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Effect of ignition energy on the initial propagation of ethanol/air laminar premixed flames: An experimental study Mengni Zhou, Gesheng Li, Zunhua Zhang, Junjie Liang, and Linyuan Tian Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b00965 • Publication Date (Web): 24 Jul 2017 Downloaded from http://pubs.acs.org on July 28, 2017

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Effect of ignition energy on the initial propagation of ethanol/air laminar premixed flames: An experimental study Mengni Zhoua,b, Gesheng Lia,b, Zunhua Zhanga,b,*, Junjie Liangb, Linyuan Tianb a

Key Laboratory of High Performance Ship Technology (Wuhan University of Technology), Ministry

of Education, Wuhan, Hubei 430063, PR China b

School of Energy and Power Engineering, Wuhan University of Technology, Wuhan, Hubei 430063,

PR China Keywords: Ethanol; Premixed laminar flame; Initial propagation; Minimum reliable ignition energy; Reference flame radius

Abstract: The ignition energy has a significant effect on the initial unsteady evolution of flame kernel, which will affect the performance and emissions of a spark-ignition engine and the uncertainty in determining laminar burning velocity and Markstein length due to the inappropriate selection of the lower limit of flame radius. In the present study, the outwardly propagating spherical flame was employed to experimentally investigate the initial propagation characteristics of ethanol/air premixed flames at different ignition energies. The emphases were to provide a new insight into the effect of ignition energy on the initial propagation of the premixed flame and to develop a new reference flame radius (the lower limit of flame radius) for deriving laminar burning velocity and Markstein length of ethanol/air mixtures. Results show that the initial flame speed of ethanol/air mixtures increases with the increase of the ignition energy, and the spark ignition has only a marginal effect on the initial flame speed when the ignition energy increases to a critical value. 1

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Furthermore, there exists an exponential relationship between the minimum stretched flame propagation speed and the ignition energy under the experimental conditions for the ethanol/air mixtures whose Lewis number is larger than unit. According to the exponential relationship, a new parameter is defined as minimum reliable ignition energy (MRIE), and the empirical formula for calculating MRIE is developed. The corresponding maximum radius of flame affected by ignition energy at MRIE is defined as reference flame radius, which is recommended to choose the lower limit of the flame radius to reduce the uncertainty in determining laminar burning velocity induced by the spark-ignition energy.

1. INTRODUCTION Bioethanol is a renewable alternative fuel, which can relief the depletion of fossil fuels and reduce the emission of greenhouse gas.1-4 When ethanol is applied to spark-ignition (SI) engines, it is of great importance for the spark ignition system to reliably ignite the combustible mixture and generate steady propagating flame. For an SI engine, the flame inside the cylinder propagates outwardly from the location of the spark generated by the plug, and the ignition energy provided by the ignition system has a significant effect on the initial propagation of the flame, which will result in the variation of the performance and emissions of the engine. The related studies5-7 showed that when the ignition energy is not large enough, the SI engine will not operate steadily, and the cycle-to-cycle variation increases, leading to the decrease in the power performance and fuel economy of the engine, and the increase in hydrocarbon emissions from the engine. If the ignition energy is elevated, the power performance and fuel economy can be improved. However, when the ignition energy increases to a certain value, the performance and emissions of the engine affected by the ignition energy will not be obviously improved any more. Thus, this is the original motivation to investigate the effect of ignition energy on the initial propagation of the ethanol/air premixed flame. On the other hand, it is well known that the laminar burning velocity is the fundament to 2


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calculate the turbulent flame speed and further investigate the turbulent combustion8, and is one of the most important parameters to validate the chemical kinetic mechanisms9-12. The previous researches show that the effect of the ignition energy on the initial flame propagation also has an influence on the accurate measurement of the laminar burning velocity from outwardly propagating spherical flames13-16. According to the computational study of Chen et al16, the boundary of the action of ignition energy on the flame initial propagation strongly depends on the initial conditions such as the equivalence ratio, pressure, temperature, etc., and if this boundary (i.e., the lower limit of flame radius) is chosen to be a fixed value, errors will be caused in the measurement of the laminar burning velocities. Unfortunately, many open literatures17-22 report the laminar burning velocities of ethanol/air mixtures at various initial conditions using outwardly propagating spherical flame, but the range of flame radius affected by ignition energy varying with the initial conditions were not taken into account in those researches. As such, there still exists a strong motivation to study the effect of ignition energy on the initial propagation process of ethanol/air laminar premixed flames for minimizing the uncertainty introduced by ignition energy in the determination of the laminar burning velocity using outwardly propagating spherical flame. There are several literatures which reported on the effect of the ignition energy on the propagation of flames15,16,23-28. Ju’s research group fundamentally investigated the critical radius for sustained flame propagation, the impacts of parameters such as pressure, heat radiation loss and Lewis numbers on the critical flame radius and the correlation between the minimum ignition energy and the critical flame radius through theoretical23, experimental16,27,28, and numerical24,26 approaches. However, the above literatures did not give the influence range of ignition energy on flame propagation and the method how to select the lower limit of flame radius to minimize the uncertainty induced by spark-ignition energy in determining the laminar burning velocity and Markstein length. Therefore, in the present study, the outwardly propagating spherical flame were employed to 3


