Effect of the Methyl Substitution on the Combustion of Two

Mar 11, 2015 - Torr) burner-stabilized flat flames of 2-methylheptane and 3-methylheptane were investigated. Flame species were measured via time-of-f...
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Effect of the Methyl Substitution on the Combustion of Two Methylheptane Isomers: Flame Chemistry Using Vacuum-Ultraviolet (VUV) Photoionization Mass Spectrometry Hatem Selim,*,† Samah Y. Mohamed,† Arnas Lucassen,‡,§ Nils Hansen,‡ and S. Mani Sarathy† †

Clean Combustion Research Center, King Abdullah University of Science and Technology (KAUST), Thuwal, Saudi Arabia Combustion Research Facility, Sandia National Laboratories, Livermore, California 94551, United States

‡

S Supporting Information *

ABSTRACT: Alkanes with one or more methyl substitutions are commonly found in liquid transportation fuels, so a fundamental investigation of their combustion chemistry is warranted. In the present work, stoichiometric low-pressure (20 Torr) burner-stabilized flat flames of 2-methylheptane and 3-methylheptane were investigated. Flame species were measured via time-of-flight molecular-beam mass spectrometry, with vacuum-ultraviolet (VUV) synchrotron radiation as the ionization source. Mole fractions of major end-products and intermediate species (e.g., alkanes, alkenes, alkynes, aldehydes, and dienes) were quantified axially above the burner surface. Mole fractions of several free radicals were also measured (e.g., CH3, HCO, C2H3, C3H3, and C3H5). Isomers of different species were identified within the reaction pool by an energy scan between 8 and 12 eV at a distance of 2.5 mm away from the burner surface. The role of methyl substitution location on the alkane chain was determined via comparisons of similar species trends obtained from both flames. The results revealed that the change in CH3 position imposed major differences on the combustion of both fuels. Comparison with numerical simulations was performed for kinetic model testing. The results provide a comprehensive set of data about the combustion of both flames, which can enhance the erudition of both fuels combustion chemistry and also improve their chemical kinetic reaction mechanisms.

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INTRODUCTION The urgent need to enhance the engine combustion efficiency while meeting strict environmental safety regulations presents a significant challenge. This tradeoff dictates that researchers develop alternatives and additives to the typical engine fuels. Monomethylated alkanes have always been important components in numerous engine fuels, such as diesel,1 gasoline,2 JP-4,1 and S-8.3 Several research groups examined the combustion of iso-paraffins in general, but most of the effort was focused on investigating small hydrocarbon chains from C4 up to C6. For instance, Davis et al.4 measured the laminar flame speed of both n-butane and iso-butane. The results proved that n-butane has a higher flame speed (by almost 5 cm/s). They attributed the difference in flame speed to differences in combustion kinetics, since they have almost similar adiabatic flame temperatures and thermal diffusivities. This difference in combustion kinetics was studied earlier by Wilk and co-workers,5 where they examined the combustion of iso-butane and n-butane in an internal combustion engine. They found that the isomerization of RO2 is much faster in the case of n-butane, which leads to a higher overall reaction rate, compared to the iso-butane. Gersen et al.6 investigated the ignition delay of both normal and branched butane isomers under a range of pressures, temperatures, and equivalence ratios. The results consistently proved that nbutane has a shorter ignition delay, compared to iso-butane; especially in the low-temperature region, where the RO2 isomerization plays a significant role, but at temperatures above ∼900 K the ignition-delay time difference starts to diminish. Another ignition delay investigation of both butane isomers was performed by Ogura et al.,7 using reflected shock © 2015 American Chemical Society

