High-temperature Ignition Delay Times Research Articles

Effects of nozzle diameter on the characteristic time scales of diesel spray two-stage ignition under cold-start conditions

Diesel engine cold-start performances are closely related to ignition delay time which in turn is strongly influenced by the nozzle diameter (Dnoz). In this study, ignition delay time and ignition location of No. −50 diesel spray at different Dnoz are investigated by high-speed direct photography in a constant volume combustion chamber (CVCC) under cold-start conditions. The experimental results clarify the non-monotonic variation of the high-temperature ignition delay time (τHTI) with decreasing nozzle diameter. The minimum experimental high-temperature ignition delay time of about 1.85 ms is obtained when Dnoz = 160 μm. To investigate the mechanism that the influence of nozzle diameter on high-temperature ignition delay time, numerical simulations of the n-heptane spray ignition process are performed under the same conditions as optical diagnostics. Five characteristic time scales for the two-stage ignition process are defined as: physical preparation time (τphy), low-temperature pre-reaction time (Δτ1), low-temperature ignition delay time (τLTI), high-temperature pre-reaction time (Δτ2), and high-temperature ignition delay time. The physical preparation time decreases linearly with nozzle diameter caused by better spray atomization, evaporation, and air-fuel mixing. Changes in nozzle diameter barely affect low-temperature pre-reaction time, indicating that the linear shortening of low-temperature ignition delay time is attributed to the acceleration of the physical preparation process. The high-temperature pre-reaction time initially decreases and then increases with decreasing Dnoz, which is the result of a smaller scalar dissipation rate promoting the decomposition of H2O2 and inhibiting flame propagation. The combined effect of decreasing Dnoz at each stage leads to a non-monotonic trend in high-temperature ignition delay time.

Methanol oxidation up to 100 atm in a supercritical pressure jet-stirred reactor

Methanol (CH3OH) has attracted considerable attention as a renewable fuel or fuel additive with low greenhouse gas emissions. Methanol oxidation was studied using a recently developed supercritical pressure jet-stirred reactor (SP-JSR) at pressures of 10 and 100 atm, at temperatures from 550 to 950 K, and at equivalence ratios of 0.1, 1.0, and 9.0 in experiments and simulations. The experimental results show that the onset temperature of CH3OH oxidation at 100 atm is around 700 K, which is more than 100 K lower than the onset at 10 atm and this trend cannot be predicted by the existing kinetics models. Furthermore, a negative temperature coefficient (NTC) behavior was clearly observed at 100 atm at fuel rich conditions for methanol for the first time. To understand the observed temperature shift in the reactivity and the NTC effect, we updated some key elementary reaction rates of relevance to high pressure CH3OH oxidation from the literature and added some new low-temperature reaction pathways such as CH2O + HO2 = HOCH2O2 (RO2), RO2 + RO2 = HOCH2O (RO) + HOCH2O (RO) + O2, and CH3OH + RO2 = CH2OH + HOCH2O2H (ROOH). Although the model with these updates improves the prediction somewhat for the experimental data at 100 atm and reproduces well high-temperature ignition delay times and laminar flame speed data in the literature, discrepancies still exist for some aspects of the 100 atm low-temperature oxidation data. In addition, it was found that the pressure-dependent HO2 chemistry shifts to lower temperature as the pressure increases such that the NTC effect at fuel-lean conditions is suppressed. Therefore, as shown in the experiments, the NTC phenomenon was only observed at the fuel-rich condition where fuel radicals are abundant and the HO2 chemistry at high pressure is weakened by the lack of oxygen resulting in comparatively little HO2 formation.

