Effects Of Airflow On Atmospheric Pressure Nanosecond Pulsed Dielectric Barrier Discharges

With the continuing study and deepening study to the discharge plasma, plasma of different properties can be generated by different exciting methods with diverse incentives at a large range of gas pressure and applied in a broad area such as flow control, enhancement of ignition and combustion, surface modification of materials, microelectronics technology, gas purification, sewage treatment and laser technology. The non-equilibrium plasma generated by atmospheric pressure dielectric barrier discharge (DBD) attracts more and more attentions because that this kind of discharge has many advantages, such as the more flexible discharge structure, higher production efficiency, lower gas temperature, lower running cost and no requirement of a vacuum chamber. Especially the DBD driven by nanosecond pulse has more advantages than that driven by traditional alternating current (AC) power supply, such as better uniformity, higher energy efficiency, higher electron density and higher chemical activity. Therefore, the atmospheric pressure nanosecond pulsed DBDs have a broader application prospects.In most applications of atmospheric pressure gas discharge, gas flow is inevitable. Because that the discharge is excited in gas flow, it is important to grasp the interaction between the discharge and the gas flow. In recent years, the effects of gas flow on discharge plasma have attracted more and more attention and some investigations have been conducted. It is found that gas flow can significantly change the characteristics of gas discharge, such as the discharge mode, discharge intensity, discharge stability and the breakdown characteristics, etc. However, the physical mechanism of the influence is still under discussion. In this article, we carry out some work about the atmospheric pressure nanosecond pulsed DBD and the effects of airflow on the discharge as follows:1. In still air, the stable atmospheric pressure nanosecond pulsed DBD is obtained, and the influence of the gap spacing and the pulse repetition frequency (PRF) on discharge uniformity, the breakdown voltage and the discharge intensity is studied, at the same time the mechanism of the nanosecond pulsed discharge is discussed. The experimental results show that, with the increased discharge gap spacing, the breakdown voltage is increased, the discharge intensity is decreased, and the discharge uniformity is decreased, which is in line with the Paschen's law of gas discharge; when the PRF is increased, the breakdown voltage is decreased, the discharge becomes more uniform, and the discharge intensity is almost not changed. It is indicated that the "memory effect" is stronger when the PRF is higher.2. With the plate-plate electrode arrangement and airflow rate ranging from 0 to 50 m/s, the effects of airflow on the nanosecond pulsed double dielectric barrier discharge (D-DBD) are investigated at different air gap spacings and PRFs, and the physical mechanism is discussed. Meanwhile, a large volume diffuse nanosecond pulsed discharge is obtained in airflow at atmospheric pressure. By measuring the applied voltage, current and photocurrent waveforms, the discharge process and the effect of airflow on the breakdown voltage and discharge intensity of the discharge are analyzed. By acquiring the discharge images of single pulse cycle, the effect of airflow on the discharge uniformity is analyzed. By acquiring the discharge images of single pulse cycle for a plurality of successive pulse periods, the evolution of the discharge channels is analyzed. By collecting the optical emission spectra and fitting the rotational temperature of nitrogen molecule, the effect of airflow on the gas temperature is analyzed. The discipline of the nanosecond pulsed DBD in airflow is summarized, which shows that the influence of airflow on the discharge at larger gap spacing and higher PRF is more obvious.3. By using plate-plate electrode arrangement, the effects of airflow on the nanosecond pulsed single dielectric barrier discharge (S-DBD) are comparatively studied with the cathode barrier (bare anode) case and the anode barrier (bare cathode) case. By measuring the applied voltage, current and photocurrent signals using high-voltage probe, current probe and photomultiplier, respectively, the discharge process in a high-voltage pulse, the variations of the breakdown voltage and discharge intensity with the increased airflow rate are analyzed. By acquiring the single pulse cycle discharge images at different conditions, the effect of airflow on the discharge uniformity is analyzed. By collecting the optical emission spectra, the effect of airflow on the plasma temperature is analyzed. By capturing the time-resolved discharge images and the schlieren images, the impact mechanism of airflow on nanosecond pulsed DBD is discussed.4. By using line-line electrode, the effects of airflow on atmospheric pressure nanosecond pulsed volume and surface DBD are comparatively studied. At higher PRF, with the increasing airflow rate, the breakdown voltage of the volume dielectric barrier discharge (VDBD) is increased and the discharge mode transforms from filamentary to diffuse discharge; however, for surface dielectric barrier discharge (SDBD), almost no effect is found when airflow is introduced. The main reason may be that the space charge and other particles produced in SDBD are accumulated on the surface of the nylon plate, the distribution and density of which is affected by the airflow slightly. While for the superposition of VDBD and SDBD, self-organized pattern will be produced in still air, and the discharge presents diffuse when the electrode distance is larger than 10 mm, additionally, the discharge intensity is increased with the increasing airflow rate at larger electrode distance.5. At the higher PRF and larger gap spacing, the effects of high-speed airflow (0-260 m/s) on nanosecond pulsed DBD are investigated. The results show that, at gap spacing of 7 mm and PRF at 1200 Hz, the discharge at the air inlet end of the gap transforms from diffuse to filamentary mode when the airflow rate is larger than 50 m/s. The boundary between the two kinds of discharge mode moves along the airflow when the airflow rate is increased, and the discharge presents totally filamentary mode when the airflow rate is larger than 140 m/s. The breakdown voltage is increased when airflow with small rate is introduced into the gap and keeps almost constant when the airflow rate is further increased to 260 m/s. The peak current of the primary discharge is decreased with the increasing airflow rate. When the PRF is 100 Hz, with the increasing airflow rate, the breakdown voltage is decreased and the peak current is increased slightly, but the discharge presents always filamentary mode. Additionally, by using the water electrode, the movement and the interaction of the discharge channels are observed from the face side.