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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.

19 September 1923 – 14 December 2019Irvin Glassman, the Robert H. Goddard Professor of Mechanical and Aerospace Engineering, Emeritus at Princeton University, passed away on December 14, 2019, at h...

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.

This paper presents an experimental and numerical investigation of the autoignition of ethanol blended with several primary reference fuels (PRFs) and toluene reference fuels (TRFs) in a Cooperative Fuel Research (CFR) engine. Autoigniting, in-cylinder pressure traces are acquired under standard research octane number (RON) conditions. Equivalent, non-autoignitiing traces are obtained by adding a small amount of tetraethyl lead (TEL) to each fuel to suppress knock, and these are used to calibrate a two-zone engine model of autoignition. The simulated autoignition timing of the PRFs without ethanol, TRFs without ethanol, and ethanol/PRF and ethanol/TRF blends are then compared to those measured in the engine. These results suggest that the incorporation of residual NO significantly improves the agreement between simulations and experiment for all cases with no or low ethanol content. Ethanol also appears to suppress the low-temperature activity of n-heptane. Both of these results are consistent with previo...

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.

The reactivity-promoting effect of trace nitrogen oxides (NOx) on post-induction oxidation of a synthetic natural gas (2% ethane in methane) has been experimentally studied in a high-pressure laminar flow reactor (HPLFR) at 10 ± 0.1 atm, nominal reaction temperature of 818 ± 5 K, and several equivalence ratios (φ ∼ 0.5, 1.0, and 2.0). Each set of experimental measurements was simulated using several literature C0–C2 + NOx kinetic models, both recent and legacy, using approaches shown to lead to robust interpretation of present experimental conditions. Coupling between the NOx and C0–C2 submodel components of these models varies significantly in both qualitative (mechanistic) and quantitative character. A comparison among the experimental measurements and modeling results serves to highlight important kinetic features particular to application-relevant natural gas oxidation in presence of trace (∼25 ppm) NOx. Additional insight is offered by a baseline experiment with no NOx perturbation, which shows that ...

This chapter is based on our experience in characterizing and formulating surrogate compositions to emulate prevaporized and multiphase global combustion behaviors of jet fuels. It is not our purpose here to provide an exhaustive review of the literature on this subject. Our purpose is to describe a logical thought process toward the development of numerical methods that analyze the interactions of real fuels in applied combustion systems. The hypotheses emphasize the relevance of the controlling global combustion behaviors that are variable from fuel to fuel and from condition to condition as a necessary basis target set for producing model descriptions that can be used in computational engineering design tools. The major goal is to develop the lowest dimensional attributes needed to replicate these behaviors and to minimize the number of chemical species that are necessary to produce high fidelity representations of a specific real fuel. This challenge is intrinsically necessitated by the very large numbers of molecular species that comprise liquid transportation fuels. The motivations for the approach are varied: to produce information that can be used to screen new fuels as to their distinctive combustion behaviors relative to those already in use; to reformulate compositions such that experimental studies may better identify physical/chemical effects that are important to multiphase combustion; and, in addition, to the advance modeling tools that are valuable for parametric computational engineering design (which is the overarching subject of this book). This chapter emphasizes the progress made on emulating fully prevaporized combustion behaviors and surrogate formulation methods that can be adapted to consider both physical and chemical property emulations in the future. While the chapter describes concepts in terms of applications to jet fuels, the extension of similar concepts to other transportation fuels is apparent.

Part 1 (10.1021/acs.energyfuels.6b00420) of this two part series presented a computational study of NOx formation during methane and ethylene combustion, representative of small fuel fragments present in natural gas and chemical processing. The influence of fuel chemistry, reaction temperature history, and inert dilution was examined using popular models present in the literature. The present work extends the study to hydrogen-rich conditions to remove the fuel variability dependency of NOx and identify possible inconsistencies in predicting NOx during high hydrogen content fuel combustion. A comprehensive chemical kinetic model is proposed consisting of CO/H2/NOx oxidation with the full implementation of thermal, N2O, and NNH paths of NOx evolution. Predictions from the model are compared against multiple experimental data sets over a wide range of venues and operating conditions. The experimental venues include shock tube, plug flow reactor, and stirred reactor experiments that cover pressures from 1 to...

