Ar mixtures

Combustion and Flame 161 (2014) 735–747

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Combustion and Flame j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / c o m b u s t fl a m e

Experimental and kinetic study on ignition delay times of DME/H2/O2/Ar mixtures Lun Pan, Erjiang Hu ⇑, Jiaxiang Zhang, Zihang Zhang, Zuohua Huang ⇑ State Key Laboratory of Multiphase Flows in Power Engineering, Xi’an Jiaotong University, Xi’an 710049, People’s Republic of China

a r t i c l e

i n f o

Article history: Received 3 June 2013 Received in revised form 19 July 2013 Accepted 17 October 2013 Available online 7 November 2013 Keywords: Ignition delay time Hydrogen blending ratio Shock tube Non-linear effect Chemical kinetics

a b s t r a c t Ignition delay times of dimethyl ether (DME)/hydrogen/oxygen/argon mixtures (hydrogen blending ratio ranging from 0% to 100%) were measured behind reflected shock waves at pressures of 1.2–10 atm, temperature range of 900–1700 K, and for the lean (/ = 0.5), stoichiometric (/ = 1.0) and rich (/ = 2.0) mixtures. For more understanding the effect of initial parameters, correlations of ignition delay times for the lean mixtures were obtained on the basis of the measured data (XH2 6 95%) through multiple linear regression. Ignition delay times of the DME/H2 mixtures demonstrate three ignition regimes. For XH2 6 80%, the ignition is dominated by the DME chemistry and ignition delay times show a typical Arrhenius dependence on temperature and pressure. For 80% 6 XH2 6 98%, the ignition is dominated by the combined chemistries of DME and hydrogen, and ignition delay times at higher pressures give higher ignition activation energy. However, for XH2 P 98%, the transition in activation energy for the mixture was found as decreasing the temperature, indicating that the ignition is dominated by the hydrogen chemistry. Simulations were made using two available models and different results were presented. Thus, sensitivity analysis was performed to illustrate the causes of different simulation results of the two models. Subsequently, chemically interpreting on the effect of hydrogen blending ratio on ignition delay times was made using small radical mole fraction and reaction pathway analysis. Finally, highpressure simulations were performed, serving as a starting point for the future work. Ó 2013 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

1. Introduction The increasing consumption of fossil fuels and stringent emission regulations motivate the researchers to develop high-efficiency combustion technologies and look for clean alternative fuels. Homogeneous charge compression ignition (HCCI) technology has attracted great attention [1] due to its higher thermal efficiency and lower NOx and soot emissions. However, the commercialization of HCCI engine is still limited by its hardly controllable ignition timing [2] because the ignition in a HCCI engine is mainly controlled by chemical kinetics. Previous studies showed that using blending fuels (one high-octane fuel blended with one high-cetane fuel) is an effective approach to control the ignition timing in the HCCI engines. DME as a promising alternative fuel, has good ignition and combustion characteristics at low temperatures [3]. During the combustion process, DME shows a two-stage combustion and heat release phenomenon, which is similar to linear high-carbon hydrocarbons such as n-heptane [4]. To extend the operation regimes of the HCCI combustion engine fueled with ⇑ Corresponding authors. Fax: +86 29 82668789. E-mail addresses: [email protected] (E. Hu), [email protected] (Z. Huang).

DME, DME blended with a high-octane fuel is expected to control the ignition timing of HCCI combustion and meet various engine loads. Hydrogen, which has a high octane number of 130, is considered to be a clean and energy-efficient fuel for the next generation of engines and power sources due to its renewable and excellent combustion characteristics [5,6]. Recently, some studies [7,8] showed that the ignition timing of a DME HCCI engine could be effectively controlled by varying the hydrogen blending ratio. So far, experimental and kinetic modeling studies on the oxidation of neat DME and neat hydrogen have been extensively reported in engine [7–11], shock tube [12–16], jet-stirred reactor [13,17], rapid compression machine [16,18–20] as well as the measurement of the laminar flame speed [21–26]. However, studies on the oxidation of DME/H2 blends are very limited. Jeon and Bae [27] studied the characteristics of the premixed charge compression ignition (PCCI) combustion of DME and DME/H2 in a single-cylinder compression-ignition engine. Their results showed that brake mean effective pressures of DME PCCI combustion were lower than those of DME/H2 PCCI combustion, and an increase in hydrogen blending ratios led to a decrease in CO, CO2 and HC emissions. Huang et al. [28] studied the laminar burning characteristics of premixed DME/H2/air mixtures at different equivalence ratios (0.8, 1.0 and 1.2), hydrogen blending ratios (0%, 20%, 40%, 60%,

0010-2180/$ - see front matter Ó 2013 The Combustion Institute. Published by Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.combustflame.2013.10.015

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Table 1 Composition of the test mixtures. The mole fraction of DME (%)

The mole fraction of H2 (%)

The mole fraction of O2 (%)

The mole fraction of Ar (%)

