A Shock Tube Study of the Ignition of n-Heptane, n-Decane, n

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A Shock Tube Study of the Ignition of n-Heptane, n-Decane, n-Dodecane, and n-Tetradecane at Elevated Pressures Hsi-Ping S. Shen, Justin Steinberg, Jeremy Vanderover, and Matthew A. Oehlschlaeger* Department of Mechanical, Aerospace, and Nuclear Engineering, Rensselaer Polytechnic Institute, Troy, New York, USA ReceiVed December 16, 2008. ReVised Manuscript ReceiVed February 17, 2009

The ignition of n-heptane, n-decane, n-dodecane, and n-tetradecane has been investigated in a heated shock tube. n-Alkane/air mixtures at Φ ) 0.25, 0.5, and 1.0 were studied in reflected shock experiments at 9-58 atm and 786-1396 K. Ignition times were measured using a combination of endwall electronically excited OH emission and sidewall pressure measurements. The measured ignition times are compared to previous data, where available, with good agreement and to several kinetic modeling predictions. The current data and the combination of the current data with previous shock tube and rapid compression machine measurements show that any differences in reactivity for C7 and larger n-alkanes is slight, within the experimental uncertainties, for n-alkane/air mixtures with common carbon content at a large range of temperatures (650-1400 K) and elevated pressures. To our knowledge, the n-tetradecane measurements presented here are the first ignition measurements to be reported for this compound. The complete data set greatly extends the kinetic modeling targets available for large n-alkanes at elevated-pressure conditions relevant to practical combustion devices.

1. Introduction Commercial liquid transportation fuels (gasoline, jet fuel, and diesel) are comprised of a large number (thousands) of individual hydrocarbon compounds. These compounds can be classified by molecular structure and grouped into categories: normal alkanes, branched alkanes, cycloalkanes, alkenes, aromatics, naphthalenes, and oxygenated hydrocarbons, among others. Normal alkanes (n-alkanes) are significant components of jet fuel and diesel but are found to a lesser degree in gasoline due to their propensity to cause engine knock. The larger n-alkanes found in transportation fuels are well-known to readily ignite at elevated temperatures, relative to other classes of hydrocarbons (e.g., branched alkanes, cycloalkanes, and aromatics), and thus have low research octane numbers (RONs) and high cetane numbers (CN). n-Heptane, the easily ignitable primary reference fuel (PRF) for gasoline, is used to define a RON of 0, and n-hexadecane, the easily ignitable diesel reference fuel, is used to define a CN of 100. n-Alkanes readily ignite due, in part, to the rapid low-temperature peroxy chemistry that they undergo, which results in the distinct negative temperature coefficient (NTC) behavior displayed by these fuels.1,2 Due to the chemical complexity of commercial transportation fuels, efforts have been and are currently being made to develop surrogate mixtures containing a limited number of representative hydrocarbon compounds to mimic the physical and chemical properties of the complete distillate fuels. To date, suggested surrogate mixtures for gasoline, jet fuel, and diesel have in almost all cases contained an n-alkane representative(s).3-8 A * Corresponding Author: 110 Eighth St, JEC 2049, Troy, NY 12180; phone: 518-276-8115; fax: 518-276-6025; e-mail: [email protected]. (1) Westbrook, C. K. Proc. Combust. Inst. 2000, 28, 1563–1577. (2) Miller, J. A.; Pilling, M. J.; Troe, J. Proc. Combust. Inst. 2005, 30, 43–88. (3) Pitz, W. J.; Cernansky, N. P.; Dryer, F. L.; Egolfopoulos, F. N.; Farrell, J. T.; Friend, D. G.; Pitsch, H. Society of AutomotiVe Engineers (SAE) Paper 2007-01-0175, 2007.

common n-alkane representative for gasoline surrogates is n-heptane.3,4 n-Decane and n-dodecane are often chosen as the n-alkane representatives in surrogate jet fuels.5-7 Appropriate n-alkane representatives for diesel are n-tetradecane and nhexadecane (n-cetane), a diesel (cetane number) reference fuel, which have molecular weight in the range of the n-alkanes found in diesel.8 However, diesel surrogate mixtures using these compounds are not widely reported in the literature due to the size (number of species and reactions) and limited number of validated kinetic mechanisms available for these larger nalkanes. However, n-tetradecane and n-hexadecane are target n-alkane representatives for future diesel surrogate mixtures.8 There have been substantial previous efforts aimed at experimental characterization and kinetic modeling of n-alkane oxidation and ignition kinetics. However, many of the previous efforts have been dedicated to C7 or smaller n-alkanes and have been performed at pressures near 1 atm. Previous experimental measurements of kinetic targets for larger n-alkanes (C7 and larger) have been performed in shock tubes,9-23 rapid compression machines,24-30 flow reactors,31-39 jet-stirred reactors,40-51 (4) Andrae, J. C. G.; Bjo¨rnborn, P.; Cracknell, R. F.; Kalghatgi, G. T. Combust. Flame 2007, 149, 2–24. (5) Colket, M.; Edwards, J. T.; Williams, S.; Cernansky, N. P.; Miller, D. L.; Egolfopoulos, F. N.; Lindstedt, P.; Seshadri, K.; Dryer, F. L.; Law, C. K.; Friend, D. G.; Lenhert, D. B.; Pitsch, H.; Sarofim, A.; Smooke, M.; Tsang, W. American Institute of Aeronautics and Astronautics (AIAA) Paper AIAA-2007-0770, 2007. (6) Edwards, T.; Maurice, L. Q. J. Prop. Power 2001, 17, 461–466. (7) Violi, A.; Yan, S.; Eddings, E. G.; Sarofim, A. F.; Granata, S.; Faravelli, T.; Ranzi, E. Combust. Sci. Technol. 2002, 174, 399–417. (8) Farrell, J. T.; Cernansky, N. P.; Dryer, F. L.; Friend, D. G.; Hergart, C. A.; Law, C. K.; McDavid, R.; Mueller, C. J.; Pitsch, H. Society of AutomotiVe Engineers (SAE) Paper 2007-01-0201, 2007. (9) Vermeer, D. J.; Meyer, J. W.; Oppenheim, A. K. Combust. Flame 1972, 18, 327–336. (10) Burcat, A.; Farmer, R. F.; Matula, R. A. Int. Sym. Shock Tubes WaVes 1982, 13, 826–833. (11) Ciezki, H. K.; Adomeit, G. Combust. Flame 1993, 93, 421–433. (12) Colket, M. B.; Spadaccini, L. J. J. Prop. Power 2001, 17, 315– 323.

