Shock Tube Study of Ignition Delay Characteristics of n-Nonane and n

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Shock Tube Study of Ignition Delay Characteristics of n‑Nonane and n‑Undecane in Argon Jiuning He,† Kangle Yong,† Weifeng Zhang,† Ping Li,*,† Changhua Zhang,† and Xiangyuan Li‡ †

Institute of Atomic and Molecular Physics, and ‡College of Chemical Engineering, Sichuan University, Chengdu 610065, People’s Republic of China S Supporting Information *

ABSTRACT: Ignition delay times of n-nonane and n-undecane in 4% oxygen/argon have been measured behind reflected shock waves in a heated shock tube at temperatures of 1168−1600 K, pressures of 2, 10, and 20 atm, and equivalence ratios of 0.5, 1.0, and 2.0. Ignition delay times are determined by using CH* emission and pressure signals monitored at the sidewall. Results show that ignition delay times of two fuels decrease as the temperature or pressure increases, and a decrease in equivalence ratio results in a shorter ignition delay time. For fuel-lean and stoichiometric mixtures, n-nonane has ∼25%−35% longer ignition delay times than n-undecane. For fuel-rich mixtures, ignition delay times of two fuels are very close. Correlations for ignition delay times of two fuels as a function of temperature, pressure, and equivalence ratio are formulated through regression analysis. The experimental data are in good agreement with shock tube data available, and the trends of experimental data were captured well by the predictions from the LLNL and JetSurF mechanisms under conditions studied. Comparison of ignition delay times for nine n-alkanes from propane to n-undecane reveals that the n-alkanes have the similar ignition delay behavior and their ignition delay times are close to each other. Reaction path analyses and sensitivity analyses are performed to investigate the consumption of fuels and identify the important reactions in the ignition process. To our knowledge, we provide the first ignition delay time data for n-undecane at elevated pressures, and our measurements for n-nonane are at a broader range of conditions than previous studies. Current results contribute toward understanding the ignition characteristics of n-nonane and n-undecane, and they provide validation targets for corresponding kinetic mechanisms. Haylett et al.16 used an aerosol shock tube to investigate ignition delay times of three heavy n-alkanes (n-decane, ndodecane, and n-hexadecane) at temperatures from 838 K to 1381 K and pressures from 1.71 atm to 8.63 atm, and they found that n-dodecane ignition delay times exhibit a negative power law dependence, relative to equivalence ratio, at high temperatures. For n-nonane, Rotavera et al.17 measured its ignition delay times diluted in argon at pressures near 1.5 and 10.4 atm with equivalence ratios of 0.5 and 1.0 in a shock tube, and they proposed a chemical kinetic mechanism for the oxidation of this fuel. Davidson et al.18 measured ignition delay times of n-nonane in a shock tube at pressures of 1−4 atm and equivalence ratios of 0.5 and 1.0. Ignition delay data under wider conditions for n-nonane are not available. For nundecane, Rotavera et al.19 measured six ignition delay data points at 1.5 atm diluted in 99% Ar in the temperature range of 1378−1594 K; this is the only experimental study on the ignition delay of the fuel. Chemical kinetic mechanisms about n-alkanes have been widely studied.20−22 Sarathy et al.23 composed a kinetic reaction mechanism for a series of nalkanes ranging from C8 to C16, and 2-methylalkanes up to C20 have also been included in this mechanism. The n-nonane pyrolysis measurements of Zhou et al.24 were well-predicted by the mechanism. Wang et al.25 have developed a surrogate jet fuel mechanism (JetSurF), which contains a high-temperature

1. INTRODUCTION Practical fuels, such as jet fuels, are complex mixtures of many types of hydrocarbon species containing alkanes, cycloalkanes, alkenes, and aromatics.1 n-Alkanes have been identified to have the largest proportion in jet fuels (Jet A-1, Jet A, and JP-8).2 To simulate the oxidation of these practical fuels, it is common to construct surrogate mixtures consisting of a few representative hydrocarbons. n-Decane (n-C10H22) has been widely selected as being the alkane most representative of jet fuels.3−5 However, the average chemical formula for Jet A-1 and JP-8 is C11H21,6−8 which is very close (with the same carbon number) to the chemical formula of n-undecane (n-C11H24). It seems that nundecane could be preferred to represent Jet A-1 and JP-8. Furthermore, the contents of n-nonane and n-undecane in Chinese RP-3 jet fuel are more than 5% and 10% (by mass), respectively. Therefore, study on the combustion of these two long-chain n-alkanes is necessary to investigate the combustion properties of jet fuels. Experimental investigations on ignition delay times of normal alkanes from methane to octane are abundant,9−13 and the measurements on other large normal alkanes are also available, such as n-decane, n-dodecane, and n-hexadecane. However, studies on ignition delay times of n-nonane and n-undecane are scarce. Recently, Campbell et al.14 employed a shock tube to measure ignition delay times of n-heptane at 6.5 atm and temperatures ranging from 651 K to 823 K, and they obtained a concentration time history of fuel, OH, H2O, and CO2. Vasu et al.15 measured the ignition delay times and OH concentration time histories of n-dodecane oxidation at high pressures. © XXXX American Chemical Society

