Experimental and Kinetic Study on Ignition Delay Times of Liquified

Oct 31, 2014 - In this study, ignition delay times of liquified petroleum gas (LPG)/dimethyl ether (DME) (LPG consists of C3H8 and C4H10 in this work)...
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Experimental and Kinetic Study on Ignition Delay Times of Liquified Petroleum Gas/Dimethyl Ether Blends in a Shock Tube Lili Xu, Linqi Ouyang, Zhuang Geng, Hua Li, Zhen Huang,* and Xingcai Lu* Key Laboratory for Power Machinery of M.O.E, Shanghai Jiao Tong University, Shanghai 200240, People’s Republic of China ABSTRACT: In this study, ignition delay times of liquified petroleum gas (LPG)/dimethyl ether (DME) (LPG consists of C3H8 and C4H10 in this work) were measured in a shock tube at different DME blending ratios (0%, 10%, 30%, and 50%), pressures (5, 10, and 15 atm), temperatures (1100−1500 K), and equivalence ratios (0.5, 1.0, and 1.5). The chemical kinetic mechanism of LPG/DME was established based on Lawrence Livermore National Laboratory’s C1−C4 chemical kinetic mechanism (Combust. Flame 1998, 114, 192−213) and Zhao’s DME chemical kinetic mechanism (Int. J. Chem. Kinet. 2008, 40, 1−18), and its predictions agree well with experimental data. A sensitivity analysis and a reaction pathway analysis were conducted using CHEMKIN-PRO to study the impact of DME addition on the ignition and combustion process. The experimental results show that the ignition delay times of LPG/DME change linearly with increasing DME blending ratios. The sensitivity analysis shows that the number of major promoting reactions for mixtures increases, including H-abstraction and decomposition of CH3OCH3, while the sensitivity factors of the H-abstraction and the decomposition of C3H8 (reactions R115, R120, and R125) decrease with increasing DME blending ratios. The reaction pathway analysis indicates that the H-abstraction reactions play a dominant role, and the contribution rate of OH to H-abstraction increases, while that of H-radical decreases slightly in the oxidation of C3H8 and C4H10 with the increasing proportion of DME in the LPG/DME mixtures. Further analysis shows that although the growth rate of H before ignition is LPG100 > LPG50 > DME, reaction R22 in the oxidation process of mixtures makes OH accumulate rapidly in a short time, resulting in a much higher peak concentration of OH than that of H; therefore, the ignition delay times of mixtures are shorter than those of neat LPG.

1. INTRODUCTION With increasingly serious energy and environmental issues challenging the development of conventional internal combustion engines, new combustion modes, including homogeneous charge compression ignition (HCCI) and low-temperature combustion (LTC), have drawn increased attention, and a large number of studies on these have been performed. HCCI, as a promising alternative combustion technology, has been widely investigated in recent years. Its advantages include relatively high thermal efficiency, good fuel adaptability, and lower emissions of NOX and particulate matter (PM). However, the main problems of a HCCI engine are as follows: the range of loads is limited, and the ignition timing and burn rate are very difficult to accurately control.1 The notable characteristic of HCCI is that the ignition and combustion process are controlled by chemical kinetics. Long-term studies indicate that fuel characteristics are one of the most decisive factors in controlling ignition timing and extending the range of loads for HCCI combustion. It is often difficult to obtain a wide range of loads in a single-property fuel. Fuel design theory deems that mixing two fuels to alter the physical properties of the fuel is an effective means to achieve this goal.2 Therefore, the study on the control of HCCI combustion based on fuel design and management has raised widespread concern. Hashimoto3 confirmed that ethanol exhibited a moderate inhibition on the ignition of n-heptane. Lu et al.4−7 compared the effects of several inert additives methanol, ethanol, isopropanol, and methyl tert-butyl ether (MTBE) on the HCCI combustion of n-heptane. Sakai et al.8 studied the effects of toluene added to primary reference fuel (PRF). Shibata and Urushihara9,10 found dual-phase high© 2014 American Chemical Society

