Shock-Tube Study of Dimethoxymethane Ignition at High Temperatures

Jun 24, 2014 - Energy Fuels , 2014, 28 (7), pp 4603–4610 .... In the current work, because all of the ignition delay times at high temperatures are ...
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Shock-Tube Study of Dimethoxymethane Ignition at High Temperatures Changhua Zhang,† Ping Li,*,† Youliang Li,† Jiuning He,† and Xiangyuan Li‡ †

Institute of Atomic and Molecular Physics, and ‡College of Chemical Engineering, Sichuan University, Chengdu, Sichuan 610065, People’s Republic of China ABSTRACT: Autoignition delay time measurements were performed for dimethoxymethane/oxygen/argon mixtures at pressures of approximately 2, 4, and 10 atm, temperatures of 1103−1454 K, argon/oxygen dilution ratios of 11.25, 23.75, and 48.75, and equivalence ratios of 0.5, 1.0, and 2.0 in a shock-tube facility. Ignition delay times were determined using OH* emission and reflected shock pressure signals monitored at the shock-tube sidewall. The dependence of the ignition delay time upon the temperature, pressure, dilution ratio, and equivalence ratio was characterized. An empirical correlation for the ignition delay times with the experimental parameters was formulated by linear regression of these data. Experimental results were compared to the kinetic modeling predictions of two available chemical kinetic mechanisms to test the performance of mechanisms. Glaude’s mechanism yielded good agreement with the experimental results at 4 and 10 atm but underestimated the ignition delay times at 2 atm. Sensitivity analysis on ignition delay time was conducted, and the dominant reactions during the ignition process were identified. Better predictions on ignition delay times were achieved after modifying the reaction rate of selected small radical reactions. Moreover, fuel reaction pathway analysis was conducted to investigate the consumption of dimethoxymethane. the effects of adding DMM to premixed n-heptane flames and found that mole fractions of most C1−C5 hydrocarbon intermediates were reduced, while that of benzene decreased apparently. Although a great deal of experiments have been conducted on the test of the property of DMM blended in fuels as well as the premixed flame blended with DMM,10−12 very limited fundamental studies focus on the ignition and combustion characteristics of pure DMM fuel at high temperatures. Daly et al.13 have investigated the oxidation of DMM in a jet-stirred reactor at a pressure of 5.07 bar and temperatures of 800−1200 K. Concentration profiles of stable compounds were measured, and a detailed chemical model accounts quite well with the experimental results. Sinha and Thomson14 have investigated the oxidation of DMM experimentally using an opposed diffusion flame, and significant quantities of formaldehyde and methyl formate were measured. They suggested that the oxidation proceed of DMM mainly via H-abstraction and successive β scission to produce methyl radical and formaldehyde. Recently, Dias et al.15,16 have studied the premixed DMM flame, and a new DMM mechanism containing 480 reactions involving 90 chemical species was elaborated to simulate the dimethoxymethane flames. A DMM chemical kinetic mechanism was embedded in the dimethyl carbonate (DMC) mechanism proposed by Glaude et al.,17 which was used to understand DMC combustion in an opposed flow diffusion flame. The autoignition characteristics of DMM are important to diesel engine ignition control as well as the development of