10.1021/ef8011036 CCC: $40.75  2009 American Chemical Society Published on Web 03/25/2009

Ignition of N-Alkanes at EleVated Pressures

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Table 1. Elevated Pressure Ignition Time Studies for C7 and Larger n-Alkanes conditions authors Adomeit11

Ciezki and Pfahl et al.18 Gauthier et al.15 Davidson et al.23 Vasu et al.22 Zhukov et al.20 Herzler et al.16 Olchanski et al.19 Griffiths et al.27 Minetti et al.25 Tanaka et al.28 Silke et al.29 Kumar et al.30 Current study

location

device

Aachen Aachen Stanford Stanford Stanford Moscow Duisburg Technion Leeds Lille MIT Galway Case Western Rensselaer

ST ST (heated) ST (heated) ST (aerosol) ST (heated) ST (heated) ST ST (heated) RCM RCM RCM RCM RCM (heated) ST (heated)

fuel

T [K]

P [atm]

mixtures

n-heptane 750-1250 3-42 Φ ) 0.5-3.0 in air n-decane 700-1300 13-50 Φ ) 0.5-2.0 in air n-heptane 806-1115 15-60 Φ ) 1.0 in air n-dodecane 1050-1330 7 Φ ) 0.5 in 21% O2/Ar bath n-dodecane 727-1422 15-34 Φ ) 0.5, 1.0 in air n-decane 800-1300 10-80 Φ ) 0.5, 1.0 in air n-heptane 720-1100 50 Φ ) 0.1-0.4 in air n-decane 1239-1616 2-10 0.49-1.5% decane, 4.2-23.3% O2 in Ar n-heptane 600-650 7-9 Φ ) 1.0 in air n-heptane 640-940 3-4.5 Φ ) 1.0 in air n-heptane 800-880 4 Φ ) 0.2-0.5 in air n-heptane 640-960 10, 15, 20 Φ ) 1.0 in air n-decane 635-706 7-30 Φ ) 0.8 in air n-heptane, n-decane, n-dodecane, 786-1396 9-58 Φ ) 0.25, 0.5, 1.0 in air n-tetradecane

premixed flames,52-61 and non-premixed flame 62-72 and droplet 73-76 environments. Previous measurements of ignition delay times for larger n-alkanes (C7 and larger) at elevated pressures, characteristic of those found in practical combustion devices (e.g., gas turbine combustors and diesel engines), have been performed in shock tubes and rapid compression machines; a list of these previous experiments is given in Table 1. Early shock tube measurements of ignition times for larger n-alkanes at elevated pressures were reported by Ciezki and Adomeit,11 who measured ignition times at elevated pressure for n-heptane/air mixtures in an unheated shock tube, and Pfahl et al.,18 who performed experiments under similar conditions for n-decane/air mixtures in a heated shock tube. Both of these studies have been widely used as kinetic targets for heptane and decane mechanism validation.51,83,87-89,95 More recently, shock tube studies of n-heptane/air have been reported by Gauthier et al.,15 who examined similar conditions (13) Horning, D. C.; Davidson, D. F.; Hanson, R. K. J. Prop. Power 2002, 18, 363–371. (14) Imbert, B.; Catoire, L.; Chaumeix, N.; Paillard, C. J. Prop. Power 2004, 20, 415–426. (15) Gauthier, B. M.; Davidson, D. F.; Hanson, R. K. Combust. Flame 2004, 139, 300–311. (16) Herzler, J.; Jerig, L.; Roth, P. Proc. Combust. Inst. 2005, 30, 1147– 1153. (17) Smith, J. M.; Simmie, J. M.; Curran, H. J. Int. J. Chem. Kinet. 2005, 37, 728–736. (18) Pfahl, U.; Fieweger, K.; Adomeit, G. Proc. Combust. Inst. 1996, 26, 781–789. (19) Olchanski, E.; Burcat, A. Int. J. Chem. Kinet. 2006, 38, 703–713. (20) Zukhov, V. P.; Sechenov, V. A.; Starikovski, A.Yu. Combust. Flame 2008, 153, 130–136. (21) Nixon, A. C.; Ackerman, G. H.; Faith, L. E.; Hawthorn, R. D.; Henderson, H. T.; Ritchie, A. W.; Ryland, L. B. Vaporizing and endothermic fuels for adVanced engine applications. U.S. Air Force Technical Report AFAPL-RF-67-114, 1967. (22) Vasu, S. S.; Davidson, D. F.; Hong, Z.; Vasudevan, V.; Hanson, R. K. Proc. Combust. Inst. 2009, 32, 173–180. (23) Davidson, D. F.; Haylett, D. R.; Hanson, R. K. Combust. Flame 2008, 155, 108–117. (24) Griffiths, J. F.; Halford-Maw, P.; Rose, D. J. Combust. Flame 1993, 95, 291–306. (25) Minetti, R.; Carlier, M.; Ribaucour, M.; Therssen, E.; Sochet, L. R. Combust. Flame 1995, 102, 298–309. (26) Minetti, R.; Ribaucour, M.; Carlier, M.; Sochet, L. R. Combust. Sci. Technol. 1996, 113, 179–192. (27) Griffiths, J. F.; Halford-Maw, P.; Mohamed, C. Combust. Flame 1997, 111, 327–337. (28) Tanaka, S.; Ayala, F.; Keck, J. C.; Heywood, J. B. Combust. Flame 2003, 132, 219–239. (29) Silke, E. J.; Curran, H. J.; Simmie, J. M. Proc. Combust. Inst. 2005, 30, 2639–2647. (30) Kumar, K. Global combustion responses of practical hydrocarbon fuels: n-heptane, iso-octane, n-decane, n-dodecane and ethylene. Ph.D. Thesis, Case Western Reserve University, Cleveland, OH, 2007.

