energies
Article
Laminar Flame Characteristics of C1–C5 Primary
Alcohol-Isooctane Blends at Elevated Temperature
Qianqian Li *, Wu Jin and Zuohua Huang *
State Key Laboratory of Multiphase Flow in Power Engineering, Xi’an Jiaotong University, Xi’an 710049, China;
[email protected]
* Correspondence: [email protected] (Q.L.); [email protected] (Z.H.);
Tel.: +86-29-8266-5075 (Q.L. & Z.H.)
Academic Editor: Enrico Sciubba
Received: 23 April 2016; Accepted: 24 June 2016; Published: 30 June 2016
Abstract: The laminar combustion characteristics of blends of isooctane and C1–C5 primary alcohols
(i.e., methanol, ethanol, n-propanol, n-butanol and n-pentanol) were investigated using the spherical
expanding flame methodology in a constant volume chamber at various equivalence ratios and
volume fractions of alcohol. The stretch effect was removed using the nonlinear methodology.
The results indicate that the laminar flame speeds of alcohol-isooctane blends increase monotonously
with the increasing volume fraction of alcohol. Among the five alcohols, the addition of methanol
is identified to be the most effective in enhancing laminar flame speed. The addition of ethanol
results in an approximately equivalent laminar flame speed enhancement rate as those of n-propanol,
n-butanol and n-pentanol at ratios of 0.8 and 1.5, and a higher rate at 1.0 and 1.2. An empirical
correlation is provided to describe the laminar flame speed variation with the volume fraction of
alcohol. Meanwhile, the laminar flame speed increases with the mass content of oxygen in the
fuel blends. At the equivalence ratio of 0.8 and fixed oxygen content, similar laminar flame speeds
are observed with different alcohols blended into isooctane. Nevertheless, with the increase of
equivalence ratio, heavier alcohol-isooctane blends tend to exhibit higher values. Markstein lengths
of alcohol-isooctane blends decrease with the addition of alcohol into isooctane at 0.8, 1.0 and 1.2,
however they increase at 1.5. This is consistent with the behavior deduced from the Schlieren images.
Keywords: primary alcohol; isooctane; laminar flame speed; oxygen content; Markstein length;
empirical correlation
1. Introduction
Driven by energy shortages and environmental issues, there has been great interest in the
development of alternative fuels in recent years. Alcohols, considered a renewable resource, are
being extensively investigated in all fields. Unlike traditional fuels, alcohols contain oxygen, which
is beneficial for complete combustion and the reduction of toxic emissions [1–8]. Specifically, light
alcohols, e.g., methanol and ethanol, have been successfully utilized as octane boosters for traditional
fuels. Heavy alcohols have high energy contents and blend well with traditional fuels. Isooctane
is identified as the representative branched paraffin and, hence an important constituent in the
surrogates of traditional fuels [9–11], therefore, investigating alcohol-isooctane blends can provide
valuable information for understanding the potential of alcohols as alternative fuels.
Laminar flame characteristics, namely laminar flame speed and flame instability properties,
are the fundamental physico-chemical information in evaluating the flame propagation process and
turbulent flame research. In addition, laminar flame speed is widely adopted to validate chemical
kinetics models. The laminar combustion characteristics of pure isooctane flames have been extensively
investigated under various initial conditions in the past years [12–18]. Recently, studies on the laminar
Energies 2016, 9, 511; doi:10.3390/en9070511
www.mdpi.com/journal/energies
Energies 2016, 9, 511
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flames of alcohols have received increasing attention. Saeed and Stone [19] and Zhang et al. [20]
studied the laminar flames of methanol in a constant volume chamber with laminar flame speeds at
different determined initial temperatures and pressures. Togbé et al. [21] presented a set of laminar
flame speeds of n-pentanol at 0.1 MPa, 423 K to evaluate the accuracy of the n-pentanol model.
Veloo et al. [22,23] measured the laminar flame speeds of methanol, ethanol, n-propanol and n-butanol
flames in a counterflow configuration. Their results showed that methanol presents similar flame
speeds as the other alcohols for lean mixtures, but yields significantly higher values for rich mixtures.
This result was consistent with the conclusions of Beeckmann et al. [24] in a subsequent study on
C1–C4 primary alcohols at elevated pressure. Sarathy et al. [25] reviewed the recent developments
in alcohol combustion studies, and laminar flame speeds of different alcohols found in the literature
were provided [25]. In addition, a comprehensive model covering C1–C5 alcohols was developed
and adopted to simulate the laminar flame speeds at 353 K. The simulation results indicated that
methanol exhibits the highest laminar flame speed, followed by ethanol and C3–C5 primary alcohols.
Zhang et al. [26,27] determined the laminar flame speeds and Markstein lengths of n-butanol-isooctane
blends at elevated temperatures with a spherical propagating flame. They also investigated the cellular
instability characteristics by analyzing the critical Peclect numbers calculated from the Schlieren
pictures and stability theory. Broustail et al. [28,29] experimentally evaluated the laminar flame
speeds and Markstein lengths of n-butanol-isooctane and ethanol-isooctane blends at different initial
temperatures and pressures in a constant volume chamber. An empirical formula was given to relate
the laminar flame speed and the volume fraction of alcohol. Gulder et al. [30] studied the laminar flame
speeds of methanol-isooctane blends at different determined blending ratios. Their results indicated
that the laminar flame speed decreased with the addition of methanol to isooctane. Since there is
no accurate model for methanol-isooctane blend combustion, it was speculated that the reason was
that the methanol product formaldehyde may consume the intermediate radicals and suppress the
oxidation reactivity. Subsequently, Gülder et al. [31] studied the effects of blending ethanol into
isooctane on the laminar flame speed, and found the isooctane combustion was strongly promoted by
the addition of ethanol. In general, experimental research on alcohol-isooctane blends is still scarce
and no comprehensive comparative study on the blending of different alcohols into isooctane has
been performed.
As mentioned above, alcohols are different from traditional fuels due to the presence of oxygen in
alcohols. Engine tests suggest that oxygen content in the alcohol fuel is relevant to the knocking
resistance and the harmful emissions of NOx , soot and carbonyl [2,3,32]. Gautam et al. [2,3]
investigated the combustion and emission characteristics of a single-cylinder engine fueled with C1–C5
alcohol-gasoline blends. It was concluded that with the increase of oxygen content in the fuel blends,
knocking resistance is enhanced, but NOx emissions are increased significantly at high compression
ratios. Sathiyagnanam et al. [33] studied the emission characteristics of hexanol-ethanol-diesel blends
in a diesel engine. The results indicated that soot emissions decrease significantly with increasing
oxygen content. Farkade and Pathre [34] compared the effects of blending methanol, ethanol and
n-butanol into gasoline on engine performance. Their results showed that CO and HC emissions
decreased with the increase in oxygen content and the better engine performance is achieved with
oxygen contents in the blends equaling 5%. However, limited fundamental investigation has been
carried out on the effect of oxygen content on laminar combustion characteristics.
In this study, the laminar flame speeds and Markstein lengths of C1–C5 primary alcohol-isooctane
blends were acquired via a spherical propagating flame in a constant volume chamber. The data were
determined at an initial temperature of 363 K, initial pressure of 0.1 MPa, four equivalence ratios (0.8,
1.0, 1.2, 1.5) and different volume fractions of alcohol (0%, 20%, 40%, 60%, 80%, 100%). Based on the
data, the dependencies of the laminar flame speed and Markstein length on the volume fraction of
alcohol were interpreted, and a correlation describing the dependence of the laminar flame speed on
volume fraction of alcohol was provided. In addition, the effect of oxygen content on the variation of
laminar flame speed was illustrated.
Energies 2016, 9, 511
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2. Experimental Apparatus and Data Derivation
The experimental apparatus utilized to determine the laminar flame speed is composed of five
parts: a cylinder chamber with an inner diameter of 180 mm and a length of 210 mm, an ignition
system, a high speed camera (Phantom V611, Vision Research Inc., Wayne, NJ, USA), the control system
and the optical system. Two quartz windows mounted on the two sides of the chamber provide the
optical path. The high temperature of 363 K was accomplished by heating the chamber with a 1500 W
heating tape surrounding the chamber. The pressure and temperature were respectively monitored
by a pressure transducer and a thermocouple arranged on the chamber. At the beginning of each
experiment, the chamber was heated up to the target temperature and evacuated. The liquid fuel was
injected into the chamber with a microliter syringe. At least 5 min was allowed to elapse to ensure the
full evaporation of the liquid fuel. Then dry air composed of 21% O2 (>99.95%) and 79% N2 (>99.95%)
was introduced into the chamber. After waiting for at least 6 min to achieve uniform mixing, ignition
was performed using the spark electrodes located in the center of the chamber. Meanwhile, the high
speed camera was initiated to track the flame propagation. More details of the experimental setup
and procedures can be found in previous studies [35–37]. The liquid fuels (C1–C5 primary alcohols
and isooctane) involved in present study had purities over 99% and the specifications listed in Table 1.
It is seen that the heating values of alcohols are lower than that of isooctane and heavier alcohols have
higher heating values. Among the five alcohols, the oxygen content decreases monotonously with
increasing length of the carbon chain.
Table 1. Specifications of the fuels involved in this study [2,3,38].
Property
Isooctane
Methanol
Ethanol
Chemical formula
Molecular weight (g¨ mol´1 )
Oxygen content (wt%)
Density (g¨ cm´3 )
Boiling point (˝ C)
Vapor pressure at 20 ˝ C (kPa)
Flash point (˝ C)
Lower heating value (MJ¨ kg´1 )
Solubility in water
C8 H18
114.23
0
691.9
99.3
5.5
´12
44.4
Insoluble
CH4 O
32.04
49.93
792
64.7
13.02
11
19.58
Soluble
C2 H6 O
46.07
34.73
789
78.37
5.95
13
26.83
Soluble
n-Propanol
C3 H8 O
60.1
26.62
804
97
1.99
22
30.63
Soluble
n-Butanol
n-Pentanol
C4 H10 O
74.12
21.59
810
117.7
0.82
35
33.09
Slight soluble
C5 H12 O
88.15
18.15
811
137–139
0.2
49
34.65
Slight soluble
The flame radius, rf , can be directly acquired from the Schlieren pictures of flame propagation.