temperature heat release (DP-HTHR) combustion of nheptane/toluene mixtures, resulting in a longer combustion period. Because of the severe shortage of oil energy, renewable fuels and their diversity have become the main development trend in vehicle dynamics. Common alternative fuels include natural gas, liquefied petroleum gas (LPG), hydrogen, alcohols, ethers and biodiesel. LPG, whose main components are propane and butane, is one type of clean-burning fossil fuel. It can be liquefied at low pressure for storage, and its supply is adequate. LPG is usually less expensive than gasoline. LPG has been used as a fuel for light vehicles for years. In general, LPG is featured as a high octane number, good antiknock property and has good adaptability at a heavy load. DME (dimethyl ether), considered to be the cleanest alternative fuel for diesel engines, can be generated from coal, natural gas, biomass resources, and so on.11 Because DME has no C−C bond, a high oxygen content (34.8%), and a similar cetane number to diesel, and is quite easily liquefied at low temperature, it is very suitable to partly or completely replace diesel. Its other advantages include low NOX and almost no soot emission.12−16 HCCI engines fueled with DME have good combustion performance and emission characteristics at low load−high speed, but it easily knocks with heavy loads. From the point of view of ignition control and load expansion, high cetane number fuels should be used at cold Received: June 25, 2014 Revised: October 30, 2014 Published: October 31, 2014 7168

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Table 1. Experimental Research on DME Combustion study (year)

facility

reactant mixtures

P (bar)

T (K)

Pfahl et al. (1996)44 Dagaut et al. (1998)36 Zinner et al. (2008)31 Cook et al. (2009)32 Tang et al. (2012)21 Hu et al. (2013)34 Li et al. (2013)39 Zhang et al. (2013)22

ST ST ST ST ST ST ST ST

DME/air; Φ = 1.0 DME/O2/Ar; Φ = 0.5, 1.0, 2.0 DME/CH4/air; Φ = 0.3,0.5,1.0,2.0 DME/O2/Ar; Φ = 0.5, 1.0, 2.0 DME/CH4/O2/Ar,∼94%Ar; Φ = 1.0 DME/C4H10/O2/Ar; Φ = 1.0 DME/air and DME/air/N2; Φ = 0.5, 1.0, 1.5 DME/C2H6/air/Ar; Φ = 0.5, 1.0, 2

13, 40 3.5 0.8−35.7 1.6−6.6 1, 5, 10 1.2−5.3 22 2, 20

650−1250 1200−1600 913−1650 1175−1900 1134−2105 1200−1600 700−1270 1100−1500

Table 2. Compositions of Mixtures in This Study mixtures neat 90% 70% 50% 70% 70% 70% 70% 70% 70%

LPG LPG + 10% DME LPG + 30% DME LPG + 50% DME LPG + 30% DME LPG + 30% DME LPG + 30% DME LPG + 30% DME C3H8 + 30% DME LPG* + 30% DME

φ

DME (%)

C3H8 (%)

C4H10 (%)

O2 (%)

N2 (%)

P (atm)

