Article
pubs.acs.org/EF
Laminar Flame Speeds and Flame Instabilities of Pentanol Isomer−
Air Mixtures at Elevated Temperatures and Pressures
Qianqian Li, Erjiang Hu,* Xinyi Zhang, Yu Cheng, and Zuohua Huang*
State Key Laboratory of Multiphase Flow in Power Engineering, Xi’an Jiaotong University, Xi’an 710049, People’s Republic of China
S Supporting Information
*
ABSTRACT: Laminar flame speeds of three pentanol isomer (1-, 2-, and 3-pentanol)−air mixtures were measured at
equivalence ratios of 0.6−1.8, initial pressures of 0.10−0.75 MPa, and initial temperatures of 393−473 K using the outwardly
propagating spherical flame. A recently developed kinetic mechanism of 1-pentanol oxidation (Dagaut model) was used to
simulate the laminar flame speeds of 1-pentanol−air mixtures under experimental conditions. A comparison between simulation
and measurement shows that the simulation yields good agreement on the stoichiometric and fuel-rich side, but it gives lower
values on the fuel-lean side. A kinetic modeling study was performed, and several rate constants of selected elemental reactions
were modified on the basis of the sensitivity analysis. The modified model gives good prediction on the laminar flame speed
under all experimental conditions. The modified model is also validated against the jet-stirred reactor (JSR) experimental data,
and it exhibits good prediction for most species. 1-Pentanol gives the fastest laminar flame speed, followed by 3- and 2-pentanol.
2- and 3-pentanol have very close values considering the experimental uncertainty. With the increase of the pressure, the
difference in the laminar flame speed among pentanol isomers is decreased. The flame instability of three pentanol isomers was
also analyzed. 2- and 3-pentanol have similar instability behavior with a close density ratio, flame thickness, and Lewis number,
while 1-pentanol shows slightly high instability behavior. In comparison to 2- and 3-pentanol, 1-pentanol has a smaller critical
radius and Peclect number, and this suggests its high instability behavior.
Table 1. Physical Parameters of Fuels1,9,10
1. INTRODUCTION
With an increasing demand for environmental protection and
energy savings, the research of alternative clean fuels has been
attracting more and more attention. Researchers have
conducted a large number of studies to research more efficient
production of clean alternative fuels. Alcohols, as a
representative of biofuels that can be derived from biomass,
are attracting the attention from governments, industrial
sectors, and scientists. Up to now, most of the studies on
alcohols concentrate on low-carbon alcohols, such as methanol
and ethanol. Some fundamental studies on fuel pyrolysis and
oxidation kinetics, fuel ignition, and laminar flame speed as well
as the operation of alcohol−diesel and alcohol−gasoline on
engines have been conducted.1−4 With the development of
production technology, interests in propanol, butanol, and
pentanol have increased recently. Pentanol, with five carbons,
exhibits even better combustion characteristics and high energy
density. Studies by Nielsen et al.5 and Cann and Liao6 showed
that pentanol and its isomers were very promising alternative
fuels. In comparison to low-carbon alcohols, pentanol is a
better non-hygroscopic, less corrosive fuel, with a close octane
number to that of gasoline, higher heating value close to
gasoline, higher vapor pressure to make it safer in storage and
transportation, easier to mix with gasoline at various
proportions, and improved cold-start performance in engines.7,8
The physical parameters of different fuels are compared in
Table 1.
Several kinds of methods have been developed to more
efficiently produce pentanol and its isomers. Pentanol, as one
kind of biofuel, can be produced through fractional distillation
of fossil oil and fermentation. It can also be produced in the
© 2013 American Chemical Society
ethanol
oxygen content (mass %)
energy volume density
(kJ/cm3)
cetane number
octane number
(R + M)/2
lower heating value
(MJ/kg)
enthalpy of vaporization
(kJ/g)
solubility in water at
20 °C (mL/100 H2O)
nbutanol
npentanol
gasoline
0
31.87
0.35
21.11
0.22
26.9
0.18
28.38
8
100
17
87
18.2
28.9
33.1
34.65
42.5
837.86
584.19
503.69
348.88
unlimited
immisciblity
9.1
2.7
82−88
Escherichia coli and terpenoid pathway and the Fischer−
Tsopsch process.6,11−13 Engineering-scale production of
pentanol was also tested in the past several years, and further
work is underway to search for cost-effective methods for the
production of biopentanol.
