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Effect of pressure and equivalence ratio on theignition characteristics of dimethyl ether-hydrogenmixtures

Lun Pan, Erjiang Hu*, Fuquan Deng, Zihang Zhang, Zuohua Huang*

State Key Laboratory of Multiphase Flows in Power Engineering, Xi'an Jiaotong University, Xi'an 710049,

People's Republic of China

a r t i c l e i n f o

Article history:

Received 27 May 2014

Received in revised form

16 September 2014

Accepted 19 September 2014

Available online 11 October 2014

Keywords:

Ignition delay times

Hydrogen fraction

Shock tube

Equivalence ratio dependence

Pressure dependence

Chemical kinetic

* Corresponding authors. Tel.: þ86 29 826650E-mail addresses: hujiang@mail.xjtu.edu.

http://dx.doi.org/10.1016/j.ijhydene.2014.09.00360-3199/Copyright © 2014, Hydrogen Energ

a b s t r a c t

Experimental and numerical study on the effect of pressure and equivalence ratio on the

ignition delay times of the DME/H2/O2 mixtures diluted in argon were conducted using a

shock tube and CHEMKIN II package at equivalence ratios of 0.5e2.0, pressures of 1.2

e10 atm and hydrogen fractions of 0e100%. It was found that the measured ignition delay

times of the DME/H2 mixtures demonstrate three ignition regimes. For the DME/H2 mixture

at XH2 �80%, the ignition is controlled by the DME chemistry and ignition delay times

present a typical Arrhenius pressure dependence and weak equivalence ratio dependence.

For the DME/H2 mixture at 80% < XH2 < 98%, the ignition is controlled by the combined

chemistries of DME and hydrogen, and the ignition delay times give higher ignition acti-

vation energy at higher pressures and a typical Arrhenius equivalence ratio dependence.

However, for the DME/H2 mixture at XH2�98%, the ignition is controlled by the hydrogen

chemistry and ignition delay time shows complex pressure dependence and weak equiv-

alence ratio dependence. Comparison of the measurements of neat DME and neat

hydrogen with the calculations using three generally accepted mechanisms, NUIG Aramco

Mech 1.3 [1], LLNL DME Mech [2e4] and Princeton-Zhao Mech [5], shows that NUIG Aramco

Mech 1.3 gives the best predictions and can well capture the pressure and equivalence ratio

dependence at various hydrogen fractions. The sensitivity and normalized H-radicals

consumption analysis were performed using NUIG Aramco Mech 1.3 and the key reactions

that control the ignition characteristics of DME/H2 mixtures were revealed. Further

chemical kinetic analysis was made to interpret the ignition delay time dependence on

pressure and equivalence ratio at varied hydrogen fractions.

Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights

reserved.

Introduction

The increasing global demand for energy and stringent

emission regulations motivated the researches on high-

75; fax: þ86 29 82668789.cn (E. Hu), zhhuang@ma98y Publications, LLC. Publ

efficiency combustion technologies and clean alternative

fuels. Hydrogen and/or syngas (primarily a mixture of

hydrogen and carbon monoxide) are expected as the next-

generation fuels for engines and power sources because of

their greatest potential benefits to energy supply and the

il.xjtu.edu.cn (Z. Huang).

ished by Elsevier Ltd. All rights reserved.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 9 ( 2 0 1 4 ) 1 9 2 1 2e1 9 2 2 3 19213

environment [6]. Hydrogen is the most abundant element in

the earth and thus offers a virtually limitless supply that can

be mass produced either from renewable materials (biomass,

agricultural products and waste) or from fossil fuels (coal and

natural gas) [7,8]. Hydrogen also offers many favorable com-

bustion properties [9,10] including wide flammability limits,

high burning velocities and free greenhouse gas emissions. On

the other hand, however, the commercialization of hydrogen-

powered engines is still challenged by the high knock ten-

dency, high level of NOx emissions because of high combus-

tion temperatures at high loads. Practical studies on internal

combustion engine [11] and gas turbine combustors [12]

demonstrated that a blended fuel strategy using a mixture of

hydrocarbon and hydrogen (or syngas) can improve the per-

formance of these combustors and offers potential to address

the above challenges.

