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

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

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

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

*f*

and the air flow transport time *t*

be transported from upstream region to the downstream region and enhance the downstream

is derived as Eq. (1). By properly controlling the

*<sup>f</sup>* < = (*L* + 2<sup>∗</sup> *S*)/*v* (1)

 , the charged particles could

*f*

adjusted by the repetition frequency of the discharge.

256 Plasma Science and Technology - Basic Fundamentals and Modern Applications

will be blow out the downstream zone, as shown in **Figure 8d**.

*p*

The time range of air flow transport time *t*

*L*/*v* < = *t*

pulse repetition interval time *t*

discharge intensity:

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 temporally and spatially for 2000 cycles.

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

**Figure 9.** Discharge at different airflow velocities: (a) static air, (b) low speed (*t <sup>f</sup> > t p* ), (c) medium speed (*tf = t p* ), and (d) high speed (*tf < t p* ).

**4.3. Discharge difference between upstream and downstream region**

in **Figure 13**. When the pulse frequency is 2 kHz, for *t*

tuates between 45 and 48 A. When the airflow velocity increases until *t*

stream discharge exhibits an opposite trend and increases from 48 to 56 A.

in discharge intensity. In the meantime, at a lower airflow velocity (*t*

When the discharge frequency was adjusted to 200 Hz, the pulse interval time *t*

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 dif-

stream discharge fluctuates between 38 and 36 A with increased airflow speed, and for the

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

The pulse frequencies were adjusted to *f* = 2 kHz and *f* = 200 Hz, and the results are shown

upstream discharge gradually decrease from 59 to 57 A, and the downstream discharge fluc-

of the upstream discharge gradually decreases from 57 to 49 A. The amplitude of the down-

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

, the amplitude of the upstream discharge decreases from 55 to 53 A, and the down-

 , the amplitude of the upstream discharge gradually decreases from 53 to

*<sup>f</sup> > tp*

 , the current amplitudes of the

 , the amplitude

was 5 ms.

*p*

), the amplitude of

*<sup>f</sup> = tp*

Repetitive Nanosecond Volume Discharges under Airflows

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

259

*<sup>f</sup>* > *tp*

*4.3.1. Various airflow velocities*

**Figure 11.** Rotational temperature at different velocities.

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

ferent flow velocities.

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

be zero.

condition of *t*

*4.3.2. Various PRFs*

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

discharge, when *tf* = *tp*  , the intensity of the emission spectrum increases, which proves that the concentration of charged particles in the downstream region increased. At *t f* < *t p* and *t <sup>f</sup>* > *t p*  , the corresponding spectral intensity gradually decreases.

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 between the upstream and downstream discharge intensity.

Under the condition of reasonable matching between *t f* and *t p*  , the mass transfer effect of airflow 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.

**Figure 11.** Rotational temperature at different velocities.

### **4.3. Discharge difference between upstream and downstream region**