to the Ciezki and Adomeit n-heptane study in addition to gasoline surrogate mixtures, and by Herzler et al.,16 who examined lean (Φ ) 0.1-0.4) n-heptane/air mixtures. n-Decane/ air mixtures have been studied at extreme pressures up to 80 atm by Zukhov et al.20 in a heated shock tube and by Olchanski et al.19 for a variety of mixtures ranging from dilute (∼4% O2) to air-like O2 concentrations (see Table 1). Most recently, n-dodecane ignition has been investigated at elevated pressures in a heated shock tube by Vasu et al.22 and by Davidson et al.,23 who made ignition measurements for n-dodecane/O2/Ar mixtures using an aerosol shock tube method. n-Heptane ignition has also been studied in rapid compression machines by Griffiths et al.,24,27 who made measurements for Φ ) 1.0 n-heptane/air mixtures at 600-950 K and 7-9 atm; Minetti et al.,25,26 who made measurements for Φ ) 1.0 n-heptane/air mixtures at 640-940 K and 3-4.5 atm; Tanaka et al.,28 who made measurements for Φ ) 0.2-0.5 n-heptane/ air mixtures at 800-880 K and 4 atm; and Silke et al.,29 who made measurements for Φ ) 1.0 n-heptane/air mixtures at 640-960 K and 10, 15, and 20 atm. Recently Kumar30 has also reported n-decane ignition delay times measured in a heated rapid compression machine at temperatures ranging from 635 to 706 K at 7-30 atm and Φ ) 0.8. (31) Lignola, P. G.; DiMaio, F. P.; Marzocchiella, A.; Mercogliano, R.; Reverchon, E. Proc. Combust. Inst. 1988, 22, 1625–1633. (32) Ciajolo, A.; D’Anna, A.; Mercogliano, R. Combust. Sci. Technol. 1993, 90, 357–371. (33) Callahan, C. V.; Held, T. J.; Dryer, F. L.; Minetti, R.; Ribaucour, M.; Sochet, L. R.; Faravelli, T.; Gaffuri, P.; Ranzi, E. Proc. Combust. Inst. 1996, 26, 739–746. (34) Held, T. J.; Marchese, A. J.; Dryer, F. L. Combust. Sci. Technol. 1997, 123, 107–146. (35) Gaffuri, P.; Faravelli, T.; Ranzi, E.; Cernansky, N. P.; Miller, D.; D’Anna, A.; Ciajolo, A. AlCheE J. 1997, 43, 1278–1286. (36) Ciajolo, A.; D’Anna, A. Combust. Flame 1998, 112, 617–622. (37) Agosta, A.; Cernansky, N. P.; Miller, D. L.; Faravelli, T.; Ranzi, E. Exp. Therm. Fluid Sci. 2004, 28, 701–708. (38) Lenhert, D. B.; Cernansky, N. P.; Miller, D. L. Oxidataion of large molecular weight hydrocarbons in a pressurized flow reactor. In Proceedings of the 4th Joint Meeting of the U.S. Section of the Combustion Institute, Philadelphia, PA, 2005. (39) Natelson, R.; Johnson, R.; Kurman, M.; Cernansky, N.; Miller, D. Low Temperature Oxidation of Selected Jet Fuel and Diesel Fuel Components at Elevated Pressures. In Proceedings of the 5th Joint Meeting of the U.S. Section of the Combustion Institute, San Diego, CA, 2007. (40) Chakir, A.; Belliman, M.; Boettner, J. C.; Cathonnet, M. Int. J. Chem. Kinet. 1992, 24, 385–410. (41) Bales-Gueret, C.; Cathonnet, M.; Boettner, J. C.; Gaillard, F. Energy Fuels 1992, 6, 189–194. (42) Cavaliere, A.; Ciajolo, A.; D’Anna, A.; Mercolgliano, R.; Ragucci, R. Combust. Flame 1993, 93, 279–286. (43) Dagaut, P.; Reuillon, M.; Cathonnet, M. Combust. Sci. Technol. 1994, 95, 233–260.