1. INTRODUCTION Diesel engines will continue to be used in the field of commercial transportation in the future because of their superior fuel efficiency. However, they contribute significantly to particulate matter and CO emissions. It has been shown that the addition of oxygenated fuel to diesel fuel can effectively reduce the particulate matter and CO emissions.1−5 Dimethoxymethane (methylal or DMM) is considered to be a potential fuel additive. In comparison to dimethyl ether (DME), DMM has a higher quantity of oxygen, lower vapor pressure, and better solubility with diesel fuel; thus, it could be a better oxygenate additive for diesel/oxygenate blends. Diesel/DMM blends generally decrease exhaust emissions. Several studies have analyzed the effect of adding DMM on emissions of compression ignition engines or direct injection engines. Huang et al.6 have investigated the combustion and emission characteristics of a compression ignition engine. A remarkable reduction in the exhaust CO and smoke can be achieved when operating on the diesel/DMM blend, and a simultaneous reduction in both NOx and smoke can be realized at large DMM additions. Sathiyagnanam and Saravanan7 have found that the DMM additive showed an appreciable reduction of emissions, such as smoke density, particulate matter emission, and marginal increase in the performance when compared to a normal diesel run. Liu et al.8 have studied the effects of DMM blended with gas-to-liquids on particulate matter emissions from a compression ignition engine. They found that DMM addition significantly reduces the total exhaust particle mass concentrations. Song and Litzinger9 have investigated the effects of the addition of DMM to diesel fuel in an optically accessible direct injection (DI) diesel engine. Blending of DMM into diesel fuel to obtain 2 and 4 wt % oxygen in the fuel can lead to substantial reductions of exhaust soot levels, 46 and 57%, respectively. Chen et al.10 have studied © 2014 American Chemical Society

Received: April 16, 2014 Revised: June 23, 2014 Published: June 24, 2014 4603

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relative kinetic mechanism. However, to our knowledge, there is no report of ignition delay time measurement of neat gasphase DMM in the literature. The purpose of this paper is mechanism validation and fundamental research of DMM ignition. As a well-constructed mechanism, it should be applied not only under practical engine conditions but also over a wide range of conditions. In this work, we systematically measured the ignition delay times of gas-phase DMM/oxygen/argon mixtures in a shock tube. The ignition results were compared to simulations of two published mechanisms proposed by Dias et al.13 and Glaude et al.15 to validate the performance of mechanisms. Sensitivity and reaction pathway analyses were employed to identify the dominant reactions during the ignition process of DMM. Suggestions to improve the mechanism have been proposed, and the rate constants of key reactions have been improved to better match the ignition delays. In our opinion, this work is useful for researchers engaged in kinetic model development of ether oxidation.

Figure 1. Determination of the ignition delay time (mixture C, p = 2.07 atm, and T = 1151 K).

2. EXPERIMENTAL SECTION

In this work, the variation in the pressure was found to be in the range of (dp/dt)(1/p) = 0−2.5%/ms, leading to a temperature rise of 0−1.0%/ms. The pressure signals and OH* emission were recorded by a digital phosphor oscilloscope (Tektronix TDS5054B). The temperatures and pressures behind the reflected shock waves were calculated by the one-dimensional normal shock model (the shocktube code of the CHEMKIN package19) using the measured incident shock speed, the initial temperature and pressure, and the mixture composition in the driven section. The incident shock speed at the shock-tube endwall was determined from the extrapolation of three shock speed measurements made over the last 0.75 m of the driven section with four pressure transducers. Typical attenuation rates of the incident shock wave ranged from 0.5 to 2.0%/m. The 1.0 μs uncertainty in the speed measurement will lead to a temperature uncertainty of about ±8 K. The uncertainty of the measured ignition time was estimated to be within ±20% based on the combined uncertainties in the reflected shock temperature and pressure induced by the shock speed measurement and non-ideal boundary layer effect, the reactant mixture composition, and the uncertainty in determining the ignition time from the measured sidewall pressure and OH* emission signals. For modeling the ignition behavior of the DMM/oxygen/argon mixture behind the reflected shock wave, the numerical simulation was carried out using the CHEMKIN II package. In the current work, because all of the ignition delay times at high temperatures are less than 1.5 ms, the numerical simulation was conducted using a zerodimensional homogeneous reactor model with a constant volume assumption and the non-ideal boundary layer effect is not considered because the reaction time is much shorter than the diffusion time.