The stretched flame propagation speed is therefore determined by:
Sb “
drf ptq
dt
(1)
Both linear and nonlinear methodologies were developed to describe the relationship between
the stretched flame propagation speed and the stretched rate. Here a nonlinear expression [39,40] was
adopted and is expressed as:
Sb “ Sb0 ´ Sb0 Lb ¨ 2{rf
(2)
where Sb0 and Lb represents the unstretched flame speed and Markstein length, respectively, and 2/rf
is linked to the stretch rate through the equation κ = (2/rf )¨ Sb . Finally, the laminar flame speed is
deduced on the basis of mass conservation by the expression:
Su0 “ ρb Sb0 {ρu
(3)
where ρb and ρu are the densities of the burned and unburned gas, respectively.
The spark ignition and the chamber confinement are recognized to affect the accuracy of laminar
flame speed measurements, therefore a flame radius of 8–22 mm was utilized to do the data processing.
The total experimental uncertainty was evaluated according to the theory of Moffat et al. [41] which
has been widely adopted in previous studies [42–44]. It can be described as:
δSu0 “
b
pBSu0 q2 ` pt1´α{2 pνqσSu0 q2
(4)
Energies 2016, 9, 511
4 of 17
where t1–α/2 (ν) is the Student’s t value at 95% confidence interval and the freedom degree of ν;
σSu0 represents the standard deviation of Su0 . BSu0 is the total bias uncertainty of the determination
methodology
and
can be obtained from the following equation:
Energies 2016,
9, 511
4 of 16
g
fconfidence
2 and the freedom degree of ν; σ𝑆 0
n
where t1–α/2(ν) is the Student’s t value at 95%
fÿ
u
BSu0 pxiinterval
q
0
BSu0𝑆u“. e
represents the standard deviation of
𝐵𝑆u0 isp the totalubias
uncertainty of the determination
iq
Bxi
i “1
methodology and can be obtained from the following
equation:
(5)
n
S 0 ( x )
where xi represents the factor influencing Bthe= measurement
accuracy and ui is the deviation
of xi .
( u i ui )2
(5)

S
xi
´
1
i 1
Present total bias uncertainty was estimated to be 1.3–2 cm¨ s with the detailed analysis given
in [44,45].
standard
(σS0 ) arising
from data processing
cm¨ s´1of
. At
where The
xi represents
thedeviation
factor influencing
the measurement
accuracy andwas
ui is 0.3–2
the deviation
xi. each
u
−1
condition,
40–80
were utilized
to do to
thebedata
the values
of νgiven
werein40–80.
Present
total pictures
bias uncertainty
was estimated
1.3–2processing,
cm·s with thus
the detailed
analysis
−1. At each condition,
[44,45].
The standarduncertainty
deviation (σS0uwas
) arising
fromdetermined
data processing
wasEquation
0.3–2 cm·s(4)
The total
experimental
finally
with
to be approximately
1.
were utilized to do the data processing, thus the values of ν were 40–80. The total
˘2–4 40–80
cm¨ s´pictures
0
u
experimental uncertainty was finally determined with Equation (4) to be approximately ±2–4 cm·s−1.
3. Results and Discussion
3. Results and Discussion
3.1. Laminar Flame Speed
3.1. Laminar Flame Speed
Figure 1 shows the stretched flame propagation speed (Sb ) of pure isooctane and alcohol flames
Figure 1 shows the stretched flame propagation speed (Sb) of pure isooctane and alcohol flames
versus stretch rate at 0.1 MPa, 363 K and four different equivalence ratios. It is observed that isooctane
versus stretch rate at 0.1 MPa, 363 K and four different equivalence ratios. It is observed that isooctane
flames
always
present
thethe
lowest
flames,demonstrating
demonstrating
isooctane
flames
flames
always
present
lowestSbSbamong
among all
all fuel
fuel flames,
thatthat
isooctane
flames
propagates
the
slowest.
The
difference
between
the
alcohol
flames
changes
with
the
variation
of
propagates the slowest. The difference between the alcohol flames changes with the variation of
equivalence
ratio.
At
ϕ
=
0.8
and
1.0,
the
methanol
flame
exhibits
approximately
the
same
S
as
the
equivalence ratio. At  = 0.8 and 1.0, the methanol flame exhibits approximately the same Sb as the
b
ethanol
flame,
higher
b values
thanthose
those of the
alcohol
flames.
ethanol
flame,
and and
higher
Sb Svalues
than
the C3–C5
C3–C5primary
primary
alcohol
flames.
4.0
4.0
(a)
= 0.8
Tu= 363 K
2.5
2.5
2.0
-1
Pu= 0.1 MPa
-1
Sb / m  s
3.0
methanol
ethanol
n-propanol
n-butanol
n-pentanol
isooctane
1.5
1.0
0.5
0
Sb / m  s
3.0
0.0
2.0
1.5
0.5
0.0
0
100 200 300 400 500 600 700 800 900 1000

/s
4.5
(c)
3.5
4.0
3.0
3.5
(d)
3.0
Sb / m  s
-1
-1
2.5
2.0
1.5
1.0
= 1.2
2.5
2.0
1.5
= 1.5
1.0
0.5
0.0
= 1.0
1.0
100 200 300 400 500 600 700 800 900 1000

/s
4.0
Sb / m  s
(b)
3.5
3.5
0.5
0
100 200 300 400 500 600 700 800 900 1000
/s

0.0
0
100 200 300 400 500 600 700 800 900 1000

/s
Figure 1. Stretched flame propagation speed vs. stretch rate at different equivalence ratios for pure
Figure
1. Stretched flame propagation speed vs. stretch rate at different equivalence ratios for pure
isooctane and pure C1–C5 primary alcohol flames: (a)  = 0.8; (b)  = 1.0; (c)  = 1.2; and (d)  = 1.5.
isooctane and pure C1–C5 primary alcohol flames: (a) ϕ = 0.8; (b) ϕ = 1.0; (c) ϕ = 1.2; and (d) ϕ = 1.5.
A line is shown in (a) to indicate the nonlinear extrapolation.
A line is shown in (a) to indicate the nonlinear extrapolation.
At  = 1.2 and 1.5, Sb of methanol flame remains the fastest, while the Sb of the ethanol flame
At
ϕ = 1.2
and 1.5,
Sbas
ofthe
methanol
flame remains
thethe
fastest,
while the ratios,
Sb of the
ethanol flame
exhibits
a similar
value
C3–C5 primary
alcohols. At
four equivalence
no significant
difference
is observed
flame propagation
speeds of
alcohols,
although
the
exhibits
a similar
value asfor
thetheC3–C5
primary alcohols.
Atthe
theC3–C5
four primary
equivalence
ratios,
no significant
n-propanol
flame presents
higher valuesspeeds
than that
n-butanol
n-pentanol
at 0.8.
difference
is observed
for the slightly
flame propagation
ofofthe
C3–C5 and
primary
alcohols,
although the
n-propanol flame presents slightly higher values than that of n-butanol and n-pentanol at 0.8.
Energies 2016, 9, 511
5 of 17
In Figure 2, laminar flame speeds measured in this study are compared with the data5 acquired
Energies 2016, 9, 511
of 16
from the literature to evaluate the performance of the experimental apparatus. Error bars are marked to
In Figure
2, laminar
flame speedsuncertainty.
measured inFigure
this study
compared
with the
dataspeeds
acquired
indicate the
experimental
measurement
2a,bare
gives
the laminar
flame
of pure
from
theisooctane
literature to
evaluate
theMPa,
performance
of the experimental
Error
are marked
ethanol
and
flames
at 0.1
respectively.
It is seen the apparatus.
present data
arebars
in good
agreement
the experimental
measurement
uncertainty.
2a,b gives
the speeds
laminarof
flame
speeds of
with to
theindicate
data from
literatures [10,46–50].
Figure
2c givesFigure
the laminar
flame
ethanol-isooctane
pure
ethanol
and
isooctane
flames
at
0.1
MPa,
respectively.
It
is
seen
the
present
data
are
in good
blend-air mixtures versus volume fraction of ethanol at 0.1 MPa. It is seen the present data
at 363 K
agreement with the data from literatures [10,46–50]. Figure 2c gives the laminar flame speeds of
is located between the two sets of data at 348 K and 373 K measured by Rau et al. [51]. This result is
ethanol-isooctane blend-air mixtures versus volume fraction of ethanol at 0.1 MPa. It is seen the
reasonable
because of the fundamental effect of the initial temperature. To allow a direct comparison
present data at 363 K is located between the two sets of data at 348 K and 373 K measured by Rau et
0
between
the
present
data
and the
data of
Raufundamental
et al. [51],effect
an empirical
relationship
Tu1.8 [52]
u 9
al. [51]. This
result is
reasonable
because
of the
of the initial
temperature.STo
allow
was introduced
to account
forthe
thepresent
effect data
of the
temperature
on therelationship
laminar flame
a direct comparison
between
and10
theKdata
of Rau et al.difference
[51], an empirical
0
1.8
speed.
10 K difference
results
a 1–2ofcm/s
in the laminar
𝑆u It∝is𝑇ufound
[52] that
was the
introduced
to account
for theineffect
the 10decrease
K temperature
differenceflame
on thespeed.
laminar flameadjusted
speed. It isdata
found
thatwell
the 10with
K difference
results
in aFigure
1–2 cm/s
laminarflame
The temperature
agree
the present
data.
2ddecrease
shows in
thethe
laminar
flame
speed. The temperature
adjusted
data agreeequivalence
well with theratios.
presentThe
data.
Figure 2d shows
the by
speeds
of n-butanol-isooctane
blends
at different
experiment
conducted
laminar
flame
speeds
of
n-butanol-isooctane
blends
at
different
equivalence
ratios.
The
experiment
Zhang et al. [27] was at 353 K which is 10 K lower than the present temperature, thus the data of
conducted by Zhang et al. [27] was at 353 K which is 10 K lower than the present temperature, thus
Zhang
et al. [27] were adjusted to match those at 363 K. It is seen that the present data agree well
the data of Zhang et al. [27] were adjusted to match those at 363 K. It is seen that the present data
with the data of Zhang et al. [27] at 0.8 equivalence ratio after temperature adjustment. At 1.0 and
agree well with the data of Zhang et al. [27] at 0.8 equivalence ratio after temperature adjustment. At
1.5, the
data slightly deviate from the temperature-adjusted data but the deviations are within the
1.0 and 1.5, the data slightly deviate from the temperature-adjusted data but the deviations are within
experimental
uncertainty.
the experimental
uncertainty.
