Experimental and Kinetic Study on Ignition Delay Times of Dimethyl

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Experimental and Kinetic Study on Ignition Delay Times of Dimethyl Ether at High Temperatures Lun Pan, Erjiang Hu, Zeming Tian, Feiyu Yang, and Zuohua Huang Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.5b00436 • Publication Date (Web): 10 Apr 2015 Downloaded from http://pubs.acs.org on April 23, 2015

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Experimental and Kinetic Study on Ignition Delay Times of Dimethyl Ether at High Temperatures Lun Pan, Erjiang Hu*, Zeming Tian, Feiyu Yang, Zuohua Huang* State Key Laboratory of Multiphase Flows in Power Engineering, Xi’an Jiaotong University, Xi’an 710049, People’s Republic of China *Corresponding author: Telephone: 86-29-82665075. Fax: 86-29-82668789. E-mail: [email protected] (Erjiang Hu), [email protected] (Zuohua Huang). Abstract: An experimental investigation was performed on the effects of temperature, pressure, equivalence ratio and fuel concentration on ignition delay times of DME/O2/Ar mixtures behind the reflected shock wave. Experimental conditions utilized temperatures over 1000 - 1600 K, pressures of 1.2 - 20 atm, and equivalence ratios of 0.5 - 2.0 with fuel concentrations of 0.5-2.457%. The measurements showed that the DME mixtures have different global activation energies under different equivalence ratios. Thus, correlations were derived under different equivalence ratios based on all experimental data, which fits fairly well with the experimental data. Four recently developed models were compared to the measurements, and their predictabilities were thoroughly discussed. Finally, a systematic kinetic chemical analysis was performed to chemically interpret the observed equivalence ratio dependence and to ascertain the key reactions that control ignition of DME, which are the potential candidates for improvement of LLNL DME Mech. Keywords: Ignition delay time, dimethyl ether, shock tube, chemical kinetics. 1. Introduction Recent studies from combustion and engine community on internal combustion engines have focused mainly on clean alternative fuels, along with improving thermal efficiency and reducing pollutant emissions. Compared with gasoline engines, diesel engines are superior power sources due to their excellent performance, higher fuel efficiency, and lower exhaust emissions. On the other hand, diesel engines emit significantly higher levels of particulate matter (PM) and nitrogen oxides (NOx) than gasoline engines. Therefore, the continually more stringent exhaust emissions regulations drive engineers to make more effort in reducing PM and NOx emission of diesel engines. With this in mind, progressive studies of diesel engines have been conducted on alternative fuels, such as bio-hydrogen, bio-alcohols, bio-diesel, liquefied petroleum gas (LPG) and dimethyl ether (DME), in *

Corresponding authors: [email protected] ; [email protected] 1

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order to discover an optimal alternative fuel that may produce clean diesel engine emissions. As the aspect of alternatives fuels, DME is proposed as a promising alternative fuel for diesel engines due to its excellent auto-ignition properties, higher oxygen content, and good evaporation characteristics. During the combustion process, DME shows a two-stage combustion and heat release phenomenon, similar to n-heptane, which has comparable combustion properties to that of diesel. Moreover, compared to other emerging alternative fuels, dimethyl ether can be mass produced commercially from many fossil fuel feedstocks, including natural gas and coal, and from renewable feedstock sources such as biomass 1. Accordingly, there is increasing interest in DME as potential alternative fuel progressed from the middle of 1990s. Spray and engine studies have demonstrated DME as an outstanding alternative fuel with respect to combustion efficiency and pollutant emissions. Youn et al.

2

investigated the combustion

characteristics and emissions of DME and conventional diesel fuels in an electronic controlled four-cylinder ignition engine under various injection and combustion parameters. They found that the peak combustion pressure and the ignition delay of DME fuel are higher and shorter than those of diesel under the same engine load, respectively. Their experimental results also showed that DME fueled engine has lower soot, HC and CO emissions and slightly higher NOX emission than diesel fueled engine. Park et al. 3 systematically reviewed the physical and chemical fuel properties, spray atomization characteristics, combustion, and exhaust emission characteristics of DME fuel. They confirmed the results of Youn et al. 2 and concluded that DME fuel is a promising clean alternative fuel for ground transportation vehicles, and it can substitute for conventional diesel fuel in a compression ignition diesel engine. Although DME exhibits excellent engine performance, the fundamental understanding of its combustion properties is also required to aid with the designing of better DME engines. Up to date, fundamental studies of DME have been widely reported including measurements of ignition delay time, premixed flame, jet-stirred reactor and pyrolysis studies. Dagaut et al.

4, 5

studied oxidation of

DME in a Jet-Stirred Reactor (JSR) at T = 800 - 1300 K, φ = 0.2 – 2 and p = 1 - 10 atm and ignition delay time of DME at T = 1200 - 1600 K, φ = 0.5 – 2 and p = 3.5 atm. A detailed kinetic model containing 43 species and 286 reactions was proposed and it well reproduced the experimental results. They found that no higher molecular weight compound was produced during DME oxidation. Yu et al. 6 studied the laminar premixed DME–air flame at different initial temperatures (303 - 453 K), initial pressures (0.1 - 0.7 MPa), dilution ratios (0 - 25%), equivalence ratios (0.7 - 1.6). Their measurements agree well with those of the previous publications

7-10

. Pyun et al.

11

measured

time-histories of CO, CH4 and C2H4 in mixtures of 0.5%, 1%, and 2% DME in argon respectively, at

2


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temperatures of 1300–1600 K and pressures of 1.5 atm behind reflected shock waves. Their results demonstrated unsatisfactory simulation results in regards to the current model. Cool et el.

12

measured mole fractions for 21 flame species in low-pressure premixed fuel-rich (φ = 1.2, 1.68) DME/oxygen/argon flat flames. They found that present models are capable to predict their measurements. Recently, Cook et al.

13

measured high temperature ignition delay times and OH

concentration time-histories in DME/O2/Ar mixtures behind reflected shock waves for temperature range of 1175 - 1900 K, pressures from 1.6 - 6.6 bar, and equivalence ratios from 0.5 - 3.0. Their measurements confirmed that the existing models capture ignition very well. Li et al.

