Article pubs.acs.org/EF
Experimental Study of Lean Premixed CH4/N2/O2 Flames under HighFrequency Alternating-Current Electric Fields Hao Duan,† Xiaomin Wu,*,†,‡ Cong Zhang,† Yuchen Cui,† Juncai Hou,† Chao Li,† and Zhongquan Gao† †
Institute of Internal Combustion Engine, Xi’an Jiaotong University, Xi’an 710049, People’s Republic of China Shaanxi University of Technology, Hanzhong 723001, People’s Republic of China
‡
ABSTRACT: In order to study the effect of the frequency and the intensity of high-frequency alternating-current (AC) electric fields on lean combustion, the effects of high-frequency (5−30 kHz; 5 kV) and high-voltage (0−5 kV; 15 kHz) AC electric fields on lean (excess air ratio 1.2/1.4/1.6) premixed CH4/N2/O2 flames are evaluated experimentally in the paper in detail. Results show that the mean flame propagation speed increases with the electric field frequency at first. However, the transition frequency, after which the mean flame propagation speed reaches to the maximum and then tends to be stabilized, is shown to be around 25 kHz. The combustion peak pressure increases minimally at various frequencies, but the timing of it decreases apparently; additionally, the effect of frequency on the combustion pressure change is very close to that on the mean flame propagation speed. The initial duration and main duration also have a similar tendency with the increase of the frequency. Besides, with the increase of the applied voltage, the mean flame propagation speed increases nearly exponentially. Meanwhile, the combustion peak pressure increases, and the timing of it decreases. Similarly, the effect of applied voltage on the combustion pressure change is also close to that on the mean flame propagation speed, and the initial duration and main duration also declines with the increase of the applied voltage. The result shows that the combustion of lean mixtures can be enhanced effectively by highfrequency (5−30 kHz) and high-voltage (1−5 kV) AC electric fields, which has a positive meaning in the further study of the electric-field-assisted combustion theory. mechanism of DC electric fields on combustion has been researched extensively.17−24 Results show that the ionic wind effect has been recognized and accepted to explain many consequences. It has been testified that there are plenty of charged particles in a flame, contributing to an electric property. These charged particles can be affected by the body force and migrate along the electric field direction when an extra DC electric field is applied on the flame. Therefore, the momentum of ions is transferred to the neutral molecules during the collisions within the distance of the mean free path, generating a bulk flow which finally leads to the ionic wind effect. On the other hand, the effects of alternating-current (AC) electric fields on the flame propagation have also been researched by experiments and simulations25−28 for several decades, especially for stagnant flames.29−32 In fact, the mechanism of the flame propagation characteristics under an AC electric field has not been totally concluded yet. Kim and co-workers reported the effect of the low-frequency AC electric field on the laminar premixed Bunsen flame stability and explained the phenomenon they had found by the ionic wind effect caused by positive and negative ions, which is called the bi-ionic wind effect.30 Cha and Lee have investigated the effect of 1 kHz AC electric fields on the methane/propane flames in a constant volume combustion bomb, and they concluded that overall burning rates are not significantly affected by the electric field for the majority of the combustion period. The work shows that a cracked structure on the flame surface could be
1. INTRODUCTION Lean combustion has become the focus of attention as a new combustion technology because of the requirement to decrease energy consumption caused by the growing crisis of limited energy resources and the strengthening of automotive pollutant legislations since the 1970s. However, the relatively lower combustion rate of lean combustion technology restricts its progress. Thus, a number of new combustion methods have emerged to compensate for its poor dynamic performance. Among them, electric-field-assisted combustion, as one of the potential technologies to improve combustion velocity, has drawn considerable attention and enjoys a bright future no doubt. During the last century, plenty of research studies, including the flame behaviors and combustion characteristics with the influence of electric fields, have been conducted on the electricfield-assisted combustion. The effect of an electric field on a flame was first explored by Brande1 in 1814, and he discovered that the flame propagation speed varied substantially under an electric field. From then on, lots of experimental and theoretical investigations have been applied to study the effects of electric fields on the flames. So far, several types of flames have been reported, such as Bunsen flame,2,3 jet flame,4,5 flat flame,6,7 counter-flow flame,8,9 and tribrachial flame.10 As a result, it is known that combustion characteristics can be markedly modified via the interaction between electric fields and ions generated in combustion, including flame propagation,11 flame stabilization,12 heat release,13 flammability limits,14 combustion products,15,16 and so on. Direct-current (DC) electric fields are mostly employed in the exploration of the electric field assist combustion and the © 2015 American Chemical Society
Received: June 25, 2015 Revised: October 12, 2015 Published: October 28, 2015 7601
DOI: 10.1021/acs.energyfuels.5b01420 Energy Fuels 2015, 29, 7601−7611
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Figure 1. Schematic of the experimental setup.