1 1 1 1 0.5 1.5 1 1 1 1

0 0.4083 1.3367 2.4513 0.6836 1.9613 1.3367 1.3367 1.3671 1.3076

3.5275 3.3074 2.807 2.2062 1.4355 4.1188 2.807 2.807 3.1899 2.4408

0.3919 0.3675 0.3119 0.2451 0.1595 0.4576 0.3119 0.3119 0 0.6102

20.185 20.1506 20.0724 19.9784 20.5297 19.6349 20.0724 20.0724 20.051 20.0927

75.8956 75.7662 75.4721 75.1189 77.1917 73.8273 75.4721 75.4721 75.3919 75.5487

10 10 10 10 10 10 5 15 10 10

DME, as well as its mixtures, using shock tubes (see Table 1). Zinner et al.31 studied the effects of the DME blending ratio on combustion by measuring the ignition delay times of DME/ methane mixtures in a shock tube for a pressure range of 0.8− 35.7 atm, a temperature range of 913−1650 K, and equivalence ratios of 0.3, 0.5, 1.0, and 2.0. Cook et al.32 measured the ignition delay times of DME/O2/Ar mixtures for a pressure range of 1.6−6.6 bar, a temperature range of 1175−1900 K, and equivalence ratios of 0.5−3.0. Zhang et al.22 studied the combustion performance of DME/C2H6 diluted with Ar by measuring ignition delay times for pressures of 2 and 20 atm, a temperature range of 913−1650 K, and equivalence ratios of 0.5, 1.0, and 2.0. Hu et al.33,34 studied DME/propane mixtures and DME/n-butane mixtures in a shock tube and determined that the ignition delay times of mixtures decrease almost linearly with increasing DME blending ratio. Currently, studies on LPG/DME mixtures are mostly carried out in the engine. Tests and mechanisms of ignition delay time are not yet comprehensive because of a lack of relevant data. Therefore, LPG/DME mixtures are selected as the test fuel in this study. Ignition delay times are measured in a shock tube at different DME blending ratios (0%, 10%, 30%, and 50%); at pressures of 5, 10, and 15 atm; in the temperature range of 1100−1500 K; and for equivalence ratios of 0.5, 1.0, and 1.5. And ignition delay times of different components of LPG (LPG consists of 90% C3H8 and 10% C4H10 in moles; LPG* consists of 80% C3H8 and 20% C4H10 in moles) are compared. The chemical kinetic mechanism of LPG/DME was established based on Lawrence Livermore National Laboratory’s (LLNL’s) C1−C4 chemical kinetic mechanism43 and Zhao’s DME chemical kinetic mechanism.37 A sensitivity analysis and a reaction pathway analysis were conducted using CHEMKINPRO with that mechanism to determine the main primary reactions as well as the effects of DME addition on the reaction pathway.

start or low-load conditions, while high octane number fuel with good antiknock is better at heavy loads; therefore, combining two fuels whose chemical properties are opposite is fairly ideal and usually produces good results. DME is often mixed with high octane number hydrocarbon fuels, such as natural gas,17,18 hydrogen,19 methanol,20 methane,21 ethane,22 and n-butane,23 to improve ignition and combustion performance. Likewise, combining DME and LPG is a promising method to control the engine combustion and emissions. Chen et al.24 found that an HCCI engine fueled with LPG had a wider range of loads and higher efficiency and produced almost no NOX after DME was added. Marchionna et al.25 studied the combustion of DME/LPG mixtures, concluding that DME/ LPG mixtures exhibited better performance compared to neat DME. Lee et al.26 studied the effects of LPG on the performance, emission characteristics, and combustion stability of SI engines fueled with DME/LPG mixtures. Yeom and Bae27 and Jang et al.28 compared two different supply methods, LPG port injection with DME direct injection and DME port injection with LPG direct injection, and determined that the former had higher IMEP (indicated mean effective pressure) and lower emission. Thus, the studies of LPG/DME mixed fuels are very important in practice because this fuel can extend the operating range of HCCI engines and result in clean combustion. The chemical kinetic mechanism is important in the study of combustion and emission characteristics, and ignition delay time is an important basis to validate the mechanism and to establish a model.19−30 Equipment commonly used to study ignition delay times mainly includes rapid compression machines, shock tubes, and flow reactors. Shock tubes use incident and reflected shocks to compress a tested gas uniformly, nonisentropically, and adiabatically. Tests can be performed at a higher temperature and pressure range compared to those with rapid compression machine (RCM). In recent years, many experts and researchers at home and abroad have performed many tests and simulations to study 7169

