Effects of Direct-Current (DC) Electric Fields on Flame Propagation

Article pubs.acs.org/EF

Effects of Direct-Current (DC) Electric Fields on Flame Propagation and Combustion Characteristics of Premixed CH4/O2/N2 Flames Xiangwen Meng, Xiaomin Wu,* Chan Kang, Andong Tang, and Zhongquan Gao Institute of Internal Combustion Engine, Xi’an Jiaotong University, Xi’an 710049, People’s Republic of China ABSTRACT: In this work, effects of direct-current (DC) electric fields on the flame propagation and combustion characteristics of premixed CH4/O2/N2 mixtures were experimentally investigated at excess air ratios of 0.8, 1.0, and 1.2, room temperature, and atmospheric pressure. Results show that the existence of the DC electric fields significantly affects the flame propagation and combustion properties. Specifically, the flame shape becomes a prolate spheroid, with the major axis in the electric field direction as a result of the movement of positive ions by the electric body force, and a further increase in the applied voltage distorts the flame front more significantly. Additionally, the flame propagation speed in the electric field direction (Sn) and corresponding unstretched laminar burning velocity (ul) are increased as the electric field becomes more intense, and this behavior is more pronounced for lean mixtures. Finally, the initial and main combustion durations defined by the pressure evolution profiles are shortened. The peak pressure and peak rate of pressure rise are increased with the increase of the electric field intensity just for lean mixtures. The observation of the laminar burning velocity and pressure evolution behavior substantiates the potential of the electric field in enhancing lean combustion. flames, respectively. In the case of premixed flat flames, it is reported in the earlier investigations that there is no influence of a DC electric field on the laminar burning velocity.32−34 In contrast, the measurable influence of an electric field on the apparent burning velocity of propane flames was reported by Fox and Mirchandani.35 Recently, the numerical and experimental work performed by de Goey and co-workers16,17 reported that the increase of laminar burning velocity induced by the electric field could reach values up to a few percentages (in the range of 7−12.1%). Spherically expanding flames, because of their simple flame configuration, well-defined flame stretch rate and well-controlled experimentation, have been extensively used for laminar flame speed determination.36−41 However, the effects of electric fields on this flame configuration have seldom been investigated. Recently, Cha and Lee42 investigated the effects of AC electric fields on spherically expanding premixed methane/air and propane/air flames using point-to-point electrodes. Their results showed that the flame propagation speed and lean mixture combustion were enhanced by the applied electric field. In contrast, the electric field has little influence on the pressure and overall burning rate. It is noted that the studies by Cha and Lee only used an AC voltage at 1000 Hz, and they did not draw any quantitative conclusions. One objective of the present study is to investigate the effects of DC electric fields on the propagation of centrally ignited flames of CH4/O2/N2 mixtures using high-speed schlieren photography. To do so, the flame propagation speed and corresponding unstretched laminar burning velocity were obtained with and without the DC electric field. Additionally, by recording the pressure evolution history, other combustion characteristics, such as the peak pressure and maximum rate of pressure rise, were

1. INTRODUCTION In response to the energy shortage and increasingly stringent emission regulations, many studies have been conducted to develop advanced combustion systems with high efficiency and low emissions. The application of the electric field, which can be easily modulated, is considered as an attractive means to improve the combustion. The hydrocarbon flame contains a large number of ions and electrons (with a number density of 109−1012 per cm3), which are produced by chemi-ionization in the reaction zone, and these chemi-ions are believed to significantly modify the combustion behavior.1,2 The electric body force is expected to affect the flame through the interaction of electric fields and the flame ions. Then, during the collisions within the distance of the mean free path, the momentum of ions is transferred to the neutral molecules, generating bulk flow, which leads to the ionic wind effect.3−7 Therefore, a properly applied electric field can affect the combustion chemistry in the reaction zone, leading to possible enhancement of combustion. The effects of the electric field on premixed and diffusion flames have been extensively studied via experimental and theoretical investigations. Specifically, various types of flames, such as Bunsen flame,8−11 jet flame,12−15 flat flame,16,17 counterflow flame,18−20 and tribrachial flame21,22 were used, and directcurrent (DC)23−27 and alternating-current (AC)19,28−30 electric fields were presented. These investigations showed that fundamental combustion characteristics, such as flame propagation, flame stabilization, heat release, flammability limits, and soot reduction can be significantly modified through the interaction between the electric field and combustion-generated chemi-ions. However, the effects of an electric field on the laminar burning velocity, which is a parameter constantly used for chemical kinetic mechanism validation, are not yet clear. Jaggers and Von Engel31 observed that Bunsen flame speeds of hydrocarbon fuels were obviously increased with the presence of DC or AC electric fields. Marcum et al.9 and Chung et al.21 came to the same conclusion from the investigation of Bunsen and tribrachial © 2012 American Chemical Society

