Article pubs.acs.org/EF
Experimental Study of the High-Temperature Autoignition of Tetralin Haowei Wang,*,† William J. Gerken,‡ Weijing Wang,‡ and Matthew A. Oehlschlaeger‡ †
Department of Mechanical Engineering, California State University, Fullerton, California 92831, United States Department of Mechanical, Aerospace, and Nuclear Engineering, Rensselaer Polytechnic Institute, Troy, New York 12180, United States
‡
ABSTRACT: The autoignition of tetralin, a naphthenic and aromatic hydrocarbon found in jet and diesel fuels and a hydrogen donor with potential as an endothermic fuel and for use in heavy crude oil upgrading processes, has been studied in shock-tube experiments. Measurements of ignition delay times for tetralin/air mixtures at equivalence ratios of 0.5 and 1.0 were made in stationary reflected shock-heated gases at nominal pressures around 13 and 37−39 bar and temperatures from 978 to 1277 K, conditions relevant to practical combustion devices, such as internal combustion engines and jet engine gas turbine combustors. The dependence of the ignition delay time upon the temperature, equivalence ratio, and pressure in the examined condition space can be described with a simple four-parameter correlation. Measurements compared the recent kinetic model by Dagaut et al. (Dagaut, P.; Ristori, A.; Frassoldati, A.; Raravelli, T.; Dayma, G.; Ranzi, E. Energy Fuels 2013, 27, 1576−1585) to good a priori agreement between the model and experiment. Comparisons between the present autoignition measurements for tetralin and previous results for decalin, toluene, and cyclohexane illustrate the relative reactivity for compounds containing aromatic, naphthenic, and combined structures. a pressurized flow reactor (8 bar and 600−800 K). Yang and Boehman13 have investigated tetralin in a variable compression ratio motored engine, finding that tetralin exhibited little lowtemperature oxidation chemistry prior to reaching the critical conditions necessary for autoignition. Yang and Boehman13 and Yang et al.14 also made comparisons of the oxidative reactivity under engine conditions of a number of hydrocarbons containing naphthenic structures. Most recently, Dagaut et al.15 studied the oxidation of tetralin in a jet-stirred reactor (790−1400 K, 1 and 10 atm, and 1000 ppm of fuel at equivalence ratios of 0.5−1.5), measuring a large number of stable major and minor species. Dagaut et al.15 also reported a semi-detailed kinetic model (>400 species and 10 000 reactions) for tetralin oxidation, the first kinetic model for tetralin oxidation to our knowledge. This model, similar to a previously reported model for decalin,16,17 uses 30 lumped reactions to describe the consumption of tetralin and formation of fragments, which then decompose and oxidize through a hydrocarbon reaction mechanism established by Ranzi and coworkers.18,19 Here, we present ignition delay time measurements for tetralin, to our knowledge for the first time. Ignition delay times are an important validation target for kinetic model development, because they provide one means of quantifying fuel reactivity and its dependence upon conditions. Comparisons are made to the recently reported Dagaut et al. model and measurements for other aromatic and naphthenic hydrocarbons.
1. INTRODUCTION Tetralin (1,2,3,4-tetrahydronaphthalene, C10H12), a bicyclic, naphthenic, and aromatic hydrocarbon, whose structure is shown in Figure 1, is a component of jet1 and diesel2 fuels. As a
Figure 1. Structure of tetralin.