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analyze the effect of the ignition energy on the initial propagation of ethanol/air laminar premixed flames at various equivalence ratios with the initial pressure and temperature of 0.1 MPa and 383 K, respectively. And the objective of this research is to study the reliable energy to ignite ethanol/air mixtures and determine the range of the effect of ignition energy on the flame initial propagation. On this basis, the reliable lower limit of flame radius used to derive the laminar burning velocity of ethanol/air mixtures will be further investigated and developed. 2. EXPERIMENTAL METHOD 2.1. Experimental apparatus. In the present study, the experimental setup of outwardly propagating spherical flame was employed to perform the related experiments of ethanol/air flames. Details about this experimental setup and the error analyses can be found in refs 29 and 30. The schematic of the experimental setup is depicted in Figure 1. The experimental setup mainly consists of a stainless steel cylindrical constant-volume combustion chamber, a heating system, an ignition system, a data acquisition system and a schlieren optical system with a high-speed digital camera. The inner diameter of the chamber is 174 mm, and the volume of the chamber is 5.86 ± 0.005 L. Two quartz optical windows of 80mm diameter are mounted in the both sides of the chamber to provide optical access to observe the spherical flames. The outside chamber surfaces are covered with heating tapes and thermal insulation materials. Two electrodes of 1.5 mm diameter are fitted in the center of the chamber with 1.5 mm gap, and a cone-shape is rubbed on the top of the electrodes. The combustible mixtures are ignited via the two electrodes by electrical sparks in the combustion chamber. The schlieren and high-speed digital camera are utilized to record the images of the process of mixtures combustion. The frame rate is between 10000 and 40000 frames per second depending on the flame propagation speed.

4


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Figure 1. Schematic of the experimental setup. Prior to conducting the experiments, the reactants were prepared. The mole numbers of ethanol and air were computed according to the initial experimental conditions. Then, the respective partial pressures of ethanol and air were derived by the ideal gas state equation. The ethanol with purity of 99.8% was used, and the air was considered to be a mixture of 21% oxygen and 79% nitrogen in volume. Before the start of experiments, the combustion chamber was heated until the inside temperature was stabilized at the desired values by using the heating system. The initial temperature and pressure in the present study were fixed at 383 K and 0.1 MPa, respectively. The thermocouple used in the present study was calibrated by ice-water mixture, and its uncertainty is 1 K. When the initial temperature is typically 383 K ± 1 K, the relative error of the temperature is 0.5%. The initial mixture pressure is determined by a pressure transducer with an accuracy of 100Pa. Therefore the relative error of initial pressure caused by the pressure transducer is estimated to be 0.1%. After the temperature was stabilized at desired value, the combustion chamber was vacuumed. Then, the calculated volume of ethanol was injected into the chamber, and started to vaporize with the increasing pressure inside the chamber. When the reading of pressure transducer did not change, the air was filled into the chamber until the pressure of chamber was stable at 0.1 MPa. In our previous study, 29 the constituents of the mixtures in the combustion chamber were checked and 5


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verified by the gas chromatographer after completing the preparation of the reactants, and the relative error was estimated to be within 1% (air/fuel mole ratio). To ensure the homogeneity and motionless of the reactants, the mixtures were ignited after the preparation for 15 minutes. The high-speed digital camera was triggered by a signal from computer, and ignition was initiated 10 ms after the camera working to capture the entire flame propagation images. In the experiments, an inductive ignition system with a 12 V direct current (DC) battery as the power source was used to ignite the combustible mixture inside the chamber, and the ignition energy discharged by the ignition system was varied through changing the resistance of primary circuit and charging time. To reduce the energy loss, MOS transistors serving as the switches were employed to control the charge and discharge processes of the ignition system. The ignition energy is larger than the minimum ignition energy of the mixture in this present study, which aims to ensure that the mixture can be ignited successfully. During the discharge process, the voltage V and current I across the gap between the two electrodes were respectively measured by a voltage probe (Tektronix P6015A) and a current probe (Tektronix TCP0030A). The DC attenuation of the voltage probe is estimated to be 3%. The sensitivity of the current probe is as low as 1 mA and the typically DC gain error is less than 3%. Then the waves were recorded by a digital oscilloscope (Tektronix MDO3014). The current trigger style was chosen as the trigger source of the oscilloscope. Hence the duration of discharge t1-t2 was obtained from the recorded current waveform. According to the temporal variation of the voltage and current, ignition energy, Eign, can be calculated through the numerical integration by t2

t2

t1

t1

Eign = ∫ V (t ) × I (t ) dt − ∫ R × I 2 (t ) dt

(1)

where R represents the resistance of the electrodes. This measurement of ignition energy can give a repeatable and accurate data. 31,32 The energy loss of resistance dissipation has been considered in the calculation. 6


