Shock-Tube Measurements of Toluene Ignition Times and Radical

Ignition times were determined using electronically excited CH* and OH* emissions and reflected shock pressure monitored through the shock-tube sidewa...
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Shock-Tube Measurements of Toluene Ignition Times and Radical Chemiluminescent Spectra at Low Pressures Changhua Zhang,† Ping Li,*,† Junjiang Guo,‡ and Xiangyuan Li‡ †

Institute of Atomic and Molecular Physics, and ‡College of Chemical Engineering, Sichuan University, Chengdu 610065, People’s Republic of China ABSTRACT: Autoignition delay time measurements were performed for toluene/oxygen/argon mixtures at pressures of approximately 1.0 and 3.0 atm, temperatures of 1312−1713 K, oxygen mole fractions of 1.8−18.0%, and equivalence ratios of 0.5, 1.0, and 2.0 in a shock-tube facility. Ignition times were determined using electronically excited CH* and OH* emissions and reflected shock pressure monitored through the shock-tube sidewall. The dependence of the ignition delay times upon pressure, oxygen mole fraction, and equivalence ratio has been characterized. An empirical correlation for the ignition delay has been deduced by linear regression of the ignition data. Experimental results are compared to simulations of three recent chemical kinetic mechanisms for the oxidation of toluene. The overall trends are captured fairly well by the mechanisms. In addition, the important reaction pathways have been elucidated by both flux and sensitivity analyses. Ultraviolet and visible chemiluminescence of toluene combustion were measured using an intensified charge-coupled device camera coupled with a spectrometer. The transient spectra show remarkably high intensities of OH*, CH*, and C2* electronic emission bands. Rotational and vibrational resolution spectra of OH*, CH*, and C2* were clearly observed behind reflected shock waves. It was found that the peak intensity ratios of OH*/CH* and C2*/CH* are strongly related to the equivalence ratio.

1. INTRODUCTION Most commercial liquid transportation fuels are mixtures of many hydrocarbon species of variable composition (n-alkanes, isoalkanes, cycloalkanes, alkenes, and aromatics),1 and the chemical kinetic processes occurring in combustion of such fuels at high temperatures have not been sufficiently validated. For this reason, typically surrogate mixtures made up of several components from the various hydrocarbon components are chosen to mimic the properties of the complex fuels.2−4 In several proposed surrogate mixtures for gasoline, jet fuel, and diesel, toluene has been chosen as the representative aromatic component. Toluene is also formed from the oxidation and pyrolysis of hydrocarbons; its concentration is usually large in fuel-rich combustion processes, leading to the formation of polycyclic aromatic hydrocarbons (PAHs) and soot.5−7 The high-temperature combustion of toluene is an area of intense research in the combustion community not only because of its abundance in commercial fuels but also because toluene is one of the simplest derivatives of benzene and its chemistry is a foundation for large aromatics formation. Ignition and oxidation experiments of toluene have been performed in rapid compression machines (RCMs),8,9 jetstirred reactors,10−12 and shock tubes.13−18 The dependence of the ignition delay time upon the temperature, pressure, and composition is critical in describing the combustion characteristic of fuels and can provide excellent validation targets for the refinement of kinetic mechanisms. Roubaud et al.8 measured the toluene ignition delay at 907 K and 20 bar in RCMs. Mittal et al.9 measured ignition times for toluene/O2/inert mixtures in RCMs from 25 to 45 atm and found that the existing mechanisms10,11,14,19−21 for toluene fail to predict the experimental data with respect to the ignition delay and heat release. Davidson et al.15 measured ignition times for toluene/ © 2011 American Chemical Society

