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Oct 8, 2014 - State Key Laboratory of Multiphase Flows in Power Engineering, Xi,an Jiaotong ..... of the H radical are on the wane at elevated pressur...
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Comparative Study of Experimental and Modeling Autoignition of Cyclohexane, Ethylcyclohexane, and n‑Propylcyclohexane Zemin Tian, Yingjia Zhang,* Feiyu Yang, Lun Pan, Xue Jiang, and Zuohua Huang* State Key Laboratory of Multiphase Flows in Power Engineering, Xi’an Jiaotong University, Xi’an, Shaanxi 710049, People’s Republic of China S Supporting Information *

ABSTRACT: Ignition delay times were measured for cyclohexane, ethylcyclohexane, and n-propylcyclohexane at atmospheric pressure, equivalence ratios of 0.5, 1.0, and 2.0, and temperatures of 1110−1650 K behind reflected shock waves with a fixed fuel concentration of 0.5%. Computational simulations were made using three generally accepted mechanisms, yielding acceptable agreements with the current measurements at the tested equivalence ratios. Nonetheless, there is a slight overprediction at ϕ = 2.0 for n-propylcychexane. Ethylcyclohexane and n-propylcyclohexane have shorter ignition delay times than cyclohexane at high temperatures. However, this difference decreases with the decrease in the temperature. Simulation and comparison to previous data indicate that the oxidation rates of ethylcyclohexane, n-propylcyclohexane, and n-butylcyclohexane are in the order of npropylcyclohexane > ethylcyclohexane ≈ n-butylcyclohexane. Kinetic analysis is performed to obtain insight into the observation.

1. INTRODUCTION Worldwide use of fossil fuels increases the need to understand their oxidation mechanisms. Cycloalkanes are the important compositions of not only traditional vehicle fuels, such as gasoline and diesel, but also jet fuels, such as Jet-A/Jet-A1/JP8.1−3 It is also found that cycloalkanes produce soot precursors more easily than chain alkanes.4 Thus, they have attracted much attention, and detailed chemistry has been established. Cyclohexane (CH) has the simplest molecular structure among all cycloalkanes with a six-membered ring and has received many investigations. Recently, Serinyel et al.5 detected 34 intermediates in the CH oxidation using a jet-stirred reactor (JSR) at a pressure of 1.07 atm, temperatures from 500 to 1100 K, and equivalence ratios of 0.5, 1.0, and 2.0. The negative temperature coefficient (NTC) was captured, and good predictions were achieved using an updated kinetic mechanism. Vranckx et al.6 measured the ignition delay times for a CH/O2/ N2/Ar mixture in a rapid compression machine (RCM) at pressures up to 40 atm and temperatures between 680 and 910 K to observe the NTC behavior. Their measurements were compared to the predictions of several mechanisms, and differences in predictions were interpreted. In addition, Daley et al.7 studied the CH ignition characteristics behind reflected shock waves at temperatures of 847−1379 K, pressures of 11− 61 atm, and ϕ = 1.0, 0.5, and 0.25. They compared and analyzed the results with four mechanisms. In addition, methylcyclohexane (MCH)8−10 has also been widely investigated in the past decade. In comparison to MCH, only limited studies of ethylcyclohexane (ECH), n-propylcyclohexane (PCH), and n-butylcyclohexane (BCH) are available. For ECH, Husson et al.11 investigated the elementary reactions at low temperatures covering the NTC area and quantitatively identified 47 intermediates using a jet-stirred reactor (JSR). Their results showed that the proposed detailed kinetic model could well-predict the key species mole fractions. Vanderover and Oehlschlaeger12 performed measurements of ignition delay © 2014 American Chemical Society