Received: June 7, 2012 Revised: October 9, 2012 Published: October 9, 2012 6612

dx.doi.org/10.1021/ef300972g | Energy Fuels 2012, 26, 6612−6620

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Figure 1. Schematic of the experimental setup.

investigated to assess the possible dependence of these parameters upon the electric fields. Finally, flame responses to the electric fields for different excess air ratios were compared to further understand the electric-field-assisted combustion.

2. EXPERIMENTAL SECTION 2.1. Experimental Setup. Figure 1 shows the sketch of the experimental system. It consists of a cylindrical constant-volume combustion bomb with a diameter of 130 mm and length of 130 mm, an ignition control system, a fuel supply system, a data acquisition system, an optical schlieren system, and an applied electric field system. The details of the combustion bomb together with the applied electric field system are shown in Figure 2. Two quartz windows are mounted on the front and

Figure 3. Configuration of the mesh electrode (mm).

Figure 2. Structure of the constant volume combustion chamber. back sides of the bomb to provide optical access. A high-speed digital camera (HG-100K) operating at 10 000 frames per second is used to record the flame propagation process. The schlieren method is employed to identify the flame front based on the density gradient. The ignition electrodes are arranged vertically in the center of the bomb and are embedded by the polytetrafluoroethylene (PTFE) insulating layer. After ignition, the ignition electrodes are both connected to the ground. To apply electric fields inside the chamber, two parallel stainlesssteel-made mesh electrodes (with the outer diameter of 60 mm) are mounted horizontally, 35 mm away from the tips of ignition electrodes. Applied voltage generated by a high-voltage power supply (Wisman DEL30N45) is connected to the electrodes. To avoid perturbing the burnt gas flow field, the mesh electrodes are manufactured to have a square mesh size of 8.5 × 8.5 mm, and the width of the mesh wire is 0.8 mm, as shown in Figure 3. In this study, CH4/O2/N2 flames with excess air ratios of 0.8, 1.0, and 1.2 are investigated at room temperature and atmospheric pressure. Seven electric voltages (0, −2.5, −5, −7.5, −10, −12, and −15 kV) are applied to each flame to study the effects of the electric field.

Figure 4. Distribution of the electric field strength at U = −10 kV in the absence of a flame [(a) vector distribution and (b) magnitude distribution]. The combustible mixture is prepared by sequentially introducing CH4 and O2/N2 synthesis gas (21% O2 and 79% N2 by volume) with the 6613


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Figure 5. Schlieren photographs of flame propagation at various applied voltages and excess air ratios.

corresponding partial pressures, which are controlled by a U-tube mercury manometer. A time delay of 150 s is used to make sure that the fuel and 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 a piezoelectric pressure transducer (Kistler 7061B) with a resolution of 0.01 kPa and a precision of 0.5%. In addition, each experiment is repeated at least 3 times for the same condition, and excellent repeatability was achieved. 2.2. Data Processing. 2.2.1. Determination of the Laminar Burning Velocity. For the spherically expanding flame, the stretched flame propagation speed, Sn, is derived from the flame radius, ru, versus time

