effect of pressure and equivalence ratio on the ignition...
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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: [email protected].
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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