JSAE 20159235
SAE 2015-01-1933
Study on the laminar characteristics of ethanol, n-butanol
and n-pentanol flames
Qianqian Li*, Yu Cheng, Wu Jin, Zhaoyang Chen, Zuohua Huang
State Key Laboratory of Multiphase Flow in Power Engineering, Xi’an Jiaotong University, Xi’an 710049, P.R.
China
Copyright © 2015 SAE Japan and Copyright © 2015 SAE International
ABSTRACT
Due to serious energy crisis and pollution problem,
interest in research of the alternative fuels is increasing
over the world. Alcohol fuels are always considered to
be promising alternative fuels. Lower alcohols owning
high octane number is good octane enhancer for SI
(Spark ignition) engine, however is difficult to be used
in CI (Compression Ignition) engines. Higher alcohols
like pentanol with higher energy content, poor water
solubility and higher cetane number are good choice
for the CI engines. In this study, laminar flame
behaviors of ethanol-air, n-butanol-air and n-pentanolair mixtures at 393 K and 0.1 MPa are compared and
analyzed with the spherical propagating flames.
Comparison of the laminar flame speeds measured in
the previous studies (Li et al.) show that laminar flame
speed of ethanol is the fastest with slower flame speed
of n-butanol and n-pentanol at lean mixture. At rich
mixture, three alcohols present very close values. The
effective Lewis number of n-pentanol is the biggest,
and then n-butanol and ethanol. The difference among
the three fuels is decreasing with the increase of
equivalence ratio. However, all the values are bigger
than one and indicating diffusively stable flame front.
Flame thicknesses of three alcohol fuels are very close,
while the wave number decreases in the order of npentanol, n-butanol and ethanol and shows the
hydrodynamic instability is enhanced with the carbon
number increasing. Combining with the schlieren
pictures, it is seen that ethanol has the more stable
flame front at very rich mixture, which indicates
hydrodynamic instability plays the dominant role at the
rich condition.
INTRODUCTION
The energy shortage and the serious environmental
problem greatly promoted the research on the
alternative fuels as the bio-fuels. Bio-alcohols are
generally considered to be the promising alternative
fuels, and were widely studied in the past years. Low
alcohol like ethanol with relatively low cost and high
octane number was paid close attention as the
gasoline additive. Engine research indicates that the
alcohol additive significantly decrease the HC, CO
emissions, as well as the particular emission [1-3]. Low
alcohol gasoline have been widely sold in the market.
However, low energy content and high hygroscopicity
bring big challenge in the further development.
Therefore, when the low alcohol blended with
PRF(primary reference fuel mainly indicates gasoline
and diesel), additional solvent are needed which may
be high alcohol fuels for their high energy and good
solubility in PRF[4].
In recent years, research on high alcohol is increasing
especially pentanol. Pentanol was initially studied as
additive in gasoline engine. Gautam et al. [5, 6]
indicated when pentanol added into gasoline increases,
the CO and HC emission decrease while the knocking
resistance is enhanced, implying this fuel might be
more suitable as the CI engine additive. Actually,
studies developed by Wei et al.[7] and CamposFernandez et al.[8] showed that blending fuel of 1pentanol/diesel runs without problems even with 30%
n-pentanol blended, behaving similar engine
performance as pure diesel and even much better
combustion characteristic. Besides, the particular
emission is significantly decreased both in the mass
concentration and the number concentration[7]. These
studies provide promising future in application and
promote the fundamental studies.
