Available online at www.sciencedirect.com
Proceedings of the Combustion Institute 36 (2017) 1279–1286
www.elsevier.com/locate/proci
Experimental and kinetic study of pentene isomers and
n-pentane in laminar flames
Yu Cheng, Erjiang Hu∗, Xin Lu, Xiaotian Li, Jing Gong, Qianqian Li,
Zuohua Huang∗
State Key Laboratory of Multiphase Flow in Power Engineering, School of Energy & Power Engineering, Xi’an Jiaotong
University, Xi’an 710049, China
Received 4 December 2015; accepted 8 August 2016
Available online 6 October 2016
Abstract
Laminar flame speeds of three pentene isomers (1-pentene, 2-pentene and 2-methyl-2-butene) and npentane are investigated at equivalence ratios of 0.7–1.6, initial pressures of 1–4 atm, and initial temperatures
of 353–433 K using a constant volume combustion bomb. Results show that the laminar flame speeds increase
in the order of 2-methyl-2-butene, n-pentane, 2-pentene, and 1-pentene. A recently published model on pentane isomers (NUI-PI) has been optimized by refining the submodels of the 1-pentene and 2-methyl-2-butene.
This optimized model yields reasonable agreement with the experimental data except over-predictions for 2pentene. The analysis indicates the discrepancy of laminar flame speeds between 1-pentene and n-pentane is
mainly caused by the thermal effect, different from the discrepancy between 1-pentene and 2-methyl-2-butene
which mainly results from the chemical kinetic effect. The kinetic effect is further investigated employing the
sensitivity and reaction path analyses. The analyses reveal that 1-pentene generates the H-radical precursor
ethyl radical, while 2-methyl-2-butene produces large amount of the H-consuming branching intermediates
(IC4 H8 , AC5 H9 –C, CC5 H9 -B and B13DE2MJ) and presents the weaker H regenerating ability. In addition,
compared with 1-pentene, 2-methyl-2-butene yields larger amount of methyl radical which would block the
whole reaction process.
© 2016 by The Combustion Institute. Published by Elsevier Inc.
Keywords: Pentene isomers; n-Pentane; Laminar flame speed; Chemical kinetics
1. Introduction
Gasoline is a complex mixture and consists of
different hydrocarbons. To reduce the complexity
∗
Corresponding author. Fax: +86 29 82668789.
E-mail addresses: [email protected] (E. Hu),
[email protected] (Z. Huang).
of simulation, the primary reference fuel composed
of n-heptane and iso-octane has been widely used
to emulate the characteristics of gasoline. With
the development of new combustion techniques
and evolution of oil industry, more component
classes should be counted to improve the simulation accuracy. Alkenes are one kind of the important gasoline components and intermediate products of alkanes. Thus alkenes are considered as
http://dx.doi.org/10.1016/j.proci.2016.08.026
1540-7489 © 2016 by The Combustion Institute. Published by Elsevier Inc.
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Y. Cheng et al. / Proceedings of the Combustion Institute 36 (2017) 1279–1286
surrogate fuel [1] as well and hereby an accurate
alkene model is essential to establish effective gasoline surrogate models. Among the alkenes, pentene
takes the largest proportion, accounting for about
8.04% in the 93# (RON) gasoline from China. Thus
it is of great importance to carry out research on
pentene isomers and several studies have been conducted on pentene isomers.
Prabhu et al. [2] studied the oxidation of
1-pentene with a plug flow reactor between 657 K
and 714 K. Alatorre et al. [3] conducted an experimental and simulation work of 1-pentene-O2 -Ar
flame at 50 mbar in the fuel-rich condition. Minetti
et al. [4] then compared the ignition delay time of
1-pentene and n-pentane between 600 K and 900 K
in a rapid compression machine (RCM). Based
on these data, a model was developed by Ribaucour et al. [5] in the following work. Subsequently,
Touchard et al. [6] investigated the high temperature (from 1130 K to 1620 K) and low temperature
(from 600 K to 900 K) auto-ignition of 1-pentene
using a shock tube and a RCM. A detailed kinetic
model was also generated by a mechanism producing software EXGAS. Further work on the ignition
behaviors of C5 –C6 linear alkenes were then conducted experimentally and numerically by Mehl et
al. [7]. Recently, Westbrook et al. [8] conducted the
first systematic study on the branched pentene, 2methyl-2-butene, with a shock tube and jet-stirred
reactor (JSR), providing a detailed chemical kinetic
model of 2-methyl-2-butene.
