Shock Tube Measurements and Modeling Study on the Ignition Delay

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Shock Tube Measurements and Modeling Study on the Ignition Delay Times of n-Butanol/Dimethyl Ether Mixtures Zhuang Geng, Lili Xu, Hua Li, Jiaxing Wang, Zhen Huang, and Xingcai Lu Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/ef5009727 • Publication Date (Web): 27 May 2014 Downloaded from http://pubs.acs.org on May 30, 2014

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Shock Tube Measurements and Modeling Study on the Ignition Delay Times of n-Butanol/Dimethyl Ether Mixtures Zhuang Geng, Lili Xu, Hua Li, Jiaxing Wang, Zhen Huang, and Xingcai Lu ∗ Key Laboratory for Power Machinery of M.O.E, Shanghai Jiao Tong University, Shanghai 200240, PR China

ABSTRACT: The ignition delay times of different n-butanol/dimethyl ether (DME) mixtures (DME mole ratios of 100%, 80%, 60%, 40%, and 0%) were studied behind reflected shock waves at equivalence ratios of 0.5, 1.0, and 1.5; pressures of 6.0 bar, 10 bar, and 15 bar; and temperatures of 1150 K-1650 K. The effects of a carrier gas (nitrogen or argon) on the ignition delay times of single and blended fuels were also studied. The chemical kinetic mechanism of DME/n-butanol was established based on Zhao’s DME chemical kinetic mechanism (Int. J. Chem. Kinet 2008; 40:1-18) and Strathy’s n-butanol chemical kinetic mechanism (Combust and Flame 2012; 159: 2028–2055), which can accurately predict the ignition delay times of both single and blended fuels. Experimental results show that the ignition delay time of DME is longer than that of n-butanol when the temperature is high (>1150 k) and that the ignition delays of blended fuels increase with an increased blending ratio of DME. However, the relationship between ignition delay time and the blending ratio is non-linear. The main factor affecting the blended fuel’s ignition delay is temperature, and with every 200 oC

increase in temperature, the ignition delay times is reduced by an order of magnitude. Additionally, pressure has a large

effect on the ignition delay times. The results of reaction path analysis of blended fuels show that with an increasing DME blending ratio, the contribution rate of OH-radicals to H-abstraction decreases during the oxidation of n-butanol, and the contribution rate of H-radicals to H-abstraction of n-butanol increases slightly. However, with an increasing DME blending ratio, the pyrolysis of DME decreases, but the contribution rate of CH3 and OH to the H-abstraction of DME increases.

∗

Corresponding author. Tel: +86-21-34206039, Fax: +86-21-34205949. E-mail address: [email protected] (X. C. Lu) 1

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Key words: Ignition; N-butanol; DME; Binary fuels; Chemical Kinetics

1. INTRODUCTION Because of the growing shortage of oil resources and increasing environmental pollution problems, researchers around the world are seeking for clean alternative engine fuels and new efficient clean combustion modes. Since the late 1990s, the new generation combustion modes, which is represented by homogeneous charge compression ignition (HCCI) combustion, has attracted a wide spread attention. HCCI combustion has the similar thermal efficiency to the traditional diesel combustion model and has low NOx and soot emissions [1, 2]. However, the HCCI combustion mode differs from the traditional one by having no direct ignition control means, and its ignition and combustion processes are determined by the chemical reaction kinetics of the fuels [3]. Therefore, the HCCI combustion model is suffering from narrow operation range, high pressure rise under a high load, and high HC and CO emissions under a low load. To overcome these difficulties, concepts such as Stratified Charge Compression Ignition (SCCI), Premixed Compression Ignition (PCI), Low Temperature Combustion (LTC), and Partially Premixed Compression Ignition (PPCI) were proposed, with the working mechanism of these concepts still dominated by chemical reaction kinetics.

Currently, it has been realized that a single fuel (or fuels with fixed physicochemical properties) cannot achieve optimal efficiency and emission in the full engine operating range; fuel design and management must be used to control the advanced engine combustion mode [4]. Thus, new combustion modes have been proposed by many people. For example, Dual Fuel Sequence Combustion (DFSC) mode proposed by Lu et al, takes advantage of heat and active radicals that are produced by high cetane number fuels during the compression process to trigger and control the combustion of high octane number fuels, such as gasoline, which can achieve smokeless, ultra-low NOx emission over wide operating ranges [5-6]. Reitz et al. proposed the Reactivity Controlled Compression Ignition (RCCI) mode, which uses port injection of pure gasoline and direct injection of gasoline containing a small amount of di-t-butyl peroxide (DTBP) in the cylinder, can achieve a total efficiency of 57% [7-8]. These research activities show that future 2


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development direction of advanced internal combustion engine modes is toward multi-component fuel combustion based on fuel design.

On the other hand, no effort has been spared on the study of new alternative fuels. Currently, ethanol, which is used to partially replace gasoline or as an octane number additive in gasoline, has been widely applied internationally, but its fermentation processing mainly comes from a variety of food crops, which is a potential problem for human foods. Recently, a new biofuel, biological butanol, has been received strongly interest in worldwide [9-12]. Butanol is a suitable gasoline alternative because of its many advantages, such as the ability to mingle with diesel or gasoline in any proportion without the occurrence of phase separation, an octane number close to gasoline, a high oxygen content (21.6%), a heat value close to gasoline and diesel (9% lower), and the many sources from which it can be fermented (sugar, starch, and lignin converted from crops in a process similar with ethanol production). Biodiesel has begun to replace diesel in Europe and the USA in recent years. Use of a biodiesel engine can significantly reduce CO, HC and smoke emissions, but NOx will rise slightly [13]. Biodiesel is derived from the transesterification reaction between vegetable oil or animal fat and alcohol (methanol or ethanol), which are also used in the food supply, creating competition problems. Therefore, there has been an effort to develop new diesel alternative fuels. Many researches confirm that DME (dimethyl ether), which is obtained by methanol dehydration, has a high cetane number and contains a high amount of oxygen, and because methanol has many sources and a low cost, is an ideal alternative fuel for diesel [14-15].

Current alternative fuels can only partly substitute gasoline or diesel because of their small production level. Additionally, to meet the low emission and high combustion efficiency standards within the scope of full load, fuel design and management have become the most common methods of advanced combustion modes for controlling ignition timing and combustion rate [16]. Because butanol has a high octane number and a high amount of oxygen, the dual-fuel combustion mode or blend fuel combustion, such as butanol-diesel and butanol-biodiesel, created great interest for researchers, and research on the ignition and flame characteristics of butanol-containing blend fuels is very important.

