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The experimental evaluation of a methodology for surrogate fuel formulation to emulate gas phase combustion kinetic phenomena

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Abstract

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:

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... The HyChem mechanism for Jet A has 48 species and 254 reactions, whereas the HyChem mechanism for C1 has 42 species and 286 reactions, making these reaction mechanisms moderately complex. Figure 2 compares predictions of the laminar flame speed, , extinction strain rate, , and ignition delay time, , between dodecane, Jet A, and C1 with experimental data from [65][66][67][68][69][70][71]. Numerical simulations are made using Chemkin [72] under the conditions reported in [65][66][67][68][69][70][71]. ...
... Figure 2 compares predictions of the laminar flame speed, , extinction strain rate, , and ignition delay time, , between dodecane, Jet A, and C1 with experimental data from [65][66][67][68][69][70][71]. Numerical simulations are made using Chemkin [72] under the conditions reported in [65][66][67][68][69][70][71]. Laminar flames at 1 atm and equivalence ratios ranging from 0.6 to 1.6 are studied to extract at = 403 K. ...
... On the other hand, the HyChem mechanism for C1 slightly underpredicts the flame speeds, especially at lean conditions, showing a maximum error of 18% at = 0.7. Some discrepancies between experiment and numerical simulations become evident when comparing the Jet A HyChem results for with those from Dooley et al. [67], Hui et al. [68], and Kumar et al. [69]. ...
Article
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The aviation sector is facing a massive change in terms of replacing the currently used fossil jet fuels (Jet A, JP5, etc.) with non-fossil jet fuels from sustainable feedstocks. This involves several challenges and, among them, we have the fundamental issue of current jet engines being developed for the existing fossil jet fuels. To facilitate such a transformation, we need to investigate the sensitivity of jet engines to other fuels, having a wider range of thermophysical specifications. The combustion process is particularly important and difficult to characterize with respect to fuel characteristics. In this study, we examine premixed and pre-vaporized combustion of dodecane, Jet A, and a synthetic test fuel, C1, based on the alcohol-to-jet (ATJ) certified pathway behind an equi-lateral bluff-body flameholder, spray combustion of Jet A and C1 in a laboratory combustor, and spray combustion of Jet A and C1 in a single-sector model of a helicopter engine by means of numerical simulations. A finite rate chemistry (FRC) large eddy simulation (LES) approach is adopted and used together with small comprehensive reaction mechanisms of around 300 reversible reactions. Comparison with experimental data is performed for the bluff-body flameholder and laboratory combustor configurations. Good agreement is generally observed, and small to marginal differences in combustion behavior are observed between the different fuels.
... The second-generation surrogate utilizes n-dodecane as a primary alkane species instead of n-decane, keeps a revised isooctane model, and removes toluene in favor of n-propyl benzene and TMB. 7,12 Combustion experimental data were collected in a highpressure shock tube for both Jet A and the new surrogate fuel in a high-pressure shock tube. Post-combustion products for both fuels were compared, and good agreement was found for mole fractions of carbon monoxide, carbon dioxide, light species with 1 to 3 carbons, and ignition delay times at temperatures ranging between 879 and 1733 K and pressures between 16 and 27 atm. ...
... For the selection of the Jet A surrogate used in this work, we recognize the effectiveness of the second-generation surrogate proposed by Dooley et al. 12 However, the underlying design principle for both firstand second-generation surrogates, that is, the importance of functional groups in the selection of surrogate components as stated by Dooley et al., 12 are also considered important for the final surrogate selection. The surrogate composition used in this work is enriched with aromatic components compared to the surrogate reported by Dooley et al. 12 and has a molar composition of 0.287/0.235/ ...
... For the selection of the Jet A surrogate used in this work, we recognize the effectiveness of the second-generation surrogate proposed by Dooley et al. 12 However, the underlying design principle for both firstand second-generation surrogates, that is, the importance of functional groups in the selection of surrogate components as stated by Dooley et al., 12 are also considered important for the final surrogate selection. The surrogate composition used in this work is enriched with aromatic components compared to the surrogate reported by Dooley et al. 12 and has a molar composition of 0.287/0.235/ ...
... A great amount of ignition, pyrolysis and oxidation studies for various hydrocarbons has been generated in shock tubes by various research groups [17][18][19]31,32,[36][37][38][39][40][41]. Furthermore, shock tubes have also been adopted to compare ignition characteristics of surrogate compounds to those of a target fuel [42][43][44][45][46][47][48]. The high temperature and high pressure in shock tube make this method less relevant to hydrocarbon LTO of interest in the present study. ...
... For this reason, RCMs have been often used to validate autoignition behavior of well formulated surrogate mixtures as compared to that of target practical fuels [44,[46][47][48][54][55][56]. RCMs may not be an ideal device to study in-situ species analysis for hydrocarbon oxidation due to a difficulty in controlling optical diagnostic equipment caused by intensive vibrations that occur during operation [57,58]. ...
... Then, the chemical kinetic mechanisms of the individual components will be combined and significant cross-linked reactions between mechanisms will be also recognized. As a means to better formulate the surrogate mixtures for vapor phase combustion, Dryer and co-workers developed an a priori methodology, which requires only limited knowledge of the molecular class composition of the specific real fuel [44,[46][47][48]. The method is based on an assumption that a much smaller pool of distinct chemical functionalities can be generated after an initial reaction. ...
Thesis
The autoignition characteristics of individual hydrocarbon species studied in motored engine can provide a better understanding of the autoignition process and complex fuels for homogeneous spark and compression ignition engines, whether the interest is understanding and preventing knock or controlling autoignition. In both instances, there is a critical need to comprehend how fuel molecular structure either retards or promotes autoignition reactivity. This understanding ultimately contributes to the development of kinetic mechanisms, which are needed for simulation of reacting flows and autoignition processes. For this reason, the dissertation discusses autoignition data on i) three pentane isomers (n-pentane, neo-pentane and iso-pentane), ii) ethyl-cycloahexane and its two isomers (1,3-dimethyl-cyclohexane and 1,2-dimethyl-cyclohexane), and iii) diisobutylene in primary reference fuels. looking for their chemical structural impacts on the ignition process. Particularly for exploring the low and intermediate temperature regions, the motored variable compression ratio engine, developed from a Cooperative Fuel Research (CFR) Octane Rating engine, provided a good platform. Analyses of the stable intermediates in the CFR engine exhaust at various end of compression pressures and temperatures can help to identify reaction pathways through which different compounds prefer to autoignite. The approach of those studies is to conduct a systematic investigation of the autoignition, which can provide useful input for qualitative and semi-quantitative validation of kinetic mechanisms for oxidation of target chemical compounds. Finally, the dissertation is further extended to an experimental validation of jet aviation fuel surrogates, potentially emulating a series of physical and chemical ignition processes in diesel engines, with an emphasis on the needs for detailed auto-ignition characteristics of various individual hydrocarbon species.
... Metcalfe et al. [9] developed the detailed chemical kinetic mechanism of toluene oxidation and its combustion intermediates and extensively verified various experimental objectives and conditions for toluene, benzene, cyclopentadiene, and phenol. Dooley et al. [10,11] used empirical correlations among fuel characteristics to develop alternative surrogates for jet fuel mixtures. To predict the wide-range combustion characteristics of real fuels, the influence of cycloalkane functional groups on the combustion kinetics of alternative mixtures has also been studied [12]. ...
... Table 1 shows the combustion characteristics of these six surrogates. These characteristics were designed to constrain the basic molecular characteristics related to combustion dynamics, so that the numerical simulation results of the surrogates mixture, including the laminar flame speed and ignition delay time, would be similar to the basic combustion phenomena of real fuel, [11]. Among these combustion characteristic indexes, the Derived Cetane Number (DCN) was related to the flammability of the fuel, and was derived from the ignition delay time [23]. ...
... Molecular weight (MW) was related to the diffusion characteristics of the fuel in the gas phase. If the difference in the molecular weight between the surrogates and the real fuel is too large, it may cause deviations in the results when the simulated flame is extinguished [11]. The Lower Heating Values (LHV) represent the amount of heat released during fuel combustion [24]. ...
Article
In this research, the composition and proportion of surrogates were first determined according to the composition of the petrochemical and renewable fuels. Then, the CHBR model in CHEMKIN-Pro software was used to verify the ignition delay time of HRJ, JP-5, and HRD. The differences in the ignition delay time of the fuels under an equivalence ratio of 1.0, different pressures (8, 11, 30 bar), and a pressure of 20 bar, and different equivalence ratios (0.5, 1.0, 1.5) are discussed. Among the three types of aviation fuel, the ignition delay time of HRJ in the low-temperature range was the shortest, while that of JP-5 was the longest. The average ignition delay time of HRJ in the low-temperature range under different equivalence ratios and pressures was approximately 59% and 57% lower than that of JP-5, respectively. On the other hand, the average ignition delay time of HRD in the low-temperature range at different equivalence ratios and pressures was 45% and 55% lower than that of petrochemical diesel, respectively. The ignition delay time of all of the fuels was shorter when the pressure was increased.
... In addition to gas-phase combustion behavior, surrogate fuels should also emulate real fuels' physical properties important in injection, atomization, vaporizing, and mixing processes. Unfortunately, physical and chemical properties result from fuel structures, so limiting the number of components in surrogate formulations is extremely difficult [5]. Each individual component also needs to have a reliable kinetic mechanism available to be put together, and important crossreactions included [4]. ...
... 5 shows the apparent heat release rate (AHRR) from experiment and simulation and the in-cylinder temperature from simulation. For n-heptane at CR = 6.0, the predicted timings of LTHR from all three mechanisms were late. ...
Thesis
An accurate combustion model for real-world fuels is a key part of internal combustion engine simulation and design to reduce pollutants and greenhouse gas emissions. However, existing chemical kinetic mechanisms are not adequately accurate, especially for low-temperature oxidation (LTO). In this study, the autoignition of a jet fuel surrogate and pure components were investigated in a motored engine via coupled experiment and simulation to validate and improve the oxidation kinetic mechanism. A multizone model was developed to simulate homogeneous charge compression ignition (HCCI) combustion in a modified CRF octane rating engine. In a simulation study for three pentane isomers, the multizone model was accurate for autoignition simulation and effective for kinetic mechanism validation. An existing mechanism for pentane isomers was accurate in predicting reactivity, heat release, and oxidation intermediate species for all three pentane isomers. However, the low-temperature reactivity for iso-pentane was slightly underpredicted, with a 32.2% underprediction for the first-stage fuel consumption. Among the oxidation intermediate species, cyclic ethers were overpredicted by a minimum of 58.9%, while chain-branching products acetaldehyde or acetone were underpredicted by more than 57.5%. 2-Pentene and 2-methyl-2-butene production was overpredicted by 104% and 126%, showing that concerted elimination reaction rates and the effect of H-atom availability need to be improved. In addition, acetone production should be negligible but was significantly overpredicted during iso-pentane oxidation. Chain-branching pathways following the first and second O2 addition to the tertiary carbon were overestimated and needed to be eliminated in the kinetic mechanism. The ignition properties of the jet fuel surrogate and its pure components were investigated through coupled engine experiments and simulation. The UM-3 Jet-A surrogate showed strong low-temperature reactivity, which the kinetic mechanism, SKE360, successfully captured. However, the global reactivity was underpredicted due to underestimated n-dodecane and decalin reactivity, and underestimated toluene oxidation. In the surrogate mixture, iso-cetane, decalin, and toluene oxidation were significantly enhanced by n-dodecane low-temperature oxidation. The radical pool from n-dodecane low-temperature oxidation enhances H-atom abstraction, fuel radical formation, and production of small intermediate species via consecutive beta-scission reactions. This oxidation enhancement was successfully predicted for iso-cetane and decalin, but underpredicted by 79.1% for toluene. For the pure components of the jet fuel surrogate, existing mechanisms underestimated n-dodecane reactivity, and their different reaction rates led to significant differences in reactivity prediction. Low-temperature oxidation was insignificant during iso-cetane oxidation at the test conditions, but was overpredicted by the jet fuel surrogate mechanism SKE360. Reaction rates for R + O2 <=> RO2 reactions need to be improved to eliminate O2 addition to the iso-cetane fuel radical in alkene formation. Decalin reactivity was underpredicted by SKE360. The production of benzene, cyclohexadiene, and cyclohexene was overpredicted by more than 10 times during decalin oxidation, showing the opening of one ring in this bicyclic alkane molecule was overestimated. The main oxidation pathways following the C-C bond breaking between the two tertiary carbons were missing and need to be added. In this work, motored engine experimental measurements were for the first time used for quantitative evaluations of kinetic mechanisms. The method developed in this study and the ignition data generated improved our fundamental understanding of combustion chemistry. Our discussions provided directions for future mechanism development.
... The large number of molecules and different molecular structures lead to complex combustion pathways that are difficult to model. However, it has been found, in previous studies [5][6][7], that the combustion behavior of real fuels can be predicted by using surrogate mixtures that have similar combustion property targets (CPTs), such as H/C ratio, threshold sooting index (TSI), DCN, molecular weight (MW), and chemical functional group distributions as the real fuels. ...
Presentation
Ignition propensities of toluene reference fuels (TRF, mixtures of n-heptane, iso-octane, and toluene) have been investigated by measuring their derived cetane number (DCN) in an Ignition Quality Tester (IQT). Considering the potential impact of NO, which is typically introduced by exhaust gas recirculation in a reciprocal internal combustion engine for increasing the thermal efficiency, DCN values of TRF mixtures were measured with and without NO addition. The measured DCN values of TRF mixtures are found to monotonically increase initially with NO addition until ~ 300 ppm of NO in air and decrease slightly with further addition of NO up to 1000 ppm. To identify the major route for NO interaction, quantitative structure-property relationship (QSPR) regression models for DCN, RON, and MON were developed by employing the chemical functional group descriptor with DCN database established in this study and RON/MON measurement available in literature. QSPR analysis suggests that the major NO impacts originate from the interaction between NO and (CH2)n chemical functional group stemming from n-heptane. Inter-correlative natures among DCN, RON, and MON were further analyzed through QSPR regression models.