Sn = dru/dt

Figure 6. Schematic diagram of the redefined flame radius. where ρb and ρu are the densities for the burned and unburned gases, respectively. ρu can be obtained according to the initial state, and ρb is determined from thermal equilibrium calculation. 2.2.2. Determination of Combustion Characteristics. Through the recorded pressure evolution history, the peak pressure and maximum rate of pressure rise can be obtained. In addition, the initial duration (tid) and main duration (tmd), which are defined as the time interval from the spark ignition to the timing of 10% of pressure rise and the time interval from the timing of 10−90% of pressure rise, respectively, can also be calculated. The rate of change of the combustion characteristics (ΔC) from the case of no input voltage to the case of a specific input voltage (=u kV) is defined as follow:

(1)

where ru is the effective radius of the flame and t is the elapsed time from spark ignition. The flame stretch rate, α, indicates the expansion of the flame front area (Af), for the spherically outward expanding flame. This is defined as α=

d(ln A f ) 1 dA f 2 = = Sn dt A f dt ru

ΔC =

(3)

where Sl is the unstretched flame propagation speed and Lb refers to the Markstein length. Sl is obtained as the intercept value of Sn at α = 0 in the plot of the Sn−α curve, and Lb is the slope of the curve. The laminar burning velocity ul is deduced from Sl ul = ρb S l /ρu

(5)

where C is presented for specific combustion characteristics [stretched flame propagation speed (Sn), the unstretched laminar burning velocity (ul), etc.]. Cu and C0 are the value of the specific combustion characteristics in the case when the applied voltage is u and 0 kV, respectively.

(2)

A linear relationship exists between the flame speed and the flame stretch rate S l − Sn = L bα

Cu − C0 × 100% C0

3. RESULTS AND DISCUSSION 3.1. Electric Field Distribution. The electric field distribution between the ignition electrodes and the mesh electrodes is simulated using ANSOFT Maxwell 2D (version 12) software. The electric field direction and the magnitude with the voltage

(4) 6614


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Figure 7. Flame radius versus combustion time at various applied voltages [(a) rich mixture, (b) stoichiometric mixture, and (c) lean mixture].

Figure 8. Stretched flame propagation speed versus flame radius at different applied voltages [(a) rich mixture, (b) stoichiometric mixture, and (c) lean mixture)].

of −10 kV are displayed in Figure 4. It is seen that the electric field strength vector points horizontally from the ignition electrodes (ground) to the mesh electrodes (negative). Moreover, the magnitude of the electric field strength is nearly constant (about 106 V/m) in the middle region between the mesh electrodes. The magnitude of the electric field at the tips of the mesh electrodes is the largest because of the largest curvature at this location. However, the outside surface of the mesh electrodes used in this study is so smooth that no discharge caused by the tips is found. The actual electric field applied on the flame would be much more complex with the interaction of the electric field and ions in the reaction zone.29,32 Therefore, the input values of applied voltage are used to describe the magnitude of the electric field strength (E) because they are proportionally related.43 3.2. Influence of the Electric Field on the Flame Shape. The typical flame propagation processes with the voltages of 0, −5, −10, and −15 kV are illustrated in Figure 5. In the absence of

the applied voltage, the flame shape is spherical and propagates smoothly in both horizontal and vertical directions, regardless of the variation in the excess air ratio. When a DC voltage of −5 kV is applied, the flame front in the direction of the electric field is obviously stretched. However, the flame front in the vertical direction is nearly unaffected. Thus, the flame shape becomes a prolate spheroid, with the major axis in the horizontal direction. A further increase in the applied voltage distorts the flame even more significantly, as seen for the −15 kV voltage case, in which the flame outward propagation velocity is so high that the flame front is almost going to split at the location of the electrode pair. These results are in accordance with the electric field strength vector distribution shown in Figure 4. The modification in the flame shape with the presence of the electric field is mainly attributed to the electric body force generated by the field-induced ion−molecule collisions. The observed effects of the electric field on the flame confirm that 6615