XH2=0 / = 0.5 / = 1.0 / = 2.0

0.677 1.309 2.457

0 0 0

4.060 3.927 3.686

95.264 94.764 93.857

XH2=50% / = 0.5

0.566

0.566

3.964

94.904

XH2=80% / = 0.5 / = 1.0 / = 2.0

0.380 0.694 1.183

1.521 2.778 4.734

3.802 3.472 2.959

94.296 93.056 91.124

XH2=90% / = 0.5

0.246

2.211

3.686

93.858

XH2=95% / = 0.5 / = 1.0 / = 2.0

0.144 0.252 0.402

2.734 4.780 7.638

3.597 3.145 2.513

93.525 91.824 89.447

XH2=98% / = 0.5 / = 1.0 / = 2.0

0.064 0.111 0.173

3.143 5.417 8.489

3.528 3.040 2.382

93.265 91.432 88.956

XH2=100% / = 0.5 / = 1.0 / = 2.0

0 0 0

3.472 5.917 9.132

3.472 2.959 2.283

93.056 91.124 88.584

Mixtures

80% and 100%) and atmospheric pressure using a constant volume bomb. Their results showed that the flame propagation speed, laminar burning velocity, and mass burning rate were increased with increasing the hydrogen addition. In addition, increasing hydrogen addition would advance the timing at the peak pressure and shorten the combustion duration. Chen et al. [29] studied the laminar premixed fuel-rich (/ = 1.5) DME/H2/O2/Ar flames with different hydrogen blending ratios (0%, 20%, 40%, 60%, and 80%) using the tunable synchrotron vacuum-ultraviolet photoionization and molecular-beam mass spectrometry. They found that the flame temperature in the flame zone was decreased with increasing hydrogen blending ratio due to decreasing the volume heat value of the mixture and increasing the heat transfer to the burner. Liu et al. [30] chemically examined the chemical composition of

Fig. 1. Typical end-wall pressure and OH⁄ chemiluminescence measurements for lean 5%DME/95%H2 at p = 10.41 atm and T = 1069 K.

Fig. 2. Comparison between present measured data and the measured data from reference for 4%H2/2%O2/94%Ar at p = 3.5 atm and / = 1.0.

laminar premixed low-pressure DME/H2/O2/Ar flames. Their results showed that the effect of hydrogen addition became more significant when hydrogen blending ratio exceeds 40%. Ignition delay times are important data to the development and validation of the chemical kinetic models. Meanwhile, it is also an important parameter to control the HCCI ignition timing. Up to now, little data on ignition delay times of DME/H2 mixtures were reported, and the effect of hydrogen addition on DME is still not well understood. In this study, ignition delay times of DME/H2/ O2/Ar mixtures were measured in a shock tube over a wide range of temperatures and pressures. Correlations of ignition delay times of lean mixtures were obtained on the basis of the measured data through multiple linear regression. Meanwhile, simulations were conducted using the Zhao model [31] and the NUIG Aramco Mech 1.3 [32]. Furthermore, sensitivity analysis was performed to reveal potential reasons for different simulation results by using the two above models. Finally, small radical analysis and reaction pathways were made to interpret the effect of hydrogen blending ratio on the ignition of DME/H2 binary fuel.

Fig. 3. Comparison between current data and the literature data for 1% neat DME at p = 3.3 atm and / = 0.5 (the definition of present work and the Cook et al. work use OH⁄ and the Dagaut et al. paper used CO2 ).

L. Pan et al. / Combustion and Flame 161 (2014) 735–747

2. Experimental and numerical approaches All measurements were conducted in a shock tube, which has been described in details previously [33]. The shock tube with a diameter of 11.5 cm is divided into a 4 m long driver section and a 5.3 m long driven section by polyethylene terephthalate (PET) diaphragms. In this study, PET diaphragms with thickness of 0.025 mm and 0.2 mm were respectively chosen for measurements at 1.2 and 10 atm. The leak rate of driven section was measured to be less than 0.01 torr/min, which can be ignored compared to 50–500 torr fuel mixtures. Fuel mixtures were prepared in a 128 L mixing tank and allowed to mix for at least 12 h by molecular diffusion. A high-accuracy pressure transmitter (ROSEMOUNT 3051) was used to measure the partial pressure of each component. The tank was evacuated to 106 bar prior to the mixture introduction. Table 1 lists the composition of seven mixtures in this study. Mixture components are DME (purity > 99.99%, HPLC), hydrogen (purity > 99.99%, HPLC), oxygen (purity > 99.999%) and argon (purity > 99.999%). The ignition delay time is defined as the time interval between the incident shock wave arrival at the end-wall and the extrapolation of the maximum rate of the end-wall OH⁄ chemiluminescence curve to the zero line, as shown in Fig. 1. The onset of ignition was also monitored by using the reflected shock pressure at the

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end-wall, which is consistent with the OH⁄ chemiluminescence. The end-wall incident shock velocity was obtained by linear extrapolation of three time intervals recorded by three time counters (FLUKE, PM6690), which were triggered by four fast-response piezoelectric pressure transducers (PCB, 113B26) located along the last 1.30 m of the driven section with the same length interval of 30 cm. The end-wall OH⁄ chemiluminescence was detected by a photomultiplier (HAMAMATSU CR 131) at 306 ± 10 nm with a half band filter located at the end-wall. All pressure and chemiluminescence signals were recorded by a digital acquisition instrument (YOKOGAWA DL750). The post-reflected shock temperatures were calculated using the software Gaseq [34]. All thermodynamic data of DME, hydrogen, argon and oxygen were obtained from the Burcat database [35]. The uncertainty of the temperature in this study was estimated to be ±25 K. It should be noted that for the high temperature condition (s < 1.5 ms), the ideal constant U, V assumption [36] is reasonable for performing simulation. However, for intermediate-low temperature (s > 1.5 ms), the effect of facility (dp/dt) becomes more significant and therefore was considered in calculation. The typical facility dependent pressure rise was calculated to be 4%/ms. Simulations of ignition delay times were carried out using the constantvolume, adiabatic and zero-dimensional reactor in CHEMKIN II

Fig. 4. Ignition delay times of DME/H2 mixtures (Red lines and symbols: 1.2 atm; blue lines and symbols: 10 atm). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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Fig. 4 (continued)

package [37] with the SENKIN/VTIM approach [38]. The simulated ignition delay time is defined as the time interval between the beginning of reaction and the peak of OH⁄ mole fraction. Due to the very sharp increase of OH⁄ mole fraction, this theoretical definition is practically consistent with the definition in the experiment.

