Flame kernel development with radiofrequency oscillating plasma ignition

Abstract

In this paper, radio frequency (RF) oscillating plasma discharge is investigated under various initial pressure up to 5 atm in a constant-volume combustion chamber. The oscillating plasma is suppressed by elevated pressure both in lengths and branch numbers. The ignition performance under elevated background pressure is investigated, besides the results are compared with the spark events with similar ignition energy. Under ambient conditions, the oscillating plasma discharge generates multiple streamers with much longer length than a spark gap, resulting in a much bigger initial flame kernel. Under elevated background pressures, fewer streamers are observed with much smaller sizes, thus the advantage of oscillating plasma discharge over spark discharge is compromised. The prolonged duration of oscillating plasma discharge consistently demonstrates a positive impact on flame propagation speed, but neither prolonged duration nor enhanced discharge current has a noticeable impact on flame kernel growth for the spark ignition cases. Both oscillating plasma and spark are used to treat non-combustible propane-air mixtures under background pressure from 1 to 5 atm.

Advances and Challenges in Combustion Control of Radio Frequency Oscillating Plasma Discharge

“…The direct images in Figure 6 show the oscillating plasma discharge in Nitrogen under various background pressures [16]. Theoretically, the oscillating plasma discharge under vacuum conditions tends to generate relatively homogeneous plasma.…”

“…An in-house built RF oscillating plasma discharge control system has been used to generate oscillating plasma discharge with adjustable discharge frequency, voltage, and duration [16,22]. The system consists of a singleelectrode igniter, a resonator coil, an RF transformer, and power drive electronics.…”

“…The voltage supply on the primary side is kept consistent in this paper as 60 V. The discharge current and voltage are measured with a Pearson 411 current probe and Tektronix P6015 high voltage probe, respectively. The simplified diagram of the in-house built RF oscillating plasma control system and the electrical waveforms using the logarithmic scale are presented in Figure 3 [16].…”

“…The residence time of OH is typically below 100 μs, as observed in the nanosecond non-equilibrium plasma ignition experiments conducted by Wu [14], and repetitive discharge is often used for nanosecond pulsed non-equilibrium plasma ignition [15]. In comparison, the RF oscillating plasma ignition can continuously produce non-equilibrium plasma in milliseconds duration to compensate for the decay of previously formed radicals until the success of ignition [16]. Third, as the effect of an electric field on flame propagation has been extensively reported in the past, post-ignition flame propagation can be accelerated due to the ionic wind effect near the ignitor [17].…”

“…Figure3. Experimental circuit and electrical waveforms of the oscillating plasma ignition system[16].…”