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Ignition delay characteristic and kinetic investigation of DME/n-pentane binary mixture: interpreting the effect of equivalence ratio and DME blending Xue Jiang, Fuquan Deng, Feiyu Yang, and Zuohua Huang Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b02682 • Publication Date (Web): 06 Feb 2018 Downloaded from http://pubs.acs.org on February 11, 2018

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Ignition delay characteristics and kinetic investigation of DME/n-pentane binary mixtures: interpreting the effect of equivalence ratio and DME blending Xue Jiang *, Fuquan Deng, Feiyu Yang, and Zuohua Huang*

State Key Laboratory of Multiphase Flow in Power Engineering, Xi’an Jiaotong University, Xi’an 710049, People’ s Republic of China

KEYWORDS: auto-ignition delay, dual-fuel, DME, n-pentane

ABSTRACT: Measurements of the auto-ignition delays of DME enriched n-pentane mixtures were conducted at the equivalence ratios of 0.5 - 2.0, pressure of 20 atm and temperatures of 1100 - 1600 K using a shock tube, new data were provided for dual-fuel engine design, kinetic model development, and computational simulation. A recently published Pentane isomer Model was validated and been used for kinetic analysis. It is found that the auto-ignition delay of n-pentane becomes longer as the equivalence ratio (F/A) increases. However, for DME, the dependence on equivalence ratio is inverse to that of n-pentane. Auto-ignition delays of DME/n-pentane mixtures become shorter with the increased DME proportion, except for the φ =0.5 conditions where the auto-ignition delays of DME and n-pentane are identical.

1. INTRODUCTION

In recent years, Dimethyl ether (DME) as a substitute fuel for liquid petroleum gas has received much attention. It can be produced from coal, biomass, methanol and

ACS Paragon Plus Environment

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residual oil and others. DME has a high oxygen content of about 35% by mass, and short carbon chain, thus generates very low PM, NOx, and CO emissions during combustion 1. DME also has some other merits, such as low boiling point can achieve instantaneous vaporization after injection 2. Practically, DME has a high cetane number, high reactivity, and good auto-ignition properties enabling its use as an ignition enhancer 3. Nowadays, DME is being used as a promising alternative fuel in household, industry, diesel engine, petrol engine, and gas turbine 3 applications.

Dual-fuel combustion strategies are considered to have great potential to enhance engine efficiency and reduce emissions4-5 . Natural gas has a high octane number, low soot and HC emission, and low cost which have been widely used in IC engines and gas turbines. It is a mixture of hydrocarbons, the main component is methane while the C2-C5 alkanes are also included. DME and natural gas have different reactivities 6, the addition of DME into natural gas will influence the ignition delay profiles. Previous researches

7-8

showed that when fueling SI engine with DME/gasoline

blends, the thermal efficiency was improved and the HC and NOx emissions are significantly reduced. The heat release rate during the low temperature oxidation was also enhanced with DME blending. Besides, the engine fuel consumption at idle also reduced and the combustion duration of the gasoline engine decreased with DME addition. Research has shown that in a natural gas /DME dual fuel CI engine, DME addition can extend the engine load range and improve thermal efficiency 9. In addition, previous studies also confirmed that for the HCCI engines, increasing DME proportions to control the ignition timing is feasible

10-14


. Studies were conducted to

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investigate the performances of DME fueling in gas turbines

15-17

. It is found that

DME is a clean and efficient gas turbines fuel, firing DME leads to lower exhaust temperature and less NOx and CO relative to methane, however, result in the increased fuel nozzle temperature. Thus, in consideration of the reliability of the DME - enriched gas turbine, it is necessary to remold the fuel nozzle.