3. Results and discussion In this section, two replicated condition from previous literature for DME and hydrogen were firstly repeated for comparison using our shock tube apparatus. And then, correlation of lean mixtures was developed based on the measured data. In addition, simulation work was conducted using two available models and subsequently kinetic studies were made. Finally, high-pressure simulations served as a starting point for future work was presented. 3.1. Comparison to the previous data To confirm the reliability of the current apparatus and the credibility of our measured data, two replicated condition from previous literature for DME and hydrogen were first repeated for

the comparison using our shock tube apparatus. Detailed description is given below. A lot of ignition delay times of hydrogen have been measured behind reflected shock wave over a wide range of conditions in the past couple of decades. Recently, Pang et al. [39] was reported ignition delay times of hydrogen over a wide temperature range. Figure 2 shows the comparison between the measured data with those by Pang et al. [39] for 4%H2/2%O2/Ar at p = 3.5 atm. It can be seen that present measured data fairly agree with that of Pang et al. over the entire temperature range. Nevertheless, it is noted that only the high temperature ignition delay times (s < 2 ms) of Pang et al. [39] were employed for comparison.

Table 2 Correlation parameters for lean DME/hydrogen mixtures. Mixture

p (atm)

A

n

Ea (kcal/mol)

R2

XH2=0% XH2=50% XH2=80% XH2=90%

1.2, 10 1.2, 10 1.2, 10 1.2 10 1.2 10

2.99E04 1.90E04 2.38E04 7.59E04 4.04E05 6.58E03 4.49E06

0.337 0.393 0.422 – – – –

38.5 39.2 37.6 33.1 38.0 25.9 41.5

0.998 0.998 0.997 0.997 0.999 0.996 0.998

XH2=95%

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For DME, Cook et al. [12] and Dagaut et al. [13] have measured ignition delay times of DME diluted with argon. Figure 3 gives the comparison between the measured data of this study and those in the literatures [12,13] for 1% DME at temperatures of 1100–1700 K, equivalence ratio of 0.5. To facilitate the comparison, a pressure index of 0.66 suggested by Cook et al. [12] was used to normalize all the experimental data to 3.3 atm. As shown in the figure, the measured values agree fairly well with those of Cook et al., but give higher value than those of Dagaut et al. [13] at relatively lower temperature, leading to a higher activation energy. The lower activation energy of Dagaut et al. [13] is probably caused by the definition of ignition delay time. In Dagaut’s study, the end-wall CO2 emission was used for the determination of the ignition delay time rather than OH⁄ emission. 3.2. Measured ignition delay times To more directly understand the effect of initial parameters, correlations of ignition delay times for lean (/ = 0.5) DME/H2/O2/ Ar mixtures with different hydrogen blending ratios were proposed, as shown in Fig. 4. The experimental data are provided in the Supplementary material. For the DME/H2 mixtures with XH2 6 80%, as shown in Fig. 4a–c, the ignition delay times of the blending fuel exhibit a strong

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Arrhenius dependence like that of neat DME, indicating that the ignition process is dominated by the DME chemistry. Thus, correlation is given based on the experimental data through multiple linear regression using the following formula:

s ¼ Apn expðEa =RTÞð0% 6 X H2 6 95%Þ

ð1Þ

where s is ignition delay time in microseconds, p is pressure in atms, Ea is global activation energy in kcal/mol, T is temperature in K, and R = 1.986  103 kcal/mol1 K1 is universal gas constant. Correlation parameters are summarized in Table 2. Negative pressure dependence indicates that the ignition delay times are decreased with the increase of pressure. In addition, their absolute values increase with increasing the hydrogen blending ratio, suggesting the increased effect of pressure. For the DME/H2 mixtures with 80% 6 XH2 6 98%, as shown in Fig. 4d and e, ignition delay times also exhibit the typical Arrhenius dependence on temperature. Thus, correlations were also formulated. However, ignition delay times at higher pressure show higher ignition activation energy, which is more pronounced as increasing hydrogen blending ratios. This behavior is attributed to the competition of reactions: R1 (H + O2 <=> OH + H) and R9 (H + O2 + (M) <=> HO2 + (M)), R433 (CH3OCH3 + H <=> CH3OCH2 + H2). The latter two reactions inhibit the ignition activity and

Fig. 5. Comparison between experimental and modeling results for the lean (/ = 0.5) mixtures (Symbols: measurement; solid lines: Zhao model; dash dot lines: NUIG Aramco Mech 1.3).

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Fig. 6. Comparison between experimental and modeling results for the stoichiometric (/ = 1.0) and rich (/ = 2.0) mixtures at p = 10 atm (symbols: measurement; solid lines: Zhao model; dash dot lines: NUIG Aramco Mech 1.3).

Fig. 7. Ignition delay times as a function of the hydrogen blending ratio at p = 10 atm (symbols: measurement; solid lines: Zhao model; dash lines: NUIG Aramco Mech 1.3).

Fig. 8. Normalized ignition delay time reduction parameter versus the DME blending ratio.

L. Pan et al. / Combustion and Flame 161 (2014) 735–747

Fig. 9. Sensitivity analysis for lean hydrogen mixture at T = 1150 K and p = 10 atm using NUIG Aramco Mech 1.3 and Zhao model.

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Fig. 10. Rate constants for reaction H + O2(+M) <=> HO2(+M) in Zhao model and NUIG Aramco Mech 1.3.

