In uence of the N /O Volumetric Ratio on the Behavior of a...

Influence of the N2/O2 Volumetric Ratio on the

Behavior of a SDBD Plasma Actuator

Audier P.∗, Rabat H.† and Hong D.‡

GREMI, UMR 7344 CNRS/University of Orléans, 14 rue d’Issoudun, 45067 Orléans Cedex 2, France

Leroy A.§

PRISME, University of Orléans, 8 rue Léonard de Vinci, 45072 Orléans Cedex 2, France

This paper is focused on studying the influence of nitrogen and oxygen on the behaviorof a surface dielectric barrier discharge (SDBD) actuator. Experimental characterizationshave been conducted under several ratio of N2/O2 gas mixtures and in ambient air. Ionicwind velocity profiles have been measured for different positions above the dielectric sur-face, electrical power measurements and ICCD imaging of plasma filaments in oxygen,nitrogen and air are presented. Experimental results show that ionic wind velocity ishigher in pure oxygen than in pure nitrogen and thus confirm the main role of oxygen ions.They also show that the presence of water molecules in ambient air induces a strongerionic wind velocity than in dry air. ICCD imaging illustrates different shapes of plasmadepending on the gas composition.

Nomenclature

∆P Pressure difference, Paρ Gaz density, kg.m−3

A Constant based on the geometry and the dielectric thickness, W.m−1.Hz−1.kV−2

FHV High voltage frequency, HzP Pressure value, PaPatm Atmospheric pressure, PaPelec Electrical consumed power, W.m

−1

Q Transferred charge, CU Velocity value, m.s−1

V0 Threshold value of the plasma ignition, kVVHV High voltage amplitude, kV

∗PhD Student, University of Orléans, GREMI, 14 rue d’Issoudun 45067 Orléans Cedex 2 France†Research Engineer, University of Orléans, GREMI, 14 rue d’Issoudun 45067 Orléans Cedex 2 France‡Professor, University of Orléans, GREMI, 14 rue d’Issoudun 45067 Orléans Cedex 2 France§Associate Professor, University of Orléans, PRISME, 8 rue Léonard de Vinci, 45072 Orléans Cedex 2 France

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American Institute of Aeronautics and Astronautics

I. Introduction

Surface dielectric barrier discharges (SDBD) are currently studied in aerodynamic research where they areused as actuators for active flow control1–4. They generate non-thermal plasma at atmospheric pressure,which directly acts on the surrounding gas momentum and induces low-velocity airflow, called ionic wind,close to the dielectric surface. Previous works have already insisted on the ability of such actuators to controlflow separations5,6 or to control the boundary layer laminar-to-turbulent transition7–9. The SDBD evolvesas a set of microdischarges repeating each half-cycle of the applied high voltage sine waveform: streamerdischarge during the positive-going half-cycle and diffuse discharge during the negative-going one. Becauseof its complexity, the mechanism of charge separation and momentum transfer of ions and electrons withneutral gas molecules is currently under investigation.

In previous experiments, optical phase-resolved techniques used to measure the time evolution of velocityshowed that the velocity value is higher during the negative-going half-cycle10. The influence of the oxygenconcentration in actuator efficiency was observed11. Enloe12 and Leonov13 confirmed the important role ofnegative ions during the negative-going half-cycle by performing time-resolved measurements of the plasma-induced momentum. They observed that main contribution to momentum transfer occurs during this half-cycle which was confirmed by numerical simulation14. Soloviev15 recently proposed an analytical estimationof the plasma-induced thrust and explained that the main contribution to body force is governed by theaccumulated volumetric charge of negative ions during the negative voltage half-cycle. In order to contributeto investigate the importance of the molecular oxygen, we have performed experiments with a plasma actuatoroperating in a controlled atmosphere.

In this paper, we present an experimental characterization of a thin SDBD conducted under severalratio of N2/O2 gas mixtures and in ambient air. Firstly the ionic wind velocity is measured, resulting intime-averaged velocity profiles for different positions above the dielectric surface. Then electrical powermeasurements are realized and ICCD imaging of plasma filaments in oxygen, nitrogen and ambient air ispresented for each half period of the AC applied voltage. The qualitative behavior of the plasma is describedfor each half-period.

II. Experimental set-up

As shown in Fig.1, the design of the SDBD consisted of two thin copper electrodes of 80µm in thickness(6 mm wide in the x-direction) mounted on both sides of a dielectric panel. The air-exposed electrode isconnected to an AC power supply (Trek), the grounded one is placed below the dielectric with a gap of 3 mmand encapsulated. The dielectric is composed of Mylar covered on each side by one layer of polyimide filmrepresenting a total thickness of 0.5 mm.

