Energy Fuels 2010, 24, 3840–3849 Published on Web 06/25/2010
: DOI:10.1021/ef100074v
Flame Propagation and Extinction Characteristics of Neat Surrogate Fuel Components Kamal Kumar* and Chih-Jen Sung Department of Mechanical Engineering, University of Connecticut, Storrs, Connecticut 06269 Received January 21, 2010. Revised Manuscript Received May 3, 2010
An experimental and computational investigation into the flame propagation and extinction characteristics of neat hydrocarbon components relevant to real transportation fuels is conducted. Pure hydrocarbon fuels corresponding to linear alkane, branched alkane, aromatic, and cyclic alkane are studied. The atmospheric pressure laminar flame speeds for the neat fuels, over a range of equivalence ratios, are measured and compared at varying preheat temperatures. A comparison of the present experimental results to the limited reported experimental data and the computed values, obtained using kinetic mechanisms available in the literature, is also carried out. Additionally, flame extinction studies, both computational and experimental, are presented and discussed. An attempt is further made to assess the performance of some recently developed reaction mechanisms for n-alkane oxidation. Further, the sensitivity of the computed flame response to the kinetic rate constants as well as the transport parameters is evaluated. The present data set is expected to provide insight into the relative response of various hydrocarbon fuel classes for high-temperature flame conditions.
In this work, we present a comparative study of the laminar flame speeds and extinction stretch rates for four different classes of hydrocarbons typically present in real transportation fuels. The classes of hydrocarbon fuels considered are linear alkane, branched alkane, aromatic, and cycloalkane. The components of interest include n-heptane, iso-octane, toluene, n-decane, n-dodecane, and methylcyclohexane. Moreover, n-heptane, iso-octane, and toluene are commonly used surrogate components for gasoline. The binary blends of n-heptane and iso-octane are referred to as the primary reference fuels (PRF), and the ternary mixtures that include toluene are called toluene reference fuels (TRF).6 In addition, kerosene-type fuels, such as Jet-A and JP-8, are represented by surrogate mixtures that typically include n-decane, n-dodecane, and methylcyclohexane.10 Furthermore, the experimentally obtained results for the laminar flame speeds and extinction stretch rates are compared to the computed values using kinetic schemes reported in the literature. Sensitivity analysis is also conducted to identify the key reaction steps responsible for flame propagation and extinction. The sensitivity of the computed flame response to the transport parameters is further evaluated as well. In the following sections, we shall first describe the experimental setup and the methodologies for determining the laminar flame speed and extinction stretch rate. Subsequently, the experimental and computed results will be presented, followed by sensitivity analysis and a discussion on the results.
Introduction The approach of using a surrogate fuel comprising a small number of neat hydrocarbons to simulate a real transportation fuel, such as gasoline, jet fuel, and diesel, is a common practice in combustion research.1-9 For a properly chosen surrogate, the development of relevant kinetic schemes for predicting the combustion properties of the target real fuel also becomes tractable by having a selected number of components in the surrogate fuel. Clearly, the accuracy of a surrogate model hinges on the comprehensive kinetic submodels for neat components. Hence, the logical starting point in the process of understanding complex real fuels is characterizing the neat constituents. One of the important validation targets for combustion mechanisms has been the laminar flame speed. The laminar flame speed is a global marker for the reactivity and exothermicity of a given fuel/oxidizer mixture. Another important global validation target for the flame environment is the extinction stretch rate. The extinction stretch rate represents a kinetics-affected phenomenon and characterizes the interaction between a characteristic flame/flow time and a chemical time. *To whom correspondence should be addressed. E-mail: kamal@ engr.uconn.edu. (1) Minetti, R.; Carlier, M.; Ribaucour, M.; Therssen, E.; Sochet, L. R. Symp. (Int.) Combust., [Proc.] 1996, 26, 747–753. (2) Violi, A.; Yan, S.; Eddings, E. G.; Sarofim, A. F.; Granata, S.; Faravelli, T.; Ranzi, E. Combust. Sci. Technol. 2002, 174, 399–417. (3) Tanaka, S.; Ayala, F.; Keck, J. C. Combust. Flame 2003, 133, 467– 481. (4) Gauthier, B. M.; Davidson, D. F.; Hanson, R. K. Combust. Flame 2004, 139, 300–311. (5) 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. (6) Andrae, J.; Johansson, D.; Bj€ ornbom, P.; Risberg, P.; Kalghatgi, G. Combust. Flame 2005, 140, 267–286. (7) Lenhert, D. B.; Miller, D. L.; Cernansky, N. P.; Owens, K. G. Combust. Flame 2009, 156, 549–564. (8) Sakai, Y.; Miyoshi, A.; Koshi, M.; Pitz, W. J. Proc. Combust. Inst. 2009, 32, 411–418. (9) Bieleveld, T.; Frassoldati, A.; Cuoci, A.; Faravelli, T.; Ranzi, E.; Niemann, U.; Seshadri, K. Proc. Combust. Inst. 2009, 32, 493–500. r 2010 American Chemical Society
Experimental Specifications The laminar flame speeds and extinction stretch rates of the fuel/O2/N2 mixtures are determined using the counterflow twinflame technique, as demonstrated in our previous studies.11-13 (10) Schulz, W. D. Prepr.;Am. Chem. Soc., Div. Pet. Chem. 1991, 37, 383–392. (11) Hirasawa, T.; Sung, C. J.; Joshi, A.; Yang, Z.; Wang, H.; Law, C. K. Proc. Combust. Inst. 2002, 29, 1427–1434. (12) Huang, Y.; Sung, C. J.; Eng, J. A. Combust. Flame 2004, 139, 239–251. (13) Kumar, K.; Sung, C. J. Combust. Flame 2007, 151, 209–224.