Table 3 Kinetic parameters of reaction H + O2(+M)<=>HO2(+M). Reaction

Kinetic parameters

References

a (Zhao model)

H + O2(+M) <=> HO2(+M) 1.475E+12 0.60 0.000E+00 LOW/ 9.042E+19–1.50 4.922E+02/ TROE/ 0.5 1E30 1E+30/H2/3.0/ H2O/16/ O2/ 1.1/ CO/2.7/ CO2/5.4/ HE/1.2/ H + O2(+M)<=>HO2(+M) 4.650E+012 0.440 0.0 LOW/ 1.737E+019–1.230 0.0/TROE/ 6.700E001 1.000E030 1.000E+030 1.000E+030/H2/ 1.30/ CO/ 1.90/ CO2/ 3.80/ HE/ 0.00/ H2O/ 10.00/ AR/ 0.00/CH4/ 2.00/ C2H6/ 3.00/ H + O2(+AR)<=>HO2(+AR) 4.650E+012 0.440 0.0 LOW/ 6.810E+018–1.200 0.0/TROE/ 7.000E001 1.000E030 1.000E+030 1.000E+030/ H + O2(+HE)<=>HO2(+HE) 4.650E+012 0.440 0.0 LOW/ 9.192E+018–1.200 0.0/TROE/ 5.900E001 1.000E030 1.000E+030 1.000E+030/

[46]

a1 (NUIG Aramco Mech 1.3)

a2 (NUIG Aramco Mech 1.3) a3 (NUIG Aramco Mech 1.3)

[47]

[47,48]

[47]

become dominant reactions at higher pressures and lower temperatures, whereas the former promotes the ignition [40]. When further increasing hydrogen blending ratios (XH2 P 98%), the transition in activation energy for the mixture was found as increasing the temperature, indicating that the ignition is dominated by the hydrogen chemistry. This behavior was also observed in previous literature [40,41]. 3.3. Numerical simulations Simulations were performed using two available chemical models: the Zhao model [31] and NUIG Aramco Mech 1.3 [32]. Both models contain the detailed H2/O2 and DME chemistries. The Zhao model [31], including 55 species and 290 reactions, was developed for DME on the basis of the RRKM/master equation method. It has been validated against various experimental targets including the flow reactor [42,43], jet stirred reactor (JSR) [13,17], pyrolysis results [31], ignition delay time [13,17], and laminar flame speeds [26,44,45]. The NUIG Aramco Mech 1.3 [32] developed in 2013 by Curran group consists of 253 species and 1542 reactions. Figure 5 gives the comparison between the measured and calculated ignition delay times using both Zhao model and NUIG Aramco Mech 1.3 for the lean (/ = 0.5) DME/H2 mixtures at pressures of 1.2

Fig. 11. Effect of the rate constant of reaction H + O2(+M) <=> HO2(+M) on the Zhao model.

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and 10 atm. Both models can qualitatively capture the effect of hydrogen addition. Prediction by Zhao model gives reasonable agreement with the experimental data at relatively high temperature at 1.2 atm at different hydrogen blending ratios, as shown in Fig. 5a. However, it over-predicts the experimental data at relatively low temperature. This behavior becomes more obvious as the hydrogen blending ratio is increased. At 10 atm, as shown in Fig. 5b, Prediction by Zhao model is good for neat DME. However, as hydrogen blending ratio is increased, it shows a remarkable over-prediction. NUIG Aramco Mech 1.3 yields fairly good agreement for all mixtures at 1.2 atm and 10 atm, as shown in Fig. 5c and d, and only a slight over-prediction is presented at relatively lower temperature. For further validation the models and investigation of the effect of hydrogen addition, additional data of the stoichiometric (/ = 1.0) and rich (/ = 2.0) mixtures were measured for five different blending ratio (XH2 = 0%, 80%, 95%, 98%, 100%) at p = 10 atm, as shown in Fig. 6, as well as the calculated values. It is also observed that NUIG Aramco Mech 1.3 agrees well with the measured data and Zhao model gives an ever-increasing over-prediction on the measured data as hydrogen addition. This prediction behaviors are similar to that of lean mixtures at p = 10 atm. To quantitatively illustrate the performance of the models and the effect of hydrogen blending ratio, a comparison between the simulation results and data obtained by correlations as a function of hydrogen blending ratio is present in Fig. 7 for three temperatures (T = 1150 K, 1300 K, 1450 K) at p = 10 atm. It is noted that correlations do not give for hydrogen blending ratios of 98% and 100%. It is observed that hydrogen addition exhibits the non-linear influence on the ignition delay times. With the increase of hydrogen blending ratio, the ignition delay time shows an initially gradual decrease and then a steep decrease when hydrogen blending ratio is larger than 50%. To further illustrate the non-linear effect, a normalized ignition delay time reduction parameter, D, which illustrates the sensitivity of the ignition delay time to the hydrogen blending ratio, is defined as:

D¼

sXH2  sDME sDME

Mech 1.3 can better capture this non-linear influence than the Zhao model, as shown in Fig. 5. In general, simulation results show that the Zhao model gives reasonable agreement with experimental data for neat DME. With the increase of hydrogen blending ratio, the performance of Zhao

ð2Þ

where XH2 is the hydrogen blending ratio, sX H2 is the ignition delay time at the hydrogen blending ratio of XH2, and sDME is the ignition delay time of pure DME. Figure 8 depicts the effect of the blending ratio on D at the conditions corresponding to Fig. 7. This effect is found to be more obvious at T = 1150 K. In addition, NUIG Aramco

Fig. 12. Mole fractions of free radicals for different hydrogen blending ratios at / = 0.5, T = 1150 K and p = 10 atm.

Fig. 13. Computed species profiles for three mixtures at / = 0.5, p = 10 atm and T = 1150 K.

