Figure
1.
Plasma-controlled flame experimental system.
| Citation: |
Xiwei HU. Special issue on the 70th anniversary of HUST[J]. Plasma Science and Technology, 2022, 24(12): 120101. DOI: 10.1088/2058-6272/acaa94
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Nonequilibrium plasma-assisted combustion is valuable in power applications such as scramjet engines and aeroengines due to its ability to extend the ignition limit, shorten the ignition delay, stabilize flames, improve combustion efficiency, reduce pollutant emissions, and realize fuel reforming [1]. In the past 20 years, this technology has received extensive international attention and has been the topic of much research [2–5]. According to incomplete statistics, during the period from 2019 to 2023 alone, more than 8,000 papers on this topic were published worldwide (according to search results in ScienceDirect [6]).
Various related studies of nonequilibrium plasma-assisted combustion have focused on the effect of plasma on flames. Various flames (e.g., premixed and nonpremixed flames), gas fuel (e.g., methane, ethylene, propane, and ammonia), and plasma discharge (high-frequency alternating current (AC), nanosecond pulsed, gliding arc, and dielectric barrier discharges) are studied [7‒12]. Some typical recent studies include as follows. Regarding premixed flames, Ding et al used a needle-to-needle electrode configuration to study the effect of the generated plasma on a methane/air premixed flame and measured the transient dynamics of O and H atoms using femtosecond two-photon-absorption laser-induced fluorescence (fs-TALIF) [13]. Kim et al designed a dielectric barrier discharge reactor to investigate the effect of non-thermal plasma on a lean-premixed methane/air flames. In their study, a cylindrical woven stainless steel mesh electrode served as the ground electrode. The adjustable reactor discharge parameters were the applied AC voltage and its frequency [14]. In a study based on the same type of premixed methane/air flame, De Giorgi et al constructed a high-frequency AC-excited coaxial discharge configuration using a burner wall and a central long rod. The results showed that the lean-burn flame-out limit could be increased by a maximum of 79.4% [15]. Given the emphasis on energy savings and emission reductions, ammonia and biomass fuels have received widespread attention. Cheo et al designed an ammonia/air premixed swirling burner and used a nanosecond pulsed source to discharge the premixed gas to generate plasma; the plasma effectively broadened the lean-burn and flame-out limits and reduced NOX emissions [16]. Paulauskasa et al revealed that nanosecond pulsed dielectric barrier discharge (DBD) plasma effectively enhanced combustion in premixed biomass fuel flames, especially in the absence of oxygen supplementation [17]. Regarding nonpremixed flames, Liao and Kuo used an AC source to drive a coaxial DBD burner and compared the effect of plasma generated by the discharge of propane or N2+O2 on a nonpremixed laminar flow lifted flame [18]. Zare et al [19] designed a coaxial nanosecond pulsed discharge device based on a methane fuel rocket engine injector. Although the methane and air formed a nonpremixed flame, discharge occurred only at the injector mouth, that is, in the mixture of methane and air, which successfully stabilized the flame. Earlier, our team also carried out a series of studies on the control of jet flames by nonequilibrium plasmas [20–23], and investigated different discharge working fluids, actuation voltages, and actuation modes.
In the field of plasma-assisted flame manipulation, most previous studies considered only easy-to-adjust parameters, such as the applied voltage, discharge frequency, and pulse width. However, few studies have considered changes in electrode configuration. From the perspective of future applications of plasma burners, it is very important to optimize the electrode configuration to realize better plasma flame control. For example, replacing traditional plate solid electrodes with mesh electrodes or hollow electrodes can reduce the weight of plasma burners, which is especially important for aircraft propulsion systems. Thus, in this study, the effects of mesh electrodes with different densities and copper foil plate electrodes on plasma flame control were compared by changing the configuration parameters of the external electrode on the plasma injector. In addition, our previous experiments were all based on ceramic insulating dielectric barrier materials. However, ceramic is opaque, which means that when it is used as the dielectric barrier of a plasma injector, it is impossible to observe the inner discharge and combustion phenomena from different angles, precluding full understanding of the volume discharge morphology of the injector. Therefore, the ceramic components used in our previous experiments were replaced with quartz components of the same size and configuration.