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Haas et al. [17] used the ignition quality tester (IQT) to measure the derived cetane number (DCN) of different proportions of butanol/heptane mixtures for studying the reaction activity of the mixed fuels. Dagaut et al. [18] studied the reaction mechanism of butanol/heptane mixtures in the jet-stirred reactors (JSR). Saisirirat et al. [19] added ethanol or 1-butanol in heptane to compare the effect of two types of alcohol-fuel on the ignition of the mixed fuels. Togbe et al. [20] studied the combustion characteristics, the reactants, the intermediates and the products of methyl octanoate-butanol mixed fuel in the JSR at a pressure of 10 bar, temperatures of 560-1190 K, a residence time of 10.7 ms, and equivalence ratios of 0.5-2. Karwat et al. [21-22] sampled and analyzed 20% butanol/heptane and 50% butanol/n-heptane (stoichiometric mixture, dilution of 5.64) in the rapid compression machine (RCM) at compressed pressure of 9 atm and temperature of 700 K and compared these mixtures with n-heptane. Additionally, Zhang et al. [23] measured the ignition delay times of different proportions of n-heptane/butanol mixtures in shock tubes at high temperatures (1200-1500 K), equivalent ratios of 0.5 and 1, and pressures of 2 atm and 10 atm. Yang et al. [24-25] studied the ignition delay times of the binary blend fuels of butanol and its isomers with n-heptane in a rapid compression machine and expounded on the mechanism of interaction of the two types of fuel mixtures.

Because DME has a high cetane number, it is usually blended or combined with fuels with high octane number to improve the combustion efficiency and emissions of the engine. Lee et al. studied the soot emission characteristics of n-heptane/DME and LPG/DME mixtures [26-27]. Liu et al. used the dual fuel mode of DME and natural gas to realize high efficiency RCCI combustion [28]. Cha et al. studied the combustion characteristics of gasoline/DME mixtures [29], and Chen et al. studied the combustion characteristics of the dual fuel of methanol and DME [30]. Additionally, Wang et al. studied the PCCI combustion of the dual fuel of diesel and DME [31]. Because DME shows high reactivity in low temperature region, the proportion of DME in the blend fuels has an important role in the mechanism of ignition, combustion process and emissions formation. Therefore, the study of the combustion reaction mechanism of DME-containing mixtures is very important. Xi'an Jiao Tong University used shock tube to measure the ignition delay

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times of DME/methane [32], DME/ethane [33], DME/propane [34] and DME/butane [35-37] mixtures and analyzed the effect of DME on the reaction path of the mixtures. Frassoldati et al. [38] studied the difference in low-pressure premixed flames between propylene/ethanol and propylene/DME mixtures. Chen, et al. [39] studied the high temperature ignition and laminar flame speed of flame propagation of methane/DME mixtures. Lowry et al. [40] studied flame stability and the laminar flame speed of methane/DME mixtures. According to fuel design theory, butanol contributes to the inhibition knock under high load because it has a high octane number. DME can promote combustion in a wide operating range because it has a high cetane number. Thus, butanol/DME mixtures can achieve efficient and clean combustion in a wide range of engine loads. However, there have been no studies on butanol/DME mixtures so far. Therefore, in this study, experiments in shock tubes were first conducted to examine the effects of DME ratio, pressure, and equivalence ratio on the ignition delay times of DME/n-butanol mixtures. Through sensitivity analysis and reaction path analysis, this paper reveals the main primary reactions affecting the ignition delay times of blend fuels and the evolutions of the main reaction pathways with different DME ratios. Moreover, this research will helpful to provide the theoretical foundation for the efficient clean combustion of DME/butanol mixtures.

2. Experimental approach Figure 1 show the shock tube, which consists of a 6 m driver section, a 5 m driven section and a diaphragm section. The inner diameter of the shock tube is 90 mm, and the diaphragm is a polyethylene terephthalate (PET) diaphragm. Five pressure transducers (PCB 113B26) are used to record the arrival time of the shock waves; the intervals among the transducers are 333 mm. One of the five pressure transducers is used to trace the pressure of the reflected shock wave, which is located 20 mm from the end wall of the shock tube. The OH* signal, whose wavelength is 307 nm, is detected by a photomultiplier (Hamamatsu, R928). All the pressure signals and the OH* signal are recorded and saved by an oscillograph. Many experimental results show that the decay rate of the shock wave in the shock tube is less than 3%. All 5


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the valves in the shock tube are Swagelok valves to ensure excellent sealing. The shock tube theoretical formulas are used to calculate temperature and pressure behind the reflected shock wave [41]. The temperature error behind the reflected shock wave can be controlled at approximately 25 K.

Fuel mixtures were prepared in a 210 L stainless-steel tank according to Dalton’s laws of partial pressure. To ensure sufficient blending of different fuels, the prepared mixture was allowed to settle at least 12 hours. Before the preparation of gas mixture, a Nanguang vacuum pump was used to pump the tank lower than 3×10-2 Pa, and the liquid fuel was weighed using an analytical balance (Mettler Toledo) with a weighing accuracy of 0.001%. The experiments used 99.999% helium as a driver gas, and the argon and oxygen used are of the same 99.999% purity. The purities of the DME and n-butanol used are higher than 99.5%. Before the experiments, the driver section, the driven section, and the entire pipeline were pumped to lower than 3×10-2 Pa, and the leakage rate of the shock tube was less than 0.1-0.5 Pa/min. Table 1 shows the detailed mixture compositions for all the test conditions. All the DME/n-butanol/oxygen/argon mixtures (φ=0.5, 1.0, 1.5, XO2/XAr=21:79) were further diluted with argon (20% mixture/80% argon).

The ignition delay time is measured according to the pressure of the reflected shock wave and the change in the OH* signal. Previous studies have shown that the emission signal measurement method is the most simple and reliable method to measure the ignition delay times of mixtures [42-43]. The ignition delay time is defined as the time interval between the time that the reflected shock wave reaches the fixed point and the fastest growing starting point of the OH* signal, as shown in figure 2.

3. Kinetic model validation 3.1

Reliability of the Ignition delay

To ensure the reliability of the experimental results of this shock tube (denoted as SJTU in the paper), the ignition delay times of n-heptane and butanol were measured and compared with the experimental data from Stanford University and McGill University. Figure 3(a) shows the experimental results of SJTU and Stanford University for the ignition delay 6


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times of n-heptane at an equivalence ratio of 1.0, a pressure of 10 bar, and temperatures of 1100-1350 K. Experiments used N2 as the carrier gas, and the molar ratio of n-heptane/O2/N2 was 1.874%/20.6%/77.526%. The test results are consistent with the Stanford data [44] in the temperature range of 1250-1350 K, and they obey the Arrhenius rules in the temperature range of 1100-1350 K. To further verify the reliability of the shock tube, the experimental data of this paper were compared to the data from McGill University [45], as shown in figure 3(b). Ar was used as the carrier gas, and the conditions were a ratio of 1, a pressure of 10 bar, temperatures of 1150-1550 K, and a molar ratio of n-butanol/O2/Ar of 0.8%/4.7%/94.5%. As seen from the graph, the experimental data of SJTU and McGill are in good agreement. In summary, the experimental results of this shock tube are reliable.