air mixtures in shock tubes at pressures from 14 to 59 atm and found that the activation energy was significantly different for two equivalence ratios. They pointed out that two toluene models (Pitz et al.22 and Dagaut et al.23) significantly overpredict the ignition delay times at Φ = 1. Shen et al.18 measured the autoignition of toluene/air mixtures at temperatures of 1021−1400 K and pressures of 10−61 atm behind reflected shock waves. The ignition results disagree with the findings by Davidson et al.15 and Mittal et al.,9 and the results were compared to the predictions of the three kinetic models (Pitz et al.,24 Andrae et al.,25 and Sakai et al.26). A new ignition study by Vasu et al.17 refuted the finding by Shen et al.;18 the new ignition times show good agreement with the earlier data by Davidson et al.15 and show differences to the RCM data by Mittal et al.9 Ignition delay times and OH radical concentration profiles of toluene/O2/Ar mixtures were measured by Vasudevan et al.16 behind reflected shock waves with pressures of 1.5−5.0 atm and low fuel fractions of 0.025−0.5%; a global correlation for ignition delay time was proposed. The radicals, such as the hydroxyl radical (OH) and methylidyne radical (CH), are important intermediates in hydrocarbon combustion chemistry. OH is a reliable indicator of flame zones, flow structure, and temperature. On the other hand, the CH radical is involved in a wide range of combustion chemistry because of its rapid addition and subtraction reactions with numerous species.27,28 The CH radical can easily enter into the carbon−hydrogen bond, which is an important process in hydrocarbon growth chemistry, leading to soot particles. A detailed knowledge about the production and Received: October 17, 2011 Revised: December 19, 2011 Published: December 23, 2011 1107

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respectively. The pressure and light signals were transferred and recorded by a digital phosphor oscilloscope (Tektronix TDS5054B). The ignition delay time in this study is defined as the time interval between the arrival of the reflected shock wave detected by the pressure transducer and the onset of CH* or OH* emissions from the sidewall. An example ignition delay time measurement is shown in Figure 1. The rise of CH* and OH* emissions and the sidewall

the destruction of the intermediate radicals is necessary to understand the fundamental combustion chemistry. Chemiluminescent emissions of intermediate radicals have been recognized as an interesting alternative to monitor and control the combustion process. It has been shown that the emissions from electronically excited states of intermediate radicals, such as OH, CH, and C2, in hydrocarbon combustion can be related to the chemical reaction rate and heat release rate.29−33 The ratio of the OH/CH signal in premixed flames shows a strong dependence upon the equivalence ratio and very little change with respect to all other parameters.33 In contrast to OH/CH, the role of C2 in the combustion process remains relatively unclear in the hydrocarbon combustion mechanisms. In this work, we report the autoignition delay results of gasphase toluene/oxygen/argon mixtures in a shock tube. Experiments were performed at high temperatures and low pressures with various oxygen mole fractions and equivalence ratios. The results are compared to simulations of recent published mechanisms developed by Andrae et al.,34 Sakai et al., 35 and Narayanaswamy et al. 36 To investigate the chemiluminescent characteristic of toluene combustion behind reflected shock waves, ultraviolet−visible (UV−vis) emission spectra have been obtained using a transient spectrum measurement system. To our knowledge, the high-resolution radical emission spectral measurements presented here are the first performed in a shock tube.

Figure 1. Example of toluene/O2/Ar experimental pressure and radical emission profiles showing the determination of the ignition delay time. pressure occur at almost the same time, showing the same ignition delay times. Although the rising step of the CH* and OH* signals matches very well, the OH* radical survives longer than the CH* radical. Because of the uncertainty in incident shock-wave velocity measurements, the uncertainty in determining the ignition time from the pressure and emission, and the uncertainty in the temperature and pressure change by non-ideal gas dynamic effects, the ignition time uncertainty is estimated to be less than ±20% in the current work. To validate our methodology, ignition times of n-heptane have been measured and the results are in good agreement with experiments by Horning et al.37 The comparison is presented in Figure 2.