hydrogen donor, it also has advantageous properties as an endothermic fuel3 and for use in hydrocracking processes for heavy crude oil and biomass pyrolysis oil upgrading.4,5 Because of the interest of tetralin as an endothermic fuel, a number of studies have investigated tetralin pyrolysis kinetics. Tetralin unimolecular decomposition was studied at hightemperature single-pulse shock-tube conditions (1000−1400 K) by Tsang and Cui,6 who determined rates for channels resulting in the formation of ethylene + benzocyclobutene and o-allyltoluene. Both Stewart et al.7 and Yu and Eser8 investigated the supercritical pyrolysis of tetralin in flow reactors observing naphthalene and methylindane as major products, with increased naphthalene and decreased methylindane yields with an increasing temperature. Li et al.9 studied the pyrolysis of tetralin under sub-atmospheric pressure flow reactor conditions (30 Torr and 850−1500 K), finding major products of dihydronaphthalenes, naphthalene, indene, indenyl radicals, and styrene. Detailed kinetic models for tetralin pyrolysis have been developed by Bounaceur et al.,10 Poutsma,11 and Li et al.9 Previous studies of the oxidation of tetralin are more limited. Lenhert et al.12 investigated the low-temperature oxidation of jet fuel surrogate mixtures containing tetralin as a component in © 2013 American Chemical Society
Received: June 22, 2013 Revised: July 30, 2013 Published: August 1, 2013 5483
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2. EXPERIMENTAL SECTION Measurements of ignition delay times were made in stationary reflected shock-heated gases in the heated high-pressure shock tube at Rensselaer. The facility, described in detail by Oehlschlaeger and coworkers,20,21 is comprised of a 4.1 m long driven section and a 2.6 m long driver and has a constant 5.7 cm inner diameter. For measurements of ignition delay times for tetralin/air mixtures, the shock-tube driven section and the mixture preparation vessel were heated to temperatures from 383 to 390 K. The axial uniformity of the shock-tube driven section wall temperature is routinely measured; an example from the present study is shown in Figure 2. Axial
Figure 3. Example of the incident shock velocity measurement. model 603B1), flush mounted in the driven section side wall at a location 2 cm from the end wall, and electronically excited OH emission around 300−320 nm, monitored with a filtered (UG-5 Schott glass) photodetector (Thorlabs model PDA36A). An example of the ignition delay measurement is shown in Figure 4. The example shows
Figure 2. Example of the internal wall temperature profile for the heated driven shock-tube section. temperature uniformity of the driven section is important because temperature stratifications in the initial uncompressed test gas result in temperature stratifications in the reflected shock-compressed test gas very near the shock-tube end wall, because of the drastic compression provided by the incident and reflected shock waves, which can lead to undesired inhomogeneities in the spatial distribution of reactivity in the reflected shock region. Tetralin/air mixtures were prepared external to the shock tube in a mixing vessel containing a rotating vane for gas mixing. Mixtures were formulated by injecting liquid tetralin and allowing it to fully vaporize in the heated mixing vessel (390 K), followed by the addition of N2 and O2 from high-pressure gas cylinders. Mixture fractions were determined manometrically, and mixtures were allowed to mechanically mix for a minimum of 20 min. Tetralin was from Aldrich at 99% purity, and O2 and N2 were from Noble Gas Solutions at 99.995% purity. Shock waves were generated by filling the driver section with helium and bursting polycarbonate diaphragms using a fixed knife edge. The incident shock velocity was measured with five piezoelectric pressure transducers (PCB model 112A05) spaced over the last meter of the driven section and four high-speed counter timers (Phillips model PM6666) to provide time intervals for shock passage. The incident shock velocity attenuates as the wave propagates through the driven section because of viscous gasdynamics; an example of the measured incident shock velocity profile is shown in Figure 3. The shock velocity is linearly extrapolated to the end wall to determine the velocity at the end wall test location. The post-incident and post-reflected shock conditions are determined using the normal shock relations with the end wall shock velocity and mixture thermodynamic properties, calculated using the Goos et al.22 thermodynamic data set as inputs. Uncertainties in reflected shock temperature and pressure are ±1 and ±1.5% (2σ confidence interval) with contributions from uncertainties in the measured shock velocity, initial conditions (temperature, pressure, and mixture fractions), and thermodynamic properties. Ignition delay time determinations were made using measurements of pressure, monitored with a piezoelectric pressure transducer (Kistler
Figure 4. Example of the tetralin/air ignition delay time measurement. Profiles are side-wall pressure (2 cm from shock-tube end wall) and OH* emission viewed through the end wall.