waves. The results confirmed the longer ignition delay of isobutane over that of n-butane. They attributed this finding to the formation of more highly reactive atomic hydrogen in the nbutane reaction pool, compared to iso-butane. In addition, the formation of the resonantly stable species C3H5 from the hydrogen abstraction of propene, formed while burning isobutane, slows the overall reaction kinetics. Similar studies were conducted to analyze the chemical kinetics of the low-temperature combustion of methyl pentanes, e.g., the work of Cullis et al.8,9 (2-methylpentane), Barat et al.10,11 (3-methylpentane), and Fish et al.12,13 (2methylpentane). On the other hand, investigations of the combustion of 2-methylhexane varied from the examination of its ignition by Silke et al.,14 and the development of detailed reaction mechanism by Westbrook et al.15,16 Recently, a few research groups started to extensively examine the combustion of 2-methylheptane. Kahandawala et al.3 investigated the ignition delay times of surrogate fuel of synjet (2methylheptane) and JP-8 (80% n-heptane and 20% toluene) and its effects on the soot and PAHs formation. Sarathy and coworkers17 developed a comprehensive reaction mechanism for 2-methylalkanes from C7 to C20. They validated the mechanism against several experimental findings for 2-methylalkanes, such as ignition-delay time, laminar flame speed, jet-stirred reactor measurements,17 and opposed-flow diffusions flame data.18 A modified version of this mechanism will be used for the Received: December 16, 2014 Revised: March 6, 2015 Published: March 11, 2015 2696

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Figure 1. Schematic diagram of the experimental setup.

validation of the experimental findings presented in this paper, where we examined the low-pressure combustion of 2methylheptane (C 8 H 18 -2) and one of its isomers, 3methylheptane (C8H18-3). 3-Methylheptane combustion kinetics has also been investigated in shock tubes and rapid compression machines,19 jet-stirred reactors,20 opposed flow diffusion flames,21,22 and premixed laminar flames.21 The goal of the present work is to identify and quantify the major species, intermediate species, as well as isomers in premixed laminar flames of 2-methylheptane and 3-methylheptane. This work reveals the effects of the methyl substitution location on combustion chemistry of iso-paraffins.

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different energies were conducted in order to differentiate between isobaric species in the reaction zone (9.0, 9.5, 9.7, 10.0, 10.5, 11.0, 11.5, 12.3, 13.2, 14.35, 15.4, 16.2, and 16.65 eV). One energy scan was conducted, between 8 and 12 eV, at a fixed location of 2.5 mm from the burner surface for species identification. Output signals were converted to mole fractions using the methodology described elsewhere.26,27 Corrections for elements isotopes, 2H, 13C, and 18O, were done before calculating the mole fractions. Correction for fragment ions generated from fuel and some intermediate species, which their cold-gas energy scans were available from calibration, were conducted as described elsewhere.26 Photon counts were calibrated using a photodiode collecting the light after passing through the ionization volume. The quantum efficiency of the photodiode was calibrated at the National Institute for Standards and Technology (NIST). Photoionization cross sections were taken from the literature.28−38 Species mole fractions were calculated using the signal of the nearest burner scan to the species ionization threshold, to avoid fragmentation due to excess photon energy. Table 1 shows details about mole fraction calculations including the energy at which species were calculated, which isomers were considered for calculations and the references for the photoionization cross sections. Argon generally was used as the reference species for mole fraction calculations of signals obtained from photon energies above 15.6 eV, H2O was used until 13.2 eV, oxygen for 12.3 eV, ethylene until 11 eV, methyl for 10.5 eV, and 1,3-butadiene for energies below 10.5 eV. Isomers with overlapped signals were identified by the difference in their ionization energy.39,40 However, in some cases, when the interfered signals emanate from two species that have very close photoionization onsets, it was not possible to distinguish between them; this will be discussed in detail for each case in the Results and Discussion section. The energy of the synchrotron radiation was calibrated using the resonating autoionization bands of O2+ between 12 eV and 13 eV,42−44 comparing our results with the findings of Holland et al.,45 we found that eV(calibrated) = eV(measured) + 0.02 eV. Since the rate of heat loss and the probe perturbation effects are significant within the vicinity of the burner surface, the results of the first 1 mm are excluded. Subsequently, the presented mole fraction profiles were shifted toward the burner surface by 1 mm.46,47 The temperature profiles were obtained using the correlation between the perturbed temperature at

