Innovative Three-Electrode Design for Definition of Multiple Dielectric Barrier Discharge Actuators

Transcription

1 Non thermal plasma Paper #P Innovative Three-Electrode Design for Definition of Multiple Dielectric Barrier Discharge Actuators N. Benard, J. Jolibois, A. Mizuno and E. Moreau Abstract For about 10 years, surface DBDs have been widely successfully used as plasma actuators in subsonic airflow control applications. However the extension length of a single DBD is limited to about two centimeters, which could limit its use to small-scale applications. One way to extend the plasma actuation surface consists in using several single surface DBD in series, energized by zero phase delayed or phase shifted high voltages. However the mutual interaction between successive discharges limits the effect of such standard multi-dbd actuators. Then this paper deals with a new design for large-scale flow control applications. It consists in replacing each single two-electrode DBD by a three-electrode DBD where the third electrode acts as a shield between two successive DBDs. Experimental measurements by Laser Doppler Velocimetry (L.D.V.) and pressure probe show that the cross-talk phenomenon can be strongly reduced, resulting in a constant electric wind velocity above the multi-dbd actuator. Index Terms dielectric barrier discharge, plasma actuator, electric wind, flow control S I. INTRODUCTION EVERAL flow control technologies have emerged recently. These are considered to show sufficient promise that they might usefully be applied to control industrial flow applications. Among these active flow control devices, actuators based on non-thermal surface plasma discharge (such as Dielectric Barrier Discharge, DBD) are identified as being suitable for subsonic flow control. Initially dedicated to the control of boundary layer and separated airflow such the one observed above airfoil model [1]-[2], the benefits of such EHD actuators have also been experimentally verified for jet mixing [3], noise control [4] and cavity flow manipulation [5]. However, non-thermal plasma actuator requires further N. Benard is with the Laboratoire d Etudes Aérodynamiques, University of Poitiers, Bld Marie et Pierre Curie, Téléport 2, Chasseneuil Futuroscope, France (phone: , fax: , nicolas.benard@lea.univ-poitiers.fr). J. Jolibois is with the Laboratoire d Etudes Aérodynamiques, University of Poitiers, Bld Marie et Pierre Curie, Téléport 2, Chasseneuil Futuroscope, France ( jerome.jolibois@lea.univ-poitiers.fr). A. Mizuno is with the Department of Ecological Engineering, Toyohashi University, Japan ( mizuno@eco.tut.ac.jp) E. Moreau is with the Laboratoire d Etudes Aérodynamiques, University of Poitiers, Bld Marie et Pierre Curie, Téléport 2, Chasseneuil Futuroscope, France ( eric.moreau@lea.univ-poitiers.fr). improvements to demonstrate sufficient effectiveness for large scale wind tunnel tests. For instance, actuation by plasma remains currently limited to Reynolds numbers typically lower than To overcome this limit, one could consider new control strategies related to adjusted perturbations of the natural instabilities of the baseline flow as an excitation at the right frequency can improve the effects of actuation [6]-[7]-[8]. However, this approach has not yet demonstrated an increase in the control ability at high Reynolds number (larger than ). Another direction could consist of increasing the electromechanical conversion of the actuator. Optimization of the electric wind, responsible for the flow control effect, requires to fully understand the involved physical phenomena. The current knowledge about surface plasma discharge is increasing thanks numerous experimental studies on the different discharge modes occurring over an AC cycle [9]- [10], the non-stationary produced electric wind [11]-[12], the electrical characteristics [13]-[14] or the mean body force production [15]. Numerical simulations are also helpful to precise the contribution of the filamentary and glow discharge regimes in the local airflow production [16]-[17]-[18]. All these data contribute to a deeper knowledge but the developed computational approach are not yet mature to be used as an efficient tool for electric wind optimization. A way to increase the induced airflow and extend the plasma surface consists in using multiple DBD in series. Such method was widely used in case of laminar to turbulent transition, by streamwise or spanwise excitations [19]-[20]- [21]-[22]. Array of DBDs was also used to control separated airflow [23]-[24]-[25] and jet flow [26] by phased actuation resulting in travelling waves, and by simultaneous actuation. The velocity range of effective control was not increased despite that it was demonstrated that asynchronous operation of four DBDs in series can increase the electric wind by a cumulative effect [11]. In most studies where multiple DBDs are used, the limited effects often reported may result from the interaction between the electrodes when the successive DBDs are placed at short distance. This was reported in [11] and recently qualified as cross-talk phenomenon in [27]. The cross-talk between electrodes when used in array of plasma reduces the benefits of multiple DBDs concept. Another demonstration of the backward flow can be found in [28]. In this study, the effects of different size of the active electrode