... These small molecules contain a large number of free radicals and are called radical pools. 37 Similarly, when the inhibitor is added, it will first be thermally decomposed or oxidized to form many small molecules, which includes a large number of small molecule radicals. In the radical pool, the radicals generated by the inhibitor will interact with the radicals generated by the fuel, and there will also be a large amount of thermal interaction. ...
Article
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C6F12O (Novec 1230) is one of the most potential substitutes of Halon 1301. However, the study of inhibition/promotion effect of C6F12O addition on aviation kerosene is rarely reported, which greatly limits the development and application of C6F12O in oil fire accidents. In this study, numerical research was conducted to study the inhibition/promotion effect of C6F12O addition by a newly developed and optimized RP-3/C6F12O coupling skeletal mechanism. A novel methodology based on fictitious species was proposed and adopted to identify the physical dilution and thermal effects as well as the chemical radical and thermal interaction effects of C6F12O addition. It is observed that both inhibition and promotion effects can be exhibited because the chemical effect includes radical and thermal interactions at different equivalence ratios. In order to explore the kinetic reasons, the reaction path analyses were conducted. The results indicate that the HF formation reactions and the O2 consumption fluorine-containing reactions, as well as the reactions of H2O + F = OH + HF and C3F7 + O2 = C3F7O + O, were the key inhibition and promotion reaction routes, respectively. Compared with methane, the lower H/C ratio of RP-3 makes it more suitable for the use of C6F12O to suppress combustion.
... However, ignition and flame stabilization are challenging issues in a scramjet [1][2][3][4][5]. The residence time of the air in a combustor (t flow ≈ 0.5 ms) is even shorter than the typical self-ignition time of fuel (t ig ≈ 1-2 ms) [6][7][8]. In the 1990s, the Russian Central Institute of Aviation Motors (CIAM) first used the cavity as a flame stabilizer in a hydrogenfueled dual-mode scramjet flight experiment jointly conducted with France [9,10]. ...
Article
Full-text available
Since flame stability is the key to the performance of scramjets, scramjet combustion mode and instability characteristics were investigated by using the POD method based on a cavity-stabilized scramjet. Experiments were developed on a directly connected scramjet model that had an inlet flow of Mach 2.5 with a cavity stabilizer. CH* chemiluminescence, schlieren, and a wall static pressure sensor were employed to observe flow and combustion behavior. Three typical combustion modes were classified by distinguishing averaged CH* chemiluminescence images of three ethylene fuel jet equivalence ratios. The formation reason was explained using schlieren images and pressure characteristics. POD modes (PDMs) were determined using the proper orthogonal decomposition (POD) of sequential flame CH* chemiluminescence images. The PSD (power spectral density) of the PDM spectra showed large peaks in a frequency range of 100–600 Hz for three typical stabilized combustion modes. The results provide oscillation characteristics of three scramjet combustion modes.
... For calculation of cetane number, a mixture of hydrocarbon which includes cetane (hexadecane) and isooctane (2, 2, 4, 4, 6, 8, 8-heptamethylnonane) is determined by gas chromatography and mass spectrometry. This hydrocarbon affects the ignition delay of the engine [9]. Determining cetane number requires a complex test engine and highly skilled operators which is a costly method. ...
Article
Full-text available
Availability and higher gasoline price have attracted the attention of researchers towards alternative fuels. Plastic is produced from the byproduct of gasoline products, which possesses a higher possibility of recycling the waste plastic as an alternative fuel. Research conducted on plastic fuel shows that a diesel engine can run with 100% plastic oil. The present work is focused on the effect of distilled plastic oil on the cetane index as the cetane index is a major fuel property of diesel that affects the ignition quality and exhaust emissions of the engine. For the measurement of the cetane index, two standards are followed and they are ASTM D4737 and ASTM D976. It is found that Crude plastic oil produced from thermal pyrolysis of waste plastic possesses a wide variety of hydrocarbon i.e. lower to higher hydrocarbon. From the fractional distillation of crude plastic oil at three temperature ranges 200◦C, 290◦C and up to the final boiling point, it gave petrol grade oil and diesel grade oil (both low and high grade). Also, it was found that the recovery of distilled high-grade plastic oil is higher than other distilled crude plastic oil. Along with this, crude plastic oil, as well as high-grade plastic oil, have a higher cetane index than the diesel available in the market. Similarly, blending diesel with high-grade plastic oil up to 20% by volume and with low-grade plastic oil up to 10% by volume increases the cetane index of fuel.
... Thus, prior to measure the DCN of crude's distillation cuts, the ability of the IQT to measure the DCN of whole crudes was investigated using Crude 1 in the facility of the IQT manufacturer ( Advanced Engine Technology Ltd), since the reactivity potentials (DCN) of crude oils or crude oil fractions were never measured before and the nature of crude oil as a whole (vary viscous liquid) may causes some issues for the IQT hardware. The DCN of whole Crude 1 was successfully determined to be 52.6 at AET facility, which is comparable to the DCN's of petroleum derived jet fuels recently reported[58,59]. ...
Thesis
Full-text available
As combined cycle gas turbines for power generation operating on light distillate fuels provide a sufficient energy, there is a global trend in using heavy liquids (e.g. HFO, crude oils) for direct-firing in gas turbine. However, using these heavy fuels imposes many challenges due to their wide range of physical/chemical properties, which control near-limit combustion behaviors, such lean blow-out and flashback. In addition, the ignition propensity of these heavy fuels is not reported in crude assays, and utilizing a simple, with small sample, and quick methodology to characterize the ignition propensity is important in determining the fuel quality. In this work, the derived cetane number (DCN) of whole crudes and their distillation cuts are measured. Four crudes were distilled into four fractions as reported in crude assays: light naphtha, heavy naphtha, kerosene, and light gas oil. The DCN of each fraction was measured and compared using an ignition quality tester (IQT), where the DCN values for all the crude cuts increase from 23-35 for lighter fractions to 50-60 for heaver one. This variation of DCN values over their distillation curves was observed before (for distillate fuels, e.g. Petroleum-derived fuels) to be linked to strongly influence near limit combustion behaviors (e.g. LBO) through preferential vaporization, which suggests that preferential vaporization will play a significant role when using whole crudes in gas turbine applications. To further analyze such a behavior, 1H and 13C NMR spectra were acquired to characterize the chemical functional groups controlling the DCN and their distinctive influence. Based on a chemical functional groups approach, A QSPR regression model was developed indicating that the n-paraffinic CH2 group has the most influence in determining the global ignition propensity of crude oils. The analysis also suggests that chemical reactivity (DCN) of crude oils can be roughly estimated based on key-functional groups determined from 1H and 13C NMR analysis. Finally, to investigate the impact of preferential vaporization on flame flashback, a spray burner was developed which has the ability to control the extent of fuel vaporization. Experiment was first conducted at burner temperature of 700 K with various n-alkanes and iso-alkanes, where two distinct flame flashback behaviors were observed, propagation and ignition-driven flashback. Two binary mixture were formulated that have identical chemical reactivity for fully vaporized fuel/air mixture but exhibit a drastic difference of ignition propensity under partially vaporized conditions. To observe the role of preferential vaporization effectively, the experiment was conducted at 450 K using those two mixtures. One of the Crude oils was used to investigate crude oil flashback behaviors in the spray burner.
... The selected compositions of the surrogate fuel should not only cover the corresponding class in the F-T fuel 30 T A B L E 1 Properties comparisons between the F-T fuel and the surrogate fuel ...
Article
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Based on the analysis of the combustion process of liquid fuel in the gas turbine combustor, we suggest that a comprehensive surrogate fuel should be developed according to the basic physical and chemical properties that affect the diffusion combustion process. The present study took the carbon–hydrogen mole ratio, molar weight, density, viscosity, surface tension, initial boiling point, and lower heating value, as the target properties. The optimal direction method was utilized to obtain the mixture composition ratio which has the minimum deviation on the basic physical and chemical properties compared with the Fischer–Tropsch (F‐T) fuel. This mixture, taken as the comprehensive surrogate fuel of the F‐T fuel, was composed of 15% n‐decane, 67% n‐tetradecane, 13% iso‐octane, and 5% methylcyclohexane in mole fraction. Further experiments were conducted to investigate the pollutant emissions of the F‐T fuel and the surrogate fuel in a gas turbine combustor. The experimental results showed that the surrogate fuel can well predict the EI(CO) and EI(NOx) of the F‐T fuel in the turbine combustor, and the difference in EI(UHC) is mainly influenced by the fuel evaporation property. (1) We developed the comprehensive surrogate fuel for the coal‐based Fischer–Tropsch (F‐T) fuel, composed of 15% n‐decane, 67% n‐tetradecane, 13% iso‐octane, and 5% methylcyclohexane in mole fraction. (2) The surrogate fuel can well simulate the atomization properties of the F‐T fuel due to the similar density, surface tension, and viscosity of the surrogate fuel and the F‐T fuel under identical conditions. (3) The surrogate fuel can better simulate the evaporation properties of the F‐T fuel due to the similar initial boiling point and final boiling point of the surrogate fuel and the F‐T fuel.
... Identifying and modeling each of these pathways accurately is a challenge that has attracted significant interest from the chemical kinetic research community for the past few decades. The HyChem approach for modeling kinetics of real fuels was one of the major successful modeling strategies that was borne out of decades worth of research [1,2]. The HyChem approach was developed at Stanford and has been demonstrated to be remarkably successful in predicting the oxidation kinetics of distillate fuels at high temperatures [3]. ...
Conference Paper
Full-text available
The complexity of modeling the oxidation kinetics of conventional transportation and aviation fuels stems from the immense number of unique components that constitute them. Unsurprisingly, these components give rise to a multitude of possible chemical pathways. Identifying and modeling each of these pathways accurately is a challenge that has attracted significant interest from the chemical kinetic research community for the past few decades. The HyChem approach for modeling kinetics of real fuels was one of the major successful modeling strategies that was borne out of decades worth of research [1,2]. The HyChem approach was developed at Stanford and has been demonstrated to be remarkably successful in predicting the oxidation kinetics of distillate fuels at high temperatures [3]. The HyChem approach models the fuel, regardless of its composition, as a single molecule. Recognizing the distinct difference in the time scale of decomposition of heavier components of the fuel into smaller and simpler (foundational) hydrocarbons, and their oxidation, the modeling approach attempts to describe the overall oxidation kinetics by an amalgamation of a simple, lumped fuel pyrolysis model and a detailed foundational fuel model. In doing so, the model implicitly assumes a cause-and-effect relationship between the identity of decomposition products, which is intimately linked to the composition and structure of the fuel, and global combustion properties like ignition delay times (IDT) and flame speeds. The parameters of the lumped fuel decomposition model are inferred from speciation measurements conducted in carefully designed shock tube and flow reactor experiments. Despite its successes in modeling the high temperature (>1100 K over pressures of 1-60 atm) kinetics of a wide variety of fuels [4-6], the efficacy of the HyChem approach in modeling and predicting low-temperature oxidation of fuels has not been demonstrated yet. A strategy geared towards this was described by Xu et al. [4]. They adopted a reduced skeletal model to describe low-temperature oxidation based on the formulation developed by Bikas and Peters [7]. The model parameters were tuned to match the second stage IDTs measured in shock tubes for stoichiometric, and lean fuel/oxidizer mixtures. There are two major issues with this approach: 1) The skeletal model is based upon the state of knowledge of low-temperature oxidation of n-alkanes in early 2010s, 2) The model parameters were directly inferred from a global combustion property (IDT), which goes against the philosophy of the HyChem approach, renders the model susceptible to inaccuracies in IDT data (for example, due to inhomogeneous ignition), and incapable of predicting the timing of first stage ignition and low-temperature heat release (LTHR) accurately. The updated skeletal model for oxidation of alkanes based on these recent studies that incorporates these pathways is shown in fig. 1. Recent synchrotron, and photoionization-mass spectroscopy studies with flames have revealed the formation of highly oxygenated transient and stable species containing more than four oxygen atoms [8]. The formation of these species could only be explained by reaction pathways that would enable the addition of a third oxygen molecule to the ketohyroperoxy species (QOOH) upon their isomerization to P(OOH)2 via internal H-abstraction. It is worth noting that these additional channels could also contribute to chain branching and raise the overall reactivity of the fuel under certain conditions [9,10]. Therefore, it is imperative for modelers to account for these recent advances in the knowledge of oxidation of hydrocarbons in developing chemical kinetic models for distillate fuels. This work attempts to do so by augmenting the low-temperature HyChem model of Xu et al. by addition of four fuel dependent species and seven more reactions to the model. Modifications were also made to the reactions retained from the Xu et al. model to enable inference of model parameters from shock tube experiments. The details of these experiments are described in the following sections of the paper. These experiments have already been performed and reported [11,12] with four neat fuels, i.e., n-heptane, n-octane, n-decane, and 2-methylhexane. Additional experiments were conducted with a three-component gasoline surrogate containing n-heptane, iso-octane, and toluene (TPRF 60). The augmented HyChem model was developed for each of these fuels based, and its performance was compared against second stage IDT measurements reported in the literature. The models show good agreement with the measurements and pave the path for application of this modeling approach to real fuels.
... Stagni et al. [13] recently investigated the role of preferential evaporation on the ignition behaviour of a monodisperse kerosene spray cloud in a homogeneous gas phase. A four-component Jet-A surrogate [14] at DCN (derived cetane number) standard conditions, i.e. T g = 833 K and p g = 22 . 1 atm, was used. ...