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It should be pointed out that the slight variations in the spark discharge will cause the small difference of ignition energy even in the same resistor and charging time. Therefore, the ignition energy is the average value of the same energy level under the same initial conditions. Considering the uncertainty from the oscilloscope recordings, the measurement errors of the probes, and the numerical integration error, the overall uncertainty in measuring the ignition energy can be estimated to be less than 5% by using the uncertainty propagation formula33. In the present study, each experiment was repeated at least three times at the same experimental energy level, and a good repeatability was observed. 2.2. Data Processing. A computer program developed in MATLAB language in our laboratory34 was used to process the schlieren flame pictures obtained through the above experimental setup to derive the parameters such as stretched flame speed and laminar burning velocity, which can simplify the processing and reduce the human bias in extracting the flame radius. In the computer program, the Canny edge detection method35 was applied to measure the flame radius, Rf. The stretched flame speed is derived from the correlation between the flame radius and the time through Sb = dRf/dt. The relative rate of the change of the flame surface area is defined as the stretch rate, K = 2Sb/Rf. The unstretched flame speed S0b and Markstein length Lb can be determined from the stretched values by using the extrapolation model. Although several extrapolation models36-41 have been developed, the present study is concentrated on the effect of ignition energy on the initial propagation of laminar premixed flames rather than the accuracy of those extrapolation models. Therefore, only three representative extrapolation models are employed in the present study, see Table 1. The laminar burning velocity, S0u , is finally obtained from the unstretched flame speed via S0u = ρbS0b /ρu. The ρb and ρu are the densities of the burned and unburned gases at the experimental conditions, respectively. Thermal radiation has not been taken into consideration when evaluating ρb42. The effect of cylindrical confinement of the combustion vessel can be neglected when the flame radius is less than

7


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30 percent of the wall radius43, and for the present apparatus the valid upper limit range of flame radius for flame speed measurement needs to be less than 26 mm. Thus, in this experiment measurement, the upper flame radii used for the extrapolation to derive the laminar burning velocity are less than 25 mm. Table 1. Extrapolation models. Model

Equations

LS36

Sb = Sb0 − Lb K

NQ37

 S b   Sb  2 Lb K  0  ln  0  = − 0 Sb  S b   Sb 

LC38

Sb = S b0 − 2 Lb / Rf

2

Notes Linear model based on stretch

2

Quasi-steady nonlinear model

Linear model based on curvature

3. RESULTS AND DISCUSSION 3.1 Effects of ignition energy on the initial propagation process of the flames. Figure 2 shows the schlieren images of spark discharge in the air under ignition energy of 18 mJ at atmospheric temperature and pressure. It is seen that the spark expands outwardly with the passage of time, and simultaneously interacts with surrounding media. This is due to the fact that plasma kernel with thermal gradient is generated during the spark discharge period, and meanwhile the energy is transferred and radiated to the surrounding media. As a result, the outwardly expanding blast wave and inhomogeneous temperature field are formed which result in the interaction with the surrounding media. Therefore, the ignition energy introduced by spark discharge will have a significant effect on the initial propagation process of flames.

8


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Figure 2. Schlieren images of spark discharge in the air at atmospheric temperature and pressure under the ignition energy of 18 mJ with frame rate of 100000 fps. In addition, as found by Eisazadeh-Far et al.44, the kernel radii for pure air and methane/air mixtures are almost identical during early stage of flame initiation, indicating that the early schlieren images cannot be treated as the real expanding flame images. Therefore, in the very early stage the speed extracted from schlieren images is not true flame speed but a propagation speed of the temperature or plasma kernel front. Moreover, in order to clearly demonstrate the effect of ignition energy on the initial propagation of ethanol/air laminar premixed flames, the data with relatively higher stretch has been removed and only the figures of the stretched flame propagation speed as a function of stretch rate are presented in the following sections. Stretched flame propagation speed as a function of stretch rate of ethanol/air premixed flames at different ignition energies and equivalence ratios are presented in Figure 3. It is appeared from Figures 3a-f that with the evolution of the flame (manifested as the continually decreasing stretch rates in the Figure 3), the stretched flame propagation speed decreases first and then increases, and a local minimum stretched flame propagation speed Sb,min can be found with the equivalence ratios ranging from 0.8 to 1.3. However, a dramatically different story can be found for the flame propagation at the equivalence ratio of 1.5 in the Figure 3g, where the stretched flame propagation

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speed monotonically reduces with the decreasing stretch rate subsequent to the establishment of flame kernel. This phenomenon is primarily because that the abilities of thermal and mass diffusion of the mixtures are different at the different equivalence ratios, the relationship between the two abilities can be characterized by the effective Lewis number, Leeff, and its calculation formula can be referred to the refs 45 and 46.

3.4

(a)

51 mJ 31 mJ 20 mJ 10 mJ 0.5 mJ

2.7 2.4

Stretched Flame Propagation Speed, Sb (m/s)


3.0

2.1 1.8 1.5 1.2 0.9

φ = 0.8

76 mJ 62 mJ 43 mJ 34 mJ 24 mJ 10 mJ 5 mJ

(b) 3.2 3.0 2.8 2.6 2.4 2.2 2.0 1.8

φ = 0.9

1.6

0.6 0

150

300

450

600

750

900

1050

0

250

500

4.2

4.2

3.9


(c) 44 mJ 32 mJ 22 mJ 10 mJ 0.5 mJ

3.6 3.3 3.0 2.7 2.4 2.1

φ = 1.0

1.8

750

1000

1250

1500

1750

Stretch Rate, K (1/s)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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(d) 51 mJ 31 mJ 20 mJ 8 mJ 1 mJ

3.9

3.6

3.3

3.0

2.7

φ = 1.1 2.4

1.5 0

200

400

600

800

1000

1200

1400

1600

1800

0

250

500


750

1000

1250

1500

1750

2000


10


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(e)

54 mJ 44 mJ 34 mJ 19 mJ 7 mJ

4.0 3.8



4.2

3.6 3.4 3.2 3.0 2.8

φ = 1.2

3.6

(f)

56 mJ 44 mJ 33 mJ 20 mJ 7 mJ

3.4 3.2 3.0 2.8 2.6

φ = 1.3 2.4

2.6 0

300

600

900

1200

1500

1800

2100

0

250

500


Stretched Flame Propagation Speed, S b (m/s)

