Evolution of H-TALIF Signal After Nanosecond Discharges in Methane-Air Mixtures

Femtosecond two-photon-absorption laser-induced fluorescence (TALIF) was used to measure the spatial and temporal evolution of atomic hydrogen (H) after single and multiple nanosecond discharges. Experiments were conducted at ambient pressure and temperature in mixtures of 4% and 2% methane by volume in air. Results show that the spatial and temporal distribution of the H-TALIF signal is primarily influenced by the hydrodynamics related to the discharge-induced flow field. However, questions remain regarding the decrease in the H signal observed in the inter-electrode region at earlier times, typically 1~2 μs, following the ~10-ns discharge. In comparing the 4% to the 2% methane-air mixture results, it was found that reducing the fuel concentration lowered the H-TALIF signal at each delay time but did not impact the spatial distribution, an expected result since methane acted as the source of H in all these cases. Finally, the H-TALIF signal was recorded after the second, fifth, and tenth discharge pulse in a burst of discharges operating at a 200 kHz repetition rate. A decrement in signal in the inter-electrode region was observed at early times after each pulse in the sequence. The spatial extent of the H-TALIF signal grew with each pulse in the train, predominantly due to the hydrodynamics related to consecutive nanosecond discharges and discharge-induced jetting motion. Combined with computational models, these results can provide valuable insights to the plasma-assisted combustion community regarding the effectiveness of nanosecond discharges for ignition and flame holding.