**3.2. Discharge density characteristics under airflows**

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

**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

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

with a bandwidth of 1 GHz.

(e) *v* = 100 m/s. Exposure time is 1/1250 s.

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

250 Plasma Science and Technology - Basic Fundamentals and Modern Applications

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

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 dynamics in the air gap.

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 3.** Applied voltage and current waveforms for discharges under airflows.

with the airflow speed increasing from 0 to 20 m/s, follows with a rapid and big change from −32 to −47 A, and then becomes gradually relative stable as the airflow speed is bigger than 80 m/s. It is mostly concerned that the difference between second pulse current values is about

Repetitive Nanosecond Volume Discharges under Airflows

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

253

With the airflow added into the gap space, the volume discharge modes vary from filament to diffuse modes, and the induced two-stage pulse currents are also influenced. These behaviors may be attributed to the combined action of two effects. On one hand, the airflows breathe more species into the discharge volume from upstream to downstream, especially the excited metastable state, which causes a decrease in the number of initial electrons. On the other hand, the airflows remove accumulated charges on the surface of the dielectric, which is favorable

As the first pulse current is arising, a bigger voltage pulse with a 5 ns rise time, as well as a stronger electric field, is being imposed on the electrodes as one discharge driver. Since the pulse action time is too small, airflows in this period can be considered as "frozen" and without any flow mobility. Therefore, the first pulse currents nearly maintain consistency for different airflow speeds. However, at the second pulse current arising, accumulated particles near the electrodes induce a space electric field as another discharge driver. Since this driver always acts for several milliseconds, airflow mobility must be considered in this period. As indicated in **Figure 5**, the second pulse currents for quiescent air are bigger than that of airflows with speeds less than 40 m/s. Since airflows breathe more species, especially under excited metastable states, out of the discharge volume, the loss of heavy particles under the force of airflows is unfavorable for the development of discharge. Under airflows with a speed of more than 40 m/s, the second pulse currents are always bigger than that of quiescent air. It is concerned with the distribution change of the accumulated charges on the surface of the dielectrics by the force of airflows, which is favorable for the development of discharge. With the speed increasing to more than 80 m/s, the second pulse currents become

20A for different airflows.

**Figure 5.** Second pulse currents under airflows at different PRFs.

for the development of discharge.

**Figure 4.** First pulse currents under airflows at different PRFs.

from 0 to 20 m/s, the amplitude of the current rapidly increases from 33.1 to 36.3 A at a PRF of 3800 Hz and then slowly decreases to about 34 A. It goes across a reverse "V"s curve with a turning point at the flow velocity of 20 m/s. The change of first pulse current values imposed by airflows is 4 A. Importantly, there are some factors leading to measurement errors in this works, which mainly include the pressure instability of air supplied to the wind channel, the measurement errors of airflow speed with the pilot tube, and the high frequency noises concerning with nanosecond discharges.

It is clearly shown in **Figure 5** that the second pulse currents are influenced by the addition of airflows to discharge zone. The second pulse currents almost occur at a same time point. At a PRF of 1800 Hz, the second pulse current has a rapid but small change from −34 to −32 A

**Figure 5.** Second pulse currents under airflows at different PRFs.

from 0 to 20 m/s, the amplitude of the current rapidly increases from 33.1 to 36.3 A at a PRF of 3800 Hz and then slowly decreases to about 34 A. It goes across a reverse "V"s curve with a turning point at the flow velocity of 20 m/s. The change of first pulse current values imposed by airflows is 4 A. Importantly, there are some factors leading to measurement errors in this works, which mainly include the pressure instability of air supplied to the wind channel, the measurement errors of airflow speed with the pilot tube, and the high frequency noises

It is clearly shown in **Figure 5** that the second pulse currents are influenced by the addition of airflows to discharge zone. The second pulse currents almost occur at a same time point. At a PRF of 1800 Hz, the second pulse current has a rapid but small change from −34 to −32 A

concerning with nanosecond discharges.

**Figure 4.** First pulse currents under airflows at different PRFs.

**Figure 3.** Applied voltage and current waveforms for discharges under airflows.

252 Plasma Science and Technology - Basic Fundamentals and Modern Applications

with the airflow speed increasing from 0 to 20 m/s, follows with a rapid and big change from −32 to −47 A, and then becomes gradually relative stable as the airflow speed is bigger than 80 m/s. It is mostly concerned that the difference between second pulse current values is about 20A for different airflows.

With the airflow added into the gap space, the volume discharge modes vary from filament to diffuse modes, and the induced two-stage pulse currents are also influenced. These behaviors may be attributed to the combined action of two effects. On one hand, the airflows breathe more species into the discharge volume from upstream to downstream, especially the excited metastable state, which causes a decrease in the number of initial electrons. On the other hand, the airflows remove accumulated charges on the surface of the dielectric, which is favorable for the development of discharge.

As the first pulse current is arising, a bigger voltage pulse with a 5 ns rise time, as well as a stronger electric field, is being imposed on the electrodes as one discharge driver. Since the pulse action time is too small, airflows in this period can be considered as "frozen" and without any flow mobility. Therefore, the first pulse currents nearly maintain consistency for different airflow speeds. However, at the second pulse current arising, accumulated particles near the electrodes induce a space electric field as another discharge driver. Since this driver always acts for several milliseconds, airflow mobility must be considered in this period. As indicated in **Figure 5**, the second pulse currents for quiescent air are bigger than that of airflows with speeds less than 40 m/s. Since airflows breathe more species, especially under excited metastable states, out of the discharge volume, the loss of heavy particles under the force of airflows is unfavorable for the development of discharge. Under airflows with a speed of more than 40 m/s, the second pulse currents are always bigger than that of quiescent air. It is concerned with the distribution change of the accumulated charges on the surface of the dielectrics by the force of airflows, which is favorable for the development of discharge. With the speed increasing to more than 80 m/s, the second pulse currents become gradually relatively stable, which seems unaffected by airflows. The detailed effects under such high airflow speeds are very interesting and are also taken into considerations in the future works.

