Shock Tube Study of Methylcyclohexane Ignition over a Wide Range

Energy & Fuels 2009, 23, 175–185

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Shock Tube Study of Methylcyclohexane Ignition over a Wide Range of Pressure and Temperature Subith S. Vasu,* David F. Davidson, Zekai Hong, and Ronald K. Hanson Mechanical Engineering Department, Stanford UniVersity, Stanford, California 94305 ReceiVed August 22, 2008. ReVised Manuscript ReceiVed September 24, 2008

Ignition delay times were measured for gas-phase methylcyclohexane (MCH)/O2/argon and MCH/air mixtures behind reflected shock waves. Initial postshock conditions covered temperatures of 795-1560 K, pressures of 1-50 atm, fuel concentrations of 0.25-2%, and equivalence ratios (φ) of 0.5-2.0. Ignition delay times were measured using side-wall pressure and CH* and OH* emission measurements. Current measurements complement past high-pressure rapid compression machine results, are in good agreement with past lowpressure shock tube data, and significantly extend the pressure range of available shock tube ignition time data. Detailed comparisons of experimental data with predictions of available MCH mechanisms are presented, and comparisons of shock tube MCH ignition delay times to those of other important jet fuel surrogates and cyclo-alkanes are discussed.

1. Introduction Recent efforts to understand and quantify the combustion properties of jet fuels, especially at engine-relevant conditions,1 and the selection of suitable surrogate fuels that contain a computationally tractable number of components2 have highlighted the importance of studying various chemical classes that are present in jet fuels. Cyclo-alkanes (naphthenes) are an important chemical class present not only in jet fuels but also in other practical fuels, such as gasoline and diesel, and may constitute up to 20% by volume of jet fuels, such as Jet-A/JetA1/JP-8, around 60% by volume of RP-1, and more than 40 wt % of diesel fuels.3-6 Cyclo-alkanes in diesel fuels have been found to influence particulate matter (PM) emissions,7 and the soot precursor production potential of cyclo-alkanes (via the formation of polycyclic aromatics) is much higher than that of normal and iso-paraffin compounds.8 For many years, the combustion and kinetic behavior of straight-chain and branched * To whom correspondence should be addressed. E-mail: subith@ stanford.edu. (1) Vasu, S. S.; Davidson, D. F.; Hanson, R. K. Combust. Flame 2008, 152, 125–143. (2) Colket, M.; Edwards, T.; Williams, S.; Cernansky, N. P.; Miller, D. L.; Egolfopoulos, F.; Lindstedt, P.; Seshadri, K.; Dryer, F. L.; Law, C. K.; Friend, D.; Lenhert, D. B.; Pitsch, H.; Sarofim, A.; Smooke, M.; Tsang, W. In 45th American Institute of Aeronautics and Astronautics (AIAA) Aerospace Sciences Meeting, Reno, NV, 2007; paper AIAA 20070770. (3) Edwards, T.; Maurice, L. Q. J. Propul. Power 2001, 17 (2), 461– 466. (4) Edwards, T. In 38th American Institute of Aeronautics and Astronautics (AIAA)/American Society of Mechanical Engineers (ASME)/Society of Automotive Engineers (SAE)/American Society for Engineering Education (ASEE) Joint Propulsion Conference, Indianapolis, IN, 2002; paper AIAA-2002-3874. (5) Edwards, T. J. Prop. Power 2003, 19 (6), 1089–1107. (6) Briker, Y.; Ring, Z.; Iacchelli, A.; McLean, N.; Rahimi, P. M.; Fairbridge, C.; Malhotra, R.; Coggiola, M. A.; Young, S. E. Energy Fuels 2001, 15, 23–37. (7) Nakakita, K.; Takasu, S.; Ban, H.; Ogawa, T.; Naruse, H.; Tsukasaki, Y.; Yeh, L. I. SAE Tech. Pap. Ser., Society of Automotive Engineers (SAE), Warrendale, PA, 1998; paper 982494. (8) Zhang, R. H.; Eddings, E. G.; Sarofim, A. F. Proc. Combust. Inst. 2007, 31, 401–409.

alkanes has received significant attention, while cyclo-alkanes, on the other hand, have received, until recently, only scant attention.9 Methylcyclohexane (C7H14) is one of the simplest cycloalkanes and is widely used to represent the cyclo-alkane fraction of jet fuel surrogates.10-15 It has also been proposed as a fuel for scramjets, because, in the presence of a catalyst, MCH can be endothermically dehydrogenated to form toluene and hydrogen, thus providing a significant heat sink (∼2190 kJ/kg) for cooling the engine.16 Very few studies concerning MCH combustion exist that can be used for surrogate fuel development, although recently there have been a variety of experimental and kinetic modeling studies conducted for other types of cyclo-alkanes.9,17-25 The kinetics of cyclo-alkanes is regarded (9) Simmie, J. M. Prog. Energy Combust. Sci. 2003, 29, 599–634. (10) Violi, A.; Yan, S.; Eddings, E. G.; Sarofim, A. F.; Granata, S.; Faravelli, T.; Ranzi, E. Combust. Sci. Technol. 2002, 174 (11-12), 399– 417. (11) Agosta, A.; Cernansky, N. P.; Miller, D. L.; Faravelli, T.; Ranzi, E. Exp. Therm. Fluid Sci. 2004, 28 (7), 701–708. (12) Cooke, J. A.; Belluci, M.; Smooke, M. D.; Gomez, A.; Violi, A.; Faravelli, T.; Ranzi, E. Proc. Combust. Inst. 2005, 30, 439–446. (13) Humer, S.; Frassoldati, A.; Granata, S.; Faravelli, T.; Ranzi, E.; Seiser, R.; Seshadri, K. Proc. Combust. Inst. 2007, 31, 393–400. (14) Pitz, W. J.; Naik, C. V.; Mhaoldu´in, T. N.; Westbrook, C. K.; Curran, H. J.; Orme, J. P.; Simmie, J. M. Proc. Combust. Inst. 2007, 31, 267–275. (15) Westbrook, C. K. Proc. Combust. Inst. 2000, 28, 1563–1577. (16) Taylor, P. H.; Rubey, W. A. Energy Fuels 1998, 2, 723–728. (17) Silke, E. J.; Pitz, W. J.; Westbrook, C. K.; Ribaucour, M. J. Phys. Chem. A 2007, 111 (19), 3761–3775. (18) Sirjean, B.; Buda, F.; Hakka, H.; Glaude, P. A.; Fournet, R.; Warth, V.; Battin-Leclerc, F.; Ruiz-Lopez, M. Proc. Combust. Inst. 2007, 31, 277– 284. (19) El Bakali, A.; Braun-Unkhoff, M.; Dagaut, P.; Frank, P.; Cathonnet, M. Proc. Combust. Inst. 2000, 28, 1631–1638. (20) Ristori, A.; Dagaut, P.; El Bakali, A.; Cathonnet, M. Combust. Sci. Technol. 2001, 165, 197–228. (21) Dayma, G.; Glaude, P. A.; Fournet, R.; Battin-Leclerc, F. Int. J. Chem. Kinet. 2003, 35 (7), 273–285. (22) Lemaire, O.; Ribaucour, M.; Carlier, M.; Minetti, R. Combust. Flame 2001, 127, 1971–1980. (23) Buda, F.; Heyberger, B.; Fournet, R.; Glaude, P. A.; Warth, V.; Battin-Leclerc, F. Energy Fuels 2006, 20, 1450–1459.

