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Experimental study of positive and negative DC electric fields in lean premixed spherically expanding flames
Fuel 193 (2017) 22–30
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Full Length Article
Experimental study of positive and negative DC electric fields in lean premixed spherically expanding flames Chao Li, Xiaomin Wu ⇑, Yiming Li, Xuxing Wei School of Energy and Power Engineering, Xian Jiaotong University, Xi’an 710049, People’s Republic of China
h i g h l i g h t s Positive and negative DC electric fields show a great effect on lean combustion. Positive voltage is more efficient than the negative one in enhancing the combustion. Stability of the spherical flames at high voltage becomes poor. The negative applied voltage is more likely to cause the flame instability.
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Article history: Received 15 September 2016 Received in revised form 25 November 2016 Accepted 1 December 2016
Keywords: Lean combustion Electric field assisted combustion Bi-ionic wind effect Spherically expanding flame
a b s t r a c t This paper investigates the effects associated with applying both positive and negative DC electric fields to premixed CH4/N2/O2 spherically expanding flames with excess air ratio k = 1.2/1.4/1.6 in a constant volume combustion chamber. The results show that, both positive and negative DC electric fields enhance the combustion rates. Firstly, the mean flame propagation speed increases in the electric field direction and decreases in a direction perpendicular to the electric field with the increase of applied voltages. This effect is most significant at lambda = 1.6, and decreases with the excess air. Secondly, the combustion peak pressure under both positive and negative electric fields increases with the applied voltage, while the peak time decreases. At the same voltage value, the pressure increase and the burning time shortening are more pronounced under a positive voltage. applied voltage. Third, under a lower voltage and a smaller excess air, the flame is basically stable. However, when the applied voltage and the excess air increase to a high value (10 kV, k = 1.6), the stability of the flame front will rapidly becomes worse. Ó 2016 Elsevier Ltd. All rights reserved.
1. Introduction Electric field assisted combustion, as a kind of potential combustion assisted technology, has become of great interest in lots of combustion field. Present techniques in this field are based on the application of DC/AC electrical potential on various flames. Using these techniques, flames could be stabilized [1], propagation speed and burning velocity could be enhanced [2–4], and low soot formation can be achieved under the same conditions [5–7], especially when combined with lean-burn technology [8]. The classical explanation of the DC electric field acting on the flame is the ionic wind effect [9–11]. It has been demonstrated that there are plenty of ions and electrons in reaction zones [12,13] which lead to an electrical property of the flame front, so when ⇑ Corresponding author at: 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. E-mail address: [email protected] (X. Wu). http://dx.doi.org/10.1016/j.fuel.2016.12.001 0016-2361/Ó 2016 Elsevier Ltd. All rights reserved.
an electric field is applied, these charged particles are affected by the electric body force and migrate. The momentum and energy of the charged particles would be transferred to other neutral molecules by the collisions, and then generating a bulk flow which finally leads to the ionic wind effect. The ionic wind effect has been recognized for long and accepted as a principal explanation to the effects observed when electric fields are applied to Bunsen flame [14–18], jet flame [19,20], flat flame [21–25], droplet flame [26], counter-flow flame [27,28]. In the just cited works, the attention was focused on the new flame equilibrium state induced by the electric field, so the ionic distribution [29], the change of the chemical reaction path [25,30] and the additional mass diffusion [25,31] were the main research objects. A few researchers used the ionic wind theory to explain the electric field effect on the spherically expanding flame [32,33], but the explanation may not be totally suitable, because the spherically expanding flame is a kind of transient flame, which could not provide a sufficient time (at least 10 ms according to Marcum [16] and
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Nomenclature p pmax t tp rh rv
vh vv
pressure (kPa) peak pressure (kPa) time (ms) arrival time of peak pressure (ms) effective horizontal flame radius (mm) effective vertical flame radius (mm)
k K Le
Kim [18]) for the ions and the electrons to generate the new electrical equilibrium state in each moment. In the premixed transient flames, a systematic study of the positive and negative electric fields has not been carried out. Thus, it is not clear whether positive and negative fields exert similar effects. Our paper is an experimental study on effects invoked by applying both positive and negative DC electric fields to premixed CH4/N2/O2 transient flame by experiment. Spherically expanding flames, as one of the most important transient flames, with simple flame configuration, well-defined flame stretch rate and wellcontrolled experimentation, are selected for this study. By comparing the flame propagation speed, the combustion pressure, the burning time and the flame stability, the effect of both positive and negative DC electric fields on the transient spherical flame is investigated. 2. Experimental setup The experimental system consists of seven parts, including a constant-volume combustion chamber, a fuel supply, an ignition control circle, an optical Schlieren system, a high-speed camera, a pressure acquisition and a high-voltage supply system, as shown in Fig. 1. The constant-volume combustion bomb, shown in Fig. 2, made out of carbon steel, is a cylinder with a diameter of 130 mm and a length of 130 mm. The chamber is lined with an insulating sleeve made of polytetrafluoroethylene (PTFE). Two ignition electrodes covered with PTFE are placed at the vertical center and they are ground after the ignition. A pair of quartz glasses with 30 mm thick is installed on both sides of the combustion chamber to provide an optical path with a diameter of 80 mm. A high-speed digital cam-
horizontal propagation speed (m s1) vertical propagation speed (m s1) excess air coefficient mobility (cm2 s1 V1) Lewis number
era (HG-100K) with a shooting speed of 5000 frames per second is used to record the flame propagation process. The pressure in the chamber is collected by a piezoelectric pressure sensor (Kistler 4075A10), which has an error of less than ±0.3% and a collection frequency of 20 kHz. Two high-voltage mesh electrodes with the outer diameter of 60 mm, are installed horizontally and symmetrically placed 35 mm away from the ignition electrodes. They are connected to the high-voltage DC power supply (Wisman DEL15P15 and Wisman DEL30N45) with the voltage range from 0 kV to ±10 kV, so that the positive and the negative DC electric field can be generated between the ignition electrodes and the mesh electrodes. In this study, CH4/O2/N2 mixtures are investigated and they are initially 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 pressure gauge. A time delay of 60 s is used 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 chamber is recorded by the piezoelectric pressure sensor. In addition, each experiment is repeated at least 5 times for the same condition to decrease experimental error. 3. Results and discussion 3.1. Pictures of the flame propagation Fig. 3 is the picture of the flame propagation with the positive and negative voltages applied on the mesh electrodes under
Fig. 1. Schematic of the experimental setup.
