*4.3.1. Various airflow velocities*

The comparison of discharge currents between upstream and downstream discharges is shown in **Figure 12**. The current peak is used to illustrate the discharge intensity under different flow velocities.

For *t <sup>f</sup> > t p* , the amplitude of the upstream discharge decreases from 55 to 53 A, and the downstream discharge fluctuates between 38 and 36 A with increased airflow speed, and for the condition of *t <sup>f</sup> = t p*  , the amplitude of the upstream discharge gradually decreases from 53 to 45 A. For the downstream discharge, the amplitude exhibits an opposite trend, changing from 36 to 46 A. With increasing airflow velocity, the amplitude of the upstream discharge current continues to decrease, and the increasing rate of the downstream discharge current tends to be zero.

### *4.3.2. Various PRFs*

**Figure 10.** Normalized intensity discharge at different velocities: (a) upstream and (b) downstream.

concentration of charged particles in the downstream region increased. At *t f* < *t*

The dependence of rotational temperature of the upstream and downstream discharges on airflow rates is shown in **Figure 11**. It can be seen that the rotational temperature is approximately 390 K when the discharge is excited in the static air. The rotational temperature decreases to approximately 320 K as the airflow velocity increases gradually to 80 m/s. The low gas temperature may be attributed to two reasons. One is that the duty cycle of the pulse power supply is low; another is that more energy is delivered to the energetic electrons. The gas temperature is substantially the same in the upstream and downstream regions. This result shows that the gas temperature under airflow is not the key factor that causes the difference

> *f* and *t p*

plays a dominant role in the upstream and downstream discharges. The particles generated by the upstream discharge can be transported to the downstream discharge region. The combined effect of flow transport and pre-ionization of charged particles enhances the downstream discharge.

corresponding spectral intensity gradually decreases.

258 Plasma Science and Technology - Basic Fundamentals and Modern Applications

between the upstream and downstream discharge intensity.

Under the condition of reasonable matching between *t*

 , the intensity of the emission spectrum increases, which proves that the

*p* and *t <sup>f</sup>* > *t p*  , the

 , the mass transfer effect of airflow

discharge, when *tf* = *tp*

The pulse frequencies were adjusted to *f* = 2 kHz and *f* = 200 Hz, and the results are shown in **Figure 13**. When the pulse frequency is 2 kHz, for *t <sup>f</sup> > tp*  , the current amplitudes of the upstream discharge gradually decrease from 59 to 57 A, and the downstream discharge fluctuates between 45 and 48 A. When the airflow velocity increases until *t <sup>f</sup> = tp*  , the amplitude of the upstream discharge gradually decreases from 57 to 49 A. The amplitude of the downstream discharge exhibits an opposite trend and increases from 48 to 56 A.

When the discharge frequency was adjusted to 200 Hz, the pulse interval time *t p* was 5 ms. Because of the low pulse repetition frequency, the charged particles in the space completed the diffusion and recombination process within the pulse interval time, causing the reduction in discharge intensity. In the meantime, at a lower airflow velocity (*t <sup>f</sup>* > *tp* ), the amplitude of

process of increasing airflow velocity until *t*

the gas velocity reaching 50 m/s (i.e., *t*

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

*f1 = t p*

discharge currents are almost enhanced with the increased gas flow velocity.

**Figure 14.** Discharge current amplitude at different distances: (a) *L* = 20 mm and (b) *L* = 50 mm.

**4.4. Interaction between upstream and downstream discharges**

concentration of positive charges declines in the space.

motion paths is shown in **Figure 15**, and the analysis is as follows.

region were blown to the downstream discharge region. The peak value of the upstream discharge current decayed gradually from 50 to 45A, and the corresponding downstream discharge current amplitude gradually increased to 43 A. When the electrode spacing was adjusted to 50 mm, with

region could be transported to the downstream discharge region. The upstream and downstream