Article
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The droplet evaporation and autoignition behaviour of a three-component kerosene surrogate is numerically investigated for a wide range of ambient pressures and temperatures representing realistic aero-engine conditions. Vitiated air with different levels of dilution is considered to represent mixing of air with combustion products. Particular attention is given to the analysis of multi-component fuel effects at the various operating conditions. The investigation also considers the impact of fuel preheating and multiple initial droplet diameters, and extends to the quantification of NOx emissions at the droplet scale level. Considering pure evaporation, results show that preferential evaporation and variation of the droplet composition are mainly affected by the gas temperature. An increase of the pressure generally increases the duration of the droplet heat-up period and reduces the effects of preferential evaporation, especially when high temperatures are considered. Autoignition in vitiated air is strongly influenced by both the level of dilution and ambient pressure, with the latter playing an important role in determining the value of the initial droplet diameter below which no autoignition occurs. Lower pressures generally make the kerosene droplet more resistant to autoignition for the same level of dilution or gas temperature. Droplet preheating mainly affects the heat-up period and can be used as a design parameter to control the autoignition delay time. NOx levels are substantially related to the gas-phase temperature and the existence of a flame at the droplet scale. Potential implications for the design of future low-emission combustor technologies are discussed with an emphasis on the fuel preparation.
... Ignition and flame stabilization is challenging work in a scramjet [1][2][3]. The residence time of air in a combustor (t flow ≈ 0.5 ms) is even shorter than the typical self-ignition time of fuel (t ig ≈ 1-2 ms) [4][5][6]. Traditional passive flame stabilization methods (such as cavity and plate flame stabilization) stabilize a flame in a vortex structure to achieve the purpose of stable combustion, which is dominated by supersonic inflow and formed passively. The interaction between the instability of the inflow and the combustion affects the flame structure [7][8][9][10][11]. ...
Article
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To examine the plasma-assisted combustion of a scramjet, a microwave-enhanced gliding arc plasma method was proposed in this study, and the flame structure and combustion instability were observed. The mechanism of plasma-assisted combustion was obtained via a Bunsen experiment, and then the influence on supersonic combustion was obtained on a direct-connected scramjet. The active species of the flame was determined via optical emission spectroscopy, and the flame temperature was measured with a thermocouple. The luminous intensity of the OH radicals in the flame increased ninefold when the flame temperature was increased to 1573 K, but the luminous intensity of CH* and C2 was not obviously changed with the excitation of arc plasma. Moreover, the DC arc plasma had no effect on the rotation and the vibration temperature of OH radicals under these experimental conditions. In the range of microwave energy less than 800 W, there was no typical change in the intensity of the radicals; however, when the microwave power was up to 1000 W, the effect became obvious. When plasma was applied to the scramjet, the plasma caused the pre-combustion shock train to move forward, and the initial and stable position of the flame was transferred from the cavity shear layer to the front of the fuel jet. These results clearly show that plasma free radical mechanisms cause changes to combustion modes.
... • Combustion is also a process of consuming distinct chemical kinetic functionalities of the fuels. [86] • Combustion kinetics is temperature sensitive, which originated from the fact that chemical reaction are mostly temperature sensitive too. It is commonly accepted that the kinetics at high and low temperature are significantly different. ...
Thesis
Reducing CO2 and pollutant emission is the essential challenge when dealing with climate change problems. In the transport sector, exhaust gas recirculation (EGR) technology is often used in turbocharged gasoline spark ignition (SI) engines to increase fuel economy, inhibit knock tendency, and reduce NOx emissions. However, high EGR ratios are still difficult to achieve, as they result in reduced heat release and engine stability. As increasing turbulence level and advance spark ignition systems could not bring sufficient improvements at such extreme conditions, growing interest is cast onto the combustion chemistry under high dilution. The present work aims to understand the combustion chemistry of highly-diluted gasoline premixed flames and to establish a detailed kinetic mechanism by multi-scale modeling to predict combustion characteristics with sufficient accuracy at highly-diluted conditions.This work adopts a multi-scale modeling approach, and targets on the laminar flame speed (SL) of a gasoline surrogate, which is named toluene reference fuel with ethanol addition (TRFE) and consist of isooctane, n-heptane, toluene, and ethanol. For micro-scale modeling, the reaction between ketene and hydroxyl radical, which might be important to the SL at highly-diluted conditions, is studied theoretically using ab initio electronic structure methods for the potential energy surface (PES) and Rice–Ramsperger–Kassel–Marcus Theory coupled with Master Equation (RRKM/ME) for the rate coefficients. Detailed PES is obtained, dominant pathways are identified, and their phenomenological rate coefficients are derived to be utilized in combustion modeling. For macro-scale modeling, firstly, important kinetic, thermodynamic, and transport parameters to the laminar flame speed at highly-diluted conditions, are firstly identified using sensitivity analysis based on a starting mechanism. Sensitive reactions are found to mostly involve HO2, C2--C3 species and fuel radicals. Secondly, in the sub-mechanisms where these reactions lies, diluted flames of the corresponding fuels are studied and chemical detail of the dilution effects are explored. The starting mechanism is updated by state-of-the-art kinetics parameters found in the literature for each sub-mechanisms. Finally, a detailed mechanism suitable for laminar flame speed calculations at highly-diluted conditions is established after validation. A mathematical SL correlation is generated for the use in computational fluid dynamic (CFD) simulations.
... En particulier pour les statoréacteurs à combustion supersonique (scramjet), les conditions extrêmes de fonctionnement rendent difficile le mélange aircarburant, l'allumage, la stabilisation de la flamme et le contrôle de la température [27,28]. Pour des nombres de Mach très élevés, le temps de résidence du fluide dans le moteur est court (0,5 ms) inférieur au temps typique d'allumage des carburants utilisés en propulsion (entre 1 et 2 ms) [29]. On rencontre le même problème lorsque la combustion est établie mais le temps de résidence du fluide dans le moteur est inférieur au temps nécessaire pour réaliser une réaction complète. ...
Thesis
La thèse présente une étude de l’interaction entre les plasmas froids nanosecondes et les ondes de combustion en vue d'améliorer la détonabilité de mélanges gazeux. Elle rapproche deux domaines de la physique aux échelles de temps caractéristiques différentes (nanoseconde vs. micro/milliseconde). Elle vise à démontrer l'existence d'un lien de causalité entre la pré-dissociation d’un mélange gazeux par plasma et la réduction des temps et longueur caractéristiques des réactions de combustion. Cette étude de l'effet du plasma sur la déflagration et la détonation est expérimentale et numérique. Après des rappels contextuels et des travaux antérieurs, nous donnons un bref résumé des phénoménologies de la détonation et de la déflagration dans les gaz et les deux définitions la détonabilité, soit, (1) la facilité pour la détonation à se propager dans des conditions données de confinement et (2) la rapidité avec laquelle la détonation s’établit à partir d’une flamme. La première est liée à la taille de la cellule de détonation caractérisant l'instabilité intrinsèque de sa zone de réaction établie. La deuxième est liée à la longueur de transition déflagration-détonation. Nous analysons également le plasma nanoseconde et son rôle dans la dissociation d’espèces dans un gaz. Nous proposons et testons un schéma cinétique, et sa procédure numérique, pour simuler l’effet du plasma dans un mélange combustible. Nous utilisons ces résultats de simulation comme paramètres initiaux d'un code de calcul de longueurs chimiques de la zone de réaction selon le modèle ZND de la détonation. Nous réalisons nos expériences dans des tubes de section carrée. Les méthodes de mesure sont l'imagerie ICCD, la strioscopie, la chimiluminescence, des enregistrements sur plaque à dépôt de carbone et des capteurs de pression dynamique, et un shunt avec courant de retour (BCS). Dans la série d'expériences à la cellule de détonation, nous démontrons que l’application d’un plasma nanoseconde devant un front de détonation établi diminue d'un facteur 2 la largeur des cellules de détonation de mélanges H2:O2:Ar, H2:O2, CH4:H2:O2:Ar et CH4:O2:AR à des pressions initiales entre 100 et 200 mbar. Dans la série d'expériences dédiée à la TDD, nous développons un système d’électrodes multi-canaux pour l’amorçage de la déflagration. Nous comparons la flamme qu’elle génère à celle issue d’une bougie d’allumage classique. La flamme induite par le plasma évolue vers le régime de détonation plus rapidement, à des distance plus courtes et à des pressions plus basses pour des mélanges H2:O2 à des pressions entre 200 mbar à 600 mbar. Pour les deux séries d’expériences nous avons caractérisé le plasma en portant une attention particulière à son efficacité (dépôt d’énergie et homogénéité) en fonction de la pression initiale. Nous démontrons ainsi une causalité entre plasma, temps d’induction chimique et détonabilité. Notre étude approfondit la compréhension du rôle des plasmas nanosecondes couplé aux ondes de combustion. Elle souligne l’intérêt de poursuivre cette approche nécessaire à la mise au point de dispositifs plasmas adaptés aux phénomènes de dynamique des détonations.Mots clés : Plasma nanoseconde, détonation, déflagration, détonabilité, temps d’induction chimique.
... Most of the combustion and modeling work with jet fuel surrogates have been in aviation-related hardware and test rigs, 2,4,[9][10][11][12][13][16][17][18]24,49,58 but there are some studies that have focused on jet fuel combustion in diesel engine applications. 20,22,23,29,59 The use of military jet fuel in military diesel engine enables fuel flexibility in operational situations. ...
Article
Full-text available
In this work, military jet fuel JP-5 surrogates were formulated and tested in comparison to a nominal JP-5 fuel. Combustion experiments were conducted in an advanced engine technology (AET) ignition quality tester (IQT) and a Yanmar L100W Tier 4 diesel engine due to the potential use of jet fuel in diesel engines in military situations. The surrogate development process began with determining the fuel chemical composition based on analyses of 256 JP-5 fuel samples. The physical and chemical properties of density, viscosity, flash point, surface tension, speed of sound, and distillation behavior guided the selection of the surrogate components and their composition. JP-5 differs from other aviation fuels in its properties, but most importantly in flash point, which is higher for safety purposes. Surrogates were prepared from n-dodecane, n-butylbenzene, 1-methylnaphthalene, tetralin, trans-decalin, iso-cetane, and n-butylcyclohexane as representatives of seven of the nine major chemical categories found in jet fuel. The mass fraction of each compound in the surrogates that fell within the range for that chemical class was found in real JP-5 fuels. After optimizing the surrogates for physical and chemical properties, six surrogates were selected for combustion testing in the Yanmar diesel engine, one of which was specifically selected for a low-derived cetane number (DCN). This surrogate performed poorly in the Yanmar engine. Four of the remaining five surrogates performed similarly to the baseline JP-5 in the diesel engine in terms of values and variability of ignition delay, rate of heat release, peak pressure, and the crank angle at which 50% of the fuel is burned. Of the six surrogates tested, the best one in terms of physical properties, chemical properties, and combustion behavior was the one that contained 0.2421, 0.1503, 0.0500, 0.0141, 0.0121, 0.2532, and 0.2782 mass percentages of n-dodecane, n-butylbenzene, 1-methylnaphthalene, tetralin, trans-decalin, iso-cetane, and n-butylcyclohexane, respectively.
... These materials and their compositions were selected based on the ability to formulate a surrogate that would mimic many of the properties of Jet A/A-1. The final formulation of the mixture largely followed previous work on formulating surrogate compositions targeting aviation fuels [27][28][29][30][31]. Naturally, material purity, cost, and availability also played a role in these decisions. ...
Article
Full-text available
Already low volume (<1mL) test methods facilitate the development of sustainable aviation fuel platforms and higher fidelity computational methods. Here a novel technique with two-dimensional gas chromatography (GCxGC) and Vacuum Ultraviolet (VUV) identification is used to characterize fuel composition and determine properties compared to previous work. Ten properties are predicted, including the temperature dependence of density, viscosity, thermal conductivity, and heat capacity. Property predictions incorporate uncertainty quantification (UQ) from analyte quantification (UQ1), root property uncertainty (UQ2), and the uncertainty associated with isomeric variance (UQ3), when an analyte is not identified via VUV. Comparisons to a previous method illustrate the ability of VUV identification to increase the fidelity of property predictions and decrease uncertainties. This method is applied to a surrogate intended to mimic the first-order properties and composition of a representative Jet A/A-1. In addition to nominal and temperature-dependent properties, the derived cetane number (DCN) of the surrogate is calculated for the distillation fraction evolved. The DCN there is shown to vary across the fraction of fuel distilled. Collectively, this method documents a process to prescreen novel sustainable aviation fuel candidates, facilitate the development of chemical process models, and automate property determinations for computational fluid dynamics.
... The RON and MON of gasoline-ethanol mixtures have also been predicted using the aforementioned functional groups along with the hydroxyl (OH) group [19]. Dooley et al. [20] employed the methyl (CH 3 ), methylene (CH 2 ) and benzyl (C 6 H 5 CH 2 ) groups to express the composition of jet fuel surrogates in the form of the three functional groups and found that these groups dictate the formation of the radical pool. Sumathi et al. [21,22] used the alkyl and free radical groups to predict the reaction rates of various reaction sets appearing in a number of kinetic models by using the functional group additivity approach. ...
Article
Full-text available
Gasoline is one of the most important distillate fuels obtained from crude refining; it is mainly used as an automotive fuel to propel spark-ignited (SI) engines. It is a complex hydrocarbon fuel that is known to possess several hundred individual molecules of varying sizes and chemical classes. These large numbers of individual molecules can be assembled into a finite set of molecular moieties or functional groups that can independently represent the chemical composition. Identification and quantification of groups enables the prediction of many fuel properties that otherwise may be difficult and expensive to measure experimentally. In the present work, high resolution 1H nuclear magnetic resonance (NMR) spectroscopy, an advanced structure elucidation technique, was employed for the molecular characterization of a gasoline sample in order to analyze the functional groups. The chemical composition of the gasoline sample was then expressed using six hydrocarbon functional groups, as follows: paraffinic groups (CH, CH2 and CH3), naphthenic CH-CH2 groups and aromatic C-CH groups. The obtained functional groups were then used to predict a number of fuel properties, including research octane number (RON), motor octane number (MON), derived cetane number (DCN), threshold sooting index (TSI) and yield sooting index (YSI).