Table 1. Key Characteristics of the High-Voltage Power Supply input voltage
output voltage
frequency
pulse width
power
220 ± 10% V (AC); 50 Hz
0−20 kV (DC) plus 0−20 kV (AC)
≤50 kHz
0−6 μs
20 W
Figure 2. Structure of the constant volume combustion chamber and mesh electrodes.
observed, which could enhance flame propagation speed for the initial period of combustion and led to reduction in the flame initiation and combustion duration times.31 Zhang et al. have researched the flame behavior and the emissions of nonpremixed methane/air flames under a 10 kHz AC electric field and found that the impact of the AC electric field on the flames relies on different mechanisms in various voltage ranges.32 It can be observed that the mechanisms of the flame propagation under a low-frequency and a high-frequency AC electric field are not the same. The effect of AC electric fields on a transient flame was rarely studied before, and the mechanism to explain the behavior of a transient flame under an AC electric field is still unknown. So it is necessary to carry out further study of the propagation of a stagnant flame under an AC electric field. Considering this, the paper mainly focuses on the effects of high-frequency AC electric fields on the premixed CH4/N2/O2 lean combustion by changing the frequency (f) and the applied voltage (U). Because methane plays an important role in human life, it is necessary to choose CH4 as the object of the present study. Meanwhile, as one of the most important transient flames, the spherically expanding flame, which can be generated in a
constant-volume combustion bomb, has been used in the study because of its simple flame configuration, well-defined flame stretch rate and well-controlled experimentation. In addition, an effort was made to provide a reasonable explanation of the experimental results.
2. EXPERIMENTAL SETUP Figure 1 shows the experimental system, which consists of seven parts, including a constant-volume combustion bomb system, a fuel supply system, an ignition control system, an optical schlieren system, a highspeed camera system, a pressure acquisition system, and a high-voltage supply system. The constant-volume combustion bomb, cast by carbon steel, is a cylinder of 130 mm in diameter and 130 mm in length with an insulating sleeve made of polytetrafluoroethylene (PTFE). A pair of 30 mm thick quartz glasses is installed on both sides of the bomb to provide an optical path. A high-speed digital camera (HG-100 K) with a shooting speed of 5000 frames per second is employed to record the flame propagation process. The bomb pressure is measured by a piezoelectric pressure sensor (Kistler7061B), which has an error of less than ±0.5%. The high-frequency AC electric fields are generated by the high-voltage power supply (WismanWPS20P20), the characteristics of which are listed in Table 1. Applied voltage (the applied voltages on the AC electric fields are all RMS voltages) generated by 7602
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Figure 3. Magnitude and vector distribution of electric field intensity (U = 5 kV; f = 15 kHz).
Table 2. Ep0, Et0, Epm, and Etm (Voltage Peaks/Troughs; U = 1−5 kV; f = 15 kHz) U/kV
1
2
3
60 60 68 68
119 119 136 136
179 179 203 203
f/kHz Ep0/kV·m−1 Et0/kV·m−1 Epm/kV·m−1 Etm/kV·m−1
4
5
239 239 271 271
298 298 339 339
5
15
5
10
12.5
17.5
20
25
30
298 298 339 339
298 298 339 339
298 298 339 339
298 298 339 339
298 298 339 339
298 298 339 339
298 298 339 339
the power supply is connected to the high-voltage electrodes, which can lead to a high-voltage electric field. The ignition electrodes, embedded by the PTFE insulating layer, are arranged vertically in the center of the bomb, and they are both connected to the ground after ignition. Meanwhile, the bomb is always grounded in the experiment. Two stainless-steel-made high-voltage mesh electrodes (60 mm in outer diameter) are mounted horizontally in the center of the bomb to apply electric fields. The structure of the mesh electrodes and the installation of them within the bomb are presented in Figure 2. In this study, CH4/O2/N2 flames are investigated at room temperature and atmospheric pressure. The combustible mixture is prepared by sequentially introducing CH4 and O2/N2 synthesis gas (21% O2 and 79% N2 by volume) with the corresponding partial pressures, which are monitored by a U-tube mercury manometer. A time delay of 150 s is set to make sure that the fuel and the oxidizer are perfectly mixed and there is no flow before ignition. The mixture is then ignited by the ignition electrodes. During combustion, the pressure inside the bomb is recorded by the piezoelectric pressure sensor. In addition, each experiment is repeated at least three times for the same condition for excellent repeatability.