14

and Phafl 15

measured ignition delay times of DME/air/N2 mixtures under pressures of 13 − 40 bar and temperatures from 697 to 1239 K for ϕ = 0.5, 1.0, and 1.5 in shock tubes . They found that DME shows a notable negative temperature coefficient (NTC) behavior. Using a detailed kinetic model, they presented a detailed kinetic analysis of ignition characteristics of DME from high to low temperature. Beyond these fundamental studies, initial theoretical investigations 16-21 have also been reported. A number of kinetic models

4, 22-28

were developed from the experimental and theoretical

findings of these investigations. Ignition delay time and its dependence on temperature, pressure, equivalence ratio, and fuel concentration are of great importance for validation and refinement of kinetic chemistry models 29. Although high-temperature ignition delay time measurements of DME have been reported, significant difference was also observed as shown in Fig. 1 as provided by Pan et al.

30

. Moreover,

previous measurements covered limited conditions regarding pressures, equivalence ratios, and fuel concentrations. The predictability of present models is not well identified, as reviewed clearly above. With the above points in mind, the primary goal of this study is to provide reliable ignition delay times of DME using a well-developed shock tube, and to extend the available experimental database of DME to a greater range of equivalence ratios, pressures, and fuel concentrations. The predictability of several recently developed kinetic models for DME oxidation and combustion is evaluated by comparing the measured ignition delay times. Finally, kinetic investigation on the ignition chemistry through radicals and sensitivity analysis will be conducted to gain further insight on ignition chemistry of DME. 2. Experimental setup and numerical approaches 2.1 Experimental details The stainless high pressure shock tube used in this experiment demonstrates an updated version of the shock tube described in detail by Zhang et al.

31

and will be briefly introduced here. The

shock tube has a diameter of 14.3 cm and a 4 m long driver section which is separated from the 5.3 m 3


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long driven section by a 0.06 m long double diaphragm section. The detailed description of this shock tube can be found in everywhere

29-35

. Four piezoelectric pressure transducers (PCB 113B26)

located along the last 1.3 meter of the test section with an equal distance were used to determine the velocity of the incident shock wave. The incident shock velocity at end-plate is determined by linear extrapolation of these three incident shock velocities to the end-plate, taking shock wave attenuation into account. The Gaseq equilibrium program

36

together with the end-plate incident velocity, initial

temperature, and thermodynamic data of the reactant mixtures is used to calculate reflected temperatures (T5) using the usual one-dimensional equations. Uncertainty of temperature was analyzed from our previous study 30. Reflected pressures (p5) were obtained by another piezoelectric pressure transducer (PCB 113B03) installed at end-plate. The OH* emission at 306 ±10 nm is selected by a narrow filter embedded in a photomultiplier (HAMAMATSU CR 131) at end-plate. The ignition delay time was defined as the time interval between the steep rise in pressure due to the arrival of the shock wave at the end-plate and the extrapolation of the maximum slope of OH* emission to the zero signal level as shown in Fig. 2. Fuel mixtures were prepared in a 128 L stain-less tank based on Dalton's partial-pressure laws and deliberately standing for overnight to ensure sufficient mixing by molecular diffusion. Before preparing mixtures, the whole tank and delivery lines were vacuumed below 1 Pa. Simultaneously, considering relatively smaller mole fraction of DME and oxygen, a portion of argon (about 30 kPa) was first added into the tank. DME, oxygen, and argon were then added by desired mole fraction. This procedure is aimed to reduce error of gas distribution. The partial pressure of each intake process was carefully controlled by a high-accuracy transmitter (ROSEMOUNT 3051). Detailed composition of the test mixtures are provided in Table 1. Test fuel mixtures were prepared according to the method in the previous study 30. 2 The fuel/O2/Ar mixtures (ϕ = 0.5, XO2 / XAr = 21%/79%) were diluted with argon (20% mixture/80% argon). The purities of helium, argon, oxygen, and DME are 99.999%, 99.995%, 99.995%, 99.995%, respectively. Simulation of ignition delay time and kinetic analysis were performed using Senkin Chemkin II package

37

in the

38

. The calculated ignition delay time is defined as the time interval between

zero and the maximum rate of the temperature rises (maximum dT/dt). SENKIN/VTIM approach

39

was employed to consider facility pressure rise with a pressure rise of 4%/ms, which has been experimentally determined in previous studies 29, 30, 32, 33. 3. Results and Discussion 3.1 Measurements and correlation As aforementioned, ignition delay times of DME were measured behind the reflected shock wave temperatures over 1000 - 1600 K, pressures of 1.2 - 20 atm, equivalence ratios of 0.5 - 2.0 and 4


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fuel concentrations of 0.5 - 2.457%. The measurement results show that ignition delay times give a typical Arrhenius dependence on temperature, pressure, and fuel concentration, while global activation energy is decreased with increasing equivalence ratio. Previously, Davison et al.

40

elaborated the derivation of ignition delay time correlation of hydrocarbon fuel in detail. Based on their study and the above discussions, the following format was developed:

τ = ApaXfuelbexp(Ea/RT)

(1) where τ is ignition delay time in µs, p is pressure in atm, Xfuel is fuel mole fraction and T is temperature in K, R = 1.986×10-3 kcal/(mole⋅K) is the universal gas constant, and Ea is the global activation energy in kcal/mol, A, a, and b are constant. Therefore, based on the measured data, correlations were developed through multiple linear regressions:

φ = 0.5:

τ = 1.81*10-4p-0.37XDME-1.21exp(38599/RT)

（2）

φ = 1.0:

τ = 5.82*10-4p-0.56XDME-1.43exp(38301/RT)

（3）

φ = 2.0:

τ = 8.27*10-3p-0.58XDME-1.37exp(33535/RT)

（4）

It is seen that the global activation energy is decreased with increasing equivalence ratios, especially on the fuel-rich side, indicating that the fuel-rich mixture is easier ignited. The global activation energy in this study is lower than that of Cook et al.

13

. Regression coefficients (R2) of this

correlation are higher than 0.97, indicating strong regression results. Fig. 3 plots the comparison between the correlated data and all measured data under different equivalence ratio at p = 1.2, 4.0, 10.0 and 20.0 atm. It can be seen that the correlation data matches fairly well with the measured data. So far, high temperature ignition delay times of DME diluted in argon behind reflected shock waves have been measured by Cook et al.

