Article
pubs.acs.org/EF
Comparative Study of Experimental and Modeling Autoignition of
Cyclohexane, Ethylcyclohexane, and n‑Propylcyclohexane
Zemin Tian, Yingjia Zhang,* Feiyu Yang, Lun Pan, 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 were measured for cyclohexane, ethylcyclohexane, and n-propylcyclohexane at atmospheric
pressure, equivalence ratios of 0.5, 1.0, and 2.0, and temperatures of 1110−1650 K behind reflected shock waves with a fixed fuel
concentration of 0.5%. Computational simulations were made using three generally accepted mechanisms, yielding acceptable
agreements with the current measurements at the tested equivalence ratios. Nonetheless, there is a slight overprediction at ϕ =
2.0 for n-propylcychexane. Ethylcyclohexane and n-propylcyclohexane have shorter ignition delay times than cyclohexane at high
temperatures. However, this difference decreases with the decrease in the temperature. Simulation and comparison to previous
data indicate that the oxidation rates of ethylcyclohexane, n-propylcyclohexane, and n-butylcyclohexane are in the order of npropylcyclohexane > ethylcyclohexane ≈ n-butylcyclohexane. Kinetic analysis is performed to obtain insight into the observation.
1. INTRODUCTION
Worldwide use of fossil fuels increases the need to understand
their oxidation mechanisms. Cycloalkanes are the important
compositions of not only traditional vehicle fuels, such as
gasoline and diesel, but also jet fuels, such as Jet-A/Jet-A1/JP8.1−3 It is also found that cycloalkanes produce soot precursors
more easily than chain alkanes.4 Thus, they have attracted much
attention, and detailed chemistry has been established.
Cyclohexane (CH) has the simplest molecular structure
among all cycloalkanes with a six-membered ring and has
received many investigations. Recently, Serinyel et al.5 detected
34 intermediates in the CH oxidation using a jet-stirred reactor
(JSR) at a pressure of 1.07 atm, temperatures from 500 to 1100
K, and equivalence ratios of 0.5, 1.0, and 2.0. The negative
temperature coefficient (NTC) was captured, and good
predictions were achieved using an updated kinetic mechanism.
Vranckx et al.6 measured the ignition delay times for a CH/O2/
N2/Ar mixture in a rapid compression machine (RCM) at
pressures up to 40 atm and temperatures between 680 and 910
K to observe the NTC behavior. Their measurements were
compared to the predictions of several mechanisms, and
differences in predictions were interpreted. In addition, Daley
et al.7 studied the CH ignition characteristics behind reflected
shock waves at temperatures of 847−1379 K, pressures of 11−
61 atm, and ϕ = 1.0, 0.5, and 0.25. They compared and
analyzed the results with four mechanisms.
In addition, methylcyclohexane (MCH)8−10 has also been
widely investigated in the past decade. In comparison to MCH,
only limited studies of ethylcyclohexane (ECH), n-propylcyclohexane (PCH), and n-butylcyclohexane (BCH) are available.
For ECH, Husson et al.11 investigated the elementary reactions
at low temperatures covering the NTC area and quantitatively
identified 47 intermediates using a jet-stirred reactor (JSR).
Their results showed that the proposed detailed kinetic model
could well-predict the key species mole fractions. Vanderover
and Oehlschlaeger12 performed measurements of ignition delay
© 2014 American Chemical Society
times for ECH/air mixtures in a shock tube at pressures of 12
and 50 atm, temperatures of 881−1319 K, and equivalence
ratios of 0.25, 0.5, and 1.0. However, they did not present
kinetic analysis. Ristori et al.13 measured concentration profiles
of reactants, intermediates, and final products of PCH at
temperatures of 950−1250 K, ϕ = 0.5−2.0, and pressure of 1.0
atm in a JSR and proposed a detailed mechanism for PCH.
