**4. Atmospheric pressure volume discharges under upstream and downstream airflows**

The previous chapters demonstrate that the volume discharge mode is influenced by airflows and the discharge intensity decreases with the increase of airflow velocities. In this chapter, a type of discharge device with an upstream and downstream structure is provided to investigate the interactions between airflows and discharges. The upstream and downstream discharges under airflow include the generation and transport of charged particles.

where the pulse interval time is denoted as *tp*

are shown in **Figure 9**.

rally and spatially for 2000 cycles.

For *t <sup>f</sup> > tp*

For *t <sup>f</sup>* ≤ *tp*

high speed (*tf < t*

*p* ).

stream regions is *L*, the width of the electrode plate is *S*, and the airflow velocity is *v* (m/s).

With a pulse repetitive frequency of 1000 Hz, the applied voltage and current waveforms of the DBD volume discharge can be detected, which has been descripted in Section 3.1. By changing the air velocity, the discharge mode characteristics under different airflow velocities

For discharge in a static condition, as shown in **Figure 9(a)**, the upstream and downstream discharge exhibits a filament discharge mode. In the middle region of the electrode, the filaments exhibit a vertical distribution, and the path of the filaments close to the edge of the

from the original vertical state to the curved state along the direction of airflow, and some

and the paths of the discharge filaments at the edge of the metal electrodes are attracted by the central discharge region. The discharge filaments shrink to the inside region of discharge, and

The normalized emission spectra for the upstream and downstream discharges are provided as **Figure 10**. Due to the weak luminescence intensity of volume discharges, the spectrometer exposure time was set to 2 s, which means that the emission intensity was averaged tempo-

With increasing airflow velocity, the intensity of the upstream discharge emission spectrum decreases, which corresponds to a decrease in discharge intensity. As for downstream

> *<sup>f</sup> > t p*

), (c) medium speed (*tf = t*

*p* ), and (d)

 , the discharge filaments move along the direction of flow, the filament path changes

 , with increasing airflow velocity, most of the discharge area exhibits a uniform state,

**4.1. Discharge mode characteristics under upstream and downstream airflows**

electrode shows a curved state because of the edge effect of the metal electrode.

parts of the discharge area become uniform. The result is shown in **Figure 9(b)**.

the discharge region exhibits a pinched state, as shown in **Figure 9(c)** and **(d)**.

**Figure 9.** Discharge at different airflow velocities: (a) static air, (b) low speed (*t*

**4.2. Discharge spectrum characteristics under upstream and downstream airflows**

 , the distance between the upstream and down-

Repetitive Nanosecond Volume Discharges under Airflows

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

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**Figure 8.** Schematic of discharges under upstream and downstream constructure with different airflow velocities.

The generation of charged particles is related to external pulse excitation, and the transport of particles can be influenced by the effect of airflow. Furthermore, the upstream and downstream discharge in the airflow channel is a multi-time scale problem, including the transport time of airflow and the time of charged particle generation. The characteristic time of airflow is related to the movement path and the airflow velocity, and the generation of particles is adjusted by the repetition frequency of the discharge.

By matching the transport effect of airflow and the pre-ionization of charged particles, some of the particles generated by the upstream discharge are transported to the downstream region, and those particles play a pre-ionization role in the downstream discharge, which causes the enhancement of the downstream discharge intensity. For the purpose of further recognizing the relationship between upstream and downstream discharge under airflow, the behavior of the particles were analyzed as schematically shown in **Figure 8**. In the quiescent air, the discharge products always stay only in the upstream zone, as shown in **Figure 8(a)**; as the airflow is injected, the discharge products produced by upstream zone will be transported to the downstream zone, as shown in **Figure 8(b)** and **8(c)**; as the further increasing of airflow velocity, the discharge product will be blow out the downstream zone, as shown in **Figure 8d**.

The time range of air flow transport time *t f* is derived as Eq. (1). By properly controlling the pulse repetition interval time *t p* and the air flow transport time *t f*  , the charged particles could be transported from upstream region to the downstream region and enhance the downstream discharge intensity:

$$\mathcal{L}/\upsilon \ll = t\_{\uparrow} \ll = (\mathcal{L} + \mathcal{D}^\* \mathcal{S})/\upsilon \tag{1}$$

where the pulse interval time is denoted as *tp*  , the distance between the upstream and downstream regions is *L*, the width of the electrode plate is *S*, and the airflow velocity is *v* (m/s).