L. Pan et al. / Combustion and Flame 161 (2014) 735–747

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model becomes moderate, which may be caused by the relatively poor H2/O2 sub-mechanism. The NUIG Aramco Mech 1.3 well predicts the measured data under all conditions. Thus, sensitivity analysis was performed to ascertain key reactions responsible for poor prediction by Zhao model in the following section. Then, small radical and reaction pathway analysis were made chemically to interpret the effect of hydrogen addition on DME/H2 mixtures. As discussed above, the non-linear effect is most obvious at T = 1150 K, thus T = 1150 K was selected as the reference temperature for the following kinetic analysis. 3.4. Chemical kinetic analysis 3.4.1. Sensitivity analysis As shown in Figs. 5 and 6, both models used in this work are in reasonable agreement with the measured data of DME; nevertheless, only NUIG Aramco Mech 1.3 can quantitatively capture the effect of hydrogen addition but Zhao model shows ever-increasing over-prediction as increasing hydrogen blending ratio. Therefore, it may be inferred that the poor performance of Zhao model is primarily caused by the relatively poor H2/O2 sub-mechanism, as mentioned above. Furthermore, Zhao model gives more accurate prediction at p = 1.2 atm than the prediction at p = 10 atm, suggesting poor prediction is more likely relevance to pressure-dependence reactions. To identify the key reactions responsible for the poor prediction, sensitivity analysis of hydrogen using both Zhao model and NUIG Aramco Mech 1.3 were conducted at p = 10 atm, T = 1150 K and / = 0.5. The elaborated definition of sensitivity analysis can be found in the previous literature [41]. Figure 9 presents 10 most sensitive reactions using two models. After systematically examined the rate constants and sensitivity coefficients, it is found that among these 10 most sensitive reactions, the chain propagation reaction:

H þ O2 ðþMÞ () HO2 ðþMÞ

ðaÞ

gives the vast difference sensitivity coefficient and reaction rate between Zhao model and NUIG Aramco Mech 1.3. In NUIG Aramco Mech 1.3, this reaction is written into three reactions:

H þ O2 ðþMÞ () HO2 ðþMÞ

ða1Þ

H þ O2 ðþARÞ () HO2 ðþARÞ

ða2Þ

H þ O2 ðþHEÞ () HO2 ðþHEÞ

ða3Þ

Table 3 gives the kinetic parameters for above key reactions. It is noted that enhanced third-body efficiencies of AR and HE in reaction (a1) in NUIG Aramco Mech 1.3 are declared as zero. As a pressure-dependent reaction, the rate constants of these reactions in two models are written in TROE form. The rate constant of (a) in Zhao model was taken from Cobos et al. [46], whereas new fits of (a1)–(a3) determined by Fernandes et al. [47] and Bates et al. [48] were employed in NUIG Aramco Mech 1.3, as shown in Table 3. The rate constants of these reactions are compared in Fig. 10. It is observed that reaction rate (a) for AR in Zhao model is approximately two times higher than those (a2) in NUIG Aramco Mech 1.3. To assess the influence of these reactions, the reaction (a) in Zhao model was replaced by reactions a1, a2, and a3 in NUIG Aramco Mech 1.3, as shown in Fig. 11. It is observed that the modified Zhao model yields considerably better agreement with experimental data especially for pure hydrogen and at p = 10 atm. Additionally, the other reactions with high sensitivity coefficient in Zhao model were replaced with those in the NUIG Aramco Mech 1.3. However, limited effect on ignition was found. Therefore, it is inferred that higher rate constant of reaction (a) mainly leads to ever-increasing over-prediction on mixtures as increasing

Fig. 14. Normalized consumption rates of H radicals from reactions R1 and R433 for lean mixtures at p = 10 atm and T = 1150 K.

Fig. 15. Normalized mole fractions of DME and hydrogen for lean mixtures at p = 10 atm and T = 1150 K. (Solid lines: DME; dash lines: hydrogen).

hydrogen blending ratio. Nevertheless, the rate constant of reaction a1, a2, and a3 in NUIG Aramco Mech 1.3 may not be definitely

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accurate; thus, further high-level theoretical studies on these rates constant are recommended. 3.4.2. Small radical mole fraction analysis As discussed above, an important aspect of this study is to interpret the non-linear effect of hydrogen addition on ignition delay times of DME/H2 mixtures. One way to interpret these results is that addition of small amount of DME can significantly retard the ignition of hydrogen. To clarify this, the small radical mole fraction analysis is performed versus DME blending ratio. It is noted that small radical analysis and the following reaction pathway analysis based on both NUIG Aramco Mech 1.3 and modified Zhao model have been obtained and the results are essentially the same. Therefore, results by NUIG Aramco Mech 1.3 were given below. Figure 12 gives the effect of DME blending ratio on the evolution profiles of the total radical pool (sum of H, O, OH, HO2, CH3, C2H5 radicals) for all mixtures at 10 atm and 1150 K. Results show that increasing the DME blending ratio decreases the total radical pool concentration and remarkably retards the peak of total radical pool, resulting in an increased ignition delay times. Particularly, a significant decrease in concentration and retarding of the peak of total radical pool are observed for a small amount of DME blending ratio (i.e. 2%). When DME blending ratio is further increased, and the effect becomes small, a similar phenomenon to the non-linear variation of ignition delay time. To clarify the radical pool development and its effect on the ignition, the computed species profiles of neat hydrogen, neat DME and their binary mixture (98%H2/2%DME) are presented in Fig. 13. An explicit steep increase of H, OH, and O radicals is observed in Fig. 13a for the neat hydrogen. It is well known that the oxidation of hydrogen is initiated by the reaction R13 (H2+O2 <=> H+HO2). But it gives small contribution to the total radical pool in the induction period. With the production of H from R13, a large amount of H, O, and OH radicals are then produced by the following three reactions: R1 (H+O2 <=> O + OH), R2 (O + H2 <=> H+OH), R3 (OH+H2 <=> H + H2O). Compared to the neat hydrogen, the increase of H, O, and OH for the DME is much slower, as shown in Fig 13b. H radicals are