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2. EXPERIMENTAL APPROACH

for predicting experimental results. Therefore, Zhao’s DME mechanism is selected in this study. Based on the two preceding mechanisms, the mechanism of LPG/DME (Mod SJTU) was established by adding Zhao’s DME submechanism to the LLNL mechanism, which contains 177 components and 845 elementary reactions. Figure 2 shows a comparison of the current experimental and computed results with the literature data. Ignition delay times of 0.2% C3H8 + 1% O2 + 98.8% Ar (P = 4 atm) are given in Figure 2a. The solid line represents the computed results of Mod SJTU. The dashed line represents the computed results of the LLNL mechanism. These two lines coincide, and they also agree well with the experimental results from the University of Texas at Austin.38 Although the solid line is a little low when the temperature is higher than 1600 K, it is still very close to the experimental results and the computed results of the LLNL mechanism. Figure 2b gives the ignition delay times of 1.31% DME + 3.93% O2 + 94.77% Ar (P = 20 atm). The solid line represents the computed results of Mod SJTU. The dashed line represents the computed results of Zhaòs DME mechanism. Likewise, the two lines basically coincide, and they also coincide well with the experimental result from Xi’an Jiao Tong University (XJTU).33 The preceding results indicate that Mod SJTU predicts ignition delay times of pure DME and pure C3H8 very well, which all agree well with the experimental results. Thus, Mod SJTU is verified to be accurate, and it can be used to compute ignition delay times of mixtures and analyze the main elementary reactions and the change of pathways. 3.2. Ignition Delay Time Measurements. Figure 3 shows the ignition delay times of neat LPG, 90% LPG/10% DME, 70% LPG/30% DME, and 50% LPG/50% DME (LPG100, LPG90, LPG70, and LPG50) at an equivalence ratio of 1.0 and a pressure of 10 atm. The figure illustrates that the ignition delay time of neat LPG is the longest, while that of the mixtures decreases as the DME blending ratio increases, indicating that DME has a promoting effect on the ignition of mixtures. As shown in Figure 3, when the temperature is lower than 1200 K, the computed results for LPG100 are slightly greater than the experimental results, while the computed results of LPG70 and LPG50 are a bit lower than the experimental results. Overall, the computed results and experimental results basically coincide over the temperature range of this study. Figure 4 shows the variation of the ignition delay times of 70% LPG/30% DME (LPG70) at different pressures and equivalence ratios. The ignition delay times of LPG70 increase with decreasing pressure at an equivalence ratio of 1.0 and pressures of 5, 10, and 15 atm. This is mainly because the increasing pressure increases the reaction rate, which makes ignition occur more easily.39 Moreover, the three curves at different pressures are almost parallel, demonstrating that pressure has a negligible effect on activation energy. Figure 4b shows the ignition delay times of LPG70 at a pressure of 10 atm and equivalence ratios of 0.5, 1.0, and 1.5. As seen from the figure, the ignition delay times of LPG70 increase with decreasing equivalence ratio. This is mainly because the relative concentration of fuel decreases as the mixtures become leaner, resulting in less H-radical and a slower reaction rate.40 Furthermore, the difference between the ignition delay times of LPG70 becomes increasing smaller with increasing temperature. This demonstrates that the sensitivity of the ignition delay time to the equivalence ratio weakens at high temperature. This is maybe because the high-temperature environment

In this study, all measurements were carried out in a shock tube whose description and reliability analysis were given in detail in a previous study.35 The homogeneous fuel mixtures were prepared in a 210 L stainless steel tank according to Dalton’s law of partial pressure. All fuels are gases at environmental temperature, and then they can become sufficiently mixed by being settled down for at least 2 h. The shock tube is divided into a 6 m long driver section and a 5 m long driven section by polyethylene terephthalate (PET) diaphragms. The photomultiplier tube (PMT; HamamatsuR928), installed 20 mm away from the endwall of the shock tube, is used to detect the OH* signal, whose wavelength is 307 nm. The time that the shock wave arrives is measured by five integrated circuit piezoelectric (ICP) pressure transducers (PCB 113B26), the interval among which is 333 mm. The pressure signals and the OH* signals are all recorded and saved by an oscilloscope. Five time counters are used to record the time when the shock arrives to calculate the velocity of the shock. A vacuum pump is used to create a vacuum in the driven section, driver section, and all tubes so that the pressure is lower than 3 × 10−2 Pa, and the leakage rate is less than 0.1−0.5 Pa/min. In this work, the uncertainty of the temperature behind the reflected shock waves is less than 25 K. Helium at a purity of 99.999% is used as the driver gas in this study. The purities of nitrogen and oxygen used here are both 99.999%. The purity of DME is 99.5%. The purities of propane and n-butane are both 99.9%. The composition of the LPG/DME mixtures (φ = 0.5, 1.0, 1.5; XO2:XN2 = 21:79), and the test conditions are shown in Table 2. In this study, neat LPG, 90% LPG/10% DME (mole ratios), 70% LPG/30% DME and 50% LPG/50% DME are referred to as LPG100, LPG90, LPG70, and LPG50, respectively. The definition of the ignition delay time is shown in Figure 1. It is defined as the interval between the time that the reflected wave arrives