EXPERIMENT AND COMPUTATIONAL METHODS

1. Experimental. The experiments in this study were conducted at the flame endstation at the Advance Light Source (ALS) at the Lawrence Berkeley National Laboratory. The ALS provides tunable monochromatic synchrotron radiation in the UV range, which is used for photoionization of the sampled gas; details about the ALS and flame endstation are given elsewhere.23−25 A schematic of the experimental setup is presented in Figure 1. The setup consists of a McKenna burner with a diameter of 60 mm, which is used to stabilize a laminar premixed flat flame. Oxygen and fuel are premixed upstream of the burner under stoichiometric conditions and diluted with argon to 50 mol % of the total mixture. Flow rates were the same for both flames: oxygen, 1.795 slm; fuel (liquid), 1.037 mL/min; and argon, 1.95 slm. The burner is mounted on a translational stage to allow gas sampling at different sampling locations. The burner is located within a vacuum chamber at pressure of 20 Torr, where the gas sampling occurs via a 0.4 mm orifice of a quartz cone. The sampled gases molecular beam is formed and collimated by passing through the quartz and the skimmer cones, and then subjected to tunable synchrotron radiation for ionization. The ions are then analyzed by a 1.3-m time-of-flight (TOF) mass spectrometer with a mass resolution of m/Δm ≈ 3500. Quantification of flame speciation was obtained via axial scanning of the burner at fixed photon energies (burner scans). Burner scans at 2697

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Table 1. List of Species, Method of Mole Fraction Calculation, Source of PICS, and Energy at Which the Species Were Calculated (Argon Was Calculated from the Overall Mass Balance) formulaa

mass (amu)

species

method

IE (eV)

H2 CH3 CH4 H2O C2H2 C2H3 CO C2H4 HCO C2H6 CH2O O2 C3H3 C3H4-p C3H4-a C3H5 C3H6 CO2 CH3CHO C3H8 C4H6-1 C4H6-1,3 C4H8-1 C4H8-2 i-C4H8 C2H5CHO C4H10 C5H102MH C5H103MH C6H122MH C6H123MH C7H142MH C7H143MH C8H18-2 C8H18-3

2.015 15.023 16.031 18.01 26.016 27.024 27.995 28.031 29.003 30.047 30.011 31.99 39.023 40.031 40.031 41.039 42.047 43.99 44.026 44.063 54.047 54.047 56.063 56.063 56.063 58.042 58.078 70.078 70.078 84.094 84.094 98.11 98.11 114.141 114.141

hydrogen methyl methane water acetylene vinyl radical carbon monoxide ethylene formyl radical ethane formaldehyde oxygen propargyl propyne allene allyl radical propene carbon dioxide acetaldehyde propane 1-butyne 1,3-butadiene 1-butene 2-butene iso-butene propanal n-butane 1-pentene 3-methyl-3-butene 4-methyl-1-pentene 1-hexene 1-heptene 2-methyl-1-hexene 2-methylheptane 3-methylheptane

H atom mass balance C2H4 reference species H2O reference species H atom mass balance H2O reference species C4H6 reference species C atom mass balance H2O reference species C4H6 reference species H2O reference species C2H4 reference species O atom mass balance C4H6 reference species CH3 reference species C4H6 reference species C4H6 reference species H2O reference species C atom mass balance CH3 reference species H2O reference species CH3 reference species H2O reference speciesf C4H6 reference species C4H6 reference species C4H6 reference species C4H6 reference species C2H4 reference species CH3 reference species CH3 reference species CH3 reference species CH3 reference species C4H6 reference species C4H6 reference species C2H4 reference species C2H4 reference species

15.4 9.84 12.61 12.62 11.4 8.25 14.01 10.52 8.12 11.52 10.88 12.06 8.67 10.36 9.69 8.18 9.73 13.7 10.23 10.94 10.18 9.072 9.55 9.11 9.22 9.96 10.53 9.49 9.1 N.A. 9.44 N.A. N.A. 9.75e 9.75e