Previous fundamental combustion studies on the hydrogen

enriched hydrocarbons were widely conducted in the past

years including the ignition delay times using shock tubes

[13,14] and rapid compression machines [15], laminar flame

speeds using the spherically expanding flame method [16,17],

the stagnation flamemethod [18,19] and the heat flux method

[20], radical concentrations [21] andnumericalmodelingworks

[22,23]. Lifshitz et al. [24]were the first to experimentally report

the high temperature ignition delays of methane/hydrogen

mixtures using a shock tube. Subsequently, a large number of

studies have focused on methane/hydrogen [25e30]. More

recently, a more systematic investigation on auto-ignition

characteristics of methane/hydrogen mixtures was conduct-

ed by Zhang et al. [13], and three auto-ignition regimes were

identified. There are relatively small investigations on

hydrogen enrichedhydrocarbonhigher thanmethane [31e35].

These experimental data are of great worthy for the validation

of the hydrocarbon kinetic mechanism. The experimental

studies showed that addition of hydrogen to hydrocarbons

could shorten the ignition delay time [14,32,36] at high tem-

perature and increase the flame speed [16,37,38]. It is clear that

the previous studies on the ignition delay times mainly

concentrated on the influence of hydrogen addition, and little

data are available on the effect of pressure and equivalence

ratio. The ignition delay time studies on hydrogen/methane by

Zhang et al. [13], hydrogen/natural gas byHerzler et al. [39] and

hydrogen/propane by Tang et al. [33] indicated that the

dependence of ignition delay times on equivalence ratio and

pressure changed at different hydrogen fractions.

DME is a promising alternative fuel for the compression-

ignition engines because of its potential low HC and soot

emissions compared with the traditional fuels (gasoline,

kerosene). During the combustion process, DME shows a two-

stage combustion and heat release phenomenon, similar to

those of large straight hydrocarbons (e.g. n-heptane). It was

suggested that the hydrogen/DME blend was a prospective

approach to achieve the cleaner combustion [40]. However, up

to now, only a few studies on DME/H2 blends have been re-

ported. Previously, authors have reported the influence of

hydrogen addition on the ignition delay times of DME/

hydrogen blends in the previous work [36]. The objective of

this paper is to analyze the effect of pressure and equivalence

ratio on the ignition delay times of the DME/H2 blends.

Meanwhile, the comparison of the experimental data with

some widely used models was made to evaluate models per-

formance at varied pressures and equivalence ratios. Sensi-

tivity analysis was conducted to interpret the effect of

pressure and equivalence ratio on the ignition delay times at

varied hydrogen fractions.

Experimental and numerical approaches

Experimental setup

All measurements were made in a shock tube that has been

provided in details in the previous publication [41]. The

schematic of the shock tube is shown in Fig. 1. In brief, the

shock tube with an internal diameter of 11.5 cm is separated

into a 4 m long driver section and 5.3 m driven section by a

0.07 m long double-diagram (polyethene terephthalate dia-

grams) section. Diaphragms of different thicknesses were

chosen, depending on the magnitude of the nominal reflected

pressure. Prior to each experiment, the fragmentized mem-

branes are flashed away by high-purity argon and the whole

tube is evacuated to a pressure below 10�5 bar prior to the

reactant mixture is added. The leak rate is regarded as negli-

gible as compared to the magnitude of the reactant mixtures

(5e70 kPa). The reactantmixtures, as provided in Table 1, were

prepared in a 128 L stainless steel tank and settled designedly

for at least 12 h bymolecular diffusion. The partial pressure of

each constituent was monitored by a high-accuracy pressure

transmitter (ROSEMOUNT 3051). In this study, all of the fuel

mixtures (fuel/oxygen/argon, XO2=XAr ¼ 21%/79%) were

diluted with a same dilution ratio of 4 (80% argon/20%

mixture). Purities of hydrogen, DME, oxygen and argon are all

higher than 99.99%.

Four fast-response sidewall pressure transducers (PCB

113B26) are installed along the last 1.3 m of the driven section

with fixed intervals (300 mm). Three time counters (FLUKE,

PM6690) are used to record the time interval of the adjacent

transducers when the shock wave is arrived, and then the

incident shock wave velocities are calculated correspond-

ingly. The incident shock velocity at endwall is obtained by

extrapolating the incident shock velocity profile to the end-

wall. The reflected temperatures (T5) are calculated from

incident shock velocity at endwall by using chemical equilib-

rium software Gaseq [42] and the reflected pressures (p5) are

determined by a pressure transducers (PCB 113B03) mounted

at the end-wall. The uncertainty of T5 was evaluated accord-

ing to the work by Petersen et al. [43]. The calculated tem-

perature errors for T5 ¼ 1200 K (M ¼ 2.17) and 1600 K (M ¼ 2.55)

are about 7 K and 11 K, respectively. These small temperature

errors lead to less than 10% experimental error of ignition

delay times. The definition of the ignition delay time shown in

Fig. 2 is the same as in Ref. [41]. The OH* chemiluminescence

chosen by a narrow filter centered at 307 ± 10 nm is deter-

mined by a photomultiplier (Hamamatsu, CR131) mounted at

the endwall.

