# Ionic wind produced by an electro-aerodynamic pump based on corona and dielectric barrier discharges

Étienne Defoort, Nicolas Bénard, Éric Moreau

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Étienne Defoort, Nicolas Bénard, Éric Moreau. Ionic wind produced by an electro-aerodynamic pump based on corona and dielectric barrier discharges. Journal of Electrostatics, 2017, 88, pp.35-40. 10.1016/j.elstat.2017.01.021. hal-03636690

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# Ionic wind produced by an electro-aerodynamic pump based on corona and dielectric barrier discharges

E. Defoort, N. Benard and E. Moreau, Université de Poitiers (Pprime, CNRS, ISAE-ENSMA), 86962 Futuroscope, France

## Abstract

The present study aims at showing the ability of ac discharges to be used as fanless blower in a rectangular channel. First, a 10-mm-high channel is used with four different types of ac atmospheric air discharges: an ac wire-to-plate corona discharge, two different ac wire-to-plate dielectric barrier discharges (DBD) and a surface DBD. The electrical and mechanical characteristics of the four different discharges are investigated and compared. They highlight that the best geometrical configuration is the wire-to-plate DBD with a thick dielectric, that allowed us to induce ionic wind velocity up to 3.3 m/s and flow rate per unit spanwise length of 24 liters per second per meter. Secondly, this optimized configuration is reduced in size (2-mm-high channel) and the effect of this downscaling on the plasma actuator performances is studied, showing that the efficiency decreases with the channel height.

## 1 Introduction

The application of a high voltage between two electrodes produces gas ionization around the sharp edges of those electrodes. This effect is known as corona discharge and occurs generally between an active high voltage needle or wire and a grounded plate. Thanks to Coulomb forces, the produced ions drift toward the grounded electrode under the effect of the electric field and they transfer their momentum to neutral gas molecules, producing a gas flow usually called ionic or electric wind. Plasma actuators are systems that use this electroaerodynamic (or electrofluid-dynamic) phenomenon to convert electrical energy into mechanical energy. Ionic wind has been widely studied for many applications such as airflow control [1-6], thermal transfer enhancement [7-8], aerospace propulsion [9-11] or simply fanless blowers [12-18].

Theory of electrofluid-dynamics in gas starts with the publications of Robinson in 1961 and 1964 who investigated the possibility for dc corona discharges to perfect fanless electrostatic blowers [12, 13]. Since, several others research groups have been investigating the electrical and mechanical characteristics of those dc corona-
based actuators, this in order to increase the electric wind velocity, to enhance the resulting flow rate or to improve the electro-mechanical conversion efficiency.

The first goal of the present experimental study is to show the ability of ac discharges to be used as fanless blower in a 10-mm high rectangular channel. The electrical and mechanical characteristics of four different types of plasma actuators are experimentally investigated. Then the best configuration is reduced in size (2-mm-high channel) and the effect of this downscaling on the plasma actuator performances is studied.

## 2 Experimental set-up

Figure 1 shows a sketch of the four configurations used in the first part of this study. The first configuration (Fig.1a) is a standard wire-to-plate ac corona discharge. The gap between the active electrode and both grounded ones is equal to 10 mm. The channel, made with two 2.5-mm-thick PMMA flat plates, is 10-mm-high and 150 mm in spanwise. The active electrode is a tungsten wire with a diameter of 100 µm and both plates are 300-µm-thick copper tapes. The second design (Fig.1b) is also a wire-to-plate configuration, but a thin Kapton dielectric layer has been added on both grounded electrodes (in yellow in Fig.1b). Then this discharge is a volume dielectric barrier discharge (DBD). This design will be called “thin DBD” because the kapton tape is very thin (60-µm-thick). The third design (Fig.1c) is also a wire-to-plate dielectric barrier discharge but the dielectric barrier is thicker since the grounded electrodes have been placed on the outer wall of the 2.5-mm-thick PMMA plates. This design will be called “thick DBD”. Note that the grounded electrodes are encapsulated in an epoxy resin to avoid any discharge ignition at the external wall of the PMMA plates (in grey in Fig.1c). Finally, the last case (Fig.1d) is a double surface dielectric barrier discharge (surface DBD) with planar electrodes flush mounted on both sides of the PMMA plates. The first part of this study aims at comparing the electromechanical characteristics of these four configurations. In a second part, a design with a 2-mm-high channel will be studied. All the geometrical parameters of this last design will be indicated latter in the paper.
<figure><img src="image_2.png" /><figcaption>Figure 1. Side views of the four configurations : ac corona discharge (a), thin DBD (b), thick DBD (c) and surface DBD (d). Blue is for PMMA channel wall, red for electrodes (tungsten wire or copper tape), yellow for kapton tape and grey for epoxy resin. All the scales are in millimeters.</figcaption></figure>

