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

Abstract

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 s, and a pulse repetition frequencies 1 Hz 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.