times for ECH/air mixtures in a shock tube at pressures of 12 and 50 atm, temperatures of 881−1319 K, and equivalence ratios of 0.25, 0.5, and 1.0. However, they did not present kinetic analysis. Ristori et al.13 measured concentration profiles of reactants, intermediates, and final products of PCH at temperatures of 950−1250 K, ϕ = 0.5−2.0, and pressure of 1.0 atm in a JSR and proposed a detailed mechanism for PCH. Other studies on PCH using a JSR and laminar flame have also been reported.14−16 In addition, Dubois et al.17 measured the ignition delay time of PCH/O2/Ar mixtures at 10 and 20 atm, with equivalence ratios of 0.2, 0.3, 0.4, 0.5, 1.0, and 1.5, ranging the temperatures of 1250−1800 K in a shock tube. They also tested the laminar flame speed at an initial temperature of 403 K and initial pressure of 1 atm with equivalence ratios between 0.6 and 1.75. The experimental data were well-modeled with a combined mechanism. Crochet et al.18 measured the ignition delay times of lean PCH/air (ϕ = 0.3, 0.4, and 0.5) at temperatures of 620−930 K and pressures of 0.45−1.34 MPa in a RCM; NTC behaviors were observed; and major decomposition pathways of PCH were given. For BCH, Conroy et al.19 measured the ignition delay times with air dilution at pressures of 10 and 30 atm, equivalence ratios of 0.3, 0.5, 1.0, and 2.0, and temperatures of 950−1430 K in a heated shock tube. Recently, some researchers conducted the comparative studies on the combustion characteristics of cycloalkanes. For example, Hong et al.20 measured the ignition delay times of CH, MCH, and BCH at pressures of 1.5 and 3.0 atm and ϕ = 0.5 and 1.0, in which they summarized the ignition delay time in the order of MCH > CH ≈ BCH. Wu et al.21 and Ji et al.22 measured the laminar flame speeds of CH and monoalkylated CHs, and the reasons for larger flame speed of CH than those of monoalkylated CHs were analyzed. Although Received: June 24, 2014 Revised: October 7, 2014 Published: October 8, 2014 7159

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many studies were reported on CH combustion, very few studies focused on the effect of branching chains on the decomposition of naphthenes. Thus, it is worth studying both experimental and kinetic analysis. In this study, measurements of ignition delay times for CH, ECH, and PCH were conducted at p = 1.1 atm, T = 1110− 1650 K, and ϕ = 0.5, 1.0, and 2.0. The experimental results are helpful for the understanding of the influences of branching chains on the decomposition of naphthenes and modifying the kinetic mechanism.

Table 1. Compositions and Pressures of Test Mixtures for Fuel/O2/Ar fuel

ϕ

fuel (%)

O2 (%)

Ar (%)

p (atm)

CH

2.0 1.0 0.5 1.0 2.0 1.0 0.5 2.0 1.0 0.5

0.5 0.5 0.5 0.444 0.5 0.5 0.5 0.5 0.5 0.5

2.25 4.5 9 4 3 6 12 3.375 6.5 13

97.25 95 90.5 95.556 96.5 93.5 87.5 96.125 93 96.5

1.1 1.1 1.1 3.0 1.1 1.1 1.1 1.1 1.1 1.1

ECH

2. EXPERIMENTAL SECTION

PCH

Measurements of ignition delay times were performed in a stainlesssteel shock tube, of which the detailed description has been presented in previous publications.23,24 Here, only a brief introduction is given. The shock tube is separated into a 4 m long driver section and a 5.3 m long driven section by a double diaphragm machine. Four fast response pressure transducers (PCB 113B26) installed over the end part of the driven section are used to measure the local incident shock velocities. A piezoelectric pressure transducer with acceleration compensation (PCB 113B03) and a photomultiplier with a wavelength of 307 ± 10 nm through a narrow filter are sited at the endwall to detect the reflected shock pressure and local OH* chemiluminescence. Fuel mixtures of 98% purity, oxygen (99.999%), and argon (99.999%) are manometrically prepared in a 128 L stainless-steel mixing tank, and no less than 12 h wait allows for homogeneous diffusion. The ignition delay time is defined as the interval between the arrival of the incident shock wave and the intersection of the steepest tangent line of the OH* chemiluminescence signal with the baseline, as shown in Figure 1. The temperature behind the reflected shock wave is determined by a chemical equilibrium program Gaseq25 with an uncertainty of about 15 K.

O2/Ar mixtures, as shown in Figure 2. In addition, the ignition delay time of CH is also measured at ϕ = 1.0 and 3.0 atm with

Figure 2. Comparison of CH ignition delay times of this study to those by Hong et al.20