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: DOI:10.1021/ef100074v
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The velocity field is obtained by means of digital particle image velocimetry (DPIV). On the basis of the measured flow field, the axial velocity profile along the nozzle central axis is extracted along with the radial velocity profile at the reference location. The reference location is chosen as the location of axial velocity minimum upstream of the flame. This axial velocity minimum is also defined as the reference stretch-affected flame speed. Because the radial velocity profile at the reference location varies linearly with the radial distance, the stretch rate K is determined on the basis of twice the value of the radial velocity gradient.11 On the basis of the variation of reference flame speed with K, the unstretched laminar flame speed can be determined by the methodology of either linear or nonlinear extrapolation to zero stretch rate.11-13 Because the Karlovitz numbers for the experimental conditions investigated herein are typically less than 0.1, the accuracy of linear extrapolation can be reduced to be within the experimental uncertainty.14,15 Thus, the linearly extrapolated laminar flame speeds are reported herein. The DPIV details as applied to the current experiments have been documented elsewhere.11-13 Further, the flow control and mixture preparation system for handling high-boiling-point, lowvapor-pressure, liquid fuels has been detailed and demonstrated in an earlier study.13 Additional details about the measured velocity profiles and data reduction procedure can be found in the supporting information. The same experimental configuration is used to determine the extinction stretch rate as well. Counterflow twin flames are first established close to the point of extinction, and the stretch rate is then slowly increased by gradually increasing the overall flow rate through the opposing burners, until extinction is observed. Furthermore, a sequence of images captured just before the extinction is processed to determine the stretch rate from the velocity maps. The extinction stretch rate values reported herein are the averages deduced from velocity maps obtained from repeated runs. Again, the reported stretch rate is based on twice the radial velocity gradient at the axial velocity minimum point ahead of the flame. This resulting stretch rate is found to be consistent with the maximum absolute value of the axial velocity gradients along the nozzle centerline upstream of the flame. When the experimental results of laminar flame speeds and extinction stretch rates are presented, the associated standard deviation is shown as an error bar in the figures.
Table 1. Kinetic Schemes Used in the Present Study fuel
kinetic model
n-heptane iso-octane toluene n-decane n-dodecane
21-23 22 and 24 11 25 and 28-31 28 and 29
flame response curve defines the extinction limit. At this turning point, the computed maximum negative value of the axial velocity gradient ahead of the flame is used to determine the extinction stretch rate. All flame simulations use mixtureaverage transport equations. Table 1 lists the reaction mechanisms used in flame calculations. Specifically, the detailed mechanisms of n-heptane and iso-octane developed by Curran et al.21,22 are employed to calculate the laminar flame speeds for the two fuels but are not used for extinction calculations because of their large size. For computational ease, the extinction stretch rates for n-heptane and iso-octane are obtained using the mechanisms by Seiser et al.23 and Hasse et al.,24 respectively. The mechanism by Seiser et al.23 was based on the detailed n-heptane mechanism by Curran et al.21 and is able to reproduce closely the computed laminar flame speed using the detailed mechanism by Curran et al.,21,22 as will be shown in due course. The mechanism by Hasse et al.24 is found to well-predict the present experimental laminar flame speeds of iso-octane. With regard to the flame calculations for n-decane and toluene, the mechanisms by Bikas and Peters25 and Hirasawa et al.11 are used, respectively. It may be noted that the toluene chemistry in the mechanism by Hirasawa et al.11 was based on the studies by Djurisic et al.26,27 The recently developed mechanisms for high-temperature oxidation of n-decane and n-dodecane taken from JetSurF28 and Dryer29 are also used for laminar flame speed calculations. It has to be pointed out that the reaction mechanism by Dryer29 is a refined model based on the earlier work by Zhao et al.30 In addition to the aforementioned semidetailed mechanisms, we have further used the detailed mechanism by Westbrook et al.31 to calculate the laminar flame speeds for n-decane/air mixtures. This detailed reaction mechanism by Westbrook et al.31 described the pyrolysis and oxidation of n-alkane hydrocarbons from n-octane to n-hexadecane and included both high- and low-temperature reaction pathways.
