# Manipulation of a grid-generated mixing with an active honeycomb dielectric barrier plasma discharge

Nicolas Bénard, Akira Mizuno, Éric Moreau

► To cite this version:

Nicolas Bénard, Akira Mizuno, Éric Moreau. Manipulation of a grid-generated mixing with an active honeycomb dielectric barrier plasma discharge. Applied Physics Letters, 2015, 107 (23), pp.233508. 10.1063/1.4937406. hal-03636773

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# Manipulation of a grid-generated mixing with an active honeycomb dielectric barrier plasma discharge

N. Benard¹, A. Mizuno² and E. Moreau¹

¹Institut PPRIME (CNRS UPR 3346, Université de Poitiers, ENSMA)
Dpt Fluide, Thermique et Combustion
Bld Marie et Pierre Curie
Téléport 2, BP 30179, 86962 Futuroscope, France

²Toyohashi University of Technology
Department of Environment and Life Science
Tenpaku-cho, Toyohashi 441-8580, Japan

## Abstract

This study defines and characterizes a new system for turbulent and scalar mixing enhancement. This system is made of an active grid composed of 121 holes where the gas flows. A high voltage is applied between printed electrodes and embedded ground electrodes in order to produce a non-thermal surface discharge at the hole exits. The goal is to modify the flow downstream of the active grid. First, electrical and optical characterizations of the actuator are proposed. Secondly, it is shown that the discharge strongly modifies the flow distribution of the jets exhausting, and the flow change depends on the high voltage applied to the active plasma grid. A minimization of the potential core by 40% is reported when discharge frequency corresponds to jet column mode instability.

Turbulent mixing of scalar is important for a large number of practical and fundamental situations. Systems such as passive grids [1] or multiple jet configurations [2] have theoretically and empirically demonstrated that the obvious power-law decay of the turbulent components can be modified. Homogeneous and isotropic turbulence can quickly develop downstream of a fractal grid [3] or multiple grids in series [4], these two passive systems producing a better mean flow homogeneity and a high turbulent intensity while maintaining a low pressure drop. Another method for increasing the mixing of turbulent flow is the use of multiple jets. For instance, reduction of pollutant emissions (such as NOx) can be achieved by stabilizing a flame thanks to the mixing improvement caused by a fuel jet confined by lateral oxygen jets [5]. A more complex system made of 6×6 interacting jets has been investigated in [6], which demonstrates a merge of the jets into a single jet after a certain distance. A lateral
deflection is caused by mutual entrainment between the jets and a low pressure region at the
center of the jet array.

The present paper defines and demonstrates how the surface plasma discharges can be useful
for mixing increase downstream of an active ‘grid like’ or ‘multi-jet like’ geometry. The use
of non-thermal surface plasma discharge for flow control is now more than an emerging field
[7, 8, 9, 10]. It involves more and more research groups, but the typical surface plasma
actuator with asymmetric electrodes, as it was originally defined by Roth [11], is still the most
used configuration.

The present paper aims at investigating a different type of electrode arrangement in order to
tackle new applicative and fundamental problems dedicated to mixing enhancement. This
type of complex electrode arrangement has been originally designed for gas cleaning by
combination of non-thermal plasma and catalysts [12, 13]. The proposed research is an
introduction of this new actuator regarding its electrical and optical characteristics, as well as
preliminary flow measurements. The primary objective is to identify the effectiveness of such
an active actuator for manipulating a turbulent flow. Indeed, this actuator can open new
research direction taking advantages of the high frequency forcing capability of plasma
discharges.

The geometry of the plasma discharge used in this paper is a new design compared to
traditional surface DBD plasma actuator used in flow control applications. The system is
made of a perforated ceramic plate (thickness of 1.27 mm) in which the grounded electrode is
embedded in the dielectric material while the high voltage electrode is printed on one of the
surfaces (Figure 1). The honeycomb has 121 holes with an inner diameter Φ of 2 and a height
of 1.27 mm. The distance between the centers of two successive holes is 3.05 mm. Both
electrodes are made of nickel with a thickness of about 10 µm. The active (high voltage)
electrode is made of 121 circular rings with an internal diameter of 2.7 mm, these rings being
all connected together. The distance between the edge of an active electrode and the edge of
the corresponding hole designed for the gas passage is 350 µm. By applying a sufficient AC
high-voltage to the active electrode, surface discharge takes place at the surface of the
perforated plate and at the inner wall of each hole. There is then one single discharge per hole.
By analogy with the geometries used for investigating fundamental grid turbulence, one can
calculate a grid solidity ratio σ (defined as the ratio of the surface obstructed by the grid to the
total area of the grid [14]) that is here equal to 0.78.

