Fuel 161 (2015) 78–86
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Fuel
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Comparative study on the explosion characteristics of pentanol
isomer–air mixtures
Qianqian Li ⇑, Yu Cheng, Wu Jin, Zuohua Huang ⇑
State Key Laboratory of Multiphase Flow in Power Engineering, Xi’an Jiaotong University, Xi’an 710049, People’s Republic of China
h i g h l i g h t s
Explosion parameters of four pentanol isomer–air mixtures were measured.
Influence of initial conditions on explosion characteristics were discussed.
Pressure oscillation occurs at the rich mixture and high pressure.
a r t i c l e
i n f o
Article history:
Received 30 May 2015
Received in revised form 8 August 2015
Accepted 11 August 2015
Keywords:
Pentanol isomers
Explosion characteristic
Pressure oscillation
Combustion phase
a b s t r a c t
A comparative study was experimentally performed on the explosion characteristics of four pentanol
isomer–air mixtures (n-pentanol, 3-methyl-1-butanol, 2-methyl-1-butanol, 2-methyl-2-butanol), at
various initial temperatures and initial pressures. The explosion parameters of explosion pressure, maximum rate of pressure rise, combustion duration and combustion development period were measured.
The influence of initial conditions on the explosion characteristics were discussed. Results show that
the peak explosion pressure is linear function of the reciprocal of initial temperature, but the maximum
rate of pressure rise is insensitive to the temperature variation. With the initial pressure elevated from
0.1 to 0.25 MPa, the peak explosion pressure increases significantly, but the increase rate is decelerated
when the pressure is further increased. Among the four pentanol isomer–air mixtures, in the order of
n-pentanol, 2-methyl-1-butanol, 3-methyl-1-butanol and 2-methyl-2-butanol, the peak explosion pressure and maximum rate of pressure rise decrease while the combustion duration and flame development
period increase, reflecting the decreasing flame speed. Difference among the isomers tends to be
decreased for the peak explosion pressure while increased for the maximum rate of pressure rise with
the increase of initial pressure. Pressure oscillation occurs at the rich mixture and high pressure, resulting
in short combustion duration, but influencing the flame development period little.
Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction
Alcohols provide promising potential in improving pollutant
emissions and reducing dependency on traditional fossil fuels. Previous research suggest that blending alcohol into traditional fuels
benefit the complete combustion in engines and hence the reduction of HC, CO and soot emissions [1–6]. Low alcohols, such as
methanol and ethanol, can be mixed with gasoline as octane
improver for their high octane number. However, low alcohols
are challenging to be used on compression ignition (CI) engine,
since additional ignition assistant is necessary [3]. Besides, the
engine needs to be redesigned when fueled with low alcohols
⇑ Corresponding authors. Tel.: +86 29 82665075; fax: +86 29 82668789.
E-mail addresses: [email protected] (Q. Li), [email protected]
(Z. Huang).
http://dx.doi.org/10.1016/j.fuel.2015.08.027
0016-2361/Ó 2015 Elsevier Ltd. All rights reserved.
due to the problems of hygroscopicity and corrosivity. Recently,
considerable studies were conducted on the high alcohol of pentanols which are found to well satisfy the requirements of CI
engine. Specifically, pentanol fuel own high energy content, relatively high cetane number and good miscibility with traditional
fuel [7,8]. Engineering-scale production of pentanol are being
developed in more advanced and cheaper way, motivating related
research in all fields [9,10].
Pentanol is among the flammable and combustible liquid fuels,
and the fuel vapor probably burn or explode especially in the case
of a fire source. Thus, the safety issue claims high concern over the
fuel transportation, storage and usage, and a thorough investigation on explosion characteristics of pentanol is required at various
initial conditions. Fundamentally, the explosion behavior in closed
vessels is characterized by the key parameters of explosion
pressure and rate of pressure rise. These data of gaseous fuels as
79
Q. Li et al. / Fuel 161 (2015) 78–86
cated the isomers are prone to ignite in the order of n-pentanol,
2-methyl-1-butanol and 3-methyl-1-butanol. Li et al. [23] studied
the laminar combustion characteristics of n-, 2-, 3-pentanol–air
mixtures with spherical propagating flame in a cylinder vessel
and found n-pentanol has the highest flame speed. The combustion
behavior in a constant vessel can also be recorded by the combustion pressure with which the explosion parameters of explosion
pressure, maximum rate of pressure rise and combustion phase
can be determined to evaluate the explosion hazard. However, no
research have been published regarding to the effect of chemical
structure on the explosion characteristics of pentanol isomers.
