**3.1. Discharge mode characteristics under airflows**

of energy and momentum occurs between plasmas and airflows. Therefore, the atmospheric pressure discharge plasma is actually in a typical multi-field couple system, and the coupling

As the couple interaction between plasmas and airflows, the plasmas macroscopically exhibit a fluid state property, the distribution of plasma particles is influenced by the heat and mass transfer from airflows, and discharge modes and discharge intensities are also changed. As a simultaneous inverse role, the energy release by discharge can cause impulsive interference and thermal effect on airflows, and a change of airflow field distribution can be generated. The airflow transport effect determines the distribution of uncharged particles, and such distribution provides an ionization condition, thus affecting the discharge breakdown. The transfer of heat and mass from airflows provides a new factor on the plasma diffusion, and the discharge energy dissipation and discharge plasmas also provide an active control of airflow distribution. To be sure, discharge plasmas under airflows have undergone a fundamental change. With the presence of airflows, discharge plasmas are more dominant with a strong interaction

The couple between plasmas and airflows has been considered as the interaction between discharge plasma dynamics and gas dynamics, from a view of time and space scales. For discharge plasmas, the topical time scales include as follows: the establishment of electric field and the generation of plasma (nanoseconds), the dissipation and the quenching of plasma (nanoseconds and/or microseconds), and the life of charged particles (seconds and/or hours). For airflows, the typical time scale is mainly determined by airflow conditions, such as the transport time of airflow (microseconds and/or milliseconds) and the convection heat transfer time (milliseconds and/or seconds) [2]. For discharge plasmas, the typical space scale is about a small size (micrometers and/or millimeters), such as the mean free path and the thickness of plasma sheath. For airflows, the typical space scale is about a big size (millimeters and/ or meters), such as the thickness of boundary layer [3]. Under such couple with a multi-time and a multi-space scale, it is necessary to recognize the phenomenon and mechanism of dis-

Plasma flow control is a type of active flow control technology based on discharge plasma technologies, which is advantageous of little power, quick response, and perfect actuation. Russians, Americans, and other research groups [4–20] have done an in-depth study on plasma flow control, as well as the interaction between plasmas and airflows, to improve the aerodynamic characteristics and promote the scientific basis for efficiency. Discharge plasmas applied to flow control mainly include surface discharges [4–14] and volume discharges [15–20]. Surface discharges are used to flow separation control by DBD discharges with a momentum exchange to neutral airflows, which generate a complex pattern of quasi-planar and spherical compression wave [4–8], as well as which are related to a strong demand on stable discharges within the flow boundary layer [7–14]. Volume discharges are applied in MHD flow control to achieve the acceleration and deceleration of airflow, which require

interaction between airflows and discharges is of extensive concerns [1].

248 Plasma Science and Technology - Basic Fundamentals and Modern Applications

between plasmas and airflows.

charges under airflows.

**2. Background for discharges under airflows**

The experimental system is shown in **Figure 1**, which includes an air wind tunnel driven by a fan, a nanosecond pulse generator, discharge system, and measurement system. By changing the speed of the fan, the flow velocity at the end of the wind tunnel can be adjusted with a maximum value of up to 200 m/s. A pitot tube is used to measure the flow velocity of the airflow. The plate-plate electrodes are set in a horizontal and parallel manner. The two electrodes are composed of stainless steel plates with a thickness of 2 mm. The electrode edges were fully polished in order to avoid the point discharge occurring at the electrode edges. The two dielectrics are made of mica with a permittivity *εr* = 6 and a thickness of 1 mm. The discharge system is installed at the downstream of subsonic wind tunnel exit with the flow direction perpendicular to the electrode surface. The applied voltage has repetitive pulses with a fixed pulse width of 5 ns and a maximum amplitude of 50 kV with a rise time of 5 ns, corresponding to the frequency ranged from 100 Hz to 3.5 kHz, respectively. The voltage and

cannot be easily observed with the naked eye in the discharge volume. The diffuse and homogeneous discharge mostly occurs at the middle region, and the filament discharge occurs mainly at the inlet region and partly at exit region of the channel. The filament channels on the two sides may be connected to the electric field concentration occurred at electrode edges. With the increasing of airflow speeds, the volume discharge modes vary from filament to diffuse modes. Further improving the airflow speed to 50 m/s, as shown in **Figure 2(d)**, a diffuse discharge also

Repetitive Nanosecond Volume Discharges under Airflows

http://dx.doi.org/10.5772/intechopen.81919

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occurs at the middle discharge region, as well as with a reduction of luminous intensity.

discharge becomes unstable and almost fades away at a PRF less than 220 Hz.

the two electrodes, and the second pulse currents arise in our experiments.

**3.2. Discharge density characteristics under airflows**

dynamics in the air gap.