10.1021/ef800694g CCC: $40.75  2009 American Chemical Society Published on Web 11/14/2008

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as critical to properly represent the reactivity of gasoline- and kerosene-based fuels.19 Previous researchers studied some aspects of the kinetics of MCH in a variety of different experimental facilities, such as, low-temperature oxidation in a flow tube,26 unimolecular decomposition in a pyrolysis apparatus,27 oxidation in a sparkignited single-cylinder engine,28 high-temperature (1050-1200 K) pyrolysis and oxidation in a turbulent flow reactor,29 oxidation in a pressurized flow reactor,30 ignition delay times (τign) in rapid compression machines (RCM),14,31,32 decomposition in laminar diffusion flames,33 and ignition in shock tubes.34,35 Zeppieri et al.29 (experimental conditions: 1100-1160 K, 1 atm, and φ ) 1.3) and Kaiser et al.28 (engine operating conditions: 1500-2500 rpm and φ ) 0.9-1.15) noted the substantial presence of 1,3-butadiene during MCH oxidation. Granata et al.36 reported a semi-detailed model of MCH pyrolysis (validated using the measurements of Zeppieri et al.29) and included cyclo-alkanes as reference components to broaden the capabilities of their surrogate model for heavy practical fuels, such as jet fuels, kerosene, and diesel oils. Granata et al.36 pointed out that their modeling study was specifically constrained by the lack of experimental data on the oxidation of MCH. Pitz et al.14 investigated MCH combustion in a RCM at temperatures of 680-980 K and pressures of 10-20 atm. Measured ignition delay times in their study exhibited negativetemperature-coefficient-type (NTC) behavior, and the Pitz et al.14 MCH mechanism predictions showed reasonable agreement with their data. Recently, Mittal and Sung32 measured RCM ignition delay times for MCH/O2/N2/Ar mixtures at temperatures of 680-905 K, pressures of 14.8-25.2 atm, and equivalence ratios (φ) of 0.5-1.5. Mittal and Sung32 noted substantial quantitative discrepancy between their experimental results and the predictions by the Pitz et al.14 MCH mechanism. Hawthorn and Nixon34 measured τign in dilute MCH/O2/Ar mixtures behind incident shock waves in the range of 1200-1480 K, 0.61-1.7 atm, and XMCH ) 0.01-0.17%, for φ ) 0.1-2.1. Recently, Orme et al.35 measured τign behind reflected shock waves in MCH/O2/Ar mixtures (P ) 1-4 atm, T ) 1200-2200 K, φ ) 0.5-2.0, and XMCH ) 0.5-1%) and found good agreement with the results of Hawthorn and Nixon.34 Orme et al.35 also presented a detailed kinetic model for MCH oxidation, (24) Zhang, H. R.; Huynh, L. K.; Kungwan, N.; Yang, Z.; Zhang, S. J. Phys. Chem. A 2007, 111 (19), 4102–4115. (25) Granata, S.; Faravelli, T.; Ranzi, E. Combust. Flame 2003, 132, 533–544. (26) Garner, F. H.; Petty, D. S. Trans. Faraday Soc. 1951, 47, 877– 896. (27) Brown, T. C.; King, K. D. Int. J. Chem. Kinet. 1989, 21, 251–266. (28) Kaiser, E. W.; Siegl, W. O.; Cotton, D. F.; Anderson, R. W. EnViron. Sci. Technol. 1992, 26 (8), 1581–1586. (29) Zeppieri, S.; Brezinsky, K.; Glassman, I. Combust. Flame 1997, 108, 266–286. (30) Agosta, A. Development of a chemical surrogate for JP-8 aviation fuel using a pressurized flow reactor. M.S. Thesis, Drexel University, Philadelphia, PA, 2002. (31) Tanaka, S.; Ayala, F.; Keck, J. C.; Heywood, J. B. Combust. Flame 2003, 132, 219–239. (32) Mittal, G.; Sung, C.-J. An experimental study of autoignition of methylcyclohexane. In Central States Section of The Combustion Institute (CSSCI) 2008 Spring Technical Meeting, Tuscaloosa, AL, 2008. (33) McEnally, C. S.; Pfefferle, L. D. Proc. Combust. Inst. 2005, 30, 1425–1432. (34) Hawthorn, R. D.; Nixon, A. C. AIAA J. 1966, 4, 513–520. (35) Orme, J. P.; Curran, H. J.; Simmie, J. M. J. Phys. Chem. A 2006, 110, 114–131. (36) Granata, S.; Faravelli, T.; Ranzi, E. Combust. Flame 2003, 132, 533–544.

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which was built on the reaction scheme developed by Laskin et al.37 for 1,3-butadiene. High-pressure (above 4 atm) shock tube ignition delays of MCH have not been reported thus far. Shock tubes can be nearly ideal devices for studying ignition phenomena because they can provide relatively well-controlled step changes in temperature and pressure and well-defined time zero and ignition delay times and are generally not affected by impurities or surface or transport problems, at least in large-diameter tubes with good vacuum characteristics. Also, shock tubes can reproduce nearly identical pressures and temperatures from experiment to experiment. Shock tube ignition time measurements of MCH mixtures can thus provide critical validation targets for refinement of MCH kinetic mechanisms. In this work, we measured τign of MCH mixtures over a wide range of conditions, including pressures of 1-50 atm, temperatures of 795-1560 K, equivalence ratios of 0.5-2.0, and for two different bath gases, N2 and Ar, using two different shock tube facilities. The τign values were measured behind reflected shocks using side-wall pressure and OH* and CH* emission diagnostics. The new data are compared to past results and the predictions of several current kinetic mechanisms, including one JP-8 surrogate mechanism (Ranzi et al.38), where MCH is an important JP-8 surrogate component, and two MCH mechanisms (Pitz et al.14 and Orme et al.35). 2. Experimental Method 2.1. High-Pressure Shock Tube Experiments. Ignition delay times in MCH/air mixtures were measured behind reflected shock waves using Stanford’s high-purity, heated, high-pressure shock tube (HPST). The shock tube driver section is 3 m long with a 7.5 cm internal diameter. He was used as the driver gas. Diaphragms were made of aluminum of 1-2 mm thickness with cross-scribing. The stainless-steel driven section of the HPST, which is 5 m long with a 5 cm internal diameter, can be heated up to 200 °C uniformly along its length (to (3 °C). This heating prevents condensation of the fuel/air mixture and allows for large fuel loading of low-vaporpressure fuels, such as MCH in the current work. Before introduction of the test mixture, ultimate pressures of less than 1.33 × 10-3 Pa and leak-outgasing rates of less than 1.33 × 10-2 Pa/min were typically achieved using a turbo-molecular pump. A fuller description of the shock tube is provided by Petersen.39 The incident shock speeds were determined using six fastresponse piezo-electric pressure transducers (PCB 113A), spaced at approximately 30 cm intervals over the last 2 m of the shock tube, and five time-interval counters. Uncertainty in the time interval measurement was (0.3 µs. The velocity of the incident shock at the end wall was then determined by extrapolation, with an uncertainty of (0.3%. Typical shock attenuation rates, defined as the normalized slope of axial velocity extrapolated to the end wall (in %/m), ranged from 0.7 to 2.5%/m for the current experiments. Fill pressure (P1) was monitored using two static pressure transducers (Setra Model 280E), and a K-type thermocouple protruding slightly from the side-wall midway along the driven section was used to determine the preshock test mixture temperature (T1). Using P1 and T1, initial conditions behind the reflected shock wave (temperature T5 and pressure P5) were determined by the onedimensional normal shock equations and the Sandia thermodynamic (37) Laskin, A.; Wang, H.; Law, C. K. Int. J. Chem. Kinet. 2000, 32, 589–614. (38) Ranzi, E. Complete mechanism (low and high temperature). Available at http://www.chem.polimi.it/CRECKModeling/kinetic.html (accessed 2006). (39) Petersen, E. L. A shock tube and diagnostics for chemistry measurements at elevated pressures with application to methane ignition. Ph.D. Thesis, Mechanical Engineering Department, Stanford University, Stanford, CA, 1998.

