| Atmospheric pressure dielectric barrier discharge is widely used in ozone generation,surface modification,biological medicine and aerospace due to these advantages of no vacuum equipment,simple experimental device and large-scale production.In general,atmospheric pressure dielectric barrier discharge driven by AC power can present filamentary discharge.Many studies show that compared with filamentary discharge,diffuse discharge has better treatment effect and higher efficiency for many industrial applications.Therefore,how to improve discharge uniformity and stability has become one of the key issues concerned by many researchers.At present,it has been found that nanosecond pulsed discharge has better uniformity and stability due to its steep rising edge and narrow pulse width of pulse voltage.Applying airflow or magnetic field can improve discharge uniformity.Thus it is of great guiding value for the generation of diffuse discharge in atmospheric air to clarify the action mechanism of airflow and magnetic field on dielectric barrier discharge.The effects of airflow alone or magnetic field alone on dielectric barrier discharge has been studied,but the synergistic effect of airflow and magnetic field on dielectric barrier discharge are rarely reported.In this paper,discharge plasma in atmospheric air is generated by dielectric barrier discharge driven by nanosecond pulses power.The effect of airflow and the synergistic effect of airflow and magnetic field on discharge characteristics in atmospheric air are studied.The research works have been carried out:1.The effects of airflow on unipolar nanosecond pulsed traditional dielectric barrier discharge structure and three-electrodes dielectric barrier discharge structure with a metal mod in atmospheric air are studied by capturing discharge images,simulating electric field distribution and measuring electrical characteristics.Experimental results show that discharge uniformity are improved when airflow is introduced into discharge gap.The traditional dielectric barrier discharge in airflow is fine filamentary discharge.However,three-electrodes dielectric barrier discharge structure in airflow is diffuse discharge.The diffuse discharge is very stable and is not affected by airflow rates.It is found from continuous single period discharge images that relatively uniform discharge area starts from electrode edge and moves gradually toward downstream under the action of airflow.When relatively uniform discharge area moves to the vicinity of metal rod,a stable diffuse discharge is formed.Then relatively uniform discharge area continues to move towards downstream and eventually disappears downstream.According to the analysis,relatively uniform discharge area starts from electrode edge due to the "edge effect".For the generation of diffuse discharge,airflow plays a role of improving discharge uniformity.The metal rod plays a role of stabilizing discharge uniformity.Besides,diffuse discharge can be expanded by an array of metal rod.2.Comparative study on three-electrodes dielectric barrier discharge structure excited by unipolar versus bipolar pulses in 0-50 m/s airflow are carried out.The results indicate that unipolar and bipolar pulsed discharges in airflow are diffuse,however,the effects of voltage on discharges are different.For unipolar nanosecond pulsed discharge,only primary discharge occurs at the rising edge of pulse voltage.Diffuse discharge first appears near the anode at lower voltage.As the voltage increases,diffuse discharge develops from anode to cathode.For bipolar nanosecond pulsed discharge,the primary and secondary discharges occur at the rising edge and falling edge of pulse voltage,respectively.At lower voltage,diffuse discharge first appears around the metal rod,and gradually expands outward as the voltage increases.Using the spectral line intensity ratio technique,results present that the electric field near anode of unipolar nanosecond pulsed discharge is stronger than that near the cathode,and the electric field around the metal rod of bipolar nanosecond pulsed discharge is also stronger than that in the surrounding area.It is analyzed that the distributions of residual charges in unipolar and bipolar nanosecond pulsed discharges are different due to the absence or presence of the secondary discharge,in turn cause the differences of local electric field.Besides,as pulse repetition rate decreases,diffuse discharge driven by bipolar nanosecond pulses expands outwards.It is considered that when the pulse repetition rate is lower,the interval time between adjacent pulses is longer,the "memory effect" of space charges is weakened.The residual charges has longer time to spread toward the surrounding area,as a result,diffuse discharge area in lower pulse repetition rate is larger.3.The effects of airflow,magnetic field and the synergistic effect of airflow with magnetic field on bipolar nanosecond pulsed three-electrodes dielectric barrier discharge are studied by capturing discharge images with different exposure time and measuring the voltage-current waveforms,photocurrent waveforms and optical emission spectra.The results show that discharge is enhanced by introducing airflow at lower voltage.As airflow rate increases,surface discharge can transform into diffuse volume discharge.Discharge can also be enhanced by applying magnetic field,which is manifested as the enhancement of surface discharge.Under the synergistic effect of airflow and magnetic field,discharge is further enhanced,so that the transition from surface discharge to diffuse volume discharge occurs at a smaller airflow rate.Using the spectral line intensity ratio technique,result presents that the reduced electric field and electron temperature of nanosecond pulsed DBD under the synergistic effect of airflow and magnetic field are higher than that in airflow,which leads to more ionizations.Under the synergistic effect of airflow and magnetic field,diffuse discharge area increases as the pulse repetition rate decreases,which is ascribed to the higher peak voltage and weaken "memory effect" of space charges at lower PRF.It is concluded that diffuse discharge is easier to generate in airflow,magnetic field and lower PRF.4.Experimental study of micropore dielectric barrier discharge driven by unipolar nanosecond pulses under different airflow velocities and pulse repetition rates is carried out by capturing discharge images and measuring voltage-current waveforms and optical emission spectra.The results show that when the PRF is 1000 Hz,micropore dielectric barrier discharge in static air is diffuse discharge except the position of micropore.It is observed from the continuous single period discharge images that at the beginning of discharge,micropore dielectric barrier discharge is filamentary discharge.As the development of discharge,filamentary discharge at the micropore becomes stronger,which forces the surrounding filaments to move towards both sides of electrode.Many discharge filaments are overlapped and coupled together in a very small area.After dozens of pulse cycles,a stable diffuse discharge is formed.When the PRF is 100 Hz,micropore dielectric barrier discharge is also filamentary discharge at the beginning of discharge,but with the development of discharge,filamentary discharge at the micropore does not gradually strengthen,leading to no diffuse discharge after dozens or hundreds of cycles.When airflow is introduced,filamentary discharge at the micropore is extinguished,in the whole space,micropore dielectric barrier discharge is fine filamentary discharge.It is analyzed that diffuse discharge is ascribed to the overlapping of many discharge filaments.At lower PRF,the interval time of adjacent pulses is longer,the"memory effect" of space charges is weaken.The residual charges left by previous discharge has no effect on next pulsed discharge,which makes filamentary discharge at the micropore does not strengthen and discharge filaments do not overlap.Thus diffuse discharge is not formed.Similarly,micropore dielectric barrier discharge in airflow is not diffuse discharge,which is related to extinguished filamentary discharge at the micropore.The stronger filamentary discharge at the micropore,as a disturbance term,forces many discharge filaments to overlap together and forms diffuse discharge. |
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