PICS source

energy (eV)b

c 31 33 37 24 30 c 32 34 32 35

16.65 11 14.35 14.35 13.2 9.5 14.35 13.2 9.5 13.2 11 14.35 9.5 10.5 10 9.5 13.2 14.35 10.5 13.2 10.5 9.5 10 9.5 9.5 10 11 10.5 10.5 10.5 10.5 10 10 11.5 11.5

ref ref ref ref ref

ref ref ref ref c ref 30 ref 24 ref 38 ref 36 ref 32 c ref 63 ref 23 ref 33 ref 38 ref 33 ref 33 ref 33 ref 33 ref 33 ref 33 ref 38 estimatedd ref 38 estimatedd estimatedd c c

a The superscripts 2MH and 3MH indicate the different isomers that were considered for the calculation of the species mole fractions. bThe energy at which the time-of-flight signal was considered for the mole fraction calculations. cSpecies PICS was not used in the mole fraction calculations. d PICS was estimated based on species with similar chemical structure. eSpecies IE was determined from the energy scan of this work. fMachine constant correction was used to compensate for using two signals at two different energies (the adopted H2O signal was at 13.2 eV).

the sampling location and the pressure of the first stage behind the skimmer cone.47,48 The proportionality constant was obtained from the temperature measurement at axial distance of 30 mm, using Pt− Pt13%Rh R-type thermocouple with alumina coating and a bead diameter of 0.51 mm. Radiation correction was done by assuming an emissivity of 0.2 and a Nusselt number (Nu) of 2, considering the thermocouple junction to be a sphere.49 Uncertainty of the calculated mole fractions is dependent on several factors including the error in photoionization cross section (PICS), fragmentation patterns, and experimental uncertainties. For major species and stable intermediate species with well-known PICS, the uncertainty range is 15%−30%;41 however, for radicals and species with heavy overlaps, the uncertainty can be up to a factor of 2. 2. Computational. The thermodynamics data of 2-methylheptane and 3-methylheptane were recalculated with new group values,50 using the THERM software.51,52 Attention was given to the optical isomers and the effect of the non-next-nearest neighbor interactions (NNI) (gauche interaction), as proposed by Sabbe et al.53,54 The presence of the branched methyl group in 2-methylheptane (or 3-methylheptane) adds one alkane gauche interaction (AG) between the tertiary and the adjacent secondary carbon site illustrated by carbon 2 and carbon 3, respectively in Figure 2. However, the radicals of these species are

Figure 2. Structure of 2-methylheptane and 3-methylheptane. treated differently based on the radical site. If a radical exists in either C2 or C3, the radical is neglected and one gauche is assumed. This is termed radical gauche 1 interaction (RG1). Radical sites which neighbor C2 or C3 (C1 and C4, respectively) and form a 60° dihedral angle with the branched methyl lead to radical gauche 2 interaction (RG2). Here, the number of gauche interactions that are taken into account is, AG − RG2; thus, no gauche effect is added. Radicals on the remaining sites have a negligible effect on gauche correction. The chemical kinetics mechanism was also modified. The base chemistry had been updated using the recent AramcoMech 1.3 from Metcalfe et al.,55 as used by Sarathy et al.56 Moreover, the H atom abstraction reaction by OH was updated using the next-nearestneighbor (NNN) estimation method proposed by Cohen et al.57 This method assigns site-specific rate constant based on the number of the 2698

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Figure 3. Rate constant of H atom abstraction by OH from primary, secondary, and tertiary sites.

Figure 4. Mole fraction profiles of the major species of 2-methylheptane flame; symbols represent experimental data and lines represent numerical data.

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Figure 5. Mole fraction profiles of the major species of 3-methylheptane flame; symbols represent experimental data and lines represent numerical data.

Figure 6. Mole fraction profiles of alkanes (C1−C4) of 2-methylheptane flame (left) and 3-methylheptane flame (right); symbols represent experimental data and lines represent numerical data. stoichiometric conditions, 20 Torr of pressure with 50% dilution of argon. The flame temperature was specified using the temperature profiles obtained experimentally. Simulations accounted for thermal diffusion and were resolved with over 250 grid points (GRAD 0.1 and CURV 0.2). In the following sections, the agreement between the simulations and experiments is discussed qualitatively and quantitatively. The simulation is in good qualitative agreement if the shape of the species profile closely matches the experimental profile, and good quantitative agreement is obtained if the predicted maximum mole fraction is within a factor 2 of the measured maximum mole fraction.