Computational Specifications and Kinetic Models Used Numerical modeling of the laminar flame speed is performed using the PREMIX16 code, in conjunction with the CHEMKIN17 and TRANSPORT18 packages. With regard to the simulations of the axisymmetric counterflow twin flames, the associated governing equations follow the plug-flow formulation by Kee et al.,19 while the flame response curves are generated using the one-point temperature controlling method by Nishioka et al.20 The extinction turning point of the
(21) Curran, H. J.; Gaffuri, P.; Pitz, W. J.; Westbrook, C. K. Combust. Flame 1998, 114, 149–177. (22) Curran, H. J.; Gaffuri, P.; Pitz, W. J.; Westbrook, C. K. Combust. Flame 2002, 129, 253–280. (23) Seiser, R.; Pitsch, H.; Seshadri, K.; Pitz, W. J.; Curran, H. J. Symp. (Int.) Combust., [Proc.] 2000, 28, 2029–2037. (24) Hasse, C.; Bollig, M.; Peters, N.; Dwyer, H. A. Combust. Flame 2000, 122, 117–129. (25) Bikas, G.; Peters, N. Combust. Flame 2001, 126, 1456–1475. (26) Djurisic, Z. M. M.Sc. Thesis, University of Delaware, Newark, DE, 1999. (27) Djurisic, Z. M.; Joshi, A. V.; Wang, H. Detailed kinetic modeling of benzene and toluene combustion. U.S. Sections Second Joint Meeting of the Combustion Institute; Berkeley, CA, 2001. (28) Sirjean, B.; Dames, E.; Sheen, D. A.; You, X.-Q.; Sung, C.; Holley, A. T.; Egolfopoulos, F. N.; Wang, H.; Vasu, S. S.; Davidson, D. F.; Hanson, R. K.; Pitsch, H.; Bowman, C. T.; Kelley, A.; Law, C. K.; Tsang, W.; Cernansky, N. P.; Miller, D. L.; Violi, A.; Lindstedt, R. P. A high-temperature chemical kinetic model of n-alkane oxidation. JetSurF, version 0.2, Sept 8, 2008. (29) Dryer, F. L. Personal communication, Princeton University, Princeton, NJ, May 6, 2008. (30) Zhao, Z.; Li, J.; Kazakov, A.; Dryer, F. L.; Zeppieri, S. P. Combust. Sci. Technol. 2005, 177, 89–106. (31) Westbrook, C. K.; Pitz, W. J.; Herbinet, O.; Curran, H. J.; Silke, E. J. Combust. Flame 2009, 156, 181–199.
(14) Vagelopoulos, C. M.; Egolfopoulos, F. N.; Law, C. K. Symp. (Int.) Combust., [Proc.] 1994, 25, 1341–1347. (15) Chao, B. H.; Egolfopoulos, F. N.; Law, C. K. Combust. Flame 1997, 109, 620–638. (16) Kee, R. J.; Grcar, J. F.; Smooke, M. D.; Miller, J. A. A FORTRAN Program for Modeling Steady Laminar One-Dimensional Premixed Flames; Technical Report SAND85-8240, 1985. (17) Kee, R. J.; Rupley, F. M.; Miller, J. A. Chemkin-II: A FORTRAN Chemical Kinetics Package for the Analysis of Gas-Phase Chemical Kinetics; Technical Report SAND89-8009, 1989. (18) Kee, R. J.; Dixon-Lewis, G.; Warnatz, J.; Coltrin, M. E.; Miller, J. A. A FORTRAN Computer Code Package for the Evaluation of GasPhase, Multicomponent Transport Properties; Technical Report SAND868246B, 1986. (19) Kee, R. J.; Miller, J. A.; Evans, G. H.; Dixon-Lewis, G. Symp. (Int.) Combust., [Proc.] 1988, 22, 1479–1494. (20) Nishioka, M.; Law, C. K.; Takeno, T. Combust. Flame 1996, 104, 328–342.
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Figure 3. Comparison of experimental and computed laminar flame speeds for toluene/air mixtures.
Figure 1. Comparison of experimental and computed laminar flame speeds for n-heptane/air mixtures.
Figure 2. Comparison of experimental and computed laminar flame speeds for iso-octane/air mixtures.
For the laminar flame speed calculations, the portions of the full mechanism describing C11-C16 kinetics were removed, leading to a n-decane mechanism of 940 species and 3887 reactions.31 This subset was made available by the authors of the mechanism.31 Unfortunately, simulations for n-dodecane using the complete mechanism by Westbrook et al.31 are not feasible at this time, owing to the large size of the complete mechanism (2115 species and 8157 reactions). Although several studies and kinetic schemes for methylcyclohexane oxidation in homogeneous environments do exist,32-34 no simulations have been carried out for methylcyclohexane flames, because the associated database for transport properties is not readily available.
Figure 4. Comparative experimental laminar flame speeds for toluene reference fuel components at (a) Tu = 400 K and (b) Tu = 470 K.
