Shock-Tube Study on Ethylcyclohexane Ignition

Jul 24, 2014 - Analysis was conducted to kinetically interpret the effects of the ... content by weight in diesel fuel is up to 40%.1−3 It has been ...
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Shock-Tube Study on Ethylcyclohexane Ignition Zemin Tian, Yingjia Zhang,* Lun Pan, Jiaxiang Zhang, Feiyu Yang, 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 of ethylcyclohexane with a fixed fuel concentration of 0.5% were measured behind reflected shock wave at pressures of 1.1−10.0 atm, equivalence ratios of 0.5−2.0, and temperatures between 1000 and 1700 K. The measured ignition delay times were compared to predictions by the mechanism JetSurF2.0, and fairly good agreements were presented under the test conditions. The ignition delay time of the fuel-rich mixture was suggested to overwhelm that of the fuellean mixture when tested at a low temperature and an elevated pressure. Analysis was conducted to kinetically interpret the effects of the pressure, temperature, equivalence ratio, and fuel concentration on the time histories of the OH radical and intermediate species of hydrocarbons. Results showed that the suppression of reaction H + O2 ⇄ OH + O by fuel-concerned radicals played an important role. Sensitivity analyses were carried out to provide insight into the controlling reactions on the ignition of ethylcyclohexane.

1. INTRODUCTION Worldwide use of fossil fuels brings some problems, among which environmental pollution is particularly significant. Plenty of work has been performed on the fundamental combustion research for the renewable, eco-friendly alternative energy sources and various kinds of transportation fuels. However, the dominant role of fossil fuel is not changed at present, especially for the vehicle. Thus, much attention has been paid to cyclohexanes (CHs) because they belong to the most important components of fossil fuels. Jet fuels, such as Jet A/ Jet A1/JP-8, contain up to 20% CHs (by volume), and the content by weight in diesel fuel is up to 40%.1−3 It has been reported that Canada’s oil-sand-derived fuel serving as a potential energy resource is composed of large cycloalkanes.4 In addition, oxidation of cycloalkanes in the oxygen-lacking circumstance can produce many aromatics, which act as the soot precursor. Therefore, they are also of great importance in the formation of particulate matter (PM).5 Recently, studies for a better understanding of oxidation of cycloalkanes were conducted, especially for CH and methylcyclohexane (MCH). Sirjean et al.6 developed a hightemperature mechanism for CH with the EXGAS software and measured the ignition delay times at 7.3−9.5 atm, equivalence ratios of 0.5−2.0, and temperatures from 1230 to 1840 K with the mixtures containing 0.5 and 1.0% fuel. They found that the ignition of CH was much faster than that of cyclopentane, and this experimental observation was wellreproduced by their kinetic mechanism. Silke et al.4 developed a detailed kinetic mechanism for CH at both high and low temperatures. General rules of reaction rate constants were proposed and applied to other CHs in their study. The mechanism was validated against plenty of kinetic targets, including ignition delay time, species profile, and laminar flame speed, and fairly good agreements were obtained. Furthermore, Daley et al.7 measured the ignition delay times of the CH/air mixtures at pressures of 11−61 atm, equivalence ratios of 1.0, 0.5, and 0.25, and temperatures from 847 to 1379 K in a shock © 2014 American Chemical Society