In present study, explosion hazard of four pentanol isomer–air
(n-pentanol,
3-methyl-1-butanol,
2-methyl-1-butanol
and
2-methyl-2-butanol) mixtures were assessed in a constant volume
vessel. The explosion parameters of peak explosion pressure and
maximum rate of pressure rise were presented at the initial temperatures ranging from 393 to 473 K and initial pressures ranging
from 0.1 to 0.75 MPa. The explosion behavior difference among the
isomers were examined taking the effect of initial conditions into
account. Finally, the combustion phase parameters of the
isomer–air mixtures were determined to provide fundamental
reference for the practical engine timing control.
hydrogen, natural gas and acetylene, etc. [11–15] have been extensively measured, but very limited data were reported for liquid
fuels, especially for alcohols. Zhang et al. [16] conducted the explosion characteristics of methanol–air mixtures at different initial
temperatures, pressures and dilution ratios through determining
the variation of explosion pressure, normalized mass burning rate
and combustion phase parameters. Cammarota et al. [17] reported
the explosion behaviors of hydrogen–ethanol–air mixtures at
elevated temperatures and various hydrogen blending ratios in a
cylinder vessel. Chang et al. [18] determined the deflagration index
and maximum rate of pressure rise of various toluene/methanol
blends in a closed spherical vessel. Gao et al. [19] discussed the
influence of different igniters on the explosion characteristics of
1-Octadecanol dust. There were also researches conducted on the
other liquid fuels. Razus et al. [20] measured the explosion pressures of n-pentane-, n-hexane-, cyclohexane- and benzene–air
mixtures in three cylinder vessels, and analyzed the influence of
initial pressure, fuel concentration and heat loss on the explosion
pressures. Flasińska et al. [21] experimentally assessed the explosion risk of C6–C8 hydrocarbons in a 20 L spherical vessel by determining the explosion parameters of peak explosion pressure,
maximum rate of pressure rise, lower explosion limit and upper
explosion limit. However, explosion characteristics of pentanol–
air mixtures were not involved so far.
It is noted that pentanol have isomers possessing various chemical structures, and these isomers were suggested to exhibit different engine performances and combustion characteristics. Tang
et al. [22] studied the auto-ignition characteristics of three pentanol isomers at high temperature and normal pressure, and indi-
6
The experiments were performed with the experimental setup
adopted in previous studies [14,24] where detailed description
has been given. The setup consists pressure acquisition system,
heating system, and a constant volume vessel with the ignitor cen-
40
(a)
5
φ = 1.0
T0= 433 K
φ = 1.0
T0= 433 K
-1
P0= 0.10 MPa
3
NP
3M1B
2M1B
2M2B
2
1
0
(b)
30
(dP/dt) / MPa⋅s
4
P/P0
2. Experimental apparatus and data acquisition
P0= 0.10 MPa
20
NP
3M1B
2M1B
2M2B
10
0
0
10
20
30
40
50
60
70
0
80
7
6
(dP/dt) / MPa⋅s
P/P0
-1
φ = 1.0
T0= 433 K
P0= 0.50 MPa
4
NP
3M1B
2M1B
2M2B
3
2
1
0
20
30
40
50
180
(c)
5
10
60
time after ignition start / ms
time after ignition start / ms
150
φ = 1.0
T0= 433 K
120
P0= 0.50 MPa
(d)
NP
3M1B
2M1B
2M2B
90
60
30
0
0
10
20
30
40
50
60
time after ignition start / ms
70
80
-30
0
20
40
60
80
time after ignition start / ms
Fig. 1. Explosion pressure and rate of pressure rise versus time at elevated temperatures and pressures for four pentanol isomer–air mixtures. (Abbreviations of the four
isomers are NP for n-pentanol, 3M1B for 3-methyl-1-butanol, 2M1B for 2-methyl-1-butanol and 2M2B for 2-methyl-2-butanol.)
