Elevated OH production from NPHFD and its effect on ignition

Ignition in flowing mixtures of methane and air at 100 kPa and 295 K initial temperature was studied using a nanosecond-pulsed high-frequency discharge ignition source. Simultaneous time-resolved 50-kHz schlieren imaging and planar laser-induced fluorescence (PLIF) measurements of OH generation in the discharge and developing ignition kernel were performed across a range of pulse repetition frequencies (PRF) in the methane-air flow. The impact of PRF on the quantity and distribution of OH and the connection to ignitability was examined. Results indicated that a significant volume of OH was generated in the discharge, and the magnitude of the said volume was a strong function of PRF. The intensity of the OH-PLIF signal during and shortly after the discharge was elevated for PRF ≥ 10 kHz, indicating that both the volume and concentration of OH were built up at high PRF. This accumulation of OH radicals in the inter-electrode region was directly correlated with high ignition probability, with higher PRF leading to faster OH accumulation in a step-wise fashion. In cases in which the PRF was below a certain threshold (< 10 kHz), OH accumulation did not occur. This condition, along with lower discharge temperatures, was believed to be directly responsible for reduced ignition probability.