2. EXPERIMENTAL SECTION Experiments were carried out in a shock tube with a constant inner diameter of 100 mm. The shock tube is built of stainless steel and is comprised of a 4.0 m driver section and a 5.0 m driven section. Two sections were separated by a double diaphragm. Polycarbonate diaphragms with thicknesses of 0.025 and 0.05 mm were used to obtain the reflected shock pressures of 1 and 3 atm, respectively. In the current work, helium gas was used as the driver gas. The shock tube was evacuated to ∼10−2 Torr with a vacuum pump system prior to each experiment. Toluene/oxygen/argon mixtures were made in a 40 L stainless-steel mixing tank. Chemically pure toluene was used to prepare the mixture. Prior to preparing the mixture to be investigated, the mixer and delivery lines were evacuated to ∼10−2 Torr. Toluene was injected into the mixing tank; after evaporation, oxygen and argon gases were added to prepare the desired fraction. The initial pressure of gas-phase fuel and oxygen were measured by a thin-film capacitance manometer. Dilute argon gas was monitored by a pressure gauge. To ensure consistency and homogeneity, sufficient time was allowed to elapse to ensure adequate mixing. To avoid saturation and adsorption effects, toluene partial pressures were kept significantly below the room-temperature saturation vapor pressure. The incident shock velocity is measured by three piezoelectric pressure transducers (PCB 113B) with rise times of ≤1 μs spaced over the last 50 cm of the test section. The transducers are flush-mounted in the shock-tube sidewall. The two incident shock velocities are linearly extrapolated to the shock-tube endwall to determine the incident shock velocity. Typical attenuation rates for the experiments ranged from 0.7 to 2.0%/m. The ignition temperature T5 and pressure P5 of the reflected shock wave were determined by the onedimensional normal shock equations using the measured initial temperature T1 and pressure P1 in the driven section, the measured incident shock wave velocity, and the thermodynamic properties of the reactant mixture. The light emission during the combustion process was exported using two quartz optical fibers located at the same crosssection as the last pressure transducer, being 15 mm away from the shock-tube endwall. Two monochromators coupled with photomultiplier tubes were set to 431 nm to detected the CH* chemiluminescence from the A2Δ−X2Π transition and 307 nm to the OH* chemiluminescence from the A 2Σ+−X2Π transition,

Figure 2. Comparison of measured ignition delay times of n-heptane with results by Horning et al.37 at 2.0 atm, 4% O2, and Φ = 1.0. A time-gated 256 × 1024 element intensified charge-coupled device (CCD) camera (Princeton Instruments) coupled with a spectrometer (Acton Spectra-Pro-275) was used to record the transient spectra during the combustion process. The emission light was exported by a quartz optical fiber, dispersed by the spectrometer, and finally detected by the intensified CCD camera with a detection range from 200 to 900 nm. The 1200 g/mm and 150 g/mm gratings were used to acquire the UV−vis electronic spectrum with different resolutions. The spectrometer was calibrated using a mercury lamp. The trigger, exposure time, and camera gain were carefully manipulated to obtain the transient spectrum. 1108

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3. RESULTS AND DISCUSSION 3.1. Ignition Delay Times. Ignition delay times for toluene/oxygen/argon mixtures have been measured at temperatures of 1312−1713 K, pressures of 1.0−3.0 atm, and oxygen mole fractions of 1.8−18.0%, with equivalence ratios of 0.5, 1.0, and 2.0. A total of 73 data are summarized in Table 1. Table 1. Ignition Delay Times of Toluene P5 (atm) 0.98 0.99 1.03 1.01 1.05 1.07 1.02 1.04 1.09 1.08 1.07 1.05 0.97 0.99 0.99 0.97 1.00 0.99 1.00 1.03 1.05 1.04 1.03 1.04 1.05 2.69 2.70 2.78 2.83 2.92 2.85 2.82 2.88 2.86 2.94 2.93 2.95 2.98 3.07 3.07

T5 (K)

delay time (μs)

18.0% O2, Φ = 0.5 1312 1791 1348 1178 1367 1112 1376 929 1418 612 1427 522 1447 455 1450 337 1489 224 1518 201 1544 138 1574 95 1.8% O2, Φ = 1.0 1417 1895 1444 1460 1460 1276 1483 1013 1519 718 1524 663 1547 478 1582 367 1597 355 1605 224 1644 174 1656 155 1711 91 1401 1649 1423 984 1430 822 1454 773 1476 599 1483 520 1494 497 1508 389 1521 376 1535 322 1549 255 1565 233 1592 192 1621 128 1634 102

P5 (atm) 1.16 1.20 0.99 1.06 1.12 1.16 1.18 2.85 2.91 2.84 2.82 3.05 2.91 2.95 3.06 3.10 0.94 0.95 0.97 0.98 0.93 0.95 0.97 0.96 0.97 0.97 0.96 1.00 1.06 1.02 1.04 1.05 1.08

T5 (K)

delay time (μs)

9.0% O2, Φ = 1.0 1388 1620 1419 1236 1457 771 1510 444 1587 240 1647 120 1662 114 1355 1138 1374 834 1395 661 1400 615 1454 375 1482 213 1503 194 1568 130 1580 103 4.5% O2, Φ = 2.0 1451 1627 1458 1411 1477 1494 1486 1246 1495 860 1501 944 1532 778 1543 719 1576 474 1583 513 1593 407 1625 257 1641 234 1653 167 1678 148 1685 123 1713 107