next neighbor of the specific site under consideration. The subscripts in Figure 2 show the NNN of each site. The rate constants of P1, P2, S 01 , S 11′ , and S 11 are obtained from the measurements of Sivaramakrishnan et al.58 at an extended range of temperatures, whereas S02, S21 T001, and T101 are obtained from the high-temperature measurements in the work of Badra et al.59 Figure 3 shows a comparison between updated rate constants and those used in the original mechanism by Sarathy et al.,17 which are originally from Orme et al.60 The plots show a close comparison (maximum 20% difference) of site-specific rate rules to the general rate rules, particularly in secondary rates for the full range of temperature, while abstractions from primary and tertiary sites show up to 50% difference in rates especially at high temperature. Further validations for the new model against flame speeds of previous findings, the updated rate rules of OH + fuel, and the updated reaction model are included in the Supporting Information. Simulations were conducted using the PREMIX code in CHEMKIN PRO software.61 For both fuels, flame was set under

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RESULTS AND DISCUSSION 1. Major Species. Major species mole fraction profiles (Ar, C8H18-2 or C8H18-3, O2, CO, CO2, H2O, and H2) in 2methylheptane and 3-methylheptane flames are presented in Figures 4 and 5, along with the temperature profiles. No major differences appear for the major species profiles between the 2700

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Figure 7. Mole fraction profiles of C2H2, C2H4, and C3H6 of 2-methylheptane flame (left) and 3-methylheptane flame (right); symbols represent experimental data and lines represent numerical data.

Figure 8. Reaction pathway diagram for 2-methylheptane oxidation at T = 815 K (italicized text) and T = 1200 K (bold text).

two flames, and thus their global reactivity is similar. These results are consistent with previous studies that demonstrated similar experiments for 2- and 3-methylheptane in premixed22 and counterflow diffusion21 flames. The argon mole fraction shows a consistent decrease until ∼3 mm, which is due to the increase in the intermediate species mole fractions. An almostconstant profile is shown onward with a minimal increase. Numerical simulations showed acceptable agreement in

comparison to the experimental results for all of the major species. 2. Intermediate Species. The intermediate species presented in this section are the stable intermediate hydrocarbons which are formed due to the breakdown of the long hydrocarbon chain in the reaction pool. 2.1. Alkanes. Small alkanes measured in both flames are presented in Figure 6. Good agreement between numerical and experimental data was attained specifically in terms of trends. 2701

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Figure 9. Reaction pathway diagram for 3-methylheptane oxidation at T = 820 K (italicized text) and T = 1190 K (bold text).

Figure 10. Mole fraction profiles butene isomers of 2-methylheptane flame (left) and 3-methylheptane flame (right); symbols represent experimental data and lines represent numerical data.

2.2. Alkenes and Alkynes. Combustion of both flames produced a series of alkenes from C2 up to C7, in addition the formation of acetylene was evident. Figure 7 presents the mole fractions of acetylene, ethylene, and propylene of both flames. No prominent differences were observed between the two flames for C2H2 or C2H4 mole fractions. However, the major

From a quantitative point of view, the model successfully captured the mole fractions of CH4, C2H6, and C4H10; however, the mole fractions of C3H8 was underestimated. Identification of the butane isomers was not achievable, because of the near ionization energy of normal butane and iso-butane.62,63 2702

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Figure 11. Mole fraction profiles of C5−C7 alkenes of 2-methylheptane flame (left) and 3-methylheptane flame (right); symbols represent experimental data and lines represent numerical data.