According to the process of pulse discharge, the electrons move to the anode under the effect of pulse voltage, and the ions remain nearly static in the time scale of nanoseconds; the discharge current is formed by electron migration. The electrons cover the surface of the dielectric, and a large amount of ions are accumulated in the discharge space. At the falling edge of the voltage pulse, when the applied voltage potential is lower than the potential generated by the positive charge space, some electrons move toward the cathode and form a reverse current. During the movement, the electrons are neutralized with some positive ions, and the

Because of the different time scales of pulse discharge and airflow, the airflow cannot directly affect the process of pulse discharge. The rising time of the pulse discharge takes place in a few nanoseconds, and the duration of current is in the tens nanoseconds. For particles, the diffusion and recombination of neutral and charged particles take place on a micro- and millisecond time scale, and the lifetime of the metastable particles produced by the discharge in short gaps is thought to have a significant high concentration of some hundreds of microseconds to milliseconds. Therefore, the airflow can affect only the discharge distribution of particles during the pulse interval times.

When the airflow is applied in the discharge space, the distribution and movement of particles in the discharge region could be influenced by the airflow; a schematic of the particle

Metastable particles are transported by the drag force of the airflow and move from the upstream discharge area to the downstream discharge area, which particles play the pre-ionization role to

 , particles generated in the upstream discharge

Repetitive Nanosecond Volume Discharges under Airflows

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261

), the particles generated in the upstream discharge

**Figure 12.** Discharge current amplitude of the upstream and downstream regions.

**Figure 13.** Discharge current amplitude at different PRFs: (a) PRF = 2 kHz and (b) PRF = 200 Hz.

the discharge current in the upstream region attenuated from 44 to 35 A, and the downstream discharge current weakened from 38 to 30 A.

#### *4.3.3. Various distances between upstream and downstream*

The conditions of *L* = 20 mm and *L* = 50 mm were studied. The experimental results for *f* = 1 kHz are shown in **Figure 14**. When the discharge electrode spacing was adjusted to *L* = 20 mm, for *t <sup>f</sup>* > *t p*  , the charged particles generated by the upstream discharge could not transit to the downstream discharge region. The peak value of the upstream discharge current decayed from 54 to 50 A, and the amplitude of the downstream discharge current fluctuated between 37 and 39 A. During the

**Figure 14.** Discharge current amplitude at different distances: (a) *L* = 20 mm and (b) *L* = 50 mm.

process of increasing airflow velocity until *t <sup>f</sup>* = *t p*  , particles generated in the upstream discharge region were blown to the downstream discharge region. The peak value of the upstream discharge current decayed gradually from 50 to 45A, and the corresponding downstream discharge current amplitude gradually increased to 43 A. When the electrode spacing was adjusted to 50 mm, with the gas velocity reaching 50 m/s (i.e., *t f1 = t p* ), the particles generated in the upstream discharge region could be transported to the downstream discharge region. The upstream and downstream discharge currents are almost enhanced with the increased gas flow velocity.

#### **4.4. Interaction between upstream and downstream discharges**

the discharge current in the upstream region attenuated from 44 to 35 A, and the downstream

**Figure 13.** Discharge current amplitude at different PRFs: (a) PRF = 2 kHz and (b) PRF = 200 Hz.

The conditions of *L* = 20 mm and *L* = 50 mm were studied. The experimental results for *f* = 1 kHz are shown in **Figure 14**. When the discharge electrode spacing was adjusted to *L* = 20 mm, for *t*

the charged particles generated by the upstream discharge could not transit to the downstream discharge region. The peak value of the upstream discharge current decayed from 54 to 50 A, and the amplitude of the downstream discharge current fluctuated between 37 and 39 A. During the

*<sup>f</sup>* > *t p*  ,

discharge current weakened from 38 to 30 A.