2 Non thermal plasma Paper #P on the flow patterns and the spatial modes of a proper orthogonal decomposition are investigated. The differences reported may directly result from partial ionization at the rear edge of the air-exposed electrode when the length of the active electrode is too short to avoid a mutual interaction with the grounded electrode. However, Teflon tape placed at the edge of the electrodes of opposite polarity can significantly reduce the unwished backward flow responsible for suppression of the cumulative effect [11]. This handmade method, however, is not strictly reproducible and affects the conditions of the boundary layer. The present paper is focused on the development of a new geometrical design limiting the mutual interactions between electrodes when used in an array of DBDs. Then, the first part of this paper deals with the electromechanical comparison between a typical two electrode DBD and one composed of a three electrodes geometry. The actuators are first electrically characterized. Then, measurements of the time-averaged produced flow are performed by using a laser doppler velocimetry system and pressure probe. The study of the three-electrode geometry in standalone configuration is followed by the demonstration of such design as a promising geometrical arrangement for multiple DBDs in series. II. EXPERIMETNAL SETUP The first set of experiments consists in investigating a DBD geometry emphasizing the cross-talk effect and the three electrodes configuration supposed to reduce the production of backward flow. A second set of experiments is performed to validate the three electrodes geometry for the development of an efficient array of plasma with a small gap between the successive DBDs. A. Actuator designs For the study of the standalone DBDs, the electrodes and the dielectric setups of both investigated configurations are sketched in Fig. 1 (slice views taken at z=0, center of the electrodes in the spanwise direction). The air-exposed electrodes are 5 mm-length (in the x direction) whereas the grounded ones are 25 mm-length. Both are made of aluminium foil. The dimension in the spanwise direction (i.e. z direction) is 70 mm. The first single DBD setup (Fig. 1a) is the baseline case and will be referred as S2DBD (Single twoelectrode DBD). The dielectric is composed of three layers including a 3-mm-thick PMMA plate and a 50-µm-thick Kapton film fit together by a 10-µm-thick epoxide polymer layer. This design results in two surface plasma discharges on both side of the air-exposed electrode, then two electric winds are produced, as illustrated by both narrows in Fig. 1a. The design of the three electrodes DBD (referred as S3DBD, Single three-electrode DBD) is more complex. The airexposed and grounded electrodes are similar to the ones of Fig. 1a but a third electrode is placed below the Kapton film. This electrode is embedded in the epoxyde polymer and is electrically linked to the air-exposed electrode. Due to its electrical potential, the embedded electrode acts as a shield between the air-exposed electrode and the grounded one. This Figure 1: Sketch of (a) S2DBD and (b) S3DBD in standalone configuration (the sizes are not to scale). Figure 2: Electrode arrangements for the studied multiple DBDs panel. Fig. a), c) and e) refer to the S2DBD whereas Fig. b), d) and f) are composed of three electrodes arrangements (S3DBD).