tube. They indicated that all mechanisms used overpredicted the experimental results under the tested conditions. For MCH, Orme et al.8 proposed a detailed mechanism, and Pitz et al.9 subsequently developed this kinetic model by adding a new low-temperature mechanism. Vanderover and Oehlschlaeger10 measured the ignition delay times for MCH at high pressures (12 and 50 atm) with ϕ = 0.25, 0.5, and 1.0. Hong et al.11 performed a comparative study of CH, MCH, and nbutylcyclohexane (BCH) at a high temperature. They measured the ignition delay times of the three fuels at ϕ = 0.5−1.0 and p = 1.0−3.0 atm with temperatures from 1220−1450 K. In addition, flux analyses were made to explain the reasons for the order of ignition delay times (MCH > CH ≈ BCH). Meanwhile, other investigations on CH and MCH using various experimental approaches, such as rapid compression machine (RCM) and jet-stirred reactor (JSR), were reported.12−15 In comparison to studies of CH and MCH, cycloalkanes with longer alkyl chains have not been studied enough. Conroy et al.16 measured the ignition delay times for BCH using the mixtures diluted in air at 10 and 30 atm and equivalence ratios of 0.3, 0.5, 1.0, and 2.0 but did not make kinetic analyses. In addition, previous studies also concerned n-propylcyclohexane (PCH) through JSR,17−19 laminar flame,20 and RCM.21 However, very limited studies were performed on the autoignition of ethylcyclohexane (ECH). Vanderover and Oehlschlaeger10 measured the ignition delay times of ECH at pressures of 15 and 50 atm and equivalence ratios of 0.25, 0.5, and 1.0. Their experiments covered the temperature range of 680−1650 K. Various intermediate species were detected, and good predictions by a detailed mechanism were observed when JSR was employed by Husson et al.22 to study the ECH at 800 Torr, 500−1100 K, and ϕ = 0.25, 1.0, and 2.0. Additionally, Ji Received: May 4, 2014 Revised: July 23, 2014 Published: July 24, 2014 5505

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et al.23 measured the laminar flame speeds of ECH in the counterflow configuration at the atmospheric pressure, temperature of 353 K, and wide range of equivalence ratios. Up to now, there are limited studies on the oxidation of longer alkyl-substituted CHs, especially for ECH. In this study, the ignition delay times of ECH diluted in argon were measured at pressures of 1.1, 5.0, and 10 atm, equivalence ratios of 0.5, 1.0, and 2.0, and temperatures from 1000 to 1700 K. On the basis of the previous research, the oxidation characteristics of ECH ignition were discussed at both low and elevated pressures, including the effects of the temperature and equivalence ratio, which deepens the understanding of oxidation of hydrocarbons theoretically. Considering some atmospherically fuel-burning equipment, such as oil boiler, scramjet, and pulse detonation engine (PDE), it is meaningful in practice to account for the oxidation of hydrocarbon at low pressure as well. In addition, this work supplements the data of ignition delay time for ECH. The data are important fundamental parameters to validate the oxidation mechanism at a low pressure, providing the basis for the development of a high-pressure oxidation mechanism.

Figure 1. Definition of the ignition delay time. zero baseline. The temperature behind the reflected shock wave was calculated using the reflected shock model in the software Gaseq26 with an uncertainty of about ±25 K. In addition, the uncertainty in the measured ignition delay time was estimated to be 15%, which resulted from the uncertainty in the shock attenuation and non-ideal shock reflection. The data with error bars are plotted in Figure 4.

2. EXPERIMENTAL SECTION The shock tube with 11.5 cm diameter has been described in details in the previous publications.24,25 It consists of a 4.0 m driver section, a 4.8 m driven section, and a double-diaphragm machine. Different thicknesses of the polyester terephthalate (PET) films were chosen to obtain different reflected wave conditions. High-purity helium (99.999%) and nitrogen (99.999%) were charged into the driver section as the driver gas. The driven section was evacuated to pressure below 1.0 Pa using a Nanguang vacuum system before the reactant mixture was added. Four fast-response pressure transducers (PCB 113B26) are mounted at the side wall at constant intervals of 300 mm along the last 1.5 m driven section. A photomultiplier (Hamamstu, CR131) and piezoelectric pressure transducer with acceleration compensation (PCB 113B03) are located at the endwall. The photomultiplier with a filter narrowly centered at 307 ± 10 nm was used to capture the OH* chemiluminescence, while the pressure transducer was used to detect the reflected shock pressure. All signals were collected with a digital recorder (Yokogawa, Scopecorder DL750). Fuel mixtures were manometrically prepared in a 128 L stainless-steel tank with a vacuum system and a highly accurate vacuum meter. Following the injection of ECH (99.5%) into the evacuated tank, the regulated amount of oxygen (99.999%) and argon (99.999%) was charged. The detailed compositions of test mixtures are listed in Table 1. A typical profile of pressure behind the reflected shock wave and OH* emission history is given in Figure 1. Pressure remains almost constant until the onset of ignition, corresponding to the eruption of OH* emission. Ignition delay time is defined as the time interval between the arrival of the reflected shock wave at the end wall and the extrapolation of the steepest rise in the OH* emission signal to the

