Hydroxyl Radical Kinetics in Repetitively Pulsed Hydrogen–Air Nanosecond Plasmas

Absolute hydroxyl radical (OH) concentration is determined in stoichiometric hydrogen-air mixtures at P = 54-94 torr and initial temperature of T = 100°C-200°C, which are both functions of time, after the application of a single approximately 25-ns-duration approximately 20-kV discharge pulse and 60 μs after the final pulse of a variable-length burst of pulses, using single-photon laser-induced fluorescence (LIF). Relative LIF signal levels are put on an absolute number density scale by means of calibration with a standard atmospheric-pressure near-adiabatic Hencken flat-flame burner. By obtaining OH LIF data in both the plasma and the flame and correcting for differences in the collisional quenching and vibrational energy transfer rates, absolute OH number density has been determined. For a single discharge pulse, the absolute OH temporal profile is found to rise rapidly during the initial ~0.1 ms after discharge initiation and decay relatively slowly, with a characteristic time scale of ~1 ms. In repetitive burst mode, the absolute OH number density is observed to rise rapidly during the first approximately ten pulses (0.25 ms) and then level off to a near steady-state plateau. In all cases, a large secondary rise in OH number density is also observed, which is clearly indicative of ignition, with ignition time ranging from 5 to 10 ms, for initial temperatures of 100 °C and 200°C and pressures in the range of 54-94 torr. Plasma kinetic modeling predictions capture this trend quantitatively, using both a full 22-hydrogen-air-chemical-reaction set and a reduced 9-reaction set.