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C. Li et al. / Fuel 193 (2017) 22–30
Fig. 2. Electrode arrangement in the combustion chamber.
(a) =1.2
(b) =1.4
(c) =1.6 Fig. 3. Pictures of the flame propagation (left positive, right negative).
0/±5/±10 kV for excess air coefficients k = 1.2/1.4/1.6. In order to facilitate the analysis, the pictures have been cut and put together, where the left side of each figure corresponds to the positive applied voltage, and the right side of each figure to the negative voltage.
From this figure, it can be seen that in the case of no electric field, the flames are spherical and propagating smoothly to the unburned area in both horizontal and vertical directions under k = 1.2/1.4/1.6, as expected in the normal methane/air mixture under laminar conditions.
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When the +5 kV and 5 kV are applied, the figure shows that the flames are stretched in the horizontal direction and inhibited in the vertical direction, which leads to an elliptical shape of the flame front. The flame size is also slightly different for the +5 kV and 5 kV voltages. These figures clearly reflect that the overall flame size, including the flame radius in horizontal and vertical directions, at a given time under +5 kV is always larger than that under 5 kV. Thus the positive voltage exerts a more pronounced effect than negative voltage. This phenomenon is consistent for k = 1.2/1.4/1.6. When the applied voltages are increased to +10 kV and 10 kV, the flame still maintains an elliptical shape under k = 1.2/1.4, and the flame stretch and compression tendencies in the horizontal and vertical directions are consistent with the results under +5 kV and 5 kV above. However, under k = 1.6, the flame is no longer elliptical. It is clear that the flame shape has been strongly altered in the electric field, and the oval flame surface gradually becomes a rectangle shape with unique three branches, especially under the lean (k = 1.6) and high voltage (10 kV) conditions. 3.2. Flame radius and mean flame propagation speed To quantify the elliptical flame shape we introduce an effective P P flame radius defined as r h ¼ 6i¼1 ri =6 and r v ¼ 6i¼1 hi =6 for the horizontal and vertical directions. respectively. As shown in Fig. 4, in order to account for flame front curvature, we use radii values within 15° angle from both the horizontal and vertical directions. The values of ri and hi are taken from the Schlieren pictures, as shown in Fig. 4. The flame radii in the two directions are then converted to the mean flame propagation speed in the two directions respectively. The effective flame radius in the horizontal direction is defined from 6 mm to 25 mm [34,35] to avoid the influence of the ignition energy, the mesh electrodes and initial deformation. The same duration is also used to determine the effective flame radius in the vertical direction. The mean flame propagation speeds in horizontal direction and vertical directions are defined as v h ¼ ðrh25 rh6 Þ=ðt25 t6 Þ and v v ¼ ðrv 25 rv 6 Þ=ðt25 t6 Þ, and the results are given in Fig. 5 and Table 1.
Fig. 4. Definition of the effective flame radius.
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Fig. 5 and Table 1 show clearly that with the increase of applied voltage, the flame propagation speed shows a significant increase in the horizontal direction and a less pronounced decrease in the vertical direction. Besides, under the same electric field intensity, the propagation speed under positive applied voltage in the two directions is always larger than under the negative voltage. For instance, the horizontal flame propagation speed increases to 1.52 m s1 and 2.01 m s1 under +5/+10 kV and k = 1.2, while it reaches only 1.51 m s1 and 1.76 m s1 under 5/10 kV and k = 1.2. 3.3. Combustion pressure In a constant volume combustion bomb, the increase rate of the combustion pressure is often used to reflect the mass burning rate. For this reason, the pressure data in the combustion chamber under electric field are also presented. Fig. 6 shows the pressure data in the combustion chamber during the combustion process under different voltages and different excess air coefficients. The peak pressure (pmax), the arrival time of peak pressure (tp), and the changing rate compared with the non-voltage cases, are shown in Table 2. Table 2 data show the relationship between the combustion pressure, the voltage amplitude, the polarity effect and the excess air coefficient. First, under a constant excess air coefficient, the pmax increases and the tp is remarkably advanced compared with that in the absence of the electric field. The higher the input voltage, the greater is effect of the applied electric field on combustion pressure. Second, under the same absolute value of the applied voltage, the increase rate of the pmax and the shortening rate of the tp under the positive applied voltage is always higher than that under the negative voltage. Third, under the same applied voltage, the excess air coefficient k also had a certain effect on the combustion pressure. The leaner the premixed fuel, the higher is the increase rate of the pmax and the shortening rate of the tp is. 3.4. Mechanism discussion The above results have shown that both positive and negative applied voltages increase flame propagation and combustion pressure. However, the combustion promotion mechanism is quite different due to the different charged species that be driven to the premixed zones by the applied voltage. When the positive voltage is applied to the mesh electrodes, the direction of the electric field is from the mesh electrodes (positive voltage) to the ignition electrodes (grounded). In this case, the electric field force drive the negative ions and electrons from the flame front to the unburned premixed zone. Due to the higher concentration [13] and the higher mobility of the electrons [36] comparing with the negative ions, researchers generally believe that the electrons is dominant in negatively charged particles. It has been already shown that the electrons present in the premixed zone excite N2 and O2 [22,37], as shown in Eqs. (1) and (2).