Figure 1. Definition of ignition delay time. at the last pressure transducer, which is defined by the step of the local pressure signal, and the intercept of the maximum slope of the OH* emission profile with the baseline.

3. RESULTS AND ANALYSIS 3.1. Chemical Kinetics of LPG/DME Blends. The zerodimensional, adiabatic, and constant-volume reactor in CHEMKIN-PRO software is used to simulate and analyze the combustion of LPG/DME mixtures in this study. Because C3H8 and C4H10 are typical hydrocarbon fuels and have been broadly studied, their mechanisms are relatively mature. The representative C1−C4 mechanism published by LLNL is selected as the LPG mechanism. There are numerous mechanisms for DME. Dagaut et al.36 and Cook et al.32 considered Zhao’s DME mechanism37 as the best mechanism 7170

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Figure 2. Comparison of the current experimental and computed results with the literature data (points for experimental data, lines for computed data).

The compositions of LPG are different for its different sources. Thus, this work investigated the effect of C4H10 fraction in LPG on the ignition delay times. Then, three LPG fuels, which consist of neat C3H8, 90% C3H8/10% C4H10 (represented as LPG in this work), 80% C3H8/20% C4H10 (represented as LPG* in this work) are compared. Figure 5 shows the ignition delay times of three different mixtures, 30% DME/70% C3H8, 30% DME/70% LPG, and 30% DME/70% LPG*, at an equivalence ratio of 1.0 and a pressure of 10 atm. As seen from the figure, when the temperature is higher than 1200 K, the ignition delay time of 30% DME/70% C3H8 is the longest, 30% DME/70% LPG is the second longest, and 30% DME/70% LPG* is the shortest. When the temperature is lower than 1200 K, there are no significant differences between them. This indicates that the increasing the proportion of C4H10 in LPG (or decreasing the proportion of C3H8) inhibits ignition. This is mainly because the high proportion of propane reduces the heat release of the low-temperature reactions and consequently delays the onset of high-temperature reactions,41 which delays ignition. This effect is not significant when the temperature is lower than 1200 K. Figure 6 gives the relation between the ignition delay times and the DME blending ratio at different temperatures and

Figure 3. Comparison of the experimental and computed data of the ignition delay time for LPG/DME blends at different DME blending ratios (points for experimental data, lines for computed data).

has provided enough energy for the ignition of the mixtures, making the ignition delay times be closer.

Figure 4. Comparison of the experimental and computed data of the ignition delay time for 70% LPG + 30% DME at different pressures and equivalence ratios (points for experimental data, lines for computed data). 7171

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delay time showed a linear decrease dependence on the variation of the DME blending ratios. To quantitatively understand the influence of the DME blending ratio on the ignition delay time, the reduction rate (RR) of the ignition delay time is defined as τLPG − τXDME RR/% = × 100 τLPG where τLPG is the ignition delay time of the LPG, XDME is the DME blending ratio, and τXDME is the ignition delay time at XDME. As seen from Figure 6, the variation of the DME blending ratio has a significant effect on the reduction rate, indicating that ignition delay times of the mixtures decrease rapidly with increasing DME blending ratio. However, the temperature and pressure have negligible effects on the reduction rate. 3.3. Sensitivity Analysis of Different Mixtures. To further analyze the specific elementary reactions affecting the ignition of LPG/DME, sensitivity analyses were conducted using CHEMKIN-PRO and Mod SJTU. Figure 7 shows the 15 most sensitive reactions for LPG100, LPG70, and LPG50 at a temperature of 1350 K, a pressure of 10 atm, and an equivalence ratio of 1.0. The sensitivity coefficients of the ignition delay time are normalized for comparison. The reactions with a positive sensitivity coefficient inhibit ignition, and those with a negative sensitivity coefficient promote ignition.