Fundamentally, the auto-ignitions of DME have been extensively investigated using shock tubes and rapid compression machines (RCMs)

18-20

, and the pyrolysis

characteristics of DME were studied in the shock tube, jet-stirred reactor, and flow reactor

21-23

. In addition, the laminar flame speeds of DME were obtained with

different experimental methods

24-27

. Previous studies also note that the composition

of natural gas may vary a lot depending on the geographic origins thus influence its ignition characteristics and lead to significant changes in engine operation control 28-29. Therefore, in order to guide practical application of DME/natural gas dual-fuel combustion, several studies intended to find out the ignition delay profiles of DME enriched C1 - C4 alkanes have been conducted. For methane, the effect of DME addition on the auto-ignition property was numerically studied by Chen et. al.

30

at

high temperature (1200 -1400 K). Later, shock tube and RCM measurements of the auto-ignition characteristics of DME enriched methane were conducted

31-32

. It is

found that DME addition can lead to a nonlinear ignition enhancement of methane ignition under both low and high temperature regimes

31-32

. The auto-ignition of

methane was significantly promoted even by adding a tiny amount of DME. Additionally, DME blending can non-linearly promote the auto-ignition of propane


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33-34

and n-butane in the low temperature and the NTC region

. The non-linear

influences of dual-fuel auto-ignitions of other hydrocarbon mixtures have been reported in previous literature35-37. However, with DME blending, the nearly linear promoting effect on the auto-ignition of propane and n-butane was presented in the high temperature shock tube measurements 38-39.

So far, most of the related studies are focusing on the ignition characteristics of 39-42

DME enriched light carbon alkanes mixtures (C1 - C4 alkanes)

. It is still essential

to systematic study the ignition delays of DME/higher order alkane blends as an extension of the previous researches, such as DME/n-pentane mixtures. n-Pentane is the component of many transportation fuels, such as natural gas and gasoline

43-44

.

The understanding of the auto-ignition characteristics of DME/n-pentane binary mixtures can be used to guide practical applications, to develop DME/natural gas dual-fuel kinetic models, and are useful for CFD modeling of turbulent combustion. Therefore, firstly, from the perspective of the practical application, it is quite necessary to find out if DME addition can inhibit and/or promote the ignition of n-pentane over a broad range of equivalence ratios (lean to rich). Thus, in this work, ignition delays of DME enriched n-pentane mixtures were systematically measured at different DME blending ratios and equivalence ratios. Another purpose is to conduct the rate of production analysis, sensitivity analysis, and mole fraction analysis to clarify the effect of DME addition and the influence of equivalence ratio on the auto-ignition delays of n-pentane under the wide range of conditions.


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2. METHODOLOGY

2.1 Shock tube measurement

All the ignition delay data were obtained in a shock tube device, as shown in Fig. 1. A detailed introduction to this test bench can be found in our previous study 42. The n-pentane/ “air” (XO2/Xar = 21%/79%) mixtures were diluted by 80% argon. Mixture gas component is shown in Table.1. The onset of ignition is determined by the slope of the CH* chemiluminescence, as shown in Fig. 2. , the fuel mixtures were settled for 12 hours for sufficient mixing. The standard root-sum-squares method 45 was used to determine the temperature uncertainty which is 20 K in this study. It leads to 20% uncertainty in ignition delay times 46.

2.2. Kinetic Simulations Gaseq software 47 was used to calculate the gas state. Chemical kinetic study was conducted by using SENKIN code 48 of CHEMKIN II 49 program. A typical pressure rise rate (dP/Pdt )

50

of this shock tube is 4.2%/ms

42

were considered in simulations.

In the numerical simulations, the auto-ignition delay was determined according to the variation of OH radical concentration.

The recently published Pentane isomers Model

51

, which includes 697 species

and 3216 reactions was validated. The DME sub-mechanism in the Pentane isomers Model was recently updated based on the study of Burke


32

, in which the

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pressure-dependent rates for the oxygen addition reactions of DME, and the decomposition of CH3OCH2, CH2OCH2O2H, and O2CH2OCH2O2H were acquired by conducting the Quantum-Rice-Ramsperger-Kassel calculation.