The SENKIN code [44] in the CHEMKIN II package [45]

associated with the SENKIN/VTIM approach [46] was chosen

to calculate the ignition delay times of DME/H2 mixtures. The

SENKIN/VTIM approach was applied to consider the non-ideal

effect which will significantly affect the computational

Fig. 1 e Schematic of the shock tube.

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 9 ( 2 0 1 4 ) 1 9 2 1 2e1 9 2 2 319214

ignition delay time for the long induced time. The computa-

tional ignition delay time is defined as the time interval be-

tween zero and the instant of maximum rate of temperature

rise (max dT/dt), which is consistent with the definition of the

experiment. The facility dependent pressure rise dp/dt was

found to have a typical value of 4%/ms [31,36,47] and was

included in the computations.

Results and discussions

The ignition delay times of DME/H2 mixtures with hydrogen

fractions from 0 to 100% were measured behind the reflected

shock waves at pressures of 1.2e10 atm, equivalence ratios of

0.5e2.0 and temperatures of 900e1700 K. The measured igni-

tion delay times in this study are summarized in the supporting

material. In this section, the effect of pressure and equivalence

Table 1 e Compositions of the test mixture.

Mixtures The mole fractionof DME (%)

The mole fractionof H2 (%)

100%DME 0.677 0.000

1.309 0.000

2.457 0.000

50%DME/50%H2 0.566 0.566

1.072 1.072

1.936 1.936

20%DME/80%H2 0.380 1.521

0.694 2.778

1.183 4.734

10%DME/90%H2 0.246 2.211

0.438 3.939

0.718 6.463

5%DME/95%H2 0.144 2.734

0.252 4.780

0.402 7.638

2%DME/98%H2 0.064 3.143

0.111 5.417

0.173 8.489

100%H2 0.000 3.472

0.000 5.917

0.000 9.132

ratio on the ignition delay times of DME/H2 blends at varied

hydrogen fractions will be firstly analyzed. Then, comparisons

of the measured ignition delay times with three generally

accepted mechanisms are made. Finally, chemical interpreta-

tion on the effect of pressure and equivalence ratio on the

ignition delay times at varied hydrogen fractions is presented.

Mechanism selected

For numerical simulation, the three generally accepted

mechanisms, the NUIG Aramco Mech 1.3 [1], the LLNL DME

Mech [2e4] and the Princeton-Zhao Mech [5] are considered.

NUIG Aramco Mech 1.3 was developed in 2013 by the Com-

bustion Chemistry Center of National University of Ireland by

incorporating new reaction rates in its early version, in which

253 species and 1542 elementary reactions are involved. LLNL

DME Mech, which includes 80 species and 351 elementary

The mole fractionof O2 (%)

The mole fractionof Ar (%)

Equivalenceratio (f)

4.060 95.264 0.5

3.927 94.764 1.0

3.686 93.857 2.0

3.964 94.904 0.5

3.751 94.105 1.0

3.388 92.740 2.0

3.802 94.297 0.5

3.472 93.056 1.0

2.959 91.124 2.0

3.686 93.857 0.5

3.282 92.341 1.0

2.693 90.126 2.0

3.597 93.525 0.5

3.145 91.824 1.0

2.513 89.447 2.0

3.528 93.265 0.5

3.040 91.432 1.0

2.382 88.956 2.0

3.472 93.056 0.5

2.959 91.124 1.0

2.283 88.584 2.0

Fig. 3 e Comparison of measured and simulated ignition

delay times using various DME mechanisms.

Fig. 2 e Definition of ignition delay time.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 9 ( 2 0 1 4 ) 1 9 2 1 2e1 9 2 2 3 19215

reactions, was developed on the basis of oxidation of DME by

Lawrence Livermore National Laboratory. Princeton-Zhao

Mech, involving 55 species and 290 reactions, was developed

for DME oxidation on the basis of the RRKM/master equation

method. All of the models contain the detailed hydrogen and

DME oxidation chemistries and have been extensively vali-

dated against the experimental data, such as ignition delay

times and laminar flame speed of DME. However, none of

these models has been validated against the ignition delay

times of DME/H2 mixtures.