This study presents several interesting findings that are valuable for understanding how different electrode configurations affect plasma-assisted flame stability.
The overall structure of the plasma-controlled flame experimental system is shown in figure 1 and includes four parts: the plasma injector, the propellant supply system, the high-voltage discharge control and measurement system, and the combustion control and diagnosis system. The gas sources used were high-pressure methane and air. The methane purity was 99.99%, the air used was high-purity dry air, and the volume ratio of N2 to O2 was 79:21. The flow rates of fuel and oxidant in the propellant pipeline supply system were controlled by volume flow controllers (Chengfeng Flowmeter Corp. LZB-3WB for air and LZB-6WB for methane), which were calibrated for air and methane gas with an accuracy of 2.5‰. The whole system is set up in an atmospheric pressure environment. More details can be found in references [20‒23].
The design of the plasma injector is based on the coaxial shear injector of liquid oxygen/methane rocket engine. The entire injector consists of a slender 304 stainless steel tube in the center, a quartz insulating sleeve as the middle layer, and a 304 stainless steel gas collection cavity as the outer layer. Each component is shown in figure 2. Quartz glass is the purest form of SiO2 and has good corrosion resistance, temperature resistance, optical characteristics and electrical insulation. The quartz glass used in the experiments is a gas-refined glass with a relative permittivity of 3.70 that can withstand continuous operation. Its long-term working maximum temperature is 1383 K, and its short-term working maximum temperature is 1523 K [24]. To reduce processing difficulty, the quartz sleeve was treated to achieve a completely transparent state in the discharge area, while the threads and flange sections are opaque and frosted to avoid affecting the observation of the entire discharge space and the recess zone of the injector.
Figure 3 presents the internal structure and assembly of the entire plasma injector. The central tube, which acts as a grounded electrode, has an inner diameter of 5 mm and an outer diameter of 7 mm. The exit end of the tube has a bevel at an angle of 30° to the axis; the exit section of the quartz tube has an inner diameter of 9 mm and an outer diameter of 15 mm; and the high-voltage electrode is a 304 stainless steel mesh electrode or copper foil plate electrode with the same dimensions, which is laid on the outer surface of the quartz tube exit section and has an axial length of 50 mm. The exit face of the central injector is flush with the high-voltage electrode, and the distance of this interface from the quartz tube exit is 5 mm.
The fuel (methane) flows out through the central tube. The oxidant (air) first fills the gas-collecting cavity and then enters the injector at the annular seam from four symmetrical circular holes with a diameter of 2.83 mm; these holes are located on the wall of the quartz tube in the coverage area of the high-voltage electrode at the exit section of the annular seam. Finally, in the premixed region, mass exchange between the methane and air plasma jets occurs through the shear layer, providing basic conditions for ignition to form a diffusion flame. More details can be found in references [20, 22].
The high-voltage AC power supply is composed of an HVAC1-30AS high-frequency high-voltage AC power supply developed by the Institute of Electric Engineering, Chinese Academy of Sciences, a voltage regulator, and a remote control computer (figure 4). The input voltage of the power supply is 220 V AC, the output voltage is 0‒30 kV, and the discharge frequency fv = 1‒50 kHz. According to previous experimental results [20], discharge frequency fv = 10 kHz was selected. The high-voltage output end is connected to the anode through a high-voltage wire. The system can run continuously for a long time.
The injector discharge voltage was obtained using a Tektronix P6015A high-voltage probe. The measurable voltage range is 0‒40 kV, the bandwidth is 75 MHz, and the voltage variation ratio is 1000. The discharge current was measured through a current coil (Person Model 6595) with an output variation ratio of 0.5 V/A. The measurement peak is 1000 A. The current and voltage waveforms were recorded using a Tektronix 2024B oscilloscope with a bandwidth and sampling frequency of 2 GHz and 10 GS/s, respectively. Figure 5 shows the relevant equipment.
Images of the discharge and flame were acquired using a Canon 600D SLR digital camera with a Canon EF50 mm f/1.8 STM fixed-focus lens. Continuous shooting mode was used to take five groups of photos for each working condition. Since the CH radical is an important intermediate reaction product in the combustion of hydrocarbon fuels such as methane [25], it was selected to represent the flame intensity. A wavelength range of 415‒440 nm was selected for the measurement of CH* radicals. CH* chemiluminescence images of the flames were acquired by installing a narrowband filter in front of the camera lens, and the flame structural parameters and flame intensity distribution were obtained. Details are given in references [20, 22].