Fundamental study on primary alcohols has been
conducted in many ways, and the combustion
characteristics are widely discussed with various
experimental setup with data measured as ignition
delay, laminar flame speed, etc. Liao et al.[9] and
Bradley et al.[10] measured the laminar flame speed
of ethanol-air mixture with the initial pressure extended
to 1.4 MPa. Bradley et al.[10] indicated that with the
initial pressure increasing, the flame front became
unstable and wrinkled due to the thermo-diffusive
instability and hydrodynamic instability. The wrinkled
flame front will further enhance the flame propagation
speed. Gu et al.[11] measured the laminar flame speed
of n-butanol and its isomers over wide equivalence
ratio range with the initial pressure elevated to be 0.75
MPa. Difference among butanol isomers were
analyzed. Besides, Gu et al.[12] developed the study
on the laminar combustion characteristics of n-butanol-
diluent-air mixtures with the dilution of nitrogen at
different dilution ratios. Li et al.[13] investigated the
laminar combustion characteristics of n-pentanol-air
mixtures, obtaining the laminar flame speed and
analyzing the flame instabilities over wide pressure
conditions. Meanwhile, the data of the laminar flame
speed was used to validate the n-pentanol model
proposed by Togbé et al.[14] to further improve the
model. Comparison among the primary alcohol fuels
were developed. Veloo et al.[15] determined the
laminar flame speed and extinction rate of methanol,
ethanol and n-butanol flames with counterflow
experimental setup at 0.1 MPa. Noorani et al.[16]
studied the high temperature ignition characteristics of
C1-C4 primary alcohols behind the shock waves.
Therefore, previous study paid close attention on the
combustion data like the flame speed or the ignition
delay, while little concerned the discrepancy of the
flame front phenomenon among the primary alcohol
flames.
This study aims to illustrate the difference on the flame
instability characteristics of the primary alcohols,
ethanol, n-butanol and n-pentanol flames at
atmospheric pressure and 393 K with the spherical
propagating flame in a constant chamber. Flame
instability parameters like the Markstein length, the
Lewis number, wavenumber and flame thickness were
calculated. Combining the flame schlieren pictures, the
flame instability characteristics were analyzed.
Additionally, the laminar flame speeds of the three
primary alcohol-air mixtures were summarized with
data from different references.
EXPERIMENTAL APPARATUS AND
PROCEDURES
the chamber. The heating tape surrounding the
chamber to supply the heat to increase the
temperature, which was monitored with thermocouple.
When the chamber was achieved the initial
temperature, the chamber was vacuumed and the
liquid alcohol was injected into the chamber with
microsyringe. Five minutes was waited to achieve the
complete vaporization. The air used here is the mixture
of 79% nitrogen and 21% oxygen with the purity over
99.95%. When the mixture was prepared, the mixture
was ignited with the electrodes located in the center of
the chamber. At the same time, the data acquisition
system was started with the schlieren photos taken.
Each condition was repeated for at least three times to
ensure the reproducibility. When the alcohol was
changed, the chamber was flushed repeatedly with the
dry air to avoid the combustion residues of the last fuel.
RESULTS AND DISCUSSION
Laminar flame speeds of the three primary alcohol
flames have been respectively reported in previous
studies [13, 17] and were summarized in Fig. 2 to make
the comparison. It is seen that ethanol has the fastest
flame speed with the equivalence ratio smaller than 1.2.
Difference is the most significant around 1.1. At richer
mixtures, three alcohol flames exhibit very close
values. N-butanol and n-pentanol flames exhibit close
values over all equivalence ratio range. The laminar
flame speed of iso-octane measured by Li et al.[18]
from the same group is plotted as well, and the values
are obviously lower than those of alcohol flames.
Faster flame speed will reduce the combustion
duration in the engine and be favor of improving the
efficiency.
0.7
0.6
0.4
Tu= 393 K
0
Su / ms-1
0.5
0.3
Pu= 0.10 MPa
ethanol
1-butanol
1-pentanol
iso-octane by Li et al.
0.2
0.1
0.0
0.6
0.8
1.0
1.2
1.4
Equivalence ratio 
1.6
1.8
Fig. 2. Laminar flame speeds of three alcohol and isooctane flames versus the equivalence ratio at 0.1
MPa and 393 K.
Fig. 1. Experimental setup
As seen in Fig. 1, the experimental setup is composed
of various parts, the ignition system, the constant
volume chamber, the heating system, the data
acquisition system and the temperature and pressure
control system. The constant chamber is a cylindrical
chamber in stainless steel with the diameter of 180 mm.