Received: May 10, 2016 Revised: October 9, 2016

A

DOI: 10.1021/acs.energyfuels.6b01122 Energy Fuels XXXX, XXX, XXX−XXX

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is shown in Figure 1. In this work, the variation in the pressure was found to be in the range of (dP/dt)(1/P) = 0−2.5%/ms, the effect of

chemical model of n-alkane up to n-dodecane. Compared to these theoretical studies, experimental studies on the ignition delay times of n-nonane and n-undecane are needed. Considering the lack of ignition data for n-nonane and nundecane, ignition delay times of two fuels were measured under wider conditions in the present work. The experimental conditions spanned as follows: temperatures of 1168−1600 K, pressures of 2, 10, and 20 atm, and equivalence ratios (Φ) of 0.5, 1.0, and 2.0. The effects of temperature, pressure, and equivalence ratio on the ignition time were analyzed and discussed for understanding the ignition characteristics of two fuels. Comparison of current data with available n-nonane experimental data and model predictions for two fuels are performed. In addition, a series of n-alkane ignition delay times from the present study and from literature are compared. Finally, reaction path analyses and sensitivity analyses are performed to investigate the consumption of fuels and identify the important reactions in the ignition process of n-nonane and n-undecane.

2. EXPERIMENTAL SECTION

Figure 1. Example of n-undecane/O2/Ar ignition delay time measurement.

Ignition delay time measurements were performed behind reflected shock waves in a heated stainless steel shock tube. The shock tube is composed of a 6 m driver and 5 m driven sections. Two sections were separated by a double diaphragm structure. Diaphragms of different gauges were selected, to obtain various reflected shock pressures. Details of this facility and associated optical diagnostics have been described in the previous publication.26 High-purity gases of O2, Ar, and He (all >99.999%), and fuels with a purity level of 99% were used. Test gas mixtures were prepared beforehand in a heated 40 L mixing tank. The tank was maintained at 150 and 170 °C, for n-nonane and nundecane, respectively, which have low vapor pressures at room temperature. The shock tube was heated to 80 °C for n-nonane mixtures and 120 °C for n-undecane mixtures to prevent condensation of the fuels during experiments. Mixtures of n-nonane/O2/Ar and nundecane/O2/Ar, covering equivalence ratios of 0.5, 1.0 and 2.0, as given in Table 1, were prepared.

this pressure variation on the ignition delay is negligible in the present high-temperature measurements. The uncertainty of ignition delay time is caused by the uncertainty in reflected pressure and temperature, mixture composition, the determination of the ignition time from pressure and CH* emission signals. The largest uncertainty is due to the uncertainty in reflected shock temperature. The uncertainty in the incident shock velocity at the endwall is estimated at ±0.7%. The uncertainties in the temperature and pressure behind the reflected shock wave are estimated to be ±0.9% and ±2.2% (by one-dimensional normal shock relations), respectively. Combining all of the uncertainties, the overall uncertainty in measured ignition delay times is estimated within ±15% in the current work.

3. RESULTS AND DISCUSSION Ignition delay times for n-nonane and n-undecane have been measured at pressures of ∼2, 10, and 20 atm over temperatures of 1168−1600 K, with equivalence ratios of Φ = 0.5, 1.0, and 2.0 in 4% oxygen/argon. The results are listed in Tables 2a and 2b and are presented in Figures 2 and 3. For each fuel, it is observed that ignition delay times exhibit an exponential temperature dependence:

Table 1. Molar Composition of Mixtures Composition (mol %) Φ

n-nonane

O2

Ar

n-undecane

O2(%)

Ar

0.5 1.0 2.0

0.143 0.286 0.571

4.0 4.0 4.0

95.857 95.714 95.429

0.118 0.235 0.471

4.0 4.0 4.0

95.882 95.765 95.529

⎛E ⎞ τign ∝ exp⎜ a ⎟ ⎝ RT ⎠

Incident shock wave speeds were measured using four piezoelectric pressure transducers (Model PCB113B), which were equally spaced alongside the test section. The distance of each two pressure transducers was 18.9 cm. The shock wave speeds at the end wall location were obtained by extrapolating the three wave speeds. Typical shock wave attenuation rates ranged from 0.5%/m to 2%/m for current experiments. Using initial pressure (P1) and temperature (T1), conditions behind the reflected shock wave (pressure P5 and temperature T5) were obtained by the one-dimensional normal shock wave equations. Light emission measurements were performed using a grating monochromator (Zolix, Model Omni-λ3009) coupled with a photomultiplier tube (PMT). The monochromator was set to 431 nm to capture CH* emission from the A2Δ−X2∏ transition. Ignition delay time (τign) was defined as the time interval between the arrival of the reflected shock wave and the onset of ignition at the side-wall observation location (15 mm from the end wall). The arrival of the reflected shock wave was indicated by the point of reflectedshock bifurcation in the rising pressure signal, and the onset of ignition was marked by the steepest increase of the CH* emission.27 An example of typical profiles of shock wave pressure and CH* emission