... Thus, the construction method of surrogate fuel becomes a hotspot of research, such as the representative composition substitution method 16 and parameter matching substitution method. 17 They were widely adopted in the formulation of surrogate fuel mixtures. However, in those substitution methods, the formulation is realized by matching some phenomenological and macroscopic property characters, which is highly empirical and crude. ...
Article
Full-text available
RP-3 kerosene is the most widely used aviation kerosene in China, and research on its chemical-kinetic mechanism is significant for understanding the combustion characteristics. Based on a novel four-component surrogate fuel consisting of n-dodecane, 2,5-dimethylhexane, 1,3,5-trimethylbenzene, and decalin (54, 22, 14, and 10% by mole), the detailed chemical-kinetic mechanism of the corresponding RP-3 surrogate fuel with 1333 species and 6803 reactions has been developed and then reduced to 145 species and 818 reactions for high-temperature conditions. After that, the merged surrogate mechanism of surrogate fuel was validated by various experimental data sets for each individual surrogate component. Then, the surrogate mechanism was validated by comparing the simulation and experimental data of the ignition delay times, species concentrations in a jet-stirred reactor, and laminar flame speeds. Good agreements between simulations and experiments were observed. In addition, using the sensitivity analysis method, the key reactions of RP-3 surrogate fuels were compared and analyzed. In summary, the mechanism developed in this study can accurately predict the ignition, oxidation, and flame propagation characteristics of RP-3 aviation kerosene. The novel surrogate model can help deeply understand the combustion characteristics of RP-3 aviation kerosene, and it is used for high-precision numerical simulation of combustion reaction flow.
Article
We perform spatially resolved measurements of temperature, gaseous species up to three-ring Polycyclic Aromatic Hydrocarbons (PAHs), and soot in atmospheric pressure counterflow diffusion flames. First, we characterize fully a baseline ethylene flame and then a toluene-seeded flame in which an aliquot of ethylene in the feed stream is replaced with 3500 ppm of prevaporized toluene. The goal is twofold: to investigate the impact of a common reference fuel component of surrogates of transportation fuels and bypass the main bottleneck to soot formation from aliphatic fuels, that is, the formation of the first aromatic ring. The composition of the fuel and oxidizer streams are adjusted to maintain a constant stoichiometric mixture fraction and global strain rate, thereby ensuring invariance of the temperature-time history in the comparison between the two flames and decoupling the chemical effects of the fuel substitution from other factors. Major combustion products and critical radicals are fixed by the baseline flame, and profiles of critical C2-C5 species precursors to aromatic formation are invariant in both flames. On the other hand, doping with toluene boosts the aromatic content and soot volume fraction, increasing the mole fraction of benzenoid structures and soot volume fraction by a factor of 2 or 3, relative to the baseline ethylene flame. This finding is consistent with the expectation that the formation of the first aromatic ring is no longer a bottleneck to soot formation in the doped flame. In addition, toluene bypasses completely benzene formation, opening a radical recombination pathway to soot precursors through the production of C14H14 (via dimerization of benzyl radical) and pyrene (through dimerization of indenyl radical).
Article
Hydrogen (H2) aircraft have been proposed for the “Fly Net Zero” target. The combination of jet fuel and H2 was offered to successfully operate the jet engine with H2 without further combustor modification. The current work used hydro-processed renewable jet fuel (HRJ) and experimentally investigated its combustion properties and H2 addition in the constant volume combustion chamber (CVCC). Experiments were performed at the initial temperature (Tini) from 600 K to 818 K, initial pressure (Pini) 10, 15 bar and equivalence ratio (φ) = 0.5. Normal spray ignition (SPI) and direct injection spark ignition (DISI) were studied. Overall, HRJ has a shorter ignition delay (ID) for SPI. At 600 K, adding 10% and 20% H2 to HRJ increased ID to 12.68% and 38.46%, respectively. As Tini rose to 725 K, increment in ID was shortened to 5.96% and 20.47% with 10% and 20% H2 addition, respectively. For DISI, at 600 K, adding 10% and 20% H2 to HRJ shortens ID to 3.56% and 4.92%, respectively. When comparing the ID of DISI with the SPI, ID was shortened to 84.64% and 91.77% for 10% and 20% H2 addition, respectively. Emission results showed that adding H2 reduced CO2, while NOx emissions were increased. Ansys Chemkin-Pro was used to run numerical simulations of a zero-dimensional (0-D) closed homogeneous batch reactor model (CHBR). An existing HRJ mechanism was adopted, and H2 elementary reaction rate constant factors were updated for better model predictions. The updated model was under prediction with the experimental ID periods. The simulation results showed that the reaction H2+OH = H + H2O is the primary cause for consuming OH radicals with H2 addition, leading to an increase in fuel ID. At higher temperatures, two major reactions (HO2 + H2 = H2O2 + H, and HO2+HO2 = H2O2 + O2) followed by H2O2 = OH + OH are accountable for reproducing OH radicals. Thus, the fuel ID was shorter.
Article
This study investigated the laminar burning characteristics of bio-aviation fuel candidate produced from corn stover lignin via catalytic hydrodeoxygenation (HDO). The experiments were conducted at equivalence ratios of 0.7–1.4, initial pressures of 1 bar, 2 bar, and 4 bar, and initial temperatures of 443 K and 473 K. The laminar burning velocity (LBV), explosion pressure, maximum pressure-rise rate, and combustion duration were studied. The LBV was studied using the constant volume (CVM) and constant pressure (CPM) methods. The CVM results were significantly higher than that of CPM in the presence of cellular flame. In other cases, the deviations of both were within 10 %. The analysis indicated that the LBV of the bio-aviation fuel candidate at certain equivalence ratios was lower than that of Jet A-1, RP-3, and n-decane. However, the LBV of the bio-aviation fuel candidate was greater than that of n-decane in fuel rich mixtures. A 12-term power-law correlation was determined to quantify the relationship between the LBV of the bio-aviation fuel candidate and the equivalence ratio, initial temperature, and initial pressure. The maximum deviation between the experimental data and the correlation was 11.85 %, and the average deviation was 2.95 %. Finally, the effects of the initial conditions on the explosion characteristics of the bio-aviation fuel candidate were analyzed. The explosion pressure, maximum pressure-rise rate, and deflagration index were linearly and positively correlated with the initial pressure; however, the latter two were insensitive to the initial temperature. Furthermore, a quantitative correlation between the explosion characteristics and the equivalence ratio and initial pressure was determined.
Article
Derived cetane number (DCN), Research and Motor Octane Numbers (RON and MON) have been fundamentally analyzed using Quantitative Structure-Property Relationship (QSPR) regression models with key chemical functional groups. Both RON and MON exhibit strong sensitivities to the abundances of (CH2)n and benzyl-type groups but lack sensitivity to the CH3 group, most dominant in real gasolines. Residual and EGR gases contain NOx known to synergize with fuel autoignition chemistry. Two TRF mixtures having high and low aromatic content but sharing the same RON and MON values were used to evaluate NOx coupling effects. DCN measurements with NO addition were found to be strongly correlated with the abundance of the (CH2)n group. Similar experiments of 200 ppm NO in a Rapid Compression Machine show promotion (inhibition) of ignition for the high (low) aromatic TRF fuel. Kinetic modeling attributes the promotion to the NONO2 interconversion reactions, NO + HO2 = NO2 + OH, CH3 + NO2 = CH3O + NO and NO2 + H = NO + OH. The inhibitive effect relates specifically to low temperature kinetics and high NO loading conditions, leading to the formation of meta-stable species (e.g. CH3 + NO2 (+M) = CH3NO2 (+M)) that decelerate the rate of conversion of HO2 to more reactive OH radicals. The coupling of NO with real gasolines depends on chemical composition and temperature conditions not only encompassed by RON and MON criteria, but by the chemical functional group characteristics. The relevance of this finding to the significance of preferential vaporization of multi-component gasolines on low-speed pre-ignition (LSPI) is discussed. Within the context of chemical functional group distributions of five distillation cuts of a marketed ethanol-free gasoline determined by NMR spectroscopy, the analyses identify considerable variations of key functionalities with fuel distillation properties, indicating chemical kinetic autoignition behaviors that are dependent on preferential vaporization.
Article
Real jet fuels have hundreds of individual hydrocarbon species with varying physical and chemical properties. Surrogate fuels containing mixtures of a small number of components have been developed to emulate the behaviors of the parent fuels. Often surrogates are derived solely based on the gas-phase combustion behavior and thus are not constrained to match physical phenomena, such as atomization and vaporization. Thus, these fuels can subsequently exhibit different behaviors than the parent fuel they are designed to emulate, especially when introduced in a two-phase system. This work explores the ability of selected established and newly developed/proposed surrogate jet fuels to mimic the properties necessary for the two-phase conversion process, including the vaporization behavior of real fuel spray. The droplet lifetime/vaporization dynamics is measured using a single droplet levitating device (acoustic levitator). The newly developed surrogates show a satisfying emulation of the parent jet fuel's physical and chemical properties, including SMD, CN, LHV, MW, and vaporization evolution. Most notably, the binary solution consisting of n-dodecane and toluene predicted the physical and chemical properties of the parent fuel closely, with a total maximum deviation from the selected targeted fuel properties ∼12.6% compared to ∼52% of the well-established previously developed Dooley 1st surrogate.
Article
This work develops a new surrogate fuel for RP-3 kerosene. Based on five optimizing targets (molecular weight, hydrogen/carbon ratio, cetane number, lower heating value and threshold sooting index) to emulate the chemical and physical properties of RP-3 kerosene, the surrogate fuel was determined to contain 22% n-dodecane, 15% iso-cetane, 48% decalin and 15% 1,2,4-trimethylbenzene. The laminar burning velocities of RP-3 kerosene and its surrogate fuel were investigated at the unburnt temperature of 413, 443, 473, 503 K, pressure of 1, 2, 5 bar and equivalence ratio of 0.6 ∼ 1.6 in a constant volume combustion bomb. The results show that the laminar burning velocities of the surrogate fuel were in good agreement with that of RP-3 kerosene. Meanwhile, a kinetic model was adopted, which well predicted the laminar burning velocities of RP-3 kerosene and the surrogate fuel. Finally, the flame propagation process and flame instability of RP-3 kerosene under elevated temperatures and pressures conditions were analyzed. It was found that the surrogate fuel was slightly different from RP-3 kerosene in terms of the combustion instability under fuel-rich conditions.
Article
A novel methodology for constructing a unified surrogate model for complex pyrolysis products was developed in this work. The concept of the unified surrogate model refers to the use of same representative components for different products, while their distinctions are reflected by content differences. Based on this method, a unified surrogate model was successfully developed for three different pyrolysis products, which contained eight components, i.e., hydrogen, methane, ethylene, and propane as pyrolysis gases and cyclohexene, toluene, decalin, and n-dodecane as pyrolysis liquids. Analysis results indicate that the model not only agrees well with the key properties of different targeted products but also successfully distinguishes their discrepancies. The experimental results of the laminar flame speeds obtained in this study further proved the accuracy of the model under various cracking and combustion conditions. The results also demonstrate that the concept and method of a unified surrogate model for different pyrolysis products are reasonable and feasible. It can be applied to other studies on the unified surrogate model for other complex pyrolysis products.
Article
Liquid transportation fuels are composed of a wide range of molecular structures and weights, therefore exhibiting a relatively large distillation temperature range. When fuel chemical properties change along with the distillation temperature curve, preferential vaporization effects could play a role in near-limit combustion behaviors. The objective of this study is to experimentally evaluate the role of preferential vaporization on flame flashback behaviors. A unique spray burner is developed to control the extent of fuel spray vaporization by adjusting flow rates and/or the spray injection location from the burner exit. Spray characteristics are comprehensively determined using Phase Doppler Particle Analyzer. Two binary component mixtures are formulated (n-octane/iso-cetane and iso-octane/n-hexadecane) to exhibit common combustion behaviors in the fully vaporized condition but have considerably different preferential vaporization characteristics. Identical flashback behaviors of two mixtures are observed for fully pre-vaporized conditions by setting the burner temperature at 700 K, including both propagation- and ignition-driven flashback behaviors. Partially vaporized conditions are investigated at two global equivalence ratios (1.0 and 1.4) by setting the burner temperature at 450 K. The flashback behaviors for both global equivalence ratio conditions are found to be affected by the preferential vaporization characteristics represented by laminar flame speeds of the vaporized fuel mixture composition. The relative significance of local flow perturbation induced by instantaneous fuel droplet evaporation near the flame surface has been also investigated by analyzing planar laser-induced fluorescence images, as well as considering the changes of Markstein length with the extent of fuel vaporization. Finally, the relative contributions of local laminar flame speed representing local fuel vapor deposit, local flow perturbation, and preferential vaporization are evaluated through feature sensitivity analyses.