N2 ðu ¼ 0Þ þ e ¼ N2 ðu ¼ 1Þ þ e
ð1Þ
N2 ðu ¼ 1Þ þ O2 ðu ¼ 0Þ ¼ N2 ðu ¼ 0Þ þ O2 ðu ¼ 1Þ
ð2Þ
This is a chain reaction, which means the accelerated electrons that be driven to the premixed region could stimulate N2 and O2 by collisions, where e is the accelerated electrons and u = 0 or 1 indicates the molecule state as normal or excited. Then the oxidation reaction could be promoted with the activated O2, and the total chemical reaction rate and the flame propagation speed in the horizontal direction could be significantly enhanced. The flame propagation shows a slight inhibition in the vertical direction, because the electric field is horizontal applied. That is to say the
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(a)
=1.2
(b) =1.4
(c) =1.6 Fig. 5. Mean propagation speed in horizontal and vertical directions.
Table 1 Mean propagation speeds and their change rate. k
U (kV)
v h
D v h (%)
D v v (%)
1.2
0 +5 +10 5 10
1.31 1.52 2.01 1.51 1.76
1.31 1.30 1.29 1.21 1.09
– 16.0 53.4 15.3 34.4
– 0.8 1.5 7.6 16.8
1.4
0 +5 +10 5 10
0.83 1.11 1.35 1.02 1.28
0.83 0.71 0.65 0.70 0.60
– 33.7 62.7 22.9 54.2
– 14.5 21.7 15.7 27.7
1.6
0 +5 +10 5 10
0.46 0.79 1.10 0.68 0.98
0.46 0.46 0.42 0.43 0.34
– 77.7 139.1 47.8 113.0
– 0 8.7 6.5 26.1
(m s1)
electrons in the vertical direction are still horizontally driven, so the excitation effect could not be formed in the vertical premixed zones.
v v
(m s1)
When the negative voltage is applied to the mesh electrodes, the direction of the electric field is from the ignition electrode (grounded) to the mesh electrodes (negative voltage). In the
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(a)
(b)
(c) Fig. 6. Pressure data.
Table 2 Pressure record. k
U (kV)
pmax (kPa)
tp (ms)
Dpmax (%)
Dtp (%)
1.2
0 +5 +10 5 10
593 603 614 601 603
103.3 95.1 91.0 96.6 96.3
– 1.68 3.54 1.35 1.68
– 7.94 12.00 6.49 6.78
1.4
0 +5 +10 5 10
518.4 527.5 532.5 519.7 520.5
142.9 143.6 133.7 145.0 135.9
– 1.76 2.71 0.25 0.41
– 1.61 6.44 2.03 4.90
1.6
0 +5 +10 5 10
393.4 420.9 438.4 400.0 415.9
300.0 232.3 222.0 253.8 224.8
– 7.00 11.44 1.68 5.72
– 22.57 26.00 15.40 25.07
horizontal direction, the main positive ions, such as CHO+ and H3O+ [12], formed at the flame front are driven to the unburned premixed zone by the electric field force, which is known as the ionic wind effect. Major positive ions have a similar radius and mass compared with other intermediate molecules, so when the positive
ions are accelerated by the electric field, the mass and heat transfer between the flame front and the premixed zone is generated by ionic and molecular collisions. Then, the temperature and internal energy of the premixed zone can be promoted with the thermal and mass diffusion, so that the chemical reactions proceed faster,
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(a)
(b) Fig. 7. Flame deformation under the condition of 10/+10 kV and k = 1.6.