Fig. 17. Branching ratio for DME consumption for lean mixtures at p = 10 atm and T = 1150 K.

initially produced by the reactions R91 (CH3O(+M) <=> CH2O+H(+M)) and R30 (HCO(+M) <=> H+CO+(M)). Reactions R91 and R30 have much lower reaction rates and produce less H radicals than those from the reactions R2 and R3 in the oxidation of neat hydrogen. In addition, the produced H radicals are readily consumed by the reaction R433 (CH3OCH3+H <=> CH3OCH2+H2). Therefore, the H radical gives a very slow increase before the ignition. As a result, the O and OH radicals also increase very slowly due to the limited rate of reactions R1, R2, and R3. For the binary mixture of 98%H2/2%DME, as shown in Fig. 13c, H, O, and OH radicals present relatively slower increase than those of neat hydrogen. As discussed above, reaction R433 is more readily to react with H radicals than reaction R1. Thus, even a small amount of DME addition can compete for a considerable quantity of H radical that produced by the oxidation of hydrogen and dramatically slows down the increase of H radical. Subsequently, reactions R1, R2, and R3 are inhibited compared to neat hydrogen

Fig. 16. Reaction pathway diagram for lean DME/H2 mixture at T = 1150 K and p = 10 atm using NUIG Aramco Mech 1.3 (Blue: neat DME; Red: 50%DME/50%H2; Purple: 2%DME/98%H2; Black: 100%H2). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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To further study the non-linear effect of hydrogen addition, the normalized consumption rates of H radicals by reactions R1 and R433 in the induction period for all mixtures at 10 atm and 1150 K are given in Fig. 14. It is observed that H radicals are predominantly consumed by reaction R1 for the neat hydrogen. When DME blending ratio is small, the consumption of H radicals by the reaction R433 is significantly increased while that of reaction R1 is decreased with the increase of DME blending ratio. Subsequently, reactions R2 and R3 are inhibited, leading to the increased ignition delay times. However, this effect is weakened at higher DME blending ratios. Therefore, compared to the mixtures with high DME blending ratio, ignition delay times are more sensitive to the DME addition at small DME blending ratio, leading to the non-linear ignition variation as shown in Fig. 7. In addition, it is noted that H-atoms in the exponential growth of radicals plays an important role in this work, therefore, residual H-atoms from the decomposition of larger hydrocarbon and residual gas may affect the ignition delay time [49]. However, high-purity air is used to wash the shock tube before each experiment and the whole shock tube was evacuated to pressure below 106 bar which means residual gas and the residual H-atom is negligible compared to 50–500 torr fuel mixtures. Figure 15 shows the normalized fuel mole fraction for all mixtures at p = 10 atm and T = 1150 K. The normalized fuel mole fraction, which illustrates the consumption rate of a fuel, is defined as:

Xm ¼

nm NAR  nAR Nm

ð3Þ

here Xm is normalized mole fraction (m = DME or H2), n is mole fraction of a fuel (DME or H2) and N refers to initial reactant mole fraction. For all binary mixtures, DME is consumed much faster than hydrogen. In addition, an anomalous behavior of an obvious increase of hydrogen in the initial induction period is observed for the DME blending ratio of 50%. This behavior is also due to the competition between reactions R433 and R1. Reaction R433 competes with reaction R1 for large amount of H radicals to yield hydrogen. This competition also inhibits the hydrogen consumption by reactions R2 and R3. Thus, the total hydrogen production rate is faster than the hydrogen consumption rate in the initial induction period, resulting in the increase of hydrogen.

Fig. 18. High-pressure simulations of ignition delay time for DME/H2/air mixtures at p = 40 atm, / = 0.5, 1.0, 2.0.

and consequently lower the whole system reactivity, leading to an increased ignition delay time. It is important to note that through competition for H radicals between reactions R433 and R1, the consumption of DME is promoted whereas the consumption of hydrogen is inhibited.

3.4.3. Reaction pathway analysis In order to better understand the role of hydrogen addition, a series of reaction pathway analysis for lean mixtures using NUIG Aramco Mech 1.3 were performed at T = 1150 K, p = 10 atm and the timing of 20% fuel consumption, with the emphasis on the effect of hydrogen addition. Figure 16 shows main fuel flux of four representative mixtures: neat DME; 50%DME/50%H2; 2%DME/ 98%H2; 100%H2. For this condition, DME consumption of all these four mixtures is almost wholly ascribed to H-abstraction by small radicals like H, OH, CH3, HO2, O, while a small amount of DME fuel directly suffers the molecular pyrolysis to form the methyl (CH3) and methoxy (CH3O) radicals through bond dissociation, as shown in Fig. 16. Nevertheless, initial reaction path which affects the composition of radical pool and reactivity of the system produces marked variations with hydrogen addition. Take the case of DME consumption, it is observed in Figs. 16 and 17 that the branching ratio of H-abstraction by H radicals (R433: CH3OCH3+H <=> CH3OCH2+H2) increases dramatically and the branching ratio of Habstraction by OH (R432: CH3OCH3 + OH <=> CH3OCH2 + H2O) and CH3 (R437: CH3OCH3 + CH3 <=> CH3OCH2 + CH4) decreases distinctly with hydrogen addition increasing. It is understandable that the decreasing branching ratio for R437 is due to the reduction of DME concentration. Simulations suggest that the increasing branching ratio for R433 is because of the increasing amount of H radicals produced by reaction sequence: R1 (H+O2 <=> O+OH),

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R2 (O+H2 <=> H+OH), R3 (OH+H2 <=> H+H2O) as increasing hydrogen addition. Although the number of OH radicals is also increased as hydrogen addition, more OH radicals are consumed by hydrogen, as shown in Fig. 16, consequently leading to the reduction on branching ratio of R432. In addition, main production and consumption paths of H radical are also changed with different DME or hydrogen blending ratio. Take the case of consumption paths, when 2% DME is added, 11.7% H radicals is consumed by DME through R433. When DME blending ratio is increased from 50% to 100%, the consumption of H radicals was only changed from 44.6% to 46.4%. This further demonstrates the non-linear effect of hydrogen addition on ignition delay time of DME/H2 mixtures.