Article
1-Methylnaphthalene (1-MN) is a typical diaromatic component in petroleum-based fuels and has been widely chosen to represent aromatic fractions in surrogate fuels of diesel and jet fuels. In this study, experimental and modeling studies were performed to investigate the formation of typical polycyclic aromatic hydrocarbons (PAHs) and soot in coflow methane/air diffusion flames doped with 1-methylnaphthalene/n-dodecane mixtures, and explore chemical effects of 1-methylnaphthalen on key soot formation steps. The laser-induced incandescence (LII) and laser-induced fluorescence (LIF) techniques were used to obtained the soot volume fraction and relative concentration of typical PAHs along the flame centerline, respectively. A novel apparatus-independent indicator, yield aromatic index (YAI), was first proposed to quantify the formation tendency of a specific aromatic product in a flame. It can be found that the addition of 1-methylnaphthalene in the fuel mixtures has a significantly promoting effect on soot formation processes. The correlation between the mass fraction of 1-methylnaphthalene and sooting tendency shows a strong linear relationship. As to the PAH formation tendency, YAIA2R5 have the smaller deviations from corresponding YSIs than YAIA2&A3 and YAIA4, which indicating that A2R5 can be a type of important species leading to soot inception and have strong contribution to soot formation. Through the chemical kinetic analysis, A2R5 is proved to be a key coupling point of two A4 formation pathways, which start from n-dodecane and 1-methylnaphthalene, respectively. As a small amount of 1-methylnaphthalene is introduced into the fuel, the dominant approach to A2R5 rapidly shifts to a more efficient pathway, and is continually enhanced with increasing 1-methylnaphthalene at a moderate trend. The results reveal that 1-methylnaphthalene in the fuel can stride over the rate limiting step of the soot formation process of non-aromatic fuels, eventually leading to an obviously increasing sooting tendency. This study aims to provide useful information for the development of soot nucleation mechanisms and the control of soot formation processes.
Article
Burning of dual fuel of diesel and natural gas is a promising technique in compression ignition engine in the aspects of fuel economy and reduced emission. Effects of blending of natural gas with diesel fuel are investigated numerically and experimentally with respect to ignition delay, heat production, and pollutant emission. First, major properties of diesel are measured at high pressure of 40 bar, which is an engine-relevant condition, and thereby, a surrogate fuel with higher accuracy is formulated, which has properties closer to diesel than the other existing surrogate fuels. Especially, as one of key properties, high molecular weight of diesel is considered critically in formulating a surrogate fuel. Ignition delay times of several mixtures of diesel and natural gas (NG), of which mixture ratios of 100/0%, 60/40%, and 20/80%, are measured by a shock tube in the ranges of temperature from 770 K to 1,000 K and of equivalence ratios from 0.5 to 2.0. Measured ignition delay is compared with that calculated by numerical simulations with the surrogate fuel formulated in this study for its validation. Finally, combustion simulations of HCCI (homogeneous charge compression ignition) are conducted with the mixture of diesel and NG and compared with available experimental data. And thereby, optimal mixture ratio of the dual fuel is suggested in terms of ignition time and pollutant emission acceptable in HCCI engines.
Article
Alkyl aromatics comprise a significant portion of real fuels. Among various alkyl aromatics, the C9H12 aromatic fuels including n-propylbenzene and trimethylbenzenes are representative alkyl aromatics, which are widely detected in real fuels, and they are widely employed as surrogate compounds in modeling real fuels. Thus, the combustion kinetic study of C9H12 fuels is necessary and urgent for fuel combustion. In this work, comparative experimental and kinetic modeling study of the high-temperature ignition of three C9H12 fuels is performed. New ignition delay time (IDT) measurements are carried out in a high-pressure shock tube (HPST) for 1,2,4-trimethylbenzene and 1,3,5-trimethylbenzene. The studied pressure is 2, 5 and 10 bar, the equivalence ratios are 0.5, 1.0 and 2.0, and the temperature range is from 1090 K to 1600 K for IDT in HPST. The experimental results are simulated using an updated detailed kinetic mechanism. Reaction path analysis and sensitivity analysis are performed to provide insight into the chemical kinetics controlling the ignition of the three C9H12 fuels. The present experimental data set and kinetic model results should be valuable to improve our understanding of the combustion chemistry of alkyl aromatics and to offer practical guidance for the surrogate model development of real fuels.
Article
The laminar burning velocities (LBVs) of n-decane/toluene/air mixtures were measured at elevated temperatures using externally heated diverging channel (EHDC) method. The effects of initial temperature (up to 650 K), toluene blending ratio (RC7H8 = 0 to 1.0 by liquid volume) and equivalence ratio (ϕ = 0.6 to 1.4) on the LBVs of n-decane/toluene/air mixtures have been investigated. Numerical computation with a detailed reaction mechanism was used to further analyze the effects of potential interactions between n-decane and toluene. It was observed that the LBVs of n-decane/toluene/air mixtures increase with the increase of initial temperature, while the LBVs decrease with the increase of toluene blending ratio. The effects of toluene on the LBVs through chemical reactions, heat and mass transfer were analyzed in detail. Moreover, the temperature compensation method, which separates the effect of adiabatic flame temperature and contribution from chemical and disunion, was used in analysis. The effects of the toluene blending on the LBV is mainly attributed to chemical term, and the thermal effect have a second order importance. The reaction pathways and sensitivity analysis show that the LBVs of stoichiometric mixtures is less sensitive to kinetic coupling. The distinct flame propagation properties of the n-decane/toluene blended fuels are determined by the distributions of reaction rates of the crucial intermediate species and radicals.
Article
The mechanism of low-density jet mixing enhancement by a pulsed laser plasma is investigated through a combined numerical and experimental method. A helium jet is injected in parallel to a Mach 2.4 supersonic airstream, and interacts with an oblique shock generated by a compression ramp of 20 deg. The helium jet Mach numbers are set to be 1.01 and 1.80 for the sonic and supersonic fuel jets in the scramjet combustor, respectively. For jet mixing enhancement control, the laser pulses at frequencies ranging from 5 to 25 kHz are introduced inside the jet. The experimental results show that laser pulses significantly improve the jet mixing with jet width increased by 30% seen from the averaged schlieren images. Three-dimensional unsteady Reynolds-averaged Navier–Stokes equations are solved, and the numerical results further reveal that the jet mixing enhancement is strongly associated with large-scale vortex rings stimulated by the laser pulses interacting with shock waves. These large-scale vortices prompt the jet cross-sectional area and mixing efficiency two times larger. However, the total pressure recovery coefficient is reduced by 0.16% downstream. Meanwhile, the turbulent kinetic energy of jet flow is significantly increased and favorable for the jet mixing.
Article
In this paper, based on a model of the five-component surrogate fuel, the authors have studied the relationship between temperature and saturated vapor pressure of the RP-3 aviation kerosene fuel, and derived an experimental function of this relationship. Then, the Antoine equation of the RP-3 aviation kerosene has been obtained by fitting the experimental data points of the function, and the resulting equation has been verified. The results of this study can provide some theoretical background for engineering practices and further theoretical studies of liquid mixtures.
Article
The temperature sensitivity of n-propylbenzene and 1,2,4-trimethylbenzene on soot formation in coflow diffusion flames was assessed. Cases with air temperatures at 300K (LT), 473K (MT), and 673K (HT) were established. Soot volume fractions and primary particle diameters were measured by Laser-induced incandescence. Soot temperatures were measured by rapid thermocouple insertion with correction by backward extrapolation. Soot yield also increased with temperature. Compared to alkanes and alkenes, alkylbenzenes exhibited much lower temperature sensitivity. The model suggested that elevating the reactant temperature did not significantly affect the production of soot precursor PAH in alkylbenzene flames, but altered the buoyancy-induced acceleration, which subsequently determined the time available for soot growth. Soot formation was promoted by extending the available time. To isolate the impact of fuel temperature, a case with heated fuel and unheated air (FHT) was also assessed. It is found that raising the fuel temperature affected soot formation more along the centerline than on the wing. This is suggested to be related to the earlier soot inception for FHT along the centerline.
Article
The need to replace fossil fuels with biofuels has become necessary due to increasing release of greenhouse gases and particulate matter by combusting fossil fuels. Biofuels are better options compared to fossil-based fuels due to the availability of cheap and abundant renewable feedstock. Due to large number of possible fuel structures clustered in databases, solo experimental search strategies cannot identify a clean and efficient fuel molecule, and application of computer-aided approaches are necessary with integration of product and production design for fuel molecules of single- and multi-species, mass- and energy-based production pathway screening for costs, and emission estimation models. Optimization of biofuel production processes can surge fuel availability and development of surrogate fuel formulation and property modelling that would improve combustion efficiency of fuels in the engine infrastructure. In this review, we have taken more synergetic approach and analysed the microbial biofuel production processes utilizing 1) metabolic engineering tools on diverse microbes 2) chemo-catalytic pathways and 3) fuel design manoeuvre that is attracting much attention, and thus are extensively discussed in this review. The review emphasized that the utilization of new/modern chemo-catalytic refunctionalization of fuel molecules using new catalysts and enzymes have not only enhanced fuel yields but have led to the production of various novel advanced energy molecules from biomasses and microbes. The contribution of this review is that it highlights the current status of microbial fuels, metabolic engineering, fuel design and production of tailored made fuels, and potential future applications of microbial fuels in transport and energy sectors.
Conference Paper
View Video Presentation: https://doi.org/10.2514/6.2022-1258.vid Lift-off heights of three well-studied reference jet fuels and three surrogate jet fuels with similar pre-vaporized combustion behavior are measured in a piloted spray burner using chemiluminescence imaging. The spray burner consists of a central tube of two-phase flow, resembling an open-pipe atomizer, and a surrounding hydrogen/air pilot flame to assist ignition of the liquid fuel. It is found that the lift-off behaviors of the reference jet fuels are generally comparable, with the main differences occurring for the most transient condition. However, the surrogate fuels do not reproduce the lift-off heights of the reference fuels due to discrepancies in the spray behaviors between the two. In particular, the differences in viscosity are inferred to be the primary reason for the variations in the flame heights. This investigation confirms that the physical properties of the liquid fuel cannot be neglected in the formulation of jet fuel surrogates.
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Gasoline/Hydrogenated catalytic biodiesel blending fuel is a prospective alternative fuel for gasoline compression ignition (GCI) engine which has the potential to solve the ignition difficulties of GCI especially at low load conditions. However, the autoignition behavior of gasoline/hydrogenated catalytic biodiesel blends is unknown. In this study, the ignition delay times of gasoline/hydrogenated catalytic biodiesel (HCB) blends were measured in a heated rapid compression machine over a wide range of conditions (the temperature range of 655–870 K, pressures of 10 and 15 bar, equivalence ratios of 0.5–1.5). Experimental results show that the measured ignition delay time of the gasoline/HCB blends exhibits negative-temperature coefficient behaviors under the investigated conditions. The total ignition delay times diminish with rising pressure, equivalence ratio and HCB ratio in the blends. The simulation results of ignition delay times for gasoline/HCB blends using a detailed mechanism are compared with experimental data. The detailed model can predict the trends of ignition delay time well during low-to-intermediate temperature region. Finally, kinetic analyses were performed under different compressed pressure, temperature, equivalence ratios and blending ratios. The dominating reactions and decisive factors of ignition are explored to gain a deeper understanding of the ignition characteristic of gasoline/HCB blends. It is found that the autoignition of fuel is completely caused by HCB related reactions at low temperature, which explains why the blending of HCB can expand the working condition downward from the perspective of chemical kinetic.
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Large Eddy Simulations (LES) are performed to compute the sensitivity of a conventional (A-2) and an alternate bio-jet (C-1) fuel to Lean Blowout (LBO). A realistic aviation gas turbine engine combustor configuration is considered. Reliable experimental LBO data and OH* chemiluminescence data for the conventional and alternate jet fuel in the combustor configuration have recently become available. The present work utilizes a highly automated, on-the-fly meshing strategy, along with adaptive mesh refinement, to demonstrate the feasibility of capturing the realistic combustion processes. A Lagrangian framework, with initial conditions specified using measurements of spray statistics, is used to model the fuel spray. Newly developed compact reaction mechanisms based on fuel surrogates are validated for the A-2 and the C-1 fuels. The compact reaction mechanisms are implemented using a detailed finite rate chemistry solver. Spray statistics computed by the present LES simulations compare well with available measurements at stable flame conditions near the lean blowout limit. The computed shape of the stable flame as represented by line integrated OH concentrations compares well with the experimental OH* chemiluminescence data. Lean blowout is reached by gradually decreasing the fuel flow rate in the computations, similar to that in the experiments. The results of the LES simulations effectively capture the fuel composition effects and estimate the sensitivity of the LBO limits to the fuel type. The computed trends in LBO limits agree within engineering accuracy with the experimental results for conventional and alternative aviation fuels. The methodology for predicting the fuel composition effects on the lean blowout limits in a fully resolved realistic, complex combustor is established for the first time. Link to full paper: https://authors.elsevier.com/a/1eI2w6CY3ylqwH
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Fuel surrogates are simplified models that mimic the combustion characteristics of very complex real transportation fuels and enable a detailed description of the computational system for the targeted real fuels. Current efforts in surrogate development focus on matching multiple target properties of the real fuel using numerical optimization of species compositions. A way to solve the multi-objective optimization problem is to employ the weighted-sum approach with arbitrarily assigned weights. In this paper, we propose a novel approach to reduce such arbitrariness by incorporating physical information into the surrogate optimization process, leveraging uncertainties from experimental measurements and mixture property predictions to determine the weight of each target property. The underlying principle of this approach is assigning low weight for properties with high uncertainty since a tight emulation of that property is not necessary. We propose formulations that convert relative uncertainties of each target property into weights for multi-objective surrogate optimization, which penalize target properties with high uncertainties. The method is flexible enough to accommodate not only multiple uncertainty sources, but also users’ preferences and the sensitivity of weights to uncertainty variations can be readily adjusted. The proposed method is applied to formulate surrogate mixtures for three reference target jet fuels (Jet-A POSF-10325, JP-8 POSF-10264, JP-5 POSF-10289), which have considerably different hydrocarbon compositions and properties. The results show that the surrogates developed by the uncertainty-based weight method emulate the target properties of fuels very closely, and they are able to capture the compositional characteristics of the target fuels as well.