An improved low and high-temperature dimethyl ether kinetic model for the combustion atmospheres with high CO2 concentration

The Dimethyl ether (DME) kinetic model is significant to understand the DME combustion in the atmosphere containing high CO2 concentration, such as pressurized oxy-fuel combustion process and exhaust gas recirculation, and design the combustor. In this paper, the ignition delay times (IDTs) of DME were measured in a shock tube under the conditions of equivalence ratios of 0.5, 1 and 1.5, temperature ranges from 1007 K to 1340 K, and pressures of 2 and 11 atm. A detailed kinetic model of DME named as OXYDME was put forward based on the OXY-Aramco model, and was validated by the low and high temperature IDTs, laminar flame speeds, and species profiles measured in this work and those from literatures in atmospheres of O2/Ar/CO2, O2/N2, O2/N2/He/CO2 and O2/CO2. The OXYDME model was compared with OXY-Aramco, Liu-DME model in detail. The effects of CO2 on the DME ignition are very weak and not sensitive to the temperature and pressure. The reason for the weak effects of CO2 is that the chemical effects of CO2 promote ignition, which counteracts the inhibition effects caused by the physical properties of CO2. The chaperon effects of CO2 dominate the chemical effects. Meanwhile, the effects of the reactions with CO2 are very small.

An experimental and kinetic modeling study of NOx sensitization on methane autoignition and oxidation

An experimental and kinetic modeling study of the influence of NOx (i.e. NO2, NO and N2O) addition on the ignition behavior of methane/‘air’ mixtures is performed. Ignition delay time measurements are taken in a rapid compression machine (RCM) and in a shock tube (ST) at temperatures and pressures ranging from 900–1500 K and 1.5–3.0 MPa, respectively for equivalence ratios of 0.5–2.0 in ‘air’. The conditions chosen are relevant to spark ignition and homogeneous charge compression ignition engine operating conditions where exhaust gas recirculation can potentially add NOx to the premixed charge. The RCM measurements show that the addition of 200 ppm NO2 to the stoichiometric CH4/oxidizer mixture results in a factor of three increase in reactivity compared to the baseline case without NOx for temperatures in the range 600–1000 K. However, adding up to 1000 ppm N2O does not show any appreciable effect on the measurements. The promoting effect of NO2 was found to increase with temperature in the range 950–1150 K, while the sensitization effect decreases at higher pressures. The experimental results measured are simulated using NUIGMech1.2 comprising an updated NOx sub-chemistry in this work. A kinetic analysis indicates that the competition between the reactions ĊH3 + NO2 ↔ CH3Ȯ + NO and ĊH3 + NO2 (+M) ↔ CH3NO2 (+M), the former being a propagation reaction and the latter being a termination reaction governs NOx sensitization on CH4 ignition. Recent calculations by Matsugi and Shiina (A. Matsugi, H. Shiina, J. Phys. Chem. A. 121 (2017) 4218–4224) for the nitromethane formation reaction CH3 + NO2 (+M) ↔ CH3NO2 (+M), together with the recently calculated rate constants for HONO/HNO2 reactions significantly improve ignition delay time predictions in the temperature range 600–1000 K. Furthermore, the experiments with NO addition reveal a non-monotonous sensitization impact on CH4 ignition at lower temperatures with NO initially acting as an inhibitor at low NO concentrations and then as a promoter as NO concentrations increase in the mixture. This non-monotonous trend is attributed to the role of the chain-termination reaction ĊH3 + NO2 (+M) ↔ CH3NO2 (+M) and the impact of NO on the transition to the chain-branching steps CH2O + HȮ2 ↔ HĊO + H2O2, H2O2 (+M) ↔ ȮH + ȮH (+M), HĊO ↔ CO + Ḣ followed by CO + O2 ↔ CO2 + Ö and Ḣ + O2 ↔ Ö + ȮH. NUIGMech1.2 is systematically validated against the new ignition delay measurements taken here together with species measurements and high temperature ignition delay time data available in the literature for CH4/oxidizer mixtures diluted with NO2/N2O/NO and is observed to accurately capture the sensitization trends.