0.5
0.7
(b)
(a)
0.6
0.4
ethanol
isooctane
S0u / ms
0.4
0
Su / m  s
-1
-1
0.5
Pu= 0.1 MPa
0.3
Present data, 363 K
Liao et al., 358 K [46]
Egofopoulos et al., 363 K [47]
G lder et al., 358 K [48]
0.2
0.1
0.0
0.8
1.0
1.2
1.4
Equivalence ratio 
1.6
1.8
0.6
0.8
n-butanol+isooctane
0.44
Present data, 363 K
Rau et al., 373 K [51]
Rau et al., tem adjustment
Rau et al., 348 K [51]
0.32
0.0
0.2
1.4
1.6
= 1.0
(d)
= 0.8
0.40
0.48
0.36
1.2
0.48
ethanol+isooctane
0.40
1.0
Equivalence ratio 
0.56
(c)
S0u / ms-1
S0u / ms-1
Present data, 363 K
Baloo et al., 363 K [49]
Sileghem et al., 358 K [10]
Dirrenberger et al., 358 K [50]
0.1
0.52
0.28
Pu= 0.1 MPa
0.2
0.0
0.6
0.60
0.56
0.3
= 1.0
Pu= 0.1 MPa
0.4
0.6
0.8
Volume fraction of ethanol
1.0
0.32
= 1.5
0.24
0.16
Pu= 0.1 MPa
Present data (363 K)
Zhang et al. (353 K) [27]
Zhang et al. (tem adjustment)
0.08
0.00
0.0
0.2
0.4
0.6
0.8
Volume fraction of n-butanol
1.0
Figure 2. Laminar flame speeds of alcohol-isooctane blends acquired in present study and in
Figure
2. Laminar flame speeds of alcohol-isooctane blends acquired in present study and in
literatures [10,27,46–51] at 0.1 MPa: (a) pure ethanol; (b) pure isooctane; (c) ethanol-isooctane blends;
literatures [10,27,46–51] at 0.1 MPa: (a) pure ethanol; (b) pure isooctane; (c) ethanol-isooctane blends;
and (d) n-butanol-isooctane blends.
and (d) n-butanol-isooctane blends.
Figure 3 plots the laminar flame speeds of C1–C5 primary alcohol-isooctane blends at 363 K, 0.1
MPa
and3 different
equivalence
ratios. speeds
Error bars
just indicated
methanol-isooctane
blendsat
for363 K,
Figure
plots the
laminar flame
of are
C1–C5
primaryfor
alcohol-isooctane
blends
clarity.
laminar
flame speeds
exhibit
nonlinear
with
addition of alcohol
to for
0.1 MPa
andThe
different
equivalence
ratios.
Errorabars
are justincrease
indicated
forthe
methanol-isooctane
blends
isooctane.
The
increase
rate
depends
on
the
kind
of
alcohol
considered.
This
is
consistent
with
the
clarity. The laminar flame speeds exhibit a nonlinear increase with the addition of alcohol to isooctane.
results obtained by Zhang et al. [27] and Broustail et al. [28,29]. It also provides support for the
The increase rate depends on the kind of alcohol considered. This is consistent with the results obtained
conclusions derived on engine tests that the combustion duration was decreased with the alcohols
by Zhang et al. [27] and Broustail et al. [28,29]. It also provides support for the conclusions derived
blended into gasoline. The methanol addition into isooctane is identified to be the most effective at
on engine tests that the combustion duration was decreased with the alcohols blended into gasoline.
The methanol addition into isooctane is identified to be the most effective at promoting the flame
propagation at different equivalence ratios. Therefore, a pure methanol flame gives the fastest flame
Energies
2016, 2016,
9, 5119, 511
Energies
6 of 166 of 17
promoting the flame propagation at different equivalence ratios. Therefore, a pure methanol flame
speedgives
among
different
alcohol
flames.
difference
between
and between
the remaining
alcohols
the the
fastest
flame speed
among
theThe
different
alcohol
flames. methanol
The difference
methanol
and the
remaining
alcohols
is within
3 cm/s
at the
equivalence
ratio of
greater
at 1.0,
is within
3 cm/s
at the
equivalence
ratio
of 0.8,
but
much greater
at0.8,
1.0,but
1.2much
and 1.5.
This
result is
1.2 and
1.5. the
Thisresults
result is
consistent
theetresults
obtained
Veloo et al.et[22,23]
Beeckmann
consistent
with
obtained
bywith
Veloo
al. [22,23]
and by
Beeckmann
al. [24]and
under
rich mixture
et
al.
[24]
under
rich
mixture
conditions.
Under
lean
conditions,
Veloo
et
al.
[22]
and
Beeckmann
et
conditions. Under lean conditions, Veloo et al. [22] and Beeckmann et al. [24] found that methanol
al. [24] found that methanol flame exhibits similar laminar flame speeds to ethanol, n-propanol and
flame exhibits similar laminar flame speeds to ethanol, n-propanol and n-butanol flames, which is
n-butanol flames, which is slightly different from the present result. In the present study, the addition
slightly different from the present result. In the present study, the addition of ethanol results in a similar
of ethanol results in a similar laminar flame speed enhancement as those of n-propanol, n-butanol
laminar
speedatenhancement
asratios
thoseofof0.8
n-propanol,
and n-pentanol
theand
equivalence
andflame
n-pentanol
the equivalence
and 1.5, andn-butanol
a higher enhancement
rate at
at 1.0
1.2.
ratiosThis
of 0.8
and
1.5, and
higher
enhancement
rate by
at 1.0
and 1.2.
result
agrees well with the
result
agrees
well a
with
the conclusions
obtained
Broustail
et al.This
[28,29]
for ethanol-isooctane
conclusions
obtained by Broustail
and n-butanol-isooctane
blends. et al. [28,29] for ethanol-isooctane and n-butanol-isooctane blends.
0.50
methanol+isooctane
ethanol+isooctane
n-propanol+isooctane
n-butanol+isooctane
n-pentanol+isooctane
0.40
0.65
(a)
(b)
0.60
Pu= 0.1 MPa
S0u / ms-1
S0u / ms-1
0.45
= 0.8
Tu= 363 K
0.35
0.50
0.45
0.30
0.25
= 1.0
0.55
0.0
0.2
0.4
0.6
0.8
Volume fraction of alcohol
0.40
1.0
0.0
0.2
0.4
0.6
0.8
Volume fraction of alcohol
0.50
0.65
1.0
(d)
(c)
0.45
0.60
= 1.2
0.55
S0u / ms-1
S0u / ms-1
0.40
0.50
= 1.5
0.35
0.30
0.25
0.45
0.20
0.40
0.0
0.2
0.4
0.6
0.8
Volume fraction of alcohol
0.15
1.0
0.0
0.2
0.4
0.6
0.8
Volume fraction of alcohol
1.0
Figure 3. Laminar flame speeds of the alcohol-isooctane blends vs. volume fraction of alcohol at 363 K,
Figure
3. Laminar flame speeds of the alcohol-isooctane blends vs. volume fraction of alcohol at 363 K,
0.1 MPa, and different equivalence ratios: (a)  = 0.8; (b)  = 1.0; (c)  = 1.2; and (d)  = 1.5. Lines are
0.1 MPa, and different equivalence ratios: (a) ϕ = 0.8; (b) ϕ = 1.0; (c) ϕ = 1.2; and (d) ϕ = 1.5. Lines are
calculated using Equation (8) in Section 3.4.
calculated using Equation (8) in Section 3.4.
3.2. Interpretation on the Variation of Laminar Flame Speeds
3.2. Interpretation on the Variation of Laminar Flame Speeds
Laminar flame speed is closely linked to the properties of thermal, diffusive and chemical
kinetics,
be described
[53,54]:
Laminarwhich
flamecan
speed
is closelyaslinked
to the properties of thermal, diffusive and chemical kinetics,
which can be described as [53,54]:
u Su0  (λ / Cp )Le[exp(  Ea / R0Tad )]
(6)
0
where Le, Ea and Tad denotes the
number, the activation energy and adiabatic temperature, (6)
ρu SLewis
u 9pλ{Cp qLerexpp´Ea {R0 Tad qs
respectively; and λ/Cp is the density-compensated thermal diffusivity. Tad is calculated based on
thermal
Lewis number
(Le) number,
is definedthe
as the
ratio of thermal
diffusivity
and
where
Le, Eaequilibrium.
and Tad denotes
the Lewis
activation
energy and
andmass
adiabatic
temperature,
derived by:
respectively; and λ/Cp is the density-compensated thermal diffusivity. Tad is calculated based on
λ
thermal equilibrium. Lewis number (Le) is defined
as the ratio of thermal and mass diffusivity
Le 
(7) and
 C D
derived by:
in which λ is the thermal conductivity of the mixture,λ Cp is the specific heat of the mixture and Dm is
Le “
(7)
the mass diffusivity of the deficient species. The Lennard-Jones
potential data of the blends involved
ρu Cp Dm
u
p
m
in which λ is the thermal conductivity of the mixture, Cp is the specific heat of the mixture and Dm is
the mass diffusivity of the deficient species. The Lennard-Jones potential data of the blends involved in
the determination of Dm were calculated based on the mass average of the pure fuels. The terms of Tad
Energies 2016, 9, 511
Energies 2016, 9, 511
Energies 2016, 9, 511
7 of 16
7 of 16
7 of 17
TadT/adK/ K
in the determination of Dm were calculated based on the mass average of the pure fuels. The terms of
inadthe
of Dm the
were
calculated
on the
average
of the pure
fuels. The Figure
terms of
T
anddetermination
(λ/Cp)Le represent
effects
of thebased
thermal
andmass
diffusive
properties,
respectively.
4
T
ad and (λ/Cp)Le represent the effects of the thermal and diffusive properties, respectively. Figure 4
and (λ/C
)Lealcohol-isooctane
represent the effects
of the
and diffusive
properties,
4 shows
shows
Tadpof
blends
at thermal
the equivalence
ratio of
1.2, 363 Krespectively.
and 0.1 MPa.Figure
It is observed
shows
Tdecreases
ad of alcohol-isooctane blends at the equivalence ratio of 1.2, 363 K and 0.1 MPa. It is observed
T
blends at with
the equivalence
ratio
of 1.2, 363
K andis0.1
MPa.by
It is
observed
Tad
that
Tadalcohol-isooctane
monotonously
the addition
of alcohols,
which
caused
the
smaller that
heating
ad of
that
T
ad decreases monotonously with the addition of alcohols, which is caused by the smaller heating
decreases
with
the addition
of alcohols,Lower
whichalcohols
is causedhave
by thesmaller
smallerheating
heating values
values
values
of monotonously
C1–C5 alcohols
compared
to isooctane.
values
oftoalcohols
C1–C5
compared
toTisooctane.