The comparisons between the measured and predicted auto-ignition properties of the test mixtures at the pressure of 20 atm, the equivalence ratios of 0.5, 1.0 and 2.0 are shown in Fig. 3 and Fig.7. Note that the Pentane isomer Model gave good predictions on the auto-ignition delays of both neat fuels and their mixtures within the whole experimental range.

3. RESULTS AND DISCUSSION

3.1 Influence of Equivalence Ratio

Figs. 3 (1) - (3) show the influence of equivalence ratio on the ignition delay times of the XDME = 0% (pure n-pentane), XDME = 100% (pure DME), and XDME = 50% mixtures at the pressure of 20 atm. For comparison, both the experimental and the simulation results from the Pentane isomer Model are plotted in Fig. 3, note that the model predictions agree well with the measurements at all conditions.

For the neat n-pentane mixtures at 20 atm, as shown in Fig. 3 (1), the lean mixture ignites faster instead of the rich ones at high temperatures, indicating that the raise of the F/A equivalence ratio will lead to longer ignition delays in current conditions. Moreover, this effect of equivalence ratio becomes gradually weaker at lower temperatures. At 20 atm, the simulated ignition delay time of n-pentane


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intersects at one point at about 1125 K, indicating the same ignition delay times at various equivalence ratios. Previous studies also reported the inversion in the effect of equivalence ratio on the auto-ignition delays of methane 52, propane 39, n-butane 38 and 53

methane/propane mixtures .

In contrast, for the XDME = 100% mixtures at 20 atm, as shown in Fig. 3 (2), the rich DME mixture is the fastest to ignite, while the lean DME mixture shows the lowest reactivity. The auto-ignition delays increase as φ decreases from 2.0 to 0.5. Moreover, it is worth noting that the influence of equivalence ratio on DME auto-ignition is much notable at low temperatures.

For the XDME = 50% mixture, in Fig. 3 (3), combined effects of DME and n-pentane kinetics were observed, the n-pentane-like properties were observed under high temperatures (T > 1225 K), the increasing equivalence ratio leads to the increased auto-ignition delays. However, at relatively low temperatures (T < 1250 K), the DME-like tendency on ignition was obtained, the increase of equivalence ratio can promote the ignition.

3.1.1 Rate-of-Production (ROP) Analysis

ROP analysis of the DME and n-pentane mixtures were conducted at a time of 20% fuel consumption, φ = 0.5, 1.0, and 2.0, p = 20 atm, and T = 1125 and 1425 K, as shown in Fig. 4 (1) and (2).

Note that for the XDME = 100% mixtures at 1125 K, as gives in Fig.4 (1), the


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DME molecules were primarily (the proportion is 95.7% - 96.1% at various equivalence ratios) expended via the H-atom abstraction by the H, OH, CH3, and HO2 radicals and yield the methoxymethyl radical (CH3OCH2). Ultimately, the generated CH3OCH2 radical almost entirely undergoes the decomposition reaction and forms the CH3 radical and formaldehyde (CH2O). In addition to the molecule H-atom abstraction pathway, a small amount of DME (2.8% to 3.6%) consumed through the uni-molecular decomposition (produceed CH3O and CH3 radical). In general, the H-atom abstraction is the dominating pathway of DME consumption while the CH3 radical and CH2O are the main products of DME oxidation from both decomposition and H-atom abstraction pathways.

For the n-pentane (XDME = 0%) mixture, as shown in Fig. 4 (2), at 1425 K, the n-pentane is mainly depleted by the H-atom abstraction from the primary, secondary and tertiary C - H bond (accounting for approximate 70% in total), and yield the n-pentyl radical (C5H11-1), sec-pentyl radicals (C5H11-2), and 3-pentyl radicals (C5H11-3), respectively. Among which, the proportion of the C5H11-2 radical is the largest among the three, followed by the C5H11-1 radical, while the proportion of the C5H11-3 radical is the least, owing to the different bond dissociation energy of the primary, secondary and tertiary hydrogen atoms. Also, note that the uni-molecular decomposition pathway accounts for nearly one-third of the n-pentane consumption. The n-pentane molecule mostly decomposes to yield the C2H5 radical and NC3H7 radical, which accounting for 23.7% - 26% under different equivalence ratios. Meanwhile, small numbers of which also formed the PC4H9 radical and CH3 radical


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through direct decomposition.

