Article
pubs.acs.org/EF
Shock-Tube Study on Ethylcyclohexane Ignition
Zemin Tian, Yingjia Zhang,* Lun Pan, Jiaxiang Zhang, Feiyu Yang, Xue Jiang, and Zuohua Huang*
State Key Laboratory of Multiphase Flows in Power Engineering, Xi’an Jiaotong University, Xi’an, Shaanxi 710049, People’s Republic
of China
S Supporting Information
*
ABSTRACT: Ignition delay times of ethylcyclohexane with a fixed fuel concentration of 0.5% were measured behind reflected
shock wave at pressures of 1.1−10.0 atm, equivalence ratios of 0.5−2.0, and temperatures between 1000 and 1700 K. The
measured ignition delay times were compared to predictions by the mechanism JetSurF2.0, and fairly good agreements were
presented under the test conditions. The ignition delay time of the fuel-rich mixture was suggested to overwhelm that of the fuellean mixture when tested at a low temperature and an elevated pressure. Analysis was conducted to kinetically interpret the
effects of the pressure, temperature, equivalence ratio, and fuel concentration on the time histories of the OH radical and
intermediate species of hydrocarbons. Results showed that the suppression of reaction H + O2 ⇄ OH + O by fuel-concerned
radicals played an important role. Sensitivity analyses were carried out to provide insight into the controlling reactions on the
ignition of ethylcyclohexane.
1. INTRODUCTION
Worldwide use of fossil fuels brings some problems, among
which environmental pollution is particularly significant. Plenty
of work has been performed on the fundamental combustion
research for the renewable, eco-friendly alternative energy
sources and various kinds of transportation fuels. However, the
dominant role of fossil fuel is not changed at present, especially
for the vehicle. Thus, much attention has been paid to
cyclohexanes (CHs) because they belong to the most
important components of fossil fuels. Jet fuels, such as Jet A/
Jet A1/JP-8, contain up to 20% CHs (by volume), and the
content by weight in diesel fuel is up to 40%.1−3 It has been
reported that Canada’s oil-sand-derived fuel serving as a
potential energy resource is composed of large cycloalkanes.4
In addition, oxidation of cycloalkanes in the oxygen-lacking
circumstance can produce many aromatics, which act as the
soot precursor. Therefore, they are also of great importance in
the formation of particulate matter (PM).5
Recently, studies for a better understanding of oxidation of
cycloalkanes were conducted, especially for CH and methylcyclohexane (MCH). Sirjean et al.6 developed a hightemperature mechanism for CH with the EXGAS software
and measured the ignition delay times at 7.3−9.5 atm,
equivalence ratios of 0.5−2.0, and temperatures from 1230 to
1840 K with the mixtures containing 0.5 and 1.0% fuel. They
found that the ignition of CH was much faster than that of
cyclopentane, and this experimental observation was wellreproduced by their kinetic mechanism. Silke et al.4 developed a
detailed kinetic mechanism for CH at both high and low
temperatures. General rules of reaction rate constants were
proposed and applied to other CHs in their study. The
mechanism was validated against plenty of kinetic targets,
including ignition delay time, species profile, and laminar flame
speed, and fairly good agreements were obtained. Furthermore,
Daley et al.7 measured the ignition delay times of the CH/air
mixtures at pressures of 11−61 atm, equivalence ratios of 1.0,
0.5, and 0.25, and temperatures from 847 to 1379 K in a shock
© 2014 American Chemical Society
tube. They indicated that all mechanisms used overpredicted
the experimental results under the tested conditions.
For MCH, Orme et al.8 proposed a detailed mechanism, and
Pitz et al.9 subsequently developed this kinetic model by adding
a new low-temperature mechanism. Vanderover and Oehlschlaeger10 measured the ignition delay times for MCH at high
pressures (12 and 50 atm) with ϕ = 0.25, 0.5, and 1.0. Hong et
al.11 performed a comparative study of CH, MCH, and nbutylcyclohexane (BCH) at a high temperature. They measured
the ignition delay times of the three fuels at ϕ = 0.5−1.0 and p
= 1.0−3.0 atm with temperatures from 1220−1450 K. In
addition, flux analyses were made to explain the reasons for the
order of ignition delay times (MCH > CH ≈ BCH).
