Shock tube measurements of ignition delay times for the butanol isomers

Ignition delay times of the four isomers of butanol were measured behind reflected shock waves over a range of experimental conditions: 1050–1600 K, 1.5–43 atm, and equivalence ratios of 1.0 and 0.5 in mixtures containing 4% O₂ diluted in argon. Additional data were also collected at 1.0–1.5 atm in order to replicate conditions used by previous researchers. Good agreement is seen with past work for 1-butanol ignition delay times, though our measured data for the other isomers were shorter than those found in some previous studies, especially at high temperatures. At most conditions, the ignition delay time increases for each isomer in the following order: 1-butanol, 2-butanol and i-butanol nearly equal, and t-butanol. In addition, t-butanol has a higher activation energy than the other three isomers. In a separate series of high-pressure experiments, ignition delay times of 1-butanol in stoichiometric air were measured at temperatures as low as 800 K. At temperatures below 1000 K, pre-ignition pressure rises as well as significant rollover of ignition delay times were observed. Modeling of all collected data using several different chemical kinetic mechanisms shows partial agreement with the experimental data depending on the mechanism, isomer, and conditions. Only the mechanism developed by Vranckx et al. [1] partially explains the rollover and pre-ignition observed in stoichiometric experiments in air.     

Experimental and modeling study on ignition delay of ammonia/methane fuels

Ammonia mixed with methane is a potential clean fuel for engine applications toward a low carbon economy. Studies are scarce on ignition phenomenon for ammonia/methane fuels in literature. In the present study, the ignition characteristics for ammonia–methane–air mixtures have been investigated by both experimental measurements and numerical simulations. Ignition processes of a 60%ammonia/40%methane (mol%) fuel blend were investigated with shock-tube experiments. Measurements of the ignition delay times were performed behind reflected shock waves for such fuel/air mixtures with different equivalence ratios of 0.5, 1, and 2, at pressures around 2 and 5 atm within the temperature range of 1369 to 1804 K. Experimental results were then compared with numerical prediction results employing detailed kinetic mechanism, which showed satisfactory agreement within most of the range of the temperatures, equivalence ratios, and pressures investigated. Within the temperature range of 1300 to 1900 K, pressure range of 1 to 10 atm, equivalence ratio range of 0.5 to 2, and methane proportion range of 0% to 50% in fuel blends, the impacts of temperature, pressure, equivalence ratio, and methane additive were simulated on the ignition delay times of the fuel blends based upon the numerical model. It was found that the improvement of ammonia/methane ignition is significant with the increase of temperature, pressure, and methane additive while it is relatively not sensitive to equivalence ratio within the studied conditions. This suggests a promising potential of such fuel blends in real engine application. In addition to the calculations, reaction sensitivity analyses were also performed to have a deep insight into the observed differences between ammonia/methane/air ignition delay times with variation of conditions.     

Pyrolysis and oxidation of decalin at elevated pressures: A shock-tube study

Ignition delay times and ethylene concentration time-histories were measured behind reflected shock waves during decalin oxidation and pyrolysis. Ignition delay measurements were conducted for gas-phase decalin/air mixtures over temperatures of 769–1202 K, pressures of 11.7–51.2 atm, and equivalence ratios of 0.5, 1.0, and 2.0. Negative-temperature-coefficient (NTC) behavior of decalin autoignition was observed, for the first time, at temperatures below 920 K. Current ignition delay data are in good agreement with past shock tube data in terms of pressure dependence but not equivalence ratio dependence. Ethylene mole fraction and fuel absorbance time-histories were acquired using laser absorption at 10.6 and 3.39 μm during decalin pyrolysis for mixtures of 2200–3586 ppm decalin/argon at pressures of 18.2–20.2 atm and temperatures of 1197–1511 K. Detailed comparisons of these ignition delay and species time-history data with predictions based on currently available decalin reaction mechanisms are presented, and preliminary suggestions for the adjustment of some key rate parameters are made.     

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 1350 K and at pressures around 10 and 20 atm. 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.     