Although some research have been done on the
oxidation and ignition chemistries of pentene isomers, up to now, laminar flame studies mainly focus on the lighter alkenes from C2 to C4 [9–13] and
little attention has been devoted to pentene isomers in laminar flames. Laminar flame speed is one
of the most important combustion parameter because it is not only the input data for the turbulent
flame speed calculation but also one validation for
the chemical reaction mechanisms. Although Farrell et al. [14] measured the laminar flame speeds of
pentene isomers using a constant volume combustion vessel, the flame speed was determined from
a thermodynamic analysis of the pressure profile.
They pointed out that the pressure results were
about 10% higher than the schlieren results in the
methane case and the difference of pressure and
schlieren results should be taken into consideration in the validation of mechanisms. Thus, it is
necessary to carry out more experimental study on
pentene isomers employing different methodology.
Since n-pentene has similar molecular structure
with n-pentane except the existence of the double
bond, the comparison in the laminar flame speeds
of n-pentene and n-pentane helps to clarify the role
of double bond.
In the present study, laminar flame speeds of
1-pentene (C5 H10 -1), 2-pentene (C5 H10 -2), 2methyl-2-butene (BC5 H10 , where B indicates the
position of the C=C double bond) and n-pentane
(NC5 H12 ) were measured using a constant volume
bomb to validate the kinetic model. A comparative
study was also conducted to illustrate the roles
of different aspects in laminar flame speeds, including a comparison of 1-pentene and n-pentane
which helps to understand the difference between
alkanes and alkenes and a comparison between
pentene isomers that reveals role of the branching
structures.
2. Experimental specifications and numerical
approach
Laminar flame speeds were measured using a
combustion bomb of which details can be found in
Refs. [15–17] and only a brief description is given
here. The cylindrical bomb was heated by the electrical heating tape wrapped around it. Temperature was measured by a thermocouple with an accuracy of ±3 K and pressure was monitored by a
pressure transmitter. Before each experiment, the
bomb was evacuated by a pump and heated to the
experiment temperature, then the fuel was injected
through a valve to the bomb with micro-syringes
instantaneously. Flame radii from 8 to 24 mm were
used in the data processing to avoid the effects
of ignition and pressure rise [18]. Nonlinear extrapolation method [19] was adopted in the data
post-processing to eliminate the stretch effect. The
purities of the fuels in present study are no less
than 98.5%, which are shown in Table S1 in Supplemental materials as well as other properties of
the tested fuels. The laminar flame speeds reported
in this study along with associated conditions are
also available in Supplemental materials. The uncertainty of the laminar flame speed is estimated to
be about 2–4 cm/s with the method of Moffat et al.
[20] and can be attributed to mixture preparation
which is affected by different factors. The temperature error could result in an uncertainty of ±0.5%
in the laminar flame speed and pressure error
could lead to an uncertainty of ±1%. Real pressure
changes after the fuel injection into the combustion
bomb were compared with the theoretical fuel pressures to ensure the full evaporation of the fuels and
the equivalence ratio error is estimated to be ±(2–
3)%. Besides, the experiments were repeated three
times at each condition and the standard deviation
is taken into consideration. Details about the data
post-processing are given in Ref. [21].
Laminar flame speeds were calculated with
PREMIX code [22] combined with CHEMKIN-II
[23]. For present calculation, the mixture-averaged
model was used and the Sorret effect was taken
into consideration. Both GRAD and CURV values
were set as 0.02 and the final number of points in
the grid was about 900. The NUI-PI-Modify model
was reduced systemically by the Princeton ChemRC software [24]. The reduced model was validated
against the detailed one on the ignition delay time
Y. Cheng et al. / Proceedings of the Combustion Institute 36 (2017) 1279–1286
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at wide ranges of temperature, pressure and equivalence ratio.
3. Results and discussions
3.1. Optimization and validation of the kinetic
model
The reaction mechanism used herein is based on
the recently proposed NUI pentane isomer (NUIPI) model [25], which has been validated by the
ignition delay times of pentane isomers obtained
with both RCM and shock-tube facility as well as
the laminar flame speed of n-pentane. However,
this model is lack of some important reactions for
the pentene isomers and some branching ratios are
not proper, according to the reaction pathway analyses of 1-pentene and 2-methyl-2-butene as shown
in Fig. S1 in Supplemental Materials. Thus, modifications have been made on the submodels of
1-pentene and 2-methyl-2-butene by adding some
missing reactions and updating some reaction rate
constants, forming a new kinetic model named as
the NUI-PI-Modify model (available in Supplemental Materials).