To reduce the shielding of the discharge area and maintain the uniformity of discharge in the entire circular seam of the injector, the outer surface of the quartz tube is wrapped with 304 stainless steel square mesh electrode. The four mesh electrodes were labeled 16EL, 20EL, 40EL and 200EL, and the corresponding mesh holes have a size of 1.5 mm×1.5 mm, 1.0 mm×1.0 mm, 0.5 mm×0.5 mm and 0.1 mm×0.1 mm, respectively. The external dimensions of the mesh electrodes are the same as those of the copper foil electrode. Mesh electrodes with different specifications were used to investigate the effect of the density of the metal mesh (i.e., the mesh hole size) on the plasma discharge and flame control. In addition, for comparison, an experiment was carried out in which a copper foil electrode was laid on the outside of the quartz tube. The copper foil electrode can represent the situation in which the mesh hole density of the mesh electrode approaches infinity.
A previous related study on methane-air diffusion flame manipulation [20] demonstrated that conditions of ˙mair = 2.5 L/min (uair = 1.66 m/s) and ˙mCH4 = 0.5 L/min (uCH4 = 0.42 m/s, i.e. ER = 1.9) have a significant flame stabilizing effect. Thus, these working conditions were selected here. Under these conditions, the flame is lifted far from the injector mouth with severe vibration and is on the verge of extinguishing. Air discharge was generated in high-frequency AC continuous mode. The effects of five external electrodes at different voltage amplitudes on an unstable methane-air diffusion flame were investigated.
Figure 6 presents the characteristic volt-ampere curves of air discharge when metal mesh and copper foil were used as external electrodes under different air flow velocities and discharge voltages. For the quartz dielectric barrier discharge, the voltage and current waveforms of the five external electrodes all present a typical filamentary discharge. Discharge occurs once on the rising edge and once on the falling edge of the voltage.
With still air and at Ud = 10 kV, the discharge of the mesh electrodes is very weak under all conditions, the current is mostly less than 20 mA, and the number of pulses is low; in comparison, the current amplitude and number of pulses are higher for the copper foil electrode. When the voltage is increased to 12 kV, the discharge current and the number of current pulses increase significantly; the current amplitude is approximately 50 mA for the mesh electrode and 100 mA for the copper foil electrode. Under the conditions of uair = 2.65 m/s and Ud = 10 kV, the current amplitude and pulse number of the different electrodes significantly increase compared with those under still air, indicating that the discharge becomes more intense. When the voltage reaches 12 kV, the current amplitude exhibits the opposite trend. That is, a large flow velocity corresponds to a smaller discharge current, thus weakening the discharge but causing little change in the number of current pulses. The opposite changes at Ud = 10 kV and Ud = 12 kV indicate that there is a discharge actuation intensity threshold UT. When the voltage is higher than UT, the gas flow velocity is greater, and the discharge is greater. When the voltage is lower than UT, the flow velocity decreases, and the discharge becomes stronger. For this experiment, 10 kV < UT < 12 kV.
The influence of flow velocity on discharge can be attributed to the flow residence time and the applied voltage, which determine active species production and cumulative heating [26], and the number of micro-discharge channels. A voltage of 10 kV is very close to the breakdown voltage of air (which is approximately 8‒9 kV), so at this voltage, the original discharge is very weak. As the gas velocity increases (from 0 m/s to 2.65 m/s), the number of air particles treated by the discharge within a certain duration increases. Thus, an enhancement of the discharge occurs (exhibits more pulses and larger amplitude of current). However, at a voltage of 12 kV, the number of micro-discharge channels remains unchanged, while the flow residence time decreases as the gas velocity increases, leading to a reduction in the cumulative effect of the discharge. Thus, an weakening of the discharge occurs.