Meanwhile, other investigations on CH and MCH using
various experimental approaches, such as rapid compression
machine (RCM) and jet-stirred reactor (JSR), were
reported.12−15
In comparison to studies of CH and MCH, cycloalkanes with
longer alkyl chains have not been studied enough. Conroy et
al.16 measured the ignition delay times for BCH using the
mixtures diluted in air at 10 and 30 atm and equivalence ratios
of 0.3, 0.5, 1.0, and 2.0 but did not make kinetic analyses. In
addition, previous studies also concerned n-propylcyclohexane
(PCH) through JSR,17−19 laminar flame,20 and RCM.21
However, very limited studies were performed on the
autoignition of ethylcyclohexane (ECH). Vanderover and
Oehlschlaeger10 measured the ignition delay times of ECH at
pressures of 15 and 50 atm and equivalence ratios of 0.25, 0.5,
and 1.0. Their experiments covered the temperature range of
680−1650 K. Various intermediate species were detected, and
good predictions by a detailed mechanism were observed when
JSR was employed by Husson et al.22 to study the ECH at 800
Torr, 500−1100 K, and ϕ = 0.25, 1.0, and 2.0. Additionally, Ji
Received: May 4, 2014
Revised: July 23, 2014
Published: July 24, 2014
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et al.23 measured the laminar flame speeds of ECH in the
counterflow configuration at the atmospheric pressure, temperature of 353 K, and wide range of equivalence ratios.
Up to now, there are limited studies on the oxidation of
longer alkyl-substituted CHs, especially for ECH. In this study,
the ignition delay times of ECH diluted in argon were
measured at pressures of 1.1, 5.0, and 10 atm, equivalence ratios
of 0.5, 1.0, and 2.0, and temperatures from 1000 to 1700 K. On
the basis of the previous research, the oxidation characteristics
of ECH ignition were discussed at both low and elevated
pressures, including the effects of the temperature and
equivalence ratio, which deepens the understanding of
oxidation of hydrocarbons theoretically. Considering some
atmospherically fuel-burning equipment, such as oil boiler,
scramjet, and pulse detonation engine (PDE), it is meaningful
in practice to account for the oxidation of hydrocarbon at low
pressure as well. In addition, this work supplements the data of
ignition delay time for ECH. The data are important
fundamental parameters to validate the oxidation mechanism
at a low pressure, providing the basis for the development of a
high-pressure oxidation mechanism.
Figure 1. Definition of the ignition delay time.
zero baseline. The temperature behind the reflected shock wave was
calculated using the reflected shock model in the software Gaseq26
with an uncertainty of about ±25 K. In addition, the uncertainty in the
measured ignition delay time was estimated to be 15%, which resulted
from the uncertainty in the shock attenuation and non-ideal shock
reflection. The data with error bars are plotted in Figure 4.
2. EXPERIMENTAL SECTION
The shock tube with 11.5 cm diameter has been described in details in
the previous publications.24,25 It consists of a 4.0 m driver section, a 4.8
m driven section, and a double-diaphragm machine. Different
thicknesses of the polyester terephthalate (PET) films were chosen
to obtain different reflected wave conditions. High-purity helium
(99.999%) and nitrogen (99.999%) were charged into the driver
section as the driver gas. The driven section was evacuated to pressure
below 1.0 Pa using a Nanguang vacuum system before the reactant
mixture was added. Four fast-response pressure transducers (PCB
113B26) are mounted at the side wall at constant intervals of 300 mm
along the last 1.5 m driven section. A photomultiplier (Hamamstu,
CR131) and piezoelectric pressure transducer with acceleration
compensation (PCB 113B03) are located at the endwall. The
photomultiplier with a filter narrowly centered at 307 ± 10 nm was
used to capture the OH* chemiluminescence, while the pressure
transducer was used to detect the reflected shock pressure. All signals
were collected with a digital recorder (Yokogawa, Scopecorder
DL750). Fuel mixtures were manometrically prepared in a 128 L
stainless-steel tank with a vacuum system and a highly accurate vacuum
meter. Following the injection of ECH (99.5%) into the evacuated
tank, the regulated amount of oxygen (99.999%) and argon (99.999%)
was charged. The detailed compositions of test mixtures are listed in
Table 1.