generally overpredict the laminar flame speed for both nheptane and iso-octane flames, especially in the equivalence ratio range of φ = 1.0-1.2. A somewhat fair agreement with experimental data is observed under fuel-lean conditions (φ = 0.7-0.9). The detailed mechanisms21,22 are also unable to capture the unburned mixture temperature dependence of the laminar flame speed, as the disagreement between the experimental and computational results is seen to worsen with increasing preheat temperature. On the other hand, the mechanism by Hasse et al.24 is shown to adequately simulate the present experimental data in the stoichiometric and rich regions for the iso-octane/air flames. Although the effect of preheat is also properly captured by the mechanism by Hasse et al.,24 there is some mismatch in the fuel-lean region. In Figure 3, the experimental laminar flame speeds for toluene/air mixtures are presented and compared to the computed values using the mechanism by Hirasawa et al.11
Laminar Flame Speed Results The experimental and computed laminar flame speed results for the primary reference fuels, n-heptane and iso-octane, are shown in Figures 1 and 2, respectively. It is seen from Figure 1 that the mechanism by Seiser et al.23 reproduces closely the computed laminar flame speeds as the detailed n-heptane mechanism by Curran et al.21,22 Moreover, Figures 1 and 2 show that the detailed mechanisms by Curran et al.21,22 (32) Zeppieri, S.; Brezinsky, K.; Glassman, I. Combust. Flame 1997, 108, 266–286. (33) Granata, S.; Faravelli, T.; Ranzi, E. Combust. Flame 2003, 132, 533–544. (34) Pitz, W. J.; Naik, C. V.; Mhaold uin, T. N.; Westbrook, C. K.; Curran, H. J.; Orme, J. P.; Simmie, J. M. Proc. Combust. Inst. 2007, 31, 267–275.
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Figure 5. Comparison of experimental and computed laminar flame speeds for n-decane/air mixtures using (a) semi-detailed mechanisms and (b) a detailed mechanism.
results shown in Figure 5a using various mechanisms for n-decane are limited to Tu = 400 K for the sake of clarity, while a similar trend is also observed at Tu = 470 K. It can be seen from Figure 5a that the mechanisms by Bikas and Peters25 and Dryer29 predict the current experimental data for n-decane/air mixtures reasonably well. The mechanism by Zhao et al.30 also simulates the experiments well, except at fuel-rich conditions. Further, the JetSurF mechanism28 is found to underpredict the current data for the entire range of equivalence ratios. It has to be pointed out that the experimental laminar flame speeds mostly lie between the predictions obtained from the various kinetic models. The detailed n-decane mechanism by Westbrook et al.31 is also found to be in good agreement with the present data, as shown in Figure 5b. In particular, this detailed mechanism is found to reproduce adequately the dependence of the laminar flame speed upon the preheat temperature when compared to the other two detailed mechanisms21,22 for n-heptane and iso-octane. The n-dodecane flame speed results shown in Figure 6 also demonstrate that the current experimental data are nearly midway between the span of simulated data points. Nevertheless, the current n-dodecane results are best predicted by the mechanism by Dryer.29 It must also be noted that the laminar flame speeds for n-dodecane/air mixtures are slightly lower as compared to n-decane/air mixtures under similar conditions. An attempt is further made to correlate the dependence of the laminar flame speed (Sou) upon the mixture preheat temperature (Tu) in the form of (Sou(Tu, φ)/Sou(T0, φ)) = (Tu/T0)n, where T0 is the lowest unburned mixture temperature investigated for a given fuel/air equivalence ratio (φ). The current linearly extrapolated experimental data for n-heptane, isooctane, n-decane, and toluene can be correlated well with n in the range of 1.67-1.85. Here, the exponent n is obtained P P by minimizing the sum of the squares of the errors, φ Tu[Sou(T0, φ) - (Sou(Tu, φ)/(Tu/T0)n)]2, for the mixture conditions investigated. Figure 7 demonstrates that the correlated laminar flame speed, (Sou(Tu, φ)/(Tu/T0)n), reduces the experimental data to a single data set as a function of the equivalence ratio. Recognizing that cycloalkanes constitute an important component of jet fuels, the laminar flame speeds of methylcyclohexane/air mixtures are also obtained. Figure 8 plots the experimental results for this cycloalkane component at Tu = 400 K. In addition, Figure 8 shows the comparative results for the neat kerosene surrogate components, including
Figure 6. Comparison of experimental and computed laminar flame speeds for n-dodecane/air mixtures.
Note that the laminar flame speed data at a preheat temperature of Tu = 298 K are nonlinearly extrapolated values, which were taken from Hirasawa et al.11 It can be seen from Figure 3 that the mechanism is unable to capture the laminar flame speeds at higher preheat temperatures and shows a significant underprediction, although at ambient temperature, laminar flame speeds agree well. A comparison of the experimental results for the selected gasoline surrogate components is presented in Figure 4. Figure 4 shows and compares the laminar flame speeds for the three neat components, n-heptane, iso-octane, and toluene, for two unburned mixture temperatures of Tu = 400 and 470 K. Here, the error bars have been omitted for clarity. It is of interest to note that while the linear alkane (n-heptane) has the highest laminar flame speed, the branched alkane (isooctane) and the aromatic (toluene) have lower but similar laminar flame speeds. This trend is seen to hold at higher preheat temperatures as well. Such information can be beneficial when considering the formulation of surrogate mixtures to represent gasoline combustion. It may also be noted that saturated alkanes are generally considered to be cleaner burning compared to aromatics, but an aromatic component such as toluene is frequently used to improve the octane rating of gasoline. Figures 5 and 6 plot the experimental and computed laminar flame speeds for the two linear alkane components, n-decane and n-dodecane, respectively, both considered to be the major components of kerosene-type fuels. The computed 3843
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Figure 8. Comparative experimental laminar flame speeds for kerosene surrogate components at Tu = 400 K.