4% O2 mole fraction in this study, which reproduces the experimental condition in the previous study.20 The data from the current study and the previous study20 at 3.0 atm and 4% O2 mole fraction are also plotted in Figure 2. The results show that the data from both studies are in good consistency. 3.1. Selected Kinetic Mechanisms and Numerical Predictions. It is well-known that several detailed kinetic mechanisms have been developed to interpret the oxidation of CH, three of which are selected here to reproduce the current measurements. One is the kinetic mechanism developed by Silke et al.;26 it includes 1081 species and 4268 reactions and involves both low- and high-temperature chemistries for CH. The model by Silke et al. has been validated against the ignition delay times of the mixture of CH/O2/N2 for the pressure ranges of 7−9 and 11−14 atm and temperature ranges of 650− 900 K obtained in RCM in the study by Lemaire et al.27 In addition, some intermediate species measured at 727 K and 7.4 atm were also reproduced. Another one is the kinetic mechanism given by Sirjean et al.28 on the basis of a mechanism generator software EXGAS and has also been validated against various data. It gives good predictions on ignition delay times at pressures of 1.0−10.0 atm and temperatures above 1000 K.20,28 The third one is the kinetic mechanism of JetSurF2.0 constructed by Wang et al.29 This model composes 348 species and 2163 reactions and has been widely validated and accepted.6,7,11,20 Despite the combined

Figure 1. Definition of the ignition delay time.

3. RESULTS AND DISCUSSION This study measures ignition delay times of CH, ECH, and PCH at a pressure of 1.1 atm, equivalence ratios of 0.5, 1.0, and 2.0, and temperatures from 1100 to 1650 K with a fixed fuel concentration of 0.5% and makes the numerical simulations using kinetic mechanisms. Compositions of all tested mixtures are listed in Table 1, and the measured data are provided in Table S1 of the Supporting Information. The current data for the stoichiometric 0.5% CH/O2/Ar mixture approximately at 1.1 atm were compared to the work by Hong et al.,20 who obtained data at 1.5 atm for 0.444% CH/ 7160

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mechanism for PCH in the study by Dubois et al.,17 JetSurF2.0 is used for ECH and PCH. In addition, all simulations were conducted using the CHEMKIN II package30 with SENKIN code,31 and the ignition delay times are determined by the time interval between zero and the maximum rate of the temperature rise (maximum dT/dt). An average pressure rise of 4%, namely, dp/dt, was considered in the simulations to model the physical influence of the shock tube.32 As shown in Figure 3a, the three selected mechanisms generally exhibit the acceptable predictions for the ignition delay times of CH under the test temperature conditions. However, the mechanism by Sirjean et al. gives overprediction up to 80% at lower temperatures for the fuel-lean mixture (ϕ = 0.5). It is probably due to the lack of some important reactions of intermediate hydrocarbons in the mechanism by Sirjean et al. For the fuel-lean mixture, the reactions of intermediate hydrocarbons plus small radicals become more important. The ignition will be delayed in shortage of some relevant oxidation reactions. The mechanisms by Silke et al. and JetSurF2.0 show better performance in reproducing the results under all test conditions. It can be noted that both mechanisms by Sirjean et al. and Silke et al. predict higher apparent activation energy of the ignition delay than JetSurF2.0. Moreover, at high temperatures, the selected mechanisms, except JetSurF2.0, give slight underprediction at high temperatures, especially the mechanism by Silke et al. For ECH and PCH, only JetSurF2.0 can reproduce the ignition delay times, as shown in panels b and c of Figure 3. However, it should be noted that there is an obvious overprediction for the rich PCH mixture (ϕ = 2.0), in contrast with the good agreements at lean and stoichiometric equivalence ratios. 3.2. Comparison between CH, ECH, and PCH. Hong et al.20 also compared the ignition delay time of CH, ECH, and BCH under similar conditions to this study. They proposed three factors in the difference of delay times among those fuels, namely, the unimolecular reactions, the regenerating ability of the H radical, and the production of CH3. Although CH and MCH have equally slow unimolecular reaction rates, CH is able to regenerate more H radical, leading to a shorter ignition delay time of CH. BCH has a faster unimolecular dissociation reaction rate, and this compensates for the less regenerated H radical. As a result, BCH exhibits a comparative ignition delay time to CH. However, it can be noticed by reviewing the previous work that the ignition delay time of BCH was shorter than that of CH at high temperatures for the fuel-lean mixture. Figure 4 shows the ignition delay times of CH and BCH at p = 3.0 atm and ϕ = 1.0 and p = 1.5 atm and ϕ = 0.5 measured by Hong et al.20 It clearly shows that BCH has a shorter ignition delay time when the temperature rises to 1350 K at ϕ = 0.5. In this study, similar behavior is also observed in the comparison of ignition delay times between CH, ECH, and PCH at ϕ = 0.5 and 1.0, as shown in Figure 5. At a temperature less than 1450 K, the ignition delay time of CH becomes closer and closer to those of ECH and PCH, but it gives a much longer ignition delay than those of ECH and PCH when the temperature is higher than 1450 K. Thus, it can be inferred that the stability of the cyclic structure of CH has effects on ignition at high temperatures. 3.2.1. Influence of the Molecular Structure at High Temperatures. To understand the ignition chemistry of CH, ECH, and PCH at high temperatures, the JetSurF2.0 mechanism was employed to conduct flux analyses because only JetSurF2.0 involves all submechanisms of the current