13

and Dagaut et al. 5. Therefore, the correlations

formulated in this work are worthwhile to be used to compare the data of Cook et al. and Dagaut et al., as shown in Figs. 4 (a-b). Interestingly, the correlated data give a slight higher value than that of Cook’s study at higher temperature. This observation is different to our previous study 30, in which the experimental data agrees fairly well with the measurements of Cook’s study for the entire temperature range at p = 3.3 atm and φ = 1.0. It is believed that this different observation is due to the wider experimental pressure and temperature in here. For Dagaut et al.’s measurements at p = 3.5 atm, the current correlations show higher values, especially at relatively higher temperatures at φ = 1.0 and 2.0. However, these two data sets match well at φ = 0.5. The differences seen in this study and Dagaut’s study may be attributed to differences in views of the definitions of ignition delay time and the diameter of shock tube. Petersen et al.

41

and Davidson et al.

42

pointed out that a smaller

diameter gives lower global activation energy due to the more profound boundary layer effect. In addition, end-wall CO2 emission was employed to determine the ignition delay time in Dagaut’s 5


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study rather than end-wall OH* emission in this study. 3.2 Model performance In this study, ignition delay times of DME were measured over a wide range of conditions, which are of great value for testing the predictability of existing models. Up to date, a number of kinetic models for describing oxidation of DME have been proposed and four recently developed models, including NUIG Mech_56.54

23

, NUIG Aramco Mech 1.3

28

, LLNL DME Mech

25-27

and

Zhao DME Mech 24, were chosen to predict the measured data here. All of these models contain the detailed DME oxidation mechanism. NUIG Mech_56.54, involving 113 species and 710 reactions, was developed based on the work of H2/CO sub-mechanism of Kéromnès et al. 43, the C1–C2 base sub-mechanism of Metcalfe et al. 28 and the recently published propene mechanism of Burke et al. 44, 45

. The model has been validated by a large array of data from JSR, flow reactor, rapid compression

machine (RCM), shock tube, shock-tube speciation, flame speed, and flame speciation. NUIG Aramco Mech 1.3 including 253 species and 1542 reactions was developed by the Combustion Chemistry Centre in National University of Ireland Galway in 2013 and fully funded by Saudi Aramco. The model characterizes the kinetic and thermochemical properties of a large number of C1-C4 based hydrocarbon and oxygenated fuels over a wide range of experimental conditions. LLNL DME Mech was constructed by Los Alamos National Laboratory, which has been validated against experimental results from burner-stabilized flames, flow reactors, stirred reactors and shock tubes. This model contains 80 species and 351 reactions. The last model, Zhao DME model, consists of 290 reversible reactions amongst 55 species and is developed in a hierarchal manner. Thus, the evaluation of these models on ignition delay times of DME is necessary. Figs. 5-7 plot the comparison between the measured ignition delay times with the calculations with NUIG Mech_56.54, NUIG Aramco Mech 1.3, LLNL DME Mech and Zhao DME Mech. Pressure dependence on ignition delay time of DME oxidation was investigated at three equivalence ratios (0.5, 1.0 and 2.0). Ignition delay times versus temperature at different equivalence ratios and nominal pressures of 1.2, 4.0, 10.0 and 20 atm are plotted in Fig. 5. It is observed that the ignition delay time decreases with increasing pressure due to higher fuel and oxygen concentration at higher pressure, as expected. The negative pressure exponent in ignition delay time correlations also evidences this trend. It is seen that NUIG Mech_56.54, NUIG Aramco Mech 1.3 and Zhao DME Mech capture quantitatively the overall trend of pressure dependence. LLNL DME Mech predicts fairly well with the measured data at p = 4.0, 10.0 and 20 atm, but it shows an under-prediction p = 1.2 atm, indicating that LLNL DME Mech entails weak pressure dependence. Besides pressure dependence, equivalence ratio dependence was also investigated at two different pressures (p = 1.2 and 20 atm), as shown in Fig. 6. Ignition delay times of DME shows 6


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different global activation energies at different equivalence ratios. At p = 1.2 atm, the lean mixture is more reactive at a temperature range of 1400-1650 K and the rest of the investigated temperature range (1200-1400 K) exhibits a convergence of measurements at all equivalence ratios, indicating that equivalence ratios gives small effect in temperature range of 1200 - 1400 K. All these four models could capture well with the dependence of equivalence ratio at p = 1.2 atm. Again, NUIG Mech_56.54, NUIG Aramco Mech 1.3 and Zhao DME Mech capture quantitatively the overall trend of equivalence ratio dependence and LLNL DME Mech under-predicts the ignition delay times at p = 1.2 atm, as shown in Fig. 6a. For p = 20 atm, the rich mixture have shortest ignition delay time at the entire investigated temperature range, which is more distinct at lower temperatures. This time,the four models well quantitatively capture the overall trend of equivalence ratio dependence. The equivalence ratio dependence indicates that DME gives different equivalence ratio sensitivity under different temperatures. The equivalence ratio dependence of DME will be chemically interpreted in detail in the following section. To further validate the predictability of the models over a wide range of conditions, dependence of fuel concentration with fuel concentrations of 1%, 1.309% and 2% is plotted at p = 20 atm and φ = 1.0, as shown in Fig. 7. Similar to other hydrocarbon fuels, the ignition delay time of DME is decreased with increasing fuel concentrations due to increased fuel and oxygen concentration, a similar mechanism for pressure dependence. The negative pressure exponent in the ignition correlations and the models all demonstrate this trend. It is seen that only NUIG Mech_56.54 and Zhao DME Mech capture well the dependence of fuel concentration, while the NUIG Aramco Mech 1.3 and LLNL DME Mech show significant over-prediction with the measurements with fuel concentrations of 2%. Overall, NUIG Mech_56.54, NUIG Aramco Mech 1.3 and Zhao DME Mech have a perfect predictability to predict the measured data, while LLNL DME Mech predicts weak pressure dependence, suggesting further optimization. Considering that NUIG Mech_56.54 incorporates the latest reaction rates of C0-C3 species as mentioned earlier and thus was chosen as the base model to conduct chemical interpretations in the following section. 3.3 Chemical interpretations 3.3.1 Interpretation on equivalence ratio dependence As afore-discussed, DME exhibits different global activation energies under different equivalence ratios. An easy way to interpret this result is that DME shows different equivalence ratio sensitivity at different temperatures. To elucidate potential reasons for this from the chemical kinetic point of view, Figs. 8-9 present the radical profiles and sensitivity analysis of DME/Ar/O2 under