3. RESULTS AND DISCUSSION 3.1. Data and Correlation. The ignition delay times are given in Figure 2 and are available in Table S1 of the Supporting Information. A formula (eq 1) suggested by Horning et al.27 is used to make the correlations under various conditions using the multiple linear regression method τ = Aϕαpγ exp(Ea /RT )

where τ is the ignition delay time (μs), α and γ are exponent factors for equivalence ratio and pressure (atm), respectively, Ea is the total activation energy (kcal/mol), R (=1.986 kcal mol−1 K−1) is the universal gas constant, and T is the temperature (K). Figure 2 shows the measured ignition delay times of ECH mixtures at three pressures under the fuel-lean (ϕ = 0.5), fuelstoichiometric (ϕ = 1.0), and fuel-rich (ϕ = 2.0) conditions. In general, the global activation energies are around 31.7−35.7 kcal/mol. The ignition delay time is correlated as τ = (1.3 ± 0.3) × 10−3ϕ1.43 ± 0.04p−0.53 ± 0.02 exp((34.6 ± 0.5)/RT )

O2 (%)

Ar (%)

ϕ

P5 (atm)

0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5

6 6 6 3 3 3 12 12 12

93.5 93.5 93.5 96.5 96.5 96.5 87.5 87.5 87.5

1.0 1.0 1.0 2.0 2.0 2.0 0.5 0.5 0.5

1.1 5.0 10.0 1.1 5.0 10.0 1.1 5.0 10.0

(2)

where physical variables and their units are the same as those in eq 1. This correlation can be used to empirically estimate the ignition delays within the tested conditions. Other available ignition delay times for ECH can be found in the study by Vanderover and Oehlschlaeger.10 They conducted the experiments at 12 and 50 atm in the dilution of air. Namely, the fraction of oxygen is 20.65−20.92%, and that of ECH is 0.44% at ϕ = 0.25, 0.86% at ϕ = 0.5, and 1.72% at ϕ = 1.0. Despite the difference in the mixture compositions, the comparisons at ϕ = 1.0 and 0.5 are depicted in Figure 3. Obviously, the ignition delay times are considerably shorter in comparison to the current data. Large discrepancy is presented even at the pressures of 10 and 12 atm. A gentler slope at p = 50 atm indicates a rise in the reactivity of the mixture. Moreover, at ϕ = 1.0, the exponent factor of pressure dependence (γ) in eq 1 is about −0.98 in the literature10 but about −0.52 in this work.

Table 1. Mixture Compositions in This Study ECH (%)

(1)

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Figure 3. Comparison of measured results with previous data at (a) ϕ = 1.0 and (b) ϕ = 0.5.