A typical profile of pressure behind the reflected shock wave and
OH* emission history is given in Figure 1. Pressure remains almost
constant until the onset of ignition, corresponding to the eruption of
OH* emission. Ignition delay time is defined as the time interval
between the arrival of the reflected shock wave at the end wall and the
extrapolation of the steepest rise in the OH* emission signal to the
3. RESULTS AND DISCUSSION
3.1. Data and Correlation. The ignition delay times are
given in Figure 2 and are available in Table S1 of the
Supporting Information. A formula (eq 1) suggested by
Horning et al.27 is used to make the correlations under various
conditions using the multiple linear regression method
τ = Aϕαpγ exp(Ea /RT )
where τ is the ignition delay time (μs), α and γ are exponent
factors for equivalence ratio and pressure (atm), respectively, Ea
is the total activation energy (kcal/mol), R (=1.986 kcal mol−1
K−1) is the universal gas constant, and T is the temperature
(K). Figure 2 shows the measured ignition delay times of ECH
mixtures at three pressures under the fuel-lean (ϕ = 0.5), fuelstoichiometric (ϕ = 1.0), and fuel-rich (ϕ = 2.0) conditions. In
general, the global activation energies are around 31.7−35.7
kcal/mol. The ignition delay time is correlated as
τ = (1.3 ± 0.3) × 10−3ϕ1.43 ± 0.04p−0.53 ± 0.02
exp((34.6 ± 0.5)/RT )
O2 (%)
Ar (%)
ϕ
P5 (atm)
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
6
6
6
3
3
3
12
12
12
93.5
93.5
93.5
96.5
96.5
96.5
87.5
87.5
87.5
1.0
1.0
1.0
2.0
2.0
2.0
0.5
0.5
0.5
1.1
5.0
10.0
1.1
5.0
10.0
1.1
5.0
10.0
(2)
where physical variables and their units are the same as those in
eq 1.
This correlation can be used to empirically estimate the
ignition delays within the tested conditions. Other available
ignition delay times for ECH can be found in the study by
Vanderover and Oehlschlaeger.10 They conducted the experiments at 12 and 50 atm in the dilution of air. Namely, the
fraction of oxygen is 20.65−20.92%, and that of ECH is 0.44%
at ϕ = 0.25, 0.86% at ϕ = 0.5, and 1.72% at ϕ = 1.0. Despite the
difference in the mixture compositions, the comparisons at ϕ =
1.0 and 0.5 are depicted in Figure 3. Obviously, the ignition
delay times are considerably shorter in comparison to the
current data. Large discrepancy is presented even at the
pressures of 10 and 12 atm. A gentler slope at p = 50 atm
indicates a rise in the reactivity of the mixture. Moreover, at ϕ =
1.0, the exponent factor of pressure dependence (γ) in eq 1 is
about −0.98 in the literature10 but about −0.52 in this work.
Table 1. Mixture Compositions in This Study
ECH (%)
(1)
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Figure 3. Comparison of measured results with previous data at (a) ϕ
= 1.0 and (b) ϕ = 0.5.
3.2. Numerical Simulation. Only one detailed mechanism,
JetSurF2.0 developed by Wang Group,29 is available for
simulating the autoignition of ECH, although various
mechanisms for CH and MCH have been developed. This
mechanism including 348 species and 2163 reactions contains
oxidation chemistry for normal alkanes up to n-dodecane.
Moreover, this model involves the submechanisms for CH/
MCH/ECH/BCH and has been validated extensively with
high-temperature data, except the submechanism for ECH.
However, no study was reported after Husson et al.22 simulated
their measured mole fractions of major species at low
temperatures using this mechanism. Here, JetSurF2.0 is used
to simulate and interpret the experimental results. The
simulation was conducted with a zero-dimensional and constant
volume adiabatic model of the SENKIN code30 in the
CHEMKIN II package.31 The calculated ignition delay time
is defined as the time interval between the start of simulation
and the time of steepest rise in the temperature (dT/dt).
In this study, both current and previous10 data are compared
to the numerical results. A significant deviation in simulation at
a low temperature occurs on the assumption of constant U and
V.32 An average pressure rise (dp/dt) of 4%/ms is taken into
account. The predictions with and without consideration of
pressure rise are plotted in Figure 4. Consideration of the
pressure rise obviously reduces the numerical ignition delay
times when the ignition time is over 500 μs. Figure 5 gives the
Figure 2. Ignition delay times and correlations for ECH at 1.1, 5.0, and
10 atm and equivalence ratios of (a) 0.5, (b) 1.0, and (c) 2.0.