Other studies on PCH using a JSR and laminar flame have also
been reported.14−16 In addition, Dubois et al.17 measured the
ignition delay time of PCH/O2/Ar mixtures at 10 and 20 atm,
with equivalence ratios of 0.2, 0.3, 0.4, 0.5, 1.0, and 1.5, ranging
the temperatures of 1250−1800 K in a shock tube. They also
tested the laminar flame speed at an initial temperature of 403
K and initial pressure of 1 atm with equivalence ratios between
0.6 and 1.75. The experimental data were well-modeled with a
combined mechanism. Crochet et al.18 measured the ignition
delay times of lean PCH/air (ϕ = 0.3, 0.4, and 0.5) at
temperatures of 620−930 K and pressures of 0.45−1.34 MPa in
a RCM; NTC behaviors were observed; and major
decomposition pathways of PCH were given. For BCH,
Conroy et al.19 measured the ignition delay times with air
dilution at pressures of 10 and 30 atm, equivalence ratios of 0.3,
0.5, 1.0, and 2.0, and temperatures of 950−1430 K in a heated
shock tube. Recently, some researchers conducted the
comparative studies on the combustion characteristics of
cycloalkanes. For example, Hong et al.20 measured the ignition
delay times of CH, MCH, and BCH at pressures of 1.5 and 3.0
atm and ϕ = 0.5 and 1.0, in which they summarized the ignition
delay time in the order of MCH > CH ≈ BCH. Wu et al.21 and
Ji et al.22 measured the laminar flame speeds of CH and
monoalkylated CHs, and the reasons for larger flame speed of
CH than those of monoalkylated CHs were analyzed. Although
Received: June 24, 2014
Revised: October 7, 2014
Published: October 8, 2014
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many studies were reported on CH combustion, very few
studies focused on the effect of branching chains on the
decomposition of naphthenes. Thus, it is worth studying both
experimental and kinetic analysis.
In this study, measurements of ignition delay times for CH,
ECH, and PCH were conducted at p = 1.1 atm, T = 1110−
1650 K, and ϕ = 0.5, 1.0, and 2.0. The experimental results are
helpful for the understanding of the influences of branching
chains on the decomposition of naphthenes and modifying the
kinetic mechanism.
Table 1. Compositions and Pressures of Test Mixtures for
Fuel/O2/Ar
fuel
ϕ
fuel (%)
O2 (%)
Ar (%)
p (atm)
CH
2.0
1.0
0.5
1.0
2.0
1.0
0.5
2.0
1.0
0.5
0.5
0.5
0.5
0.444
0.5
0.5
0.5
0.5
0.5
0.5
2.25
4.5
9
4
3
6
12
3.375
6.5
13
97.25
95
90.5
95.556
96.5
93.5
87.5
96.125
93
96.5
1.1
1.1
1.1
3.0
1.1
1.1
1.1
1.1
1.1
1.1
ECH
2. EXPERIMENTAL SECTION
PCH
Measurements of ignition delay times were performed in a stainlesssteel shock tube, of which the detailed description has been presented
in previous publications.23,24 Here, only a brief introduction is given.
The shock tube is separated into a 4 m long driver section and a 5.3 m
long driven section by a double diaphragm machine. Four fast
response pressure transducers (PCB 113B26) installed over the end
part of the driven section are used to measure the local incident shock
velocities. A piezoelectric pressure transducer with acceleration
compensation (PCB 113B03) and a photomultiplier with a wavelength
of 307 ± 10 nm through a narrow filter are sited at the endwall to
detect the reflected shock pressure and local OH* chemiluminescence.
Fuel mixtures of 98% purity, oxygen (99.999%), and argon (99.999%)
are manometrically prepared in a 128 L stainless-steel mixing tank, and
no less than 12 h wait allows for homogeneous diffusion.
The ignition delay time is defined as the interval between the arrival
of the incident shock wave and the intersection of the steepest tangent
line of the OH* chemiluminescence signal with the baseline, as shown
in Figure 1. The temperature behind the reflected shock wave is
determined by a chemical equilibrium program Gaseq25 with an
uncertainty of about 15 K.