3.2 Model development and validation

A mixed DME/n-butanol mechanism was constructed according to the method proposed by L.R.Cancino et al. [46], which has been used by many researchers [47-48]. In this paper, the DME mechanism of Zhao [46] and the n-butanol mechanism of Sarathy [49] were combined to form the DME/n-butanol blend fuel mechanism. The n-butanol mechanism of Sarathy can be applied to a wide temperature range and various isomers. The construction process of the mechanism is to put the DME mechanism and n-butanol in the same file and delete the repeated chemical reactions in the n-butanol mechanism. The new blending mechanism consists of 433 species and 2416 reactions. Chemkin-Pro software and a zero-dimensional and constant-volume adiabatic model were used to perform the ignition delay time simulations, sensitivity analysis, and the reaction pathway analysis.

In general, a detailed and accurate mechanism of blend fuel is able to predict not only the ignition delay times of the mixtures but also that of the single-component fuel. Figure 4 shows the comparison of the calculated and experimental ignition delay times of the blend fuels mechanism and the neat fuel mechanism. In Figure 4 (a), DME/O2/Ar mixtures are at an equivalence ratio of 1.0, a pressure of 10 bar, and temperatures of 1150-1600 K. Experimental and calculated results are consistent very well. Additionally, as seen from the figure, the calculated results of the blend fuel mechanism 7


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are in better agreement with the experimental results than Zhao’s mechanism at high temperature. However, at low temperature, the calculated results for the blend fuel mechanism have some deviations but are still close to the experimental values and the calculated results for the Zhao’s mechanism. In Figure 4 (b), n-butanol/O2/Ar mixtures are at an equivalence ratio of 1.0, a pressure of 10 bar, and temperatures of 1200-1500 K. The experimental and calculated results are agreement very well, and the results of the blend fuel mechanism are better than those of Sarathy’s mechanism over the entire temperature range.

4. Results and discussion Figure 5 shows the ignition delay times of neat DME, 80% DME/20% n-butanol, 60% DME/40% n-butanol, 40% DME/60% n-butanol, and neat butanol (referred to as M100, M80, M60, M40, and M0, respectively) at an equivalence ratio of 1.0 and a pressure of 10 bar. As seen from the figure, the ignition delay time of neat DME is the longest, whereas the ignition delay time of neat butanol is shortest over the experimental temperature range of this paper. As the n-butanol proportion increases, the ignition delay times of the mixtures decrease gradually. Additionally, the calculated results of the blend fuel mechanism are consistent with the experimental results, which can also accurately predict the ignition delay times of DME/n-butanol mixtures for a wide range of temperatures.

The main factors affecting the ignition delay times of the blend fuels include not only the respective proportion of fuels and temperature but also the pressure and the equivalence ratio. Fig 6 shows the effect of the equivalence ratio and the pressure on the ignition delay times of 60% DME/40% n-butanol mixtures (M60). As shown in figure 6(a), at an equivalence ratio of 1 and pressures of 6 bar, 10 bar and 15 bar, the ignition delay times of M60 decrease with the increase of pressure, and the three curves are almost parallel, which shows that pressure shows negligible effect on the activation energy. Figure 6 (b) shows the ignition delay times of M60 at a pressure of 10 bar and equivalence ratios of 1.0, 1.5 and under 0.5. As seen from the figure, the ignition delay times decrease with the increase of equivalent ratio when the temperature is in the range of 1100-1300 K. At temperatures higher than 1300 K, the ignition delay times change in a 8


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small ranges. This result is consistent with previous studies [33] and shows that in the high temperature phase, the ignition delay times of DME/n-butanol mixtures change little with the equivalence ratio, which is mainly because high temperature conditions can provide enough energy for the chemical reactions.

For the shock tube experiments, the thermal properties of the carrier gas have some effects on the measured results. Currently, nitrogen or argon is widely used as a carrier gas. Figure 7 compares the effect of the carrier gas on the ignition delay times of a single fuel and a binary mixed fuel. Figure 7 (a) shows the ignition delay times of DME/O2/Ar (1.309%/3.927%/94.764%) and DME/O2/N2 (1.309%/3.927%/94.764%) at an equivalence ratio of 1.0 and a pressure of 10 bar. As seen from the chart, the experimental results are consistent with the calculated results, and the ignition delay times of DME/O2/N2 are longer than those of DME/O2/Ar. Figure 7 (b) compares the ignition delay times of 80% DME/20% n-butanol/O2/Ar (0.8822%/0.2204%/3.9664%/94.931%) with those of 80% DME/20% n-butanol/O2/N2 (0.8822%/0.2204%/3.9664%/94.931%), which are similar to Figure 7 (a). These results are also consistent with previous research results [51]. Because the heat capacity of Ar is smaller than the heat capacity of N2, the shock tube using Ar as the dilution gas will dissipate less heat than the shock tube using N2, thus providing better conditions for the blend fuel to ignite.

Figure 8 shows the change in ignition delay times for DME blending ratios with different temperatures and pressures. In generally, the ignition delay times are not completely consistent with the Arrhenius laws in the high temperature regions, and it is difficult to keep the temperature behind the reflected shock wave at a constant value. However, as previously stated, the calculated results of the blending mechanism of this paper are consistent with the experimental results; thus, the calculated results of the blend fuel mechanism of this paper are used for comparison. It can be seen from the figure that the relationship between the ignition delay times of the DME/n-butanol mixtures and the DME blending ratios is not linear. The change in ignition delay times of the mixtures with DME blending ratios above 50% is faster than the change in ignition delay times of the mixtures with DME blending ratios less than 50%. Additionally, as

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shown in Figure 8, the main factor affecting the ignition delay time in the high temperature range is temperature level. For each temperature rise of 200°C, ignition delay times are reduced by an order of magnitude. Additionally, the effect of pressure cannot be ignored.