Venderover and Oehlschlaeger10 gave the value of −0.99 for
MCH, and Vasu et al.28 gave the value of −0.98 for MCH,
under similar test conditions. The values for CH and
cyclopentane provided by Daley et al.7 turned out to be
around 1.0 again. However, the value of about −0.55 was
acquired by fitting the ignition delay time of MCH at 1.0−4.0
atm and ϕ = 1.0 in the study by Orme et al.8 and our
unpublished measured results for CH and MCH at 1.0−5.0
atm. This is attributed to the high pressure and fuel- and O2rich mixture.
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Figure 5. Validation of the JetSurF 2.0 mechanism against data by
Venderover et al.10 Symbols, experimental measurements; lines,
simulations.
mechanism at a low temperature, just like the motivating
influence of peroxy species chemistry on ignition at an elevated
pressure and a moderate temperature in the study by Daley et
al.7 It should be noted that the ignition delay time at 12 atm
may have larger uncertainty. There is doubt that the previous
data may be too limited.
3.3. Chemical Kinetic Interpretation of ECH Ignition.
From the discussion above, it can be seen that the JetSurF2.0
mechanism well predicts the ignition delay times of ECH at
1.0−10.0 atm and can also capture the dependence upon the
equivalence ratio at an elevated pressure. In this work, the
ignition delay time is increased with increasing the equivalence
ratio at a fixed fuel concentration. Hong et al.11 also found a
similar observation when keeping the oxygen concentration
constant in their study. However, Vanderover and Oehlschlaeger10 obtained a different conclusion, which was also
found in other investigations on hydrocarbons.7,33,34 All of
these studies were performed at pressures up to 50 atm with an
almost fixed O2 fraction. In general, if the oxygen concentration
is altered to obtain various equivalence ratios, a fuel-rich
mixture shows a longer ignition delay time. However, if the fuel
concentration is changed to obtain different equivalence ratios,
a fuel-lean mixture presents the faster ignition. Actually, this
feature can be well-reproduced using JetSurF2.0. Without loss
of generality, the ignition delay times of ECH at 5.0 and 50 atm,
equivalence ratios of 0.5 and 2.0, and temperatures from 1100
Figure 4. Validation against the mechanism with the current data.
Symbols, measurements; lines, simulations.
comparison between the measured data in ref 10 and the
simulated results using the JetSurF2.0 mechanism. Despite the
pressure variation of 2−4%/ms in their study, dp/dt has limited
influence on the simulation, as shown in the dot line in Figure
5b, because all measurements are performed at an ignition time
less than 1.0 ms.
The predictions agree well with the current data but depart
from the previous data. However, the experimental data in the
literature10 always overpredict under the test conditions,
especially at 12 atm, as shown in Figure 5. The inaccuracy at
50 atm may be contributed to the poor performance of the
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to 1900 K with constant 6% O2 were simulated, as shown in
Figure 6. Results show that a fuel-rich mixture exhibits a shorter
Figure 6. Simulation of ignition delay times for ECH at 5.0 and 50
atm, with ϕ = 0.5 and 2.0.
ignition delay time when the temperature decreases at 50 atm.
To clarify this influence in details, the kinetic analyses will be
made for the ECH ignition at temperatures of 1150 and 1550
K.
3.3.1. Moderate Temperature. It is well-known that
oxidation and decomposition pathways are different at different
temperatures. The temperature of 1150 K is selected as a
representative of the moderate temperatures. The histories of
the temperature and mole fractions of ECH and C2H4 for 5 and
50 atm at this temperature are plotted in Figure 7. The mole
fraction of C2H4 collapses immediately, and the temperature
rises rapidly, when ECH is completely consumed in the ignition
process. However, both fuel-lean and fuel-rich mixtures have a
comparable decomposition rate of ECH at 5.0 atm, but the
mole fraction of ECH decreases faster at ϕ = 2.0 than that at ϕ
= 0.5 when the pressure is up to 50 atm. In other words, the
rate of consumption at ϕ = 2.0 is higher than that at ϕ = 0.5 at
the pressure of 50 atm.