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In addition to the previous experimental investigations for C7 and larger n-alkanes, there have been many kinetic modeling studies.19,33,34,40-51,60,77-95 Mechanism development efforts for larger n-alkanes have predominately focused on n-heptane and n-decane, although mechanisms have been developed for n-alkanes as large as n-hexadecane by several authors.48,49,77,95 Here we present ignition time measurements for n-heptane, n-decane, n-dodecane, and n-tetradecane in air mixtures for pressures from 9 to 58 atm and temperatures ranging from 786 to 1396 K. Experiments were performed for Φ ) 0.25, 0.5, and 1.0 for n-heptane and n-decane and Φ ) 0.5 and 1.0 for n-dodecane and n-tetradecane. These experiments provide validation targets for kinetic modeling for larger n-alkanes, for which there is little data in the literature, at conditions of relevance to practical combustion devices. The current experi(44) Dagaut, P.; Reuillon, M.; Cathonnet, M. Combust. Sci. Technol. 1994, 103, 349–359. (45) Dagaut, P.; Reuillon, M.; Boettner, J.-C.; Cathonnet, M. Proc. Combust. Inst 1994, 25, 919–926. (46) Dagaut, P.; Reuillon, M.; Cathonnet, M. Combust. Flame 1995, 101, 132–140. (47) Zeppieri, S. P.; Klotz, S. D.; Dryer, F. L. Proc. Combust. Inst. 2000, 28, 1587–1595. (48) Fournet, R.; Battin-Leclerc, F.; Glaude, P. A.; Judenherc, B.; Warth, V.; Come, G. M.; Scacchi, G.; Ristori, A.; Pengloan, G.; Dagaut, P.; Cathonnet, M. Int. J. Chem. Kinet. 2001, 33, 574–586. (49) Ristori, A.; Dagaut, P.; Cathonnet, M. Combust. Flame 2001, 125, 1128–1137. (50) Dagaut, P.; El Bakali, A.; Ristori, A. Fuel 2006, 85, 944–956. (51) Biet, J.; Hakka, M. H.; Warth, V.; Glaude, P.-A.; Battin-Leclerc, F. Energy Fuels 2008, 22, 2258–2269. (52) Doute, C.; Delfau, J.-L.; Vovelle, C. Combust. Sci. Technol. 1995, 106, 327–344. (53) Doute, C.; Delfau, J.-L.; Akrich, R.; Vovelle, C. Combust. Sci. Technol. 1997, 124, 249–276. (54) Doute, C.; Delfau, J.-L.; Vovelle, C. Combust. Sci. Technol. 1997, 130, 269–313. (55) El Bakali, A.; Delfau, J.-L.; Vovelle, C. Combust. Sci. Technol. 1998, 140, 69–91. (56) El Bakali, A.; Delfau, J.-L.; Vovelle, C. Combust. Flame 1999, 118, 381–398. (57) Ingemarsson, A. T.; Pedersen, J. R.; Olsson, J. O. J. Phys. Chem. A 1999, 103, 8222–8230. (58) Inal, F.; Senkan, S. M. Combust. Flame 2002, 131, 16–28. (59) Xue, H.; Aggarwal, S. K. Combust. Flame 2003, 132, 723–741. (60) Zhao, Z.; Li, J.; Kazakov, A.; Dryer, F. L.; Zeppieri, S. P. Combust. Sci. Technol. 2004, 177, 89–106. (61) Kumar, K.; Sung, C. J. Combust. Flame 2007, 151, 209–224. (62) Davis, S. G.; Law, C. K. Proc. Combust. Inst. 1998, 27, 521–527. (63) Seiser, R.; Pitsch, H.; Seshadri, K.; Pitz, W. J.; Curran, H. J. Proc. Combust. Inst. 2000, 28, 2029–2037. (64) Seiser, R.; Seshadri, K.; Piskernik, E.; Linan, A. Combust. Flame 2000, 122, 339–349. (65) Cooke, J. A.; Bellucci, M.; Smooke, M. D.; Gomez, A.; Violi, A.; Faravelli, T.; Ranzi, E. Proc. Combust. Inst. 2005, 30, 439–446. (66) Humer, S.; Frassoldati, A.; Granata, S.; Faravelli, T.; Ranzi, E.; Seiser, R.; Seshadri, K. Proc. Combust. Inst. 2007, 31, 393–400. (67) Skjoth-Rasmussen, M. S.; Braun-Unkhoff, M.; Naumann, C.; Frank, P. Experimental and numerical study of n-decane chemistry. In Proceedings of the 1st European Combustion Meeting, Orleans, France, 2003. (68) Huang, Y.; Sung, C. J.; Eng, J. A. Combust. Flame 2004, 139, 239–251. (69) Holley, A. T.; Dong, Y.; Andac, M. G.; Egolfopoulos, F. N. Combust. Flame 2006, 144, 448–460. (70) Hamins, A.; Seshadri, K. Combust. Flame 1987, 68, 295–307. (71) Petarca, L.; Marconi, F. Combust. Flame 1989, 78, 308–325. (72) Seiser, R.; Truett, L.; Trees, D.; Seshadri, K. Proc. Combust. Inst. 1998, 27, 649–657. (73) Nayagam, V.; Haggard, J. B., Jr.; Colantonio, R. O.; Marchese, A. J.; Dryer, F. L.; Zhang, B. L.; Williams, F. A. AIAA J. 1998, 36, 1369– 1378. (74) Suekane, T.; Yasutomi, K.; Hirai, S. Combust. Flame 2001, 126, 1599–1601. (75) Tanabe, M.; Kono, M.; Sato, J.; Koenig, J.; Eigenbrod, C.; Dinkelacker, F.; Rath Zarm, H. J. Combust. Sci. Technol. 1995, 108, 103– 119. (76) Tanabe, M.; Bolik, T.; Eigenbrod, C.; Rath Zarm, H. J.; Sato, J.; Kono, M. Proc. Combust. Inst. 1996, 26, 1637–1643.

