Nanosecond plasma enhanced H₂/O₂/N₂ premixed flat flames

The effect of nanosecond pulsed plasma discharges on a laminar, lean (φ = 0.5) premixed H₂/O₂/N₂ flame is studied at low pressure (25 Torr) using a novel plasma-flame facility, non-intrusive laser diagnostics, and high-fidelity numerical simulations. Spatially-resolved, quantitative OH mole fraction and temperature measurements are performed with and without a burst of 200 discharge pulses using laser-induced fluorescence. Measured temperatures increase by ∼20% in both the pre-heat and post-flame zones with the use of the plasma discharge. In addition, OH mole fractions increase by as much as 500% in the preheat zone and by 40% in the post-flame gases. Simulations are conducted with a one-dimensional, multiscale, pulsed-discharge model with detailed plasma-combustion kinetics to develop additional insight into the complex plasma and flame interactions. Good agreement between measured and predicted OH and temperature profiles provides confidence in the model framework. The reduced electric field, E/N, varies inversely with number density during each pulse. A significant fraction of the input energy is consumed during electron impact ionization processes in the high E/N (700-1000 Td) regions downstream of the flame. Lower E/N values (100-700 Td) in the lower-temperature preheat zone promote efficient generation of radicals and excited species via electron impact processes, as well as by collisional quenching of excited states. The plasma action results in a significant increase in O and H atom density, with the peak values increasing by a factor of 6 and 4 respectively. In presence of the discharge, species and temperature profiles shift upstream (closer to the burner) by approximately 0.2 cm. Simulations show that electron impact dissociation and excitation processes in the plasma have a major impact on the observed temperature and species profile displacement, such that Joule heating alone does not account for the observed effects of the plasma.