Along with other nitrogen oxides, nitric oxide (NO) is a regulated air pollutant that is primarily produced as a result of combustion processes. This pollutant is produced in every combustion system that uses oxidizer mixtures containing N2 and/or fuels that contain organically bound nitrogen. To design combustors to minimize NO emissions, high-fidelity computational models for NO production and NO/NO2 interconversion are required. To improve model performance and understanding of NO production pathways, a computational parametric study is performed to investigate the effects of fuel chemistry, reaction temperature history, and inert gas dilution on NO production in methane and ethylene combustion. Predictions using popular models from the literature are compared for premixed laminar flame conditions as well as against previously reported stirred reactor measurements. Differences in the predictions and their relationships to the NOx submodel and hydrocarbon chemistry are investigated. We find that signifi...

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.

As alternative jet fuels continue to be developed, their impact on combustor performance remains of utmost importance. Alternative jet fuels generally contain few aromatics and differ in alkylated compositions, yielding different chemical and physical properties from those of conventional jet fuels; understanding how these property differences impact combustor performance near limiting conditions is important in certifying their use in blending with petroleum derived fuels or as complete substitutes. Ignition and extinction properties that are associated with Lean Blowout (LBO) are areas of focus for jet fuel certification as they are important safety metrics bounding combustor stability. Previous results for 23 different test fuels in a referee combustor show a strong correlation of Lean Blowout (LBO) with fuel Derived Cetane Number (DCN). This previous study involved fuels with compositions similar to conventional fuels. However, fuels with properties differing significantly from conventional fuels were found to have a weaker correlation with DCN and higher LBO equivalence ratios overall. The surrogate fuels and blends that show the largest discrepancy from the earlier correlation were blends involving highly volatile, low DCN components such as iso-octane prevalent in the early stages of distillation, and less volatile, high DCN normal alkane components such as n-hexadecane, prevalent in the final stages of distillation. Thus, significant differences in fuel reactivity along the distillation curve from those of conventional petroleum derived fuels appeared to exhibit differing LBO character. From these observations, three hypotheses, preferential vaporization, relative droplet lifetimes, and thermal quenching, are proposed and investigated by utilizing the available data. Using normalized power law regressions, distillation simulation methods and Quantitative Structure Property Relation (QSPR) results, the DCN at 34% distillation recovery show a stronger correlation with LBO than the DCN determine for the fuel itself. In this paper, we apply findings to propose fuel compositions to investigate the noted hypotheses by utilizing reactive low molecular weight molecules and a less reactive high molecular weight fuel. The suggested fuel to stress test this hypothesis is a blend of 30 (molar)% n-heptane and 70 (molar)% Gevo Alcohol-to-Jet (ATJ), which is essentially composed of (primarily) 2,2,4,6,6 iso-dodecane and isocetane. If preferential vaporization is significant, then this fuel should be more stable than the “DCN-Law,” i.e. fuels are no more stable than the corresponding DCN allows, would predict.

The ‘FLame EXtinguishment’ (FLEX) program conducted by NASA on board the International Space Station (ISS) has been assisting in developing fire-safety protocols for low gravity applications through microgravity droplet combustion experiments. A wide range of fuels including alcohols and alkanes have been studied in different ambient conditions that also encompass the use of various diluent species and concentrations. A prime focus of the work has been to observe the relative effectiveness of atmospheric composition and pressure changes on fire suppression under ‘reduced’ gravity conditions. Here, detailed numerical simulations are performed to investigate the combustion and extinction characteristics of isolated sphero-symmetric 1.0-2.0 mm diameter methanol droplets burning in xenon (Xe) enriched environments. Comparisons of diluent behaviors under identical conditions using argon (Ar), carbon dioxide (CO2) and helium (He) as the alternative diluent to nitrogen are also reported. The predictions are compared against ISS experiments with good agreement and with less satisfactory agreement with the results published earlier by Shaw et al. (2012). Xenon as diluent rather than nitrogen results in reduced burning rate, larger extinction diameter and counter intuitively, prolonged burning time. The limiting oxygen index (LOI) for xenon is found to decrease significantly from that found with argon, carbon dioxide or helium. The numerical analyses indicate that the lower thermal diffusivity of xenon is the principal factor responsible for the remarkably lower LOI. Water accumulation within the methanol droplet and its relevance to the extinction process is also discussed. It is concluded that the combined observation of elevated peak gas temperature, its slow decay due to minimal diffusive heat loss and the exceptionally lower LOI value associated with xenon as a diluent all detract from its utility for suppressing fire concerns in reduced gravity applications.