For a certain flow velocity and voltage, as the mesh of the external electrode becomes denser (i.e., the mesh hole size decreases), the discharge current gradually increases, and the number of current pulses increases. The current and pulse number of the copper foil electrode are greater than those of the mesh electrodes. As the hole size approaches zero, a mesh electrode can be considered equivalent to a sheet electrode such as a copper foil electrode; thus, the copper foil electrode also follows this trend. The change in the number of current pulses reflects the increase in the number of micro-discharge channels, which is caused by an increase in the number of metal particles per unit area. Because each point on the internal electrode will seek the shortest path to form the discharge path with the external electrode. In a cross section along the axial direction of the injector, the distance along the radius is the shortest. As the mesh of the external electrode becomes sparser, the number of cross sections that can construct the shortest path will inevitably decrease, and thus, the number of micro-discharge channels will change. The current amplitude decreases because the shortest path cannot be obtained across some cross sections. During discharge, a plasma channel forms at the second best point on the external electrode. Thus, the discharge gap becomes larger, but the applied voltage remains the same, and the current decreases.
The breakdown voltage gradually decreases as the mesh of the external electrode becomes denser. For example, when the air flow velocity is 2.65 m/s, the breakdown voltages of the five electrodes from sparse to dense are 9.0, 8.9, 8.9, 8.4, and 8.1 kV, respectively. However, the breakdown voltage of a ceramic insulating dielectric plasma injector with a copper foil electrode is only approximately 6.0 kV under these working conditions [20]. That is, a ceramic insulating dielectric plasma injector is easier to discharge, which is consistent with the pattern that the larger the dielectric constant is, the higher the ionization degree is under the same voltage [27].
The use of a quartz insulating dielectric plasma injector in combination with a mesh electrode allowed the discharge morphology to be observed at different locations in the axial direction. Figure 7 shows the cold air discharge and combustion state discharge of the quartz dielectric plasma injector with a 16EL metal external electrode, an air flow velocity of 1.66 m/s, and a methane flow velocity of 0.42 m/s. The discharge is deep purple and is uniformly distributed along the axis of the injector. The flame is anchored at the injector mouth under an applied voltage of 16 kV. Changing the mesh density of the external electrode has little effect on the plasma color.
Figure 8 shows the CH* chemiluminescence images of the flames, which can reflect the location and basic structure of the flames. The shooting parameters used to photograph the images of the flames were as follows: camera shutter speed, 1/20 s; sensitivity, ISO = 800; and aperture, f = 1.8.
According to the flame CH* chemiluminescence distributions, the height of the flame root and the flame length generally decrease as the voltage increases. Note that at Ud = 8 kV, the flame stability is also improved as a result of the increase in local conductivity and the decrease in breakdown voltage. At Ud = 12 kV, the flame root is completely attached to the injector mouth in most cases. At Ud = 14 kV, the flame length decreases further, and the flame becomes more stable.
However, as the external mesh electrode becomes denser, slight sparks are increasingly likely to occur at the flame under 14 kV. When the flame root falls to the recessed section of the injector (i.e., flashback occurs), creepage phenomenon is observed. Thus, the voltage applied to the injector has a maximum value, and this value decreases as the external electrode becomes denser. Among the five external electrodes, the copper foil has the lowest maximum value, because a denser electrode corresponds to a larger discharge current under a certain voltage.
In addition, at high voltages such as Ud = 12 kV and 14 kV, the flame intensities obtained for the 40EL, 200EL, and copper foil electrodes are generally greater than those for the 16EL and 20EL electrodes, and the strong CH* chemiluminescence zone at the root of the flame has a wider radical distribution. In particular, the area of the red CH* chemiluminescence zone is significantly larger for the 200EL electrode and the copper foil electrode than the other electrodes at 14 kV, indicating that the discharge effect is stronger. This is consistent with the results in section 3.1, that is, a denser mesh electrode corresponds to a stronger discharge under a certain voltage. Thus, the local combustion is enhanced by the strong discharge.
The variations in four flame characteristic parameters, i.e., flame center height, representative flame length, and average and maximum CH* chemiluminescence intensity under different external electrodes, are shown in figure 9. The error bars in figure 10 represent the maximum value and minimum value among the five data points obtained under a certain working condition.