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Figure 9. Unstretched laminar burning velocity versus different applied voltages.

positive ions are responsible for the ion−molecule collisions. The outward acceleration of the positive ion by the electric body force causes the flame to propagate more rapidly in the horizontal direction compared to that in the vertical direction. Consequently, the asymmetric flame front was observed. 3.3. Influence of the Electric Field on the Flame Speed. With the presence of the applied voltage, the flame shape changes from spherical to ellipsoidal, as seen in Figure 5. To clarify the flame deformation and better investigate the influence of the electric field on the flame, it is necessary to define an effective flame radius and the overall stretch rate in the study. As shown in Figure 6, the flame radius was defined as the mean value of the flame radius from the ignition center to the flame front at the set polar angles of 0°, ±15°, ±165°, and 180°, representatively, described in eq 6 n

ru =

∑ i=1

ri , n

n=6 (6)

where ru is the mean flame radius (mm) and ri is the flame radius of a specific polar angle (mm). Besides, the values of the flame radius for further processing are limited between 5 and 26 mm to avoid the influence of ignition energy and pressure variation.44,45 The history of the flame radius is plotted in Figure 7. It is seen that the flame radius increases approximately linearly with time in the absence of applied voltage for each excess air ratio. As applied voltage increases, the flame radius increases monotonically. For the specific applied voltage of −15 kV, the corresponding times when ru increases to 25 mm are 13.07, 7.60, and 10.15 ms for the rich, stoichiometric, and lean mixtures, respectively. The timing is advanced by 39.61, 27.52, and 43.62%, respectively, when compared to the case of no electric field, as seen from Figure 7. The effect of DC electric fields on the stretched flame propagation speed (Sn) of an expanding flame is investigated by varying the applied voltage, as shown in Figure 8. For a given excess air ratio, Sn increases obviously, while it increases nonlinearly with the increase of the applied voltage. The maximum values of Sn are 2.57, 3.9, and 2.81 m/s, occurring at −15 kV, for rich, stoichiometric, and lean mixtures, respectively. They are 2.30, 1.84, and 2.33 times larger than that of the case of no applied voltage, respectively. Additionally, the influence of electric fields on Sn increases with the flame propagation. The field-induced flame stretch should be responsible for the increase of the stretch flame

Figure 10. Combustion pressure rise characteristics versus different applied voltages after ignition start [(a) rich mixture, (b) stoichiometric mixture, and (c) lean mixture].

propagation speed. Moreover, a DC electric field shows a similar behavior to an AC electric field42 upon increasing the flame propagation speed. According to the theory of an outwardly propagating spherical flame, the unstretched laminar burning velocity, which is obtained at zero stretch, depends upon the flame stretch rate.46 Figure 9 shows the effect of the applied voltage on the unstretched laminar burning velocity (ul). With the absence of the applied voltage, the values of ul are 0.29, 0.36, and 0.27 m/s for rich, stoichiometric, and lean mixtures, respectively, similar to others cited in the literature.39,46 Besides, ul increases monotonically with the increase of the applied voltage. Meanwhile, for a given applied voltage, the increase of ul for lean mixtures is more pronounced in comparison to rich and stoichiometric mixtures. For example, with the increase of the DC voltage from 0 to −15 kV, the incremental percentage increases are 65.27, 67.17, and 91.62% for rich, stoichiometric, and lean mixtures, 6616


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Table 1. Combustion Characteristics and Their Corresponding Rates of Change for Different Applied Voltages and Excess Air Ratios λ

U (kV)

Pmax (kPa)

tp (ms)

tid (ms)

tmd (ms)

ΔPmax (%)

Δtp (%)

Δtid (%)

Δtmd (%)

0.8

0 −2.5 −5 −7.5 −10 −12 −15 0 −2.5 −5 −7.5 −10 −12 −15 0 −2.5 −5 −7.5 −10 −12 −15

569.64 571.13 572.93 564.84 567.77 567.21 573.21 626.88 618.23 630.79 622.29 626.60 623.10 623.19 527.65 537.90 551.48 549.45 552.23 553.67 559.15