Figure 1. Typical heated shock tube inner wall temperature profiles (driven section).

ments also provide an assessment of the influence n-alkane chain length has on reactivity and an assessment of several kinetic oxidation mechanisms found in the literature. The measurements are compared to n-alkane mechanisms developed by Curran et al.83 and Westbrook et al.95 at Lawrence Livermore National Laboratory (LLNL), by Ranzi et al.89 at Politecnico di Milano, and by Biet et al.51 at Nancy Universite´. The ignition time measurements presented here for n-tetradecane are the first, to our knowledge, to be reported in the literature. The measurements for the other compounds extend the database of kinetic targets available for these large n-alkanes. 2. Experimental Method Ignition times for n-alkane/air mixtures were measured in the externally heated high-pressure shock tube (constant inner diameter of 5.7 cm, driven section length of 4.11 m, and driver section length of 2.59 m) at Rensselaer Polytechnic Institute, which has been previously described.96,97 The shock tube, mixing manifold, and vessel temperature were chosen for a particular set of experiments in order to provide sufficient vapor pressure of the n-alkane of interest for mixture preparation and to prevent condensation on the shock tube walls. Experiments for n-heptane mixtures were performed with the shock tube, mixing manifold, and vessel at room temperature, whereas for n-decane, n-dodecane, and n-tetradecane the shock tube, mixing manifold, and vessel were maintained at uniform temperatures of 50-70, 100-120, and 120-160 °C, respectively. The shock tube temperature uniformity was routinely monitored, with nonuniformity within (2 °C for all but the highest temperatures; for n-tetradecane experiments performed at a shock tube temperature of 160 °C the measured temperature nonuniformity was (4 °C. The shock tube temperature uniformity was maintained by zone heater controllers previously described.97 See Figure 1 for shock tube driven section temperature profiles measured with a (77) Chevalier, C.; Pitz, W. J.; Warnatz, J.; Westbrook, C. K.; Melenk, H. Proc. Combust. Inst. 1992, 24, 93–101. (78) Ranzi, E.; Gaffuri, P.; Faravelli, T.; Dagaut, P. Combust. Flame 1995, 103, 91–106. (79) Lindstedt, R. P.; Maurice, L. Q. Combust. Sci. Technol. 1995, 107, 317–353. (80) Nehse, M.; Warnatz, J.; Chevalier, C. Proc. Combust. Inst. 1996, 26, 773–780. (81) Ranzi, E.; Faravelli, T.; Gaffuri, P.; Garavaglia, E.; Goldaniga, A. Ind. Eng. Chem. Res. 1997, 36, 3336–3344. (82) Glaude, P. A.; Warth, V.; Fournet, R.; Battin-Leclerc, F.; Schacchi, G.; Come, G. M. Int. J. Chem. Kinet. 1998, 30, 949–959. (83) Curran, H. J.; Gaffuri, P.; Pitz, W. J.; Westbrook, C. K. Combust. Flame 1998, 114, 149–177.



Figure 2. Example ignition time measurements.

type-K thermocouple. n-Alkane/air mixtures with equivalence ratios of 0.25 (only for n-heptane and n-decane), 0.5, and 1.0 were prepared using 99+% pure liquid n-heptane, n-decane, n-dodecane, and n-tetradecane obtained from Sigma Aldrich and O2 at 99.995% purity and N2 at 99.995% purity. The liquid n-alkanes were degassed prior to mixture preparation and introduced to the evacuated mixing vessel via vaporization. Mixtures were allowed to mix, via a magnetically powered vane assembly located in the mixing vessel, for anywhere from 15 min to 3 h prior to shock tube experiments. No dependence of measured ignition times on the mixing duration was observed. With the short mixing durations and lack of dependence of measured ignition time on mixing duration it can be safely assumed that the mixtures were not influenced by decomposition or other reaction in the heated mixing vessel. Ignition times were determined in the reflected shock region using electronically excited OH emission viewed through the shock tube endwall and pressure measurements made using a piezoelectric transducer located in the sidewall at a location 2 cm from the endwall. Example ignition time measurements are shown in Figure 2. Reflected shock conditions were determined using the normal shock relations. The uncertainty in the reflected shock temperatures and pressure are estimated at 1.5 and 2.0%, respectively. The complete data set presented here spans a temperature range from 786 to 1396 K and a pressure range from 9 to 58 atm. Ignition time measurements were performed using the traditional helium driver gas technique from 42 to 1687 µs. Ignition time measurements were limited on the short end by the ability to accurately determine the ignition times from the measured emission and pressure traces and on the long end by the reflected shock test time. For Φ ) 1.0 n-heptane experiments we report three experiments with measured ignition times up to 3048 µs that were performed using a tailored driver mixture (20% N2/He). Note that this facility displays a nonideal gasdynamic increase in pressure, due to boundary layer shock attenuation, of approximately dP/dt ) ∼2%/ ms for test times shorter than 4 ms. Assuming an isentropic relationship between pressure and temperature changes this results in a temperature change of approximately dT/dt ) ∼0.5%/ms for the mixtures studied here. As discussed by Hanson and coworkers22,98-100 and Chaos and Dryer,101 these nonideal gasdynamic changes in pressure and temperature can complicate the interpretation of ignition time measurements made at long test times. For the n-alkane results presented here the influence of gasdynamic pressure changes on the measured ignition times is small, we estimate that the observed pressure increase (2%/ms) will shorten an ignition time of 1.0 ms by at most 5% and shorten an ignition time of 3.0 ms by at most 10%, based on kinetic simulations for n-alkane/air ignition with time varying pressure and temperature histories. The estimated influence of gasdynamic variation in (84) Glaude, P. A.; Warth, V.; Fournet, R.; Battin-Leclerc, F.; Scacchi, G.; Come, G. M. Int. J. Chem. Kinet. 1999, 30, 949–959. (85) Lindstedt, R. P.; Maurice, L. Q. J. Prop. Power 2000, 16, 187– 195.

conditions on ignition time is relatively small for n-alkanes because of the smaller ignition time dependence on temperature in and near the negative temperature coefficient regime where ignition times are longer. A complete tabulation of all ignition time measurements is given in the Supporting Information.