Shock Tube Study of MCH Ignition

Energy & Fuels, Vol. 23, 2009 177

Figure 1. Experimental setup for emission (L ) lens; PDA 55 ) photodiode).

database of Kee et al.,40 including thermodynamic values for MCH taken from ref 35. Pressure in the test region was monitored using a piezo-electric pressure transducer (Kistler model 603B1) located 10 mm from the end wall. Temperatures and pressures calculated in this study assumed full vibrational relaxation of the shocked gases in both the incident and reflected regimes in accordance with the observations made by Vasu et al.1 and Gauthier et al.41 Mixtures of research-grade (g99.5% pure) MCH (from SigmaAldrich) and high-purity synthetic air (Praxair 79% N2 and 21% O2, >99.999%) were prepared in a 12.8 L, magnetically stirred, thermally insulated, stainless-steel mixing tank. The mixing tank and connecting gas lines to the shock tube were heated to avoid condensation of the fuel and to achieve higher fuel concentrations. In the present study, the mixing tank and connecting gas lines to the shock tube were heated to 110 °C (because the fuel here is at a higher partial pressure than in the shock tube itself prior to the incident shock) and the shock tube was used in both an unheated and heated (105 °C) configuration. The mixing tank was wellinsulated to avoid any spatial temperature non-uniformity, thereby avoiding surface temperature variation and local condensation points inside the tank. Details about the fuel/oxidizer mixing and some discussion about the effects of different mixing times for fuel/ oxidizer mixtures inside the tank and different shock tube temperatures on the ignition delay time measurements are provided in Vasu et al.1 The emission from OH* was detected using a lens/slit/filter setup with a Thorlabs PDA55 detector and Schott Glass UG-5 filter with >95% transmission at 306 nm, at an observation window located at the same axial location as the side-wall pressure transducer (see Figure 1). The axial spatial resolution of the detector system (95% transmission at 306 nm) and CH* (430 nm band-pass filter; fwhm ) 10 nm), detected using a lens/slit setup with Thorlabs PDA55 detectors at the same axial location. The emission setup used was similar to that shown in Figure 1. The onset of ignition was defined by the peak of the CH* and OH* emission signals because this definition gave more consistent and unambiguous indication of the ignition event. Additionally, both the CH* and OH* peaks gave the same ignition delay times as shown in Figure 3. Note that when compared to the high-pressure experiments (e.g., in Figure 2), in the lowpressure experiments (e.g., in Figure 3), pressure is a rather weak indicator of the ignition event. Here again, the overall uncertainty (44) Vasudevan, V.; Davidson, D. F.; Hanson, R. K. J. Phys. Chem. A 2005, 109, 3352–3359. (45) Vasudevan, V.; Davidson, D. F.; Hanson, R. K. Proc. Combust. Inst. 2005, 30, 1155–1163. (46) Horning, D. C.; Davidson, D. F.; Hanson, R. K. J. Propul. Power 2002, 18, 363–371. (47) Herbon, J. T.; Hanson, R. K.; Golden, D. M.; Bowman, C. T. Proc. Combust. Inst. 2002, 29, 1201–1208.


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in the ignition delay time measurements was less than (10%, which was dominated primarily by uncertainties in the reflected temperature (generally less than (1% and mainly because of uncertainties in determining the end-wall velocity).

3. Results and Discussion In comparing experimental data to model predictions of three available mechanisms, the calculations were performed assuming homogeneous, adiabatic conditions behind reflected shock waves, with the common constant-internal-energy, constantvolume constraint (constant U,V) using CHEMKIN 4.1.1.48 Reflected shock wave experiments can, in most cases, be designed to produce stationary, uniform mixtures, and behave like near-ideal constant-volume reactors up to the time of ignition. Typically, for short ignition delay times (less than about 2 ms), the constant U,V constraint is a good assumption for the purpose of ignition delay time calculations (see Davidson and Hanson49 and Petersen and Hanson50 for further discussion on this topic). The predicted OH profiles are used to derive τign using three mechanisms: (1) the Pitz et al.14 MCH mechanism, 1001 species, 4436 reactions, (2) the Orme et al.35 MCH mechanism, 190 species, 904 reactions, and (3) the Ranzi et al.38 JP-8 surrogate mechanism, 280 species, 7800 reactions. It should be noted that the Pitz et al.14 mechanism contained submechanisms for toluene, benzene, C1-C6, and cyclopentadiene, developed by the same authors and adapted from the high-temperature MCH reactions from ref 35. At longer test times, even in the absence of the reaction, the reflected shock pressure (P5) sometimes increases slowly (approximately linearly with time in our facility). This rise is caused by non-ideal effects, such as incident shock attenuation, boundary layer growth, and interaction of the reflected shock wave with the side-wall boundary layer (see Petersen and Hanson50 for details about the non-idealities in our HPST). Recently, we have developed a model, CHEMSHOCK,51 which combines CHEMKIN with an isentropic compression of the test gas mixture consistent with the actual pressure measured during the experiment. This CHEMSHOCK model was employed to describe some of the ignition phenomena (not necessarily just the non-ideal effects) associated with current experiments, as will be discussed later in this paper. The MCH ignition delay time results are presented in the following order: (3.1) low-pressure results and discussion and (3.2) high-pressure results and discussion. 3.1. Low-Pressure Results and Discussion. Low-pressure ignition delay time measurements in MCH/O2 diluted with argon were conducted over a range of conditions: 1.3-2.9 atm, 1225-1560 K, MCH mole fraction (XMCH) from 0.25 to 1%, and equivalence ratios from 0.5 to 2. In most cases, the fuel vapor concentrations inside the shock tube (after filling with test gases) were measured using the mid-IR laser absorption (48) Kee, R. J.; Rupley, F. M.; Miller, J. A.; Coltrin, M. E.; Grcar, J. F.; Meeks, E.; Moffat, H. K.; Lutz, A. E.; Dixon-Lewis, G.; Smooke, M. D.; Warnatz, J.; Evans, G. H.; Larson, R. S.; Mitchell, R. E.; Petzold, L. R.; Reynolds, W. C.; Caracotsios, M.; Stewart, W. E.; Glarborg, P.; Wang, C.; McLellan, C. L.; Adigun, O.; Houf, W. G.; Chou, C. P.; Miller, S. F.; Ho, P.; Young, P. D.; Young, D. J.; Hodgson, D. W.; Petrova, M. V.; Puduppakkam, K. V. CHEMKIN Release 4.1.1, Reaction Design, San Diego, CA, 2007. (49) Davidson, D. F.; Hanson, R. K. Int. J. Chem. Kinet. 2004, 36 (9), 510–523. (50) Petersen, E. L.; Hanson, R. K. Shock WaVes 2001, 10, 405–420. (51) Li, H.; Owens, Z. C.; Davidson, D. F.; Hanson, R. K. Int. J. Chem. Kinet. 2008, 40, 189–198. (52) Klingbeil, A. E.; Jeffries, J. B.; Hanson, R. K. Proc. Combust. Inst. 2007, 31, 807–815.