O2/Ar mixtures, as shown in Figure 2. In addition, the ignition
delay time of CH is also measured at ϕ = 1.0 and 3.0 atm with
Figure 2. Comparison of CH ignition delay times of this study to
those by Hong et al.20
4% O2 mole fraction in this study, which reproduces the
experimental condition in the previous study.20 The data from
the current study and the previous study20 at 3.0 atm and 4%
O2 mole fraction are also plotted in Figure 2. The results show
that the data from both studies are in good consistency.
3.1. Selected Kinetic Mechanisms and Numerical
Predictions. It is well-known that several detailed kinetic
mechanisms have been developed to interpret the oxidation of
CH, three of which are selected here to reproduce the current
measurements. One is the kinetic mechanism developed by
Silke et al.;26 it includes 1081 species and 4268 reactions and
involves both low- and high-temperature chemistries for CH.
The model by Silke et al. has been validated against the ignition
delay times of the mixture of CH/O2/N2 for the pressure
ranges of 7−9 and 11−14 atm and temperature ranges of 650−
900 K obtained in RCM in the study by Lemaire et al.27 In
addition, some intermediate species measured at 727 K and 7.4
atm were also reproduced. Another one is the kinetic
mechanism given by Sirjean et al.28 on the basis of a
mechanism generator software EXGAS and has also been
validated against various data. It gives good predictions on
ignition delay times at pressures of 1.0−10.0 atm and
temperatures above 1000 K.20,28 The third one is the kinetic
mechanism of JetSurF2.0 constructed by Wang et al.29 This
model composes 348 species and 2163 reactions and has been
widely validated and accepted.6,7,11,20 Despite the combined
Figure 1. Definition of the ignition delay time.
3. RESULTS AND DISCUSSION
This study measures ignition delay times of CH, ECH, and
PCH at a pressure of 1.1 atm, equivalence ratios of 0.5, 1.0, and
2.0, and temperatures from 1100 to 1650 K with a fixed fuel
concentration of 0.5% and makes the numerical simulations
using kinetic mechanisms. Compositions of all tested mixtures
are listed in Table 1, and the measured data are provided in
Table S1 of the Supporting Information.
The current data for the stoichiometric 0.5% CH/O2/Ar
mixture approximately at 1.1 atm were compared to the work
by Hong et al.,20 who obtained data at 1.5 atm for 0.444% CH/
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mechanism for PCH in the study by Dubois et al.,17 JetSurF2.0
is used for ECH and PCH. In addition, all simulations were
conducted using the CHEMKIN II package30 with SENKIN
code,31 and the ignition delay times are determined by the time
interval between zero and the maximum rate of the temperature
rise (maximum dT/dt). An average pressure rise of 4%, namely,
dp/dt, was considered in the simulations to model the physical
influence of the shock tube.32
As shown in Figure 3a, the three selected mechanisms
generally exhibit the acceptable predictions for the ignition
delay times of CH under the test temperature conditions.
However, the mechanism by Sirjean et al. gives overprediction
up to 80% at lower temperatures for the fuel-lean mixture (ϕ =
0.5). It is probably due to the lack of some important reactions
of intermediate hydrocarbons in the mechanism by Sirjean et al.
For the fuel-lean mixture, the reactions of intermediate
hydrocarbons plus small radicals become more important.
The ignition will be delayed in shortage of some relevant
oxidation reactions. The mechanisms by Silke et al. and
JetSurF2.0 show better performance in reproducing the results
under all test conditions. It can be noted that both mechanisms
by Sirjean et al. and Silke et al. predict higher apparent
activation energy of the ignition delay than JetSurF2.0.
Moreover, at high temperatures, the selected mechanisms,
except JetSurF2.0, give slight underprediction at high temperatures, especially the mechanism by Silke et al. For ECH and
PCH, only JetSurF2.0 can reproduce the ignition delay times,
as shown in panels b and c of Figure 3. However, it should be
noted that there is an obvious overprediction for the rich PCH
mixture (ϕ = 2.0), in contrast with the good agreements at lean
and stoichiometric equivalence ratios.