The experimental apparatus and its validation have been described in detail elsewhere.18 Briefly, all ignition measurements were carried out in a stainless-steel shock tube with a constant inner diameter of 10 cm. The shock tube is separated into a 4.0 m long driver section and a 5.0 m long driven section by the double polycarbonate diaphragms. Diaphragms of different thickness were used to achieve different strengths of shock waves. The shock tube can be evacuated to the pressure below 10−2 Torr by a vacuum pump system. Helium was used as the driver gas with a purity of 99.99%. DMM/oxygen/argon mixtures were prepared in advance in a 40 L stainless-steel tank. The purity of the used oxygen, argon, and DMM is better than 99.99, 99.99, and 99%, respectively. The desired mixtures were allowed to settle at least 3 h before the experiment to ensure sufficient mixing. The composition of the mixtures employed in this study is given in Table 1, in which Φ is the equivalence ratio and D is

Table 1. List of DMM/O2/Ar Mixtures mixture

Φ

DMM (%)

O2 (%)

Ar (%)

Ar/O2 ratio, D

A B C D E

1 1 1 2 0.5

1 0.5 2 1.98 0.503

4 2 8 3.96 4.02

95 97.5 90 94.06 95.477

23.75 48.75 11.25 23.75 23.75

the dilution ratio (the mole ratio of the argon concentration/oxygen concentration). The percentage value in Table 1 is in mole fraction. Ignition was monitored by the reflected shock pressure and OH* radical chemiluminescence signals at the shock-tube sidewall. The reflected shock pressure was traced by a pressure transducer (PCB113B), which was located at 15 mm upstream of the endflange of the shock tube. The sidewall measurement results in a shorter ignition delay time from the endwell data because of the different speeds between the reflected wave and combustion wave. The significance of this sidewall error decreases with a decreasing distance from the endwall and with a decreasing temperature. Moreover, a higher fuel/oxidizer concentration (lower dilution) will lead to a faster combustion wave speed and, as a result, a larger discrepancy between the sidewall and endwall. For present diagnostics located 15 mm from the endwall and for reflected shock temperatures less than 1450 K, the discrepancy is estimated to be within 20 μs for all measurements and has been included in the uncertainty analysis. OH* emission at the wavelength of 307 nm was obtained at the same location by a monochromator coupled with a photomultiplier. The ignition delay time is defined as the time interval between the arrival of the reflected shock wave and the sharp rise of the OH* chemiluminescence signal, as shown in Figure 1.

3. RESULTS AND DISCUSSION Ignition delay times of DMM/oxygen/argon mixtures have been measured at temperatures of 1103−1454 K, pressures of 2−10 atm, argon/oxygen dilution ratios of 11.25−48.75, and equivalence ratios of 0.5, 1.0, and 2.0. The original data are summarized in Table 2. Figures 2−4 present the ignition delay times for DMM/oxygen/argon mixtures at different test conditions as well as the numerical predictions of two kinetic mechanisms. In the temperature range investigated, the ignition delay times show exponential dependence upon the inverse temperature. This denotes an Arrhenius behavior that is generally observed for other fuel ignitions at high temperatures.20,21 The effect of the pressure on ignition is studied at average pressures of 2, 4, and 10 atm for mixture A. The influence of the equivalence ratio is also studied at 2 and 4 atm using lean, rich, and stoichiometric mixtures at a dilution ratio of 23.75. To investigate the effect of dilution, experiments were 4604

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Table 2. Measured DMM Ignition Delay Times p (atm) 2.05 1.80 2.09 1.93 2.03 2.05 2.00 2.05 2.03 2.03 4.08 4.01 4.23 4.08 4.07 3.91 4.05 4.02 4.06 9.99 9.29 9.76 10.89 9.90 10.11 9.92 11.15 1.80 2.06 2.02 2.03 2.05 2.01 2.00 1.97 1.98 1.99 4.01 3.92 3.83 3.87 3.98 3.92 3.90 4.00 4.16 4.12

T (K) mix A 1162 1179 1188 1198 1255 1280 1292 1352 1371 1393 1159 1178 1188 1216 1250 1265 1320 1352 1387 1120 1167 1184 1208 1214 1250 1304 1306 mix B 1232 1236 1247 1266 1300 1310 1350 1397 1422 1454 1197 1220 1242 1256 1295 1309 1332 1374 1412 1413