The unimolecular initiation reaction rate constant for 1-pentene (C5 H10 -1<=>C2 H5 +C3 H5 -A)
is considered to be similar to that for 1-butene
(C4 H8 -1<=>C3 H5 -A+CH3 ) since both of the
two reactions occur in the allylic site. However,
the rate constant for 1-pentene is much smaller
than that for 1-butene in NUI-PI model. Thus
this rate constant is replaced by the one given by
Mehl et al. [7]. Besides, the unimolecular initiation
reactions breaking the C–H bond are also considered. The β-scission reactions for the pentenyl
radicals (C5 H9 1-4 and C5 H9 1-5) which break the
C–H bonds to generate the pentadienes, as well
as the following reactions for the 1,4-pentadiene
(C5 H8 1-4) are also added in present model.
For the BC5 H10 submodel, the rate constants of the unimolecular decomposition reactions, which break the vinylic C–C bonds,
are over estimated in the NUI-PI model. In
the NUI-PI-Modify model, the rate constant
of BC5 H10 <=>CH3 + C4 H7 2-2 is obtained by
analogy with that of the decomposition reaction for IC4 H8 (IC4 H8 <=>C3 H5 -T+CH3 ) [26].
The rate constant of BC5 H10 <=>CH3 + C4 H7 I1 is half of the former one considering the
molecular structure. Given low BDE for the 9
allylic C–H bonds in BC5 H10 , the unimolecular
initiation reactions BC5 H10 <=>AC5 H9 –C + H
and BC5 H10 <=>CC5 H9 -B + H are added in
the NUI-PI-Modify model. Besides, the reactions of CC5 H9 -B<=>B13DE2M+H and
CH3 +C4 H6 <=>CC5 H9 -A were also supplemented in the NUI-PI-Modify model. For the
reaction C2 H3 +C3 H4 -A<=>B13DE2MJ, we
use analogous rate constant of CH3 +C3 H4 -
Fig. 1. Comparison of the laminar flame speeds of npentane [28–30]. (Symbols: Measurements; Lines: Calculations with NUI-PI-Modify model).
A<=>IC4 H7 [26]. All the modified reactions
along with their references are available in Table
S4 in Supplemental Materials.
The experimental and simulated ignition delay times of 1-pentene, 2-methyl-2-butene and npentane [27] at p = 10 atm, φ = 1.0 are shown
in Figure S2 in Supplemental Materials. It can
be seen that NUI-PI-Modify model yields more
accurate predictions than NUI-PI model on 1pentene. Although the ignition delay time of 2methyl-2-butene is slightly over-estimated by NUIPI-Modify model with temperature below 1300 K,
the results on the whole are acceptable. Generally
speaking, the simulations reasonably agree with the
experimental results for all these three fuels.
3.2. System validation
Figure 1 shows the comparison between present
laminar flame speeds of n-pentane (353 K, 1 atm)
and those in literature [28–30]. Present experimental data agree fairly well with data of Kelley et al.
[28] using a constant pressure combustion bomb
over a wide range of equivalence ratios, indicating the reliability of the present experimental apparatus. Compared with the experimental results of
Ji et al. [30] measured with counterflow configuration, our data are relatively lower in the fuel-lean
side and slightly higher when the equivalence ratio is greater than 1.1. In addition, the data of Ji
et al. [30] peak around φ ≈1.05 while present data
peak around φ ≈1.1. This equivalence ratio shifting of laminar speed between data from counterflow configuration and combustion bomb has also
been reported in previous liquid fuel studies [31,32].
Measurements of Dirrenberger et al. [29] using
the heat flux method are relatively higher than
present results and this may result from the higher
initial experimental temperature in the study of
Dirrenberger et al. [29]. The calculated results using
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Fig. 3. Experimental and simulated laminar flame speeds
of 1-pentene, 2-pentene, 2-methyl-2-butene, and npentane/air flames at 353 K, 1 atm.
Fig. 2. Experimental and calculated laminar flame speeds
of 1-pentene/air flames at (a) T = 353 K, p = 1, 2, 4 atm;
(b) T = 353, 393, 433 K, p = 1 atm.