Up to now, only limited studies on fundamental combustion
of pentanol were reported. Kinetic mechanisms of n-pentanol
and isopentanol were developed by Dagaut and co-workers7,14
They extended the reaction mechanisms from previous scheme
for the oxidation of lighter hydrocarbons or alcohols. Stable
species concentration profiles were measured in a jet-stirred
reactor (JSR) at 10 atm and various equivalence ratios, and the
Received: November 22, 2012
Revised: December 26, 2012
Published: January 2, 2013
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2. EXPERIMENTAL AND NUMERICAL APPROACHES
proposed model could give a good prediction. Engine studies
on alcohol fuels containing pentanol were also reported.9,15,16
To provide a greater understanding of the combustion behavior
and to extend the application of pentanol, a further
fundamental study on pentanol is necessary.
Fuel property varies with its structure. Fuel isomers possess
the same thermodynamic and transport properties but different
chemical properties. Previous studies have been reported on the
propanol and butanol isomers. Veloo and Egolfopoulos13
studied the laminar flame speed and extinction strain of
propanol isomers. Their experimental results showed that npropanol demonstrated a higher laminar flame speed and
extinction strain than those of isopropanol. Through sensitivity
and reaction path analyses, they summarized two major
contributors to this difference. One contributor is higher
concentrations of propene in the isopropanol flame, which
results in relatively unreactive allyl radicals. Another contributor
is higher concentrations of formaldehyde reacting readily to
form formyl radicals, which are overall reactivity enhancers.
Veloo and Egolfopoulos17 and Gu et al.18 conducted the kinetic
modeling of laminar flame speed for four butanol isomers. Both
studies indicated that n-butanol gave the fastest flame speed,
followed by isobutanol and sec-butanol, while tert-butanol gave
the slowest flame speed. Moss et al.19 and Sarathy et al.20
developed the oxidation kinetics for the four butanol isomers.
Dominant reaction pathways were provided, and simulations
with mechanisms were compared to experimental data,
including premixed laminar flame speed, ignition delay, and
premixed flat flame species profiles. A reasonably good
agreement was achieved. Pentanol has eight isomers because
of the different positions of −OH and −CH3. Some physical
properties of pentanol isomers, such as viscosity, density, and
self-diffusion, have been studied in the past decade.1,21−23 Very
recently, thermal decomposition of pentanol was reported.8
Temperature- and pressure-dependent Rate constants for the
dominant reaction channels were computed, and the difference
with butanol was analyzed.
In this study, an experimental and kinetic study on laminar
flame characteristics of the three pentanol isomer (1-, 2-, and 3pentanol)−air mixtures was conducted. As shown in Figure 1,
2.1. Experimental Approach. The experiment apparatus has
been described in detail in refs 24 and 25. A brief description is given
here. The experimental apparatus consists of four parts, including a
heating system, a photography acquisition system, an ignition system,
and a constant volume combustion bomb. A high-speed camera with a
speed of 10 000 frames per second was used to record the flame
images. The bomb was heated with the heating tape wrapped around
the bomb. When the bomb is heated to the experimental temperature,
the liquid fuel is injected into the bomb with a syringe. Oxygen and
nitrogen are introduced according to their partial pressures. Ignition
starts after keeping for 10 min. To ensure a full vaporization of liquid
fuel, the partial pressure of liquid fuel in all mixtures is below 70% of
its saturated vapor pressure. Experiment tests show that 10 min is
sufficient for the vaporization of pentanol isomers and to form a
homogeneous mixture. For each condition, experiments are repeated 3
times to ensure its repeatability.