Figs. 3 and 4 give the comparisons between the measured

and model predicted ignition delay times of DME and

hydrogen, respectively. For DME, as shown in Fig. 3, NUIG

Aramco Mech 1.3 and Princeton-Zhao Mech predict moder-

ately well the ignition delay times of neat DME, and only a

slight overprediction is exhibited at relatively lower temper-

ature for the investigated conditions. Prediction by LLNL DME

Mech agrees fairly well with the ignition delay times at

p ¼ 10 atm over all equivalence ratios. However, it shows an

ever-increasing underprediction with the decrease of pres-

sure, indicating that LLNL DME Mech gives weaker pressure-

dependence compared with that of the measurements. This

may attributed to the uncertainty of rate coefficients of the

unimolecular reactions in Princeton-Zhao model. For neat

hydrogen, as shown in Fig. 4, NUIG Aramco Mech 1.3 shows

perfect predictions over the entire range of the studied con-

ditions, while LLNL DME Mech and Princetion-Zhao Mech

underpredict the ignition delay time, especially at lower

temperature and higher pressure. The underprediction by

LLNL DME Mech and Princeton-Zhao Mech is largely affected

by the reaction HþO2ðþMÞ⇔HO2ðþMÞ [36].In general, NUIG Aramco Mech 1.3 can predict fairly well

the ignition delay times of both neat DME and neat hydrogen

under the test conditions. This mechanism thus was used to

simulate the ignition delay time and make the kinetic in-

terpretations of the DME/H2 mixture in the following sections.

Ignition delay times analysis

The effect of pressure on ignition delay times under stoi-

chiometric condition along with the mechanism predictions

by NUIG Aramco Mech 1.3 is shown in Fig. 5. The effect of

pressure under lean and rich conditions can also found in the

supporting material. The mechanism shows fairly well

agreement at various hydrogen fractions across the entire

Fig. 4 e Comparison of measured and simulated ignition

delay times using different hydrogen mechanisms.

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 9 ( 2 0 1 4 ) 1 9 2 1 2e1 9 2 2 319216

temperature and pressure range tested. For the DME/H2 mix-

tures at XH2 �80%, as shown in Fig. 5aec, the ignition delay

times exhibit a clear Arrhenius dependence on temperature at

all pressures under the tested conditions. Ignition delay time

decreases with the increase of pressure. Parallel lines of

ignition delay time versus the inverse temperature at different

pressures are demonstrated, indicating that for these

hydrogen additions, the overall activation energy of different

pressures are quite equivalent. The global activation energies

at XH2 ¼ 0, 50, 80% usingmultiple linear regressionmethod are

39.4 kcal/mol, 38.5 kcal/mol and 37.4 kcal/mol, respectively

(R2 > 0.99). The decrease of the global activation energy with

the increase of hydrogen fraction indicates that hydrogen

addition can promote the ignition of DME/H2 mixtures. For the

DME/H2 mixtures at 80% < XH2 < 98%, as shown in Fig. 5d and

e, the ignition delay time still gives the Arrhenius dependence

on temperature. However, ignition delay times at higher

pressure exhibits higher ignition activation energy, which is

more distinct when increasing the hydrogen fraction. For

these hydrogen addition, ignition delay times still decrease

with the increase of pressure, but the behavior becomes

moderated as temperature is decreased. When further

increasing hydrogen blending ratios (XH2 � 98%), as shown in

Fig. 5f and g, ignition delay time no longer follows the Arrhe-

nius dependence on temperature. A complex pressure

dependence on ignition delay time is exhibited as tempera-

ture is decreased, and this behavior was also observed in the

previous publications [14,32,48].

