Kinetics of low-temperature hydrogen oxidation and ignition by repetitively pulsed nonequilibrium plasmas

Two photon and single photon laser induced fluorescence of atomic oxygen and the OH radical, respectively, complemented with UV ICCD imaging, is used to study low-temperature plasma assisted fuel oxidation kinetics in repetitive nanosecond pulse discharges in hydrogen-air mixtures at 40 Torr pressure. Air and premixed fuel-air mixtures are excited by a burst of high-voltage nanosecond pulses at a 10 kHz or 40 kHz pulse repetition rate and burst repetition rate of 10 Hz. The number of pulses in the burst is varied from one pulse to a few hundred pulses. Time-resolved relative OH concentration measurements are in good agreement with predictions of a new hydrogen-air plasma chemistry model which incorporates non-equilibrium plasma discharge processes, low temperature H2 - air chemistry, non-empirical scaling of nanosecond discharge pulse energy coupled to the plasma, and quasi-onedimensional conduction heat transfer. Kinetic model prediction of a significant reduction in O atom concentration in the presence of hydrogen has been confirmed qualitatively by the experimental data. Kinetic sensitivity analysis shows that in room temperature hydrogen-air discharges, OH formation and decay, as well as the initial heating rate are controlled by the three process sequence: O + HO₂ + M → HO ₂ + M, O + HO₂ → OH + O₂ and OH + H ₂ → H₂O + H, essentially without radical chain branching. At intermediate temperatures, 500 - 600 K, significant chain branching, with associated additional energy release, occurs in reactions of O with HO₂, as well as in O + H₂ → OH + H reaction. Both chain branching and net exothermic heat release in plasma chemical reactions becomes more pronounced at higher temperatures, eventually resulting in ignition. Sensitivity analysis also shows that generation of radicals in the plasma is key to low-temperature plasma chemical fuel oxidation and associate heat release, while ignition is primarily controlled by the well known chain branching sequence O + H₂ → OH + H and H + O₂ → OH + O.