Figure 4. Low-pressure ignition delay time results for 1% MCH/O2/ Ar, 1.5 atm, and φ ) 1.0. Data (ranged from 1.3 to 2.9 atm, from 0.25 to 1% fuel concentration, and equivalence ratios from 0.5 to 2.0) were scaled using the expression given in the text (eq 1). Constant U,V modeling results shown used the (1) Ranzi et al.,38 (2) Pitz et al.,14 and (3) Orme et al.35 mechanisms.

technique described in Klingbeil et al.52 A total of 67 experiments were performed, and all of the low-pressure data can be correlated using an expression (along with standard errors of the least-squares estimates of parameters) of the form τign (µs) ) 7.5 × 10-8(0.4P (atm)-0.98(0.13 × XMCH-0.82(0.08φ1.47(0.09exp(25560 ( 900/T (K)) (1) Ignition times of MCH, scaled to a pressure of 1.5 atm using eq 1 are shown in Figure 4. The ignition delay times vary with pressure in an expected manner but appear to have a strong dependence upon the equivalence ratio in this temperature range. It should be noted that, over a wide equivalence ratio range, the equivalence ratio dependence may not be accurately represented by a simple logarithmic relationship. This is because ignition time correlations developed under rich conditions will result in higher equivalence ratio sensitivity than those based on lean conditions (see Horning et al.).46 The activation energy obtained for MCH (50.8 kcal/mol) is slightly higher than obtained for n-alkanes (46.6 kcal/mol from n-butane to n-decane) under similar conditions by Horning et al.46 From our current high- and low-pressure results, it is seen that the global activation energies for ignition increase with decreasing pressure, which is similar to the observations in n-heptane by Ciezki et al.53 The low-pressure data can also be correlated in a form explicitly showing the dependence upon the fuel and oxidizer concentrations τign)Z[MCH]0.55(0.11[O2]-1.58(0.13exp(25800 ( 1250/T (K)) (2) where the parameter Z is simply a constant determined from the regression analysis. It should be noted that there is consistency between the two correlation forms (eqs 1 and 2); i.e., both correlations result in essentially the same pressure dependence and activation energy. In eq 2, the sum of fuel and oxygen concentration sensitivities result in pressure dependence, which is close to that given explicitly by eq 1. In the current experiments (according to eq 2), for the same temperatures and pressures, increasing the oxygen concentration [O2] from 5.25% (φ ) 2.0) to 10.5% (φ ) 1.0) for a constant MCH concentration (53) Ciezki, H. K.; Adomeit, G. Combust. Flame 1993, 93, 421–433.


Energy & Fuels, Vol. 23, 2009 179 Table 1. Summary of Current High-Pressure Ignition Time Results in MCH/Air (O ) 1.0)a

Figure 5. Comparison of current low-pressure shock tube ignition delay time results in MCH/O2/Ar (φ ) 1.0) with past data: Hawthorn and Nixon34 at 1.02 atm, Orme et al.35 at 1 atm, current data (1% MCH/ O2/Ar) scaled to 1 atm using the correlation given in the text (eq 1).

[MCH] ) 1% leads to almost a factor of 3 reduction in ignition times. On the other hand, increasing the MCH concentration from 0.5% (φ ) 0.5) to 1% (φ ) 1.0) for a constant oxygen concentration of 10.5% gives only a 50% increase in ignition times. This difference can be explained by analyzing the competition between fuel and oxygen molecules for the reactive radicals produced from the fuel molecule. The positive power dependence on [fuel] and the large negative power dependence on [O2], such as in eq 2, is common to most hydrocarbons.46 These trends are a result of the suppression and promotion of the H + O2 chain branching reaction; i.e, an increase in [fuel] reduces [H] because fuel molecules scavenge the H atom through H-abstraction reactions, but an increase in [O2] promotes chain branching. Figure 5 compares the current low-pressure results and past shock tube measurements by Hawthorn and Nixon34 and Orme et al.35 Below about 1380 K for φ ) 1, the current data appear to be in good agreement in magnitude with the results of both Hawthorn and Nixon34 and Orme et al.35 The agreement between these three data sets (current, Hawthorn and Nixon,34 and Orme et al.35) is encouraging, considering that there may exist possible reasons for small variation in ignition time results among these three shock tube methods, which include (1) different ignition delay time detection scheme and definitions and (2) different diameter shock tubes used (smaller diameter shock tubes are prone to larger uncertainties in shock temperatures). The onset of ignition was identified as the first detectable rise of the light trace from its baseline by Hawthorn and Nixon,34 the maximum rise in the rate of emission by Orme et al.,35 and the peak of the CH* and OH* emission signals by the current authors. Hawthorn and Nixon34 measurements were performed behind incident shock waves, and they used a 7.62 cm square test section. Orme et al. conducted their measurements behind reflected shock waves using a 10.24 cm internal diameter shock tube (compared to the 15.24 cm internal diameter tube used in current experiments). Above 1380 K, current data and Orme et al.35 appear to be diverging but the ignition times are close to or less than 100 µs, and typically, for very short times such as this, τign measurements in shock tubes have larger uncertainties related to the definition and determination of the ignition delay process.

P1 (psi)

T1 (C)

Vshock (mm/µs)

attenuation (%/m)

T5 (K)

P5 (atm)

τign (µs)

29.57 27.37 26.42 25.32 24.76 23.24 22.77 20.22 19.53 17.52 16.01 15.27 14.37 13.58 12.07 12.24 11.42 14.97 13.95 12.82 12.55 11.25 10.04 8.83 11 7.89 29.5 25.48 25.97 25.54 25.46

22.4 22.4 22.3 21.9 21.9 22.8 22 22.7 22.6 22.7 22.5 22.3 22.8 21.9 22.45 21.9 22.5 103.6 104 104.15 103.5 103.8 104.2 104.3 104.2 103.7 104 104 103.7 103.5 104.15

0.7631 0.7864 0.7938 0.8061 0.8085 0.8218 0.8338 0.8509 0.8542 0.8815 0.8855 0.92 0.926 0.938 0.9446 0.9605 0.9627 0.8105 0.8199 0.8281 0.8428 0.8679 0.8731 0.9053 0.9126 0.9378 0.8357 0.8393 0.8638 0.8666 0.8765

1.755 1.905 1.832 1.906 1.873 1.958 1.835 1.974 1.93 2.05 2.183 1.916 2.082 1.993 2.216 2.167 2.504 0.807 0.74 0.977 1.119 1.039 2.4 2.341 0.863 2.36 0.931 0.828 0.922 0.857 0.877

795 827 837 854 857 876 893 918 922 962 968 1019 1029 1046 1057 1081 1085 912 926 937 958 994 1002 1049 1060 1098 948 953 988 992 1007

48.12 49.061 48.817 49.233 48.607 47.79 49.236 46.384 45.37 44.84 41.586 44.618 42.714 42.168 38.173 40.813 38.235 20.456 19.768 18.765 19.508 19.219 17.458 17.246 22.049 17.28 44.524 38.995 43.708 43.469 44.839

1620 1384 1337 1393 1509 1484 1238 1185 1084 664 722 345 318 223 226 133 136 1995 1940 1485 1226 842 637 411 358 273 670 696 440 418 337

a Mixture: MCH ) 1.96%, O ) 20.60%, and N ) 77.44%. V 2 2 shock is the incident shock velocity at the end wall.