80
Q. Li et al. / Fuel 161 (2015) 78–86
2400
2300
Tad/ K
2200
2100
433 K, 0.10 MPa
2000
NP
3M1B
2M1B
2M2B
1900
1800
0.6
0.8
1.0
1.2
1.4
1.6
1.8
Equivalence ratio φ
Fig. 2. Adiabatic temperature at 433 K and 0.1 MPa for four pentanol isomer–air
mixtures.
trally located. The vessel is a stainless cylinder with the diameter of
180 mm and the length of 210 mm, equipped with gas inlet and
outlet, a pressure transducer, a pressure transmitter, thermocouple
and heating tapes. During explosions, the pressure in the combustion vessel was tracked by the pressure transducer (Kistler 7001)
and recorded by an oscilloscope together with a Charge Amplifier
(Kistler 5011). The pressure transducer works at a sample rate of
100 kHz. The air is supplied as the mixture of 79% N2 (>99.95%)
and 21% O2 (>99.95%). The purity of all pentanol isomers are over
99% except 2-methyl-1-butanol over 98%.
The experiments of four pentanol isomer–air mixtures were all
performed at the equivalence ratios of 0.6–1.8, the initial temperatures of 393, 433, 473 K and the initial pressures of 0.1, 0.25, 0.5,
0.75 MPa. For each condition, the vessel was evacuated down to
3 kPa before the test. In the case of changing fuel, the vessel needs
to be flushed with dry air repeatedly to avoid the influence of the
fuel residual of last experiment. After the mixture was admitted
into the vessel and achieved uniform, the ignition electrodes were
started. Meanwhile, the oscilloscope was initiated to record the
pressure history during the combustion process in the vessel. With
the recorded combustion pressure–time (P–t) curve, the important
explosion parameters of rate of pressure rise (dP/dt) and maximum
rate of pressure rise (dP/dt)max were determined. Additionally, the
combustion pressure normalized with initial pressure, namely the
explosion pressure (P/P0) as well as the peak value of explosion
pressure, Pmax/P0 were also obtained. The combustion phase
parameters of combustion duration (tc) and flame development
period (td) were calculated as well. The combustion duration
adopts the definition of the time necessary to reach the peak explosion pressure [15] and the flame development period is defined as
the time interval between ignition and 7% pressure rise [25].
3. Results and discussions
3.1. Pressure history
Fig. 1 plots the explosion pressure (P/P0) and rate of pressure rise
(dP/dt) versus time at 1.0, 433 K and two initial pressures for the four
7.2
7.2
(a)
T0= 433 K
6.3
P0= 0.10 MPa
5.4
Pmax/P0
Pmax/P0
6.3
4.5
NP
3M1B
2M1B
2M2B
3.6
2.7
(b)
0.6
0.8
1.0
1.2
5.4
NP
3M1B
2M1B
2M2B
4.5
3.6
1.4
1.6
2.7
1.8
0.6
0.8
1.6
1.8
(d)
6.3
5.4
T0= 433 K
Pmax/P0
Pmax/P0
1.4
7.2
(c)
6.3
P0= 0.50 MPa
4.5
NP
3M1B
2M1B
2M2B
3.6
2.7
1.2
Equivalence ratioφ
Equivalence ratioφ
7.2
1.0
T0= 433 K
P0= 0.25 MPa
0.6
0.8
1.0
1.2
1.4
1.6
Equivalence ratioφ
5.4
NP
3M1B
2M1B
2M2B
4.5
T0= 433 K
P0= 0.75 MPa
3.6
1.8
2.7
0.6
0.8
1.0
1.2
1.4
1.6
1.8
Equivalence ratioφ
Fig. 3. Comparison among peak explosion pressures of four pentanol isomer–air mixtures versus equivalence ratio at different initial pressures and 433 K.