Figure 3. Effect of the pressure on the ignition time and comparison of experimental data to mechanisms.

the measured data, as presented latter in eq 1. Ignition times have been found to scale with a relationship τ ∼ P−0.64. In Figure 3, all data have been normalized to P = 1.0 or 3.0 atm. The ignition delays decrease with an increasing post-reflected shock pressure, which is commonly observed for hightemperature hydrocarbon ignition. Figure 3 also shows the comparison between the current ignition data and the models by Andrae et al.,34 Sakai et al.,35 and Narayanaswamy et al.36 The mechanism by Andrae et al. underpredicts the ignition delay time. Both mechanisms by Sakai et al. and Andrae et al. predict a weaker dependence of the ignition time upon pressure. The mechanism by Narayanaswamy et al. agrees well with the current results. The initial oxygen mole fraction normally has a marked effect on the magnitude of the ignition delay times. Experiments were devised to test the dependence upon the oxygen fraction by increasing the oxygen fraction from 1.8 to 9.0% and keeping the equivalence ratio at 1.0. The results presented in Figure 4 show

Figure 4. Effect of the oxygen mole fraction on the ignition time and comparison of experimental data to mechanisms.

that the ignition delay time decreases with an increasing oxygen mole fraction, therefore showing negative oxygen mole fraction dependence. This is quite a typical behavior for hydrocarbon oxidation at high temperatures. This trend is close to that predicted by mechanisms. However, all mechanisms predicted a stronger dependence of the ignition time upon the oxygen mole fraction. Another important factor affecting the ignition delay is the equivalence ratio. In this work, ignition delay times for toluene/ oxygen/argon mixtures have been measured at equivalence

Figures 3−7 present the ignition delay times for toluene/ oxygen/argon mixtures. In the temperature range investigated, the ignition time measurements show exponential dependence upon the inverse temperature. This denotes an Arrhenius behavior that is generally observed for other hydrocarbons.37 Figure 3 shows the effect of the pressure on the ignition delay time as a function of the post-reflected shock temperature T5 for a stoichiometric mixture with an oxygen mole fraction of 9.0%. Pressure variations from the average pressure have been corrected with a power law from a regression analysis of all of 1109

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ratios of 2.0, 1.0, and 0.5 with the pressure of 1.0 atm. As shown in Figure 5, decreasing the equivalence ratio causes a significant

Figure 6. Experimental data scaled to P = 1.0 atm, XO2 = 9.0%, and Φ = 1.0 using correlation.

Current high-fuel concentration ignition data are compared to the low-fuel concentration results by Vasudevan et al.,16 which scaled to the same condition using their correlation. As seen from Figure 7, the comparison shows good agreement.

Figure 5. Effect of the equivalence ratio on the ignition time and comparison of experimental data to mechanisms.

reduction in the ignition delay times, showing a positive equivalence ratio dependence. This phenomenon was already observed for linear alkanes.37 The explanation given for linear alkanes can be used here to explain the trend. It has been suggested that, prior to ignition, there is a competition between the fuel and molecular oxygen for the H reaction,38 as shown by the following two reactions:

H + O2 → OH + O

(R1)

H + C6H5CH3 → C6H5CH2 + H2

(R2)

Reaction R1 is a highly effective chain-branching reaction that accelerates the formation of two reactive radicals, while reaction R2 transforms a reactive radical H into less reactive species. As the oxygen mole fraction increases with decreasing equivalence ratios, R1 is promoted and R2 is inhibited. Thus, with a decreasing equivalence ratio, the reactivity of the mixture increases and the ignition delay times decrease. This behavior is also observed from the predictions of mechanisms, as shown in Figure 5. At low pressures, the activation energy does not vary with the equivalence ratio. This phenomenon is consistent with the low-pressure results by Vasudevan et al.16 but in contrast with the high-pressure results by Davidson et al.,15 where they found that the activation energy was significantly different for two equivalence ratios. A multiple regression analysis has been performed for all data collected in this work, and a regression expression defining the experimental ignition delay time of toluene has been obtained −5 −0.64

τ = 2.71 × 10 P

exp(46970/RT )

XO2

Figure 7. Comparison between experimental ignition times and results from mechanisms. P = 1.0 atm, XO2 = 9.0%, and Φ = 1.0.

