气流对微秒脉冲滑动放电特性的影响

Effect of flow rate on the characteristics of repetitive microsecond-pulse gliding discharges

脉冲电源驱动的滑动放电能够在大气压下产生高能量、高功率密度的低温等离子体.为了研究微秒脉冲电源在针-针电极结构中产生滑动放电的特征,本文采用电压幅值为0—30kV,脉冲宽度约8μs,脉冲重复频率为1—3000 Hz的微秒脉冲电源,通过测量电压、电流波形和拍摄放电图像,研究了微秒脉冲滑动放电的电特性.实验结果表明,随着施加电压的增加微秒脉冲滑动放电存在三种典型的放电模式:电晕放电、弥散放电和类滑动放电.不同放电模式的电压、电流波形和放电图像之间差异显著.脉冲重复频率对微秒脉冲滑动放电特性有影响,表现为当气体流量较小(2 L/min)时,类滑动放电的放电通道随着脉冲重复频率的增大逐渐集中,而当气体流量较大(16 L/min)时,类滑动放电的放电通道随着脉冲重复频率的增大逐渐分散.不同气流下重复频率对滑动放电特性的影响与放电中粒子的记忆效应和气流的状态有关.

Gliding discharges driven by microsecond-pulse power supply can generate non-thermal plasmas with high energy and high power density at atmospheric pressure. However, the flowing air significantly influences the characteristics of the microsecond-pulse gliding discharges in a repetitive mode. In this paper, in order to obtain the characteristics of the microsecond-pulse gliding discharges in a needle-to-needle gap, a microsecond-pulse power supply with an output voltage up to 30 kV, a pulse width ～8 μs, and a pulse repetition frequencies 1–3000 Hz is used to investigate the electrical characteristics of gliding discharges by analyzing the voltage-current waveforms and obtaining the discharge images. Experimental results show that there are three typical discharge modes in the microsecond-pulse gliding discharges as the applied voltage increases, i.e. corona discharge, diffuse discharge, and gliding-like discharge. Both voltage-current waveforms and the discharge images at different discharge modes have significantly different behaviors. Corona discharge only exists near the positive electrode with a small radius of curvature. Diffuse discharges behave as the overlapped plasma channels bridge the entire gap. The channel of diffuse discharge is full of gap, which starts from the positive electrode, spreads in all directions, and ends at the negative electrode. Gliding-like discharge behaves as a continuous spark channeling, showing a continuous spark, which is discharging strongly and influenced by flow rates. Furthermore, both pulse repetition frequency （PRF） and flow rate remarkably affects the characteristics of microsecond-pulse gliding discharges. When the flow rate is small （2 L/min）, the spark channels of gliding-like discharge gradually concentrate with the increase of the PRF. However, when the flow rate is larger （16 L/min）, the spark channels of gliding-like discharge behave dispersively when the PRF increases. In our opinion, different characteristics of microsecond-pulse gliding discharge at different flow rates are closely related to the memory effect of the residual particles in the discharges and the state of the air flow. When the flow rate is small （2 L/min）, the air flow is stable, and the discharge is generated in a laminar flow state. In this case, the memory effect of particles in repetitive microsecond-pulse gliding discharges dominates the formation of the discharges. These particles could enhance the electric field strength for the next pulse. Because the time interval between two pulses at high PRF is shorter than that at low PRF, there are fewer particles leaving the air gap at high PRF. Thus, memory effect is more significant at high PRF. As a result, the channel of spark discharge concentrates with the increase of the PRF. When the flow rate increases to 16 L/min, the calculated Reynolds number increases to 2864, indicating the transition from laminar state to turbulence state. The residual particles are more likely to escape from the gap. Thus, memory effect slightly affects the characteristics of the microsecond-pulse gliding discharges. In this case, the state of the air flow dominates the formation of the discharge. The spark channels spread towards the top in the direction of the gas flow, making the region of the spark channels gradually disperse as the PRF increases.