For a spherically expanding flame, the raw flame radius, rf(t), is
derived directly from the photography. The flame propagation speed is
derived by Sb = drf/dt. The flame stretch rate is calculated by κ = 2Sb/
rf. At the early stage of flame propagation, there exists a linear
relationship between the flame propagation speed and stretch rate, S0b
− Sb = Lbκ.26 Through linear regression, the unstretched flame
propagation speed, S0b, and Markstein length, Lb, are obtained. The
laminar flame speed is obtained through S0u = (ρbS0b)/ρu based on mass
conservation across the flame front. ρu and ρb are the unburned and
burned gas densities, respectively. The adiabatic temperature, Tad, and
gas density are calculated through thermal equilibrium. The radii used
to derive the laminar flame speed in this paper are in the range of 6−
25 mm to eliminate the effects of ignition energy and pressure
rise.25,27,28 In addition, the radius is restricted by the occurrence of the
cellular structure. All data of laminar flame speeds of pentanol isomers
are provided in the Supporting Information of this paper.
2.2. Kinetic Model. Simulation on laminar flame speeds was
performed using the Chemkin and Premix codes.29,30 The kinetic
model (Dagaut model) used in this study was developed on the basis
of the measured species concentration in JSR and the previously
proposed C1−C4 alcohol oxidation mechanism.7 It contains 2099
reactions and 261 species. In addition, modification on the Dagaut
model was made to obtain better predictions on the measured laminar
burning velocities and in comparison to JSR experimental data.
3. RESULTS AND DISCUSSION
3.1. Laminar Flame Speed of 1-Pentanol and Kinetic
Analysis. Figure 2 shows the variation of the flame radius
versus time for the stoichiometric 1-pentanol−air mixture at
different initial temperatures and pressures. The linear
relationship between flame radius and time is presented. An
increasing temperature or a decreasing pressure will increase
the flame propagation speed.
Figure 3 shows the stretched flame propagation speeds
versus stretch rate at different initial temperatures and pressures
for the stoichiometric 1-pentanol−air mixtures. A linear
relationship between the stretched flame propagation speed
and stretch rate is presented. The stretch rate decreases with
the increase of the flame radius. Flame propagation speeds up
as the flame develops. When the radius tends toward infinity,
the stretch rate reaches zero. The unstretched flame
propagation speed is derived through extrapolation of the
stretched flame propagation speed versus stretch rate to the
zero stretch point.
Figure 4 gives both measured and simulated laminar flame
speeds of 1-pentanol−air mixtures versus equivalence ratio at
three initial temperatures and four initial pressures. A
reasonable prediction is presented at the stochiometric and
rich mixtures. Simulation underpredicts the laminar flame speed
Figure 1. Chemical structures of 1-, 2-, and 3-pentanol isomers.
these three pentanol isomers have a similar structure with the
straight carbon chain and one hydroxyl attached. The only
difference is the position of hydroxyl attached to the carbon.
Experiments were performed at equivalence ratios of 0.6−1.8,
three initial temperatures (393, 433, and 473 K), and four initial
pressures (0.1, 0.25, 0.5, and 0.75 MPa), using an outwardly
propagating spherical flame and high-speed schlieren photography. First, laminar flame speeds of 1-pentanol at various
temperatures and pressures were measured. The kinetic model
was modified on the basis of the recently developed mechanism
by Togbé et al.7 Second, a comparison on the laminar flame
speed of three pentanol isomers was made. Finally, the flame
instability of three pentanol isomers was analyzed.
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Figure 2. Flame radius versus time.
Figure 4. Laminar flame speeds versus equivalence ratio. Symbols,
measurement; lines, calculation.
Figure 5 gives the relatioship between the adiabatic
temperature, Tad, and equivalence ratio. The adiabatic temperature is relatively sensitive to temperature rather than pressure.
This suggests that temperature contribution to the laminar
flame speed is stronger than pressure. Sensitivity analysis in
Figure 3. Stretched flame propagation speed versus stretch rate.
at the lean mixture side but represents the tendency well.