A comparative study of the oxidation characteristics of cyclohexane, methylcyclohexane, and n-butylcyclohexane at high temperatures

Ignition delay times were measured behind reflected shock waves for cyclohexane, methylcyclohexane, and n-butylcyclohexane at 1.5 and 3 atm, equivalence ratios near 1 and 0.5, and temperatures between 1280 and 1480 K. The observed ignition delay times can be summarized as follows: methylcyclohexane > n-butylcyclohexane ≈ cyclohexane. Several reasons are suggested to explain the ordering of the ignition delay times for these three naphthenes. We believe that this work provides the first set of ignition delay time data for n-butylcyclohexane. In addition, H₂O and OH time-histories were recorded during the oxidation of cyclohexane, methylcyclohexane, n-butylcyclohexane, iso-octane and n-heptane under similar test conditions. OH time-histories near time zero are distinctive for each type of fuel studied, and these early-time OH profiles provides critical insight into the influence of molecular structures on ignition behavior, particularly in the case of the cycloalkanes. Comparisons of measured time-histories with simulations from recent cycloalkane oxidation mechanisms are also presented.     

An experimental and kinetic modeling study of n-propanol and i-propanol ignition at high temperatures

Ignition delay times of n- and i-propanol mixtures in argon-diluted oxygen were measured behind reflected shocks. Experimental conditions are: temperatures from 1100 and 1500 K, pressures from 1.2 to 16.0 atm, fuel concentrations of 0.5%, 0.75%, 1.0%, and equivalence ratios of 0.5, 1.0 and 2.0. A detailed kinetic model consisting of 238 species and 1448 reactions was developed to simulate the ignition of the two propanol isomers, with the computed ignition delay times agreeing well with the present measured results as well as the literature data at other conditions. Further validation of the kinetic mechanism was conducted by comparing the simulated results with measured JSR data and laminar flame speeds, and reasonable agreements were achieved for all test conditions. Moreover, reaction pathway analysis indicated that n-propanol mainly produces ethenol, ethene and propene, while i-propanol primarily produces acetone and propene. Finally, sensitivity analysis demonstrated that some fuel-species reactions can be found in the most important reactions for both propanols, and these are mainly the H-abstraction reactions.     

Ignition delay times of low-vapor-pressure fuels measured using an aerosol shock tube

Gas-phase ignition delay times were measured behind reflected shock waves for a wide variety of low-vapor-pressure fuels. These gas-phase measurements, without the added convolution with evaporation times, were made possible by using an aerosol shock tube. The fuels studied include three large normal alkanes, n-decane, n-dodecane and n-hexadecane; one large methyl ester, methyl decanoate; and several diesel fuels, DF-2, with a range of cetane indices from 42 to 55. The reflected shock conditions of the experiments covered temperatures from 838 to 1381 K, pressures from 1.71 to 8.63 atm, oxygen concentrations from 1 to 21%, and equivalence ratios from 0.1 to 2. Ignition delay times were measured using sidewall pressure, IR laser absorption by fuel at 3.39 μm, and CH^* and OH^* emission. Measurements are compared to previous studies using heated shock tubes and current models. Model simulations show similar trends to the current measurement except in the case of n-dodecane/21% O₂/argon experiments. At higher temperatures, e.g. 1250 K, the measured ignition delay times for these mixtures are significantly longer in lean mixtures than in rich mixtures; current models predict the opposite trend. As well, the current measurements show significantly shorter ignition delay times for rich mixtures than the model predictions.     

Effect of argon dilution on the pre-ignition oxidation kinetics of benzene

Experimental and modelling studies on the high-temperature oxidation of benzene have been done to study the effect of argon dilution on its pre-ignition thermochemical kinetics. The ignition delay times of stoichiometric C₆H₆/O₂ mixtures are measured behind reflected shock waves at a total pressure of 4 bar with varying argon dilution (0–95%) for a wide range of temperatures (650–2200 K). A kinetic scheme with 60 elementary reactions among 23 reacting species is proposed which is able to predict the experimental results obtained in this as well as earlier studies. The model was subjected to a sensitivity analysis to identify the significant reactions. The effects of diluent concentration on the reaction rates and energy release history in the vicinity of the induction times are reported. Correlations are deduced to predict the ignition delay times. The thermal and kinetic effects of argon concentration on the ignition activation energy and induction times are examined.     