7


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various equivalence ratios at p = 20 atm for T = 1200 and 1400 K using NUIG Mech_56.54. A strong evidence for relative reactivity of DME/O2/Ar mixtures under different equivalence ratios for different temperatures is plotted in Fig. 8. Although the fuel-rich mixture shows the highest total radical evolution profile at early ignition induction time, the mixtures under various equivalence ratios reach the peak of total radical almost at the same time at higher temperature (T = 1400 K), resulting in almost equal ignition delay times. Comparatively, at low temperature (T =1200 K), calculation results show that increasing equivalence ratios increase the total radical pool concentration and distinctly advances the peak of total radical pool, resulting in an decreased ignition delay times. Consequently, the different equivalence ratio sensitivity under different temperatures leads to the different global activation energies under different equivalence ratios. To further interpret the different equivalence ratio sensitivity of DME/O2/Ar mixtures under different equivalence ratios, sensitivity analysis using NUIG Mech_56.45 is given in Fig. 9. It is observed that at higher temperature (T = 1400 K), the chain branching reaction R1: H+O2=OH+O plays the dominant role in ignition, as shown in Fig. 9a. The fuel dissociation reaction R426: CH3OCH3(+M)=CH3+CH3O(+M) shows the second highest normalized sensitivity coefficient, but compared to reaction R1, its role is much less important. Therefore, the ignition at higher temperature is characterized by reaction R1 and the fuel lean, stoichiometric and rich mixtures have almost equal oxygen concentrations as listed in Table 1, leading to almost equal reaction rate of R1. Consequently, DME/O2/Ar mixtures under different equivalence ratios have almost equal ignition delay times. As temperature decreased, as shown in Fig. 9b, the normalized sensitivity coefficient of reaction R1 decreased significantly as compared to that of higher temperature and reaction R426. At lower temperature (T = 1200 K), the sensitivity analysis evidenced that DME dissociation reaction R426 has even higher sensitivity coefficients than the main chain branching reaction H + O2 = OH + O, especially at higher equivalence ratio. As provided in table 1, the DME concentration is increased with increasing equivalence ratio, while oxygen concentrations remain equal. Therefore, the fuel-rich mixture has a higher reaction rate of DME dissociation reaction R426 than fuel-lean mixture, leading to a shorter ignition delay time at higher equivalence ratio. The more significant decreasing of normalized sensitivity coefficient decreasing of reaction R1 than reaction R426 as decreasing temperature could be explained by its more temperature-independent, as shown in Fig. 10. All in all, it is concluded that the more temperature-independent of reaction R1 than R426 leads to different global activation energies under different equivalence ratios. 3.3.2 Optimization of LLNL DME Mech Another important aspect in this work is to improve LLNL DME Mech, as mentioned earlier. NUIG Aramco Mech 1.3, NUIG Mech_56.54 and Zhao DME Mech predict well with the measured 8


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ignition delay times of DME, while LLNL DME Mech shows fairly well prediction at high pressure and an under-prediction at low pressure. To ascertain the important reactions in controlling ignition characteristics of DME mixture, sensitivity analysis were conducted at T = 1300 K, φ = 1.0 and p = 1.2 and 20 atm using NUIG Mech_56.54 and LLNL DME Mech, as shown in Fig. 11. Sensitivity analysis using both NUIG Mech_56.54 and LLNL DME Mech reveals almost the same sensitive reactions, such as chain branching reaction R1, DME dissociation reaction R426 and etc. Considering the relative maturity of small radical reactions, it is inferred that the poor prediction of LLNL DME Mech is largely a result of fuel-specific reactions. Based on sensitivity analysis, three fuel-specific reactions were identified to have strong influence on ignition: R426: CH3OCH3(+M)CH3+CH3O(+M) R428: CH3OCH3+HCH3OCH2+H2 R433: CH3OCH3+CH3CH3OCH2+CH4. To figure out which reactions are responsible for the poor predictions, rate constants of those reactions from both NUIG Mech_56.54 and LLNL DME Mech are plotted at p = 1.2 atm in Fig. 12. It is observed that rate constants of the reaction R426 and R433 in LLNL DME Mech are three times higher and two times lower than the corresponding one in NUIG Mech_56.54 at p = 1.2 atm. Therefore, it is quite possible that the higher rate constant of the reaction R426 in LLNL DME Mech contributes to their under-predictions and different rate constants of the reaction R433 in the two models may also contribute to their different predictions. To evaluate the effect of these reactions, rate constants of the reaction R426 and R433 in LLNL DME Mech was replaced by the corresponding one in NUIG Mech_56.54, as shown in Fig. 13. It is shown that the modified LLNL DME Mech with new reaction constant of R426 from NUIG Mech_56.54 shows fairly good agreement with the measured ignition delay times, while further refinement of the LLNL DME Mech with reaction constant of R433 from NUIG Mech_56.54 gives no significant improvement. Other highly sensitive reactions were also examined and were found to have little effect on the performance of LLNL DME Mech. Therefore, it is concluded that the poor prediction at p = 1.2 atm by LLNL DME Mech is largely come from the uncertainly of reaction R426 and considering the rate constants of this reaction this elementary reaction are roughly estimated, a further study on high-level ab initio calculation is appreciated to further improve the accuracy of DME kinetics. 4. Conclusions High temperature ignition delay times of DME/O2/Ar were experimentally measured behind the reflected shock wave over temperatures of 1000 – 1600 K, pressures of 1.2 – 20 atm and equivalence ratios of 0.5 – 1.0 with fuel concentration of 0.5 – 3.0%. The main results are summarized below: 9