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Energy & Fuels

2.6

750

1000

1250

1500

1750


(g) 42 mJ 30 mJ 7 mJ 1.7 mJ

2.5 2.4 2.3 2.2 2.1 2.0 1.9 1.8 φ = 1.5

1.7 0

150

300

450

600

750

900

1050


Figure 3. Effects of ignition energy on the initial propagation characteristics of ethanol/air laminar premixed flame at different equivalence ratios, P = 1 MPa and Tu = 383 K. The effective Lewis number of ethanol/air mixtures at different equivalence ratios under the experimental conditions can be seen in Figure 4. The effective Lewis number decreases with the increasing equivalence ratio. And the effective Lewis numbers are larger than unit at the equivalence ratios from 0.8 to 1.3, indicating that the heat loss due to thermal diffusion is larger than the heat gain from the mass diffusion in the spherical flame front. As a result, after ignition, the heat generated by chemical reactions inside the ignition area is less than the energy dissipated outside the flame by conduction and radiation, and therefore the stretched flame propagation speed initially drops. While the flame surface area enlarges with the development of the flame, the combustible mixture which is 11


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involved into the flame front will increase, and consequently the heats generated by chemical reactions also get a rise. Hence the stretched flame propagation speeds eventually hold the increasing trend after passing through the local minimum stretched flame propagation speed. When the equivalence ratio increases to 1.5, its effective Lewis number is less than unit, indicating that the reverse will be hold. Therefore, the stretch strengthens the flame, the stretched flame propagation speed declines with the decreasing stretch rate. Consequently, the minimum stretched flame propagation speed does not exist. 1.8

Effective Lewis Number, Leeff

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1.6

1.4

1.2

1.0

0.8 0.6

0.8

1.0

1.2

1.4

1.6

Equivalence Ratio φ

Figure 4. Effective Lewis number of ethanol/air mixtures at different equivalence ratios P = 1 MPa and Tu = 383 K. As illustrated in the Figures 3a-g, it can be also seen that the stretched flame propagation speed during the initial period strongly depends on the ignition energy, while the flame speeds gradually coincide well with the development of flame. It indicates that the influence scope of the ignition energy on the flame propagation speed is mainly limited in the initial propagation of flame. Taking the Figure 3b as an example, when the ignition energy increases from 5 mJ to 10 mJ, or from 10 mJ to 24 mJ, the initial stretched flame propagation speed increases obviously. While comparing the stretched flame propagation speed at the ignition energies of 34 mJ, 43 mJ, 62 mJ, and 76 mJ, it can

12


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be found that the effect of the increased ignition energy on the initial stretched flame propagation speed becomes weaker and weaker. The results indicate that the initial stretched flame propagation speed increases with the increase of the ignition energy, but the increase rate decreases with the increasing ignition energy. When the ignition energy is above a certain value, the effect on the initial stretched flame propagation speed will not be obviously enhanced any more. 3.2 Minimum reliable ignition energy (MRIE). The above experimental results show that the mixtures whose effective Lewis number is larger than unit, there exists the minimum stretched flame propagation speed, Sb,min, which is sensitive to the ignition energy. Therefore, in this section, the Sb,min is selected to quantitatively discuss the effect degree of ignition energy on the initial stretched flame propagation speed. 1.6

Minimum Stretched Flame Propagation Speed, Sb,min (m/s)

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2.2 2.1

1.4

2.0 1.2

1.9

φ = 0.9

φ = 0.8 1.0

1.8 0 10 20 30 40 50 60 70 80 90

0

10 20 30 40 50 60 70 80

2.6

2.85

2.4

2.80 2.75

2.2

φ = 1.0

φ = 1.1

2.70

2.0 0

10

20

30

40

50

60

70

0

10

20

30

40

50 3.0

2.95 2.9 2.90

2.8 2.7

2.85

φ = 1.2

φ = 1.3

2.6

2.80 10

20

30

40

50

60 0

10

20

30

40

50

60

Ignition Energy, Eign (mJ)

Figure 5. Variation of minimum stretched flame propagation speed with various ignition energies for ethanol/air mixtures at different equivalence ratios, P = 1 MPa and Tu = 383 K. Variation of minimum stretched flame propagation speed of ethanol/ air mixtures with various ignition energies at different equivalence ratios are plotted in Figure 5. It should be pointed out that 13


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the Sb,min is the average value of at least three experimental results at the same initial conditions. It is observed from the Figure 5 that the Sb,min increases as ignition energy rises, whereas the increase rate reduces. Specifically, there exists an exponential relationship between the Sb,min and ignition energy at the experimental conditions, as the red solid lines shown in the Figure 5. The exponential curves are used to fit the experimental data via the formula S b, min = α − β × γ

Eign

（2）

(MIE < Eign < 90 mJ)

where Sb,min represents the minimum stretched flame propagation speed in m/s. Eign indicates the ignition energy in mJ. Because the ignition energy provided by the ignition system for a spark-ignition engine mainly concentrates on the range of 30-80 mJ47, the experimental ignition energy is above the minimum ignition energy of ethanol/air mixture (MIE) but less than 90 mJ in this study. α stands for a constant which is a mathematical limit value of Sb,min and has no practical meaning. β also represents a constant. γ denotes an ignition factor and its value is less than unity. The coefficients of the exponential fitting formula at various equivalence ratios are presented in Table 2. Table 2. Coefficients of the exponential-fitting formula. ϕ