NUI-PI-Modify model are also plotted in Fig. 1 for
comparison and good agreement is achieved with
present experimental data.
3.3. Laminar flame speeds of 1-pentene
Figure 2 shows the measured and simulated
laminar flame speeds of 1-pentene-air mixtures versus equivalence ratio at different initial pressures
(1, 2 and 4 atm) and initial temperatures (353, 393
and 433 K). The model shows fairly well agreement
with the experimental data at 353 K. With the increase of initial temperature, the discrepancy between the simulations and experiments increases
with a maximum discrepancy of 4 cm/s observed at
433 K. Generally, simulation results using the NUIPI-Modify model show satisfactory agreement with
the experimental results of 1-pentene.
3.4. Comparison of C5 laminar flame speeds
Figure 3 shows the measured laminar flame
speeds of 1-pentene, 2-pentene, 2-methyl-2-butene
and n-pentane at 353 K, 1 atm. It can be seen that
the flame speeds of pentene isomers decrease in
the order of 1-pentene, 2-pentene and 2-methyl-2butene with the difference between 1-pentene and
2-pentene smaller than 2 cm/s. Similar behavior of
the pentene isomers has been reported by Farrell
et al. [14]. Compared with 1-pentene, n-pentane exhibits slower laminar flame speeds.
The simulation results of the laminar flame
speeds using the NUI-PI-Modify model are also
plotted in Fig. 3. It can be seen that this model gives
reasonable predictions for 1-pentene, n-pentane,
and 2-methyl-2-butene while for 2-pentene, the
model over-estimates the laminar flame speed
about 6 cm/s in the stoichiometric condition. Moreover, the model fails to describe the flame speed
ranking of 1-pentene and 2-pentene, implying that
the flame oxidation chemistry of 2-pentene maybe
not appropriately described. Thus the 2-pentene/air
flame is not analyzed in the following sections. Considering the consistency of NUI-PI-Modify model
with the experiments for 1-pentene, n-pentane and
2-methyl-2-butene, this model is adopted to interpret the flame speed discrepancy among the three
species.
3.4.1. Thermal effects
To clarify the effect of thermodynamic on the
ranking of laminar flame speeds, adiabatic flame
temperature of the three fuels are depicted in
Fig. 4. As expected, n-pentane has the lowest adiabatic flame temperature (2302 K in the stoichiometric condition). The pentene isomers show relative high adiabatic flame temperature, 2343 K and
2333 K for 1-pentene and 2-methyl-2-butene in the
stoichiometric condition, respectively. The fastest
flame speed of 1-pentene among the three fuels may
be attributed to its highest flame temperature.
In order to separate the influence of thermodynamic factor and identify the fundamental roles in
Y. Cheng et al. / Proceedings of the Combustion Institute 36 (2017) 1279–1286
Fig. 4. Adiabatic flame temperature of 1-pentene, 2methyl-2-butene and n-pentane/air flames at 353 K,
1 atm.
Fig. 5. Computed laminar flame speed of BC5 H10 /N2 /O2
mixture and NC5 H12 /N2 /O2 mixture with the same adiabatic flame temperature to that of C5 H10 -1/air mixture by reducing nitrogen concentration at 353 K, 1 atm.
Measured and computed flame speeds of C5 H10 -1/air,
BC5 H10 /air and NC5 H12 /air are plotted for comparison.
laminar flame speed ranking of the three C5 fuels,
laminar flame speeds of n-pentane and 2-methyl-2butene with the same adiabatic flame temperature
to that of 1-pentene are calculated by reducing the
concentration of N2 in the mixtures, as shown in
Fig. 5. And the mole fractions of N2 in adjusted
NC5 H12 /N2 /O2 and BC5 H10 /N2 /O2 mixtures corresponding to Fig. 5 are available in Table S5 in
Supplemental materials. With the same adiabatic
flame temperature, n-pentane shows comparative
laminar flame speed with 1-pentene, indicating that
the lower adiabatic flame temperature is primarily
responsible for the slower laminar flame speed of
n-pentane than 1-pentene. Ranzi et al. [33] reported
that at constant flame temperature, laminar flame
speeds for C2 species maintain the original order of
alkanes < alkenes and this order changes for C3 and
C4 species, becoming alkenes < alkanes. It can also
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be observed that at the constant flame temperature,
such difference between C4 species is smaller than
that of C3 species in the study of Ranzi et al. [33]. In
present study, 1-pentene and n-pentane have comparative laminar flame speeds. It suggests that with
the increase of carbon chain length, the difference
of laminar flame speeds of alkanes and alkenes at
constant flame temperature decreases and the difference of alkenes and n-alkanes resulting from the
chemical kinetics in laminar flame speeds may be
weakened. The dominant reason for the laminar
flame speed discrepancies between larger carbon nalkenes and n-alkanes is considered to be associated
with the differences in corresponding flame temperatures resulting from the different H/C ratios.