3 Non thermal plasma Paper #P configuration should reduce the ionization at the rear side of the air-exposed electrode (at x=-2.5 mm) and then cancel or limit at least the backward flow production. The oversize of the grounded electrode (comparatively to typical DBD used in flow control [1]-[2]) allows proving the efficiency of the tested design. This will result in a severe test for the S3DBD configuration as the backward flow will be emphasized. It could be expected that the effectiveness of the design can be more impressive when used in array of plasma where distance between electrodes are higher. In a second set of experiments, arrays of four DBDs in series are designed. S2DBD and S3DBD arrangements compose the plasma arrays (see Fig. 2). The gap, d, between two successive single DBD is investigated for values of 5, 10 and 15 mm (refers to Fig. 2 for the definition of d). It is expected that the backward flow is significantly stronger with a small gap (Fig. 2a) than for a larger gap distance as the one shown in Fig. 2e (d=15 mm). To estimate the efficiency of the S3DBD to reduce the cross-talk effects, the experiments are reproduced for the three gap values as illustrated in Fig. 2b, 2d and 2f. B. Electrical setup The actuator is operated using a 1 khz carrier frequency at voltages of 28, 32, 36 and 40 kv pp. The waveform of the electric signal is a sinusoid (f AC =1 khz) synthesized by a generator function (Hameg, HM-8131) and then amplified by a High-Voltage generator (Trek 20/20C). All the measurements are performed in initially quiescent air. The electrical behaviour is observed with a digital oscilloscop (Lecroy, Wavesurfer 424, 2 Gs/s, bandwidth of 200 MHz) by a 33 nf capacitor (connected in series with the grounded electrode) used to measure the charge-voltage evolution. III. RESULTS A. Standalone DBDs Electrical characteristics Fig. 3 presents the Q-V curves for both the S2DBD and S3DBD arrangements at 28 to 40 kv pp. As reported by Manley [30], the power consumption can be estimated by computing the inner surface of the Lissajous curve. Here, it appears that the change in the electrode arrangement does not significantly modify the consumed power. At 40 kv pp, an injected power of about 0.9 W/cm is measured whereas 28 kv pp conduces to a consumption of 0.3 W/cm. Fig. 3 also highlights that the S2DBD produces a more filamentary discharge in the streamer regime (i.e. the positive-going-cycle) compared to the S3DBD. The intensity of the transferred charge is correlated with the length and the number of filaments [31]. A decrease in the transferred charge is observed for the S3DBD arrangements being probably due to shorter streamers in space on the third embedded electrode side. In this region, the charge build-up over the dielectric surface and the electric field are certainly reduced because the third electrode acts as a shield between the air-exposed electrode and the grounded one. This results in a reduction of the ionization rate which limit streamer formation. The third electrode constricts the filamentary discharge in a smaller area limiting the presence of long and strength current filaments. C. Electric wind measurements The local airflow produced by the plasma actuator is measured by two ways. In the standalone DBD experiments, the electric wind is measured by a mono-component Laser Doppler Velocimetry (LDV) system. A full description of this device can be found in [29]. Here, the LDV is used to measure the mean induced airflow in the x direction. Acquisitions of data points at 8-14 khz are performed and timeaveraged to obtain the final estimation of the electric wind at the measurement location (displacement volume of 0.6 mm 3 ). Small droplets resulting from atomized pharmaceutical oil (Shell, Ondina 15) are used as reflective tracer particles. The particle size (d 2-5 µm) suggests that these particles should follow the airflow with negligible deviation due to charging effects. For the DBDs in array configuration, the timeaveraged flow velocity is measured by a Pitot probe made of glass (d Pitot =500 µm). This probe is connected to a manometer (Furness, FC014). The mean velocity is computed over data samples recorded at 1 khz. Figure 3: Voltage versus charge for (a) S2DBD at 28 kv pp (b) S2DBD at 36 kv pp, (c) S3DBD at 28 kv pp and (d) S3DBD at 36 kv pp. B. Standalone DBDs Electric wind measurements The first acquisitions consist in measuring the U x velocity component of the electric wind (velocity in the x direction) along the dielectric layer at constant distance from the wall (y=1 mm). The actuator is operated at voltages from 28 up to 40 kv pp (Fig. 4).

4 Non thermal plasma Paper #P Figure 4: Velocity component in the x direction at y=1 mm (x=0 corresponds to the middle width position of the air-exposed electrode). For the S2DBD geometry, results indicate that the produced airflow is symmetric for negative and positive x locations and the velocity increases with the applied voltage (maximal velocity of 2.4 and 3.8 m/s at V=28 and 40 kv pp, respectively). For the S3DBD, the electric wind is no more symmetric along the x direction. The third electrode conduces to a strong decrease of the backward flow velocity measured for negative values of x. The maximal reduction is equal to 64, 55, 48 and 49% at voltages of 28, 32, 36 and 40 kv pp, respectively. Figure 5: Velocity component of backward flow at x=-5 mm when voltages of 28 and 40 kv pp are applied. However, a complete cancellation of the backward flow is not achieved. Results plotted in Fig. 4 indicate that the electric wind in the positive x direction is also affected by the third electrode: It decreases slightly but this effect is minimized with an increasing high voltage. A way to characterize the electric wind resulting from a surface plasma discharge consists in calculating the mass flow rate promoted by the actuator initially in quiescent air. To compute such quantities, velocity profiles along the y direction are required. Backward flow velocity for x=-5 mm (that is at the rear edge of the air-exposed electrode) for V=28 and 40 kv pp is plotted in Fig. 5. As illustrated, the backward flow velocity is cancelled as the potential difference between the air-exposed and the embedded electrodes is equal to zero. Further upstream (x=-10 mm, Fig. 6), the embedded electrode is too short to form an effective shield. Results suggest that, for x<-7.5 mm, the electric field above the dielectric surface is high enough to initiate and sustain the discharge. However, the momentum transfer initiated by the S3DBD is still reduced compared to the S2DBD configuration. The flow in the positive x direction is shown in Fig. 7 to demonstrate that the electric wind is not strongly affected by the presence of the third electrode. To evaluate the global flow changes, the mass flow rates per unit length of electrode (in g.s -1.m -1 ) are computed at x=±5 and ±10 mm (Fig. 8). For the positive x locations, the mass flow rates confirm that an increase in the applied voltage leads to an increase of the promoted momentum transfer. Moreover, the S3DBD allows a small improvement of the actuator effectiveness in producing mass flow rate (maximal gain of 25% at 36 kv pp ). At x=-5 mm, the mass flow rate remains very weak as suspected from the velocity profiles plotted in Fig. 5. At x=-10 mm, the induced mass flow rate is not negligible (7.4 g.m -1.s -1 at 40 kv pp for S2DBD), however the

