Fast gas heating in a nitrogen-oxygen discharge plasma: I. Kinetic mechanism

A model of fast heating of nitrogen-oxygen mixtures excited by a gas discharge in a broad range of reduced electric fields E/N is presented. It is shown that in air at E/N <= 200 Td the main contribution to gas heating occurs due to dissociation reactions by electron impact of O2 molecules and due to processes of quenching of electronically excited N(2)(B(3)Pi g, C(3)Pi(u), a' (1)Sigma(-)(u)) molecules by oxygen and excited O((1)D) atoms by nitrogen. At E/N > 400 Td, dissociation reactions by electron impact of N(2) molecules are dominant as well as the processes involving charged particles. The fraction of discharge energy converted to fast gas heating does not exceed 40%. An analysis of the experimental data on fast air heating in discharges at high reduced electric fields E/N is given. It was shown that, in a broad range of reduced electric fields, a fixed fraction of discharge powe eta(E) spent on the excitation of electronic degrees of freedom, ionization and dissociation of molecules is converted to fast heating of nitrogen-oxygen mixtures. In air, the value of eta(E) is about 30 +/- 3%. The value of eta(E) diminishes with decreasing share of oxygen in a mixture. The significant role of heat release in the pooling reactions of N(2)(A(3)Sigma(+)(u), upsilon) molecules for fast gas heating in pure nitrogen and in nitrogen with small admixtures of oxygen is demonstrated. The simulation results agree with experimental data at E/N < 200 Td within the range of oxygen content delta = 0-20%.