*4.3.3. Various distances between upstream and downstream*

**Figure 12.** Discharge current amplitude of the upstream and downstream regions.

260 Plasma Science and Technology - Basic Fundamentals and Modern Applications

According to the process of pulse discharge, the electrons move to the anode under the effect of pulse voltage, and the ions remain nearly static in the time scale of nanoseconds; the discharge current is formed by electron migration. The electrons cover the surface of the dielectric, and a large amount of ions are accumulated in the discharge space. At the falling edge of the voltage pulse, when the applied voltage potential is lower than the potential generated by the positive charge space, some electrons move toward the cathode and form a reverse current. During the movement, the electrons are neutralized with some positive ions, and the concentration of positive charges declines in the space.

Because of the different time scales of pulse discharge and airflow, the airflow cannot directly affect the process of pulse discharge. The rising time of the pulse discharge takes place in a few nanoseconds, and the duration of current is in the tens nanoseconds. For particles, the diffusion and recombination of neutral and charged particles take place on a micro- and millisecond time scale, and the lifetime of the metastable particles produced by the discharge in short gaps is thought to have a significant high concentration of some hundreds of microseconds to milliseconds. Therefore, the airflow can affect only the discharge distribution of particles during the pulse interval times.

When the airflow is applied in the discharge space, the distribution and movement of particles in the discharge region could be influenced by the airflow; a schematic of the particle motion paths is shown in **Figure 15**, and the analysis is as follows.

Metastable particles are transported by the drag force of the airflow and move from the upstream discharge area to the downstream discharge area, which particles play the pre-ionization role to

**Author details**

**References**

2011. pp. 55-96

Jingfeng Tang, Liqiu Wei\* and Daren Yu

\*Address all correspondence to: weiliqiu@hit.edu.cn

Harbin Institute of Technology, Harbin, People's Republic of China

sped flow control, AIAA 2012 3137. Reston: AIAA; 2012

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Applied Physics. 2007;**40**(3):605-636

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**Figure 15.** Schematic of particle motion paths.

the downstream discharge. For the charged ions in space, the ions move along the airflow direction mainly under the effect of the drag force of airflow and the electrostatic force from the surface electrons [15, 16]. According to the literature, during a pulse interval time of approximately 1 ms, the electric field strength between the surface electrons and the space charges is several kV/cm, and the ion density *ni* is approximately 10<sup>12</sup> cm−<sup>3</sup> . The estimated electric field force of the ions is approximately 1.6 × 10−<sup>2</sup> N. In terms of the bulk flow, the conservation of momentum of the bulk fluid is described by the Navier-Stokes equation. The air density is approximately 1.29 kg/m<sup>3</sup> , the volume of the discharge area is approximately 5 mm × 30 mm × 30 mm, and the pressure difference between inlet and outlet is nearly 0.5% of the atmospheric pressure. The estimated flow field force to the ions is approximately 1.2 × 10−<sup>1</sup> N, and the ratio of the flow force and the electrical field force is approximately 8:1. For the ions in the gap, the distance of ions moved along the y-axis is approximately 2 mm during a pulse interval of 1 ms, which is less than the gap distance. The estimation illustrates that the ions are blown to the downstream area during the pulse interval time under the coupling force of the airflow and electric field.

Furthermore, the space charges have different velocities at different regions in the airflow channel. In the region near the dielectric, the space charges have a lower velocity because of the drag force in the boundary layer and the larger electrostatic force from the surface charges. However, the space charges in the middle region of the gap have the fastest velocities and the smaller electrostatic force, and the drag force from the airflow plays a dominant role compared with the electrostatic force of the surface electrons. The curved discharge channel under the low-speed condition also illustrates that the effect of the drag force from the mainstream area is stronger than the electrostatic force between the surface electrons and the space charges; this phenomenon is also discussed in the literature [15, 22]. Therefore, both the estimation results and the discharge phenomenon illustrate that some particles, such as metastable particles and ions, could transit to the downstream discharge area during the pulse interval time; these particles act as seed electrons to the downstream discharge, and the downstream discharge intensity is enhanced.