However, the activation energy (Ea) is different at three equivalence ratios. Therefore, a single correlation for ignition delay time cannot be formulated, but correlations at Φ = 0.5, 1.0, and 2.0, in terms of pressure and temperature, can be obtained, respectively. Thus, an Arrhenius-like expression, ⎛E ⎞ τign = AP nXO2 m exp⎜ a ⎟ ⎝ RT ⎠

is suitable to correlate the measured data at different equivalence ratios Φ, where τign is the ignition delay time (in μs), P the reflected shock pressure (in atm), XO2 the oxygen concentration, R the universal gas constant (R = 1.986 × 10−3 kcal mol−1 K−1), and T the reflected shock temperature (in Kelvin). Since current data were measured at an oxygen mole fraction of 4%, XO2 is constant here and the formula above is reduced to B

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Energy & Fuels Table 2a. Measured Ignition Delay Time (τign) for n-Nonanea at Different Equivalence Ratios Φ = 0.5

a

Φ = 1.0

Φ = 2.0

T5 (K)

P5 (atm)

τign (μs)

T5 (K)

P5 (atm)

τign (μs)

T5 (K)

P5 (atm)

τign (μs)

1229 1264 1276 1298 1327 1359 1360 1375 1379 1430 1480 1232 1241 1269 1270 1286 1307 1343 1366 1372 1220 1243 1259 1275 1308 1321 1328 1374

2.22 2.1 2.11 2.11 2.11 2.12 2.18 2.18 2.06 2.23 2.15 11.03 9.29 10.22 10.37 10 10 10.27 10.35 10.81 21.35 20.9 21.6 20.12 20.7 20.4 20.14 21.02

1482 1044 850 671 456 266 280 195 188 105 61 755 654 375 450 316 225 122 96 83 650 398 330 230 150 121 111 64

1258 1285 1295 1302 1342 1348 1402 1426 1468 1519 1211 1227 1244 1285 1306 1307 1312 1357 1365 1379 1386 1413 1195 1222 1245 1268 1317 1335 1363 1369

2.05 2.09 2.12 2.15 2.41 2.3 2.08 2.03 2.06 2.12 10.48 10.66 10.49 11.15 10.9 10.09 10.12 9.92 10.26 11.04 10.67 10.21 19.99 20.69 21.14 19.88 21.21 20.1 21.14 21.06

1680 980 970 795 466 476 272 195 130 79 1030 740 709 484 355 387 306 209 181 181 165 105 758 610 463 339 195 169 114 93

1317 1339 1382 1407 1451 1483 1544 1577 1600 1253 1260 1309 1329 1350 1356 1368 1409 1440 1453 1199 1242 1250 1268 1306 1340 1358 1397

2 2.04 2.06 2.04 2.05 2 1.99 2.07 2.05 11.34 10.99 10.37 9.03 9 9.68 9.3 9.54 10.05 10.22 20.34 20.86 20.24 20.24 20.88 21.22 20.88 20.87

1076 918 610 500 320 230 140 100 80 740 624 441 396 265 265 274 172 100 96 846 471 424 325 228 155 130 93

n-Nonane/4% O2/Ar.

⎛E ⎞ τign = AP n exp⎜ a ⎟ ⎝ RT ⎠

is observed that the ignition delay times of the three n-alkanes are very close to each other at 2 atm. This seems that, at low pressure and under fuel-rich conditions, carbon chain length has no significant effect on the ignition delay times of n-alkanes. There are three approaches to investigate the effect of equivalence ratio Φ on the ignition delay time τign: keep the fuel concentration constant,28 keep the oxygen concentration constant,29 and keep the ratio of diluent gas to oxygen constant.30 Current work investigated the effect of equivalence ratio on ignition delay times of n-undecane by the second way, as shown in Figure 3. It is observed that ignition delay times taken at higher equivalence ratio are longer than those taken at lower equivalence ratio under all pressures studied. That is, reactivity decreases (ignition delay time increases) Φ increases from 0.5 to 2.0 when the oxygen concentration is fixed at 4%. Referring to the effect of Φ on the ignition delay, Tian et al.28 found that the τign value for ethylcyclohexane increases as Φ increases at 1.1 atm, and decreases as Φ increases at pressures up to 50 atm when keeping the fuel concentration constant. Similar behavior has been observed for the ignition delay times of hydrocarbon/air mixtures. For example, the ignition delay times of n-propylbenzene/air increase as Φ increases at low pressures and high temperatures, and decrease as Φ increases at high pressures.31 In the current work, the fuel-lean mixture exhibits the highest activation energy, whereas the fuel-rich mixture exhibits the lowest one (shown in Table 3). In addition, a convergence for the ignition delay times of three

