Comparative study on the explosion characteristics of pentanol isomer–air mixtures

Fuel 161 (2015) 78–86

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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

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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. Flasin´ska 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.)

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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.

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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.

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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

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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.

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(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). References [1] Gautam M, Martin DW, Carder D. Emissions characteristics of higher alcohol/gasoline blends. Proc Inst Mech Eng A – J Power 2000;214:165–82. [2] Yasar A. Effects of alcohol-gasoline blends on exhaust and noise emissions in small scaled generators. Metalurgija 2010;49:335–8. [3] You F, Li G, Gao X. Study on reformed ethanol engine; 2007. [4] Yucesu HS, Topgul T, Cinar C, Okur M. Effect of ethanol-gasoline blends on engine performance and exhaust emissions in different compression ratios. Appl Therm Eng 2006;26:2272–8. [5] Zhao H, Ge Y, Hao C, Han X, Fu M, Yu L, et al. Carbonyl compound emissions from passenger cars fueled with methanol/gasoline blends. Sci Total Environ 2010;408:3607–13. [6] Surisetty VR, Dalai AK, Kozinski J. Alcohols as alternative fuels: an overview. Appl Catal A-Gen 2011;404:1–11. [7] Campos-Fernandez J, Arnal JM, Gomez J, Lacalle N, Dorado MP. Performance tests of a diesel engine fueled with pentanol/diesel fuel blends. Fuel 2013;107:866–72. [8] Wei L, Cheung CS, Huang Z. Effect of n-pentanol addition on the combustion, performance and emission characteristics of a direct-injection diesel engine. Energy 2014;70:172–80. [9] Cann AF, Liao JC. Pentanol isomer synthesis in engineered microorganisms. Appl Microbiol Biotechnol 2010;85:893–9. [10] Cann AF, Liao JC. Production of 2-methyl-1-butanol in engineered Escherichia coli. Appl Microbiol Biotechnol 2008;81:89–98.

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