Figure 7 also presents the predictions of three recent mechanisms. The mechanism by Andrae et al. underpredicts the ignition delay time. For quantitative comparison, similar correlations were obtained from the predicted results of these mechanisms. The correlation formulas are as follows. Narayanaswamy et al.:

τ = 4.04 × 10−6P −0.63XO2−0.67 Φ0.76 exp(48290/RT )

−0.36 0.65

Φ

(2)

Sakai et al.:

τ = 1.00 × 10−6P −0.39XO2−0.56Φ0.40

(1)

where τ is the ignition delay time in microseconds, P is the pressure behind the reflected shock wave in atmospheres, XO2 is the oxygen mole fraction, Φ is the equivalence ratio, Ea is the global activation energy (=46.97 kcal/mol), T is the temperature behind the reflected shock wave in Kelvin, and R is the universal gas constant (=1.986 × 10−3 kcal mol−1 K−1). The ignition times are scaled to a common condition (P = 1.0 atm, XO2 = 9.0%, and Φ = 1.0) shown in Figure 6, illustrating the adequacy of the correlation for representing the data over the range of experimental conditions (r2 = 97.6%).

exp(59290/RT )

(3)

Andrae et al.:

τ = 4.24 × 10−7P −0.40XO2−0.74 Φ0.49 exp(53264/RT )

(4)

The effects of the pressure and equivalence ratio on the ignition delay of the mechanism by Narayanaswamy et al. are τ ∼ P−0.63 and τ ∼ Φ0.76, which agree well with our experiment results τ ∼ 1110

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P−0.64 and τ ∼ Φ0.65, respectively. The other two mechanisms underpredicted these effects. All three mechanisms overpredicted the effect of the oxygen mole fraction. Both mechanisms by Sakai et al. and Andrae et al. predict higher global activation energy. The global activation energy of the mechanism by Narayanaswamy et al. is 48.2 kcal/mol, which agrees with the current measurement of 47.0 kcal/mol. To understand the chemistry controlling the autoignition of toluene under shock-tube conditions, an analysis of the reaction flux was carried out for a stoichiometric mixture of 9% oxygen in Ar at 1.0 atm and 1400 K. As seen in Figure 8, benzyl radical

noxy radical (o-, m-, and p-OA1CH3), and cresol (o-, m-, and pHOA1CH3), respectively. The reaction flux analyses of the benzyl radical, methylphenyl, benzene (A1), cresol, and methylphenoxy radical are illustrated in Figure 9. Obviously, the benzyl radical, methylphenyl, cresol, and methylphenoxy radical can mainly change to benzene or phenyl radical (A1−), which decomposed into several productions containing five-membered rings. The rings of these productions are opened to form smaller molecules and free radicals. To identify important reactions relevant to ignition, sensitivity analysis has been performed for toluene at 1400 K equivalence ratios of 0.5, 1.0, and 2.0 using the mechanism by Narayanaswamy et al. For the sensitivity analysis on the ignition delay (τ), the rate constant of each reaction, ki, is individually doubled. The change in the ignition delay, [τ(2ki) − τ(ki)]/ τ(ki), is taken as the sensitivity of that particular reaction. Figure 10 demonstrates such a sensitivity analysis. A positive

Figure 8. Reaction flux analysis for toluene using the mechanism by Narayanaswamy et al.36 at Φ = 1.0, 9% oxygen, 1400 K, and 1.0 atm.

(A1CH2) and three isomers of methylphenyl (o-, m-, and pA1CH3*) are produced from toluene through hydrogenabstraction reactions and benzene can be generated through a hydrogen atom substituting the CH3 group of toluene. A bit of methylphenoxy radical (o-, m-, and p-OA1CH3) and cresol (o-, m-, and p-HOA1CH3) can also be formed. In Figure 8, lumped {A1CH3*}, lumped {OA1CH3}, and lumped {HOA1CH3} represent methylphenyl (o-, m-, and p-A1CH3*), methylphe-

Figure 10. Sensitivity of different reactions to the ignition delay times, calculated with the mechanism by Narayanaswamy et al.36 for toluene at P = 1.0 atm, T = 1400 K, and XO2 = 9.0%, with Φ = 0.5, 1.0, and 2.0.