The corresponding correlation parameters for present ignition delay times of n-nonane and n-undecane measured at Φ = 0.5, 1.0, and 2.0 are listed in Table 3, where r2 is the correlation coefficient. Figure 2 shows the effect of reflected shock pressure on ignition delay times of two fuels at three equivalence ratios. It can be seen that the correlation results agree well with the experimental data. Current data have been scaled to three pressures (2, 10, and 20 atm) using τign ∝ Pn, where corresponding values of n are given in Table 3. As is evident in the figure, ignition delay times decrease as the pressure increases, which is caused by the increase in molecule concentration with increasing pressure. It is observed that the ignition delay times of n-undecane are ∼25%−35% shorter than that of n-nonane under the same pressures and temperatures for fuel-lean and stoichiometric mixtures. However, for fuel-rich mixtures, ignition delay times of two fuels are very close at pressures of 2, 10, and 20 atm. It shows that the length of carbon chain has a weak effect on ignition delay times under the fuel-rich condition. Being curious about whether other nalkanes have the same behavior, the ignition delay times of nheptane in 4% oxygen/argon at P = 2 atm with Φ = 2.0 were also measured. The results are appended in Figure 2c, and the raw data are listed in Table 2S in the Supporting Information. It C

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Energy & Fuels Table 2b. Measured Ignition Delay Time (τign) for n-Undecanea at Different Equivalence Ratios Φ = 0.5

a

Φ = 1.0

Φ = 2.0

T5 (K)

P5 (atm)

τign (μs)

T5 (K)

P5 (atm)

τign (μs)

T5 (K)

P5 (atm)

τign (μs)

1227 1250 1261 1296 1330 1351 1362 1402 1440 1206 1226 1230 1258 1284 1303 1325 1346 1370 1377 1185 1201 1221 1243 1243 1253 1291 1328 1359 1390

2.56 2.57 2.48 2.47 2.52 2.6 2.66 2.49 2.56 10.55 10.41 10.06 10.41 10.32 10.36 10.17 10.29 10.58 10.35 22.22 21.49 20.78 20.73 19.51 20.75 20.91 20.95 20.12 20.56

1355 1055 900 580 345 205 209 129 85 900 500 650 380 285 200 170 104 72 66 750 580 420 350 310 276 180 90 66 45

1252 1305 1332 1333 1373 1385 1392 1433 1433 1463 1467 1218 1250 1264 1281 1300 1351 1365 1384 1168 1202 1234 1236 1267 1283 1332 1348 1408

2.56 2.7 2.65 2.56 2.57 2.59 2.62 2.55 2.44 2.43 2.51 10.27 9.2 10.49 10.36 10.94 10.29 10.67 9.97 20.63 20.65 19.58 19.54 20.76 19.94 20.6 20.6 21.14

1175 665 556 468 278 244 283 156 134 95 88 767 636 480 390 256 149 115 99 920 640 353 367 265 220 121 108 51

1311 1338 1360 1436 1440 1444 1487 1493 1530 1572 1202 1223 1224 1271 1317 1329 1357 1392 1414 1419 1449 1203 1239 1265 1316 1340 1370 1387

2.46 2.57 2.51 2.6 2.6 2.49 2.52 2.53 2.55 2.7 9.63 9.78 9.8 9.76 10.38 9.86 10.37 10.33 9.85 9.96 9.99 20.15 20.34 20.15 19 19.61 20.2 20.03

1280 995 730 388 368 366 260 220 155 109 1550 1039 1035 597 390 336 266 175 140 138 101 830 568 410 215 140 110 87

n-Undecane/4% O2/Ar.

Table 3. Correlation Parameters for n-Nonane and n-Undecane Ignition Delay Times Measured at 4% Oxygen Mole Fractiona

a

equivalence ratio, Φ

pressure, P (atm)

A

0.5 1.0 2.0

2−20 2−20 2−20

4.26 × 10−6 3.01 × 10−4 1.11 × 10−3

0.5 1.0 2.0

2−20 2−20 2−20

1.02 × 10−5 1.06 × 10−4 1.53 × 10−3

n n-nonane −0.56 ± 0.03 −0.54 ± 0.04 −0.74 ± 0.04 n-undecane −0.60 ± 0.04 −0.66 ± 0.05 −0.87 ± 0.04

activation energy, Ea (kcal mol−1)

correlation coefficient, r2

49.53 ± 1.82 39.34 ± 1.15 37.64 ± 1.85

0.996 0.980 0.987

47.09 ± 1.84 42.07 ± 2.44 37.68 ± 1.37

0.995 0.981 0.992

Using the expression τign = APn exp[Ea/(RT)].