(5) An anomalous behavior of an obvious increase of hydrogen in the initial induction period is found for the DME blending ratio of 50% which results from the competition between reactions R1 and R433. (6) Reaction pathway analysis not only confirms the competition between reactions of R1 and R433, but also shows that the branching ratio of R433 increases dramatically with the hydrogen addition. (7) High-pressure simulations of DME/H2/air shows that hydrogen addition also gives the non-linear effect on ignition delay time at low temperature, resembling to the ignition behavior at high temperature. However, hydrogen inhibits auto-ignition of the mixtures at low temperature.

3.5. High-pressure simulations Low temperature auto-ignition studies at high pressure are also of particular interest, owing to its more relevant to practical engine conditions. Considering technical and safety factors in present shock tube, ignition delay time at low temperature and high pressure was not measured in this study. However, it is our core work in the future and once finished building our new high-pressure shock tube, this work will be done. In addition, high-pressure simulations of DME/H2/air mixture using NUIG Aramco Mech 1.3 were presented serving as a starting point for future work. Figure 18 shows the simulations at p = 40 atm, / = 0.5, 1.0, 2.0 and hydrogen blending ratio ranging from 0% to 100%. It is observed that neat DME shows notable NTC behaviors resemble to n-heptane. Compared to neat DME, neat hydrogen shows much shorter ignition delay times at high temperature, but longer ignition delay times and no observable NTC behavior at low temperature. In addition, hydrogen addition gives the non-linear effect on ignition delay time of DME/H2/air mixtures at both high temperature and low temperature; nevertheless, hydrogen addition displays opposite influence: it respectively enhances and suppresses the ignition of DME/H2/air mixtures at high temperature and low temperature. As such opposing influence, both experimental and kinetic studies using shock tube or rapid compression machine are recommended. 4. Conclusions Ignition delay times of DME/H2/O2/Ar mixtures were measured behind reflected shock waves, with emphasizing on the effect of hydrogen blending ratio. Kinetic study was also made using the available models. Main results are summarized as follows. (1) Correlations of ignition delay times of lean mixtures were given based on the measured data. The ignition characteristics of the DME/H2 mixtures can be classified into three regimes depending on hydrogen blending ratio. (2) The measured ignition delay times of DME/H2 mixtures give an initially gradual decrease and then a steep decrease with the increase of hydrogen blending ratio, revealing a non-linear effect of hydrogen addition on the ignition delay times. Simulation results show that NUIG Aramco Mech 1.3 can better capture this non-linear effect than Zhao model. (3) Sensitivity analysis shows that poor prediction by Zhao model is probably caused by reaction H+O2(+M) <=>HO2(+M). After replacing the rate constant of this reaction with those in NUIG Aramco Mech 1.3, the modified Zhao model yields significant better prediction. (4) Small radical analysis reveals that the non-linear effect is controlled by the competition between reactions of R1 and R433. Reaction R433 is more readily to react with H radicals than reaction R1.