Figure 9. Reaction flux analyses for the benzyl radical, methylphenyl, benzene, cresol, and methylphenoxy radical using the mechanism by Narayanaswamy et al.36 at Φ = 1.0, 9% oxygen, 1400 K, and 1.0 atm. 1111

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A high-resolution spectrometer was used to check the spectral features of CH*, OH*, and C2* radicals. Detailed spectra are shown in Figure 12. The OH (0,0) bands at 306.8 nm (R2) and 309.1 nm (Q2) and the OH (1,1) bands at 312.3 nm (R1) were clearly observed. The asterisked peak at 314.4 nm is proven to be the CH (0−0) B2Σ−−X2Π band head. The Q branch of CH (0,0) at 431.4 nm was seen as a band head, and the weak Q branch of CH (2,2) at 432.3 nm was also observed. In the C2* spectrum, the vibrational bands of (0,0), (1,1), and (2,2) were clearly observed.

sensitivity implies that ignition delay increases with an increasing rate constant of a particular reaction. The most sensitive reaction is the H + O2 chain-branching reaction. Both reactions R1 and R2 exhibit very high sensitivity; they have opposite effects on the ignition delay prediction. Also important are the toluene decomposition reaction, the reactions of toluene with H, O, and OH radicals, and the reactions involving benzyl radical. The sensitivity does not vary remarkably according to the equivalence ratio, which may lead to the same global activation energy at low pressure. 3.2. Chemiluminescent Spectra. The chemiluminescent spectra of toluene/O2/Ar combustion were measured using an intensified CCD camera coupled with a spectrometer behind reflected shock waves. As presented in Figure 11, intermediate

4. CONCLUSION The autoignition of gas-phase toluene mixtures with oxygen/ argon has been studied behind reflected shock waves at high temperatures and low pressures. The effects of the pressure, temperature, equivalence ratio, and oxygen mole fraction on the ignition time have been investigated. The obtained data have been compared to simulations using three different toluene combustion models. The trends are in close agreement with that predicted by mechanisms. An empirical correlation for ignition delay has been deduced from the experimental data to be τ = 2.71 × 10−5P−0.64XO2−0.36ϕ0.65 exp(46970/RT). The major reaction pathways of toluene oxidation and the important reactions in the ignition process have been investigated by reaction flux analysis and sensitivity analysis, respectively. For the equivalence ratios studied in this work, the sensitivity does not change remarkably. Transient emission spectra of toluene combustion have been obtained. It was found that the OH*/CH* and C2*/CH* ratios are not sensitive to the ignition pressure, temperature, and oxygen fraction but are sensitive to the equivalence ratio. When the equivalence ratio increases, the C2*/CH* ratio increases and the OH*/CH* ratio decreases. High-resolution spectra of OH*, CH*, and C2* have been obtained, and the vibrational and rotational structures of these radicals have been clearly observed.

Figure 11. Emission spectra of toluene behind reflected shock waves. P = 1.0 atm, and T = 1500 K.

diatomic radicals CH*, OH*, and C2* were observed above a certain level of background emission. The OH 0−0 band (A2Σ+−X2Π at 307 nm), two CH 0−0 bands (B2Σ−−X2Π at 387 nm and A2Δ−X2Π at 431 nm) and C2 0−0 band (A3Πg− X3Πu at 516 nm) were clearly observed. Additional C2 1−0 and 0−1 bands (A3Πg−X3Πu at 474 and 564 nm, respectively) were collected as well. The absolute intensities of CH*, OH*, and C2* radicals change according to the ignition conditions. However, we found that the intensity ratios OH*/CH* and C2*/CH* were not sensitive to the ignition pressure, temperature, and oxygen fraction; only the equivalence ratio affects the intensity ratios dramatically, as shown in Figure 11. To compare the relative intensity of each peak, spectra were normalized according to the CH* signal. When the equivalence ratio increases from 0.5 to 2.0 and the C2*/CH* ratio increases from 16 to 50%, the OH*/CH* ratio decreases from 17 to 10%.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected].

ACKNOWLEDGMENTS The authors acknowledge the support from the National Natural Science Foundation of China under Grant 91016002. REFERENCES

(1) William, J. P.; Charles, J. M. Prog. Energy Combust. Sci. 2011, 37, 330−350.

Figure 12. High-resolution emission spectra of CH*, OH*, and C2* radicals. P = 1.0 atm, T = 1500 K, and Φ = 2.0. 1112

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dx.doi.org/10.1021/ef201611a | Energy Fuels 2012, 26, 1107−1113