**1. Introduction**

Recently, atmospheric pressure discharge plasma has been considered for many applications, such as airflow control [1–20], material modification [21–31], air purification [31–49], and so on. With the difference from low-pressure discharge plasmas, atmospheric pressure discharge plasmas are usually operated under an open environment, and the collision between ions and electrons is very frequent. Such strong elastic and/or inelastic collision produces a significant chemical effect and corresponding thermal effect. In addition, air as the media of the discharge process is usually in a flow state, such with an airflow state with different velocities. The various particles in the plasmas always perform a macro-overall movement, and the exchange

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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

high-intensity volume discharges under airflows condition, as well as which essentially need

Repetitive Nanosecond Volume Discharges under Airflows

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Plasma material processing is promising for material modification and its industrial applications. Efficient discharges are important demands to realize the surface modification and functional structure construction [21–31]. The moderate power density and the uniform energy distribution are beneficial to material modification. However, under a gas flowing condition, discharges can easily transit from a stable state into an unstable state, which can cause a disaster to the industrial application of plasma material processing. In order to obtain a uniform and stable discharge, discharges under airflows are employed to excite plasmas. Work groups [29] illustrate the surface modification of polyimide films by the discharge under airflow, it is found that the plasma at a homogeneous DBD is evenly distributed than at a filamentary DBD, and by the more efficient introduction of atomic oxygen to the PP surface

Plasma air purification attracts widespread attention in recent years, mainly related with corona discharges under an air supply channel. Plasma air purification has been developed in many applications, including electrostatic precipitation [32–40], industrial gas exhaust treatment [41–44], and indoor air purification [45–49]. Work groups [35] illustrate that the electric power and the energy loss of corona discharges highly depend on airflow velocities, and

In addition, discharges under airflows are of complicated technical challenges, and the mech-

In order to get a better understanding of airflow effects on volume discharge characteristics, this chapter presents the study of nanosecond pulse volume discharges in high-speed

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

discharge enhancement methods under MHD airflow environment [15–19].

corona discharge modes are also related to airflow conditions.

anism and its characteristics need a deep and wide investigation.

**3.1. Discharge mode characteristics under airflows**

**3. Atmospheric pressure volume discharges under airflows**

in the case of homogeneous DBD.

airflows.

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 between plasmas and airflows.

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 discharges under airflows.