Figure 5. Comparison of the experimental and computed data of the ignition delay time for the DME/LPG blends for different blending ratios (points for experimental data, lines for computed data).

pressures. Because Mod SJTU predicts the experimental results very well, the computed results of Mod SJTU are used for the analysis. As seen from the figure, the ignition delay times of LPG/DME decrease linearly with increasing DME blending ratio at three pressures (6, 10, and 15 atm) and temperatures (1200, 1350, and 1500 K). These behaviors are the same as in the study of Hu et al.,33 who claim that the logarithmic ignition

Figure 6. Impact of different DME blending ratios on ignition delay times. 7172

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R605: CH3 + O2 = CH 2O + OH

(4)

R125: C3H8 + H = n‐C3H 7 + H 2

(5)

R607: CH3 + CH3 ( +M) = C2H6 ( +M)

(6)

R120: C3H8 + OH = n‐C3H 7 + H 2O

(7)

R612: CH3 + HO2 = CH4 + O2 Figure 7. Normalized sensitivity of the ignition delay time for varied DME blending ratios at T = 1350 K, P = 10 atm, and φ = 1.

R20: HO2 + HO2 = H 2O2 + O2

R2: (1)

R606: CH3 + HO2 = CH3O + OH

(2)

R115: C3H8 ( +M) = C2H5 + CH3 ( +M)

(9)

R2 has the largest negative temperature coefficient and produces a large number of active OH. It is dominant in promoting ignition. R606 and R605, typical chain-branching reactions, consume CH3 and produce OH, increasing the concentration of the radical pool. The decomposition of C3H8 (R115), producing CH3 and C2H5, is also one of the promoting reactions. R125, competing with R2 for H and with R115 for C3H8, has the largest positive temperature coefficient and is dominant in inhibiting ignition. R120 also competes with R115 for C3H8, consuming OH and decreasing the concentration of the radical pool. R607, R612, and R20 consume active CH3 and HO2, producing stable C2H6, CH4, H2O2, and O2, and also inhibit ignition.

For LPG100, the major reactions affecting ignition are as follows:

O2 + H = O + OH

(8)

(3)

Figure 8. Reaction pathways for LPG100, LPG50, and DME at T = 1350 K, P = 10 atm, and φ = 1 at 20% fuel consumption. 7173

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Figure 9. Mole fraction of some small radicals in the ignition and combustion processes of the LPG/DME mixtures.

OH, CH3, HO2, and O, to produce n-C3H7 and iso-C3H7 with H abstracted, and the other is the remaining C 3 H 8 decomposing into C2H5 + CH3 by the third body (M) participation. n-C3H7 is consumed by β-scission reactions to produce C2H4 + CH3. Most of the iso-C3H7 is consumed to produce C3H6 + H, while the remainder reacts with O2 to produce C3H6 + HO2. H-abstraction reactions, accounting for 93.4%, play a dominant role in the consumption of C3H8. OH and H, the maximum contribution to the H-abstraction reaction, account for 36.2% and 38.7%, respectively. C4H10 is consumed in three pathways: one is C4H10 reacting with small radicals, such as H, OH, CH3, HO2, and O, to produce para-C4H9 and sec-C4H9 with H-abstracted; the second is C4H10 decomposing into n-C3H7 + CH3; and the third is C4H10 decomposing into two C2H5. para-C4H9 and sec-C4H9 are consumed by β-scission reactions to produce C2H5 + C2H4 and C3H6 + CH3, respectively. n-C3H7 is consumed by βscission reactions to produce C2H4 + CH3. H-abstraction reactions, accounting for 56.11%, also play a dominant role in the consumption of C4H10. OH, and H, the maximum contributions to the H-abstraction reaction, account for 33.34% and 13.3%, respectively. For neat DME, most CH3OCH3 react with small radicals, such as H, OH, CH3, HO2, and O, to produce CH3OCH2 with H abstracted, and the remainder decomposes into CH3 + CH3O. CH3OCH2 is consumed by β-scission reactions to produce CH2O + CH3.The H-abstraction reaction, accounting