An improved 2-pentanone low to high-temperature kinetic model using Bayesian Optimization algorithm

The ignition delay times (IDTs) of 2-pentanone (methyl propyl ketone, MPK) were measured at equivalence ratios of 0.5, 1.0, and 1.5, pressures of 1 bar and 5 bar, and temperatures ranging between 1227 and 1571 K. A MPK low to high-temperature model was constructed on the basis of Pieper model and Fenard model. The improvement of the model is that the rate constants of sixteen MPK decomposition and hydrogen abstraction reactions (R1–R16) obtained by the analogy-based method, were globally optimized by the Bayesian Optimization algorithm using the high-temperature IDTs. The optimized model well predicts the laminar flame speeds (measured by Li et al.) and the IDTs and species profiles in low-temperature (measured by Fenard et al.). There is no overfitting during the optimization process. Therefore, the optimization method and the optimized MPK model are reliable. The comparisons of the optimized model with Pieper model and Fenard model were performed by the reaction pathway analysis and sensitivity analysis for the predictions of the MPK profile, the low and high-temperature IDTs and the laminar flame speeds. Because of the difference in the molecular structure between MPK and 2-butanone, remaining the branching ratio among three MPK decompositions (R1–R3) and the ratio among the rate constants of the MPK hydrogen abstraction reactions by OH, H and CH3 unchanged is unnecessary in the development of the MPK model using the analogy-based method, while the branching ratio of the hydrogen abstraction reactions at carbon 1, 3, and 5 remains unchanged. The competition among the decomposition and hydrogen abstraction reactions of MPK is intricate and crucial to the low and high-temperature oxidation. It is necessary to globally optimize the rate constants of R1-R16 based on the multidimensional experimental results, such as low and high-temperature IDTs, laminar flame speeds, and species profiles.

Large-eddy simulation of dual-fuel spray ignition at different ambient temperatures

Here, a finite-rate chemistry large-eddy simulation (LES) solver is utilized to investigate dual-fuel (DF) ignition process of n-dodecane spray injection into a methane–air mixture at engine-relevant ambient temperatures. The investigated configurations correspond to single-fuel (SF) ϕCH4= 0 and DF ϕCH4= 0.5 conditions for a range of temperatures. The simulation setup is a continuation of the work by Kahila et al. (2019, Combustion and Flame) with the baseline SF spray setup corresponding to the Engine Combustion Network (ECN) Spray A configuration. First, ignition is investigated at different ambient temperatures in 0D and 1D studies in order to isolate the effect of chemistry and chemical mechanism selection to ignition delay time (IDT). Second, 3D LES of SF and DF sprays at three different ambient temperatures is carried out. Third, a reaction sensitivity analysis is performed to investigate the effect of ambient temperature on the most sensitive reactions. The main findings of the paper are as follows: (1) DF ignition characteristics depend on the choice of chemical mechanism, particularly at lower temperatures. (2) Addition of methane to the ambient mixture delays ignition, and this effect is the strongest at lower temperatures. (3) While the inhibiting effect of methane on low- and high-temperature IDT’s is evident, the time difference between these two stages is shown to be only slightly dependent on temperature. (4) Reaction sensitivity analysis indicates that reactions related to methane oxidation are more pronounced at lower temperatures. The provided quantitative results indicate the strong ambient temperature sensitivity of the DF ignition process.

Measurements of the High Temperature Ignition Delay Times and Kinetic Modeling Study on Oxidation of Nitromethane

ABSTRACTIgnition delay times of the nitromethane (NM)/O2/Ar mixtures were measured behind reflected shock waves in the temperature range of 1200–2000 K, pressures of 2.0 and 10.0 atm, equivalence ratios of 0.5, 1.0, and 2.0, and fuel concentrations of 0.5%, 1.0%, and 2.0%. OH emission intensity and pressure evolution were used for the ignition delay time determination. Two-stage ignition behavior was observed in the experiments. The ignition delay time decreases sharply with pressure increasing and equivalence ratio decreasing. In addition, a sub-model including 24 updated reactions for nitromethane oxidation was proposed based on ab initio calculation with CCSD(T)/CBS//B3LYP/6–311 + G(3DF, 3P) level. The present model and the Mathieu’s model (Mathieu et al. Fuel 182(2016): 597) were then validated against present measured ignition delay times, as well as some laminar flame speed data and ignition data in the literature. The present model shows better or comparable predictions with the experimental data. The main species concentration evolution and reaction analysis were presented, based on which the experimentally observed two-stage ignition behavior was interpreted.