For all the experiments, a sinusoidal 1 kHz ac high voltage is applied to the active electrode with the help of a TREK amplifier (30 kV – 20 mA). The applied voltage amplitude begins at 2kV and can grows up to 18kV, depending of the electrical breakdown limit of the configuration. Electrical power consumption is determined with the help of a capacitor to plot the charge-voltage curve. Then integration of this curve gives the energy consumed during one ac period, the electrical power corresponding to this energy multiplied by the voltage frequency. The signal generation and the power calculation are made with a PXI from National Instrument, integrating the software LabVIEW and communicating by RJ45 with an oscilloscope (HDO6054, Teledyne LeCroy, 500 MHz). For each applied voltage amplitudes, the electrical power is averaged over 10 instantaneous measurements.

Velocity measurements are made with a home-made glass Pitot tube (internal diameter of 500 µm) mounted on a three-dimensional displacement system (precision of 10 µm) controlled from a computer running LabVIEW. The Pitot tube is connected on a FC014 micromanometer from Furness controls, which have a precision better than 0.01%. Each velocity acquisition is the mean of 3000 measures with a sampling rate of 1 kHz. In such a condition, the RMS values were never more than a few percent. To characterize the ionic wind produced by the different configurations, vertical profiles are measured at 1 mm downstream of the outlet of the channel, at mid-spanwise, assuming that the ionic jet is two-dimensional. Then the induced flow rate and mechanical power are calculated by integration of the velocity profiles and normalized by the channel spanwise length.
Moreover, particle image velocimetry (P.I.V.) measurements have been conducted in order to visualize the whole ionic wind jet produced by the thick DBD design, in a channel of 10 mm in height and then in a 2-mm-high channel. The PIV system is composed by a high-resolution camera (Imager LX 16 MP, 4000 × 4000 pixels²), a single head Nd:Yag laser (Quantel Big Sky), a triggering unit (EG, R&D Vision) and a PC running Davis V8.2 Lavision software. The laser sheet is placed parallel to the flow, at mid-span. The plasma actuators are placed inside a closed box in which quiescent air is seeded with the help of a smoke generator (TM 210, Topas) using a dielectric Ondina oil. The mean diameter of the oil droplets used as fluid tracers is 0.3 µm. The velocity is computed using a cross-correlation algorithm with adaptive multi-passes, interrogation windows of 64× 64 to 16×16 pixels and an overlap set to 50%. Each mean velocity field is the average of 1000 instantaneous fields to ensure the convergence.

# 3 Results and discussion

## 3.1 Electrical and mechanical measurements in the 10-mm-high channel

Figure 2 shows the velocity profiles measured with the four configurations. First, let us compare the three first designs that produce a volume discharge from the high voltage wire electrode toward the channel exit (Figures 2a, 2b and 2c). For these three designs, one can remark that the jet velocity profiles are nearly similar at 6, 8 and 10 kV. Their maximum velocities, obtained at the center of the channel exit, are respectively equal to about 0.5, 0.75 and 1.5 m/s. The volume discharges produce a nearly parabolic jet that flatten at low voltage and form a central peak at 10 kV. Surprisingly, the presence of a dielectric barrier between the wire and the grounded electrodes seems to have no significant effect on the produced jet since the plasma physics should be modified by the thin dielectric barrier (Fig. 2b). For the corona discharge and the thin DBD, the high voltage is limited to 10 kV because sparks occur for voltages above this threshold. With the thick DBD, the voltage can be increased up to 18 kV, allowing us to raise the ionic wind jet velocity up to about 3.3 m/s. With the surface DBD (Fig. 2d), the jet is fully different because the ionic wind is induced at both the internal walls of the channel. Because the thickness of both wall ionic wind jets is smaller than 5 mm, the resulting jet at the channel outlet has a velocity close to zero at the channel center. Finally, one can remark that the momentum (proportional to the area under profiles) is much larger for the volume discharges than for the surface DBD.
<figure><img src="image_3.png" /><figcaption>Figure 2. Velocity profiles for the ac corona discharge (a), the thin DBD (b), the thick DBD (c) and the surface DBD (d) for different voltage values.</figcaption></figure>