<figure><img src="image_2.png" /><figcaption>Figure 1 : Photograph, sketch and dimensions of the honeycomb DBD</figcaption></figure>

A high voltage amplifier (Trek, model 30/40 mA) is used to apply a gain of 3,000 V/V to the input voltage provided by a function generator. The input signal is a sine waveform applied to the active electrode at the surface of the honeycomb. The applied peak-to-peak voltage does not exceed 14 kVpp for AC frequencies (fAC) from a few hundred of Hz up to 2000 Hz.
Top-views images of the plasma discharge (viewed from the active surface) are proposed in Figure 2 for two $f_{AC}$ frequency values. In the present paper, a certain attention is paid to the influence of $f_{AC}$ due to the advantages offered by the plasma discharges in attaining the momentum transfer at single as well as multiple frequencies [10]. Indeed, such approach should permit in a near future to explore new exciting active control scenario in grid-generated turbulence and multi-jet burners.

As seen in Figure 2, at the lower frequency ($f_{AC}=100$ Hz) the plasma discharge is already uniformly ignited and it covers all the surface of the honeycomb at the exception of the holes and the conductive air-exposed electrode. At the highest applied frequency ($f_{AC}=2000$ Hz), all the cells of the system are fully powered with a strong light emission. A zoomed view indicates that the plasma extends from the edge of the air-exposed electrode up to the border of the hole where a brighter region forms a ring. The whole surface of this region is ionized and some strong filaments, connecting the edge of the active electrode to the borders of the hole, are visible regardless of the applied frequency. At the highest $f_{AC}$, air ionization can also be observed beyond the diameter of the hole confirming that the plasma extends up to a certain distance inside the holes.
<figure><img src="image_3.png" /><figcaption>Figure 2: Front view of the plasma discharge for two f<sub>AC</sub> (14 kV<sub>pp</sub>).</figcaption></figure>

In order to propose a brief description of the electrical characteristics of this particular
discharge, the voltage and current time signals are recorded with a high-resolution
oscilloscope (Lecroy HDO6054). The discharge current is measured by a shunt resistor
(100Ω) connected to the grounded electrode. A typical trace of voltage and current time
evolution is illustrated in Figure 3 for an applied voltage of 14 kV<sub>pp</sub> and a frequency of 2000
Hz. This plot confirms that the honeycomb discharge behaves as a typical dielectric barrier
discharge with two remarkable regimes. On one hand, numerous positive current peaks with
amplitude of about 20-70 mA are observed during the positive-going cycle of the voltage due
to the streamer propagation (positive discharge). On the other hand, the negative-going cycle
of the voltage results in small amplitude negative current peaks caused by a glow-like
negative plasma regime. A measurement of the consumed electrical power indicates that a
maximum consumption of 14.3 W is achieved when the actuator is operated at 14kVpp and
frequency fAC of 2000 Hz.

As it was predicted by the temporal trace of the current, two plasma discharges form over one
period of the AC cycle. In order to observe both positive and negative discharges, the front
view visualizations by fast imaging have been performed (Figure 3). An EMICCD camera
(Princeton PIMAX 4) with a 200 mm Nikkor objective and a Nikon teleconverter has been
placed in front of the honeycomb to capture a zoomed region of 9×9 mm². The camera is
gated according to the positive and negative-going cycles. The positive-going cycle of the
voltage corresponds to a corona-like discharge covering the distance from the active electrode
edges to the border of the holes. This discharge corresponds to some intense streamers with
strong light emission (responsible for the high amplitude positive current peaks)
superimposed on the current caused by the ion drift.