3.1.2 Sensitivity Analysis

Fig. 5 (1) and (2) depict the 12 most sensitive elementary reactions on ignition of DME and n-pentane at φ = 0.5, 1.0, and 2.0 and p = 20 atm predicted by the Pentane isomer Model.

S=

τ ( 2ki ) − τ (0.5ki ) 1.5τ (ki )

Eq.(1)

Where S is the sensitivity coefficient, ki is the specific rate coefficient, τ is the auto-ignition delay. If the S is negative, it indicates the reaction promotes the ignition and vice versa.

As illustrated in the sensitivity analysis in Fig.5 (1), for the XDME = 0% mixture (n-pentane) at 1425 K, it can be seen that the auto-ignition delay is extremely sensitive to H + O2 O + OH (R1). Meanwhile, the most ignition-inhibiting reaction is the H-atom abstraction of n-pentane, NC5H12 + H C5H11-2 + H2 (R2313). Since the chain-propagation reaction (R2313) competes for H radical with the

chain-branching

reaction

(R1),

it

is

showing

the

most

significant

ignition-inhibiting effect and reduces the system reactivity.

To further understand the influence of the equivalence ratio, Fig.6 (1) gives the main consumption flux of H radical for the XDME = 0% mixture (n-pentane) at 1425 K, 20 atm, and 20% fuel consumption (corresponding to ROP analyses of n-pentane).


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Note that the H radicals are mainly consumed through the reaction (R1); as well as the H-atom abstractions of n-pentane through (R2312), (R2313) and (R2314), among which the reaction (R2313) accounted for the largest proportion due to the relatively weaker secondary C-H bond, this observation agreed with the ROP analysis in Fig. 4. As shown in Fig. 6 (1), a gradually reduced consumption proportion of H radical through (R1), from 14.86% to 4.62%, was observed as the equivalence increases from 0.5 to 2.0. This is due to that, on the one hand, compared to the lean mixture, the oxygen concentration of the rich mixture reduced. On the other hand, the n-pentane concentration of the rich mixture was higher relative to the lean mixture, this leads to the more frequent chain-propagation from the H-atom abstractions of n-pentane. The reaction (R2313) took up to 39.81% of the total H radical rate of consumption at the equivalence of 2.0, this percentage was higher relative to the stoichiometric and lean conditions (36.39% and 32.87%, respectively). Thus, due to the above reasons, the auto-ignition of n-pentane is inhibited by the increasing equivalence ratios.

For the XDME = 100% mixtures (DME) at 1150 K, Fig. 5 (2), despite the auto-ignition is still susceptible to the reaction O2 + H O + OH (R1), the DME molecule relevant H-atom abstraction reaction, CH3OCH3 + CH3 CH3OCH2 + CH4 (R474) is the most important chain branching reaction. Other CH3 radical relevant reactions, namely the CH2O + CH3 HCO + CH4 (R76), CH3 + HO2 CH4 +O2 (R147), and CH3 + CH3 (+M) C2H6 (+M) (R190), are all highly sensitive reactions that affect DME ignition. Note the most significant ignition-inhibition reaction of n-pentane ignition at 1425 K (R2313), and the most