Article
A three-component surrogate model was developed to emulate both the physical and chemical characteristics of RP-3 kerosene for aircraft spark ignition (SI) engine. Due to the similar carbon number and physical–chemical properties, n-decane (C10H22), 1,3,5-trimethylbenzene (C9H12) and iso-dodecane (IC12H26) were selected as surrogate fuel (46.31%/25%/28.69% by mole, respectively). The surrogate well captured the target properties of RP-3, including RON, CN, H/C ratio, MW, LHV, liquid density, kinematic viscosity and surface tension. Furthermore, a skeletal mechanism of RP-3 was developed based on the provided surrogate model. The RP-3 sub-mechanism only included 65 species and 200 reactions which was efficient and compact for complex CFD simulation. The mechanism of each pure component and RP-3 surrogate had been verified with the ignition delay time data under various conditions. The measured species concentrations and laminar flame speed data were also used to test the kinetic mechanism. Surrogate model and skeletal sub-mechanism were implemented under evaporating spray environment, Bunsen burner and SI engine mode by CFD method. The acceptable agreement between experiments and simulations proved the reliability of the skeletal mechanism of RP-3 surrogate in SI engine simulation.
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In the present work, algae-derived biocrude was collected from the hydrothermal liquefaction of algae slurry, and the mixtures of algae-derived biocrude and RP-3 aviation kerosene was used as aviation substitute fuel named as microalgae oil/RP-3 blend. In order to understand the laminar burning velocity (LBV) of microalgae oil/RP-3, experiments have been carried out in a constant volume chamber (CVC) to investigate the effects of initial temperature, initial pressure and microalgae oil addition on the LBV of microalgae oil/RP-3 blend over a wide equivalence ratio range from 0.8 to 1.4. Experimental result shows that with the increase of initial temperature and microalgae oil addition, the LBV of microalgae oil/RP-3 mixture increases, while with the increase of initial pressure the LBV of microalgae oil/RP-3 mixture decreases. According to the method of Metghachi, the relationship between the LBV and its influence factors was obtained, the exponents of αT(ϕ), βP(ϕ) and γR(ϕ) indicate that the change of initial temperature and pressure have less effect on LBV around equivalence ratio of 1.1 compared with other equivalence ratio conditions, while the change of microalgae oil addition has more significant effect to improve LBV at equivalence ratio of 1.1.
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Full-text available
Ignition delay and oxidation of two jet aviation fuels, Jet A-1 and its blended fuel with a bio-jet fuel in half, are investigated by experiments and numerical simulations. From their major combustion properties, derived cetane number and molecular weight of the blended fuel, Jet50-Bio50, are higher than those of Jet A-1, and its H/C ratio and threshold sooting index are lower because more n-alkanes are contained in a bio-jet fuel and aromatic compounds are not. The surrogate fuels of the two jet fuels are constructed for numerical simulations of their ignition and oxidation. Early ignition of the blended fuel measured in a shock tube experiment is investigated by comparing the speciation profiles of several products from the two fuels, and their global reactivity is measured in a laminar flow reactor. Oxidation of the blended fuel is initiated at a lower temperature than Jet A-1, and reaction pathways of the two fuels are analyzed at two temperatures of 600 and 1100 K, respectively. At a low temperature of 600 K, reaction pathways of the major surrogate components for the two fuels are significantly different, while they are almost the same at high temperatures. The active radical of OH is produced more by the oxidation of Jet50-Bio50, and its oxidation is initiated at a lower temperature than Jet A-1, leading to earlier ignition. At low temperatures, the difference between initiation times of oxidation of the two fuels is much larger than at high temperatures. At both temperatures, production rates of the major reaction steps, where OH is produced, are higher in Jet50-Bio50 than in Jet A-1.
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A Discrete Component Model (DCM), based on the analytical solutions to heat transfer and species diffusion equations, together with the Abramzon-Sirignano model are applied to analyse the droplet heating and evaporation of Jet A kerosene and its surrogates. The models are implemented into MFSim code, which opens the way for modelling of the droplet heating and evaporation process alongside other spray processes. The composition of Jet A fuel used in the analysis, with 61 components split into 7 hydrocarbon groups, is described. This composition is approximated by twelve previously developed surrogates. The number of components in these surrogates varies between two and nine, which is expected to lead to a significant reduction in CPU requirements for calculation of droplet heating and evaporation, when compared to surrogates typically used to describe Jet A droplets. The prediction ability of the MFSim code, with new models implemented into it, is validated against available experimental results. The surrogates best able to predict droplet evaporation time and temperature of the Jet A fuel with 61 components are identified. It is shown that the number of terms in the series of analytical solutions for temperature and species mass fractions can be considerably reduced without affecting the accuracy of calculations.
Article
Conventional transportation fuels used in aviation (jet fuel) or in ground transportation (gasoline, diesel) contain multitude of hydrocarbon components and are difficult to be modeled, if one has to consider each of the component present. A typical approach is the definition of a fuel surrogate with a limited number of fuel components. In this context, a single semi-detailed high temperature reaction kinetic mechanism is presented in this work, which contains all the important molecular classes required for the detailed surrogate modeling of a hydrocarbon fuel. The appeal of the mechanism is the suitability for a broad range of technical fuels covering gasoline, diesel and jet fuels. The reaction mechanism for hydrocarbon combustion is consisted of 238 species and 1814 reactions and is rigorously validated for 70 neat hydrocarbon components over a wide range of experimental conditions including combustion setups such as shock-tubes, laminar flames, jet-stirred and flow reactors. The purpose of this study is to provide a single reaction model that (1) includes variety of hydrocarbon molecules of varying degree of complexity and carbon numbers, (2) has capability to model a spectrum of different fuels, initially aviation fuels, and (3) is compact to apply both in simple (fundamental kinetic investigations) and complex geometries (CFD studies) of combustion system enabled through customized mechanism reductions. The ultimate goal is to resolve the fuel differences using the model predictions obtained from the reaction mechanism that will supply parameters for fuel design and optimization of fuels. Extensive supporting information is available in this work.
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Kinetic effects of aromatic molecular structures for jet fuel surrogates on the extinction of diffusion flames have been investigated experimentally and numerically in the counterflow configuration for toluene, n-propylbenzene, 1,2,4-trimethylbenzene, and 1,3,5-trimethylbenzene. Quantitative measurement of OH concentration for aromatic fuels was conducted by directly measuring the quenching rate from the emission lifetimes of OH planar laser induced fluorescence (LIF). The kinetic models for toluene and 1,2,4-trimethylbenzene were validated against the measurements of extinction strain rates and LIF measurements. A semi-detailed n-propylbenzene kinetic model was developed and tested. The experimental results showed that the extinction limits are ranked from highest to lowest as n-propylbenzene, toluene, 1,2,4-trimethylbenzene, and 1,3,5-trimethylbenzene. The present models for toluene and n-propylbenzene agree reasonably well with the measurements, whereas the model for 1,2,4-trimethylbenzene under-estimates extinction limits. Kinetic pathways of OH production and consumption were analyzed to investigate the impact of fuel fragmentation on OH formation. It was found that, for fuels with different molecular structures, the fuel decomposition pathways and their propagation into the formation of radical pool play an important role to determine the extinction limits of diffusion flames. Furthermore, OH concentrations were found to be representative of the entire radical pool concentration, the balance between chain branching and propagation/termination reactions and the balance between heat production from the reaction zone and heat losses to the fuel and oxidizer sides. Finally, a proposed “OH index,” was defined to demonstrate a linear correlation between extinction strain rate and OH index and fuel mole fraction, suggesting that the diffusion flame extinctions for the tested aromatic fuels can be determined by the capability of a fuel to establish a radical pool in a manner largely governed by molecular structure.
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The oxidation of kerosene Jet-A1 and that of n-decane have been studied experimentally in a jet-stirred reactor at atmospheric pressure and constant residence time, over the high temperature range 900–1300K, and for variable equivalence ratio (0.5≤ϕ≤2). Concentration profiles of the reactants, stable intermediates, and final products have been obtained by probe sampling followed by on-line and off-line GC analyses. The oxidation of neat n-decane and of kerosene in these conditions was modeled using a detailed kinetic reaction mechanism (209 species and 1673 reactions, most of them reversible). The present model was successfully used to simulate the structure of a fuel-rich premixed n-decane–oxygen–nitrogen flame. In the modelling, kerosene was represented by four surrogate model fuels: 100% n-decane, n-decane-n-propylbenzene (74%/26% mol), n-decane-n-propylcyclohexane (74%/26% mol), and n-decane-n-propylbenzene-n-propylcyclohexane (74%/15%/11% mol). The 3-component model fuel was the most appropriate for simulating the JSR experiments. It was also successfully used to simulate the structure of a fuel-rich premixed kerosene–oxygen–nitrogen flame.
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Full-text available
The oxidation of methyl formate (CH3OCHO) has been studied in three experimental environments over a range of applied combustion relevant conditions: A detailed chemical kinetic model has been constructed, validated against, and used to interpret these experimental data. The kinetic model shows that methyl formate oxidation proceeds through concerted elimination reactions, principally forming methanol and carbon monoxide as well as through bimolecular hydrogen abstraction reactions. The relative importance of elimination versus abstraction was found to depend on the particular environment. In general, methyl formate is consumed exclusively through molecular decomposition in shock tube environments, while at flow reactor and freely propagating premixed flame conditions, there is significant competition between hydrogen abstraction and concerted elimination channels. It is suspected that in diffusion flame configurations the elimination channels contribute more significantly than in premixed environments. © 2010 Wiley Periodicals, Inc. Int J Chem Kinet 42: 527–549, 2010
Article
Laminar flame speed measurements are carried out for premixed iso-octane/air and n-heptane/air mixtures under conditions of atmospheric pressure, equivalence ratios ranging from 0.7 to 1.4, and unburned mixture temperatures of 298, 360, 400, and 470 K using the counterflow flame technique. These experiments employ the digital particle image velocimetry technique to characterize the two-dimensional flow field upstream of the flame. As such, the reference stretch-affected flame speed and the imposed stretch rate can be simultaneously determined. By systematically varying the imposed stretch rate, the corresponding laminar flame speed is obtained by linearly extrapolating to zero stretch rate. In addition, the effect of nitrogen dilution level on the laminar flame speed is investigated by varying the nitrogen molar percentage in the oxidizer mixture from 78.5 to 80.5%. These results are further used for the determination of overall activation energies at different equivalence ratios. The experimental laminar flame speeds are subsequently compared with the computed values using two iso-octane reaction mechanisms and two n-heptane reaction mechanisms available in the literature, followed by discussion and sensitivity analysis.
Article
The oxidation of a kerosene fuel and of a mixture of 3 hydrocarbons (79% n-undecane—10% n-propylcyclohexane—11% 1,2,4-trimethylbenzene) was studies in a jet-stirred flow reactor in the temperature range 873–1033 K at atmospheric pressure. The concentrations of molecular species were measured at different extents of reaction by gas chromatography. The reaction products formed during the oxidation of the kerosene and the ternary mixture were identical. The main hydrocarbon intermediates were ethylene, propene, methane, 1-butene, 1,3-butadiene and ethane. Several other unsaturated hydrocarbon were also detected as minor products: 1-pentene, 1-hexene, 1-heptene, 1-octene, 1-nonene and 1-dencene and also 1,3-pentadiene, cyclopentadiene, benzene, toluene and xylene. In both experiments, the concentration profiles of molecular species were very similar, indicating that the mixture of 3 hydrocarbons from C9 to C11 belonging to 3 different chemical families (n-alkanes, cyclanes and aromatics) is representative of the kerosene studied. On the basis of the experimental observation of a low concentration level for large hydrocarbon intermediates, quasi-global chemical kinetic reaction mechanisms were developed to reproduce the experimental data. These models involve a few global molecular reactions for the oxidation-pyrolysis of the initial fuel molecules and a detailed mechanism for the oxidation of the small intermediate hydrocarbons.
Article
Laminar flame speeds of n-decane/air and n-dodecane/air mixtures are measured using the counterflow twin-flame configuration at preheat temperatures ranging from 360 to 470 K and equivalence ratios ranging from 0.7 to 1.4. Extinction stretch rate measurement as a function of equivalence ratio is also carried out for fuel/O{sub 2}/N{sub 2} mixtures with [N{sub 2}/(O{sub 2} + N{sub 2})] = 0.84 by mole and preheat temperature of 400 K. All experiments are conducted under atmospheric pressure conditions. In addition, the overall activation energies of n-decane/air mixtures at varying equivalence ratios are deduced. The experimental data for laminar flame speeds and extinction stretch rates are also simulated using chemical kinetic mechanisms available in the literature. Comparison of the experimental and computed results demonstrates the deficiencies of the existing mechanisms. Although sensitivity analysis is performed to identify the most sensitive reactions pertinent to laminar flame speed and extinction limit, the results are unable to assess the adequacy of the chemistry involving large hydrocarbons. (author)
Article
Currently, modeling the combustion of aviation fuels, such as JP-8 and JetA, is not feasible due to the complexity and compositional variation of these practical fuels. Surrogate fuel mixtures, composed of a few pure hydrocarbon compounds, are a key step toward modeling the combustion of practical aviation fuels. For the surrogate to simulate the practical fuel, the composition must be designed to reproduce certain pre-designated chemical parameters such as sooting tendency, H/C ratio, autoignition, as well as physical parameters such as boiling range and density. In this study, we focused only on the sooting characteristics based on the Threshold Soot Index (TSI). New measurements of TSI values derived from the smoke point along with other sooting tendency data from the literature have been combined to develop a set of recommended TSI values for pure compounds used to make surrogate mixtures. When formulating the surrogate fuel mixtures, the TSI values of the components are used to predict the TSI of the mixture. To verify the empirical mixture rule for TSI, the TSI values of several binary mixtures of candidate surrogate components were measured. Binary mixtures were also used to derive a TSI for iso-cetane, which had not previously been measured, and to verify the TSI for 1-methylnaphthalene, which had a low smoke point and large relative uncertainty as a pure compound. Lastly, surrogate mixtures containing three components were tested to see how well the measured TSI values matched the predicted values, and to demonstrate that a target value for TSI can be maintained using various components, while also holding the H/C ratio constant. (author)
Article
The autoignition of CH aromatic/air mixtures (ortho-xylene, meta-xylene, para-xylene, and ethylbenzene in air) has been studied in a shock tube at temperatures of 941-1408 K, pressures of 9-45 atm, and equivalence ratios of =1.0 and 0.5. Ignition times were determined using electronically excited OH emission and pressure measurements. The measurements illustrate the differences in reactivity for the CH aromatics under the studied conditions. Ethylbenzene was by far the most reactive CH aromatic with ignition times a factor of two to three shorter than the xylenes. The xylene isomers exhibited ignition times that were similar, with o-xylene the most reactive, p-xylene the least reactive, and m-xylene just slightly more reactive than p-xylene. The p-xylene ignition times are almost identical to previous measurements for toluene at the same conditions. The differences in reactivity can be attributed to the C-H and C-C bond strengths in the alkyl side chains and the proximity of the methyl groups in the xylenes. These results represent the first ignition measurements for CH aromatics at the elevated-pressure moderate-temperature conditions studied, providing needed targets for kinetic modeling at engine-relevant conditions. Kinetic modeling illustrates the importance of the methylbenzyl + HO reaction and indicates further study of this reaction is warranted. (author)
Article
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.