#### **3.3. Conditional boundary characteristics under airflows**

The boundary conditions of DBD volume discharges under airflows velocities are investigated. With the addition of airflows, no matter for any voltage, both the initiation and the extinction pulse repetition frequency need to be kept to a specified value to maintain the presence of discharge. Under a particular applied voltage and when the pulse repetitive frequency is rather low, any discharge does not occur; nevertheless, a current is detected due to the existence of the displacement electric field. With the growth of pulse repetitive frequency, the discharge can be established with an obvious change of discharge current.

The initiation and extinction boundaries are identified by making a subtraction of detected currents between discharge on and off, as shown in **Figures 6** and **7**. To improve the accuracy, the airflow and the applied voltage are kept constant as soon as possible while with an adjustable PRF, and all data are acquired with 10 times measuring. As the airflow velocity accelerates, the initiation and extinction value of PRFs is increased. In a still air, when the applied voltage reaches to 17 kV, the corresponding PRFs needed to reach about 1 kHz, which is almost an order of magnitude larger than the value under 27 kV. The lower is the applied voltage, the larger are the needed PRFs as well as the difference of PRFs. The initiation and extinction PRFs are almost inversely proportional to the applied voltage value under a certain airflow velocity. Once the airflow velocity attains 100 m/s under 17 kV, the discharge is not detected even though the PRF is up to the maximum value of applied value. As a special case of contrast, the initiation and extinction PRFs show a slight dependence on the velocities

under an applied voltage of 27 kV. It can be concluded that the boundary PRFs are decided by

Repetitive Nanosecond Volume Discharges under Airflows

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

255

In still air, mostly free electrons move rapidly toward the anode under the applied electric field. Many ions and metastable particles are produced by the complicated collisions between various particles in the discharge zone. Considering the interval time between two voltage pulses, only metastable particles with a lifetime of the order of several seconds can survive as residual seeds, while most ions have quenched. So, the increase of PRF can raise the concentration of residual seeds, which make the discharge breakdown more easily to occur. With the addition of the airflows, a large number of seeds are blown away from the discharge region, which makes a difficulty on the next pulsed discharge as well as leads to a growth of initiation PRFs. However, with the applied voltage growing, the free electrons obtain more energy from the electric field, and seed particles deposit on the dielectric plate. Owing to the existence of the viscous boundary layer, the seed particles are hardly moved away by the airflows. As a result under 27 kV, only a slight dependence of the initiation and extinction PRFs is observed with the airflow velocities.

the combined action between the applied voltages and the injected airflows.

**Figure 7.** Extinction frequencies versus airflow speeds.

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

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.

**downstream airflows**

**Figure 6.** Initiation frequencies versus airflow speeds.

**Figure 7.** Extinction frequencies versus airflow speeds.

gradually relatively stable, which seems unaffected by airflows. The detailed effects under such high airflow speeds are very interesting and are also taken into considerations in the

The boundary conditions of DBD volume discharges under airflows velocities are investigated. With the addition of airflows, no matter for any voltage, both the initiation and the extinction pulse repetition frequency need to be kept to a specified value to maintain the presence of discharge. Under a particular applied voltage and when the pulse repetitive frequency is rather low, any discharge does not occur; nevertheless, a current is detected due to the existence of the displacement electric field. With the growth of pulse repetitive frequency, the

The initiation and extinction boundaries are identified by making a subtraction of detected currents between discharge on and off, as shown in **Figures 6** and **7**. To improve the accuracy, the airflow and the applied voltage are kept constant as soon as possible while with an adjustable PRF, and all data are acquired with 10 times measuring. As the airflow velocity accelerates, the initiation and extinction value of PRFs is increased. In a still air, when the applied voltage reaches to 17 kV, the corresponding PRFs needed to reach about 1 kHz, which is almost an order of magnitude larger than the value under 27 kV. The lower is the applied voltage, the larger are the needed PRFs as well as the difference of PRFs. The initiation and extinction PRFs are almost inversely proportional to the applied voltage value under a certain airflow velocity. Once the airflow velocity attains 100 m/s under 17 kV, the discharge is not detected even though the PRF is up to the maximum value of applied value. As a special case of contrast, the initiation and extinction PRFs show a slight dependence on the velocities

**3.3. Conditional boundary characteristics under airflows**

254 Plasma Science and Technology - Basic Fundamentals and Modern Applications

**Figure 6.** Initiation frequencies versus airflow speeds.

discharge can be established with an obvious change of discharge current.

future works.

under an applied voltage of 27 kV. It can be concluded that the boundary PRFs are decided by the combined action between the applied voltages and the injected airflows.

In still air, mostly free electrons move rapidly toward the anode under the applied electric field. Many ions and metastable particles are produced by the complicated collisions between various particles in the discharge zone. Considering the interval time between two voltage pulses, only metastable particles with a lifetime of the order of several seconds can survive as residual seeds, while most ions have quenched. So, the increase of PRF can raise the concentration of residual seeds, which make the discharge breakdown more easily to occur. With the addition of the airflows, a large number of seeds are blown away from the discharge region, which makes a difficulty on the next pulsed discharge as well as leads to a growth of initiation PRFs. However, with the applied voltage growing, the free electrons obtain more energy from the electric field, and seed particles deposit on the dielectric plate. Owing to the existence of the viscous boundary layer, the seed particles are hardly moved away by the airflows. As a result under 27 kV, only a slight dependence of the initiation and extinction PRFs is observed with the airflow velocities.