For LPG70, its major inhibiting reactions are basically the same as those for LPG100, but the major promoting reactions are only somewhat similar to those for LPG100. Likewise, R2 still has the largest negative temperature coefficient, and it is dominant in promoting ignition. R794 produces active CH3 and CH3O. CH3 then reacts with CH3OCH3 molecules, producing CH3OCH2 and CH4 (R797). With increasing CH3O produced by R794 and R606, CH3O produces CH2O with H abstracted. CH2O then reacts with OH, HO2, and CH3, abstracts H, and produces H2O, H2O2, CH4, and HCO (R599, R601, and R602). H2O2 in R22 rapidly produces two active OH and increases the concentration of the radical pool further, promoting ignition. For LPG50, its major sensitive reactions to ignition are basically the same as those for LPG70. Compared to LPG100 and LPG70, the sensitivity of R115, R120, and R125 decrease, indicating that the promoting effects of C3H8 decomposition and the inhibiting effects of C3H8 H-abstraction decrease with increasing DME blending ratios. 3.4. Reaction Pathway Analysis of Different Mixtures. To further explore how DME blending ratios affect ignition, reaction pathway analyses on LPG100, LPG50, and neat DME were conducted at a temperature of 1350 K, a pressure of 10 atm, an equivalence ratio of 1.0, and the timing of 20% fuel consumption. The computed results are shown in Figure 8. For neat LPG, C3H8 is consumed in two pathways: one is where most of the C3H8 reacts with small radicals, such as H, 7174

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LPG50 > LPG100; therefore, the ignition delay time of DME is the shortest, LPG50 is second, and LPG100 is the longest.

for 94.8%, is the main consumption pathway of CH3OCH3. CH3, the maximum contribution to the H-abstraction reaction, accounts for 69.73%. For the LPG50 mixtures, the consumption of C3H8 is still dominated by H-abstraction reactions. However, the contribution rate of OH increases to 44.34%, while that of H drops slightly to 35.86%. The proportion of H-abstraction reactions and the main consumption pathway of C4H10 increase when compared to neat LPG. In addition, the contribution of OH rises to 40.3%, while that of H drops slightly to 11.44%. The proportion of H-abstraction reactions, the main consumption pathway of CH3OCH3, also increases compared to neat DME. In addition, the contribution of CH3 drops significantly to 55.37%, but it is still the main H-abstraction reaction. Figure 9 gives the mole fraction of some small radicals (H, OH, O, HO2, and CH3) under the same conditions as Figure 9. As seen from the figure, the ignition timing increases with increasing DME blending ratio. For LPG100, this is approximately 170 μs; for LPG50, it is approximately 80 μs; and for DME, it is approximately 40 μs. The growth processes of the small radicals of the three mixtures are almost the same. All of them reach the peak around ignition timing. To compare, the concentrations of CH3 and HO2 were increased 3-fold. OH has the highest peak concentration, and that of H is the second highest. The peak concentration of O is slightly lower than that of H. However, the peak concentration of CH3 is far lower than that of OH, and that of HO2 is the lowest. For the OH peak concentration, DME > LPG50 > LPG100. For the CH3 peak concentration, the variation is the same with OH: DME > LPG50 > LPG100. For neat LPG, the H-abstraction reaction is dominated by OH and H in the oxidation process of C3H8 and C4H10. For mixtures, the proportion of OH participating in the oxidation process of C3H8 and C4H10 increases, while that of H decreases slightly; therefore, the ignition delay times of the mixtures are shorter than those of neat LPG. For neat DME, its Habstraction reaction is dominated by CH3. For mixtures, the proportion of CH3 participating in the H-abstraction reaction decreases significantly; therefore, the ignition delay times of mixtures are longer than those of neat DME. Figure 9d gives the concentration variance of H in the oxidation process of three fuels. As seen from the figure, the growth rates of H are different before ignition. That of LPG100 is the fastest, LPG50 is the second, and DME is the slowest. Through the preceding pathway analysis, it can be determined that, for neat LPG, 93.4% of C3H8 is consumed in Habstraction reactions and generate iso-C3H7, among which 70% decomposes into propylene and H, leading to the rapid growth of H. For neat DME, H comes mainly from the decomposition of a small amount of CH3O [CH3O (+M) = CH2O + H (+M)];42 therefore, the growth of H is relatively very slow. For LPG50, as shown in Figure 9d, the growth rate of H is in between that of LPG100 and DME. This is mainly because part of the H generated in the C3H8 oxidation process of the mixture is in competition with CH3OCH3 for H-abstraction reactions, similar to Zhang’s analysis of DME/ethane.22 The difference is that the sensitivity coefficient of R22 (H2O 2 = OH + OH) gradually increases as the DME blending ratio increases (as shown in Figure 7), resulting in the accumulation of OH and a far higher peak concentration than that of H; therefore, the reactivity of the whole system is greatly improved, and the ignition becomes easier. The relation of the OH peak concentration of the different mixtures is DME >