3. RESULTS AND DISCUSSION 3.1. Mechanism Discussion. Three different kinds of effects are considered to explain the flame behaviors observed under AC electric fields: (1) the thermal effect; (2) the ionic wind effect; (3) the electrical−chemical effect.29−32 The thermal effect means that when the current in the electric field is sufficient, the heat energy that transformed from the electric energy will be assimilated.33−35 In the experiment, the current in the flame during the combustion is very small and only exists between the ignition electrodes at the ignition start. Meanwhile, the ignition energy is slight and limited, and thus, the thermal effect is not taken into consideration in contemporary work. The ionic wind effect mentioned above is essentially a kind of particle movement caused by collision. These particles take a directed migration under a DC electric field and then collide with other particles to create a directional
Figure 4. Pictures of the flames ( f = 5−30 kHz; U = 5 kV; λ = 1.2/ 1.4/1.6).
volume flow, which is recognized as the ionic wind effect.28 However, when a high-frequency AC electric field is applied, the polarity of the field and the direction of the electric field force suffered by the charged particles change alternately, leading to a nondirectional migration of the particles. 7603
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the 3D simulation capacity of the ANSOFT Maxwell 14.0 software was used to quantitatively study the intensities of the electric fields generated by high-voltage electrodes with various voltages. As a result, the distributions of the electric fields (with the voltage of 5 kV and the frequency of 15 kHz) generated in the longitudinal section of the bomb center in the peaks and troughs are shown in Figure 3. Since the bomb and the electric fields are both symmetrical in the horizontal direction, only the magnitude distribution in the left and the vector distribution in the right are presented. When comparing Figure 3a and 3b, we can discover that the electric field distributions are almost the same in cases of the voltage peaks and troughs. The electric fields are almost symmetrical in the vertical direction; the electric field intensity near the ignition electrode tip is the maximum; the electric field intensity in the horizontal direction is approximately between 1 × 105 V/m and 2 × 105 V/m and gradually decreases with the increase of the distance from the center of the bomb. We also can find from the figure that both of the electric fields of the voltage peaks and troughs are along the horizontal direction but in the opposite direction. A parameter d (mm) is defined as the horizontal distance toward to the center (positive to the right and negative to the left). When |d| < 6 mm, the flame ignition energy has a significant impact on the development of the flame;39 when |d| > 25 mm, both the pressure change and the temperature change in the bomb are so large that their impact on the flame development cannot be ignored, and the structure of the bomb also shows a great effect. Thus, the computation area is selected as |d| = 6−25 mm, when calculating electric field parameters (the values of flame propagation characteristics for further processing are limited in the same area). The mean electric field intensity of the area in the peaks (Ep0/kV·m−1) and troughs (Et0/kV·m−1) are investigated as the reflection of the electric field for quantitative study, so are the maximum values of those in the peaks (Epm/kV·m−1) and troughs (Etm/kV·m−1). Simulation results are shown in Table 2, and it can be noted that Ep0 and Et0 or Epm and Etm are always the same at a constant voltage of 5 kV, regardless of the variation in the frequency. Thus, we can conclude that the changing frequency of the electric field will not contribute to the change of electric field intensity. Additionally, all these parameters increase linearly with the applied voltage at a frequency of 15 kHz, meaning that the electric field intensity also increases linearly with the applied voltage.
Figure 5. Schematic diagram of the defined flame radius.
