1 Cavity Flow Ignition and Flameholding in Ethylene-Air by a Repetitively Pulsed Nanosecond Discharge1

The results demonstrate that repetitive nanosecond pulse plasma cavity ignition of ethylene-air flows at moderate discharge pressures, P=0.2-0.25 atm, occurs via formation of multiple arc filaments in the fuel-air plasma. Plasma filamentation occurs in the nanosecond pulse discharge in a fuel-air mixture, although air plasma remains diffuse and low-temperature until the fuel was added. At these conditions, it is difficult to isolate thermal ignition in high-temperature filaments from nonequilibrium plasma chemical fuel oxidation and ignition due to radical generation in the plasma. After ignition occurs in the cavity, the flame couples out to the main flow, extinguishes, and is reignited within a few tens of milliseconds. Main flow ignition frequency at P=150 torr decreases from 30 Hz at u=35 m/sec to approximately 10 Hz at u=65 m/sec. It is demonstrated that that repetitive ignition occurs due to a slow rate of mixing between the main flow and the cavity, which results in combustion products remaining in the cavity for extended periods of time. To counter this effect, a small fraction of the main premixed fuel-air flow was injected into the cavity. At higher flow velocities, u=50-70 m/sec, this approach considerably reduced delay time between ignition events and increased burned fuel fraction. However, no continuous flame was sustained either in the cavity or in the main flow. Comparison of repetitive nanosecond pulse discharge ignitio