to the branched alkane and the aromatic component studied in this work. On the basis of the current experimental results, we conclude that, for similar carbon numbers, aromatic (toluene, C7H8) and branched alkane (iso-octane, C8H18) burn the slowest, n-alkane (n-heptane, C7H16) burns the fastest, and cycloalkane (methylcyclohexane, C7H14) is midway. It is well-known that certain coal-derived alternative fuels are largely made up of saturated branched alkanes,35 while their petroleum-derived counterparts have a significant amount of cycloalkane and aromatic components. Hence, the present observation regarding the flame propagation responses for different molecule classes can be of use in constructing surrogate fuel blends. Comparative Laminar Flame Speed Results In this section, we compare selected stretch-corrected laminar flame speed data reported in recent literature. We limit our comparison to n-heptane, iso-octane, and n-decane and focus specifically on results for preheated fuel/oxidizer mixtures. The aforementioned fuels have been studied by multiple research groups using different techniques and offer a chance to assess the unique features of the reported data. Stretch-corrected laminar flame speeds for n-heptane have been reported by Davis and Law,36 Kwon et al.,37 Smallbone et al.,38 and Ji et al.39 Similar data for iso-octane flames have been reported by Davis and Law,36 Kwon et al.,37 and Bradley et al.40 Among the recent stretch-corrected laminar flame speed data for n-decane are the studies by Zhao et al.30 and Ji et al.39 The counterflow technique was used in the experiments by Davis and Law,36 Smallbone et al.,38 and Ji et al.,39 while the outwardly propagating flame was used in the studies by Kwon et al.37 and Bradley et al.40 The data reduction techniques for obtaining the unstretched laminar flame speed are specific to each apparatus and investigation. It must be pointed out that the conterflow configuration data used for comparison are based on linearly extrapolated values to zero stretch rate, except for the work of Figure 7. Correlated laminar flame speeds, (Sou(Tu, φ)/(Tu/T0)n), of n-heptane/air (n = 1.67; T0 = 298 K), iso-octane/air (n = 1.85; T0 = 298 K), n-decane/air (n = 1.75; T0 = 360 K), and toluene/air (n = 1.82; T0 = 400 K) mixtures as a function of the equivalence ratio. T0 is the lowest unburned mixture temperature investigated for a given fuel/air composition.
(35) Huber, M. L.; Smith, B. L.; Ott, L. S.; Bruno, T. J. Energy Fuels 2008, 22, 1104–1114. (36) Davis, S. G.; Law, C. K. Symp. (Int.) Combust., [Proc.] 1998, 27, 521–527. (37) Kwon, O. C.; Hassan, M. I.; Faeth, G. M. J. Propul. Power 2000, 16, 513–522. (38) Smallbone, A. J.; Liu, W.; Law, C. K.; You, X. Q.; Wang, H. Proc. Combust. Inst. 2009, 32, 1245–1252. (39) Ji, C.; Dames, E.; Wang, Y. L.; Wang, H.; Egolfopoulos, F. N. Combust. Flame 2010, 157, 277–287. (40) Bradley, D.; Hicks, R. A.; Lawes, M.; Sheppard, C. G. W.; Woolley, R. Combust. Flame 1998, 115, 126–144.
n-decane, n-dodecane, and methylcyclohexane. Note that, while cycloalkane (methylcyclohexane) has a lower laminar flame speed than those of linear alkanes (n-decane and n-dodecane), the flame speed of methylcyclohexane is still higher compared 3844
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Figure 10. Comparison to literature experimental data for isooctane/air mixtures.
Figure 9. Comparison to literature experimental data for n-heptane/ air mixtures. Figure 11. Comparison to literature experimental data for n-decane/air mixtures.