3.2. Comparison between CH, ECH, and PCH. Hong et
al.20 also compared the ignition delay time of CH, ECH, and
BCH under similar conditions to this study. They proposed
three factors in the difference of delay times among those fuels,
namely, the unimolecular reactions, the regenerating ability of
the H radical, and the production of CH3. Although CH and
MCH have equally slow unimolecular reaction rates, CH is able
to regenerate more H radical, leading to a shorter ignition delay
time of CH. BCH has a faster unimolecular dissociation
reaction rate, and this compensates for the less regenerated H
radical. As a result, BCH exhibits a comparative ignition delay
time to CH. However, it can be noticed by reviewing the
previous work that the ignition delay time of BCH was shorter
than that of CH at high temperatures for the fuel-lean mixture.
Figure 4 shows the ignition delay times of CH and BCH at p =
3.0 atm and ϕ = 1.0 and p = 1.5 atm and ϕ = 0.5 measured by
Hong et al.20 It clearly shows that BCH has a shorter ignition
delay time when the temperature rises to 1350 K at ϕ = 0.5.
In this study, similar behavior is also observed in the
comparison of ignition delay times between CH, ECH, and
PCH at ϕ = 0.5 and 1.0, as shown in Figure 5. At a temperature
less than 1450 K, the ignition delay time of CH becomes closer
and closer to those of ECH and PCH, but it gives a much
longer ignition delay than those of ECH and PCH when the
temperature is higher than 1450 K. Thus, it can be inferred that
the stability of the cyclic structure of CH has effects on ignition
at high temperatures.
3.2.1. Influence of the Molecular Structure at High
Temperatures. To understand the ignition chemistry of CH,
ECH, and PCH at high temperatures, the JetSurF2.0
mechanism was employed to conduct flux analyses because
only JetSurF2.0 involves all submechanisms of the current
Figure 3. Ignition delay times and prediction for (a) CH by
JetSurF2.0,29 Silke et al.,26 and Sirjean et al.28 mechanisms, (b) ECH
by JetSurF2.0, and (c) PCH by JetSurF2.0 at 1.1 atm. The uncertainty
is 15%.
study objects. Simulations of CH ignition delay times by the
mechanisms of Silke et al. and Sirjean et al. are also included in
Figure 5. In comparison to JetSurF2.0, the mechanism by Silke
et al. gives a shorter prediction in ignition delay time at high
temperatures (>1500 K) and a longer prediction in ignition
delay time at low temperatures (<1450 K). It is noted that the
unimolecular reactions for CH in the three mechanisms are
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should be pointed out that these two pathways in the
mechanisms by JetSurF2.0 and Sirjean et al. have little influence
on ignition. In contrast, according to Silke et al., the CH
molecule directly decomposes into methyl and 1-penten-6-yl
(cC6H12 = C5H9 + CH3), which is proven to be essential to the
oxidation of CH in the mechanism by Silke et al.
Previous pyrolysis studies reported the decomposition of
CH. For example, Kiefer et al.33 reported that CH was mainly
isomerized to 1-hexene based on their experimental and
theoretical analyses. The mole fraction of 1-hexene was
detected by Serinyel et al.5 and Wang et al.34 Moreover,
Granata et al.35 also introduced the production of 1-hexene by
CH. Hence, the adoption of reaction: cC6H12 = C5H9 + CH3 in
the mechanism by Silke et al. seems inaccurate. Moreover, the
mechanism by Sirjean et al. gives clearly longer prediction at
low temperatures. Therefore, the JetSurF2.0 mechanism is used
to analyze the CH oxidation.
The oxidation of CH, ECH, and PCH at 1600 K for the
stoichiometric mixtures was analyzed to interpret the
observation above. Histories of the temperature and fuel
mole fraction are displayed in Figure 6. The result indicates that
the ignition delay time of ECH is much shorter than that of CH
and slightly longer than that of PCH. Note that a longer
ignition delay time is actually spent on the oxidation of
hydrocarbon intermediates. It spends much more time in
decomposing CH (≈35 μs) than in decomposing PCH and
ECH (≈9 μs), which partly brings the longer ignition delay
time for CH.