τ (μs) 1471 1193 1028 885 452 322 290 171 116 106 1148 825 725 556 330 254 134 92 62 1372 702 416 283 280 180 91 79 1187 1074 843 710 531 444 269 166 130 92 1436 770 662 547 333 253 166 106 79 71

p (atm) 2.07 2.04 2.09 1.98 1.94 2.02 1.95 2.03 2.02 3.87 3.91 3.94 4.07 4.09 4.16 4.05 1.98 1.99 1.94 1.96 1.86 2.03 1.97 2.00 1.98 2.03 3.85 3.91 3.98 3.97 3.91 3.98 3.97 1.90 1.98 2.03 1.97 1.99 1.98 1.94 1.95 1.99 1.99 4.03 4.01 3.91 4.01 3.93 4.05 4.04

T (K) mix C 1151 1180 1207 1217 1230 1272 1275 1301 1335 1120 1139 1173 1177 1217 1240 1258 mix D 1137 1163 1180 1207 1254 1268 1280 1313 1333 1366 1103 1131 1165 1206 1247 1286 1323 mix E 1197 1199 1245 1254 1295 1323 1346 1374 1411 1449 1174 1210 1255 1299 1317 1360 1400

τ (μs) 911 704 493 453 409 214 168 144 100 1178 868 424 422 232 190 152

Figure 2. Effect of the pressure on the ignition delay time for mixture A.

have been corrected with a power law relationship τ ∼ P−0.66 obtained from a regression analysis of all of the measured data, as presented latter in eq 1. It is observed that the ignition delay time decreases with the increasing post-reflected shock pressure. The increasing pressure leads to the increasing relative reactivity because of the increasing molecular concentration. Figure 2 also shows the comparison between current ignition data and the simulations based on the mechanisms by Dias et al.15 and Glaude et al.17 The mechanism by Dias et al. markedly overpredicts the ignition delay time. The mechanism results by Glaude et al. agree well with the measured ignition delay times, but this mechanism predicts a weaker dependence of ignition time upon the pressure. 3.2. Effect of the Equivalence Ratio. The rate of reaction, which is subject to the law of mass action, is influenced by the concentrations of the reactants in the system. The presence of the dilute gas affects the concentration of DMM and oxygen. Therefore, it is necessary to test the kinetic mechanisms under various levels of dilution at the same equivalence ratio. The effect of the equivalence ratio on ignition delay times is depicted in Figure 3 by comparing the ignition delays of mixtures A, D, and E (Φ = 1.0, 2.0, and 0.5, respectively) at a fixed dilution ratio of 23.75 at 2 and 4 atm. Figure 3 shows that the ignition delay time decreases apparently as the equivalence ratio increases. That is, the fuel-lean mixture is less reactive. This is reasonable, because in these mixtures, the concentration of oxygen is almost the same and the equivalence ratio increases from 0.5 to 2.0 with the increasing DMM concentration from about 0.5 to 2%. The increasing DMM concentration leads to a significant decreasing in the ignition delay times; as a result, negative equivalence ratio dependence upon the equivalence ratio is observed. Both mechanisms by Dias et al. and Glaude et al. can catch this dependence. However, the mechanism by Dias et al. significantly overpredicts the ignition delay times. The mechanism predictions by Glaude et al. are in good agreement with the measured values at 4 atm, but this mechanism predicts shorter ignition times at 2 atm, especially in the Φ = 0.5 case. 3.3. Effect of the Dilution Ratio. The dilution ratio has an evident effect on the ignition delay time. Experiments were designed to test the quantitive dependence upon the dilution ratio by changing the oxygen and DMM fractions to obtain three different dilution ratios (D = 11.25, 23.75, and 48.75) and keeping the equivalence ratio under stoichiometric conditions. The results of 2 and 4 atm are demonstrated in Figure 4. As expected, an increase in the dilution ratio leads to the reduction of the reactant concentration; as a result, the ignition delay time