81
Q. Li et al. / Fuel 161 (2015) 78–86
6.5
1.8
P /MPa
0.8
1.0
1.4
(a)
P0= 0.1 MPa
Pmax/P0
5.0
4.5
4.0
380
400
420
440
460
480
T0 / K
6.5
0.7
1.1
1.5
6.0
0.8
1.2
1.6
0.9
1.3
(b)
1.0
1.4
5.5
5.0
4.5
4.0
φ = 1.2
φ = 1.3
φ = 1.4
φ = 1.5
φ = 1.6
1.0
0.9
1.3
5.5
(a)
1.2
0.8
1.2
1.6
3M1B
1.6
1.4
0.7
1.1
1.5
6.0
Pmax/P0
pentanol isomer–air mixtures. Abbreviations are applied in the
figures as NP for n-pentanol, 3M1B for 3-methyl-1-butanol, 2M1B
for 2-methyl-1-butanol and 2M2B for 2-methyl-2-butanol. All the
explosion pressure curves present a similar behavior that the pressure almost keeps constant at the initial stage of the flame propagation and then increases sharply until reaching its peak. At the initial
pressure of 0.1 MPa, the peak explosion pressure (Pmax/P0) slightly
decreases in sequence of n-pentanol, 2-methyl-1-butanol,
3-methyl-1-butanol and 2-methyl-2-butanol. This decreasing order
remains for the maximum rate of pressure rise but with significant
difference among the isomer–air mixtures. To find out the fundamental reason, the adiabatic flame temperatures (Tad) of four
isomer–air mixtures at 433 K and 0.1 MPa were plotted in Fig. 2.
The adiabatic flame temperature was deduced based on thermal
equilibrium in combustion. At the same equivalence ratio,
n-pentanol has the highest Tad corresponding to the biggest
Pmax/P0, and 2-methyl-2-butanol has the lowest Tad and exhibit the
lowest Pmax/P0. 2-Methyl-1-butanol and 3-methyl-1-butanol have
close values of Tad, resulting in approximate Pmax/P0. In addition,
the time interval between ignition and peak explosion pressure is
prolonged in the order of n-pentanol, 2-methyl-1-butanol,
3-methyl-1-butanol and 2-methyl-2-butanol, demonstrating the
decreasing flame propagation speed. The combined effects of the
adiabatic temperature and flame speed thus result in the significant
difference of (dP/dt)max among the isomer–air mixtures.
3M1B
2.1
2.2
2.3
2.4
P0= 0.1 MPa
2.5
2.6
1000 / T0 / K
-1
NP
T0= 433 K
0.6
Fig. 5. Peak explosion pressure at different equivalence ratios and 0.1 MPa for
3-methyl-1-butanol–air mixtures.
P0= 0.25 MPa
0.4
0.2
0
20
40
60
80
100
120
time after ignition start / ms
Table 1
Coefficients of linear correlations between the peak explosion pressure and the
reciprocal value of initial temperature for four pentanol isomer–air mixtures at
P0 = 0.1 MPa.
4.0
(b)
3.5
3.0
P /MPa
2.5
φ = 0.8
φ = 1.0
φ = 1.2
φ = 1.4
2.0
1.5
NP
T0= 433 K
P0= 0.50 MPa
1.0
0.5
0.0
0
20
40
60
80
100
120
time after ignition start / ms
Fig. 4. Combustion pressure versus time at different conditions for n-pentanol–air
mixtures.
a
b
r 2n
/ = 0.8
NP
3M1B
2M1B
2M2B
2.23
2.107
1.767
2.055
1.124
1.133
1.272
1.087
0.993
0.999
0.983
0.993
/ = 1.0
NP
3M1B
2M1B
2M2B
1.183
1.003
1.370
1.425
1.807
1.862
0.721
1.616
0.997
0.994
0.996
0.979
/ = 1.2
NP
3M1B
2M1B
2M2B
1.512
1.843
1.945
2.156
1.791
1.585
1.446
1.268
0.990
0.993
0.998
0.995
/ = 1.4
NP
3M1B
2M1B
2M2B
1.4
1.185
1.739
1.697
1.755
0.777
1.627
1.568
0.999
0.983
0.999
0.986
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Q. Li et al. / Fuel 161 (2015) 78–86
6
0.7
1.1
5
0.9
1.3
3M1B
T0= 433 K
3
1
1
0.2
0.3
0.4
0.5
0.6
0.7
0.1
0.2
0.3
P0 / MPa
6
5
5
Pmax/P0
Pmax/P0
6
4
3
2
0.0
0.1
0.2
0.3
0.8
1.1
1.4
0.4
0.5
0.6
0.7
7
(c)
0.7
1.0
1.3
0.4
0.8
P0 / MPa
7
0.6
0.9
1.2
1.0
3M1B
0
0.0
0.8
0.8
1.4
T0= 433 K
3
2
0.1
0.6
1.2
4
2
0
0.0
(b)
5
Pmax/ MPa
Pmax/ MPa
4
6
(a)
NP
T0= 433 K
0.5
0.6
0.7
(d)
4
0.6
0.9
1.2
3
2
0.0
0.8
0.1
0.7
1.0
1.3
0.2
0.3
0.8
1.1
0.4
0.5
3M1B
T0= 433 K
0.6
0.7
0.8
P0 / MPa
P0 / MPa
Fig. 6. Peak combustion pressure and peak explosion pressure at different equivalence ratios and 433 K for two pentanol isomer–air mixtures.