Because of the similar consuming rates of PCH and ECH,
the major attention is focused on the comparison of the
ignition of ECH to that of CH. Figure 7 depicts the rates of
consumption of CH and ECH and presents the decomposition
pathways. CH is consumed by either direct C−C bond fission
to form 1-hexene (path 1) or H-abstraction reactions by small
radicals to form the cyclohexyl radical (path 2). Likewise, C−C
bond fission and H-abstraction reactions are the major
decomposing pathways for ECH. Each reaction pathway is
classified on the basis of the reaction sites. The dissociation
reaction of the H atom (R → R• + H) has little influence and,
thus, is ignored.
Figure 8 displays the normalized rates of each pathway along
with the consumed CH and ECH. Although the majority of CH
is initially decomposed via C−C bond fission (path 1), the
consuming rate peaks when H-abstraction reactions (path 2)
flourish. Particularly, at the time of 50% consumption of CH,
path 1 only accounts for 20% of the total consuming rate. In
contrast, the unimolecular decomposition reactions significantly
promote the consumption of ECH. A total of 50% ECH still
undergoes C−C bond fission (path 1) when 50% ECH is
consumed. According to the comparison to ECH, the
unimolecular pyrolysis of CH exhibits a smaller effect on the
consumption of CH at high temperatures, which provides
evidence of the stable structure of the unsubstituted ring for
CH. As a result, ECH can be dissociated much faster than CH
at high temperatures, facilitating the ignition of ECH because
the unimolecular reactions are essential at high temperatures.
Also, it should be noted that the influence is notable only at
high temperatures. For PCH, almost the same profile is
observed.
Moreover, CH produces much more 1-hexene than ECH
and PCH, contributing to the longer delay time of CH. Figure
7a shows the decomposition process of 1-hexene, which rapidly
cracks to allyl (aC3H5) plus propyl (nC3H7). This was
Figure 4. Comparison of ignition delay times between CH and BCH
mixtures containing 4% O2 using data by Hong et al.20
Figure 5. Comparison of ignition delay times among CH, ECH, and
PCH at 1.1 atm with equivalence ratios of (a) ϕ = 1.0 and (b) ϕ = 0.5.
different, as listed in Table 2. In the JetSurF2.0 mechanism, a
C−C bond of the CH molecule is broken to form 1-hexene
(cC6H12 = C6H12), while a C6H12 diradical (C6H12-16) is
produced because of C−C bond homolysis in the mechanism
by Sirjean et al. Then, the C6H12 diradical forms 1-hexene. It
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Table 2. Comparison of Rate Constants of CH Unimolecular Reactions in Mechanisms by JetSurF2.0, Sirjean et al., and Silke et
al.
mechanism
JetSurF2.0
Sirjean et al.
Silke et al.
reaction
A (cm3 mol−1 s−1)
cC6H12 = C6H12
cC6H12 = C6H12-16
cC6H12 = C5H9 + CH3
5.01 × 10
3.1 × 1017
1.0 × 1016
16
rate at 1600 K (cm3 mol−1 s−1)
n
Ea (cal)
0
0
0
8.82 × 10
8.9 × 104
7.10 × 104
4
4.56 × 104
2.13 × 105
2.05 × 106
reaction for CH (R1897: CH ⇄ 1-hexene) has a remarkable
inhibiting effect on the ignition of CH, while similar reactions
for ECH and PCH are not present. Furthermore, the oxidation
of CH suffers from strong inhibition through reaction R321: H
+ aC3H5 (+M) ⇄ C3H6, while that of ECH and PCH seems
exempt from it.
Thus, the longer ignition delay time of CH than those of
ECH and PCH at high temperatures is mainly owing to the
increase in unimolecular decomposition of alkylated CHs and
the abundant productions of allyl and propene during CH
oxidation.