1421 1035 756 584 390 304 238 184 128 111 1572 1061 702 416 208 112 86 1381 1230 901 669 486 364 287 171 137 85 1432 755 390 268 177 121 66

also carried out at argon/oxygen ratios of 11.25, 23.75, and 48.75 at pressures of 2 and 4 atm. 3.1. Effect of the Pressure. Figure 2 shows the effect of the pressure on the ignition delay time for a stoichiometric mixture with an argon/oxygen ratio of 23.75 at high temperatures. Pressure variations from the average pressure 4605

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Figure 3. Effect of the equivalence ratio on the ignition delay time at (a) 2.0 atm and (b) 4.0 atm.

Figure 4. Effect of the dilution ratio on the ignition delay time at (a) 2.0 atm and (b) 4.0 atm, with Φ = 1.0.

Figure 5. Sensitivity analysis for DMM ignition at T = 1200 K.

decreases with a decreasing dilution ratio, therefore showing positive dilution ratio dependence. This trend is in close agreement with the predicted results of mechanisms. However, the mechanism by Dias et al. predicts much higher ignition times at both 2 and 4 atm. The mechanism predictions by Glaude et al. are in agreement with measured ignition times at 4 atm. However, the calculated ignition delay times are shorter than the measured results at 2 atm. To develop a correlation for DMM ignition, a multiple regression analysis has been performed for all ignition data collected in this work and a regression expression defining the experimental ignition delay time of DMM has been obtained

τ = (8.14 ± 3.1) × 10−6Φ(−0.56 ± 0.06)D(0.79 ± 0.05)p(−0.66 ± 0.05) exp(39.4 ± 1.0/RT )

(1)

where τ is the ignition delay time in μs, p is the pressure behind the reflected shock wave in atm, Φ is the equivalence ratio, D is the dilution ratio, Ea = 39.4 kcal/mol is the global activation energy, T is the temperature behind the reflected shock wave in K, and R = 1.986 × 10−3 kcal mol−1 K−1 is the universal gas constant. The dependence of the ignition delay time upon the pressure is τ ∼ p−0.66, which is the same as that of DME measured by Cook et al.22 Meanwhile, the global activation energy of DMM reasonably agrees with that of DME. The 4606

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similarity ignition property could be due to the molecular structure of DMM being similar to that of DME. 3.4. Sensitivity Analysis. Because the model by Glaude et al. yields good agreement with the measured ignition delay times of DMM, to identify important reactions relevant to the ignition process, sensitivity analysis has been performed for DMM using the mechanism by Glaude et al. in this study at T = 1200 K with equivalence ratios of 0.5, 1.0, and 2.0. For the sensitivity analysis on the ignition delay (τ), the rate constant of each reaction, ki, is individually doubled. The change in the ignition delay, [τ(2ki) − τ(ki)]/τ(ki), is taken as the sensitivity of that particular reaction. Figure 5 presents such a sensitivity analysis result with the 17 most sensitive reactions. A positive sensitivity coefficient implies that the ignition delay time increases with increasing the rate constant of a particular reaction. As shown in Figure 5a, at Φ = 1.0, each main reaction yields similar sensitivity at different pressures of 2, 4, and 10 atm.

Figure 6. Comparison of sensitivity at Φ = 1.0, p = 4 atm, and D = 23.75.

Figure 7. Comparison of experimental data to predictions of the mechanism by Glaude et al. and the modified mechanism (short dashed lines, mechanism by Glaude et al.; solid lines, modified mechanism). 4607

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Figure 10. Reaction pathway analysis for DMM of mixture B at p = 2 atm, T = 1250 K, and 20% fuel consumption using the modified mechanism.