According to the ICCD images, only one streamer occurs per hole observed in the field of
view. However, that is corroborated by a current peak analysis (Figure 3) showing that over
10 AC cycles, the mean number of positive current peaks is 121±4 during one streamer period
(data not shown). For the negative-going cycle of the voltage, the current trace evokes a glow
regime. The fast imaging indicates that many straight filaments are initiated for this period of
the voltage signal as in a conventional DBD [10] (with light intensity three times smaller than
those observed during the streamer regime). They connect the edge of the active electrode to
the border of the hole, several of them being observed per hole.
<figure><img src="image_4.png" /><figcaption>Figure 3: Time evolution of the total current through the honeycomb discharge, current peak analysis and front view of the plasma discharge by short time exposure (200 µs) with ICCD camera. The voltage is 14 kVpp and the frequency 2000 Hz.</figcaption></figure>

The primary purpose of this new plasma discharge concerns about the active manipulation of turbulent jet flows. In order to identify the potential benefit of this system, it has been implemented in a circular open-air type wind-tunnel. A detailed description of a similar round free jet facility can be found in [15], with the only difference being the honeycomb plasma discharge placed at the exhaust of the convergent nozzle. The local velocity ($U_0$) at the center of one hole of the honeycomb plasma actuator is fixed at 10 m/s (Reynolds numbers of $Re_\phi=1970$). For a Reynolds number of about 2000, a round jet becomes turbulent very close to the exit as, for example, discussed and illustrated in [16]. At a distance of 30 mm on the centerline of the plasma device, pressure probe measurement indicates that the velocity ($U_{0\_core}$) is reduced to 7.5 m.s$^{-1}$.
Flow visualizations are performed using a short-pulsed Nd:YLF laser (Terra PIV 527-100-M) synchronized with a fast camera (Photron SA-Z). This optical diagnostic is operated at 1300 Hz. Dielectric oil (Ondina 915) is atomized into fine droplets to permit the laser light scattering (particles with mean diameter of 0.36 µm, measured by an aerosol spectrometer).

For each of the tests, a flow sequence of 5000 images has been recorded. The data processing on the raw images consists of computing the minimum intensity at each pixel location in all recordings and then of subtracting the minimum intensity from each images of the dataset. In a final stage, the averaged grey level of the 5000 images is computed. The images of the mean flow without plasma discharge for streamwise and cross-stream flow visualizations are presented in Figure 4. The mean flow visualizations indicate that the multi-jet quickly evolves as a single jet with a potential core whose length can be easily estimated. A virtual diameter, D, can also be defined from the flow images. The global jet at the exit of the multi-jet system corresponds to a virtual diameter D of about 28 mm (see Figure 4). In the initial region of the global jet, the individual jets can be observed up to a few centimeters and a significant deflection of the peripheral jets is revealed. Beyond this location, and in the inner part of the global jet, the individual jets interact with the others. In the present flow configuration, the jet column extends up to 165 mm. By considering the flow velocity $U_{0\_core}$, the Reynolds number based on the virtual diameter is about 14000. The potential core of the global jet extends up to 5.6D (with an uncertainty on its determination based on a grey level analysis of ±0.2D), a value in agreement with numerical and experimental results (~6D) for a single turbulent jet [17, 18]. Finally, the present investigation demonstrates that multi-jet configuration rapidly evolves into a single jet due to deflection of peripheral jets towards the centerline of the device as in [6].
<figure><img src="image_5.png" /><figcaption>Figure 4: Streamwise (including a zoomed view of the initial region) and cross-stream visualizations of the mean flow downstream of the multi-jet plasma device (the actuator is turned off). Dimensions are normalized by the virtual diameter D.</figcaption></figure>

The flow, in case of an actuation by the DBD plasma actuator at frequencies f<sub>AC</sub> of 100, 500 and 2000 Hz, is shown in Figure 5. The far-field turbulence cannot be resolved, but the reduction in the potential core length of the jet can be observed because of a higher spreading rate of the annular turbulent shear layer developing at the periphery of the jet. Here, a maximal reduction of 30% is observed for a plasma discharge operating at f<sub>AC</sub>=100 Hz (3.8D). The flow control mechanism cannot be elucidated by the present measurement system. However, for every hole, one can suspect that the produced ionic wind is mainly radially distributed and directed from the circular edges of the surface electrode to the center
of each hole as suggested by the intense light emission at the surface of the honeycomb.
Moreover, ionic wind can also occur along the inner surface of the holes. Here, the flow
produced by the discharges is supposed to promote a strong shearing in the initial region of
the single jet issuing from each hole. Because, the produced ionic wind linearly increases with
fAC [7, 8, 9, 10], a better control effectiveness is expected for an excitation at fAC=2000 Hz.
Here, it is shown that the larger influence on the flow is obtained for low frequency forcing
despite of a produced EHD force that is not maximized. One might expect to get much better
results at lower fAC (i.e., lower than 100 Hz). In fact, as for a round turbulent single jet [19],
the flow issuing from the present multi-jet system is highly sensitive to the frequency of the
periodic perturbations imposed in the initial region of the jet. In the present flow
configuration, the most effective forcing at fAC=100 Hz is close to the well-known best
periodic forcing that excites the instability of the jet column mode or preferred mode of a
turbulent jet [20]. Based on a forcing frequency of 100 Hz, the corresponding Strouhal
number StD (with $St_D = \frac{f \times D}{U_{0\_core}}$) is StD=0.37, a value in the 0.3-0.4 range reported many
times in the literature [21]. Unfortunately, the present flow diagnostic is only qualitative and
cannot provide detailed information regarding the flow control mechanism but the better
influence for periodic forcing at StD=0.37 clearly indicates that the control effect relates to an
excitation of the instability modes of the jet column. This suggests that an actuation with a
forcing frequency lower than 100 Hz would not be more effective.