and finally leading to a larger flame propagation speed. The mechanism of the propagation inhibition in the vertical direction is similar to that in the positive voltage, because the positive ions in the vertical direction are still horizontally driven due to the horizontal electric field, so the chemical reaction rate in this region is decreased due to the decrease of positive ion concentration and then reduce the flame propagation speed in the vertical direction. The promotional efficiency by the positive applied voltage (excitation effect of the electrons) is always higher than that by the negative applied voltage (ionic wind effect of the positive ions), which is contrary to effects observed in stagnant flames [16,22,23,25]. Previous works [16,18] had demonstrated that the ionic wind needs a time (at least 10 ms) to develop. In the stagnant flame, there is no doubt that the ionic wind effect is fully developed due to the formation of new equilibrium state. However, in the transient propagation flame, the response time may be insufficient. In other words, the ionic wind effect on the transient flame is always in a weaker state. The excitation effect is seldom affected by this reason, because the mobility of electrons is much higher than that of the ions (electrons: K = 4000 cm2 s1 V1 [31], ions: K = 1 cm2 s1 V1 [36]), so even in a very short time, the electrons could still affect the whole premixed area, which means excitation effects are always fully developed, no matter in the stagnant flame or in the transient propagation flame. For this reason, it is plausible that the full developed excitation effect is more efficient than the non-fully developed ionic wind effect to promote the combustion. This is our explanation for the effects observed in our experiments. 3.5. Flame stability In the spherically expanding flame, the flame stability is usually affected by two aspects: the hydrodynamic and diffusive-thermal instabilities [38–41]. In the case of no electric field, the spherical flame propagates smoothly, since the flame is laminar and propagates through a uniform mixture, which means the hydrodynamic instability is pretty low. Besides, the Lewis number (Le), which is defined as the ratio of heat diffusivity of the mixture to the mass diffusivity of the premixed methane-air, is always near one, no matter what is the excess air k = 1.2/1.4/1.6 [39]. Thus, the diffusive-thermal instability would not magnify or reduce the (potentially) existing flame instability due to the critical state of
the heat and mass transfer ratio (Le = 1). For this reason, the spherical methane-air flames do not show an instabilities in the absence of the electric field. When the electric field is applied, the situation changes. In the hydrodynamic instability part, the spherical flame spreads in all directions around, while the electric field is just horizontally applied, so both the ionic wind and the excitation effects of the negative and the positive voltages are unequal along the flame surface [42]. Thus, the combustion rate varies along the flame front due to unequal ionic wind and the excitation effects. For this reason, the hydrodynamic instability is produced, and the flame deformation occurs. In the diffusive-thermal instability, the negative applied voltage (ionic wind effect) promotes the mass diffusivity between the flame front and the premixed zone, while the thermal diffusivity was slightly reduced due to the ionic and molecular accumulation at the boundary between the two regions, so the Le in this case is less than 1, which means the diffusive-thermal instability can be promoted with the negative applied voltage. Thus, the short disturbances, such as the flame deformation by the non-uniform ionic concentration and mass diffusion, are amplified and the hydrodynamic instability is enhanced by diffusion effects. On the contrary, the positive applied voltage (excitation effect) promotes the thermal diffusion and inhibits the mass diffusion due to the opposite electric field compared with the negative applied voltage, so the Le in this case is higher that 1. Thus, the diffusive-thermal instability is reduced, and any other instabilities are dumped. The most serious flame deformation appears under the condition of 10/+10 kV and k = 1.6, as shown in Fig. 7, which can be ascribed to two reasons. First, the intensity of both the ionic wind effect and the excitation effect increase with the increase of the applied voltage, so the flame instability caused by the electric field also increases with a higher applied voltage. Second, when the premixed fuel is under the leanest condition (k = 1.6), the flame propagation takes more time due to its slow speed. This lengthens the period of the applied electric field influence on the combustion process. According to the theory of ionic wind effect [43], the amount of accelerated/excited neutral molecules which have collided with the positive ions/electrons and gained momentum/energy from them is increased when the available time is lengthened, and this can markedly enhance the effect of the electric field even the flame instability.
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The ionic wind effect (negative voltage) is more likely to lead to instability of the flame compared with the excitation effect (positive voltage) because the ionic flow could enhance both the hydrodynamic instability and the diffusive-thermal instability (Le < 1), so that the unstable flame deformation could make a further development with the flame propagation. For instance, the flame front under the negative voltage cracks and splits at the very beginning (about 16 ms), and then gradually produces a obvious threebranch structure on the flame front at 22 ms, as shown in Fig. 7 (a). The excitation effect (positive applied voltage) does not cause a strong flame instability, because it only increases the hydrodynamic instability, but reduces the diffusive-thermal instability (Le > 1). So flame instability is not obviously magnified, and the flame shape just acquire a rectangular shape from 12 ms to 22 ms, as shown in Fig. 7(b). 4. Conclusions The paper experimentally evaluates the effect of positive and negative voltage applied to lean (with the excess air ratios of 1.2, 1.4 and 1.6) premixed CH4/O2/N2 flames. Several main Conclusions are as follows: (1) The flame propagation speed have been enhanced in the electric field direction (horizontal direction), while the flame propagation speed have been inhibited in the direction perpendicular to the electric field (vertical direction). The effects are more pronounced when the applied voltage is larger and the premixed mixture is leaner. (2) The applied voltage increases the combustion pressure. The combustion peak pressure increases and the arrival time of the peak pressure decreases with the increase of the applied voltage. Both the positive and negative electric field can effectively improve the mass burning rate of the spherically expanding flame in the constant volume combustion chamber, and the positive applied voltage should be a better choice because of its higher combustion promoting efficiency. (3) The mechanism of electric field assisted combustion is quite different for positive and negative electric fields. Under the positive voltages, the flame propagation enhancement is driven by the electrons moving towards the premixed zone and promoting the oxidation chain reactions. Under the negative voltage, the flame propagation enhancement is due to the intensification of heat and mass transfer between the reaction zone and the premixed zone by the positive ions. (4) The application of the electric field causes an instability on the flame surface, especially under ±10 kV and k = 1.6, because the ionic wind and the excitation effect produce a hydrodynamic instability in both negative and positive applied voltages. Higher voltages and the lower flame propagation speeds both enhance this effect. Notes The authors declare no competing financial interest. Conflicts of interest The authors declare no conflict of interest. Acknowledgment The authors gratefully acknowledge the National Natural Science Foundation of China (Grants No.: 51476126).