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1). The measurements show that global activation energy of DME is decreased with increasing equivalence ratios, and thus correlations were developed through multiple linear regression under various equivalence ratios. Using these correlations, the current data was compared to the measurements of Cool et al. and Dagaut et al. and it is found that the current data does not match previous data well, suggesting more accuracte measurements are needed. 2). Measured ignition delay times were evaluated with four recently developed models. NUIG Mech_56.54, NUIG Aramco Mech 1.3 and Zhao DME Mech have a perfect predictability to predict the measured data, while the comparison results showed that LLNL DME Mech predicts weak pressure dependence, indicating further optimization is worthwhile. 3). The total radical evolution profiles showed that the mixtures under various equivalence ratios reach the peak of total radical almost at the same time at higher temperature and comparatively, increasing equivalence ratio increases the total radical pool concentration and distinctly advances the peak of total radical pool at lower temperature. Sensitivity analyses at different equivalence ratios using NUIG Mech_56.54 showed that the greater temperature-independence of reaction R1 than R426 leads to different global activation energies under different equivalence ratios. 4). Sensitivity analysis was conducted to ascertain the key reactions that are responsible for poor prediction of LLNL DME Mech. It is found that remarkable under-prediction at p = 1.2 atm by LLNL DME Mech is probably caused by uncertainty of reaction rate of reaction R426 and further study of rate constants of this reaction is highly recommended. Acknowledgements This work is supported by the National Natural Science Foundation of China (51306144, 91441118) and the National Basic Research Program (2013CB228406). Authors also appreciate the funding support from the Fundamental Research Funds for the Central Universities.

References 1. Kim, H. J.; Lee, K. S.; Lee, C. S., A study on the reduction of exhaust emissions through HCCI combustion by using a narrow spray angle and advanced injection timing in a DME engine. Fuel Process Technol. 2011, 92, (9), 1756-1763. 2. Youn, I. M.; Park, S. H.; Roh, H. G.; Lee, C. S., Investigation on the fuel spray and emission reduction characteristics for dimethyl ether (DME) fueled multi-cylinder diesel engine with common-rail injection system. Fuel Process Technol. 2011, 92, (7), 1280-1287. 3.

Park, S. H.; Kim, H. J.; Lee, C. S., Macroscopic spray characteristics and breakup performance 10


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of dimethyl ether (DME) fuel at high fuel temperatures and ambient conditions. Fuel 2010, 89, (10), 3001-3011. 4. Dagaut, P.; Boettner, J.-C.; Cathonnet, M., Chemical kinetic study of dimethylether oxidation in a jet stirred reactor from 1 to 10 ATM: Experiments and kinetic modeling. Symp. (Int.) Combust. 1996, 26, (1), 627-632. 5. Dagaut, P.; Daly, C.; Simmie, J. M.; Cathonnet, M., The oxidation and ignition of dimethylether from low to high temperature (500–1600 K): Experiments and kinetic modeling. Symp. (Int.) Combust. 1998, 27, (1), 361-369. 6. Yu, H.; Hu, E.; Cheng, Y.; Zhang, X.; Huang, Z., Experimental and numerical study of laminar premixed dimethyl ether/methane–air flame. Fuel 2014, 136, (0), 37-45. 7. de Vries, J.; Lowry, W. B.; Serinyel, Z.; Curran, H. J.; Petersen, E. L., Laminar flame speed measurements of dimethyl ether in air at pressures up to 10 atm. Fuel 2011, 90, (1), 331-338. 8. Wang, Y. L.; Holley, A. T.; Ji, C.; Egolfopoulos, F. N.; Tsotsis, T. T.; Curran, H. J., Propagation and extinction of premixed dimethyl-ether/air flames. Proc. Combust. Inst. 2009, 32, (1), 1035-1042. 9. Daly, C. A.; Simmie, J. M.; Würmel, J.; DjebaÏli, N.; Paillard, C., Burning velocities of dimethyl ether and air. Combust. Flame 2001, 125, (4), 1329-1340. 10. Zhao, Z.; Kazakov, A.; Dryer, F. L., Measurements of dimethyl ether/air mixture burning velocities by using particle image velocimetry. Combust. Flame 2004, 139, (1–2), 52-60. 11. Pyun, S. H.; Ren, W.; Lam, K.-Y.; Davidson, D. F.; Hanson, R. K., Shock tube measurements of methane, ethylene and carbon monoxide time-histories in DME pyrolysis. Combust. Flame 2013, 160, (4), 747-754. 12. Cool, T. A.; Wang, J.; Hansen, N.; Westmoreland, P. R.; Dryer, F. L.; Zhao, Z.; Kazakov, A.; Kasper, T.; Kohse-Höinghaus, K., Photoionization mass spectrometry and modeling studies of the chemistry of fuel-rich dimethyl ether flames. Proc. Combust. Inst. 2007, 31, (1), 285-293. 13. Cook, R. D.; Davidson, D. F.; Hanson, R. K., Shock tube measurements of ignition delay times and OH time-histories in dimethyl ether oxidation. Proc. Combust. Inst. 2009, 32, (1), 189-196. 14. Li, Z.; Wang, W.; Huang, Z.; Oehlschlaeger, M. A., Dimethyl Ether Autoignition at Engine-Relevant Conditions. Energy Fuels 2013, 27, (5), 2811-2817. 15. Pfahl, U.; Fieweger, K.; Adomeit, G., Self-ignition of diesel-relevant hydrocarbon-air mixtures under engine conditions. Symp. (Int.) Combust. 1996, 26, (1), 781-789. 16. Francisco, J. S., On the competition between hydrogen abstraction versus C-O bond fission in initiating dimethyl ether combustion. Combust. Flame 1999, 118, (1–2), 312-316. 17. Guo, H.; Sun, W.; Haas, F. M.; Farouk, T.; Dryer, F. L.; Ju, Y., Measurements of H2O2 in low temperature dimethyl ether oxidation. Proc. Combust. Inst. 2013, 34, (1), 573-581. 18. Sivaramakrishnan, R.; Michael, J. V.; Wagner, A. F.; Dawes, R.; Jasper, A. W.; Harding, L. B.; Georgievskii, Y.; Klippenstein, S. J., Roaming radicals in the thermal decomposition of dimethyl ether: Experiment and theory. Combust. Flame 2011, 158, (4), 618-632. 19. Cook, R. D.; Davidson, D. F.; Hanson, R. K., High-Temperature Shock Tube Measurements of Dimethyl Ether Decomposition and the Reaction of Dimethyl Ether with OH. J. Phys. Chem. A 2009, 113, (37), 9974-9980. 20. Good, D. A.; Francisco, J. S., Tropospheric Oxidation Mechanism of Dimethyl Ether and Methyl Formate. J. Phys. Chem. A 2000, 104, (6), 1171-1185.