Rate coefficients for the reaction H + NO2 → OH + NO (R1) have been determined over the nominal temperature and pressure ranges of 737-882 K and 10-20 atm, respectively, from measurements in two different flow reactor facilities: one laminar and one turbulent. Considering the existing database of experimental k1 measurements, the present conditions add measurements of k1 at previously unconsidered temperatures between ∼820-880 K, as well as at pressures that exceed existing measurements by over an order of magnitude. Experimental measurements of NOx-perturbed H2 oxidation have been interpreted by a quasi-steady state NOx plateau (QSSP) method. At the QSSP conditions considered here, overall reactivity is sensitive only to the rates of R1 and H + O2 + M → HO2 + M (R2.M). Consequently, the ratio of k1 to k2.M may be extracted as a simple algebraic function of measured NO2, O2, and total gas concentrations with only minimal complication (within measurement uncertainty) due to treatment of overall gas composition M that differs slightly from pure bath gas B. Absolute values of k1 have been determined with reference to the relatively well-known, pressure-dependent rate coefficients of R2.B for B = Ar and N2. Rate coefficients for the title reaction determined from present experimental interpretation of both laminar and turbulent flow reactor results appear to be in very good agreement around a representative value of 1.05 × 10(14) cm(3) mol(-1) s(-1) (1.74 × 10(-10) cm(3) molecule(-1) s(-1)). Further, the results of this study agree both with existing low pressure flash photolysis k1 determinations of Ko and Fontijn (J. Phys. Chem. 95 3984) near 760 K as well as a present fit to the theoretical expression of Su et al. (J. Phys. Chem. A 106 8261). These results indicate that, over the temperature range considered in this study and up to at least 20 atm, net chemistry due to stabilization of the H-NO2 reaction intermediate to form isomers of HNO2 may proceed at negligible rates compared to R1.

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.

Typically, real jet fuels are composed primarily of normal-, iso-, and cycloalkanes, along with significant fractions of alkyl aromatics. This study elucidates fundamentally the influence of the cycloalkane functionality on the fully prevaporized global combustion kinetic behavior of a fuel mixture composed of normal- and isoparaffin and alkyl aromatic fractions. Methylcyclohexane is chosen as a representative of this generic functional class. A n-decane/iso-octane/toluene mixture previously applied in surrogate fuel research is used as the reference functional class composition for this study. A second fuel mixture is derived by incorporating ∼25 mol % methylcyclohexane into this reference component mixture in such a manner that four selected combustion property targets, derived cetane number, hydrogen/carbon ratio, molecular weight, and threshold sooting index, of both the reference fuel and the new mixture are the same. In so doing, the intention is to regulate the gas-phase oxidative reactivity of bot...

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.

We have demonstrated previously that a (surrogate fuel) mixture of known pure hydrocarbon species that closely matches four combustion property targets (the derived cetane number (DCN), the hydrogen to carbon molar ratio (H/C), the threshold soot index (TSI), and the average molecular weight) of a specific jet fuel, displays fully prevaporized global combustion kinetic behaviors that are closely consistent. Here, we demonstrate a similar result can be obtained by formulating surrogate hydrocarbon fluid mixtures from distillation cuts of molecular class hydrocarbons or even real gas turbine fuels (for which the specific molecular species classes are no more than qualitatively known). Fully prevaporized chemical reactivities of hydrocarbon fluid surrogate mixtures and real jet fuels are compared using a high pressure flow reactor at 12.5 atm pressure, over the temperature range 500–1000 K, at stoichiometric conditions, and for the same fixed molar carbon content. Results are reported for two different real ...

This paper presents a combined experimental and numerical study of a modified Cooperative Fuel Research (CFR) engine that allows both the Research and Motor octane numbers (RON and MON) of any arbitrary Liquefied Petroleum Gas (LPG) mixture to be determined. The design of the modified engine incorporates modern hardware that enables accurate metering of different LPG mixtures, together with measurement of the in-cylinder pressure, the air-fuel ratio and the engine-out emissions. The modified CFR engine is first used to measure the octane numbers of dif ferent LPG mixtures. The measured octane numbers are shown to be similar to the limited data acquired using the now withdrawn Motor (LP) test method (ASTM D2623). The volumetric efficiency, engine-out emissions and combustion efficiency for twelve alternative LPG mixtures are then compared with equivalent data acquired with the standard CFR engine operating on a liquid fuel. Finally, the modified CFR engine is modelled using GT-Power. The full engine model contains empirical sub-models of the intake and exhaust systems, the gas exchange processes, the flame propagation and the in-cylinder heat transfer . The calibrated combustion models are used to determine the residual gas fraction and crank angle resolved mass fraction burned histories during octane rating for both gaseous and liquid fuels. Overall, this analysis suggests that the performance of the modified CFR engine is consistent with that of the standard engine operating on a conventional, liquid fuel.

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.