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Full Length Article
Experimental study of positive and negative DC electric fields in lean premixed spherically expanding flames Chao Li, Xiaomin Wu ⇑, Yiming Li, Xuxing Wei School of Energy and Power Engineering, Xian Jiaotong University, Xi’an 710049, People’s Republic of China
h i g h l i g h t s Positive and negative DC electric fields show a great effect on lean combustion. Positive voltage is more efficient than the negative one in enhancing the combustion. Stability of the spherical flames at high voltage becomes poor. The negative applied voltage is more likely to cause the flame instability.
a r t i c l e
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Article history: Received 15 September 2016 Received in revised form 25 November 2016 Accepted 1 December 2016
Keywords: Lean combustion Electric field assisted combustion Bi-ionic wind effect Spherically expanding flame
a b s t r a c t This paper investigates the effects associated with applying both positive and negative DC electric fields to premixed CH4/N2/O2 spherically expanding flames with excess air ratio k = 1.2/1.4/1.6 in a constant volume combustion chamber. The results show that, both positive and negative DC electric fields enhance the combustion rates. Firstly, the mean flame propagation speed increases in the electric field direction and decreases in a direction perpendicular to the electric field with the increase of applied voltages. This effect is most significant at lambda = 1.6, and decreases with the excess air. Secondly, the combustion peak pressure under both positive and negative electric fields increases with the applied voltage, while the peak time decreases. At the same voltage value, the pressure increase and the burning time shortening are more pronounced under a positive voltage. applied voltage. Third, under a lower voltage and a smaller excess air, the flame is basically stable. However, when the applied voltage and the excess air increase to a high value (10 kV, k = 1.6), the stability of the flame front will rapidly becomes worse. Ó 2016 Elsevier Ltd. All rights reserved.
1. Introduction Electric field assisted combustion, as a kind of potential combustion assisted technology, has become of great interest in lots of combustion field. Present techniques in this field are based on the application of DC/AC electrical potential on various flames. Using these techniques, flames could be stabilized [1], propagation speed and burning velocity could be enhanced [2–4], and low soot formation can be achieved under the same conditions [5–7], especially when combined with lean-burn technology [8]. The classical explanation of the DC electric field acting on the flame is the ionic wind effect [9–11]. It has been demonstrated that there are plenty of ions and electrons in reaction zones [12,13] which lead to an electrical property of the flame front, so when ⇑ Corresponding author at: 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. E-mail address: [email protected] (X. Wu). http://dx.doi.org/10.1016/j.fuel.2016.12.001 0016-2361/Ó 2016 Elsevier Ltd. All rights reserved.
an electric field is applied, these charged particles are affected by the electric body force and migrate. The momentum and energy of the charged particles would be transferred to other neutral molecules by the collisions, and then generating a bulk flow which finally leads to the ionic wind effect. The ionic wind effect has been recognized for long and accepted as a principal explanation to the effects observed when electric fields are applied to Bunsen flame [14–18], jet flame [19,20], flat flame [21–25], droplet flame [26], counter-flow flame [27,28]. In the just cited works, the attention was focused on the new flame equilibrium state induced by the electric field, so the ionic distribution [29], the change of the chemical reaction path [25,30] and the additional mass diffusion [25,31] were the main research objects. A few researchers used the ionic wind theory to explain the electric field effect on the spherically expanding flame [32,33], but the explanation may not be totally suitable, because the spherically expanding flame is a kind of transient flame, which could not provide a sufficient time (at least 10 ms according to Marcum [16] and
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Nomenclature p pmax t tp rh rv
vh vv
pressure (kPa) peak pressure (kPa) time (ms) arrival time of peak pressure (ms) effective horizontal flame radius (mm) effective vertical flame radius (mm)
k K Le
Kim [18]) for the ions and the electrons to generate the new electrical equilibrium state in each moment. In the premixed transient flames, a systematic study of the positive and negative electric fields has not been carried out. Thus, it is not clear whether positive and negative fields exert similar effects. Our paper is an experimental study on effects invoked by applying both positive and negative DC electric fields to premixed CH4/N2/O2 transient flame by experiment. Spherically expanding flames, as one of the most important transient flames, with simple flame configuration, well-defined flame stretch rate and wellcontrolled experimentation, are selected for this study. By comparing the flame propagation speed, the combustion pressure, the burning time and the flame stability, the effect of both positive and negative DC electric fields on the transient spherical flame is investigated. 2. Experimental setup The experimental system consists of seven parts, including a constant-volume combustion chamber, a fuel supply, an ignition control circle, an optical Schlieren system, a high-speed camera, a pressure acquisition and a high-voltage supply system, as shown in Fig. 1. The constant-volume combustion bomb, shown in Fig. 2, made out of carbon steel, is a cylinder with a diameter of 130 mm and a length of 130 mm. The chamber is lined with an insulating sleeve made of polytetrafluoroethylene (PTFE). Two ignition electrodes covered with PTFE are placed at the vertical center and they are ground after the ignition. A pair of quartz glasses with 30 mm thick is installed on both sides of the combustion chamber to provide an optical path with a diameter of 80 mm. A high-speed digital cam-
horizontal propagation speed (m s1) vertical propagation speed (m s1) excess air coefficient mobility (cm2 s1 V1) Lewis number
era (HG-100K) with a shooting speed of 5000 frames per second is used to record the flame propagation process. The pressure in the chamber is collected by a piezoelectric pressure sensor (Kistler 4075A10), which has an error of less than ±0.3% and a collection frequency of 20 kHz. Two high-voltage mesh electrodes with the outer diameter of 60 mm, are installed horizontally and symmetrically placed 35 mm away from the ignition electrodes. They are connected to the high-voltage DC power supply (Wisman DEL15P15 and Wisman DEL30N45) with the voltage range from 0 kV to ±10 kV, so that the positive and the negative DC electric field can be generated between the ignition electrodes and the mesh electrodes. In this study, CH4/O2/N2 mixtures are investigated and they are initially 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 pressure gauge. A time delay of 60 s is used 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 chamber is recorded by the piezoelectric pressure sensor. In addition, each experiment is repeated at least 5 times for the same condition to decrease experimental error. 3. Results and discussion 3.1. Pictures of the flame propagation Fig. 3 is the picture of the flame propagation with the positive and negative voltages applied on the mesh electrodes under
Fig. 1. Schematic of the experimental setup.