H, O, OH, and HO2 are mainly chain-branching radicals to
accelerate the oxidation of hydrocarbons. Their production and
consumption can provide some key information in the fuel
oxidation process. To interpret the effects of these free radicals
on the oxidation of ECH, a detailed analysis on the rate of
production (ROP) of the OH radical will be conducted using
JetSurF2.0 in the next section. Figure 8 shows the ROP analysis
of OH at the pressures of 5.0 and 50 atm with the equivalence
ratios of 0.5 and 2.0 during the period before ECH ignition.
Three major reactions contribute to the generation of the OH
radical; they are H + O2 ⇄ OH + O (R1), 2OH (+M) ⇄ H2O2
(R14), and CH3 + HO2 ⇄ CH3O + OH (R92).
Figure 9 depicts the ROP time history, where reactions R14
and R92 contribute more to the production of OH than
reaction R1 prior to the ignition. Moreover, this contribution
becomes remarkable at a pressure of 50 atm and an equivalence
ratio of 2.0. It is the reaction R1 that triggers the burst of OH
and initiates the ignition in all conditions. As shown in Figure
7b, the rates of decomposition of ECH and consumption of
intermediates for the fuel-rich mixture were faster than that for
the fuel-lean mixture at the pressure of 50 atm. It is clear that
the fuel-rich mixture presents greater total productivity of OH
Figure 7. Calculated temperature and mole fractions of ECH and
C2H4 at 1150 K, at pressures of (a) 5 and (b) 50 atm and equivalence
ratios of 0.5 (0.25% ECH, 6% O2, and 93.75% Ar) and 2.0 (1% ECH,
6% O2, and 93% Ar). Solid lines, ϕ = 0.5; dashed lines, ϕ = 2.0. Black
lines, mole fraction of ECH; blue lines, mole fraction of C2H4; red
lines, temperature.
than the fuel-lean mixture, as shown in the dash-dotted lines in
Figure 8b. The net OH production rate is restrained by the fuel
consumption, which needs small radicals. Namely, the fuel-rich
mixture will consume more radicals, resulting in the nearly
paralleled net ROPs of OH between fuel-lean and fuel-rich
mixtures at 50 atm, as shown in Figure 8a. Reactions R14 and
R92 are more productive at ϕ = 2.0 than that at ϕ = 0.5, as
shown in Figure 9b, and this contributes to the greater OH
ROP for the fuel-rich mixture at 50 atm. In contrast to the
behavior at 5 atm, as shown in Figure 9a, the production rates
of R14 and R92 show distinct superiority over that of R1 at ϕ =
2.0 because fuel and hydrocarbon intermediates are intensified
because an elevated pressure promotes the three-body
reactions.
However, R14 and R92 lose their lead in chain branching to
R1 when the pressure falls to 5.0 atm, as shown in Figure 9a. As
a result, OH production rates in the fuel-lean and fuel-rich
mixtures turn out to be equal when only the initial stage (<750
μs) is considered, as depicted in Figure 8b. On the other hand,
because more fuel consumes more radicals, the lean mixture has
a greater net OH production rate in Figure 8a. As shown in
solid lines in Figure 9a, the rate of R1 at ϕ = 0.5 is larger than
that at ϕ = 2.0, even at the initial stage. This is mainly
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Figure 9. Major reactions responsible for the production of the OH
radical at 1150 K. Solid lines, ϕ = 0.5; dotted lines, ϕ = 2.0. Red lines,
H + O2 ⇄ OH + O (R1); blue lines, 2OH ⇄ H2O2 (R14); black lines,
CH3 + HO2 ⇄ CH3O + OH (R92).
Figure 8. Net and total rates of production of OH at 1150 K: (a) net
rate of production and (b) total rate of production.
attributed to the less amount of oxygen because the mole
fractions of O2 for both equivalence ratios are almost equal,
while the contents of H atoms are obviously different, as shown
in Figure 10. In general, at the pressure of 5.0 atm, free radicals
are harder to accumulate and ignition delays for the fuel lean
mixture because fuel decomposition fails to be intensified and
more free radicals are demanded. Curran et al.35 also pointed
out that the radicals formed directly from a parent fuel are in
charge of chain branching at a low temperature. This
dependence is strengthened by increasing the pressure and
equivalence ratio.