On the other hand, a significant difference still exists between the flame speeds of 2-methyl-2-butene
and 1-pentene even with the same adiabatic flame
temperature, demonstrating that the laminar flame
speed difference between 1-pentene and 2-methyl2-butene mainly results from the chemical kinetic
aspect rather than the thermodynamic aspect. The
ignition delay times results in Fig. S2 also show that
1-pentene and n-pentane have the comparative reactivity while 2-methyl-2-butene shows the lower
reactivity compared with 1-pentene.
3.4.2. Reaction path analysis
To provide insight into the chemical kinetic effect on the difference in laminar flame speeds of
1-pentene and 2-methyl-2-butene, the reaction path
analysis of the stoichiometric flame is conducted at
353 K and 1 atm, as shown Fig. 6. The main initial
path way of fuel cracking for pentene isomers is the
H-abstraction reactions, generating various pentenyl radicals. Meanwhile, pentene isomers can also
be consumed through the unimolecular decomposition reaction. Apart from the H-abstraction
and unimolecular decomposition reactions which
are also common in alkanes, pentene isomers undergo the radical-addition reactions on the double
bond, forming pentyl radicals. Most of the generated pentyl and pentenyl radicals will then undergo
the β-scission reactions. For the pentyl radicals, the
β-scission is mainly carried on the C–C bond; for
pentenyl radicals, the breakage can be either on
the C–C bond to crack into smaller species or on
the C–H bond to produce the pentadiene and H
radical. The pentadiene will then break into much
smaller intermediates or combine with the H, OH,
O radicals to produce pentenyl radicals in other
forms.
As shown in Fig. 6a for 1-pentene, the unimolecular decomposition mainly occurs in the allylic
site, accounting for 20.7%. Most of 1-pentene is
consumed through H-abstraction reactions and
generates the C5 H9 1-3, C5 H9 1-4 and C5 H9 1-5
radicals. The generated pentenyl radicals would
further break down through the β-scission rule. For
C5 H9 1-3, C5 H9 1-5 radicals, the β-scission mainly
occurs on the C–C bond .While for C5 H9 1-4, due
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allylic pentenyl radicals would further decompose
via β-scission, producing 1, 2-butadiene (C4 H6 12)
and CH3 as well as iso-prene (B13DE2M) and H
atom through the following reactions.
B13DE2M+H <=> AC5 H9 –C
(R1)
C4 H6 12+CH3 <=> AC5 H9 –C
(R2)
B13DE2M+H <=> CC5 H9 −B
(R3)
Part of the generated B13DE2M can produce other allylic pentenyl radicals (CC5 H9 -A and
AC5 H9 -D) and about 63.8% of it is consumed
through H-abstraction reactions in the allylic site,
B13DE2M+H <=> B13DE2MJ+H2
(R4)
B13DE2M+OH <=> B13DE2MJ+H2 O
(R5)
These H-abstraction reactions consume active
radicals (H, OH) and generate the iso-prenyl radical (B13DE2MJ) which would successively recombine with H atom regenerating B13DE2M,
H+B13DE2MJ <=> B13DE2M
Fig. 6. Reaction path analysis for the stoichiometric 1pentene and 2-methyl-2-butene/air flames at 353 K and 1
atm.
to the large activation energy required for the formation of the vinyl radical, the β-scission mainly
occurs on the C–H bond and produces pentadiene
(C5 H8 1-3 and C5 H8 1-4). C5 H8 1-3 and C5 H8 1-4
will then undergo the addition reaction and form
C5 H9 1-3 and C5 H9 1-5 radicals, respectively.