Ji et al.39 The laminar flame speed data for the work of Ji et al.39 were based on a simulation-aided extrapolation technique, as discussed in their investigation. Figure 9a compares the stretch-corrected laminar flame speeds of n-heptane/air mixtures in the unburned temperature range of Tu = 350-360 K, while Figure 9b compares the room-temperature data (Tu = 298 K). It is seen from Figure 9b that the room-temperature data for the current study and those by Davis and Law,36 both on the basis of the counterflow technique, agree well and exhibit a similar trend on both the fuel-lean and fuel-rich sides. The maximum difference between the two experimental data sets, under similar conditions, is 3.9 cm/s for Sou ∼ 40 cm/s. The data from Kwon et al.,37 obtained using an outwardly propagating flame configuration, are significantly lower, except for φ = 1.4, as demonstrated in Figure 9b. The data sets with preheat, compared in Figure 9a, show some interesting trend. The current data are seen to be in good agreement with that by Ji et al.39 on the fuel-lean side and up to the stoichiometric condition. However, the data by Ji et al.39 drop rather sharply on the fuel-rich side. Note that the current data are slightly higher compared to Ji et al.39 on account of the slight difference in the preheat temperature and differing extrapolation techniques used. Further comparing the results by Ji et al.39 to those by Smallbone et al.38 shows that the linearly extrapolated data38 are lower than the simulation-aided, nonlinearly extrapolated data39 in the range of φ = 0.8-1.2, when one would expect things to be the opposite. Also note that the data by Smallbone et al.38 are significantly lower compared to the current data. However, beyond φ = 1.2, the laminar flame speed data by Ji et al.39 become the lowest. It has been reported by Ji et al.39 that their extrapolation technique gives lower values
compared to the linear extrapolation method. The differences can between 5-20% on the fuel-rich side and depend upon the equivalence ratio. Even after considering the differences in data reduction techniques and preheat temperature variation, the differences between the current data set and that by Ji et al.,39 on the fuel-rich side, cannot be fully explained and require further investigation. The comparative data for iso-octane/air mixtures are shown in Figure 10. There is very good agreement between the current data and the data by Davis and Law36 under ambient temperature conditions, using the counterflow configuration with linear extrapolation. As with the n-heptane case, the results by Kwon et al.37 for Tu = 298 K, obtained from an outwardly propagating flame configuration, are lower. At higher preheat temperatures, the data by Bradley et al.40 show a fair agreement with the current data near the stoichiometric region. Comparative results for the n-decane/air mixture shown in Figure 11 were all obtained using the counterflow flame configuration. The current results and those by Ji et al.39 are in fair agreement in the lean-stoichionetric region. There is, however, a significant mismatch on the fuel-rich side. The higher preheat temperature data are in very good qualitative agreement with the work by Zhao et al.30 In particular, both of the data sets exhibit a similar profile for the variation in laminar flame speed under fuel-rich conditions, which is very different from the study by Ji et al.39 Furthermore, as demonstrated in Figure 5b, the current data are in good agreement with computations using a detailed kinetic mechanism by Westbrook et al.31 3845
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The key conclusions that can be drawn from the comparative data are summarized in the following. The current data is in both qualitative and quantitative agreement with previous counterflow data by Davis and Law36 and Zhao et al.30 However, there is a significant discrepancy between the current data set and that by Ji et al.39 on the fuel-rich side. The n-heptane data by Smallbone et al.38 are much lower compared to two other data sets under similar conditions, using a similar experimental configuration. In addition, it is somewhat difficult to make a direct comparison between the reported laminar flame speed values obtained using different techniques, namely, the outwardly propagating flames and the counterflow flames. On the basis of the computed results presented earlier and the comparative experimental data, it is observed that the computations using detailed mechanism are in qualitative agreement with the study by Davis and Law,36 Zhao et al.,30 and the current data for the hydrocarbon fuels, in terms of the dependence of the laminar flame speed upon the equivalence ratio. We further note that, wherever computed results agree well with our current data, they do tend to capture the equivalence ratio and preheat temperature dependence quite well. Interestingly, even for the toluene mechanism by Hirasawa et al.,11 which does not show good quantitative agreement at elevated preheat temperatures, one can clearly see that the curves seem to be translated and that the dependence upon the equivalence ratio is captured quite well by both experiments and simulations. Recognizing that there exists only one true value of the laminar flame speed, for a given mixture condition of pressure, preheat, and composition, and that the current comparisons show a wide variation in reported data, further studies on the merits of experimental techniques and data reduction methodologies are clearly warranted.
Figure 12. Comparison of experimental and computed extinction stretch rates for gasoline surrogate components at Tu = 400 K. The molar ratio of N2/(N2 þ O2) in the oxidizer is 0.84.
Figure 13. Comparison of experimental and computed extinction stretch rates for n-decane/O2/N2 mixtures, with the molar ratio of N2/(N2 þ O2) being 0.84 and Tu = 400 K.
Extinction Stretch Rate Results The extinction stretch rate is also an important parameter relevant to combustor performance. It may be linked to practical concepts, such as flameout or the ability to relight. In this work, we have experimentally and computationally investigated the stretch-induced extinction limits for the neat hydrocarbons relevant to surrogate gasoline and kerosene fuels. The performance of several chemical kinetic mechanisms with regard to the experimental data is assessed. Computationally, by plotting the flame response parameter, such as maximum flame temperature, as a function of stretch rate, the flame strength decreases as the extinction turning point is approached. As mentioned earlier, the computational extinction stretch rate corresponds to the turning point in the flame response curve. The entire set of computed response curves can be found in the Supporting Information. The experimental and computational results of extinction stretch rates for gasoline surrogate components are summarized and compared in Figure 12. With respect to the experimental data, it can be seen that toluene (aromatic) and iso-octane (branched alkane) have similar extinction stretch rates over the conditions investigated. The highest extinction stretch rate is obtained for the n-heptane (linear alkane) fuel. Thus, the trend observed for extinction stretch rates is consistent with that for the laminar flame speeds. It is also seen from Figure 12 that only the mechanism by Hasse et al.24 is capable of yielding good extinction stretch rate predictions for iso-octane flames. The n-heptane mechanism by
Figure 14. Comparison of experimental and computed extinction stretch rates for n-dodecane/O2/N2 mixtures, with the molar ratio of N2/(N2 þ O2) being 0.84 and Tu = 400 K.