Moreover, the frequencies of the fields are very high in the paper, and the collision response time (which is in the order of 10 ms)30,36 is much larger than the half cycle (less than 0.1 ms); therefore, the charged particles do not have enough time to be accelerated, and directional volume flow will not be generated in the flame reaction zone. In other words, the ionic wind effect has little effect on the flame propagation under high-frequency AC electric fields, which has been mentioned by Kim et al.30 Thus, the ionic wind effect was also ignored in the present paper. As a consequence, in the current study, only the electrical− chemical effect is considered to analyze the flame behavior at high-frequency AC electric fields. In an AC electric field, particles achieve energy mainly by the conversation of the electromagnetic radiation. Among them, some particles absorbed sufficient energy and are then excited, directly affecting partial reactions of the combustion process, which is termed as the electrical−chemical effect.31,37,38 Because it directly impacts the reactions of the combustion progress, the electrical−chemical effect can affect the flame propagation markedly under a high-frequency electric field. 3.2. Electric Field Simulation. The properties of the highfrequency AC electric fields in the bomb are determined by the high-voltage mesh electrodes and the voltage and frequency of the field, so the simulation on the field is necessary. In this part,
Figure 6. Flame radius versus elapsed time (( f = 5−30 kHz; U = 5 kV; λ = 1.2/1.4/1.6)). 7604
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Figure 7. Flame propagation speed versus flame radius (( f = 5−30 kHz; U = 5 kV; λ = 1.2/1.4/1.6)).
discussed quantitatively. Additionally, the combustion characters were also studied in the experiment. 3.3.1. Pictures of the Flame Propagation. Figure 4 shows the pictures of the flame propagation with the voltages of 0 and 5 kV for λ = 1.2/1.4/1.6 at various frequencies. When the applied voltage is 0 kV, the flames are spherical and propagate smoothly to the unburned area in both horizontal and vertical directions, and when the applied voltage is 5 kV, the flames are all stretched in the horizontal direction for various frequencies, especially with the excess air ratio of 1.6. However, as is shown in the figure, the flames change little in the vertical direction, resulting in elliptical shapes. 3.3.2. Flame Propagation Speed. Since the flame shape change in the vertical direction is difficult to observe, only the flame propagation in the horizontal direction is investigated. The flame radius in the paper is defined as r = ∑i 6= 1ri/6, and the value of ri is directly decided by the schlieren pictures, as is shown in Figure 5. Figure 6 shows the flame radius versus the elapsed time after ignition start (t) at various frequencies. It can be pointed out that the flame radius almost linearly increases with time when the applied voltage and the excess air ratio are constant. With the increase of the frequency at a constant excess air ratio, the growth of r is accelerated, too. For example, the time when r increases to 25 mm is 18.64 ms without the electric field at an excess air ratio of 1.2, whereas it decreases to 15.21 ms when the electric field is applied with the applied voltage of 5 kV and the frequency of 30 kHz, reduced by 18.40%. Besides, the tendency is particularly evident when the excess air ratio is 1.6. That is probably because for a larger value of excess air ratio, the mixtures are leaner and the combustion
Figure 8. Mean flame propagation speed versus AC electric field frequency (( f = 5−30 kHz; U = 5 kV; λ = 1.2/1.4/1.6)).
3.3. Effects of Frequency of High-Frequency AC Electric Field on Lean Combustion. In this part, the effect of high-frequency AC electric field with various frequencies on flame propagation is experimentally studied. The applied voltage on the high-voltage electrodes is constant (5 kV), and the frequency of the AC electric field is changed from 5 to 30 kHz in the experiment, with excess air ratios (λ) of 1.2, 1.4, and 1.6. As for the results, the flame propagation pictures were analyzed first. And then the flame radius and speed were