Experimental and kinetic study on ignition delay times of DME/H2/O2/Ar mixtures

Ignition delay times of dimethyl ether (DME)/hydrogen/oxygen/argon mixtures (hydrogen blending ratio ranging from 0% to 100%) were measured behind reflected shock waves at pressures of 1.2–10 atm, temperature range of 900–1700 K, and for the lean (? = 0.5), stoichiometric (? = 1.0) and rich (? = 2.0) mixtures. For more understanding the effect of initial parameters, correlations of ignition delay times for the lean mixtures were obtained on the basis of the measured data (X_H2 ? 95%) through multiple linear regression. Ignition delay times of the DME/H₂ mixtures demonstrate three ignition regimes. For X_H2 ? 80%, the ignition is dominated by the DME chemistry and ignition delay times show a typical Arrhenius dependence on temperature and pressure. For 80% ? X_H2 ? 98%, the ignition is dominated by the combined chemistries of DME and hydrogen, and ignition delay times at higher pressures give higher ignition activation energy. However, for X_H2 ? 98%, the transition in activation energy for the mixture was found as decreasing the temperature, indicating that the ignition is dominated by the hydrogen chemistry. Simulations were made using two available models and different results were presented. Thus, sensitivity analysis was performed to illustrate the causes of different simulation results of the two models. Subsequently, chemically interpreting on the effect of hydrogen blending ratio on ignition delay times was made using small radical mole fraction and reaction pathway analysis. Finally, high-pressure simulations were performed, serving as a starting point for the future work.     

A high pressure shock tube study of n-propylbenzene oxidation and its comparison with n-butylbenzene

Ignition delay times have been measured for mixtures of n-propylbenzene in air (≈21% O₂, ≈79% N₂) at equivalence ratios of 0.29, 0.48, 0.96 and 1.92 and at reflected shock pressures of 1, 10 and 30 atm in a heated high-pressure shock tube over a wide temperature range (1000–1600 K). The effects of reflected shock pressure and of equivalence ratio on ignition delay time were determined and common trends highlighted. Simulations were carried out using the n-propylbenzene sub-mechanism contained in an n-butylbenzene reaction mechanism available in the literature. This kinetic model was improved by including pressure dependent reactions which were not in place previously and the addition of the NUI Galway C₀–C₄ sub-mechanism. These simulations showed very good agreement with the experimental data. Additionally a comparison is made with experimental data previously obtained and published for n-butylbenzene over the same range of conditions and common trends are highlighted.     

Effects of N2O addition on the ignition of H2–O2 mixtures: Experimental and detailed kinetic modeling study

Ignition delay times of H₂/O₂ mixtures highly diluted with Ar and doped with various amounts of N₂O (100, 400, 1600, 3200 ppm) were measured in a shock tube behind reflected shock waves over a wide range of temperatures (940–1675 K). The pressure range investigated during this work (around 1.6, 13 and 32 atm) allows studying the effect of N₂O on hydrogen ignition at pressure conditions that have never been heretofore investigated. Ignition delay times were decreased when N₂O was added to the mixture only for the higher nitrous oxide concentrations, and some changes in the activation energy were also observed at 1.5 and 32 atm. When it occurred, the decrease in the ignition delay time was proportional to the amount of N₂O added and depended on pressure and temperature conditions. A detailed chemical kinetics model was developed using kinetic mechanisms from the literature. This model predicts well the experimental data obtained during this study and from the literature. The chemical analysis using this model showed that the decrease in the ignition delay time was mainly due to the reaction N₂O + M ? N₂ + O + M which provides O atoms to strengthen the channel O + H₂ ? OH + H.     

Shock tube study of ethylamine pyrolysis and oxidation

Ethylamine (CH₃CH₂NH₂) pyrolysis and oxidation were studied using laser absorption behind reflected shock waves. For ethylamine pyrolysis, NH₂ time-histories were measured in 2000 ppm ethylamine/argon mixtures. For ethylamine oxidation, ignition delay times, and NH₂ and OH time-histories were measured in ethylamine/O₂/argon mixtures. Measurements covered the temperature range of 1200–1448 K, with pressures near 0.85, 1.35 and 2 atm, and fuel mixtures with equivalence ratios of 0.75, 1 and 1.25 in 0.2%, 0.8% and 4% O₂/argon. Simulations using the recent Li et al. mechanism gave significantly shorter ignition delay times and higher intermediate radical species concentrations than the experimental results. The reaction rate constants for the two major ethylamine decomposition pathways were modified in the Li et al. mechanism to improve the prediction of the time-histories of NH₂ and OH in ethylamine pyrolysis. In addition, recommendations from recent studies of ethylamine + OH reactions were implemented. With these modifications, the Modified Li et al. mechanism provides significantly improved agreement with the species time-history measurements and the ignition delay time data.     