As shown in Fig. 6b, 2-methyl-2-butene will
undergo addition reaction with H radical and
produce CC5 H11 and BC5 H11 radical, which would
further decompose through β-scission, producing
a methyl radical and butene. Due to the unique
molecular structure of 2-methyl-2-butene, all
the C–C bonds are in the vinylic site and 9 out
of 10 C–H bonds are at the allylic sites. Thus
the unimolecular decomposition reactions which
break the C–C bonds take quite small proportions (less than 0.2%) and are hence omitted in
Fig. 6. The H-abstraction reactions, accounting for
68.7% (45.8%+22.9%) in total, dominate the consumption of BC5 H10 and produce two resonantly
stabilized allylic pentenyl radicals (AC5 H9 –C and
CC5 H9 -B). A small amount of the AC5 H9 –C and
CC5 H9 -B radicals will produce BC5 H10 through
the recombination reaction with the H and OH
radicals. The transformations between the allylic
pentenyl radicals (AC5 H9 –C and CC5 H9 -B) and
BC5 H10 scavenge the active radical species and
inhibit the reaction process. Most of the stabilized
(R6)
Thus the transformation between B13DE2M
and B13DE2MJ radical behaves as sinks of the active radical species (H, OH) and reduces the global
activity.
When compared the chemical kinetic effects
on the difference of 1-pentene and 2-methyl-2butene, two factors are given into consideration.
The first factor is the generation and consumption
of the H radical. Due to the existence of C=C
bond, C5 H10 -1 can be easily consumed through the
unimolecular decomposition reaction with the allylic C–C bond broken, forming the H-precursor
C2 H5 radical and stable C3 H5 -A radical. While
BC5 H10 are more likely to break the allylic C–H
bond through H abstraction reactions and produce
the resonantly stabilized allylic radicals (AC5 H9 –C
and CC5 H9 -B). For C5 H10 -1, C2 H5 radical yielded
from the unimolecular decomposition reactions of
C5 H10 -1 and β-scission reaction of C5 H11 -2 can
rapidly decompose and produce H radical, promoting the most important chain branching reaction H+O2 <=>OH+O. For both C5 H10 -1 and
BC5 H10 , the formation of the resonantly stabilized
allylic radicals (C3 H5 -A, AC5 H9 –C, CC5 H9 -B and
B13DE2MJ) can act as the sink of the H radical
since they can consume the H radical and produce
the alkenes (C3 H6 , BC5 H10 and B13DE2M) which
would further undergo the H abstraction reactions
to regenerate the resonantly stabilized allyllic radicals. Although C5 H10 -1 would produce the stable
C3 H5 -A radical, plenty of the AC5 H9 –C, CC5 H9 B and B13DE2MJ are also generated in BC5 H10 .
Besides, the isobutene produced from the decomposition of the BC5 H11 would further abstract H
Y. Cheng et al. / Proceedings of the Combustion Institute 36 (2017) 1279–1286
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Fig. 7. Mole fractions of H and CH3 of stoichiometric 1pentene and 2-methyl-2-butene/air flames at 353 K, 1 atm.
radical and produce the stable isobutyl radical.
For both C5 H10 -1 and BC5 H10 , the transformation between different pentenyl radicals through
the pentadiene can be considered to contribute the
amount of the H radical little since each time one
H radical is produced along with the generation
of a pentadiene and then one H radical is consumed in the production of the pentenyl radical.
Generally speaking, C5 H10 -1 shows better H regenerating ability when compared with BC5 H10 .
The second factor, which might not be as important as the first one but can still be considered as a supplement, is the production of the
methyl radical. From the path way analyses above,
2-methyl-2-butene will generate large amount of
CH3 radical which would recombine into stable
ethane (CH3 +CH3 (+M)<=>C2 H6 (+M)) or produce CH4 (CH3 +HO2 <=>CH4 +O2 ), removing
radicals from the reaction system and slowing
down the overall reaction rate.
Mole fractions of the H and CH3 radicals
of 1-pentene and 2-methyl-2-butene stoichiometric
flames at 353 K, 1 atm are plotted to extend and
validate the analyses on the fuel reactivity above,
as shown in Fig. 7. It can be seen that 1-pentene
exhibits higher H and lower CH3 mole fraction, indicating the relative higher reactivity of 1-pentene
compared with 2-methyl-2-butene. These analyses
confirm the reaction path way analyses presented
above.
The lower flame speeds of the branched hydrocarbons compared to their normal isomers have
been reported in previous literature, including alkanes isomers [13], alcohol isomers [21,31,32] and
alkene isomers [12,14]. Davis and Law et al. [13] indicated that molecular branching of fuels would result in higher concentrations of stable branched intermediates and thus lower global activity.