The two correlations presented allow for the testing of MCH mechanisms to predict changes in τign with pressure, fuel concentration, equivalence ratio, and temperature. Current results have been compared to the predictions of the Orme et al.,35 the Ranzi et al.,38 and the Pitz et al.14 mechanisms (see Figure 4). All mechanisms predict lower activation energies than the current data, with the Pitz et al.14 mechanism giving the closest (40.4 kcal/mol) agreement. In general, all three mechanisms predict (quantitatively) the current low-pressure data below 3 atm very well. 3.2. High-Pressure Results and Discussion. All highpressure τign data for MCH/air (mixture: MCH ) 1.96%, O2 ) 20.60%, N2 ) 77.44%, and φ ) 1.0) in the range T ) 795-1098 K and P ) 17.2-49.2 atm are summarized in Table 1. The high-pressure τign data for two pressures near 20 and 45 atm, both for an equivalence ratio of 1, are plotted in Figures 6 and 7. There does not appear to be a significant difference in τign measurements (see Figure 6) when the shock tube was heated to T1 ) 105 °C or when the shock tube was kept at room temperature (T1 ) 22.5 °C). Note that. in all figures, data labeled as heated refer to experiments when the shock tube was kept at 105 °C; otherwise, the shock tube was kept at room temperature (T1 ) 22.5 °C). Current data are characterized by small scatter, and we have used the experimentally found (τign ∼ P-0.865) relation to scale τign to nominal pressures in Figure 6 and to 45 atm in Figure 7. It should be noted that the pressure dependence of the ignition delay time is expected to vary with temperature, and researchers have used different pressure scaling in different temperature regimes for similar hydrocarbons (see Yates et al.54). (54) Yates, A. D. B.; Swarts, A.; Viljoen, C. L. SAE Techn. Pap. Ser., Society of Automotive Engineers, Warrendale, PA, 2005; paper 2005-012083.


Figure 6. High-pressure MCH/air ignition delay time results (φ ) 1.0, MCH ) 1.96%, O2 ) 20.60%, and N2 ) 77.44%). High-pressure data scaled to 20 or 45 atm using τign ∼ P-0.865. Low-pressure data scaled to 1.5 atm using eq 1 (φ ) 1.0 and MCH ) 1.962%). Modeling shows τign predictions at 20 atm. Constant U,V modeling results at 20 atm shown using Orme et al.,35 Ranzi et al.,38 and Pitz et al.14 mechanisms.

Figure 7. High-pressure MCH/air ignition delay time results near 45 atm (φ ) 1.0, MCH ) 1.96%, O2 ) 20.60%, and N2 ) 77.44%) and pressure scaling. Data (9) scaled to 45 atm using τign ∼ P-0.865. The gray solid line is a fit through data. Constant U,V modeling results at 45 atm shown using Orme et al.,35 Ranzi et al.,38 and Pitz et al.14 mechanisms.

Accordingly, the current data follows τign ∼ P-0.75 dependence above 912 K (without including the NTC region data points). Because of the changing activation energy in the data, a standard Arrhenius regression expression of current data is not possible; however, in the temperature region above 912 K, an overall activation energy of 24.9 kcal/mol was obtained using the Arrhenius expression. In Figures 6 and 7, at low temperatures (less than about 880 K), ignition delay times show negativetemperature-coefficient-type (NTC) behavior. Linear and branched alkanes and their mixtures (for example, n-heptane and iso(55) Fieweger, K.; Blumenthal, R.; Adomeit, G. Combust. Flame 1997, 109, 599–619.

Vasu et al.

Figure 8. Current high-pressure MCH/air (φ ) 1.0, MCH ) 1.96%, O2 ) 20.60%, and N2 ) 77.44%) τign results. Current data scaled to 20 or 45 atm using τign ∼ P-0.865. RCM data (unheated) from Pitz et al.14 Lines are constant U,V modeling predictions using the Pitz et al.14 mechanism: (1) 10 atm, (2) 20 atm, and (3) 45 atm.

Figure 9. Current high-pressure MCH/air (φ ) 1.0, MCH ) 1.96%, O2 ) 20.60%, and N2 ) 77.44%) τign results. HPST data scaled to 20 or 45 atm using τign ∼ P-0.865. RCM data (unheated) from Pitz et al.14 Solid lines are fit through data at respective pressures. Low-pressure data scaled to 1.5 atm using eq 1 (φ ) 1.0 and MCH ) 1.962%).

octane as in Fieweger et al.55 and Gauthier et al.41) and jet fuels (see Vasu et al.1) strongly show this “S-shaped” dependence in air. Also shown in Figure 6 are the low-pressure data from the current study extrapolated to the following conditions: 1.96% MCH, φ ) 1.0, and 1.5 atm. The near-unity pressure dependence (P-1) of the lower-pressure data and the higher-pressure data extends up to 1560 K. Our results complement earlier ignition time studies of MCH by Pitz et al.14 in a RCM (see Figures 8 and 9), and the combined data sets provide kinetic targets in the range from 650 to 1560 K. The RCM τign values are less reproducible (have more scatter than the current high-pressure data) when time scales reach near 50 ms; see, for example, the observations of Dooley et al.56 By examining all of the data from 45 to 10 atm, it is evident that, in the intermediate temperature range near


Figure 10. High-pressure cyclo-alkanes/air (φ ) 1.0) τign results. Mixtures: MCH ) 1.96%, O2 ) 20.60%, and N2 ) 77.44%; cyclohexane ) 2.28%, O2 ) 20.53%, and N2 ) 77.19%; and cyclo-pentane ) 2.72%, O2 ) 20.44%, and N2 ) 76.84%. Current MCH data scaled to 45 atm using τign ∼ P-0.865. Cyclo-hexane (scaled using τign ∼ P-1.1 and cyclo-pentane (scaled using τign ∼ P-0.9) data from Daley et al.58 Solid lines are fit through data.

850 K, the pressure appears to have the most pronounced influence on the measured ignition delay times. However, on the basis of the RCM measurements, at low temperatures near 700 K, ignition times are almost independent of the pressure. It should be noted that the Pitz et al.14 RCM measurements used three different diluents (100% N2, 50% N2/50% Ar, and 100% Ar) to access their temperature range. According to a recent study by Wu¨rmel et al.,57 the choice of diluent gas affects RCM τign measurements. Wu¨rmel et al.57 reported that the effect of adding argon to nitrogen (changing the diluent gases is a standard procedure used to achieve various compressed temperatures in most RCM facilities) is to decelerate ignition in a RCM at the same compressed temperature and fuel and oxygen concentration compared to pure N2 as a diluent. This increased ignition time is due to the enhanced cooling of Ar in the postcompression period and could be attributed to the effect of the heat capacity of the bulk carrier gas (see Wu¨rmel et al.57 and Davidson and Hanson49). Such an effect could explain the slightly higher activation energy at temperatures (above 833 K) shown by the RCM data at 10 atm than the current data at 20 and 45 atm (see Figure 9). Another observation from the modeling results in Figure 8 is that, as the temperature is decreased, occurrence of NTC behavior is only slightly delayed but stronger at lower pressures near 10 atm than at 45 atm. Daley et al.58 recently measured τign in cyclo-pentane/air and cyclo-hexane/air mixtures at high pressures and high temperatures in a shock tube (their conditions are similar to current experiments in MCH). A comparison of the high-pressure shock tube τign results for three cyclo-alkanes/air are shown in Figure 10 (for 45 atm and φ ) 1.0). Current MCH pressure scaling of τign ∼ P-0.865 and Daley et al.58 pressure scaling of τign ∼ P-1.1 for cyclo-hexane and τign ∼ P-0.9 for cyclo-pentane were used in Figure 10. It is interesting to note that the activation energies (56) Dooley, S.; Curran, H. J.; Simmie, J. M. Combust. Flame 2008, 153, 2–32. (57) Wu¨rmel, J.; Silke, E. J.; Curran, H. J.; O Conaire, M. S.; Simmie, J. M. Combust. Flame 2007, 151, 289–302. (58) Daley, S. D.; Berkowitz, A. M.; Oehlschlaeger, M. A. A shock tube study of cyclopentane and cyclohexane ignition at elevated pressures. Int. J. Chem. Kinet. 2008, 40, 624–634.