Two quartz windows with 80 mm diameter were
embedded in the two sides to provide the optical
access. The inlet and outlet valves were arranged on
Flame front instabilities are dominant by three
mechanisms, the buoyancy instability, the diffusionalthermal instability and the hydrodynamic instability.
The buoyancy instability is always considered near
flammability limits which is not in the range of present
conditions. The diffusional-thermal instability resulting
in the non-equal diffusion between the heat and the
mass, is qualified by Lewis number defined as the ratio
of the thermal diffusivity and the mass diffusivity. The
hydrodynamic instability is triggered by the density
jump across the flame front, characterized by the flame
 LeO



 LeF


thickness and wavenumber. Besides, Markstein length
obtained
through
the
linear
regression,
S  Sb  Lb , is a parameter illustrating the
0
b
sensitivity of the laminar flame speed to stretch and
reflecting the overall flame instability characteristics.
Fig. 3 gives the Markstein length (Lb) of three alcohol
flames versus the equivalence ratio at 0.1 MPa and
393 K. Higher Markstein length indicates more stable
flame front. Obviously, Markstein length monotonically
decreases with the increase of the equivalence ratio
due to the discrepancy on the diffusive properties for
different mixtures. Thus, the flame front tends unstable
from lean to rich conditions. No significant difference is
observed among the three alcohol flames at lean
mixture, while Markstein length of ethanol flame is
slightly higher than the else two alcohol flame at
extremely rich mixture of 1.6 to 1.8, demonstrating the
relatively more stable flame front of ethanol flame.
3.2
2.4
Tu= 393 K
Pu= 0.10 MPa
Lb/ mm
1.6
0.8
0.0
ethanol
n-butanol
n-pentanol
-0.8
Leeff
 (1   ) LeF
2 
 1
 (1   ) LeO
2 
 1
(1)
in which  =and Ea(Tb-Tu)/RTb2. is the
equivalence ratio, Ea the activation energy and
Zeldovich number. LeF and LeO respectively
represents the Lewis number of the thermal diffusivity
of the mixture to the mass diffusivities of the fuel and
oxidizer. It is seen that Leeff decreases with the
increase of the equivalence ratio, indicating the
thermal-diffusive instability is enhanced with the
mixture becoming richer. This conclusion is consistent
with that of Markstein length, indicating the variation of
Markstein length with the equivalence ratio is mainly
resulting from the diffusion properties. Among the
three alcohol-air flames, Leeff decreases in the order of
n-pentanol, n-butanol and ethanol at a fixed
equivalence ratio. The difference is significant at lean
side and gradually decreases with the mixture
becoming richer. At very rich mixture of 1.8, three
mixtures exhibit almost the same value. This is
because of the increasingly dominant role of diffusivity
of oxidizer at richer mixture. Generally, Leeff is bigger
than 1.0, indicating the flame front is thermal-diffusive
stable.
0.25
-1.6
0.8
1.0
1.2
1.4
1.6
Equivalence ratio 
1.8
0.20
Tu= 393 K
Pu= 0.10 MPa
Fig. 3. Markstein length of three alcohol flames
versus the equivalence ratio at 0.1 MPa and 393 K.
0.15
ethanol
n-butanol
n-pentanol
f
0.6
0.10
2.4
2.2
Pu= 0.10 MPa
2.0
0.00
1.8
Leeff
0.05
Tu= 393 K
ethanol
n-butanol
n-pentanol
1.6
0.6
0.8
1.0
1.2
1.4
Equivalence ratio 
1.6
1.8
Fig. 5. Flame thickness of three alcohol flames versus
the equivalence ratio at 0.1 MPa and 393 K.