3.4.3. Sensitivity analysis
To further understand the key reactions related to laminar flame speed, sensitivity analyses
of stoichiometric 1-pentene/air and 2-methyl-2-
Fig. 8. Sensitivity analyses for the stoichiometric 1pentene and 2-methyl-2-butene/air flames at 353 K and
1 atm.
butene/air flames are conducted at 353 K, 1 atm.
As expected, both C5 H10 -1 and BC5 H10 flames are
mostly sensitive to small hydrocarbon chemistry
and the chain branching reaction H + O2 <=>
O + OH has the largest sensitivity coefficient as the
order of 0.30. In order to help better understand
the role of the fuel-specific reactions in the flames,
only these reactions are shown in Fig. 8. Obviously,
the sensitivity coefficients for BC5 H10 are larger
than those for C5 H10 -1, indicating the thermal and
chemical stability of 2-methyl-2-butene. For both
C5 H10 -1 and BC5 H10 , the H-abstraction reactions
for the fuels show negative sensitivities. Consistent with the analysis above, the unimolecular
decomposition reactions for C5 H10 -1 have positive
effect on the flame speed. For BC5 H10 , the reaction
R6 (H+B13DE2MJ <=> B13DE2M) indicates
negative sensitivity since it consumes the active
H radical and produces B13DE2M, which goes
to B13DE2MJ by consuming another H radical.
Thus, the reaction B13DE2MJ <=> C2 H3 +C3 H4 A competing with reaction R6, and B13DE2M+H
<=> AC5 H9 -D competing with the H-abstraction
reaction, present positive sensitivity. The hydrogen
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addition reaction BC5 H10 +H <=> BC5 H11 also
shows relatively high negative sensitivity for it
consumes H radical and produces BC5 H11 which
would further decompose into CH3 radical and
isobutene.
4. Conclusions
The laminar flame speeds of three pentene isomers (1-pentene, 2-pentene and 2-methyl-2-butene)
and n-pentane were measured using a constant
volume combustion bomb at three initial temperatures (353, 393 and 433 K) and pressures (1, 2
and 4 atm). Results show that 1-pentene has the
highest flame speeds, followed by 2-pentene, and
then n-pentane and 2-methyl-2-butene. The kinetic
model on pentane isomers recently published by
NUI [25] was optimized, forming a new kinetic
model named as NUI-PI-Modify model. Calculations and analyses were carried out using the
NUI-PI-Modify model. The analyses show that
the lower adiabatic flame temperature of n-pentane
is mainly responsible for the slower laminar flame
speed of n-pentane than that of 1-pentene, while
the effect of chemical kinetics plays the dominant
role in the lower flame speed of 2-methyl-2-butene
than 1-pentene. 1-Pentene is found to produce
the C2 H5 radical and have better H regenerating ability. While 2-methyl-2-butene are mostly
prone to generate the H-consuming branching
intermediates (IC4 H8 , AC5 H9 –C, CC5 H9 -B and
B13DE2MJ) as well as CH3 radical, decreasing
the overall reactivity. The combination effect of all
these aspects lead to the lower laminar flame speed
of BC5 H10 compared with C5 H10 -1.
Acknowledgment
This study is supported by the National Natural Science Foundation of China (91441118,
91441203, and 91541107). Authors also appreciate the funding support from the Fundamental Research Funds for the Central Universities and State
Key Laboratory of Engines (SKLE201502).
Supplementary materials
Supplementary material associated with this article can be found, in the online version, at doi:
10.1016/j.proci.2016.08.026.
References
[1] G. Kukkadapu, K. Kumar, C.J. Sung, M. Mehl,
W.J. Pitz, Proc. Combust. Inst. 34 (1) (2013) 345–352.
[2] S.K. Prabhu, R.K. Bhat, D.L. Miller, N.P. Cernansky, Combust. Flame 104 (4) (1996) 377–390.
[3] G.G. Alatorre, H. Bohm, B. Atakan, K. Kohse-Hoinghaus, Z. Phys. Chem. 215 (8) (2001) 981–995.
[4] R. Minetti, A. Roubaud, E. Therssen, M. Ribaucour, L. Sochet, Combust. Flame 118 (1) (1999)
213–220.
[5] M. Ribaucour, R. Minetti, L.R. Sochet, Proc. Combust. Inst. 27 (1) (1998) 345–351.