2 atm, using the rule of τign ∝ P−0.53, given by them and shown in Figure 4a. Comparisons between current corresponding results with those of Davidson et al. show that two sources of data are in very good agreement with each other. This verified that our equipment and procedure are performing satisfactorily and current results for n-alkanes are reliable. Besides, Rotavera et al.17 have measured ignition delay times of n-nonane diluted in argon with oxygen mole fractions of 0.966% and 0.933% at P = 1.5 and 10.4 atm with Φ = 0.5 and 1.0. The ignition delay time data from Rotavera et al. are plotted in Figure 4b, together with current corresponding data. Comparisons show that two sets of data have approximately the same sensitivity for ignition delay to temperature, and the mixtures with an oxygen mole fraction of 4% is faster to ignite than those with oxygen mole fractions of 0.966% and 0.933% at the same pressure and

mixtures is observed at low temperatures and a pressure of 20 atm. Therefore, it can be predicted that there is a crossover for the ignition delay times of three mixtures at higher pressures, and, with continually increasing pressure, fuel-rich mixtures will have the shortest ignition delay times. From the present and previous studies, it seems that ignition delay times of hydrocarbons have an opposite dependence on equivalence ratio at low and high pressures. The effect of Φ on the ignition delay time for n-nonane is same as for n-undecane, and the details are shown in Figure S1 in the Supporting Information. 3.1. Comparison with Previous Experimental Data. Davidson et al.18 have measured the ignition delay times of nnonane/4% O2/Ar behind reflected shock waves at high temperatures of 1150−1550 K, low pressures of 1−4 atm, and equivalence ratios of Φ = 0.5 and 1.0. Their data were scaled to D

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Figure 3. Effects of equivalence ratio (Φ) on ignition delay time (τign) of n-undecane/O2/Ar at (a) 2 atm, (b) 10 atm, and (c) 20 atm. Symbols represent measured values; lines represent correlations results.

Moreover, there are a great deal of previous measurements of ignition delay times for other normal alkanes in argon, which cover wide range of temperatures, pressures, and equivalence ratios. These previous experiments are summarized in Table 4, and Figure 5 shows such a series of n-alkane ignition delay times from the present study and the literature.18,32 Because these measurements were carried out under different conditions, their scaling rules of τign ∝ P−0.55XO2−0.63 and τign ∝ P−0.55 were respectively used to scale the data of Horning et al.32 and Davidson et al.18 to a common condition (XO2 = 4% and 2 atm), for comparison. As shown in Figure 5, ignition delay times of any two fuels with adjacent carbon numbers exhibit very small discrepancies,

Figure 2. Comparisons of ignition delay times for n-nonane/O2/Ar and n-undecane/O2/Ar mixtures at (a) Φ = 0.5, (b) Φ = 1.0, and (c) Φ = 2.0. Symbols represent experimental data; lines represent correlations.

equivalence ratio. All data together exhibit a conventional negative power-law dependence of ignition delay time on oxygen mole concentration τign ∝ XO2−n. For n-undecane ignition delay times, the previous six data from Rotavera et al.19 are measured at a very low fuel concentration of 0.0556% (current work: 0.235%) in volumetric percentage with Φ = 1.0; their data are not compared with the present data here. E

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Figure 4. Comparisons of current n-nonane/O2/Ar ignition delay times with experimental data from Davidson et al.18 and Rotavera et al.17

Table 4. Experimental Conditions and Ignition Delay Times for n-Alkanes/O2/Ar in Shock Tubes fuel

XO2 (mol %)

equivalence ratio, Φ

pressure (atm)

temperature (K)

delay, τign (μs)

ref

propane n-butane n-pentane n-hexane n-heptane n-octane n-decane

20 3.25−13 4.0 4.0 2.2−19.7 4.0 3.1

1.0 0.5−2.0 0.5−1.0 0.5−1.0 0.5−2.0 0.5−1.0 1.0

1.12−1.26 1.03−3.81 1.62−3.75 1.67−3.60 1.14−5.71 1.87−3.81 1.22−1.26

1376−1504 1352−1734 1261−1533 1237−1475 1329−1676 1252−1455 1397−1516

93−357 83−475 49−1845 78−2706 86−488 104−1569 124−480

Horning et al.32 Horning et al.32 Davidson et al.18 Davidson et al.18 Horning et al.32 Davidson et al.18 Horning et al.32

Figure 5. Comparisons of n-alkanes/O2/Ar ignition delay times from present study and previous experiments at Φ = 0.5 and 1.0. All data have been scaled to 4% O2 and 2 atm.

3.2. Comparison with Model Predictions. Westbrook et al.23 developed a detailed chemical kinetic mechanism (LLNL) to describe the pyrolysis and oxidation of nine n-alkanes, from n-octane to n-hexadecane. A surrogate jet-fuel mechanism (JetSurF) has also been composed by Wang et al.,25 which covers n-alkanes up to n-dodecane, in addition to cyclohexane and monoalkylated cyclohexanes, up to n-butyl-cyclohexane. These two mechanisms were used here to predict ignition delay times of n-nonane/O2/Ar and n-undecane/O2/Ar. Predicted values were calculated with CHEMKIN software,33 using a constant-volume, adiabatic, and zero-dimensional reactor model (constant U, V). The comparison results at Φ = 0.5, 1.0, and 2.0 are respectively shown in Figure 6. The JetSurF mechanism can perfectly predict n-nonane and n-undecane

and are even indistinguishable. For fuel-lean mixtures, a systematic decrease of 40% in ignition delay times is found between n-butane and n-undecane. Moreover, most of these data have similar activation energies (Ea ≈ 47 kcal/mol). The details about the activation energy of each set of data are given in the Supporting Information (Tables S3 and S4). For stoichiometric mixtures, a systematic decrease of 60% in the ignition delay times is found from propane to n-undecane, and the corresponding activation energies vary between 41.20 kcal/ mol and 50.24 kcal/mol. Comparison results indicates that these n-alkanes have similar ignition delay behaviors and their ignition delay times are similar to each other at high temperatures. F