3.3.2. High Temperature. Curran et al.35 pointed out that
the reaction H + O2 ⇄ OH + H becomes dominant in the
chain-branching process with an increase in the temperature.
Thus, the temperature of 1550 K is chosen to analyze the effect
of temperature variation on the ECH ignition.
Figure 11 shows the time-dependent temperature and mole
fractions of ECH and C2H4 for both fuel-lean and fuel-rich
mixtures during the ignition process at 1550 K and 5.0 atm. It is
observed that ECH disappears swiftly with similar speeds under
both fuel-lean and fuel-rich conditions. In comparison to ECH
consumption, the oxidation of hydrocarbon intermediates takes
a longer time before ignition. As discussed above, small radicals,
such as OH, affect the oxidation. The major reactions
Figure 10. Mole fraction of H and O2 for ECH at 1150 K and 5.0 atm.
associated with OH production are depicted in Figure 12. In
comparison to that at 1150 K, reaction R14: 2OH (+M) ⇄
H2O2 (+M) is ruled out and reaction R92: CH3 + HO2 ⇄
CH3O + OH is suppressed. Reaction H + O2 ⇄ OH + H,
nevertheless, plays a significant role in producing the OH
radical, especially at ϕ = 0.5. This implies that the offspring
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According to the analysis above, it is understood why the
fuel-lean mixture is easily ignited at a high temperature. The
advantage of alkyl radicals provided by the fuel fades away.
Adversely, the less the fuel is in the mixture, the easier it is
depleted and the mixture will be ignited.
3.4. Sensitivity Analysis. To examine the effect of the
dominant elementary reactions on the ECH ignition, the
sensitivity analyses were made at 5.0 atm, 1400 K, and three
equivalence ratios. The normalized sensitivity coefficient is
defined as
S=
τ(2ki) − τ(0.5ki)
1.5τ(ki)
(3)
where τ is the ignition delay time and ki is the reaction rate
coefficient of the ith reaction.
A total of 17 reactions with the largest sensitivity coefficients
are identified, as shown in Figure 13. As expected, the
sensitivity coefficient of the reaction R1 is 3 times greater than
that of the second largest reaction at 1400 K, indicating its large
influence on the ignition at a high temperature. It is noted that
the ignition-promoting reactions are identical in each mixture,
while some key ignition-inhibiting reactions are significantly
different. For example, the ignition at ϕ = 0.5 is promoted by
reaction R35: HCO + M ⇄ CO + H + M and inhibited by
reaction R37: HCO + O2 ⇄ CO + HO2. However, this is
opposite at ϕ = 2.0. Actually, reaction R35 competes for the
HCO species with R37, which brings the opposite effects on
the ECH ignition. At ϕ = 2.0, radical HO2 is responsible for the
oxidation during the initial stage, while radical H is responsible
at ϕ = 0.5. The HO2 radical produced through R37 favors the
generation of the OH radical via R92: CH3 + HO2 = CHO +
OH and, thus, promotes the global reaction rate and accelerates
the ignition for the fuel-rich mixture. Reaction R35 consumes
HCO and produces the less important H radical, limiting the
amount of HCO to be consumed via R37. Therefore, at ϕ =
2.0, R37 promotes the ignition, while R35 slows the ignition.
R35 and R37 behave adversely at ϕ = 0.5. In addition, reaction
R20: HO2 + OH ⇄ H2O + O2 has the largest positive
sensitivity coefficient at ϕ = 0.5 and 1.0 but is very small at ϕ =
2.0. Instead, R35 has an inhibiting influence on the ignition for
the fuel-rich mixture. For the fuel-lean mixture, the OH radical
is a key chain-branching radical and the HO2 radical is
important as well. Thus, it is reasonable that R20, which
simultaneously annihilates these two radicals, is the most
significant terminal reaction. For the fuel-rich mixture, HO2
tends to be produced more easily and the importance of OH
radical fades, resulting in the relegation of R20. Because of the
competition between R35 and R37, R35 outstands. In addition,
there are also the reactions associated with ethylene (C2H4), as
shown in Figure 13. Dissociation of ethylene to form vinyl
radical (C2H3) through the attack of H and OH tends to
shorten the ignition delay. In the case of the fuel-rich mixture,
reaction R248: C2H4 + O ⇄ CH3 + HCO becomes an ignition
inhibitor. This is ascribed to the less importance of the
subsequent reactions of this pathway in ignition compared to
other pathways that ethylene undergoes.