[6] S. Touchard, F. Buda, G. Dayma, P.A. Glaude,
R. Fournet, F. Battin-Leclerc, Int. J. Chem. Kinet. 37
(8) (2005) 451–463.
[7] M. Mehl, W.J. Pitz, C.K. Westbrook, K. Yasunaga,
C. Conroy, H.J. Curran, Proc. Combust. Inst. 33 (11)
(2011) 201–208.
[8] C.K. Westbrook, W.J. Pitz, M. Mehl, et al., J. Phys.
Chem. A 119 (28) (2015) 7462–7480.
[9] G. Jomaas, X.L. Zheng, D.L. Zhu, C.K. Law, Proc.
Combust. Inst. 30 (1) (2005) 193–200.
[10] M.I. Hassan, K.T. Aung, O.C. Kwon, G.M. Faeth, J.
Propul. Power 14 (4) (1998) 479–488.
[11] K. Kumar, G. Mittal, C.J. Sung, C.K. Law, Combust.
Flame 153 (3) (2008) 343–354.
[12] P. Zhao, W. Yuan, H. Sun, et al., Proc. Combust. Inst.
35 (1) (2014) 309–316.
[13] S.G. Davis, C.K. Law, Combust. Sci. Technol. 140
(1-6) (1998) 427–449.
[14] J.T. Farrell, R.J. Johnston, I.P. Androulakis. SAE Paper 2004-01-2936.
[15] Q. Li, E. Hu, X. Zhang, Y. Cheng, Z. Huang, Energy
Fuels 27 (2) (2013) 1141–1150.
[16] X. Gu, Z. Huang, S. Wu, Q. Li, Combust. Flame 157
(12) (2010) 2318–2325.
[17] X. Zhang, C. Tang, H. Yu, Q. Li, J. Gong, Z. Huang,
Energy Fuels 27 (4) (2013) 2327–2335.
[18] M.P. Burke, Z. Chen, Y. Ju, F.L. Dryer, Combust.
Flame 156 (4) (2009) 771–779.
[19] A.P. Kelley, C.K. Law, Combust. Flame 156 (9)
(2009) 1844–1851.
[20] R.J. Moffat, Exp. Therm. Fluid Sci. 1 (1) (1988) 3–17.
[21] Q. Li, C. Tang, Y. Cheng, L. Guan, Z. Huang, Energy
Fuels 29 (8) (2015) 5334–5348.
[22] R.J. Kee, J.F. Grcar, M.D. Smooke, J.A. Miller, A
FORTRAN Program for Modeling Steady Laminar
One-Dimensional Premixed Flames, Sandia National
Laboratories, 1985 Sandia Report SAND85-8240.
[23] R.J. Kee, F.M. Rupley, J.A. Miller, Chemkin-II: A
Fortran Chemical Kinetics Package for the Analysis of
Gas-Phase Chemical Kinetics, Sandia National Laboratories, 1989 Sandia Report SAND89-8009.
[24] W. Sun, Z. Chen, X. Gou, Y. Ju, Combust. Flame 157
(7) (2010) 1298–1307.
[25] J. Bugler, B. Marks, O. Mathieu, et al., Combust.
Flame 163 (2015) 138–156.
[26] C.W. Zhou, Y. Li, E. O’Connor, et al., Combust.
Flame 167 (2016) 353–379.
[27] Y. Cheng, E. Hu, F. Deng, et al., Fuel 172 (2016)
263–272.
[28] A.P. Kelley, A.J. Smallbone, D.L. Zhu, C.K. Law,
Proc. Combust. Inst. 33 (1) (2011) 963–970.
[29] P. Dirrenberger, L. Hervé, R. Bounaceur,
P.A. Glaude, F. Battin-Leclerc, Energy Fuels 29
(1) (2014) 398–404.
[30] C. Ji, E. Dames, Y.L. Wang, H. Wang, F.N. Egolfopoulos, Combust. Flame 157 (2) (2010) 277–287.
[31] F. Wu, C.K. Law, Combust. Flame 160 (12) (2013)
2744–2756.
[32] P.S. Veloo, F.N. Egolfopoulos, Proc. Combust. Inst.
33 (1) (2011) 987–993.
[33] E. Ranzi, A. Frassoldati, R. Grana, et al., Prog. Energy Combust. Sci. 38 (4) (2012) 468–501.