relatively vast literature on the application of flow reactors for generating kinetic validation data. The less frequently discussed influences of reactor simulation assumptions on modeling predictions are addressed through examples for which the kinetic behavior of specific reactant combinations may cause experimental observations to depart locally from plug flow behavior.

The global combustion characteristics of 2,6,10-trimethyl dodecane (trimethyl dodecane), a synthetic fuel candidate species, have been experimentally investigated by measuring extinction limits for strained laminar diffusion flames at 1 atm and reflected shock ignition delays at 20 atm. The Derived Cetane Number (DCN) of trimethyl dodecane, (59.1) and Hydrogen/Carbon (H/C) ratio (2.133) are very close to the DCN and H/C ratio of a previously studied synthetic aviation fuel, S-8 POSF 4734 (S-8) and its surrogate mixture composed of n-dodecane/iso-octane (58.9 and 2.19, respectively). Identical high temperature global kinetic reactivities are observed in all experiments involving the aforementioned compounds. However, at temperatures below ∼870 K, the S-8 surrogate mixture has ignition delay times approximately a factor of two faster. A chemical functional group analysis identifies that the methylene (CH2) to methyl (CH3) ratio globally correlates the low temperature alkylperoxy radical reactivity for these large paraffinic fuels. This result is further supported experimentally, by comparing observations using a surrogate fuel mixture of n-hexadecane (n-cetane) and 2,2,4,4,6,8,8-heptamethyl nonane (iso-cetane) that shares the same methylene-to-methyl ratio as trimethyl dodecane, in addition to the same DCN and H/C ratio. Measurements of both diffusion flame extinction and reflected shock ignition delays show that the n-cetane/iso-cetane model fuel has very similar combustion behavior to trimethyl dodecane at all conditions studied. A kinetic modeling analysis on the model fuel suggests the formation of alkylhydroperoxy radicals (QOOH) to be strongly influenced by the absence or presence of the methyl and methylene functional groups in the fuel chemical structure. The experimental observations and analyses suggest that paraffinic based fuels having high DCN values may be more appropriately emulated by further including the CH2 to CH3 ratio as an additional combustion property target, as DCN alone fails to fully distinguish the relative reaction characteristics of low temperature kinetic phenomena.

Recent experimentally observed two stage combustion of n-heptane droplets in microgravity is numerically studied. The simulations are conducted with detailed chemistry and transport in order to obtain insight into the features controlling the low temperature second stage burn. Predictions show that the second stage combustion occurs as a result of chemical kinetics associated with classical premixed “Cool Flame” phenomena. In contrast to the kinetic interactions responsible for premixed cool flame properties, those important to cool flame droplet burning are characteristically associated with the temperature range between the turnover temperature and the hot ignition. Initiation of and continuing second stage combustion involves a dynamic balance of heat generation from diffusively controlled chemical reaction and heat loss from radiation and diffusion. Within the noted temperature range, increasing reaction temperature leads to decreased chemical reaction rate and vice versa. As a result, changes of heat loss rate are dynamically balanced by heat release from chemical reaction rate as the droplet continues to burn and regress in size. At reaction temperatures below the turnover, heat loss over takes the heat release rate and extinction occurs. Should heat release exceed heat loss as the temperature increases to that for hot ignition, initiation of a high temperature burning phase may be possible. Parametric study on factors leading to initiation of the second stage burning phenomena are studied. Results show that both carbon dioxide and helium diluents can promote initiation of low temperature burning at smaller initial drop diameters than found with nitrogen as diluent. Small amounts of carbon dioxide and helium in the ambient is sufficient to activate the phenomena. The chemical kinetics dictating the second stage combustion and extinction process is also discussed.