mole fraction in 3-methylheptane flame. The photoionization efficiency (PIE) revealed important information about the type of isomers of some of the species such as C5H10, as shown in Figure 12. For 2-methylheptane flame, the ion signal onset was

remark to be noticed is that C3H6 maximum concentration for 2-methylheptane flame is almost double in comparison with that for 3-methylheptane flame. This same trend is also captured by the numerical simulations. Figure 8 shows the strong channel created by the H-abstraction from C1 in 2methylheptane to eventually form C3H6 and C3H7, while the analogous reaction sequence in Figure 9 is weaker for 3methylheptane, especially at high temperatures. Formation of C4H8 isomers was also evident in the results; and with the additional information provided by the energy scan, we managed to calculate the mole fractions of each C4H8 isomer. For the 2-methylheptane flame, the H-abstraction from C2 or C5 followed by C3−C4 scission can form 1-butene and iso-butene; while for 3-methylheptane, C3−C4 bond cleavage can form 1-butene and 2-butene. Accordingly, Figure 10 presents the mole fractions of butene isomers of each flame. The results are in good agreement with the numerical simulations. Formation of higher alkenes C5−C7 also was evident as shown in Figure 11. All three profiles are showing high mole fraction near the burner surface, which might be attributed to some species fragmentations, which are not experimentally detectable. In addition, the results at 0 mm are actually measured at 1 mm, but the data was shifted toward the burner. The prominent difference between the two flames is that for 2methylheptane C5H10 is the dominant among the three alkenes; on the contrary, C6H12 possess the highest mole fraction in case of 3-methylheptane flame. This trend was also verified numerically, but quantitatively the mole fractions were underestimated by more than a factor of 2 for the 2methylheptane flame and a factor of 3 for the 3-methylheptane flame. The mechanistic pathways shown in Figures 8 and 9 interpret this difference between the two fuels, where the outcome of the H-abstraction from C4 mostly forms C5H10, in the case of the 2-methylheptane flame. However, with the change in the methyl group position in 3-methylheptane, the βscission forms mostly C6H12. For 2-methylheptane flame, C6H12 mole fraction is the lowest; this can be credited to the fact that it requires the scission of a strong primary carbon− carbon bond. For the same reason, C5H10 showed the lowest

Figure 12. Photoionization efficiency curve for different C5H10 isomers.

attained at ∼9.5 eV, which is credited to the formation of 1pentene at 9.49 eV33 and/or 2-methyl-3-butene at 9.52 eV.38 The contribution of each isomer was not distinguished, because of their close IE. In the case of 3-methylheptane, the signal onset was observed at ∼9.1 eV, which is mainly attributed to the presence of 3-methyl-3-butene.38 This is confirmed by the reaction pathways diagrams of both fuels. The PIE curve with the contribution of each isomer is included in the Supporting Information. 2.3. Oxygenated Compounds (Aldehydes, Ketones, Allylic Alcohols, and Enols). Figure 13 shows the PIE curve of m/z = 58 for both flames, where several oxygenated species existed. 2703

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In the case of other oxygenated species, determination of isomers was not possible, because of the similar ionization thresholds and the numerous possible isomers. For example, IE of acetone is 9.7 eV and 2-propen-1-ol has 9.67 eV. Subsequently, the signal obtained is possibly a contribution of both species, even though acetone is expected to be more dominant. 2.4. Dienes. The final group of intermediate stable species that we identified was dienes and many of their isomers. Figure 15 shows the photoionization efficiency curve of C4H6 isomers

Figure 13. Photoionization efficiency curve for m/z = 58 isomers.

Formation of oxygenated species has been evident in flame studies as intermediate species, especially formaldehyde, acetaldehyde, ethenol, and acetone. However, enols were only observed in flames in the past decade.64 Different reaction pathways were presented in the literature to highlight the onset of the oxygenated species in flames of nonoxygenated fuels; for instance, formation of formyl radical due to the reaction of atomic oxygen and ethylene,65 reaction of methoxy radical with alkane to form alcohol,65 reaction between alkene and hydroxyl radical to form enols,66 and enol tautomerization to form ketones.67 Figure 14 describes the aldehydes formed in the reaction pool of both flames. The results revealed that the mole fraction of formaldehyde is 1 order of magnitude higher than that of other aldehydes. The numerical simulations successfully capture the trends of experimental data, especially for formaldehyde. However, the predicted results underestimated propanal.