To further analyze the effect of the DME ratio on the ignition delay times of DME/n-butanol mixtures, the reaction pathways of the neat DME (M100), 60% DME/40% n-butanol (M60), and neat n-butanol (M0) at a temperature of 1400 K, pressure of 10 bar, and equivalence ratio of 1.0 were studied. The time at 20% fuel consumption was chosen as the computation time, and the calculation results are shown in Figure 9. The dehydrogenation reaction of neat n-butanol occurs mainly by the OH, H, HO2, CH3, O, and the third body (M) participation, in which C4H8OH-1 and C4H8OH-3 almost completely decomposition to C2H3OH+C2H5 and CH2OH+C3H6, respectively, at high temperature. However, neat DME mainly has two types of reaction at high temperature: pyrolysis to CH3O+ CH3 and a dehydrogenation reaction in which CH3, H, OH, HO2, and CH3 radicals participate. The contribution rate of CH3 to the H-abstraction is 67.92%, whereas the contribution rate of OH is only 6.58%. Thus, the ignition delay time of DME is slightly longer than that of n-butanol at high temperature. For DME/n-butanol mixtures, the dehydrogenation reaction ratio of OH in alpha and beta positions decreases from 23.59% and 8.09% to 17.36% and 6.02%, respectively. The ratio of n-butanol that participates in the H-abstraction reaction with OH and generates C4H8OH-4 decreased from 7.83% to 5.75%. For DME in the M60 mixtures, the proportion of participation of CH3 in direct pyrolysis and dehydrogenation reactions decreased significantly. However, the dehydrogenation reaction in which H and OH participate increased.

Figure 10 shows the mole fractions of some free radicals (H, OH, O, HO2, CH3) with time at the same conditions as Figure 9. As seen from the figure, for neat DME, neat n-butanol, and M60 mixtures, the peak values of the selected radicals appears around the ignition timing, and the concentration of O is slightly lower than that of H and OH. The concentrations of the CH3 and HO2 radicals are much lower than the concentrations of H, OH, and O. According to figure 9, the dehydrogenation of DME occurs mainly by the participation of CH3, but the peak concentration is very low;

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thus, the ignition delay time of DME is longer. For n-butanol, the dehydrogenation reaction occurs mainly by OH and H radicals, and the peak concentration of the two types of free radicals is higher; thus, the ignition delay time is shorter. For the DME/n-butanol mixtures, the concentration of the five small radicals is close to that of neat fuel; however, the proportion of OH in n-butanol’s dehydrogenation oxidation process in the mixed fuel decreases significantly, and the contribution rate of H increases. The dehydrogenation reaction of DME in mixed fuels via CH3 decreases significantly, and the contribution rate of OH increases significantly. Therefore, from a macro point of view, ignition delay times of mixed fuels are slightly longer than those of neat n-butanol. Figure 10 (d) shows the CH3 concentration change of several mixtures (M100, M80, M60, M40, M0). DME produces a large amount of CH3 by high-temperature decomposition, and the activity of CH3 is weaker than that of OH and H radicals; thus, the ignition delay of neat DME is longer than that of other fuels (M80, M60, M40, M0).

To further explore the main primitive reactions in DME/n-butanol mixtures that affect ignition, Chemkin-Pro software was used to conduct the sensitivity analysis of different DME/n-butanol mixtures. The following formula can be used to calculate sensitivity index.

Si =

τ（2ki）（0. -τ 5ki） 1.5τ（ki） ,

where

Si is the sensitivity index of the ignition delay time, τ is ignition delay time, and ki is the rate coefficient

of the ith reaction. A negative sensitivity index indicates ignition promotion. Figure 11 shows the 15 most sensitive reactions for the DME/n-butanol mixtures (M100, M80, M60 and M0) at an equivalence ratio of 1.0, a temperature of 1400 K, and a pressure of 10 bar. The results are shown in Figure 11.

For neat DME (M100), the following reactions that have the greatest influence on the ignition delay:

H+O2=O+OH

(R1)

CH3OCH3=CH3+CH3O

(R499)

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CH3+HO2=CH3O+OH

(R172)

CH3OCH3+H=CH3OCH2+H2

CH3+CH3+(M)= C2H6+(M)

CH3+HO2=CH4+O2

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(R501)

(R225)

(R173)

Among them, reaction R1 is a typical branch chain reaction that produces OH radicals, which are very active and can effectively shorten the ignition delay time. Reaction R499 also has a large negative sensitivity coefficient, which produces a large amount of CH3 and contributes to the H-abstraction of DME, thus playing a large role in promoting ignition. Additionally, reaction R172 produces a highly active OH and a weaker CH3O by the consumption of two weak active radicals, CH3 and HO2. Reaction R501 has the greatest positive sensitivity coefficient, which consumes a large amount of H and competes with R1 to win H radicals, thus inhibiting the ignition of DME. Reaction R225 consumes a large amount of CH3 and restrains the H-abstraction of DME. Reaction R173 consumes CH3 and HO2 and also hinders the reaction of other small radicals to restrain the ignition of DME.

For neat n-butanol (M0), the same reactions, R1 (H + O2 = O + OH) and R172 (CH3 + HO2 = CH3O + OH), have the highest negative sensitivity coefficients. These two reactions generate a large amount of OH, which can effectively promote other reactions and contribute to the ignition of n-butanol. Reaction R1614 (NC4H9OH (+ M) = NC3H7 + CH2OH (+ M)) is the decomposition reaction of n-butanol; thus, it has a direct effect on the ignition of n-butanol. The products (NC3H7 and CH2OH) of reaction R1614 can further decompose to form small groups and provide

intermediate

materials

for

other

reactions.

Reactions

R19

(HO2+OH=H2O+O2)

and

R612

(C3H5-A+H(+M)=C3H6(+M)) have the greatest positive sensitivity coefficients, which consume a large amount of OH and

H.

Reactions

R612

(C3H5-A+H(+M)=C3H6(+M)),

R151

(CH3+H(+M)=CH4(+M)),

and

R1124

(NC4H9OH+H=C4H8OH-2+H2) have a strong inhibitory effect on the ignition of n-butanol.

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When mixing n-butanol in DME (M80, M60), reactions that stimulate ignition are similar to those of M100 and M0 mixtures. Reactions R1, R172 and R49 have larger negative sensitivities, whereas reactions R19, R173, R225 and R501 have larger positive sensitivities. From the sensitivity analyses, when n-butanol is mixed in DME, the negative sensitivity coefficients of R35 (HCO+O2=CO+HO2), R76 (CH2O+OH=HCO+H2O), R80 (CH2O+HO2=HCO+H2O2), R172 (CH3+HO2=CH3O+OH), R499 (CH3OCH3=CH3+CH3), and R505 (CH3OCH3+CH3=CH3OCH2+CH4) decrease; however,

the

negative

sensitivity

coefficients

of

reactions

R649

(C3H4-A+H=C3H5-A),

R1613

(NC4H9OH(+M)=C2H5+PC2H4OH(+M)), and R1614 (NC4H9OH(+M)=NC3H7+CH2OH(+M)) increase. Moreover, when blending n-butanol in DME, it is easier for DME/n-butanol mixtures to ignite.