3.2.2. Influence of the Molecular Structure at Low
Temperatures. With the decrease in temperature (<1250 K),
the ignition delay times of CH, ECH, and PCH tend to fall in
the order of CH ≈ ECH > PCH, as shown in Figure 5. It is
because unimolecular reactions with large activation energy
become less important and the H-abstraction reactions start to
dominate the ignition chemistry. Figure 10 gives the consuming
rate versus percentage of consumed fuel for CH, ECH, and
PCH at 1150 K, 1.1 atm, and ϕ = 1.0. CH is consumed quickly
at the beginning and then is depleted at a modest rate, while
other fuels have steady consuming rates. In the first part when
less than 50% fuel is consumed, the H-abstraction reactions by
H radical dominate the ignition chemistry, as shown in Figure
11. Hence, the regenerative capacity of the H radical is a
significant factor in fuel consumption.20 Figure 12 presents the
total H production rates over normalized consumed fuel
concentrations for CH, ECH, and PCH. Obviously, CH
produces more H radicals than other fuels during the first part
because of the special products of CH. As pointed out by Hong
et al.,20 the major product of CH, cyclohexyl (cycC6H11) can
form hexenyl, followed by the production of ethyl (C2H5) and
butenyl (C4H7), which are key predecessors of the H radical.
Then, the H radical stimulates the fuel decomposition, leading
to continuous regeneration of the H radical.
Figure 6. Ignition time histories for CH, ECH, and PCH at 1600 K
and ϕ = 1.0, including the fuel mole fraction and temperature. The
dash lines represent the temperature.
confirmed by Tsang et al.36 and Kiefer et al.33 The
concentration of propyl keeps a much lower level compared
to that of allyl, as illustrated in Figure 9. The reason is that
propyl will be quickly decomposed to methyl (CH3) and
ethylene (C2H4). With respect to allyl, over 40% is consumed
through reaction R321: H + aC3H5 (+M) ⇄ C3H6 to form
propylene (C3H6) and over 40% propylene again forms aC3H5,
which entirely serves as a terminal step. Ji et al.22 and Wu et
al.21 reported that the substituted CHs have laminar flame
speeds of 4−5 cm/s smaller than that of CH. They attributed it
to the large amount of propene and allyl produced by the
substitution group. According to their analyses, propene and
allyl act as a net sink for the H radical, which is consistent with
the discussion above. Moreover, on the basis of the sensitivity
analyses for CH, ECH, and PCH, the initial alkene-forming
Figure 7. Decomposition pathways at 1600 K. Path 1, C−C fission; path 2, H abstraction for (a) CH and (b) ECH.
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Figure 10. Consumption rates of cycloalkanes (CH, MCH, ECH,
PCH, and BCH) at 1150 K and ϕ = 1.0.
Figure 8. Rates of main fuel consumption reactions at 1600 K for (a)
CH and (b) ECH. The total rate is normalized by the maximum
decomposition rate, and the rates of each type of reaction are
normalized by the instantaneous total consuming rate.
Figure 11. Normalized H abstraction with the H atom rate. It is the
ratio of the total rates of H abstraction with H reactions to the total
fuel decomposition rates at 1150 K and ϕ = 1.0.
Figure 9. Mole fractions of aC3H5 and nC3H7 for CH decomposition
at 1600 K.
Figure 12. Total rates of production (ROPs) of the H radical for
cycloalkanes (CH, MCH, ECH, PCH, and BCH).
However, the amount of the H radical used in fuel
decomposition is decreased afterward because the increasingly
accumulated intermediate hydrocarbons consume more and
more H radical. Then, the rates of CH consumption and H
production are lowered, resulting in the peak for CH lines in
Figures 10 and 12. With respect to the substituted cycloalkanes,
there is no pathway to rapidly generate the H radical, and thus,
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no peak is presented. It should be pointed out that the
JetSurF2.0 mechanism lacks the reactions of alkylcyclohexyl
that produce the H radical and cycloolefins, such as R1905
(cC6H11 ⇄ cC6H10 + H), for all alkylated cycloalkanes.