97.80 94.50 90.60 88.05 85.30 83.35 79.00 57.40 56.50 54.05 53.80 53.65 52.85 50.15 95.25 85.95 78.20 77.80 75.25 73.30 68.70

40.85 39.05 37.30 35.60 34.05 31.25 28.85 22.85 22.35 21.20 20.50 19.85 19.25 17.75 37.10 33.50 29.60 28.25 26.15 25.60 23.05

40.30 39.60 39.00 38.45 36.90 37.45 35.55 26.25 25.85 25.05 24.60 24.35 24.20 23.75 42.55 39.50 35.45 35.55 35.00 34.70 32.55

0.00 0.26 0.58 −0.84 −0.33 −0.43 0.63 0.00 −1.38 0.62 −0.73 −0.04 −0.60 −0.59 0.00 1.94 4.52 4.13 4.66 4.93 5.97

0.00 −3.37 −7.36 −9.97 −12.78 −14.78 −19.22 0.00 −1.57 −5.84 −6.27 −6.53 −7.93 −12.63 0.00 −9.76 −17.90 −18.32 −21.00 −23.04 −27.87

0.00 −4.41 −8.69 −12.85 −16.65 −23.50 −29.38 0.00 −2.19 −7.22 −10.28 −13.13 −15.75 −22.32 0.00 −9.70 −20.22 −23.85 −29.51 −31.00 −37.87

0.00 −1.74 −3.23 −4.59 −8.44 −7.07 −11.79 0.00 −1.52 −4.57 −6.29 −7.24 −7.81 −9.52 0.00 −7.17 −16.69 −16.45 −17.74 −18.45 −23.50

1.0

1.2

voltage increases from 0 to −15 kV. For stoichiometric and rich mixture cases, the peak pressure is kept almost the same for different applied electric fields. However, pmax increases slightly with the increase of the applied voltage just for lean mixtures. When U is greater than or equal to −5 kV, the increase rate of pmax is larger than 4% for all lean mixtures with the presence of the applied voltage. The maximum increment is 5.97% when the applied voltage is increased from 0 to −15 kV. The increase of the peak pressure with the presence of applied voltage at lean conditions is most likely due to improved heat release of combustion in the bomb under the influence of an applied electric field. With the same mixture composition and initial condition, the peak pressure mostly depends upon the combustible gas motion and heat loss to the bomb wall. Figure 11 shows a comparison of the combustible gas motion for U = 0 and −15 kV at lean conditions. As shown in Figure 11a, without an electric field, the spherical flame front reaches the bomb wall immediately and then shortens contact duration between the flame front and bomb wall. As a result, heat loss from the flame to the bomb wall is minimized. However, for the case of U = −15 kV, the electric field can affect the combustion in two ways, as shown in Figure 11b. First, the turbulence is strongly enhanced by the flame colliding with the bomb wall with more rapid speed under the influence of an applied electric field. Thus, the mixing and heat transfer of burned and unburned gases are promoted, which leads to improved combustion and shortened combustion duration. Second, only some portion of the flame front touches the bomb wall first for the non-spherically propagating flame stretched by an applied electric field. Therefore, the larger contact duration yields more heat loss compared to the U = 0 kV case. Meanwhile, the increased heat-transfer rate to the wall caused by the turbulence enhances heat loss. The comprehensive result of the two aspects could be responsible for the increase of the peak pressure caused by an applied electric field. The effectiveness of applying DC voltage on the initial duration (tid) and main duration (tmd) is demonstrated in Table 1. Both the tid and tmd are shortened significantly with the increase

Figure 11. Schlieren photographs of combustible gas motion for lean mixtures at (a) U = 0 kV and (b) U = −15 kV.