5. Conclusion This paper studied the ignition delay times of different DME/n-butanol mixtures (DME mole ratio is 100%, 80%, 60%, 40%, and 0%, respectively) behind reflected shock waves at equivalence ratios of 0.5, 1.0, and 1.5, pressures of 6.0 bar, 10 bar, and 15 bar, and temperatures of 1150-1650 K. Additionally, ignition delay times of M60 mixtures were measured with different pressures, temperatures, and carrier gases (Ar and N2). The mechanism of the DME/n-butanol mixtures was established based on the DME mechanism of Zhao and the n-butanol mechanism of Sarathy, and sensitivity and reaction path analyses were performed. In this paper, some conclusions can be obtained:

(1) At high temperature (>1200 K), the ignition delay of DME is longer than that of n-butanol. Thus, with the increase in the DME mixed ratio, the ignition delay times of the DME/n-butanol mixtures increase, but the relationship between ignition delay time and mixing proportion is nonlinear.

(2) The main factor influencing the ignition delay of the DME/n-butanol mixtures is temperature level, and with every 200°C increase in temperature, the ignition delay time of the mixtures decreases by an order of magnitude. (3) In this paper, the DME/n-butanol oxidation reaction kinetics mechanism was built, which not only predicts the ignition delay of a single fuel but also predicts the ignition delay times of binary fuel mixtures in wide ranges of 13


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Page 14 of 30

temperature, pressure, and mixture ratio.

(4) The experimental results of the effect of a carrier gas (Ar or N2) for a single fuel composition and fuel mixtures show that the ignition delay is shorter with argon as the carrier gas than with nitrogen.

(5) The reaction path and the concentration analysis of the main reactants indicate that the ignition delay time of DME is longer under high temperature because the pyrolysis of DME produces relatively high levels of methyl, but with weak activity. The H-abstraction of n-butanol occurs by the dehydrogenation reaction of OH and H in the alpha and gamma position, respectively. The concentration of the two radicals is higher and the activity is stronger; thus, lead to a shorter ignition delay.

(6) For the oxidation process of DME/n-butanol mixtures, the contribution rate of OH to the dehydrogenation of n-butanol decreases, and the contribution rate of H to the dehydrogenation of n-butanol increases slightly. However, for the oxidation of DME in the mixtures, the pyrolysis of DME decreases, but the contribution rate of CH3 and OH to the dehydrogenation of DME increases significantly. Thus, for DME/n-butanol mixtures, the rate of DME oxidation increases, whereas the rate of n-butanol decreases.

(7) Sensitivity analysis shows that the ignition of DME/n-butanol mixtures relies more on the reactions that produce small radicals (H, OH, O2, HO2, and CH3), such as H+O2=O+OH, CH3OCH3=CH3+CH3O, and CH3+HO2=CH3O+OH.

ASSOCIATED CONTENT Supporting Information The chemical kinetic mechanism and experimental data of DME/n-butanol. This material is available free of

charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION

14


Page 15 of 30

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Energy & Fuels

Corresponding Author *Telephone: 86-21-34206039. E-mail: [email protected].

Notes The authors declare no competing financial interest.

Acknowledgement This work was supported by the National 973 Major Basic Project (2013CB228405).

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[2]

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Splitter, D.; Reitz, R.; Hanson, R. High Efficiency, Low emissions RCCI combustion by use of a fuel additive. SAE International, 2010-01-2167.

[8]

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Atsumi, S.; Hanai, T.; Liao, J. C. Non-fermentative pathways for synthesis of branched-chain higher alcohols as biofuels. Nature, 2008, 451,86-90.

[10] Dürre, P. Biobutanol: An attractive biofuel. Biophys. J . 2007, 2, 1525-1534. [11] Kohse-Hoinghaüs, K.; Obald, P.; A. Cool, T. A.; Kasper, T.; Hansen, N.; Qi, F.; Westbrook, C. K.; Westmoreland, P. R. Biofuel Combustion Chemistry: From Ethanol to Biodiesel. Angew. Chem. Int. Ed. 2010, 49, 3572-3597. [12] J, Chao.; Yao, M. F.; Liu, H. F.; Lee, C. F.; Ji, J. Progress in the production and application of n-butanol as a biofuel . Renew Sust Energy Rev 2011, 15, 4080-4106. [13] Zhu, L.; Cheung, C. S.; Zhang, W. G.; Huang, Z. Emissions characteristics of a diesel engine operating on biodiesel and biodiesel blended with ethanol and methanol. Sci. Total Environ. 2010, 408, 914-921. [14] Arcoumanis, C.; Bae, C.; Crookes, R.; Kinoshita, E. The potential of dimethyl ether (DME) as an alternative fuel for