4. CONCLUSION In this study, ignition delay times of LPG/DME mixtures (LPG consists of 90% C3H8 and 10% C4H10 in moles) were measured using a shock tube at different DME blending ratios (0%, 10%, 30%, and 50%), pressures (5, 10, and 15 atm), temperatures (1100−1500 K), and equivalence ratios (0.5, 1.0, and 1.5). The chemical kinetic mechanism of LPG/DME was established based on LLNL’s C1−C4 mechanism and Zhao’s DME mechanism. Sensitivity analyses and reaction pathway analyses were conducted using CHEMKIN-PRO. Some main conclusions are as follows: (1) Ignition delay times decrease with increasing DME blending ratios at the same pressure and equivalence ratio. Ignition delay times increase with decreasing equivalence ratios at the same pressure and DME blending ratio in the temperature range of 1100−1400 K. When the temperatures become increasingly high, the differences of the ignition delay times decrease at three different equivalence ratios. Ignition delay times decrease with increasing pressures at the same DME blending ratios and equivalence ratios. And increasing the proportion of C4H10 in LPG (or decreasing the proportion of C3H8) inhibits ignition. (2) The chemical kinetic mechanism of LPG/DME was established based on LLNL’s C1−C4 mechanism and Zhao’s DME mechanism, and its predictions agree well with experimental data. (3) Ignition delay times of LPG/DME change linearly with increasing DME blending ratio at three different pressures and temperatures. (4) The sensitivity analysis shows that the number of major promoting reactions for mixtures increases, including H abstraction and decomposition of CH3OCH3, while the sensitivity factors of the H abstraction and decomposition of C3H8 (R115, R120, and R125) decrease with increasing DME blending ratios. (5) The reaction pathway analysis indicates that Habstraction reactions play the dominant role. For mixtures, the contribution rate of OH increases, while that of H decreases slightly in the oxidation of C3H8 and C4H10 compared to neat LPG, resulting in shorter ignition delay times of mixtures compared to neat LPG. (6) Further analysis shows that although the growth rate of H before ignition is LPG100 > LPG50 > DME, reaction R22 in the oxidation of mixtures makes OH accumulate rapidly in a short time, resulting in a much higher peak concentration of OH compared to H; therefore, the ignition delay times of mixtures are shorter than that of neat LPG.

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AUTHOR INFORMATION

Corresponding Authors

*(X.L.) Tel.: +86-21-34206039. E-mail: [email protected]. *(Z.H.) Tel.: +86-21-34206039. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Key Basic Research Projects of Science and Technology Commission of Shanghai (Grant 7175

dx.doi.org/10.1021/ef5014133 | Energy Fuels 2014, 28, 7168−7177

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