Moreover, several excitation conditions are selected for the detailed investigation of airflow effects, in which the applied voltage amplitude is chosen from 10 to 30 kV; PRF is selected from 100 to 3800 Hz, and airflow speed is changed from 0 to 200 m/s, respectively. In a quiescent air or under a low speed, it presents filament and inhomogeneous violet discharges in a large volume, as likely shown in **Figure 2(a)**. At such airflow speeds, the number of the bright filaments is slightly increased with only increasing the applied voltage, as a process of the pinch of several filaments. However, the change of discharges cannot be clearly observed with only changing PRFs. With a speed less than 30 m/s, there are less filaments in the volume, and the change of discharge luminance is relatively small with changing the applied voltages and PRFs. With the airflow speed increasing higher, for example, a speed of 50 m/s as shown in **Figure 2(d)**, the relative uniform discharge in a large volume is promoted. Importantly, the

As a nanosecond pulse is applied to a plate-plate gap, initial electrons are accelerated by the electric field, and an avalanche process is followed under the electron multiplication of collision ionization. Fast electrons with high energy can run away from the head of the critical avalanche and dominate in the subsequent development of the critical avalanche. When the head of the critical avalanche reaches near to the anode, a discharge bridge is built up between the two dielectrics as well as electrodes, and a discharge current runs through the gap space, which can be represented as the first pulse current. When the applied pulse is gone, more electrons accumulate in the dielectric surface near the anode, interact with interaction of accumulated positive particles near the cathode, and build up a strong electric field imposed on the discharge space. Such space electric field will induce another avalanche process between

With an applied pulse voltage of 18 kV and a pulse repetitive frequency of 1800 Hz, the applied voltage and current waveforms under different airflows are shown in **Figure 3**. It is provided that the plate-plate DBD discharge is characterized by a series of two-stage pulse currents. In contrast with the unipolar pulse of applied voltage, the discharge current behaves bipolar and consists of both positive and negative pulses. The discharge current distributes irregularly, which can be attributed to the random nature of breakdown and complicated

There is a series of two-stage pulse currents for each nanosecond pulsed discharge. Even considering the existence of discharge delays, the first pulse currents always occur with a same breakdown voltage. Furthermore, it can be drawn from **Figure 4** that the first pulse currents increase first and then reduce with airflow speeds. With the flow speed increasing

**Figure 1.** Schematic of experimental setup.

**Figure 2.** Discharges at different airflow speeds. (a) Flow speed *v* = 0 m/s, (b) *v* = 10 m/s, (c) *v* = 35 m/s, (d) *v* = 50 m/s, and (e) *v* = 100 m/s. Exposure time is 1/1250 s.

current are measured by using a high-voltage probe and a Rogowski coil with a response time of less than 1 ns. The voltage and current signals are recorded by a digitalized oscilloscope with a bandwidth of 1 GHz.

The typical luminous discharge images under airflows are shown in **Figure 2**. In the quiescent air (i.e., the flow velocity is 0 m/s), a multichannel and inhomogeneous violet discharge is present in the discharge volume. The discharge filaments are straight, and the filament foots are randomly and extensively distributed on the dielectric surface, as shown in **Figure 2(a)**. Increasing the flow velocity to 10 m/s, the number of the bright filaments is slightly reduced, but the change of glow component cannot be clearly observed, as shown in **Figure 2(b)**. When the flow velocity varies from 10 to 20 m/s, the filament number is gradually reduced, and the change of discharge luminance and distribution are relatively small. When the airflow with a speed of 35 m/s is introduced into the volume discharge, as shown in **Figure 2(c)**, interestingly, a diffuse discharge in a large volume is promoted. The unsteady nature of the filamentary part of the discharge cannot be easily observed with the naked eye in the discharge volume. The diffuse and homogeneous discharge mostly occurs at the middle region, and the filament discharge occurs mainly at the inlet region and partly at exit region of the channel. The filament channels on the two sides may be connected to the electric field concentration occurred at electrode edges. With the increasing of airflow speeds, the volume discharge modes vary from filament to diffuse modes. Further improving the airflow speed to 50 m/s, as shown in **Figure 2(d)**, a diffuse discharge also occurs at the middle discharge region, as well as with a reduction of luminous intensity.

Moreover, several excitation conditions are selected for the detailed investigation of airflow effects, in which the applied voltage amplitude is chosen from 10 to 30 kV; PRF is selected from 100 to 3800 Hz, and airflow speed is changed from 0 to 200 m/s, respectively. In a quiescent air or under a low speed, it presents filament and inhomogeneous violet discharges in a large volume, as likely shown in **Figure 2(a)**. At such airflow speeds, the number of the bright filaments is slightly increased with only increasing the applied voltage, as a process of the pinch of several filaments. However, the change of discharges cannot be clearly observed with only changing PRFs. With a speed less than 30 m/s, there are less filaments in the volume, and the change of discharge luminance is relatively small with changing the applied voltages and PRFs. With the airflow speed increasing higher, for example, a speed of 50 m/s as shown in **Figure 2(d)**, the relative uniform discharge in a large volume is promoted. Importantly, the discharge becomes unstable and almost fades away at a PRF less than 220 Hz.