It has been suggested that radiation heat loss may be a large source of experimental uncertainty in flame speed measurements using the outwardly propagating spherical flame method. Thermal radiation is usually not considered in interpretation of these experiments, yet it may contribute significantly to uncertainty especially for model-constraining conditions at low flame temperature and high pressure. In the present work, a conservative analytical estimate of the effects of radiation heat loss is derived and validated against detailed numerical simulations. A solver with a graphical interface is provided in the Supplemental material to allow implementation of these analytical results. The analytical estimate considers the radiation induced burned gas motion as well as the decreasing flame temperature due to conduction to the radiating burned gas and radiation loss from the flame zone. The results show that previous measurements of hydrogen flame speeds at low flame temperature by Burke et al. (2010) [3] are minimally affected by radiation, but flames with low flame speeds can be strongly inhibited by radiative loss. Future laminar spherical flame measurements and interpretation of existing determinations with low adiabatic flame speeds must include consideration of radiation effects to avoid large uncertainties.

This paper reports the Research (RON) and Motor (MON) Octane Numbers of ethanol blended with production gasoline, four gasoline surrogates, n-heptane, isooctane and toluene. The ethanol concentration was varied from zero to 100%, resulting in a clear picture of the variations of the RONs and MONs in all cases. Of initial interest are the RONs and MONs of ethanol blended with an Australian production gasoline and with several US production gasolines. The observed differences then prompt a systematic study of the variation in the RONs and MONs of ethanol blended with four gasoline surrogates, as well as with n-heptane, isooctane and toluene. Both n-heptane, isooctane and their Primary Reference Fuels (PRFs) are shown to blend synergistically with ethanol, whilst toluene blends antagonistically. Consistent with these trends, a progressive increase in the toluene content in Toluene Reference Fuels (TRFs) of a constant RON results in increasingly linear ethanol/TRF blending. Together, these results show that the antagonism of ethanol’s blending with toluene acts against its synergism with isooctane and n-heptane, and more broadly suggest that the antagonism of ethanol’s blending with aromatics may act against its synergism with paraffins. If correct, this explains trends observed both in the literature and in this study, and has implications for fuel design.

Trimethylbenzenes have been suggested as useful components for the formulation of simple hydrocarbon mixtures that quantitatively emulate the gas-phase combustion behaviour of real liquid transportation fuels as model or surrogate fuels. To facilitate this application, various combustion properties of 1,3,5-trimethylbenzene (mesitylene) have been characterised experimentally and a new chemical kinetic model for its combustion constructed. Experimental determinations of 1,3,5-trimethylbenzene reflected shock ignition delay, laminar burning velocities and high-pressure flow reactor oxidative reactivity profiles are presented. These data allow for the testing of a detailed kinetic model, developed by direct analogy to, and incorporating as a subcomponent, a recent comprehensively tested kinetic model for toluene oxidation [Metcalfe WK, Dooley S, Dryer FL. Energy Fuels 2011; 25: 4915–4936]. Model calculations are also compared against data pertinent to 1,3,5-trimethylbenzene combustion phenomena from the published literature. The modelling approach allows for the accurate reproduction of the global combustion phenomena of ignition delay, burning velocity, diffusive and premixed strained extinction limits and flow reactor reactivity, with some noted shortcomings. Analyses of the constructed model suggest that the mechanism of 1,3,5-trimethylbenzene combustion occurs through the formation of 3,5-dimethylbenzaldehyde and 1,2-bis(3,5-dimethylphenyl) ethane as the major stable intermediate species, with relative proportions depending on the conditions of the particular reacting environment.

This paper presents an experimental study of the Research (RON) and Motor (MON) octane numbers of Liquefied Petroleum Gas (LPG). A comprehensive set of RON and MON data for mixtures of propane, propylene (propene), n-butane and iso-butane are presented, using a method that is consistent with the currently active ASTM Research and Motor test methods for liquid fuels. Empirical models which relate LPG composition to its RON and MON are then developed, such that the simplest relationships between the constituent species’ mole fractions and the mixture octane rating are achieved. This is used to determine the degree of non-linearity between the composition and the RON and MON of different LPG mixtures. Finally, implications for LPG fuel quality standards are discussed briefly, as part of a suggested, more substantial undertaking by the community which also revisits the standard test procedures for measuring the RON and MON of LPG.