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compression-ignition engines: A review. Fuel 2008, 87 (7), 1014−1030. [15] Park, S. H.; Lee, C. S. Combustion performance and emission reduction characteristics of automotive DME engine system. Prog. Energy Combust. Sci. 2013, 39(1), 147-168. [16] Andrae, J. C. G. Comprehensive chemical kinetic modeling of toluene reference fuels oxidation. Fuel 2013, 107: 740-748. [17] Haas, F. M.; Ramcharan, A.; Dryer, F. L. Relative Reactivities of the Isomeric Butanols and Ethanol in an Ignition Quality Tester. Energy Fuels 2011, 25, 3909-16. [18] Dagaut, P.; Togbe, C. Experimental and Modeling Study of the Kinetics of Oxidation of Butanol-n-Heptane Mixtures in a Jet-stirred Reactor. Energy Fuels 2009, 23, 3527-3535. [19] Saisirirat, P.; Foucher, F.; Chanchaona, S.; Mounaım-Rousselle, C. Spectroscopic Measurements of Low-Temperature Heat Release for Homogeneous Combustion Compression Ignition (HCCI) n-Heptane/Alcohol Mixture Combustion. Energy Fuels 2010, 24, 5404–5409. [20] Togbe, C.; Dayma, G.; Mze-Ahmed, A.; Dagaut, P. Experimental and Modeling Study of the Kinetics of Oxidation of Simple Biodiesel−Biobutanol Surrogates: Methyl Octanoate−Butanol Mixtures. Energy Fuels 2010, 24, 3906–3916. [21] Karwat, D. M. A.; Wagnon, S. W.; Wooldridge, M. S.; Westbrook, C. K. On the Combustion Chemistry of n-Heptane and n-Butanol Blends. J. Phys. Chem. A 2012, 116, 12406–12421. [22] Karwat, D. M. A.; Wagnon, S. W.; Teini, P. D.; Wooldridge, M. S. On the Chemical Kinetics of n-Butanol: Ignition and Speciation Studies. J. Phys. Chem. A 2011, 115, 4909–4921. [23] Zhang, J. X.; Niu, S. D.; Zhang, Y. J. ; Tang, C. L. ; Jiang, X.; Hu, E. J.; Huang, Z. H. Experimental and modeling study of the auto-ignition of n-heptane/n-butanol mixtures. Combust. Flame 2013, 160, 31-39. [24] Yang, Z.; Qian, Y.; Yang, X.; Wang, Y.; Wang, Y.; Huang, Z.; Lu, X. C. Autoignition of n-Butanol/n-Heptane Blend Fuels in a Rapid Compression Machine under Low-to-Medium Temperature Ranges. Energy Fuels 2013, 27, 7800−7808. [25] Yang, Z.; Wang, Y.; Yang, X.; Qian, Y.; Lu, X. C.; Huang, Z.; Autoignition of butanol isomers/n-heptane blend fuels on a rapid compression machine in N2/O2/Ar mixtures. Sci China Tech Sci 2014, 57: 461-470 [26] Lee, S.; Oh, S.; Choi, Y.; Kang, K. Performance and emission characteristics of a CI engine operated with n-Butane blended DME fuel. Appl. Therm. Eng. 2011, 31,1929-1935. [27] Lee, S.; Oh, S.; Choi, Y.; Kang, K. Effect of n-Butane and propane on performance and emission characteristics of an SI engine operated with DME-blended LPG fuel. Fuel 2011, 90, 1674-1680. [28] Liu, J.; Yang, F. Y.; Wang, H. W.; Ouyang, M. G. Numerical study of hydrogen addition to DME/CH4 dual fuel RCCI engine. Int. J. Hydrogen Energy 2012, 37, 8688-8697. [29] Cha, J.; Kwon, S.; Kwon, S.; Park, S. Combustion and emission characteristics of a gasoline-dimethyl ether dual-fuel engine. Proceedings of The Institution Of Mechanical Engineers Part D-Journal Of Automobile Engineering 2012, 226(D12), 1667-1677. [30] Chen, Z.; Yao, M.; Zheng, Z. Q.; Zhang, Q. C. Experimental and Numerical Study of Methanol/Dimethyl Ether Dual-Fuel Compound Combustion. Energy Fuels 2009, 23, 2719–2730. [31] Wang, Y.; Zhao, Y. W.; Yang, Z. L.. Dimethyl ether energy ratio effects in a dimethyl ether-diesel dual fuel premixed charge compression ignition engine. Appl. Therm. Eng. 2013, 54, 481-487. [32] Tang, C. L.; Wei, L. J.; Zhang, J. X.; Man, X. J.; Huang, Z. H. Shock Tube Measurements and Kinetic Investigation on the Ignition Delay Times of Methane/Dimethyl Ether Mixtures. Energy Fuels 2012, 26 (11), 6720–6728. [33] Zhang, J. X.; Hu, E. J.; Pan, L.; Zhang, Z. H.; Huang, Z. H. Shock-Tube Measurements of Ignition Delay Times for the Ethane/Dimethyl Ether Blends. Energy Fuels 2013, 27 (10), 6247–6254. [34] Hu, E. J.; Zhang, Z. H.; Pan, L.; Zhang, J. X.; Huang, Z. H. Experimental and Modeling Study on Ignition Delay Times of Dimethyl Ether/Propane/Oxygen/Argon Mixtures at 20 bar. Energy Fuels 2013, 27 (7), 4007–4013.

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[35] Hu, E. J.; Jiang, X.; Huang, Z. H, Zhang, J. X.; Zhang, Z. H.; Man, X. J. Experimental and Kinetic Studies on Ignition Delay Times of Dimethyl Ether/n-Butane/O2/Ar Mixtures. Energy Fuels 2013, 27 (1), 530–536. [36] Jiang, X.; Zhang, Y. J.; Man, X. J.; Pan, L.; Huang, Z. H. Shock Tube Measurements and Kinetic Study on Ignition Delay Times of Lean DME/n-Butane Blends at Elevated Pressures. Energy Fuels 2013, 27 (10), 6238–6246. [37] Jiang, X.; Zhang, Y. J.; Man, X. J.; Pan, L.; Huang, Z. H. Experimental and Modeling Study on Ignition Delay Times of Dimethyl Ether/n-Butane Blends at a Pressure of 2.0 MPa. Energy Fuels 2014, 28 (3):2189–2198 [38] Frassoldati, A.; Faravelli, T.; Ranzi, E.; Kohse-Höinghaus, K.; Westmoreland, P. R. Kinetic modeling study of ethanol and dimethyl ether addition to premixed low-pressure propene-oxygen-argon flames. Combust. Flame 2011, 158, 1264-1276. [39] Chen, Z.; Qin, X.; Ju, Y. G.; Zhao, Z. W.; Chaos, M.; Dryer, F. L. High temperature ignition and combustion enhancement by dimethyl ether addition to methane–air mixtures. Proc. Combust. Ins.2007, 1, 1215-1222. [40]

Lowry, W. B.; Serinyel, Z.; Krejci, M. C.; Curran, H. J.; Bourque, G. ; Petersen E.L. Effect of methane-dimethyl ether fuel blends on flame stability, laminar flame speed, and Markstein length. Proc. Combust. Ins. 2011, 33, 929-937.