For 2000 Hz, the ionic wind produced by the plasma significantly increases compared to the
one at 100 Hz [7, 8, 9, 10]. However, only a minor influence is observed here. As for a round
turbulent single jet, the flow issuing from the multi-jet system is highly sensitive to the
frequency of the periodic perturbations imposed in the initial region of the jet. The Strouhal
number $St_D$ (with $St_D = \frac{f \times D}{U_{0\_core}}$) corresponding to a physical forcing at 100 Hz is $St_D=0.37$, a forcing condition close to the well-known best periodic forcing that excites the instability of the jet column mode or preferred mode of a turbulent jet [19].

Knowing that the use of 'burst modulation' is an effective method for maximizing the amplitude of the imposed periodic perturbations [10], this type of actuation is conducted for a sinusoidal waveform with voltage amplitude of 14 kVpp and fAC frequency of 2000 Hz, the actuator being turned on and off by a gate function operated at frequency fBM (with 50% duty-cycle). The frequency fBM imposed by the burst modulation ranges here from 5 up to 400 Hz ($St_D=0.018$ to 1.49). The potential core lengths estimated by grey level analysis and a few visualizations of the mean flow are introduced in Figure 5. This figure confirms that the thickness of the shear layer, its spreading rate and thus the length of the potential core are modified accordingly to the forcing frequency introduced by the burst modulation. For the worst cases, the length of the potential core remains similar to the one reported for the natural flow. However, a minima is found at $St_D=0.37$ ($f_{BM}=100$ Hz) for which the maximal reduction of the potential core length reaches approximately 40% by comparison with the natural flow. This result supports a control mechanism scenario related to an excitation of the jet column mode where forcing influence is primarily related to the frequency of the imposed perturbation rather than to its amplitude. This further confirms that continuous forcing at fAC lower than 100 Hz would not be more effective. However, as already observed for the control of separated turbulent shear layer, the use of burst modulation instead of a continuous AC signal improves the influence of the discharge on the characteristics of the mean jet flow. Here, as for the continuous forcing mode, the reduction in the potential core is caused by a rapid growing of the annular shear layer and higher mixing rate is expected beyond the end of the potential core. The influence of the discharge on the isotropic character of the turbulent
flow field, on the Reynold stress components, on the integral length scales or on the decay
law along the centerline of the global jet has to be explored by quantitative measurements in
future investigations.

<figure><img src="image_6.png" /><figcaption>Figure 5: Streamwise fast visualizations of the flow downstream of the multi-jet plasma device and influence of $St_D$ on the length of the potential core for continuous forcing (flow forcing by $f_{AC}$) and burst modulation (flow forcing by $f_{BM}$).</figcaption></figure>

The present study has defined an original device dedicated to the enhancement of mixing mechanisms and the manipulation of turbulent flow. These preliminary measurements confirm the potential of this device. The reduction in the potential core length is significant (-40%) which suggests remarkable modifications in the far-field region. It is clearly shown that the plasma actuator can affect the spatial distribution of the mean flow fields. However, the question of an active manipulation of the turbulent scales and energy cascade by such actuator
is not solved here. A lot of studies by time-averaged and time-resolved measurements have to be conducted on this specific actuator. In particular, the response of a turbulent flow to a multi-frequency actuation will be investigated experimentally. The primary objective will be a precise control of the mixing-rates but it is expected that this device will help in emphasizing on certain length-scales of the turbulent flow to stimulate embedded small scale combustion processes.

## Acknowledgment

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). This work was partly supported by a Grant-in-Aid for Scientific Research (KAKENHI 24108005) from the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan. The authors are also grateful for Kyocera Co.

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