This study presents an experimental effort and detailed numerical simulation of the burning process of an ester-based biodiesel fuel droplet – methyl decanoate (MD). The experiments are carried out using test droplets that are anchored to small SiC support structures (14 lm diameter) and that burn in an ambience subjected to a low gravity level to promote spherical symmetry in the droplet burning process. The initial droplet diameters are 0.53–0.57 mm and the combustion gas is normal atmospheric pressure air. A detailed numerical simulation of the burning process is also presented that features detailed MD combustion chemistry, radiative heat transfer, species diffusion, and phase change effects to predict the evolution of droplet and flame diameter. The analysis also incorporates a model for heat transfer through the droplet support structure. Predicted droplet and flame diameters are shown to agree within the range of uncertainty of the experimental data lending support to the transport model and combustion kinetics incorporated in the simulation. Effects of the tether fiber’s size, thermal conductivity, and Nusselt number on the droplet burning data are also examined. Results show that the fiber properties within the range investigated in this study do not significantly affect the burning process. Species and temperature distributions during the transient process of MD droplet combustion are detailed in this study. These comparisons and predictions for multi-phase combustion of MD that have not been seen in the literature provide validation for MD models associated with chemical kinetics and multi-physics and are therefore valuable for the study of esterbased biodiesel combustion.

Combustion characteristics of isolated sub-millimeter sized methyl butanoate (MB) droplets are studied at low gravity (10−4 m/s2) in a 1.2 s drop tower. In the experiments, droplets were grown and deployed onto the intersection of two 14 μm silicon carbide fibers in a cross-string arrangement and exposed to symmetrically placed spark ignition sources. The initial droplet diameter was fixed at 0.54 ± 0.01 mm, and experiments were carried out in room temperature air at atmospheric pressure. Detailed measurements of the evolution of droplet diameter, flame standoff ratio and burning rate are reported. The experimental results are compared against predictions from a comprehensive time-dependent, sphero-symmetric droplet combustion simulation that includes detailed gas phase chemical kinetics, spectrally resolved radiative heat transfer, multi-component diffusive transport, full thermal property variations and tether fiber perturbation effects. The predicted combustion characteristics of MB are also compared with n-heptane droplets of nearly identical sizes over a range of oxygen concentrations. The results show that predicted burning histories, burning rates and flame standoff ratios are in excellent agreement with the measurements. The average burning rates and flame temperatures for both fuels were found to be similar even though the heat of combustion of n-heptane is higher by a factor of ∼1.6. However, the average flame standoff ratio for MB was found to be significantly smaller than for n-heptane, due to the presence of additional oxygen atoms in the parent fuel. Important differences in the diffusion flame chemistries of the two fuels are also discussed.

H 2 O 2 is one of the most important species in dimethyl ether (DME) oxidation, acting not only as a marker for low temperature kinetic activity but also responsible for the “hot ignition” transition. This study reports, for the first time, direct measurements of H 2 O 2 and CH 3 OCHO, among other intermediate species concentrations in helium-diluted DME oxidation in an atmospheric pressure flow reactor from 490 to 750 K, using molecular beam electron-ionization mass spectrometry (MBMS). H 2 O 2 measurements were directly calibrated, while a number of other species were quantified by both MBMS and micro gas chromatography to achieve cross-validation of the measurements. Experimental results were compared to two different DME kinetic models with an updated rate coefficient for the H + DME reaction, under both zero-dimensional and two-dimensional physical model assumptions. The results confirm that low and intermediate temperature DME oxidation produces significant amounts of H 2 O 2 . Peroxide, as well as O 2 , DME, CO , and CH 3 OCHO profiles are reasonably well predicted, though profile predictions for H 2 /CO 2 and CH 2 O are poor above and below ∼625 K, respectively. The effect of the collisional efficiencies for the H + O 2 + M = HO 2 + M reaction on DME oxidation was investigated by replacing 20% He with 20% CO 2 . Observed changes in measured H 2 O 2 concentrations agree well with model predictions. The new experimental characterizations of important intermediate species including H 2 O 2 , CH 2 O and CH 3 OCHO, and a path flux analysis of the oxidation pathways of DME support that kinetic parameters for decomposition reactions of HOCH 2 OCO and HCOOH directly to CO 2 may be responsible for model under-prediction of CO 2 . The H abstraction reactions for DME and/or CH 2 O and the unimolecular decomposition of HOCH 2 O merit further scrutiny towards improving the prediction of H 2 formation.