[41] Gaydon, A. G.; D.Sc; F.R.S.; Hurle, I. R. The shock tube in high-temperature chemical physics. Chapman and Hall LTD.London.1963, 13-27. [42] Petersen, E. L.; Hall, J. M.; Smith, S. D.; Vries, J.de; Amadio, A.R.; Crofton, M.W. Ignition of lean methane-based fuel blends at gas turbine pressures. J. Eng. Gas Turbines Power 2007, 129,937-944. [43] Richard, M. J. A.; Hall, J. M.; Petersen, E. L. Effect of silane addition on acetylene ignition behind reflected shock waves. Proc. Combust. Ins. 2005, 30, 1915-1923. [44] Gauthier, B. M.; Davidson, D. F.; Hanson, R.K. Shock tube determination of ignition delay times in full-blend and surrogate fuel mixtures. Combust. Flame 2004, 3, 300-311. [45] Noorani, K. E.; Akih-Kumgeh, B.; Bergthorson, F. M. Comparative high temperature shock tube ignition of C1-C4 primary alcohols. Energy Fuels 2010, 24, 5834-5843. [46] Cancino, L. R.; Fikri, M.; Oliveira, A. A. M.; Schulz, C. Autoignition of gasoline surrogate mixtures at intermediate temperatures and high pressures: Experimental and numerical approaches. Proc. Combust. Ins. 2009, 32, 501-508. [47] Cancino, L. R.; Fikri, M.; Oliveira, A. A. M.; Schulz, C. Ignition delay times of ethanol containing multi-component gasoline surrogates: shock-tube experiments and detailed modeling. Fuel 2011, 3, 1238-1244. [48] Cancino L R. Development and Application of Detailed Chemical Kinetics Mechanisms for Ethanol and Ethanol Containing Hydrocarbon Fuels. Doctoral thesis, Federal University of Santa Catarina, Florianopolis, Brazil, 2009. [49] Zhao, Z. W.; Chaos, M.; Kazakov, A.; Dryer, F. L. Thermal decomposition reaction and a comprehensive kinetic model of dimethyl ether. Int. J. Chem. Kinet. 2008, 40, 1-18. [50] Sarathy, S. M.; Vranckx, S.; Yasunaga, K.; Mehl, M.; Oßwald, P.; Metcalfe, W. K.; Westbrook, C. K.; Pitz, W. J.; Kohse-Höinghaus, K.; Fernandes R. X.; Curran, H. J. A comprehensive chemical kinetic combustion model for the four butanol isomers. Combust. Flame 2012, 159(6), 2028–2055. [51] Würmel, J.; Silke, E. J.; Curran, H. J.; Ó Conaire, M. S.; Simmie, J. M. The effect of diluent gases on ignition delay times in shock tube and in the rapid compression machine. Combust. Flame 2007, 151, 289–302.

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Page 18 of 30

Table 1 Mixture composition in this study Figure 1. Schematic of the shock tube and mixture preparing system. Figure 2. Definition of ignition delay time (80% DME/20% n-butanol, φ=1, P=10 bar). Figure 3. Comparison of the current results with the literature data (φ=1, P=10 bar). Figure 4. Comparison of the current experimental and calculated results with the literature data at φ=1. Figure 5. Experimental and calculated results of the stoichiometric DME/n-butanol blends at φ=1.0, P=10 bar (Exps are the experimental results, and Mods are the calculated results for the blending model). Figure 6. Experimental and calculated results for 60% DME/40% n-butanol (M60) at different pressures and equivalence ratios. Figure 7. Effects of the carrier gases (Ar or N2) on the ignition delay times of DME and DME/n-butanol blend fuel at φ=1.0, P=10 bar. Figure 8. Ignition delay times versus DME blending ratio Figure 9. Reaction pathways for three fuel mixtures (M100, M60 and M0) at T=1400 K, P=10 bar, φ=1.0, and 20% fuel consumption. Figure 10. Mole fractions of some free radicals (H, O, OH, HO2, and CH3) for fuel mixtures (M100, M80, M60, M40, M0) at φ=1.0, T=1400 K, p=10 bar. Figure 11. Normalized sensitivity of the ignition delay for various DME blending ratios at φ=1.0, T=1400 K,P=10 bar.

18


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Energy & Fuels

Table 1 Mixture composition in this study n-butanol

O2

E%

%

%

1.0

1.3090

0

80%DME+20%n-butanol

1.0

0.8822

60%DME+40%n-butanol

1.0

40%DME+60%n-butanol 60%DME+40%n-butanol

Mixtures

Equivalence ratio (φ)

Neat DME

DM

Ar%

P(bar)

3.9270

94.7640

10

0.2204

3.9664

94.9310

10

0.5716

0.3809

3.9994

95.0481

10

1.0

0.3355

0.5030

4.0239

95.1376

10

0.5

0.2928

0.1951

4.0992

95.4129

10

60%DME+40%n-butanol

1.5

0.8375

0.5581

3.9080

94.6964

10

60%DME+40%n-butanol

1.0

0.5716

0.3811

4.0015

95.0457

6

60%DME+40%n-butanol

1.0

0.5714

0.3810

4.0002

95.0474

15

Neat n-butanol

1.0

0

0.6761

4.0565

95.2674

10

19


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Figure 1. Schematic of the shock tube and mixture preparing system.

20


Page 21 of 30

20

Endwall OH* emission

15

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

Endwall pressure 10

Reflected wave 5

t = 488µ s 0 600

800

1000

1200

1400

Time(µs) Figure 2. Definition of ignition delay time (80% DME/20% n-butanol, φ=1, P=10 bar).

21


Energy & Fuels

10000

10000

n-butanol_McGill n-butanol_SJTU

Ignition delay times(µs)

n-heptane_Stanford n-heptane_SJTU

Ignition delay times(µ s)

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 30

1000

100

10 0.70

1000

100

10

1

0.75

0.80

0.85

0.90

0.95

0.65

-1

0.70

0.75

1000/T(K

1000/T(K )

(a)

-1

0.80

0.85

0.90

)

(b)

Figure 3. Comparison of the current results with the literature data (φ=1, P=10 bar).

22


Page 23 of 30

Exp,SJTU_n-butannol_10bar Exp,XJTU_n-butannol_10bar Sarathy model_10bar new blend model_10bar

1000

Ignition delay time(µs)

1000

Ignition delay time(µs)

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

100

100

Exp,SJTU_DME_10bar Exp,XJTU_DME_10bar Zhao model_10bar new blend model_10bar

10 0.65

0.70

0.75

-1 1000/T(K )

0.80

0.85

10

0.65

0.70

(a)

0.75

-1 1000/T (K )

0.80

0.85

(b)

Figure 4. Comparison of the current experimental and calculated results with the literature data at φ=1.

23


Energy & Fuels

1000


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 30

Exp_M100 Exp_M80 Exp_M60 Exp_M40 Exp_M0 Mod_M100 Mod_M80 Mod_M60 Mod_M40 Mod_M0

100

10 0.65

0.70

0.75

0.80

0.85

-1 1000/T(K ) Figure 5. Experimental and calculated results of the stoichiometric DME/n-butanol blends at φ=1.0, P=10 bar (Exps are the experimental results, and Mods are the calculated results for the blending model).