However, they must have a slight effect on the ignition because
of the good prediction performance of the mechanism.
In consideration of previous work, which shows that the
ignition delay times of CH, MCH, and BCH are in the order of
MCH > CH ≈ BCH at low temperatures,20 it is implied that
the order of ignition delay times of CH, MCH, ECH, PCH, and
BCH is MCH > ECH ≈ BCH ≈ CH > PCH, indicating the
effect of the length of the branching chain on ignition delay.
Figures 10 and 12 also include MCH and BCH to interpret this
effect. A comparison of the CH ignition delay to those of ECH
and PCH has been discussed above. CH is no longer focused in
the following. It is shown that PCH shows the largest
consumption rate, and BCH gives the smallest consumption
rate. Although the consumption rate of MCH approximates
that of ECH initially, it decreases significantly later, and MCH
is the last to disappear. This means that the decomposition of
PCH is fastest, that of MCH is slowest, and those of ECH and
BCH fall in the middle. Moreover, the generating rate of the H
radical is found to be a major factor in all fuel dissociations. For
all fuels, the majority of the H radical is produced by C2H5 in
the first part (as shown in Figure S1 of the Supporting
Information). The difference in the production of C2H5 is
strongly responsible for the difference in fuel consumption
rates.
Figure 13 depicts the main pathways of C2H5 formation in
the ignition processes of ECH, PCH, and BCH. In this study,
the moment of 20% consumption of fuel was selected to plot
the oxidation pathway. Contributions to the production of
C2H5 are marked in black; the percentages of the dissociation
of fuel and intermediates are in red and green, respectively. It is
observed that the generation of C2H5 is strongly dependent
upon the specific molecule of fuel. In Figure 13a, about 43%
ECH is consumed via H abstraction at sites 2 and 3, forming
ECH-R2 and ECH-R3, which are the precursors of 71% C2H5
(employing similar nomenclature as in the work by Hong et
al.20). Particularly, ECH-R2 produces twice the amount of ethyl
as that from ECH-R3. However, only 4.4% ECH is directly
decomposed to form 10.6% ethyl. Less than 10% ECH
generates cyclohexyl (cycC6H11), which is a well-known Hradical origin. In comparison, PCH seems to have fewer
pathways in C2H5 production, as shown in Figure 13b. It can be
seen that 33.7% C2H5 is produced by PCH-R1, which is the
most outstanding pathway of production of C2H5. Additionally,
91.4% PCH-R1, which is formed by 17.6%, undergoes this
pathway. This means that about 16.1% PCH can generate C2H5
through two steps. In contrast, only 7% ECH can generate
C2H5 through two steps (ECH → ECH-R2 → C2H5). Other
pathways in the PCH oxidation profile also tend to be more
effective than those in ECH oxidation. In other words, PCH
has a branch and is easier to regenerate the H radical.
Additionally, PCH can yield more cyclohexyl (15.2%).
Consequently, PCH demonstrates better H-regeneration ability
than ECH. For BCH in Figure 13c, the appearance of butyl
(pC4H9) tends to largely inbibit the production of ethyl, even
though a certain amount of cyclohexyl (16.1%) is produced. As
a result, the rate of H regeneration of BCH is the lowest among
these fuels. In a word, complex pathways tend to slow the
production of ethyl. Therefore, the consuming rates of ECH,
PCH, and BCH are in the order of PCH > ECH ≈ BCH, and
Figure 13. C2H5 generation pathways at 1150 K and 20%
consumption of (a) ECH, (b) PCH, and (c) BCH. Red means the
pathways of fuel consumption. Green means the consuming pathways
of intermediates. Black means the pathways of production of ethyl.