This work investigates experimentally and numerically the kinetic effects of water vapor addition on the burning rates of H 2 , H 2 /CO mixtures, and C 2 H 4 from 1 atm to 10 atm at flame temperatures between 1600 K and 1800 K. Burning rates were measured using outwardly propagating spherical flames in a nearly constant pressure chamber. Results show good agreement with newly updated kinetic models for H 2 flames. However, there is considerable disagreement between simulations and measurements for H 2 /CO and C 2 H 4 flames at high pressure and high water vapor dilution. Both experiments and simulations show that water vapor addition causes a monotonic decrease in mass burning rate and the inhibitory effect increases with pressure. For hydrogen flames, water vapor addition reduces the critical pressure above which a negative pressure dependence of the burning rate is observed. However, for C 2 H 4 flames, the burning rate always increases with pressure. The results also show that water vapor addition has the same effect as a pressure increase for H 2 and H 2 /CO flames, shifting the reaction zone into a narrower window at higher temperatures. For all fuels, water vapor addition increases OH formation via H 2 O + O while reducing the overall active radical pool for hydrogen flames. For C 2 H 4 , the additional HO 2 production pathway through HCO results in a dramatic difference in pressure dependence of the burning rate from that observed for hydrogen. The present work provides important additions to the experimental database for syngas and C 0 –C 2 high pressure kinetic model validations.

Quasi-steady burning and extinction of droplets are of interest from both fundamental and application viewpoints. The latter is related to combustor performance and fire safety issues in reduced gravity environments. Influences of diluent in the atmosphere on isolated droplet combustion characteristics including extinction provide insights to fire extinguishment phenomena and the effectiveness of various diluents as fire suppressants. Extinction of pure methanol and methanol–water droplets ranging from 1.5 to 7 mm size, for varying levels of ambient carbon-dioxide, helium and oxygen concentration – burning in a quiescent microgravity environment were studied numerically to compare the effectiveness of fire suppressant diluent selection and determining the limiting oxygen index. The results show distinct regimes of diffusive and radiative extinction. The transition from diffusive to radiative extinction is strongly influenced by the ambient diluent selection, especially by carbon dioxide concentration. Results for helium as the diluent showed increased burning rate and extinction due to diffusive heat loss. An “ extinction characteristic ” correlation is proposed that depends on burning rate, ambient diffusivity and flame standoff ratio. Recent methanol droplet experiments conducted over a wide range of operating conditions onboard the International Space Station were found to yield results that agree well with the proposed “ extinction characteristic ” correlation.

A surrogate fuel is formulated in an a priori manner through a combustion property matching technique to emulate the gas phase chemical kinetic combustion phenomena of S-8 POSF 4734, an alternative aviation fuel derived from natural gas via the Fischer–Tropsch process. A fundamental concept is described which identifies n-dodecane and iso-octane as being appropriate surrogate fuel components for the non-aromatic synthetic fuels. The performance of the formulated 51.9/48.1 mole % n-dodecane/iso-octane mixture as a surrogate for the target real fuel is evaluated by the measurement of a series of combustion phenomena exhibited by both fuels including: (1) The oxidative reactivity of stoichiometric mixtures of each fuel in O2/N2 at 12.5 atm and 500–1050 K, for a residence time of 1.8 s at a fixed carbon content of 0.3% using a variable pressure flow reactor. (2) The autoignition behavior of stoichiometric mixtures of each fuel in air at compressed conditions of 667–1223 K and ∼20 atm by the reflected shock technique. (3) The strained extinction limits of diffusion flames of each fuel at 1 atm. The performance of available kinetic models for n-dodecane/iso-octane mixtures is evaluated by analysis of their computations of this experimental data. Furthermore, the impact of oxidation kinetics unique to the mono methylated alkanes which are the dominant molecular structure in synthetic fuels is examined by an experimental study involving the formulation of an n-decane/iso-octane mixture as a surrogate fuel for 2-methyl heptane, a proposed model molecule for such real fuel components.