a gradual rise in pressure, following incident and reflected shock heating, over the course of the induction period prior to the rapid rise in pressure and OH emission associated with ignition. The slight pressure gradient, because of viscous gasdynamics, has been measured to be (dP/dt)(1/P0) = 2−3% ms−1 for experiments at the range of conditions reported here. After an induction period, the pressure OH emission signals rapidly rise at the onset of ignition. The ignition time has been defined as the time period between the reflection of the shock at the end wall, determined from the passage of the shock wave at the transducer location 2 cm upstream of the end wall, and the onset of ignition at the end wall, defined by extrapolating the maximum slope in the end wall OH emission signal to the baseline. The uncertainty in ignition delay is estimated at ±20% based on the dependence of ignition delay times upon the temperature, pressure, and reactant fuel/air mixture composition, the uncertainties in those metrics, and also the uncertainty in determining ignition delay from the measured OH* emission and pressure profiles. 5484
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τ = 2.8 × 10−9ϕ−0.66P−0.68 exp(15300 K/T )
3. RESULTS AND DISCUSSION Ignition delay measurements for tetralin/air mixtures were carried out at equivalence ratios (ϕ) of 0.5 and 1.0 and at the temperatures and pressures specified in Table 1. The raw
(s)
where the pressure P is in bar. In Figure 6, all of the experiments have been scaled to a common condition using the
Table 1. Ignition Delay Time Measurements for Tetralin/Air Mixturesa tetralin/air, ϕ = 0.5: 0.8015 mol % tetralin, 20.67% O2, and 78.36% N2
a
tetralin/air, ϕ = 1.0: 1.590 mol % tetralin, 20.84% O2, and 77.74% N2
P (bar)
T (K)
τ (μs)
P (bar)
T (K)
τ (μs)
14.8 14.6 13.0 14.9 14.1 13.6 13.7 12.2 12.3 36.0 38.8 36.7 38.2 38.1 35.3 34.9 34.9 35.0
1073 1092 1100 1111 1120 1121 1171 1206 1243 1019 1058 1079 1083 1120 1142 1180 1184 1209
1099 918 864 579 553 552 345 233 157 1226 785 523 499 283 238 179 163 117
13.8 14.1 13.9 12.6 11.6 13.7 13.2 13.2 12.4 41.0 40.6 38.8 36.9 38.2 38.6 38.3 38.0
1036 1077 1125 1134 1178 1195 1202 1248 1277 978 993 1036 1055 1102 1103 1125 1136
1327 648 387 365 246 192 166 95 75 1176 1020 647 469 236 240 162 142
Figure 6. Ignition delay time measurements scaled to ϕ = 1 and 13 bar using the correlation given in the text.
correlation to demonstrate its effectiveness in describing the present data (standard deviation of data about correlation is ±8.5%). Comparisons are made in Figure 7 between the present ignition delay times and the only known tetralin oxidation
τ is the ignition delay time.
Figure 7. Comparison of the present tetralin/air ignition delay time measurements (symbols) to modeling predictions from the Dagaut et al. kinetic model (lines).
Figure 5. Ignition delay time measurements for tetralin/air mixtures with linear best fits.