Seiser et al.23 severely overpredicts the extinction stretch rates of n-heptane flames. On the other hand, the toluene extinction stretch rates are significantly underpredicted by the mechanism by Hirasawa et al.11 Furthermore, both the experimental and computed extinction stretch rates are found to peak on the fuel-rich side for the three components of gasoline surrogate. We next present and compare the experimental and computed stretch-induced extinction limits for n-alkane components relevant to kerosene-type fuels, as shown in Figures 13 and 14. It is first noted that the measured extinction stretch 3846
Energy Fuels 2010, 24, 3840–3849
: DOI:10.1021/ef100074v
Kumar and Sung
Figure 16. Bar plot showing the distribution of the laminar flame speed model output to the variation in pure species collision diameters using the mechanism by Bikas and Peters25 for n-decane/air mixtures at Tu = 400 K.
potential flow,19 the extinction stretch rate sensitivity is calculated on the basis of the radial pressure gradient H. Specifically, the normalized sensitivity of the radial pressure gradient with respect to the ith reaction is defined as SH,i = (ki/H)(∂H/∂ki). Note that the positive (negative) value of SH,i indicates an increase (a decrease) in the extinction stretch rate with an increasing ki. Figure 15 shows the sensitivity analysis results on the laminar burning flux and extinction stretch rate conducted for stoichiometric n-decane flames. To facilitate the comparison, the normalized sensitivity coefficients as defined above are scaled with respect to their respective maximum values for both the laminar mass burning flux and extinction stretch rate. It can be seen from Figure 15 that in general the sensitivity to the laminar burning flux and the extinction stretch rate is of the same sign for the important reactions identified in the sensitivity analysis. In fact, the magnitude of the relative sensitivity coefficients is also comparable. A notable exception is the reaction of carbon monoxide with the hydroxyl radicals. This reaction is relatively more important to the flame extinction limits. Because this reaction is one of the main contributors to the overall heat release, which in turns affects the flame temperature, it plays an increasingly significant role for near-limit flames that are more temperaturesensitive. It is well-recognized that the flame processes are influenced by both chemistry as well as transport. Having discussed the chemistry sensitivity of the results for flame propagation and extinction, the effect of transport is examined next. Recent studies38,41 have indicated the importance of such an analysis for hydrocarbon flames. To evaluate the degree of influence of transport properties on the computed results, the model response to variation in pure species collision diameters is evaluated. In particular, all of the pure species collision diameters (σi) are varied simultaneously within a 10% range on either side of its base value specified in the original transport database. The value for a particular σi is assumed to be uniformly distributed in the (10% range, and a random value within this range is chosen. Multiple model evaluations are conducted, aiming to obtain insight into the input-output relationship. Note that the pure species collision diameters
Figure 15. Relative sensitivity comparison for the laminar mass burning flux and extinction limit of stoichiometric n-decane flames using (a) the JetSurF mechanism28 and (b) the mechanism by Bikas and Peters.25
rates for n-dodecane/O2/N2 flames are found to be slightly lower than those for n-decane/O2/N2 flames, consistent with the measured laminar flame speed trend. Figures 13 and 14 demonstrate that, in comparison to the computed results from some earlier kinetic schemes,25,30 the latest kinetic scheme for higher order hydrocarbon combustion, JetSurF,28 provides the best match with the experimental data for the extinction of both n-decane and n-dodecane flames. Specific improvement is observed on the fuel-rich side, where the other mechanisms25,30 significantly overpredicted the current extinction stretch rate data. The JetSurF28 mechanism, however, underpredicts the current experimental data when used for laminar flame speed predictions. Currently, we are unable to do a comparison of extinction stretch rate results to the literature data because either they are not available or they have been carried out in non-adiabatic configurations. Sensitivity Analysis A sensitivity analysis on reaction rate constants is conducted for the results of laminar mass burning flux and extinction stretch rate using the mechanisms by Bikas and Peters25 and JetSurF.28 The normalized sensitivity of the laminar mass burning flux (m0 = unburned mixture density Sou) is defined as ∂ ln(m0)/∂ ln(ki), where ki is the reaction rate constant of the ith reaction. Recognizing that the eigenvalue in the plug-flow formulation is the radial pressure gradient19 and that the magnitude of the radial pressure gradient scales with the square of the stretch rate in the corresponding fully developed
(41) Holley, A. T.; You, X. Q.; Dames, E.; Wang, H.; Egolfopoulos, F. N. Proc. Combust. Inst. 2009, 32, 1157–1163.
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Energy Fuels 2010, 24, 3840–3849
: DOI:10.1021/ef100074v
Kumar and Sung
Figure 17. Scatter plot showing the laminar flame speed model output to the variation in pure species collision diameters using the mechanism by Bikas and Peters25 for n-decane/air mixtures at Tu = 400 K.