First, regarding the flame center height, as shown in figure 9(a), the increase in voltage from 0 kV to 8 kV causes the largest change in height. According to the slopes of each curve, the copper foil electrode has the greatest effect, followed by the 200EL, 400EL, and 20EL mesh electrodes, and the flame height of the 16EL mesh electrode decreases the least compared to its initial level. As the electrodes become denser (i.e., 16EL, 20EL, 40EL, 200EL, and copper foil electrodes, in sequence), the voltages resulting in the greatest decrease in flame center height (corresponding to the steepest section of the curve in the voltage interval of 8‒14 kV) are 12, 12, 10, 10, and 10 kV, respectively. Considering the heights for each working condition at 0‒8 kV, a mesh external electrode with smaller holes is more conducive to stabilizing the flame under a lower voltage.
Regarding the representative flame length, because ignition occurs below the originally lifted-off flame, the length increases at Ud = 8 kV with different external electrodes. Additionally, the flame length decreases as the voltage increases. According to reference [20], the flame length decreases as the equivalence ratio increases under a certain air flow velocity. Thus, the discharge plasma acts as an increase in equivalence ratio. As the electrode density increases, the voltages resulting in the sharpest reduction in representative flame length (i.e., the steepest section of the curve) are 12, 14, 12, 12, and 10 kV, respectively. This indicates that using a denser electrode with a lower voltage can significantly shorten the flame length. This finding is consistent with the effect of the electrode on the flame center height; i.e., at a certain voltage, an electrode with smaller mesh holes has a greater effect on flame length.
The change in the average CH* chemiluminescence intensity of the flame in response to voltage is less prominent than the changes in flame center height and representative flame length. However, in general the intensity increases as the mesh hole size decreases. Since the average CH* chemiluminescence intensity reflects the intensity of the flame, it is better to choose a dense mesh or copper foil external electrode for plasma-assisted combustion.
The maximum CH* chemiluminescence of the flame first decreases and then increases with increasing voltage for each electrode working condition, which is different from the trend in flame length. Note that a decrease in flame length can be considered equivalent to an increase in equivalence ratio. An increase in equivalence ratio will lead to an increase in flame power based on the flame power calculation formula; therefore, the maximum CH* chemiluminescence substantially increases, and vice versa. Considering the randomness of the transient value of the maximum CH* chemiluminescence intensity, the flame fluctuates fiercely with variable structure before it becoming completely stable. Thus, only the maximum CH* chemiluminescence intensity of a fully stabilized flame at Ud = 14 kV was compared to the intensity at Ud = 0 kV (i.e., no discharge), and the intensities at intermediate voltage amplitudes were not analyzed. The increases in the maximum CH* chemiluminescence intensity are approximately 15.8%, 18.6%, 16.3%, 32.7%, and 38.8%, respectively, with decreasing electrode mesh hole size. This is consistent with the variation in the average CH* chemiluminescence intensity of the flame, indicating that using an external electrode with a smaller mesh hole size makes for enhancing the combustion of the plasma injector.
According to the variations in the four flame characteristic parameters, Ud = 12 kV is assumed to be the optimal actuation voltage for all five electrode schemes in this study. Under this voltage, the plasma injector can not only stabilize the original unstable flame, but also enhance combustion.
The discharge power and cost-effectiveness ratio are essential considerations for practical application of plasma injectors. Therefore, the average discharge power of the injector was obtained based on equation (1), where n is the number of data points recorded by the oscilloscope; ∆t is the time interval of adjacent data points; and U and I are the transient discharge voltage and the current across the electrodes, respectively. The voltage across the electrodes was measured with a Tektronix (P6015A) high-voltage probe. The current through the electrodes was measured with a Pearson coil (Model 6595). Both signals were recorded simultaneously with a Tektronix oscilloscope (2024B, 2 GHz). Details can also be found in references [20, 22].