kinetic model, the recent model reported by Dagaut et al.15 Ignition delay simulations were performed with the Daguat et al. model using CHEMKIN-PRO,23 using the closed adiabatic reactor model. Ignition delay times were defined from the modeled ground-state OH profiles using the maximum gradient method also used to define ignition from the experimental OH* profiles. Comparisons between the Dagaut et al. model and the present experiments illustrate generally good agreement. The model predicts the measured temperature dependence and the high pressure (37−39 bar) ignition delay data
experimental data are displayed in Figure 5 on Arrhenius axes with linear best fits and are given in Table 1. These data and linear best fits illustrate constant apparent activation energy for the studied temperature range (978−1277 K) and experimental scatter that is appreciably less than the ±20% uncertainties. For the range of conditions studied, the ignition delay times decrease with an increasing pressure and equivalence ratio, and the data set is well-described using a simple four-parameter correlation determined through regression analysis 5485
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within the ±20% experimental uncertainties. At 13 bar, the model−experiment deviation is a bit larger, with the model predicting ignition delay at most 50% longer than the experiment. Given that the Dagaut et al. model was validated using jet-stirred reactor (JSR) experiments at slightly lower pressures (1 and 10 atm) and significantly less fuel loading (1000 ppm) than the present study, we find the model− experiment comparisons shown in Figure 7 quite good. The relative agreement suggests that the Dagaut et al. model is suitable for predicting reactivity at the high-pressure high-fuelloading conditions found in internal combustion and gas turbine engines. Reaction flux analysis was carried out using the Dagaut et al. model to determine the important tetralin-specific reaction pathways as contained in the model. At the present temperatures ( decalin > cyclohexane. The large difference between toluene and cyclohexane, 5486
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(20) Shen, H. P. S.; Vanderover, J.; Oehlschlaeger, M. A. Combust. Flame 2008, 155, 739−755. (21) Wang, H.; Warner, S. J.; Oehlschlaeger, M. A.; Bounaceur, R.; Biet, J.; Glaude, P. A.; Battin-Leclerc, F. Combust. Flame 2010, 157, 1976−1988. (22) Goos, E.; Burcat, A.; Ruscic, B. Ideal Gas Thermochemical Database with Updates from Active Thermochemical Tables; ftp://ftp. technion.ac.il/pub/supported/aetdd/thermodynamics and http:// garfield.chem.elte.hu/Burcat/burcat.html (accessed 2012). (23) Reaction Design, Inc. CHEMKIN-PRO, Release 15101; Reaction Design, Inc.: San Diego, CA, 2010. (24) Shen, H. P. S.; Vanderover, J.; Oehlschlaeger, M. A. Proc. Combust. Inst. 2009, 32, 165−172. (25) Daley, S. M.; Berkowitz, A. M.; Oehlschlaeger, M. A. Int. J. Chem. Kinet. 2008, 40, 624−634. (26) Wang, H.; Oehlschlaeger, M. A.; Dooley, S.; Dryer, F. L. A shock tube and kinetic modeling study of the autoignition of npropylbenzene. Proceedings of the U.S. National Meeting on Combustion; Atlanta, GA, March 20−23, 2011. (27) Shen, H. P. S.; Steinberg, J.; Vanderover, J.; Oehlschlaeger, M. A. Energy Fuels 2009, 23, 2482−2489.
ignition delay upon pressure and equivalence ratio and Arrhenius exponential dependence upon inverse temperature. The data are compared to the recently reported detailed kinetic model for tetralin oxidation by Dagaut et al.,15 with good a priori agreement between the model and the present ignition delay measurements. Reaction flux analysis performed with the Dagaut et al. model indicates that tetralin is nearly wholly consumed via H abstraction reactions under the present conditions to form tetralyl radicals, which decompose to form naphthalene and to a lesser extent indene. Comparisons of the tetralin ignition delay measurements to measurements made at common conditions for other compounds containing naphthenic and aromatic structures illustrate the dramatic promoting effect that the naphthenic ring has on the relatively unreactive aromatic ring and the mild stabilization that the addition of a second naphthenic ring or aromatic ring has on the monocyclic cyclohexane. The present data set should be valuable for the future development and validation of kinetic models for tetralin and other bicyclic hydrocarbon compounds.
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AUTHOR INFORMATION
Corresponding Author
*Telephone: 657-278-7913. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the U.S. Air Force Office of Scientific Research (Grant FA9550-11-1-0261), with Dr. Chiping Li as the technical monitor.
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REFERENCES
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