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Energy & Fuels

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21. Pophristic, V.; Goodman, L., Influence of Protonation on Internal Rotation of Dimethyl Ether. J. Phys. Chem. A 2000, 104, (14), 3231-3238. 22. Curran, H. J.; Pitz, W. J.; Westbrook, C. K.; Dagaut, P.; Boettner, J. C.; Cathonnet, M., A wide range modeling study of dimethyl ether oxidation. Int. J. Chem. Kinet. 1998, 30, (3), 229-241. 23. Burke, U.; Somers, K. P.; O’Toole, P.; Zinner, C. M.; Marquet, N.; Bourque, G.; Petersen, E. L.; Metcalfe, W. K.; Serinyel, Z.; Curran, H. J., An ignition delay and kinetic modeling study of methane, dimethyl ether, and their mixtures at high pressures. Combust. Flame 2015, 162, (2), 315-330. 24. Zhao, Z.; Chaos, M.; Kazakov, A.; Dryer, F. L., Thermal decomposition reaction and a comprehensive kinetic model of dimethyl ether. Int. J. Chem. Kinet. 2008, 40, (1), 1-18. 25. Curran, H. J.; Fischer, S. L.; Dryer, F. L., The reaction kinetics of dimethyl ether. II: Low-temperature oxidation in flow reactors. Int. J. Chem. Kinet. 2000, 32, (12), 741-759. 26. Fischer, S. L.; Dryer, F. L.; Curran, H. J., The reaction kinetics of dimethyl ether. I: High-temperature pyrolysis and oxidation in flow reactors. Int. J. Chem. Kinet. 2000, 32, (12), 713-740. 27. Kaiser, E. W.; Wallington, T. J.; Hurley, M. D.; Platz, J.; Curran, H. J.; Pitz, W. J.; Westbrook, C. K., Experimental and Modeling Study of Premixed Atmospheric-Pressure Dimethyl Ether−Air Flames. J. Phys. Chem. A 2000, 104, (35), 8194-8206. 28. Metcalfe, W. K.; Burke, S. M.; Ahmed, S. S.; Curran, H. J., A Hierarchical and Comparative Kinetic Modeling Study of C1 − C2 Hydrocarbon and Oxygenated Fuels. Int. J. Chem. Kinet. 2013, 45, (10), 638-675. 29. Pan, L.; Zhang, Y.; Tian, Z.; Yang, F.; Huang, Z., Experimental and Kinetic Study on Ignition Delay Times of iso-Butanol. Energy Fuels 2014, 28, (3), 2160-2169. 30. Pan, L.; Hu, E.; Zhang, J.; Zhang, Z.; Huang, Z., Experimental and kinetic study on ignition delay times of DME/H2/O2/Ar mixtures. Combust. Flame 2014, 161, (3), 735-747. 31. Zhang, Y.; Huang, Z.; Wei, L.; Zhang, J.; Law, C. K., Experimental and modeling study on ignition delays of lean mixtures of methane, hydrogen, oxygen, and argon at elevated pressures. Combust. Flame 2012, 159, (3), 918-931. 32. Pan, L.; Hu, E.; Deng, F.; Zhang, Z.; Huang, Z., Effect of pressure and equivalence ratio on the ignition characteristics of dimethyl ether-hydrogen mixtures. Int. J. Hydrogen Energy 2014, 39, (33), 19212-19223. 33. Pan, L.; Zhang, Y.; Zhang, J.; Tian, Z.; Huang, Z., Shock tube and kinetic study of C2H6/H2/O2/Ar mixtures at elevated pressures. Int. J. Hydrogen Energy 2014, 39, (11), 6024-6033. 34. Zhang, J.; Niu, S.; Zhang, Y.; Tang, C.; Jiang, X.; Hu, E.; Huang, Z., Experimental and modeling study of the auto-ignition of n-heptane/n-butanol mixtures. Combust. Flame 2013, 160, (1), 31-39. 35. Zhang, Y.; Jiang, X.; Wei, L.; Zhang, J.; Tang, C.; Huang, Z., Experimental and modeling study on auto-ignition characteristics of methane/hydrogen blends under engine relevant pressure. Int. J. Hydrogen Energy 2012, 37, (24), 19168-19176. 36. Morley, C. Gaseq v0.76; http://www.gaseq.co.uk. 37. Lutz, A. E.; Kee, R. J.; Miller, J. A. SENKIN: A Fortran Program for Predicting Homogeneous Gas Phase Chemical Kinetics with Sensitivity Analysis; Sandia National Laboratories:: Albuquerque, NM, 1988; SAND87-8248. 38. Kee, R. J.; Rupley, F. M.; Miller, J. A. CHEMKIN-II: A Fortran Chemical Kinetics Package for the Analysis of Gas-Phase Chemical Kinetics; Sandia National Laboratories: Albuquerque, NM, 12