3.2. Numerical Simulation. Only one detailed mechanism, JetSurF2.0 developed by Wang Group,29 is available for simulating the autoignition of ECH, although various mechanisms for CH and MCH have been developed. This mechanism including 348 species and 2163 reactions contains oxidation chemistry for normal alkanes up to n-dodecane. Moreover, this model involves the submechanisms for CH/ MCH/ECH/BCH and has been validated extensively with high-temperature data, except the submechanism for ECH. However, no study was reported after Husson et al.22 simulated their measured mole fractions of major species at low temperatures using this mechanism. Here, JetSurF2.0 is used to simulate and interpret the experimental results. The simulation was conducted with a zero-dimensional and constant volume adiabatic model of the SENKIN code30 in the CHEMKIN II package.31 The calculated ignition delay time is defined as the time interval between the start of simulation and the time of steepest rise in the temperature (dT/dt). In this study, both current and previous10 data are compared to the numerical results. A significant deviation in simulation at a low temperature occurs on the assumption of constant U and V.32 An average pressure rise (dp/dt) of 4%/ms is taken into account. The predictions with and without consideration of pressure rise are plotted in Figure 4. Consideration of the pressure rise obviously reduces the numerical ignition delay times when the ignition time is over 500 μs. Figure 5 gives the

Figure 2. Ignition delay times and correlations for ECH at 1.1, 5.0, and 10 atm and equivalence ratios of (a) 0.5, (b) 1.0, and (c) 2.0.

Venderover and Oehlschlaeger10 gave the value of −0.99 for MCH, and Vasu et al.28 gave the value of −0.98 for MCH, under similar test conditions. The values for CH and cyclopentane provided by Daley et al.7 turned out to be around 1.0 again. However, the value of about −0.55 was acquired by fitting the ignition delay time of MCH at 1.0−4.0 atm and ϕ = 1.0 in the study by Orme et al.8 and our unpublished measured results for CH and MCH at 1.0−5.0 atm. This is attributed to the high pressure and fuel- and O2rich mixture. 5507

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Figure 5. Validation of the JetSurF 2.0 mechanism against data by Venderover et al.10 Symbols, experimental measurements; lines, simulations.

mechanism at a low temperature, just like the motivating influence of peroxy species chemistry on ignition at an elevated pressure and a moderate temperature in the study by Daley et al.7 It should be noted that the ignition delay time at 12 atm may have larger uncertainty. There is doubt that the previous data may be too limited. 3.3. Chemical Kinetic Interpretation of ECH Ignition. From the discussion above, it can be seen that the JetSurF2.0 mechanism well predicts the ignition delay times of ECH at 1.0−10.0 atm and can also capture the dependence upon the equivalence ratio at an elevated pressure. In this work, the ignition delay time is increased with increasing the equivalence ratio at a fixed fuel concentration. Hong et al.11 also found a similar observation when keeping the oxygen concentration constant in their study. However, Vanderover and Oehlschlaeger10 obtained a different conclusion, which was also found in other investigations on hydrocarbons.7,33,34 All of these studies were performed at pressures up to 50 atm with an almost fixed O2 fraction. In general, if the oxygen concentration is altered to obtain various equivalence ratios, a fuel-rich mixture shows a longer ignition delay time. However, if the fuel concentration is changed to obtain different equivalence ratios, a fuel-lean mixture presents the faster ignition. Actually, this feature can be well-reproduced using JetSurF2.0. Without loss of generality, the ignition delay times of ECH at 5.0 and 50 atm, equivalence ratios of 0.5 and 2.0, and temperatures from 1100

Figure 4. Validation against the mechanism with the current data. Symbols, measurements; lines, simulations.

comparison between the measured data in ref 10 and the simulated results using the JetSurF2.0 mechanism. Despite the pressure variation of 2−4%/ms in their study, dp/dt has limited influence on the simulation, as shown in the dot line in Figure 5b, because all measurements are performed at an ignition time less than 1.0 ms. The predictions agree well with the current data but depart from the previous data. However, the experimental data in the literature10 always overpredict under the test conditions, especially at 12 atm, as shown in Figure 5. The inaccuracy at 50 atm may be contributed to the poor performance of the 5508

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to 1900 K with constant 6% O2 were simulated, as shown in Figure 6. Results show that a fuel-rich mixture exhibits a shorter