Acknowledgments This work is supported by the National Natural Science Foundation of China (51306144 and 51136005), and the National Basic Research Program (2013CB228406). Authors also appreciate the funding support from the Fundamental Research Funds for the Central Universities and State Key Laboratory of Engines (SKLE201302). Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.combustflame. 2013.10.015. References [1] M. Yao, Z. Zheng, H. Liu, Prog. Energy Combust. Sci. 35 (2009) 398–437. [2] N.P. Komninos, C.D. Rakopoulos, Renew. Sustain. Energy Rev. 16 (2012) 1588– 1610. [3] T.A. Semelsberger, R.L. Borup, H.L. Greene, J. Power Sources 156 (2006) 497– 511. [4] N. Peters, G. Paczko, R. Seiser, K. Seshadri, Combust. Flame 128 (1–2) (2002) 38–59. [5] M. Balat, Int. J. Hydrogen Energy 33 (2008) 4013–4029. [6] M.Ay. Midilli, I. Dincer, M.A. Rosen, Renew. Sustain. Energy Rev. 9 (2005) 255– 271. [7] C. Ji, S. Wang, Int. J. Hydrogen Energy 35 (2010) 1453–1462. [8] M.Y. Kim, S.H. Yoon, B.W. Ryu, C.S. Lee, Fuel 87 (2008) 2779–2786. [9] B. Shin, Y. Cho, D. Han, S. Song, K.M. Chun, Int. J. Hydrogen Energy 36 (2011) 6281–6291. [10] J. Song, Z. Huang, X. Qiao, W. Wang, Energy Convers. Manage. 45 (2004) 2223– 2232. [11] S. Wang, C. Ji, J. Zhang, B. Zhang, Int. J. Hydrogen Energy 36 (2011) 11164– 11173. [12] R.D. Cook, D.F. Davidson, R.K. Hanson, Proc. Combust. Inst. 32 (2009) 189–196. [13] P. Dagaut, C. Daly, J.M. Simmie, M. Cathonnet, Symp. (Int.) Combust. 27 (1998) 361–369. [14] R.D. Cook, D.F. Davidson, R.K. Hanson, J. Phys. Chem. A 113 (2009). [15] Z. Hong, A. Farooq, E.A. Barbour, D.F. Davidson, R.K. Hanson, J. Phys. Chem. A 113 (2009) 12919–12925. [16] A. Kéromnès, W.K. Metcalfe, K.A. Heufer, N. Donohoe, A.K. Das, C.-J. Sung, J. Herzler, C. Naumann, P. Griebel, O. Mathieu, M.C. Krejci, E.L. Petersen, W.J. Pitz, H.J. Curran, Combust. Flame 160 (2013) 995–1011. [17] P. Dagaut, J.-C. Boettner, M. Cathonnet, Symp. (Int.) Combust. 26 (1996) 627– 632. [18] K. Das, C.-J. Sung, Y. Zhang, G. Mittal, Int. J. Hydrogen Energy 37 (2012) 6901– 6911. [19] G. Mittal, M. Chaos, C.-J. Sung, F.L. Dryer, Fuel Process. Technol. 89 (2008) 1244–1254. [20] E. Toulson, C.M. Allen, D.J. Miller, H.J. Schock, T. Lee, Energy Fuel 24 (2010) 3510–3516. [21] M. Kuznetsov, S. Kobelt, J. Grune, T. Jordan, Int. J. Hydrogen Energy 37 (2012) 17580–17588. [22] J. Pareja, H.J. Burbano, Y. Ogami, Int. J. Hydrogen Energy 35 (2010) 1812–1818. [23] J. Pareja, H.J. Burbano, A. Amell, J. Carvajal, Int. J. Hydrogen Energy 36 (2011) 6317–6324. [24] S. Ravi, E.L. Petersen, Int. J. Hydrogen Energy 37 (2012) 19177–19189. [25] Z. Huang, Q. Wang, J. Yu, Y. Zhang, K. Zeng, H. Miao, D. Jiang, Fuel 86 (2007) 2360–2366. [26] X. Qin, Y. Ju, Proc. Combust. Inst. 30 (2005) 233–240.

L. Pan et al. / Combustion and Flame 161 (2014) 735–747 [27] J. Jeon, C. Bae, Int. J. Hydrogen Energy 38 (2013) 265–273. [28] Z. Huang, G. Chen, C. Chen, H. Miao, X. Wang, D. Jiang, Energy Fuel 22 (2008) 967–971. [29] Z. Chen, L. Wei, X. Gu, Z. Huang, T. Yuan, Y. Li, Z. Tian, Energy Fuel 24 (2010) 1628–1635. [30] J. Liu, H. Wang, M. Ouyang, Int. J. Hydrogen Energy 36 (2011) 15860–15867. [31] Z. Zhao, M. Chaos, A. Kazakov, F.L. Dryer, Int. J. Chem. Kinet. 40 (2008) 1–18. [32] W.K. Metcalfe, S.M. Burke, S.S. Ahmed, H.J. Curran, Int. J. Chem. Kinet. 45 (10) (2013) 638–675. [33] J. Zhang, L. Wei, X. Man, X. Jiang, Y. Zhang, E. Hu, Z. Huang, Energy Fuels 26 (2012) 3368–3380. [34] C. Morley, Gaseq v0.76, . [35] E. Goos, A. Burcat, B. Ruscic, . [36] D.F. Davidson, R.K. Hanson, Int. J. Chem. Kinet. 36 (2004) 510–523. [37] R.J. Kee, F.M. Rupley, J.A. Miller, Chemkin-II: A Fortran Chemical Kinetics Package for The Analysis of Gas-Phase Chemical Kinetics, Report No. SAND898009, Sandia National Laboratories, 1989. [38] M. Chaos, F.L. Dryer, Int. J. Chem. Kinet. 42 (3) (2010) 143–150.

747

[39] G.A. Pang, D.F. Davidson, R.K. Hanson, Proc. Combust. Inst. 32 (1) (2009) 181– 188. [40] X. Man, C. Tang, L. Wei, L. Pan, Z. Huang, Int. J. Hydrogen Energy 38 (2013) 2523–2530. [41] Y. Zhang, Z. Huang, L. Wei, J. Zhang, C.K. Law, Combust. Flame 159 (2012) 918– 931. [42] H.J. Curran, S.L. Fischer, F.L. Dryer, Int. J. Chem. Kinet. 32 (12) (2000) 741–759. [43] S.L. Fischer, F.L. Dryer, H.J. Curran, Int. J. Chem. Kinet. 32 (12) (2000) 713–740. [44] C.A. Daly, J.M. Simmie, J. Würmel, N. DjebaÏli, C. Paillard, Combust. Flame 125 (4) (2001) 1329–1340. [45] Z. Zhao, A. Kazakov, F.L. Dryer, Combust. Flame 139 (1–2) (2004) 52–60. [46] C.J. Cobos, H. Hippler, J. Troe, J. Phys. Chem. 89 (2) (1985) 342–349. [47] R.X. Fernandes, K. Luther, J. Troe, V.G. Ushakov, Phys. Chem. Chem. Phys. 10 (29) (2008) 4313–4321. [48] R.W. Bates, D.M. Golden, R.K. Hanson, C.T. Bowman, Phys. Chem. Chem. Phys. 3 (12) (2001) 2337–2342. [49] J. Urzay, N. Kseib, D.F. Davidson, G. Iaccarino, R.K. Hanson, Combust. Flame 161 (1) (2014) 1–15, http://dx.doi.org/10.1016/j.combustflame.2013.08.012.