Figure 11. τign results (from HPST) in Jet-A and major singlecomponent surrogate fuels (φ ) 1.0). HPST data scaled to 20 atm using respective pressure scaling for individual fuels (see the text). Solid lines are fit through data at 20 atm: iso-octane and toluene from Davidson et al.,60 n-dodecane from Vasu et al.,61 and Jet-A from Vasu et al.1

for these three cyclo-alkanes are nearly the same, i.e, 23.9 kcal/ mol for cyclo-pentane, 24.9 kcal/mol for MCH, and 27.6 kcal/ mol for cyclo-hexane. τign in MCH falls between the data for the other two cyclo-alkanes (see Figure 10), and cyclo-hexane τign values are approximately half of those for cyclo-pentane. This suggests that reactivity of these fuels (for φ ) 1.0) is in the order cyclo-hexane > MCH > cyclo-pentane, which could be attributed to the relative stability of primary cyclo-alkyl radicals formed from these fuels and their propensity to yield H atoms (see Sirjean et al.18 for details on the high-temperature reactivity of cylco-alkanes above T > 1200 K). However, it should be noted that our above observation is true only in the temperature region (above 900 K), where the influence of NTC chemistry is minimal. It is clear from Figure 10 that MCH shows NTC behavior at higher temperatures than compared to cyclohexane at 45 atm. This indicates that propensity for RO2 radical isomerization (and thereby NTC behavior) is higher in MCH and that MCH would be more reactive in the NTC region than cyclo-hexane. Similar conclusions in the NTC region can be reached by comparing the RCM ignition delays of MCH (Pitz et al.14) and cyclo-hexane (Lemaire et al.22), both at pressures near 10 atm. Additionally, at high temperatures and low pressures (see section 3.1), the activation energy obtained in argon-dilute mixtures is almost 2 times that observed at high pressures and moderate temperatures (such as results in this section) with air as the oxidizer. Daley et al.58 observed a similar difference in activation energy, consistent with most hydrocarbon fuels in the literature (such as toluene, n-heptane, iso-octane, and cyclopentane), and attributed this difference because of the influence from the NTC chemistry in high-pressure, moderate-temperature studies. Current MCH measurements are part of our larger goal of building a reference database59 of jet fuel and surrogate component ignition delay times. Shown in Figure 11 is a comparison of high-pressure shock tube ignition delay times (all from the Stanford HPST) for φ ) 1.0 in air (synthetic: 79% N2 and 21% O2) at 20 atm for jet fuel and important jet fuel (see refs 2, 3, and 10) surrogate components. Apart from current (59) Davidson, D. F.; Hanson, R. K. Available at http://hanson. stanford.edu/news.htm (accessed 2006).


MCH data (scaled using τign ∼ P-0.865), Figure 11 includes data for the following fuels: Jet-A from Vasu et al.1 (scaled using τign ∼ P-1), iso-octane and toluene from Davidson et al.60 (scaled using τign ∼ P0.79 and τign ∼ P-0.93, respectively), and n-dodecane from Vasu et al.61 (scaled using τign ∼ P-1). In the hightemperature region (above 960 K), toluene shows the longest ignition time (aromatics are known to be least reactive), and Jet-A, n-dodecane, and MCH have the shortest ignition time. Ignition time results for iso-octane, which is also a main component of gasoline surrogates and a primary reference fuel, lie in between (note that τign results in iso-octane are very close to those of gasoline measured by Gauthier et al.41). n-Dodecane, which is the main n-alkane component of most jet fuel surrogates (see Edwards and Maurice3 and Violi et al.10) and has physical and chemical properties very similar to that of jet fuels, is very reactive for the conditions shown. It should be noted that the starting temperature for the NTC behavior of MCH is very similar to that of jet fuels and above 800 K (Figure 11), and among all of the single-component fuels considered here, MCH has the closest ignition time to that of Jet-A at high pressures. Large n-alkanes (here n-dodecane), which are main components of jet fuel surrogate mixtures, ignite faster under mild conditions than small or branched chain alkanes in the NTC region and show the NTC behavior much earlier (higher temperatures) and exhibit stronger NTC behavior than jet fuel (see Figure 11). Peroxy chemistry plays a more important role in longer straight-chain alkanes, and large n-alkanes ignite faster under milder conditions than branched or cyclo-alkanes (see Miller et al.62). Results in Figure 11 support our earlier conclusion (Vasu et al.1,61) that, in the hightemperature region, using a single-component surrogate (such as MCH above 800 K or n-dodecane above 1000 K) may be adequate to represent certain combustion characteristics (such as ignition delay time) of jet fuels. However, it may be necessary to use multicomponent surrogates in simple surrogate mixtures for jet fuels that can accurately reproduce τign data in the entire temperature regime (especially in the NTC region below 900 K). Model predictions using the mechanisms of Ranzi et al.,38 Pitz et al.,14 and Orme et al.35 are presented in Figures 6 and 7 for 20 and 45 atm, respectively. All mechanisms are able to predict the low-pressure ignition results (Figure 4); however, their predictions are significantly different at high pressures. The Ranzi et al.38 mechanism gives the closest agreement, and the Orme et al.35 mechanism predictions are the farthest from data at high temperatures. In general, all mechanisms predict longer ignition delay times than experimental results, and none of the mechanisms, except the Pitz et al.14 MCH mechanism, exhibits the NTC behavior. However, at the peak near 870 K (before the NTC trend starts in both the 45 atm data and the Pitz et al.14 model), ignition delay times predicted by the Pitz et al.14 mechanism are approximately 5 times larger than data. At temperatures higher than 912 K, the global activation energies for ignition and pressure dependence according to various mechanisms are as follows: 34.0 kcal/mol and τign ∼ P-0.77 (Orme et al.35), 29.5 kcal/mol and τign ∼ P-0.87 (Pitz et al.14), and 32.9 kcal/mol and τign ∼ P-0.76 (Ranzi et al.38), whereas the experimental values are 24.9 kcal/mol and τign ∼ P-0.75. (60) Davidson, D. F.; Gauthier, B. M.; Hanson, R. K. Proc. Combust. Inst. 2005, 30, 1175–1182. (61) Vasu, S. S.; Davidson, D. F.; Hong, Z.; Vasudevan, V.; Hanson, R. K. n-Dodecane oxidation at high pressures: Measurements of ignition delay times and OH concentration time histories Proc. Combust. Inst. 2009, 32, in press, doi: 10.1016/j.proci.2008.05.006. (62) Miller, J. A.; Pilling, M. J.; Troe, J. Proc. Combust. Inst. 2005, 30, 43–88.

Vasu et al.