| P=WT=n∑i=1UiIiΔtnΔt=n∑i=1UiIin. | (1) |
The average discharge power of the plasma injector under different external electrode conditions is listed in table 1. The plasma injector power with the copper foil electrode at Ud = 16 kV is not given because creepage occurred. Obviously, the discharge power increases with increasing voltage for a certain flow velocity. The maximum power reaches 48.6 W, while the minimum power is only 6.1 W. The injector power first increases and then decreases as the air velocity increases at Ud = 10 kV for all electrodes. However, at Ud ⩾ 12 kV, in general, a high velocity corresponds to a low power. This can be attributed to the threshold UT (within 10‒12 kV), as discussed in section 3.2. The threshold UT determines the change in discharge current. For a certain air flow velocity and voltage, electrodes with smaller mesh holes produce a higher power. Among the tested electrodes, the copper foil electrode has the highest power. For example, the power of the copper foil electrode at a voltage of 14 kV is 0.2 W higher than that of the 40EL mesh electrode at a voltage of 16 kV under no-flow conditions. Moreover, the difference in power between the copper foil electrode and the mesh metal electrodes is significantly greater than that between mesh electrodes with different hole sizes, under the same conditions.
| 10 kV | 12 kV | 14 kV | 16 kV | ||
| 16EL mesh electrode | 0.00 m/s | 6.1 | 17.5 | 38.1 | 46.9 |
| 1.33 m/s | 10.3 | 16.8 | 26.5 | 35.5 | |
| 2.65 m/s | 7.9 | 15.5 | 26.3 | 39.1 | |
| 20EL mesh electrode | 0.00 m/s | 6.0 | 15.6 | 37.3 | 45.0 |
| 1.33 m/s | 9.8 | 20.7 | 31.2 | 40.2 | |
| 2.65 m/s | 9.2 | 17.3 | 25.0 | 37.2 | |
| 40EL mesh electrode | 0.00 m/s | 6.8 | 18.4 | 31.4 | 46.5 |
| 1.33 m/s | 9.7 | 18.1 | 28.2 | 41.9 | |
| 2.65 m/s | 9.7 | 15.2 | 28.8 | 38.4 | |
| 200EL mesh electrode | 0.00 m/s | 6.9 | 17.6 | 37.6 | 48.6 |
| 1.33 m/s | 11.5 | 20.6 | 29.6 | 41.2 | |
| 2.65 m/s | 11.9 | 20.4 | 28.0 | 40.2 | |
| Copper foil electrode | 0.00 m/s | 9.9 | 35.8 | 46.7 | − |
| 1.33 m/s | 15.2 | 24.7 | 33.8 | − | |
| 2.65 m/s | 14.4 | 23.4 | 37.2 | − |
DownLoad:
CSV
The cost-effectiveness ratio is defined as the ratio of the discharge power with an optimal voltage to the corresponding flame power [22]. At the optimal excitation voltage Ud = 12 kV, the cost-effectiveness ratios of the different electrodes are 4.9% (16EL mesh electrode), 5.7% (20EL mesh electrode), 5.5% (40EL mesh electrode), 6.2% (200EL mesh electrode), and 7.2% (copper foil electrode), based on the injector power (air flow velocity of 1.66 m/s, not listed in table 1) and flame power. This indicates that an electrode with a smaller mesh hole size generally yields a higher cost-effectiveness ratio of the plasma injector. Since the cost-effectiveness ratio of the copper foil external electrode is higher than those of the mesh electrodes, a mesh electrode presents more advantageous within the range of studied working conditions.
In addition, according to the results given in reference [20], the cost-effectiveness ratios of ceramic dielectric barrier plasma injectors are within the range of 8.9%‒35.5% under the same conditions as those used in this study. Thus, compared with ceramic dielectric barrier plasma injectors, the cost-effectiveness ratio obtained with quartz as a barrier material is lower when other conditions are the same.
In view of its application for aero-engines, the material, manufacturing cost, and long-term operating costs of the plasma injector are all required to discuss. Here a preliminary discussion is provided according to our experience:
(1) For the electrodes, the acceptable material general includes copper, stainless steel; for the dielectric barrier material, the acceptable material general includes ceramic and quartz; for the manufacturing, the manufacture of ceramic or quartz takes most of the cost, while the manufacture of metal is very cheap. The specific cost would fluctuate based on the market. In this study, the material and manufacture of the injector costs about $1200. However, through mass production, its costs would decrease a lot, compared with our costs.
(2) Other main cost comes from the power supply, as the power supply we used is a universal high voltage power supply for a wide extent of scientific research. Nowadays, it is believed that the power supply required can be integrated to a small and light circuit board with a much cheaper price than the equipment we used.