200
200
(a)
(b)
-1
P0= 0.10 MPa
150
(dP/dt)max / MPa⋅s
(dP/dt)max / MPa⋅s
-1
T0= 433 K
NP
3M1B
2M1B
2M2B
100
50
0
0.6
0.8
1.0
1.2
1.4
1.6
150
T0= 433 K
P0= 0.50 MPa
100
NP
3M1B
2M1B
2M2B
50
0
1.8
0.6
0.8
400
the same data as shown in (b)
-1
1.2
1.4
1.6
1.8
(c)
350 but in larger scale of ordinate
(dP/dt)max / MPa⋅s
1.0
Equivalence ratio φ
Equivalence ratio φ
300
250
T0= 433 K
200
P0= 0.50 MPa
NP
3M1B
2M1B
2M2B
150
100
50
0
0.6
0.8
1.0
1.2
1.4
1.6
1.8
Equivalence ratio φ
Fig. 7. Maximum rate of pressure rise versus equivalence ratio at 433 K and two initial pressures for four pentanol isomer–air mixtures.
83
Q. Li et al. / Fuel 161 (2015) 78–86
Fig. 6 gives the peak combustion pressure (Pmax) and peak
explosion pressure (Pmax/P0) versus initial pressure at 433 K and
different equivalence ratios. It is seen Pmax linearly increases with
the increase of initial pressure. The slopes indicating the increase
rate highly depends on the concentration of fuel–air mixture, similar to the gaseous fuel–air mixtures studied previously, e.g. pro-
Table 2
Fit parameters of linear correlations of (dP/dt)max = f(P0) for pentanol isomer–air
mixtures at different equivalence ratios.
a
b
r 2n
NP
3M1B
2M1B
2M2B
11.11
10.921
10.861
9.672
129.74
120.922
129.086
105.694
0.985
0.982
0.975
0.977
/ = 1.0
NP
3M1B
2M1B
2M2B
9.773
9.206
8.32
10.678
277.31
242.667
263.859
227.16
0.998
0.991
0.998
0.995
/ = 1.2
NP
3M1B
2M1B
2M2B
8.602
10.247
12.623
4.032
322.482
253.794
244.404
256.208
0.998
0.98
0.983
0.998
65
0.7
1.1
1.5
0.8
1.2
1.6
0.9
1.3
1.7
1.0
1.4
1.8
(a)
(dP/dt)max / MPa⋅s
-1
52
P0= 0.10 MPa
3M1B
39
26
13
0
380
400
420
440
460
480
T0 / K
300
0.6
0.9
1.2
-1
250
(dP/dt)max / MPa⋅s
Fig. 3 gives the variation of peak explosion pressure with equivalence ratio for pentanol isomer–air mixtures at different initial pressures and 433 K. At normal pressure and 433 K as shown in Fig. 3a,
the peak explosion pressure decreases in the order of n-pentanol,
2-methyl-1-butanol, 3-methyl-1-butanol and 2-methyl-2-butanol
with 2-methyl-1-butanol and 3-methyl-1-butanol giving approximate values. The difference of peak explosion pressure among the
isomers is more significant for rich mixtures than for lean mixtures.
This is because Pmax/P0 greatly related to the adiabatic temperature.
As shown in Fig. 2, it is seen Tad differences among the isomers are
easier to be distinguished at rich mixture than at lean mixture.
Particularly, flame speed is quite low at the extremely rich mixture,
causing large heat loss to the vessel wall and greatly reducing
Pmax/P0. With the initial pressure elevated, the difference of Pmax/P0
among the isomers tends to be decreased. In addition, it is observed
Pmax/P0 presents abnormal increase for n-pentanol–air mixture at
1.4 and 0.25 MPa as well as for all isomer–air mixtures at mixtures
richer than 1.2 and 0.5 MPa. This phenomenon is arisen from the
pressure oscillation as shown in Fig. 4.