Figure 12. Measured and predicted (Ranzi et al.38 and Pitz et al.14) pressure-time histories.

In comparison to the other two mechanisms, the Pitz et al.14 mechanism provides slightly better agreement with the experimental pressure dependence and activation energy. However, the Pitz et al.14 mechanism does not predict their RCM measurements very well (see Figure 8) using the constant U,V approach, which was also used by Pitz et al.14 to develop their mechanism by validating against their RCM data. It should be noted that the constant U,V approach may be incorrect for RCM environments and various other approaches (according to Wu¨rmel et al.,57 there is currently no off-the-shelf simulation tool available that allows for the realistic description of combustion in a RCM) have been used in the literature to model RCM experiments mainly because of the heat loss and compression stroke effects in RCMs (see Silke et al.17 and Wu¨rmel et al.57 for details on this topic). Vasu et al.1 found that the Ranzi et al.38 mechanism applied to the Violi et al.10 JP-8 surrogate mixture (which has 10% MCH) predicted a NTC trend similar to that seen in their JetA/air τign data near 20 atm (although the mechanism predicted stronger NTC behavior than the data). However, the Ranzi et al.38 JP-8 mechanism does not predict the NTC behavior seen in MCH (see Figures 6 and 7). Because peroxy chemistry is critical in the NTC region, this observation suggest that the MCH submechanism included in the current detailed Ranzi et al.38 JP-8 mechanism needs to be improved (to accurately model the current MCH and Vasu et al.1 Jet-A data in the NTC region) by adding or modifying reactions for the peroxy reaction channels of the MCH oxidation. Of considerable importance is the finding by Pitz et al.14 that the RCM τign data in the NTC regime for MCH, where MCH oxidation proceeds via the peroxy channels, was found to be greatly influenced by the calculated RO2 (here methylcyclohexylperoxy radical) isomerization rates. Specifically, the Pitz et al.14 computations using n- and isoalkane-based estimates of methylcyclohexylperoxy radical isomerization rate constants predicted ignition delay times too long compared to their experiments. The high-temperature Orme et al.35 mechanism, which was not designed to capture the NTC trend, had previously been validated for temperatures higher than 1058 K only). Comparisons of an experimental pressure profile with the Ranzi et al.38 and the Pitz et al.14 model results are presented in Figure 12 for a 46.4 atm test condition (also shown in Figure 2). These comparisons serve to confirm that, up to the point of


ignition, the constant U,V assumption is a good representation of the shock tube behavior (Davidson and Hanson49). However, simple constant U,V calculations do not capture the experimentally measured pressure oscillations caused by the blast wave after ignition. Because of the nature of the constant U,V assumption, predictions by both mechanisms achieve the same final pressure plateau (at ∼5 ms). The measured pressure profiles at some conditions, including Figure 12, show a weak, approximately linear pressure ramp starting just before ignition, which is defined as the ratio DP/ Dt in Figure 12. This type of behavior has been observed by others in various other fuels in high-pressure shock tube ignition studies53,55,60,63-67 and in RCM ignition studies.68,69 Note that there is no observable pre-ignition rise in the measured OH* emission signal (see Figure 2) for this case. Various hypotheses for this pressure increase (DP/Dt) have been advanced, including chemical heat release before ignition (either at the measurement location or elsewhere in the reflected shock region), different modes or regimes of ignition, multidimensionality of ignition, etc.55,60,68 Hence, the pre-ignition phenomena (or DP/Dt) may or may not be a result of homogeneous chemistry alone (here homogeneous refers to uniformity at a given axial location and not necessarily the entire reflected shock zone) and could be a result of various non-uniform phenomena (such as axial or even three-dimensional inhomogeneities, which might arise in temperature, radical concentration, or particles in the test gas) acting in addition to chemical heat release (see Blumenthal et al.67). In the shock tube, the main cause of temperature inhomogeneities in the reflected shock region is likely to be the interaction of the reflected shock wave with the boundary layer arising from the incident shock (according to Weber et al.70 and Fieweger et al.55). Also, in the current experiments at high-pressures and all temperatures, ignition events can be classified as “strong ignition” (see Oppenheim et al.71,72), which is characterized by large pressure peaks immediately after the main ignition event and by the following large-amplitude pressure oscillations seen in Figure 12 (this is consistent with “strong ignition” observations in n-heptane/air by Ciezki et al.53 and in iso-octane/air by Fieweger et al.55 in their shock tube ignition studies). For the current MCH experiments, some qualitative observations can be made about the nature and magnitude of DP/Dt. Figure 13 provides pressure profiles from HPST experiments for different temperatures (T5) and two pressure regions (20 and 45 atm). We found that the DP/Dt values are slightly lower for heated shock tube experiments for the same P5 and T5 compared to room-temperature experiments and do not show any strong (63) Herzler, J.; Jerig, L.; Roth, P. Combust. Sci. Technol. 2004, 176, 1627–1637. (64) Fieweger, K.; Blumenthal, R.; Adomeit, G. Proc. Combust. Inst. 1994, 25, 1579–1585. (65) Fieweger, K.; Blumenthal, R.; Adomeit, G. In Proceedings of the 19th International Symposium on Shock WaVes; Brun, R., Dumitrescu, L. Z., Eds.; Springer-Verlag: Berlin, Germany, 1993; Vol. 2, pp 161-166. (66) Pfahl, U.; Fieweger, K.; Adomeit, G. Proc. Combust. Inst. 1996, 25, 781–789. (67) Blumenthal, R.; Fieweger, K.; Komp, K. H.; Adomeit, G. Combust. Sci. Technol. 1996, 113-114, 137–166. (68) Walton, S. M.; He, X.; Zigler, B. T.; Wooldridge, M. S.; Atreya, A. Combust. Flame 2007, 150, 246–262. (69) He, X.; Donovan, M. T.; Zigler, B. T.; Palmer, T. R.; Walton, S. M.; Wooldridge, M. S.; Atreya, A. Combust. Flame 2005, 142, 266–275. (70) Weber, Y. S.; Oran, W. S.; Boris, J. P.; Anderson, J. D., Jr. In Proceedings of the 20th International Symposium on Shock WaVes; Sturtevant, B., Shepherd, J. W., Hornung, H. G., Eds.; World Scientific: Hackensack, NJ, 1995; Vol. 1, pp 801-806. (71) Oppenheim, A. K. Philos. Trans. R. Soc., A 1985, 31 (5), 471– 508. (72) Vermeer, D. J.; Meyer, J. W.; Oppenheim, A. K. Combust. Flame 1972, 18, 327–336.


Figure 13. HPST pressure-time histories for MCH/air (φ ) 1.0) ignition. (A) Near 45 atm and (B) near 20 atm.

dependency upon pressure, i.e., no variation with P5 when T5 is constant (not shown here). However, at 795 K (the lowest temperature near 45 atm, which is labeled as 3 in Figure 13), the pressure starts to behave more like a two-stage ignition than showing a linear DP/Dt as defined in Figure 12. It should also be noted that, above 950 K, the duration (Dt) of pressure ramp decreases with an increasing temperature in MCH/air mixtures (current experiments), and this trend is consistent with observations made by Pfahl et al.66 in their high-pressure shock tube experiments performed in stoichiometric R-methylnaphthalene/ air mixtures at 12.8 atm. Modeled pressure-time histories for MCH/air at 20 atm using the Pitz et al.14 mechanism (shown in Figure 14) do not fully duplicate the experimental DP/Dt; however, a two-stage pressure increase at low temperatures (725 and 775 K) and an approximately linear increase in pressure P5 at high temperatures (825 and 875 K) are observed. As seen from Figure 14, in modeled pressures, typically even with the simple constant U,V assumption, the P5 rises to about 25 atm (starting from 20 atm) before ignition because of pre-ignition chemical heat release alone. These modeling results provide potentially important evidence that the observed DP/Dt behavior is not only due to non-ideal shock tube effects (including inhomogeneous reaction). The effect of DP/Dt on τign may need to be taken into consideration when comparing modeled and experimental results especially at longer ignition delay times (see Pfahl et al.66). Although developed to deal principally with facility-dependent


Vasu et al.