Lowerwith
alcohols
have
heating
values
of C1–C5
compared
to to
isooctane.
have
smaller
heating
values
referring
to
referring
Table 1,alcohols
thus leading
lower
adLower
of theiralcohols
blends
isooctane
forsmaller
a fixed
volume
fraction
referring
to
Table
1,
thus
leading
to
lower
T
ad of their blends with isooctane for a fixed volume fraction
Table
1,
thus
leading
to
lower
T
of
their
blends
with
isooctane
for
a
fixed
volume
fraction
of
alcohol.
of alcohol.
ad
of alcohol.
2260
2260
2250
2250
2240
2240
2230
2230
2220
2220
2210
2210
2200
2200
2190
2190
= 1.2
== 1.2
T
363 K
u
Tu=
= 0.1
363MPa
K
P
u
Pu= 0.1 MPa
methanol+isooctane
methanol+isooctane
ethanol+isooctane
ethanol+isooctane
n-propanol+isooctane
n-propanol+isooctane
n-butanol+isooctane
n-butanol+isooctane
n-pentanol+isooctane
n-pentanol+isooctane
0.0
0.2
0.4
0.6
0.8
of alcohol
0.0
0.2Volume
0.4fraction0.6
0.8
Volume fraction of alcohol
1.0
1.0
Figure
Figure 4.
4. Adiabatic
Adiabatic temperature
temperature of
of alcohol-isooctane
alcohol-isooctane blends
blends vs.
vs. volume
volumefraction
fractionof
ofalcohol
alcoholat
at363
363 K,
K,
Figure
4.and
Adiabatic
temperature
ofofalcohol-isooctane
blends vs. volume fraction of alcohol at 363 K,
0.1
MPa
the
equivalence
ratio
1.2.
0.1 MPa and the equivalence ratio of 1.2.
0.1 MPa and the equivalence ratio of 1.2.
Figures 5 and 6 show the density compensated thermal diffusivity (λ/Cp) and Lewis number (Le)
and66show
showthe
thedensity
densitycompensated
compensated
thermal
diffusivity
(λ/C
Lewis
number
p ) and
Figures
55 and
thermal
diffusivity
p) and
Lewis
number
(Le)
of five
alcohol-isooctane
blends
at 363 K, 0.1 MPa,
respectively.
λ/C(λ/C
p and Le decrease with the
(Le)five
of alcohol-isooctane
five alcohol-isooctane
blends
atK,363
K,
0.1 respectively.
MPa, respectively.
λ/C
and Le with
decrease
pdecrease
of
blends
at
363
0.1
MPa,
λ/C
p and
Le
the
increasing blending ratio of alcohol, and the values decrease more for lower alcohols. Therefore,
with
the
increasing
blending
ratio
of
alcohol,
and
the
values
decrease
more
for
lower
alcohols.
increasing blendingblends
ratio of
alcohol,
and the
values
decrease
more for lower alcohols. Therefore,
methanol-isooctane
have
the lowest
values
of λ/C
p and Le for a fixed volume fraction of alcohol.
Therefore, methanol-isooctane
blends
have the
lowest
values
ofLe
λ/C
and
Le volume
for a fixed
volume
methanol-isooctane
blends
have
the
lowest
values
of
λ/C
p and
forp aprimary
fixed
fraction
of fraction
alcohol.
This demonstrates that methanol is more diffusive than the other
alcohols
and isooctane.
of alcohol.
This demonstrates
thatismethanol
is morethan
diffusive
thanprimary
the other
primary
alcohols
and
This
demonstrates
that
methanol
more
diffusive
the
other
alcohols
and
isooctane.
Combining with the result of Tad, it is concluded that both thermal and diffusive factors exert negative
isooctane.
Combining
with
the
result
of
T
,
it
is
concluded
that
both
thermal
and
diffusive
factors
exert
Combining
with theflame
resultspeed
of Tad, with
it is concluded
that both
thermal
and diffusive
factors
exert negative
effects
on laminar
theadincreasing
volume
fraction
of alcohol.
Moreover,
such a
negative
effects
on laminar
flame with
speedthe
with
the increasing
volume
fraction
of alcohol.
Moreover,
such
effects
on
laminar
flame
speed
increasing
volume
fraction
of
alcohol.
Moreover,
such
a
negative effect is identified to be the strongest for the methanol-isooctane blends which, however,
a
negative
effect
is
identified
to
be
the
strongest
for
the
methanol-isooctane
blends
which,
however,
negative
is laminar
identified
to bespeed.
the strongest
foritthe
methanol-isooctane
blends which,
have
the effect
highest
flame
Therefore,
is speculated
that the chemical
kinetichowever,
effect is
Therefore, itit is
is speculated
speculated that
that the
the chemical
chemical kinetic
kinetic
have
the
highest
laminar
flame
speed.
Therefore,
primarily responsible for the laminar flame speed variation with the addition of alcohols. effect is
primarily responsible for the laminar flame speed
speed variation
variation with
with the
the addition
addition of
of alcohols.
alcohols.
-5
2.9x10
-5
2.9x10
-5
-1
C
/ kg/m
C
s s
/ kg/m
p p
-1
2.8x10
-5
2.8x10
-5
2.7x10
-5
2.7x10
-5
2.6x10
-5
2.6x10
-5
2.5x10
-5
2.5x10
= 1.2
== 1.2
T
363 K
u
T
=
363MPa
K
u
P = 0.1
u
Pu= 0.1 MPa
methanol+isooctane
methanol+isooctane
ethanol
+isooctane
ethanol+isooctane
n-propanol
+isooctane
n-propanol+isooctane
n-butanol+isooctane
n-butanol+isooctane
n-pentanol
+isooctane
n-pentanol+isooctane
0.0
0.2
0.4
0.6
0.8
0.0
0.2 Volume
0.4fraction
0.6of alcohol
0.8
Volume fraction of alcohol
1.0
1.0
Figure 5. Density-compensated thermal diffusivity of alcohol-isooctane blend-air mixtures at 363 K,
Figure
5.and
Density-compensated
thermal
Figure
Density-compensated
thermal
diffusivity of
of alcohol-isooctane
alcohol-isooctane blend-air
blend-air mixtures
mixtures at
at 363
363 K,
K,
0.1
MPa5.
the equivalence ratio
of 1.2. diffusivity
0.1
of 1.2.
1.2.
0.1 MPa
MPa and
and the
the equivalence
equivalence ratio
ratio of
The laminar flame speed differences between the five alcohol-isooctane blends are inherently
Thebylaminar
flamecombustion
speed differences
between
the
fivefuels.
alcohol-isooctane
are kinetics
inherently
caused
the distinct
chemistries
of the
pure
Therefore, theblends
chemical
of
The laminar
flame speed differences
between
the
five alcohol-isooctane
blends
are inherently
caused
by
the
distinct
combustion
chemistries
of
the
pure
fuels.
Therefore,
the
chemical
kinetics
of
pure
alcohols
isooctane
were comparatively
Figure
7 showsthe
thechemical
simulations
of C1–
caused
by theand
distinct
combustion
chemistries ofinvestigated.
the pure fuels.
Therefore,
kinetics
of
pure
alcohols
and
isooctane
were
comparatively
investigated.
Figure
7
shows
the
simulations
of
C1–
C5
primary
and isooctane
versus equivalence
ratio Figure
at 363 7Kshows
and 0.1
The simulations
pure
alcoholsalcohols
and isooctane
were comparatively
investigated.
theMPa.
simulations
of C1–C5
C5 primary
alcohols
and
isooctane
versus equivalence
ratio at CHEMKIN
363 K and 0.1
MPa. The
simulations
were
carried
out and
with
the
PREMIX
[55] coupling
package
[55–58].
Inwere
the
primary
alcohols
isooctane
versuscode
equivalence
ratio atwith
363 K and 0.1 MPa.
The simulations
were carried out with the PREMIX code [55] coupling with CHEMKIN package [55–58]. In the
Energies 2016, 9, 511
8 of 17
Energies 2016, 9, 511
8 of 16
carried
out2016,
with9, the
Energies
511 PREMIX code [55] coupling with CHEMKIN package [55–58]. In the simulations,
8 of 16
thewell
Soret
as well as the mixture-averaged
were
used.
Here the
we adopted
the simulations,
Soret effect as
aseffect
the mixture-averaged
transport weretransport
used. Here
we
adopted
mechanism
simulations,
effect
as well
as the
transport
were
used.
Here wethe
adopted
the
mechanism
ofSoret
C1–C5
alcohol
combustion
developed
Sarathy
et al.
[25]
to alcohols’
simulate
alcohols’
of C1–C5
alcoholthe
combustion
developed
bymixture-averaged
Sarathy
et al.by[25]
to simulate
the
flames
and the
the
mechanism
of
C1–C5
alcohol
combustion
developed
by
Sarathy
et
al.
[25]
to
simulate
the
alcohols’
and theproposed
PRF mechanism
proposed
by Chaos
et al. [59]
simulateflames.
the isooctane
flames.
is
PRFflames
mechanism
by Chaos
et al. [59]
to simulate
the to
isooctane
It is seen
thatItunder
flames
the PRF
mechanism
proposed
by Chaos et al. [59]curves
to simulateslightly
the isooctane flames.
It is of
thatand
under
conditions,
the methanol
than those
leanseen
conditions,
thelean
methanol
simulation
curvessimulation
are slightly lowerare
than thoselower
of ethanol,
similar
to
seen that
undertolean
conditions,
the methanol
simulation
curves
arethan
slightly
lower
than thoseUnder
of
ethanol,
similar
those
of
the
C3–C5
primary
alcohols
and
higher
those
of
isooctane.
thoseethanol,
of the C3–C5
primary
and
higheralcohols
than those
of isooctane.
Under
rich conditions,
similar to
those ofalcohols
the C3–C5
primary
and higher
than those
of isooctane.
Under the
rich conditions, the simulated methanol curves are the highest, followed by ethanol, n-propanol, nsimulated
methanol
curves
are
the
highest,
followed
by
ethanol,
n-propanol,
n-butanol
and
n-pentanol,
rich conditions, the simulated methanol curves are the highest, followed by ethanol, n-propanol,
nbutanol and n-pentanol, and isooctane flame gives the lowest values. This result accurately captures
and isooctane
flame
gives the
values.