Besides the effect of pressure, the effect of equivalence

ratio on the ignition delay times was also discussed at pres-

sure of 4 atm, as shown in Fig. 6. Again, the mechanism gives

reasonably well agreement across the temperature range for

stoichiometric mixture. For XH2 �80%, as shown in Fig. 6aec,

the ignition delay times exhibit typical Arrhenius dependence

on temperature at all equivalence ratios but weak equivalence

ratio dependence. It is noted that ignition delay times at

higher equivalence ratio exhibits slightly lower ignition acti-

vation energy, indicating that the fuel-rich mixtures are more

reactive. At XH2 ¼ 0% and 50% as shown in Fig. 6a and b,

crossing points at 1410 K and 1350 K are presented respec-

tively when temperature shifts to the lower range. When

hydrogen fraction increases to 80%, as shown in Fig. 6c, the

ignition delay times of three mixtures converge to 1170 K,

indicating that theremust be a crossing point at 1170 K.When

temperature is higher than that of the crossing point, the

ignition delay times exhibits weak negative effect upon the

equivalence ratio. For the DME/H2 mixtures at

80% < XH2 � 98%, as shown in Fig. 6def, the ignition delay time

still gives the Arrhenius dependence on temperature at all

equivalence ratios. Ignition delay time increases remarkably

with the increase of equivalence ratio. Parallel lines of ignition

delay time versus the inverse temperature at different

equivalence ratios are presented, indicating that for these

hydrogen fractions, the overall activation energies at different

equivalence ratios are equivalent. The global activation en-

ergies at XH2 ¼ 90, 95 and 98% using multiple linear regression

method are 36.3 kcal/mol, 34.4 kcal/mol and 33.8 kcal/mol,

respectively (R2 > 0.99). When further increasing hydrogen

blending ratios (XH2 > 98%), as shown in Fig. 6g, the influence

of equivalence ratio on ignition delay time is much weak.

Through the above analysis, NUIG Aramco Mech 1.3, which

gives reasonably well predictions for the DME/H2 mixtures at

the studied conditions, can be used to analyze the pressure and

equivalence ratio dependence on the ignition delay times of

DME/H2 at various hydrogen fractions in the following section.

Fig. 5 e Effect of pressure on ignition delay times for stoichiometric DME/H2 mixtures.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 9 ( 2 0 1 4 ) 1 9 2 1 2e1 9 2 2 3 19217

Fig. 6 e Effect of equivalence ratio on ignition delay times for DME/H2 mixtures at p ¼ 4 atm.

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 9 ( 2 0 1 4 ) 1 9 2 1 2e1 9 2 2 319218

Fig. 7 e Sensitivity coefficients versus pressure for

stoichiometric DME/H2 mixture at XH2 ¼ 95%.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 9 ( 2 0 1 4 ) 1 9 2 1 2e1 9 2 2 3 19219

Chemical kinetic interpretations on the effect of pressure andequivalence ratio

Together with our previous study [36], we note that the igni-

tion characteristics of DME/H2 blends can be divided into three

regimes on the basis of hydrogen fractions, i.e. the DME

chemistry dominated ignition (DCDI, XH2 � 80%), the com-

bined chemistries of DME and hydrogen dominated ignition

(CCDHDI, 80% < XH2 < 98%) and the hydrogen chemistry

dominated ignition (HCDI, XH2 � 98%). In the DCDI system, the

ignition chemistry of the mixture resembles to that of neat

DME. In the CCDHDI system, the ignition is controlled by the

combined chemistries of DME and hydrogen. In the HCDI

system, the ignition chemistry of the mixture resembles to

that of neat hydrogen. These ignition behaviors are attributed

to the different ignition chemistries of the mixtures. To

interpret the effect of pressure and equivalence ratio on the

ignition delay times of varied hydrogen fraction mixtures, the

sensitivity study was performed to identify the dominant re-

actions associated with the ignition of DME/H2 blends under

various conditions. Definition of sensitivity was given in the

previous publication [13].

Interpretation on the effect of pressureFor the DCDI system, the ignition delay time presents

extremely high sensitivity to the chain branching reaction R1:

HþO2⇔OþOH and the dimethyl ether molecular decompo-

sition reaction R431: CH3OCH3ðþMÞ⇔CH3 þ CH3OðþMÞ [49].

Reaction R431 promotes the ignition because it decomposes

into the methyl and methoxy radicals which can further pro-

duce theH radicals through reactions R91 ðCH3OðþMÞ⇔CH2OþHðþMÞÞ and R30 ðHCOðþMÞ⇔Hþ COþ ðMÞÞ. These reactions

are the major initial sources of radicals [36,50]. Thus, the re-

action rates of reactions R431 and R1 are increased with the

increase of pressure because of the higher absolute DME and

oxygen concentration at higher pressure, leading to the

decrease of the ignition delay time. As a result, the high

pressure-mixture gives the shorter ignition delay times

compared to those of low pressure-mixture. For the HCDI

system, this complex pressure dependence was also observed

Fig. 8 e Normalized rate of consumption of H radicals from

R1 and R433 at XH2 ¼ 95%, f ¼ 1.0.