A shock tube study of ignition delay in the combustion of ethylene

Ethylene combustion was investigated behind reflected shock waves. The experimental conditions covered a temperature range of 1000–1650 K, at pressures of 2, 10 and 18 atm, equivalence ratios of 3 and 1, for several mixture compositions using argon as the diluent (93%, 96% and 98% (vol)). In all experiments, dwell times were kept in the range of 7.55–7.85 ms by using a suitable argon–helium mixture as the driver gas. Ignition delay times were determined from the onset of visible broadband emission observed at the end plate of the shock tube. In selected experiments ignition delay times were also determined by simultaneous measurement of chemiluminescence emissions of CH^* and OH^*. In relatively concentrated ethylene/oxygen mixtures with 93% argon (vol), the results show an indiscernible difference between ignition delay times over the ranges of pressure and equivalence ratio tested. In more dilute mixtures (with 98% and 96% argon), longer ignition delay times were observed and there was a noticeable variation of delay times as a function of pressure; with an increase in pressure having the effect of shortening the delay time and an increase in the apparent activation energy. Modeling results using USC Mech II (Wang et al., 2007 [31]) based kinetic model, SERDP PAH model 0.1, developed by Wang and Colket, show good agreement with experiments under stoichiometric and fuel-rich conditions at low pressures. At high pressures for fuel-rich mixtures, optimized version of USC Mech II model (Wang et al., 2009 [36]) had to be used to produce good agreement between calculated ignition delay times and the experimental results. The results of this study are consistent with literature data. The present work extends the existing ethylene ignition delay experimental data set to high pressure and fuel-rich domain, the conditions that are critical for soot and polycyclic aromatic hydrocarbons (PAHs) formation.     

An experimental study of the autoignition characteristics of conventional jet fuel/oxidizer mixtures: Jet-A and JP-8

Ignition delay times of Jet-A/oxidizer and JP-8/oxidizer mixtures are measured using a heated rapid compression machine at compressed charge pressures corresponding to 7, 15, and 30 bar, compressed temperatures ranging from 650 to 1100 K, and equivalence ratios varying from 0.42 to 2.26. When using air as the oxidant, two oxidizer-to-fuel mass ratios of 13 and 19 are investigated. To achieve higher compressed temperatures for fuel lean mixtures (equivalence ratio of ∼0.42), argon dilution is also used and the corresponding oxidizer-to-fuel mass ratio is 84.9. For the conditions studied, experimental results show two-stage ignition characteristics for both Jet-A and JP-8. Variations of both the first-stage and overall ignition delays with compressed temperature, compressed pressure, and equivalence ratio are reported and correlated. It is noted that the negative temperature coefficient phenomenon becomes more prominent at relatively lower pressures. Furthermore, the first-stage-ignition delay is found to be less sensitive to changes in equivalence ratio and primarily dependent on temperature.     

Analysis of ignition delay by taking Di-tertiary-butyl peroxide as an additive in a dual fuel diesel engine using hydrogen as a secondary fuel

Ignition delay (ID) is one of the important parameters that make influenced on the combustion process inside the cylinder. This ignition delay affects not only the performances but also the noise and emissions of the engine. In this regards the experiments were conducted on single cylinder 4–stroke compression ignition research diesel engine, power 3.50 kW at constant speed 1500 rpm Kirloskar model TV1 with base fuel as diesel and hydrogen as secondary fuel with and without Di-tertiary-butyl-peroxide (DTBP). Experiments were conducted to measure the ignition delay of the dual fuel diesel (DFD) engine at different load conditions and substitution of diesel by hydrogen with or without DTBP and then it was compared with predicted ID given by Hardenberg-Hase equation and modified Hardenberg-Hase equation.The experimental values of ignition delay were compared with theoretical ignition delay which was predicted on the basis of Hardenberg-Hase equation by considering mean cylinder temperature, pressure, activation energy and cetane number and variations are found in between 6.60% and 21.22%. While, the Hardenberg-Hase equation was modified (by considering variation in activation energy) for DFD engine working on diesel as primary fuel and hydrogen as secondary fuel shows variations 1.20%–11.96%. Furthermore, with DTBP it gives variation up to 18.01%. It was found that ID decreases with increase in percentage of DTBP and hydrogen in air-fuel mixture. This might be due to the cetane improver nature of DTBP, pre-ignition reaction rate and energy release rate of hydrogen fuel. The polytropic index get increased by addition of (Di-tert butyl peroxide) DTBP. Similarly, 5% Di tertiary butyl peroxide reduces Ignition delay.     