Experimental and numerical investigation of atmospheric laminar premixed n-butane flames in sooting conditions

Experimental and numerical investigation of atmospheric laminar premixed n-butane/O2/N2 flames operating at two equivalence ratios (ϕ=2.16 and 2.32) has been performed. Both flames produce soot particles. Mole fraction of reactants, stable intermediate and combustion products have been obtained using a quartz microprobe coupled to gas chromatography and infrared spectroscopy technique analysis. The temperature profiles were obtained by Laser induced fluorescence of nitric oxide. The experimental results showed that a weak variation of equivalence ratio in sooting conditions introduces an increase of benzene and some unsaturated aliphatic species peak mole fraction. Except for benzene, the peak species mole fraction increase does not exceed a factor of two. A new reaction kinetic model is proposed to predict gaseous species mole fraction. In addition to the good agreement observed with our experimental data base, the proposed mechanism predicts well the detailed data obtained in low pressure rich n-butane premixed flames and high temperature ignition delay times reported in the literature.Reactions path analysis revealed that the first aromatic ring is controlled by C4+C2 (nC4H5+C2H2=C6H6+H) and C3+C3 (self recombination of C3H3 radicals) reactions routes. C4+C2 reaction path predominates in the reaction zone while C3+C3 path tends to predominate in the post flame region. The impact of the equivalence ratio on both reactions and the main differences between our and three selected literature models are discussed.

Theoretical Kinetics Analysis for Ḣ Atom Addition to 1,3-Butadiene and Related Reactions on the Ċ4H7 Potential Energy Surface

The oxidation chemistry of the simplest conjugated hydrocarbon, 1,3-butadiene, can provide a first step in understanding the role of polyunsaturated hydrocarbons in combustion and, in particular, an understanding of their contribution toward soot formation. On the basis of our previous work on propene and the butene isomers (1-, 2-, and isobutene), it was found that the reaction kinetics of Ḣ-atom addition to the C═C double bond plays a significant role in fuel consumption kinetics and influences the predictions of high-temperature ignition delay times, product species concentrations, and flame speed measurements. In this study, the rate constants and thermodynamic properties for Ḣ-atom addition to 1,3-butadiene and related reactions on the Ċ4H7 potential energy surface have been calculated using two different series of quantum chemical methods and two different kinetic codes. Excellent agreement is obtained between the two different kinetics codes. The calculated results including zero-point energies, single-point energies, rate constants, barrier heights, and thermochemistry are systematically compared among the two quantum chemical methods. 1-Methylallyl (Ċ4H71-3) and 3-buten-1-yl (Ċ4H71-4) radicals and C2H4 + Ċ2H3 are found to be the most important channels and reactivity-promoting products, respectively. We calculated that terminal addition is dominant (>80%) compared to internal Ḣ-atom addition at all temperatures in the range 298-2000 K. However, this dominance decreases with increasing temperature. The calculated rate constants for the bimolecular reaction C4H6 + Ḣ → products and C2H4 + Ċ2H3 → products are in excellent agreement with both experimental and theoretical results from the literature. For selected C4 species, the calculated thermochemical values are also in good agreement with literature data. In addition, the rate constants for H atom abstraction by Ḣ atoms have also been calculated, and it is found that abstraction from the central carbon atoms is the dominant channel (>70%) at temperatures in the range of 298-2000 K. Finally, by incorporating our calculated rate constants for both Ḣ atom addition and abstraction into our recently developed 1,3-butadiene model, we show that laminar flame speed predictions are significantly improved, emphasizing the value of this study.