Fig. 4 plots the combustion pressure at different equivalence
ratios and two initial pressures. At the two given initial pressures,
strong pressure oscillation can be observed for specific equivalence
ratios. The oscillation starts before the explosion pressure reaches
the peak, and exhibits the biggest oscillation amplitude around the
peak explosion pressure. This phenomenon always occurs for rich
mixtures at high pressures in present study, and this behavior
was also reported at both lean and rich mixture sides for hydrogen
and ethylene [26,27]. As illustrated in Fig. 4, the oscillation magnitude increases with the elevated pressure. Specifically, the biggest
oscillation amplitude at the equivalence ratio of 1.4 increases from
0.2 to 0.5 MPa with the initial pressure elevated from 0.25 to
0.5 MPa. In Fig. 4a, the pressure histories were plotted for five different equivalence ratios at 0.25 MPa. At 1.2 and 1.3, the pressure–
time curves are smooth, taking the rule for most experimental conditions as shown in Fig. 1. When the equivalence ratio is increased
to be 1.4 and 1.5, significant pressure oscillation is presented and
the magnitude is even bigger at 1.5. However, the oscillations disappear at further richer mixture of 1.6 which is approaching the
flammability limit. Similar behavior was also observed for hydrogen [26]. Previous study indicated the explosion pressure reaches
the peak after the flame front approached the vessel wall, and
these oscillations are caused by acoustic interactions between
the combustion front and the chamber wall [28,29]. For the rich
mixture at high pressure, the flame front is wrinkled and easily
developed to be turbulent type in response to the flame front instability mechanism. The collision with the vessel wall further
enhances the intensity of the turbulence, leading to the sharp
increase of the local flame deflagration speed. The pressure oscillation curve was smoothed with Savitzky–Golay method using 21
points of window and second polynomial order, as introduced in
previous studies [14,26]. The smoothed curve was used to do the
further analysis.
Fig. 5 shows the peak explosion pressure as functions of initial
temperature and the reciprocal value of initial temperature under
the normal pressure for 3-methyl-1-butanol–air mixture. With the
increase of initial temperature, the peak explosion pressure
decreases monotonously due to the reducing burning charge and
heat release amount. Besides, a linear correlation is exhibited
between the explosion pressure and the reciprocal value of initial
temperature, which can be expressed as, Pmax =P 0 ¼ a þ b=T 0 . This
correlation is applicable for the else isomer–air mixtures with
the coefficients, a, b and the determination coefficient, r 2n , presented in Table 1 at normal pressure and different equivalence
ratios. Such a linear relationship was also reported for the explosion pressures of different gaseous–air mixtures, including
methane [30], LPG [31], propane [32], etc.
0.7
1.0
1.3
(b)
0.8
1.1
200
3M1B
150
T0= 433 K
100
50
0
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
P0 / MPa
Fig. 8. Maximum pressure rise rate of 3-methyl-1-butanol–air mixtures at different
initial conditions.
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Q. Li et al. / Fuel 161 (2015) 78–86
pane [30,32], methane [30], LPG [31]. However, a disagreement
was observed for dimethyl-ether–air mixtures [33] that the linear
correlations between the peak explosion pressure and initial pressure present almost the same slopes. The peak explosion pressure,
Pmax/P0, which is the dimensionless peak combustion pressure,
steeply increases with the initial pressure increasing from 0.1 to
0.25 MPa. However, the increment is decelerated with the initial
pressure further increased, and even shows negative trend at lean
mixtures of 0.6 and 0.7, as indicated in Fig. 6c and d.