Figure 9. Combustion pressure versus elapsed time (( f = 5−30 kHz; U = 5 kV; λ = 1.2/1.4/1.6)). 7605
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Table 3. Mean Flame Propagation Speeds and the Relative Change Rates (f = 5−15 kHz; U = 5 kV; λ = 1.2/1.4/1.6) 5 kV −1
S̅r/m·s
ΔS̅r/%
λ
0 kV
5 kHz
7.5 kHz
10 kHz
12.5 kHz
15 kHz
20 kHz
25 kHz
30 kHz
1.2 1.4 1.6 1.2 1.4 1.6
1.14 0.77 0.51 0 0 0
1.37 1.00 0.85 19.42 30.04 66.70
1.38 1.06 0.91 20.46 37.07 77.75
1.41 1.08 0.95 23.05 40.04 85.30
1.41 1.12 0.97 23.64 44.58 89.82
1.42 1.14 0.98 24.60 47.71 91.08
1.43 1.16 1.00 25.37 49.82 94.99
1.44 1.18 1.01 25.60 52.07 97.58
1.44 1.17 1.01 25.55 52.00 97.89
speed is slower, which provide more time for the electrical− chemical effect to develop, and the enhancement of the flame propagation may also be improved with the excess air ratio. For example, when the frequency applied is 30 kHz, the times with excess air ratios of 1.4 and 1.6 reduced by 33.73 and 44.03%, much greater than that at the excess air ratio of 1.2. The flame propagation speed, defined as the moving velocity of the flames relative to the stationary combustion wall, can be calculated from the flame pictures. The flame propagation speed here is calculated as Sr = dr/dt, where r is the flame radius, and t is the elapsed time after ignition start. Figure 7 shows that the variation of the flame propagation speed increases with the radius at various frequencies. Table 2 shows the mean flame propagation speeds (Sr̅ ; the average value of Sr during the area r = 6−25 mm) and the relative change rates (ΔSr̅ ; the relative change rate of Sr̅ when compared to that without the electric field). The figure shows that S̅r presents an increase tendency with the frequency. It is possible that the increasing of the frequency can increase the electromagnetic energy and result in producing more excited particles in preflame zone, which promote the electrical−chemical effect and then improve the flame propagation in the bomb. However, it cannot be ignored that Sr̅ does not increase unlimitedly with the frequency applied on the electric field, meaning that although the electrical−chemical effect can be improved by increasing the frequency of the electric fields, it will be stabilized when the frequency reaches a critical value, which is defined as the transition frequency in the paper. It may be due to the concentration of the mixtures, which has a positive correlation with the number of the excited particles. When the number of the excited particles has reached the maximum, the transition frequency will very likely exist. Figure 8 presents the variation of Sr̅ with the increase of f, and three fitting curves are shown in the figure to describe the relationships between S̅r and f at various excess air ratios
Figure 10. Pictures of the flames ( f = 15 kHz; U = 0−5 kV; λ = 1.2/ 1.4/1.6).
Figure 11. Flame radius versus elapsed time (f = 15 kHz; U = 0−5 kV; λ = 1.2/1.4/1.6). 7606
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Figure 12. Flame propagation speed versus flame radius ( f = 15 kHz; U = 0−5 kV; λ = 1.2/1.4/1.6).
Table 4. Combustion Characteristics and the Relative Change Rates (f = 5−15 kHz; U = 5 kV; λ = 1.2/1.4/1.6) λ
U/kV
f/kHz
pmax/kPa
tp/ms
tid/ms
tmd/ms
Δpmax/%
Δtp/%
Δtid/%
Δtmd/%
1.2
0 5
1.4
0 5
1.6
0 5
/ 5 7.5 10 12.5 15 20 25 30 / 5 7.5 10 12.5 15 20 25 30 / 5 7.5 10 12.5 15 20 25 30
560 578 583 581 586 585 584 580 579 474 508 504 508 510 511 512 514 516 401 442 446 451 454 456 455 457 454
91.3 80.3 78.1 77.3 76.3 76.0 75.1 75.2 74.9 132.9 119.4 117.1 116.4 115.7 115.2 114.5 110.1 110.6 211.1 166.4 164.2 162.0 157.7 158.4 154.6 150.8 151.0
41.2 35.0 34.8 35.3 35.0 35.0 34.2 34.5 34.2 52.8 50.4 50.5 50.9 50.9 49.3 48.7 48.5 47.3 78.8 67.5 62.8 63.4 63.0 62.9 60.4 60.3 58.9
37.5 33.1 32.3 31.2 30.8 30.4 30.7 30.1 29.9 59.5 53.8 52.2 51.4 50.3 49.8 50.4 49.3 50.1 83.7 75.1 74.9 75.5 73.2 71.7 72.8 71.8 71.8
0.00 3.21 4.11 3.75 4.64 4.46 4.29 3.57 3.39 0.00 7.17 6.33 7.17 7.59 7.81 8.02 8.44 8.86 0.00 10.22 11.22 12.47 13.22 13.72 13.47 13.97 13.22
0.00 −12.05 −14.46 −15.33 −16.43 −16.76 −17.74 −17.63 −17.96 0.00 −10.16 −11.89 −12.42 −12.94 −13.32 −13.84 −17.16 −16.78 0.00 −21.17 −22.22 −23.26 −25.30 −24.96 −26.76 −28.56 −28.47