Figure 3. Ignition delay times and prediction for (a) CH by JetSurF2.0,29 Silke et al.,26 and Sirjean et al.28 mechanisms, (b) ECH by JetSurF2.0, and (c) PCH by JetSurF2.0 at 1.1 atm. The uncertainty is 15%.

study objects. Simulations of CH ignition delay times by the mechanisms of Silke et al. and Sirjean et al. are also included in Figure 5. In comparison to JetSurF2.0, the mechanism by Silke et al. gives a shorter prediction in ignition delay time at high temperatures (>1500 K) and a longer prediction in ignition delay time at low temperatures ( CH ≈ BCH at low temperatures,20 it is implied that the order of ignition delay times of CH, MCH, ECH, PCH, and BCH is MCH > ECH ≈ BCH ≈ CH > PCH, indicating the effect of the length of the branching chain on ignition delay. Figures 10 and 12 also include MCH and BCH to interpret this effect. A comparison of the CH ignition delay to those of ECH and PCH has been discussed above. CH is no longer focused in the following. It is shown that PCH shows the largest consumption rate, and BCH gives the smallest consumption rate. Although the consumption rate of MCH approximates that of ECH initially, it decreases significantly later, and MCH is the last to disappear. This means that the decomposition of PCH is fastest, that of MCH is slowest, and those of ECH and BCH fall in the middle. Moreover, the generating rate of the H radical is found to be a major factor in all fuel dissociations. For all fuels, the majority of the H radical is produced by C2H5 in the first part (as shown in Figure S1 of the Supporting Information). The difference in the production of C2H5 is strongly responsible for the difference in fuel consumption rates. Figure 13 depicts the main pathways of C2H5 formation in the ignition processes of ECH, PCH, and BCH. In this study, the moment of 20% consumption of fuel was selected to plot the oxidation pathway. Contributions to the production of C2H5 are marked in black; the percentages of the dissociation of fuel and intermediates are in red and green, respectively. It is observed that the generation of C2H5 is strongly dependent upon the specific molecule of fuel. In Figure 13a, about 43% ECH is consumed via H abstraction at sites 2 and 3, forming ECH-R2 and ECH-R3, which are the precursors of 71% C2H5 (employing similar nomenclature as in the work by Hong et al.20). Particularly, ECH-R2 produces twice the amount of ethyl as that from ECH-R3. However, only 4.4% ECH is directly decomposed to form 10.6% ethyl. Less than 10% ECH generates cyclohexyl (cycC6H11), which is a well-known Hradical origin. In comparison, PCH seems to have fewer pathways in C2H5 production, as shown in Figure 13b. It can be seen that 33.7% C2H5 is produced by PCH-R1, which is the most outstanding pathway of production of C2H5. Additionally, 91.4% PCH-R1, which is formed by 17.6%, undergoes this pathway. This means that about 16.1% PCH can generate C2H5 through two steps. In contrast, only 7% ECH can generate C2H5 through two steps (ECH → ECH-R2 → C2H5). Other pathways in the PCH oxidation profile also tend to be more effective than those in ECH oxidation. In other words, PCH has a branch and is easier to regenerate the H radical. Additionally, PCH can yield more cyclohexyl (15.2%). Consequently, PCH demonstrates better H-regeneration ability than ECH. For BCH in Figure 13c, the appearance of butyl (pC4H9) tends to largely inbibit the production of ethyl, even though a certain amount of cyclohexyl (16.1%) is produced. As a result, the rate of H regeneration of BCH is the lowest among these fuels. In a word, complex pathways tend to slow the production of ethyl. Therefore, the consuming rates of ECH, PCH, and BCH are in the order of PCH > ECH ≈ BCH, and

Figure 13. C2H5 generation pathways at 1150 K and 20% consumption of (a) ECH, (b) PCH, and (c) BCH. Red means the pathways of fuel consumption. Green means the consuming pathways of intermediates. Black means the pathways of production of ethyl. Blue means the pathways of production of n-butyl.

