Ionization behind Shock Waves in Nitrogen-Oxygen Mixtures

It is shown theoretically and experimentally that the initial oxygen concentration has a strong influence on the ionization rate in nitrogen-oxygen mixtures. The theoretical results are obtained by solving the chemical rate equations together with the hydrodynamical conservation equations. Ion density profiles are measured in a shock tube using various electrostatic probes. The impurity level in the test gas due to outgassing of the shock tube walls is estimated to be less than 1 ppm. the present paper ionization rates in binary gas mixtures of trogen and oxygen are studied for shock Mach numbers between Ms = 7 and Ms = 17. Various recent papers on ionization rates in air1'2 have shown that the predominant carrier of positive charges in the Mach number range considered here is the NO+ ion which is obtained mainly from the reaction N + O±+NO+ +e The purpose of the present paper is to investigate the influence of the initial oxygen concentration on the ionization relaxation time and on the equilibrium values of the ion density, the temperature and the mass density. Theoretical ion density profiles for the Mach number range of Ms = 7 to Ms = 20 are compared with shock tube measurements for various nitrogen-oxygen mixtures.