Figure 15. Photoionization efficiency curve for C4H6 isomers.

in both flames, where the results showed that 1,2-butadiene and 2-butyne did not form in the reaction pool, while 1,3-butadiene and 1-butyne are the dominant isomers. This trend has been found in previous studies, even though the fuels that have been used were different; cyclopentene,68 benzene,69 allene,70 and propane.27 Mole fractions of C4H6 isomers are shown in Figure 16. The model does not differentiate between C4H6 isomers, thus the comparisons between experiments and numerical

Figure 14. Mole fraction profiles of aldehydes of 2-methylheptane flame (left) and 3-methylheptane flame (right); symbols represent experimental data and lines represent numerical data. 2704

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Figure 16. Mole fraction profiles of C4H6 isomers of 2-methylheptane flame (left) and 3-methylheptane flame (right); symbols represent experimental data and lines represent numerical data.

Figure 17. Mole fraction profiles of C3H4 isomers of 2-methylheptane flame (left) and 3-methylheptane flame (right); symbols represent experimental data and lines represent numerical data.

fractions qualitatively; however, the C3H5 mole fraction was overestimated quantitatively. The resonantly stable radical C3H5 showed a signal onset at ∼8.2 eV, and thus it is identified as an allyl radical (H2CC−CH2) with IE of 8.18 eV.30 Formation of C3H5 can take place through the reaction of C4H6 with atomic oxygen to form C3H5 and HCO,72,73 and C3H6 reaction with atomic hydrogen.

predictions were shown for the lumped summation of C4H6. The results revealed that the mole fraction of C4H6 was underestimated by a factor of 2. Separation of the C3H4 isomers, allene (H2CCCH2) and propyne (H3C−CCH), has been achieved in previous findings,71 where an abrupt increase in the signal from C3H4 is obtained at ∼10.3 eV, which is the IE of propyne. Figure 17 describes the mole fractions of both allene and propyne isomers. Numerical results showed better agreement for the 2methylheptane flame. 3. Radicals. Several important radicals within the reaction pool were determined using TOF mass spectrometry. Figure 18 present the significant radicals obtained within the reaction pool of both flames. The trends of radical species are similar between both flames where CH3 and C3H3 have the highest mole fractions. Numerical simulations well predict both mole

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CONCLUSIONS Flame chemistry of stoichiometric 2- and 3-methylheptane flames was investigated using a time-of-flight (TOF) mass spectrometer and vacuum-ultraviolet (VUV) synchrotron ionization. Comparisons of end-products, intermediate species, and free radicals were conducted with a specific focus on the role of the methyl branch on the n-heptane. Results of the major species did not show prominent differences between 2705

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Figure 18. Mole fraction profiles of the radicals obtained from 2-methylheptane flame (left) and 3-methylheptane flame (right); symbols represent experimental data and lines represent numerical data.

both flames. Intermediate species of alkanes, alkenes, alkynes, aldehydes, ketones, enols, and dienes were identified for both flames. The high-resolution TOF mass spectrometer allowed us to segregate the peaks of alkenes, some oxygenated species, and radicals. However, separation of oxygenated species was not achievable in several cases due to the numerous possible isomers and/or the close ionization energies. Several free radicals were also obtained from both flames (e.g., CH3, HCO, C2H3, C3H3, and C3H5). The results proved major differences between intermediate species mole fractions and isomers of both flames, which are mainly attributed to the change in the methyl group location on the normal chain. Finally, numerical comparison showed good agreement with the end-products in particular. However, some discrepancies were found for the intermediate species and the free radicals.

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the Director, Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy, under Contract No. DEAC02-05CH11231. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the National Nuclear Security Administration under Contract No. DE-AC04-94-AL85000.

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ASSOCIATED CONTENT

S Supporting Information *

This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Present Address §

Department for Thermophysical Quantities, Physikalisch Technische Bundesanstalt (PTB), 38116 Braunschweig, Germany.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge funding support from the Clean Combustion Research Center and from Saudi Aramco, under the FUELCOM program. The measurements were performed within the “Flame Team” collaboration at the Advanced Light Source (ALS), Lawrence Berkeley National Laboratory, Berkeley, USA, and we thank the students and postdocs for the help with the data acquisition. The experiments at the Advanced Light Source (ALS) have profited from the expert technical assistance of Paul Fugazzi. The ALS is supported by 2706

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