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Energy & Fuels

1989; SAND89-8009. 39. Chaos, M.; Dryer, F. L., Chemical-kinetic modeling of ignition delay: Considerations in interpreting shock tube data. Int. J. Chem. Kinet. 2010, 42, (3), 143-150. 40. Davidson, D. F.; Oehlschlaeger, M. A.; Herbon, J. T.; Hanson, R. K., Shock tube measurements of iso-octane ignition times and OH concentration time histories. Proc. Combust. Inst. 2002, 29, (1), 1295-1301. 41. Petersen, E. L.; Hanson, R. K., Nonideal effects behind reflected shock waves in a high-pressure shock tube. Shock Waves 2001, 10, (6), 405-420. 42. Davidson, D. F.; Hanson, R. K., Interpreting shock tube ignition data. Int. J. Chem. Kinet. 2004, 36, (9), 510-523. 43. Kéromnès, A.; Metcalfe, W. K.; Heufer, K. A.; Donohoe, N.; Das, A. K.; Sung, C.-J.; Herzler, J.; Naumann, C.; Griebel, P.; Mathieu, O.; Krejci, M. C.; Petersen, E. L.; Pitz, W. J.; Curran, H. J., An experimental and detailed chemical kinetic modeling study of hydrogen and syngas mixture oxidation at elevated pressures. Combust. Flame 2013, 160, (6), 995-1011. 44. Burke, S. M.; Metcalfe, W.; Herbinet, O.; Battin-Leclerc, F.; Haas, F. M.; Santner, J.; Dryer, F. L.; Curran, H. J., An experimental and modeling study of propene oxidation. Part 1: Speciation measurements in jet-stirred and flow reactors. Combust. Flame 2014, 161, (11), 2765-2784. 45. Burke, S. M.; Burke, U.; Mc Donagh, R.; Mathieu, O.; Osorio, I.; Keesee, C.; Morones, A.; Petersen, E. L.; Wang, W.; DeVerter, T. A.; Oehlschlaeger, M. A.; Rhodes, B.; Hanson, R. K.; Davidson, D. F.; Weber, B. W.; Sung, C.-J.; Santner, J.; Ju, Y.; Haas, F. M.; Dryer, F. L.; Volkov, E. N.; Nilsson, E. J. K.; Konnov, A. A.; Alrefae, M.; Khaled, F.; Farooq, A.; Dirrenberger, P.; Glaude, P.-A.; Battin-Leclerc, F.; Curran, H. J., An experimental and modeling study of propene oxidation. Part 2: Ignition delay time and flame speed measurements. Combust. Flame 2015, 162, (2), 296-314.

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 25

List of Tables and Figures Table 1 Composition of the test mixtures Mixture

φ

% DME % O2

% Ar

p (atm)

1

0.5

0.677

4.060 95.264

1.2, 4.0, 10.0, 20.0

2

0.5

0.500

3.000 96.500

20

3

0.5

1.000

6.000 93.000

20

4

1

1.309

3.927 94.764

1.2, 4.0, 10.0, 20.0

5

1

1.000

3.000 96.000

20

6

1

2.000

6.000 92.000

20

7

2

2.457

3.686 93.857

1.2, 4.0, 10.0, 20.0

8

2

1.000

1.500 98.500

20

9

2

2.000

3.000 95.000

20

14


Page 15 of 25

3

10

2

Pan et al.(2014) Dagaut et al. (1998) Cook et al. (2009)

Ignition delay time (us)

10

0.65

0.70

0.75

0.80

-1

0.85

0.90

1000/T (K ) Fig. 1. Comparison of the literature data for 1%DME/3%O2/96%Ar at p = 3.3 atm ((the definition of Pan et al. and the Cook et al. work use OH* and the Dagaut et al. work use CO2*). 0.4

4

End-wall pressure signal End-wall OH* signal 0.3

3

dp/dt = 4% Relative signal

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

0.2

2

t = 2657 us

0.1

1

0.0

1000

0

2000

3000

4000

5000

Time (us) Fig. 2. Typical endwall pressure and OH* emmision measurements with corresponding ignition delay time for 1.309% DME at p = 10.16 atm, φ = 1.0 and T = 1136 K.

15


Energy & Fuels

5

10

4

φ = 0.5 φ = 1.0 φ = 2.0

3

10

a

b

τ / (p XDME)

10

2

10

Symbols: Measurements Lines: Correlations

1

10 0.6

0.7

0.8

-1

0.9

1.0

1000/T (K )

Fig. 3. Comparison of correlations with all measured data of DME under different equivalence ratio at p = 1.2, 4.0, 10.0 and 20.0 atm. 10


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 25

4

10

3

10

2

p = 1.6 bar p = 1.8 bar p = 3.3 bar p = 5.4 bar p = 6.6 bar

Symbols: Data of Cook et al. (2009) Lines: Correlation in this study

1

10 0.65

0.70

0.75

-1

1000/T (K )

16


0.80

0.85

Page 17 of 25

4

10


φ = 0.5 φ = 1.0 φ = 2.0 3

10

2

10

Symbols: Data of Dagaut et al. (1998) Lines: Correlation in this study 1

10 0.60

0.65

0.70

0.75

0.80

-1

0.85

1000/T (K )

Fig. 4. Comparison of the correlated data with measured data of DME from Cook et al. and Dagaut et al..


NUIG Mech_56.54

NUIG Aramco Mech 1.3

3

10

p = 1.2 atm p = 4.0 atm p = 10.0 atm p = 20.0 atm Symbols: measurements Solid lines: Model simulations

2

10

LLNL DME Mech


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

Zhao DME Mech

3

10

2

10

0.65

0.70

0.75

0.80

0.85

0.65 0.90

0.70

0.75

-1

0.80 -1

1000/T (K )

1000/T (K )

(a) φ = 0.5

17


0.85

0.90

Energy & Fuels

NUIG Mech_56.54


3


10


2

10

LLNL DME Mech

Zhao DME Mech

3


10

2

10

0.65

0.70

0.75

0.80

0.85

0.65 0.90

0.70

0.75

-1

0.80

0.85

0.90

-1

1000/T (K )

1000/T (K )

(b) φ = 1.0


NUIG Mech_56.54


3

10


2

10

Zhao DME Mech

LLNL DME Mech Ignition delay time (us)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 25

3

10

2

10

0.60

0.65

0.70

0.75

0.80

0.85

0.90

0.60 0.95

0.65

0.70

-1

0.75

0.80

0.85

-1

1000/T (K )

1000/T (K )

(c) φ = 2.0 Fig. 5. Effect of pressure on ignition delay times of DME. .

18


0.90

0.95

Page 19 of 25

NUIG Mech_56.54


3


10

φ = 0.5 φ = 1.0 φ = 2.0

2

10

Symbols: measurements Solid lines: Model simulations

Zhao DME Mech

LLNL DME Mech 3


10

2

10

0.60

0.65

0.70

0.75

0.80

0.60 0.85

0.65

0.70

-1

0.75

0.80

0.85

0.85

0.90

-1

1000/T (K )

1000/T (K )

(a) p = 1.2 atm NUIG Aramco Mech 1.3

NUIG Mech_56.54 3


10

φ = 0.5 φ = 1.0 φ = 2.0

2

10

Symbols: measurements Solid lines: Model simulations

Zhao DME Mech

LLNL DME Mech 3

10


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

2

10

0.65

0.70

0.75

0.80

0.85

0.65 0.90

0.70

0.75

-1

0.80 -1

1000/T (K )

1000/T (K )

(b) p = 20.0 atm Fig. 6. Effect of equivalence ratio on ignition delay times of DME.