0.00 −15.05 −15.53 −14.32 −15.05 −15.05 −16.99 −16.26 −16.99 0.00 −4.55 −4.36 −3.60 −3.60 −6.63 −7.77 −8.14 −10.42 0.00 −14.34 −20.30 −19.54 −20.05 −20.18 −23.35 −23.48 −25.25
0.00 −11.73 −13.87 −16.80 −17.87 −18.93 −18.13 −19.73 −20.27 0.00 −9.58 −12.27 −13.61 −15.46 −16.30 −15.29 −17.14 −15.80 0.00 −10.27 −10.51 −9.80 −12.54 −14.34 −13.02 −14.22 −14.22
more clearly. The fitting curves in the figure have a maximum relative error of 0.65%, meaning that the curves can greatly reflect the relationship between Sr̅ and f. According to the curves, we can find that Sr̅ almost reaches the maximum when f increases to 25 kHz for all excess air ratio conditions; that is, the transition frequency could be estimated to be around 25 kHz. Moreover, the maximum mean flame propagation speeds also can be calculated by eqs 1−3 of the curves listed below, which are 1.44, 1.18, and 1.01 m/s for λ = 1.2, 1.4 and 1.6, respectively. Sr = 1.43503 −
1.33828 12.93946
π 2
e
⎛ f − 1.45889 ⎞2 ⎟ − 2⎜ ⎝ 12.93946 ⎠ (λ
Sr = 1.17827 −
35.72258 30.40915
π 2
e
⎛ f + 23.00542 ⎞2 ⎟ − 2⎜ ⎝ 30.40915 ⎠ (λ
= 1.4) (2)
Sr = 1.01003 −
121309.33594 51.99433
π 2
e
⎛ f + 107.58718 ⎞ ⎟ − 2⎜ ⎝ 51.99433 ⎠
2
(λ = 1.6) (3)
3.3.3. Combustion Characteristics. Figure 9 clearly shows the effect of an electric field on the combustion pressure curves. The peak pressure (pmax) and the timing of the peak pressure (tp) are listed in Table 3. As can be seen in the figure, the pressure increases sharply after the short initial combustion stage (about 25 ms) for all cases. For all excess air ratio conditions, pmax increases apparently under a 5 kV electric field,
= 1.2) (1) 7607
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Table 5. Mean Flame Propagation Speeds and the Relative Change Rates ( f = 15 kHz; U = 0−5 kV; λ = 1.2/1.4/1.6) 15 kHz S̅r/m·s−1
ΔS̅r/%
λ
0 kV
1 kV
2 kV
3 kV
4 kV
5 kV
1.2 1.4 1.6 1.2 1.4 1.6
1.14 0.77 0.51 0 0 0
1.16 0.80 0.54 1.82 3.85 6.34
1.18 0.83 0.58 3.42 7.36 14.37
1.23 0.94 0.67 7.69 21.07 32.04
1.31 1.00 0.84 14.70 29.72 63.68
1.42 1.14 0.98 24.60 47.71 91.08
flame, which induces quicker flame propagation and shorter combustion durations. It can be seen that tmd is reduced by 20.42, 20.85, 21.37, 22.15, 21.89, 22.24, 25.28, and 23.28% with an excess air ratio of 1.6 when the frequency increases from 5 to 30 kV, respectively. Noticeably, the results on initial duration and main duration fit well with that on the combustion pressure, indicating that the high-frequency AC electric fields can significantly affect lean combustion. 3.4. Effects of Voltage of High-frequency AC Electric Fields on Lean Combustion. In this part, the effect of applied voltage on flame propagation is experimentally studied. The electric field frequency is constant (15 kHz), and the applied voltage of the AC electric field is changed from 0 kV to 5 kV in the experiment, with the excess air ratios of 1.2/1.4/1.6. 3.4.1. Pictures of Flame Propagation. Figure 10 shows the pictures of the flame propagation at various applied voltages under 15 kHz AC electric fields. As the applied voltage increases, the flames are stretched apparently in the horizontal direction for all excess air ratios conditions. However, the flames shape change in the vertical direction is still very slight. In fact, the electric field intensity increases with the applied voltage. A direct result may be the increase of the electromagnetic energy absorbed by the charged particles could lead to the promotion of the electrical−chemical effect and the improvement of the flame shape change. 3.4.2. Flame Propagation Speed. Figure 11 shows the flame radius versus the elapsed time after ignition start at various applied voltages. For a constant excess air ratio, with the increase of applied voltage, the growth of the flame radius is accelerated significantly. For example, the value of the time when r increases to 25 mm is 18.63 ms when the applied voltage is 0 kV with the excess air ratio of 1.2, whereas it decreases to 18.47, 18.18, 17.46, 16.46, and 15.26 ms when the
Figure 13. Mean flame propagation speed versus applied voltage (f = 15 kHz; U = 0−5 kV; λ = 1.2/1.4/1.6).