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 9 ( 2 0 1 4 ) 1 9 2 1 2e1 9 2 2 319220

in the previous publications [13,14,32,39]. Ignition of the HCDI

mixture is dominated by a pair of competing reactions: R1: HþO2⇔OþOH and R9: HþO2ðþMÞ⇔HO2ðþMÞ. The former reac-

tion promotes the ignition, while the latter inhibits the igni-

tion. Moreover, reaction R9 is more dominant at higher

pressures because of its pressure-dependent behavior and

Fig. 9 e Sensitivity coefficients versus equivalence ratio for DME

lower temperatures because of the lower activation energy,

leading to the complex ignition delay time dependence on

pressure [39].

To explain the pressure dependence behavior for the

CCDHDI system, Fig. 7 gives the highest normalized sensitivity

coefficients as a function of pressure for the stoichiometric

/H2 mixture at XH2 ¼ 0, 95, 100%, T ¼ 1250 K and p ¼ 4 atm.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 9 ( 2 0 1 4 ) 1 9 2 1 2e1 9 2 2 3 19221

DME/H2 mixture at XH2 ¼ 95% and temperatures of 1000 K and

1500 K. At both temperatures, the most inhibiting reaction at

three pressures is the H-abstraction reaction of DME R433:

CH3OCH3 þH⇔CH3OCH2 þH2. This reaction consumes the

highly reactive H radicals and generates the stable hydrogen

molecule, resulting in the reduction of the system reactivity.

The most promoting reaction is the chain branching reaction

R1: HþO2⇔OþOH, which consumes one radical but pro-

duces two radicals. It is noted that reaction R433 competes the

H radicals with chain branching reaction R1. Additionally, at

1500 K, the normalized sensitivity index of the above two re-

actions are increased with the increase of pressure, indicating

that the ignition is more dominant by these two reactions at

higher pressures at T ¼ 1500 K. At 1000 K, the normalized

sensitivity index of the above two reactions are generally

decreased with the increase of pressure, indicating that they

are less dominant at higher pressures at T ¼ 1000 K.

To further understand the role of these two reactions in the

ignition chemistry of DME/H2 blends, the normalized con-

sumption rates of H radicals by reactions R1 and R433 at the

timing of 20% fuel consumption is provided in Fig. 8. At higher

temperature (T ¼ 1500 K), the consumption of H radicals by

reaction R1 (~45%) is nearly equal to the contribution of R433.

Consequently, the high pressure-mixture gives the shorter

ignition delay times because of the higher absolute oxygen

concentration which increases the rate of reaction R1. At

lower temperature (T ¼ 1000 K), a large amount of H radicals

are still consumed through R1 (15%) and, thus, the ignition

delay times are decreased with the increase of pressure

because of the higher oxygen concentration. However, as

shown in Fig. 8, the contribution of R433 becomes obvious in

comparison to the consumption rate of R1 at low temperature

and high pressure, and this leads to the pressure effect at low

temperature is less than at high temperature system. Conse-

quently, the ignition delay times at higher pressure exhibits

higher ignition activation energy for the CCDHDI system.

These results are consistent to studies by Man et al. [32] in the

study of the C3H8/H2 mixtures.

Interpretation on the effect of equivalence ratioTo clarify the ignition chemistry of DME/H2 mixtures at

different equivalence ratios, Fig. 9 provides the reactions with

the highest normalized sensitivity coefficients for the mix-

tures at XH2 ¼ 0, 95 and 100% at three equivalence ratios,

pressure of 4 atm and temperature of 1250 K. For the DCDI

system, as shown in Fig. 9a, the ignition delay time is highly

sensitive to the reactions R1, R431 and R437:

CH3OCH3⇔CH3OCH2 þ CH4. These three reactions are the

ignition promoting reactions because reaction R1 is the most

important chain-branching reaction in the hydrocarbon

combustion system and reactions R431 and 437 produce the

CH3 and CH3O radicals which can further produce the H rad-

icals through reactions R91 and R30, as discussed above. Re-

action R1 becomes dominant at fuel-lean mixture because of

the higher oxygen concentration whereas reactions R431 and

R437 become dominant at fuel-rich mixtures because of the

higher DME concentration. The combined influence of these

reactions leads to theweak ignition delay time dependence on

equivalence ratio for the DCDI system. For the HCDI system,

besides reaction R1, the hydrogen-specific reaction R2

ðOþH2⇔HþOHÞ and R3 ðOHþH2⇔HþH2OÞ also exhibits

highly sensitivity to the ignition delay time, as shown in

Fig. 9c. Similarly, reaction R1 becomes dominant at fuel-lean

mixtures because of the higher oxygen concentration

whereas reactions R2 and R3 become dominant at fuel-rich

mixtures because of the higher hydrogen concentration.