This result
accurately
captures
differences
between
butanol and
n-pentanol,
andlowest
isooctane
flame gives
the lowest
values. This
result the
accurately
captures
the differences between the pure fuels observed in the experimental data.
the differences
between
fuels observed
the pure
fuels observed
in the
the pure
experimental
data.in the experimental data.
3.2
3.2
0.94
0.94
(a)
(a)
0.92
0.92
2.4
2.4 = 0.8
= 0.8
Tu==363
2.0
363KK
2.0 T
u
PP
=
0.1
MPa
u = 0.1 MPa
1.6
u
1.6
methanol+isooctane
methanol+isooctane
ethanol+isooctane
1.2
ethanol+isooctane
1.2
n-propanol+isooctane
n-propanol+isooctane
0.8
n-butanol+isooctane
0.8
n-butanol+isooctane
n-pentanol+isooctane
n-pentanol+isooctane
0.4
0.4
0.0
0.2
0.4
0.6
0.8
0.0
0.2
0.4
0.8
Volume fraction
fraction of alcohol
Volume
alcohol
0.90
0.90
Le
Le
Le
Le
2.8
2.8
(b)
(b)
0.88
0.88
==
1.21.2
0.86
0.86
0.84
0.84
0.82
0.82
0.80
0.80
1.0
1.0
0.0
0.0
0.20.2 0.40.4 0.60.6 0.8 0.8 1.0 1.0
Volume
fraction
of alcohol
Volume
fraction
of alcohol
0.94
(c)(c)
0.92
0.92
Le
Le
0.90
0.90
0.88
0.88
0.86
0.86
= 1.5
= 1.5
0.84
0.84
0.82
0.82
0.80
0.80
0.0
0.0
0.2
0.4
0.6
0.8
0.2Volume0.4
0.8
fraction0.6
of alcohol
Volume fraction of alcohol
1.0
1.0
Figure 6. Lewis number of C1–C5 primary alcohol-isooctane blends at different equivalence ratios: (a)
Figure
6. Lewis
number
primaryalcohol-isooctane
alcohol-isooctane
blends
at different
equivalence
Figure
6. Lewis
numberof
of C1–C5
C1–C5 primary
blends
at different
equivalence
ratios: ratios:
(a)
 = 0.8; (b)  = 1.0; and (c)  = 1.2.
(a) ϕ
=
0.8;
(b)
ϕ
=
1.0;
and
(c)
ϕ
=
1.2.
 = 0.8; (b)  = 1.0; and (c)  = 1.2.
0.75
0.75
0.60
0.60
S0u / Sm0u /s-1ms-1
0.45
0.45
0.30
0.30
0.15
0.15
0.00
0.7
0.00
0.7
0.8
0.9
0.8
0.9
methanol
ethanol
methanol
n-propanol
ethanol
n-butanol
n-propanol Tu= 363 K
n-pentanol
n-butanol PT
= 0.1 MPa
u u= 363 K
isooctane
n-pentanol
Pu= 0.1 MPa
isooctane
1.0
1.1 1.2 1.3
1.4 1.5
Equivalence ratio 
1.0
1.1
1.2
1.3
1.4
1.5
1.6
1.6
Equivalence
ratio  at 363 K and 0.1 MPa using the
Figure 7. Simulations of C1–C5 primary alcohols
and isooctane
mechanisms
proposed
by
Sarathy
et
al.
[25]
and
Chaos
et
al.
[59], respectively.
Figure 7. Simulations of C1–C5 primary alcohols and isooctane
at 363 K and 0.1 MPa using the
Figure 7. Simulations of C1–C5 primary alcohols and isooctane at 363 K and 0.1 MPa using the
mechanisms proposed by Sarathy et al. [25] and Chaos et al. [59], respectively.
mechanisms
proposed
Sarathy
et al. [25]
and Chaos
et al.species
[59], respectively.
Sensitivity
analysisbywas
performed
to identify
the key
and reactions highly sensitive to
the laminar flame speed variation. The sensitivity coefficient, SFi, is evaluated by the expression
Sensitivity
analysis was performed to identify the key species and reactions highly sensitive to
𝑘 𝜕𝑆 0
Sensitivity
analysis
was
performed
identifyof
the
key species
and
reactions
highly
sensitive
𝑆𝐹𝑖laminar
= 0𝑖 u ,flame
wherespeed
ki represents
theThe
rateto
constant
reaction
i.SFFigure
8 depicts
sensitivity
the
variation.
sensitivity
coefficient,
i, is evaluated
bythethe
expression
𝑆u 𝜕𝑘𝑖
𝑘 𝜕𝑆 0 flame speed variation. The sensitivity coefficient, SFi , is evaluated by the expression
to the
laminar
𝑆𝐹 = 𝑖 u , where ki represents the rate constant of reaction i. Figure
8 depicts the sensitivity
𝑖
SFi “
0
0
k i 𝑆uB S𝜕𝑘
u𝑖
,
Su0 B k i
where ki represents the rate constant of reaction i. Figure 8 depicts the sensitivity
Energies 2016, 9, 511
9 of 17
Energiesof
2016,
9 ofMPa
16
coefficients
the9, 511
laminar flame speeds of the pure alcohols and isooctane at 363 K, 0.1
and
the equivalence ratio of 1.2. It is seen that the laminar flame speeds of different fuels are primarily
coefficients of the laminar flame speeds of the pure alcohols and isooctane at 363 K, 0.1 MPa and the
sensitive
to the kinetics of H and small hydrocarbons. Fuel-specific reactions are not included,
equivalence ratio of 1.2. It2 is seen that the laminar flame speeds of different fuels are primarily
suggesting
theytoare
the rate-limiting
reactions.
This Fuel-specific
result is consistent
the
observations
sensitive
thenot
kinetics
of H2 and small
hydrocarbons.
reactions with
are not
included,
on many
other
hydrocarbons
[14,45,52].
Nevertheless,
it
is
noted
that
the
laminar
flame
of
suggesting they are not the rate-limiting reactions. This result is consistent with the observationsspeeds
on
many
other
hydrocarbons
[14,45,52].
Nevertheless,
it
is
noted
that
the
laminar
flame
speeds
of
heavier
heavier hydrocarbons (the heavier alcohols and isooctane) are more sensitive to the kinetics of stable
heavier alcohols and isooctane) are more sensitive to the kinetics of stable species
species hydrocarbons
like C2 H2 , C(the
3 H6 and iC4 H8 . This indicates that the effects of the distributions of stable species
like C2H2, C3H6 and iC4H8. This indicates that the effects of the distributions of stable species on
on laminar flame speed are fairly significant for heavy alcohols and isooctane. The distributions of
laminar flame speed are fairly significant for heavy alcohols and isooctane. The distributions of these
these key
species revealed in the sensitivity analyses will change as different alcohols are blended into
key species revealed in the sensitivity analyses will change as different alcohols are blended into
isooctane,
and finally
result
in the
variation
flamespeed.
speed.
isooctane,
and finally
result
in the
variationofoflaminar
laminar flame
(a)
methanol
ethanol
n-propanol
n-butanol
n-pentanol
isooctane
iC4H8<=>iC4H7+H
C3H6+OH<=>C3H5-A+H2O
C3H6+H<=>C2H4+CH3
C3H5-A+H(+M)<=>C3H6(+M)
CH3+HO2<=>CH3O+OH
 = 1.2
Tu= 363 K
CH3+H(+M)<=>CH4(+M)
Pu= 0.1 MPa
C2H3+H<=>C2H2+H2
CH2O+H(+M)<=>CH2OH(+M)
C3H6+OH<=>aC3H5+H2O
C2H3+H<=>C2H2+H2
H+C2H4(+M)<=>C2H5(+M)
CH2+O2=>2H+CO2
O+CH3=>H+H2+CO
HCO+M<=>H+CO+M
HCO+OH<=>CO+H2O
 = 1.2
HCO+H2O<=>H+CO+H2O
HCO+H<=>CO+H2
Tu= 363 K
2CH3<=>H+C2H5
HCO+M<=>H+CO+M
CO+OH<=>CO2+H
Pu= 0.1 MPa
2CH3(+M)<=>C2H6(+M)
CH+O2<=>O+HCO
HO2+OH<=>H2O+O2
HO2+CH3<=>OH+CH3O
H2+O2<=>H+HO2
OH+CO<=>H+CO2
HO2+H<=>OH+OH
OH+CH3<=>CH2*+H2O
H+O2(+M)<=>HO2(+m)
H+HCO<=>H2+CO
H+CH3(+M)<=>CH4(+M)
H+OH+M<=>H2O+M
OH+H2<=>H+H2O
H+OH+M<=>H2O+M
H+O2<=>O+OH
-0.2
-0.1
0.0
(b)
iC4H7<=>aC3H4+CH3
C3H6+H<=>C3H5-A+H2
H+O2<=>O+OH
0.1
0.2
0.3
0.4
0.5
-0.2
-0.1
0.0
0.1
0.2
0.3
0.4
0.5
Figure 8. Normalized rate constant sensitivity coefficients on laminar flame speed for (a) pure C1–C5
Figure 8.
Normalized rate constant sensitivity coefficients on laminar flame speed for (a) pure C1–C5
primary alcohols and (b) pure isooctane at 363 K, 0.1 MPa and the equivalence ratio of 1.2.
primary alcohols and (b) pure isooctane at 363 K, 0.1 MPa and the equivalence ratio of 1.2.
Figure 9 plots the mole fractions of key species in different fuel flames at 363 K, 0.1 MPa and the
equivalence ratio of 1.2. The concentrations of H and OH are positively relevant to the laminar flame
Figure 9 plots the mole fractions of key species in different fuel flames at 363 K, 0.1 MPa and
speed. Obviously, the mole fractions of H, OH in alcohol flames are significantly higher than those
the equivalence ratio of 1.2. The concentrations of H and OH are positively relevant to the laminar
in isooctane flame, which explains why alcohols exhibit higher laminar flame speeds than isooctane.
flame speed.
the mole
fractionsthe
ofmole
H, fractions
OH in alcohol
flames
are significantly
With the Obviously,
addition of alcohols
into isooctane,
of O and OH
are increased,
promoting higher
than those
in isooctane
which the
explains
why alcohols
higher
the overall
reactivityflame,
and enhancing
flame propagation.
The exhibit
mole fractions
of laminar
H and OHflame
in C3–speeds
C5 primaryWith
alcohol
flames
are similar,
and lower
than
those in methanol
andfractions
ethanol flames,
whichOH are
than isooctane.
the
addition
of alcohols
into
isooctane,
the mole
of O and
is
consistent
with
the
laminar
flame
speed
conclusion.