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Figure 6. Comparisons of current data with predictions for n-nonane/O2/Ar and n-undecane/O2/Ar ignition delay times. Solid lines represent predictions from the JetSurF mechanism,25 and dashed lines represent predictions from the LLNL mechanism.23

ignition delay times under fuel-lean conditions, but overpredicts the experimental data under fuel-rich conditions. For n-nonane, the LLNL mechanism predicts the ignition delay times well at Φ = 0.5 and P = 10 and 20 atm, but its predictions at 2 atm are ∼25% shorter than the measured data. Under stoichiometric conditions, the LLNL mechanism underestimates the ignition delay times of n-nonane by 10%−35% (Figure 6b); however, the JetSurF mechanism accurately captures the measured data under these conditions. In contrast, the JetSurF mechanism overpredicts the ignition delay times of n-nonane by a factor of ∼1.5 at Φ = 2.0; however, the LLNL mechanism correctly simulates the ignition delay time data under these conditions. For n-undecane, the ignition delay data at Φ = 0.5, 1.0, and 2.0, the predictions from the LLNL mechanism have good agreement with the measured ignition delay times at pressures

of 10 and 20 atm. However, at 2 atm, the LLNL mechanism predictions for n-undecane are shorter than the measured data, with a maximum deviation of 35% observed at the three equivalence ratios. Generally, the trends of experimental data were captured well by the predictions from both the LLNL and JetSurF mechanisms under all conditions studied. 3.3. Reaction Path Analysis and Sensitivity Analysis. To investigate the important reactions responsible for the consumption of the fuels and the formation of products, reaction path analyses were performed in the ignition processes of n-nonane and n-undecane by using the LLNL mechanism, respectively. Figures 7 and 8 respectively show the analysis results for the fuel-lean mixtures of n-nonane and n-undecane at T = 1330 K and P = 2.0 atm. As seen in Figure 7, a total of 73% n-nonane is consumed primarily by hydrogen abstraction by G

DOI: 10.1021/acs.energyfuels.6b01122 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels

process, the n-undecane ignition produces more types of radicals, and most of the reaction types and reaction channels are the same as n-nonane. In summary, at high temperature, the consumption processes of two fuels are mainly controlled by the hydrogen abstraction reaction of OH and H radicals from fuels and C−C bond breaking reactions. Sensitivity analyses were carried out by using the LLNL mechanism to identify the important reactions in the ignition process of n-nonane/O2/Ar and n-undecane/O2/Ar mixtures. The percent change in ignition delay time, [τign(2ki) − τign(ki)]/τign(ki), is taken as the sensitivity coefficient of the ith reaction to the ignition delay time, where ki is the rate coefficient of that reaction.34 A negative sensitivity coefficient suggests that the ignition delay time decreases as the rate constant increases, implying a promoting influence on ignition, and vice versa. Figure 9 demonstrates the results of the sensitivity analysis for n-nonane and n-undecane, for Φ = 0.5, 1.0, and 2.0, at 1330 K and 2 atm, respectively. As shown in Figure 9, among 16 high-sensitivity elementary reactions, the chain-branching reaction H + O2 ↔ O + OH exhibits the strongest sensitivity and has the highest promotion effect on the ignition for both fuels. Its sensitivity coefficient is 2-fold−4-fold greater than those of other promoting reactions, indicating that it is a dominant reaction in the ignition process of the fuels at high temperatures. This reaction also is a major source of hydroxyl radicals (OH) that are mainly responsible for the consumption of n-nonane and n-undecane through hydrogen abstraction reactions. Besides, the reaction C3H5 + HO2 ↔ C3H5O + OH exhibits a large negative sensitivity coefficient and has relatively strong promoting influence on the ignition of two fuels. Because this reaction consumes the relative stable allyl radical (C3H5) and generates the hydroxyl radical and C3H5O, and the production of C3H5O can further produce the active H atom through the reaction of C3H5O ↔ C2H3CHO + H. Moreover, because of the consumption of the relative stable hydroperoxy radical (HO2) from CH3 + HO2 ↔ CH3O + OH, this reaction also shows relatively strong promoting effect on the ignition. Reactions that involve the hydroperoxyl radical (HO2) and the hydroxyl radical to form H2O and molecular oxygen, the formation of the stable allyl radical from the decomposition of propylene (C3H6), and H abstraction by OH and H from propylene exhibit positive sensitivity coefficients and have relatively strong inhibiting influence on the ignition of n-nonane and n-undecane. Overall, the main promoting and inhibiting reactions for the ignition of

Figure 7. Reaction path analysis for the ignition of n-nonane at 1330 K, 2 atm, and Φ = 0.5.

Figure 8. Reaction path analysis for the ignition of n-undecane at 1330 K, 2 atm, and Φ = 0.5.