and it also increases with the electric field frequency; nonetheless, the improvement is quite limited, especially when the excess air ratio is 1.2, and pmax is almost unchanged with various high frequencies (for the largest change of 4.64%). However, tp shows a more obvious downtrend with the frequency. It can be seen that tp is advanced by 21.17, 22.22, 23.26, 25.30, 24.96, 26.76, 28.56, and 28.47% with an excess air ratio of 1.6 when the frequency increases from 5 to 30 kV, respectively. It demonstrates that the increase of the frequency can accelerate the peak pressure and the combustion process apparently, especially the latter. As is shown in the figure, the peak pressures with various frequencies are all larger than that without the electric fields, which is most likely due to the promoted heat release of combustion in the bomb. Since the mixture is enhanced by the flame colliding with the bomb wall because of the accelerated speed under the influence of electric fields, the mixing and heat transfer of burned and unburned gases are promoted, which leads to improved combustion and shortened combustion duration. The effectiveness of applying high-voltage AC electric fields on the initial duration (tid) and main duration (tmd) is also demonstrated in Table 3. It can be seen that both tid and tmd are shortened significantly with high-frequency AC electric fields, and with the increase of the frequency, the tendencies are similar to that of the combustion pressure. The application of high-frequency AC electric fields could result to a stretch of the
Figure 14. Combustion pressure versus elapse time ( f = 15 kHz; U = 0−5 kV; λ = 1.2/1.4/1.6). 7608
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Energy & Fuels Table 6. Combustion Characteristics and the Relative Change Rates (f = 15 kHz; U = 0−5 kV; λ = 1.2/1.4/1.6) λ
U/kV
f/kHz
pmax/kPa
tp/ms
tid/ms
tmd/ms
Δpmax/%
Δtp/%
Δtid/%
Δtmd/%
1.2
0 1 2 3 4 5 0 1 2 3 4 5 0 1 2 3 4 5
/ 5
560 562 565 572 580 585 474 495 501 504 507 511 401 416 420 428 449 456
91.3 86.4 83.0 83.4 80.4 76.0 132.9 128.3 125.0 122.6 120.2 115.2 211.1 194.7 190.6 178.5 167.7 158.4
41.2 38.3 37.4 35.9 35.7 35.0 52.8 51.6 51.2 50.4 49.8 49.3 78.8 78.1 77.5 73.2 69.4 62.9
37.5 35.6 34.7 33.9 32.1 30.4 59.5 59.1 56.1 57.4 53.6 49.8 83.7 82.4 80.6 76.7 72.6 71.7
0.00 0.36 0.89 2.14 3.57 4.46 0.00 4.43 5.70 6.33 6.96 7.81 0.00 3.74 4.74 6.73 11.97 13.72
0.00 −5.37 −9.09 −8.65 −11.94 −16.76 0.00 −3.46 −5.94 −7.75 −9.56 −13.32 0.00 −7.77 −9.71 −15.44 −20.56 −24.96
0.00 −7.04 −9.22 −12.86 −13.35 −15.05 0.00 −2.27 −3.03 −4.55 −5.68 −6.63 0.00 −0.89 −1.65 −7.11 −11.93 −20.18
0.00 −5.07 −7.47 −9.60 −14.40 −18.93 0.00 −0.67 −5.71 −3.53 −9.92 −16.30 0.00 −1.55 −3.70 −8.36 −13.26 −14.34
1.4
1.6
/ 5
/ 5
applied voltages are 1, 2, 3, 4, and 5 kV, reduced by 0.86, 2.43, 6.28, 11.68, and 18.10%, respectively. Besides, the tendency is particularly evident when the excess air ratio is 1.6. For example, when the applied voltage is 5 kV, the times reduced by 18.10 and 32.02% with excess air ratios of 1.2 and 1.4, respectively, although it reduced by as much as 42.87% with the excess air ratio of 1.6. Figure 12 displays the flame propagation speed versus the radius at various applied voltages, and the mean flame propagation speeds and the relative change rates are shown in Table 4. According to the figure, S̅r increases with the applied voltage, which could be explained by the reason that the electrical−chemical effect obviously increases with the applied voltage. It should be further noted that the growth rates of S̅r increases with the excess air ratio at a constant applied voltage. This is probably because with the increases of the excess air ratio, the flame propagation speed decreases, resulting in the increase of the combustion duration. Thus, the electromagnetic energy absorbed by the ions in the flame front also increases and the electrical−chemical effect is enhanced, leading to the increase of the amplification of the flame propagation speed. It can be noted that S̅r or ΔSr̅ increases limitedly in the lowvoltage area (1 and 2 kV). However, it is greatly improved in the high-voltage area (3, 4, and 5 kV). Figure 13 presents the variation of S̅r with the increase of U, and three fitting curves are shown in the figure to describe the relationships between Sr̅ and U at various excess air ratios more clearly. The fitting curves in the figure have a maximum relative error of 3.35%, meaning that the curves can greatly reflect the relationship between Sr̅ and U. It can be seen that the S̅r−U curves, described as eqs 4−6, respectively, are not linear but exponential for all excess air ratios, which indicates that the electrical−chemical effect probably increases with the electric field intensity exponentially. It is likely that the electrical− chemical effect directly influences the reaction rate of some reactions in combustion, which could promote the propagation of flame, and therefore, with the augmentation of the applied voltage, S̅r increases approximate exponentially. Thus, although S̅r increases limitedly at the voltage of 1 or 2 kV, it could obtain a great improvement at the voltage of 3, 4, or 5 kV.