The laminar flame speeds and premixed extinction limits of n -propylbenzene, 1,2,4-trimethylbenzene, 1,3,5-trimethylbenzene, and toluene have been studied experimentally to assess the effects of different alkyl substitutions to the benzene ring on flame propagation and extinction. The experiments were carried out in a twin-flame counterflow setup under atmospheric pressure. The laminar flame speeds of fuel/air mixtures at two unburned mixture temperatures of 400 K and 470 K were determined over an equivalence ratio range of ϕ = 0.7–1.4. Additionally, the extinction stretch rates of fuel/O 2 /N 2 mixtures at an unburned mixture temperature of 400 K were measured over an equivalence ratio range of ϕ = 0.8–1.6, with an oxidizer composition of 16% O 2 and 84% N 2 by mole. The experimental laminar flame speeds and extinction stretch rate values were compared to simulated results, for each fuel, using detailed kinetic models available in the literature. The simulation results were found to be in reasonable agreement with the current experimental data, except for 1,2,4-trimethylbenzene, where the model under-predicts the extinction limits significantly. Sensitivity and flux analyses were conducted to identify reactions and species to which the computed results were most sensitive.

A detailed kinetic model for the oxidation of the biodiesel surrogate, methyl decanoate, has been developed and tested against a broad range of experimental data. The methyl decanoate model consists of both low and high temperature oxidation chemistry. It has been constructed strictly through the extension of the chemical kinetic and thermochemical parameters used to describe the oxidation of the better-understood small methyl ester, methyl butanoate. The constructed model is tested in an a priori manner by the computation of all of the appropriate experimental data available for methyl decanoate oxidation. The results show a generally improved performance of the present model relative to that of literature models which have generally been constructed based on similarity to alkane oxidation reaction kinetics. Chemical path flux analyses of all available methyl decanoate models are analyzed and the results reveal that the fuel oxidation pathways exhibit completely different chemical mechanisms depending on the modeling prescriptions of the kinetic and thermochemical parameters. In particular, there is a wide degree of variability in the fate prescribed to the methyl ester functionality. In addition, experimental analysis of diffusion flame extinctions for methyl butanoate and methyl decanoate reveals that the high temperature reactivity of methyl butanoate is similar to that of methyl decanoate by introducing a concept of transport-weighted enthalpy. Consequently, the present modeling work and experimental analysis suggest that further studies of small methyl ester systems, such as methyl butanoate are required in order to improve the model fidelity of large biodiesel like methyl esters.

The extinction limits of methyl butanoate, n-heptane, and methyl butanoate/n-heptane diffusion flames have been measured as a function of fuel mole fraction with nitrogen dilution in counterflow with air. On a mole fraction basis, methyl butanoate diffusion flames are observed to have a much lower extinction strain rate than n-heptane diffusion flames and the extinction strain rate of n-heptane/methyl butanoate diffusion flames is observed to increase significantly as the n-heptane fraction is increased. Based on previous works, detailed chemical kinetic models to describe the high temperature oxidation of these fuel mixtures are assembled, tested and reduced. When the transport properties of ester species are re-evaluated by means of a thorough literature review, numerical computations of extinction generally reproduce experimental results for the pure fuels as well as for mixtures. An in-depth analysis of the kinetic model computations reveals that the extinction behaviour of both fuels is due to (1) fuel energy content affects and (2) the chemical kinetic potential of each fuel to produce the hydroperoxy radical. Comparatively, in n-heptane flames reactive ethyl radicals and ethylene are the major intermediates formed, but in methyl butanoate flames the major intermediates are formyl radicals and formaldehyde. In all flames studied, increased strain rates affect an increased interaction of formyl and/or vinyl radicals with molecular oxygen leading to a transition from hydrogen atom production at low strain rates, to the production of large quantities of the hydroperoxy radical at higher strain rates. The formation of the hydroperoxy radical induces extinction in each flame by directly interfering with the important radical chain branching and exothermic elementary reactions of H atoms and OH radicals that are dominant in weakly strained flames. It is postulated that the similar inhibitive effect of methyl butanoate fuelled flames will also be observed for more biodiesel like, larger n-alkyl esters when compared to equivalent n-alkanes. The diffusive extinction limits of methyl decanoate diffusion flames are also measured and show reactivity comparable to n-heptane diffusion flames by a molar comparison.