Ignition delay time correlation of fuel blends based on Livengood-Wu description

In this work, a universal methodology for ignition delay time (IDT) correlation of multicomponent fuel mixtures is reported. The method is applicable over wide ranges of temperatures, pressures, and equivalence ratios. n-Heptane, iso-octane, toluene, ethanol and their blends are investigated in this study because of their relevance to gasoline surrogate formulation. The proposed methodology combines benefits from the Livengood-Wu integral, the cool flame characteristics and the Arrhenius behavior of the high-temperature ignition delay time to suggest a simple and comprehensive formulation for correlating the ignition delay times of pure components and blends. The IDTs of fuel blends usually have complex dependences on temperature, pressure, equivalence ratio and composition of the blend. The Livengood-Wu integral is applied here to relate the NTC region and the cool flame phenomenon. The integral is further extended to obtain a relation between the IDTs of fuel blends and pure components. Ignition delay times calculated using the proposed methodology are in excellent agreement with those simulated using a detailed chemical kinetic model for n-heptane, iso-octane, toluene, ethanol and blends of these components. Finally, very good agreement is also observed for combustion phasing in homogeneous charge compression ignition (HCCI) predictions between simulations performed with detailed chemistry and calculations using the developed ignition delay correlation.

Shock Tube Ignition Delay Data Affected by Localized Ignition Phenomena

ABSTRACTShock tubes have conventionally been used for measuring high-temperature ignition delay times of approximately O(1 ms). In the last decade or so, the operating regime of shock tubes has been extended to lower temperatures by accessing longer observation times. Such measurements may potentially be affected by some non-ideal phenomena. The purpose of this work is to measure long ignition delay times for fuels exhibiting negative temperature coefficient and to assess the impact of shock tube non-idealities on ignition delay data. Ignition delay times of n-heptane and n-hexane were measured over the temperature range of 650–1250 K and pressures near 1.5 atm. Driver gas tailoring and long length of shock tube driver section were utilized to measure ignition delay times as long as 32 ms. Measured ignition delay times agree with chemical kinetic models at high (>1100 K) and low (<700 K) temperatures. In the intermediate temperature range (700–1100 K), however, significant discrepancies are observed between the measurements and homogeneous ignition delay simulations. It is postulated, based on experimental observations, that localized ignition kernels could affect the ignition delay times at the intermediate temperatures, which lead to compression (and heating) of the bulk gas and result in expediting the overall ignition event. The postulate is validated through simple representative computational fluid dynamic simulations of post-shock gas mixtures, which exhibit ignition advancement via a hot spot. The results of the current work show that ignition delay times measured by shock tubes may be affected by non-ideal phenomena for certain conditions of temperature, pressure, and fuel reactivity. Care must, therefore, be exercised in using such data for chemical kinetic model development and validation.

High temperature ignition delay time of DME/n-pentane mixture under fuel lean condition

Ignition delay times were measured behind reflect shock waves for the lean DME/n-pentane fuel blends with DME blending ratios from 0% to 100% over a wide range of conditions: pressures from 2 to 20atm, temperatures from 1100 to 1600K and equivalence ratio of 0.5. Correlations of ignition delay times of DME/n-pentane were deduced from the experimental data using multiple linear regression. Comparison between measured ignition delay times and numerical predictions by five available kinetic mechanisms, namely NUI Galway pentane isomer model, JetSurF2.0 model, LLNL n-alkanes Model, NUI Galway Mech_56.54 Model and Wang’s DME model, were conducted. NUI Galway pentane isomer model shows good predictions for ignition delay times of DME, n-pentane and their blends which was conducted in this study to interpret the effect of DME addition on the ignition chemistry of n-pentane.