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significant ignition-promotion reaction of DME at 1125 K (R474), are of precisely the same type: H-atom abstraction of fuel molecule producing an R radical. For the DME mixture, reaction (R474) was competing for the CH3 radical with the important ignition inhibition reactions, (R147) and (R190), thus increases the system reactivity. Fig. 6 (2) shows the CH3 radical rate of consumption for the XDME = 100% mixture at 1125 K, 20 atm, and 20% fuel consumption. As φ increased from 0.5 to 2.0, due to the increased DME concentration, the proportion of the CH3 radical consumption through reaction (R474) increased from 24.88% to 28.65%, indicating a more pronounced ignition-promoting effect through the reaction (R474). In the meantime, the CH3 radical rates of consumption through the ignition-inhibiting reactions (R147) and (R190), all gradually decrease with the increase of equivalent ratio. Therefore, for the DME mixtures, the system reactivity increases with the equivalent ratio.

3.2 Influence of DME Addition

The influence on auto-ignition of adding DME to n-pentane at φ = 0.5, 1.0 and 2.0, p = 20 atm are shown in Fig. 7 (1) - (3). It is noted that the auto-ignition delays of XDME = 100%, 50%, and 0% mixtures are almost the same at ϕ = 0.5. However, for the ϕ = 1.0 and 2.0 mixtures, the XDME = 100% mixture ignites faster than the XDME = 0% mixture and the DME addition can obviously enhance the auto-ignition of n-pentane. Especially, the effect of DME addition becomes more significant as the equivalence ratio increases.

Fig. 8 (1) and (2) illustrates auto-ignition delay in response to the DME


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percentage at different equivalence ratios and temperatures, respectively. It can be seen from Fig.8 (1) that at 20 atm, 1425 K, the auto-ignition of n-pentane was insensitive to DME blending under the lean condition. Meanwhile, as the equivalence ratio increases, the effect of DME blending on n-pentane ignition becomes prominent. At the equivalence of 2.0, as the XDME increases from 0% to 100%, the auto-ignition delay time decreases from 164 µs to 50 µs, dropped by about a factor of 3 outside of the uncertainty. Similarly, at different temperatures and the equivalence of 2.0 condition, as shown in Fig. 8 (2), DME addition leads to the decrease of auto-ignition delays in all the temperatures. As the XDME increases from 0% to 100%, the auto-ignition delays at 1325 K, 1425 K, and 1525 K are all reduced by about a factor of 3, from 446 µs to 137 µs, 164 µs to 50 µs, and 66 µs to 22 µs, respectively.

Fig. 9 illustrates the

high temperature ignition of methane

31

, propane

39

,

n-butane 38, and n-pentane in response to DME percentage at the equivalence ratio of 1.0, from the literature and the present study. Despite those comparisons were conducted under temperatures and pressures, the influence trend of DME blending on the ignition delay features of those alkanes can be illustrated. Apparently, the ignition of DME is much faster than methane, the addition of which leads to an obvious non-linear effect on methane ignition, note that the promotion on auto-ignition is more pronounced at lower DME blending levels. While for the propane, n-butane, and n-pentane mixtures, the ignition delays are much faster than methane and the effects of DME addition become less prominent. It is found that for the high temperature oxidation of methane30-31 , CH4 + O2


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CH3 + HO2 is the chief reaction governing the initiation of the radical pool. This reaction can not obviously promote the ignition until the concentration of HO2 and CH3 are high enough30-31. The addition of DME lead to fast accumulation of the CH3 radical (through the reaction CH3OCH3 CH3 + CH3O) and activated chain-branching reaction channel of CH3 + HO2 CH3O + OH, thus obviously shorten the auto-ignition. For higher order alkanes, the effects of fuel blending on the auto-ignition become less obvious. This is because, for higher order alkanes, the oxygen addition reaction and the uni-molecular decomposition are important initial pathways38-39, 41. Since methane has the strong C-H bond, the reaction rate of CH4 + O2 CH3 + HO2 is much slower relative to the similar type reaction and the decomposition for the higher order alkanes30-31, the effect of DME addition becomes less pronounced for propane, n-butane, and n-pentane. 3.2.1 Sensitivity Analysis

Fig. 10 depicts the 12 most important sensitive reactions for XDME = 0%, 30%, 50%, 70% and 100% mixtures at T = 1425 K, p = 20 atm and φ = 2.0.