Figure
9c
shows
the
mole
fractions
of
CH
in
increased, promoting the overall reactivity and enhancing the flame propagation. The mole3 fractions
different
fuel
flames.
It
is
observed
that
isooctane
flame
has
the
highest
CH
3 concentration, followed
of H and OH in C3–C5 primary alcohol flames are similar, and lower than those in methanol and
by C2–C5 primary alcohols and methanol flames. CH3 can be mainly consumed through two
ethanol flames, which is consistent with the laminar flame speed conclusion. Figure 9c shows the
terminating reactions, CH3 + H (+ M) <=> CH4 (+ M) and CH3 + CH3 <=> C2H6. Therefore, a high
mole fractions
of CH
in3 will
different
flames.
is observed that isooctane flame has the highest
concentration
of 3CH
reducefuel
the H
and CHIt
3 concentrations and inhibit the flame propagation.
CH3 concentration,
followed
by
C2–C5
primary
alcohols
andthan
methanol
flames.
CH
There is a much higher concentration of CH3 in ethanol flames
in methanol
flames,
contributing
3 can be mainly
to thethrough
lower flame
of ethanol reactions,
than methanol.
the mole
C3HCH
6. The
consumed
twospeed
terminating
CHFigure
(+shows
M) <=>
CH4fractions
(+ M) ofand
3 + H9d
3 + CH3
of Ca3Hhigh
6 increases
as the alcohol
heavier,
and
isooctane
the highest
<=> C2 concentration
H6 . Therefore,
concentration
of becomes
CH3 will
reduce
the
H andflame
CH3 has
concentrations
and
3H6 concentration. C3H6 are primarily consumed by C3H6 + H <=> C3H5 − A + H2 and C3H6 + OH <=>
inhibit C
the
flame propagation. There is a much higher concentration of CH3 in ethanol flames than
C3H5 − A + H2O, as noted in Figure 8. Thus a high concentration of C3H6 retards the overall reaction
in methanol flames, contributing to the lower flame speed of ethanol than methanol. Figure 9d
and the flame propagation. In summary, the species retarding the overall reactivity tends to exhibit
shows higher
the mole
fractions in
ofthe
C3flames
H6 . The
concentration
of much
C3 H6higher
increases
as theflames.
alcohol
becomes
concentrations
of heavier
alcohols and
in isooctane
With
heavier,theand
isooctane
flamealcohols
has theinto
highest
C3 Hthe
consumed
additions
of various
isooctane,
concentrations ofCthe
are changed
in
6 concentration.
3 Hspecies
6 are primarily
extent,
laminar
kinetic in
analyses
by C3 H6different
+ H <=>
C3 H5finally
´ A +causing
H2 andthe
C3 H
<=> speed
C3 H5 distinctions.
´ A + H2 O,The
as noted
Figureabove
8. Thus a
6 + OHflame
provide strong of
support
our chemical
kineticsreaction
considerations.
high concentration
C3 H6forretards
the overall
and the flame propagation. In summary,
the species retarding the overall reactivity tends to exhibit higher concentrations in the flames
of heavier alcohols and much higher in isooctane flames. With the additions of various alcohols
into isooctane, the concentrations of the species are changed in different extent, finally causing the
laminar flame speed distinctions. The kinetic analyses above provide strong support for our chemical
kinetics considerations.
Energies 2016, 9, 511
10 of 17
Energies 2016, 9, 511
10 of 16
Mole fraction 103
12
methanol
ethanol
n-propanol
n-butanol
n-pentanol
isooctane
H
10
8
8
 = 1.2
Tu= 363 K
2
6
4
2
Pu= 0.1 MPa
0
0.0
0.1
0.2
Distance / cm
0.3
0
0.0
0.4
35
30
(b)
OH
6
4
10
(a)
Mole fraction 103
14
0.2
Distance / cm
0.3
50
(c)
CH3
0.1
0.4
(d)
C3H6
4
Mole fraction 10
Mole fraction 104
40
25
20
15
10
20
10
5
0
0.05
30
0.06
0.07
0.08 0.09 0.10
Distance / cm
0.11
0.12
0
0.04
0.06
0.08
Distance / cm
0.10
0.12
Figure 9. Mole fraction profiles of key species for pure isooctane and alcohol fuels at 363 K, 0.1 MPa
Figure
9. Mole fraction profiles of key species for pure isooctane and alcohol fuels at 363 K, 0.1 MPa
and equivalence ratio of 1.2: (a) H; (b) OH; (c) CH3; and (d) C3H6.
and equivalence ratio of 1.2: (a) H; (b) OH; (c) CH3 ; and (d) C3 H6 .
3.3. The Effect of Oxygen Content
3.3. The Effect of Oxygen Content
To learn the effect of oxygen content on the laminar flame characteristics, the laminar flame
To
learn
the alcohol-isooctane
effect of oxygenblends
content
the laminar
the laminar
speeds
of the
wereon
plotted
in Figureflame
10 as acharacteristics,
function of the mass
content offlame
oxygen
in
the
blends
at
363
K,
0.1
MPa
and
four
different
equivalence
ratios.
At
all
equivalence
ratios,
speeds of the alcohol-isooctane blends were plotted in Figure 10 as a function of the mass
content
with
different
alcohols
added
into
isooctane,
laminar
flame
speeds
increase
significantly
with
the
of oxygen in the blends at 363 K, 0.1 MPa and four different equivalence ratios. At all equivalence
increasing
oxygenalcohols
content. added
It confirms
conclusion
reported
by Gautam
and Martin
[2] that the with
ratios,
with different
intothe
isooctane,
laminar
flame
speeds increase
significantly
combustion interval decreases with the increasing oxygen content in the fuel. At  = 0.8 and fixed
the increasing oxygen content. It confirms the conclusion reported by Gautam and Martin [2] that the
oxygen content, laminar flame speeds are similar for the blends of isooctane and different alcohols.
combustion interval decreases with the increasing oxygen content in the fuel. At ϕ = 0.8 and fixed
With the increase of equivalence ratio, deviations appear between different blends and tend to be
oxygen
content,
flame
speeds
similar
for the blends
of isooctane
and different
alcohols.
greater.
At a laminar
fixed oxygen
content,
theare
blends
of isooctane
and heavier
alcohols exhibit
faster flame
With speed,
the increase
equivalence ratio,
deviations
different blends and tend to be
namely,of
methanol-isooctane
blends
have theappear
slowest between
flame speed.
greater. At a fixed oxygen content, the blends of isooctane and heavier alcohols exhibit faster flame
3.4.namely,
Markstein
Length and Flame Instability
speed,
methanol-isooctane
blends have the slowest flame speed.
Markstein length, Lb, extracted along with the unstretched flame propagation speed,
characterizes the thermal-diffusive instability properties of the flame front and reflects the response
of laminar flame
speed
to the stretch
effect
valuesflame
indicate
a stable flame
front
while
Markstein
length,
Lb , extracted
along
with[54].
thePositive
unstretched
propagation
speed,
characterizes
negative
values
indicate
an
unstable
flame
front.
Figure
11a
shows
the
Markstein
length
of n-flame
the thermal-diffusive instability properties of the flame front and reflects the response of laminar
propanol-isooctane blends versus volume fraction of n-propanol with the data listed in Table S1 in
speed to the stretch effect [54]. Positive values indicate a stable flame front while negative values
the Supplementary Materials. It is observed that Lb shows a decreasing trend at  = 0.8, 1.0 and 1.2
indicate an unstable flame front. Figure 11a shows the Markstein length of n-propanol-isooctane
with the addition of n-propanol into isooctane, demonstrating the thermal-diffusive instability is
blends
versus volume fraction of n-propanol with the data listed in Table S1 in the Supplementary
enhanced. At  = 1.5, Lb increases significantly, and the thermal-diffusive instability is suppressed.
Materials.
is observed
that
Lb shows
decreasing
trend at ϕ = blends
0.8, 1.0with
andthe
1.2data
withprovided
the addition
of
SimilarItbehavior
is also
observed
for athe
other alcohol-isooctane
in
n-propanol
into isooctane,
demonstrating
the Figure
thermal-diffusive
ϕ = 1.5,
Tables S2–S5
in the Supplementary
Materials.
11b plots Lb asinstability
the functionisofenhanced.
equivalenceAt
ratio
Lb increases
significantly,
and the
thermal-diffusive
is suppressed.
Similar
is
for pure isooctane
and C1–C5
primary
alcohol flames atinstability
363 K and 0.1
MPa and the data
can bebehavior
found
in Tables S1–S5
in the
Supplementary
Materials.
Isooctane
exhibits
higher in
Lb than
theS2–S5
alcoholin the
also observed
for the
other
alcohol-isooctane
blends
withflame
the data
provided
Tables
flames underMaterials.
lean conditions
it yields
at extremely
rich conditions.
Therefore,
Supplementary
Figurewhile
11b plots
Lb lower
as thevalues
function
of equivalence
ratio for pure
isooctane
3.4. Markstein Length and Flame Instability
and C1–C5 primary alcohol flames at 363 K and 0.1 MPa and the data can be found in Tables S1–S5 in
the Supplementary Materials. Isooctane flame exhibits higher Lb than the alcohol flames under lean
conditions while it yields lower values at extremely rich conditions. Therefore, crosses exist between
Energies
Energies 2016,
2016, 9,
9, 511
511
11
11of
of16
17
Energies 2016, 9, 511
11 of 16
crosses exist between the Lb– curve of isooctane and those of alcohols at the critical equivalence
the Lb(*).
–ϕ curve
of isooctane
and
those of alcohols
at than
the critical
equivalence
ratios
(ϕ*). Specifically,
at
ratios
Specifically,
at the
equivalence
ratio less
*, Lb of
increases
the critical
addition
of alcohol
crosses
exist
between the
Lb–
curve of isooctane
and those
alcoholswith
at the
equivalence
the
equivalence
ratio
less
than
ϕ*,
L
increases
with
the
addition
of
alcohol
into
isooctane,
and
the
into
isooctane,
and the thermal-diffusive
instability
is suppressed.
At the
equivalence
ratio
b
ratios
(*). Specifically,
at the equivalence
ratio less than
*, Lb increases
with
the addition
of greater
alcohol
thermal-diffusive
instability
is addition
suppressed.