3. Results The ignition time results are displayed on Arrhenius axes in Figures 3 and 4 with comparison to previous shock tube and rapid compression machine studies performed at elevated pressures. We estimate the uncertainty in measured ignition time at (20% based on contributions from (1) the uncertainty in determination of ignition time based on the ignition time definition and measured pressure and OH emission, (2) uncertainties in the initial reflected shock conditions (mixture composition, temperature, and pressure), and (3) estimated uncertainty due to changes in temperature and pressure due to nonideal gasdynamic effects. In Figures 3 and 4 the ignition times exhibit clear negative temperature coefficient (NTC) behavior at the lower temperatures studied (T < 1000 K) as observed in the previous experiments displayed in the figures11,15,16,18,22,25,27,29 and predicted by kinetic modeling studies.51,83,88,89,95 Additionally, the (86) Battin-Leclerc, F.; Fournet, R.; Glaude, P. A.; Judenherc, B.; Warth, V.; Come, G. M.; Scacchi, G. Proc. Combust. Inst. 2000, 28, 1597–1605. (87) Bikas, G.; Peters, N. Combust. Flame 2001, 126, 1456–1475. (88) Buda, F.; Bounaceur, R.; Warth, V.; Glaude, P. A.; Fournet, R.; Battin-Leclerc, F. Combust. Flame 2005, 142, 170–186. (89) Ranzi, E.; Frassoldati, A.; Granata, S.; Faravelli, T. Ind. Eng. Chem. Res. 2005, 44, 5170–5183. (90) Touchard, S.; Fournet, R.; Glaude, P. A.; Warth, V.; Battin-Leclerc, F.; Vanhove, G.; Ribaucour, M.; Minetti, R. Proc. Combust. Inst. 2005, 30, 1073–1081. (91) Dagaut, P.; Cathonnet, M. Prog. Energy Combust. Sci. 2006, 36, 48–92. (92) Moreac, G.; Blurock, E. S.; Mauss, F. Combust. Sci. Technol. 2006, 178, 2025–2038. (93) Muharam, Y.; Warnatz, J. Phys. Chem. Chem. Phys. 2007, 9, 4218– 4229. (94) Battin-Leclerc, F. Prog. Energy Combust. Sci. 2008, 34, 440–498. (95) Westbrook, C. K.; Pitz, W. J.; Herbinet, O.; Curran, H. J.; Silke, E. J. Combust. Flame 2009, 156, 181–199. (96) Shen, H.-P. S.; Vanderover, J.; Oehlschlaeger, M. A. Combust. Flame 2008, 155, 739–755. (97) Shen, H.-P. S. Oehlschlaeger, M. A. Combust. Flame 2008, 11, 015, to appear, doi: 10.1016/j.combustflame. (98) Vasu, S. S.; Davidson, D. F.; Hanson, R. K. Combust. Flame 2008, 152, 125–143. (99) Pang, G.; Davidson, D. F.; Hanson, R. K. Proc. Combust. Inst. 2009, 32, 181–188. (100) Li, H.; Owens, Z. C.; Davidson, D. F.; Hanson, R. K. Int. J. Chem. Kinet. 2008, 40, 89–98. (101) Chaos, M.; Dryer, F. L. Int. J. Chem. Kinet., to appear.

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Figure 3. Ignition time measurements for n-heptane/air and n-decane/air mixtures with comparison to previous shock tube and rapid compression machine studies. All literature and current data scaled to the listed pressures using τ ∝ P-1 to account for deviations in experimental pressure.

ignition times decrease with increasing equivalence ratio and pressure. For all four n-alkanes studied and at temperatures greater than that where NTC behavior begins (T > 1000 K), the ignition time dependence on pressure follows the inverse relationship (τ ∝ P-1) previously observed and employed for n-alkanes by other authors.22,23,98 In the NTC regime (T < 1000 K) the ignition times exhibit dependence on pressure that is stronger than τ ∝ P-1 (i.e., τ ∝ P-a where a > 1). This is due to the strong dependence of ignition times in the NTC regime on the low temperature peroxy oxidation pathway, which is strongly pressure dependent (see discussion in the next section). Ciezki and Adomeit11 have previously observed strong ignition time dependence on pressure in the NTC for n-heptane. In Figures 3 and 4 all the data has been scaled, due deviations in reflected shock pressure, to the common pressures given in the

legend using τ ∝ P-1. Although in the NTC the τ ∝ P-1 dependence is weaker than that observed experimentally, the deviations in reflected shock pressure for a given data set are not large enough for the pressure scaling to significantly influence the comparisons made in Figures 3 and 4. Additionally, the current and previous data are not comprehensive enough to estimate the change in ignition time dependence on pressure with temperature in the NTC. The agreement of the current ignition time measurements with previous studies performed at elevated pressures shown in Figures 3 and 4 is fairly good in the cases where the measurements are at similar pressures. The n-heptane data is in good agreement with the previous shock tube studies of Ciezki and Adomeit11 and Gauthier et al.15 at Φ ) 1.0; and the Herzler et al.16 results at Φ ) 0.2, 0.3, and 0.4 are in accord with the



Figure 4. Ignition time measurements for n-dodecane/air and n-tetradecane/air mixtures. The n-dodecane measurements are compared to the previous shock tube measurements of Vasu et al. 22 and Davidson et al.23 All literature and current data scaled to the listed pressures using τ ∝ P-1 to account for deviations in experimental pressure.