1.4
1.2
1.0
0.8
0.6
0.8
1.0
1.2
1.4
Equivalence ratio 
1.6
1.8
Fig. 4. Effective lewis number of three primary alcohol
flames versus the equivalence ratio at 0.1 MPa and
393 K.
Fig. 4 shows the effective Lewis number of three
alcohol flames versus the equivalence ratio at 0.1 MPa
and 393 K. The overall effective Lewis number of the
mixture, Leeff, is a weighted average value of LeF and
LeO, given as [19],
Fig. 5 and Fig. 6 respectively illustrates the flame
thickness and wavenumber of the alcohol mixtures at
0.1 MPa and 393 K over equivalence ratio of 0.6 to 1.8.
Wave number is calculated through the equation
as[19],
   3   2  
(2)
 1
Where  is the density ratio of the unburned to
DL 
burned mixture. The two parameters all characterize
the hydrodynamic instability property. Higher value of
flame thickness and smaller wavenumber indicate the
hydrodynamic instability is inhibited. As seen in Fig. 5,
the flame front thickness are very close for three
alcohol flames, while the wavenumber decreases in
the order of n-pentanol, n-butanol and ethanol. N-
pentanol flame has very close values with n-butanol
flame which is significantly higher than ethanol flame.
Therefore, n-pentanol is the most hydrodynamic
unstable, and ethanol is relatively hydrodynamic stable.
decrease with the increase of the equivalence ratio.
No significant difference is observed for Markstein
length among the three alcohol flames except for
the very rich mixture, where ethanol exhibits higher
values. The effective Lewis number decreases in
the order of n-pentanol, n-butanol and ethanol.
However, the effective Lewis number is bigger
than 1.0 at most conditions, indicating the flame
front is diffusively stable.
1.55
1.50
1.45
1.40
Tu= 393 K
DL
1.35
(2) Three alcohol flames exhibit very close flame
Pu= 0.10 MPa
thickness, and the wavenumber decreases in the
order of n-pentanol, n-butanol and ethanol,
indicating n-pentanol flame is the most
hydrodynamic unstable.
1.30
ethanol
n-butanol
n-pentanol
1.25
1.20
(3) Flame propagation schlieren pictures show the
1.15
1.10
0.6
0.8
1.0
1.2
1.4
Equivalence ratio 
1.6
flame front of ethanol flame is the most stable
while cracks arise in the flame front of n-butanol
and n-pentanol flames. Combining the parameters
calculated, it is inferred the hydrodynamic
mechanism plays the significant role in the flame
propagation at very rich mixture.
1.8
Fig. 6 Wavenumber of three alcohol flames versus
the equivalence ratio at 0.1 MPa and 393 K.
Fig. 7 shows the schlieren pictures of different flame
radius at the equivalence ratio of 1.6, 0.1 MPa and 393
K for three alcohol flames. At this rich mixture, the
ethanol flame keeps smooth flame front during the
flame propagation. N-butanol and n-pentanol flame
front is in smooth state at the very initial stage, while
some cracks arise and grow during the flame
propagation, and the flame front become unstable. The
above flame instability parameters indicate the
hydrodynamic instability is enhanced in the order of npentanol, n-butanol and ethanol while the thermodiffusive instability is weakened in the same order, it is
inferred that the difference on the instability
characteristic among the alcohol flames is dominant by
the hydrodynamic mechanism.
rf=15 mm 20 mm 25 mm 30 mm 35 mm
ACKNOWLEDGMENTS
This work is supported by the National Natural Science
Foundation of China (Grant No. 51406159, 91441203
and 50876085), the National Basic Research Program
(2013CB228406), and the China Postdoctoral Science
Foundation (2014M560774).
REFERENCES
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Ethanol
N-butanol
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Fig. 7. Schlieren pictures of three alcohol flames at
the equivalence ratio of 1.6, 0.1 MPa and 393 K.
CONCLUSION
5.
Laminar flame instability characteristics of three
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MPa and 393 K over wide equivalence ratio range in a
constant chamber. Flame instability parameters as
Markstein length, flame thickness, Lewis number, and
wavenumber were calculated. The main conclusion
were as follows.
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CONTACT
Corresponding author: Qianqian Li
E-mail: [email protected]
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BUTANOL ON ENGINE EFFICIENCY AND EMISSIONS USING A DIRECT-INJECTION,
SPARK-IGNITION ENGINE. Proceedings of the 2009 Spring Technical Conference of
the Asme Internal Combustion Engine Division2009. 157-165.