19


Energy & Fuels


NUIG Mech_56.54 3


10

XDME = 1.0% 2

XDME = 1.309%

10

XDME = 2.0% Symbols: measurements Solid lines: Model simulations

Zhao DME Mech

LLNL DME Mech 3


10

2

10

0.65

0.70

0.75

0.80

0.85

0.65 0.90

0.70

0.75

-1

0.80

0.85

0.90

-1

1000/T (K )

1000/T (K )

Fig. 7. Effect of fuel concentration on ignition delay times of DME. 0.01

φ = 0.5 φ = 1.0 φ = 2.0

Temperature (K)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 25

1E-3

1E-4 1

10

100

Time (us) (a) T =1400 K

20


1000

Page 21 of 25

0.1

φ = 0.5 φ = 1.0 φ = 2.0

0.01

Temperature (K)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

1E-3

1E-4

1E-5

1E-6 0 10

1

2

10

3

10

4

10

10

Time (us) (b) T = 1200 K Fig. 8. Mole fractions of free radicals for different equivalence ratios at p = 20 atm for T = 1200 and 1400 K. R146:CH3+HO2CH4+O2

φ = 2.0 φ = 1.0 φ = 0.5

R189:CH3+CH3(+M)C2H6(+M) R428: CH3OCH3+HCH3OCH2+H2

R30: HCO+MH+CO+M R433:CH3OCH3+CH3CH3OCH2+CH4 R76: CH2O+OHHOCH2O R75:CH2O+CH3HCO+CH4 R145: CH3+HO2CH3O+OH R426:CH3OCH3(+M)CH3+CH3O(+M) R1: H+O2O+OH

-0.6

-0.5

-0.4

-0.3

-0.2

-0.1

0.0

0.1

0.2

0.3

Normalized sensitivity coefficient (a) T =1400 K

21


0.4

0.5

0.6

Energy & Fuels

R146:CH3+HO2CH4+O2 R189:CH3+CH3(+M)C2H6(+M) R428: CH3OCH3+HCH3OCH2+H2 R30: HCO+MH+CO+M R433:CH3OCH3+CH3CH3OCH2+CH4 R76: CH2O+OHHOCH2O R75:CH2O+CH3HCO+CH4 R145: CH3+HO2CH3O+OH R426:CH3OCH3(+M)CH3+CH3O(+M) R1: H+O2O+OH

-0.4

-0.3

-0.2

-0.1

0.0

0.1

0.2

0.3

0.4

Normalized sensitivity coefficient (b) T = 1200 K Fig. 9. Sensitivity analysis for DME at p = 20 atm, φ = 1.0, T = 1200 and 1400 K under various equivalence ratios using NUIG Mech_56.54. 13

4.5x10

13

4.0x10

Reaction R1 Reaction R426 p = 20 atm

13

3.5x10

-3

-1

-1

Rate coefficient (cm mol K )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 25

13

3.0x10

13

2.5x10

13

2.0x10

13

1.5x10 1000

1250

1500

1750

T (K) Fig. 10. Comparison of rate constants of the reaction R1 and R426.

22


2000

Page 23 of 25

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

R146:CH3+HO2CH4+O2

p = 20.0 atm p = 1.2 atm

R189:CH3+CH3(+M)C2H6(+M) R428: CH3OCH3+HCH3OCH2+H2

R75:CH2O+OHHOCH2O R433:CH3OCH3+CH3CH3OCH2+CH4 R30: HCO+MH+CO+M R75:CH2O+CH3HCO+CH4 R145: CH3+HO2CH3O+OH R1: H+O2O+OH R426:CH3OCH3(+M)CH3+CH3O(+M)

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

Normalized sensitivity coefficient (a) LLNL DME Mech

R189:CH3+CH3(+M)C2H6(+M) R31: HCO+O2CO+HO2 R146:CH3+HO2CH4+O2 R433:CH3OCH3+CH3CH3OCH2+CH4 R76:CH2O+HO2HCO+H2O2 R30: HCO+MH+CO+M R75:CH2O+CH3HCO+CH4 R145: CH3+HO2CH3O+OH R426:CH3OCH3(+M)CH3+CH3O(+M) R1: H+O2O+OH

-0.4

-0.2

0.0

0.2

0.4

Normalized sensitivity coefficient (b) NUIG Mech_56.54 Fig. 11. Fig. 9. Sensitivity analysis for DME at T = 1300 K, φ = 1.0, and p = 1.2 and 20 atm using NUIG Mech_56.54 and LLNL DME Mech.

23


Energy & Fuels

. 1E17 R426: CH3OCH3(+M)CH3O+CH3(+M) NUIG Mech_56.54

1E16

-3

-1

-1


LLNL DME Mech

1E15

1E14 1000

1250

1500

1750

2000

1750

2000

T (K) (a) R426 1E15 R428: CH3OCH3+HCH3CH2+H2 LLNL DME Mech NUIG Mech_56.54

1E14

-3

-1

-1


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 25

1E13

1E12 1000

1250

1500

T (K) (b) R428

24


Page 25 of 25

1E14 R433: CH3OCH3+CH3CH3OCH2+CH4 NUIG Mech_56.54

1E13

-3

-1

-1


LLNL DME Mech

1E12

1E11 1000

1250

1500

1750

2000

T (K) (c) R433 Fig. 12. Comparison of rate constants of the reaction R426, R428 and R433 in NUIG Mech_56.54 and LLNL DME Mech.

p = 1.2 atm p = 20.0 atm


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

10

3

10

2

Symbols: Experimental data Solid lines: Original LLNL DME Mech Dash Lines: Modified LLNL DME Mech(R426) Dot Lines: Modified LLNL DME Mech(R426+R433)

0.65

0.70

0.75

0.80

-1

0.85

0.90

1000/T (K ) Fig. 13 Effect of the reaction R426 and R433 on performance of LLNL DME Mech.

25


0.95

Experimental and Kinetic Study on Ignition Delay Times of Dimethyl

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