Blue means the pathways of production of n-butyl.
this corresponds inversely to the behavior of ignition delay
times. To verify this assumption, ignition delay times of BCH at
ϕ = 1.0, 1.5 atm, and 4% O2 from Hong et al.20 are used to
make the comparison to those of PCH measured in the same
test conditions and definition (primary data are provided in
Table S2 of the Supporting Information). Figure 14 shows that
both simulated and experimental results give a shorter ignition
delay time in PCH, even considering the uncertainty of 15%.
With respect to MCH, because it has weak H-regeneration
ability, as analyzed in the study by Hong et al.,20 its ignition
delay time is the longest. In conclusion, the abstraction with the
H radical, which is highly influenced by the regenerating rate of
H, is crucial to the consumption of these fuels in the initial
stage, while the regeneration of H is largely dominated by the
special molecular structure.
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ignition delay times of PCH and BCH from Hong et al.20 in
identical conditions also verifies the simulation.
■
ASSOCIATED CONTENT
S Supporting Information
*
Measured ignition delay times of CH, ECH, and PCH, where p
is the pressure in atmosphere, T is the temperature in kelvin,
and τ is ignition delay time in microseconds (Table S1),
measured ignition delay times of PCH with 4% O2 at ϕ = 1.0
for comparison to those of BCH from Hong et al., with τ1
referring to the definition with endwall pressure and τ2 referring
to the definition with sidewall pressure (Table S2), and
normalized rate of production of R245 (C2H5 ⇄ C2H4 + H)
for ECH, PCH, and BCH at 1150 K (Figure S1). This material
is available free of charge via the Internet at http://pubs.acs.org.
■
Figure 14. Comparison of ignition delay times of BCH from Hong et
al.20 and PCH with 4% O2 concentration at 1.5 atm and ϕ = 1.0.
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].
What should be mentioned is that BCH, PCH, and BCH
tend to have a comparative ignition delay at elevated pressures
(>40 atm) on the basis of JetSurF2.0 because the OH radical
instead of the H radical becomes significant to the
decomposition of fuel. Additionally, the majority of OH is
produced by HO2-related reactions. This means that the effects
of the H radical are on the wane at elevated pressures.
Therefore, the different H-regeneration abilities among
alkylated CHs (ECH, PCH, and BCH) have little influence
on ignition, resulting in comparable ignition delay times for
them.
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)
and the National Basic Research Program (2013CB228406).
The authors also appreciate the funding support from the
Fundamental Research Funds for the Central Universities and
the State Key Laboratory of Engines (SKLE201305).
■
4. CONCLUSION
Ignition delay times of CH, ECH, and PCH were measured at
pressures of 1.1 atm, equivalence ratios of 0.5, 1.0, and 2.0, and
temperatures ranging from 1110 to 1650 K with a fixed fuel
concentration of 0.5%. Good agreement was obtained in the
comparison of the measured CH ignition delay times to those
by Hong et al.20 For CH, three available mechanisms (Sirjean,
Silke, and JetSurF2.0) used to make the simulation and
satisfactory agreements with experiments were achieved.
However, the difference from CH unimolecular reactions
between mechanisms by Silke et al. and JetSurF2.0 leads to
different predictions at high temperatures. For ECH and PCH,
the only available JetSurF2.0 well-predicts the experimental
data, except for the overprediction in the case of PCH at ϕ =
2.0.
In addition, the branching chain favoring the unimolecular
reactions causes the shorter ignition delay time of ECH and
PCH than that of CH at high temperatures (T > 1450 K) when
these reactions dominate the fuel decomposition. At lower
temperatures, the consumption of fuel largely relays on the Habstraction reactions, and thus, the H radical becomes a major
stimulus of CH decomposition. It is also noticed that the
ignition delay times of ECH, PCH, and BCH are in the order of
PCH < ECH ≈ BCH, which is attributed to the molecule
structure of each fuel. Because PCH has shorter and simpler
pathways to generate the H radical than others, it enables
quicker H-radical generation. Relatively complex pathways of H
production in BCH oxidation lead to difficulty in quickly
producing the H radical. As a result, the ignition delay time of
BCH is obviously longer than that of PCH. A comparison of
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