24


Page 25 of 30

ϕ =1.0 Exp_M60_6 bar Exp_M60_10 bar Exp_M60_15 bar Mod_M60_6 bar Mod_M60_15 bar Mod_M60_10 bar

p =10 bar 1000


1000


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

100

Exp_M60_1.0 Exp_M60_0.5 Exp_M60_1.5 Mod_M60_1.5 Mod_M60_1.0 Mod_M60_0.5

100

10

10 0.65

0.70

0.75

0.80

0.65

0.85

0.70

0.75

0.80

0.85

-1 1000/T(K )

-1 1000/T(K )

(a)

(b)

Figure 6. Experimental and calculated results for 60% DME/40% n-butanol (M60) at different pressures and equivalence ratios.

25


Energy & Fuels

10000


1000


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 26 of 30

1000

100

Exp_M100_N2 Exp_M100_Ar Mod_M100_N2 Mod_M100_Ar

100

Exp_M80_N2 Exp_M80_Ar Mod_M80_N2 Mod_M80_Ar 10

10 0.70

0.75

-1 1000/T(K )

0.80

0.85

0.65

0.70

0.75

1000/T(K

(a)

-1

0.80

0.85

)

(b)

Figure 7. Effects of the carrier gases (Ar or N2) on the ignition delay times of DME and DME/n-butanol blend fuel at φ=1.0, P=10 bar.

26


Page 27 of 30

10000 100 80

ϕ = 1.0, P =10bar T=1250K T=1450K T=1650K

1000

60

40

ϕ = 1.0, T=1450K P=6bar P=10bar P=15bar

20 0

20

40

60

80

100



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

100

10

1 0

20

DME blending ratio/%

40

60

80

100

DME blending ratio/%

(a)

(b) Figure 8. Ignition delay times versus DME blending ratio.

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Page 28 of 30

M100

NC3H7+CH2OH

M0

(+H)12.31% (+O)1.59% (+M) (+OH)8.09% 6.08% (+HO2)0.13% (+CH3)0.36%

(+H)11.05% (+O)0.94% (+OH)23.59% (+HO2)1.72% (+CH3)1.1%

C4H8OH-1

(+H)7.57% 100% (+O)1.17% (+OH)7.83% CH2OH+C3H6 (+HO2)0.19% (+CH3)0.19%

52.79%

CH3OCH2 100%

C4H8OH-4

C4H8OH-2

47.21% 14.08%

CH3+CH3O

(+H)11.31% (+O)1.85% (+OH)6.58% (+HO2)0.62% (+CH3)67.92%

C4H8OH-3

NC4H9OH

(+H)8.48% 100% (+O)1.23% (+OH)5.98% C2H3OH+C2H5 (+HO2)0.16% (+CH3)0.23%

11.72%

CH3OCH3

85.92%

C3H5OH+CH3 C4H8-1+OH PC4H9O C2H4+PC2H4OH

CH3+CH2O

NC3H7+CH2OH

M60

C4H8OH-1

CH3OCH3

(+M)

C4H8OH-2

(+H)8.11% 100% (+O)0.54% (+OH)5.75% CH2OH+C3H6 (+HO2)0.24% (+CH3)0.76%

CH3+CH3O

(+H)18.25% (+O)5.08% (+OH)26.79% (+HO2)1.15% (+CH3)42.73%

C4H8OH-3

NC4H9OH

(+H)9.01% 100% (+O)0.54% (+OH)4.36% C2H3OH+C2H5 (+HO2)0.24% (+CH3)0.79% 52.78%

6.0%

(+H)13.04% (+O)0.76% (+OH)6.02% 14.55% (+HO2)0.06% (+CH3)0.91%

(+H)11.68% (+O)0.42% (+OH)17.36% (+HO2)1.18% (+CH3)3.69%

CH3OCH2 100%

C4H8OH-4

47.22% 13.41%

86.59%

CH3+CH2O C3H5OH+CH3 C4H8-1+OH PC4H9O C2H4+PC2H4OH

Figure 9. Reaction pathways for three fuel mixtures (M100, M60 and M0) at T=1400 K, P=10 bar, φ=1.0, and 20% fuel consumption.

28


Page 29 of 30

Mole fraction

3.0x10

2.0x10

1.0x10

-3

4.0x10

-3

M100 H O OH HO2 CH3

-3

-3

-3

-3

0.0 50

M0 H O OH HO2 CH3

3.0x10

Mole fraction

4.0x10

-3

2.0x10

-3

1.0x10

0.0 75

100

125

150

175

200

50

75

100

Time(µs)

(a)

150

175

200

(b) 0.0005

4.0x10

M60 H O OH HO2 CH3

M100 M80 M60 M40 M0

0.0004

CH3 mole fraction

-3

3.0x10

-3

2.0x10

-3

1.0x10

0.0 50

125

Time(µs)

-3

Mole fraction

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

0.0003

0.0002

0.0001

0.0000 75

100

125

150

175

200

0

50

100

150

200

250

Time(µs)

Time(µs)

(c)

(d)

Figure 10. Mole fractions of some free radicals (H, O, OH, HO2, and CH3) for fuel mixtures (M100, M80, M60, M40, M0) at φ=1.0, T=1400 K, p=10 bar.

29


Energy & Fuels

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 30 of 30

R1624.NC4H9OH+H=C4H8OH-2+H2 R1614.NC4H9OH(+M)=NC3H7+CH2OH(+M) R1613.NC4H9OH(+M)=C2H5+PC2H4OH(+M) R1612.NC4H9OH(+M)=CH3+C3H6OH(+M) R649.C3H4-A+H=C3H5-A R621.C3H6+OH=C3H5-A+H2O R612.C3H5-A+H(+M)=C3H6(+M) R505.CH3OCH3+CH3=CH3OCH2+CH4 R501.CH3OCH3+H=CH3OCH2+H2 R500.CH3OCH3+OH=CH3OCH2+H2O R499.CH3OCH3=CH3+CH3O R389.C2H3+O2=O+CH2CHO R354.C2H4+OH=C2H3+H2O R225.CH3+CH3(+M)=C2H6(+M) R173.CH3+HO2=CH4+O2 R172.CH3+HO2=CH3O+OH R153.CH4+OH=CH3+H2O R152.CH4+H=CH3+H2 R151.CH3+H(+M)=CH4(+M) R80.CH2O+HO2=HCO+H2O2 R76.CH2O+OH=HCO+H2O R35.HCO+O2=CO+HO2 R34.HCO+M=H+CO+M R19.HO2+OH=H2O+O2

M0 M60 M80 M100

R1.H+O2=O+OH

-1.0

-0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4

Sensitive index Figure 11. Normalized sensitivity of the ignition delay for various DME blending ratios at φ=1.0, T=1400 K, P=10 bar.

30


Shock Tube Measurements and Modeling Study on the Ignition Delay

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