Note that for all the mixtures, the most-promoting reaction has the highest negative sensitivity index at 1425 K is the still H + O2 O + OH (R1), indicating the system reactivity is sensitive to the oxygen concentration. With the DME addition, the ignition becomes less sensitive to (R1). Meanwhile, for XDME = 100% mixture, the decomposition reaction CH3OCH3 (+M) CH3 + CH3O (+M) (R468) also has a


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high negative sensitivity index, the absolute value of sensitivity coefficient of which increases with DME addition. This indicates that for neat DME and DME enriched fuel mixture, besides (R1), the decomposition of DME molecule is another important chain-branching channel, which will accelerate the accumulation of free radicals and enhance system reactivity, which is consistent with previous findings on the ignition of DME enriched methane30 .

In addition, the ignition delays of the XDME = 0% - 100% mixtures are also sensitive to the chain-propagation reactions CH3 + HO2 CH3O + OH (R146). Through this reaction, the less reactive CH3 radical was consumed through (R146) while the active OH radical was produced, thus the system reactivity was increased. In contrary, the chain termination-reaction CH3 + HO2 CH4 + O2 (R147) is competing for CH3 and HO2 radicals with reaction (R146) and yield stable CH4 and O2 thus play important roles in inhibiting the ignition of various DME/n-pentane mixtures. As soon as DME is added to n-pentane, the production of the CH3 radical will be accelerated through both the pyrolysis of DME and the beta-scission of the CH3OCH2 radical

31

. Moreover, the concentration of CH2O, which is the major

intermediate species during DME ignition 41, increases with DME addition, which will result in an increased amount of the HO2 radical ( as discussed in detail the mole fraction analysis). Ultimately, the oxidation of the CH3 radical and HO2 radical is accelerated through (R146) and led to the reduced auto-ignition delay.

Note that the reaction NC5H12+H C5H11-2 + H2 (R2313), is showing the


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most significant ignition-inhibiting effect for neat n-pentane and binary mixtures. As XDME increases, the decreased concentration of n-pentane leads to the decreased reaction rate of (R2313) and decreased inhibition of ignition.

3.2.2 Radical Mole Fraction

During high temperature ignition of hydrocarbons, H, O, OH, HCO, CH3, and HO2 are chain carriers which play important roles major product of DME oxidation

34

30-31

. Moreover, the CH2O is the

. Therefore, to analyze the influence of DME

addition to n-pentane in the homogeneous system, the evolution profiles of the H, O, OH, HCO, CH3, HO2, and CH2O were analyzed at the equivalence ratio of 2.0, 1425 K and 20 atm, as shown in Fig. 11 (1) - (7).

Generally, with DME addition, the concentrations of the H, O, OH, HO2, HCO, and CH3 radicals all increased and the formation of these radicals in the ignition induction time becomes faster. Similar results have been obtained in the ignition of DME enriched methane

30

. This means that the DME addition can obviously

accelerate the accumulation of small radicals and increase the concentration of radical pool, therefore leads to an enhanced reactivity and decreased auto-ignition delay. Also note that the H, O, and OH concentrations presented a single peak profile, Fig. 11 (1) - (3), the maximum concentrations emerged at higher temperatures, almost consistent with the time of ignition. Meanwhile, CH3 radical profiles of the DME/ n-pentane mixtures gave the multi-peak characteristic, shown in Fig. 11 (4). Again, for the HCO and HO2 radicals, Fig. 11 (5) - (6), the production and consumption of which are


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mostly happened before ignition, indicating that those radicals are important for the radical pool build-up during the early stage. Additionally, the CH2O is also formed in the initial stage, and almost entirely consumed before ignition, as shown in Fig. 11 (7).

