Characterizing nanosecond-pulsed high-frequency and DC Arc discharges in terms of ignition probability

Ignition probability and flame kernel development were measured in methane–air mixtures at equivalence ratios (ϕ) of 0.47 – 0.55 with a flow velocity of 5 m/s using a nanosecond-pulsed high-frequency discharge (NPHFD) exciter and a DC arc discharge (conventional) exciter. The ignition performance of both excitation systems was compared across multiple average power conditions, namely 110, 180, and 375 W. Data was collected at two additional average power conditions (35 and 53 W) for the NPHFD exciter. A novel metric to define a successful ignition event was demonstrated in that ignition probability was based on an area growth rate criterion that must be met within a given amount of time. In total, four minimum growth rate values (0.001, 10, 20, and 30 mm²/ms) and four characteristic time intervals (6 – 9, 9 – 12, 12 – 15, and 15 – 18 ms) were utilized to distinguish between an ignition success and failure. At the most lenient definition of a successful ignition event (lowest growth rate criterion in the longest characteristic time interval), the conventional exciter outperformed the NPHFD exciter in terms of ignition probability at leaner equivalence ratios. The poor performance of the NPHFD exciter at lean conditions was due to localized quenching of the flame kernel early in its development from the entrainment of cold reactants between pulses and the onset of jetting motion that further split the reacting region. As the growth rate threshold for a successful ignition event was increased, the ignition probability curves for all test conditions shifted to higher equivalence ratios, indicating that a more reactive mixture was needed to sufficiently grow the kernels to meet the criteria. However, the ignition probability curves for the NPHFD exciter were less sensitive overall to increasing the growth rate threshold for a successful ignition event due to its ability to generate flame kernels with rapid growth rates at higher equivalence ratios. Decreasing the characteristic time interval did not have a large impact on ignition probability for either exciter until the minimum growth rate criterion was increased to 20 mm²/ms, upon which the conventional exciter exhibited a greater drop in ignition probability at higher equivalence ratios.