Article
Jet A and JP-8 are kerosene fuels used in aviation and consist of complex mixtures of higher order hydrocarbons, including alkanes, cycloalkanes, and aromatic molecules. The objectives of the current work are to develop a surrogate mixture to represent JP-8 fuels and to discuss a general detailed chemical kinetic model for jet fuels, which is suitable for future reduction. Asurrogate blend of six pure hydrocarbons is found to adequately simulate the distillation and compositional characteristics of a practical JP-8. A hierarchically constructed kinetic model already available for the oxidation of alkanes and simple aromatic molecules (benzene, toluene, ethylbenzene, xylene, etc.) is extended to include methylcyclohexane and tetralin as new reference fuel components. The kinetic model is validated through comparisons with experimental data for the pure components and it is also used to verify and predict the structures of laminar premixed flames of different pure components as well as conventional kerosene fuels.
Article
Surrogate mixtures of one to 10 hydrocarbons that have similar properties to aviation fuels are desirable for experimental and computational tractability and reproducibility, However, aviation fuels, such as Jet A, JP-8, and JP-5, contain hundreds of hydrocarbons, This paper describes appropriate "surrogate" mixtures to reproduce the behavior of multicomponent aviation fuels. Surrogate mixtures from the literature and their applicability to various situations are summarized.
Article
A surrogate fuel comprised of 14 pure hydrocarbons is formulated based on the distillation curve and compound class composition of a petroleum-derived JP-4. The goal is to establish a fuel of controlled composition for modeling, and for the study of fuel property and chemical composition effects in the combustion of JP-4 fuels. Spatially resolved interferometric measurements of droplet size and droplet velocity are obtained and compared for both the petroleum and surrogate JP-4 in a nonreacting spray chamber. Measurements are also obtained for a high aromatic JP-5 of purposefully disparate properties. The performance of these three fuels is then compared in a swirl-stabilized, spray-atomized model laboratory combustor where in-flame measurements of velocity and temperature are acquired and compared.
Article
A rapid compression machine (RCM) has been designed and fabricated for the purpose of chemical kinetics studies at elevated pressures and temperatures. The present RCM is pneumatically driven and hydraulically actuated and stopped. Stroke of the machine varies from 7 to 10 inches and clearance is also adjustable. Compression ratio of up to 21 can be obtained. The optically-accessible reaction chamber is equipped with sensors for the measurements of pressure and temperature. In addition, a rapid sampling apparatus is incorporated in the reaction chamber for determining species concentration at specific post-compression time. A deliberately machined crevice on the cylindrical surface of the piston has been optimized, using STAR-CD CFD package, in order to suppress the formation of the roll-up vortex and provide a homogeneous core of reaction mixture. Temperature mapping using planar laser induced fluorescence of acetone shows that roll-up vortex is indeed suppressed by using the present creviced piston. Experiments with either inert gases or reactive mixtures demonstrate the reproducibility of pressure traces. Compression process is also shown to be very rapid and free from any significant mechanical vibrations. Measurements show that highly repeatable compressed conditions of up to 50 bar and greater than 1000 K can be obtained. A numerical model accounting for heat loss is also developed to simulate the RCM data. This work documents the design and operation of the present RCM as well as establishes its suitability for combustion studies.
Article
Liquid fuels, such as gasolines and jet and diesel fuels, are usually refined products from the processing of crude oil. Their composition is mainly based on major physical properties and combustion performance indexes. For these reasons, real transportation fuels contain thousands of compounds that greatly vary with the feedstock origins, the seasons, and the economic factors that are imposed by the refinery. Regardless of this complexity, the chemical species contained in the fuels belongs to only four hydrocarbon classes: linear or branched alkanes, alkenes, cycloalkanes, and aromatics. Moreover, the physical properties ( such as vapor pressure and flash point) and the combustion properties ( such as octane or cetane numbers and smoke point) are regularly variable with composition. On these bases, it is viable to define surrogate mixtures to reproduce the most important chemical and physical properties of real transportation fuels. These surrogate fuels are then very useful both for the design of more reproducible experimental tests and for the development of reliable kinetic models, which are always projected to a deeper understanding of combustion processes. This paper analyzes some critical features in the definition of surrogates and in the development of detailed kinetic schemes of the pyrolysis and combustion of liquid fuels and also discusses experiments and simulation results obtained under very different conditions. These examples not only relate to ideal reactors ( such as plug flow, jet stirred, shock tube, or rapid compression devices), but also concern the knock propensity of hydrocarbon mixtures in internal combustion engines as well as the combustion behavior of liquid fuel droplets and the structure of premixed and diffusion flames.
Article
Autoignition of Jet-A and mixtures of benzene, hexane, and decane in air has been studied using a heated shock tube at mean post-shock pressures of 8.5±1atm within the temperature range of 1000–1700K with the objective of identifying surrogate fuels for aviation kerosene. The influence of each component on ignition delay time and on critical conditions required for strong ignition of the mixture has been deduced from experimental observations. Correlation equation for Jet-A ignition times has been derived from the measurements. It is found that within the scatter of experimental data dilution of n-decane with benzene and n-hexane leads to slight increase in ignition times at low temperatures and does not change critical temperatures required for direct initiation of detonations in comparison with pure n-decane/air mixtures. Ignition times in 20% hexane/80% decane (HD), 20% benzene/80% decane (BD) and 18.2% benzene/9.1% hexane/72.7% decane (BHD) mixtures at temperature range of T≅1450–1750K correlate well with induction time of Jet-A fuel suggesting that these mixtures could serve as surrogates for aviation kerosene. At the same time, HD, BD and BHD surrogate fuels demonstrate a stronger autoignition and peak velocities of reflected shock front in comparison with Jet-A and n-decane/air mixtures.
Article
The oxidation of a commercial B30 (30% FAME by vol.) bio-Diesel fuel and a B30 bio-Diesel surrogate fuel (49% n-decane, CAS 124-18-5; 21% 1-methylnaphthalene, CAS 90-12-0; 30% methyl octanoate, CAS 111-11-5, in mole) was performed using a pressurized fused-silica jet-stirred reactor under the same initial experimental conditions (560–1030K, 6 and 10atm, equivalence ratios of 0.25–1.5, 10,300ppm of carbon). The results of this series of experiments consisted of concentration profiles of reactants, stable intermediates and products measured as a function of temperature by low-pressure probe sampling followed by Fourier transform infrared absorption spectrometry and gas chromatography analyses. The results obtained with the commercial and surrogate B30 mixtures were compared with each other, showing that the mixture n-decane/1-methylnaphthalene/methyl octanoate 49/21/30% in mole is an excellent simple B30 Diesel fuel surrogate. A detailed chemical kinetic reaction mechanism consisting of 7748 reactions involving 1964 species was proposed based on previous chemical kinetic reaction mechanisms for the oxidation of n-decane, methyl octanoate, and 1-methylnaphthalene under similar conditions. The kinetic modeling showed reasonable agreement between the present data and computations over the entire range of conditions considered in this study.
Article
Experimental and numerical studies are carried out to develop a surrogate that can reproduce selected aspects of combustion of kerosene. Jet fuels, in particular Jet-A1, Jet-A, and JP-8 are kerosene type fuels. Surrogate fuels are defined as mixtures of few hydrocarbon compounds with combustion characteristics similar to those of commercial fuels. A mixture of n-decane 80% and 1,2,4-trimethylbenzene 20% by weight, called the Aachen surrogate, is selected for consideration as a possible surrogate of kerosene. Experiments are carried out employing the counterflow configuration. The fuels tested are kerosene and the Aachen surrogate. Critical conditions of extinction, autoignition, and volume fraction of soot measured in laminar non premixed flows burning the Aachen surrogate are found to be similar to those in flames burning kerosene.A chemical-kinetic mechanism is developed to describe the combustion of the Aachen surrogate. This mechanism is assembled using previously developed chemical-kinetic mechanisms for the components: n-decane and 1,2,4-trimethylbenzene. Improvements are made to the previously developed chemical-kinetic mechanism for n-decane. The combined mechanisms are validated using experimental data obtained from shock tubes, rapid compression machines, jet stirred reactor, burner stabilized premixed flames, and a freely propagating premixed flame. Numerical calculations are performed using the chemical-kinetic mechanism for the Aachen surrogate. The calculated values of the critical conditions of autoignition and soot volume fraction agree well with experimental data. The present study shows that the chemical-kinetic mechanism for the Aachen surrogate can be employed to predict non premixed combustion of kerosene.
Article
This paper presents soot processes of a blend of 23% m-xylene and 77% n-dodecane, which has been selected by several working groups as a surrogate for jet fuel. Fuel sprays were injected into high-temperature, high-pressure ambient conditions that are representative of practical engine combustion. Simultaneous laser extinction (KL) measurement and planar laser-induced incandescence imaging were performed to derive the in situ soot volume fraction. Also, soot particles were extracted from different positions within the reacting jet by means of a thermophoretic probe, and analyzed using transmission electron microscopy (TEM) to clarify the soot structure and its correlation with the measured soot volume fraction. The same measurements were repeated for the conventional jet fuel to understand the overall performance of the selected surrogate fuel. The soot volume fraction results show that, at fixed ambient conditions, the surrogate fuel produces more soot than the conventional jet fuel. The TEM images show that the soot aggregates are more agglomerated, which may not be easily eliminated by in-cylinder oxidation. The total number of primary particles and the mean primary particle size are higher for the surrogate fuel, consistent with the soot volume fraction trend. Considering that there is similar lift-off length between fuels, the differences in soot level and morphology are caused by molecular structure effects, such as a higher aromatic content. The quantitative soot database obtained from the present study offers data for the validation of soot kinetic models, particularly at high temperature and pressure conditions where little fundamental data exist.
Article
Detailed flame simulations require the use of both chemical kinetics and molecular transport. Compared to kinetics, notably less work has been done to assess the validity of transport properties and their effect on the results of flame simulations. In the present study, transport parameters were proposed for n-alkanes up to tetradecane and 1-alkene up to dodecene, based on the correlations of corresponding states. The effect of molecular diffusion was illustrated by considering the phenomena of propagation and extinction for flames of n-dodecane, a main component of surrogates of practical jet fuels. Sensitivity analyses were performed on the reaction rates and binary diffusion coefficients and the relative importance of chemical kinetics and transport properties on flame propagation and extinction was quantified. The effect of carbon number on the extinction of non-premixed n-alkane flames was addressed also based on transport effects.
Article
For modeling the combustion of aviation fuels, consisting of very complex hydrocarbon mixtures, it is often necessary to use less complex surrogate mixtures. The various surrogates used to represent kerosene and the available kinetic data for the ignition, oxidation, and combustion of kerosene and surrogate mixtures are reviewed. Recent achievements in chemical kinetic modeling of kerosene combustion using model-fuels of variable complexity are also presented. (c) 2005 Elsevier Ltd. All rights reserved.
Article
The auto-ignition features of 11 alkylbenzenes in a rapid compression machine have been compared for stoichiometric mixtures in the lower temperature region (600–900 K), and at compressed pressures up to 25 bar, by following pressure traces and light emission. They are classified in two groups. Toluene, m-xylene, p-xylene, and 1,3,5-trimethylbenzene ignite only above 900 K and 16 bar. o-Xylene, ethylbenzene, 1,2,3-trimethylbenzene, 1,2,4-trimethylbenzene, n-propylbenzene, 2-ethyltoluene, and n-butylbenzene ignite at much lower temperature and pressure. The second group shows a complex phenomenology similar to alkanes and alkenes when submitted to adapted conditions of reactant concentrations. Ignition in two steps and negative temperature dependence of ignition delays are observed in favorable cases. Some of them show a low-temperature luminescence. Ignition features of o-xylene and n-butylbenzene are similar, in spite of their dissimilar molecular structure. The higher degree of reactivity of the second group is ascribed to the close proximity and/or length of their alkyl chains.
Article
A rational threshold soot index (TSI) varying from 0 to 100 is defined for evaluating the onset of soot formation in both premixed and diffusion flames. It is shown that all of the data in the literature on either premixed or diffusion flames, taken by many techniques, are consitent with respect to molecular structure for each of the two types of flames. There is also a closer similarity between the effect of molecular structure on soot formation in premixed and diffusion flames than previously thought. The use of TSI permits one to use all of the literature data to interpret molecular structure effects and thus arrive at rules for predicting the effect of molecular structure for compounds which have not yet been measured or to correlate the results from one experimental sytem with another. If a correlation can be demonstrated between the effect of molecular structure on soot formation in laboratory and in practical systems, then TSIs will be useful to the synfuels program for defining the desired fuel components to be prepared from a given feedstock.