5 Non thermal plasma Paper #P Figure 6: Velocity component of backward flow at x=-10 mm when voltages of 28 and 40 kv pp are applied. Figure 7: Velocity component of the electric wind at x=10 mm when voltages of 28 and 40 kv pp are applied. Figure 8: Mass flow rate induced by the surface discharge for S2DBD and S3DBD at x=±5 and ±10 mm. reduction due to the third electrode is significant (reduction of 50% at 40 kv pp ). Regardless of the applied voltage, the mass flow rate is systematically halved. All the presented results indicate that S3DBD can strongly modify the spatial distribution and the velocity of the produced electric wind. The backward flow can not be fully cancelled in the chosen configuration. However, the retained configurations are designed to emphasize the cross-talk process, then it can be expected that better results will be observed with multiple DBDs designs. C. Multi-DBDs Electric wind measurements In this paper, six multi-dbds setups are tested for voltages of 28 kv pp to 40 kv pp (see Fig. 2 for the setups description). The investigated parameter is the gap, d, between two successive single DBDs. Three different gaps are compared (d=5, 10 and 15 mm). The S2DBD and S3DBD configurations are individually tested for each gap value. The objective is to measure the electric wind above the dielectric layer (at y = 0.3 mm) to demonstrate the effects of possible mutual interactions between a grounded electrode and the air-exposed of the next single DBD. Experiments are systematically reproduced with S3DBD to quantify the benefits of using this geometry. For d = 5 mm (Fig. 9), the S2DBD fails to maintain a continuous electric wind. The plasma discharge of each S2DBD is initiated resulting in velocity increase few millimetres downstream of each air-exposed electrode. However, the velocity systematically drops to zero at the right extremity of the grounded electrodes. This clearly suggests a strong mutual interaction between the grounded electrode and

6 Non thermal plasma Paper #P Figure 9: Electric wind velocity for multi-dbds configuration with a gap d=5 mm (at y=0.3 mm). Figure 10: Electric wind velocity for multi-dbds configuration with a gap d= 10 mm (at y=0.3 mm). the next air-exposed electrode. This interaction leads to a plasma discharge producing a backward flow significant enough to cancel the velocity production due to the previous discharge. This interaction is also confirmed by the velocity promoted by the second and third S2DBD composing the actuator array. Usually, an increase of the voltage applied to a single DBD results in a velocity increase. Here, the electric wind velocity produced by the second and third S2DBD does not increase with the voltage. When the upstream conditions are free of perturbations, as it is the case for the first S2DBD, the voltage dependence can be observed (induced airflow of 2 and 2.5 m/s at 28 and 40 kv pp, respectively). A same observation can be formulated for the last S2DBD which does not suffer of a backward flow due to a next single DBD. For the S3DBD, results totally differ as the electric wind remains always greater than 1 m/s. This clearly indicates that the