respectively. This observation indicates that the lean combustion stability can be remarkably enhanced with the application of an electric field. 3.4. Influence of the Electric Field on Combustion Characteristics. Figure 10 clearly shows the effect of an electric field on the combustion pressure curves. The peak pressure (pmax) and the timing of pmax (tp) are listed in Table 1. As seen in Figure 10, in the main combustion period, pressure increases sharply after the short initial combustion stage (about 20 ms) for all cases. For all excess air ratio conditions, the pressure trace shows a more rapid rise for the rapid combustion stage and an obvious advanced tp when the DC voltage is applied. It can be seen that tp is advanced by 27.87, 19.22, and 12.63% for the lean, rich, and stoichiometric mixtures, respectively, as the applied 6617


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Figure 12. Rate of the pressure rise at various applied voltages after ignition start [(a) rich mixture, (b) stoichiometric mixture, and (c) lean mixture].

of the applied voltage. The applied voltage causes a flame front stretch, which induces the quicker flame propagation and results in the shorter combustion duration. For rich, stoichiometric, and lean mixtures, the decrement from 0 to −15 kV of tid is −29.38, −22.32, and −37.87% and that of tmd is −11.79, −9.52, and −23.50%, respectively. Thus, the applied voltage is especially effective in shortening initial duration compared to main duration. In comparison to the rich and stoichiometric mixtures, the effect of the applied voltage on the initial and main durations is more pronounced at lean conditions. These results also confirm that electric fields significantly affect the stretch flame propagation speed and unstretched laminar burning velocity. Figure 12a shows shot-by-shot variation of a stoichiometric mixture of CH4/O2/N2 in the absence of an electric field, demonstrating little deviation during the combustion process. In the absence of the electric field, the rate of the pressure rise curve has a single peak, as shown in Figure 12. The stoichiometric and lean mixtures have the maximum and minimum peak values, respectively. With the electric field present, bimodal curves appear, and the pressure rises remarkably in the initial stage of combustion. The presence of the first peak may be explained by the increasing heat loss from the flame to the bomb wall because of turbulence and large contact duration as the electric field is applied. The pressure rise is retarded remarkably by the large heat loss, which indicates that the rate of pressure rise is decreased. This leaves behind the first peak of the curve of the pressure rise rate. As we all know, the second peak of the rate of the pressure rise was caused by combustion, which was the same for cases without applied voltage. It is noted that, as the applied voltage increases, the maximum rate of the pressure rise increases for lean mixture combustion, indicating that the DC electric field is more effective for lean mixtures, and this observation again substantiates the potential of an electric field in enhancing the lean combustion stability. With sufficiently high voltage (−15 kV),

the maximum rate of pressure rise can be increased by 22.60% compared to the condition with no applied voltage.

4. CONCLUSION The effects of a DC electric field on flame shape, flame propagation, burning velocity, and pressure of CH4/O2/N2 premixed spherically propagating flames were studied experimentally. The main conclusions are summarized as follows: (1) The simulation results of the electrostatic field in the geometry defined by the mesh electrodes and ignition electrode show that the electric field strength in the space of mesh electrodes is approximately homogeneous and only exists in the horizontal direction. The flame surface is remarkably stretched by the applied voltage in the direction of the electric field, while there is nearly no change in the flame shape in the vertical direction. (2) The flame propagation speed and unstretched laminar burning velocity increase significantly with the increase of the applied voltage. The effect of the applied voltage on these two characteristics is more pronounced for lean mixtures compared to rich and stoichiometric mixtures. (3) With the increase of the applied voltage, the combustion pressure and pressure rise rate for all mixtures are increased at the early stage of flame propagation. Additionally, the timing of the peak pressure occurrence is advanced, and the initial and main durations are shortened. For lean mixtures, the pressure peak and pressure rise rate peak are enhanced with the presence of the applied voltage.

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AUTHOR INFORMATION

Corresponding Author

*Fax: +86-29-82668789. E-mail: [email protected]. Notes

The authors declare no competing financial interest. 6618


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ACKNOWLEDGMENTS The authors gratefully acknowledge the National Natural Science Foundation of China (Grants 50876087 and 51176150).

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