HO2 + HO2 ) H2O2 + O2

(R1)

H2O2(+M) ) OH + OH(+M)

(R2)

and

Figure 14. Computed pressure-time histories for MCH/air ignition at 20 atm, φ ) 1.0, Pitz et al.14 mechanism.

non-idealities in shock tubes, the CHEMSHOCK51 program from our laboratory can be applied along with the measured pressure profiles to assess the effect of DP/Dt on the modeled τign. A similar approach was used by Pang et al.,73 and they concluded that the use of the experimental pressure trace and the CHEMSHOCK model more accurately modeled the reflectedshock ignition process in hydrogen than the traditional approach using CHEMKIN with a constant U,V constraint. This new approach allows for combined facility-dependent effects and energy release phenomena in the reflected shock environment. It should be noted that CHEMSHOCK assumes stationary, homogeneous conditions within the test gas volume monitored but does not require axial homogeneity throughout the reflected shock region. The CHEMSHOCK ignition delay times predicted by the Orme et al.35 mechanism using the measured pressure profile are only 15% higher than the experimentally measured τign value at P5 ) 20.46 atm and T5 ) 912 K (note that this P5 and T5 represent an extreme case where τign is long). In the current simulations, the pressure was approximated by assuming a linear extrapolation beyond the experimental ignition point (with the same slope, i.e., DP/Dt) until simulation showed ignition (see Pang et al.73 for details). Because CHEMSHOCK is a zero-dimensional approach, this latter assumption may force the modeled gases to artificially ignite faster because of the extended increase in pressure (and temperature) beyond the actual experimental ignition point. Because the experimental pressure trace does not differentiate between a rise (i.e., DP/ Dt) as a result of non-ideal gasdynamics or energy release from chemical reaction (somewhere in the reflected shock region), caution should be applied in drawing kinetic conclusions about predictability of MCH kinetic mechanisms when used with the CHEMSHOCK modeling approach. Efforts are currently underway in our laboratory to develop and validate more advanced versions (one-dimensional and above) of the CHEMSHOCK program to model shock tube experiments. Ignition delay time sensitivity analyses were performed using both the Pitz et al.14 and the Orme et al.35 mechanisms. The key reactions that influence the ignition times at both 910 and 1110 K (for P ) 20 atm) are nearly the same. Specifically, the formation and decomposition of H2O2 via (73) Pang, G. A.; Davidson, D. F.; Hanson, R. K. Experimental study and modeling of shock tube ignition delay times for hydrogen-oxygenargon mixtures at low temperatures. Proc. Combust. Inst. 2009, 32, in press, doi: 10.1016/j.proci.2008.06.014.

are the dominant reactions influencing τign. We have not tried to adjust the mechanisms by modifying k1 and k2 within their uncertainty limits; however, the Baulch et al.74 review estimated an uncertainty factor of 2.51 for k1, and according to ref 75, no direct measurements have been performed for R1 above 600 K and very few experimental data exist for k2. Recently, He et al.69 provided a best-fit value for k2 using their ignition studies of iso-octane at high pressures. Apart from reactions R1 and R2, H2O2 formation via H-abstraction reactions from MCH by HO2 radical, i.e., MCH + HO2 ) H2O2 + methylcyclohexyl isomers (three isomers formed at primary, secondary, and tertiary sites), have considerable impact on the predictions of mechanisms, and these rate constants only have been estimated and not measured. Hence, the above-mentioned reactions (especially R1 and R2) should be measured directly to enable accurate modeling of MCH ignition times at engine-relevant conditions (such as those presented in the current work). Hydrocarbon ignition is, to a large extent, controlled by the chemistry of the small transient radical pool (H, OH, CH3, etc.), and in particular, very little or no information is available for these species. At higher pressures, most current mechanisms have been validated only against the measured yields of the more stable intermediates or against ignition delay times alone and small radical species time-history data are needed for complete mechanism validation. We have performed measurements of OH concentration time histories using laser absorption techniques to this end, and the results of these experiments are published elsewhere.76 4. Conclusions We have measured ignition delay times in MCH/O2 mixtures over a wide range of conditions, including pressures of 1-50 atm, temperatures of 795-1560 K, equivalence ratios of 0.5-2.0, and for two different bath gases N2 and Ar, using two different shock tube facilities. Correlations of the current low-pressure data (below 3 atm) show a strong dependence upon the equivalence ratio and oxygen concentration. In addition, we found good agreement between the ignition delay time predictions of several current mechanisms and the current low-pressure experiments. The current high-pressure measurements were characterized by relatively low scatter, exhibited “strong ignition” features, showed NTC behavior at 45 atm for temperatures below 880 K, and are fit by a pressure dependence of τign ∼ P-0.865. In the high-temperature region above 912 K, an overall activation energy of 24.9 kcal/mol was obtained. These high-pressure measurements complement earlier ignition time studies of MCH (74) Baulch, D. L.; Cobos, C. J.; Cox, R. A.; Frank, P.; Hayman, G.; Just, Th.; Kerr, J. A.; Murrells, T.; Pilling, M. J.; Troe, J.; Walker, R. W.; Warnatz, J. J. Phys. Chem. Ref. Data 1994, 23, 847–1033. (75) Manion, J. A.; Huie, R. E.; Levin, R. D.; Burgess, D. R., Jr.; Orkin, V. L.; Tsang, W.; McGivern, W. S.; Hudgens, J. W.; Knyazev, V. D.; Atkinson, D. B.; Chai, E.; Tereza, A. M.; Lin, C.-Y.; Allison, T. C.; Mallard, W. G.; Westley, F.; Herron, J. T.; Hampson, R. F.; Frizzell, D. H. NIST Chemical Kinetics Database. NIST Standard Reference Database 17 (Web Version), Release 1.4.2, Data Version 08.09; National Institute of Standards and Technology (NIST), Gaithersburg, MD, available at http://kinetics. nist.gov/. (76) Vasu, S. S.; Davidson, D. F.; Hanson, R. K. OH time-histories during oxidation of n-heptane and methylcyclohexane at high pressures and temperatures. Combust. Flame 2008 (doi: 10.1016/j.combustflame.2008.09.006).


in a RCM by Pitz et al.14 and extend both the pressure and temperature range of available MCH ignition time data. Finally, a comparison of the current MCH measurements and earlier fuel surrogate measurements from our laboratory indicates that MCH may be an effective single-component surrogate for jet fuel ignition times at high pressures over a wide temperature range that includes the NTC regime above 800 K.

Energy & Fuels, Vol. 23, 2009 185 Acknowledgment. This research was sponsored by the Army Research Office (ARO), with Dr. Ralph Anthenien, Jr. as the technical monitor. The authors thank Nihir Parikh and Dr. Adam Klingbeil of Stanford University for their help with low-pressure experiments. EF800694G

Shock Tube Study of Methylcyclohexane Ignition over a Wide Range

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