α

β

γ

0.8

1.54478

0.45955

0.93011

0.9

2.15228

0.32147

0.9278

1.0

2.58655

0.46833

0.89812

1.1

2.85494

0.17374

0.84611

1.2

2.96032

0.22184

0.93948

1.3

2.96580

0.40217

0.94628

It can be seen in Table 3 that when the provided ignition energy reaches a special value, the corresponding value for Sb,min reaches the 99% of the theoretical limit value of Sb,min (i.e., the α in eq 2 ). After that, if the ignition energy increases by 1000 mJ, the absolute value of the increment of Sb,min is no more than 0.03 m/s. In other words, the average increment of Sb,min is less than 10-4 (m/s)

14


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per 1 mJ increment of ignition energy. It indicates that during that period, the effect of the increased ignition energy on the Sb,min is marginal. In addition, the ignition energy of applicable scope of the equation 2 is between MIE-90 mJ, so the maximum value of eq 2 is the corresponding Sb,min at the ignition energy of 90 mJ, Sb,min,90. It can be found in Table 3 that the relative error between Sb,min and α is about a magnitude order of 10-4. Therefore, the corresponding ignition energy value at which Sb,min is equal to 99% of Sb,min,90 can be defined as the minimum reliable ignition energy (MRIE), above which the influence of the increased ignition energy on the initial propagation speed of ethanol/air laminar premixed flame becomes extremely weak. It suggests that the MRIE can not only effectively improve the initial flame propagation speed but also avoid wasting ignition energy. Then the MRIE is essentially different from the MIE which is just the minimum energy required for successful ignition of combustible mixture. According to the definition of the MRIE and eq 2, an empirical formula can be obtained to calculate the MRIE, Er,ign = ln((α − 0.99 S b,min,90 ) / β ) / ln(γ ) .

Table 3. Effect of large ignition energy on the minimum stretched flame propagation speed. （Sb,min,E0.99α +1000 -99%α）

(α − Sb,min,90 )

99%α （m/s）

S b,min,E 0.99α +1000

S b,min,E 0.99α +1000 − 99%α

(m/s)

（m/s）

∆E

0.8

1.52933

1.54478

0.01544

1.54478E-05

1.54410

0.000438081

0.9

2.13075

2.15228

0.02152

2.15228E-05

2.15190

0.000175847

1.0

2.56068

2.58655

0.02586

2.58655E-05

2.58652

1.14268E-05

1.1

2.82639

2.85494

0.02854

2.85494E-05

2.85493

1.78944E-08

1.2

2.93071

2.96032

0.02960

2.96032E-05

2.95951

0.000272007

1.3

2.936142

2.96580

0.02965

0.000029658

2.96300

0.000941971

ϕ

(m/s)/mJ

S b,min,90

(m/s)

α

Note: E0.99α represents the corresponding ignition energy value at which Sb,min is equal to 99% of α. S b,min,E0.99α +1000 denotes the corresponding minimum stretched flame propagation speed calculated by

eq 2 when ignition energy is the sum of E0.99α and 1000 mJ. Figure 6 gives the minimum reliable ignition energy of ethanol/air mixtures at different 15


Energy & Fuels

equivalence ratios. It is seen that the MRIE decreases at first and then increases with the increase of equivalence ratio, which presents a presenting V-shape trend, indicating that there are significant differences in the effects of the same ignition energy on the initial flame propagation speed of ethanol/air mixtures at different equivalence ratios. Furthermore, for an SI engine, the effect of ignition energy on the performance and emissions of the engine is caused primarily by its effect on the initial flame propagation speed. Therefore, the existence of MRIE can partly explain why the performances and emissions of the engine affected by the ignition energy will not be obviously improved any more when the ignition energy increases to a certain value. 60 Minimum Reliable Ignition Energy, Er,ign (mJ)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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50

40

30

20

10 0.7

0.8

0.9

1.0

1.1

1.2

1.3

1.4


Figure 6. Minimum reliable ignition energy of ethanol/air mixtures at various equivalence ratios, P = 1 MP and Tu = 383 K. 3.3 Effects of ignition energy on the measurement of laminar burning velocity and Markstein length. The effect degree of flame propagation speed affected by ignition energy is different at various ignition energies and equivalence ratios. Therefore, choosing a fixed flame radius as a lower limit used in data processing, error will be caused in the measurement of laminar burning velocity and Markstein length via the outwardly propagating spherical flame. The flame trajectories under three different ignition energies for ethanol/air mixtures at the equivalence ratio of 0.8 are depicted in Figure 7a. It is observed that those different flame trajectories 16


Page 17 of 26

almost converge into a single curve at different flame radii, indicating that the range of flame propagation speed affected by ignition energy is different at the different ignition energy. The flame radius marked with a circle in the Figure 7a where the flame speed trajectories initially merge can be considered as the maximum flame radius affected by the smaller ignition energy, Rign, and its value

2.4

Maximum flame radius affected by ignition energy, Re(mm)

varies with different ignition energies.