= 2.0 and a moderate coefficient at Φ = 1.0, but it has a slight effect on the ignition delay at Φ = 0.5. The reaction HCO + M = H + CO + M shows the highest positive sensitivity coefficient at Φ = 2.0 and a moderate coefficient at Φ = 1.0 but has a weak negative sensitivity coefficient at Φ = 0.5. The sensitivity coefficient of reactions related to DMM does not change dramatically as the equivalence ratio changes. As shown in Figures 2−4, the mechanism by Glaude et al. underestimates the ignition delay time at some conditions, especially at 2 atm, indicating that the rate of certain main reactions in the mechanism need to be improved. Sensitivity analysis results in Figure 5 show that the important reactions are small radical reactions and the reactions related to DMM. We did not consider improving the rate constant of DMMrelated reactions because no further theoretical or experimental value was reported. The rate constant of the H + O2 = O + OH reaction has been studied widely, and it is out of our consideration in this work. The important CH3 + HO2 reaction and the equivalence ratio sensitive reactions HCO + O2 = CO + HO2 and HCO + M = H + CO + M are considered here. Details of modifications of these four reactions are summarized as follows: (1) The rate of the chain-branching reaction CH3 + HO2 = CH3O + OH and the rate of the chain termination reaction CH3 + HO2 = CH4 + O2 are modified to 6.8 × 1012 and 4.4 × 1012 cm3 mol−1 s−1, respectively, as measured by Hong et al.24 (2) The rate coefficient of HCO + M = H + CO + M is modified to 4.8 × 1017T−1.2 exp(−74.2 kJ mol/RT) cm3 mol−1 s−1, as reported by Friedrichs et al.25 (3) The rate

Figure 8. Comparison of sensitivity analysis for DMM ignition using the mechanism by Glaude et al. to the modified mechanism at T = 1200 K.

During the ignition process, the dominate reactions are the unimolecular decomposition reactions and the H-abstraction reactions of DMM as well as reactions between small species. The promoting decomposition reaction of DMM, DMM = CH3OH + CH3 + HCO, exhibits the highest negative sensitivity index. The main chain-branching reactions H + O2 = O + OH and CH3 + HO2 = CH3O + OH have moderately high negative sensitivity coefficients. The chain termination reaction CH3 + HO2 = CH4 + O2 plays an inhibition role on the reactivity and has the highest positive effect to the DMM ignition. The sensitivity of DMM = CH3OH + CH3 + HCO decreases as the temperature increases at T > 1100 K, while that of H + O2 = O + OH increases as the temperature increases. At high temperatures, where T > 1300 K, we found that H + O2 = O + OH is the most sensitive reaction, as shown in Figure 6. Similar temperature-dependent sensitivity results are observed for other fuels, such as propanal.23 The sensitivity analysis shows that some important reactions between small species vary when the equivalence ratio changes, as shown in Figure 5b. The promoting reaction HCO + O2 = CO + HO2 shows a strong negative sensitivity coefficient at Φ

Figure 9. Measured and modeling mole fraction profiles of species in the DMC opposed-flow diffusion flame. The horizontal axis is the distance from the fuel port in centimeters (scatter, experimental results;17 solid line, modified mechanism results; and dashed line, mechanism results by Glaude et al.). 4608