current Φ ) 0.25 and 0.5 data. The Φ ) 1.0 n-heptane data is in fairly good accord with the recent rapid compression machine data of Silke et al.29 The current n-decane Φ ) 0.25 data is the first of its kind. Although the n-decane Φ ) 0.5 at 12 and 50 atm cannot be directly compared to the 80 atm Zhukov et al.20 data, the Φ ) 1.0 n-decane data at 11 atm is in very good agreement with the previous measurements of Pfahl et al.18 at 12 atm. The current Φ ) 1.0 n-decane data at 40 atm cannot be directly compared to the 50 atm data of Pfahl et al. and the 80 atm data of Zhukov et al. because of the strong dependence of ignition time on pressure in the NTC regime. Additionally, the correlation developed by Olchanski and Burcat to describe their hightemperature n-decane ignition time measurements (1239-1616 K and 2-10 atm) is in very good agreement with the current data at conditions where the two studies overlap; this includes the current data at Φ ) 0.5 and 1.0 data at 12 and 11 atm, respectively. Figure 4 illustrates agreement between the current n-dodecane data with the 20 atm measurements of Vasu et al.;22 the current Φ ) 0.5 and 1.0 n-dodecane data at 40 atm are the first n-dodecane ignition measurements at pressures in excess of 20 atm, to our knowledge. Additionally, the n-tetradecane measurements are the first, to our knowledge, for any n-alkane larger than C12. 4. Comparison of n-Alkane Reactivity The current ignition time measurements for n-heptane, n-decane, n-dodecane, and n-tetradecane allow an assessment of the influence of n-alkane chain length on reactivity at the

elevated pressure conditions studied. In Figure 5 all of the ignition time measurements for Φ ) 0.5 n-alkane/air mixtures are displayed on Arrhenius axes for pressures of 12 and 40 atm. Again, as in Figure 3 and 4, the data in Figure 5 is scaled to 12 and 40 atm using τ ∝ P-1 to account for differences in experimental reflected shock pressure. The Figure 5 comparisons show that the ignition times for all four n-alkanes at Φ ) 0.5 fall within bands representing (30% in measured ignition time. Additionally, the error bars for each measured ignition time ((20%) overlaps with measurements for other n-alkanes. These results show that any differences in reactivity for the four n-alkanes are slight and are not discernible within our experimental uncertainties. The indiscernible difference in reactivity for the n-alkanes illustrated in Figure 5 is common for all conditions studied. In Figure 6 all of the current Φ ) 1.0 n-alkane ignition time measurements performed near 12 atm are compared to previous shock tube and rapid compression machine measurements for n-heptane, n-decane, and n-dodecane made at similar pressures. Again, all of the data is scaled to 12 atm using τ ∝ P-1 to account for deviations in pressure. The compilation of the current data and that of the previous studies illustrates that all of the data, except the n-heptane data of Ciezki and Adomeit,11 falls within a band representing (40% in ignition time over the complete temperature range displayed (625-1430 K), indicating that any differences in reactivity are slight. There is, perhaps, a slight decrease in reactivity with increasing chain length in Figure 6. However, the decrease is slight and this observation is influenced by the ignition time data for n-heptane reported by Ciezki and Adomeit,11 which is longer than both

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Figure 5. Comparison of ignition times for n-heptane/air, n-decane/air, n-dodecane/air, and n-tetradecane/air at Φ ) 0.5.

Figure 7. Comparison of the data shown above in Figure 7 with kinetic modeling predictions for n-alkane/air ignition from Curran et al.83 and Westbrook et al.95 (LLNL), Ranzi et al.89 (Milano), and Biet et al.51 (Nancy).

Figure 6. Comparison of current n-alkane/air ignition measurements at 12 atm with previous measurements made near 12 atm. The solid lines represent a (40% band in ignition time, which most of the data falls within.

the current and Gauthier et al.15 n-heptane data. Additionally, the coupled uncertainty resulting from the comparison of ignition time measurements made in different facilities using different techniques and the uncertainties resulting from applying τ ∝ P-1 scaling at all temperatures, which as discussed before is oversimplistic, certainly is near (40% in ignition time. Comparison of the combined data for n-heptane, n-decane, n-dodecane, and n-tetradecane from this and previous studies with kinetic modeling predictions is made in Figure 7. The comparisons are made for Φ ) 1.0 n-alkane/air mixtures at pressures near 12 atm with modeling predictions from the C7-C16 n-alkane mechanisms of Curran et al.83 and Westbrook et al.95 (LLNL); the C5-C16 n-alkane mechanism from Ranzi et al.,89 (Politecnico di Milano) who uses a lumped approach to reduce the number of intermediate species and reactions; and the C7-C16 n-alkane mechanism of Biet et al.,51 developed using the EXGAS routine for automatic mechanism generation (Nancy Universite´). The n-heptane and n-tetradecane predictions of Curran et al. and Westbrook et al. and the n-heptane and n-decane predictions of Ranzi et al. are in best accord with the measured ignition

times. The predictions of the LLNL mechanisms (Curran et al.83 and Westbrook et al.95) for n-heptane and n-tetradecane show no difference in reactivity at high and low temperatures and a slight decrease in ignition time in the NTC with increasing chain length. The maximum predicted difference in ignition time between n-heptane and n-tetradecane by the LLNL mechanisms is 30% in the NTC region. The Ranzi et al.89 predictions for n-heptane and n-decane show negligible difference in reactivity for temperatures less than 750 K and deviation at lower temperatures with n-decane ignition times 50% shorter than those for n-heptane at 650 K. On the other hand, the Biet et al.51 mechanism shows a large increase in reactivity with increasing n-alkane chain length, particularly in the NTC regime (750-1000 K) where the ignition times for n-tetradecane are as much as a factor of 7 shorter than those for n-heptane. This is in disagreement with the combined data shown in Figures 6 and the other mechanisms shown in Figure 7. The comparisons of experimental n-alkane ignition times displayed in Figures 5-7 indicate that the reactivity of n-alkane/air mixtures for C7 and larger n-alkanes vary little with n-alkane chain length. These mixtures all have approximately common carbon content regardless of the length of the n-alkane chain. For example, a Φ )1.0 n-heptane/air mixture contains 1.874% molar n-heptane (nC7H16) whereas a Φ )1.0 n-tetradecane/air mixture the contains 0.9677% molar n-tetradecane (nC14H30). Although the n-heptane mixture contains approximately a factor of 2 more fuel