Stretched Flame Propagation Speed, Sb(m/s)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

(a)

2.2 2.0 1.8 1.6 1.4

Ignition Energy

1.2

φ = 0.8

9.5

(b)

9.0 8.5 8.0 7.5 7.0 6.5

MRIE

φ = 0.8

6.0

1.0 2

3

4

5

6

7

8

9

10

11

12

0

10

20

30

40

50

60

70

80

90

Ignition Energy, Eign (mJ)

Flame Radius, R (mm)

Figure 7. Range of flame propagation speed affected by ignition energy for ethanol/air mixture at ϕ = 0.8, P = 1 MPa and Tu = 383 K. Furthermore, Figure 7b gives the variation of Rign with ignition energy for ethanol/air mixture at the equivalence ratio of 0.8. As described in the Figure 7b, the Rign rises with the increasing ignition energy under the range of experimental ignition energy, but the increase rate decreases with the increase of ignition energy, which is similar with the effect of ignition energy on the initial flame propagation speed. When the ignition energy approximately reaches the minimum reliable ignition energy of 47 mJ, the effectiveness of continuous increasing ignition energy on Rign becomes extremely weak. Therefore, it is considered that after the ignition energy reaches MRIE, the increase rate of Rign can be neglected, and the same phenomenon also can be found at other equivalence ratios. Therefore, the corresponding Rign at the MRIE can be employed to characterize the maximum radius of flame propagation speed affected by ignition energy, which is defined as the reference flame 17


Energy & Fuels

radius (RFR) in the present study. 0.45

14 13

0.40 12 0.35 11 0.30

10 9

0.25

Flame Thichness δ (mm)

Reference Flame Radius (mm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 26

8 0.20 7 6 0.7

0.8

0.9

1.0

1.1

1.2

1.3

0.15 1.4


Figure 8. Reference flame radius and flame thickness for ethanol/air mixtures at different equivalence ratios, P = 1 MPa and Tu = 383 K. Figure 8 gives the reference flame radius and flame thickness versus the equivalence ratio for ethanol/air mixtures, and the flame thickness can be derived via temperature profile48,49

δ =(Tb − Tu ) / ( dT / dR ) max . It is seen that the reference flame radius (RFR) varies non-monotonically with the increase of equivalence ratio and reaches the minimum at the equivalence ratio of 1.1. Due to the fact that the flame thickness has a significant effect on the duration of initial flame transition period16, the range of flame propagation speed affected by ignition energy also correlates with the flame thickness. As shown in the Figure 8, the flame thickness rises first and then drops with the increasing equivalence ratio, which has similarity to the tendency of the FRF at different equivalence ratios, implying that the reference flame radius given in the present study is reasonable to some extent. Figure 9 plots the laminar burning velocities and Markstein lengths as a function of the different selected lower limit of flame radius for ethanol/air mixtures under different extrapolation models at the equivalence ratios of 0.8 and 1.3, which is derived from the experimental data at the ignition energy above the MRIE. In addition, the RFR shown in the Figure 9 represents the maximum flame 18


Page 19 of 26

radius affected by ignition energy. As shown in Figure 9, the S0u and Lb vary with different selected lower limit of flame radius under all of the extrapolation models, and the tendency of variation is almost similar. It is suggested that regardless of which extrapolation model is employed to determine the S0u and Lb, the effect of ignition energy on the initial propagation of laminar premixed flame should be considered when the lower limit of flame radius is selected. Consequently, if the spherically expending flame experiment is conducted with the large ignition energy, while a small flame radius is selected as the lower limit used in data processing, the additional error will be caused in the measurement of laminar burning velocity and Markstein length. For instance, choosing 5-6 mm as the RfL will cause that the absolute error of S0u is more than 0.5m/s at the equivalence ratio of 0.8 using the NQ extrapolation model. Moreover, the relative error of Lb even exceeds 10% at the equivalence ratio of 1.3. Therefore, choosing RFR as the RfL can simplify the experimental operation process and minimize the uncertainty induced by spark-ignition energy in determining the laminar burning velocity and Markstein length via outwardly propagating spherical flame. 45.5

1.20

(a)

φ = 0.8

48

(b)

φ = 0.8

1.15

44.5 46 44.0

45

43.5

42.5 5.0

44

NQ LC LS 5.5 6.0

6.5 7.0 7.5

1.10 Markstein Length, Lb (mm)

47

43.0

2.2 2.0

45.0

0

Laminar Burning Velocity, Su(cm/s)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

1.8

1.05 1.00

1.6

0.95 1.4

0.90 0.85

1.2

NQ LC LS

0.80

43

RFR 8.0 8.5

9.0

42 9.5 10.0 10.5 11.0 11.5

Selected Lower Limit of Flame Radius, RfL(mm)

0.75 0.70 5.5

6.0

6.5

1.0 RFR 7.0

7.5

8.0

8.5

9.0

0.8 9.5 10.0 10.5 11.0 11.5


19


Energy & Fuels

0.55

54.5 (c)

φ = 1.3

(d)

φ = 1.3

Markstein Length, Lb (mm)

0.50 Laminar Burning Velocity, S0u(cm/s)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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54.0