2.
Kohse-Hoeinghaus, K., et al., Biofuel Combustion Chemistry: From Ethanol to Biodiesel.
Angewandte Chemie-international Edition - ANGEW CHEM INT ED, 2010. 49(21): p.
3572-3597.
3.
Yucesu, H.S., et al., Effect of ethanol-gasoline blends on engine performance and
exhaust emissions in different compression ratios. Applied Thermal Engineering, 2006.
26(17-18): p. 2272-2278.
4.
Sathiyagnanam, A.P., C.G. Saravanan, and M. Gopalakrishnan, Hexanol-ethanol diesel
blends on DI-diesel engine to study the combustion and emission. Proceedings of the
World Congress on Engineering 2010. 2.
5.
Gautam, M. and D.W. Martin, Combustion characteristics of higher-alcohol/gasoline
blends. Proceedings of the Institution of Mechanical Engineers Part a-Journal of Power
and Energy, 2000. 214(A5): p. 497-511.
6.
Gautam, M., D.W. Martin, and D. Carder, Emissions characteristics of higher
alcohol/gasoline blends. Proceedings of the Institution of Mechanical Engineers Part aJournal of Power and Energy, 2000. 214(A2): p. 165-182.
7.
Wei, L., C.S. Cheung, and Z. Huang, Effect of n-pentanol addition on the combustion,
performance and emission characteristics of a direct-injection diesel engine. Energy,
2014. 70(0): p. 172-180.
8.
Campos-Fernandez, J., et al., Performance tests of a diesel engine fueled with
pentanol/diesel fuel blends. Fuel, 2013. 107(0): p. 866-872.
9.
Liao, S.Y., et al., Determination of the laminar burning velocities for mixtures of ethanol
and air at elevated temperatures. Applied Thermal Engineering, 2007. 27(2-3): p. 374380.
10.
Bradley, D., M. Lawes, and M.S. Mansour, Explosion bomb measurements of ethanol–
air laminar gaseous flame characteristics at pressures up to 1.4MPa. Combustion and
Flame, 2009. 156(7): p. 1462-1470.
11.
Gu, X.L., et al., Laminar burning velocities and flame instabilities of butanol isomers-air
mixtures. Combustion and Flame, 2010. 157(12): p. 2318-2325.
12.
Gu, X.L., Q.Q. Li, and Z.H. Huang, Laminar Burning Characteristics of Diluted NButanol/Air Mixtures. Combustion Science and Technology, 2011. 183(12): p. 1360-1375.
13.
Li, Q., et al., Laminar Flame Speeds and Flame Instabilities of Pentanol Isomer–Air
Mixtures at Elevated Temperatures and Pressures. Energy & Fuels, 2013. 27(2): p. 11411150.
14.
Togbe, C., et al., Experimental and detailed kinetic modeling study of 1-pentanol oxidation
in a JSR and combustion in a bomb. Proceedings of the Combustion Institute, 2011. 33:
p. 367-374.
15.
Veloo, P.S., et al., A comparative experimental and computational study of methanol,
ethanol, and n-butanol flames. Combustion and Flame, 2010. 157(10): p. 1989-2004.
16.
Noorani, K., B. Akih-Kumgeh, and J. Bergthorson, Comparative High Temperature Shock
Tube Ignition of C1−C4 Primary Alcohols. Energy & Fuels, 2010. 24(11): p. 5834-5843.
17.
Li, Q., et al., Measurements of laminar flame speeds and flame instability analysis of 2methyl-1-butanol–air mixtures. Fuel, 2013. 112(0): p. 263-271.
18.
ianqian, L., et al., Laminar Flame Speeds of DMF/Iso-octane-Air-N2/CO2 Mixtures.
Energy & Fuels, 2012. 26(1): p. 917-925.
19.
Matalon, M., Flame dynamics. Proceedings of the Combustion Institute, 2009. 32(1): p.
57-82.