Minimized Skeletal Mechanism for Methyl Butanoate Oxidation and Its Application to the Prediction of C3–C4Products in Nonpremixed Flames: A Base Model of Biodiesel Fuels

Kinetic database of methyl butanoate (MB) combustion has been widely used as a base component for formulating kinetic mechanisms describing oxidation of larger methyl esters for biodiesel fuel surrogates. In this study, the detailed mechanism of Dooley et al. (Dooley, S.; Curran, H. J.; Simmie, J. M. Combust. Flame 2008, 153, 2–32) is minimized without empirical adjustment of rate constants in elementary reactions, using a combination of methods including a path flux analysis, removal of individual species, and peak molar concentration analysis. In the first phase, a basic skeletal mechanism with 38 species and 170 reactions for the oxidation of MB is developed, toward understanding the capability and limits of the reduced descriptions on high-temperature ignition delay times and major species profiles in 1-D counterflow flames. A comprehensive analysis of the decomposition pathways associated with preserved and removed reactions provides a scheme for mechanism developers with a clear foundation for creating new skeletal mechanisms of large methyl ester combustion. In the second phase, the MB skeletal mechanism takes into account additional 10 species and 33 reactions in order to predict experimentally measured products in a coflow nonpremixed methane/air flame doped with methyl butanoate. This 48-species skeletal mechanism combined with a 2-D computational fluid dynamic (CFD) flamelet model, for the first time, is able to predict the magnitude and shape of seven C3–C4 intermediate products, which are comparable to the experimental results. Moreover, the present skeletal mechanism, compared with published skeletal mechanisms, not only features a significant reduction in computational cost but also retains more significant products mass-spectrometrically determined.

The high-temperature autoignition of biodiesels and biodiesel components

Ignition delay time measurements are reported for two reference fatty-acid methyl ester biodiesel fuels, derived from methanol-based transesterification of soybean oil and animal fats, and four primary constituents of all methyl ester biodiesels: methyl palmitate, methyl stearate, methyl oleate, and methyl linoleate. Experiments were carried out behind reflected shock waves for gaseous fuel/air mixtures at temperatures ranging from 900 to 1350K and at pressures around 10 and 20atm. Ignition delay times were determined by monitoring pressure and ultraviolet chemiluminescence from electronically-excited OH radicals. The results show similarity in ignition delay times for all methyl ester fuels considered, irrespective of the variations in organic structure, at the high-temperature conditions studied and also similarity in high-temperature ignition delay times for methyl esters and n-alkanes. Comparisons with recent kinetic model efforts are encouraging, showing deviations of at most a factor of two and in many cases significantly less.

High temperature ignition delay times of C5 primary alcohols

Ignition delay times of the three C5 primary alcohol isomers (n-pentanol, iso-pentanol and 2-methyl-1-butanol) were measured behind reflected shock waves. Experiments were conducted in the temperature range of 1100–1500K, pressures of 1.0 and 2.6atm, equivalence ratios of 0.25, 0.5 and 1.0, and O2 concentration in the fuel/O2/Ar mixtures varying from 3.75% to 15%. Measurements show that the ignition delay time and the global activation energy of the three isomers both decrease in the order of iso-pentanol, 2-methyl-1-butanol, and n-pentanol. Chemical kinetic mechanisms for n-pentanol (Mech NP) and iso-pentanol (Mech IP), recently developed by Dagaut and co-workers, were used to model the respective ignition delay times. Results show that Mech NP yields close agreement at the equivalence ratio of 0.25, but the agreement is moderated with increasing equivalence ratio. Mech IP yields fairly close agreements at relatively higher temperatures but over-predicts the measurements by 50% at relatively lower temperatures for the three equivalence ratios studied. A new 2-methyl-1-butanol high temperature mechanism was proposed and validated against the ignition delay data. Sensitivity analysis for both n-pentanol and iso-pentanol showed the dominance of small radical reactions. Reaction pathway analysis aided further scrutiny of the fuel-specific reactions in Mech NP, leading to refinement of the kinetic model, and improved agreement between the predicted and measured ignition delay times as well as the jet-stirred reactor results.