53.5

53.0

52.5 5.5

NQ LC LS 6.0

6.5

7.5

8.0

8.5

9.0

9.5

0.40 0.35 0.30

NQ LC LS

0.25

RFR

7.0

0.45

10.0

10.5

0.20 5.5

6.0


6.5

7.0

RFR

7.5

8.0

8.5

9.0

9.5

10.0 10.5 11.0


Figure 9. Laminar burning velocity and Markstein length as a function of the different selected lower limit of flame radius for ethanol/air mixtures under different extrapolation models at equivalence ratios of 0.8 and 1.3, P = 1 MPa, Tu = 383 K. In addition, it also can be observed in the Figure 9 that when the chosen RfL is getting close to RFR, the variations of the S0u and Lb become smaller. But after that the S0u and Lb still decrease with the increase of RfL. The result suggests that arbitrarily choosing a large flame radius as the lower limit of flame radius during the determination of the S0u and Lb is unreasonable. It is because that if the flame radius selected as the RfL is too large, the valid raw data that can be used in data processing will be reduced, which will also cause the increase of uncertainty in stretch extrapolation to get the S0u and Lb. Therefore, in order to derive more accurate the S0u and Lb, the reference flame radius should be used to determine the lower limit of the flame radius to reduce the ignition effects on the determining laminar burning velocity and Markstein length. 5. CONCLUSIONS The effect of ignition energy on the initial propagation characteristics of ethanol/air premixed flames at the initial temperature of 383 K and the pressure of 0.1 MPa is presented by using outwardly propagating spherical flames. The quantitative determination of the effect of ignition

20


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Energy & Fuels

energy on the flame propagation is performed in this study, and the main conclusions are summarized as follows: The initial stretched flame propagation speed increases with the increase of ignition energy, but the increase rate reduces with the increasing ignition energy. The effect of the same ignition energy on the initial flame propagation speed for ethanol/air mixtures is different at different equivalence ratios. The exponential correlation between the minimum stretched flame propagation speed and the experimental ignition energy can be found for ethanol/ air mixtures whose Lewis number is larger than unit. There exists a minimum reliable ignition energy (MRIE) above which the effect of the increasing ignition energy on the initial flame propagation is marginal. The MRIE can be defined as the corresponding ignition energy value at which the minimum stretched flame propagation speed is equal to 99% of its limit value under experimental condition and the empirical formula of E r,ign = ln((α − 0.99 S b,min,90 ) / β ) / ln(γ ) is developed to calculate the MRIE.

The maximum flame radius affected by the MRIE is defined as reference flame radius (RFR), which can characterize the maximum radius of flame propagation speed affected by ignition energy. It is demonstrated that the reference flame radius can be used to minimize the uncertainty induced by ignition energy in the determination of the laminar burning velocity and Markstein length of ethanol/air mixtures from the outwardly propagating spherical flame. Finally, we have to point out that, although the situation of the engine-like combustion is turbulent, the laminar flame is of still great importance to get insight into the turbulent combustion, anyway. Moreover, the effect of the ignition energy on the initial flame propagation at the condition of forced flow is the next step which is underway in our group.

21


Energy & Fuels

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Page 22 of 26

AUTHOR INFORMATION Corresponding Author *Telephone: +86-27-86523305. Fax: +86-27-86523305. E-mail: [email protected] (Z. Zhang) Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS This work was financially supported by National Natural Science Foundation (Grant NO. 51509198 and 51479149). The authors sincerely acknowledge “Fuels & Combustion Research Group in Wuhan University of Technology” for the assistance with setting up and testing experimental facilities. ABBREVIATIONS Nomenclature V

Voltage, V

I

Current, A

t

Times, s

R

Resistance, Ω

Eign

Ignition energy, mJ

Rf

Flame radius, m

A

Flame surface area, m2 22


Page 23 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

Sb

Stretched flame speed, m/s

K

Flame stretch rate, 1/s

S0b

Unstretched flame speed, m/s

S0u

Laminar burning velocity, cm/s

Lb

Markstein length, mm

Leeff

Effective Lewis number

Sb,min

Minimum stretched flame propagation speed, m/s

Sb,min,90

The corresponding Sb,min at the ignition energy of 90 mJ, m/s

E0.99α

The corresponding ignition energy at which the Sb,min is equal to 99% of its limit value, mJ

S b,min,E0.99α +1000

The corresponding Sb,min when ignition energy is the sum of E0.99α and 1000 mJ, m/s

Er,ign

Minimum reliable ignition energy, mJ

Rign

Maximum flame radius affected by ignition energy, mm

T

Temperature, K

P

Pressure, MPa

Subscript u

Unburned gas

b

Burned gas

Greek Symbols ρ

Density, g/cm3

α

Mathematical limit value of Sb,min, m/s

β

Constant

γ

Ignition factor 23


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

δ

Page 24 of 26

Flame thickness, mm

Acronyms SI

Spark-ignition

DC

Direct current

MRIE

Minimum reliable ignition energy

RFR

Reference flame radius

REFERENCES (1) Galb, M.; Zacchi, G. App.l Microbiot. Biot. 2002, 59, 618-628. (2) Taherzadeh, M. J.; Karimi, K.; BioResources 2007, 2, 707-738. (3) Scholl, A. L.; Menegol, D.; Pitarelo, A. P.; etc. Bioresour. Technol. 2015, 192, 228-237. (4) Luo, M.; Liu, D. Int. J. Hydrogen Energy 2017, 42, 3813-3823. (5) Song, J.; Sunwoo, M. Proc. Inst. Mech. Eng., Part D 2002, 216, 505-512. (6) Dale, J. D.; Checkel, M. D.; Smy, P. R. Prog. Energy Combust. Sci. 1997, 23, 379-398. (7) Gu, Y.; Fang, Z. Experimental Study on Different Ignition System Matching Different Spark Plug Gap. Proceedings of the 2009 International Conference on Computational Intelligence and Natural Computing; China, Wuhan, June 6-7, 2009. (8) Clavin, P.; Williams, F. A. J. Fluid Mech. 1982, 116, 251-282. (9) Egolfopoulos, F. N.; Hansen, N.; Ju. Y.; etc. Prog. Energy Combust. Sci. 2014, 43, 36-67. (10) Law, C. K.; Sung, C. J.; Wang, H.; etc. AIAA J. 2003, 41, 1629-1646. (11) Gu, X.; Li, Q.; Huang, Z.; etc. Energy Convers. Manage. 2011, 52, 3137-3146. (12) Tang, C.; Zhang, Y.; Huang, Z. Renewable Sustainable Energy Rev. 2014, 30, 195-216. (13) Chen, Z. Combust. Flame 2015, 162, 2442-2453. (14) Xiouris, C.; Ye, T.; Jayachandran, J.; etc. Combust. Flame 2016, 163, 270-283. (15) Lawes, M.; Sharpe, G. J.; Tripathi, N.; etc. Fuel 2016, 186, 579-586.