Article
Understanding of the impact of kinetic and transport coupling on the combustion characteristics for alkane and aromatics blended fuels is of great importance to construct a reliable surrogate fuel model for JP-8 fuels. The diffusion flame structures and extinction limits of n-decane/toluene/nitrogen mixtures were studied experimentally and computationally through the use of counterflow diffusion flames with nitrogen dilution. The impact of toluene addition to n-decane on the extinction limit, OH distribution, and maximum heat release were investigated. The results showed that as the toluene blending ratio was increased, the extinction strain rate decreased significantly. It was also found that the extinction limit depends linearly on the radical pool for the blended fuels. Planar laser induced fluorescence measurements of OH showed that the maximum concentration of OH found experimentally was more sensitive to toluene addition than found in numerical computations. The maximum chemical heat release rate showed that CO + OH → CO2 + H was one of the dominant exothermic reaction pathways, indicating that the extinction limits are strongly coupled with OH concentration. Numerical simulations demonstrated that the radical pool decreased, particularly the OH and H concentration via abstraction reactions of toluene, led to rapid approach to the extinction limits. The extinction strain rate of the blended fuels was analyzed and based on the variation of the radical pool concentration, which can be represented by the H/C ratio of blended fuels. Copyright © 2009 by the American Institute of Aeronautics and Astronautics, Inc.
Article
Experimental and kinetic modeling of kerosene-type fuels is reported in the present work with special emphasis on the low-temperature oxidation phenomenon relevant to gas turbine premixing conditions. Experiments were performed in an atmospheric pressure, tubular flow reactor to measure ignition delay time of kerosene (fuel-oil No. 1) in order to study the premature autoignition of liquid fuels at gas turbine premixing conditions. The experimental results indicate that the ignition delay time decreases exponentially with the equivalence ratio at fuel-lean conditions. However for very high equivalence ratios (>2), the ignition delay time approaches an asymptotic value. Equivalence ratio fluctuations in the premixer can create conditions conducive for autoignition of fuel in the premixer as the gas turbines generally operate under lean conditions during premixed prevaporized combustion. Ignition delay time measurements of stoichiometric fuel-oil No. 1/air mixture at 1 atm were comparable with that of kerosene type Jet-A fuel available in the literature. A detailed kerosene mechanism with approximately 1400 reactions of 550 species is developed using a surrogate mixture of n-decane, n-propylcyclohexane, n-propylbenzene, and decene to represent the major chemical constituents of kerosene, namely n-alkanes, cyclo-alkanes, aromatics, and olefins, respectively. As the major portion of kerosene-type,fuels consists of alkanes, which are relatively more reactive at low temperatures, a detailed kinetic mechanism is developed for n-decane oxidation including low temperature reaction kinetics. With the objective of achieving a more comprehensive kinetic model for n-decane, the mechanism is validated against target data for a wide range of experimental conditions available in the literature. The data include shock tube ignition delay time measurements, jet-stirred reactor reactivity profiles, and plug-flow reactor species time-history profiles. The kerosene model predictions agree fairly well with the ignition delay time measurements obtained in the present work as well as the data available in the literature for Jet A. The kerosene model was able to reproduce the low-temperature preignition reactivity profile of JP-8 obtained in a flow reactor at 12 atm. Also, the kerosene mechanism predicts the species reactivity profiles of Jet A-1 obtained in a jet-stirred reactor fairly well.
Article
Experimental soot threshold results obtained using binary and ternary hydrocarbon mixtures were analyzed to obtain two empirical rules for estimating the onset of soot formation for fuel blends in atmospheric pressure, laminar premixed and diffusion flames. The results suggest that the onset of soot formation for a fuel mixture of known composition can be accurately calculated from soot threshold data for the pure compounds.
Article
Kinetic modeling can prove to be a powerful tool in the analysis of many systems. It has already been applied to a wide variety of chemical engineering problems, including gas phase and liquid phase pyrolysis, polymer thermal degradation, oxidative coupling and several other chemical processes. Extended kinetic schemes are now being used with increasing frequency in practical applications and most of them are available in the technical literature. Their dimensions and complexity justify the adoption of analogy rules and other simplifying assumptions within the different chemical reaction classes. The levels of simplification have to be carefully evaluated to make them coherent with the final aim of the model. Owing to the huge amount of possible isomers for large hydrocarbons, it is usually necessary to lump a large number of real components into a properly selected number of equivalent components. Consequently, the corresponding elementary reactions are also grouped into equivalent or lumped reactions.
Article
A single pulse shock tube has been designed and constructed in order to achieve extremely high pressures and temperatures to facilitate gas-phase chemical kinetic experiments. Postshock pressures of greater than 1000 atmospheres have been obtained. Temperatures greater than 1400 K have been achieved and, in principle, temperatures greater than 2000 K are easily attainable. These high temperatures and pressures permit the investigation of hydrocarbon species pyrolysis and oxidation reactions. Since these reactions occur on the time scale of 0.5–2 ms the shock tube has been constructed with an adjustable length driven section that permits variation of reaction viewing times. For any given reaction viewing time, samples can be withdrawn through a specially constructed automated sampling apparatus for subsequent species analysis with gas chromatography and mass spectrometry. The details of the design and construction that have permitted the successful generation of very high-pressure shocks in this unique apparatus are described. Additional information is provided concerning the diaphragms used in the high-pressure shock tube. © 2001 American Institute of Physics.
Article
The preignition reactivity behavior of five JP-8 samples, three Jet-A samples, one JP-7 sample, and four JP-8 surrogate mixtures was studied in a pressurized flow reactor in the low and intermediate temperature regime (600–800 K). A strong Negative Temperature Coefficient (NTC) behavior was observed to start near 692 K for all the fuels. No significant differences in the low-temperature oxidation behavior were observed between Jet-A and JP-8, suggesting that the two fuels can be used interchangeably for surrogate development. Two surrogates stood out as the best to mimic the low temperature reactivity of JP-8, the Hex-12 surrogate by Sarofim and coworkers (Eddings et al., 2005) and the S1 surrogate by Agosta (2002). The results showed that extreme care must be taken to ensure that the sample used for surrogate development is of average composition and properties, otherwise severe mistuning of the surrogate could occur.
Article
Experiments are conducted on n-decane and JP-10 flames, stabilized between two counterflowing streams—fuel stream and oxidizer stream. The fuel stream is a mixture of prevaporized fuel and nitrogen, and the oxidizer stream is air. Critical conditions of extinction are measured for n-decane and JP-10. Concentration profiles of , and hydrocarbons from C2 to C6 are measured. These measurements are made by removing gas samples from the flame using a quartz microprobe and analyzing the samples using a gas chromatograph. Temperature profiles are measured using a thermocouple. Numerical calculations are performed using detailed kinetic models to predict the flame structure and critical conditions of extinction and autoignition. The predicted values of the critical conditions of extinction and autoignition for n-decane agree well with experimental data for extinction and with previously measured autoignition data. For JP-10, for a given mass fraction of fuel in the fuel stream, the predicted strain rates at extinction are higher than the measurements. The predicted autoignition temperatures, however, agree well with previously measured experimental data. The predicted profiles and maximum mass fractions of the major species are found to agree with the measurements. There are, however, differences between the predicted and measured mass fractions of those species that have more than three carbon atoms. In general, the concentrations of saturated hydrocarbons in the n-decane flame are higher than those in the JP-10 flame.
Article
An experimental investigation of the autoignition for various n-decane/oxidizer mixtures is conducted using a rapid compression machine, in the range of equivalence ratios of ϕ=0.5–2.2, dilution molar ratios of N2/(O2 + N2) = 0.79–0.95, compressed gas pressures of PC=7–30 bar, and compressed gas temperatures of TC=635–770 K. The current experiments span a temperature range not fully investigated in previous autoignition studies on n-decane. Two-stage ignition, characteristic of large hydrocarbons, is observed over the entire range of conditions investigated, as demonstrated in the plots of raw experimental pressure traces. In addition, experimental results reveal the sensitivity of the first-stage and total ignition delays to variations in fuel and oxygen mole fractions, pressure, and temperature. Predictability of two kinetic mechanisms is compared against the present data. Discrepancies are noted and discussed, which are of direct relevance for further improvement of kinetic models of n-decane at conditions of elevated pressures and low-to-intermediate temperatures.
Article
As part of a large-scale thermophysical property measurement project, the global decomposition kinetics of the aviation turbine fuel Jet A was investigated. Decomposition reactions were performed at 375, 400, 425, and 450 °C in stainless steel ampule reactors. At each temperature, the extent of decomposition was determined as a function of time by gas chromatography. These data were used to derive global pseudo-first-order rate constants that approximate the overall decomposition rate of the mixture. Decomposition rate constants ranged from 5.9 × 10−6 s−1 at 375 °C to 4.4 × 10−4 s−1 at 450 °C. These rate constants are useful for planning property measurements at high temperatures. On the basis of the amount of time required for 1% of the sample to decompose (t0.01), we found that allowable instrument residence times ranged from about 0.5 h at 375 °C to less than 1 min at 450 °C. The kinetic data were also used to derive Arrhenius parameters of A = 4.1 × 1012 s−1 and Ea = 220 kJ·mol−1. In addition to the decomposition kinetics, we have also done a GC-MS analysis of the vapor phase that is produced during the thermal decomposition measurements.
Article
The distillation (or boiling) curve of a complex fluid is a critically important indicator of the bulk behavior or response of the fluid. For this reason, the distillation curve, usually presented graphically as the boiling temperature against the volume fraction distilled, is often cited as a primary design and testing criterion for liquid fuels, lubricants, and other important industrial fluids. While the distillation curve gives a direct measure of fluid volatility fraction by fraction, the information the curve contains can be taken much further; there are numerous engineering and application-specific parameters that can be correlated to the distillation curve. When applied to liquid motor fuels, for example, one can estimate engine starting ability, drivability, fuel system icing and vapor lock, the fuel injector schedule, and fuel autoignition, etc. It can be used in environmental applications as a guide for blending virgin stock with reclaimed oil, guiding the formulation of product that will be suitable in various applications. Moreover, the distillation curve can be related to mutagenicity and the composition of the pollutant suite. It is therefore desirable to enhance or extend the usual approach to distillation curve measurement to allow optimal information content. In this paper, we present several modifications to the measurement of distillation curves that provide (1) temperature and volume measurement(s) of low uncertainty and, most important, (2) a composition-explicit data channel in addition to the usual temperature−volume relationship. This latter modification is achieved with a new sampling approach that allows precise qualitative as well as quantitative analyses of each fraction, on the fly. The analysis is done by gas chromatography coupled with specific or universal detectors. This second modification is the most significant change, since it is composition that is the most important underlying parameter that governs curve shape.
Article
A theoretical method for predicting the octane number of pure hydrocarbon liquids is presented. The method is based on a structural group contribution approach and requires no experimental procedure or knowledge of the physical or chemical properties only the chemical structure of the molecule. The proposed model is simple and can predict the research and motor octane numbers of more than 200 pure hydrocarbon liquids with an average deviation of 4 and 5.7, respectively. The results of two different sets of structural groups derived from the Joback group contribution approach are tested and compared. The method is notable for the absence of any theoretical procedure which has previously been used to estimate the pure-component octane number. In addition, the method has the potential advantage of synthesis of additional hydrocarbons with knock measurements as a major objective.
Article
For pure compounds a simple group counting scheme is used to predict the cetane numbers of normal and branched paraffins and singly substituted alkylbenzenes. To extend the counting scheme to hydrocarbon mixtures, carbon-13 nuclear magnetic resonance (¹³C NMR) is used. ¹³C NMR is sensitive to the local environment, up to three to four carbon atoms away, of each carbon atom. Intramolecular reactions that are important for ignition kinetics imply that molecular fragments of three or four carbon atoms must be considered. The authors show that group concentrations derived from ¹³C NMR spectra are useful in predicting the cetane number of hydrocarbon mixtures.
Article
In part 1 (10.1021/ef100496j) of this series of two papers, we presented an evaluation strategy that can be applied to surrogate mixtures for finished fuels. This strategy uses the advanced distillation curve approach to evaluate the surrogate in terms of physicochemical authenticity or how well the surrogate represents the chemical and physical properties of the finished fuel. While this protocol can be applied to any surrogate family, of particular interest here are surrogates for Jet-A/JP-8. The volatility was studied in detail as described in part 1 (10.1021/ef100496j), whereas here, we focus on density, speed of sound, and viscosity. We calculated these properties for the common Jet-A/JP-8 surrogates and Jet-A, with the National Institute of Standards and Technology (NIST) REFPROP program (which incorporates equations of state and a transport property model). We then used REFPROP as a surrogate mixture design tool and developed a simple, three-component surrogate mixture (n-dodecane, n-tetradecane, and 1,2,4-trimethylbenzene with mass fractions of 0.31, 0.38, and 0.31, respectively). This mixture was subsequently formulated in the laboratory and measured with the advanced distillation curve approach. We found the agreement with the theory to be excellent (within 1.5 °C), and we also found that the ability of such a simple mixture to represent Jet-A/JP-8 was also excellent. Comparisons made to the calculated density, speed of sound, and viscosity were also excellent.
Article
Because of the complexities involved in measuring and modeling the performance and properties of finished fuels, the fuel science community must often use surrogate mixtures as substitutes, especially in the absence of consensus standard mixtures. While surrogate mixtures are often formulated on the basis of the ability of a particular mixture to reproduce a particular property, there is usually a desire to employ surrogate mixtures that are physicochemically authentic. This means that, provided that the primary purpose is satisfied, researchers are inclined to choose mixtures that have physical and chemical properties appropriate to the finished fuel. In this paper, we apply the advanced distillation curve method as a means to evaluate the physicochemical authenticity of surrogate mixtures. While the strategy outlined here can be used for any family of surrogates, we apply it to surrogate mixtures for Jet-A/JP-8. Mixtures were divided into two groups: (1) simple surrogate mixtures with up to three components and (2) complex surrogate mixtures with more than three components. We found that the modified Aachen surrogate (among the simple fluids) and the Schultz surrogate (among the complex fluids) had the best physicochemical authenticity.