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Fig. 2. Electrode arrangement in the combustion chamber.
(a) =1.2
(b) =1.4
(c) =1.6 Fig. 3. Pictures of the flame propagation (left positive, right negative).
0/±5/±10 kV for excess air coefficients k = 1.2/1.4/1.6. In order to facilitate the analysis, the pictures have been cut and put together, where the left side of each figure corresponds to the positive applied voltage, and the right side of each figure to the negative voltage.
From this figure, it can be seen that in the case of no electric field, the flames are spherical and propagating smoothly to the unburned area in both horizontal and vertical directions under k = 1.2/1.4/1.6, as expected in the normal methane/air mixture under laminar conditions.
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When the +5 kV and 5 kV are applied, the figure shows that the flames are stretched in the horizontal direction and inhibited in the vertical direction, which leads to an elliptical shape of the flame front. The flame size is also slightly different for the +5 kV and 5 kV voltages. These figures clearly reflect that the overall flame size, including the flame radius in horizontal and vertical directions, at a given time under +5 kV is always larger than that under 5 kV. Thus the positive voltage exerts a more pronounced effect than negative voltage. This phenomenon is consistent for k = 1.2/1.4/1.6. When the applied voltages are increased to +10 kV and 10 kV, the flame still maintains an elliptical shape under k = 1.2/1.4, and the flame stretch and compression tendencies in the horizontal and vertical directions are consistent with the results under +5 kV and 5 kV above. However, under k = 1.6, the flame is no longer elliptical. It is clear that the flame shape has been strongly altered in the electric field, and the oval flame surface gradually becomes a rectangle shape with unique three branches, especially under the lean (k = 1.6) and high voltage (10 kV) conditions. 3.2. Flame radius and mean flame propagation speed To quantify the elliptical flame shape we introduce an effective P P flame radius defined as r h ¼ 6i¼1 ri =6 and r v ¼ 6i¼1 hi =6 for the horizontal and vertical directions. respectively. As shown in Fig. 4, in order to account for flame front curvature, we use radii values within 15° angle from both the horizontal and vertical directions. The values of ri and hi are taken from the Schlieren pictures, as shown in Fig. 4. The flame radii in the two directions are then converted to the mean flame propagation speed in the two directions respectively. The effective flame radius in the horizontal direction is defined from 6 mm to 25 mm [34,35] to avoid the influence of the ignition energy, the mesh electrodes and initial deformation. The same duration is also used to determine the effective flame radius in the vertical direction. The mean flame propagation speeds in horizontal direction and vertical directions are defined as v h ¼ ðrh25 rh6 Þ=ðt25 t6 Þ and v v ¼ ðrv 25 rv 6 Þ=ðt25 t6 Þ, and the results are given in Fig. 5 and Table 1.
Fig. 4. Definition of the effective flame radius.
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Fig. 5 and Table 1 show clearly that with the increase of applied voltage, the flame propagation speed shows a significant increase in the horizontal direction and a less pronounced decrease in the vertical direction. Besides, under the same electric field intensity, the propagation speed under positive applied voltage in the two directions is always larger than under the negative voltage. For instance, the horizontal flame propagation speed increases to 1.52 m s1 and 2.01 m s1 under +5/+10 kV and k = 1.2, while it reaches only 1.51 m s1 and 1.76 m s1 under 5/10 kV and k = 1.2. 3.3. Combustion pressure In a constant volume combustion bomb, the increase rate of the combustion pressure is often used to reflect the mass burning rate. For this reason, the pressure data in the combustion chamber under electric field are also presented. Fig. 6 shows the pressure data in the combustion chamber during the combustion process under different voltages and different excess air coefficients. The peak pressure (pmax), the arrival time of peak pressure (tp), and the changing rate compared with the non-voltage cases, are shown in Table 2. Table 2 data show the relationship between the combustion pressure, the voltage amplitude, the polarity effect and the excess air coefficient. First, under a constant excess air coefficient, the pmax increases and the tp is remarkably advanced compared with that in the absence of the electric field. The higher the input voltage, the greater is effect of the applied electric field on combustion pressure. Second, under the same absolute value of the applied voltage, the increase rate of the pmax and the shortening rate of the tp under the positive applied voltage is always higher than that under the negative voltage. Third, under the same applied voltage, the excess air coefficient k also had a certain effect on the combustion pressure. The leaner the premixed fuel, the higher is the increase rate of the pmax and the shortening rate of the tp is. 3.4. Mechanism discussion The above results have shown that both positive and negative applied voltages increase flame propagation and combustion pressure. However, the combustion promotion mechanism is quite different due to the different charged species that be driven to the premixed zones by the applied voltage. When the positive voltage is applied to the mesh electrodes, the direction of the electric field is from the mesh electrodes (positive voltage) to the ignition electrodes (grounded). In this case, the electric field force drive the negative ions and electrons from the flame front to the unburned premixed zone. Due to the higher concentration [13] and the higher mobility of the electrons [36] comparing with the negative ions, researchers generally believe that the electrons is dominant in negatively charged particles. It has been already shown that the electrons present in the premixed zone excite N2 and O2 [22,37], as shown in Eqs. (1) and (2).