A methodology for the formulation of surrogate fuels for the emulation of real fuel gas phase combustion kinetic phenomena pertinent to gas turbine combustion is described and tested. A mixture of n -dodecane/ iso -octane/1,3,5-trimethylbenzene/ n -propylbenzene is formulated in a predictive manner to exhibit the same gas phase combustion phenomena of a target Jet-A fuel by the sharing of fundamentally significant combustion property targets in addition to a prescribed commonality of chemical kinetically controlling intermediate species. The appropriateness of the surrogate formulation technique is demonstrated by the experimental measurement of various gas phase combustion kinetic phenomena of the proposed surrogate mixture and of the target Jet-A fuel: (1) A variable pressure flow reactor is used to chart the chemical reactivity of a stoichiometric mixture of surrogate fuel/O 2 /N 2 at 12.5 atm and 500–1000 K, for a residence time of 1.8 s at a fixed carbon content of 0.3%. (2) The autoignition behavior of stoichiometric mixtures of surrogate fuel in air is measured with a shock tube at 667–1223 K at ∼20 atm and also with a rapid compression machine at 645–714 K at compressed pressures of 21.7 atm. (3) Detailed measurements of the intermediate species formed in the high temperature oxidation of the target fuel and in the oxidation of the surrogate fuel are performed with a shock tube for reaction times of 1.23–3.53 ms at 18–35 atm and 901–1760 K for 0.0808/0.158/0.1187 mole% mixtures of C/H/O 2 . (4) The laminar burning velocity and strain extinction limits of premixed mixtures of surrogate fuel in O 2 /N 2 are determined by the counter flow twin flame technique. These phenomena are also determined for premixed mixtures of the target fuel and for a previously proposed surrogate fuel composed of n -decane/ iso -octane/toluene in O 2 /N 2 . (5) The high temperature chemical reactivity and chemical kinetic–molecular diffusion coupling of the surrogate fuel is evaluated by measurement of the strained extinction limits of diffusion flames. (6) The propensity of surrogate and real fuel to form soot is tested by laser extinction measurements of the soot volume fractions formed by each fuel in a wick-fed laminar flame diffusion burner as a function of the radial distance of each flame. These experimental data are compared to those previously reported at identical conditions for the target Jet-A fuel and for a similar n -decane/ iso -octane/toluene surrogate fuel. A conceptual theory of real fuel oxidation is proposed and the similarity of the exhibited combustion phenomena of all three fuels is analyzed and interpreted in this context in order to (a) further evaluate the proposed strategy to surrogate fuel formulation and the appropriateness of the proposed theory to real fuel oxidation, (b) evaluate the appropriateness of the proposed n -dodecane/ iso -octane/1,3,5-trimethylbenzene/ n -propylbenzene mixture as a surrogate fuel for the target Jet-A fuel, and (c) to provide direction for the development of a tractable numerical modeling framework to compute real fuel multiphase combustion phenomena.

The combustion chemistry of tertiary -butanol is studied experimentally in a high pressure flow reactor and in counterflow diffusion flames. Princeton Variable Pressure Flow Reactor results show that t -butanol does not exhibit low temperature chemistry, and thus has no negative temperature coefficient behavior under the studied conditions. The onset of gas phase chemistry at high pressure occurs at ∼780 K. Over the temperature range of 780–950 K, t -butanol primarily reacts through hydrogen abstraction − alkyl or alkoxy radical beta-scission pathways to form methyl and propen-2-ol, which likely tautomerizes in the sampling system to form acetone. A species sampling study of a t -butanol counterflow diffusion flame reveals that the high temperature consumption routes of t -butanol lead to the stable intermediates isobutene, acetone, and methane, with isobutene existing in the highest concentrations. The extinction limits of t -butanol, isobutene, acetone, and methane diffusion flames are also reported. On a transport-weighted enthalpy basis, t -butanol extinguishes more readily than any of its primary intermediates, signifying that it is kinetically less resistant to extinction than the products of its initial reactions. Numerical simulation of these t -butanol flames reveals that the isobutene and acetone chemistry sub-models significantly affect the computed extinction limits. Improvement in the current understanding of isobutene oxidation kinetics, in particular, appears necessary to developing reliable kinetic models for t -butanol combustion.