Laminar flame speeds reach their peak values at an equivalence
ratio of 1.1. A comparison suggests that further improvement
on the kinetic model is required to make it more accurately
predict at lean mixture. The decreasing trend in the laminar
flame speed becomes weakening as the pressure increases.
Figure 5. Adibatic temperature versus equivalence ratio.
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Figure 6 shows that main-chain branching reactions and
termination reactions are sensitive to temperature and pressure.
Competition between chain branching reactions and chain
termination reactions determines the laminar flame speed
variation tendency. Experimental results implicate that the
effect of chain branching reactions increases with the increase of
the temperature and pressure.
On the basis of the sensitivity analysis from the effect of the
equivalence ratio, as shown in Figure 6a, a modification is made
on the Dagaut model. Several reaction rates are updated in
values by referencing the latest literature. The modified rate
constants of the targeted reactions are listed in Table 2.
Simulations on laminar flame speeds were conducted using
the modified model. Figure 7 gives the comparison between the
Figure 7. Laminar flame speed versus equivalence ratio. Symbols,
measurement; lines, calculation with the modified model.
simulated and measured laminar flame speeds. It can be seen
that the modified model can accurately predict the laminar
flame speeds at all equivalence ratios, temperatures, and
pressures.
To validate the modified model for 1-pentanol combustion
besides laminar flame speed, the modified model was also
validated against the JSR data by Togbé et al.7 A comparison
between modified model and Dagaut model predictions on
species concentration was also made. Figure 8 gives the
Figure 6. Sensitivity analysis.
Table 2. Comparison of Reaction Rates between the Dagaut and Modified Models
Dagaut model
A
H + O2 + M = HO2 + M
(1)31
C2H 2 + O = HCCO + H
(2)32
C2H3 + O2 = CH 2CHO + O
(3)33
n
new model
Ea
8.00 × 1017
−0.8
0
H2O, 16.25; CO, 1.875; CO2, 3.75; CH4, 16.25;
C2H6, 16.25; and H2, 2.5
4.62 × 104
2.6
656
3.53 × 1017
−1.36
5580
1144
A
n
Ea
4.65 × 1012
0.44
0.00 × 100
20
LOW, 6.366 × 10 , −1.72, and 5.248 × 102; TROE, 0.5,
1 × 10−30, and 1 × 1030; H2, 2.0; H2O, 14; O2, 0.78; CO,
1.9; CO2, 3.8; AR, 0.67; and HE, 0.8
1.63 × 107
2
1900
5.50 × 1014
−0.611
5.26 × 103
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Figure 8. Experimental and computed species concentration profiles at ϕ = 1.0, p = 10 atm, and τ = 0.7 s. Symbols, experiment; solid line, modified
model; dashed line, Dagaut model.
MPa. 1-Pentanol gives a higher laminar flame speed than those
of 3- and 2-pentanol. This is consistent with the previous study
that n-alcohol gives the highest laminar flame speed among all
isomers.34 2- and 3-pentanol give almost the same laminar
flame speeds.
To further analyze laminar flame behaviors of three pentanol
isomers, extended experiments were carried out at an initial
temperature of 433 K and four initial pressures, as shown in
Figure 12. Consistent with the behavior in Figure 11, 1pentanol gives the highest laminar flame speed among three
pentanol isomers at the elevated pressures. At a pressure of 0.1
MPa, the difference between 1- and 2- and 3-pentanol is
increased when the temperature changes from 393 to 433 K. At
elevated pressures, the difference among three pentanol isomers
is decreased. An increasing temperature increases the laminar
flame speed, while an increasing pressure decreases the laminar
flame speed.
The adiabatic flame temperature, Tad, is a very important
parameter for the laminar flame. It has a positive relationship
with the laminar flame speed. Figure 13 gives the adiabatic
comparison between model predictions and experimental data
for the stoichiometric 1-pentanol−air mixture at p = 10 atm and
τ = 0.7 s. Predictions on species, such as O2, CO, CH2O,
pentanol, and n-pentanol, with the modified model are slightly
improved in comparison to those with the Dagaut model.