Consequently, the ignition delay times at both fuel-lean and

fuel-rich mixtures give the comparable values, leading to a

weak equivalence ratio behavior for HCDI system. Compared

to HCDI system, although the ignition delay time of CCDHDI

system is still sensitive to reactions R1, R2 and R3, the DME

fuel-specific reactions R433 and R432 also show high sensi-

tivity because of the DME addition, as shown in Fig. 9b. Re-

actions R433 and R432 have higher reaction rates under the

fuel-rich mixtures because of the higher DME molecular

concentration in the fuel-rich mixtures. Reactions R432 and

R433 are the inhibiting reactions because they consume

reactive radicals (H and OH) and generate the stable molecule

(H2 and H2O). As a result, ignition delay times of CCDHDI

system are decreased with the increase of equivalence ratio.

Conclusions

Study on the effect of pressure and equivalence ratio on the

ignition delay times of DME/H2/O2/Ar was conducted at pres-

sures of 1.2e10 atm, equivalence ratios of 0.5e2.0 and

hydrogen fractions of 0e100%. Main conclusions are sum-

marized as follows:

1). NUIG Aramco Mech 1.3 gives the best prediction on the

ignition delay times of neat hydrogen and DME, and can

well capture the pressure and equivalence ratio

dependence.

2). Ignition delay times of the DME/H2 mixtures demon-

strate three ignition regimes. They are, the DME

chemistry dominated ignition (DCDI, XH2 � 80%), the

combined chemistries of DME and hydrogen dominated

ignition (CCDHDI, 80% < XH2 < 98%), and the hydrogen

chemistry dominated ignition (HCDI, XH2 � 98%). Igni-

tion delay time show different pressure and equiva-

lence ratio dependence within different ignition

regimes.

3). For the DCDI system, ignition delay times show a typical

Arrhenius pressure dependence and weak equivalence

ratio dependence. Ignition delay time is highly sensitive

to the reactions R1, R431 and R437. Reaction rates of

reactions R431, R437 and R1 are increased because of the

higher absolute DME and oxygen concentration at

higher pressure, leading to the decrease of the ignition

delay time with the increase of pressure. Reaction R1

becomes dominant under fuel-lean condition because

of the higher oxygen concentration whereas reactions

R431 and R437 become dominant under fuel-rich con-

dition because of the higher DME concentration. The

combined influence of these reactions leads to a weak

ignition delay time dependence on equivalence ratio for

the DCDI system.

4). For the DHCCDI system, ignition delay times give higher

ignition activation energy at higher pressures and a

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 9 ( 2 0 1 4 ) 1 9 2 1 2e1 9 2 2 319222

typical Arrhenius equivalence ratio dependence.

Sensitivity and H-radical consumption analysis show

that pressure dependence behavior is from the

competition between reactions of R1 and R433. R433

inhibits the ignition activity and become dominant re-

action at higher pressures and lower temperatures,

whereas the R1 promotes the ignition. Sensitivity

analysis shows that the decrease of the ignition delay

times of CCDHDI system with the increase of equiva-

lence ratio is controlled by competition between re-

actions R433 and R1.

5). For the HCDI system, ignition delay time gives complex

dependence on pressure and weak dependence on

equivalence ratio. The complex dependence on pres-

sure is controlled by the competition between reactions

of R1 and R9 and the weak dependence on equivalence

ratio is from the competition between reactions of R1

and R2, R3.

Acknowledgments

This work is supported by the National Natural Science

Foundation of China (51306144, 51136005), the National Basic

Research Program (2013CB228406) and the State Key Labora-

tory of Engines at Tianjin University (SKLE201302). Authors

also appreciate the funding support from the Fundamental

Research Funds for the Central Universities.

Appendix A. Supplementary data

Supplementary data related to this article can be found at

http://dx.doi.org/10.1016/j.ijhydene.2014.09.098.

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