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coefficient of HCO + O2 = CO + HO2 is modified to 13.45 × 1012 exp(−0.4 kcal mol/RT) cm3 mol−1 s−1, as adopted in GRIMech 3.0.26 Figure 7 presents the comparison between current measured data and the predicted ignition delay times simulated from the mechanism by Glaude et al. and the modified mechanism. It clearly exhibits that the modified mechanism shows better agreement with the experiments and can well-predict the ignition delay time at almost all experimental conditions, especially at 2 atm. The modified mechanism shows a stronger pressure dependence (Figure 7a) and equivalence ratio dependence (panels b and c of Figure 7) as well as a stronger dilution ratio dependence (panels d and e of Figure 7). Figure 8 shows the comparison of sensitivity analysis for DMM ignition using the mechanism by Glaude et al. to the modified mechanism at T = 1200 K. The sensitivities of the promoting reaction HCO + O2 = CO + HO2 and the inhibiting reaction HCO + M = H + CO + M decrease in the modified mechanism. The chain-branching reaction CH3 + HO2 = CH3O + OH shows much higher sensitivity to DMM ignition. Other reactions show similar sensitivity to the original mechanism by Glaude et al. Figure 9 presents the comparison between the results of the mechanisms and the experiments for species profiles in the DMC opposed-flow diffusion flame.17 For major species, such as DMM, CO2, CO, and CH4, no noticeable discrepancy was found between the results of the modified mechanism and the original mechanism. The modified mechanism predicts the ethane profile better but predicts the ethane and acetylene profiles worse. 3.5. Reaction Pathway Analysis. Reaction pathway analysis was performed using the modified mechanism to investigate the important reaction pathways of DMM. The consumption rate was analysis at 20% fuel consumption for mixture B at p = 2.0 atm and T = 1250 K. As seen in Figure 10, the main consumption reactions of DMM are the unimolecular decomposition and H-abstraction reactions. The decomposition of DMM is mainly via DMM = CH3OH + CH3 + HCO (33.3%). About 8.5% portion of DMM decomposes via decomposition of the C−O bond in DMM = CH3OCH2O + CH3 , followed by the dehydrogenation reaction from CH3OCH2O to form methyl formate CH3OCHO. Another unimolecular decomposition of DMM (2.0%) is the cleavage of the C−O bond to form methoxy methyl radical CH3OCH2 and methoxy radical CH3O. The H-abstraction reactions by H, O, OH, CH3, and HO2 radicals at the primary carbon or the central carbon of DMM generate CH3OCH2OCH2 and CH3OCHOCH3, respectively. The H-abstraction at the primary carbon produces a CH3OCH2OCH2 radical (31.1%), which successively dissociates via β scission to produce methoxy methyl radical CH3OCH2 and formaldehyde CH2O. The H-abstraction at the central carbon produces a CH3OCHOCH3 radical (25.1%), which will, under further dissociation, form methyl formate CH3OCHO and CH3.

The conclusions are summarized as follows: (1) The effects of pressure, equivalence ratio, and dilution ratio were investigated. An empirical correlation of ignition delay times as a function of experimental parameters has been formulated by multiple regression analysis. (2) Current results were used to test the performance of proposed mechanisms. Numerical predictions of ignition delay times have been made with two developed kinetic mechanisms of DMM under the experimental conditions. The predictions of the mechanism by Glaude et al. agree reasonably well with the measurements at 4 atm but underestimated the ignition delay time at 2 atm. (3) Sensitivity analysis has been conducted using the mechanism by Glaude et al. The important reactions are mainly small radical reactions and the DMM-related reactions. The sensitivity does not change according to the pressure changing but has a dramatic change according to the equivalence ratio changing for some small radical reactions. (4) The reaction rates of four key small radical reactions have been improved, and the modified mechanism shows remarkable improvement to the prediction on the ignition delay time of DMM. Reaction pathway analysis has been performed using the modified mechanism to investigate the fuel consumption pathways. The main consumption reactions of DMM are the unimolecular decomposition and H-abstraction reactions.

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AUTHOR INFORMATION

Corresponding Author

*Telephone: 86-28-85405515. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the support from the National Natural Science Foundation of China under Grant 91016002.

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REFERENCES

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4. CONCLUSION The ignition delay characteristic of DMM/oxygen/argon has been systematically studied behind the reflected shock waves at high temperatures. The experiments were conducted in the temperature range between 1103 and 1454 K, pressures from about 2 to 10 atm, equivalence ratios of 0.5, 1.0, and 2.0, and dilution ratios of 11.25, 23.75, and 48.75. To our knowledge, this work is the first report of DMM ignition in a shock tube. 4609

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dx.doi.org/10.1021/ef500853v | Energy Fuels 2014, 28, 4603−4610