N2 ðu ¼ 0Þ þ e ¼ N2 ðu ¼ 1Þ þ e
ð1Þ
N2 ðu ¼ 1Þ þ O2 ðu ¼ 0Þ ¼ N2 ðu ¼ 0Þ þ O2 ðu ¼ 1Þ
ð2Þ
This is a chain reaction, which means the accelerated electrons that be driven to the premixed region could stimulate N2 and O2 by collisions, where e is the accelerated electrons and u = 0 or 1 indicates the molecule state as normal or excited. Then the oxidation reaction could be promoted with the activated O2, and the total chemical reaction rate and the flame propagation speed in the horizontal direction could be significantly enhanced. The flame propagation shows a slight inhibition in the vertical direction, because the electric field is horizontal applied. That is to say the
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(a)
=1.2
(b) =1.4
(c) =1.6 Fig. 5. Mean propagation speed in horizontal and vertical directions.
Table 1 Mean propagation speeds and their change rate. k
U (kV)
v h
D v h (%)
D v v (%)
1.2
0 +5 +10 5 10
1.31 1.52 2.01 1.51 1.76
1.31 1.30 1.29 1.21 1.09
– 16.0 53.4 15.3 34.4
– 0.8 1.5 7.6 16.8
1.4
0 +5 +10 5 10
0.83 1.11 1.35 1.02 1.28
0.83 0.71 0.65 0.70 0.60
– 33.7 62.7 22.9 54.2
– 14.5 21.7 15.7 27.7
1.6
0 +5 +10 5 10
0.46 0.79 1.10 0.68 0.98
0.46 0.46 0.42 0.43 0.34
– 77.7 139.1 47.8 113.0
– 0 8.7 6.5 26.1
(m s1)
electrons in the vertical direction are still horizontally driven, so the excitation effect could not be formed in the vertical premixed zones.
v v
(m s1)
When the negative voltage is applied to the mesh electrodes, the direction of the electric field is from the ignition electrode (grounded) to the mesh electrodes (negative voltage). In the
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(a)
(b)
(c) Fig. 6. Pressure data.
Table 2 Pressure record. k
U (kV)
pmax (kPa)
tp (ms)
Dpmax (%)
Dtp (%)
1.2
0 +5 +10 5 10
593 603 614 601 603
103.3 95.1 91.0 96.6 96.3
– 1.68 3.54 1.35 1.68
– 7.94 12.00 6.49 6.78
1.4
0 +5 +10 5 10
518.4 527.5 532.5 519.7 520.5
142.9 143.6 133.7 145.0 135.9
– 1.76 2.71 0.25 0.41
– 1.61 6.44 2.03 4.90
1.6
0 +5 +10 5 10
393.4 420.9 438.4 400.0 415.9
300.0 232.3 222.0 253.8 224.8
– 7.00 11.44 1.68 5.72
– 22.57 26.00 15.40 25.07
horizontal direction, the main positive ions, such as CHO+ and H3O+ [12], formed at the flame front are driven to the unburned premixed zone by the electric field force, which is known as the ionic wind effect. Major positive ions have a similar radius and mass compared with other intermediate molecules, so when the positive
ions are accelerated by the electric field, the mass and heat transfer between the flame front and the premixed zone is generated by ionic and molecular collisions. Then, the temperature and internal energy of the premixed zone can be promoted with the thermal and mass diffusion, so that the chemical reactions proceed faster,
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C. Li et al. / Fuel 193 (2017) 22–30
(a)
(b) Fig. 7. Flame deformation under the condition of 10/+10 kV and k = 1.6.