The plasma is obtained by applying a sinusoidal high-voltage with amplitude VHV ≤ 15 kV and frequencyof FHV =1 kHz. All the experiments have been conducted at atmospheric pressure in a 0.1 m

3 tank equippedwith optical access for ICCD camera imaging. A pressure probe of 1000 mbar in full range (Baratron MKS)is used to adjust the volumetric ratio of N2/O2. Voltage and current are measured with a high voltage probeand a Rogowski coil probe. The charge transferred by the whole circuit is measured using a 47 nF capacitoras shown in Fig.1. All electrical signals are recorded with an oscilloscope.

The induced low-velocity airflow is measured using a total pressure glass probe having a 0.5 mm en-trance diameter section, moving along three axes X, Y and Z and connected to a pressure transducer (GEDruck LPM 9481 0.2 mbar) as illustrated in Fig.1. The pressure transducer provides simultaneously thepressure difference between the overpressure P associated with the airflow and the atmospheric pressurePatm measured in the tank. Airflow velocity is computed by using the following equation 1.

U =

√2 · ∆Pρ

(1)

Where ∆P=P-Patm and ρ is the gas density. Velocity profiles are plotted for a given axis at a fixed pointin the plane defined by the other axes (i.e. along the Z-direction for x=6 mm and y=0 mm) and correspondto a time-averaged value of the flow velocity.

Images of the discharge have been acquired with an ICCD camera (Andor, iStar DH734) equipped with a

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60 mm focal lens (Nikon AF Micro Nikkor). A triggering system is used to record images at specified times.

(a) Perspective view of the SDBD actuator (b) Set-up for velocity measurements

Figure 1. Sketch of the SDBD actuator and set-up for velocity measurements.

III. Experimental results

A. Influence of N2/O2 volumetric ratio on ionic wind velocity

First series of measurements were achieved to compare the velocity induced by discharges operating inambient air (relative humidity of 20%) and artificial dry air mixed with dry gases (80% of N2 and 20% ofO2). Note that all the experimental data have been obtained by averaging velocities over several minutes.Velocity profiles along the three directions are presented in Fig.2. Results show that a relative humidityof 20% in ambient air induces a 10% higher velocity than in dry air. Previous works16,17 have underlinedthe influence of relative humidity on the induced velocity and showed that the ionic wind velocity could bereduced or increased when the discharge was operated in a wide range of relative humidity (40% to 98%).This difference of velocity is particularly noticeable at each space position probed in the y-direction velocityprofile (Fig.2(b)) where y=0 mm corresponds to the middle of the active electrode in the transverse direction.The un-homogeneity in this velocity profile is due to the roughness of the active electrode edge13.

(a) X-direction (y=2 mm, z=0.25 mm) (b) Y-direction (x=6 mm, z=0.25 mm) (c) Z-direction (x=6 mm, y=2 mm)

Figure 2. Ionic wind velocity UX along the three directions, comparison between ambient air and dry air.VHV =12kV and FHV =1kHz.

Second series of measurements are dedicated to the characterization of velocity profiles along the X-direction indicated in Fig.1. Velocity evolution is plotted in Fig.3 as a function of X for three differentgas mixtures: artificial dry air (80% of N2 and 20% of O2), pure nitrogen and pure oxygen with the sameelectrical parameters (VHV =12 kV and FHV =1 kHz). For the three gases, velocity increases from the edgeof the upper electrode, reaches a maximum and then decreases. One can notice that the maximum for dryair is the highest and the one for pure nitrogen is the lowest. The velocity in pure oxygen is higher than thatin pure nitrogen (about twice), but lower than that in dry air (15% less). Otherwise, the velocity reach itsmaximum more quickly in pure oxygen than in dry air or in pure nitrogen where the ionic wind diffusion area

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is longer (the positions of velocity maximum are 3.5, 6 and 7.5 mm respectively). This confirms that greaterionic wind is induced in oxygen than in nitrogen, even if ionic wind produced in nitrogen is not negligible.

Figure 3. Ionic wind velocity UX along the X-direction. VHV =12kV and FHV =1kHz, y=0mm and z=0.25mm.

Third series of measurements are dedicated to the characterization of velocity profiles along the Z-direction. Velocity evolution is plotted in Fig.4 as a function of z for three X-values corresponding tothe beginning, the middle and the end of the grounded electrode. Note that y=2 mm corresponds to themaximum of induced velocity recorded in Fig.2(b). In pure nitrogen, the induced velocity is always about70% lower than in the other mixtures. In pure oxygen, the induced velocity is two times higher than indry air over the gap (x=0 mm to x=3 mm) but becomes about 20% lower just after the beginning of thegrounded electrode (x=4 mm). Furthermore, we can notice that whatever the mixture, the Z-position of themaximum velocity increases as x increases.