At the equivalence
ratio greater than
ϕ*, Lb decreases
with
than
*,
L
b decreases
with
the
of
alcohol,
and
the
thermal-diffusive
instability
is
enhanced.
into isooctane, and the thermal-diffusive instability is suppressed. At the equivalence ratio greater
the addition
ofconfirms
alcohol,the
andresult
the thermal-diffusive
instability
isinenhanced.
Thisof
behavior
confirms the
This
behavior
reported
by
Zhang
et
al.
[27]
their
research
n-butanol-isooctane
than *, Lb decreases with the addition of alcohol, and the thermal-diffusive instability is enhanced.
result reported
by
Zhang
et
al.
[27] in their
research
of n-butanol-isooctane
blends
with
of ϕ*
blends
with the
value
ofthe
*result
identified
to be
From
11b,
theresearch
values
of
forthe
thevalue
blends
of
This behavior
confirms
reported
by1.3.
Zhang
et Figure
al. [27] in
their
of *
n-butanol-isooctane
identified and
to bedifferent
1.3. From
Figure 11b,
the valuesto
ofbe
ϕ*1.2–1.3
for the which
blendsare
of isooctane
andresult
different
alcohols
isooctane
alcohols
are
estimated
close
to
the
of
Zhang
blends with the value of * identified to be 1.3. From Figure 11b, the values of * for the blends et
of
are[27].
estimated to be 1.2–1.3 which are close to the result of Zhang et al. [27].
al.
isooctane and different alcohols are estimated to be 1.2–1.3 which are close to the result of Zhang et
al. [27].
0.42
0.34
0.32
0.32
0.30
0.30
0.65
0
= 0.8
Tu= 363 K
= 0.8
Pu= 0.1 MPa
Tu= 363 K
0.36
0.34
0.0
0.1
0.2
0.3
0.4
Mass content of oxygen
0.1
0.2
0.3
0.4
Mass content of oxygen
0.65
0.60
0.42
0.5
= 1.0
0.0
0.0
0.1
0.2
0.3
0.4
Mass content of oxygen
0.1
0.2
0.3
0.4
Mass content of oxygen
0.5
0.5
(d)
0.50
0.45
(c)
(d)
0.45
0.40
0
-1
S0u / mSsu-1/ ms
-1
0
S0u / mSsu-1/ ms
0.50
(c)
= 1.0
0.48
0.45
0.5
0.60
0.55
0.55
0.50
= 1.2
0.50
0.45
= 1.2
0.45
0.40
0.51
0.48
0.45
0.42
Pu= 0.1 MPa
0.0
(b)
0.54
0.51
-1
0.38
0.36
(a)
0
S0u / mSsu-1/ ms
-1
0.40
0.38
(b)
0.57
0.54
S0u / mSsu-1/ ms
0.42
0.40
0.57
(a)
methanol+isooctane
ethanol+isooctane
methanol+isooctane
n-propanol+isooctane
ethanol+isooctane
n-butanol+isooctane
n-propanol+isooctane
n-pentanol+isooctane
n-butanol+isooctane
n-pentanol+isooctane
0.40
0.35
0.35
0.30
0.30
0.25
= 1.5
0.25
0.20
= 1.5
0.20
0.15
0.1
0.2
0.3
0.4
0.5
0.0
0.1
0.2
0.3
0.4
0.5
Mass content of oxygen
Mass content of oxygen
0.40
0.15
0.0
0.1
0.2
0.3
0.4
0.5
0.0
0.1
0.2
0.3
0.4
0.5
Mass
content
of oxygen
Mass content
of oxygen
Figure 10. Laminar
flame
speeds
of alcohol-isooctane blends vs. mass content
of oxygen
at 363 K, 0.1
0.0
Figure 10. Laminar flame speeds of alcohol-isooctane blends vs. mass content of oxygen at 363 K,
MPa,
andand
different
equivalence
ratios: (a) (a)
= 0.8;
= 1.0;
 =(c)
1.2;
and
 = 1.5.ϕ = 1.5.
0.1 MPa,
different
equivalence
ϕ =(b)
0.8;(b)
ϕ =(c)
1.0;
ϕ content
= 1.2;(d)
and
Figure
10. Laminar
flame
speeds ofratios:
alcohol-isooctane
blends
vs. mass
of (d)
oxygen at 363 K, 0.1
MPa,
0.25 and different equivalence ratios: (a)  = 0.8; (b)  = 1.0; (c)  = 1.2; and (d)  = 1.5.
0.10
0.15
0.05
0.10
0.00
0.05
-0.05
0.00
-0.10
-0.05
-0.15
-0.10
-0.20
-0.15
-0.20
n-propanol+isooctane
n-propanol+isooctane
= 0.8
= 1.0
= 1.2
0.8
=

=
1.0
= 1.5
= 1.2
= 1.5
Tu= 363 K
0.25
(a)
(a)
Lb / cm
Lb / cm
Lb / cm
Lb / cm
0.20
0.25
0.15
0.20
Tu= 363 K
0.15
0.20
Pu= 0.1 MPa
Tu= 363 K
0.10
0.15
Pu= 0.1 MPa
(b)
0.05
0.10
methanol
ethanol
methanol
n-propanol
-0.05
ethanol
n-butanol
0.00
Pu= 0.1 MPa
n-propanol
n-pentanol
-0.10
-0.05
n-butanol
isooctane
n-pentanol
-0.15
-0.10
isooctane
0.0
0.2
0.4
0.6
0.8
1.0
0.7 0.8 0.9 1.0 1.1 1.2 1.3
Volume fraction of n-propanol
-0.15
Equivalence ratio 
0.0
0.2
0.4
0.6
0.8
1.0
0.7 0.8 0.9 1.0 1.1 1.2 1.3
Volumelengths
fraction of
of n-propanol
Equivalence
ratio
11. Markstein
n-propanol-isooctane blends and pure fuels
at 363 K,
0.1
Pu= 0.1 MPa
Tu= 363 K
(b)
0.20
0.25
0.00
0.05
1.4
1.5
1.6
1.4
1.5
1.6
Figure
MPa: (a) Lb
of n-propanol-isooctane blends; and (b) Lb of pure isooctane and alcohol flames.
Figure
11. Markstein
lengths of
ofn-propanol-isooctane
n-propanol-isooctaneblends
blendsand
andpure
pure
fuels
363
MPa:
Figure 11.
Markstein lengths
fuels
at at
363
K, K,
0.10.1
MPa:
(a)(a)
Lb L
ofb
of
n-propanol-isooctane
blends;
and
(b)
L
b of pure isooctane and alcohol flames.
n-propanol-isooctane
and (b)instability
Lb of pure isooctane
andthe
alcohol
flames. images for the alcoholTo
better recognizeblends;
the flame
properties,
Schlieren
isooctane
blendsrecognize
at different
blending
(fv,acl) properties,
and the equivalence
ratio ofimages
1.5 are shown
Figure
To better
the
flame ratios
instability
the Schlieren
for theinalcohol12.
For pure
isooctane,
cracksblending
clearly appear
the flame front. With the increasing alcohol blending
isooctane
blends
at different
ratios (fon
v,acl) and the equivalence ratio of 1.5 are shown in Figure
12. For pure isooctane, cracks clearly appear on the flame front. With the increasing alcohol blending
Energies 2016, 9, 511
12 of 17
To better recognize the flame instability properties, the Schlieren images for the alcohol-isooctane
blends
at different
blending ratios (f v,acl ) and the equivalence ratio of 1.5 are shown in Figure
Energies 2016,
9, 511
12 of12.
16
For pure isooctane, cracks clearly appear on the flame front. With the increasing alcohol blending
ratio, the number of cracks on the flame front displays a decreasing tendency. This demonstrates that
the flame instability on the flame front is suppressed with the addition of alcohol. In particular, pure
methanol and ethanol flames even present smooth flame fronts. Moreover, slight differences can be
seen in that the methanol-isooctane blend with 80% methanol presents a smooth flame front, while
there are cracks on the flame
flame front
front of
of the
the ethanol-isooctane
ethanol-isooctane blend
blend with
with 80%
80% ethanol.
ethanol. Therefore, the
addition of methanol is the most effective in suppressing
suppressing the flame instability.
instability. This result is consistent
with the Markstein length behavior
behavior observed
observed in
in Figure
Figure 11.
11.
Figure 12.
12. Schlieren
and
Figure
Schlieren images
images of
of C1–C5
C1–C5 primary
primary alcohol-isooctane
alcohol-isooctane blends
blends at
at 363
363 K,
K, 0.1
0.1 MPa
MPa and
equivalence
ratio
of
1.5.
equivalence ratio of 1.5.
3.5. Laminar
3.5.
Laminar Flame
Flame Speed
Speed Correlation
Correlation
In fuel
fuel blending
blending research,
research,an
anequation
equationgiving
givingaccurate
accurateestimation
estimationonon
the
laminar
flame
speeds
In
the
laminar
flame
speeds
of
of
the
blends
is
indispensable.
As
mentioned
above,
the
laminar
flame
speeds
of
alcohol-isooctane
the blends is indispensable. As mentioned above, the laminar flame speeds of alcohol-isooctane blends
blends nonlinear
exhibit nonlinear
thefraction
volumeoffraction
alcohol.
An correlation
empirical correlation
exhibit
variationvariation
with the with
volume
alcohol.ofAn
empirical
describing
describing
the
dependence
of
laminar
flame
speed
on
the
volume
fraction
of
alcohol
the dependence of laminar flame speed on the volume fraction of alcohol is given as: is given as:
1
Su0,fva =
1  f v,alc 1 f v,alc
0
Su, f va “ 1´0 f  0 f
Su,Cv,alc
Su,C H v,alc
H
O
0 8 18 ` S0x y
Su,C
H
u,C Hy O
(8)
(8)
x
8 18
0
0
where fv,alc represents the blending ratio of alcohol in liquid volume, 𝑆u,C
and 𝑆u,C
indicate
8 H18
𝑥 H𝑦 O
0
0
where
f
represents
the
blending
ratio
of
alcohol
in
liquid
volume,
S
and
S
indicate
v,alc
the laminar flame speeds of pure isooctane and alcohol flames underu,C
the
conditions
u,Cx Hy O (initial
8 Hsame
18
the
laminar flame
of and
pureequivalence
isooctane and
alcohol
flames under
same conditions
temperature,
initialspeeds
pressure
ratio),
respectively.