24


Page 25 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

(16) Chen, Z.; Burke, M. P.; Ju. Y. Proc. Combust. Inst. 2009, 32, 1253-1260. (17) Broustail, G.; Seers, P.; Halter, F.; etc. Fuel 2011, 90, 1-6. (18) Liao, S.; Jiang, D.; Huang, Z.; etc. Appl. Therm. Eng. 2007, 27, 374-380. (19) Bradley, D.; Lawes, M.; Mansour, M. S. Combust. Flame 2009, 156, 1462-1470. (20) Tran, L. S.; Glaude, P. A.; Fournet, R.; etc. Energy Fuels 2013, 27, 2226-2245. (21) Badawy, T.; Williamson, J.; Xu, H. Fuel 2016, 183, 627-640. (22) Jiang, Y.; Xu, H.; Ma, X.; etc. Fuel, 2017, 190, 10-20. (23) Chen, Z.; Ju, Y. Combust. Theory Modell. 2007, 11, 427-453. (24) Chen, Z. Studies on the initiation, propagation, and extinction of premixed flames. Ph.D. Thesis, Princeton University, 2009. (25) Kelley, A. P.; Jomaas, G.; Law, C. K. Combust. Flame 2009, 156, 1006-1013. (26) Chen, Z.; Burke, M. P.; Ju, Y. Proc. Combust. Inst. 2011, 33, 1219-1226. (27) Kim, H. H.; Won, S. H.; Santner, J.; etc. Proc. Combust. Inst. 2013, 34, 929-936. (28) Santner, J. S.; Won, S. H.; Ju, Y. Proc. Combust. Inst. 2017, 36, 1457-1465. (29) Zhang, Z.; Li, G.; Ouyang, L.; etc. Int. J. Hydrogen Energy 2011, 36, 13194-13206. (30) Liang, J.; Li, G.; Zhang, Z.; etc. Energy Fuels 2014, 28, 4754-4761. (31) Randeberg, E.; Olsen, W.; Eckhoff, R. K. J. Electrost. 2006, 64, 263-272. (32) Huang, C.; Shy. S.; Liu, C.; etc. Proc. Combust. Inst. 2007, 31, 1401-1409. (33) Harris, D. C. Quantitative chemical analysis, 8th ed.; Macmillan: New York, 2010; pp 51-75. (34) Li, G.; Liang, J.; Zhang, Z.; etc. Energy Fuels 2015, 29, 4549-4556. (35) Wu, F.; Jomaas, G.; Law, C. K. Proc. Combust. Inst. 2013, 34, 937-945. (36) Wu, C. K.; Law, C. K. Symp. (Int.) Combust. 1985, 20, 1941-1949. (37) Kelley, A. P.; Law, C. K. Combust. Flame 2009, 156, 1844-1851. (38) Chen, Z. Combustion and Flame, 2011, 158, 291-300. (39) Kelley, A. P.; Bechtold, J. K.; Law, C. K. J. Fluid Mech. 2012, 691, 26-51. 25


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 26

(40) Wu, F.; Liang, W.; Chen, Z.; etc. Proc. Combust. Inst. 2015, 35, 663-670. (41) Liang, W.; Wu, F.; Law, C. K. Proc. Combust. Inst. 2017, 36, 1137-1143. (42) Liu, J.; Zhang, X. Wang, T., etc. Int. J. Hydrogen Energy 2015, 40, 8475-8483. (43) Burke, M. P.; Chen, Z.; Ju, Y.; etc. Combust. Flame 2009, 156, 771-779. (44) Eisazadeh-Far, K.; Parsinejad, F.; Metghalchi, H.; etc. Combust. Flame 2010, 157, 2211-2221. (45) Addabbo, R.; Bechtold, J. K.; Matalon, M. Proc. Combust. Inst. 2002, 29, 1527-1535. (46) Law, C. K.; Jomaas, G.; Bechtold, J. K. Proc. Combust. Inst. 2005, 30, 159-167. (47) Brandt, M.; Hettinger, A.; Schneider, A.; etc. Extension of Operating Window for Modern Combustion Systems by High Performance Ignition. Proceedings of the 3rd International Conference on Ignition Systems for Gasoline Engines; Berlin, Germany, November 3-4, 2016. (48) Sun, Z.; Liu, F.; Bao, X.; etc. Int. J. Hydrogen Energy 2012, 37, 7889-7899. (49) Poinsot, T.; Yeynante, D. Theoretical and numerical combustion, 3rd ed.; Flourtown: RT Edwards Inc, 2012; p 57.

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