Fig. 7 gives the maximum rate of pressure rise of pentanol isomer–air mixtures versus equivalence ratio at 1.0, 433 K and two different initial pressures. The maximum rate of pressure rise is largely
dependent on the vessel volume, and plays significant role in the
assessment of explosion hazard. At 0.1 MPa and 433 K as indicated
in Fig. 7a, (dP/dt)max decreases in the order of n-pentanol,
2-methyl-1-butanol, 3-methyl-1-butanol and 2-methyl-2-butanol,
and the latter two mixtures exhibit approximate values. The maximum (dP/dt)max is obtained around 1.1. Richer or leaner mixtures
exhibit lower values of (dP/dt)max. When the initial pressure is elevated to 0.5 MPa as shown in Fig. 7b, (dP/dt)max dramatically
increases and the differences among the isomers are magnified especially around 1.1. However, when the data at 0.5 MPa were plotted in
a larger scale of ordinate range as illustrated in Fig. 7c, an intense
fluctuation is presented for (dP/dt)max at rich mixtures. This phenomenon is explained by the pressure oscillation illustrated above,
which is fundamentally caused by the intensified flame instability
and the sharp increase of flame speed.
Fig. 8 gives the variation of maximum rate of pressure rise with
initial temperature and initial pressure for 3-methyl-1-butanol–air
mixture. With the increase of initial temperature, (dP/dt)max varies
little at fixed equivalence ratio. With the increase of initial pressure, (dP/dt)max linearly increases with the steepest slope obtained
at 1.1. This knowledge indicates the explosion hazard is insensitive
to the variation of temperature, but take big risk at high pressure
especially around the equivalence ratio of 1.1. Similar results have
been obtained for other flammable fuels including propane [34],
hydrogen [14], ethylene [12], etc. The maximum rate of pressure
rise can be correlated with the initial pressure in linear expression
of ðdP=dt Þmax ¼ a þ b P0 . This linear correlation holds for the four
pentanol isomer–air mixtures at different equivalence ratios. As
shown in Table 2, the intercept, a, the slope, b, and the determination coefficients, r 2n , of the linear correlations between the maximum rate of pressure rise and initial pressure are listed for
pentanol isomer–air mixtures at 433 K and three equivalence
ratios.
3.2. Combustion phase
The combustion phase characteristic is significantly related to
the combustion in the internal engine, affecting the heat release
and eventually the engine efficiency and emissions. The optimized
combustion phase largely depends on the fuel physical and chemical properties. Isomers have different chemical structures, thus
fundamentally determining the combustion phase parameters of
the isomers will be of great importance. Present study adopt the
combustion duration (tc) and flame development period (td) to
characterize the combustion phase. The two parameters were
determined according to the definitions in references [15,25].
350
350
(a)
P0= 0.10 MPa
NP
3M1B
2M1B
2M2B
200
150
150
100
50
50
0.8
1.0
1.2
1.4
1.6
NP
3M1B
2M1B
2M2B
200
100
0.6
P0= 0.50 MPa
250
tc / ms
tc / ms
250
0.6
1.8
(b)
T0= 433 K
300
T0= 433 K
300
0.8
1.0
1.2
1.4
1.6
140
140
(c)
120
P0= 0.10 MPa
NP
3M1B
2M1B
2M2B
80
60
20
20
1.2
NP
3M1B
2M1B
2M2B
60
40
1.0
P0= 0.50 MPa
80
40
0.8
T0= 433 K
100
td / ms
td / ms
(d)
120
T0= 433 K
100
0.6
1.8
Equivalence ratio φ
Equivalence ratio φ
1.4
Equivalence ratio φ
1.6
1.8
0.6
0.8
1.0
1.2
1.4
1.6
1.8
Equivalence ratio φ
Fig. 9. Combustion duration and flame development period of four pentanol isomer–air mixtures versus equivalence ratio at 433 K and two initial pressures.
85
Q. Li et al. / Fuel 161 (2015) 78–86
270
3M1B
240
420
(a)
(b)
0.6
1.2
350
210
tc / ms
150
1.0
1.6
1.2
1.8
280
tc / ms
0.8
1.4
180
P0= 0.10 MPa
120
60
3M1B
T0= 433 K
210
70
400
420
440
460
480
0.0
0.1
0.2
0.3
T0 / K
0.5
0.6
0.7
0.8
175
(c)
3M1B
100
0.6
1.2
150
90
0.8
1.4
80
1.0
1.6
1.2
1.8
125
P0= 0.10 MPa
td / ms
td / ms
0.4
P0 / MPa
110
70
1.0
1.6
140
90
30
380
0.8
1.4
60
100
0.8
1.4
(d)
1.0
1.6
3M1B
T0= 433 K
75
50
40
50
30
25
20
380
400
420
440
460
480
0
0.0
0.1
T0 / K
0.2
0.3
0.4
0.5
0.6
0.7
0.8
P0 / MPa
Fig. 10. Combustion duration and flame development period at different initial conditions for 3-methyl-1-butanol–air mixtures.