speed. This reduction in σN2 also leads to a higher fuelmixture diffusion coefficient. The results of panels of c and d of Figure 17 further show the sensitivity to collision parameters for oxygen (deficient reactant) and hydrogen (stable intermediary) in the fuel-rich case. In comparison to σO2, σH2 exhibits a relatively small but visible influence on the laminar flame speed under fuel-rich conditions. Similar transport sensitivity results showing the variance of the stretched flame response are presented in Figure 18. The baseline is a flame solution near extinction, while the scatter represents the change in the resulting stretch rate by keeping the same flame location as the baseline flame when all of the σi are varied randomly at the same time, within the stated interval. As such, the higher (lower) resulting stretch rate indicates an increase (a decrease) in the extinction stretch rate because of the change in σi. Similar to the laminar flame speed case, the fuel-rich extinction shows a larger variation when compared to its fuel-lean counterpart. Inspection of the scatter plots shows that, unlike the laminar flame speed case, the collision parameter for the parent fuel significantly influences the computed stretch rate values, especially for the fuellean and stoichiometic (cf. Figure 19b) conditions. Panels a and c of Figure 19 show an increasing trend of the extinction stretch rate with an increasing collision diameter of nitrogen (decreasing thermal diffusivity of the mixture) for stoichiometric and fuel-rich cases, respectively. The smaller mixture thermal diffusivity reduces the thermal energy loss out of the reaction zone of the counterflow flame, resulting in a higher extinction stretch rate. On the other hand, the opposite trend is observed for the effect of σn-C10H22 in the stoichiometric case (Figure 19b) and the effect of σO2 under fuel-rich conditions (Figure 19d). Recognizing that the species-mixture diffusivity increases with decreasing σi, the trend shown in panels b and d of Figure 19 is because the increased diffusion of the controlling/deficient reactant to the reaction zone that leads to stronger burning.
Figure 18. Distribution of the counterflow flame model output to the variation in pure species collision diameters using the mechanism by Bikas and Peters25 for n-decane/air mixtures at Tu = 400 K. The base case corresponds to a near-extinction flame condition.
serve as input to the evaluation of binary diffusion coefficients based on the appropriate combination rule. The results of such a variation on the laminar flame speeds of n-decane/air mixtures are shown in Figure 16. A greater spread is observed for the fuel-rich flame as compared to the fuel-lean one. As a first approximation, useful information about the degree of influence of a particular σi can be obtained by inspection of scatter plots, as shown in Figure 17. It is found that the laminar flame speed values are most sensitive to the collision parameter for nitrogen, for both fuel-rich and fuel-lean cases, while being almost insensitive to the parent fuel. One likely reason for the dominant effect of nitrogen is the variation in mixture thermal diffusivity. As the value of σN2 is lowered, the mixture thermal diffusivity is significantly increased, leading to an enhancement of the laminar flame 3848
Energy Fuels 2010, 24, 3840–3849
: DOI:10.1021/ef100074v
Kumar and Sung
Figure 19. Scatter plot showing the counterflow flame model output to the variation in pure species collision diameters using the mechanism by Bikas and Peters25 for n-decane/air mixtures at Tu = 400 K.
aromatic exhibit the lowest laminar flame speeds, although their values are quite similar. Moreover, the n-alkane flames are more resistant to stretch-induced extinction as compared to branched alkane and aromatic flames. Furthermore, the sensitivity analysis yields some interesting insight into the influence of kinetics as well as transport properties. It is found that both the flame propagation and extinction phenomena are very sensitive to kinetics as well as transport. The range of variation for a given level of perturbation in the kinetics and transport parameters has also been quantified.
Conclusions Laminar flame speeds and extinction stretch rates have been experimentally obtained for typical surrogate components of gasoline and kerosene-type fuels at atmospheric pressure conditions. Among the gasoline surrogate components studied, n-heptane exhibits the highest laminar flame speed, while the laminar flame speeds of iso-octane and toluene are found to be quite similar. The same trend holds true for extinction stretch rates of gasoline surrogate components. Detailed kinetic schemes for n-heptane and iso-octane by Curran et al.21,22 are found to overpredict the experimental data. On the other hand, the iso-octane mechanism by Hasse et al.24 well-predicts the present experimental results of laminar flame speeds and extinction stretch rates. The results for key kerosene surrogate components show that n-decane has the highest laminar flame speed, followed by n-dodecane and methylcyclohexane. Despite that, the laminar flame speeds and the extinction stretch rates for n-decane and n-dodecane are considered to be quite similar. There is a fair prediction of laminar flame speeds by some of the recent kinetic schemes, although the extinction stretch rates are generally overpredicted. In addition, the recent detailed kinetic scheme by Westbrook et al.31 adequately captures the trend for n-decane laminar flame speeds, as both a function of the equivalence ratio and the mixture preheat temperature. It is found that n-alkanes exhibit the highest flame speeds followed by cycloalkane. The branched alkane and the
Acknowledgment. This work has been supported by the Air Force Office of Scientific Research under Grant FA9550-07-10515 and the National Aeronautics and Space Administration under Grant NNX07AB36A. We also acknowledge Prof. Fred Dryer of Princeton University for providing the reaction mechanism of reference 29 prior to publication. Supporting Information Available: Typical axial velocity profile along the nozzle centerline and radial velocity profile at the reference location identified in panel a (Figure S1), reference flame speed as a function of the stretch rate for all fuels tested at Tu = 400 K (Figure S2), experimental results demonstrating the top-hat velocity profile in the core region of the nozzle flow (Figure S3), and computed response of maximum flame temperature to variations in the stretch rate for reaction mechanisms used in the current study (Figure S4). This material is available free of charge via the Internet at http://pubs.acs.org.
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