Predictions on other species, such as C2H4, CH4, and H2, with
the modified model are greatly improved. Simulations on
species concentration profiles for the lean and rich 1-pentanol−
air mixtures were also performed, as shown in Figures 9 and 10,
respectively. For the lean mixture (ϕ = 0.5), both Dagaut and
modified models can accurately predict the CH4 concentration.
The modified model improves the prediction on C2H4.
Prediction on H2 is largely improved with the modified
model. For the rich mixture (ϕ = 2.0), both modified and
Dagaut models give the approximate prediction on species
concentrations of CH4 and H2, while the modified model
improves the prediction on the species concentration of C2H4.
3.2. Laminar Flame Speeds of Three Pentanol
Isomers. Figure 11 shows laminar flame speeds versus
equivalence ratio of three pentanol isomers at 393 K and 0.1
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Figure 11. Laminar flame speeds versus equivalence ratio for three
pentanol isomers.
flame temperatures for three pentanol isomers. A small
difference in the adiabatic flame temperature is presented. An
enlargened graph shows that the adiabatic flame temperature of
1-pentanol is higher than those of 2- and 3-pentanol. Almost no
difference between 2- and 3-pentanol is demonstrated. The
behavior of the adiabatic flame temperature among three
pentanol isomers is consistent with the behavior of the laminar
flame speed among three pentanol isomers.
The molecular structure plays an important role in the
laminar flame speed. Previous studies covered some kinds of
hydrocarbon and alcohol isomers.13,17,34,35 In comparison to
OH branched-chain alcohols, OH straight-chain alcohols give a
higher reaction rate.13,17,34,35 Their influences on the following
reactions contribute to the higher reaction rate of OH straightchain alcohols:
(a) In OH straight-chain alcohol flames, notably higher
concentrations of formaldehyde are formed, which, in turn, lead
to the formation of more formyl radicals. Formyl radicals are a
very important chain-branching reaction through R1 to
generate H and CO and contribute to the increase in the
reaction rate.
Figure 9. Experimental and computed species concentration profiles at
ϕ = 0.5, p = 10 atm, and τ = 0.7 s. Symbols, experiment; solid line,
modified model; dashed line, Dagaut model.
HCO + M = H + CO + M
(R1)
(b) OH branched-chain alcohols produce more CH3 radicals,
which, in turn, consume H through recombination reaction R2.
CH3 + H + M → CH4 + M
(R2)
(c) Carbons are easily consumed through molecular
dehydration and form stable species of alkene and water
when hydroxy (OH) attaches on β-carbon and γ-carbon
compared to α-carbon. This will decrease the reaction rate.
3.3. Flame Instability. Both diffusional−thermal instability
and intrinsic hydrodynamic instability were analyzed in this
study. Intrinsic hydrodynamic instability caused by a density
jump across the flame front becomes more important at an
elevated pressure. It can be analyzed by the density ratio and
flame front thickness.36,37 The flame front thickness and density
ratio are calculated via equations lf = v/S0u and σ = ρu/ρb. A
decrease in the flame front thickness and an increase in the
density ratio enhance the hydrodynamic instability. Diffusional−thermal instability caused by non-equal diffusion
between heat and mass can be assessed by the Lewis number
(Le), the ratio of mixture thermal diffusivity/mass diffusivity.28,38−41
Markstein length, Lb, is a parameter characterizing the effect
of stretch on flame propagation and reflects flame instability. A
negtive value of Markstein length corresponds to an unstable
Figure 10. Experimental and computed species concentration profiles
at ϕ = 2.0, p = 10 atm, and τ = 0.7 s. Symbols, experiment; solid line,
modified model; dashed line, Dagaut model.
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Figure 12. Laminar flame speeds versus equivalence ratio at elevated pressures for three pentanol isomers.
Figure 14. Markstein lengths versus equivalence ratio for three
pentanol isomers.
Figure 13. Adabatic flame temperature versus equivalence ratio for
three pentanol isomers.