this corresponds inversely to the behavior of ignition delay times. To verify this assumption, ignition delay times of BCH at ϕ = 1.0, 1.5 atm, and 4% O2 from Hong et al.20 are used to make the comparison to those of PCH measured in the same test conditions and definition (primary data are provided in Table S2 of the Supporting Information). Figure 14 shows that both simulated and experimental results give a shorter ignition delay time in PCH, even considering the uncertainty of 15%. With respect to MCH, because it has weak H-regeneration ability, as analyzed in the study by Hong et al.,20 its ignition delay time is the longest. In conclusion, the abstraction with the H radical, which is highly influenced by the regenerating rate of H, is crucial to the consumption of these fuels in the initial stage, while the regeneration of H is largely dominated by the special molecular structure. 7165

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ignition delay times of PCH and BCH from Hong et al.20 in identical conditions also verifies the simulation.

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ASSOCIATED CONTENT

S Supporting Information *

Measured ignition delay times of CH, ECH, and PCH, where p is the pressure in atmosphere, T is the temperature in kelvin, and τ is ignition delay time in microseconds (Table S1), measured ignition delay times of PCH with 4% O2 at ϕ = 1.0 for comparison to those of BCH from Hong et al., with τ1 referring to the definition with endwall pressure and τ2 referring to the definition with sidewall pressure (Table S2), and normalized rate of production of R245 (C2H5 ⇄ C2H4 + H) for ECH, PCH, and BCH at 1150 K (Figure S1). This material is available free of charge via the Internet at http://pubs.acs.org.

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Figure 14. Comparison of ignition delay times of BCH from Hong et al.20 and PCH with 4% O2 concentration at 1.5 atm and ϕ = 1.0.

AUTHOR INFORMATION

Corresponding Authors

*Telephone: 86-29-82665075. Fax: 86-29-82668789. E-mail: [email protected]. *Telephone: 86-29-82665075. Fax: 86-29-82668789. E-mail: [email protected].

What should be mentioned is that BCH, PCH, and BCH tend to have a comparative ignition delay at elevated pressures (>40 atm) on the basis of JetSurF2.0 because the OH radical instead of the H radical becomes significant to the decomposition of fuel. Additionally, the majority of OH is produced by HO2-related reactions. This means that the effects of the H radical are on the wane at elevated pressures. Therefore, the different H-regeneration abilities among alkylated CHs (ECH, PCH, and BCH) have little influence on ignition, resulting in comparable ignition delay times for them.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China (51206132, 51136005, and 51121092) and the National Basic Research Program (2013CB228406). The authors also appreciate the funding support from the Fundamental Research Funds for the Central Universities and the State Key Laboratory of Engines (SKLE201305).

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4. CONCLUSION Ignition delay times of CH, ECH, and PCH were measured at pressures of 1.1 atm, equivalence ratios of 0.5, 1.0, and 2.0, and temperatures ranging from 1110 to 1650 K with a fixed fuel concentration of 0.5%. Good agreement was obtained in the comparison of the measured CH ignition delay times to those by Hong et al.20 For CH, three available mechanisms (Sirjean, Silke, and JetSurF2.0) used to make the simulation and satisfactory agreements with experiments were achieved. However, the difference from CH unimolecular reactions between mechanisms by Silke et al. and JetSurF2.0 leads to different predictions at high temperatures. For ECH and PCH, the only available JetSurF2.0 well-predicts the experimental data, except for the overprediction in the case of PCH at ϕ = 2.0. In addition, the branching chain favoring the unimolecular reactions causes the shorter ignition delay time of ECH and PCH than that of CH at high temperatures (T > 1450 K) when these reactions dominate the fuel decomposition. At lower temperatures, the consumption of fuel largely relays on the Habstraction reactions, and thus, the H radical becomes a major stimulus of CH decomposition. It is also noticed that the ignition delay times of ECH, PCH, and BCH are in the order of PCH < ECH ≈ BCH, which is attributed to the molecule structure of each fuel. Because PCH has shorter and simpler pathways to generate the H radical than others, it enables quicker H-radical generation. Relatively complex pathways of H production in BCH oxidation lead to difficulty in quickly producing the H radical. As a result, the ignition delay time of BCH is obviously longer than that of PCH. A comparison of

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dx.doi.org/10.1021/ef501389f | Energy Fuels 2014, 28, 7159−7167