Fig. 9 gives the combustion duration and flame development
period of the isomer–air mixtures versus equivalence ratio at
433 K and different pressures. At the normal pressure, tc and td of
different mixtures acquire the minimum value around 1.1, reflecting the flames propagate the fastest around this equivalence ratio.
At a high pressure of 0.5 MPa, the combustion duration of all isomer–air mixtures are decreased when the equivalence ratio is
increased from 0.6 to 1.1, and then vary little with the equivalence
ratio further increased. However, the flame development period
acquires the minimum value around 1.1 with longer td observed
at richer mixtures, which is different from the result of the combustion duration. At the high pressure and rich mixture, the flame
front is easily developed to be turbulent type as a response to the
enhanced flame instability, resulting in the acceleration of flame
propagation and short time necessary to reach the peak explosion
pressure. As td and tc is respectively defined as the time necessary
to reach 7% pressure rise and peak explosion pressure, it is inferred
that, by the end of the flame development period, the flame front
has not been developed to be full turbulence and the acceleration
of flame propagation can be neglected. At all initial pressures,
n-pentanol–air mixture has the shortest tc and td and 2-methyl2-butanol–air mixture has the longest tc and td, revealing the flame
speed of n-pentanol–air and 2-methyl-2-butanol–air mixture is the
fastest and the slowest, respectively. Significant difference among
the isomers are observed for both tc and td at the normal pressure
and the extremely rich mixture. This is because the heat loss is
large but different for the isomers near flammable limit, further
increasing the tc and td differences of the isomers.
Fig. 10 gives the combustion duration (tc) and flame development period (td) of 3-methyl-1-butanol–air mixture at different
initial conditions. It is seen the combustion duration and flame
development period exhibit similar behavior that they almost linearly increase with the decrease of initial temperature and the
increase of initial pressure. The slopes of these linear relations
are always the biggest at flammability limits, such as equivalence
ratios of 0.6 and 1.8. This phenomenon holds for all equivalence
ratios as well as the else three isomer–air mixtures.
4. Conclusions
The explosion characteristics of four pentanol isomer–air mixtures were comparatively studied at elevated initial temperatures
and initial pressures in a constant vessel, covering wide equivalence
ratio range of 0.6–1.8. The differences among the isomer–air
mixtures as well as the effect of initial conditions on the explosion
characteristics were analyzed. The main conclusions are summarized as follows.
(1) For one certain pentanol fuel, with the increase of initial
temperature, the peak explosion pressure, combustion duration and flame development period all linearly decrease,
while the maximum rate of pressure rise varies little. The
peak explosion pressure sharply increases with the initial
pressure increasing from 0.1 to 0.25 MPa, but the increase
rate is decreased when the initial pressure is further
increased. The maximum rate of pressure rise, combustion
duration and flame development period linearly increase
with the increase of initial pressure, reflecting the decreasing flame propagation speed.
86
Q. Li et al. / Fuel 161 (2015) 78–86
(2) Among the four pentanol isomer–air mixtures, the peak
explosion pressure and maximum rate of pressure rise
decrease in the order of n-pentanol, 2-methyl-1-butanol, 3methyl-1-butanol and 2-methyl-2-butanol at 0.1 MPa. With
the increase of initial pressure, the differences among the
isomer–air mixtures are decreased for the peak explosion
pressure but increased for the maximum rate of pressure
rise. Besides, n-pentanol and 2-methyl-2-butanol–air mixture respectively presents the shortest and the longest combustion duration and flame development period, indicating
the fastest and slowest flame speed.
(3) Pressure oscillation occurs for the four isomer–air mixtures
at 0.5 MPa and equivalence ratios 1.3, 1.4, as well as for npentanol–air mixtures at 0.25 MPa and 1.4. This is caused
by the acoustic interactions between the vessel wall and
the turbulent combustion front. The pressure oscillation significantly increases the explosion pressure and maximum
rate of pressure rise, and shortens the combustion duration,
but influences the flame development period little.
Acknowledgements
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).
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