These results show that the induced velocity in air is mostly due to the contribution of oxygen ions asdiscussed in previous works12,13. However nitrogen ions contribution is not negligible especially over the endof the grounded electrode.

(a) y=2 mm and x=3 mm (b) y=2 mm and x=6 mm (c) y=2 mm and x=9 mm

Figure 4. Ionic wind velocity UX along the Z-direction for three N2/O2 ratios. VHV =12kV and FHV =1kHz.

B. Influence of N2/O2 volumetric ratio on power consumption

In order to describe and compare the electrical characteristics of the plasma actuator in various gases, theconsumed electrical power Pelec has been computed as a function of the high voltage by using a capacitorin series with the actuator. Results are presented in the Fig.5 for three various gas mixtures: artificial air,pure nitrogen and pure oxygen. A common method18, consisting in calculating the Q− VHV cycle area, hasbeen used to estimate the power consumption :

Pelec = FHV

∫cycle

VHV · dQ (2)

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Experimental data curves have been fitted by the following power law function according to our experi-mental configuration7. Where V0 is the threshold value of the plasma ignition and A is a constant based ongeometry, dielectric thickness and gas composition.

Pelec = A · FHV (VHV − V0)2 (3)

Figure 5. Electrical power against the high voltage amplitude for three N2/O2 ratios, FHV =1kHz.

In Fig.5, the electrical power consumption is plotted against the high voltage amplitude. We can noticethat the power consumption differs when gas changes, i.e. the electrical power consumption is more importantin pure nitrogen and less important in pure oxygen than in dry air. The threshold value of the plasma ignitionalso differs for each gas (V0 and A values are presented for each gas in Tab.1). The plasma ignition earlyoccurs in pure nitrogen and lately in pure oxygen than in air. Note that even if the induced velocity inambient air is 10% higher than in dry air, power consumption does not differ.

Table 1. Power law parameters versus gas mixture

Gas mixture V0(kV) A(W.m−1.Hz−1.kV−2)

Air (80/20 vol ratio) 5.5 2 10−3

Pure nitrogen 4.25 1.65 10−3

Pure oxygen 6.5 2.5 10−3

C. Influence of N2/O2 volumetric ratio on plasma morphological characteristics

Images of plasma discharges created in nitrogen, oxygen and dry air are separately shown in the three nextsections for positive and negative half cycles. The evolution of plasma filaments is illustrated in reversedcolor for different high voltage amplitudes (6-14 kV). ICCD images have been recorded with a gate widthof 200µs and a delay of 100µs after the beginning of each half-cycle which is enough to accumulate several100 ns time-scale microdischarges12. Note that ionic wind direction goes from the active electrode to thegrounded one.

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1. ICCD imaging of plasma filaments in nitrogen

• Positive half-cycle: In pure nitrogen, plasma filaments with a streamer-like structure extend acrossthe electrode gap and almost over all the grounded electrode for a 6 kV amplitude. The filamentspropagate straight over the gap and start branching at the beginning of the passive electrode (x=3 mm).Only some plasma filaments are present and the discharge distribution is not homogeneous at all. Byincreasing the high voltage amplitude, the number of filaments increases and their length can passthe grounded electrode: the discharge distribution becomes more homogeneous. Filaments can branchfrom their ignition site (x=0 mm) and have tortuous trajectories. Furthermore, many plasma filamentsignite at same location and follow same ionized channel along the gap, resulting also in light intensityincrease.

• Negative half-cycle: The discharge consists of diffuse filaments, propagating only across the gap,with a luminous part near the exposed electrode. When the high voltage increases, the number offilaments also increases but the discharge distribution remains non homogeneous and less luminous incomparison with the positive half-cycle.

Figure 6. Plasma morphology in pure nitrogen against the high voltage amplitude. F=1kHz. Gate width of200µs with a delay of 100µs after the beginning of each half cycle.

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2. ICCD imaging of plasma filaments in oxygen

• Positive half-cycle: In pure oxygen, plasma ignition occurs later than in pure nitrogen (Fig.6); thelength, the number and the luminosity of plasma filaments also increase as the high voltage increases.However, filaments are shorter and broader than in pure nitrogen and their structures are more straightwithout branching parts. For VHV ≥ 10 kV, filaments reach the end of the grounded electrode andmany common broad ionized channels are noticeable along the gap, resulting in light intensity increase.