From these velocity profiles, the maximum velocity is determined and the jet flow rate Q (l/s per meter), the mechanical power P<sub>mech</sub> per unit length (W/m) and the electro-mechanical conversion efficiency can be determined by using :

$$Q = \int_{y} v(y) dy$$

$$P_{\text{mech}} = \frac{1}{2} \rho \int_{y} v^{3}(y) dy$$

$$\eta = \frac{P_{\text{mech}}}{P_{\text{elec}}}$$

where v(y) is the ionic wind velocity versus y location (m/s), ρ the air mass density, P<sub>elec</sub> is the electrical power consumption per unit length of electrode (W/m).

First, Fig.3a highlights that the electrical power increases with the applied voltage V, following the well-known Townsend expression P<sub>elec</sub> = CV(V-V<sub>0</sub>) where C is a constant and V<sub>0</sub> the voltage for which the discharge ignites. More, the power
consumption of the corona discharge and the thin DBD are equal, showing that the thin layer of Kapton does modify neither the produced ionic wind, neither the power consumption. The consumption of the thick DBD is a little bit higher and the one of the surface DBD is twice. Fig. 3b and 3c show that both velocity and flow rate increase nearly linearly with the applied voltage and that they are not significantly affected by the presence of the dielectric because the values obtained with the corona discharge, the thin DBD and the thick DBD are similar. On the contrary, the velocity and the flow rate induced by the surface DBD are smaller. Finally, Fig. 3d shows the efficiency of the four designs. One can see that it is very small compared to the one of Moreau and Touchard in the case of dc corona between a needle and a grid configuration [15]. Indeed, it seems that the use of a wire active electrode is not relevant for momentum production because the discharge is not as stable as the one established with a needle.

<figure><img src="image_4.png" /><figcaption>Figure 3. Electrical power (a), maximum velocity (b), flow rate (c) and efficiency (d) versus applied voltage for the four configurations.</figcaption></figure>

## 3.2 Thick DBD with $H = 2$ mm and 10 mm

The thick DBD being the most relevant and efficient configuration, one focus on this discharge, the goal being now to investigate the effect of a downscaling of the plasma actuator. For that, a new channel is perfected. This configuration is similar to
the one presented in Fig. 1c, but at a lower scale. The channel is now 2-mm-high and 20-mm-long. It is still made with two 2.5-mm-thick PMMA plates, with a spanwise length of 35 mm. The grounded electrodes are 5-mm-wide and the gap between them and the active wire is equal to 5 mm. Fig. 4 shows the velocity profiles of both thick volume DBD actuators. Obviously, the smallest actuator has a narrower jet but its overall shape remains parabolic. However, the induced velocity is higher, highlighting that the local momentum production is important because the skin-friction developing at the channel wall significantly counteracts on the momentum production in small channel. Fig. 5 presents the electrical power consumption, the maximum velocity, the resulting flow rate and the electro-mechanical efficiency as a function of the applied high voltage. First, it shows that the electrical power is higher (at a given voltage value) with the thin channel because the electric field is reinforced by a reduced electrode gap. Secondly, the maximum velocity is higher but the flow rate is weaker because the channel height is divided by five. Finally, efficiency is weak because this discharge is penalized by a high consumed power while the mechanical power is small due to the size of the channel. One can notice the particular point for a voltage amplitude of 18 kV, an amplitude for that a sudden increase in electrical power is observed probably due to a transition to a filamentary regime.