The extinction limits of diffusion flames have been measured experimentally and computed numerically for fuels of three different molecular structures pertinent to surrogate fuel formulation: n-alkanes, alkyl benzenes, and iso-octane. The focus of this study is to isolate the thermal and mass transport effects from chemical kinetic contributions to diffusion flame extinction, allowing for a universal correlation of extinction limit to molecular structure. A scaling analysis has been performed and reveals that the thermal and mass transport effects on the extinction limit can be normalized by consideration of the enthalpy flux to the flame via the diffusion process. The transport-weighted enthalpy is defined as the product of the enthalpy of combustion per unit mole of fuel and the inverse of the square root of fuel molecular weight. The chemical kinetic contribution provided by the specific fuel chemistry has thus been elucidated for tested individual component and multi-component surrogate fuels. A chemical kinetic flux analysis for n-decane flames shows that the production/consumption rates of the hydroxyl (OH) radical govern the heat release rate in these flames and therefore play significant roles in defining the extinction limit. The rate of OH formation has been defined by considering the OH concentration, flame thickness, and flow strain rate. A fuel-specific radical index has been introduced as a concept to represent and quantify the kinetic contribution to the extinction limit owing to the fuel-specific chemistry. A relative radical index scale, centered on the radical index of a series of n-alkanes which are observed and fundamentally explained to be common, is established. A universal correlation of the observed extinction limits of all tested fuels has been obtained through a combined metric of radical index and transport-weighted enthalpy. Finally, evidence as to the validity of the fundamental arguments presented is provided by the success of the universal correlation in predicting the extinction limits of the multi-component mixtures typical of surrogate fuels.

Tethered methanol droplet combustion in carbon dioxide enriched environment is simulated using a transient one-dimensional spherosymmetric droplet combustion model that includes the effects of tethering. A priori numerical predictions are compared against recent experimental data. The numerical predictions compare favorably with the experimental results and show significant effects of tethering on the experimental observations. The presence of a relatively large quartz fiber tether increases the burning rate significantly and hence decreases the extinction diameter. The simulations further show that the extinction diameter depends on both the initial droplet diameter and the ambient concentration of carbon dioxide. Increasing the droplet diameter and ambient carbon dioxide concentration both of them lead to a decrease in the burning rate and increase in the extinction diameter. The influence of ambient carbon dioxide concentration on extinction shows a sharp transition in extinction for larger size droplets ( d o > 1.5 mm) due to a change in the mode of extinction from diffusive to radiative control. In addition predictions from the numerical model is compared against a recently developed simplified theoretical model for predicting extinction diameter for methanol droplets, where the presence and heat transfer contribution of the tether is not taken into account implicitly. The numerical results suggest some limitation in the theoretical modeling assumptions for favorable comparisons with the experimental data.

The oxidation of n -decane/oxygen/nitrogen is studied at stoichiometric conditions of 1000 ppm fuel in the Princeton variable pressure flow reactor at temperatures of 520–830 K and pressures of 8 and 12.5 atm. The overall oxidative reactivity of n -decane is observed in detail to show low temperature, negative temperature coefficient (NTC) and hot ignition regimes. Detailed temporal speciation studies are performed at reactor initial temperatures of 533 K and 740 K at 12.5 atm pressure and 830 K at 8 atm pressure. Significant amounts of large olefins are produced at 830 K, at conditions of transition from NTC to hot ignition behavior. The predictions using available chemical kinetic models for n -decane oxidation are compared against each other and the experiments. Only the kinetic models of Westbrook et al., Ranzi et al., and Biet et al. capture the NTC behavior exhibited by n -decane. However, each of these models yields varying disparities in the mechanistic predictions of major intermediate species, including ethylene and formaldehyde. Analyses of the Westbrook et al. model are compared with the new data. The predicted double-peaked species yield of ethylene, a behavior not found for the other models or in the experimental observations results from deficiencies in the C 2 chemistry. Mechanistic validation information about fuel oxidation chemistry is also provided by the measurement of various larger carbon number alkene isomers at 830 K and 8 atm. The modeling analysis suggests that in addition to n -alkyl beta-scission chemistry, alkyl peroxy radical chemistry contributes significantly to the formation of these alkenes. Specific reaction pathways and rate constants which affect the computation of these observations are discussed.