• Negative half-cycle: The plasma ignition occurs also later than in pure nitrogen but filamentsstructure is very different. Instead of diffuse spots, only some straight filaments appear between thetwo electrodes. When the high voltage increases, the number of filaments increases but the dischargestay non homogeneous at all. Strong filaments straightly propagate across the electrode gap andover the grounded electrode with a high light intensity. When the high voltage amplitude exceeds10 kV, these strong ionized channels branch when they reach the middle of the grounded electrode(x=6 mm). This phenomenon has already been observed in ambient air in previous works as negativespark breakdown19–21.

Figure 7. Plasma morphology in pure oxygen against the high voltage amplitude. F=1kHz. Gate width of200µs with a delay of 100µs after the beginning of each half cycle.

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3. ICCD imaging of plasma filaments in dry air

• Positive half-cycle: Plasma filaments with a streamer-like structure propagate in dry air as inpure nitrogen (Fig.6) and their length, number and luminosity increase as the high voltage increases.However, filaments are less luminous and shorter than in pure nitrogen.

• Negative half-cycle: For VHV ≤ 8 kV, the discharge consists of diffuse filaments with a luminouspart near the exposed electrode as in pure nitrogen. When the high voltage increases, the filamentsstructure becomes less diffuse and more luminous. Filaments are longer and extend over the middleof the grounded electrode (x=6 mm). For VHV ≥ 12 kV, strong filaments appear as in pure oxygen(Fig.7)and straightly propagate with a high light intensity. These ionized channels are ended witha large branching area propagating along the edge of the grounded electrode (Fig.8). Negative sparkbreakdowns can be observed when VHV ≥ 14 kV.

Figure 8. Plasma morphology in air against the high voltage amplitude. F=1kHz. Gate width of 200µs witha delay of 100µs after the beginning of each half cycle.

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IV. Conclusion

This study was focused on the influence of nitrogen and oxygen on the behavior of a SDBD plasmaactuator. Velocity and power consumption measurements were performed in a controlled atmosphere andresults show:

• The relative humidity of 20% in ambient air induces a 10% higher velocity than in dry air.

• The velocity in pure oxygen is higher than in pure nitrogen (about twice), but lower than in dry air(15% less), confirming the important role played by oxygen ions. Nevertheless, the contribution ofnitrogen in velocity is not negligible.

• The power consumed is more important when the discharge is driven in pure nitrogen where the plasmaextension is longer and the number of filaments is higher than in pure oxygen. Moreover the plasmaignition occurs early in nitrogen and lately in oxygen.

ICCD images of plasma filaments in oxygen, nitrogen and air were presented for both positive and negativeperiod of the AC applied voltage. During a positive half-cycle, we can observe that the size, the number andthe light intensity of filaments are more important in nitrogen than in oxygen. This can be explain that powerconsumption is higher in pure nitrogen. However, the negative half-cycle is more active in oxygen wherenegative spark breakdowns start to appear when the voltage exceeds 12 kV. Previous studies have underlinedthe great predominance of the negative half-cycle in the transfer of momentum to the surrounding air and theinfluence of the oxygen concentration in actuator efficiency10–13. Leonov13 observes no momentum transferin nitrogen for high voltage with negative polarity, whereas it forms the main contribution in air. Thusphase-averaged velocity measurements should be performed in nitrogen and in oxygen to further analyse themechanisms of induced momentum transfer. The results presented here confirm the main role of oxygen ionsand are expected to supply informations for future experimental investigations and numerical simulations toimprove the potential of such actuator for flow control.

References

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15Soloviev, V. R., “Analytical estimation of the thrust generated by a surface dielectric barrier discharge,” Journal ofPhysics D: Applied Physics, Vol. 45, No. 2, 2012.

16Benard, N., Balcon, N., and Moreau, E., “Electric Wind Produced by a Surface Dielectric Barrier Discharge OperatingOver a Wide Range of Relative Humidity,” 47th AIAA Aerospace Sciences Meeting, Vol. 488, 2009.

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18Pons, J., Moreau, E., and Touchard, G., “Asymmetric surface dielectric barrier discharge in air at atmospheric pressure:electrical properties and induced airflow characteristics,” Journal of Physics D: Applied Physics, Vol. 38, No. 19, 2005.

19Larigaldie, S., “Spark propagation mechanisms in ambient air at the surface of a charged dielectric. I. Experimental: Themain stages of the discharge,” J. Appl. Phys., Vol. 61, No. 1, 1987, pp. 90.

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21Joussot, R., Boucinha, V., Weber, R., and Hong, D., “Negative Spark Leaders on a Surface DBD Plasma Actuator,”Plasma Science, IEEE Transactions on, Vol. 39, No. 11, nov. 2011, pp. 2194 –2195.

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