Panels a and b of Figure 14 show the sensitivity analysis at
1100 K and two pressures (5.0 and 50 atm), respectively. At p =
5.0 atm, reaction R19: HO2 + HO2 = O2 + H2O2, which does
not appear at 1400 K, will play a main ignition-inhibiting role at
all three equivalence ratios. It strongly terminates HO2 to form
more stable O2 and H2O2, indicating the considerable
Figure 11. Calculated temperature and mole fractions of ECH and
C2H4 at 1550 K, 5.0 atm, and equivalence ratios of 0.5 and 2.0. Solid
lines, ϕ = 0.5; dashed lines, ϕ = 2.0. Black lines, mole fraction of ECH;
blue lines, mole fraction of C2H4; red lines, temperature.
Figure 12. Major reactions responsible for production of OH at 1550
K.
radicals of a parent fuel devote less to the oxidation process.
Thus, rich fuel again becomes a kind of a burden for the
ignition, and the mixture of a high equivalence ratio undergoes
a longer ignition delay time.
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Figure 14. Sensitivity analysis for ECH at 1100 K at (a) 5.0 atm and
(b) 50 atm.
4. CONCLUSION
Ignition delay times were measured behind reflected shock
waves for ECH at the pressures of 1.1, 5.0, and 10.0 atm,
equivalence ratios of 0.5, 1.0, and 2.0, and temperatures from
1050 to 1650 K. Numerical simulation was made with the
mechanism JetSurF2.0. Good agreement between the experimental and simulated results was realized. Data published in
previous literature10 at 12 and 50 atm diluted with air were
compared to the simulations. Overprediction by the mechanism
is attributed to the weak performance at a low temperature.
In comparison to the fuel-lean mixture, the fuel-rich mixture
has shorter ignition delay times at a low temperature and high
pressure. Reaction H + O2 = OH + H is dominant at the high
temperature, while reactions related to HO2 are dominant at
the low temperature. The fuel-rich mixture is more reactive
because of its more production of HO2, which is facilitated by a
high pressure and concentration of fuel at intermediate and low
temperatures. However, this benefit for fuel-rich mixtures is
weakened at a high temperature, and more fuel content requires
more time to be consumed, resulting in the longer ignition
delay time for the fuel-rich mixture.
Sensitivity analysis shows that the promoting effects of
reaction R1 (H + O2 = OH + H) on the ignition at a high
temperature goes down with the decrease in the temperature.
Elevating the pressure at a low temperature further restrains the
promoting effectiveness. At a high temperature, reactions R35
Figure 13. Sensitivity analysis for ECH at 1400 K and 5.0 atm with
equivalence ratios of (a) 0.5, (b) 1.0, and (c) 2.0 and fixed O2 mole
fraction of 0.5%.
significance of HO2. R20 only strongly inhibits the ignition at ϕ
= 0.5 and 1.0. However, this becomes different as the pressure
increases. Reaction R1 is highly suppressed. The fuel-related
reactions turn to be more important. At p = 50 atm, new
reactions, such as R1727: ECH + HO2 ⇄ C2H5S3×cC6H10 and
R1728: ECH + HO2 ⇄ C2H5S3×cC6H10 appear. Moreover, the
low-temperature reaction R2703 also exhibits a fairly strong
effect on the ignition, indicating its importance under lowtemperature chemistry at an elevated pressure.
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■
ASSOCIATED CONTENT
S Supporting Information
*
Ignition delay times of ECH (Table S1). This material is
available free of charge via the Internet at http://pubs.acs.org.
■
AUTHOR INFORMATION
Corresponding Authors
*Telephone: 86-29-82665075. Fax: 86-29-82668789. E-mail:
[email protected].
*Telephone: 86-29-82665075. Fax: 86-29-82668789. E-mail:
[email protected].
Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS
This work is supported by the National Natural Science
Foundation of China (51206132, 51136005, and 51121092),
the National Basic Research Program (2013CB228406), and
the China Postdoctoral Science Foundation (2013T60876).
The authors also appreciate the funding support from the
Fundamental Research Funds for the Central Universities and
State Key Laboratory of Engines (SKLE201305).
■
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