Table 3. Instability Parameters of Pentanol Isomers at 393 K
and 0.1 MPa
flame front structure, and a positive value of Markstein length
corresponds to a stable flame front structure. A high Markstein
length indicates a highly stable flame front as the flame
develops. Figure 14 gives the Markstein lengths of three
pentanol isomers at 0.1 MPa and 393 K. The Markstein length
decreases monotonically with the increase of the equivalence
ratio. Markstein lengths of three pentanol isomers give close
values, and this indicates the same flame front stability and/or
instability behavior for the three pentanol isomers.
Table 3 gives the parameters that reflect the flame instability
for three pentanol isomers at 393 K, 0.1 MPa, and various
equivalence ratios. They are flame front thickness lf, density
ratio σ of unburned mixture/burned mixture, and Lewis
number. Close values in parameters are presented at a fixed
equivalence ratio among three pentanol isomers, especially for
2- and 3-pentanol. 1-Pentanol gives a smaller flame thickness
and higher density ratio than those of the other two isomers,
ϕ
0.8
1.2
1.6
fuel
lf (mm)
σ
Le
1-pentanol
2-pentanol
3-pentanol
1-pentanol
2-pentanol
3-pentanol
1-pentanol
2-pentanol
3-pentanol
0.0471
0.0527
0.0513
0.039
0.0424
0.0414
0.1076
0.1106
0.1217
5.6624
5.6404
5.6379
6.4661
6.4334
6.4297
6.1691
6.1188
6.1131
2.4137
2.4249
2.4268
0.9248
0.9311
0.9322
0.8840
0.8917
0.8931
reflecting its higher hydrodynamic instability behavior than
those of the other two isomers. Meanwhile, Le of 1-pentanol is
smaller, which indicates its higher diffusional−thermal instability behavior. Thus, 1-pentanol has a higher flame front
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instability tendency than those of the other two isomers. The
flame front just reflects the diffusional−thermal instability
because hydrodynamic instability is not significant at
atmospheric pressure. The difference is small, which is also
demonstrated in the photos. Figure 15 gives the flame
Figure 16. Schlieren photos of three pentanol isomers at elevated
pressures (Tu = 433 K and ϕ = 1.1).
cellular structure at the flame front increases the flame front
area, leading to a rapid increase in flame speed. This
phenomenon easily occurs at an elevated pressure. Bechtold
and Matalon42 defined the onset radius of the cellular flame
front structure as the critical radius, Rcr. In this study, we adopt
the method by Jomaas et al.43 to give the Rcr. A dimensionless
critical Peclet number, Pecr, which is the ratio of Rcr to flame
thickness, lf, is also used to reflect flame instability. Figure 17
Figure 15. Schlieren photos of three pentanol isomers at Tu = 393 K,
pu = 0.1 MPa, and ϕ = 1.6.
propagation photos of three pentanol isomers at a rich mixture
(ϕ = 1.6). The photos show similar flame front morphology at
the same flame radius among three pentanol isomers. Some
cracks appear at the beginning of flame propagation, but the
number of cracks does not increase.
Table 4 gives the instability characterizing parameters of
three pentanol isomers at 433 K and ϕ = 1.1 at four initial
Table 4. Instability Parameters of Pentanol Isomers at 433 K
and ϕ = 1.1
Pu (MPa)
0.10
0.25
0.50
0.75
fuel
lf (mm)
σ
Le
1-pentanol
2-pentanol
3-pentanol
1-pentanol
2-pentanol
3-pentanol
1-pentanol
2-pentanol
3-pentanol
1-pentanol
2-pentanol
3-pentanol
0.0395
0.0431
0.0416
0.0189
0.0203
0.0199
0.0112
0.0118
0.0114
0.0092
0.0099
0.0088
5.9283
5.9059
5.9033
5.9727
5.9484
5.9456
5.9988
5.9731
5.9702
6.0112
5.9849
5.9819
0.9396
0.9433
0.9443
0.9396
0.9433
0.9443
0.9396
0.9433
0.9443
0.9396
0.9433
0.9443
pressures. The corresponding schlieren photos at a specified
flame radius are provided in Figure 16. With the increase of the
pressure, the flame thickness (lf) is decreased and the density
ratio (σ) is increased, with little variation in the Lewis number
(Le). This reflects the enhanced hydrodynamic instability and
insusceptible diffusional−thermal instability as pressure is
increased. At low pressure (0.1 MPa), a smooth flame front
is presented during flame propagation. At a pressure of 0.25
MPa, cracks appear on the flame front surface. When the
pressure is further increased (0.5 and 0.75 MPa), the flame
front surface tends to be more unstable and a cellular structure
occurs. At an elevated pressure, the cellular structure of 1pentanol is stronger than those of the other two isomers.