For the fuel-rich mixtures, the increasing DME blending ratio will lead to the increased CH2O production, as shown in Fig. 11 (7). This is because the CH2O is more readily formed for DME rather than n-pentane during the high temperature oxidation2, 9, 54. The HCO radical are mainly originated from CH2O (as shown in the ROP analysis in Supporting Information), which undergoes the H-atom abstraction CH2O + H HCO + H2 (R74) to form the HCO radical. Those HCO radicals are mostly consumed in the early-stage, as the concentration of the HCO radical is sufficiently high, the chain-branching reaction HCO + O2 CO + HO2 (R32) becomes more pronounced and leads to the increased concentration of the HO2 in the radical pool. Meanwhile, as DME addition increases, the mole fraction of the CH3 radical also increases, as shown in Fig. 11 (4). When the concentrations of the CH3 and HO2 radical are both high enough, the reaction CH3 + HO2 CH3O + OH (R146) will become indispensable in radical pool development, the CH3 and the HO2 radicals were consumed through this reaction, and the OH radicals were produced to accelerate the ignition.

On the other hand, the fuel/air equivalence ratio was fixed when changing the DME blending ratio. Actually, with the increasing of DME addition level, the C/O


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atom ratios of the various mixtures gradually decreases from 0.625 to 0.5. Accordingly, the concentrations of the O, OH, HO2 and HCO radicals during the ignition inducing time all obviously increased as the DME blending ratio increased.

4. CONCLUSION

In this study, the measurements on the auto-ignition delays of n-pentane enriched by DME were conducted in a shock tube.

(1) New ignition delays of DME enriched n-pentane were provided for dual-fuel engine design, kinetic model development, and computational simulation. It is found that the predictions of the Pentane isomers Model agree well with the measurements.

(2) Auto-ignition delays of the n-pentane mixtures increases as the equivalence ratio increases. However, the opposite tendency is observed on DME ignition. This can be attributed to different dominating kinetics. For the n-pentane mixture, auto-ignition is sensitive to H + O2 O + OH (R1), while the chain-propagation reaction (R2313) competes for H radical with (R1) and inhibit the auto-ignition. Thus, the decreased the oxygen concentration and increased n-pentane concentration of the rich mixture leads to longer auto-ignition delay. For the DME mixture, however, the DME molecule relevant H-atom abstraction reaction (R474) is the most important chain branching reaction, which competes for CH3 radical with the main ignition inhibition reactions, (R147) and (R190). As φ increased from 0.5 to 2.0, due to the increased DME concentration, the proportion of the CH3 radical consumption through reaction (R474) increased, thus increases the system reactivity.


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(3) DME addition shows a negligible effect on the auto-ignition of n-pentane under lean conditions (ϕ = 0.5). However, as soon as DME is added in n-pentane, auto-ignition delays become shorter under ϕ = 1.0 and 2.0 conditions. This is mainly due to the following reasons: firstly, the stoichiometric and rich DME mixtures have higher reactivities and ignite faster than n-pentane at high temperatures, thus the ignition of n-pentane is enhanced by DME addition. Additionally, the decomposition of DME molecule provided the alternative chain branching channel besides H + O2 O + OH (R1), promoting the development of radical pool and the system reactivity. Moreover, DME addition leads to the increased production of the CH3 radical and CH2O, increased CH2O production can result in an increased HO2 radical production, thus accelerated ignition from the reaction CH3 + HO2 CH3O + OH (R146) and promote reactivity.

AUTHOR INFORMATION Corresponding Author * xuejiang1128@ xjtu.edu.cn (X. Jiang). * zhhuang@ xjtu.edu.cn (Z. Huang).

Tel: 0086-29-82665075

Funding Sources This work is supported by National Natural Science Foundation of China [grant numbers 51506164 and 91441203] and China Postdoctoral Science Foundation [grant


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number 2015M570831]. The supports from Xi’an Jiaotong University and the Fundamental Research Funds for the Central Universities are also appreciated.

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