Ignition properties of methane/hydrogen mixtures in a rapid compression machine

We investigate changes in the combustion behavior of methane, the primary component of natural gas, upon hydrogen addition by characterizing the autoignition behavior of methane/hydrogen mixtures in a rapid compression machine (RCM). Ignition delay times were measured under stoichiometric conditions at pressures between 15 and 70 bar, and temperatures between 950 and 1060 K; the hydrogen fraction in the fuel varied between 0 and 1. The ignition delay times in methane/hydrogen mixtures are well correlated with the ignition delay times of the pure fuels by using a simple mixing relation reported in the literature. Simulations of the ignition delay times using various chemical mechanism are also reported. The mechanism given by Petersen et al. shows excellent agreement with the measurements for all mixtures studied. Initial results on fuel–lean mixtures show a modest effect of equivalence ratio on the delay times.     

Shock tube determination of ignition delay times in full-blend and surrogate fuel mixtures

Autoignition characteristics of n-heptane/air, gasoline/air, and ternary surrogate/air mixtures were studied behind reflected shock waves in a high-pressure, low-temperature regime similar to that found in homogeneous charge compression ignition (HCCI) engine cycles. The range of experiments covered combustion of fuel in air for lean, stoichiometric, and rich mixtures (Φ=0.5, 1.0, 2.0), two pressure ranges (15-25 and 45-60 atm), temperatures from 850 to 1280 K, and exhaust gas recirculation (EGR) loadings of (0, 20, and 30%). The ignition delay time measurements in n-heptane are in good agreement with the shock tube study of Fieweger et al. (Proc. Combust. Inst. 25 (1994) 1579-1585) and support the observation of a pronounced, low-temperature, NTC region. Strong agreement was seen between ignition delay time measurements for RD387 gasoline and surrogate (63% iso-octane/20% toluene/17% n-heptane by liquid volume) over the full range of experimental conditions studied. Ignition delay time measurements under fuel-lean (Φ=0.5) mixture conditions were longer than with Φ=1.0 mixtures at both the low- (15-25 atm) and high- (45-60 atm) pressure conditions. Ignition delay times in fuel-rich (Φ=2.0) mixtures for both gasoline and surrogate were indistinguishable in the low-pressure (15-25 atm) range, but were clearly shorter at high-pressures (45-60 atm). EGR loading affected the ignition delay times similarly for both gasoline and surrogate, with clear trends indicating an increase in ignition delay time with increased EGR loading. This data set should provide benchmark targets for detailed mechanism validation and refinement under HCCI conditions.     

The high temperature oxidation of pyrrole and pyridine; ignition delay times measured behind reflected shock waves

The ignition delay times for mixtures of pyrrole and oxygen and pyridine and oxygen were measured behind reflected shock waves, using argon as a diluent. Ignition delay times, defined with respect to pressure rise, were measured for mixtures comprising of 0.5-1.0% fuel and 1.5-14.5% oxygen, between 200 and 560 kPa within the temperature regime of 1100-1800 K. Results for the ignition times for the compounds studied can be fitted to the following global regression equations (times in s, concentrations in mol cm^-3):

     

Auto-ignition of toluene-doped n-heptane and iso-octane/air mixtures: High-pressure shock-tube experiments and kinetics modeling

Toluene is often used as a fluorescent tracer for fuel concentration measurements, but without considering whether it affects the auto-ignition properties of the base fuel. We investigate the auto-ignition of pure toluene and its influence on the auto-ignition of n-heptane and iso-octane/air mixtures under engine-relevant conditions at typical tracer concentrations. Ignition delay times τ_ign were measured behind reflected shock waves in mixtures with air at φ = 1.0 and 0.5 at p = 40 bar, over a temperature range of T = 700–1200 K and compared to numerical results using two different mechanisms. Based on the models, information is derived about the relative influence of toluene on τ_ign on the base fuels as function of temperature. For typical toluene tracer concentrations ?10%, the ignition delay time τ_ign changes by less than 10% in the relevant pressure and temperature range.