OH, H, and O radicals from five different H positions to form different nonyl radicals (n-C9H19-1 to n-C9H19-5). The unimolecular decomposition reaction is also a consumption channel of the fuel via reaction n-C9H20 = C7H15 + C2H5 (12%). Thereafter, these productions of nonyl radicals and heptyl radical (n-C7H15-1) mainly form low-carbon straightchain 1-alkyl and 1-olefins radicals through C−C bond breaking reactions. The reaction pathway analyses for n-undecane show that 76% of the n-undecane consumption proceeds mainly through hydrogen abstraction by free radicals. Most of these hydrogen abstractions are by OH, H, and O to form undecyl radicals (n-C11H23-1 to n-C11H23-5), all of which subsequently decompose to small 1-alkyl and 1-olefins radicals. In addition, a fraction of n-undecane (10%) is consumed through a decomposition reaction to form n-C9H19-1 and C2H5 radicals. Compared to n-nonane consumptions during the ignition

Figure 9. Sensitivity analysis results calculated using the LLNL mechanism at 1330 K and 2 atm (a) for n-nonane and (b) for n-undecane. H

DOI: 10.1021/acs.energyfuels.6b01122 Energy Fuels XXXX, XXX, XXX−XXX

Energy & Fuels

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ACKNOWLEDGMENTS The authors acknowledge the support from the National Natural Science Foundation of China, under Grant No. 91441132.

n-nonane and n-undecane are the same and their sensitivity coefficients are very similar at the three equivalence ratios, which may be the reason for the similar ignition characteristics and close ignition delay times for both fuels.

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4. CONCLUSION The ignition delay times (τign) of n-nonane/O2/Ar and nundecane/O2/Ar have been systematically studied in a heated shock tube over a wide range of conditions, including pressures of 2, 10, and 20 atm, temperatures of 1168−1600 K, and equivalence ratios (Φ) of 0.5, 1.0, and 2.0. Results show that the τign values of two fuels decrease as the temperature or pressure increase, and a decrease in Φ results in a decrease in τign. Ignition delay times that are ∼25%−35% longer for nnonane than n-undecane were observed under fuel-lean and stoichiometric conditions. For fuel-rich mixtures, ignition delay times of two fuels are very close. Correlations of ignition delay times, as a function of temperature and pressure for two fuels, were obtained through regression analysis. Current data for nnonane are in good agreement with previous shock tube data. Both the LLNL and JetSurF mechanisms can well predict present experimental trends for the ignition delay times of two fuels under the conditions studied. Comparison of the ignition delay times of nine n-alkanes, from propane to n-undecane, reveals that these n-alkanes have similar ignition delay behaviors and their ignition delay times are similar to each other. Through the reaction path analysis, we find that n-nonane and n-undecane consumption processes at high temperatures are mainly controlled by hydrogen abstraction reaction of OH and H radicals from fuels and C−C bond breaking reactions. Sensitivity analyses show that the chain branching reaction H + O2 ↔ O + OH is the most promoting reaction in the hightemperature ignitions of n-nonane and n-undecane. Reactions related to the formation of the relative stable allyl radical exhibit positive sensitivity coefficients and have a strong inhibiting influence on the ignition of n-nonane and n-undecane. The main promoting and inhibiting reactions for n-nonane and nundecane ignitions are the same and their sensitivity coefficients are very close at three equivalence ratios. The ignition delay data of n-undecane at 10 and 20 atm are first reported here, and new ignition delay data under a wider range of ignition conditions for n-nonane are provided. Current results are useful for understanding the ignition properties of nnonane and n-undecane, and they provide validation targets for the corresponding kinetic mechanisms of two fuels.