Sr = 0.02728eU/2.05641 + 1.11570 (λ = 1.2)
(4)
Sr = 0.08090eU/2.90014 + 0.68751 (λ = 1.4)
(5)
Sr = 0.09081eU/2.70944 + 0.41061 (λ = 1.6)
(6)
3.4.3. Combustion Characteristics. Figure 14 clearly shows the effect of an electric field on the combustion pressure curves. The peak pressure and the timing of the peak pressure are listed in Table 5. For all excess air ratio conditions, pmax increases and tp decreases with the increase of applied voltage. It can be seen that for an excess air ratio of 1.6 when the applied voltage increases from 1 to 5 kV, pmax is improved by 3.68, 4.60, 6.85, 11.95, and 13.89% and tp is advanced by 7.81, 9.75, 15.48, 20.60, and 25.01%, respectively, which shows that the increase of the applied voltage can improve combustion and shortened combustion duration. The increase of the electric field intensity leads to the enhancement of electrical−chemical effect and then contributes to the increase of the pmax and the decrease of tp. The initial duration (tid) and main duration (tmd) is also demonstrated in Table 6. It can be seen that both tid and tmd are shortened significantly with high-frequency AC electric fields, and with the increase of the applied voltage, both of them are reduced, similar to that of the combustion pressure. It can be seen that tmd is reduced by 12.08, 15.29, 19.37, 20.50, and 21.89% with an excess air ratio of 1.6 when the applied voltage increases from 0 to 5 kV, respectively. Noticeably, the results on initial duration and main duration fit well with that on the combustion pressure, indicating that the high-frequency AC electric fields can significantly affect the combustion characters of lean combustion.
4. CONCLUSIONS The effects of high-frequency and high-voltage AC electric fields on lean combustion premixed CH4/O2/N2 flames are evaluated experimentally in the paper. Several main conclusions are summarized as follows: (1) The mean flame propagation speed increases with the electric field frequency. However, there exists a transition frequency, after which the mean flame propagation speed reaches to the maximum and then tends to be stabilized. 7609
DOI: 10.1021/acs.energyfuels.5b01420 Energy Fuels 2015, 29, 7601−7611
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Energy & Fuels
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The transition frequency is shown to be around 25 kHz. The combustion peak pressure has limited increase at various frequencies, but the timing of it decreases apparently. The effect of frequency on the combustion pressure change is very close to that on the mean flame propagation speed. Besides, the initial duration and main duration also have a similar tendency with the increase of the frequency. (2) The mean flame propagation speed increases approximately exponentially with the applied voltage. Meanwhile, the combustion peak pressure increases, and the timing of it decreases. The effect of applied voltage on the combustion pressure change is also close to that on the mean flame propagation speed. Besides, the initial duration and main duration also declined with the increase of the applied voltage. (3) Combustion of lean mixtures can be enhanced effectively by high-frequency (5−30 kHz) and high-voltage (1−5 kV) AC electric fields, which could be explained by the electrical−chemical effect. The result has a definite meaning in the further study of the electric-field-assisted combustion theory.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Fax:+86-29-82668789. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors gratefully acknowledge the National Natural Science Foundation of China (Grant Nos. 51176150 and 51476126), State Key Laboratory of Automotive Safety and Energy, Tsinghua University (Grant No. KF14122)
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REFERENCES
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