The oxidation of methyl formate (CH3OCHO), the simplest methyl ester, is studied in a series of burnerstabilized laminar flames at pressures of 22–30 Torr and equivalence ratios (U) from 1.0 to 1.8 for flame conditions of 25–35% fuel. Flame structures are determined by quantitative measurements of species mole fractions with flame-sampling molecular-beam synchrotron photoionization mass spectrometry (PIMS). Methyl formate is observed to be converted to methanol, formaldehyde and methane as major intermediate species of mechanistic relevance. Smaller amounts of ethylene and acetylene are also formed from methyl formate oxidation. Reactant, product and major intermediate species profiles are in good agreement with the computations of a recently developed kinetic model for methyl formate oxidation [S. Dooley, M.P. Burke, M. Chaos, Y. Stein, F.L. Dryer, V.P. Zhukov, O. Finch, J.M. Simmie, H.J. Curran, Int. J. Chem. Kinet. 42 (2010) 527–529] which shows that hydrogen abstraction reactions dominate fuel consumption under the tested flame conditions. Radical–radical reactions are shown to be significant in the formation of a number of small concentration intermediates, including the production of ethyl formate (C2H5OCHO), the subsequent decomposition of which is the major source of observed ethylene concentrations. The good agreement of model computations with this set of experimental data provides a further test of the predictive capabilities of the proposed mechanism of methyl formate oxidation. Other salient issues in the development of this model are discussed, including recent controversy regarding the methyl formate decomposition mechanism, and uncertainties in the experimental measurement and modeling of low-pressure flame-sampling experiments. Kinetic model computations show that worst-case disturbances to the measured temperature field, which may be caused by the insertion of the sampling cone into the flame, do not alter mechanistic conclusions provided by the kinetic model. However, such perturbations are shown to be responsible for disparities in species location between measurement and computation.

Recent optical engine studies have linked increases in NO x emissions from fatty acid methyl ester combustion to differences in the premixed autoignition zone of the diesel fuel jet. In this study, ignition of single, isolated liquid droplets in quiescent, high temperature air was considered as a means of gaining insight into the transient, partially premixed ignition conditions that exist in the autoignition zone of a fatty acid methyl ester fuel jet. Normal gravity and microgravity (10 −4 m/s 2 ) droplet ignition delay experiments were conducted by use of a variety of neat methyl esters and commercial soy methyl ester. Droplet ignition experiments were chosen because spherically symmetric droplet combustion represents the simplest two-phase, time-dependent chemically reacting flow system permitting a numerical solution with complex physical submodels. To create spherically symmetric conditions for direct comparison with a detailed numerical model, experiments were conducted in microgravity by use of a 1.1 s drop tower. In the experiments, droplets were grown and deployed onto 14 μm silicon carbide fibers and injected into a tube furnace containing atmospheric pressure air at temperatures up to 1300 K. The ignition event was characterized by measurement of UV emission from hydroxyl radical (OH*) chemiluminescence. The experimental results were compared against predictions from a time-dependent, spherically symmetric droplet combustion simulation with detailed gas phase chemical kinetics, spectrally resolved radiative heat transfer and multi-component transport. By use of a skeletal chemical kinetic mechanism (125 species, 713 reactions), the computed ignition delay period for methyl decanoate (C 11 H 22 O 2 ) showed excellent agreement with experimental results at furnace temperatures greater than 1200 K.