7 Non thermal plasma Paper #P backward flow (and then the mutual interaction) is drastically reduced. There is no cumulative effect on the produced velocity as observed by Forte et al. [11]. However a significant electric wind can be sustained over a long distance (velocity between 1 and 1.5 m/s maintained over 80 mm in the x direction). As observed from the S2DBD, the last plasma discharge does not suffer from a possible mutual interaction resulting in a higher velocity production with maximal value directly related to the applied voltage (2 and 2.75 m/s at 28 and 40 kv pp, respectively). For d = 10 mm (Fig. 10), the behavior of the electric wind is similar. The S2DBD induces alternation of positive and zero velocity. However, an increasing gap distance allows a greater plasma extension. As the maximal induced velocity occurs at the end of the plasma extension, this lower plasma constriction provokes a faster electric wind. This is also verified for the three electrodes design. As reported in Fig. 10, a gap equals to 10 mm conduces to a momentum transfer producing mean velocities of 1.75 and 2.5 m/s for high voltages at 28 kv pp and 40 kv pp, respectively. The third embedded electrode still avoids the velocity drops and leads to a nearly constant electric wind velocity over the surface. The last experiments are performed for a gap set to 15 mm (Fig. 11). With S2DBDs, the electric wind remains periodic in space. However, the region (in the x direction) associated with a velocity production is wider. It can be noticed that the length of this region linearly increases with the gap value d (8, 12, and 18 mm with positive velocity for gaps of 5, 10 and 15 mm, respectively). The distance over which there is a mutual interaction can be roughly estimated at 7~8 mm. Finally, Fig. 11 indicates that the S3DBD design conserves its capability to maintain a constant velocity. More, for 36 and 40 kv pp, a small cumulative effect is observed. The final velocity is increased by successive addition of momentum over each S3DBD composing the multiple DBD. For instance, at 40 kv pp, the first S3DBD produces a maximal velocity of 2.9 m/s, the second S3DBD does not increase this maximal velocity but maintains the electric wind constant, the third S3DBD allows a small amplification in velocity (3.25 m/s) whereas the last S3DBD conduces to a final electric wind of about 3.7 m/s. This confirms the results observed in [11], but also indicates that a cumulative effect can be performed only for specific geometrical and electrical parameters. CONCLUSION This study deals with a new design for surface plasma discharges dedicated to flow control. Large scale applications require developing plasma actuators which offer a wide control surface. In case of separated flow, small change in the surrounding airflow conditions can drastically modify the location of the separation point. To remain effective whatever the external perturbations, a successful flow control device has to consider this possible displacement of the separation point. A way to extend the plasma discharge, and then the control surface, simply consists in using array of plasmas. However, even for low scale applications, multi-dbds actuators based on conventional electrode arrangement are not systematically more effective than that of single DBD. The reasons of this lack of effectiveness are mostly related to mutual interactions between the electrodes of each DBDs when placed at a short distance. These interactions conduce to the production of a backward flow which limits the global production of electric wind. The present paper proposes a new DBD design composed of three electrodes arrangement to overcome this drawback. Two DBD actuators are compared in the first set of experiments. The first geometry is designed to emphasize the Figure 11: Electric wind velocity for multi-dbds configuration with a gap d= 15 mm (at y=0.3 mm).

8 Non thermal plasma Paper #P backward flow (S2DBD) whereas the second one proposes an additional embedded electrode dedicated to the reduction of the backward flow (S3DBD). The results demonstrate that the S3DBD design is suitable to reduce the backward flow of 50% up to 65% according to the applied high voltage. These results are promising as mutual interaction is reduced without necessarily an increase of the electrical power consumed by the actuator. Indeed, electrical measurements indicate similar power consumptions regardless of the investigated electrode design. The second set of experiments consists of applying the S3DBD arrangement for the definition of multi-dbds control devices. To investigate and highlight the limitations of this innovative design, three different gap values between the single DBDs composing the plasma array is proposed (gaps of 5, 10 and 15 mm). The results reveal that S2DBD is not suitable to compose effective multi-dbds. Indeed, for such typical DBD design, the backward flow is strong enough to provoke a cancellation of the electric wind produced by the upstream plasma discharge. This results in an alternative electric wind with positive velocity followed by zero velocity regions, regardless of the gap distance proposed here. When a S3DBD is used, results clearly differ. In this case, the mutual interaction is not fully cancelled but this new DBD design allows to maintain a constant velocity value over the whole actuation surface. More, at specific gap and electrical input values, the final velocity produced by the multi-dbds actuator results from the successive addition of momentum increasing the efficiency of the control system. The present results indicate the benefits of using a threeelectrode arrangement compared to conventional DBD designs when used in array of plasmas. 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