<figure><img src="image_5.png" /><figcaption>Figure 4. Velocity profiles for both scales of thick DBD actuators (H = 10 and 2 mm) at different voltage values.</figcaption></figure>
<figure><img src="image_6.png" /><figcaption>Figure 5. Electrical power (a), maximum velocity (b), flow rate (c) and efficiency (d) versus applied voltage for both ionic pump (H=10mm in black and H=2mm in red).</figcaption></figure>

## 3.3 Particle image velocimetry measurements

The last set of experiments aim at visualizing the ionic wind jet produced by the thick DBD design, in a channel of 10 mm in height and then in a 2-mm-high channel. Figures 5a and 5b give examples of the produced jet in the 10-mm-high channel, for voltages equal to 10 and 18 kV. As in Fig. 2c that presented velocity profiles measured with a Pitot tube, the maximum velocities are about 1.5 and 3.2 m/s at 10 and 18 kV, respectively. This shows a good agreement between the Pitot tube measurements and the PIV ones. More, the jet topology behaves nearly as a typical free jet induced by a pressure gradient; the maximum velocity is higher in the middle of the jet, the jet width increases with x and the potential core length is equal to about 60 mm (see Fig. 5b), i.e. six times the initial jet height. On the contrary, the topology of the jet at the outlet of the 2-mm-high channel differs from the previous configuration. Indeed, considering a constant mean flow at the exit of this channel equal to 1.5 m/s (Figure 5), the Reynolds number is very low (Re ≈ 16). In such a
situation, momentum diffusion is high and the shape of the mean jet is rounded
compared to large channel jet flows. The length of the potential core is quite small
(8 mm) but it cannot be directly compared to other types of micro-channel flow.
Indeed, the ionic wind induced by an ac plasma discharge is known to be highly
periodic and unsteady, and as shown in Fig.4, the velocity is not constant along the
height of the channel. However, the flow topology measured here in air highly
resembles the helium micro-jet flow of Whalley and Walsh [19] which was
manipulated also by a plasma discharge at low Reynolds number.

<figure><img src="image_7.png" /><figcaption>Figure 6. PIV velocity fields of the jet produced with a thick DBD in the 10-mm-high channel at 10 kV (a) and 18 kV (b), and in the 2-mm-high channel at 6 kV (c) and 8 kV (d). Spatial scales are in mm, Velocity scale is in m/s. Black area show the position of the channel outlet.</figcaption></figure>

**4 Conclusion**

The goal of the present paper was to compare the performances of several types
of ac discharges for generating a jet at the outlet of a channel. First, experiments
have been conducted with a 10-mm-high channel. It was shown that the ionic wind jet
produced at the channel outlet depends highly on the configuration of the electrodes.
Velocities up to 3.5 m/s have been measured. On one hand, this is weak compared
to the velocity produced by needle-to-plate or needle-to-grid dc corona discharges
such as in [15]. On the other hand, these values are higher than the ones of Tsubone
et al. [16] with a wire-to-plate dc corona discharge, and similar to the ones of
Campbell and Roy with a wire-to-plate DBD [18], highlighting that the use of needles
is certainly more efficient.
Secondly, the jet at the outlet of a 2-mm-high channel has been investigated. It was shown that the produced velocity was a little bit higher than the ones measured previously with the 10-mm-high channel, but the flow rate and the efficiency are low.

To conclude, one can say that this study was the first step of a more global work on the use of atmospheric air discharges in channels and micro-channels. Indeed, knowing that the electrostatic force becomes dominant at small scales, one can wish that, as for liquids, electro-micro-aerodynamics becomes a relevant tool for industrial applications.

## Acknowledgments

Research funded by the French Government program "Investissements d'Avenir" (LABEX INTERACTIFS, reference ANR-11-LABX-0017-01) and conducted under DGA and ANR supervision (INOPLAS, ANR-13-ASTR-0015-01). The co-authors from Institut PPRIME would like to acknowledge the NATO Applied Vehicle Technology (AVT) 254 "Assessment of plasma actuator technologies for internal flows".

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