Figure 6. Simulation of ignition delay times for ECH at 5.0 and 50 atm, with ϕ = 0.5 and 2.0.

ignition delay time when the temperature decreases at 50 atm. To clarify this influence in details, the kinetic analyses will be made for the ECH ignition at temperatures of 1150 and 1550 K. 3.3.1. Moderate Temperature. It is well-known that oxidation and decomposition pathways are different at different temperatures. The temperature of 1150 K is selected as a representative of the moderate temperatures. The histories of the temperature and mole fractions of ECH and C2H4 for 5 and 50 atm at this temperature are plotted in Figure 7. The mole fraction of C2H4 collapses immediately, and the temperature rises rapidly, when ECH is completely consumed in the ignition process. However, both fuel-lean and fuel-rich mixtures have a comparable decomposition rate of ECH at 5.0 atm, but the mole fraction of ECH decreases faster at ϕ = 2.0 than that at ϕ = 0.5 when the pressure is up to 50 atm. In other words, the rate of consumption at ϕ = 2.0 is higher than that at ϕ = 0.5 at the pressure of 50 atm. H, O, OH, and HO2 are mainly chain-branching radicals to accelerate the oxidation of hydrocarbons. Their production and consumption can provide some key information in the fuel oxidation process. To interpret the effects of these free radicals on the oxidation of ECH, a detailed analysis on the rate of production (ROP) of the OH radical will be conducted using JetSurF2.0 in the next section. Figure 8 shows the ROP analysis of OH at the pressures of 5.0 and 50 atm with the equivalence ratios of 0.5 and 2.0 during the period before ECH ignition. Three major reactions contribute to the generation of the OH radical; they are H + O2 ⇄ OH + O (R1), 2OH (+M) ⇄ H2O2 (R14), and CH3 + HO2 ⇄ CH3O + OH (R92). Figure 9 depicts the ROP time history, where reactions R14 and R92 contribute more to the production of OH than reaction R1 prior to the ignition. Moreover, this contribution becomes remarkable at a pressure of 50 atm and an equivalence ratio of 2.0. It is the reaction R1 that triggers the burst of OH and initiates the ignition in all conditions. As shown in Figure 7b, the rates of decomposition of ECH and consumption of intermediates for the fuel-rich mixture were faster than that for the fuel-lean mixture at the pressure of 50 atm. It is clear that the fuel-rich mixture presents greater total productivity of OH

Figure 7. Calculated temperature and mole fractions of ECH and C2H4 at 1150 K, at pressures of (a) 5 and (b) 50 atm and equivalence ratios of 0.5 (0.25% ECH, 6% O2, and 93.75% Ar) and 2.0 (1% ECH, 6% O2, and 93% Ar). Solid lines, ϕ = 0.5; dashed lines, ϕ = 2.0. Black lines, mole fraction of ECH; blue lines, mole fraction of C2H4; red lines, temperature.

than the fuel-lean mixture, as shown in the dash-dotted lines in Figure 8b. The net OH production rate is restrained by the fuel consumption, which needs small radicals. Namely, the fuel-rich mixture will consume more radicals, resulting in the nearly paralleled net ROPs of OH between fuel-lean and fuel-rich mixtures at 50 atm, as shown in Figure 8a. Reactions R14 and R92 are more productive at ϕ = 2.0 than that at ϕ = 0.5, as shown in Figure 9b, and this contributes to the greater OH ROP for the fuel-rich mixture at 50 atm. In contrast to the behavior at 5 atm, as shown in Figure 9a, the production rates of R14 and R92 show distinct superiority over that of R1 at ϕ = 2.0 because fuel and hydrocarbon intermediates are intensified because an elevated pressure promotes the three-body reactions. However, R14 and R92 lose their lead in chain branching to R1 when the pressure falls to 5.0 atm, as shown in Figure 9a. As a result, OH production rates in the fuel-lean and fuel-rich mixtures turn out to be equal when only the initial stage (