Figure 8. The primary reaction pathways for alkane oxidation.

molecules than the n-tetradecane mixture, due to the different chain length of these two n-alkanes the two mixtures differ in carbon atom content by only 3%. The similarity in reactivity observed experimentally and predicted by the LLNL83,95 and Ranzi et al.89 mechanisms indicates that the oxidation kinetics of n-alkanes is influenced little by chain length. A schematic indicating the major reaction pathways for alkanes is shown in Figure 8; this schematic and the below description of the n-alkane oxidation pathways is consistent with and is taken from the modeling approaches of the LLNL83,95 and Nancy51 groups. At temperatures below 1400 K the n-alkanes are primarily consumed via H-atom abstraction by small radicals (O, H, OH, HO2, CH3, and others) to produce alkyl radicals; at T > 1400 K n-alkane thermal decomposition competes. At higher temperatures (T > 900-1000 K) these alkyl radicals primarily decompose, which can be proceeded by isomerization (H-atom transfer), to produce olefins, most of which are ethylene and propene102,103 for all n-alkanes regardless of chain length. Therefore, at higher temperatures the intermediate olefin pool for all n-alkanes is similar, provided that the mixtures are of common carbon content, and therefore the measured and modeled ignition times are very similar. At lower temperatures (T < 900-1000 K) the alkyl radicals add directly to O2 to form alkylperoxy radicals (RO2) which can dissociate back to alkyl and O2 or isomerize to form hydroperoxy alkyl radicals (QOOH). The QOOH can add an additional O2 to form hydroperoxy peroxy (OOQOOH), which can quickly isomerize to a ketohydroperoxide and an OH radical. The ketohydroperoxide can decompose to form a second OH radical and another radical. In total, this reaction sequence produces three radicals from the original alkyl radical and thus chain low-temperature radical branching. This reaction sequence occurs at temperatures lower than 800 K. At moderate temperatures (700-1000 K) the low-temperature branching reaction pathway competes with the dissociation of QOOH to form different products: olefins and HO2, cyclic ethers and OH, and β-scission products and alkyl radicals. This moderate temperature pathway results in no radical branching, thus lower reactivity is observed at moderate temperatures, in (102) Tsang, W.; Walker, J. A.; Manion, J. A. Proc. Combust. Inst. 2007, 31, 141–148. (103) Horning, D. C. A study of the high-temperature autoignition and thermal decomposition of hydrocarbons. Ph.D. Thesis, Stanford University, Stanford, CA, 2001.


the negative temperature coefficient (NTC) regime, than at lower temperatures where QOOH + O2 is faster than QOOH decomposition. The competition between QOOH decomposition and QOOH + O2 is strongly pressure dependent. Hence, in the NTC regime the ignition times exhibit stronger pressure dependence than they do at higher temperature. At the transition from the NTC regime to the high-temperature regime decomposition of hydrogen peroxide, H2O2 + M f 2OH + M, begins to become fast enough to enhance the radical pool and results in the end of the NTC regime (∼900-1000 K) and a return to the traditional increase in reactivity with increasing temperature observed in the high-temperature regime (T > 1000 K). Hydrogen peroxide is formed via the sequence QOOH f olefin + HO2 followed by n-alkane + HO2 f alkyl + H2O2. The similarity of the measured ignition times in all the experimental studies shown in Figure 6 for temperatures less than 1000 K implies that the moderate- and low-temperature reaction pathways and rates are not strongly influenced by n-alkane chain length for C7 and larger n-alkanes. In particular, reactions involving internal isomerization (RO2 f QOOH and OOQOOH f OH + ketohydroperoxide), the rates of which are dependent on the length of R for smaller molecules, must not be strongly dependent on length, for C7 and larger alkyls. This conclusion is consistent with the premise that reactions proceeding through cyclic transition states typically proceed through 5-8 member rings, and in the case of larger alkanes the addition of chain length does not add probable isomerization pathways. 5. Summary New ignition time measurements for n-heptane, n-decane, n-dodecane, and n-tetradecane are reported at conditions of relevance to practical combustion devices. Measurements were made for Φ ) 0.25, 0.5, and 1.0 n-alkane/air mixtures at pressures from 9 to 58 atm and at temperatures from 786 to 1396 K. The n-dodecane experiments are performed at higher pressures than those previously studied, and the n-tetradecane data is the first of its kind, to our knowledge. At conditions where data from previous studies exists, the present study is in good agreement with those studies.11,15,16,18,19,22 The current data and the combination of the current data with previous shock tube and rapid compression machine studies indicates that the ignition times for n-alkanes are influenced little by n-alkane chain length for C7 and longer n-alkanes and for mixtures with common carbon content. The mechanisms of Curran et al.83 and Westbrook et al.95 (LLNL) and those of Ranzi et al.89 are in agreement with this finding. On the other hand, the mechanism of Biet et al.51 shows a larger influence of n-alkane chain length on reactivity than the experimental data indicate. The data presented here will allow for the validation of kinetic mechanisms for larger n-alkanes, for which there has previously been limited kinetic validation data. Acknowledgment. This work was supported by the U.S. Air Force Office of Scientific Research (Grant No. FA9550-07-1-0114) with Dr. Julian Tishkoff as technical monitor. Supporting Information Available: Experimental data in tabular form. This material is available free of charge via the Internet at http://pubs.acs.org. EF8011036

A Shock Tube Study of the Ignition of n-Heptane, n-Decane, n

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