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REFERENCES

(1) Ranzi, E. Energy Fuels 2006, 20, 1024−1032. (2) Dagaut, P.; El Bakali, A.; Ristori, A. Fuel 2006, 85, 944−956. (3) Lindstedt, R. P.; Maurice, L. Q. J. Propul. Power 2000, 16, 187− 195. (4) Dooley, S.; Won, S. H.; Chaos, M.; Heyne, J.; Ju, Y.; Dryer, F. L.; Kumar, K.; Sung, C. J.; Wang, H.; Oehlschlaeger, M. A.; Santoro, R. J.; Litzinger, T. A. Combust. Flame 2010, 157, 2333−2339. (5) Natelson, R. H.; Kurman, M. S.; Cernansky, N. P.; Miller, D. L. Fuel 2008, 87, 2339−2342. (6) Dagaut, P.; Cathonnet, M. Prog. Energy Combust. Sci. 2006, 32, 48−92. (7) Edwards, T.; Maurice, L. Q. J. Propul. Power 2001, 17, 461−466. (8) Zhukov, V. P.; Sechenov, V. A.; Starikovskiy, A. Y. Fuel 2014, 126, 169−176. (9) Koroglu, B.; Pryor, O. M.; Lopez, J.; Nash, L.; Vasu, S. S. Combust. Flame 2016, 164, 152−163. (10) de Vries, J.; Hall, J. M.; Simmons, S. L.; Rickard, M. J.; Kalitan, D. M.; Petersen, E. L. Combust. Flame 2007, 150, 137−150. (11) Sabia, P.; de Joannon, M.; Lubrano Lavadera, M.; Giudicianni, P.; Ragucci, R. Combust. Flame 2014, 161, 3022−3030. (12) Zhang, K.; Banyon, C.; Togbé, C.; Dagaut, P.; Bugler, J.; Curran, H. J. Combust. Flame 2015, 162, 4194−4207. (13) Wang, W.; Li, Z.; Oehlschlaeger, M. A.; Healy, D.; Curran, H. J.; Sarathy, S. M.; Mehl, M.; Pitz, W. J.; Westbrook, C. K. Proc. Combust. Inst. 2013, 34, 335−343. (14) Campbell, M. F.; Wang, S.; Goldenstein, C. S.; Spearrin, R. M.; Tulgestke, A. M.; Zaczek, L. T.; Davidson, D. F.; Hanson, R. K. Proc. Combust. Inst. 2015, 35, 231−239. (15) Vasu, S. S.; Davidson, D. F.; Hong, Z.; Vasudevan, V.; Hanson, R. K. Proc. Combust. Inst. 2009, 32, 173−180. (16) Haylett, D. R.; Davidson, D. F.; Hanson, R. K. Combust. Flame 2012, 159, 552−561. (17) Rotavera, B.; Petersen, E. L. Proc. Combust. Inst. 2013, 34, 435− 442. (18) Davidson, D. F.; Ranganath, S. C.; Lam, K. Y.; Liaw, M.; Hong, Z.; Hanson, R. K. J. Propul. Power 2010, 26, 280−287. (19) Rotavera, B.; Petersen, E. L. Shock Waves 2013, 23, 345−359. (20) Zeppieri, S. P.; Klotz, S. D.; Dryer, F. L. Proc. Combust. Inst. 2000, 28, 1587−1595. (21) Jahangirian, S.; Dooley, S.; Haas, F. M.; Dryer, F. L. Combust. Flame 2012, 159, 30−43. (22) Narayanaswamy, K.; Pepiot, P.; Pitsch, H. Combust. Flame 2014, 161, 866−884. (23) Sarathy, S. M.; Westbrook, C. K.; Mehl, M.; Pitz, W. J.; Togbe, C.; Dagaut, P.; Wang, H.; Oehlschlaeger, M. A.; Niemann, U.; Seshadri, K.; Veloo, P. S.; Ji, C.; Egolfopoulos, F. N.; Lu, T. Combust. Flame 2011, 158, 2338−2357. (24) Zhou, P.; Hollis, O. L.; Crynes, B. L. Ind. Eng. Chem. Res. 1987, 26, 846−852. (25) Wang, H.; Dames, E.; Sirjean, B.; Sheen, D. A.; Tangko, R.; Violi, A.; Lai, J. Y. W.; Egolfopoulos, F. N.; Davidson, D. F.; Hanson, R. K.; Bowman, C. T.; Law, C. K.; Tsang, W.; Cernansky, N. P.; Miller, D. L.; Lindstedt, R. P. JetSurF, Version 2.0. Available via the Internet at: http://melchior.usc.edu/JetSurF/JetSurF2.0; accessed Sept. 19, 2010. (26) Zhang, C.; Li, B.; Rao, F.; Li, P.; Li, X. Proc. Combust. Inst. 2015, 35, 3151−3158. (27) Hargis, J. W.; Petersen, E. L. Energy Fuels 2015, 29, 7712−7726. (28) Tian, Z.; Zhang, Y.; Pan, L.; Zhang, J.; Yang, F.; Jiang, X.; Huang, Z. Energy Fuels 2014, 28, 5505−5514. (29) Li, S.; Campos, A.; Davidson, D. F.; Hanson, R. K. Fuel 2014, 118, 398−405.

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DOI: 10.1021/acs.energyfuels.6b01122 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels (30) Eldeeb, M. A.; Akih-Kumgeh, B. Energy Fuels 2014, 28, 6618− 6626. (31) Darcy, D.; Tobin, C. J.; Yasunaga, K.; Simmie, J. M.; Würmel, J.; Metcalfe, W. K.; Niass, T.; Ahmed, S. S.; Westbrook, C. K.; Curran, H. J. Combust. Flame 2012, 159, 2219−2232. (32) Horning, D. C.; Davidson, D. F.; Hanson, R. K. J. Propul. Power 2002, 18, 363−371. (33) Kee, R. J.; Rupley, F. M.; Miller, J. A. Chemkin-II: A Fortran Chemical Kinetics Package for the Analysis of Gas-Phase Chemical Kinetics, Sandia Report, No. SAND89-8009, Sandia National Laboratories: Albuquerque, NM, 1989. (34) Kumar, K.; Mittal, G.; Sung, C. J.; law, C. K. Combust. Flame 2008, 153, 343−354.

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DOI: 10.1021/acs.energyfuels.6b01122 Energy Fuels XXXX, XXX, XXX−XXX