Model predictions of higher-order normal alkane ignition from dilute shock-tube experiments

Shock-induced oxidation of two higher-order linear alkanes was measured using a heated shock tube facility. Experimental overlap in stoichiometric ignition delay times obtained under dilute (99 % Ar) conditions near atmospheric pressure was observed in the temperature-dependent ignition trends of n-nonane (n-C9H20) and n-undecane (n-C11H24). Despite the overlap, model predictions of ignition using two different detailed chemical kinetics mechanisms show discrepancies relative to both the measured data as well as to one another. The present study therefore focuses on the differences observed in the modeled, high-temperature ignition delay times of higher-order n-alkanes, which are generally regarded to have identical ignition behavior for carbon numbers above C7. Comparisons are drawn using experimental data from the present study and from recent work by the authors relative to two existing chemical kinetics mechanisms. Time histories from the shock-tube OH* measurements are also compared to the model predictions; a double-peaked structure observed in the data shows that the time response of the detector electronics is crucial for properly capturing the first, incipient peak near time zero. Calculations using the two mechanisms were carried out at the dilution level employed in the shock-tube experiments for lean \({({\rm {\phi}} = 0.5)}\), stoichiometric, and rich \({({\rm {\phi}} = 2.0)}\) equivalence ratios, 1230–1620 K, and for both 1.5 and 10 atm. In general, the models show differing trends relative to both measured data and to one another, indicating that agreement among chemical kinetics models for higher-order n-alkanes is not consistent. For example, under certain conditions, one mechanism predicts the ignition delay times to be virtually identical between the n-nonane and n-undecane fuels (in fact, also for all alkanes between at least C8 and C12), which is in agreement with the experiment, while the other mechanism predicts the larger fuels to ignite progressively more slowly.

High-Temperature Ignition Delay Times and Kinetic Study of Furan

High-temperature ignition delay times of furan were measured behind the reflected shock waves at the elevated pressures (1.2–10.4 atm) for the mixtures with equivalence ratios of 0.5, 1.0, and 2.0 and furan concentrations of 0.25, 0.5 and 1% diluted in argon. An empirical equation was formulated to correlate the measured ignition delay times with the experimental parameters. The measured ignition delay times at 1.2 atm were compared to numerical predictions using a recently developed furan kinetic mechanism (Mech FR), and the results showed that the model yields reasonable agreement with experiments at an equivalence ratio of 2.0 but significant disagreement at the equivalence ratios of 1.0 and 0.5. A modified kinetic model was built by refining rate constants of selected reactions in Mech FR based on the sensitivity analysis results. The modified mechanism yields remarkably better performance in predicting the ignition delay times as well as the thermal decomposition results. Furthermore, the modified mechanism indicates that the most important fuel consumption path is through the unimolecular initiation reaction: furan = FA (CH2CCHCHO), instead of the H abstraction reactions, which were suggested by Mech FR.

Autoignition and burning rates of fuel droplets under microgravity

This paper presents a mathematical model for the unsteady evaporation, ignition, and combustion of isolated fuel droplets under microgravity. The model consists of a large structured system of differential algebraic equations, where the numerical complexity is due both to the stiff nature of the kinetic mechanism and to the flame structure around the droplet. A very general, detailed kinetic scheme, consisting of ∼200 species and over 5000 reactions, is used to describe the gas-phase combustion of different fuels. Several comparisons with experimental measurements, carried out under various operating conditions, confirm that the proposed model is a useful tool for characterizing low-temperature and high-temperature ignition delay times. The predicted explosion diagrams of n-alkanes, as well as their ignition delay times, agree with the experimental measurements; the various oxidation regions are closely reproduced too. In addition to this, recent experimental results, relating to the influence of the initial diameter on droplet burning rates in cold and hot environments, are also presented and discussed. Lastly, an analysis of the extinction diameters for the combustion of n-heptane droplets allows a discussion of the role of radiative heat transfer, as well as further emphasizing the importance of the low-temperature oxidation mechanisms.