(3) Moreover, considering the power consumption of this injector, it will cost little during long-term operation.
A quartz dielectric barrier discharge plasma injector was designed and tested in a nonequilibrium plasma-controlled flame experimental system. The influence of the external electrode on the plasma assisted methane-air diffusion flame manipulation was investigated. The electrical characteristics of the plasma, the flame morphology and intensity, the injector power, and the cost-effectiveness ratio were all analyzed in detail. The main conclusions are as follows:
(1) Under the same experimental conditions, the discharge volt-ampere features of the quartz plasma injector are similar to those of the ceramic injector, with multiple micro-filamentary discharges occurring at the rising edge and falling edge of the voltage curve. The breakdown voltage decreases as the density of the external electrode increases. Compared with ceramic dielectric barriers, it is more difficult for quartz dielectric barrier injectors to reach breakdown.
(2) For a certain voltage, an electrode with a smaller mesh hole size tends to lead to a larger discharge current, but the maximum voltage that can be applied to the electrode decreases; the copper foil electrode has the lowest allowed maximum voltage among the five electrodes. According to the analysis of CH* chemiluminescence images, the local flame intensity is enhanced by strong discharge, and the discharge strength increases as the electrode becomes denser. Based on the variations in the flame center height and representative length, it is better to choose an external electrode with a small mesh hole size for flame stabilization. Specifically, a smaller mesh hole size reduces the lift-off flame height, decreases the flame length, and increases the flame intensity.
(3) The injector discharge power increases as the electrode mesh hole size decreases under a certain air flow velocity and discharge voltage. The injector power reaches the highest value when copper foil is used as the external electrode. Under the optimized actuation parameters, the plasma injector with a mesh external electrode has a lower cost-effectiveness ratio than that with a copper foil external electrode, and the cost-effectiveness ratio decreases as the mesh hole size increases. In addition, the cost-effectiveness ratio obtained with quartz as a barrier material of the injector is lower than that of obtained with a ceramic barrier material when other conditions are the same.
Considering the potential applications of the plasma injectors, future work may focus on the wide adaptability of the injector and the alternative electrode materials. For example, researchers could perform experiments under different background pressures to confirm whether the proposed design is feasible for various aeroengines and then optimize the design. Different electrode materials such as tungsten, copper, and graphite should also be tested to optimize the injector.
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| 1. | Zhu, J., Li, L., Tian, Y. et al. Mutual effects between a gliding arc discharge and a premixed flame. Plasma Science and Technology, 2024, 26(12): 125505. DOI:10.1088/2058-6272/ad8120 |
Supported by: Beijing Renhe Information Technology Co., Ltd. 
| 10 kV | 12 kV | 14 kV | 16 kV | ||
| 16EL mesh electrode | 0.00 m/s | 6.1 | 17.5 | 38.1 | 46.9 |
| 1.33 m/s | 10.3 | 16.8 | 26.5 | 35.5 | |
| 2.65 m/s | 7.9 | 15.5 | 26.3 | 39.1 | |
| 20EL mesh electrode | 0.00 m/s | 6.0 | 15.6 | 37.3 | 45.0 |
| 1.33 m/s | 9.8 | 20.7 | 31.2 | 40.2 | |
| 2.65 m/s | 9.2 | 17.3 | 25.0 | 37.2 | |
| 40EL mesh electrode | 0.00 m/s | 6.8 | 18.4 | 31.4 | 46.5 |
| 1.33 m/s | 9.7 | 18.1 | 28.2 | 41.9 | |
| 2.65 m/s | 9.7 | 15.2 | 28.8 | 38.4 | |
| 200EL mesh electrode | 0.00 m/s | 6.9 | 17.6 | 37.6 | 48.6 |
| 1.33 m/s | 11.5 | 20.6 | 29.6 | 41.2 | |
| 2.65 m/s | 11.9 | 20.4 | 28.0 | 40.2 | |
| Copper foil electrode | 0.00 m/s | 9.9 | 35.8 | 46.7 | − |
| 1.33 m/s | 15.2 | 24.7 | 33.8 | − | |
| 2.65 m/s | 14.4 | 23.4 | 37.2 | − |
DownLoad:
CSV