During flame propagation, the changing on the flame front
structure will result in the variation of flame characteristics,
such as cellular structure and cracks. The occurrence of the
Figure 17. Stretched flame speed versus stretch rate.
gives the stretched flame speeds of 2- and 3-pentanol versus
stretch rate at 0.5 MPa and three equivalence ratios. The onset
of the cellular structure is presented for rich mixtures of these
two pentanol isomers. Figure 18a gives the Rcr of three
pentanol isomers versus equivalence ratio at elevated pressures
(0.5 and 0.75 MPa). Rcr decreases with the increase of the
equivalence ratio for the three pentanol isomers, indicating the
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pentanol, 1-pentanol has a smaller flame thickness and a higher
Le number and density ratio, suggesting its high flame
instability behavior. The critical radius and Peclect number,
which correspond to the onset of the cellular structure, are
smaller for the 1-pentanol flame.
■
ASSOCIATED CONTENT
■
AUTHOR INFORMATION
S Supporting Information
*
All data of laminar flame speeds for pentanol isomers (Tables
S1−S5). This material is available free of charge via the Internet
at http://pubs.acs.org.
Corresponding Author
*Telephone: 0086-29-82665075. Fax: 0086-29-82668789. Email: [email protected] (E.H.); [email protected]
(Z.H.).
Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS
This work is supported by the National Natural Science
Foundation of China (51136005 and 51206132) and the
National Basic Research Program (2013CB228406).
■
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Figure 18. Critical parameters versus equivalence ratio for three
pentanol isomers: (a) critical radius and (b) critical Peclet number.
enhancement of cellular structure development when the
mixture is enriched. For a lean mixture, the diffusional−thermal
instability is suppressed (Le > 1) and the flame is hard to
develop into the cellular structure, at least within the observable
window. 2- and 3-pentanol give the approximate critical radius,
while 1-pentanol gives a smaller critical radius. This indicates
the easily unstable flame front for 1-pentanol, which is
consistent with the observation in flame photos in Figure 17.
The decrease of Rcr corresponds to the advancement of the
onset of flame cellularity. Figure 18b gives the critical Peclet
number versus equivalence ratio for three pentanol isomers at
elevated pressures. Similar to Rcr, Pecr decreases monotonically
with the increase of the equivalence ratio. 1-Pentanol gives a
lower Pecr than those of 2- and 3-pentanol.
4. CONCLUSION
Laminar flame speeds of three pentanol isomers (1-, 2-, and 3pentanol) were measured, and kinetic analysis on 1-pentanol
was made by both Dagaut and modified models. Flame
instabilities were analyzed. The main conclusions are
summarized as follows: (1) Laminar flame speeds of 1-pentanol
increase with the increase of the temperature and the decrease
of the pressure. Predictions using the Dagaut model give lower
values than the measurement for the lean mixture, and
discrepancy is decreased with the increase of the pressure.
Good agreement between model prediction and measurement
is presented at stoichiometric and fuel-rich mixtures. (2)
Modification on the Dagaut model gives good prediction on the
laminar flame speed. (3) The laminar flame speed of 1-pentanol
gives the highest value, followed by those of 3- and 2-pentanol.
The difference between three isomers decreases with the
increase of the pressure. (4) In comparison to 2- and 31149
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Energy & Fuels
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