Such a the
correlation
is derived(initial
based
temperature,
initial
pressure
and equivalence
ratio),
Such
a correlation
derived
based
on the formula
given
by Di Sarli
and Benedetto
[60]respectively.
with the mole
fraction
replacedisby
the volume
on
the formula
given by Diwith
Sarli Equation
and Benedetto
[60] been
with the
moleinfraction
the volume
fraction.
The calculations
(8) have
plotted
Figurereplaced
3. Goodby
agreement
is
fraction.
The
calculations
with
Equation
(8)
have
been
plotted
in
Figure
3.
Good
agreement
is observed
observed at different conditions for all alcohol-isooctane blends with deviations of less than
1.7 cm/s.
at different
conditions
all alcohol-isooctane
blends with
deviations
less than
1.7speed
cm/s.variation
Broustail
et al. [29]for
proposed
an empirical Equation
to describe
the of
laminar
flame
of the alcohol-isooctane blends with volume fraction of alcohol (fv,alc), which is expressed as:
0
Su0,fva  Su,C
(
8 H18
0
Su,C
xHyO
0
Su,C
8 H18
)
f v, alc
(9)
Energies 2016, 9, 511
13 of 17
Broustail et al. [29] proposed an empirical Equation to describe the laminar flame speed variation
of the alcohol-isooctane blends with volume fraction of alcohol (f v,alc ), which is expressed as:
0
0
Su,
f va “ Su,C8 H18 p
Energies 2016, 9, 511
Energies 2016, 9, 511
0
Su,C
x Hy O
0
Su,C
8 H18
q fv, alc
(9)
13 of 16
13 of 16
The
The calculations
calculations of
of Equations
Equations (8)
(8) and
and (9)
(9) were
were plotted
plotted in
in Figure
Figure 13
13 to
to allow
allow comparisons
comparisons with
with
The calculations of Equations (8) and (9) were plotted in Figure 13 to allow comparisons with
the
the experimental
experimental data
data at
at the
the equivalence
equivalence ratios
ratios of
of 1.0
1.0 and
and 1.5.
1.5. ItIt is
is found
found that
that the
the two
two equations
equations give
give
the experimental data at the equivalence ratios of 1.0 and 1.5. It is found that the two equations give
similar
estimations
and
satisfactory
agreement
with
the
experimental
data
for
the
blends
of
isooctane
similar estimations and satisfactory agreement with the experimental data for the blends of isooctane
similar estimations and satisfactory agreement with the experimental data for the blends of isooctane
and
alcohols. However,Equation
Equation (8)gives
gives moreaccurate
accurate
estimations
than
Equation
and C2–C5 primary alcohols.
estimations
than
Equation
(9)
and
C2–C5 primary alcohols. However,
However, Equation(8)
(8) givesmore
more accurate
estimations
than
Equation
(9)
does
methanol-isooctane
blends.
does
forfor
methanol-isooctane
blends.
(9) does for methanol-isooctane blends.
/ m
S0uS/0um
s-1s-1
0.60
0.60
0.55
0.55
0.50
0.50
(a)
(a)
methanol+isooctane
methanol+isooctane
ethanol+isooctane
ethanol+isooctane
n-propanol+isooctane
n-propanol+isooctane
n-butanol+isooctane
n-butanol+isooctane
n-pentanol+isooctane
n-pentanol+isooctane
= 1.0
= 1.0
Tu= 363 K
T = 363 K
Puu= 0.1 MPa
Pu= 0.1 MPa
-1
/m
S0uS/0um
s-1s
0.65
0.65
0.45
0.45
0.40
0.40
0.0
0.0
0.2
0.4
0.6
0.8
0.2 Volume
0.4fraction
0.6of alcohol
0.8
Volume fraction of alcohol
1.0
1.0
0.50
0.50
0.45
0.45
0.40
0.40
0.35
0.35
0.30
0.30
0.25
0.25
0.20
0.20
0.15
0.15
(b)
(b)
= 1.5
= 1.5
0.0
0.0
0.2
0.4
0.6
0.8
0.2Volume
0.4fraction
0.6of alcohol
0.8
Volume fraction of alcohol
1.0
1.0
Figure
Figure 13.
13. Experimental
Experimental measurements
measurementsand
andcalculations
calculationsat
atthe
the equivalence
equivalenceratios
ratiosof
of 1.0
1.0 and
and 1.5
1.5 for
for
Figure
13.
Experimental
measurements
and
calculations
at
the
equivalence
ratios
of
1.0
and
1.5
for
C1–C5
primary
alcohol-isooctane
blends:
(a)(a)
 =ϕ1.0;
andand
(b) 
= 1.5.
Symbols:
experimental
data; solid
C1–C5
primary
alcohol-isooctane
blends:
=
1.0;
(b)
ϕ
=
1.5.
Symbols:
experimental
data;
C1–C5 primary alcohol-isooctane blends: (a)  = 1.0; and (b)  = 1.5. Symbols: experimental data; solid
lines:
of Equation
(8); dash
of Equation
(9). (9).
solid calculations
lines:
calculations
of Equation
(8); lines:
dash calculations
lines:
calculations
of Equation
lines:
calculations
of Equation
(8); dash
lines:
calculations
of Equation
(9).
More data of alcohols-isooctane blends in literatures [27–29,51] at various initial conditions were
More
More data
data of
of alcohols-isooctane
alcohols-isooctane blends
blends in
in literatures
literatures [27–29,51]
[27–29,51] at
at various
various initial
initial conditions
conditions were
were
acquired to examine the feasibility of Equation (8). The measured laminar flame speeds [27–29,51] as
acquired
measured
laminar
flame
speeds
[27–29,51]
as
acquired to
toexamine
examinethe
thefeasibility
feasibilityofofEquation
Equation(8).
(8).The
The
measured
laminar
flame
speeds
[27–29,51]
well as the calculations of Equation (8) were plotted in Figure 14. The deviations between the
well
as as
thethe
calculations
of of
Equation
as well
calculations
Equation(8)(8)were
wereplotted
plottedininFigure
Figure 14.
14. The
The deviations
deviations between
between the
the
measured data and the calculations are mainly within ±2 cm/s, demonstrating good performance of
measured
mainly within
within ˘2
±2 cm/s,
measured data and the calculations are mainly
cm/s,demonstrating
demonstratinggood
good performance
performance of
of
the equation.
the
equation.
the equation.
0.7
0.7
-1 -1
Calculated
/m
Calculated
S0uS/0um
s s
0.6
0.6
0.5
0.5
0.4
0.4
0.3
0.3
0.2
0.2
0.2
0.2
0.3
0.3
0.4
0.5
0.4 S0 / m
0.5s-1
Measured
u
0
Measured Su / ms-1
0.6
0.6
0.7
0.7
Figure 14. The measured laminar flame speeds in literatures [27–29,51] vs. the calculated values with
Figure
The
measured
laminar
flame
Figure 14.
14.(8)
The
measured
laminar
flame speeds
speeds in
in literatures
literatures [27–29,51]
[27–29,51] vs.
vs. the
the calculated
calculated values
values with
with
Equation
at various
initial
conditions.
Equation
(8)
at
various
initial
conditions.
Equation (8) at various initial conditions.
4. Conclusions
4. Conclusions
Laminar flame speeds of the blends of isooctane and C1–C5 primary alcohols were determined
Laminar flame speeds of the blends of isooctane and C1–C5 primary alcohols were determined
by the spherical propagating flame method at 363 K, 0.1MPa, four equivalence ratios (0.8, 1.0, 1.2 and
by the spherical propagating flame method at 363 K, 0.1MPa, four equivalence ratios (0.8, 1.0, 1.2 and
1.5) and various blending ratios of alcohols (0%–100%). The effects of blending different alcohols into
1.5) and various blending ratios of alcohols (0%–100%). The effects of blending different alcohols into
isooctane on the laminar flame speed as well as the flame instability characteristics were
isooctane on the laminar flame speed as well as the flame instability characteristics were
comparatively illustrated. Finally, an empirical correlation was given based on the experimental data.
comparatively illustrated. Finally, an empirical correlation was given based on the experimental data.
The main conclusions are as follows:
The main conclusions are as follows:
Energies 2016, 9, 511
14 of 17
4. Conclusions
Laminar flame speeds of the blends of isooctane and C1–C5 primary alcohols were determined
by the spherical propagating flame method at 363 K, 0.1MPa, four equivalence ratios (0.8, 1.0, 1.2 and
1.5) and various blending ratios of alcohols (0%–100%). The effects of blending different alcohols into
isooctane on the laminar flame speed as well as the flame instability characteristics were comparatively
illustrated. Finally, an empirical correlation was given based on the experimental data. The main
conclusions are as follows:
(1) The laminar flame speeds of alcohol-isooctane blends increase monotonously with the addition of
alcohol into isooctane. At a fixed alcohol blending ratio, methanol-isooctane blends present the
highest values due to the chemical kinetic effect. Ethanol addition yields a comparable laminar
flame speed enhancement as the additions of n-propanol, n-butanol and n-pentanol do at 0.8 and
1.5, but stronger enhancement at 1.0 and 1.2.
(2) The laminar flame speeds of the alcohol-isooctane blends increase significantly with the increasing
oxygen content in the blends. At the fixed oxygen content and equivalence ratio of 0.8, the
alcohol-isooctane blends exhibit similar values. At higher equivalence ratios, the addition of
heavier alcohol tends to exhibit higher laminar flame speeds.
(3) Markstein length displays a decreasing tendency with the addition of alcohol into isooctane at the
equivalence ratios of 0.8, 1.0 and 1.2, however, it increases at 1.5. Therefore, the Lb –ϕ curve of the
isooctane flame crosses with those of the pure C1–C5 primary alcohol flames. The equivalence
ratios corresponding to the crosses are approximately 1.2–1.3.
(4) An empirical correlation was provided between the laminar flame speed and the volume fraction
of alcohol based on the experimental data.
Supplementary Materials: The following are available online at www.mdpi.com/1996-1073/9/7/511/s1.
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), the China Postdoctoral
Science Foundation (2014M560774), and State Key Laboratory of Engines, Tianjin University (K2016-02).
Author Contributions: The author contributions list is as follows: Study concepts were proposed by Qianqian Li
and Zuohua Huang. Experimental studies, data processing, and the manuscript preparation were done
by Qianqian Li. Data analysis and interpretation were done by Qianqian Li, Wu Jin and Zuohua Huang.
Manuscript editing was made by Wu Jin. Manuscript revision were done by Wu Jin and Zuohua Huang.
Conflicts of Interest: The authors declare no conflict of interest.
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