and finally leading to a larger flame propagation speed. The mechanism of the propagation inhibition in the vertical direction is similar to that in the positive voltage, because the positive ions in the vertical direction are still horizontally driven due to the horizontal electric field, so the chemical reaction rate in this region is decreased due to the decrease of positive ion concentration and then reduce the flame propagation speed in the vertical direction. The promotional efficiency by the positive applied voltage (excitation effect of the electrons) is always higher than that by the negative applied voltage (ionic wind effect of the positive ions), which is contrary to effects observed in stagnant flames [16,22,23,25]. Previous works [16,18] had demonstrated that the ionic wind needs a time (at least 10 ms) to develop. In the stagnant flame, there is no doubt that the ionic wind effect is fully developed due to the formation of new equilibrium state. However, in the transient propagation flame, the response time may be insufficient. In other words, the ionic wind effect on the transient flame is always in a weaker state. The excitation effect is seldom affected by this reason, because the mobility of electrons is much higher than that of the ions (electrons: K = 4000 cm2 s1 V1 [31], ions: K = 1 cm2 s1 V1 [36]), so even in a very short time, the electrons could still affect the whole premixed area, which means excitation effects are always fully developed, no matter in the stagnant flame or in the transient propagation flame. For this reason, it is plausible that the full developed excitation effect is more efficient than the non-fully developed ionic wind effect to promote the combustion. This is our explanation for the effects observed in our experiments. 3.5. Flame stability In the spherically expanding flame, the flame stability is usually affected by two aspects: the hydrodynamic and diffusive-thermal instabilities [38–41]. In the case of no electric field, the spherical flame propagates smoothly, since the flame is laminar and propagates through a uniform mixture, which means the hydrodynamic instability is pretty low. Besides, the Lewis number (Le), which is defined as the ratio of heat diffusivity of the mixture to the mass diffusivity of the premixed methane-air, is always near one, no matter what is the excess air k = 1.2/1.4/1.6 [39]. Thus, the diffusive-thermal instability would not magnify or reduce the (potentially) existing flame instability due to the critical state of
the heat and mass transfer ratio (Le = 1). For this reason, the spherical methane-air flames do not show an instabilities in the absence of the electric field. When the electric field is applied, the situation changes. In the hydrodynamic instability part, the spherical flame spreads in all directions around, while the electric field is just horizontally applied, so both the ionic wind and the excitation effects of the negative and the positive voltages are unequal along the flame surface [42]. Thus, the combustion rate varies along the flame front due to unequal ionic wind and the excitation effects. For this reason, the hydrodynamic instability is produced, and the flame deformation occurs. In the diffusive-thermal instability, the negative applied voltage (ionic wind effect) promotes the mass diffusivity between the flame front and the premixed zone, while the thermal diffusivity was slightly reduced due to the ionic and molecular accumulation at the boundary between the two regions, so the Le in this case is less than 1, which means the diffusive-thermal instability can be promoted with the negative applied voltage. Thus, the short disturbances, such as the flame deformation by the non-uniform ionic concentration and mass diffusion, are amplified and the hydrodynamic instability is enhanced by diffusion effects. On the contrary, the positive applied voltage (excitation effect) promotes the thermal diffusion and inhibits the mass diffusion due to the opposite electric field compared with the negative applied voltage, so the Le in this case is higher that 1. Thus, the diffusive-thermal instability is reduced, and any other instabilities are dumped. The most serious flame deformation appears under the condition of 10/+10 kV and k = 1.6, as shown in Fig. 7, which can be ascribed to two reasons. First, the intensity of both the ionic wind effect and the excitation effect increase with the increase of the applied voltage, so the flame instability caused by the electric field also increases with a higher applied voltage. Second, when the premixed fuel is under the leanest condition (k = 1.6), the flame propagation takes more time due to its slow speed. This lengthens the period of the applied electric field influence on the combustion process. According to the theory of ionic wind effect [43], the amount of accelerated/excited neutral molecules which have collided with the positive ions/electrons and gained momentum/energy from them is increased when the available time is lengthened, and this can markedly enhance the effect of the electric field even the flame instability.
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The ionic wind effect (negative voltage) is more likely to lead to instability of the flame compared with the excitation effect (positive voltage) because the ionic flow could enhance both the hydrodynamic instability and the diffusive-thermal instability (Le < 1), so that the unstable flame deformation could make a further development with the flame propagation. For instance, the flame front under the negative voltage cracks and splits at the very beginning (about 16 ms), and then gradually produces a obvious threebranch structure on the flame front at 22 ms, as shown in Fig. 7 (a). The excitation effect (positive applied voltage) does not cause a strong flame instability, because it only increases the hydrodynamic instability, but reduces the diffusive-thermal instability (Le > 1). So flame instability is not obviously magnified, and the flame shape just acquire a rectangular shape from 12 ms to 22 ms, as shown in Fig. 7(b). 4. Conclusions The paper experimentally evaluates the effect of positive and negative voltage applied to lean (with the excess air ratios of 1.2, 1.4 and 1.6) premixed CH4/O2/N2 flames. Several main Conclusions are as follows: (1) The flame propagation speed have been enhanced in the electric field direction (horizontal direction), while the flame propagation speed have been inhibited in the direction perpendicular to the electric field (vertical direction). The effects are more pronounced when the applied voltage is larger and the premixed mixture is leaner. (2) The applied voltage increases the combustion pressure. The combustion peak pressure increases and the arrival time of the peak pressure decreases with the increase of the applied voltage. Both the positive and negative electric field can effectively improve the mass burning rate of the spherically expanding flame in the constant volume combustion chamber, and the positive applied voltage should be a better choice because of its higher combustion promoting efficiency. (3) The mechanism of electric field assisted combustion is quite different for positive and negative electric fields. Under the positive voltages, the flame propagation enhancement is driven by the electrons moving towards the premixed zone and promoting the oxidation chain reactions. Under the negative voltage, the flame propagation enhancement is due to the intensification of heat and mass transfer between the reaction zone and the premixed zone by the positive ions. (4) The application of the electric field causes an instability on the flame surface, especially under ±10 kV and k = 1.6, because the ionic wind and the excitation effect produce a hydrodynamic instability in both negative and positive applied voltages. Higher voltages and the lower flame propagation speeds both enhance this effect. Notes The authors declare no competing financial interest. Conflicts of interest The authors declare no conflict of interest. Acknowledgment The authors gratefully acknowledge the National Natural Science Foundation of China (Grants No.: 51476126).
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